compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

   1 // ignore-tidy-filelength
   2
   3 use super::{
   4     DefIdOrName, FindExprBySpan, Obligation, ObligationCause, ObligationCauseCode,
   5     PredicateObligation,
   6 };
   7
   8 use crate::infer::InferCtxt;
   9 use crate::traits::{NormalizeExt, ObligationCtxt};
  10
  11 use hir::def::CtorOf;
  12 use rustc_data_structures::fx::FxHashSet;
  13 use rustc_data_structures::stack::ensure_sufficient_stack;
  14 use rustc_errors::{
  15     error_code, pluralize, struct_span_err, Applicability, Diagnostic, DiagnosticBuilder,
  16     ErrorGuaranteed, MultiSpan, Style,
  17 };
  18 use rustc_hir as hir;
  19 use rustc_hir::def::DefKind;
  20 use rustc_hir::def_id::DefId;
  21 use rustc_hir::intravisit::Visitor;
  22 use rustc_hir::lang_items::LangItem;
  23 use rustc_hir::{AsyncGeneratorKind, GeneratorKind, Node};
  24 use rustc_hir::{Expr, HirId};
  25 use rustc_infer::infer::error_reporting::TypeErrCtxt;
  26 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  27 use rustc_infer::infer::{InferOk, LateBoundRegionConversionTime};
  28 use rustc_middle::hir::map;
  29 use rustc_middle::ty::error::TypeError::{self, Sorts};
  30 use rustc_middle::ty::relate::TypeRelation;
  31 use rustc_middle::ty::{
  32     self, suggest_arbitrary_trait_bound, suggest_constraining_type_param, AdtKind, DefIdTree,
  33     GeneratorDiagnosticData, GeneratorInteriorTypeCause, Infer, InferTy, InternalSubsts,
  34     IsSuggestable, ToPredicate, Ty, TyCtxt, TypeAndMut, TypeFoldable, TypeFolder,
  35     TypeSuperFoldable, TypeVisitable, TypeckResults,
  36 };
  37 use rustc_span::def_id::LocalDefId;
  38 use rustc_span::symbol::{sym, Ident, Symbol};
  39 use rustc_span::{BytePos, DesugaringKind, ExpnKind, MacroKind, Span, DUMMY_SP};
  40 use rustc_target::spec::abi;
  41 use std::ops::Deref;
  42
  43 use super::method_chain::CollectAllMismatches;
  44 use super::InferCtxtPrivExt;
  45 use crate::infer::InferCtxtExt as _;
  46 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
  47 use rustc_middle::ty::print::{with_forced_trimmed_paths, with_no_trimmed_paths};
  48
  49 #[derive(Debug)]
  50 pub enum GeneratorInteriorOrUpvar {
  51     // span of interior type
  52     Interior(Span, Option<(Option<Span>, Span, Option<hir::HirId>, Option<Span>)>),
  53     // span of upvar
  54     Upvar(Span),
  55 }
  56
  57 // This type provides a uniform interface to retrieve data on generators, whether it originated from
  58 // the local crate being compiled or from a foreign crate.
  59 #[derive(Debug)]
  60 pub enum GeneratorData<'tcx, 'a> {
  61     Local(&'a TypeckResults<'tcx>),
  62     Foreign(&'tcx GeneratorDiagnosticData<'tcx>),
  63 }
  64
  65 impl<'tcx, 'a> GeneratorData<'tcx, 'a> {
  66     // Try to get information about variables captured by the generator that matches a type we are
  67     // looking for with `ty_matches` function. We uses it to find upvar which causes a failure to
  68     // meet an obligation
  69     fn try_get_upvar_span<F>(
  70         &self,
  71         infer_context: &InferCtxt<'tcx>,
  72         generator_did: DefId,
  73         ty_matches: F,
  74     ) -> Option<GeneratorInteriorOrUpvar>
  75     where
  76         F: Fn(ty::Binder<'tcx, Ty<'tcx>>) -> bool,
  77     {
  78         match self {
  79             GeneratorData::Local(typeck_results) => {
  80                 infer_context.tcx.upvars_mentioned(generator_did).and_then(|upvars| {
  81                     upvars.iter().find_map(|(upvar_id, upvar)| {
  82                         let upvar_ty = typeck_results.node_type(*upvar_id);
  83                         let upvar_ty = infer_context.resolve_vars_if_possible(upvar_ty);
  84                         if ty_matches(ty::Binder::dummy(upvar_ty)) {
  85                             Some(GeneratorInteriorOrUpvar::Upvar(upvar.span))
  86                         } else {
  87                             None
  88                         }
  89                     })
  90                 })
  91             }
  92             GeneratorData::Foreign(_) => None,
  93         }
  94     }
  95
  96     // Try to get the span of a type being awaited on that matches the type we are looking with the
  97     // `ty_matches` function. We uses it to find awaited type which causes a failure to meet an
  98     // obligation
  99     fn get_from_await_ty<F>(
 100         &self,
 101         visitor: AwaitsVisitor,
 102         hir: map::Map<'tcx>,
 103         ty_matches: F,
 104     ) -> Option<Span>
 105     where
 106         F: Fn(ty::Binder<'tcx, Ty<'tcx>>) -> bool,
 107     {
 108         match self {
 109             GeneratorData::Local(typeck_results) => visitor
 110                 .awaits
 111                 .into_iter()
 112                 .map(|id| hir.expect_expr(id))
 113                 .find(|await_expr| {
 114                     ty_matches(ty::Binder::dummy(typeck_results.expr_ty_adjusted(&await_expr)))
 115                 })
 116                 .map(|expr| expr.span),
 117             GeneratorData::Foreign(generator_diagnostic_data) => visitor
 118                 .awaits
 119                 .into_iter()
 120                 .map(|id| hir.expect_expr(id))
 121                 .find(|await_expr| {
 122                     ty_matches(ty::Binder::dummy(
 123                         generator_diagnostic_data
 124                             .adjustments
 125                             .get(&await_expr.hir_id.local_id)
 126                             .map_or::<&[ty::adjustment::Adjustment<'tcx>], _>(&[], |a| &a[..])
 127                             .last()
 128                             .map_or_else::<Ty<'tcx>, _, _>(
 129                                 || {
 130                                     generator_diagnostic_data
 131                                         .nodes_types
 132                                         .get(&await_expr.hir_id.local_id)
 133                                         .cloned()
 134                                         .unwrap_or_else(|| {
 135                                             bug!(
 136                                                 "node_type: no type for node `{}`",
 137                                                 ty::tls::with(|tcx| tcx
 138                                                     .hir()
 139                                                     .node_to_string(await_expr.hir_id))
 140                                             )
 141                                         })
 142                                 },
 143                                 |adj| adj.target,
 144                             ),
 145                     ))
 146                 })
 147                 .map(|expr| expr.span),
 148         }
 149     }
 150
 151     /// Get the type, expression, span and optional scope span of all types
 152     /// that are live across the yield of this generator
 153     fn get_generator_interior_types(
 154         &self,
 155     ) -> ty::Binder<'tcx, &[GeneratorInteriorTypeCause<'tcx>]> {
 156         match self {
 157             GeneratorData::Local(typeck_result) => {
 158                 typeck_result.generator_interior_types.as_deref()
 159             }
 160             GeneratorData::Foreign(generator_diagnostic_data) => {
 161                 generator_diagnostic_data.generator_interior_types.as_deref()
 162             }
 163         }
 164     }
 165
 166     // Used to get the source of the data, note we don't have as much information for generators
 167     // originated from foreign crates
 168     fn is_foreign(&self) -> bool {
 169         match self {
 170             GeneratorData::Local(_) => false,
 171             GeneratorData::Foreign(_) => true,
 172         }
 173     }
 174 }
 175
 176 // This trait is public to expose the diagnostics methods to clippy.
 177 pub trait TypeErrCtxtExt<'tcx> {
 178     fn suggest_restricting_param_bound(
 179         &self,
 180         err: &mut Diagnostic,
 181         trait_pred: ty::PolyTraitPredicate<'tcx>,
 182         associated_item: Option<(&'static str, Ty<'tcx>)>,
 183         body_id: LocalDefId,
 184     );
 185
 186     fn suggest_dereferences(
 187         &self,
 188         obligation: &PredicateObligation<'tcx>,
 189         err: &mut Diagnostic,
 190         trait_pred: ty::PolyTraitPredicate<'tcx>,
 191     ) -> bool;
 192
 193     fn get_closure_name(&self, def_id: DefId, err: &mut Diagnostic, msg: &str) -> Option<Symbol>;
 194
 195     fn suggest_fn_call(
 196         &self,
 197         obligation: &PredicateObligation<'tcx>,
 198         err: &mut Diagnostic,
 199         trait_pred: ty::PolyTraitPredicate<'tcx>,
 200     ) -> bool;
 201
 202     fn check_for_binding_assigned_block_without_tail_expression(
 203         &self,
 204         obligation: &PredicateObligation<'tcx>,
 205         err: &mut Diagnostic,
 206         trait_pred: ty::PolyTraitPredicate<'tcx>,
 207     );
 208
 209     fn suggest_add_clone_to_arg(
 210         &self,
 211         obligation: &PredicateObligation<'tcx>,
 212         err: &mut Diagnostic,
 213         trait_pred: ty::PolyTraitPredicate<'tcx>,
 214     ) -> bool;
 215
 216     fn extract_callable_info(
 217         &self,
 218         hir_id: HirId,
 219         param_env: ty::ParamEnv<'tcx>,
 220         found: Ty<'tcx>,
 221     ) -> Option<(DefIdOrName, Ty<'tcx>, Vec<Ty<'tcx>>)>;
 222
 223     fn suggest_add_reference_to_arg(
 224         &self,
 225         obligation: &PredicateObligation<'tcx>,
 226         err: &mut Diagnostic,
 227         trait_pred: ty::PolyTraitPredicate<'tcx>,
 228         has_custom_message: bool,
 229     ) -> bool;
 230
 231     fn suggest_borrowing_for_object_cast(
 232         &self,
 233         err: &mut Diagnostic,
 234         obligation: &PredicateObligation<'tcx>,
 235         self_ty: Ty<'tcx>,
 236         object_ty: Ty<'tcx>,
 237     );
 238
 239     fn suggest_remove_reference(
 240         &self,
 241         obligation: &PredicateObligation<'tcx>,
 242         err: &mut Diagnostic,
 243         trait_pred: ty::PolyTraitPredicate<'tcx>,
 244     ) -> bool;
 245
 246     fn suggest_remove_await(&self, obligation: &PredicateObligation<'tcx>, err: &mut Diagnostic);
 247
 248     fn suggest_change_mut(
 249         &self,
 250         obligation: &PredicateObligation<'tcx>,
 251         err: &mut Diagnostic,
 252         trait_pred: ty::PolyTraitPredicate<'tcx>,
 253     );
 254
 255     fn suggest_semicolon_removal(
 256         &self,
 257         obligation: &PredicateObligation<'tcx>,
 258         err: &mut Diagnostic,
 259         span: Span,
 260         trait_pred: ty::PolyTraitPredicate<'tcx>,
 261     ) -> bool;
 262
 263     fn return_type_span(&self, obligation: &PredicateObligation<'tcx>) -> Option<Span>;
 264
 265     fn suggest_impl_trait(
 266         &self,
 267         err: &mut Diagnostic,
 268         span: Span,
 269         obligation: &PredicateObligation<'tcx>,
 270         trait_pred: ty::PolyTraitPredicate<'tcx>,
 271     ) -> bool;
 272
 273     fn point_at_returns_when_relevant(
 274         &self,
 275         err: &mut DiagnosticBuilder<'tcx, ErrorGuaranteed>,
 276         obligation: &PredicateObligation<'tcx>,
 277     );
 278
 279     fn report_closure_arg_mismatch(
 280         &self,
 281         span: Span,
 282         found_span: Option<Span>,
 283         found: ty::PolyTraitRef<'tcx>,
 284         expected: ty::PolyTraitRef<'tcx>,
 285         cause: &ObligationCauseCode<'tcx>,
 286         found_node: Option<Node<'_>>,
 287         param_env: ty::ParamEnv<'tcx>,
 288     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed>;
 289
 290     fn note_conflicting_closure_bounds(
 291         &self,
 292         cause: &ObligationCauseCode<'tcx>,
 293         err: &mut DiagnosticBuilder<'tcx, ErrorGuaranteed>,
 294     );
 295
 296     fn suggest_fully_qualified_path(
 297         &self,
 298         err: &mut Diagnostic,
 299         item_def_id: DefId,
 300         span: Span,
 301         trait_ref: DefId,
 302     );
 303
 304     fn maybe_note_obligation_cause_for_async_await(
 305         &self,
 306         err: &mut Diagnostic,
 307         obligation: &PredicateObligation<'tcx>,
 308     ) -> bool;
 309
 310     fn note_obligation_cause_for_async_await(
 311         &self,
 312         err: &mut Diagnostic,
 313         interior_or_upvar_span: GeneratorInteriorOrUpvar,
 314         is_async: bool,
 315         outer_generator: Option<DefId>,
 316         trait_pred: ty::TraitPredicate<'tcx>,
 317         target_ty: Ty<'tcx>,
 318         typeck_results: Option<&ty::TypeckResults<'tcx>>,
 319         obligation: &PredicateObligation<'tcx>,
 320         next_code: Option<&ObligationCauseCode<'tcx>>,
 321     );
 322
 323     fn note_obligation_cause_code<T>(
 324         &self,
 325         err: &mut Diagnostic,
 326         predicate: T,
 327         param_env: ty::ParamEnv<'tcx>,
 328         cause_code: &ObligationCauseCode<'tcx>,
 329         obligated_types: &mut Vec<Ty<'tcx>>,
 330         seen_requirements: &mut FxHashSet<DefId>,
 331     ) where
 332         T: ToPredicate<'tcx>;
 333
 334     /// Suggest to await before try: future? => future.await?
 335     fn suggest_await_before_try(
 336         &self,
 337         err: &mut Diagnostic,
 338         obligation: &PredicateObligation<'tcx>,
 339         trait_pred: ty::PolyTraitPredicate<'tcx>,
 340         span: Span,
 341     );
 342
 343     fn suggest_floating_point_literal(
 344         &self,
 345         obligation: &PredicateObligation<'tcx>,
 346         err: &mut Diagnostic,
 347         trait_ref: &ty::PolyTraitRef<'tcx>,
 348     );
 349
 350     fn suggest_derive(
 351         &self,
 352         obligation: &PredicateObligation<'tcx>,
 353         err: &mut Diagnostic,
 354         trait_pred: ty::PolyTraitPredicate<'tcx>,
 355     );
 356
 357     fn suggest_dereferencing_index(
 358         &self,
 359         obligation: &PredicateObligation<'tcx>,
 360         err: &mut Diagnostic,
 361         trait_pred: ty::PolyTraitPredicate<'tcx>,
 362     );
 363     fn note_function_argument_obligation(
 364         &self,
 365         arg_hir_id: HirId,
 366         err: &mut Diagnostic,
 367         parent_code: &ObligationCauseCode<'tcx>,
 368         param_env: ty::ParamEnv<'tcx>,
 369         predicate: ty::Predicate<'tcx>,
 370         call_hir_id: HirId,
 371     );
 372     fn point_at_chain(
 373         &self,
 374         expr: &hir::Expr<'_>,
 375         typeck_results: &TypeckResults<'tcx>,
 376         type_diffs: Vec<TypeError<'tcx>>,
 377         param_env: ty::ParamEnv<'tcx>,
 378         err: &mut Diagnostic,
 379     );
 380     fn probe_assoc_types_at_expr(
 381         &self,
 382         type_diffs: &[TypeError<'tcx>],
 383         span: Span,
 384         prev_ty: Ty<'tcx>,
 385         body_id: hir::HirId,
 386         param_env: ty::ParamEnv<'tcx>,
 387     ) -> Vec<Option<(Span, (DefId, Ty<'tcx>))>>;
 388 }
 389
 390 fn predicate_constraint(generics: &hir::Generics<'_>, pred: ty::Predicate<'_>) -> (Span, String) {
 391     (
 392         generics.tail_span_for_predicate_suggestion(),
 393         format!("{} {}", generics.add_where_or_trailing_comma(), pred),
 394     )
 395 }
 396
 397 /// Type parameter needs more bounds. The trivial case is `T` `where T: Bound`, but
 398 /// it can also be an `impl Trait` param that needs to be decomposed to a type
 399 /// param for cleaner code.
 400 fn suggest_restriction<'tcx>(
 401     tcx: TyCtxt<'tcx>,
 402     item_id: LocalDefId,
 403     hir_generics: &hir::Generics<'tcx>,
 404     msg: &str,
 405     err: &mut Diagnostic,
 406     fn_sig: Option<&hir::FnSig<'_>>,
 407     projection: Option<&ty::AliasTy<'_>>,
 408     trait_pred: ty::PolyTraitPredicate<'tcx>,
 409     // When we are dealing with a trait, `super_traits` will be `Some`:
 410     // Given `trait T: A + B + C {}`
 411     //              -  ^^^^^^^^^ GenericBounds
 412     //              |
 413     //              &Ident
 414     super_traits: Option<(&Ident, &hir::GenericBounds<'_>)>,
 415 ) {
 416     if hir_generics.where_clause_span.from_expansion()
 417         || hir_generics.where_clause_span.desugaring_kind().is_some()
 418     {
 419         return;
 420     }
 421     let generics = tcx.generics_of(item_id);
 422     // Given `fn foo(t: impl Trait)` where `Trait` requires assoc type `A`...
 423     if let Some((param, bound_str, fn_sig)) =
 424         fn_sig.zip(projection).and_then(|(sig, p)| match p.self_ty().kind() {
 425             // Shenanigans to get the `Trait` from the `impl Trait`.
 426             ty::Param(param) => {
 427                 let param_def = generics.type_param(param, tcx);
 428                 if param_def.kind.is_synthetic() {
 429                     let bound_str =
 430                         param_def.name.as_str().strip_prefix("impl ")?.trim_start().to_string();
 431                     return Some((param_def, bound_str, sig));
 432                 }
 433                 None
 434             }
 435             _ => None,
 436         })
 437     {
 438         let type_param_name = hir_generics.params.next_type_param_name(Some(&bound_str));
 439         let trait_pred = trait_pred.fold_with(&mut ReplaceImplTraitFolder {
 440             tcx,
 441             param,
 442             replace_ty: ty::ParamTy::new(generics.count() as u32, Symbol::intern(&type_param_name))
 443                 .to_ty(tcx),
 444         });
 445         if !trait_pred.is_suggestable(tcx, false) {
 446             return;
 447         }
 448         // We know we have an `impl Trait` that doesn't satisfy a required projection.
 449
 450         // Find all of the occurrences of `impl Trait` for `Trait` in the function arguments'
 451         // types. There should be at least one, but there might be *more* than one. In that
 452         // case we could just ignore it and try to identify which one needs the restriction,
 453         // but instead we choose to suggest replacing all instances of `impl Trait` with `T`
 454         // where `T: Trait`.
 455         let mut ty_spans = vec![];
 456         for input in fn_sig.decl.inputs {
 457             ReplaceImplTraitVisitor { ty_spans: &mut ty_spans, param_did: param.def_id }
 458                 .visit_ty(input);
 459         }
 460         // The type param `T: Trait` we will suggest to introduce.
 461         let type_param = format!("{}: {}", type_param_name, bound_str);
 462
 463         let mut sugg = vec![
 464             if let Some(span) = hir_generics.span_for_param_suggestion() {
 465                 (span, format!(", {}", type_param))
 466             } else {
 467                 (hir_generics.span, format!("<{}>", type_param))
 468             },
 469             // `fn foo(t: impl Trait)`
 470             //                       ^ suggest `where <T as Trait>::A: Bound`
 471             predicate_constraint(hir_generics, trait_pred.to_predicate(tcx)),
 472         ];
 473         sugg.extend(ty_spans.into_iter().map(|s| (s, type_param_name.to_string())));
 474
 475         // Suggest `fn foo<T: Trait>(t: T) where <T as Trait>::A: Bound`.
 476         // FIXME: once `#![feature(associated_type_bounds)]` is stabilized, we should suggest
 477         // `fn foo(t: impl Trait<A: Bound>)` instead.
 478         err.multipart_suggestion(
 479             "introduce a type parameter with a trait bound instead of using `impl Trait`",
 480             sugg,
 481             Applicability::MaybeIncorrect,
 482         );
 483     } else {
 484         if !trait_pred.is_suggestable(tcx, false) {
 485             return;
 486         }
 487         // Trivial case: `T` needs an extra bound: `T: Bound`.
