src/librustc_trait_selection/traits/error_reporting/suggestions.rs

   1 use super::{
   2     EvaluationResult, Obligation, ObligationCause, ObligationCauseCode, PredicateObligation,
   3     SelectionContext,
   4 };
   5
   6 use crate::autoderef::Autoderef;
   7 use crate::infer::InferCtxt;
   8 use crate::traits::normalize_projection_type;
   9
  10 use rustc_data_structures::stack::ensure_sufficient_stack;
  11 use rustc_errors::{error_code, struct_span_err, Applicability, DiagnosticBuilder, Style};
  12 use rustc_hir as hir;
  13 use rustc_hir::def::DefKind;
  14 use rustc_hir::def_id::DefId;
  15 use rustc_hir::intravisit::Visitor;
  16 use rustc_hir::lang_items;
  17 use rustc_hir::{AsyncGeneratorKind, GeneratorKind, Node};
  18 use rustc_middle::ty::{
  19     self, suggest_constraining_type_param, AdtKind, DefIdTree, Infer, InferTy, ToPredicate, Ty,
  20     TyCtxt, TypeFoldable, WithConstness,
  21 };
  22 use rustc_middle::ty::{TypeAndMut, TypeckResults};
  23 use rustc_span::symbol::{kw, sym, Ident, Symbol};
  24 use rustc_span::{MultiSpan, Span, DUMMY_SP};
  25 use std::fmt;
  26
  27 use super::InferCtxtPrivExt;
  28 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
  29
  30 #[derive(Debug)]
  31 pub enum GeneratorInteriorOrUpvar {
  32     // span of interior type
  33     Interior(Span),
  34     // span of upvar
  35     Upvar(Span),
  36 }
  37
  38 // This trait is public to expose the diagnostics methods to clippy.
  39 pub trait InferCtxtExt<'tcx> {
  40     fn suggest_restricting_param_bound(
  41         &self,
  42         err: &mut DiagnosticBuilder<'_>,
  43         trait_ref: ty::PolyTraitRef<'tcx>,
  44         body_id: hir::HirId,
  45     );
  46
  47     fn suggest_dereferences(
  48         &self,
  49         obligation: &PredicateObligation<'tcx>,
  50         err: &mut DiagnosticBuilder<'tcx>,
  51         trait_ref: &ty::PolyTraitRef<'tcx>,
  52         points_at_arg: bool,
  53     );
  54
  55     fn get_closure_name(
  56         &self,
  57         def_id: DefId,
  58         err: &mut DiagnosticBuilder<'_>,
  59         msg: &str,
  60     ) -> Option<String>;
  61
  62     fn suggest_fn_call(
  63         &self,
  64         obligation: &PredicateObligation<'tcx>,
  65         err: &mut DiagnosticBuilder<'_>,
  66         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
  67         points_at_arg: bool,
  68     );
  69
  70     fn suggest_add_reference_to_arg(
  71         &self,
  72         obligation: &PredicateObligation<'tcx>,
  73         err: &mut DiagnosticBuilder<'_>,
  74         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
  75         points_at_arg: bool,
  76         has_custom_message: bool,
  77     ) -> bool;
  78
  79     fn suggest_remove_reference(
  80         &self,
  81         obligation: &PredicateObligation<'tcx>,
  82         err: &mut DiagnosticBuilder<'_>,
  83         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
  84     );
  85
  86     fn suggest_change_mut(
  87         &self,
  88         obligation: &PredicateObligation<'tcx>,
  89         err: &mut DiagnosticBuilder<'_>,
  90         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
  91         points_at_arg: bool,
  92     );
  93
  94     fn suggest_semicolon_removal(
  95         &self,
  96         obligation: &PredicateObligation<'tcx>,
  97         err: &mut DiagnosticBuilder<'_>,
  98         span: Span,
  99         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
 100     );
 101
 102     fn return_type_span(&self, obligation: &PredicateObligation<'tcx>) -> Option<Span>;
 103
 104     fn suggest_impl_trait(
 105         &self,
 106         err: &mut DiagnosticBuilder<'_>,
 107         span: Span,
 108         obligation: &PredicateObligation<'tcx>,
 109         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
 110     ) -> bool;
 111
 112     fn point_at_returns_when_relevant(
 113         &self,
 114         err: &mut DiagnosticBuilder<'_>,
 115         obligation: &PredicateObligation<'tcx>,
 116     );
 117
 118     fn report_closure_arg_mismatch(
 119         &self,
 120         span: Span,
 121         found_span: Option<Span>,
 122         expected_ref: ty::PolyTraitRef<'tcx>,
 123         found: ty::PolyTraitRef<'tcx>,
 124     ) -> DiagnosticBuilder<'tcx>;
 125
 126     fn suggest_fully_qualified_path(
 127         &self,
 128         err: &mut DiagnosticBuilder<'_>,
 129         def_id: DefId,
 130         span: Span,
 131         trait_ref: DefId,
 132     );
 133
 134     fn maybe_note_obligation_cause_for_async_await(
 135         &self,
 136         err: &mut DiagnosticBuilder<'_>,
 137         obligation: &PredicateObligation<'tcx>,
 138     ) -> bool;
 139
 140     fn note_obligation_cause_for_async_await(
 141         &self,
 142         err: &mut DiagnosticBuilder<'_>,
 143         interior_or_upvar_span: GeneratorInteriorOrUpvar,
 144         interior_extra_info: Option<(Option<Span>, Span, Option<hir::HirId>, Option<Span>)>,
 145         inner_generator_body: Option<&hir::Body<'tcx>>,
 146         outer_generator: Option<DefId>,
 147         trait_ref: ty::TraitRef<'tcx>,
 148         target_ty: Ty<'tcx>,
 149         typeck_results: &ty::TypeckResults<'tcx>,
 150         obligation: &PredicateObligation<'tcx>,
 151         next_code: Option<&ObligationCauseCode<'tcx>>,
 152     );
 153
 154     fn note_obligation_cause_code<T>(
 155         &self,
 156         err: &mut DiagnosticBuilder<'_>,
 157         predicate: &T,
 158         cause_code: &ObligationCauseCode<'tcx>,
 159         obligated_types: &mut Vec<&ty::TyS<'tcx>>,
 160     ) where
 161         T: fmt::Display;
 162
 163     fn suggest_new_overflow_limit(&self, err: &mut DiagnosticBuilder<'_>);
 164
 165     /// Suggest to await before try: future? => future.await?
