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 hir::{Expr, HirId};
  13 use rustc_data_structures::fx::FxHashSet;
  14 use rustc_data_structures::stack::ensure_sufficient_stack;
  15 use rustc_errors::{
  16     error_code, pluralize, struct_span_err, Applicability, Diagnostic, DiagnosticBuilder,
  17     ErrorGuaranteed, MultiSpan, Style,
  18 };
  19 use rustc_hir as hir;
  20 use rustc_hir::def::DefKind;
  21 use rustc_hir::def_id::DefId;
  22 use rustc_hir::intravisit::Visitor;
  23 use rustc_hir::lang_items::LangItem;
  24 use rustc_hir::{AsyncGeneratorKind, GeneratorKind, Node};
  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::symbol::{sym, Ident, Symbol};
  38 use rustc_span::{BytePos, DesugaringKind, ExpnKind, MacroKind, Span, DUMMY_SP};
  39 use rustc_target::spec::abi;
  40 use std::ops::Deref;
  41
  42 use super::method_chain::CollectAllMismatches;
  43 use super::InferCtxtPrivExt;
  44 use crate::infer::InferCtxtExt as _;
  45 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
  46 use rustc_middle::ty::print::{with_forced_trimmed_paths, with_no_trimmed_paths};
  47
  48 #[derive(Debug)]
  49 pub enum GeneratorInteriorOrUpvar {
  50     // span of interior type
  51     Interior(Span, Option<(Option<Span>, Span, Option<hir::HirId>, Option<Span>)>),
  52     // span of upvar
  53     Upvar(Span),
  54 }
  55
  56 // This type provides a uniform interface to retrieve data on generators, whether it originated from
  57 // the local crate being compiled or from a foreign crate.
  58 #[derive(Debug)]
  59 pub enum GeneratorData<'tcx, 'a> {
  60     Local(&'a TypeckResults<'tcx>),
  61     Foreign(&'tcx GeneratorDiagnosticData<'tcx>),
  62 }
  63
  64 impl<'tcx, 'a> GeneratorData<'tcx, 'a> {
  65     // Try to get information about variables captured by the generator that matches a type we are
  66     // looking for with `ty_matches` function. We uses it to find upvar which causes a failure to
  67     // meet an obligation
  68     fn try_get_upvar_span<F>(
  69         &self,
  70         infer_context: &InferCtxt<'tcx>,
  71         generator_did: DefId,
  72         ty_matches: F,
  73     ) -> Option<GeneratorInteriorOrUpvar>
  74     where
  75         F: Fn(ty::Binder<'tcx, Ty<'tcx>>) -> bool,
  76     {
  77         match self {
  78             GeneratorData::Local(typeck_results) => {
  79                 infer_context.tcx.upvars_mentioned(generator_did).and_then(|upvars| {
  80                     upvars.iter().find_map(|(upvar_id, upvar)| {
  81                         let upvar_ty = typeck_results.node_type(*upvar_id);
  82                         let upvar_ty = infer_context.resolve_vars_if_possible(upvar_ty);
  83                         if ty_matches(ty::Binder::dummy(upvar_ty)) {
  84                             Some(GeneratorInteriorOrUpvar::Upvar(upvar.span))
  85                         } else {
  86                             None
  87                         }
  88                     })
  89                 })
  90             }
  91             GeneratorData::Foreign(_) => None,
  92         }
  93     }
  94
  95     // Try to get the span of a type being awaited on that matches the type we are looking with the
  96     // `ty_matches` function. We uses it to find awaited type which causes a failure to meet an
  97     // obligation
  98     fn get_from_await_ty<F>(
  99         &self,
 100         visitor: AwaitsVisitor,
 101         hir: map::Map<'tcx>,
 102         ty_matches: F,
 103     ) -> Option<Span>
 104     where
 105         F: Fn(ty::Binder<'tcx, Ty<'tcx>>) -> bool,
 106     {
 107         match self {
 108             GeneratorData::Local(typeck_results) => visitor
 109                 .awaits
 110                 .into_iter()
 111                 .map(|id| hir.expect_expr(id))
 112                 .find(|await_expr| {
 113                     ty_matches(ty::Binder::dummy(typeck_results.expr_ty_adjusted(&await_expr)))
 114                 })
 115                 .map(|expr| expr.span),
 116             GeneratorData::Foreign(generator_diagnostic_data) => visitor
 117                 .awaits
 118                 .into_iter()
 119                 .map(|id| hir.expect_expr(id))
 120                 .find(|await_expr| {
 121                     ty_matches(ty::Binder::dummy(
 122                         generator_diagnostic_data
 123                             .adjustments
 124                             .get(&await_expr.hir_id.local_id)
 125                             .map_or::<&[ty::adjustment::Adjustment<'tcx>], _>(&[], |a| &a[..])
 126                             .last()
 127                             .map_or_else::<Ty<'tcx>, _, _>(
 128                                 || {
 129                                     generator_diagnostic_data
 130                                         .nodes_types
 131                                         .get(&await_expr.hir_id.local_id)
 132                                         .cloned()
 133                                         .unwrap_or_else(|| {
 134                                             bug!(
 135                                                 "node_type: no type for node `{}`",
 136                                                 ty::tls::with(|tcx| tcx
 137                                                     .hir()
 138                                                     .node_to_string(await_expr.hir_id))
 139                                             )
 140                                         })
 141                                 },
 142                                 |adj| adj.target,
 143                             ),
 144                     ))
 145                 })
 146                 .map(|expr| expr.span),
 147         }
 148     }
 149
 150     /// Get the type, expression, span and optional scope span of all types
 151     /// that are live across the yield of this generator
 152     fn get_generator_interior_types(
 153         &self,
 154     ) -> ty::Binder<'tcx, &[GeneratorInteriorTypeCause<'tcx>]> {
 155         match self {
 156             GeneratorData::Local(typeck_result) => {
 157                 typeck_result.generator_interior_types.as_deref()
 158             }
 159             GeneratorData::Foreign(generator_diagnostic_data) => {
 160                 generator_diagnostic_data.generator_interior_types.as_deref()
 161             }
 162         }
 163     }
 164
 165     // Used to get the source of the data, note we don't have as much information for generators
 166     // originated from foreign crates
 167     fn is_foreign(&self) -> bool {
 168         match self {
 169             GeneratorData::Local(_) => false,
 170             GeneratorData::Foreign(_) => true,
 171         }
 172     }
 173 }
 174
 175 // This trait is public to expose the diagnostics methods to clippy.
 176 pub trait TypeErrCtxtExt<'tcx> {
 177     fn suggest_restricting_param_bound(
 178         &self,
 179         err: &mut Diagnostic,
 180         trait_pred: ty::PolyTraitPredicate<'tcx>,
 181         associated_item: Option<(&'static str, Ty<'tcx>)>,
 182         body_id: hir::HirId,
 183     );
 184
 185     fn suggest_dereferences(
 186         &self,
 187         obligation: &PredicateObligation<'tcx>,
 188         err: &mut Diagnostic,
 189         trait_pred: ty::PolyTraitPredicate<'tcx>,
 190     ) -> bool;
 191
 192     fn get_closure_name(&self, def_id: DefId, err: &mut Diagnostic, msg: &str) -> Option<Symbol>;
 193
 194     fn suggest_fn_call(
 195         &self,
 196         obligation: &PredicateObligation<'tcx>,
 197         err: &mut Diagnostic,
 198         trait_pred: ty::PolyTraitPredicate<'tcx>,
 199     ) -> bool;
 200
 201     fn check_for_binding_assigned_block_without_tail_expression(
 202         &self,
 203         obligation: &PredicateObligation<'tcx>,
 204         err: &mut Diagnostic,
 205         trait_pred: ty::PolyTraitPredicate<'tcx>,
 206     );
 207
 208     fn suggest_add_clone_to_arg(
 209         &self,
 210         obligation: &PredicateObligation<'tcx>,
 211         err: &mut Diagnostic,
 212         trait_pred: ty::PolyTraitPredicate<'tcx>,
 213     ) -> bool;
 214
 215     fn extract_callable_info(
 216         &self,
 217         hir_id: HirId,
 218         param_env: ty::ParamEnv<'tcx>,
 219         found: Ty<'tcx>,
 220     ) -> Option<(DefIdOrName, Ty<'tcx>, Vec<Ty<'tcx>>)>;
 221
 222     fn suggest_add_reference_to_arg(
 223         &self,
 224         obligation: &PredicateObligation<'tcx>,
 225         err: &mut Diagnostic,
 226         trait_pred: ty::PolyTraitPredicate<'tcx>,
 227         has_custom_message: bool,
 228     ) -> bool;
 229
 230     fn suggest_borrowing_for_object_cast(
 231         &self,
 232         err: &mut Diagnostic,
 233         obligation: &PredicateObligation<'tcx>,
 234         self_ty: Ty<'tcx>,
 235         object_ty: Ty<'tcx>,
 236     );
 237
 238     fn suggest_remove_reference(
 239         &self,
 240         obligation: &PredicateObligation<'tcx>,
 241         err: &mut Diagnostic,
 242         trait_pred: ty::PolyTraitPredicate<'tcx>,
 243     ) -> bool;
 244
 245     fn suggest_remove_await(&self, obligation: &PredicateObligation<'tcx>, err: &mut Diagnostic);
 246
 247     fn suggest_change_mut(
 248         &self,
 249         obligation: &PredicateObligation<'tcx>,
 250         err: &mut Diagnostic,
 251         trait_pred: ty::PolyTraitPredicate<'tcx>,
 252     );
 253
 254     fn suggest_semicolon_removal(
 255         &self,
 256         obligation: &PredicateObligation<'tcx>,
 257         err: &mut Diagnostic,
 258         span: Span,
 259         trait_pred: ty::PolyTraitPredicate<'tcx>,
 260     ) -> bool;
 261
 262     fn return_type_span(&self, obligation: &PredicateObligation<'tcx>) -> Option<Span>;
 263
 264     fn suggest_impl_trait(
 265         &self,
 266         err: &mut Diagnostic,
 267         span: Span,
 268         obligation: &PredicateObligation<'tcx>,
 269         trait_pred: ty::PolyTraitPredicate<'tcx>,
 270     ) -> bool;
 271
 272     fn point_at_returns_when_relevant(
 273         &self,
 274         err: &mut DiagnosticBuilder<'tcx, ErrorGuaranteed>,
 275         obligation: &PredicateObligation<'tcx>,
 276     );
 277
 278     fn report_closure_arg_mismatch(
 279         &self,
 280         span: Span,
 281         found_span: Option<Span>,
 282         found: ty::PolyTraitRef<'tcx>,
 283         expected: ty::PolyTraitRef<'tcx>,
 284         cause: &ObligationCauseCode<'tcx>,
 285         found_node: Option<Node<'_>>,
 286     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed>;
 287
 288     fn note_conflicting_closure_bounds(
 289         &self,
 290         cause: &ObligationCauseCode<'tcx>,
 291         err: &mut DiagnosticBuilder<'tcx, ErrorGuaranteed>,
 292     );
 293
 294     fn suggest_fully_qualified_path(
 295         &self,
 296         err: &mut Diagnostic,
 297         item_def_id: DefId,
 298         span: Span,
 299         trait_ref: DefId,
 300     );
 301
 302     fn maybe_note_obligation_cause_for_async_await(
 303         &self,
 304         err: &mut Diagnostic,
 305         obligation: &PredicateObligation<'tcx>,
 306     ) -> bool;
 307
 308     fn note_obligation_cause_for_async_await(
 309         &self,
 310         err: &mut Diagnostic,
 311         interior_or_upvar_span: GeneratorInteriorOrUpvar,
 312         is_async: bool,
 313         outer_generator: Option<DefId>,
 314         trait_pred: ty::TraitPredicate<'tcx>,
 315         target_ty: Ty<'tcx>,
 316         typeck_results: Option<&ty::TypeckResults<'tcx>>,
 317         obligation: &PredicateObligation<'tcx>,
 318         next_code: Option<&ObligationCauseCode<'tcx>>,
 319     );
 320
 321     fn note_obligation_cause_code<T>(
 322         &self,
 323         err: &mut Diagnostic,
 324         predicate: T,
 325         param_env: ty::ParamEnv<'tcx>,
 326         cause_code: &ObligationCauseCode<'tcx>,
 327         obligated_types: &mut Vec<Ty<'tcx>>,
 328         seen_requirements: &mut FxHashSet<DefId>,
 329     ) where
 330         T: ToPredicate<'tcx>;
 331
 332     /// Suggest to await before try: future? => future.await?
 333     fn suggest_await_before_try(
 334         &self,
 335         err: &mut Diagnostic,
 336         obligation: &PredicateObligation<'tcx>,
 337         trait_pred: ty::PolyTraitPredicate<'tcx>,
 338         span: Span,
 339     );
 340
 341     fn suggest_floating_point_literal(
 342         &self,
 343         obligation: &PredicateObligation<'tcx>,
 344         err: &mut Diagnostic,
 345         trait_ref: &ty::PolyTraitRef<'tcx>,
 346     );
 347
 348     fn suggest_derive(
 349         &self,
 350         obligation: &PredicateObligation<'tcx>,
 351         err: &mut Diagnostic,
 352         trait_pred: ty::PolyTraitPredicate<'tcx>,
 353     );
 354
 355     fn suggest_dereferencing_index(
 356         &self,
 357         obligation: &PredicateObligation<'tcx>,
 358         err: &mut Diagnostic,
 359         trait_pred: ty::PolyTraitPredicate<'tcx>,
 360     );
 361     fn note_function_argument_obligation(
 362         &self,
 363         arg_hir_id: HirId,
 364         err: &mut Diagnostic,
 365         parent_code: &ObligationCauseCode<'tcx>,
 366         param_env: ty::ParamEnv<'tcx>,
 367         predicate: ty::Predicate<'tcx>,
 368         call_hir_id: HirId,
 369     );
 370     fn point_at_chain(
 371         &self,
 372         expr: &hir::Expr<'_>,
 373         typeck_results: &TypeckResults<'tcx>,
 374         type_diffs: Vec<TypeError<'tcx>>,
 375         param_env: ty::ParamEnv<'tcx>,
 376         err: &mut Diagnostic,
 377     );
 378     fn probe_assoc_types_at_expr(
 379         &self,
 380         type_diffs: &[TypeError<'tcx>],
 381         span: Span,
 382         prev_ty: Ty<'tcx>,
 383         body_id: hir::HirId,
 384         param_env: ty::ParamEnv<'tcx>,
 385     ) -> Vec<Option<(Span, (DefId, Ty<'tcx>))>>;
 386 }
 387
 388 fn predicate_constraint(generics: &hir::Generics<'_>, pred: ty::Predicate<'_>) -> (Span, String) {
 389     (
 390         generics.tail_span_for_predicate_suggestion(),
 391         format!("{} {}", generics.add_where_or_trailing_comma(), pred),
 392     )
 393 }
 394
 395 /// Type parameter needs more bounds. The trivial case is `T` `where T: Bound`, but
 396 /// it can also be an `impl Trait` param that needs to be decomposed to a type
 397 /// param for cleaner code.
 398 fn suggest_restriction<'tcx>(
 399     tcx: TyCtxt<'tcx>,
 400     hir_id: HirId,
 401     hir_generics: &hir::Generics<'tcx>,
 402     msg: &str,
 403     err: &mut Diagnostic,
 404     fn_sig: Option<&hir::FnSig<'_>>,
 405     projection: Option<&ty::AliasTy<'_>>,
 406     trait_pred: ty::PolyTraitPredicate<'tcx>,
 407     // When we are dealing with a trait, `super_traits` will be `Some`:
 408     // Given `trait T: A + B + C {}`
 409     //              -  ^^^^^^^^^ GenericBounds
 410     //              |
 411     //              &Ident
 412     super_traits: Option<(&Ident, &hir::GenericBounds<'_>)>,
 413 ) {
 414     if hir_generics.where_clause_span.from_expansion()
 415         || hir_generics.where_clause_span.desugaring_kind().is_some()
 416     {
 417         return;
 418     }
 419     let Some(item_id) = hir_id.as_owner() else { return; };
 420     let generics = tcx.generics_of(item_id);
 421     // Given `fn foo(t: impl Trait)` where `Trait` requires assoc type `A`...