 488         let (sp, suggestion) = match (
 489             hir_generics
 490                 .params
 491                 .iter()
 492                 .find(|p| !matches!(p.kind, hir::GenericParamKind::Type { synthetic: true, .. })),
 493             super_traits,
 494         ) {
 495             (_, None) => predicate_constraint(hir_generics, trait_pred.to_predicate(tcx)),
 496             (None, Some((ident, []))) => (
 497                 ident.span.shrink_to_hi(),
 498                 format!(": {}", trait_pred.print_modifiers_and_trait_path()),
 499             ),
 500             (_, Some((_, [.., bounds]))) => (
 501                 bounds.span().shrink_to_hi(),
 502                 format!(" + {}", trait_pred.print_modifiers_and_trait_path()),
 503             ),
 504             (Some(_), Some((_, []))) => (
 505                 hir_generics.span.shrink_to_hi(),
 506                 format!(": {}", trait_pred.print_modifiers_and_trait_path()),
 507             ),
 508         };
 509
 510         err.span_suggestion_verbose(
 511             sp,
 512             &format!("consider further restricting {}", msg),
 513             suggestion,
 514             Applicability::MachineApplicable,
 515         );
 516     }
 517 }
 518
 519 impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
 520     fn suggest_restricting_param_bound(
 521         &self,
 522         mut err: &mut Diagnostic,
 523         trait_pred: ty::PolyTraitPredicate<'tcx>,
 524         associated_ty: Option<(&'static str, Ty<'tcx>)>,
 525         mut body_id: LocalDefId,
 526     ) {
 527         let trait_pred = self.resolve_numeric_literals_with_default(trait_pred);
 528
 529         let self_ty = trait_pred.skip_binder().self_ty();
 530         let (param_ty, projection) = match self_ty.kind() {
 531             ty::Param(_) => (true, None),
 532             ty::Alias(ty::Projection, projection) => (false, Some(projection)),
 533             _ => (false, None),
 534         };
 535
 536         // FIXME: Add check for trait bound that is already present, particularly `?Sized` so we
 537         //        don't suggest `T: Sized + ?Sized`.
 538         while let Some(node) = self.tcx.hir().find_by_def_id(body_id) {
 539             match node {
 540                 hir::Node::Item(hir::Item {
 541                     ident,
 542                     kind: hir::ItemKind::Trait(_, _, generics, bounds, _),
 543                     ..
 544                 }) if self_ty == self.tcx.types.self_param => {
 545                     assert!(param_ty);
 546                     // Restricting `Self` for a single method.
 547                     suggest_restriction(
 548                         self.tcx,
 549                         body_id,
 550                         &generics,
 551                         "`Self`",
 552                         err,
 553                         None,
 554                         projection,
 555                         trait_pred,
 556                         Some((ident, bounds)),
 557                     );
 558                     return;
 559                 }
 560
 561                 hir::Node::TraitItem(hir::TraitItem {
 562                     generics,
 563                     kind: hir::TraitItemKind::Fn(..),
 564                     ..
 565                 }) if self_ty == self.tcx.types.self_param => {
 566                     assert!(param_ty);
 567                     // Restricting `Self` for a single method.
 568                     suggest_restriction(
 569                         self.tcx, body_id, &generics, "`Self`", err, None, projection, trait_pred,
 570                         None,
 571                     );
 572                     return;
 573                 }
 574
 575                 hir::Node::TraitItem(hir::TraitItem {
 576                     generics,
 577                     kind: hir::TraitItemKind::Fn(fn_sig, ..),
 578                     ..
 579                 })
 580                 | hir::Node::ImplItem(hir::ImplItem {
 581                     generics,
 582                     kind: hir::ImplItemKind::Fn(fn_sig, ..),
 583                     ..
 584                 })
 585                 | hir::Node::Item(hir::Item {
 586                     kind: hir::ItemKind::Fn(fn_sig, generics, _), ..
 587                 }) if projection.is_some() => {
 588                     // Missing restriction on associated type of type parameter (unmet projection).
 589                     suggest_restriction(
 590                         self.tcx,
 591                         body_id,
 592                         &generics,
 593                         "the associated type",
 594                         err,
 595                         Some(fn_sig),
 596                         projection,
 597                         trait_pred,
 598                         None,
 599                     );
 600                     return;
 601                 }
 602                 hir::Node::Item(hir::Item {
 603                     kind:
 604                         hir::ItemKind::Trait(_, _, generics, ..)
 605                         | hir::ItemKind::Impl(hir::Impl { generics, .. }),
 606                     ..
 607                 }) if projection.is_some() => {
 608                     // Missing restriction on associated type of type parameter (unmet projection).
 609                     suggest_restriction(
 610                         self.tcx,
 611                         body_id,
 612                         &generics,
 613                         "the associated type",
 614                         err,
 615                         None,
 616                         projection,
 617                         trait_pred,
 618                         None,
 619                     );
 620                     return;
 621                 }
 622
 623                 hir::Node::Item(hir::Item {
 624                     kind:
 625                         hir::ItemKind::Struct(_, generics)
 626                         | hir::ItemKind::Enum(_, generics)
 627                         | hir::ItemKind::Union(_, generics)
 628                         | hir::ItemKind::Trait(_, _, generics, ..)
 629                         | hir::ItemKind::Impl(hir::Impl { generics, .. })
 630                         | hir::ItemKind::Fn(_, generics, _)
 631                         | hir::ItemKind::TyAlias(_, generics)
 632                         | hir::ItemKind::TraitAlias(generics, _)
 633                         | hir::ItemKind::OpaqueTy(hir::OpaqueTy { generics, .. }),
 634                     ..
 635                 })
 636                 | hir::Node::TraitItem(hir::TraitItem { generics, .. })
 637                 | hir::Node::ImplItem(hir::ImplItem { generics, .. })
 638                     if param_ty =>
 639                 {
 640                     // We skip the 0'th subst (self) because we do not want
 641                     // to consider the predicate as not suggestible if the
 642                     // self type is an arg position `impl Trait` -- instead,
 643                     // we handle that by adding ` + Bound` below.
 644                     // FIXME(compiler-errors): It would be nice to do the same
 645                     // this that we do in `suggest_restriction` and pull the
 646                     // `impl Trait` into a new generic if it shows up somewhere
 647                     // else in the predicate.
 648                     if !trait_pred.skip_binder().trait_ref.substs[1..]
 649                         .iter()
 650                         .all(|g| g.is_suggestable(self.tcx, false))
 651                     {
 652                         return;
 653                     }
 654                     // Missing generic type parameter bound.
 655                     let param_name = self_ty.to_string();
 656                     let mut constraint = with_no_trimmed_paths!(
 657                         trait_pred.print_modifiers_and_trait_path().to_string()
 658                     );
 659
 660                     if let Some((name, term)) = associated_ty {
 661                         // FIXME: this case overlaps with code in TyCtxt::note_and_explain_type_err.
 662                         // That should be extracted into a helper function.
 663                         if constraint.ends_with('>') {
 664                             constraint = format!(
 665                                 "{}, {} = {}>",
 666                                 &constraint[..constraint.len() - 1],
 667                                 name,
 668                                 term
 669                             );
 670                         } else {
 671                             constraint.push_str(&format!("<{} = {}>", name, term));
 672                         }
 673                     }
 674
 675                     if suggest_constraining_type_param(
 676                         self.tcx,
 677                         generics,
 678                         &mut err,
 679                         &param_name,
 680                         &constraint,
 681                         Some(trait_pred.def_id()),
 682                     ) {
 683                         return;
 684                     }
 685                 }
 686
 687                 hir::Node::Item(hir::Item {
 688                     kind:
 689                         hir::ItemKind::Struct(_, generics)
 690                         | hir::ItemKind::Enum(_, generics)
 691                         | hir::ItemKind::Union(_, generics)
 692                         | hir::ItemKind::Trait(_, _, generics, ..)
 693                         | hir::ItemKind::Impl(hir::Impl { generics, .. })
 694                         | hir::ItemKind::Fn(_, generics, _)
 695                         | hir::ItemKind::TyAlias(_, generics)
 696                         | hir::ItemKind::TraitAlias(generics, _)
 697                         | hir::ItemKind::OpaqueTy(hir::OpaqueTy { generics, .. }),
 698                     ..
 699                 }) if !param_ty => {
 700                     // Missing generic type parameter bound.
 701                     if suggest_arbitrary_trait_bound(
 702                         self.tcx,
 703                         generics,
 704                         &mut err,
 705                         trait_pred,
 706                         associated_ty,
 707                     ) {
 708                         return;
 709                     }
 710                 }
 711                 hir::Node::Crate(..) => return,
 712
 713                 _ => {}
 714             }
 715             body_id = self.tcx.local_parent(body_id);
 716         }
 717     }
 718
 719     /// When after several dereferencing, the reference satisfies the trait
 720     /// binding. This function provides dereference suggestion for this
 721     /// specific situation.
 722     fn suggest_dereferences(
 723         &self,
 724         obligation: &PredicateObligation<'tcx>,
 725         err: &mut Diagnostic,
 726         trait_pred: ty::PolyTraitPredicate<'tcx>,
 727     ) -> bool {
 728         // It only make sense when suggesting dereferences for arguments
 729         let ObligationCauseCode::FunctionArgumentObligation { arg_hir_id, call_hir_id, .. } = obligation.cause.code()
 730             else { return false; };
 731         let Some(typeck_results) = &self.typeck_results
 732             else { return false; };
 733         let hir::Node::Expr(expr) = self.tcx.hir().get(*arg_hir_id)
 734             else { return false; };
 735         let Some(arg_ty) = typeck_results.expr_ty_adjusted_opt(expr)
 736             else { return false; };
 737
 738         let span = obligation.cause.span;
 739         let mut real_trait_pred = trait_pred;
 740         let mut code = obligation.cause.code();
 741         while let Some((parent_code, parent_trait_pred)) = code.parent() {
 742             code = parent_code;
 743             if let Some(parent_trait_pred) = parent_trait_pred {
 744                 real_trait_pred = parent_trait_pred;
 745             }
 746
 747             let real_ty = real_trait_pred.self_ty();
 748             // We `erase_late_bound_regions` here because `make_subregion` does not handle
 749             // `ReLateBound`, and we don't particularly care about the regions.
 750             if self
 751                 .can_eq(obligation.param_env, self.tcx.erase_late_bound_regions(real_ty), arg_ty)
 752                 .is_err()
 753             {
 754                 continue;
 755             }
 756
 757             if let ty::Ref(region, base_ty, mutbl) = *real_ty.skip_binder().kind() {
 758                 let autoderef = (self.autoderef_steps)(base_ty);
 759                 if let Some(steps) =
 760                     autoderef.into_iter().enumerate().find_map(|(steps, (ty, obligations))| {
 761                         // Re-add the `&`
 762                         let ty = self.tcx.mk_ref(region, TypeAndMut { ty, mutbl });
 763
 764                         // Remapping bound vars here
 765                         let real_trait_pred_and_ty =
 766                             real_trait_pred.map_bound(|inner_trait_pred| (inner_trait_pred, ty));
 767                         let obligation = self.mk_trait_obligation_with_new_self_ty(
 768                             obligation.param_env,
 769                             real_trait_pred_and_ty,
 770                         );
 771                         if obligations
 772                             .iter()
 773                             .chain([&obligation])
 774                             .all(|obligation| self.predicate_may_hold(obligation))
 775                         {
 776                             Some(steps)
 777                         } else {
 778                             None
 779                         }
 780                     })
 781                 {
 782                     if steps > 0 {
 783                         // Don't care about `&mut` because `DerefMut` is used less
 784                         // often and user will not expect autoderef happens.
 785                         if let Some(hir::Node::Expr(hir::Expr {
 786                             kind:
 787                                 hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Not, expr),
 788                             ..
 789                         })) = self.tcx.hir().find(*arg_hir_id)
 790                         {
 791                             let derefs = "*".repeat(steps);
 792                             err.span_suggestion_verbose(
 793                                 expr.span.shrink_to_lo(),
 794                                 "consider dereferencing here",
 795                                 derefs,
 796                                 Applicability::MachineApplicable,
 797                             );
 798                             return true;
 799                         }
 800                     }
 801                 } else if real_trait_pred != trait_pred {
 802                     // This branch addresses #87437.
 803
 804                     // Remapping bound vars here
 805                     let real_trait_pred_and_base_ty =
 806                         real_trait_pred.map_bound(|inner_trait_pred| (inner_trait_pred, base_ty));
 807                     let obligation = self.mk_trait_obligation_with_new_self_ty(
 808                         obligation.param_env,
 809                         real_trait_pred_and_base_ty,
 810                     );
 811                     if self.predicate_may_hold(&obligation) {
 812                         let call_node = self.tcx.hir().get(*call_hir_id);
 813                         let msg = "consider dereferencing here";
 814                         let is_receiver = matches!(
 815                             call_node,
 816                             Node::Expr(hir::Expr {
 817                                 kind: hir::ExprKind::MethodCall(_, receiver_expr, ..),
 818                                 ..
 819                             })
 820                             if receiver_expr.hir_id == *arg_hir_id
 821                         );
 822                         if is_receiver {
 823                             err.multipart_suggestion_verbose(
 824                                 msg,
 825                                 vec![
 826                                     (span.shrink_to_lo(), "(*".to_string()),
 827                                     (span.shrink_to_hi(), ")".to_string()),
 828                                 ],
 829                                 Applicability::MachineApplicable,
 830                             )
 831                         } else {
 832                             err.span_suggestion_verbose(
 833                                 span.shrink_to_lo(),
 834                                 msg,
 835                                 '*',
 836                                 Applicability::MachineApplicable,
 837                             )
 838                         };
 839                         return true;
 840                     }
 841                 }
 842             }
 843         }
 844         false
 845     }
 846
 847     /// Given a closure's `DefId`, return the given name of the closure.
 848     ///
 849     /// This doesn't account for reassignments, but it's only used for suggestions.
 850     fn get_closure_name(&self, def_id: DefId, err: &mut Diagnostic, msg: &str) -> Option<Symbol> {
 851         let get_name = |err: &mut Diagnostic, kind: &hir::PatKind<'_>| -> Option<Symbol> {
 852             // Get the local name of this closure. This can be inaccurate because
 853             // of the possibility of reassignment, but this should be good enough.
 854             match &kind {
 855                 hir::PatKind::Binding(hir::BindingAnnotation::NONE, _, ident, None) => {
 856                     Some(ident.name)
 857                 }
 858                 _ => {
 859                     err.note(msg);
 860                     None
 861                 }
 862             }
 863         };
 864
 865         let hir = self.tcx.hir();
 866         let hir_id = hir.local_def_id_to_hir_id(def_id.as_local()?);
 867         match hir.find_parent(hir_id) {
 868             Some(hir::Node::Stmt(hir::Stmt { kind: hir::StmtKind::Local(local), .. })) => {
 869                 get_name(err, &local.pat.kind)
 870             }
 871             // Different to previous arm because one is `&hir::Local` and the other
 872             // is `P<hir::Local>`.
 873             Some(hir::Node::Local(local)) => get_name(err, &local.pat.kind),
 874             _ => None,
 875         }
 876     }
 877
 878     /// We tried to apply the bound to an `fn` or closure. Check whether calling it would
 879     /// evaluate to a type that *would* satisfy the trait binding. If it would, suggest calling
 880     /// it: `bar(foo)` → `bar(foo())`. This case is *very* likely to be hit if `foo` is `async`.
 881     fn suggest_fn_call(
 882         &self,
 883         obligation: &PredicateObligation<'tcx>,
 884         err: &mut Diagnostic,
 885         trait_pred: ty::PolyTraitPredicate<'tcx>,
 886     ) -> bool {
 887         // It doesn't make sense to make this suggestion outside of typeck...
 888         // (also autoderef will ICE...)
 889         if self.typeck_results.is_none() {
 890             return false;
 891         }
 892
 893         if let ty::PredicateKind::Clause(ty::Clause::Trait(trait_pred)) = obligation.predicate.kind().skip_binder()
 894             && Some(trait_pred.def_id()) == self.tcx.lang_items().sized_trait()
 895         {
 896             // Don't suggest calling to turn an unsized type into a sized type
 897             return false;
 898         }
 899
 900         let self_ty = self.replace_bound_vars_with_fresh_vars(
 901             DUMMY_SP,
 902             LateBoundRegionConversionTime::FnCall,
 903             trait_pred.self_ty(),
 904         );
 905
 906         let body_hir_id = self.tcx.hir().local_def_id_to_hir_id(obligation.cause.body_id);
 907         let Some((def_id_or_name, output, inputs)) = self.extract_callable_info(
 908             body_hir_id,
 909             obligation.param_env,
 910             self_ty,
 911         ) else { return false; };
 912
 913         // Remapping bound vars here
 914         let trait_pred_and_self = trait_pred.map_bound(|trait_pred| (trait_pred, output));
 915
 916         let new_obligation =
 917             self.mk_trait_obligation_with_new_self_ty(obligation.param_env, trait_pred_and_self);
 918         if !self.predicate_must_hold_modulo_regions(&new_obligation) {
 919             return false;
 920         }
 921
 922         // Get the name of the callable and the arguments to be used in the suggestion.
 923         let hir = self.tcx.hir();
 924
 925         let msg = match def_id_or_name {
 926             DefIdOrName::DefId(def_id) => match self.tcx.def_kind(def_id) {
 927                 DefKind::Ctor(CtorOf::Struct, _) => {
 928                     "use parentheses to construct this tuple struct".to_string()
 929                 }
 930                 DefKind::Ctor(CtorOf::Variant, _) => {
 931                     "use parentheses to construct this tuple variant".to_string()
 932                 }
 933                 kind => format!("use parentheses to call this {}", kind.descr(def_id)),
 934             },
 935             DefIdOrName::Name(name) => format!("use parentheses to call this {name}"),
 936         };
 937
 938         let args = inputs
 939             .into_iter()
 940             .map(|ty| {
 941                 if ty.is_suggestable(self.tcx, false) {
 942                     format!("/* {ty} */")
 943                 } else {
 944                     "/* value */".to_string()
 945                 }
 946             })
 947             .collect::<Vec<_>>()
 948             .join(", ");
 949
 950         if matches!(obligation.cause.code(), ObligationCauseCode::FunctionArgumentObligation { .. })
 951             && obligation.cause.span.can_be_used_for_suggestions()
 952         {
 953             // When the obligation error has been ensured to have been caused by
 954             // an argument, the `obligation.cause.span` points at the expression
 955             // of the argument, so we can provide a suggestion. Otherwise, we give
 956             // a more general note.
 957             err.span_suggestion_verbose(
 958                 obligation.cause.span.shrink_to_hi(),
 959                 &msg,
 960                 format!("({args})"),
 961                 Applicability::HasPlaceholders,
 962             );
 963         } else if let DefIdOrName::DefId(def_id) = def_id_or_name {
 964             let name = match hir.get_if_local(def_id) {
 965                 Some(hir::Node::Expr(hir::Expr {
 966                     kind: hir::ExprKind::Closure(hir::Closure { fn_decl_span, .. }),
 967                     ..
 968                 })) => {
 969                     err.span_label(*fn_decl_span, "consider calling this closure");
 970                     let Some(name) = self.get_closure_name(def_id, err, &msg) else {
 971                         return false;
 972                     };
 973                     name.to_string()
 974                 }
 975                 Some(hir::Node::Item(hir::Item { ident, kind: hir::ItemKind::Fn(..), .. })) => {
 976                     err.span_label(ident.span, "consider calling this function");
 977                     ident.to_string()
 978                 }
 979                 Some(hir::Node::Ctor(..)) => {
 980                     let name = self.tcx.def_path_str(def_id);
 981                     err.span_label(
 982                         self.tcx.def_span(def_id),
 983                         format!("consider calling the constructor for `{}`", name),
 984                     );
 985                     name
 986                 }
 987                 _ => return false,
 988             };
 989             err.help(&format!("{msg}: `{name}({args})`"));
 990         }
 991         true
 992     }
 993
 994     fn check_for_binding_assigned_block_without_tail_expression(
 995         &self,
 996         obligation: &PredicateObligation<'tcx>,
 997         err: &mut Diagnostic,
 998         trait_pred: ty::PolyTraitPredicate<'tcx>,
 999     ) {
1000         let mut span = obligation.cause.span;
1001         while span.from_expansion() {
1002             // Remove all the desugaring and macro contexts.
1003             span.remove_mark();
1004         }
1005         let mut expr_finder = FindExprBySpan::new(span);
1006         let Some(body_id) = self.tcx.hir().maybe_body_owned_by(obligation.cause.body_id) else { return; };
1007         let body = self.tcx.hir().body(body_id);
1008         expr_finder.visit_expr(body.value);
1009         let Some(expr) = expr_finder.result else { return; };
1010         let Some(typeck) = &self.typeck_results else { return; };
1011         let Some(ty) = typeck.expr_ty_adjusted_opt(expr) else { return; };
1012         if !ty.is_unit() {
1013             return;
1014         };
1015         let hir::ExprKind::Path(hir::QPath::Resolved(None, path)) = expr.kind else { return; };
1016         let hir::def::Res::Local(hir_id) = path.res else { return; };
1017         let Some(hir::Node::Pat(pat)) = self.tcx.hir().find(hir_id) else {
1018             return;
1019         };
1020         let Some(hir::Node::Local(hir::Local {
1021             ty: None,
1022             init: Some(init),
1023             ..