 166     fn suggest_await_before_try(
 167         &self,
 168         err: &mut DiagnosticBuilder<'_>,
 169         obligation: &PredicateObligation<'tcx>,
 170         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
 171         span: Span,
 172     );
 173 }
 174
 175 fn predicate_constraint(generics: &hir::Generics<'_>, pred: String) -> (Span, String) {
 176     (
 177         generics.where_clause.tail_span_for_suggestion(),
 178         format!(
 179             "{} {}",
 180             if !generics.where_clause.predicates.is_empty() { "," } else { " where" },
 181             pred,
 182         ),
 183     )
 184 }
 185
 186 /// Type parameter needs more bounds. The trivial case is `T` `where T: Bound`, but
 187 /// it can also be an `impl Trait` param that needs to be decomposed to a type
 188 /// param for cleaner code.
 189 fn suggest_restriction(
 190     tcx: TyCtxt<'tcx>,
 191     generics: &hir::Generics<'tcx>,
 192     msg: &str,
 193     err: &mut DiagnosticBuilder<'_>,
 194     fn_sig: Option<&hir::FnSig<'_>>,
 195     projection: Option<&ty::ProjectionTy<'_>>,
 196     trait_ref: ty::PolyTraitRef<'tcx>,
 197     super_traits: Option<(&Ident, &hir::GenericBounds<'_>)>,
 198 ) {
 199     // When we are dealing with a trait, `super_traits` will be `Some`:
 200     // Given `trait T: A + B + C {}`
 201     //              -  ^^^^^^^^^ GenericBounds
 202     //              |
 203     //              &Ident
 204     let span = generics.where_clause.span_for_predicates_or_empty_place();
 205     if span.from_expansion() || span.desugaring_kind().is_some() {
 206         return;
 207     }
 208     // Given `fn foo(t: impl Trait)` where `Trait` requires assoc type `A`...
 209     if let Some((bound_str, fn_sig)) =
 210         fn_sig.zip(projection).and_then(|(sig, p)| match p.self_ty().kind {
 211             // Shenanigans to get the `Trait` from the `impl Trait`.
 212             ty::Param(param) => {
 213                 // `fn foo(t: impl Trait)`
 214                 //                 ^^^^^ get this string
 215                 param.name.as_str().strip_prefix("impl").map(|s| (s.trim_start().to_string(), sig))
 216             }
 217             _ => None,
 218         })
 219     {
 220         // We know we have an `impl Trait` that doesn't satisfy a required projection.
 221
 222         // Find all of the ocurrences of `impl Trait` for `Trait` in the function arguments'
 223         // types. There should be at least one, but there might be *more* than one. In that
 224         // case we could just ignore it and try to identify which one needs the restriction,
 225         // but instead we choose to suggest replacing all instances of `impl Trait` with `T`
 226         // where `T: Trait`.
 227         let mut ty_spans = vec![];
 228         let impl_trait_str = format!("impl {}", bound_str);
 229         for input in fn_sig.decl.inputs {
 230             if let hir::TyKind::Path(hir::QPath::Resolved(
 231                 None,
 232                 hir::Path { segments: [segment], .. },
 233             )) = input.kind
 234             {
 235                 if segment.ident.as_str() == impl_trait_str.as_str() {
 236                     // `fn foo(t: impl Trait)`
 237                     //            ^^^^^^^^^^ get this to suggest `T` instead
 238
 239                     // There might be more than one `impl Trait`.
 240                     ty_spans.push(input.span);
 241                 }
 242             }
 243         }
 244
 245         let type_param_name = generics.params.next_type_param_name(Some(&bound_str));
 246         // The type param `T: Trait` we will suggest to introduce.
 247         let type_param = format!("{}: {}", type_param_name, bound_str);
 248
 249         // FIXME: modify the `trait_ref` instead of string shenanigans.
 250         // Turn `<impl Trait as Foo>::Bar: Qux` into `<T as Foo>::Bar: Qux`.
 251         let pred = trait_ref.without_const().to_predicate(tcx).to_string();
 252         let pred = pred.replace(&impl_trait_str, &type_param_name);
 253         let mut sugg = vec![
 254             match generics
 255                 .params
 256                 .iter()
 257                 .filter(|p| match p.kind {
 258                     hir::GenericParamKind::Type {
 259                         synthetic: Some(hir::SyntheticTyParamKind::ImplTrait),
 260                         ..
 261                     } => false,
 262                     _ => true,
 263                 })
 264                 .last()
 265             {
 266                 // `fn foo(t: impl Trait)`
 267                 //        ^ suggest `<T: Trait>` here
 268                 None => (generics.span, format!("<{}>", type_param)),
 269                 // `fn foo<A>(t: impl Trait)`
 270                 //        ^^^ suggest `<A, T: Trait>` here
 271                 Some(param) => (
 272                     param.bounds_span().unwrap_or(param.span).shrink_to_hi(),
 273                     format!(", {}", type_param),
 274                 ),
 275             },
 276             // `fn foo(t: impl Trait)`
 277             //                       ^ suggest `where <T as Trait>::A: Bound`
 278             predicate_constraint(generics, pred),
 279         ];
 280         sugg.extend(ty_spans.into_iter().map(|s| (s, type_param_name.to_string())));
 281
 282         // Suggest `fn foo<T: Trait>(t: T) where <T as Trait>::A: Bound`.
 283         // FIXME: once `#![feature(associated_type_bounds)]` is stabilized, we should suggest
 284         // `fn foo(t: impl Trait<A: Bound>)` instead.
 285         err.multipart_suggestion(
 286             "introduce a type parameter with a trait bound instead of using `impl Trait`",
 287             sugg,
 288             Applicability::MaybeIncorrect,
 289         );
 290     } else {
 291         // Trivial case: `T` needs an extra bound: `T: Bound`.
 292         let (sp, suggestion) = match super_traits {
 293             None => predicate_constraint(
 294                 generics,
 295                 trait_ref.without_const().to_predicate(tcx).to_string(),
 296             ),
 297             Some((ident, bounds)) => match bounds {
 298                 [.., bound] => (
 299                     bound.span().shrink_to_hi(),
 300                     format!(" + {}", trait_ref.print_only_trait_path().to_string()),
 301                 ),
 302                 [] => (
 303                     ident.span.shrink_to_hi(),
 304                     format!(": {}", trait_ref.print_only_trait_path().to_string()),
 305                 ),
 306             },
 307         };
 308
 309         err.span_suggestion_verbose(
 310             sp,
 311             &format!("consider further restricting {}", msg),
 312             suggestion,
 313             Applicability::MachineApplicable,
 314         );
 315     }
 316 }
 317
 318 impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
 319     fn suggest_restricting_param_bound(
 320         &self,
 321         mut err: &mut DiagnosticBuilder<'_>,
 322         trait_ref: ty::PolyTraitRef<'tcx>,
 323         body_id: hir::HirId,
 324     ) {
 325         let self_ty = trait_ref.skip_binder().self_ty();
 326         let (param_ty, projection) = match &self_ty.kind {
 327             ty::Param(_) => (true, None),
 328             ty::Projection(projection) => (false, Some(projection)),
 329             _ => return,
 330         };
 331
 332         // FIXME: Add check for trait bound that is already present, particularly `?Sized` so we
 333         //        don't suggest `T: Sized + ?Sized`.