 422     if let Some((param, bound_str, fn_sig)) =
 423         fn_sig.zip(projection).and_then(|(sig, p)| match p.self_ty().kind() {
 424             // Shenanigans to get the `Trait` from the `impl Trait`.
 425             ty::Param(param) => {
 426                 let param_def = generics.type_param(param, tcx);
 427                 if param_def.kind.is_synthetic() {
 428                     let bound_str =
 429                         param_def.name.as_str().strip_prefix("impl ")?.trim_start().to_string();
 430                     return Some((param_def, bound_str, sig));
 431                 }
 432                 None
 433             }
 434             _ => None,
 435         })
 436     {
 437         let type_param_name = hir_generics.params.next_type_param_name(Some(&bound_str));
 438         let trait_pred = trait_pred.fold_with(&mut ReplaceImplTraitFolder {
 439             tcx,
 440             param,
 441             replace_ty: ty::ParamTy::new(generics.count() as u32, Symbol::intern(&type_param_name))
 442                 .to_ty(tcx),
 443         });
 444         if !trait_pred.is_suggestable(tcx, false) {
 445             return;
 446         }
 447         // We know we have an `impl Trait` that doesn't satisfy a required projection.
 448
 449         // Find all of the occurrences of `impl Trait` for `Trait` in the function arguments'
 450         // types. There should be at least one, but there might be *more* than one. In that
 451         // case we could just ignore it and try to identify which one needs the restriction,
 452         // but instead we choose to suggest replacing all instances of `impl Trait` with `T`
 453         // where `T: Trait`.
 454         let mut ty_spans = vec![];
 455         for input in fn_sig.decl.inputs {
 456             ReplaceImplTraitVisitor { ty_spans: &mut ty_spans, param_did: param.def_id }
 457                 .visit_ty(input);
 458         }
 459         // The type param `T: Trait` we will suggest to introduce.
 460         let type_param = format!("{}: {}", type_param_name, bound_str);
 461
 462         let mut sugg = vec![
 463             if let Some(span) = hir_generics.span_for_param_suggestion() {
 464                 (span, format!(", {}", type_param))
 465             } else {
 466                 (hir_generics.span, format!("<{}>", type_param))
 467             },
 468             // `fn foo(t: impl Trait)`
 469             //                       ^ suggest `where <T as Trait>::A: Bound`
 470             predicate_constraint(hir_generics, trait_pred.to_predicate(tcx)),
 471         ];
 472         sugg.extend(ty_spans.into_iter().map(|s| (s, type_param_name.to_string())));
 473
 474         // Suggest `fn foo<T: Trait>(t: T) where <T as Trait>::A: Bound`.
 475         // FIXME: once `#![feature(associated_type_bounds)]` is stabilized, we should suggest
 476         // `fn foo(t: impl Trait<A: Bound>)` instead.
 477         err.multipart_suggestion(
 478             "introduce a type parameter with a trait bound instead of using `impl Trait`",
 479             sugg,
 480             Applicability::MaybeIncorrect,
 481         );
 482     } else {
 483         if !trait_pred.is_suggestable(tcx, false) {
 484             return;
 485         }
 486         // Trivial case: `T` needs an extra bound: `T: Bound`.
 487         let (sp, suggestion) = match (
 488             hir_generics
 489                 .params
 490                 .iter()
 491                 .find(|p| !matches!(p.kind, hir::GenericParamKind::Type { synthetic: true, .. })),
 492             super_traits,
 493         ) {
 494             (_, None) => predicate_constraint(hir_generics, trait_pred.to_predicate(tcx)),
 495             (None, Some((ident, []))) => (
 496                 ident.span.shrink_to_hi(),
 497                 format!(": {}", trait_pred.print_modifiers_and_trait_path()),
 498             ),
 499             (_, Some((_, [.., bounds]))) => (
 500                 bounds.span().shrink_to_hi(),
 501                 format!(" + {}", trait_pred.print_modifiers_and_trait_path()),
 502             ),
 503             (Some(_), Some((_, []))) => (
 504                 hir_generics.span.shrink_to_hi(),
 505                 format!(": {}", trait_pred.print_modifiers_and_trait_path()),
 506             ),
 507         };
 508
 509         err.span_suggestion_verbose(
 510             sp,
 511             &format!("consider further restricting {}", msg),
 512             suggestion,
 513             Applicability::MachineApplicable,
 514         );
 515     }
 516 }
 517
 518 impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
 519     fn suggest_restricting_param_bound(
 520         &self,
 521         mut err: &mut Diagnostic,
 522         trait_pred: ty::PolyTraitPredicate<'tcx>,
 523         associated_ty: Option<(&'static str, Ty<'tcx>)>,
 524         body_id: hir::HirId,
 525     ) {
 526         let trait_pred = self.resolve_numeric_literals_with_default(trait_pred);
 527
 528         let self_ty = trait_pred.skip_binder().self_ty();
 529         let (param_ty, projection) = match self_ty.kind() {
 530             ty::Param(_) => (true, None),
 531             ty::Alias(ty::Projection, projection) => (false, Some(projection)),
 532             _ => (false, None),
 533         };
 534
 535         // FIXME: Add check for trait bound that is already present, particularly `?Sized` so we
 536         //        don't suggest `T: Sized + ?Sized`.
 537         let mut hir_id = body_id;
 538         while let Some(node) = self.tcx.hir().find(hir_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                         hir_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, hir_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                         hir_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                         hir_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
 716             hir_id = self.tcx.hir().get_parent_item(hir_id).into();
 717         }
 718     }
 719
 720     /// When after several dereferencing, the reference satisfies the trait
 721     /// binding. This function provides dereference suggestion for this
 722     /// specific situation.
 723     fn suggest_dereferences(
 724         &self,
 725         obligation: &PredicateObligation<'tcx>,
 726         err: &mut Diagnostic,
 727         trait_pred: ty::PolyTraitPredicate<'tcx>,
 728     ) -> bool {
 729         // It only make sense when suggesting dereferences for arguments
 730         let ObligationCauseCode::FunctionArgumentObligation { arg_hir_id, call_hir_id, .. } = obligation.cause.code()
 731             else { return false; };
 732         let Some(typeck_results) = &self.typeck_results
 733             else { return false; };
 734         let hir::Node::Expr(expr) = self.tcx.hir().get(*arg_hir_id)
 735             else { return false; };
 736         let Some(arg_ty) = typeck_results.expr_ty_adjusted_opt(expr)
 737             else { return false; };
 738
 739         let span = obligation.cause.span;
 740         let mut real_trait_pred = trait_pred;
 741         let mut code = obligation.cause.code();
 742         while let Some((parent_code, parent_trait_pred)) = code.parent() {
 743             code = parent_code;
 744             if let Some(parent_trait_pred) = parent_trait_pred {
 745                 real_trait_pred = parent_trait_pred;
 746             }
 747
 748             let real_ty = real_trait_pred.self_ty();
 749             // We `erase_late_bound_regions` here because `make_subregion` does not handle
 750             // `ReLateBound`, and we don't particularly care about the regions.
 751             if self
 752                 .can_eq(obligation.param_env, self.tcx.erase_late_bound_regions(real_ty), arg_ty)
 753                 .is_err()
 754             {
 755                 continue;
 756             }
 757
 758             if let ty::Ref(region, base_ty, mutbl) = *real_ty.skip_binder().kind() {
 759                 let autoderef = (self.autoderef_steps)(base_ty);
 760                 if let Some(steps) =
 761                     autoderef.into_iter().enumerate().find_map(|(steps, (ty, obligations))| {
 762                         // Re-add the `&`
 763                         let ty = self.tcx.mk_ref(region, TypeAndMut { ty, mutbl });
 764
 765                         // Remapping bound vars here
 766                         let real_trait_pred_and_ty =
 767                             real_trait_pred.map_bound(|inner_trait_pred| (inner_trait_pred, ty));
 768                         let obligation = self.mk_trait_obligation_with_new_self_ty(
 769                             obligation.param_env,
 770                             real_trait_pred_and_ty,
 771                         );
 772                         if obligations
 773                             .iter()
 774                             .chain([&obligation])
 775                             .all(|obligation| self.predicate_may_hold(obligation))
 776                         {
 777                             Some(steps)
 778                         } else {
 779                             None
 780                         }
 781                     })
 782                 {
 783                     if steps > 0 {
 784                         // Don't care about `&mut` because `DerefMut` is used less
 785                         // often and user will not expect autoderef happens.
 786                         if let Some(hir::Node::Expr(hir::Expr {
 787                             kind:
 788                                 hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Not, expr),
 789                             ..
 790                         })) = self.tcx.hir().find(*arg_hir_id)
 791                         {
 792                             let derefs = "*".repeat(steps);
 793                             err.span_suggestion_verbose(
 794                                 expr.span.shrink_to_lo(),
 795                                 "consider dereferencing here",
 796                                 derefs,
 797                                 Applicability::MachineApplicable,
 798                             );
 799                             return true;
 800                         }
 801                     }
 802                 } else if real_trait_pred != trait_pred {
 803                     // This branch addresses #87437.
 804
 805                     // Remapping bound vars here
 806                     let real_trait_pred_and_base_ty =
 807                         real_trait_pred.map_bound(|inner_trait_pred| (inner_trait_pred, base_ty));
 808                     let obligation = self.mk_trait_obligation_with_new_self_ty(
 809                         obligation.param_env,
 810                         real_trait_pred_and_base_ty,
 811                     );
 812                     if self.predicate_may_hold(&obligation) {
 813                         let call_node = self.tcx.hir().get(*call_hir_id);
 814                         let msg = "consider dereferencing here";
 815                         let is_receiver = matches!(
 816                             call_node,
 817                             Node::Expr(hir::Expr {
 818                                 kind: hir::ExprKind::MethodCall(_, receiver_expr, ..),
 819                                 ..
 820                             })
 821                             if receiver_expr.hir_id == *arg_hir_id
 822                         );
 823                         if is_receiver {
 824                             err.multipart_suggestion_verbose(
 825                                 msg,
 826                                 vec![
 827                                     (span.shrink_to_lo(), "(*".to_string()),
 828                                     (span.shrink_to_hi(), ")".to_string()),
 829                                 ],
 830                                 Applicability::MachineApplicable,
 831                             )
 832                         } else {
 833                             err.span_suggestion_verbose(
 834                                 span.shrink_to_lo(),
 835                                 msg,
 836                                 '*',
 837                                 Applicability::MachineApplicable,
 838                             )
 839                         };
 840                         return true;
 841                     }
 842                 }
 843             }
 844         }
 845         false
 846     }
 847
 848     /// Given a closure's `DefId`, return the given name of the closure.
 849     ///
 850     /// This doesn't account for reassignments, but it's only used for suggestions.
 851     fn get_closure_name(&self, def_id: DefId, err: &mut Diagnostic, msg: &str) -> Option<Symbol> {
 852         let get_name = |err: &mut Diagnostic, kind: &hir::PatKind<'_>| -> Option<Symbol> {
 853             // Get the local name of this closure. This can be inaccurate because
 854             // of the possibility of reassignment, but this should be good enough.
 855             match &kind {
 856                 hir::PatKind::Binding(hir::BindingAnnotation::NONE, _, ident, None) => {
 857                     Some(ident.name)
 858                 }
 859                 _ => {
 860                     err.note(msg);
 861                     None
 862                 }
 863             }
 864         };
 865
 866         let hir = self.tcx.hir();
 867         let hir_id = hir.local_def_id_to_hir_id(def_id.as_local()?);
 868         match hir.find_parent(hir_id) {
 869             Some(hir::Node::Stmt(hir::Stmt { kind: hir::StmtKind::Local(local), .. })) => {
 870                 get_name(err, &local.pat.kind)
 871             }
 872             // Different to previous arm because one is `&hir::Local` and the other
 873             // is `P<hir::Local>`.
 874             Some(hir::Node::Local(local)) => get_name(err, &local.pat.kind),
 875             _ => None,
 876         }
 877     }
 878
 879     /// We tried to apply the bound to an `fn` or closure. Check whether calling it would
 880     /// evaluate to a type that *would* satisfy the trait binding. If it would, suggest calling
 881     /// it: `bar(foo)` → `bar(foo())`. This case is *very* likely to be hit if `foo` is `async`.
 882     fn suggest_fn_call(
 883         &self,
 884         obligation: &PredicateObligation<'tcx>,
 885         err: &mut Diagnostic,
 886         trait_pred: ty::PolyTraitPredicate<'tcx>,
 887     ) -> bool {
 888         // It doesn't make sense to make this suggestion outside of typeck...
 889         // (also autoderef will ICE...)
 890         if self.typeck_results.is_none() {
 891             return false;
 892         }
 893
 894         if let ty::PredicateKind::Clause(ty::Clause::Trait(trait_pred)) = obligation.predicate.kind().skip_binder()
 895             && Some(trait_pred.def_id()) == self.tcx.lang_items().sized_trait()
 896         {
 897             // Don't suggest calling to turn an unsized type into a sized type
 898             return false;
 899         }
 900
 901         let self_ty = self.replace_bound_vars_with_fresh_vars(
 902             DUMMY_SP,
 903             LateBoundRegionConversionTime::FnCall,
 904             trait_pred.self_ty(),
 905         );
 906
 907         let Some((def_id_or_name, output, inputs)) = self.extract_callable_info(
 908             obligation.cause.body_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(hir::Node::Expr(body)) = self.tcx.hir().find(obligation.cause.body_id) else { return; };
1007         expr_finder.visit_expr(&body);
1008         let Some(expr) = expr_finder.result else { return; };
1009         let Some(typeck) = &self.typeck_results else { return; };
1010         let Some(ty) = typeck.expr_ty_adjusted_opt(expr) else { return; };
1011         if !ty.is_unit() {
1012             return;
1013         };
1014         let hir::ExprKind::Path(hir::QPath::Resolved(None, path)) = expr.kind else { return; };
1015         let hir::def::Res::Local(hir_id) = path.res else { return; };
1016         let Some(hir::Node::Pat(pat)) = self.tcx.hir().find(hir_id) else {
1017             return;
1018         };
1019         let Some(hir::Node::Local(hir::Local {
1020             ty: None,
1021             init: Some(init),
1022             ..
1023         })) = self.tcx.hir().find_parent(pat.hir_id) else { return; };
1024         let hir::ExprKind::Block(block, None) = init.kind else { return; };
1025         if block.expr.is_some() {
1026             return;
1027         }
1028         let [.., stmt] = block.stmts else {
1029             err.span_label(block.span, "this empty block is missing a tail expression");
1030             return;
1031         };
1032         let hir::StmtKind::Semi(tail_expr) = stmt.kind else { return; };
1033         let Some(ty) = typeck.expr_ty_opt(tail_expr) else {
1034             err.span_label(block.span, "this block is missing a tail expression");
1035             return;
1036         };
1037         let ty = self.resolve_numeric_literals_with_default(self.resolve_vars_if_possible(ty));
1038         let trait_pred_and_self = trait_pred.map_bound(|trait_pred| (trait_pred, ty));
1039
1040         let new_obligation =
1041             self.mk_trait_obligation_with_new_self_ty(obligation.param_env, trait_pred_and_self);
1042         if self.predicate_must_hold_modulo_regions(&new_obligation) {
1043             err.span_suggestion_short(
1044                 stmt.span.with_lo(tail_expr.span.hi()),
1045                 "remove this semicolon",
1046                 "",
1047                 Applicability::MachineApplicable,
1048             );
1049         } else {
1050             err.span_label(block.span, "this block is missing a tail expression");
1051         }
1052     }
1053
1054     fn suggest_add_clone_to_arg(
1055         &self,
1056         obligation: &PredicateObligation<'tcx>,
1057         err: &mut Diagnostic,
1058         trait_pred: ty::PolyTraitPredicate<'tcx>,
1059     ) -> bool {
1060         let self_ty = self.resolve_vars_if_possible(trait_pred.self_ty());
1061         let ty = self.tcx.erase_late_bound_regions(self_ty);
1062         let owner = self.tcx.hir().get_parent_item(obligation.cause.body_id);
1063         let Some(generics) = self.tcx.hir().get_generics(owner.def_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     fn extract_callable_info(
1107         &self,
1108         hir_id: HirId,
1109         param_env: ty::ParamEnv<'tcx>,
1110         found: Ty<'tcx>,
1111     ) -> Option<(DefIdOrName, Ty<'tcx>, Vec<Ty<'tcx>>)> {
1112         // Autoderef is useful here because sometimes we box callables, etc.