1024         })) = self.tcx.hir().find_parent(pat.hir_id) else { return; };
1025         let hir::ExprKind::Block(block, None) = init.kind else { return; };
1026         if block.expr.is_some() {
1027             return;
1028         }
1029         let [.., stmt] = block.stmts else {
1030             err.span_label(block.span, "this empty block is missing a tail expression");
1031             return;
1032         };
1033         let hir::StmtKind::Semi(tail_expr) = stmt.kind else { return; };
1034         let Some(ty) = typeck.expr_ty_opt(tail_expr) else {
1035             err.span_label(block.span, "this block is missing a tail expression");
1036             return;
1037         };
1038         let ty = self.resolve_numeric_literals_with_default(self.resolve_vars_if_possible(ty));
1039         let trait_pred_and_self = trait_pred.map_bound(|trait_pred| (trait_pred, ty));
1040
1041         let new_obligation =
1042             self.mk_trait_obligation_with_new_self_ty(obligation.param_env, trait_pred_and_self);
1043         if self.predicate_must_hold_modulo_regions(&new_obligation) {
1044             err.span_suggestion_short(
1045                 stmt.span.with_lo(tail_expr.span.hi()),
1046                 "remove this semicolon",
1047                 "",
1048                 Applicability::MachineApplicable,
1049             );
1050         } else {
1051             err.span_label(block.span, "this block is missing a tail expression");
1052         }
1053     }
1054
1055     fn suggest_add_clone_to_arg(
1056         &self,
1057         obligation: &PredicateObligation<'tcx>,
1058         err: &mut Diagnostic,
1059         trait_pred: ty::PolyTraitPredicate<'tcx>,
1060     ) -> bool {
1061         let self_ty = self.resolve_vars_if_possible(trait_pred.self_ty());
1062         let ty = self.tcx.erase_late_bound_regions(self_ty);
1063         let Some(generics) = self.tcx.hir().get_generics(obligation.cause.body_id) else { return false };
1064         let ty::Ref(_, inner_ty, hir::Mutability::Not) = ty.kind() else { return false };
1065         let ty::Param(param) = inner_ty.kind() else { return false };
1066         let ObligationCauseCode::FunctionArgumentObligation { arg_hir_id, .. } = obligation.cause.code() else { return false };
1067         let arg_node = self.tcx.hir().get(*arg_hir_id);
1068         let Node::Expr(Expr { kind: hir::ExprKind::Path(_), ..}) = arg_node else { return false };
1069
1070         let clone_trait = self.tcx.require_lang_item(LangItem::Clone, None);
1071         let has_clone = |ty| {
1072             self.type_implements_trait(clone_trait, [ty], obligation.param_env)
1073                 .must_apply_modulo_regions()
1074         };
1075
1076         let new_obligation = self.mk_trait_obligation_with_new_self_ty(
1077             obligation.param_env,
1078             trait_pred.map_bound(|trait_pred| (trait_pred, *inner_ty)),
1079         );
1080
1081         if self.predicate_may_hold(&new_obligation) && has_clone(ty) {
1082             if !has_clone(param.to_ty(self.tcx)) {
1083                 suggest_constraining_type_param(
1084                     self.tcx,
1085                     generics,
1086                     err,
1087                     param.name.as_str(),
1088                     "Clone",
1089                     Some(clone_trait),
1090                 );
1091             }
1092             err.span_suggestion_verbose(
1093                 obligation.cause.span.shrink_to_hi(),
1094                 "consider using clone here",
1095                 ".clone()".to_string(),
1096                 Applicability::MaybeIncorrect,
1097             );
1098             return true;
1099         }
1100         false
1101     }
1102
1103     /// Extracts information about a callable type for diagnostics. This is a
1104     /// heuristic -- it doesn't necessarily mean that a type is always callable,
1105     /// because the callable type must also be well-formed to be called.
1106     // FIXME(vincenzopalazzo): move the HirId to a LocalDefId
1107     fn extract_callable_info(
1108         &self,
1109         hir_id: HirId,
1110         param_env: ty::ParamEnv<'tcx>,
1111         found: Ty<'tcx>,
1112     ) -> Option<(DefIdOrName, Ty<'tcx>, Vec<Ty<'tcx>>)> {
1113         // Autoderef is useful here because sometimes we box callables, etc.
1114         let Some((def_id_or_name, output, inputs)) = (self.autoderef_steps)(found).into_iter().find_map(|(found, _)| {
1115             match *found.kind() {
1116                 ty::FnPtr(fn_sig) =>
1117                     Some((DefIdOrName::Name("function pointer"), fn_sig.output(), fn_sig.inputs())),
1118                 ty::FnDef(def_id, _) => {
1119                     let fn_sig = found.fn_sig(self.tcx);
1120                     Some((DefIdOrName::DefId(def_id), fn_sig.output(), fn_sig.inputs()))
1121                 }
1122                 ty::Closure(def_id, substs) => {
1123                     let fn_sig = substs.as_closure().sig();
1124                     Some((DefIdOrName::DefId(def_id), fn_sig.output(), fn_sig.inputs().map_bound(|inputs| &inputs[1..])))
1125                 }
1126                 ty::Alias(ty::Opaque, ty::AliasTy { def_id, substs, .. }) => {
1127                     self.tcx.item_bounds(def_id).subst(self.tcx, substs).iter().find_map(|pred| {
1128                         if let ty::PredicateKind::Clause(ty::Clause::Projection(proj)) = pred.kind().skip_binder()
1129                         && Some(proj.projection_ty.def_id) == self.tcx.lang_items().fn_once_output()
1130                         // args tuple will always be substs[1]
1131                         && let ty::Tuple(args) = proj.projection_ty.substs.type_at(1).kind()
1132                         {
1133                             Some((
1134                                 DefIdOrName::DefId(def_id),
1135                                 pred.kind().rebind(proj.term.ty().unwrap()),
1136                                 pred.kind().rebind(args.as_slice()),
1137                             ))
1138                         } else {
1139                             None
1140                         }
1141                     })
1142                 }
1143                 ty::Dynamic(data, _, ty::Dyn) => {
1144                     data.iter().find_map(|pred| {
1145                         if let ty::ExistentialPredicate::Projection(proj) = pred.skip_binder()
1146                         && Some(proj.def_id) == self.tcx.lang_items().fn_once_output()
1147                         // for existential projection, substs are shifted over by 1
1148                         && let ty::Tuple(args) = proj.substs.type_at(0).kind()
1149                         {
1150                             Some((
1151                                 DefIdOrName::Name("trait object"),
1152                                 pred.rebind(proj.term.ty().unwrap()),
1153                                 pred.rebind(args.as_slice()),
1154                             ))
1155                         } else {
1156                             None
1157                         }
1158                     })
1159                 }
1160                 ty::Param(param) => {
1161                     let generics = self.tcx.generics_of(hir_id.owner.to_def_id());
1162                     let name = if generics.count() > param.index as usize
1163                         && let def = generics.param_at(param.index as usize, self.tcx)
1164                         && matches!(def.kind, ty::GenericParamDefKind::Type { .. })
1165                         && def.name == param.name
1166                     {
1167                         DefIdOrName::DefId(def.def_id)
1168                     } else {
1169                         DefIdOrName::Name("type parameter")
1170                     };
1171                     param_env.caller_bounds().iter().find_map(|pred| {
1172                         if let ty::PredicateKind::Clause(ty::Clause::Projection(proj)) = pred.kind().skip_binder()
1173                         && Some(proj.projection_ty.def_id) == self.tcx.lang_items().fn_once_output()
1174                         && proj.projection_ty.self_ty() == found
1175                         // args tuple will always be substs[1]
1176                         && let ty::Tuple(args) = proj.projection_ty.substs.type_at(1).kind()
1177                         {
1178                             Some((
1179                                 name,
1180                                 pred.kind().rebind(proj.term.ty().unwrap()),
1181                                 pred.kind().rebind(args.as_slice()),
1182                             ))
1183                         } else {
1184                             None
1185                         }
1186                     })
1187                 }
1188                 _ => None,
1189             }
1190         }) else { return None; };
1191
1192         let output = self.replace_bound_vars_with_fresh_vars(
1193             DUMMY_SP,
1194             LateBoundRegionConversionTime::FnCall,
1195             output,
1196         );
1197         let inputs = inputs
1198             .skip_binder()
1199             .iter()
1200             .map(|ty| {
1201                 self.replace_bound_vars_with_fresh_vars(
1202                     DUMMY_SP,
1203                     LateBoundRegionConversionTime::FnCall,
1204                     inputs.rebind(*ty),
1205                 )
1206             })
1207             .collect();
1208
1209         // We don't want to register any extra obligations, which should be
1210         // implied by wf, but also because that would possibly result in
1211         // erroneous errors later on.
1212         let InferOk { value: output, obligations: _ } =
1213             self.at(&ObligationCause::dummy(), param_env).normalize(output);
1214
1215         if output.is_ty_var() { None } else { Some((def_id_or_name, output, inputs)) }
1216     }
1217
1218     fn suggest_add_reference_to_arg(
1219         &self,
1220         obligation: &PredicateObligation<'tcx>,
1221         err: &mut Diagnostic,
1222         poly_trait_pred: ty::PolyTraitPredicate<'tcx>,
1223         has_custom_message: bool,
1224     ) -> bool {
1225         let span = obligation.cause.span;
1226
1227         let code = if let ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } =
1228             obligation.cause.code()
1229         {
1230             &parent_code
1231         } else if let ObligationCauseCode::ItemObligation(_)
1232         | ObligationCauseCode::ExprItemObligation(..) = obligation.cause.code()
1233         {
1234             obligation.cause.code()
1235         } else if let ExpnKind::Desugaring(DesugaringKind::ForLoop) =
1236             span.ctxt().outer_expn_data().kind
1237         {
1238             obligation.cause.code()
1239         } else {
1240             return false;
1241         };
1242
1243         // List of traits for which it would be nonsensical to suggest borrowing.
1244         // For instance, immutable references are always Copy, so suggesting to
1245         // borrow would always succeed, but it's probably not what the user wanted.
1246         let mut never_suggest_borrow: Vec<_> =
1247             [LangItem::Copy, LangItem::Clone, LangItem::Unpin, LangItem::Sized]
1248                 .iter()
1249                 .filter_map(|lang_item| self.tcx.lang_items().get(*lang_item))
1250                 .collect();
1251
1252         if let Some(def_id) = self.tcx.get_diagnostic_item(sym::Send) {
1253             never_suggest_borrow.push(def_id);
1254         }
1255
1256         let param_env = obligation.param_env;
1257
1258         // Try to apply the original trait binding obligation by borrowing.
1259         let mut try_borrowing = |old_pred: ty::PolyTraitPredicate<'tcx>,
1260                                  blacklist: &[DefId]|
1261          -> bool {
1262             if blacklist.contains(&old_pred.def_id()) {
1263                 return false;
1264             }
1265             // We map bounds to `&T` and `&mut T`
1266             let trait_pred_and_imm_ref = old_pred.map_bound(|trait_pred| {
1267                 (
1268                     trait_pred,
1269                     self.tcx.mk_imm_ref(self.tcx.lifetimes.re_static, trait_pred.self_ty()),
1270                 )
1271             });
1272             let trait_pred_and_mut_ref = old_pred.map_bound(|trait_pred| {
1273                 (
1274                     trait_pred,
1275                     self.tcx.mk_mut_ref(self.tcx.lifetimes.re_static, trait_pred.self_ty()),
1276                 )
1277             });
1278
1279             let mk_result = |trait_pred_and_new_ty| {
1280                 let obligation =
1281                     self.mk_trait_obligation_with_new_self_ty(param_env, trait_pred_and_new_ty);
1282                 self.predicate_must_hold_modulo_regions(&obligation)
1283             };
1284             let imm_ref_self_ty_satisfies_pred = mk_result(trait_pred_and_imm_ref);
1285             let mut_ref_self_ty_satisfies_pred = mk_result(trait_pred_and_mut_ref);
1286
1287             let (ref_inner_ty_satisfies_pred, ref_inner_ty_mut) =
1288                 if let ObligationCauseCode::ItemObligation(_) | ObligationCauseCode::ExprItemObligation(..) = obligation.cause.code()
1289                     && let ty::Ref(_, ty, mutability) = old_pred.self_ty().skip_binder().kind()
1290                 {
1291                     (
1292                         mk_result(old_pred.map_bound(|trait_pred| (trait_pred, *ty))),
1293                         mutability.is_mut(),
1294                     )
1295                 } else {
1296                     (false, false)
1297                 };
1298
1299             if imm_ref_self_ty_satisfies_pred
1300                 || mut_ref_self_ty_satisfies_pred
1301                 || ref_inner_ty_satisfies_pred
1302             {
1303                 if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
1304                     // We don't want a borrowing suggestion on the fields in structs,
1305                     // ```
1306                     // struct Foo {
1307                     //  the_foos: Vec<Foo>
1308                     // }
1309                     // ```
1310                     if !matches!(
1311                         span.ctxt().outer_expn_data().kind,
1312                         ExpnKind::Root | ExpnKind::Desugaring(DesugaringKind::ForLoop)
1313                     ) {
1314                         return false;
1315                     }
1316                     if snippet.starts_with('&') {
1317                         // This is already a literal borrow and the obligation is failing
1318                         // somewhere else in the obligation chain. Do not suggest non-sense.
1319                         return false;
1320                     }
1321                     // We have a very specific type of error, where just borrowing this argument
1322                     // might solve the problem. In cases like this, the important part is the
1323                     // original type obligation, not the last one that failed, which is arbitrary.
1324                     // Because of this, we modify the error to refer to the original obligation and
1325                     // return early in the caller.
1326
1327                     let msg = format!("the trait bound `{}` is not satisfied", old_pred);
1328                     if has_custom_message {
1329                         err.note(&msg);
1330                     } else {
1331                         err.message =
1332                             vec![(rustc_errors::DiagnosticMessage::Str(msg), Style::NoStyle)];
1333                     }
1334                     err.span_label(
1335                         span,
1336                         format!(
1337                             "the trait `{}` is not implemented for `{}`",
1338                             old_pred.print_modifiers_and_trait_path(),
1339                             old_pred.self_ty().skip_binder(),
1340                         ),
1341                     );
1342
1343                     if imm_ref_self_ty_satisfies_pred && mut_ref_self_ty_satisfies_pred {
1344                         err.span_suggestions(
1345                             span.shrink_to_lo(),
1346                             "consider borrowing here",
1347                             ["&".to_string(), "&mut ".to_string()],
1348                             Applicability::MaybeIncorrect,
1349                         );
1350                     } else {
1351                         let is_mut = mut_ref_self_ty_satisfies_pred || ref_inner_ty_mut;
1352                         err.span_suggestion_verbose(
1353                             span.shrink_to_lo(),
1354                             &format!(
1355                                 "consider{} borrowing here",
1356                                 if is_mut { " mutably" } else { "" }
1357                             ),
1358                             format!("&{}", if is_mut { "mut " } else { "" }),
1359                             Applicability::MaybeIncorrect,
1360                         );
1361                     }
1362                     return true;
1363                 }
1364             }
1365             return false;
1366         };
1367
1368         if let ObligationCauseCode::ImplDerivedObligation(cause) = &*code {
1369             try_borrowing(cause.derived.parent_trait_pred, &[])
1370         } else if let ObligationCauseCode::BindingObligation(_, _)
1371         | ObligationCauseCode::ItemObligation(_)
1372         | ObligationCauseCode::ExprItemObligation(..)
1373         | ObligationCauseCode::ExprBindingObligation(..) = code
1374         {
1375             try_borrowing(poly_trait_pred, &never_suggest_borrow)
1376         } else {
1377             false
1378         }
1379     }
1380
1381     // Suggest borrowing the type
1382     fn suggest_borrowing_for_object_cast(
1383         &self,
1384         err: &mut Diagnostic,
1385         obligation: &PredicateObligation<'tcx>,
1386         self_ty: Ty<'tcx>,
1387         object_ty: Ty<'tcx>,
1388     ) {
1389         let ty::Dynamic(predicates, _, ty::Dyn) = object_ty.kind() else { return; };
1390         let self_ref_ty = self.tcx.mk_imm_ref(self.tcx.lifetimes.re_erased, self_ty);
1391
1392         for predicate in predicates.iter() {
1393             if !self.predicate_must_hold_modulo_regions(
1394                 &obligation.with(self.tcx, predicate.with_self_ty(self.tcx, self_ref_ty)),
1395             ) {
1396                 return;
1397             }
1398         }
1399
1400         err.span_suggestion(
1401             obligation.cause.span.shrink_to_lo(),
1402             &format!(
1403                 "consider borrowing the value, since `&{self_ty}` can be coerced into `{object_ty}`"
1404             ),
1405             "&",
1406             Applicability::MaybeIncorrect,
1407         );
1408     }
1409
1410     /// Whenever references are used by mistake, like `for (i, e) in &vec.iter().enumerate()`,
1411     /// suggest removing these references until we reach a type that implements the trait.
1412     fn suggest_remove_reference(
1413         &self,
1414         obligation: &PredicateObligation<'tcx>,
1415         err: &mut Diagnostic,
1416         trait_pred: ty::PolyTraitPredicate<'tcx>,
1417     ) -> bool {
1418         let mut span = obligation.cause.span;
1419         let mut trait_pred = trait_pred;
1420         let mut code = obligation.cause.code();
1421         while let Some((c, Some(parent_trait_pred))) = code.parent() {
1422             // We want the root obligation, in order to detect properly handle
1423             // `for _ in &mut &mut vec![] {}`.
1424             code = c;
1425             trait_pred = parent_trait_pred;
1426         }
1427         while span.desugaring_kind().is_some() {
1428             // Remove all the hir desugaring contexts while maintaining the macro contexts.
1429             span.remove_mark();
1430         }
1431         let mut expr_finder = super::FindExprBySpan::new(span);
1432         let Some(body_id) = self.tcx.hir().maybe_body_owned_by(obligation.cause.body_id) else {
1433             return false;
1434         };
1435         let body = self.tcx.hir().body(body_id);
1436         expr_finder.visit_expr(body.value);
1437         let mut maybe_suggest = |suggested_ty, count, suggestions| {
1438             // Remapping bound vars here
1439             let trait_pred_and_suggested_ty =
1440                 trait_pred.map_bound(|trait_pred| (trait_pred, suggested_ty));
1441
1442             let new_obligation = self.mk_trait_obligation_with_new_self_ty(
1443                 obligation.param_env,
1444                 trait_pred_and_suggested_ty,
1445             );
1446
1447             if self.predicate_may_hold(&new_obligation) {
1448                 let msg = if count == 1 {
1449                     "consider removing the leading `&`-reference".to_string()
1450                 } else {
1451                     format!("consider removing {count} leading `&`-references")
1452                 };
1453
1454                 err.multipart_suggestion_verbose(
1455                     &msg,
1456                     suggestions,
1457                     Applicability::MachineApplicable,
1458                 );
1459                 true
1460             } else {
1461                 false
1462             }
1463         };
1464
1465         // Maybe suggest removal of borrows from types in type parameters, like in
1466         // `src/test/ui/not-panic/not-panic-safe.rs`.
1467         let mut count = 0;
1468         let mut suggestions = vec![];
1469         // Skipping binder here, remapping below
1470         let mut suggested_ty = trait_pred.self_ty().skip_binder();
1471         if let Some(mut hir_ty) = expr_finder.ty_result {
1472             while let hir::TyKind::Ref(_, mut_ty) = &hir_ty.kind {
1473                 count += 1;
1474                 let span = hir_ty.span.until(mut_ty.ty.span);
1475                 suggestions.push((span, String::new()));
1476
1477                 let ty::Ref(_, inner_ty, _) = suggested_ty.kind() else {
1478                     break;
1479                 };
1480                 suggested_ty = *inner_ty;
1481
1482                 hir_ty = mut_ty.ty;
1483
1484                 if maybe_suggest(suggested_ty, count, suggestions.clone()) {
1485                     return true;
1486                 }
1487             }
1488         }
1489
1490         // Maybe suggest removal of borrows from expressions, like in `for i in &&&foo {}`.
1491         let Some(mut expr) = expr_finder.result else { return false; };
1492         let mut count = 0;
1493         let mut suggestions = vec![];
1494         // Skipping binder here, remapping below
1495         let mut suggested_ty = trait_pred.self_ty().skip_binder();
1496         'outer: loop {
1497             while let hir::ExprKind::AddrOf(_, _, borrowed) = expr.kind {
1498                 count += 1;
1499                 let span = if expr.span.eq_ctxt(borrowed.span) {
1500                     expr.span.until(borrowed.span)
1501                 } else {
1502                     expr.span.with_hi(expr.span.lo() + BytePos(1))
1503                 };
1504                 suggestions.push((span, String::new()));
1505
1506                 let ty::Ref(_, inner_ty, _) = suggested_ty.kind() else {
1507                     break 'outer;
1508                 };
1509                 suggested_ty = *inner_ty;
1510
1511                 expr = borrowed;
1512
1513                 if maybe_suggest(suggested_ty, count, suggestions.clone()) {
1514                     return true;
1515                 }
1516             }
1517             if let hir::ExprKind::Path(hir::QPath::Resolved(None, path)) = expr.kind
1518                 && let hir::def::Res::Local(hir_id) = path.res
1519                 && let Some(hir::Node::Pat(binding)) = self.tcx.hir().find(hir_id)
1520                 && let Some(hir::Node::Local(local)) = self.tcx.hir().find_parent(binding.hir_id)
1521                 && let None = local.ty
1522                 && let Some(binding_expr) = local.init
1523             {
1524                 expr = binding_expr;
1525             } else {
1526                 break 'outer;
1527             }
1528         }
1529         false
1530     }
1531
1532     fn suggest_remove_await(&self, obligation: &PredicateObligation<'tcx>, err: &mut Diagnostic) {
1533         let span = obligation.cause.span;
1534
1535         if let ObligationCauseCode::AwaitableExpr(hir_id) = obligation.cause.code().peel_derives() {
1536             let hir = self.tcx.hir();
1537             if let Some(hir::Node::Expr(expr)) = hir_id.and_then(|hir_id| hir.find(hir_id)) {
1538                 // FIXME: use `obligation.predicate.kind()...trait_ref.self_ty()` to see if we have `()`
1539                 // and if not maybe suggest doing something else? If we kept the expression around we
1540                 // could also check if it is an fn call (very likely) and suggest changing *that*, if
1541                 // it is from the local crate.