 334         let mut hir_id = body_id;
 335         while let Some(node) = self.tcx.hir().find(hir_id) {
 336             match node {
 337                 hir::Node::Item(hir::Item {
 338                     ident,
 339                     kind: hir::ItemKind::Trait(_, _, generics, bounds, _),
 340                     ..
 341                 }) if self_ty == self.tcx.types.self_param => {
 342                     assert!(param_ty);
 343                     // Restricting `Self` for a single method.
 344                     suggest_restriction(
 345                         self.tcx,
 346                         &generics,
 347                         "`Self`",
 348                         err,
 349                         None,
 350                         projection,
 351                         trait_ref,
 352                         Some((ident, bounds)),
 353                     );
 354                     return;
 355                 }
 356
 357                 hir::Node::TraitItem(hir::TraitItem {
 358                     generics,
 359                     kind: hir::TraitItemKind::Fn(..),
 360                     ..
 361                 }) if self_ty == self.tcx.types.self_param => {
 362                     assert!(param_ty);
 363                     // Restricting `Self` for a single method.
 364                     suggest_restriction(
 365                         self.tcx, &generics, "`Self`", err, None, projection, trait_ref, None,
 366                     );
 367                     return;
 368                 }
 369
 370                 hir::Node::TraitItem(hir::TraitItem {
 371                     generics,
 372                     kind: hir::TraitItemKind::Fn(fn_sig, ..),
 373                     ..
 374                 })
 375                 | hir::Node::ImplItem(hir::ImplItem {
 376                     generics,
 377                     kind: hir::ImplItemKind::Fn(fn_sig, ..),
 378                     ..
 379                 })
 380                 | hir::Node::Item(hir::Item {
 381                     kind: hir::ItemKind::Fn(fn_sig, generics, _), ..
 382                 }) if projection.is_some() => {
 383                     // Missing restriction on associated type of type parameter (unmet projection).
 384                     suggest_restriction(
 385                         self.tcx,
 386                         &generics,
 387                         "the associated type",
 388                         err,
 389                         Some(fn_sig),
 390                         projection,
 391                         trait_ref,
 392                         None,
 393                     );
 394                     return;
 395                 }
 396                 hir::Node::Item(hir::Item {
 397                     kind:
 398                         hir::ItemKind::Trait(_, _, generics, _, _)
 399                         | hir::ItemKind::Impl { generics, .. },
 400                     ..
 401                 }) if projection.is_some() => {
 402                     // Missing restriction on associated type of type parameter (unmet projection).
 403                     suggest_restriction(
 404                         self.tcx,
 405                         &generics,
 406                         "the associated type",
 407                         err,
 408                         None,
 409                         projection,
 410                         trait_ref,
 411                         None,
 412                     );
 413                     return;
 414                 }
 415
 416                 hir::Node::Item(hir::Item {
 417                     kind:
 418                         hir::ItemKind::Struct(_, generics)
 419                         | hir::ItemKind::Enum(_, generics)
 420                         | hir::ItemKind::Union(_, generics)
 421                         | hir::ItemKind::Trait(_, _, generics, ..)
 422                         | hir::ItemKind::Impl { generics, .. }
 423                         | hir::ItemKind::Fn(_, generics, _)
 424                         | hir::ItemKind::TyAlias(_, generics)
 425                         | hir::ItemKind::TraitAlias(generics, _)
 426                         | hir::ItemKind::OpaqueTy(hir::OpaqueTy { generics, .. }),
 427                     ..
 428                 })
 429                 | hir::Node::TraitItem(hir::TraitItem { generics, .. })
 430                 | hir::Node::ImplItem(hir::ImplItem { generics, .. })
 431                     if param_ty =>
 432                 {
 433                     // Missing generic type parameter bound.
 434                     let param_name = self_ty.to_string();
 435                     let constraint = trait_ref.print_only_trait_path().to_string();
 436                     if suggest_constraining_type_param(
 437                         self.tcx,
 438                         generics,
 439                         &mut err,
 440                         &param_name,
 441                         &constraint,
 442                         Some(trait_ref.def_id()),
 443                     ) {
 444                         return;
 445                     }
 446                 }
 447
 448                 hir::Node::Crate(..) => return,
 449
 450                 _ => {}
 451             }
 452
 453             hir_id = self.tcx.hir().get_parent_item(hir_id);
 454         }
 455     }
 456
 457     /// When after several dereferencing, the reference satisfies the trait
 458     /// binding. This function provides dereference suggestion for this
 459     /// specific situation.
 460     fn suggest_dereferences(
 461         &self,
 462         obligation: &PredicateObligation<'tcx>,
 463         err: &mut DiagnosticBuilder<'tcx>,
 464         trait_ref: &ty::PolyTraitRef<'tcx>,
 465         points_at_arg: bool,
 466     ) {
 467         // It only make sense when suggesting dereferences for arguments
 468         if !points_at_arg {
 469             return;
 470         }
 471         let param_env = obligation.param_env;
 472         let body_id = obligation.cause.body_id;
 473         let span = obligation.cause.span;
 474         let real_trait_ref = match &obligation.cause.code {
 475             ObligationCauseCode::ImplDerivedObligation(cause)
 476             | ObligationCauseCode::DerivedObligation(cause)
 477             | ObligationCauseCode::BuiltinDerivedObligation(cause) => &cause.parent_trait_ref,
 478             _ => trait_ref,
 479         };
 480         let real_ty = match real_trait_ref.self_ty().no_bound_vars() {
 481             Some(ty) => ty,
 482             None => return,
 483         };
 484
 485         if let ty::Ref(region, base_ty, mutbl) = real_ty.kind {
 486             let mut autoderef = Autoderef::new(self, param_env, body_id, span, base_ty);
 487             if let Some(steps) = autoderef.find_map(|(ty, steps)| {
 488                 // Re-add the `&`
 489                 let ty = self.tcx.mk_ref(region, TypeAndMut { ty, mutbl });
 490                 let obligation =
 491                     self.mk_trait_obligation_with_new_self_ty(param_env, real_trait_ref, ty);
 492                 Some(steps).filter(|_| self.predicate_may_hold(&obligation))
 493             }) {
 494                 if steps > 0 {
 495                     if let Ok(src) = self.tcx.sess.source_map().span_to_snippet(span) {
 496                         // Don't care about `&mut` because `DerefMut` is used less
 497                         // often and user will not expect autoderef happens.