1113         let Some((def_id_or_name, output, inputs)) = (self.autoderef_steps)(found).into_iter().find_map(|(found, _)| {
1114             match *found.kind() {
1115                 ty::FnPtr(fn_sig) =>
1116                     Some((DefIdOrName::Name("function pointer"), fn_sig.output(), fn_sig.inputs())),
1117                 ty::FnDef(def_id, _) => {
1118                     let fn_sig = found.fn_sig(self.tcx);
1119                     Some((DefIdOrName::DefId(def_id), fn_sig.output(), fn_sig.inputs()))
1120                 }
1121                 ty::Closure(def_id, substs) => {
1122                     let fn_sig = substs.as_closure().sig();
1123                     Some((DefIdOrName::DefId(def_id), fn_sig.output(), fn_sig.inputs().map_bound(|inputs| &inputs[1..])))
1124                 }
1125                 ty::Alias(ty::Opaque, ty::AliasTy { def_id, substs, .. }) => {
1126                     self.tcx.bound_item_bounds(def_id).subst(self.tcx, substs).iter().find_map(|pred| {
1127                         if let ty::PredicateKind::Clause(ty::Clause::Projection(proj)) = pred.kind().skip_binder()
1128                         && Some(proj.projection_ty.def_id) == self.tcx.lang_items().fn_once_output()
1129                         // args tuple will always be substs[1]
1130                         && let ty::Tuple(args) = proj.projection_ty.substs.type_at(1).kind()
1131                         {
1132                             Some((
1133                                 DefIdOrName::DefId(def_id),
1134                                 pred.kind().rebind(proj.term.ty().unwrap()),
1135                                 pred.kind().rebind(args.as_slice()),
1136                             ))
1137                         } else {
1138                             None
1139                         }
1140                     })
1141                 }
1142                 ty::Dynamic(data, _, ty::Dyn) => {
1143                     data.iter().find_map(|pred| {
1144                         if let ty::ExistentialPredicate::Projection(proj) = pred.skip_binder()
1145                         && Some(proj.def_id) == self.tcx.lang_items().fn_once_output()
1146                         // for existential projection, substs are shifted over by 1
1147                         && let ty::Tuple(args) = proj.substs.type_at(0).kind()
1148                         {
1149                             Some((
1150                                 DefIdOrName::Name("trait object"),
1151                                 pred.rebind(proj.term.ty().unwrap()),
1152                                 pred.rebind(args.as_slice()),
1153                             ))
1154                         } else {
1155                             None
1156                         }
1157                     })
1158                 }
1159                 ty::Param(param) => {
1160                     let generics = self.tcx.generics_of(hir_id.owner.to_def_id());
1161                     let name = if generics.count() > param.index as usize
1162                         && let def = generics.param_at(param.index as usize, self.tcx)
1163                         && matches!(def.kind, ty::GenericParamDefKind::Type { .. })
1164                         && def.name == param.name
1165                     {
1166                         DefIdOrName::DefId(def.def_id)
1167                     } else {
1168                         DefIdOrName::Name("type parameter")
1169                     };
1170                     param_env.caller_bounds().iter().find_map(|pred| {
1171                         if let ty::PredicateKind::Clause(ty::Clause::Projection(proj)) = pred.kind().skip_binder()
1172                         && Some(proj.projection_ty.def_id) == self.tcx.lang_items().fn_once_output()
1173                         && proj.projection_ty.self_ty() == found
1174                         // args tuple will always be substs[1]
1175                         && let ty::Tuple(args) = proj.projection_ty.substs.type_at(1).kind()
1176                         {
1177                             Some((
1178                                 name,
1179                                 pred.kind().rebind(proj.term.ty().unwrap()),
1180                                 pred.kind().rebind(args.as_slice()),
1181                             ))
1182                         } else {
1183                             None
1184                         }
1185                     })
1186                 }
1187                 _ => None,
1188             }
1189         }) else { return None; };
1190
1191         let output = self.replace_bound_vars_with_fresh_vars(
1192             DUMMY_SP,
1193             LateBoundRegionConversionTime::FnCall,
1194             output,
1195         );
1196         let inputs = inputs
1197             .skip_binder()
1198             .iter()
1199             .map(|ty| {
1200                 self.replace_bound_vars_with_fresh_vars(
1201                     DUMMY_SP,
1202                     LateBoundRegionConversionTime::FnCall,
1203                     inputs.rebind(*ty),
1204                 )
1205             })
1206             .collect();
1207
1208         // We don't want to register any extra obligations, which should be
1209         // implied by wf, but also because that would possibly result in
1210         // erroneous errors later on.
1211         let InferOk { value: output, obligations: _ } =
1212             self.at(&ObligationCause::dummy(), param_env).normalize(output);
1213
1214         if output.is_ty_var() { None } else { Some((def_id_or_name, output, inputs)) }
1215     }
1216
1217     fn suggest_add_reference_to_arg(
1218         &self,
1219         obligation: &PredicateObligation<'tcx>,
1220         err: &mut Diagnostic,
1221         poly_trait_pred: ty::PolyTraitPredicate<'tcx>,
1222         has_custom_message: bool,
1223     ) -> bool {
1224         let span = obligation.cause.span;
1225
1226         let code = if let ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } =
1227             obligation.cause.code()
1228         {
1229             &parent_code
1230         } else if let ObligationCauseCode::ItemObligation(_)
1231         | ObligationCauseCode::ExprItemObligation(..) = obligation.cause.code()
1232         {
1233             obligation.cause.code()
1234         } else if let ExpnKind::Desugaring(DesugaringKind::ForLoop) =
1235             span.ctxt().outer_expn_data().kind
1236         {
1237             obligation.cause.code()
1238         } else {
1239             return false;
1240         };
1241
1242         // List of traits for which it would be nonsensical to suggest borrowing.
1243         // For instance, immutable references are always Copy, so suggesting to
1244         // borrow would always succeed, but it's probably not what the user wanted.
1245         let mut never_suggest_borrow: Vec<_> =
1246             [LangItem::Copy, LangItem::Clone, LangItem::Unpin, LangItem::Sized]
1247                 .iter()
1248                 .filter_map(|lang_item| self.tcx.lang_items().get(*lang_item))
1249                 .collect();
1250
1251         if let Some(def_id) = self.tcx.get_diagnostic_item(sym::Send) {
1252             never_suggest_borrow.push(def_id);
1253         }
1254
1255         let param_env = obligation.param_env;
1256
1257         // Try to apply the original trait binding obligation by borrowing.
1258         let mut try_borrowing = |old_pred: ty::PolyTraitPredicate<'tcx>,
1259                                  blacklist: &[DefId]|
1260          -> bool {
1261             if blacklist.contains(&old_pred.def_id()) {
1262                 return false;
1263             }
1264             // We map bounds to `&T` and `&mut T`
1265             let trait_pred_and_imm_ref = old_pred.map_bound(|trait_pred| {
1266                 (
1267                     trait_pred,
1268                     self.tcx.mk_imm_ref(self.tcx.lifetimes.re_static, trait_pred.self_ty()),
1269                 )
1270             });
1271             let trait_pred_and_mut_ref = old_pred.map_bound(|trait_pred| {
1272                 (
1273                     trait_pred,
1274                     self.tcx.mk_mut_ref(self.tcx.lifetimes.re_static, trait_pred.self_ty()),
1275                 )
1276             });
1277
1278             let mk_result = |trait_pred_and_new_ty| {
1279                 let obligation =
1280                     self.mk_trait_obligation_with_new_self_ty(param_env, trait_pred_and_new_ty);
1281                 self.predicate_must_hold_modulo_regions(&obligation)
1282             };
1283             let imm_ref_self_ty_satisfies_pred = mk_result(trait_pred_and_imm_ref);
1284             let mut_ref_self_ty_satisfies_pred = mk_result(trait_pred_and_mut_ref);
1285
1286             let (ref_inner_ty_satisfies_pred, ref_inner_ty_mut) =
1287                 if let ObligationCauseCode::ItemObligation(_) | ObligationCauseCode::ExprItemObligation(..) = obligation.cause.code()
1288                     && let ty::Ref(_, ty, mutability) = old_pred.self_ty().skip_binder().kind()
1289                 {
1290                     (
1291                         mk_result(old_pred.map_bound(|trait_pred| (trait_pred, *ty))),
1292                         mutability.is_mut(),
1293                     )
1294                 } else {
1295                     (false, false)
1296                 };
1297
1298             if imm_ref_self_ty_satisfies_pred
1299                 || mut_ref_self_ty_satisfies_pred
1300                 || ref_inner_ty_satisfies_pred
1301             {
1302                 if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
1303                     // We don't want a borrowing suggestion on the fields in structs,
1304                     // ```
1305                     // struct Foo {
1306                     //  the_foos: Vec<Foo>
1307                     // }
1308                     // ```
1309                     if !matches!(
1310                         span.ctxt().outer_expn_data().kind,
1311                         ExpnKind::Root | ExpnKind::Desugaring(DesugaringKind::ForLoop)
1312                     ) {
1313                         return false;
1314                     }
1315                     if snippet.starts_with('&') {
1316                         // This is already a literal borrow and the obligation is failing
1317                         // somewhere else in the obligation chain. Do not suggest non-sense.
1318                         return false;
1319                     }
1320                     // We have a very specific type of error, where just borrowing this argument
1321                     // might solve the problem. In cases like this, the important part is the
1322                     // original type obligation, not the last one that failed, which is arbitrary.
1323                     // Because of this, we modify the error to refer to the original obligation and
1324                     // return early in the caller.
1325
1326                     let msg = format!("the trait bound `{}` is not satisfied", old_pred);
1327                     if has_custom_message {
1328                         err.note(&msg);
1329                     } else {
1330                         err.message =
1331                             vec![(rustc_errors::DiagnosticMessage::Str(msg), Style::NoStyle)];
1332                     }
1333                     err.span_label(
1334                         span,
1335                         format!(
1336                             "the trait `{}` is not implemented for `{}`",
1337                             old_pred.print_modifiers_and_trait_path(),
1338                             old_pred.self_ty().skip_binder(),
1339                         ),
1340                     );
1341
1342                     if imm_ref_self_ty_satisfies_pred && mut_ref_self_ty_satisfies_pred {
1343                         err.span_suggestions(
1344                             span.shrink_to_lo(),
1345                             "consider borrowing here",
1346                             ["&".to_string(), "&mut ".to_string()],
1347                             Applicability::MaybeIncorrect,
1348                         );
1349                     } else {
1350                         let is_mut = mut_ref_self_ty_satisfies_pred || ref_inner_ty_mut;
1351                         err.span_suggestion_verbose(
1352                             span.shrink_to_lo(),
1353                             &format!(
1354                                 "consider{} borrowing here",
1355                                 if is_mut { " mutably" } else { "" }
1356                             ),
1357                             format!("&{}", if is_mut { "mut " } else { "" }),
1358                             Applicability::MaybeIncorrect,
1359                         );
1360                     }
1361                     return true;
1362                 }
1363             }
1364             return false;
1365         };
1366
1367         if let ObligationCauseCode::ImplDerivedObligation(cause) = &*code {
1368             try_borrowing(cause.derived.parent_trait_pred, &[])
1369         } else if let ObligationCauseCode::BindingObligation(_, _)
1370         | ObligationCauseCode::ItemObligation(_)
1371         | ObligationCauseCode::ExprItemObligation(..)
1372         | ObligationCauseCode::ExprBindingObligation(..) = code
1373         {
1374             try_borrowing(poly_trait_pred, &never_suggest_borrow)
1375         } else {
1376             false
1377         }
1378     }
1379
1380     // Suggest borrowing the type
1381     fn suggest_borrowing_for_object_cast(
1382         &self,
1383         err: &mut Diagnostic,
1384         obligation: &PredicateObligation<'tcx>,
1385         self_ty: Ty<'tcx>,
1386         object_ty: Ty<'tcx>,
1387     ) {
1388         let ty::Dynamic(predicates, _, ty::Dyn) = object_ty.kind() else { return; };
1389         let self_ref_ty = self.tcx.mk_imm_ref(self.tcx.lifetimes.re_erased, self_ty);
1390
1391         for predicate in predicates.iter() {
1392             if !self.predicate_must_hold_modulo_regions(
1393                 &obligation.with(self.tcx, predicate.with_self_ty(self.tcx, self_ref_ty)),
1394             ) {
1395                 return;
1396             }
1397         }
1398
1399         err.span_suggestion(
1400             obligation.cause.span.shrink_to_lo(),
1401             &format!(
1402                 "consider borrowing the value, since `&{self_ty}` can be coerced into `{object_ty}`"
1403             ),
1404             "&",
1405             Applicability::MaybeIncorrect,
1406         );
1407     }
1408
1409     /// Whenever references are used by mistake, like `for (i, e) in &vec.iter().enumerate()`,
1410     /// suggest removing these references until we reach a type that implements the trait.
1411     fn suggest_remove_reference(
1412         &self,
1413         obligation: &PredicateObligation<'tcx>,
1414         err: &mut Diagnostic,
1415         trait_pred: ty::PolyTraitPredicate<'tcx>,
1416     ) -> bool {
1417         let mut span = obligation.cause.span;
1418         let mut trait_pred = trait_pred;
1419         let mut code = obligation.cause.code();
1420         while let Some((c, Some(parent_trait_pred))) = code.parent() {
1421             // We want the root obligation, in order to detect properly handle
1422             // `for _ in &mut &mut vec![] {}`.
1423             code = c;
1424             trait_pred = parent_trait_pred;
1425         }
1426         while span.desugaring_kind().is_some() {
1427             // Remove all the hir desugaring contexts while maintaining the macro contexts.
1428             span.remove_mark();
1429         }
1430         let mut expr_finder = super::FindExprBySpan::new(span);
1431         let Some(hir::Node::Expr(body)) = self.tcx.hir().find(obligation.cause.body_id) else {
1432             return false;
1433         };
1434         expr_finder.visit_expr(&body);
1435         let mut maybe_suggest = |suggested_ty, count, suggestions| {
1436             // Remapping bound vars here
1437             let trait_pred_and_suggested_ty =
1438                 trait_pred.map_bound(|trait_pred| (trait_pred, suggested_ty));
1439
1440             let new_obligation = self.mk_trait_obligation_with_new_self_ty(
1441                 obligation.param_env,
1442                 trait_pred_and_suggested_ty,
1443             );
1444
1445             if self.predicate_may_hold(&new_obligation) {
1446                 let msg = if count == 1 {
1447                     "consider removing the leading `&`-reference".to_string()
1448                 } else {
1449                     format!("consider removing {count} leading `&`-references")
1450                 };
1451
1452                 err.multipart_suggestion_verbose(
1453                     &msg,
1454                     suggestions,
1455                     Applicability::MachineApplicable,
1456                 );
1457                 true
1458             } else {
1459                 false
1460             }
1461         };
1462
1463         // Maybe suggest removal of borrows from types in type parameters, like in
1464         // `src/test/ui/not-panic/not-panic-safe.rs`.
1465         let mut count = 0;
1466         let mut suggestions = vec![];
1467         // Skipping binder here, remapping below
1468         let mut suggested_ty = trait_pred.self_ty().skip_binder();
1469         if let Some(mut hir_ty) = expr_finder.ty_result {
1470             while let hir::TyKind::Ref(_, mut_ty) = &hir_ty.kind {
1471                 count += 1;
1472                 let span = hir_ty.span.until(mut_ty.ty.span);
1473                 suggestions.push((span, String::new()));
1474
1475                 let ty::Ref(_, inner_ty, _) = suggested_ty.kind() else {
1476                     break;
1477                 };
1478                 suggested_ty = *inner_ty;
1479
1480                 hir_ty = mut_ty.ty;
1481
1482                 if maybe_suggest(suggested_ty, count, suggestions.clone()) {
1483                     return true;
1484                 }
1485             }
1486         }
1487
1488         // Maybe suggest removal of borrows from expressions, like in `for i in &&&foo {}`.