1542                 err.span_suggestion(
1543                     span,
1544                     "remove the `.await`",
1545                     "",
1546                     Applicability::MachineApplicable,
1547                 );
1548                 // FIXME: account for associated `async fn`s.
1549                 if let hir::Expr { span, kind: hir::ExprKind::Call(base, _), .. } = expr {
1550                     if let ty::PredicateKind::Clause(ty::Clause::Trait(pred)) =
1551                         obligation.predicate.kind().skip_binder()
1552                     {
1553                         err.span_label(*span, &format!("this call returns `{}`", pred.self_ty()));
1554                     }
1555                     if let Some(typeck_results) = &self.typeck_results
1556                             && let ty = typeck_results.expr_ty_adjusted(base)
1557                             && let ty::FnDef(def_id, _substs) = ty.kind()
1558                             && let Some(hir::Node::Item(hir::Item { ident, span, vis_span, .. })) =
1559                                 hir.get_if_local(*def_id)
1560                         {
1561                             let msg = format!(
1562                                 "alternatively, consider making `fn {}` asynchronous",
1563                                 ident
1564                             );
1565                             if vis_span.is_empty() {
1566                                 err.span_suggestion_verbose(
1567                                     span.shrink_to_lo(),
1568                                     &msg,
1569                                     "async ",
1570                                     Applicability::MaybeIncorrect,
1571                                 );
1572                             } else {
1573                                 err.span_suggestion_verbose(
1574                                     vis_span.shrink_to_hi(),
1575                                     &msg,
1576                                     " async",
1577                                     Applicability::MaybeIncorrect,
1578                                 );
1579                             }
1580                         }
1581                 }
1582             }
1583         }
1584     }
1585
1586     /// Check if the trait bound is implemented for a different mutability and note it in the
1587     /// final error.
1588     fn suggest_change_mut(
1589         &self,
1590         obligation: &PredicateObligation<'tcx>,
1591         err: &mut Diagnostic,
1592         trait_pred: ty::PolyTraitPredicate<'tcx>,
1593     ) {
1594         let points_at_arg = matches!(
1595             obligation.cause.code(),
1596             ObligationCauseCode::FunctionArgumentObligation { .. },
1597         );
1598
1599         let span = obligation.cause.span;
1600         if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
1601             let refs_number =
1602                 snippet.chars().filter(|c| !c.is_whitespace()).take_while(|c| *c == '&').count();
1603             if let Some('\'') = snippet.chars().filter(|c| !c.is_whitespace()).nth(refs_number) {
1604                 // Do not suggest removal of borrow from type arguments.
1605                 return;
1606             }
1607             let trait_pred = self.resolve_vars_if_possible(trait_pred);
1608             if trait_pred.has_non_region_infer() {
1609                 // Do not ICE while trying to find if a reborrow would succeed on a trait with
1610                 // unresolved bindings.
1611                 return;
1612             }
1613
1614             // Skipping binder here, remapping below
1615             if let ty::Ref(region, t_type, mutability) = *trait_pred.skip_binder().self_ty().kind()
1616             {
1617                 let suggested_ty = match mutability {
1618                     hir::Mutability::Mut => self.tcx.mk_imm_ref(region, t_type),
1619                     hir::Mutability::Not => self.tcx.mk_mut_ref(region, t_type),
1620                 };
1621
1622                 // Remapping bound vars here
1623                 let trait_pred_and_suggested_ty =
1624                     trait_pred.map_bound(|trait_pred| (trait_pred, suggested_ty));
1625
1626                 let new_obligation = self.mk_trait_obligation_with_new_self_ty(
1627                     obligation.param_env,
1628                     trait_pred_and_suggested_ty,
1629                 );
1630                 let suggested_ty_would_satisfy_obligation = self
1631                     .evaluate_obligation_no_overflow(&new_obligation)
1632                     .must_apply_modulo_regions();
1633                 if suggested_ty_would_satisfy_obligation {
1634                     let sp = self
1635                         .tcx
1636                         .sess
1637                         .source_map()
1638                         .span_take_while(span, |c| c.is_whitespace() || *c == '&');
1639                     if points_at_arg && mutability.is_not() && refs_number > 0 {
1640                         // If we have a call like foo(&mut buf), then don't suggest foo(&mut mut buf)
1641                         if snippet
1642                             .trim_start_matches(|c: char| c.is_whitespace() || c == '&')
1643                             .starts_with("mut")
1644                         {
1645                             return;
1646                         }
1647                         err.span_suggestion_verbose(
1648                             sp,
1649                             "consider changing this borrow's mutability",
1650                             "&mut ",
1651                             Applicability::MachineApplicable,
1652                         );
1653                     } else {
1654                         err.note(&format!(
1655                             "`{}` is implemented for `{:?}`, but not for `{:?}`",
1656                             trait_pred.print_modifiers_and_trait_path(),
1657                             suggested_ty,
1658                             trait_pred.skip_binder().self_ty(),
1659                         ));
1660                     }
1661                 }
1662             }
1663         }
1664     }
1665
1666     fn suggest_semicolon_removal(
1667         &self,
1668         obligation: &PredicateObligation<'tcx>,
1669         err: &mut Diagnostic,
1670         span: Span,
1671         trait_pred: ty::PolyTraitPredicate<'tcx>,
1672     ) -> bool {
1673         let hir = self.tcx.hir();
1674         let node = hir.find_by_def_id(obligation.cause.body_id);
1675         if let Some(hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(sig, _, body_id), .. })) = node
1676             && let hir::ExprKind::Block(blk, _) = &hir.body(*body_id).value.kind
1677             && sig.decl.output.span().overlaps(span)
1678             && blk.expr.is_none()
1679             && trait_pred.self_ty().skip_binder().is_unit()
1680             && let Some(stmt) = blk.stmts.last()
1681             && let hir::StmtKind::Semi(expr) = stmt.kind
1682             // Only suggest this if the expression behind the semicolon implements the predicate
1683             && let Some(typeck_results) = &self.typeck_results
1684             && let Some(ty) = typeck_results.expr_ty_opt(expr)
1685             && self.predicate_may_hold(&self.mk_trait_obligation_with_new_self_ty(
1686                 obligation.param_env, trait_pred.map_bound(|trait_pred| (trait_pred, ty))
1687             ))
1688         {
1689             err.span_label(
1690                 expr.span,
1691                 &format!(
1692                     "this expression has type `{}`, which implements `{}`",
1693                     ty,
1694                     trait_pred.print_modifiers_and_trait_path()
1695                 )
1696             );
1697             err.span_suggestion(
1698                 self.tcx.sess.source_map().end_point(stmt.span),
1699                 "remove this semicolon",
1700                 "",
1701                 Applicability::MachineApplicable
1702             );
1703             return true;
1704         }
1705         false
1706     }
1707
1708     fn return_type_span(&self, obligation: &PredicateObligation<'tcx>) -> Option<Span> {
1709         let hir = self.tcx.hir();
1710         let Some(hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(sig, ..), .. })) = hir.find_by_def_id(obligation.cause.body_id) else {
1711             return None;
1712         };
1713
1714         if let hir::FnRetTy::Return(ret_ty) = sig.decl.output { Some(ret_ty.span) } else { None }
1715     }
1716
1717     /// If all conditions are met to identify a returned `dyn Trait`, suggest using `impl Trait` if
1718     /// applicable and signal that the error has been expanded appropriately and needs to be
1719     /// emitted.
1720     fn suggest_impl_trait(
1721         &self,
1722         err: &mut Diagnostic,
1723         span: Span,
1724         obligation: &PredicateObligation<'tcx>,
1725         trait_pred: ty::PolyTraitPredicate<'tcx>,
1726     ) -> bool {
1727         match obligation.cause.code().peel_derives() {
1728             // Only suggest `impl Trait` if the return type is unsized because it is `dyn Trait`.
1729             ObligationCauseCode::SizedReturnType => {}
1730             _ => return false,
1731         }
1732
1733         let hir = self.tcx.hir();
1734         let fn_hir_id = hir.local_def_id_to_hir_id(obligation.cause.body_id);
1735         let node = hir.find_by_def_id(obligation.cause.body_id);
1736         let Some(hir::Node::Item(hir::Item {
1737             kind: hir::ItemKind::Fn(sig, _, body_id),
1738             ..
1739         })) = node
1740         else {
1741             return false;
1742         };
1743         let body = hir.body(*body_id);
1744         let trait_pred = self.resolve_vars_if_possible(trait_pred);
1745         let ty = trait_pred.skip_binder().self_ty();
1746         let is_object_safe = match ty.kind() {
1747             ty::Dynamic(predicates, _, ty::Dyn) => {
1748                 // If the `dyn Trait` is not object safe, do not suggest `Box<dyn Trait>`.
1749                 predicates
1750                     .principal_def_id()
1751                     .map_or(true, |def_id| self.tcx.check_is_object_safe(def_id))
1752             }
1753             // We only want to suggest `impl Trait` to `dyn Trait`s.
1754             // For example, `fn foo() -> str` needs to be filtered out.
1755             _ => return false,
1756         };
1757
1758         let hir::FnRetTy::Return(ret_ty) = sig.decl.output else {
1759             return false;
1760         };
1761
1762         // Use `TypeVisitor` instead of the output type directly to find the span of `ty` for
1763         // cases like `fn foo() -> (dyn Trait, i32) {}`.
1764         // Recursively look for `TraitObject` types and if there's only one, use that span to
1765         // suggest `impl Trait`.
1766
1767         // Visit to make sure there's a single `return` type to suggest `impl Trait`,
1768         // otherwise suggest using `Box<dyn Trait>` or an enum.
1769         let mut visitor = ReturnsVisitor::default();
1770         visitor.visit_body(&body);
1771
1772         let typeck_results = self.typeck_results.as_ref().unwrap();
1773         let Some(liberated_sig) = typeck_results.liberated_fn_sigs().get(fn_hir_id).copied() else { return false; };
1774
1775         let ret_types = visitor
1776             .returns
1777             .iter()
1778             .filter_map(|expr| Some((expr.span, typeck_results.node_type_opt(expr.hir_id)?)))
1779             .map(|(expr_span, ty)| (expr_span, self.resolve_vars_if_possible(ty)));
1780         let (last_ty, all_returns_have_same_type, only_never_return) = ret_types.clone().fold(
1781             (None, true, true),
1782             |(last_ty, mut same, only_never_return): (std::option::Option<Ty<'_>>, bool, bool),
1783              (_, ty)| {
1784                 let ty = self.resolve_vars_if_possible(ty);
1785                 same &=
1786                     !matches!(ty.kind(), ty::Error(_))
1787                         && last_ty.map_or(true, |last_ty| {
1788                             // FIXME: ideally we would use `can_coerce` here instead, but `typeck` comes
1789                             // *after* in the dependency graph.
1790                             match (ty.kind(), last_ty.kind()) {
1791                                 (Infer(InferTy::IntVar(_)), Infer(InferTy::IntVar(_)))
1792                                 | (Infer(InferTy::FloatVar(_)), Infer(InferTy::FloatVar(_)))
1793                                 | (Infer(InferTy::FreshIntTy(_)), Infer(InferTy::FreshIntTy(_)))
1794                                 | (
1795                                     Infer(InferTy::FreshFloatTy(_)),
1796                                     Infer(InferTy::FreshFloatTy(_)),
1797                                 ) => true,
1798                                 _ => ty == last_ty,
1799                             }
1800                         });
1801                 (Some(ty), same, only_never_return && matches!(ty.kind(), ty::Never))
1802             },
1803         );
1804         let mut spans_and_needs_box = vec![];
1805
1806         match liberated_sig.output().kind() {
1807             ty::Dynamic(predicates, _, ty::Dyn) => {
1808                 let cause = ObligationCause::misc(ret_ty.span, obligation.cause.body_id);
1809                 let param_env = ty::ParamEnv::empty();
1810
1811                 if !only_never_return {
1812                     for (expr_span, return_ty) in ret_types {
1813                         let self_ty_satisfies_dyn_predicates = |self_ty| {
1814                             predicates.iter().all(|predicate| {
1815                                 let pred = predicate.with_self_ty(self.tcx, self_ty);
1816                                 let obl = Obligation::new(self.tcx, cause.clone(), param_env, pred);
1817                                 self.predicate_may_hold(&obl)
1818                             })
1819                         };
1820
1821                         if let ty::Adt(def, substs) = return_ty.kind()
1822                             && def.is_box()
1823                             && self_ty_satisfies_dyn_predicates(substs.type_at(0))
1824                         {
1825                             spans_and_needs_box.push((expr_span, false));
1826                         } else if self_ty_satisfies_dyn_predicates(return_ty) {
1827                             spans_and_needs_box.push((expr_span, true));
1828                         } else {
1829                             return false;
1830                         }
1831                     }
1832                 }
1833             }
1834             _ => return false,
1835         };
1836
1837         let sm = self.tcx.sess.source_map();
1838         if !ret_ty.span.overlaps(span) {
1839             return false;
1840         }
1841         let snippet = if let hir::TyKind::TraitObject(..) = ret_ty.kind {
1842             if let Ok(snippet) = sm.span_to_snippet(ret_ty.span) {
1843                 snippet
1844             } else {
1845                 return false;
1846             }
1847         } else {
1848             // Substitute the type, so we can print a fixup given `type Alias = dyn Trait`
1849             let name = liberated_sig.output().to_string();
1850             let name =
1851                 name.strip_prefix('(').and_then(|name| name.strip_suffix(')')).unwrap_or(&name);
1852             if !name.starts_with("dyn ") {
1853                 return false;
1854             }
1855             name.to_owned()
1856         };
1857
1858         err.code(error_code!(E0746));
1859         err.set_primary_message("return type cannot have an unboxed trait object");
1860         err.children.clear();
1861         let impl_trait_msg = "for information on `impl Trait`, see \
1862             <https://doc.rust-lang.org/book/ch10-02-traits.html\
1863             #returning-types-that-implement-traits>";
1864         let trait_obj_msg = "for information on trait objects, see \
1865             <https://doc.rust-lang.org/book/ch17-02-trait-objects.html\
1866             #using-trait-objects-that-allow-for-values-of-different-types>";
1867
1868         let has_dyn = snippet.split_whitespace().next().map_or(false, |s| s == "dyn");
1869         let trait_obj = if has_dyn { &snippet[4..] } else { &snippet };
1870         if only_never_return {
1871             // No return paths, probably using `panic!()` or similar.
1872             // Suggest `-> impl Trait`, and if `Trait` is object safe, `-> Box<dyn Trait>`.
1873             suggest_trait_object_return_type_alternatives(
1874                 err,
1875                 ret_ty.span,
1876                 trait_obj,
1877                 is_object_safe,
1878             );
1879         } else if let (Some(last_ty), true) = (last_ty, all_returns_have_same_type) {
1880             // Suggest `-> impl Trait`.
1881             err.span_suggestion(
1882                 ret_ty.span,
1883                 &format!(
1884                     "use `impl {1}` as the return type, as all return paths are of type `{}`, \
1885                      which implements `{1}`",
1886                     last_ty, trait_obj,
1887                 ),
1888                 format!("impl {}", trait_obj),
1889                 Applicability::MachineApplicable,
1890             );
1891             err.note(impl_trait_msg);
1892         } else {
1893             if is_object_safe {
1894                 // Suggest `-> Box<dyn Trait>` and `Box::new(returned_value)`.
1895                 err.multipart_suggestion(
1896                     "return a boxed trait object instead",
1897                     vec![
1898                         (ret_ty.span.shrink_to_lo(), "Box<".to_string()),
1899                         (span.shrink_to_hi(), ">".to_string()),
1900                     ],
1901                     Applicability::MaybeIncorrect,
1902                 );
1903                 for (span, needs_box) in spans_and_needs_box {
1904                     if needs_box {
1905                         err.multipart_suggestion(
1906                             "... and box this value",
1907                             vec![
1908                                 (span.shrink_to_lo(), "Box::new(".to_string()),
1909                                 (span.shrink_to_hi(), ")".to_string()),
1910                             ],
1911                             Applicability::MaybeIncorrect,
1912                         );
1913                     }
1914                 }
1915             } else {
1916                 // This is currently not possible to trigger because E0038 takes precedence, but
1917                 // leave it in for completeness in case anything changes in an earlier stage.
1918                 err.note(&format!(
1919                     "if trait `{}` were object-safe, you could return a trait object",
1920                     trait_obj,
1921                 ));
1922             }
1923             err.note(trait_obj_msg);
1924             err.note(&format!(
1925                 "if all the returned values were of the same type you could use `impl {}` as the \
1926                  return type",
1927                 trait_obj,
1928             ));
1929             err.note(impl_trait_msg);
1930             err.note("you can create a new `enum` with a variant for each returned type");
1931         }
1932         true
1933     }
1934
1935     fn point_at_returns_when_relevant(
1936         &self,
1937         err: &mut DiagnosticBuilder<'tcx, ErrorGuaranteed>,
1938         obligation: &PredicateObligation<'tcx>,
1939     ) {
1940         match obligation.cause.code().peel_derives() {
1941             ObligationCauseCode::SizedReturnType => {}
1942             _ => return,
1943         }
1944
1945         let hir = self.tcx.hir();
1946         let node = hir.find_by_def_id(obligation.cause.body_id);
1947         if let Some(hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(_, _, body_id), .. })) =
1948             node
1949         {
1950             let body = hir.body(*body_id);
1951             // Point at all the `return`s in the function as they have failed trait bounds.
1952             let mut visitor = ReturnsVisitor::default();
1953             visitor.visit_body(&body);
1954             let typeck_results = self.typeck_results.as_ref().unwrap();
1955             for expr in &visitor.returns {
1956                 if let Some(returned_ty) = typeck_results.node_type_opt(expr.hir_id) {
1957                     let ty = self.resolve_vars_if_possible(returned_ty);
1958                     if ty.references_error() {
1959                         // don't print out the [type error] here
1960                         err.delay_as_bug();
1961                     } else {
1962                         err.span_label(
1963                             expr.span,
1964                             &format!("this returned value is of type `{}`", ty),
1965                         );
1966                     }
1967                 }
1968             }
1969         }
1970     }
1971
1972     fn report_closure_arg_mismatch(
1973         &self,
1974         span: Span,
1975         found_span: Option<Span>,
1976         found: ty::PolyTraitRef<'tcx>,
1977         expected: ty::PolyTraitRef<'tcx>,
1978         cause: &ObligationCauseCode<'tcx>,
1979         found_node: Option<Node<'_>>,
1980         param_env: ty::ParamEnv<'tcx>,
1981     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
1982         pub(crate) fn build_fn_sig_ty<'tcx>(
1983             infcx: &InferCtxt<'tcx>,
1984             trait_ref: ty::PolyTraitRef<'tcx>,
1985         ) -> Ty<'tcx> {
1986             let inputs = trait_ref.skip_binder().substs.type_at(1);
1987             let sig = match inputs.kind() {
1988                 ty::Tuple(inputs) if infcx.tcx.is_fn_trait(trait_ref.def_id()) => {
1989                     infcx.tcx.mk_fn_sig(
1990                         inputs.iter(),
1991                         infcx.next_ty_var(TypeVariableOrigin {
1992                             span: DUMMY_SP,
1993                             kind: TypeVariableOriginKind::MiscVariable,
1994                         }),
1995                         false,
1996                         hir::Unsafety::Normal,
1997                         abi::Abi::Rust,
1998                     )
1999                 }
2000                 _ => infcx.tcx.mk_fn_sig(
2001                     std::iter::once(inputs),
2002                     infcx.next_ty_var(TypeVariableOrigin {
2003                         span: DUMMY_SP,
2004                         kind: TypeVariableOriginKind::MiscVariable,
2005                     }),
2006                     false,
2007                     hir::Unsafety::Normal,
2008                     abi::Abi::Rust,
2009                 ),
2010             };
2011
2012             infcx.tcx.mk_fn_ptr(trait_ref.rebind(sig))
2013         }
2014
2015         let argument_kind = match expected.skip_binder().self_ty().kind() {
2016             ty::Closure(..) => "closure",
2017             ty::Generator(..) => "generator",
2018             _ => "function",
2019         };
2020         let mut err = struct_span_err!(
2021             self.tcx.sess,
2022             span,
2023             E0631,
2024             "type mismatch in {argument_kind} arguments",
2025         );
2026
2027         err.span_label(span, "expected due to this");
2028
2029         let found_span = found_span.unwrap_or(span);
2030         err.span_label(found_span, "found signature defined here");
2031
2032         let expected = build_fn_sig_ty(self, expected);
2033         let found = build_fn_sig_ty(self, found);
2034
2035         let (expected_str, found_str) = self.cmp(expected, found);
2036
2037         let signature_kind = format!("{argument_kind} signature");
2038         err.note_expected_found(&signature_kind, expected_str, &signature_kind, found_str);
2039
2040         self.note_conflicting_closure_bounds(cause, &mut err);
2041
2042         if let Some(found_node) = found_node {
2043             hint_missing_borrow(self, param_env, span, found, expected, found_node, &mut err);
2044         }
2045
2046         err
2047     }
2048
2049     // Add a note if there are two `Fn`-family bounds that have conflicting argument
2050     // requirements, which will always cause a closure to have a type error.