 498                         if src.starts_with("&") && !src.starts_with("&mut ") {
 499                             let derefs = "*".repeat(steps);
 500                             err.span_suggestion(
 501                                 span,
 502                                 "consider adding dereference here",
 503                                 format!("&{}{}", derefs, &src[1..]),
 504                                 Applicability::MachineApplicable,
 505                             );
 506                         }
 507                     }
 508                 }
 509             }
 510         }
 511     }
 512
 513     /// Given a closure's `DefId`, return the given name of the closure.
 514     ///
 515     /// This doesn't account for reassignments, but it's only used for suggestions.
 516     fn get_closure_name(
 517         &self,
 518         def_id: DefId,
 519         err: &mut DiagnosticBuilder<'_>,
 520         msg: &str,
 521     ) -> Option<String> {
 522         let get_name =
 523             |err: &mut DiagnosticBuilder<'_>, kind: &hir::PatKind<'_>| -> Option<String> {
 524                 // Get the local name of this closure. This can be inaccurate because
 525                 // of the possibility of reassignment, but this should be good enough.
 526                 match &kind {
 527                     hir::PatKind::Binding(hir::BindingAnnotation::Unannotated, _, name, None) => {
 528                         Some(format!("{}", name))
 529                     }
 530                     _ => {
 531                         err.note(&msg);
 532                         None
 533                     }
 534                 }
 535             };
 536
 537         let hir = self.tcx.hir();
 538         let hir_id = hir.as_local_hir_id(def_id.as_local()?);
 539         let parent_node = hir.get_parent_node(hir_id);
 540         match hir.find(parent_node) {
 541             Some(hir::Node::Stmt(hir::Stmt { kind: hir::StmtKind::Local(local), .. })) => {
 542                 get_name(err, &local.pat.kind)
 543             }
 544             // Different to previous arm because one is `&hir::Local` and the other
 545             // is `P<hir::Local>`.
 546             Some(hir::Node::Local(local)) => get_name(err, &local.pat.kind),
 547             _ => None,
 548         }
 549     }
 550
 551     /// We tried to apply the bound to an `fn` or closure. Check whether calling it would
 552     /// evaluate to a type that *would* satisfy the trait binding. If it would, suggest calling
 553     /// it: `bar(foo)` → `bar(foo())`. This case is *very* likely to be hit if `foo` is `async`.
 554     fn suggest_fn_call(
 555         &self,
 556         obligation: &PredicateObligation<'tcx>,
 557         err: &mut DiagnosticBuilder<'_>,
 558         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
 559         points_at_arg: bool,
 560     ) {
 561         let self_ty = match trait_ref.self_ty().no_bound_vars() {
 562             None => return,
 563             Some(ty) => ty,
 564         };
 565
 566         let (def_id, output_ty, callable) = match self_ty.kind {
 567             ty::Closure(def_id, substs) => (def_id, substs.as_closure().sig().output(), "closure"),
 568             ty::FnDef(def_id, _) => (def_id, self_ty.fn_sig(self.tcx).output(), "function"),
 569             _ => return,
 570         };
 571         let msg = format!("use parentheses to call the {}", callable);
 572
 573         // `mk_trait_obligation_with_new_self_ty` only works for types with no escaping bound
 574         // variables, so bail out if we have any.
 575         let output_ty = match output_ty.no_bound_vars() {
 576             Some(ty) => ty,
 577             None => return,
 578         };
 579
 580         let new_obligation =
 581             self.mk_trait_obligation_with_new_self_ty(obligation.param_env, trait_ref, output_ty);
 582
 583         match self.evaluate_obligation(&new_obligation) {
 584             Ok(
 585                 EvaluationResult::EvaluatedToOk
 586                 | EvaluationResult::EvaluatedToOkModuloRegions
 587                 | EvaluationResult::EvaluatedToAmbig,
 588             ) => {}
 589             _ => return,
 590         }
 591         let hir = self.tcx.hir();
 592         // Get the name of the callable and the arguments to be used in the suggestion.
 593         let (snippet, sugg) = match hir.get_if_local(def_id) {
 594             Some(hir::Node::Expr(hir::Expr {
 595                 kind: hir::ExprKind::Closure(_, decl, _, span, ..),
 596                 ..
 597             })) => {
 598                 err.span_label(*span, "consider calling this closure");
 599                 let name = match self.get_closure_name(def_id, err, &msg) {
 600                     Some(name) => name,
 601                     None => return,
 602                 };
 603                 let args = decl.inputs.iter().map(|_| "_").collect::<Vec<_>>().join(", ");
 604                 let sugg = format!("({})", args);
 605                 (format!("{}{}", name, sugg), sugg)
 606             }
 607             Some(hir::Node::Item(hir::Item {
 608                 ident,
 609                 kind: hir::ItemKind::Fn(.., body_id),
 610                 ..
 611             })) => {
 612                 err.span_label(ident.span, "consider calling this function");
 613                 let body = hir.body(*body_id);
 614                 let args = body
 615                     .params
 616                     .iter()
 617                     .map(|arg| match &arg.pat.kind {
 618                         hir::PatKind::Binding(_, _, ident, None)
 619                         // FIXME: provide a better suggestion when encountering `SelfLower`, it
 620                         // should suggest a method call.
 621                         if ident.name != kw::SelfLower => ident.to_string(),
 622                         _ => "_".to_string(),
 623                     })
 624                     .collect::<Vec<_>>()
 625                     .join(", ");
 626                 let sugg = format!("({})", args);
 627                 (format!("{}{}", ident, sugg), sugg)
 628             }
 629             _ => return,
 630         };
 631         if points_at_arg {
 632             // When the obligation error has been ensured to have been caused by
 633             // an argument, the `obligation.cause.span` points at the expression
 634             // of the argument, so we can provide a suggestion. This is signaled
 635             // by `points_at_arg`. Otherwise, we give a more general note.