1489         let Some(mut expr) = expr_finder.result else { return false; };
1490         let mut count = 0;
1491         let mut suggestions = vec![];
1492         // Skipping binder here, remapping below
1493         let mut suggested_ty = trait_pred.self_ty().skip_binder();
1494         'outer: loop {
1495             while let hir::ExprKind::AddrOf(_, _, borrowed) = expr.kind {
1496                 count += 1;
1497                 let span = if expr.span.eq_ctxt(borrowed.span) {
1498                     expr.span.until(borrowed.span)
1499                 } else {
1500                     expr.span.with_hi(expr.span.lo() + BytePos(1))
1501                 };
1502                 suggestions.push((span, String::new()));
1503
1504                 let ty::Ref(_, inner_ty, _) = suggested_ty.kind() else {
1505                     break 'outer;
1506                 };
1507                 suggested_ty = *inner_ty;
1508
1509                 expr = borrowed;
1510
1511                 if maybe_suggest(suggested_ty, count, suggestions.clone()) {
1512                     return true;
1513                 }
1514             }
1515             if let hir::ExprKind::Path(hir::QPath::Resolved(None, path)) = expr.kind
1516                 && let hir::def::Res::Local(hir_id) = path.res
1517                 && let Some(hir::Node::Pat(binding)) = self.tcx.hir().find(hir_id)
1518                 && let Some(hir::Node::Local(local)) = self.tcx.hir().find_parent(binding.hir_id)
1519                 && let None = local.ty
1520                 && let Some(binding_expr) = local.init
1521             {
1522                 expr = binding_expr;
1523             } else {
1524                 break 'outer;
1525             }
1526         }
1527         false
1528     }
1529
1530     fn suggest_remove_await(&self, obligation: &PredicateObligation<'tcx>, err: &mut Diagnostic) {
1531         let span = obligation.cause.span;
1532
1533         if let ObligationCauseCode::AwaitableExpr(hir_id) = obligation.cause.code().peel_derives() {
1534             let hir = self.tcx.hir();
1535             if let Some(hir::Node::Expr(expr)) = hir_id.and_then(|hir_id| hir.find(hir_id)) {
1536                 // FIXME: use `obligation.predicate.kind()...trait_ref.self_ty()` to see if we have `()`
1537                 // and if not maybe suggest doing something else? If we kept the expression around we
1538                 // could also check if it is an fn call (very likely) and suggest changing *that*, if
1539                 // it is from the local crate.
1540                 err.span_suggestion(
1541                     span,
1542                     "remove the `.await`",
1543                     "",
1544                     Applicability::MachineApplicable,
1545                 );
1546                 // FIXME: account for associated `async fn`s.
1547                 if let hir::Expr { span, kind: hir::ExprKind::Call(base, _), .. } = expr {
1548                     if let ty::PredicateKind::Clause(ty::Clause::Trait(pred)) =
1549                         obligation.predicate.kind().skip_binder()
1550                     {
1551                         err.span_label(*span, &format!("this call returns `{}`", pred.self_ty()));
1552                     }
1553                     if let Some(typeck_results) = &self.typeck_results
1554                             && let ty = typeck_results.expr_ty_adjusted(base)
1555                             && let ty::FnDef(def_id, _substs) = ty.kind()
1556                             && let Some(hir::Node::Item(hir::Item { ident, span, vis_span, .. })) =
1557                                 hir.get_if_local(*def_id)
1558                         {
1559                             let msg = format!(
1560                                 "alternatively, consider making `fn {}` asynchronous",
1561                                 ident
1562                             );
1563                             if vis_span.is_empty() {
1564                                 err.span_suggestion_verbose(
1565                                     span.shrink_to_lo(),
1566                                     &msg,
1567                                     "async ",
1568                                     Applicability::MaybeIncorrect,
1569                                 );
1570                             } else {
1571                                 err.span_suggestion_verbose(
1572                                     vis_span.shrink_to_hi(),
1573                                     &msg,
1574                                     " async",
1575                                     Applicability::MaybeIncorrect,
1576                                 );
1577                             }
1578                         }
1579                 }
1580             }
1581         }
1582     }
1583
1584     /// Check if the trait bound is implemented for a different mutability and note it in the
1585     /// final error.
1586     fn suggest_change_mut(
1587         &self,
1588         obligation: &PredicateObligation<'tcx>,
1589         err: &mut Diagnostic,
1590         trait_pred: ty::PolyTraitPredicate<'tcx>,
1591     ) {
1592         let points_at_arg = matches!(
1593             obligation.cause.code(),
1594             ObligationCauseCode::FunctionArgumentObligation { .. },
1595         );
1596
1597         let span = obligation.cause.span;
1598         if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
1599             let refs_number =
1600                 snippet.chars().filter(|c| !c.is_whitespace()).take_while(|c| *c == '&').count();
1601             if let Some('\'') = snippet.chars().filter(|c| !c.is_whitespace()).nth(refs_number) {
1602                 // Do not suggest removal of borrow from type arguments.
1603                 return;
1604             }
1605             let trait_pred = self.resolve_vars_if_possible(trait_pred);
1606             if trait_pred.has_non_region_infer() {
1607                 // Do not ICE while trying to find if a reborrow would succeed on a trait with
1608                 // unresolved bindings.
1609                 return;
1610             }
1611
1612             // Skipping binder here, remapping below
1613             if let ty::Ref(region, t_type, mutability) = *trait_pred.skip_binder().self_ty().kind()
1614             {
1615                 let suggested_ty = match mutability {
1616                     hir::Mutability::Mut => self.tcx.mk_imm_ref(region, t_type),
1617                     hir::Mutability::Not => self.tcx.mk_mut_ref(region, t_type),
1618                 };
1619
1620                 // Remapping bound vars here
1621                 let trait_pred_and_suggested_ty =
1622                     trait_pred.map_bound(|trait_pred| (trait_pred, suggested_ty));
1623
1624                 let new_obligation = self.mk_trait_obligation_with_new_self_ty(
1625                     obligation.param_env,
1626                     trait_pred_and_suggested_ty,
1627                 );
1628                 let suggested_ty_would_satisfy_obligation = self
1629                     .evaluate_obligation_no_overflow(&new_obligation)
1630                     .must_apply_modulo_regions();
1631                 if suggested_ty_would_satisfy_obligation {
1632                     let sp = self
1633                         .tcx
1634                         .sess
1635                         .source_map()
1636                         .span_take_while(span, |c| c.is_whitespace() || *c == '&');
1637                     if points_at_arg && mutability.is_not() && refs_number > 0 {
1638                         // If we have a call like foo(&mut buf), then don't suggest foo(&mut mut buf)
1639                         if snippet
1640                             .trim_start_matches(|c: char| c.is_whitespace() || c == '&')
1641                             .starts_with("mut")
1642                         {
1643                             return;
1644                         }
1645                         err.span_suggestion_verbose(
1646                             sp,
1647                             "consider changing this borrow's mutability",
1648                             "&mut ",
1649                             Applicability::MachineApplicable,
1650                         );
1651                     } else {
1652                         err.note(&format!(
1653                             "`{}` is implemented for `{:?}`, but not for `{:?}`",
1654                             trait_pred.print_modifiers_and_trait_path(),
1655                             suggested_ty,
1656                             trait_pred.skip_binder().self_ty(),
1657                         ));
1658                     }
1659                 }
1660             }
1661         }
1662     }
1663
1664     fn suggest_semicolon_removal(
1665         &self,
1666         obligation: &PredicateObligation<'tcx>,
1667         err: &mut Diagnostic,
1668         span: Span,
1669         trait_pred: ty::PolyTraitPredicate<'tcx>,
1670     ) -> bool {
1671         let hir = self.tcx.hir();
1672         let parent_node = hir.parent_id(obligation.cause.body_id);
1673         let node = hir.find(parent_node);
1674         if let Some(hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(sig, _, body_id), .. })) = node
1675             && let hir::ExprKind::Block(blk, _) = &hir.body(*body_id).value.kind
1676             && sig.decl.output.span().overlaps(span)
1677             && blk.expr.is_none()
1678             && trait_pred.self_ty().skip_binder().is_unit()
1679             && let Some(stmt) = blk.stmts.last()
1680             && let hir::StmtKind::Semi(expr) = stmt.kind
1681             // Only suggest this if the expression behind the semicolon implements the predicate
1682             && let Some(typeck_results) = &self.typeck_results
1683             && let Some(ty) = typeck_results.expr_ty_opt(expr)
1684             && self.predicate_may_hold(&self.mk_trait_obligation_with_new_self_ty(
1685                 obligation.param_env, trait_pred.map_bound(|trait_pred| (trait_pred, ty))
1686             ))
1687         {
1688             err.span_label(
1689                 expr.span,
1690                 &format!(
1691                     "this expression has type `{}`, which implements `{}`",
1692                     ty,
1693                     trait_pred.print_modifiers_and_trait_path()
1694                 )
1695             );
1696             err.span_suggestion(
1697                 self.tcx.sess.source_map().end_point(stmt.span),
1698                 "remove this semicolon",
1699                 "",
1700                 Applicability::MachineApplicable
1701             );
1702             return true;
1703         }
1704         false
1705     }
1706
1707     fn return_type_span(&self, obligation: &PredicateObligation<'tcx>) -> Option<Span> {
1708         let hir = self.tcx.hir();
1709         let parent_node = hir.parent_id(obligation.cause.body_id);
1710         let Some(hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(sig, ..), .. })) = hir.find(parent_node) 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.parent_id(obligation.cause.body_id);
1735         let node = hir.find(fn_hir_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.object_safety_violations(def_id).is_empty())
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, fn_hir_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 parent_node = hir.parent_id(obligation.cause.body_id);
1947         let node = hir.find(parent_node);
1948         if let Some(hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(_, _, body_id), .. })) =
1949             node
1950         {
1951             let body = hir.body(*body_id);
1952             // Point at all the `return`s in the function as they have failed trait bounds.
1953             let mut visitor = ReturnsVisitor::default();
1954             visitor.visit_body(&body);
1955             let typeck_results = self.typeck_results.as_ref().unwrap();
1956             for expr in &visitor.returns {
1957                 if let Some(returned_ty) = typeck_results.node_type_opt(expr.hir_id) {
1958                     let ty = self.resolve_vars_if_possible(returned_ty);
1959                     if ty.references_error() {
1960                         // don't print out the [type error] here
1961                         err.delay_as_bug();
1962                     } else {
1963                         err.span_label(
1964                             expr.span,
1965                             &format!("this returned value is of type `{}`", ty),
1966                         );
1967                     }
1968                 }
1969             }
1970         }
1971     }
1972
1973     fn report_closure_arg_mismatch(
1974         &self,
1975         span: Span,
1976         found_span: Option<Span>,
1977         found: ty::PolyTraitRef<'tcx>,
1978         expected: ty::PolyTraitRef<'tcx>,
1979         cause: &ObligationCauseCode<'tcx>,
1980         found_node: Option<Node<'_>>,
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(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, ..) => {
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, ..) => {
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 mut interior_or_upvar_span = None;
2346
2347         let from_awaited_ty = generator_data.get_from_await_ty(visitor, hir, ty_matches);
2348         debug!(?from_awaited_ty);
2349
2350         // The generator interior types share the same binders
2351         if let Some(cause) =
2352             generator_data.get_generator_interior_types().skip_binder().iter().find(
2353                 |ty::GeneratorInteriorTypeCause { ty, .. }| {
2354                     ty_matches(generator_data.get_generator_interior_types().rebind(*ty))
2355                 },
2356             )
2357         {
2358             let ty::GeneratorInteriorTypeCause { span, scope_span, yield_span, expr, .. } = cause;
2359
2360             interior_or_upvar_span = Some(GeneratorInteriorOrUpvar::Interior(
2361                 *span,
2362                 Some((*scope_span, *yield_span, *expr, from_awaited_ty)),
2363             ));
2364         }
2365
2366         if interior_or_upvar_span.is_none() {
2367             interior_or_upvar_span =
2368                 generator_data.try_get_upvar_span(&self, generator_did, ty_matches);
2369         }
2370
2371         if interior_or_upvar_span.is_none() && generator_data.is_foreign() {
2372             interior_or_upvar_span = Some(GeneratorInteriorOrUpvar::Interior(span, None));
2373         }
2374
2375         debug!(?interior_or_upvar_span);
2376         if let Some(interior_or_upvar_span) = interior_or_upvar_span {
2377             let is_async = self.tcx.generator_is_async(generator_did);
2378             let typeck_results = match generator_data {
2379                 GeneratorData::Local(typeck_results) => Some(typeck_results),
2380                 GeneratorData::Foreign(_) => None,
2381             };
2382             self.note_obligation_cause_for_async_await(
2383                 err,
2384                 interior_or_upvar_span,
2385                 is_async,
2386                 outer_generator,
2387                 trait_ref,
2388                 target_ty,
2389                 typeck_results,
2390                 obligation,
2391                 next_code,
2392             );
2393             true
2394         } else {
2395             false
2396         }
2397     }
2398
2399     /// Unconditionally adds the diagnostic note described in
2400     /// `maybe_note_obligation_cause_for_async_await`'s documentation comment.
2401     #[instrument(level = "debug", skip_all)]
2402     fn note_obligation_cause_for_async_await(
2403         &self,
2404         err: &mut Diagnostic,
2405         interior_or_upvar_span: GeneratorInteriorOrUpvar,
2406         is_async: bool,
2407         outer_generator: Option<DefId>,
2408         trait_pred: ty::TraitPredicate<'tcx>,
2409         target_ty: Ty<'tcx>,
2410         typeck_results: Option<&ty::TypeckResults<'tcx>>,
2411         obligation: &PredicateObligation<'tcx>,
2412         next_code: Option<&ObligationCauseCode<'tcx>>,
2413     ) {
2414         let source_map = self.tcx.sess.source_map();
2415
2416         let (await_or_yield, an_await_or_yield) =
2417             if is_async { ("await", "an await") } else { ("yield", "a yield") };
2418         let future_or_generator = if is_async { "future" } else { "generator" };
2419
2420         // Special case the primary error message when send or sync is the trait that was
2421         // not implemented.
2422         let hir = self.tcx.hir();
2423         let trait_explanation = if let Some(name @ (sym::Send | sym::Sync)) =
2424             self.tcx.get_diagnostic_name(trait_pred.def_id())
2425         {
2426             let (trait_name, trait_verb) =
2427                 if name == sym::Send { ("`Send`", "sent") } else { ("`Sync`", "shared") };
2428
2429             err.clear_code();
2430             err.set_primary_message(format!(
2431                 "{} cannot be {} between threads safely",
2432                 future_or_generator, trait_verb
2433             ));
2434
2435             let original_span = err.span.primary_span().unwrap();
2436             let mut span = MultiSpan::from_span(original_span);
2437
2438             let message = outer_generator
2439                 .and_then(|generator_did| {
2440                     Some(match self.tcx.generator_kind(generator_did).unwrap() {
2441                         GeneratorKind::Gen => format!("generator is not {}", trait_name),
2442                         GeneratorKind::Async(AsyncGeneratorKind::Fn) => self
2443                             .tcx
2444                             .parent(generator_did)
2445                             .as_local()
2446                             .map(|parent_did| hir.local_def_id_to_hir_id(parent_did))
2447                             .and_then(|parent_hir_id| hir.opt_name(parent_hir_id))
2448                             .map(|name| {
2449                                 format!("future returned by `{}` is not {}", name, trait_name)
2450                             })?,
2451                         GeneratorKind::Async(AsyncGeneratorKind::Block) => {
2452                             format!("future created by async block is not {}", trait_name)
2453                         }
2454                         GeneratorKind::Async(AsyncGeneratorKind::Closure) => {
2455                             format!("future created by async closure is not {}", trait_name)
2456                         }
2457                     })
2458                 })
2459                 .unwrap_or_else(|| format!("{} is not {}", future_or_generator, trait_name));
2460
2461             span.push_span_label(original_span, message);
2462             err.set_span(span);
2463
2464             format!("is not {}", trait_name)
2465         } else {
2466             format!("does not implement `{}`", trait_pred.print_modifiers_and_trait_path())
2467         };
2468
2469         let mut explain_yield =
2470             |interior_span: Span, yield_span: Span, scope_span: Option<Span>| {
2471                 let mut span = MultiSpan::from_span(yield_span);
2472                 let snippet = match source_map.span_to_snippet(interior_span) {
2473                     // #70935: If snippet contains newlines, display "the value" instead
2474                     // so that we do not emit complex diagnostics.