2051     fn note_conflicting_closure_bounds(
2052         &self,
2053         cause: &ObligationCauseCode<'tcx>,
2054         err: &mut DiagnosticBuilder<'tcx, ErrorGuaranteed>,
2055     ) {
2056         // First, look for an `ExprBindingObligation`, which means we can get
2057         // the unsubstituted predicate list of the called function. And check
2058         // that the predicate that we failed to satisfy is a `Fn`-like trait.
2059         if let ObligationCauseCode::ExprBindingObligation(def_id, _, _, idx) = cause
2060             && let predicates = self.tcx.predicates_of(def_id).instantiate_identity(self.tcx)
2061             && let Some(pred) = predicates.predicates.get(*idx)
2062             && let ty::PredicateKind::Clause(ty::Clause::Trait(trait_pred)) = pred.kind().skip_binder()
2063             && self.tcx.is_fn_trait(trait_pred.def_id())
2064         {
2065             let expected_self =
2066                 self.tcx.anonymize_bound_vars(pred.kind().rebind(trait_pred.self_ty()));
2067             let expected_substs = self
2068                 .tcx
2069                 .anonymize_bound_vars(pred.kind().rebind(trait_pred.trait_ref.substs));
2070
2071             // Find another predicate whose self-type is equal to the expected self type,
2072             // but whose substs don't match.
2073             let other_pred = predicates.into_iter()
2074                 .enumerate()
2075                 .find(|(other_idx, (pred, _))| match pred.kind().skip_binder() {
2076                     ty::PredicateKind::Clause(ty::Clause::Trait(trait_pred))
2077                         if self.tcx.is_fn_trait(trait_pred.def_id())
2078                             && other_idx != idx
2079                             // Make sure that the self type matches
2080                             // (i.e. constraining this closure)
2081                             && expected_self
2082                                 == self.tcx.anonymize_bound_vars(
2083                                     pred.kind().rebind(trait_pred.self_ty()),
2084                                 )
2085                             // But the substs don't match (i.e. incompatible args)
2086                             && expected_substs
2087                                 != self.tcx.anonymize_bound_vars(
2088                                     pred.kind().rebind(trait_pred.trait_ref.substs),
2089                                 ) =>
2090                     {
2091                         true
2092                     }
2093                     _ => false,
2094                 });
2095             // If we found one, then it's very likely the cause of the error.
2096             if let Some((_, (_, other_pred_span))) = other_pred {
2097                 err.span_note(
2098                     other_pred_span,
2099                     "closure inferred to have a different signature due to this bound",
2100                 );
2101             }
2102         }
2103     }
2104
2105     fn suggest_fully_qualified_path(
2106         &self,
2107         err: &mut Diagnostic,
2108         item_def_id: DefId,
2109         span: Span,
2110         trait_ref: DefId,
2111     ) {
2112         if let Some(assoc_item) = self.tcx.opt_associated_item(item_def_id) {
2113             if let ty::AssocKind::Const | ty::AssocKind::Type = assoc_item.kind {
2114                 err.note(&format!(
2115                     "{}s cannot be accessed directly on a `trait`, they can only be \
2116                         accessed through a specific `impl`",
2117                     assoc_item.kind.as_def_kind().descr(item_def_id)
2118                 ));
2119                 err.span_suggestion(
2120                     span,
2121                     "use the fully qualified path to an implementation",
2122                     format!("<Type as {}>::{}", self.tcx.def_path_str(trait_ref), assoc_item.name),
2123                     Applicability::HasPlaceholders,
2124                 );
2125             }
2126         }
2127     }
2128
2129     /// Adds an async-await specific note to the diagnostic when the future does not implement
2130     /// an auto trait because of a captured type.
2131     ///
2132     /// ```text
2133     /// note: future does not implement `Qux` as this value is used across an await
2134     ///   --> $DIR/issue-64130-3-other.rs:17:5
2135     ///    |
2136     /// LL |     let x = Foo;
2137     ///    |         - has type `Foo`
2138     /// LL |     baz().await;
2139     ///    |     ^^^^^^^^^^^ await occurs here, with `x` maybe used later
2140     /// LL | }
2141     ///    | - `x` is later dropped here
2142     /// ```
2143     ///
2144     /// When the diagnostic does not implement `Send` or `Sync` specifically, then the diagnostic
2145     /// is "replaced" with a different message and a more specific error.
2146     ///
2147     /// ```text
2148     /// error: future cannot be sent between threads safely
2149     ///   --> $DIR/issue-64130-2-send.rs:21:5
2150     ///    |
2151     /// LL | fn is_send<T: Send>(t: T) { }
2152     ///    |               ---- required by this bound in `is_send`
2153     /// ...
2154     /// LL |     is_send(bar());
2155     ///    |     ^^^^^^^ future returned by `bar` is not send
2156     ///    |
2157     ///    = help: within `impl std::future::Future`, the trait `std::marker::Send` is not
2158     ///            implemented for `Foo`
2159     /// note: future is not send as this value is used across an await
2160     ///   --> $DIR/issue-64130-2-send.rs:15:5
2161     ///    |
2162     /// LL |     let x = Foo;
2163     ///    |         - has type `Foo`
2164     /// LL |     baz().await;
2165     ///    |     ^^^^^^^^^^^ await occurs here, with `x` maybe used later
2166     /// LL | }
2167     ///    | - `x` is later dropped here
2168     /// ```
2169     ///
2170     /// Returns `true` if an async-await specific note was added to the diagnostic.
2171     #[instrument(level = "debug", skip_all, fields(?obligation.predicate, ?obligation.cause.span))]
2172     fn maybe_note_obligation_cause_for_async_await(
2173         &self,
2174         err: &mut Diagnostic,
2175         obligation: &PredicateObligation<'tcx>,
2176     ) -> bool {
2177         let hir = self.tcx.hir();
2178
2179         // Attempt to detect an async-await error by looking at the obligation causes, looking
2180         // for a generator to be present.
2181         //
2182         // When a future does not implement a trait because of a captured type in one of the
2183         // generators somewhere in the call stack, then the result is a chain of obligations.
2184         //
2185         // Given an `async fn` A that calls an `async fn` B which captures a non-send type and that
2186         // future is passed as an argument to a function C which requires a `Send` type, then the
2187         // chain looks something like this:
2188         //
2189         // - `BuiltinDerivedObligation` with a generator witness (B)
2190         // - `BuiltinDerivedObligation` with a generator (B)
2191         // - `BuiltinDerivedObligation` with `impl std::future::Future` (B)
2192         // - `BuiltinDerivedObligation` with a generator witness (A)
2193         // - `BuiltinDerivedObligation` with a generator (A)
2194         // - `BuiltinDerivedObligation` with `impl std::future::Future` (A)
2195         // - `BindingObligation` with `impl_send (Send requirement)
2196         //
2197         // The first obligation in the chain is the most useful and has the generator that captured
2198         // the type. The last generator (`outer_generator` below) has information about where the
2199         // bound was introduced. At least one generator should be present for this diagnostic to be
2200         // modified.
2201         let (mut trait_ref, mut target_ty) = match obligation.predicate.kind().skip_binder() {
2202             ty::PredicateKind::Clause(ty::Clause::Trait(p)) => (Some(p), Some(p.self_ty())),
2203             _ => (None, None),
2204         };
2205         let mut generator = None;
2206         let mut outer_generator = None;
2207         let mut next_code = Some(obligation.cause.code());
2208
2209         let mut seen_upvar_tys_infer_tuple = false;
2210
2211         while let Some(code) = next_code {
2212             debug!(?code);
2213             match code {
2214                 ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } => {
2215                     next_code = Some(parent_code);
2216                 }
2217                 ObligationCauseCode::ImplDerivedObligation(cause) => {
2218                     let ty = cause.derived.parent_trait_pred.skip_binder().self_ty();
2219                     debug!(
2220                         parent_trait_ref = ?cause.derived.parent_trait_pred,
2221                         self_ty.kind = ?ty.kind(),
2222                         "ImplDerived",
2223                     );
2224
2225                     match *ty.kind() {
2226                         ty::Generator(did, ..) | ty::GeneratorWitnessMIR(did, _) => {
2227                             generator = generator.or(Some(did));
2228                             outer_generator = Some(did);
2229                         }
2230                         ty::GeneratorWitness(..) => {}
2231                         ty::Tuple(_) if !seen_upvar_tys_infer_tuple => {
2232                             // By introducing a tuple of upvar types into the chain of obligations
2233                             // of a generator, the first non-generator item is now the tuple itself,
2234                             // we shall ignore this.
2235
2236                             seen_upvar_tys_infer_tuple = true;
2237                         }
2238                         _ if generator.is_none() => {
2239                             trait_ref = Some(cause.derived.parent_trait_pred.skip_binder());
2240                             target_ty = Some(ty);
2241                         }
2242                         _ => {}
2243                     }
2244
2245                     next_code = Some(&cause.derived.parent_code);
2246                 }
2247                 ObligationCauseCode::DerivedObligation(derived_obligation)
2248                 | ObligationCauseCode::BuiltinDerivedObligation(derived_obligation) => {
2249                     let ty = derived_obligation.parent_trait_pred.skip_binder().self_ty();
2250                     debug!(
2251                         parent_trait_ref = ?derived_obligation.parent_trait_pred,
2252                         self_ty.kind = ?ty.kind(),
2253                     );
2254
2255                     match *ty.kind() {
2256                         ty::Generator(did, ..) | ty::GeneratorWitnessMIR(did, ..) => {
2257                             generator = generator.or(Some(did));
2258                             outer_generator = Some(did);
2259                         }
2260                         ty::GeneratorWitness(..) => {}
2261                         ty::Tuple(_) if !seen_upvar_tys_infer_tuple => {
2262                             // By introducing a tuple of upvar types into the chain of obligations
2263                             // of a generator, the first non-generator item is now the tuple itself,
2264                             // we shall ignore this.
2265
2266                             seen_upvar_tys_infer_tuple = true;
2267                         }
2268                         _ if generator.is_none() => {
2269                             trait_ref = Some(derived_obligation.parent_trait_pred.skip_binder());
2270                             target_ty = Some(ty);
2271                         }
2272                         _ => {}
2273                     }
2274
2275                     next_code = Some(&derived_obligation.parent_code);
2276                 }
2277                 _ => break,
2278             }
2279         }
2280
2281         // Only continue if a generator was found.
2282         debug!(?generator, ?trait_ref, ?target_ty);
2283         let (Some(generator_did), Some(trait_ref), Some(target_ty)) = (generator, trait_ref, target_ty) else {
2284             return false;
2285         };
2286
2287         let span = self.tcx.def_span(generator_did);
2288
2289         let generator_did_root = self.tcx.typeck_root_def_id(generator_did);
2290         debug!(
2291             ?generator_did,
2292             ?generator_did_root,
2293             typeck_results.hir_owner = ?self.typeck_results.as_ref().map(|t| t.hir_owner),
2294             ?span,
2295         );
2296
2297         let generator_body = generator_did
2298             .as_local()
2299             .and_then(|def_id| hir.maybe_body_owned_by(def_id))
2300             .map(|body_id| hir.body(body_id));
2301         let mut visitor = AwaitsVisitor::default();
2302         if let Some(body) = generator_body {
2303             visitor.visit_body(body);
2304         }
2305         debug!(awaits = ?visitor.awaits);
2306
2307         // Look for a type inside the generator interior that matches the target type to get
2308         // a span.
2309         let target_ty_erased = self.tcx.erase_regions(target_ty);
2310         let ty_matches = |ty| -> bool {
2311             // Careful: the regions for types that appear in the
2312             // generator interior are not generally known, so we
2313             // want to erase them when comparing (and anyway,
2314             // `Send` and other bounds are generally unaffected by
2315             // the choice of region). When erasing regions, we
2316             // also have to erase late-bound regions. This is
2317             // because the types that appear in the generator
2318             // interior generally contain "bound regions" to
2319             // represent regions that are part of the suspended
2320             // generator frame. Bound regions are preserved by
2321             // `erase_regions` and so we must also call
2322             // `erase_late_bound_regions`.
2323             let ty_erased = self.tcx.erase_late_bound_regions(ty);
2324             let ty_erased = self.tcx.erase_regions(ty_erased);
2325             let eq = ty_erased == target_ty_erased;
2326             debug!(?ty_erased, ?target_ty_erased, ?eq);
2327             eq
2328         };
2329
2330         // Get the typeck results from the infcx if the generator is the function we are currently
2331         // type-checking; otherwise, get them by performing a query. This is needed to avoid
2332         // cycles. If we can't use resolved types because the generator comes from another crate,
2333         // we still provide a targeted error but without all the relevant spans.
2334         let generator_data = match &self.typeck_results {
2335             Some(t) if t.hir_owner.to_def_id() == generator_did_root => GeneratorData::Local(&t),
2336             _ if generator_did.is_local() => {
2337                 GeneratorData::Local(self.tcx.typeck(generator_did.expect_local()))
2338             }
2339             _ if let Some(generator_diag_data) = self.tcx.generator_diagnostic_data(generator_did) => {
2340                 GeneratorData::Foreign(generator_diag_data)
2341             }
2342             _ => return false,
2343         };
2344
2345         let generator_within_in_progress_typeck = match &self.typeck_results {
2346             Some(t) => t.hir_owner.to_def_id() == generator_did_root,
2347             _ => false,
2348         };
2349
2350         let mut interior_or_upvar_span = None;
2351
2352         let from_awaited_ty = generator_data.get_from_await_ty(visitor, hir, ty_matches);
2353         debug!(?from_awaited_ty);
2354
2355         // The generator interior types share the same binders
2356         if let Some(cause) =
2357             generator_data.get_generator_interior_types().skip_binder().iter().find(
2358                 |ty::GeneratorInteriorTypeCause { ty, .. }| {
2359                     ty_matches(generator_data.get_generator_interior_types().rebind(*ty))
2360                 },
2361             )
2362         {
2363             let ty::GeneratorInteriorTypeCause { span, scope_span, yield_span, expr, .. } = cause;
2364
2365             interior_or_upvar_span = Some(GeneratorInteriorOrUpvar::Interior(
2366                 *span,
2367                 Some((*scope_span, *yield_span, *expr, from_awaited_ty)),
2368             ));
2369
2370             if interior_or_upvar_span.is_none() && generator_data.is_foreign() {
2371                 interior_or_upvar_span = Some(GeneratorInteriorOrUpvar::Interior(*span, None));
2372             }
2373         } else if self.tcx.sess.opts.unstable_opts.drop_tracking_mir
2374             // Avoid disclosing internal information to downstream crates.
2375             && generator_did.is_local()
2376             // Try to avoid cycles.
2377             && !generator_within_in_progress_typeck
2378         {
2379             let generator_info = &self.tcx.mir_generator_witnesses(generator_did);
2380             debug!(?generator_info);
2381
2382             'find_source: for (variant, source_info) in
2383                 generator_info.variant_fields.iter().zip(&generator_info.variant_source_info)
2384             {
2385                 debug!(?variant);
2386                 for &local in variant {
2387                     let decl = &generator_info.field_tys[local];
2388                     debug!(?decl);
2389                     if ty_matches(ty::Binder::dummy(decl.ty)) && !decl.ignore_for_traits {
2390                         interior_or_upvar_span = Some(GeneratorInteriorOrUpvar::Interior(
2391                             decl.source_info.span,
2392                             Some((None, source_info.span, None, from_awaited_ty)),
2393                         ));
2394                         break 'find_source;
2395                     }
2396                 }
2397             }
2398         }
2399
2400         if interior_or_upvar_span.is_none() {
2401             interior_or_upvar_span =
2402                 generator_data.try_get_upvar_span(&self, generator_did, ty_matches);
2403         }
2404
2405         if interior_or_upvar_span.is_none() && generator_data.is_foreign() {
2406             interior_or_upvar_span = Some(GeneratorInteriorOrUpvar::Interior(span, None));
2407         }
2408
2409         debug!(?interior_or_upvar_span);
2410         if let Some(interior_or_upvar_span) = interior_or_upvar_span {
2411             let is_async = self.tcx.generator_is_async(generator_did);
2412             let typeck_results = match generator_data {
2413                 GeneratorData::Local(typeck_results) => Some(typeck_results),
2414                 GeneratorData::Foreign(_) => None,
2415             };
2416             self.note_obligation_cause_for_async_await(
2417                 err,
2418                 interior_or_upvar_span,
2419                 is_async,
2420                 outer_generator,
2421                 trait_ref,
2422                 target_ty,
2423                 typeck_results,
2424                 obligation,
2425                 next_code,
2426             );
2427             true
2428         } else {
2429             false
2430         }
2431     }
2432
2433     /// Unconditionally adds the diagnostic note described in
2434     /// `maybe_note_obligation_cause_for_async_await`'s documentation comment.
2435     #[instrument(level = "debug", skip_all)]
2436     fn note_obligation_cause_for_async_await(
2437         &self,
2438         err: &mut Diagnostic,
2439         interior_or_upvar_span: GeneratorInteriorOrUpvar,
2440         is_async: bool,
2441         outer_generator: Option<DefId>,
2442         trait_pred: ty::TraitPredicate<'tcx>,
2443         target_ty: Ty<'tcx>,
2444         typeck_results: Option<&ty::TypeckResults<'tcx>>,
2445         obligation: &PredicateObligation<'tcx>,
2446         next_code: Option<&ObligationCauseCode<'tcx>>,
2447     ) {
2448         let source_map = self.tcx.sess.source_map();
2449
2450         let (await_or_yield, an_await_or_yield) =
2451             if is_async { ("await", "an await") } else { ("yield", "a yield") };
2452         let future_or_generator = if is_async { "future" } else { "generator" };
2453
2454         // Special case the primary error message when send or sync is the trait that was
2455         // not implemented.
2456         let hir = self.tcx.hir();
2457         let trait_explanation = if let Some(name @ (sym::Send | sym::Sync)) =
2458             self.tcx.get_diagnostic_name(trait_pred.def_id())
2459         {
2460             let (trait_name, trait_verb) =
2461                 if name == sym::Send { ("`Send`", "sent") } else { ("`Sync`", "shared") };
2462
2463             err.clear_code();
2464             err.set_primary_message(format!(
2465                 "{} cannot be {} between threads safely",
2466                 future_or_generator, trait_verb
2467             ));
2468
2469             let original_span = err.span.primary_span().unwrap();
2470             let mut span = MultiSpan::from_span(original_span);
2471
2472             let message = outer_generator
2473                 .and_then(|generator_did| {
2474                     Some(match self.tcx.generator_kind(generator_did).unwrap() {
2475                         GeneratorKind::Gen => format!("generator is not {}", trait_name),
2476                         GeneratorKind::Async(AsyncGeneratorKind::Fn) => self
2477                             .tcx
2478                             .parent(generator_did)
2479                             .as_local()
2480                             .map(|parent_did| hir.local_def_id_to_hir_id(parent_did))
2481                             .and_then(|parent_hir_id| hir.opt_name(parent_hir_id))
2482                             .map(|name| {
2483                                 format!("future returned by `{}` is not {}", name, trait_name)
2484                             })?,
2485                         GeneratorKind::Async(AsyncGeneratorKind::Block) => {
2486                             format!("future created by async block is not {}", trait_name)
2487                         }
2488                         GeneratorKind::Async(AsyncGeneratorKind::Closure) => {
2489                             format!("future created by async closure is not {}", trait_name)
2490                         }
2491                     })
2492                 })
2493                 .unwrap_or_else(|| format!("{} is not {}", future_or_generator, trait_name));
2494
2495             span.push_span_label(original_span, message);
2496             err.set_span(span);
2497
2498             format!("is not {}", trait_name)
2499         } else {
2500             format!("does not implement `{}`", trait_pred.print_modifiers_and_trait_path())
2501         };
2502
2503         let mut explain_yield =
2504             |interior_span: Span, yield_span: Span, scope_span: Option<Span>| {
2505                 let mut span = MultiSpan::from_span(yield_span);
2506                 let snippet = match source_map.span_to_snippet(interior_span) {
2507                     // #70935: If snippet contains newlines, display "the value" instead
2508                     // so that we do not emit complex diagnostics.
2509                     Ok(snippet) if !snippet.contains('\n') => format!("`{}`", snippet),
2510                     _ => "the value".to_string(),
2511                 };
2512                 // note: future is not `Send` as this value is used across an await
2513                 //   --> $DIR/issue-70935-complex-spans.rs:13:9
2514                 //    |
2515                 // LL |            baz(|| async {
2516                 //    |  ______________-
2517                 //    | |
2518                 //    | |
2519                 // LL | |              foo(tx.clone());
2520                 // LL | |          }).await;
2521                 //    | |          - ^^^^^^ await occurs here, with value maybe used later
2522                 //    | |__________|
2523                 //    |            has type `closure` which is not `Send`
2524                 // note: value is later dropped here
2525                 // LL | |          }).await;
2526                 //    | |                  ^
2527                 //
2528                 span.push_span_label(
2529                     yield_span,
2530                     format!("{} occurs here, with {} maybe used later", await_or_yield, snippet),
2531                 );
2532                 span.push_span_label(
2533                     interior_span,
2534                     format!("has type `{}` which {}", target_ty, trait_explanation),
2535                 );
2536                 if let Some(scope_span) = scope_span {
2537                     let scope_span = source_map.end_point(scope_span);
2538
2539                     let msg = format!("{} is later dropped here", snippet);
2540                     span.push_span_label(scope_span, msg);
2541                 }
2542                 err.span_note(
2543                     span,
2544                     &format!(
2545                         "{} {} as this value is used across {}",
2546                         future_or_generator, trait_explanation, an_await_or_yield
2547                     ),
2548                 );
2549             };
2550         match interior_or_upvar_span {
2551             GeneratorInteriorOrUpvar::Interior(interior_span, interior_extra_info) => {
2552                 if let Some((scope_span, yield_span, expr, from_awaited_ty)) = interior_extra_info {
2553                     if let Some(await_span) = from_awaited_ty {
2554                         // The type causing this obligation is one being awaited at await_span.