 636             err.span_suggestion_verbose(
 637                 obligation.cause.span.shrink_to_hi(),
 638                 &msg,
 639                 sugg,
 640                 Applicability::HasPlaceholders,
 641             );
 642         } else {
 643             err.help(&format!("{}: `{}`", msg, snippet));
 644         }
 645     }
 646
 647     fn suggest_add_reference_to_arg(
 648         &self,
 649         obligation: &PredicateObligation<'tcx>,
 650         err: &mut DiagnosticBuilder<'_>,
 651         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
 652         points_at_arg: bool,
 653         has_custom_message: bool,
 654     ) -> bool {
 655         if !points_at_arg {
 656             return false;
 657         }
 658
 659         let span = obligation.cause.span;
 660         let param_env = obligation.param_env;
 661         let trait_ref = trait_ref.skip_binder();
 662
 663         if let ObligationCauseCode::ImplDerivedObligation(obligation) = &obligation.cause.code {
 664             // Try to apply the original trait binding obligation by borrowing.
 665             let self_ty = trait_ref.self_ty();
 666             let found = self_ty.to_string();
 667             let new_self_ty = self.tcx.mk_imm_ref(self.tcx.lifetimes.re_static, self_ty);
 668             let substs = self.tcx.mk_substs_trait(new_self_ty, &[]);
 669             let new_trait_ref = ty::TraitRef::new(obligation.parent_trait_ref.def_id(), substs);
 670             let new_obligation = Obligation::new(
 671                 ObligationCause::dummy(),
 672                 param_env,
 673                 new_trait_ref.without_const().to_predicate(self.tcx),
 674             );
 675
 676             if self.predicate_must_hold_modulo_regions(&new_obligation) {
 677                 if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
 678                     // We have a very specific type of error, where just borrowing this argument
 679                     // might solve the problem. In cases like this, the important part is the
 680                     // original type obligation, not the last one that failed, which is arbitrary.
 681                     // Because of this, we modify the error to refer to the original obligation and
 682                     // return early in the caller.
 683
 684                     let msg = format!(
 685                         "the trait bound `{}: {}` is not satisfied",
 686                         found,
 687                         obligation.parent_trait_ref.skip_binder().print_only_trait_path(),
 688                     );
 689                     if has_custom_message {
 690                         err.note(&msg);
 691                     } else {
 692                         err.message = vec![(msg, Style::NoStyle)];
 693                     }
 694                     if snippet.starts_with('&') {
 695                         // This is already a literal borrow and the obligation is failing
 696                         // somewhere else in the obligation chain. Do not suggest non-sense.
 697                         return false;
 698                     }
 699                     err.span_label(
 700                         span,
 701                         &format!(
 702                             "expected an implementor of trait `{}`",
 703                             obligation.parent_trait_ref.skip_binder().print_only_trait_path(),
 704                         ),
 705                     );
 706
 707                     // This if is to prevent a special edge-case
 708                     if !span.from_expansion() {
 709                         // We don't want a borrowing suggestion on the fields in structs,
 710                         // ```
 711                         // struct Foo {
 712                         //  the_foos: Vec<Foo>
 713                         // }
 714                         // ```
 715
 716                         err.span_suggestion(
 717                             span,
 718                             "consider borrowing here",
 719                             format!("&{}", snippet),
 720                             Applicability::MaybeIncorrect,
 721                         );
 722                     }
 723                     return true;
 724                 }
 725             }
 726         }
 727         false
 728     }
 729
 730     /// Whenever references are used by mistake, like `for (i, e) in &vec.iter().enumerate()`,
 731     /// suggest removing these references until we reach a type that implements the trait.
 732     fn suggest_remove_reference(
 733         &self,
 734         obligation: &PredicateObligation<'tcx>,
 735         err: &mut DiagnosticBuilder<'_>,
 736         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
 737     ) {
 738         let span = obligation.cause.span;
 739
 740         if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
 741             let refs_number =
 742                 snippet.chars().filter(|c| !c.is_whitespace()).take_while(|c| *c == '&').count();
 743             if let Some('\'') = snippet.chars().filter(|c| !c.is_whitespace()).nth(refs_number) {
 744                 // Do not suggest removal of borrow from type arguments.
 745                 return;
 746             }
 747
 748             let mut suggested_ty = match trait_ref.self_ty().no_bound_vars() {
 749                 Some(ty) => ty,
 750                 None => return,
 751             };
 752
 753             for refs_remaining in 0..refs_number {
 754                 if let ty::Ref(_, inner_ty, _) = suggested_ty.kind {
 755                     suggested_ty = inner_ty;
 756
 757                     let new_obligation = self.mk_trait_obligation_with_new_self_ty(
 758                         obligation.param_env,
 759                         trait_ref,
 760                         suggested_ty,
 761                     );
 762
 763                     if self.predicate_may_hold(&new_obligation) {
 764                         let sp = self
 765                             .tcx
 766                             .sess
 767                             .source_map()
 768                             .span_take_while(span, |c| c.is_whitespace() || *c == '&');
 769
 770                         let remove_refs = refs_remaining + 1;
 771
 772                         let msg = if remove_refs == 1 {
 773                             "consider removing the leading `&`-reference".to_string()
 774                         } else {
 775                             format!("consider removing {} leading `&`-references", remove_refs)
 776                         };
 777
 778                         err.span_suggestion_short(
 779                             sp,
 780                             &msg,
 781                             String::new(),
 782                             Applicability::MachineApplicable,
 783                         );
 784                         break;
 785                     }
 786                 } else {
 787                     break;
 788                 }
 789             }
 790         }
 791     }
 792
 793     /// Check if the trait bound is implemented for a different mutability and note it in the
 794     /// final error.
 795     fn suggest_change_mut(
 796         &self,
 797         obligation: &PredicateObligation<'tcx>,
 798         err: &mut DiagnosticBuilder<'_>,
 799         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
 800         points_at_arg: bool,
 801     ) {
 802         let span = obligation.cause.span;
 803         if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
 804             let refs_number =
 805                 snippet.chars().filter(|c| !c.is_whitespace()).take_while(|c| *c == '&').count();
 806             if let Some('\'') = snippet.chars().filter(|c| !c.is_whitespace()).nth(refs_number) {
 807                 // Do not suggest removal of borrow from type arguments.
 808                 return;
 809             }
 810             let trait_ref = self.resolve_vars_if_possible(trait_ref);
 811             if trait_ref.has_infer_types_or_consts() {
 812                 // Do not ICE while trying to find if a reborrow would succeed on a trait with
 813                 // unresolved bindings.
 814                 return;
 815             }
 816
 817             if let ty::Ref(region, t_type, mutability) = trait_ref.skip_binder().self_ty().kind {
 818                 if region.is_late_bound() || t_type.has_escaping_bound_vars() {
 819                     // Avoid debug assertion in `mk_obligation_for_def_id`.