2475                     Ok(snippet) if !snippet.contains('\n') => format!("`{}`", snippet),
2476                     _ => "the value".to_string(),
2477                 };
2478                 // note: future is not `Send` as this value is used across an await
2479                 //   --> $DIR/issue-70935-complex-spans.rs:13:9
2480                 //    |
2481                 // LL |            baz(|| async {
2482                 //    |  ______________-
2483                 //    | |
2484                 //    | |
2485                 // LL | |              foo(tx.clone());
2486                 // LL | |          }).await;
2487                 //    | |          - ^^^^^^ await occurs here, with value maybe used later
2488                 //    | |__________|
2489                 //    |            has type `closure` which is not `Send`
2490                 // note: value is later dropped here
2491                 // LL | |          }).await;
2492                 //    | |                  ^
2493                 //
2494                 span.push_span_label(
2495                     yield_span,
2496                     format!("{} occurs here, with {} maybe used later", await_or_yield, snippet),
2497                 );
2498                 span.push_span_label(
2499                     interior_span,
2500                     format!("has type `{}` which {}", target_ty, trait_explanation),
2501                 );
2502                 if let Some(scope_span) = scope_span {
2503                     let scope_span = source_map.end_point(scope_span);
2504
2505                     let msg = format!("{} is later dropped here", snippet);
2506                     span.push_span_label(scope_span, msg);
2507                 }
2508                 err.span_note(
2509                     span,
2510                     &format!(
2511                         "{} {} as this value is used across {}",
2512                         future_or_generator, trait_explanation, an_await_or_yield
2513                     ),
2514                 );
2515             };
2516         match interior_or_upvar_span {
2517             GeneratorInteriorOrUpvar::Interior(interior_span, interior_extra_info) => {
2518                 if let Some((scope_span, yield_span, expr, from_awaited_ty)) = interior_extra_info {
2519                     if let Some(await_span) = from_awaited_ty {
2520                         // The type causing this obligation is one being awaited at await_span.
2521                         let mut span = MultiSpan::from_span(await_span);
2522                         span.push_span_label(
2523                             await_span,
2524                             format!(
2525                                 "await occurs here on type `{}`, which {}",
2526                                 target_ty, trait_explanation
2527                             ),
2528                         );
2529                         err.span_note(
2530                             span,
2531                             &format!(
2532                                 "future {not_trait} as it awaits another future which {not_trait}",
2533                                 not_trait = trait_explanation
2534                             ),
2535                         );
2536                     } else {
2537                         // Look at the last interior type to get a span for the `.await`.
2538                         debug!(
2539                             generator_interior_types = ?format_args!(
2540                                 "{:#?}", typeck_results.as_ref().map(|t| &t.generator_interior_types)
2541                             ),
2542                         );
2543                         explain_yield(interior_span, yield_span, scope_span);
2544                     }
2545
2546                     if let Some(expr_id) = expr {
2547                         let expr = hir.expect_expr(expr_id);
2548                         debug!("target_ty evaluated from {:?}", expr);
2549
2550                         let parent = hir.parent_id(expr_id);
2551                         if let Some(hir::Node::Expr(e)) = hir.find(parent) {
2552                             let parent_span = hir.span(parent);
2553                             let parent_did = parent.owner.to_def_id();
2554                             // ```rust
2555                             // impl T {
2556                             //     fn foo(&self) -> i32 {}
2557                             // }
2558                             // T.foo();
2559                             // ^^^^^^^ a temporary `&T` created inside this method call due to `&self`
2560                             // ```
2561                             //
2562                             let is_region_borrow = if let Some(typeck_results) = typeck_results {
2563                                 typeck_results
2564                                     .expr_adjustments(expr)
2565                                     .iter()
2566                                     .any(|adj| adj.is_region_borrow())
2567                             } else {
2568                                 false
2569                             };
2570
2571                             // ```rust
2572                             // struct Foo(*const u8);
2573                             // bar(Foo(std::ptr::null())).await;
2574                             //     ^^^^^^^^^^^^^^^^^^^^^ raw-ptr `*T` created inside this struct ctor.
2575                             // ```
2576                             debug!(parent_def_kind = ?self.tcx.def_kind(parent_did));
2577                             let is_raw_borrow_inside_fn_like_call =
2578                                 match self.tcx.def_kind(parent_did) {
2579                                     DefKind::Fn | DefKind::Ctor(..) => target_ty.is_unsafe_ptr(),
2580                                     _ => false,
2581                                 };
2582                             if let Some(typeck_results) = typeck_results {
2583                                 if (typeck_results.is_method_call(e) && is_region_borrow)
2584                                     || is_raw_borrow_inside_fn_like_call
2585                                 {
2586                                     err.span_help(
2587                                         parent_span,
2588                                         "consider moving this into a `let` \
2589                         binding to create a shorter lived borrow",
2590                                     );
2591                                 }
2592                             }
2593                         }
2594                     }
2595                 }
2596             }
2597             GeneratorInteriorOrUpvar::Upvar(upvar_span) => {
2598                 // `Some((ref_ty, is_mut))` if `target_ty` is `&T` or `&mut T` and fails to impl `Send`
2599                 let non_send = match target_ty.kind() {
2600                     ty::Ref(_, ref_ty, mutability) => match self.evaluate_obligation(&obligation) {
2601                         Ok(eval) if !eval.may_apply() => Some((ref_ty, mutability.is_mut())),
2602                         _ => None,
2603                     },
2604                     _ => None,
2605                 };
2606
2607                 let (span_label, span_note) = match non_send {
2608                     // if `target_ty` is `&T` or `&mut T` and fails to impl `Send`,
2609                     // include suggestions to make `T: Sync` so that `&T: Send`,
2610                     // or to make `T: Send` so that `&mut T: Send`
2611                     Some((ref_ty, is_mut)) => {
2612                         let ref_ty_trait = if is_mut { "Send" } else { "Sync" };
2613                         let ref_kind = if is_mut { "&mut" } else { "&" };
2614                         (
2615                             format!(
2616                                 "has type `{}` which {}, because `{}` is not `{}`",
2617                                 target_ty, trait_explanation, ref_ty, ref_ty_trait
2618                             ),
2619                             format!(
2620                                 "captured value {} because `{}` references cannot be sent unless their referent is `{}`",
2621                                 trait_explanation, ref_kind, ref_ty_trait
2622                             ),
2623                         )
2624                     }
2625                     None => (
2626                         format!("has type `{}` which {}", target_ty, trait_explanation),
2627                         format!("captured value {}", trait_explanation),
2628                     ),
2629                 };
2630
2631                 let mut span = MultiSpan::from_span(upvar_span);
2632                 span.push_span_label(upvar_span, span_label);
2633                 err.span_note(span, &span_note);
2634             }
2635         }
2636
2637         // Add a note for the item obligation that remains - normally a note pointing to the
2638         // bound that introduced the obligation (e.g. `T: Send`).
2639         debug!(?next_code);
2640         self.note_obligation_cause_code(
2641             err,
2642             obligation.predicate,
2643             obligation.param_env,
2644             next_code.unwrap(),
2645             &mut Vec::new(),
2646             &mut Default::default(),
2647         );
2648     }
2649
2650     fn note_obligation_cause_code<T>(
2651         &self,
2652         err: &mut Diagnostic,
2653         predicate: T,
2654         param_env: ty::ParamEnv<'tcx>,
2655         cause_code: &ObligationCauseCode<'tcx>,
2656         obligated_types: &mut Vec<Ty<'tcx>>,
2657         seen_requirements: &mut FxHashSet<DefId>,
2658     ) where
2659         T: ToPredicate<'tcx>,
2660     {
2661         let tcx = self.tcx;
2662         let predicate = predicate.to_predicate(tcx);
2663         match *cause_code {
2664             ObligationCauseCode::ExprAssignable
2665             | ObligationCauseCode::MatchExpressionArm { .. }
2666             | ObligationCauseCode::Pattern { .. }
2667             | ObligationCauseCode::IfExpression { .. }
2668             | ObligationCauseCode::IfExpressionWithNoElse
2669             | ObligationCauseCode::MainFunctionType
2670             | ObligationCauseCode::StartFunctionType
2671             | ObligationCauseCode::IntrinsicType
2672             | ObligationCauseCode::MethodReceiver
2673             | ObligationCauseCode::ReturnNoExpression
2674             | ObligationCauseCode::UnifyReceiver(..)
2675             | ObligationCauseCode::OpaqueType
2676             | ObligationCauseCode::MiscObligation
2677             | ObligationCauseCode::WellFormed(..)
2678             | ObligationCauseCode::MatchImpl(..)
2679             | ObligationCauseCode::ReturnType
2680             | ObligationCauseCode::ReturnValue(_)
2681             | ObligationCauseCode::BlockTailExpression(_)
2682             | ObligationCauseCode::AwaitableExpr(_)
2683             | ObligationCauseCode::ForLoopIterator
2684             | ObligationCauseCode::QuestionMark
2685             | ObligationCauseCode::CheckAssociatedTypeBounds { .. }
2686             | ObligationCauseCode::LetElse
2687             | ObligationCauseCode::BinOp { .. }
2688             | ObligationCauseCode::AscribeUserTypeProvePredicate(..)
2689             | ObligationCauseCode::RustCall => {}
2690             ObligationCauseCode::SliceOrArrayElem => {
2691                 err.note("slice and array elements must have `Sized` type");
2692             }
2693             ObligationCauseCode::TupleElem => {
2694                 err.note("only the last element of a tuple may have a dynamically sized type");
2695             }
2696             ObligationCauseCode::ProjectionWf(data) => {
2697                 err.note(&format!("required so that the projection `{data}` is well-formed"));
2698             }
2699             ObligationCauseCode::ReferenceOutlivesReferent(ref_ty) => {
2700                 err.note(&format!(
2701                     "required so that reference `{ref_ty}` does not outlive its referent"
2702                 ));
2703             }
2704             ObligationCauseCode::ObjectTypeBound(object_ty, region) => {
2705                 err.note(&format!(
2706                     "required so that the lifetime bound of `{}` for `{}` is satisfied",
2707                     region, object_ty,
2708                 ));
2709             }
2710             ObligationCauseCode::ItemObligation(_)
2711             | ObligationCauseCode::ExprItemObligation(..) => {
2712                 // We hold the `DefId` of the item introducing the obligation, but displaying it
2713                 // doesn't add user usable information. It always point at an associated item.
2714             }
2715             ObligationCauseCode::BindingObligation(item_def_id, span)
2716             | ObligationCauseCode::ExprBindingObligation(item_def_id, span, ..) => {
2717                 let item_name = tcx.def_path_str(item_def_id);
2718                 let short_item_name = with_forced_trimmed_paths!(tcx.def_path_str(item_def_id));
2719                 let mut multispan = MultiSpan::from(span);
2720                 let sm = tcx.sess.source_map();
2721                 if let Some(ident) = tcx.opt_item_ident(item_def_id) {
2722                     let same_line =
2723                         match (sm.lookup_line(ident.span.hi()), sm.lookup_line(span.lo())) {
2724                             (Ok(l), Ok(r)) => l.line == r.line,
2725                             _ => true,
2726                         };
2727                     if ident.span.is_visible(sm) && !ident.span.overlaps(span) && !same_line {
2728                         multispan.push_span_label(ident.span, "required by a bound in this");
2729                     }
2730                 }
2731                 let descr = format!("required by a bound in `{item_name}`");
2732                 if span.is_visible(sm) {
2733                     let msg = format!("required by this bound in `{short_item_name}`");
2734                     multispan.push_span_label(span, msg);
2735                     err.span_note(multispan, &descr);
2736                 } else {
2737                     err.span_note(tcx.def_span(item_def_id), &descr);
2738                 }
2739             }
2740             ObligationCauseCode::ObjectCastObligation(concrete_ty, object_ty) => {
2741                 let (concrete_ty, concrete_file) =
2742                     self.tcx.short_ty_string(self.resolve_vars_if_possible(concrete_ty));
2743                 let (object_ty, object_file) =
2744                     self.tcx.short_ty_string(self.resolve_vars_if_possible(object_ty));
2745                 err.note(&with_forced_trimmed_paths!(format!(
2746                     "required for the cast from `{concrete_ty}` to the object type `{object_ty}`",
2747                 )));
2748                 if let Some(file) = concrete_file {
2749                     err.note(&format!(
2750                         "the full name for the casted type has been written to '{}'",
2751                         file.display(),
2752                     ));
2753                 }
2754                 if let Some(file) = object_file {
2755                     err.note(&format!(
2756                         "the full name for the object type has been written to '{}'",
2757                         file.display(),
2758                     ));
2759                 }
2760             }
2761             ObligationCauseCode::Coercion { source: _, target } => {
2762                 err.note(&format!("required by cast to type `{}`", self.ty_to_string(target)));
2763             }
2764             ObligationCauseCode::RepeatElementCopy { is_const_fn } => {
2765                 err.note(
2766                     "the `Copy` trait is required because this value will be copied for each element of the array",
2767                 );
2768
2769                 if is_const_fn {
2770                     err.help(
2771                         "consider creating a new `const` item and initializing it with the result \
2772                         of the function call to be used in the repeat position, like \
2773                         `const VAL: Type = const_fn();` and `let x = [VAL; 42];`",
2774                     );
2775                 }
2776
2777                 if self.tcx.sess.is_nightly_build() && is_const_fn {
2778                     err.help(
2779                         "create an inline `const` block, see RFC #2920 \
2780                          <https://github.com/rust-lang/rfcs/pull/2920> for more information",
2781                     );
2782                 }
2783             }
2784             ObligationCauseCode::VariableType(hir_id) => {
2785                 let parent_node = self.tcx.hir().parent_id(hir_id);
2786                 match self.tcx.hir().find(parent_node) {
2787                     Some(Node::Local(hir::Local { ty: Some(ty), .. })) => {
2788                         err.span_suggestion_verbose(
2789                             ty.span.shrink_to_lo(),
2790                             "consider borrowing here",
2791                             "&",
2792                             Applicability::MachineApplicable,
2793                         );
2794                         err.note("all local variables must have a statically known size");
2795                     }
2796                     Some(Node::Local(hir::Local {
2797                         init: Some(hir::Expr { kind: hir::ExprKind::Index(_, _), span, .. }),
2798                         ..
2799                     })) => {
2800                         // When encountering an assignment of an unsized trait, like
2801                         // `let x = ""[..];`, provide a suggestion to borrow the initializer in
2802                         // order to use have a slice instead.