2555                         let mut span = MultiSpan::from_span(await_span);
2556                         span.push_span_label(
2557                             await_span,
2558                             format!(
2559                                 "await occurs here on type `{}`, which {}",
2560                                 target_ty, trait_explanation
2561                             ),
2562                         );
2563                         err.span_note(
2564                             span,
2565                             &format!(
2566                                 "future {not_trait} as it awaits another future which {not_trait}",
2567                                 not_trait = trait_explanation
2568                             ),
2569                         );
2570                     } else {
2571                         // Look at the last interior type to get a span for the `.await`.
2572                         debug!(
2573                             generator_interior_types = ?format_args!(
2574                                 "{:#?}", typeck_results.as_ref().map(|t| &t.generator_interior_types)
2575                             ),
2576                         );
2577                         explain_yield(interior_span, yield_span, scope_span);
2578                     }
2579
2580                     if let Some(expr_id) = expr {
2581                         let expr = hir.expect_expr(expr_id);
2582                         debug!("target_ty evaluated from {:?}", expr);
2583
2584                         let parent = hir.parent_id(expr_id);
2585                         if let Some(hir::Node::Expr(e)) = hir.find(parent) {
2586                             let parent_span = hir.span(parent);
2587                             let parent_did = parent.owner.to_def_id();
2588                             // ```rust
2589                             // impl T {
2590                             //     fn foo(&self) -> i32 {}
2591                             // }
2592                             // T.foo();
2593                             // ^^^^^^^ a temporary `&T` created inside this method call due to `&self`
2594                             // ```
2595                             //
2596                             let is_region_borrow = if let Some(typeck_results) = typeck_results {
2597                                 typeck_results
2598                                     .expr_adjustments(expr)
2599                                     .iter()
2600                                     .any(|adj| adj.is_region_borrow())
2601                             } else {
2602                                 false
2603                             };
2604
2605                             // ```rust
2606                             // struct Foo(*const u8);
2607                             // bar(Foo(std::ptr::null())).await;
2608                             //     ^^^^^^^^^^^^^^^^^^^^^ raw-ptr `*T` created inside this struct ctor.
2609                             // ```
2610                             debug!(parent_def_kind = ?self.tcx.def_kind(parent_did));
2611                             let is_raw_borrow_inside_fn_like_call =
2612                                 match self.tcx.def_kind(parent_did) {
2613                                     DefKind::Fn | DefKind::Ctor(..) => target_ty.is_unsafe_ptr(),
2614                                     _ => false,
2615                                 };
2616                             if let Some(typeck_results) = typeck_results {
2617                                 if (typeck_results.is_method_call(e) && is_region_borrow)
2618                                     || is_raw_borrow_inside_fn_like_call
2619                                 {
2620                                     err.span_help(
2621                                         parent_span,
2622                                         "consider moving this into a `let` \
2623                         binding to create a shorter lived borrow",
2624                                     );
2625                                 }
2626                             }
2627                         }
2628                     }
2629                 }
2630             }
2631             GeneratorInteriorOrUpvar::Upvar(upvar_span) => {
2632                 // `Some((ref_ty, is_mut))` if `target_ty` is `&T` or `&mut T` and fails to impl `Send`
2633                 let non_send = match target_ty.kind() {
2634                     ty::Ref(_, ref_ty, mutability) => match self.evaluate_obligation(&obligation) {
2635                         Ok(eval) if !eval.may_apply() => Some((ref_ty, mutability.is_mut())),
2636                         _ => None,
2637                     },
2638                     _ => None,
2639                 };
2640
2641                 let (span_label, span_note) = match non_send {
2642                     // if `target_ty` is `&T` or `&mut T` and fails to impl `Send`,
2643                     // include suggestions to make `T: Sync` so that `&T: Send`,
2644                     // or to make `T: Send` so that `&mut T: Send`
2645                     Some((ref_ty, is_mut)) => {
2646                         let ref_ty_trait = if is_mut { "Send" } else { "Sync" };
2647                         let ref_kind = if is_mut { "&mut" } else { "&" };
2648                         (
2649                             format!(
2650                                 "has type `{}` which {}, because `{}` is not `{}`",
2651                                 target_ty, trait_explanation, ref_ty, ref_ty_trait
2652                             ),
2653                             format!(
2654                                 "captured value {} because `{}` references cannot be sent unless their referent is `{}`",
2655                                 trait_explanation, ref_kind, ref_ty_trait
2656                             ),
2657                         )
2658                     }
2659                     None => (
2660                         format!("has type `{}` which {}", target_ty, trait_explanation),
2661                         format!("captured value {}", trait_explanation),
2662                     ),
2663                 };
2664
2665                 let mut span = MultiSpan::from_span(upvar_span);
2666                 span.push_span_label(upvar_span, span_label);
2667                 err.span_note(span, &span_note);
2668             }
2669         }
2670
2671         // Add a note for the item obligation that remains - normally a note pointing to the
2672         // bound that introduced the obligation (e.g. `T: Send`).
2673         debug!(?next_code);
2674         self.note_obligation_cause_code(
2675             err,
2676             obligation.predicate,
2677             obligation.param_env,
2678             next_code.unwrap(),
2679             &mut Vec::new(),
2680             &mut Default::default(),
2681         );
2682     }
2683
2684     fn note_obligation_cause_code<T>(
2685         &self,
2686         err: &mut Diagnostic,
2687         predicate: T,
2688         param_env: ty::ParamEnv<'tcx>,
2689         cause_code: &ObligationCauseCode<'tcx>,
2690         obligated_types: &mut Vec<Ty<'tcx>>,
2691         seen_requirements: &mut FxHashSet<DefId>,
2692     ) where
2693         T: ToPredicate<'tcx>,
2694     {
2695         let tcx = self.tcx;
2696         let predicate = predicate.to_predicate(tcx);
2697         match *cause_code {
2698             ObligationCauseCode::ExprAssignable
2699             | ObligationCauseCode::MatchExpressionArm { .. }
2700             | ObligationCauseCode::Pattern { .. }
2701             | ObligationCauseCode::IfExpression { .. }
2702             | ObligationCauseCode::IfExpressionWithNoElse
2703             | ObligationCauseCode::MainFunctionType
2704             | ObligationCauseCode::StartFunctionType
2705             | ObligationCauseCode::IntrinsicType
2706             | ObligationCauseCode::MethodReceiver
2707             | ObligationCauseCode::ReturnNoExpression
2708             | ObligationCauseCode::UnifyReceiver(..)
2709             | ObligationCauseCode::OpaqueType
2710             | ObligationCauseCode::MiscObligation
2711             | ObligationCauseCode::WellFormed(..)
2712             | ObligationCauseCode::MatchImpl(..)
2713             | ObligationCauseCode::ReturnType
2714             | ObligationCauseCode::ReturnValue(_)
2715             | ObligationCauseCode::BlockTailExpression(_)
2716             | ObligationCauseCode::AwaitableExpr(_)
2717             | ObligationCauseCode::ForLoopIterator
2718             | ObligationCauseCode::QuestionMark
2719             | ObligationCauseCode::CheckAssociatedTypeBounds { .. }
2720             | ObligationCauseCode::LetElse
2721             | ObligationCauseCode::BinOp { .. }
2722             | ObligationCauseCode::AscribeUserTypeProvePredicate(..)
2723             | ObligationCauseCode::RustCall => {}
2724             ObligationCauseCode::SliceOrArrayElem => {
2725                 err.note("slice and array elements must have `Sized` type");
2726             }
2727             ObligationCauseCode::TupleElem => {
2728                 err.note("only the last element of a tuple may have a dynamically sized type");
2729             }
2730             ObligationCauseCode::ProjectionWf(data) => {
2731                 err.note(&format!("required so that the projection `{data}` is well-formed"));
2732             }
2733             ObligationCauseCode::ReferenceOutlivesReferent(ref_ty) => {
2734                 err.note(&format!(
2735                     "required so that reference `{ref_ty}` does not outlive its referent"
2736                 ));
2737             }
2738             ObligationCauseCode::ObjectTypeBound(object_ty, region) => {
2739                 err.note(&format!(
2740                     "required so that the lifetime bound of `{}` for `{}` is satisfied",
2741                     region, object_ty,
2742                 ));
2743             }
2744             ObligationCauseCode::ItemObligation(_)
2745             | ObligationCauseCode::ExprItemObligation(..) => {
2746                 // We hold the `DefId` of the item introducing the obligation, but displaying it
2747                 // doesn't add user usable information. It always point at an associated item.
2748             }
2749             ObligationCauseCode::BindingObligation(item_def_id, span)
2750             | ObligationCauseCode::ExprBindingObligation(item_def_id, span, ..) => {
2751                 let item_name = tcx.def_path_str(item_def_id);
2752                 let short_item_name = with_forced_trimmed_paths!(tcx.def_path_str(item_def_id));
2753                 let mut multispan = MultiSpan::from(span);
2754                 let sm = tcx.sess.source_map();
2755                 if let Some(ident) = tcx.opt_item_ident(item_def_id) {
2756                     let same_line =
2757                         match (sm.lookup_line(ident.span.hi()), sm.lookup_line(span.lo())) {
2758                             (Ok(l), Ok(r)) => l.line == r.line,
2759                             _ => true,
2760                         };
2761                     if ident.span.is_visible(sm) && !ident.span.overlaps(span) && !same_line {
2762                         multispan.push_span_label(ident.span, "required by a bound in this");
2763                     }
2764                 }
2765                 let descr = format!("required by a bound in `{item_name}`");
2766                 if span.is_visible(sm) {
2767                     let msg = format!("required by this bound in `{short_item_name}`");
2768                     multispan.push_span_label(span, msg);
2769                     err.span_note(multispan, &descr);
2770                 } else {
2771                     err.span_note(tcx.def_span(item_def_id), &descr);
2772                 }
2773             }
2774             ObligationCauseCode::ObjectCastObligation(concrete_ty, object_ty) => {
2775                 let (concrete_ty, concrete_file) =
2776                     self.tcx.short_ty_string(self.resolve_vars_if_possible(concrete_ty));
2777                 let (object_ty, object_file) =
2778                     self.tcx.short_ty_string(self.resolve_vars_if_possible(object_ty));
2779                 err.note(&with_forced_trimmed_paths!(format!(
2780                     "required for the cast from `{concrete_ty}` to the object type `{object_ty}`",
2781                 )));
2782                 if let Some(file) = concrete_file {
2783                     err.note(&format!(
2784                         "the full name for the casted type has been written to '{}'",
2785                         file.display(),
2786                     ));
2787                 }
2788                 if let Some(file) = object_file {
2789                     err.note(&format!(
2790                         "the full name for the object type has been written to '{}'",
2791                         file.display(),
2792                     ));
2793                 }
2794             }
2795             ObligationCauseCode::Coercion { source: _, target } => {
2796                 err.note(&format!("required by cast to type `{}`", self.ty_to_string(target)));
2797             }
2798             ObligationCauseCode::RepeatElementCopy { is_const_fn } => {
2799                 err.note(
2800                     "the `Copy` trait is required because this value will be copied for each element of the array",
2801                 );
2802
2803                 if is_const_fn {
2804                     err.help(
2805                         "consider creating a new `const` item and initializing it with the result \
2806                         of the function call to be used in the repeat position, like \
2807                         `const VAL: Type = const_fn();` and `let x = [VAL; 42];`",
2808                     );
2809                 }
2810
2811                 if self.tcx.sess.is_nightly_build() && is_const_fn {
2812                     err.help(
2813                         "create an inline `const` block, see RFC #2920 \
2814                          <https://github.com/rust-lang/rfcs/pull/2920> for more information",
2815                     );
2816                 }
2817             }
2818             ObligationCauseCode::VariableType(hir_id) => {
2819                 let parent_node = self.tcx.hir().parent_id(hir_id);
2820                 match self.tcx.hir().find(parent_node) {
2821                     Some(Node::Local(hir::Local { ty: Some(ty), .. })) => {
2822                         err.span_suggestion_verbose(
2823                             ty.span.shrink_to_lo(),
2824                             "consider borrowing here",
2825                             "&",
2826                             Applicability::MachineApplicable,
2827                         );
2828                         err.note("all local variables must have a statically known size");
2829                     }
2830                     Some(Node::Local(hir::Local {
2831                         init: Some(hir::Expr { kind: hir::ExprKind::Index(_, _), span, .. }),
2832                         ..
2833                     })) => {
2834                         // When encountering an assignment of an unsized trait, like
2835                         // `let x = ""[..];`, provide a suggestion to borrow the initializer in
2836                         // order to use have a slice instead.
2837                         err.span_suggestion_verbose(
2838                             span.shrink_to_lo(),
2839                             "consider borrowing here",
2840                             "&",
2841                             Applicability::MachineApplicable,
2842                         );
2843                         err.note("all local variables must have a statically known size");
2844                     }
2845                     Some(Node::Param(param)) => {
2846                         err.span_suggestion_verbose(
2847                             param.ty_span.shrink_to_lo(),
2848                             "function arguments must have a statically known size, borrowed types \
2849                             always have a known size",
2850                             "&",
2851                             Applicability::MachineApplicable,
2852                         );
2853                     }
2854                     _ => {
2855                         err.note("all local variables must have a statically known size");
2856                     }
2857                 }
2858                 if !self.tcx.features().unsized_locals {
2859                     err.help("unsized locals are gated as an unstable feature");
2860                 }
2861             }
2862             ObligationCauseCode::SizedArgumentType(sp) => {
2863                 if let Some(span) = sp {
2864                     if let ty::PredicateKind::Clause(clause) = predicate.kind().skip_binder()
2865                         && let ty::Clause::Trait(trait_pred) = clause
2866                         && let ty::Dynamic(..) = trait_pred.self_ty().kind()
2867                     {
2868                         let span = if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span)
2869                             && snippet.starts_with("dyn ")
2870                         {
2871                             let pos = snippet.len() - snippet[3..].trim_start().len();
2872                             span.with_hi(span.lo() + BytePos(pos as u32))
2873                         } else {
2874                             span.shrink_to_lo()
2875                         };
2876                         err.span_suggestion_verbose(
2877                             span,
2878                             "you can use `impl Trait` as the argument type",
2879                             "impl ".to_string(),
2880                             Applicability::MaybeIncorrect,
2881                         );
2882                     }
2883                     err.span_suggestion_verbose(
2884                         span.shrink_to_lo(),
2885                         "function arguments must have a statically known size, borrowed types \
2886                          always have a known size",
2887                         "&",
2888                         Applicability::MachineApplicable,
2889                     );
2890                 } else {
2891                     err.note("all function arguments must have a statically known size");
2892                 }
2893                 if tcx.sess.opts.unstable_features.is_nightly_build()
2894                     && !self.tcx.features().unsized_fn_params
2895                 {
2896                     err.help("unsized fn params are gated as an unstable feature");
2897                 }
2898             }
2899             ObligationCauseCode::SizedReturnType => {
2900                 err.note("the return type of a function must have a statically known size");
2901             }
2902             ObligationCauseCode::SizedYieldType => {
2903                 err.note("the yield type of a generator must have a statically known size");
2904             }
2905             ObligationCauseCode::SizedBoxType => {
2906                 err.note("the type of a box expression must have a statically known size");
2907             }
2908             ObligationCauseCode::AssignmentLhsSized => {
2909                 err.note("the left-hand-side of an assignment must have a statically known size");
2910             }
2911             ObligationCauseCode::TupleInitializerSized => {
2912                 err.note("tuples must have a statically known size to be initialized");
2913             }
2914             ObligationCauseCode::StructInitializerSized => {
2915                 err.note("structs must have a statically known size to be initialized");
2916             }
2917             ObligationCauseCode::FieldSized { adt_kind: ref item, last, span } => {
2918                 match *item {
2919                     AdtKind::Struct => {
2920                         if last {
2921                             err.note(
2922                                 "the last field of a packed struct may only have a \
2923                                 dynamically sized type if it does not need drop to be run",
2924                             );
2925                         } else {
2926                             err.note(
2927                                 "only the last field of a struct may have a dynamically sized type",
2928                             );
2929                         }
2930                     }
2931                     AdtKind::Union => {
2932                         err.note("no field of a union may have a dynamically sized type");
2933                     }
2934                     AdtKind::Enum => {
2935                         err.note("no field of an enum variant may have a dynamically sized type");
2936                     }
2937                 }
2938                 err.help("change the field's type to have a statically known size");
2939                 err.span_suggestion(
2940                     span.shrink_to_lo(),
2941                     "borrowed types always have a statically known size",
2942                     "&",
2943                     Applicability::MachineApplicable,
2944                 );
2945                 err.multipart_suggestion(
2946                     "the `Box` type always has a statically known size and allocates its contents \
2947                      in the heap",
2948                     vec![
2949                         (span.shrink_to_lo(), "Box<".to_string()),
2950                         (span.shrink_to_hi(), ">".to_string()),
2951                     ],
2952                     Applicability::MachineApplicable,
2953                 );
2954             }
2955             ObligationCauseCode::ConstSized => {
2956                 err.note("constant expressions must have a statically known size");
2957             }
2958             ObligationCauseCode::InlineAsmSized => {
2959                 err.note("all inline asm arguments must have a statically known size");
2960             }
2961             ObligationCauseCode::ConstPatternStructural => {
2962                 err.note("constants used for pattern-matching must derive `PartialEq` and `Eq`");
2963             }
2964             ObligationCauseCode::SharedStatic => {
2965                 err.note("shared static variables must have a type that implements `Sync`");
2966             }
2967             ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
2968                 let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
2969                 let ty = parent_trait_ref.skip_binder().self_ty();
2970                 if parent_trait_ref.references_error() {
2971                     // NOTE(eddyb) this was `.cancel()`, but `err`
2972                     // is borrowed, so we can't fully defuse it.
2973                     err.downgrade_to_delayed_bug();
2974                     return;
2975                 }
2976
2977                 // If the obligation for a tuple is set directly by a Generator or Closure,
2978                 // then the tuple must be the one containing capture types.
2979                 let is_upvar_tys_infer_tuple = if !matches!(ty.kind(), ty::Tuple(..)) {
2980                     false
2981                 } else {
2982                     if let ObligationCauseCode::BuiltinDerivedObligation(data) = &*data.parent_code
2983                     {
2984                         let parent_trait_ref =
2985                             self.resolve_vars_if_possible(data.parent_trait_pred);
2986                         let nested_ty = parent_trait_ref.skip_binder().self_ty();
2987                         matches!(nested_ty.kind(), ty::Generator(..))
2988                             || matches!(nested_ty.kind(), ty::Closure(..))
2989                     } else {
2990                         false
2991                     }
2992                 };
2993
2994                 let identity_future = tcx.require_lang_item(LangItem::IdentityFuture, None);
2995
2996                 // Don't print the tuple of capture types
2997                 'print: {
2998                     if !is_upvar_tys_infer_tuple {
2999                         let msg = with_forced_trimmed_paths!(format!(
3000                             "required because it appears within the type `{ty}`",
3001                         ));
3002                         match ty.kind() {
3003                             ty::Adt(def, _) => match self.tcx.opt_item_ident(def.did()) {
3004                                 Some(ident) => err.span_note(ident.span, &msg),
3005                                 None => err.note(&msg),
3006                             },
3007                             ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }) => {
3008                                 // Avoid printing the future from `core::future::identity_future`, it's not helpful
3009                                 if tcx.parent(*def_id) == identity_future {
3010                                     break 'print;
3011                                 }
3012
3013                                 // If the previous type is `identity_future`, this is the future generated by the body of an async function.
3014                                 // Avoid printing it twice (it was already printed in the `ty::Generator` arm below).
3015                                 let is_future = tcx.ty_is_opaque_future(ty);
3016                                 debug!(
3017                                     ?obligated_types,
3018                                     ?is_future,
3019                                     "note_obligation_cause_code: check for async fn"
3020                                 );
3021                                 if is_future
3022                                     && obligated_types.last().map_or(false, |ty| match ty.kind() {
3023                                         ty::Generator(last_def_id, ..) => {
3024                                             tcx.generator_is_async(*last_def_id)
3025                                         }
3026                                         _ => false,
3027                                     })
3028                                 {
3029                                     break 'print;
3030                                 }
3031                                 err.span_note(self.tcx.def_span(def_id), &msg)
3032                             }
3033                             ty::GeneratorWitness(bound_tys) => {
3034                                 use std::fmt::Write;
3035
3036                                 // FIXME: this is kind of an unusual format for rustc, can we make it more clear?
3037                                 // Maybe we should just remove this note altogether?
3038                                 // FIXME: only print types which don't meet the trait requirement
3039                                 let mut msg =
3040                                     "required because it captures the following types: ".to_owned();
3041                                 for ty in bound_tys.skip_binder() {
3042                                     with_forced_trimmed_paths!(write!(msg, "`{}`, ", ty).unwrap());
3043                                 }
3044                                 err.note(msg.trim_end_matches(", "))
3045                             }
3046                             ty::GeneratorWitnessMIR(def_id, substs) => {
3047                                 use std::fmt::Write;
3048
3049                                 // FIXME: this is kind of an unusual format for rustc, can we make it more clear?