 820                     //
 821                     // If the self type has escaping bound vars then it's not
 822                     // going to be the type of an expression, so the suggestion
 823                     // probably won't apply anyway.
 824                     return;
 825                 }
 826
 827                 let suggested_ty = match mutability {
 828                     hir::Mutability::Mut => self.tcx.mk_imm_ref(region, t_type),
 829                     hir::Mutability::Not => self.tcx.mk_mut_ref(region, t_type),
 830                 };
 831
 832                 let new_obligation = self.mk_trait_obligation_with_new_self_ty(
 833                     obligation.param_env,
 834                     &trait_ref,
 835                     suggested_ty,
 836                 );
 837                 let suggested_ty_would_satisfy_obligation = self
 838                     .evaluate_obligation_no_overflow(&new_obligation)
 839                     .must_apply_modulo_regions();
 840                 if suggested_ty_would_satisfy_obligation {
 841                     let sp = self
 842                         .tcx
 843                         .sess
 844                         .source_map()
 845                         .span_take_while(span, |c| c.is_whitespace() || *c == '&');
 846                     if points_at_arg && mutability == hir::Mutability::Not && refs_number > 0 {
 847                         err.span_suggestion_verbose(
 848                             sp,
 849                             "consider changing this borrow's mutability",
 850                             "&mut ".to_string(),
 851                             Applicability::MachineApplicable,
 852                         );
 853                     } else {
 854                         err.note(&format!(
 855                             "`{}` is implemented for `{:?}`, but not for `{:?}`",
 856                             trait_ref.print_only_trait_path(),
 857                             suggested_ty,
 858                             trait_ref.skip_binder().self_ty(),
 859                         ));
 860                     }
 861                 }
 862             }
 863         }
 864     }
 865
 866     fn suggest_semicolon_removal(
 867         &self,
 868         obligation: &PredicateObligation<'tcx>,
 869         err: &mut DiagnosticBuilder<'_>,
 870         span: Span,
 871         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
 872     ) {
 873         let is_empty_tuple =
 874             |ty: ty::Binder<Ty<'_>>| ty.skip_binder().kind == ty::Tuple(ty::List::empty());
 875
 876         let hir = self.tcx.hir();
 877         let parent_node = hir.get_parent_node(obligation.cause.body_id);
 878         let node = hir.find(parent_node);
 879         if let Some(hir::Node::Item(hir::Item {
 880             kind: hir::ItemKind::Fn(sig, _, body_id), ..
 881         })) = node
 882         {
 883             let body = hir.body(*body_id);
 884             if let hir::ExprKind::Block(blk, _) = &body.value.kind {
 885                 if sig.decl.output.span().overlaps(span)
 886                     && blk.expr.is_none()
 887                     && is_empty_tuple(trait_ref.self_ty())
 888                 {
 889                     // FIXME(estebank): When encountering a method with a trait
 890                     // bound not satisfied in the return type with a body that has
 891                     // no return, suggest removal of semicolon on last statement.
 892                     // Once that is added, close #54771.
 893                     if let Some(ref stmt) = blk.stmts.last() {
 894                         let sp = self.tcx.sess.source_map().end_point(stmt.span);
 895                         err.span_label(sp, "consider removing this semicolon");
 896                     }
 897                 }
 898             }
 899         }
 900     }
 901
 902     fn return_type_span(&self, obligation: &PredicateObligation<'tcx>) -> Option<Span> {
 903         let hir = self.tcx.hir();
 904         let parent_node = hir.get_parent_node(obligation.cause.body_id);
 905         let sig = match hir.find(parent_node) {
 906             Some(hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(sig, ..), .. })) => sig,
 907             _ => return None,
 908         };
 909
 910         if let hir::FnRetTy::Return(ret_ty) = sig.decl.output { Some(ret_ty.span) } else { None }
 911     }
 912
 913     /// If all conditions are met to identify a returned `dyn Trait`, suggest using `impl Trait` if
 914     /// applicable and signal that the error has been expanded appropriately and needs to be
 915     /// emitted.
 916     fn suggest_impl_trait(
 917         &self,
 918         err: &mut DiagnosticBuilder<'_>,
 919         span: Span,
 920         obligation: &PredicateObligation<'tcx>,
 921         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
 922     ) -> bool {
 923         match obligation.cause.code.peel_derives() {
 924             // Only suggest `impl Trait` if the return type is unsized because it is `dyn Trait`.
 925             ObligationCauseCode::SizedReturnType => {}
 926             _ => return false,
 927         }
 928
 929         let hir = self.tcx.hir();
 930         let parent_node = hir.get_parent_node(obligation.cause.body_id);
 931         let node = hir.find(parent_node);
 932         let (sig, body_id) = if let Some(hir::Node::Item(hir::Item {
 933             kind: hir::ItemKind::Fn(sig, _, body_id),
 934             ..
 935         })) = node
 936         {
 937             (sig, body_id)
 938         } else {
 939             return false;
 940         };
 941         let body = hir.body(*body_id);
 942         let trait_ref = self.resolve_vars_if_possible(trait_ref);
 943         let ty = trait_ref.skip_binder().self_ty();
 944         let is_object_safe = match ty.kind {
 945             ty::Dynamic(predicates, _) => {
 946                 // If the `dyn Trait` is not object safe, do not suggest `Box<dyn Trait>`.
 947                 predicates
 948                     .principal_def_id()
 949                     .map_or(true, |def_id| self.tcx.object_safety_violations(def_id).is_empty())
 950             }
 951             // We only want to suggest `impl Trait` to `dyn Trait`s.
 952             // For example, `fn foo() -> str` needs to be filtered out.
 953             _ => return false,
 954         };
 955
 956         let ret_ty = if let hir::FnRetTy::Return(ret_ty) = sig.decl.output {
 957             ret_ty
 958         } else {
 959             return false;
 960         };
 961
 962         // Use `TypeVisitor` instead of the output type directly to find the span of `ty` for
 963         // cases like `fn foo() -> (dyn Trait, i32) {}`.
 964         // Recursively look for `TraitObject` types and if there's only one, use that span to
 965         // suggest `impl Trait`.
 966
 967         // Visit to make sure there's a single `return` type to suggest `impl Trait`,
 968         // otherwise suggest using `Box<dyn Trait>` or an enum.