2803                         err.span_suggestion_verbose(
2804                             span.shrink_to_lo(),
2805                             "consider borrowing here",
2806                             "&",
2807                             Applicability::MachineApplicable,
2808                         );
2809                         err.note("all local variables must have a statically known size");
2810                     }
2811                     Some(Node::Param(param)) => {
2812                         err.span_suggestion_verbose(
2813                             param.ty_span.shrink_to_lo(),
2814                             "function arguments must have a statically known size, borrowed types \
2815                             always have a known size",
2816                             "&",
2817                             Applicability::MachineApplicable,
2818                         );
2819                     }
2820                     _ => {
2821                         err.note("all local variables must have a statically known size");
2822                     }
2823                 }
2824                 if !self.tcx.features().unsized_locals {
2825                     err.help("unsized locals are gated as an unstable feature");
2826                 }
2827             }
2828             ObligationCauseCode::SizedArgumentType(sp) => {
2829                 if let Some(span) = sp {
2830                     if let ty::PredicateKind::Clause(clause) = predicate.kind().skip_binder()
2831                         && let ty::Clause::Trait(trait_pred) = clause
2832                         && let ty::Dynamic(..) = trait_pred.self_ty().kind()
2833                     {
2834                         let span = if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span)
2835                             && snippet.starts_with("dyn ")
2836                         {
2837                             let pos = snippet.len() - snippet[3..].trim_start().len();
2838                             span.with_hi(span.lo() + BytePos(pos as u32))
2839                         } else {
2840                             span.shrink_to_lo()
2841                         };
2842                         err.span_suggestion_verbose(
2843                             span,
2844                             "you can use `impl Trait` as the argument type",
2845                             "impl ".to_string(),
2846                             Applicability::MaybeIncorrect,
2847                         );
2848                     }
2849                     err.span_suggestion_verbose(
2850                         span.shrink_to_lo(),
2851                         "function arguments must have a statically known size, borrowed types \
2852                          always have a known size",
2853                         "&",
2854                         Applicability::MachineApplicable,
2855                     );
2856                 } else {
2857                     err.note("all function arguments must have a statically known size");
2858                 }
2859                 if tcx.sess.opts.unstable_features.is_nightly_build()
2860                     && !self.tcx.features().unsized_fn_params
2861                 {
2862                     err.help("unsized fn params are gated as an unstable feature");
2863                 }
2864             }
2865             ObligationCauseCode::SizedReturnType => {
2866                 err.note("the return type of a function must have a statically known size");
2867             }
2868             ObligationCauseCode::SizedYieldType => {
2869                 err.note("the yield type of a generator must have a statically known size");
2870             }
2871             ObligationCauseCode::SizedBoxType => {
2872                 err.note("the type of a box expression must have a statically known size");
2873             }
2874             ObligationCauseCode::AssignmentLhsSized => {
2875                 err.note("the left-hand-side of an assignment must have a statically known size");
2876             }
2877             ObligationCauseCode::TupleInitializerSized => {
2878                 err.note("tuples must have a statically known size to be initialized");
2879             }
2880             ObligationCauseCode::StructInitializerSized => {
2881                 err.note("structs must have a statically known size to be initialized");
2882             }
2883             ObligationCauseCode::FieldSized { adt_kind: ref item, last, span } => {
2884                 match *item {
2885                     AdtKind::Struct => {
2886                         if last {
2887                             err.note(
2888                                 "the last field of a packed struct may only have a \
2889                                 dynamically sized type if it does not need drop to be run",
2890                             );
2891                         } else {
2892                             err.note(
2893                                 "only the last field of a struct may have a dynamically sized type",
2894                             );
2895                         }
2896                     }
2897                     AdtKind::Union => {
2898                         err.note("no field of a union may have a dynamically sized type");
2899                     }
2900                     AdtKind::Enum => {
2901                         err.note("no field of an enum variant may have a dynamically sized type");
2902                     }
2903                 }
2904                 err.help("change the field's type to have a statically known size");
2905                 err.span_suggestion(
2906                     span.shrink_to_lo(),
2907                     "borrowed types always have a statically known size",
2908                     "&",
2909                     Applicability::MachineApplicable,
2910                 );
2911                 err.multipart_suggestion(
2912                     "the `Box` type always has a statically known size and allocates its contents \
2913                      in the heap",
2914                     vec![
2915                         (span.shrink_to_lo(), "Box<".to_string()),
2916                         (span.shrink_to_hi(), ">".to_string()),
2917                     ],
2918                     Applicability::MachineApplicable,
2919                 );
2920             }
2921             ObligationCauseCode::ConstSized => {
2922                 err.note("constant expressions must have a statically known size");
2923             }
2924             ObligationCauseCode::InlineAsmSized => {
2925                 err.note("all inline asm arguments must have a statically known size");
2926             }
2927             ObligationCauseCode::ConstPatternStructural => {
2928                 err.note("constants used for pattern-matching must derive `PartialEq` and `Eq`");
2929             }
2930             ObligationCauseCode::SharedStatic => {
2931                 err.note("shared static variables must have a type that implements `Sync`");
2932             }
2933             ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
2934                 let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
2935                 let ty = parent_trait_ref.skip_binder().self_ty();
2936                 if parent_trait_ref.references_error() {
2937                     // NOTE(eddyb) this was `.cancel()`, but `err`
2938                     // is borrowed, so we can't fully defuse it.
2939                     err.downgrade_to_delayed_bug();
2940                     return;
2941                 }
2942
2943                 // If the obligation for a tuple is set directly by a Generator or Closure,
2944                 // then the tuple must be the one containing capture types.
2945                 let is_upvar_tys_infer_tuple = if !matches!(ty.kind(), ty::Tuple(..)) {
2946                     false
2947                 } else {
2948                     if let ObligationCauseCode::BuiltinDerivedObligation(data) = &*data.parent_code
2949                     {
2950                         let parent_trait_ref =
2951                             self.resolve_vars_if_possible(data.parent_trait_pred);
2952                         let nested_ty = parent_trait_ref.skip_binder().self_ty();
2953                         matches!(nested_ty.kind(), ty::Generator(..))
2954                             || matches!(nested_ty.kind(), ty::Closure(..))
2955                     } else {
2956                         false
2957                     }
2958                 };
2959
2960                 let identity_future = tcx.require_lang_item(LangItem::IdentityFuture, None);
2961
2962                 // Don't print the tuple of capture types
2963                 'print: {
2964                     if !is_upvar_tys_infer_tuple {
2965                         let msg = with_forced_trimmed_paths!(format!(
2966                             "required because it appears within the type `{ty}`",
2967                         ));
2968                         match ty.kind() {
2969                             ty::Adt(def, _) => match self.tcx.opt_item_ident(def.did()) {
2970                                 Some(ident) => err.span_note(ident.span, &msg),
2971                                 None => err.note(&msg),
2972                             },
2973                             ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }) => {
2974                                 // Avoid printing the future from `core::future::identity_future`, it's not helpful
2975                                 if tcx.parent(*def_id) == identity_future {
2976                                     break 'print;
2977                                 }
2978
2979                                 // If the previous type is `identity_future`, this is the future generated by the body of an async function.
2980                                 // Avoid printing it twice (it was already printed in the `ty::Generator` arm below).
2981                                 let is_future = tcx.ty_is_opaque_future(ty);
2982                                 debug!(
2983                                     ?obligated_types,
2984                                     ?is_future,
2985                                     "note_obligation_cause_code: check for async fn"
2986                                 );
2987                                 if is_future
2988                                     && obligated_types.last().map_or(false, |ty| match ty.kind() {
2989                                         ty::Generator(last_def_id, ..) => {
2990                                             tcx.generator_is_async(*last_def_id)
2991                                         }
2992                                         _ => false,
2993                                     })
2994                                 {
2995                                     break 'print;
2996                                 }
2997                                 err.span_note(self.tcx.def_span(def_id), &msg)
2998                             }
2999                             ty::GeneratorWitness(bound_tys) => {
3000                                 use std::fmt::Write;
3001
3002                                 // FIXME: this is kind of an unusual format for rustc, can we make it more clear?
3003                                 // Maybe we should just remove this note altogether?
3004                                 // FIXME: only print types which don't meet the trait requirement
3005                                 let mut msg =
3006                                     "required because it captures the following types: ".to_owned();
3007                                 for ty in bound_tys.skip_binder() {
3008                                     with_forced_trimmed_paths!(write!(msg, "`{}`, ", ty).unwrap());
3009                                 }
3010                                 err.note(msg.trim_end_matches(", "))
3011                             }
3012                             ty::Generator(def_id, _, _) => {
3013                                 let sp = self.tcx.def_span(def_id);
3014
3015                                 // Special-case this to say "async block" instead of `[static generator]`.
3016                                 let kind = tcx.generator_kind(def_id).unwrap().descr();
3017                                 err.span_note(
3018                                     sp,
3019                                     with_forced_trimmed_paths!(&format!(
3020                                         "required because it's used within this {kind}",
3021                                     )),
3022                                 )
3023                             }
3024                             ty::Closure(def_id, _) => err.span_note(
3025                                 self.tcx.def_span(def_id),
3026                                 "required because it's used within this closure",
3027                             ),
3028                             _ => err.note(&msg),
3029                         };
3030                     }
3031                 }
3032
3033                 obligated_types.push(ty);
3034
3035                 let parent_predicate = parent_trait_ref;
3036                 if !self.is_recursive_obligation(obligated_types, &data.parent_code) {
3037                     // #74711: avoid a stack overflow
3038                     ensure_sufficient_stack(|| {
3039                         self.note_obligation_cause_code(
3040                             err,
3041                             parent_predicate,
3042                             param_env,
3043                             &data.parent_code,
3044                             obligated_types,
3045                             seen_requirements,
3046                         )
3047                     });
3048                 } else {
3049                     ensure_sufficient_stack(|| {
3050                         self.note_obligation_cause_code(
3051                             err,
3052                             parent_predicate,
3053                             param_env,
3054                             cause_code.peel_derives(),
3055                             obligated_types,
3056                             seen_requirements,
3057                         )
3058                     });
3059                 }
3060             }
3061             ObligationCauseCode::ImplDerivedObligation(ref data) => {
3062                 let mut parent_trait_pred =
3063                     self.resolve_vars_if_possible(data.derived.parent_trait_pred);
3064                 parent_trait_pred.remap_constness_diag(param_env);
3065                 let parent_def_id = parent_trait_pred.def_id();
3066                 let (self_ty, file) =
3067                     self.tcx.short_ty_string(parent_trait_pred.skip_binder().self_ty());
3068                 let msg = format!(
3069                     "required for `{self_ty}` to implement `{}`",
3070                     parent_trait_pred.print_modifiers_and_trait_path()
3071                 );
3072                 let mut is_auto_trait = false;
3073                 match self.tcx.hir().get_if_local(data.impl_def_id) {
3074                     Some(Node::Item(hir::Item {
3075                         kind: hir::ItemKind::Trait(is_auto, ..),
3076                         ident,
3077                         ..
3078                     })) => {
3079                         // FIXME: we should do something else so that it works even on crate foreign
3080                         // auto traits.
3081                         is_auto_trait = matches!(is_auto, hir::IsAuto::Yes);
3082                         err.span_note(ident.span, &msg);
3083                     }
3084                     Some(Node::Item(hir::Item {
3085                         kind: hir::ItemKind::Impl(hir::Impl { of_trait, self_ty, .. }),
3086                         ..
3087                     })) => {
3088                         let mut spans = Vec::with_capacity(2);
3089                         if let Some(trait_ref) = of_trait {
3090                             spans.push(trait_ref.path.span);
3091                         }
3092                         spans.push(self_ty.span);
3093                         let mut spans: MultiSpan = spans.into();
3094                         if matches!(
3095                             self_ty.span.ctxt().outer_expn_data().kind,
3096                             ExpnKind::Macro(MacroKind::Derive, _)
3097                         ) || matches!(
3098                             of_trait.as_ref().map(|t| t.path.span.ctxt().outer_expn_data().kind),
3099                             Some(ExpnKind::Macro(MacroKind::Derive, _))
3100                         ) {
3101                             spans.push_span_label(
3102                                 data.span,
3103                                 "unsatisfied trait bound introduced in this `derive` macro",
3104                             );
3105                         } else if !data.span.is_dummy() && !data.span.overlaps(self_ty.span) {
3106                             spans.push_span_label(
3107                                 data.span,
3108                                 "unsatisfied trait bound introduced here",
3109                             );
3110                         }
3111                         err.span_note(spans, &msg);
3112                     }
3113                     _ => {
3114                         err.note(&msg);
3115                     }
3116                 };
3117
3118                 if let Some(file) = file {
3119                     err.note(&format!(
3120                         "the full type name has been written to '{}'",
3121                         file.display(),
3122                     ));
3123                 }
3124                 let mut parent_predicate = parent_trait_pred;
3125                 let mut data = &data.derived;
3126                 let mut count = 0;
3127                 seen_requirements.insert(parent_def_id);
3128                 if is_auto_trait {
3129                     // We don't want to point at the ADT saying "required because it appears within
3130                     // the type `X`", like we would otherwise do in test `supertrait-auto-trait.rs`.
3131                     while let ObligationCauseCode::BuiltinDerivedObligation(derived) =
3132                         &*data.parent_code
3133                     {
3134                         let child_trait_ref =
3135                             self.resolve_vars_if_possible(derived.parent_trait_pred);
3136                         let child_def_id = child_trait_ref.def_id();
3137                         if seen_requirements.insert(child_def_id) {
3138                             break;
3139                         }
3140                         data = derived;
3141                         parent_predicate = child_trait_ref.to_predicate(tcx);
3142                         parent_trait_pred = child_trait_ref;
3143                     }
3144                 }
3145                 while let ObligationCauseCode::ImplDerivedObligation(child) = &*data.parent_code {
3146                     // Skip redundant recursive obligation notes. See `ui/issue-20413.rs`.
3147                     let child_trait_pred =
3148                         self.resolve_vars_if_possible(child.derived.parent_trait_pred);
3149                     let child_def_id = child_trait_pred.def_id();
3150                     if seen_requirements.insert(child_def_id) {
3151                         break;
3152                     }
3153                     count += 1;
3154                     data = &child.derived;
3155                     parent_predicate = child_trait_pred.to_predicate(tcx);
3156                     parent_trait_pred = child_trait_pred;
3157                 }
3158                 if count > 0 {
3159                     err.note(&format!(
3160                         "{} redundant requirement{} hidden",
3161                         count,
3162                         pluralize!(count)
3163                     ));
3164                     let (self_ty, file) =
3165                         self.tcx.short_ty_string(parent_trait_pred.skip_binder().self_ty());
3166                     err.note(&format!(
3167                         "required for `{self_ty}` to implement `{}`",
3168                         parent_trait_pred.print_modifiers_and_trait_path()
3169                     ));
3170                     if let Some(file) = file {
3171                         err.note(&format!(
3172                             "the full type name has been written to '{}'",
3173                             file.display(),
3174                         ));
3175                     }
3176                 }
3177                 // #74711: avoid a stack overflow
3178                 ensure_sufficient_stack(|| {
3179                     self.note_obligation_cause_code(
3180                         err,
3181                         parent_predicate,
3182                         param_env,
3183                         &data.parent_code,
3184                         obligated_types,
3185                         seen_requirements,
3186                     )
3187                 });
3188             }
3189             ObligationCauseCode::DerivedObligation(ref data) => {
3190                 let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
3191                 let parent_predicate = parent_trait_ref;
3192                 // #74711: avoid a stack overflow
3193                 ensure_sufficient_stack(|| {
3194                     self.note_obligation_cause_code(
3195                         err,
3196                         parent_predicate,
3197                         param_env,
3198                         &data.parent_code,
3199                         obligated_types,
3200                         seen_requirements,
3201                     )
3202                 });
3203             }
3204             ObligationCauseCode::FunctionArgumentObligation {
3205                 arg_hir_id,
3206                 call_hir_id,
3207                 ref parent_code,
3208                 ..
