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

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