 969         let mut visitor = ReturnsVisitor::default();
 970         visitor.visit_body(&body);
 971
 972         let typeck_results = self.in_progress_typeck_results.map(|t| t.borrow()).unwrap();
 973
 974         let mut ret_types = visitor
 975             .returns
 976             .iter()
 977             .filter_map(|expr| typeck_results.node_type_opt(expr.hir_id))
 978             .map(|ty| self.resolve_vars_if_possible(&ty));
 979         let (last_ty, all_returns_have_same_type, only_never_return) = ret_types.clone().fold(
 980             (None, true, true),
 981             |(last_ty, mut same, only_never_return): (std::option::Option<Ty<'_>>, bool, bool),
 982              ty| {
 983                 let ty = self.resolve_vars_if_possible(&ty);
 984                 same &=
 985                     !matches!(ty.kind, ty::Error(_))
 986                         && last_ty.map_or(true, |last_ty| {
 987                             // FIXME: ideally we would use `can_coerce` here instead, but `typeck` comes
 988                             // *after* in the dependency graph.
 989                             match (&ty.kind, &last_ty.kind) {
 990                                 (Infer(InferTy::IntVar(_)), Infer(InferTy::IntVar(_)))
 991                                 | (Infer(InferTy::FloatVar(_)), Infer(InferTy::FloatVar(_)))
 992                                 | (Infer(InferTy::FreshIntTy(_)), Infer(InferTy::FreshIntTy(_)))
 993                                 | (
 994                                     Infer(InferTy::FreshFloatTy(_)),
 995                                     Infer(InferTy::FreshFloatTy(_)),
 996                                 ) => true,
 997                                 _ => ty == last_ty,
 998                             }
 999                         });
1000                 (Some(ty), same, only_never_return && matches!(ty.kind, ty::Never))
1001             },
1002         );
1003         let all_returns_conform_to_trait =
1004             if let Some(ty_ret_ty) = typeck_results.node_type_opt(ret_ty.hir_id) {
1005                 match ty_ret_ty.kind {
1006                     ty::Dynamic(predicates, _) => {
1007                         let cause = ObligationCause::misc(ret_ty.span, ret_ty.hir_id);
1008                         let param_env = ty::ParamEnv::empty();
1009                         only_never_return
1010                             || ret_types.all(|returned_ty| {
1011                                 predicates.iter().all(|predicate| {
1012                                     let pred = predicate.with_self_ty(self.tcx, returned_ty);
1013                                     let obl = Obligation::new(cause.clone(), param_env, pred);
1014                                     self.predicate_may_hold(&obl)
1015                                 })
1016                             })
1017                     }
1018                     _ => false,
1019                 }
1020             } else {
1021                 true
1022             };
1023
1024         let sm = self.tcx.sess.source_map();
1025         let snippet = if let (true, hir::TyKind::TraitObject(..), Ok(snippet), true) = (
1026             // Verify that we're dealing with a return `dyn Trait`
1027             ret_ty.span.overlaps(span),
1028             &ret_ty.kind,
1029             sm.span_to_snippet(ret_ty.span),
1030             // If any of the return types does not conform to the trait, then we can't
1031             // suggest `impl Trait` nor trait objects: it is a type mismatch error.
1032             all_returns_conform_to_trait,
1033         ) {
1034             snippet
1035         } else {
1036             return false;
1037         };
1038         err.code(error_code!(E0746));
1039         err.set_primary_message("return type cannot have an unboxed trait object");
1040         err.children.clear();
1041         let impl_trait_msg = "for information on `impl Trait`, see \
1042             <https://doc.rust-lang.org/book/ch10-02-traits.html\
1043             #returning-types-that-implement-traits>";
1044         let trait_obj_msg = "for information on trait objects, see \
1045             <https://doc.rust-lang.org/book/ch17-02-trait-objects.html\
1046             #using-trait-objects-that-allow-for-values-of-different-types>";
1047         let has_dyn = snippet.split_whitespace().next().map_or(false, |s| s == "dyn");
1048         let trait_obj = if has_dyn { &snippet[4..] } else { &snippet[..] };
1049         if only_never_return {
1050             // No return paths, probably using `panic!()` or similar.
1051             // Suggest `-> T`, `-> impl Trait`, and if `Trait` is object safe, `-> Box<dyn Trait>`.
1052             suggest_trait_object_return_type_alternatives(
1053                 err,
1054                 ret_ty.span,
1055                 trait_obj,
1056                 is_object_safe,
1057             );
1058         } else if let (Some(last_ty), true) = (last_ty, all_returns_have_same_type) {
1059             // Suggest `-> impl Trait`.
1060             err.span_suggestion(
1061                 ret_ty.span,
1062                 &format!(
1063                     "use `impl {1}` as the return type, as all return paths are of type `{}`, \
1064                      which implements `{1}`",
1065                     last_ty, trait_obj,
1066                 ),
1067                 format!("impl {}", trait_obj),
1068                 Applicability::MachineApplicable,
1069             );
1070             err.note(impl_trait_msg);
1071         } else {
1072             if is_object_safe {
1073                 // Suggest `-> Box<dyn Trait>` and `Box::new(returned_value)`.
1074                 // Get all the return values and collect their span and suggestion.
1075                 if let Some(mut suggestions) = visitor
1076                     .returns
1077                     .iter()
1078                     .map(|expr| {
1079                         let snip = sm.span_to_snippet(expr.span).ok()?;
1080                         Some((expr.span, format!("Box::new({})", snip)))
1081                     })
1082                     .collect::<Option<Vec<_>>>()
1083                 {
1084                     // Add the suggestion for the return type.
1085                     suggestions.push((ret_ty.span, format!("Box<dyn {}>", trait_obj)));
1086                     err.multipart_suggestion(
1087                         "return a boxed trait object instead",
1088                         suggestions,
1089                         Applicability::MaybeIncorrect,
1090                     );
1091                 }
1092             } else {
1093                 // This is currently not possible to trigger because E0038 takes precedence, but
1094                 // leave it in for completeness in case anything changes in an earlier stage.
1095                 err.note(&format!(
1096                     "if trait `{}` was object safe, you could return a trait object",
1097                     trait_obj,
1098                 ));
1099             }
1100             err.note(trait_obj_msg);
1101             err.note(&format!(
1102                 "if all the returned values were of the same type you could use `impl {}` as the \
1103                  return type",
1104                 trait_obj,
1105             ));
1106             err.note(impl_trait_msg);
1107             err.note("you can create a new `enum` with a variant for each returned type");
1108         }
1109         true
1110     }
1111
1112     fn point_at_returns_when_relevant(
1113         &self,
1114         err: &mut DiagnosticBuilder<'_>,
1115         obligation: &PredicateObligation<'tcx>,
1116     ) {
1117         match obligation.cause.code.peel_derives() {
1118             ObligationCauseCode::SizedReturnType => {}
1119             _ => return,
1120         }
1121
1122         let hir = self.tcx.hir();
1123         let parent_node = hir.get_parent_node(obligation.cause.body_id);
1124         let node = hir.find(parent_node);
1125         if let Some(hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(_, _, body_id), .. })) =
1126             node
1127         {
1128             let body = hir.body(*body_id);
1129             // Point at all the `return`s in the function as they have failed trait bounds.