3209             } => {
3210                 self.note_function_argument_obligation(
3211                     arg_hir_id,
3212                     err,
3213                     parent_code,
3214                     param_env,
3215                     predicate,
3216                     call_hir_id,
3217                 );
3218                 ensure_sufficient_stack(|| {
3219                     self.note_obligation_cause_code(
3220                         err,
3221                         predicate,
3222                         param_env,
3223                         &parent_code,
3224                         obligated_types,
3225                         seen_requirements,
3226                     )
3227                 });
3228             }
3229             ObligationCauseCode::CompareImplItemObligation { trait_item_def_id, kind, .. } => {
3230                 let item_name = self.tcx.item_name(trait_item_def_id);
3231                 let msg = format!(
3232                     "the requirement `{predicate}` appears on the `impl`'s {kind} \
3233                      `{item_name}` but not on the corresponding trait's {kind}",
3234                 );
3235                 let sp = self
3236                     .tcx
3237                     .opt_item_ident(trait_item_def_id)
3238                     .map(|i| i.span)
3239                     .unwrap_or_else(|| self.tcx.def_span(trait_item_def_id));
3240                 let mut assoc_span: MultiSpan = sp.into();
3241                 assoc_span.push_span_label(
3242                     sp,
3243                     format!("this trait's {kind} doesn't have the requirement `{predicate}`"),
3244                 );
3245                 if let Some(ident) = self
3246                     .tcx
3247                     .opt_associated_item(trait_item_def_id)
3248                     .and_then(|i| self.tcx.opt_item_ident(i.container_id(self.tcx)))
3249                 {
3250                     assoc_span.push_span_label(ident.span, "in this trait");
3251                 }
3252                 err.span_note(assoc_span, &msg);
3253             }
3254             ObligationCauseCode::TrivialBound => {
3255                 err.help("see issue #48214");
3256                 if tcx.sess.opts.unstable_features.is_nightly_build() {
3257                     err.help("add `#![feature(trivial_bounds)]` to the crate attributes to enable");
3258                 }
3259             }
3260             ObligationCauseCode::OpaqueReturnType(expr_info) => {
3261                 if let Some((expr_ty, expr_span)) = expr_info {
3262                     let expr_ty = with_forced_trimmed_paths!(self.ty_to_string(expr_ty));
3263                     err.span_label(
3264                         expr_span,
3265                         with_forced_trimmed_paths!(format!(
3266                             "return type was inferred to be `{expr_ty}` here",
3267                         )),
3268                     );
3269                 }
3270             }
3271         }
3272     }
3273
3274     #[instrument(
3275         level = "debug", skip(self, err), fields(trait_pred.self_ty = ?trait_pred.self_ty())
3276     )]
3277     fn suggest_await_before_try(
3278         &self,
3279         err: &mut Diagnostic,
3280         obligation: &PredicateObligation<'tcx>,
3281         trait_pred: ty::PolyTraitPredicate<'tcx>,
3282         span: Span,
3283     ) {
3284         let body_hir_id = obligation.cause.body_id;
3285         let item_id = self.tcx.hir().parent_id(body_hir_id);
3286
3287         if let Some(body_id) =
3288             self.tcx.hir().maybe_body_owned_by(self.tcx.hir().local_def_id(item_id))
3289         {
3290             let body = self.tcx.hir().body(body_id);
3291             if let Some(hir::GeneratorKind::Async(_)) = body.generator_kind {
3292                 let future_trait = self.tcx.require_lang_item(LangItem::Future, None);
3293
3294                 let self_ty = self.resolve_vars_if_possible(trait_pred.self_ty());
3295                 let impls_future = self.type_implements_trait(
3296                     future_trait,
3297                     [self.tcx.erase_late_bound_regions(self_ty)],
3298                     obligation.param_env,
3299                 );
3300                 if !impls_future.must_apply_modulo_regions() {
3301                     return;
3302                 }
3303
3304                 let item_def_id = self.tcx.associated_item_def_ids(future_trait)[0];
3305                 // `<T as Future>::Output`
3306                 let projection_ty = trait_pred.map_bound(|trait_pred| {
3307                     self.tcx.mk_projection(
3308                         item_def_id,
3309                         // Future::Output has no substs
3310                         [trait_pred.self_ty()],
3311                     )
3312                 });
3313                 let InferOk { value: projection_ty, .. } =
3314                     self.at(&obligation.cause, obligation.param_env).normalize(projection_ty);
3315
3316                 debug!(
3317                     normalized_projection_type = ?self.resolve_vars_if_possible(projection_ty)
3318                 );
3319                 let try_obligation = self.mk_trait_obligation_with_new_self_ty(
3320                     obligation.param_env,
3321                     trait_pred.map_bound(|trait_pred| (trait_pred, projection_ty.skip_binder())),
3322                 );
3323                 debug!(try_trait_obligation = ?try_obligation);
3324                 if self.predicate_may_hold(&try_obligation)
3325                     && let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span)
3326                     && snippet.ends_with('?')
3327                 {
3328                     err.span_suggestion_verbose(
3329                         span.with_hi(span.hi() - BytePos(1)).shrink_to_hi(),
3330                         "consider `await`ing on the `Future`",
3331                         ".await",
3332                         Applicability::MaybeIncorrect,
3333                     );
3334                 }
3335             }
3336         }
3337     }
3338
3339     fn suggest_floating_point_literal(
3340         &self,
3341         obligation: &PredicateObligation<'tcx>,
3342         err: &mut Diagnostic,
3343         trait_ref: &ty::PolyTraitRef<'tcx>,
3344     ) {
3345         let rhs_span = match obligation.cause.code() {
3346             ObligationCauseCode::BinOp { rhs_span: Some(span), is_lit, .. } if *is_lit => span,
3347             _ => return,
3348         };
3349         if let ty::Float(_) = trait_ref.skip_binder().self_ty().kind()
3350             && let ty::Infer(InferTy::IntVar(_)) = trait_ref.skip_binder().substs.type_at(1).kind()
3351         {
3352             err.span_suggestion_verbose(
3353                 rhs_span.shrink_to_hi(),
3354                 "consider using a floating-point literal by writing it with `.0`",
3355                 ".0",
3356                 Applicability::MaybeIncorrect,
3357             );
3358         }
3359     }
3360
3361     fn suggest_derive(
3362         &self,
3363         obligation: &PredicateObligation<'tcx>,
3364         err: &mut Diagnostic,
3365         trait_pred: ty::PolyTraitPredicate<'tcx>,
3366     ) {
3367         let Some(diagnostic_name) = self.tcx.get_diagnostic_name(trait_pred.def_id()) else {
3368             return;
3369         };
3370         let (adt, substs) = match trait_pred.skip_binder().self_ty().kind() {
3371             ty::Adt(adt, substs) if adt.did().is_local() => (adt, substs),
3372             _ => return,
3373         };
3374         let can_derive = {
3375             let is_derivable_trait = match diagnostic_name {
3376                 sym::Default => !adt.is_enum(),
3377                 sym::PartialEq | sym::PartialOrd => {
3378                     let rhs_ty = trait_pred.skip_binder().trait_ref.substs.type_at(1);
3379                     trait_pred.skip_binder().self_ty() == rhs_ty
3380                 }
3381                 sym::Eq | sym::Ord | sym::Clone | sym::Copy | sym::Hash | sym::Debug => true,
3382                 _ => false,
3383             };
3384             is_derivable_trait &&
3385                 // Ensure all fields impl the trait.
3386                 adt.all_fields().all(|field| {
3387                     let field_ty = field.ty(self.tcx, substs);
3388                     let trait_substs = match diagnostic_name {
3389                         sym::PartialEq | sym::PartialOrd => {
3390                             Some(field_ty)
3391                         }
3392                         _ => None,
3393                     };
3394                     let trait_pred = trait_pred.map_bound_ref(|tr| ty::TraitPredicate {
3395                         trait_ref: self.tcx.mk_trait_ref(
3396                             trait_pred.def_id(),
3397                             [field_ty].into_iter().chain(trait_substs),
3398                         ),
3399                         ..*tr
3400                     });
3401                     let field_obl = Obligation::new(
3402                         self.tcx,
3403                         obligation.cause.clone(),
3404                         obligation.param_env,
3405                         trait_pred,
3406                     );
3407                     self.predicate_must_hold_modulo_regions(&field_obl)
3408                 })
3409         };
3410         if can_derive {
3411             err.span_suggestion_verbose(
3412                 self.tcx.def_span(adt.did()).shrink_to_lo(),
3413                 &format!(
3414                     "consider annotating `{}` with `#[derive({})]`",
3415                     trait_pred.skip_binder().self_ty(),
3416                     diagnostic_name,
3417                 ),
3418                 format!("#[derive({})]\n", diagnostic_name),
3419                 Applicability::MaybeIncorrect,
3420             );
3421         }
3422     }
3423
3424     fn suggest_dereferencing_index(
3425         &self,
3426         obligation: &PredicateObligation<'tcx>,
3427         err: &mut Diagnostic,
3428         trait_pred: ty::PolyTraitPredicate<'tcx>,
3429     ) {
3430         if let ObligationCauseCode::ImplDerivedObligation(_) = obligation.cause.code()
3431             && self.tcx.is_diagnostic_item(sym::SliceIndex, trait_pred.skip_binder().trait_ref.def_id)
3432             && let ty::Slice(_) = trait_pred.skip_binder().trait_ref.substs.type_at(1).kind()
3433             && let ty::Ref(_, inner_ty, _) = trait_pred.skip_binder().self_ty().kind()
3434             && let ty::Uint(ty::UintTy::Usize) = inner_ty.kind()
3435         {
3436             err.span_suggestion_verbose(
3437                 obligation.cause.span.shrink_to_lo(),
3438             "dereference this index",
3439             '*',
3440                 Applicability::MachineApplicable,
3441             );
3442         }
3443     }
3444     fn note_function_argument_obligation(
3445         &self,
3446         arg_hir_id: HirId,
3447         err: &mut Diagnostic,
3448         parent_code: &ObligationCauseCode<'tcx>,
3449         param_env: ty::ParamEnv<'tcx>,
3450         failed_pred: ty::Predicate<'tcx>,
3451         call_hir_id: HirId,
3452     ) {
3453         let tcx = self.tcx;
3454         let hir = tcx.hir();
3455         if let Some(Node::Expr(expr)) = hir.find(arg_hir_id)
3456             && let Some(typeck_results) = &self.typeck_results
3457         {
3458             if let hir::Expr { kind: hir::ExprKind::Block(..), .. } = expr {
3459                 let expr = expr.peel_blocks();
3460                 let ty = typeck_results.expr_ty_adjusted_opt(expr).unwrap_or(tcx.ty_error());
3461                 let span = expr.span;
3462                 if Some(span) != err.span.primary_span() {
3463                     err.span_label(
3464                         span,
3465                         if ty.references_error() {
3466                             String::new()
3467                         } else {
3468                             let ty = with_forced_trimmed_paths!(self.ty_to_string(ty));
3469                             format!("this tail expression is of type `{ty}`")
3470                         },
3471                     );
3472                 }
3473             }
3474
3475             // FIXME: visit the ty to see if there's any closure involved, and if there is,
3476             // check whether its evaluated return type is the same as the one corresponding
3477             // to an associated type (as seen from `trait_pred`) in the predicate. Like in
3478             // trait_pred `S: Sum<<Self as Iterator>::Item>` and predicate `i32: Sum<&()>`
3479             let mut type_diffs = vec![];
3480
3481             if let ObligationCauseCode::ExprBindingObligation(def_id, _, _, idx) = parent_code.deref()
3482                 && let Some(node_substs) = typeck_results.node_substs_opt(call_hir_id)
3483                 && let where_clauses = self.tcx.predicates_of(def_id).instantiate(self.tcx, node_substs)
3484                 && let Some(where_pred) = where_clauses.predicates.get(*idx)
3485             {
3486                 if let Some(where_pred) = where_pred.to_opt_poly_trait_pred()
3487                     && let Some(failed_pred) = failed_pred.to_opt_poly_trait_pred()
3488                 {
3489                     let mut c = CollectAllMismatches {
3490                         infcx: self.infcx,
3491                         param_env,
3492                         errors: vec![],
3493                     };
3494                     if let Ok(_) = c.relate(where_pred, failed_pred) {
3495                         type_diffs = c.errors;
3496                     }
3497                 } else if let Some(where_pred) = where_pred.to_opt_poly_projection_pred()
3498                     && let Some(failed_pred) = failed_pred.to_opt_poly_projection_pred()
3499                     && let Some(found) = failed_pred.skip_binder().term.ty()
3500                 {
3501                     type_diffs = vec![
3502                         Sorts(ty::error::ExpectedFound {
3503                             expected: self.tcx.mk_ty(ty::Alias(ty::Projection, where_pred.skip_binder().projection_ty)),
3504                             found,
3505                         }),
3506                     ];
3507                 }
3508             }
3509             if let hir::ExprKind::Path(hir::QPath::Resolved(None, path)) = expr.kind
3510                 && let hir::Path { res: hir::def::Res::Local(hir_id), .. } = path
3511                 && let Some(hir::Node::Pat(binding)) = self.tcx.hir().find(*hir_id)
3512                 && let parent_hir_id = self.tcx.hir().parent_id(binding.hir_id)
3513                 && let Some(hir::Node::Local(local)) = self.tcx.hir().find(parent_hir_id)
3514                 && let Some(binding_expr) = local.init
3515             {
3516                 // If the expression we're calling on is a binding, we want to point at the
3517                 // `let` when talking about the type. Otherwise we'll point at every part
3518                 // of the method chain with the type.
3519                 self.point_at_chain(binding_expr, &typeck_results, type_diffs, param_env, err);
3520             } else {
3521                 self.point_at_chain(expr, &typeck_results, type_diffs, param_env, err);
3522             }
3523         }
3524         let call_node = hir.find(call_hir_id);
3525         if let Some(Node::Expr(hir::Expr {
3526             kind: hir::ExprKind::MethodCall(path, rcvr, ..), ..
3527         })) = call_node
3528         {
3529             if Some(rcvr.span) == err.span.primary_span() {
3530                 err.replace_span_with(path.ident.span, true);
3531             }
3532         }
3533         if let Some(Node::Expr(hir::Expr {
3534             kind:
3535                 hir::ExprKind::Call(hir::Expr { span, .. }, _)
3536                 | hir::ExprKind::MethodCall(hir::PathSegment { ident: Ident { span, .. }, .. }, ..),
3537             ..
3538         })) = hir.find(call_hir_id)
3539         {
3540             if Some(*span) != err.span.primary_span() {
3541                 err.span_label(*span, "required by a bound introduced by this call");
3542             }
3543         }
3544     }
3545
3546     fn point_at_chain(
3547         &self,
3548         expr: &hir::Expr<'_>,
3549         typeck_results: &TypeckResults<'tcx>,
3550         type_diffs: Vec<TypeError<'tcx>>,
3551         param_env: ty::ParamEnv<'tcx>,
3552         err: &mut Diagnostic,
3553     ) {
3554         let mut primary_spans = vec![];
3555         let mut span_labels = vec![];
3556
3557         let tcx = self.tcx;
3558
3559         let mut print_root_expr = true;
3560         let mut assocs = vec![];
3561         let mut expr = expr;
3562         let mut prev_ty = self.resolve_vars_if_possible(
3563             typeck_results.expr_ty_adjusted_opt(expr).unwrap_or(tcx.ty_error()),
3564         );
3565         while let hir::ExprKind::MethodCall(_path_segment, rcvr_expr, _args, span) = expr.kind {
3566             // Point at every method call in the chain with the resulting type.
3567             // vec![1, 2, 3].iter().map(mapper).sum<i32>()
3568             //               ^^^^^^ ^^^^^^^^^^^
3569             expr = rcvr_expr;
3570             let assocs_in_this_method =
3571                 self.probe_assoc_types_at_expr(&type_diffs, span, prev_ty, expr.hir_id, param_env);
3572             assocs.push(assocs_in_this_method);
3573             prev_ty = self.resolve_vars_if_possible(
3574                 typeck_results.expr_ty_adjusted_opt(expr).unwrap_or(tcx.ty_error()),
3575             );
3576
3577             if let hir::ExprKind::Path(hir::QPath::Resolved(None, path)) = expr.kind
3578                 && let hir::Path { res: hir::def::Res::Local(hir_id), .. } = path
3579                 && let Some(hir::Node::Pat(binding)) = self.tcx.hir().find(*hir_id)
3580                 && let Some(parent) = self.tcx.hir().find_parent(binding.hir_id)
3581             {
3582                 // We've reached the root of the method call chain...
3583                 if let hir::Node::Local(local) = parent
3584                     && let Some(binding_expr) = local.init
3585                 {
3586                     // ...and it is a binding. Get the binding creation and continue the chain.
3587                     expr = binding_expr;
3588                 }
3589                 if let hir::Node::Param(param) = parent {
3590                     // ...and it is a an fn argument.
3591                     let prev_ty = self.resolve_vars_if_possible(
3592                         typeck_results.node_type_opt(param.hir_id).unwrap_or(tcx.ty_error()),
3593                     );
3594                     let assocs_in_this_method = self.probe_assoc_types_at_expr(&type_diffs, param.ty_span, prev_ty, param.hir_id, param_env);
3595                     if assocs_in_this_method.iter().any(|a| a.is_some()) {
3596                         assocs.push(assocs_in_this_method);
3597                         print_root_expr = false;
3598                     }
3599                     break;
3600                 }
3601             }
3602         }
3603         // We want the type before deref coercions, otherwise we talk about `&[_]`
3604         // instead of `Vec<_>`.