3050                                 // Maybe we should just remove this note altogether?
3051                                 // FIXME: only print types which don't meet the trait requirement
3052                                 let mut msg =
3053                                     "required because it captures the following types: ".to_owned();
3054                                 for bty in tcx.generator_hidden_types(*def_id) {
3055                                     let ty = bty.subst(tcx, substs);
3056                                     write!(msg, "`{}`, ", ty).unwrap();
3057                                 }
3058                                 err.note(msg.trim_end_matches(", "))
3059                             }
3060                             ty::Generator(def_id, _, _) => {
3061                                 let sp = self.tcx.def_span(def_id);
3062
3063                                 // Special-case this to say "async block" instead of `[static generator]`.
3064                                 let kind = tcx.generator_kind(def_id).unwrap().descr();
3065                                 err.span_note(
3066                                     sp,
3067                                     with_forced_trimmed_paths!(&format!(
3068                                         "required because it's used within this {kind}",
3069                                     )),
3070                                 )
3071                             }
3072                             ty::Closure(def_id, _) => err.span_note(
3073                                 self.tcx.def_span(def_id),
3074                                 "required because it's used within this closure",
3075                             ),
3076                             _ => err.note(&msg),
3077                         };
3078                     }
3079                 }
3080
3081                 obligated_types.push(ty);
3082
3083                 let parent_predicate = parent_trait_ref;
3084                 if !self.is_recursive_obligation(obligated_types, &data.parent_code) {
3085                     // #74711: avoid a stack overflow
3086                     ensure_sufficient_stack(|| {
3087                         self.note_obligation_cause_code(
3088                             err,
3089                             parent_predicate,
3090                             param_env,
3091                             &data.parent_code,
3092                             obligated_types,
3093                             seen_requirements,
3094                         )
3095                     });
3096                 } else {
3097                     ensure_sufficient_stack(|| {
3098                         self.note_obligation_cause_code(
3099                             err,
3100                             parent_predicate,
3101                             param_env,
3102                             cause_code.peel_derives(),
3103                             obligated_types,
3104                             seen_requirements,
3105                         )
3106                     });
3107                 }
3108             }
3109             ObligationCauseCode::ImplDerivedObligation(ref data) => {
3110                 let mut parent_trait_pred =
3111                     self.resolve_vars_if_possible(data.derived.parent_trait_pred);
3112                 parent_trait_pred.remap_constness_diag(param_env);
3113                 let parent_def_id = parent_trait_pred.def_id();
3114                 let (self_ty, file) =
3115                     self.tcx.short_ty_string(parent_trait_pred.skip_binder().self_ty());
3116                 let msg = format!(
3117                     "required for `{self_ty}` to implement `{}`",
3118                     parent_trait_pred.print_modifiers_and_trait_path()
3119                 );
3120                 let mut is_auto_trait = false;
3121                 match self.tcx.hir().get_if_local(data.impl_def_id) {
3122                     Some(Node::Item(hir::Item {
3123                         kind: hir::ItemKind::Trait(is_auto, ..),
3124                         ident,
3125                         ..
3126                     })) => {
3127                         // FIXME: we should do something else so that it works even on crate foreign
3128                         // auto traits.
3129                         is_auto_trait = matches!(is_auto, hir::IsAuto::Yes);
3130                         err.span_note(ident.span, &msg);
3131                     }
3132                     Some(Node::Item(hir::Item {
3133                         kind: hir::ItemKind::Impl(hir::Impl { of_trait, self_ty, .. }),
3134                         ..
3135                     })) => {
3136                         let mut spans = Vec::with_capacity(2);
3137                         if let Some(trait_ref) = of_trait {
3138                             spans.push(trait_ref.path.span);
3139                         }
3140                         spans.push(self_ty.span);
3141                         let mut spans: MultiSpan = spans.into();
3142                         if matches!(
3143                             self_ty.span.ctxt().outer_expn_data().kind,
3144                             ExpnKind::Macro(MacroKind::Derive, _)
3145                         ) || matches!(
3146                             of_trait.as_ref().map(|t| t.path.span.ctxt().outer_expn_data().kind),
3147                             Some(ExpnKind::Macro(MacroKind::Derive, _))
3148                         ) {
3149                             spans.push_span_label(
3150                                 data.span,
3151                                 "unsatisfied trait bound introduced in this `derive` macro",
3152                             );
3153                         } else if !data.span.is_dummy() && !data.span.overlaps(self_ty.span) {
3154                             spans.push_span_label(
3155                                 data.span,
3156                                 "unsatisfied trait bound introduced here",
3157                             );
3158                         }
3159                         err.span_note(spans, &msg);
3160                     }
3161                     _ => {
3162                         err.note(&msg);
3163                     }
3164                 };
3165
3166                 if let Some(file) = file {
3167                     err.note(&format!(
3168                         "the full type name has been written to '{}'",
3169                         file.display(),
3170                     ));
3171                 }
3172                 let mut parent_predicate = parent_trait_pred;
3173                 let mut data = &data.derived;
3174                 let mut count = 0;
3175                 seen_requirements.insert(parent_def_id);
3176                 if is_auto_trait {
3177                     // We don't want to point at the ADT saying "required because it appears within
3178                     // the type `X`", like we would otherwise do in test `supertrait-auto-trait.rs`.
3179                     while let ObligationCauseCode::BuiltinDerivedObligation(derived) =
3180                         &*data.parent_code
3181                     {
3182                         let child_trait_ref =
3183                             self.resolve_vars_if_possible(derived.parent_trait_pred);
3184                         let child_def_id = child_trait_ref.def_id();
3185                         if seen_requirements.insert(child_def_id) {
3186                             break;
3187                         }
3188                         data = derived;
3189                         parent_predicate = child_trait_ref.to_predicate(tcx);
3190                         parent_trait_pred = child_trait_ref;
3191                     }
3192                 }
3193                 while let ObligationCauseCode::ImplDerivedObligation(child) = &*data.parent_code {
3194                     // Skip redundant recursive obligation notes. See `ui/issue-20413.rs`.
3195                     let child_trait_pred =
3196                         self.resolve_vars_if_possible(child.derived.parent_trait_pred);
3197                     let child_def_id = child_trait_pred.def_id();
3198                     if seen_requirements.insert(child_def_id) {
3199                         break;
3200                     }
3201                     count += 1;
3202                     data = &child.derived;
3203                     parent_predicate = child_trait_pred.to_predicate(tcx);
3204                     parent_trait_pred = child_trait_pred;
3205                 }
3206                 if count > 0 {
3207                     err.note(&format!(
3208                         "{} redundant requirement{} hidden",
3209                         count,
3210                         pluralize!(count)
3211                     ));
3212                     let (self_ty, file) =
3213                         self.tcx.short_ty_string(parent_trait_pred.skip_binder().self_ty());
3214                     err.note(&format!(
3215                         "required for `{self_ty}` to implement `{}`",
3216                         parent_trait_pred.print_modifiers_and_trait_path()
3217                     ));
3218                     if let Some(file) = file {
3219                         err.note(&format!(
3220                             "the full type name has been written to '{}'",
3221                             file.display(),
3222                         ));
3223                     }
3224                 }
3225                 // #74711: avoid a stack overflow
3226                 ensure_sufficient_stack(|| {
3227                     self.note_obligation_cause_code(
3228                         err,
3229                         parent_predicate,
3230                         param_env,
3231                         &data.parent_code,
3232                         obligated_types,
3233                         seen_requirements,
3234                     )
3235                 });
3236             }
3237             ObligationCauseCode::DerivedObligation(ref data) => {
3238                 let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
3239                 let parent_predicate = parent_trait_ref;
3240                 // #74711: avoid a stack overflow
3241                 ensure_sufficient_stack(|| {
3242                     self.note_obligation_cause_code(
3243                         err,
3244                         parent_predicate,
3245                         param_env,
3246                         &data.parent_code,
3247                         obligated_types,
3248                         seen_requirements,
3249                     )
3250                 });
3251             }
3252             ObligationCauseCode::FunctionArgumentObligation {
3253                 arg_hir_id,
3254                 call_hir_id,
3255                 ref parent_code,
3256                 ..
3257             } => {
3258                 self.note_function_argument_obligation(
3259                     arg_hir_id,
3260                     err,
3261                     parent_code,
3262                     param_env,
3263                     predicate,
3264                     call_hir_id,
3265                 );
3266                 ensure_sufficient_stack(|| {
3267                     self.note_obligation_cause_code(
3268                         err,
3269                         predicate,
3270                         param_env,
3271                         &parent_code,
3272                         obligated_types,
3273                         seen_requirements,
3274                     )
3275                 });
3276             }
3277             ObligationCauseCode::CompareImplItemObligation { trait_item_def_id, kind, .. } => {
3278                 let item_name = self.tcx.item_name(trait_item_def_id);
3279                 let msg = format!(
3280                     "the requirement `{predicate}` appears on the `impl`'s {kind} \
3281                      `{item_name}` but not on the corresponding trait's {kind}",
3282                 );
3283                 let sp = self
3284                     .tcx
3285                     .opt_item_ident(trait_item_def_id)
3286                     .map(|i| i.span)
3287                     .unwrap_or_else(|| self.tcx.def_span(trait_item_def_id));
3288                 let mut assoc_span: MultiSpan = sp.into();
3289                 assoc_span.push_span_label(
3290                     sp,
3291                     format!("this trait's {kind} doesn't have the requirement `{predicate}`"),
3292                 );
3293                 if let Some(ident) = self
3294                     .tcx
3295                     .opt_associated_item(trait_item_def_id)
3296                     .and_then(|i| self.tcx.opt_item_ident(i.container_id(self.tcx)))
3297                 {
3298                     assoc_span.push_span_label(ident.span, "in this trait");
3299                 }
3300                 err.span_note(assoc_span, &msg);
3301             }
3302             ObligationCauseCode::TrivialBound => {
3303                 err.help("see issue #48214");
3304                 if tcx.sess.opts.unstable_features.is_nightly_build() {
3305                     err.help("add `#![feature(trivial_bounds)]` to the crate attributes to enable");
3306                 }
3307             }
3308             ObligationCauseCode::OpaqueReturnType(expr_info) => {
3309                 if let Some((expr_ty, expr_span)) = expr_info {
3310                     let expr_ty = with_forced_trimmed_paths!(self.ty_to_string(expr_ty));
3311                     err.span_label(
3312                         expr_span,
3313                         with_forced_trimmed_paths!(format!(
3314                             "return type was inferred to be `{expr_ty}` here",
3315                         )),
3316                     );
3317                 }
3318             }
3319         }
3320     }
3321
3322     #[instrument(
3323         level = "debug", skip(self, err), fields(trait_pred.self_ty = ?trait_pred.self_ty())
3324     )]
3325     fn suggest_await_before_try(
3326         &self,
3327         err: &mut Diagnostic,
3328         obligation: &PredicateObligation<'tcx>,
3329         trait_pred: ty::PolyTraitPredicate<'tcx>,
3330         span: Span,
3331     ) {
3332         if let Some(body_id) = self.tcx.hir().maybe_body_owned_by(obligation.cause.body_id) {
3333             let body = self.tcx.hir().body(body_id);
3334             if let Some(hir::GeneratorKind::Async(_)) = body.generator_kind {
3335                 let future_trait = self.tcx.require_lang_item(LangItem::Future, None);
3336
3337                 let self_ty = self.resolve_vars_if_possible(trait_pred.self_ty());
3338                 let impls_future = self.type_implements_trait(
3339                     future_trait,
3340                     [self.tcx.erase_late_bound_regions(self_ty)],
3341                     obligation.param_env,
3342                 );
3343                 if !impls_future.must_apply_modulo_regions() {
3344                     return;
3345                 }
3346
3347                 let item_def_id = self.tcx.associated_item_def_ids(future_trait)[0];
3348                 // `<T as Future>::Output`
3349                 let projection_ty = trait_pred.map_bound(|trait_pred| {
3350                     self.tcx.mk_projection(
3351                         item_def_id,
3352                         // Future::Output has no substs
3353                         [trait_pred.self_ty()],
3354                     )
3355                 });
3356                 let InferOk { value: projection_ty, .. } =
3357                     self.at(&obligation.cause, obligation.param_env).normalize(projection_ty);
3358
3359                 debug!(
3360                     normalized_projection_type = ?self.resolve_vars_if_possible(projection_ty)
3361                 );
3362                 let try_obligation = self.mk_trait_obligation_with_new_self_ty(
3363                     obligation.param_env,
3364                     trait_pred.map_bound(|trait_pred| (trait_pred, projection_ty.skip_binder())),
3365                 );
3366                 debug!(try_trait_obligation = ?try_obligation);
3367                 if self.predicate_may_hold(&try_obligation)
3368                     && let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span)
3369                     && snippet.ends_with('?')
3370                 {
3371                     err.span_suggestion_verbose(
3372                         span.with_hi(span.hi() - BytePos(1)).shrink_to_hi(),
3373                         "consider `await`ing on the `Future`",
3374                         ".await",
3375                         Applicability::MaybeIncorrect,
3376                     );
3377                 }
3378             }
3379         }
3380     }
3381
3382     fn suggest_floating_point_literal(
3383         &self,
3384         obligation: &PredicateObligation<'tcx>,
3385         err: &mut Diagnostic,
3386         trait_ref: &ty::PolyTraitRef<'tcx>,
3387     ) {
3388         let rhs_span = match obligation.cause.code() {
3389             ObligationCauseCode::BinOp { rhs_span: Some(span), is_lit, .. } if *is_lit => span,
3390             _ => return,
3391         };
3392         if let ty::Float(_) = trait_ref.skip_binder().self_ty().kind()
3393             && let ty::Infer(InferTy::IntVar(_)) = trait_ref.skip_binder().substs.type_at(1).kind()
3394         {
3395             err.span_suggestion_verbose(
3396                 rhs_span.shrink_to_hi(),
3397                 "consider using a floating-point literal by writing it with `.0`",
3398                 ".0",
3399                 Applicability::MaybeIncorrect,
3400             );
3401         }
3402     }
3403
3404     fn suggest_derive(
3405         &self,
3406         obligation: &PredicateObligation<'tcx>,
3407         err: &mut Diagnostic,
3408         trait_pred: ty::PolyTraitPredicate<'tcx>,
3409     ) {
3410         let Some(diagnostic_name) = self.tcx.get_diagnostic_name(trait_pred.def_id()) else {
3411             return;
3412         };
3413         let (adt, substs) = match trait_pred.skip_binder().self_ty().kind() {
3414             ty::Adt(adt, substs) if adt.did().is_local() => (adt, substs),
3415             _ => return,
3416         };
3417         let can_derive = {
3418             let is_derivable_trait = match diagnostic_name {
3419                 sym::Default => !adt.is_enum(),
3420                 sym::PartialEq | sym::PartialOrd => {
3421                     let rhs_ty = trait_pred.skip_binder().trait_ref.substs.type_at(1);
3422                     trait_pred.skip_binder().self_ty() == rhs_ty
3423                 }
3424                 sym::Eq | sym::Ord | sym::Clone | sym::Copy | sym::Hash | sym::Debug => true,
3425                 _ => false,
3426             };
3427             is_derivable_trait &&
3428                 // Ensure all fields impl the trait.
3429                 adt.all_fields().all(|field| {
3430                     let field_ty = field.ty(self.tcx, substs);
3431                     let trait_substs = match diagnostic_name {
3432                         sym::PartialEq | sym::PartialOrd => {
3433                             Some(field_ty)
3434                         }
3435                         _ => None,
3436                     };
3437                     let trait_pred = trait_pred.map_bound_ref(|tr| ty::TraitPredicate {
3438                         trait_ref: self.tcx.mk_trait_ref(
3439                             trait_pred.def_id(),
3440                             [field_ty].into_iter().chain(trait_substs),
3441                         ),
3442                         ..*tr
3443                     });
3444                     let field_obl = Obligation::new(
3445                         self.tcx,
3446                         obligation.cause.clone(),
3447                         obligation.param_env,
3448                         trait_pred,
3449                     );
3450                     self.predicate_must_hold_modulo_regions(&field_obl)
3451                 })
3452         };
3453         if can_derive {
3454             err.span_suggestion_verbose(
3455                 self.tcx.def_span(adt.did()).shrink_to_lo(),
3456                 &format!(
3457                     "consider annotating `{}` with `#[derive({})]`",
3458                     trait_pred.skip_binder().self_ty(),
3459                     diagnostic_name,
3460                 ),
3461                 format!("#[derive({})]\n", diagnostic_name),
3462                 Applicability::MaybeIncorrect,
3463             );
3464         }
3465     }
3466
3467     fn suggest_dereferencing_index(
3468         &self,
3469         obligation: &PredicateObligation<'tcx>,
3470         err: &mut Diagnostic,
3471         trait_pred: ty::PolyTraitPredicate<'tcx>,
3472     ) {
3473         if let ObligationCauseCode::ImplDerivedObligation(_) = obligation.cause.code()
3474             && self.tcx.is_diagnostic_item(sym::SliceIndex, trait_pred.skip_binder().trait_ref.def_id)
3475             && let ty::Slice(_) = trait_pred.skip_binder().trait_ref.substs.type_at(1).kind()
3476             && let ty::Ref(_, inner_ty, _) = trait_pred.skip_binder().self_ty().kind()
3477             && let ty::Uint(ty::UintTy::Usize) = inner_ty.kind()
3478         {
3479             err.span_suggestion_verbose(
3480                 obligation.cause.span.shrink_to_lo(),
3481             "dereference this index",
3482             '*',
3483                 Applicability::MachineApplicable,
3484             );
3485         }
3486     }
3487     fn note_function_argument_obligation(
3488         &self,
3489         arg_hir_id: HirId,
3490         err: &mut Diagnostic,
3491         parent_code: &ObligationCauseCode<'tcx>,
3492         param_env: ty::ParamEnv<'tcx>,
3493         failed_pred: ty::Predicate<'tcx>,
3494         call_hir_id: HirId,
3495     ) {
3496         let tcx = self.tcx;
3497         let hir = tcx.hir();
3498         if let Some(Node::Expr(expr)) = hir.find(arg_hir_id)
3499             && let Some(typeck_results) = &self.typeck_results
3500         {
3501             if let hir::Expr { kind: hir::ExprKind::Block(..), .. } = expr {
3502                 let expr = expr.peel_blocks();
3503                 let ty = typeck_results.expr_ty_adjusted_opt(expr).unwrap_or(tcx.ty_error());
3504                 let span = expr.span;
3505                 if Some(span) != err.span.primary_span() {
3506                     err.span_label(
3507                         span,
3508                         if ty.references_error() {
3509                             String::new()
3510                         } else {
3511                             let ty = with_forced_trimmed_paths!(self.ty_to_string(ty));
3512                             format!("this tail expression is of type `{ty}`")
3513                         },
3514                     );
3515                 }
3516             }
3517
3518             // FIXME: visit the ty to see if there's any closure involved, and if there is,
3519             // check whether its evaluated return type is the same as the one corresponding
3520             // to an associated type (as seen from `trait_pred`) in the predicate. Like in
3521             // trait_pred `S: Sum<<Self as Iterator>::Item>` and predicate `i32: Sum<&()>`
3522             let mut type_diffs = vec![];
3523
3524             if let ObligationCauseCode::ExprBindingObligation(def_id, _, _, idx) = parent_code.deref()
3525                 && let Some(node_substs) = typeck_results.node_substs_opt(call_hir_id)
3526                 && let where_clauses = self.tcx.predicates_of(def_id).instantiate(self.tcx, node_substs)
3527                 && let Some(where_pred) = where_clauses.predicates.get(*idx)
3528             {
3529                 if let Some(where_pred) = where_pred.to_opt_poly_trait_pred()
3530                     && let Some(failed_pred) = failed_pred.to_opt_poly_trait_pred()
3531                 {
3532                     let mut c = CollectAllMismatches {
3533                         infcx: self.infcx,
3534                         param_env,
3535                         errors: vec![],
3536                     };
3537                     if let Ok(_) = c.relate(where_pred, failed_pred) {
3538                         type_diffs = c.errors;
3539                     }
3540                 } else if let Some(where_pred) = where_pred.to_opt_poly_projection_pred()
3541                     && let Some(failed_pred) = failed_pred.to_opt_poly_projection_pred()
3542                     && let Some(found) = failed_pred.skip_binder().term.ty()
3543                 {
3544                     type_diffs = vec![
3545                         Sorts(ty::error::ExpectedFound {
3546                             expected: self.tcx.mk_ty(ty::Alias(ty::Projection, where_pred.skip_binder().projection_ty)),
3547                             found,
3548                         }),
3549                     ];
3550                 }
3551             }
3552             if let hir::ExprKind::Path(hir::QPath::Resolved(None, path)) = expr.kind
3553                 && let hir::Path { res: hir::def::Res::Local(hir_id), .. } = path
3554                 && let Some(hir::Node::Pat(binding)) = self.tcx.hir().find(*hir_id)
3555                 && let parent_hir_id = self.tcx.hir().parent_id(binding.hir_id)
3556                 && let Some(hir::Node::Local(local)) = self.tcx.hir().find(parent_hir_id)
3557                 && let Some(binding_expr) = local.init
3558             {
3559                 // If the expression we're calling on is a binding, we want to point at the
3560                 // `let` when talking about the type. Otherwise we'll point at every part
3561                 // of the method chain with the type.
3562                 self.point_at_chain(binding_expr, &typeck_results, type_diffs, param_env, err);
3563             } else {
3564                 self.point_at_chain(expr, &typeck_results, type_diffs, param_env, err);
3565             }
3566         }
3567         let call_node = hir.find(call_hir_id);
3568         if let Some(Node::Expr(hir::Expr {
3569             kind: hir::ExprKind::MethodCall(path, rcvr, ..), ..
3570         })) = call_node
3571         {
3572             if Some(rcvr.span) == err.span.primary_span() {
3573                 err.replace_span_with(path.ident.span, true);
3574             }
3575         }
3576         if let Some(Node::Expr(hir::Expr {
3577             kind:
3578                 hir::ExprKind::Call(hir::Expr { span, .. }, _)
3579                 | hir::ExprKind::MethodCall(hir::PathSegment { ident: Ident { span, .. }, .. }, ..),
3580             ..
3581         })) = hir.find(call_hir_id)
3582         {
3583             if Some(*span) != err.span.primary_span() {
3584                 err.span_label(*span, "required by a bound introduced by this call");
3585             }
3586         }
3587     }
3588
3589     fn point_at_chain(
3590         &self,
3591         expr: &hir::Expr<'_>,
3592         typeck_results: &TypeckResults<'tcx>,
3593         type_diffs: Vec<TypeError<'tcx>>,
3594         param_env: ty::ParamEnv<'tcx>,
3595         err: &mut Diagnostic,
3596     ) {
3597         let mut primary_spans = vec![];
3598         let mut span_labels = vec![];
3599
3600         let tcx = self.tcx;
3601
3602         let mut print_root_expr = true;
3603         let mut assocs = vec![];
3604         let mut expr = expr;
3605         let mut prev_ty = self.resolve_vars_if_possible(
3606             typeck_results.expr_ty_adjusted_opt(expr).unwrap_or(tcx.ty_error()),
3607         );
3608         while let hir::ExprKind::MethodCall(_path_segment, rcvr_expr, _args, span) = expr.kind {
3609             // Point at every method call in the chain with the resulting type.
3610             // vec![1, 2, 3].iter().map(mapper).sum<i32>()
3611             //               ^^^^^^ ^^^^^^^^^^^
3612             expr = rcvr_expr;
3613             let assocs_in_this_method =
3614                 self.probe_assoc_types_at_expr(&type_diffs, span, prev_ty, expr.hir_id, param_env);
3615             assocs.push(assocs_in_this_method);
3616             prev_ty = self.resolve_vars_if_possible(
3617                 typeck_results.expr_ty_adjusted_opt(expr).unwrap_or(tcx.ty_error()),
3618             );
3619
3620             if let hir::ExprKind::Path(hir::QPath::Resolved(None, path)) = expr.kind
3621                 && let hir::Path { res: hir::def::Res::Local(hir_id), .. } = path
3622                 && let Some(hir::Node::Pat(binding)) = self.tcx.hir().find(*hir_id)
3623                 && let Some(parent) = self.tcx.hir().find_parent(binding.hir_id)
3624             {
3625                 // We've reached the root of the method call chain...
3626                 if let hir::Node::Local(local) = parent
3627                     && let Some(binding_expr) = local.init
3628                 {
3629                     // ...and it is a binding. Get the binding creation and continue the chain.
3630                     expr = binding_expr;
3631                 }
3632                 if let hir::Node::Param(param) = parent {
3633                     // ...and it is a an fn argument.
3634                     let prev_ty = self.resolve_vars_if_possible(
3635                         typeck_results.node_type_opt(param.hir_id).unwrap_or(tcx.ty_error()),
3636                     );
3637                     let assocs_in_this_method = self.probe_assoc_types_at_expr(&type_diffs, param.ty_span, prev_ty, param.hir_id, param_env);
3638                     if assocs_in_this_method.iter().any(|a| a.is_some()) {
3639                         assocs.push(assocs_in_this_method);
3640                         print_root_expr = false;
3641                     }
3642                     break;
3643                 }
3644             }
3645         }
3646         // We want the type before deref coercions, otherwise we talk about `&[_]`
3647         // instead of `Vec<_>`.
3648         if let Some(ty) = typeck_results.expr_ty_opt(expr) && print_root_expr {
3649             let ty = with_forced_trimmed_paths!(self.ty_to_string(ty));
3650             // Point at the root expression
3651             // vec![1, 2, 3].iter().map(mapper).sum<i32>()
3652             // ^^^^^^^^^^^^^
3653             span_labels.push((expr.span, format!("this expression has type `{ty}`")));
3654         };
3655         // Only show this if it is not a "trivial" expression (not a method
3656         // chain) and there are associated types to talk about.
3657         let mut assocs = assocs.into_iter().peekable();
3658         while let Some(assocs_in_method) = assocs.next() {
3659             let Some(prev_assoc_in_method) = assocs.peek() else {
3660                 for entry in assocs_in_method {
3661                     let Some((span, (assoc, ty))) = entry else { continue; };
3662                     if primary_spans.is_empty() || type_diffs.iter().any(|diff| {
3663                         let Sorts(expected_found) = diff else { return false; };
3664                         self.can_eq(param_env, expected_found.found, ty).is_ok()
3665                     }) {
3666                         // FIXME: this doesn't quite work for `Iterator::collect`
3667                         // because we have `Vec<i32>` and `()`, but we'd want `i32`
3668                         // to point at the `.into_iter()` call, but as long as we
3669                         // still point at the other method calls that might have
3670                         // introduced the issue, this is fine for now.
3671                         primary_spans.push(span);
3672                     }
3673                     span_labels.push((
3674                         span,
3675                         with_forced_trimmed_paths!(format!(
3676                             "`{}` is `{ty}` here",
3677                             self.tcx.def_path_str(assoc),
3678                         )),
3679                     ));
3680                 }
3681                 break;
3682             };
3683             for (entry, prev_entry) in
3684                 assocs_in_method.into_iter().zip(prev_assoc_in_method.into_iter())
3685             {
3686                 match (entry, prev_entry) {
3687                     (Some((span, (assoc, ty))), Some((_, (_, prev_ty)))) => {
3688                         let ty_str = with_forced_trimmed_paths!(self.ty_to_string(ty));
3689
3690                         let assoc = with_forced_trimmed_paths!(self.tcx.def_path_str(assoc));
3691                         if self.can_eq(param_env, ty, *prev_ty).is_err() {
3692                             if type_diffs.iter().any(|diff| {
3693                                 let Sorts(expected_found) = diff else { return false; };
3694                                 self.can_eq(param_env, expected_found.found, ty).is_ok()
3695                             }) {
3696                                 primary_spans.push(span);
3697                             }
3698                             span_labels
3699                                 .push((span, format!("`{assoc}` changed to `{ty_str}` here")));
3700                         } else {
3701                             span_labels.push((span, format!("`{assoc}` remains `{ty_str}` here")));
3702                         }
3703                     }
3704                     (Some((span, (assoc, ty))), None) => {
3705                         span_labels.push((
3706                             span,
3707                             with_forced_trimmed_paths!(format!(
3708                                 "`{}` is `{}` here",
3709                                 self.tcx.def_path_str(assoc),
3710                                 self.ty_to_string(ty),
3711                             )),
3712                         ));
3713                     }
3714                     (None, Some(_)) | (None, None) => {}
3715                 }
3716             }
3717         }
3718         if !primary_spans.is_empty() {
3719             let mut multi_span: MultiSpan = primary_spans.into();
3720             for (span, label) in span_labels {
3721                 multi_span.push_span_label(span, label);
3722             }
3723             err.span_note(
3724                 multi_span,
3725                 "the method call chain might not have had the expected associated types",
3726             );
3727         }
3728     }
3729
3730     fn probe_assoc_types_at_expr(
3731         &self,
3732         type_diffs: &[TypeError<'tcx>],
3733         span: Span,
3734         prev_ty: Ty<'tcx>,
3735         body_id: hir::HirId,
3736         param_env: ty::ParamEnv<'tcx>,
3737     ) -> Vec<Option<(Span, (DefId, Ty<'tcx>))>> {
3738         let ocx = ObligationCtxt::new_in_snapshot(self.infcx);
3739         let mut assocs_in_this_method = Vec::with_capacity(type_diffs.len());
3740         for diff in type_diffs {
3741             let Sorts(expected_found) = diff else { continue; };
3742             let ty::Alias(ty::Projection, proj) = expected_found.expected.kind() else { continue; };
3743
3744             let origin = TypeVariableOrigin { kind: TypeVariableOriginKind::TypeInference, span };
3745             let trait_def_id = proj.trait_def_id(self.tcx);
3746             // Make `Self` be equivalent to the type of the call chain
3747             // expression we're looking at now, so that we can tell what
3748             // for example `Iterator::Item` is at this point in the chain.
3749             let substs = InternalSubsts::for_item(self.tcx, trait_def_id, |param, _| {
3750                 match param.kind {
3751                     ty::GenericParamDefKind::Type { .. } => {
3752                         if param.index == 0 {
3753                             return prev_ty.into();
3754                         }
3755                     }
3756                     ty::GenericParamDefKind::Lifetime | ty::GenericParamDefKind::Const { .. } => {}
3757                 }
3758                 self.var_for_def(span, param)
3759             });
3760             // This will hold the resolved type of the associated type, if the
3761             // current expression implements the trait that associated type is
3762             // in. For example, this would be what `Iterator::Item` is here.
3763             let ty_var = self.infcx.next_ty_var(origin);
3764             // This corresponds to `<ExprTy as Iterator>::Item = _`.
3765             let projection = ty::Binder::dummy(ty::PredicateKind::Clause(ty::Clause::Projection(
3766                 ty::ProjectionPredicate {
3767                     projection_ty: self.tcx.mk_alias_ty(proj.def_id, substs),
3768                     term: ty_var.into(),
3769                 },
3770             )));
3771             let body_def_id = self.tcx.hir().enclosing_body_owner(body_id);
3772             // Add `<ExprTy as Iterator>::Item = _` obligation.
3773             ocx.register_obligation(Obligation::misc(
3774                 self.tcx,
3775                 span,
3776                 body_def_id,
3777                 param_env,
3778                 projection,
3779             ));
3780             if ocx.select_where_possible().is_empty() {
3781                 // `ty_var` now holds the type that `Item` is for `ExprTy`.
3782                 let ty_var = self.resolve_vars_if_possible(ty_var);
3783                 assocs_in_this_method.push(Some((span, (proj.def_id, ty_var))));
3784             } else {
3785                 // `<ExprTy as Iterator>` didn't select, so likely we've
3786                 // reached the end of the iterator chain, like the originating
3787                 // `Vec<_>`.
3788                 // Keep the space consistent for later zipping.
3789                 assocs_in_this_method.push(None);
3790             }
3791         }
3792         assocs_in_this_method
3793     }
3794 }
3795
3796 /// Add a hint to add a missing borrow or remove an unnecessary one.
3797 fn hint_missing_borrow<'tcx>(
3798     infcx: &InferCtxt<'tcx>,
3799     param_env: ty::ParamEnv<'tcx>,
3800     span: Span,
3801     found: Ty<'tcx>,
3802     expected: Ty<'tcx>,
3803     found_node: Node<'_>,
3804     err: &mut Diagnostic,
3805 ) {
3806     let found_args = match found.kind() {
3807         ty::FnPtr(f) => infcx.replace_bound_vars_with_placeholders(*f).inputs().iter(),
3808         kind => {
3809             span_bug!(span, "found was converted to a FnPtr above but is now {:?}", kind)
3810         }
3811     };
3812     let expected_args = match expected.kind() {
3813         ty::FnPtr(f) => infcx.replace_bound_vars_with_placeholders(*f).inputs().iter(),
3814         kind => {
3815             span_bug!(span, "expected was converted to a FnPtr above but is now {:?}", kind)
3816         }
3817     };
3818
3819     // This could be a variant constructor, for example.
3820     let Some(fn_decl) = found_node.fn_decl() else { return; };
3821
3822     let args = fn_decl.inputs.iter().map(|ty| ty);
3823
3824     fn get_deref_type_and_refs(mut ty: Ty<'_>) -> (Ty<'_>, Vec<hir::Mutability>) {
3825         let mut refs = vec![];
3826
3827         while let ty::Ref(_, new_ty, mutbl) = ty.kind() {
3828             ty = *new_ty;
3829             refs.push(*mutbl);
3830         }
3831
3832         (ty, refs)
3833     }
3834
3835     let mut to_borrow = Vec::new();
3836     let mut remove_borrow = Vec::new();
3837
3838     for ((found_arg, expected_arg), arg) in found_args.zip(expected_args).zip(args) {
3839         let (found_ty, found_refs) = get_deref_type_and_refs(*found_arg);
3840         let (expected_ty, expected_refs) = get_deref_type_and_refs(*expected_arg);
3841
3842         if infcx.can_eq(param_env, found_ty, expected_ty).is_ok() {
3843             // FIXME: This could handle more exotic cases like mutability mismatches too!
3844             if found_refs.len() < expected_refs.len()
3845                 && found_refs[..] == expected_refs[expected_refs.len() - found_refs.len()..]
3846             {
3847                 to_borrow.push((
3848                     arg.span.shrink_to_lo(),
3849                     expected_refs[..expected_refs.len() - found_refs.len()]
3850                         .iter()
3851                         .map(|mutbl| format!("&{}", mutbl.prefix_str()))
3852                         .collect::<Vec<_>>()
3853                         .join(""),
3854                 ));
3855             } else if found_refs.len() > expected_refs.len() {
3856                 let mut span = arg.span.shrink_to_lo();
3857                 let mut left = found_refs.len() - expected_refs.len();
3858                 let mut ty = arg;
3859                 while let hir::TyKind::Ref(_, mut_ty) = &ty.kind && left > 0 {
3860                     span = span.with_hi(mut_ty.ty.span.lo());
3861                     ty = mut_ty.ty;
3862                     left -= 1;
3863                 }
3864                 let sugg = if left == 0 {
3865                     (span, String::new())
3866                 } else {
3867                     (arg.span, expected_arg.to_string())
3868                 };
3869                 remove_borrow.push(sugg);
3870             }
3871         }
3872     }
3873
3874     if !to_borrow.is_empty() {
3875         err.multipart_suggestion_verbose(
3876             "consider borrowing the argument",
3877             to_borrow,
3878             Applicability::MaybeIncorrect,
3879         );
3880     }
3881
3882     if !remove_borrow.is_empty() {
3883         err.multipart_suggestion_verbose(
3884             "do not borrow the argument",
3885             remove_borrow,
3886             Applicability::MaybeIncorrect,
3887         );
3888     }
3889 }
3890
3891 /// Collect all the returned expressions within the input expression.
3892 /// Used to point at the return spans when we want to suggest some change to them.
3893 #[derive(Default)]
3894 pub struct ReturnsVisitor<'v> {
3895     pub returns: Vec<&'v hir::Expr<'v>>,
3896     in_block_tail: bool,
3897 }
3898
3899 impl<'v> Visitor<'v> for ReturnsVisitor<'v> {
3900     fn visit_expr(&mut self, ex: &'v hir::Expr<'v>) {
3901         // Visit every expression to detect `return` paths, either through the function's tail
3902         // expression or `return` statements. We walk all nodes to find `return` statements, but
3903         // we only care about tail expressions when `in_block_tail` is `true`, which means that
3904         // they're in the return path of the function body.
3905         match ex.kind {
3906             hir::ExprKind::Ret(Some(ex)) => {
3907                 self.returns.push(ex);
3908             }
3909             hir::ExprKind::Block(block, _) if self.in_block_tail => {
3910                 self.in_block_tail = false;
3911                 for stmt in block.stmts {
3912                     hir::intravisit::walk_stmt(self, stmt);
3913                 }
3914                 self.in_block_tail = true;
3915                 if let Some(expr) = block.expr {
3916                     self.visit_expr(expr);
3917                 }
3918             }
3919             hir::ExprKind::If(_, then, else_opt) if self.in_block_tail => {
3920                 self.visit_expr(then);
3921                 if let Some(el) = else_opt {
3922                     self.visit_expr(el);
3923                 }
3924             }
3925             hir::ExprKind::Match(_, arms, _) if self.in_block_tail => {
3926                 for arm in arms {
3927                     self.visit_expr(arm.body);
3928                 }
3929             }
3930             // We need to walk to find `return`s in the entire body.
3931             _ if !self.in_block_tail => hir::intravisit::walk_expr(self, ex),
3932             _ => self.returns.push(ex),
3933         }
3934     }
3935
3936     fn visit_body(&mut self, body: &'v hir::Body<'v>) {
3937         assert!(!self.in_block_tail);
3938         if body.generator_kind().is_none() {
3939             if let hir::ExprKind::Block(block, None) = body.value.kind {
3940                 if block.expr.is_some() {
3941                     self.in_block_tail = true;
3942                 }
3943             }
3944         }
3945         hir::intravisit::walk_body(self, body);
3946     }
3947 }
3948
3949 /// Collect all the awaited expressions within the input expression.
3950 #[derive(Default)]
3951 struct AwaitsVisitor {
3952     awaits: Vec<hir::HirId>,
3953 }
3954
3955 impl<'v> Visitor<'v> for AwaitsVisitor {
3956     fn visit_expr(&mut self, ex: &'v hir::Expr<'v>) {
3957         if let hir::ExprKind::Yield(_, hir::YieldSource::Await { expr: Some(id) }) = ex.kind {
3958             self.awaits.push(id)
3959         }
3960         hir::intravisit::walk_expr(self, ex)
3961     }
3962 }
3963
3964 pub trait NextTypeParamName {
3965     fn next_type_param_name(&self, name: Option<&str>) -> String;
3966 }
3967
3968 impl NextTypeParamName for &[hir::GenericParam<'_>] {
3969     fn next_type_param_name(&self, name: Option<&str>) -> String {
3970         // This is the list of possible parameter names that we might suggest.
3971         let name = name.and_then(|n| n.chars().next()).map(|c| c.to_string().to_uppercase());
3972         let name = name.as_deref();
3973         let possible_names = [name.unwrap_or("T"), "T", "U", "V", "X", "Y", "Z", "A", "B", "C"];
3974         let used_names = self
3975             .iter()
3976             .filter_map(|p| match p.name {
3977                 hir::ParamName::Plain(ident) => Some(ident.name),
3978                 _ => None,
3979             })
3980             .collect::<Vec<_>>();
3981
3982         possible_names
3983             .iter()
3984             .find(|n| !used_names.contains(&Symbol::intern(n)))
3985             .unwrap_or(&"ParamName")
3986             .to_string()
3987     }
3988 }
3989
3990 fn suggest_trait_object_return_type_alternatives(
3991     err: &mut Diagnostic,
3992     ret_ty: Span,
3993     trait_obj: &str,
3994     is_object_safe: bool,
3995 ) {
3996     err.span_suggestion(
3997         ret_ty,
3998         &format!(
3999             "use `impl {}` as the return type if all return paths have the same type but you \
4000                 want to expose only the trait in the signature",
4001             trait_obj,
4002         ),
4003         format!("impl {}", trait_obj),
4004         Applicability::MaybeIncorrect,
4005     );
4006     if is_object_safe {
4007         err.multipart_suggestion(
4008             &format!(
4009                 "use a boxed trait object if all return paths implement trait `{}`",
4010                 trait_obj,
4011             ),
4012             vec![
4013                 (ret_ty.shrink_to_lo(), "Box<".to_string()),
4014                 (ret_ty.shrink_to_hi(), ">".to_string()),
4015             ],
4016             Applicability::MaybeIncorrect,
4017         );
4018     }
4019 }
4020
4021 /// Collect the spans that we see the generic param `param_did`
4022 struct ReplaceImplTraitVisitor<'a> {
4023     ty_spans: &'a mut Vec<Span>,
4024     param_did: DefId,
4025 }
4026
4027 impl<'a, 'hir> hir::intravisit::Visitor<'hir> for ReplaceImplTraitVisitor<'a> {
4028     fn visit_ty(&mut self, t: &'hir hir::Ty<'hir>) {
4029         if let hir::TyKind::Path(hir::QPath::Resolved(
4030             None,
4031             hir::Path { res: hir::def::Res::Def(_, segment_did), .. },
4032         )) = t.kind
4033         {
4034             if self.param_did == *segment_did {
4035                 // `fn foo(t: impl Trait)`
4036                 //            ^^^^^^^^^^ get this to suggest `T` instead
4037
4038                 // There might be more than one `impl Trait`.
4039                 self.ty_spans.push(t.span);
4040                 return;
4041             }
4042         }
4043
4044         hir::intravisit::walk_ty(self, t);
4045     }
4046 }
4047
4048 // Replace `param` with `replace_ty`
4049 struct ReplaceImplTraitFolder<'tcx> {
4050     tcx: TyCtxt<'tcx>,
4051     param: &'tcx ty::GenericParamDef,
4052     replace_ty: Ty<'tcx>,
4053 }
4054
4055 impl<'tcx> TypeFolder<'tcx> for ReplaceImplTraitFolder<'tcx> {
4056     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
4057         if let ty::Param(ty::ParamTy { index, .. }) = t.kind() {
4058             if self.param.index == *index {
4059                 return self.replace_ty;
4060             }
4061         }
4062         t.super_fold_with(self)
4063     }
4064
4065     fn tcx(&self) -> TyCtxt<'tcx> {
4066         self.tcx
4067     }
4068 }