3605         if let Some(ty) = typeck_results.expr_ty_opt(expr) && print_root_expr {
3606             let ty = with_forced_trimmed_paths!(self.ty_to_string(ty));
3607             // Point at the root expression
3608             // vec![1, 2, 3].iter().map(mapper).sum<i32>()
3609             // ^^^^^^^^^^^^^
3610             span_labels.push((expr.span, format!("this expression has type `{ty}`")));
3611         };
3612         // Only show this if it is not a "trivial" expression (not a method
3613         // chain) and there are associated types to talk about.
3614         let mut assocs = assocs.into_iter().peekable();
3615         while let Some(assocs_in_method) = assocs.next() {
3616             let Some(prev_assoc_in_method) = assocs.peek() else {
3617                 for entry in assocs_in_method {
3618                     let Some((span, (assoc, ty))) = entry else { continue; };
3619                     if primary_spans.is_empty() || type_diffs.iter().any(|diff| {
3620                         let Sorts(expected_found) = diff else { return false; };
3621                         self.can_eq(param_env, expected_found.found, ty).is_ok()
3622                     }) {
3623                         // FIXME: this doesn't quite work for `Iterator::collect`
3624                         // because we have `Vec<i32>` and `()`, but we'd want `i32`
3625                         // to point at the `.into_iter()` call, but as long as we
3626                         // still point at the other method calls that might have
3627                         // introduced the issue, this is fine for now.
3628                         primary_spans.push(span);
3629                     }
3630                     span_labels.push((
3631                         span,
3632                         with_forced_trimmed_paths!(format!(
3633                             "`{}` is `{ty}` here",
3634                             self.tcx.def_path_str(assoc),
3635                         )),
3636                     ));
3637                 }
3638                 break;
3639             };
3640             for (entry, prev_entry) in
3641                 assocs_in_method.into_iter().zip(prev_assoc_in_method.into_iter())
3642             {
3643                 match (entry, prev_entry) {
3644                     (Some((span, (assoc, ty))), Some((_, (_, prev_ty)))) => {
3645                         let ty_str = with_forced_trimmed_paths!(self.ty_to_string(ty));
3646
3647                         let assoc = with_forced_trimmed_paths!(self.tcx.def_path_str(assoc));
3648                         if self.can_eq(param_env, ty, *prev_ty).is_err() {
3649                             if type_diffs.iter().any(|diff| {
3650                                 let Sorts(expected_found) = diff else { return false; };
3651                                 self.can_eq(param_env, expected_found.found, ty).is_ok()
3652                             }) {
3653                                 primary_spans.push(span);
3654                             }
3655                             span_labels
3656                                 .push((span, format!("`{assoc}` changed to `{ty_str}` here")));
3657                         } else {
3658                             span_labels.push((span, format!("`{assoc}` remains `{ty_str}` here")));
3659                         }
3660                     }
3661                     (Some((span, (assoc, ty))), None) => {
3662                         span_labels.push((
3663                             span,
3664                             with_forced_trimmed_paths!(format!(
3665                                 "`{}` is `{}` here",
3666                                 self.tcx.def_path_str(assoc),
3667                                 self.ty_to_string(ty),
3668                             )),
3669                         ));
3670                     }
3671                     (None, Some(_)) | (None, None) => {}
3672                 }
3673             }
3674         }
3675         if !primary_spans.is_empty() {
3676             let mut multi_span: MultiSpan = primary_spans.into();
3677             for (span, label) in span_labels {
3678                 multi_span.push_span_label(span, label);
3679             }
3680             err.span_note(
3681                 multi_span,
3682                 "the method call chain might not have had the expected associated types",
3683             );
3684         }
3685     }
3686
3687     fn probe_assoc_types_at_expr(
3688         &self,
3689         type_diffs: &[TypeError<'tcx>],
3690         span: Span,
3691         prev_ty: Ty<'tcx>,
3692         body_id: hir::HirId,
3693         param_env: ty::ParamEnv<'tcx>,
3694     ) -> Vec<Option<(Span, (DefId, Ty<'tcx>))>> {
3695         let ocx = ObligationCtxt::new_in_snapshot(self.infcx);
3696         let mut assocs_in_this_method = Vec::with_capacity(type_diffs.len());
3697         for diff in type_diffs {
3698             let Sorts(expected_found) = diff else { continue; };
3699             let ty::Alias(ty::Projection, proj) = expected_found.expected.kind() else { continue; };
3700
3701             let origin = TypeVariableOrigin { kind: TypeVariableOriginKind::TypeInference, span };
3702             let trait_def_id = proj.trait_def_id(self.tcx);
3703             // Make `Self` be equivalent to the type of the call chain
3704             // expression we're looking at now, so that we can tell what
3705             // for example `Iterator::Item` is at this point in the chain.
3706             let substs = InternalSubsts::for_item(self.tcx, trait_def_id, |param, _| {
3707                 match param.kind {
3708                     ty::GenericParamDefKind::Type { .. } => {
3709                         if param.index == 0 {
3710                             return prev_ty.into();
3711                         }
3712                     }
3713                     ty::GenericParamDefKind::Lifetime | ty::GenericParamDefKind::Const { .. } => {}
3714                 }
3715                 self.var_for_def(span, param)
3716             });
3717             // This will hold the resolved type of the associated type, if the
3718             // current expression implements the trait that associated type is
3719             // in. For example, this would be what `Iterator::Item` is here.
3720             let ty_var = self.infcx.next_ty_var(origin);
3721             // This corresponds to `<ExprTy as Iterator>::Item = _`.
3722             let projection = ty::Binder::dummy(ty::PredicateKind::Clause(ty::Clause::Projection(
3723                 ty::ProjectionPredicate {
3724                     projection_ty: self.tcx.mk_alias_ty(proj.def_id, substs),
3725                     term: ty_var.into(),
3726                 },
3727             )));
3728             // Add `<ExprTy as Iterator>::Item = _` obligation.
3729             ocx.register_obligation(Obligation::misc(
3730                 self.tcx, span, body_id, param_env, projection,
3731             ));
3732             if ocx.select_where_possible().is_empty() {
3733                 // `ty_var` now holds the type that `Item` is for `ExprTy`.
3734                 let ty_var = self.resolve_vars_if_possible(ty_var);
3735                 assocs_in_this_method.push(Some((span, (proj.def_id, ty_var))));
3736             } else {
3737                 // `<ExprTy as Iterator>` didn't select, so likely we've
3738                 // reached the end of the iterator chain, like the originating
3739                 // `Vec<_>`.
3740                 // Keep the space consistent for later zipping.
3741                 assocs_in_this_method.push(None);
3742             }
3743         }
3744         assocs_in_this_method
3745     }
3746 }
3747
3748 /// Add a hint to add a missing borrow or remove an unnecessary one.
3749 fn hint_missing_borrow<'tcx>(
3750     span: Span,
3751     found: Ty<'tcx>,
3752     expected: Ty<'tcx>,
3753     found_node: Node<'_>,
3754     err: &mut Diagnostic,
3755 ) {
3756     let found_args = match found.kind() {
3757         ty::FnPtr(f) => f.inputs().skip_binder().iter(),
3758         kind => {
3759             span_bug!(span, "found was converted to a FnPtr above but is now {:?}", kind)
3760         }
3761     };
3762     let expected_args = match expected.kind() {
3763         ty::FnPtr(f) => f.inputs().skip_binder().iter(),
3764         kind => {
3765             span_bug!(span, "expected was converted to a FnPtr above but is now {:?}", kind)
3766         }
3767     };
3768
3769     // This could be a variant constructor, for example.
3770     let Some(fn_decl) = found_node.fn_decl() else { return; };
3771
3772     let arg_spans = fn_decl.inputs.iter().map(|ty| ty.span);
3773
3774     fn get_deref_type_and_refs(mut ty: Ty<'_>) -> (Ty<'_>, usize) {
3775         let mut refs = 0;
3776
3777         while let ty::Ref(_, new_ty, _) = ty.kind() {
3778             ty = *new_ty;
3779             refs += 1;
3780         }
3781
3782         (ty, refs)
3783     }
3784
3785     let mut to_borrow = Vec::new();
3786     let mut remove_borrow = Vec::new();
3787
3788     for ((found_arg, expected_arg), arg_span) in found_args.zip(expected_args).zip(arg_spans) {
3789         let (found_ty, found_refs) = get_deref_type_and_refs(*found_arg);
3790         let (expected_ty, expected_refs) = get_deref_type_and_refs(*expected_arg);
3791
3792         if found_ty == expected_ty {
3793             if found_refs < expected_refs {
3794                 to_borrow.push((arg_span, expected_arg.to_string()));
3795             } else if found_refs > expected_refs {
3796                 remove_borrow.push((arg_span, expected_arg.to_string()));
3797             }
3798         }
3799     }
3800
3801     if !to_borrow.is_empty() {
3802         err.multipart_suggestion(
3803             "consider borrowing the argument",
3804             to_borrow,
3805             Applicability::MaybeIncorrect,
3806         );
3807     }
3808
3809     if !remove_borrow.is_empty() {
3810         err.multipart_suggestion(
3811             "do not borrow the argument",
3812             remove_borrow,
3813             Applicability::MaybeIncorrect,
3814         );
3815     }
3816 }
3817
3818 /// Collect all the returned expressions within the input expression.
3819 /// Used to point at the return spans when we want to suggest some change to them.
3820 #[derive(Default)]
3821 pub struct ReturnsVisitor<'v> {
3822     pub returns: Vec<&'v hir::Expr<'v>>,
3823     in_block_tail: bool,
3824 }
3825
3826 impl<'v> Visitor<'v> for ReturnsVisitor<'v> {
3827     fn visit_expr(&mut self, ex: &'v hir::Expr<'v>) {
3828         // Visit every expression to detect `return` paths, either through the function's tail
3829         // expression or `return` statements. We walk all nodes to find `return` statements, but
3830         // we only care about tail expressions when `in_block_tail` is `true`, which means that
3831         // they're in the return path of the function body.
3832         match ex.kind {
3833             hir::ExprKind::Ret(Some(ex)) => {
3834                 self.returns.push(ex);
3835             }
3836             hir::ExprKind::Block(block, _) if self.in_block_tail => {
3837                 self.in_block_tail = false;
3838                 for stmt in block.stmts {
3839                     hir::intravisit::walk_stmt(self, stmt);
3840                 }
3841                 self.in_block_tail = true;
3842                 if let Some(expr) = block.expr {
3843                     self.visit_expr(expr);
3844                 }
3845             }
3846             hir::ExprKind::If(_, then, else_opt) if self.in_block_tail => {
3847                 self.visit_expr(then);
3848                 if let Some(el) = else_opt {
3849                     self.visit_expr(el);
3850                 }
3851             }
3852             hir::ExprKind::Match(_, arms, _) if self.in_block_tail => {
3853                 for arm in arms {
3854                     self.visit_expr(arm.body);
3855                 }
3856             }
3857             // We need to walk to find `return`s in the entire body.
3858             _ if !self.in_block_tail => hir::intravisit::walk_expr(self, ex),
3859             _ => self.returns.push(ex),
3860         }
3861     }
3862
3863     fn visit_body(&mut self, body: &'v hir::Body<'v>) {
3864         assert!(!self.in_block_tail);
3865         if body.generator_kind().is_none() {
3866             if let hir::ExprKind::Block(block, None) = body.value.kind {
3867                 if block.expr.is_some() {
3868                     self.in_block_tail = true;
3869                 }
3870             }
3871         }
3872         hir::intravisit::walk_body(self, body);
3873     }
3874 }
3875
3876 /// Collect all the awaited expressions within the input expression.
3877 #[derive(Default)]
3878 struct AwaitsVisitor {
3879     awaits: Vec<hir::HirId>,
3880 }
3881
3882 impl<'v> Visitor<'v> for AwaitsVisitor {
3883     fn visit_expr(&mut self, ex: &'v hir::Expr<'v>) {
3884         if let hir::ExprKind::Yield(_, hir::YieldSource::Await { expr: Some(id) }) = ex.kind {
3885             self.awaits.push(id)
3886         }
3887         hir::intravisit::walk_expr(self, ex)
3888     }
3889 }
3890
3891 pub trait NextTypeParamName {
3892     fn next_type_param_name(&self, name: Option<&str>) -> String;
3893 }
3894
3895 impl NextTypeParamName for &[hir::GenericParam<'_>] {
3896     fn next_type_param_name(&self, name: Option<&str>) -> String {
3897         // This is the list of possible parameter names that we might suggest.
3898         let name = name.and_then(|n| n.chars().next()).map(|c| c.to_string().to_uppercase());
3899         let name = name.as_deref();
3900         let possible_names = [name.unwrap_or("T"), "T", "U", "V", "X", "Y", "Z", "A", "B", "C"];
3901         let used_names = self
3902             .iter()
3903             .filter_map(|p| match p.name {
3904                 hir::ParamName::Plain(ident) => Some(ident.name),
3905                 _ => None,
3906             })
3907             .collect::<Vec<_>>();
3908
3909         possible_names
3910             .iter()
3911             .find(|n| !used_names.contains(&Symbol::intern(n)))
3912             .unwrap_or(&"ParamName")
3913             .to_string()
3914     }
3915 }
3916
3917 fn suggest_trait_object_return_type_alternatives(
3918     err: &mut Diagnostic,
3919     ret_ty: Span,
3920     trait_obj: &str,
3921     is_object_safe: bool,
3922 ) {
3923     err.span_suggestion(
3924         ret_ty,
3925         &format!(
3926             "use `impl {}` as the return type if all return paths have the same type but you \
3927                 want to expose only the trait in the signature",
3928             trait_obj,
3929         ),
3930         format!("impl {}", trait_obj),
3931         Applicability::MaybeIncorrect,
3932     );
3933     if is_object_safe {
3934         err.multipart_suggestion(
3935             &format!(
3936                 "use a boxed trait object if all return paths implement trait `{}`",
3937                 trait_obj,
3938             ),
3939             vec![
3940                 (ret_ty.shrink_to_lo(), "Box<".to_string()),
3941                 (ret_ty.shrink_to_hi(), ">".to_string()),
3942             ],
3943             Applicability::MaybeIncorrect,
3944         );
3945     }
3946 }
3947
3948 /// Collect the spans that we see the generic param `param_did`
3949 struct ReplaceImplTraitVisitor<'a> {
3950     ty_spans: &'a mut Vec<Span>,
3951     param_did: DefId,
3952 }
3953
3954 impl<'a, 'hir> hir::intravisit::Visitor<'hir> for ReplaceImplTraitVisitor<'a> {
3955     fn visit_ty(&mut self, t: &'hir hir::Ty<'hir>) {
3956         if let hir::TyKind::Path(hir::QPath::Resolved(
3957             None,
3958             hir::Path { res: hir::def::Res::Def(_, segment_did), .. },
3959         )) = t.kind
3960         {
3961             if self.param_did == *segment_did {
3962                 // `fn foo(t: impl Trait)`
3963                 //            ^^^^^^^^^^ get this to suggest `T` instead
3964
3965                 // There might be more than one `impl Trait`.
3966                 self.ty_spans.push(t.span);
3967                 return;
3968             }
3969         }
3970
3971         hir::intravisit::walk_ty(self, t);
3972     }
3973 }
3974
3975 // Replace `param` with `replace_ty`
3976 struct ReplaceImplTraitFolder<'tcx> {
3977     tcx: TyCtxt<'tcx>,
3978     param: &'tcx ty::GenericParamDef,
3979     replace_ty: Ty<'tcx>,
3980 }
3981
3982 impl<'tcx> TypeFolder<'tcx> for ReplaceImplTraitFolder<'tcx> {
3983     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
3984         if let ty::Param(ty::ParamTy { index, .. }) = t.kind() {
3985             if self.param.index == *index {
3986                 return self.replace_ty;
3987             }
3988         }
3989         t.super_fold_with(self)
3990     }
3991
3992     fn tcx(&self) -> TyCtxt<'tcx> {
3993         self.tcx
3994     }
3995 }