1130             let mut visitor = ReturnsVisitor::default();
1131             visitor.visit_body(&body);
1132             let typeck_results = self.in_progress_typeck_results.map(|t| t.borrow()).unwrap();
1133             for expr in &visitor.returns {
1134                 if let Some(returned_ty) = typeck_results.node_type_opt(expr.hir_id) {
1135                     let ty = self.resolve_vars_if_possible(&returned_ty);
1136                     err.span_label(expr.span, &format!("this returned value is of type `{}`", ty));
1137                 }
1138             }
1139         }
1140     }
1141
1142     fn report_closure_arg_mismatch(
1143         &self,
1144         span: Span,
1145         found_span: Option<Span>,
1146         expected_ref: ty::PolyTraitRef<'tcx>,
1147         found: ty::PolyTraitRef<'tcx>,
1148     ) -> DiagnosticBuilder<'tcx> {
1149         crate fn build_fn_sig_string<'tcx>(
1150             tcx: TyCtxt<'tcx>,
1151             trait_ref: ty::TraitRef<'tcx>,
1152         ) -> String {
1153             let inputs = trait_ref.substs.type_at(1);
1154             let sig = if let ty::Tuple(inputs) = inputs.kind {
1155                 tcx.mk_fn_sig(
1156                     inputs.iter().map(|k| k.expect_ty()),
1157                     tcx.mk_ty_infer(ty::TyVar(ty::TyVid { index: 0 })),
1158                     false,
1159                     hir::Unsafety::Normal,
1160                     ::rustc_target::spec::abi::Abi::Rust,
1161                 )
1162             } else {
1163                 tcx.mk_fn_sig(
1164                     ::std::iter::once(inputs),
1165                     tcx.mk_ty_infer(ty::TyVar(ty::TyVid { index: 0 })),
1166                     false,
1167                     hir::Unsafety::Normal,
1168                     ::rustc_target::spec::abi::Abi::Rust,
1169                 )
1170             };
1171             ty::Binder::bind(sig).to_string()
1172         }
1173
1174         let argument_is_closure = expected_ref.skip_binder().substs.type_at(0).is_closure();
1175         let mut err = struct_span_err!(
1176             self.tcx.sess,
1177             span,
1178             E0631,
1179             "type mismatch in {} arguments",
1180             if argument_is_closure { "closure" } else { "function" }
1181         );
1182
1183         let found_str = format!(
1184             "expected signature of `{}`",
1185             build_fn_sig_string(self.tcx, found.skip_binder())
1186         );
1187         err.span_label(span, found_str);
1188
1189         let found_span = found_span.unwrap_or(span);
1190         let expected_str = format!(
1191             "found signature of `{}`",
1192             build_fn_sig_string(self.tcx, expected_ref.skip_binder())
1193         );
1194         err.span_label(found_span, expected_str);
1195
1196         err
1197     }
1198
1199     fn suggest_fully_qualified_path(
1200         &self,
1201         err: &mut DiagnosticBuilder<'_>,
1202         def_id: DefId,
1203         span: Span,
1204         trait_ref: DefId,
1205     ) {
1206         if let Some(assoc_item) = self.tcx.opt_associated_item(def_id) {
1207             if let ty::AssocKind::Const | ty::AssocKind::Type = assoc_item.kind {
1208                 err.note(&format!(
1209                     "{}s cannot be accessed directly on a `trait`, they can only be \
1210                         accessed through a specific `impl`",
1211                     assoc_item.kind.as_def_kind().descr(def_id)
1212                 ));
1213                 err.span_suggestion(
1214                     span,
1215                     "use the fully qualified path to an implementation",
1216                     format!("<Type as {}>::{}", self.tcx.def_path_str(trait_ref), assoc_item.ident),
1217                     Applicability::HasPlaceholders,
1218                 );
1219             }
1220         }
1221     }
1222
1223     /// Adds an async-await specific note to the diagnostic when the future does not implement
1224     /// an auto trait because of a captured type.
1225     ///
1226     /// ```text
1227     /// note: future does not implement `Qux` as this value is used across an await
1228     ///   --> $DIR/issue-64130-3-other.rs:17:5
1229     ///    |
1230     /// LL |     let x = Foo;
1231     ///    |         - has type `Foo`
1232     /// LL |     baz().await;
1233     ///    |     ^^^^^^^^^^^ await occurs here, with `x` maybe used later
1234     /// LL | }
1235     ///    | - `x` is later dropped here
1236     /// ```
1237     ///
1238     /// When the diagnostic does not implement `Send` or `Sync` specifically, then the diagnostic
1239     /// is "replaced" with a different message and a more specific error.
1240     ///
1241     /// ```text
1242     /// error: future cannot be sent between threads safely
1243     ///   --> $DIR/issue-64130-2-send.rs:21:5
1244     ///    |
1245     /// LL | fn is_send<T: Send>(t: T) { }
1246     ///    |               ---- required by this bound in `is_send`
1247     /// ...
1248     /// LL |     is_send(bar());
1249     ///    |     ^^^^^^^ future returned by `bar` is not send
1250     ///    |
1251     ///    = help: within `impl std::future::Future`, the trait `std::marker::Send` is not
1252     ///            implemented for `Foo`
1253     /// note: future is not send as this value is used across an await
1254     ///   --> $DIR/issue-64130-2-send.rs:15:5
1255     ///    |
1256     /// LL |     let x = Foo;
1257     ///    |         - has type `Foo`
1258     /// LL |     baz().await;
1259     ///    |     ^^^^^^^^^^^ await occurs here, with `x` maybe used later
1260     /// LL | }
1261     ///    | - `x` is later dropped here
1262     /// ```
1263     ///
1264     /// Returns `true` if an async-await specific note was added to the diagnostic.
1265     fn maybe_note_obligation_cause_for_async_await(
1266         &self,
1267         err: &mut DiagnosticBuilder<'_>,
1268         obligation: &PredicateObligation<'tcx>,
1269     ) -> bool {
1270         debug!(
1271             "maybe_note_obligation_cause_for_async_await: obligation.predicate={:?} \
1272                 obligation.cause.span={:?}",
1273             obligation.predicate, obligation.cause.span
1274         );
1275         let hir = self.tcx.hir();
1276
1277         // Attempt to detect an async-await error by looking at the obligation causes, looking
1278         // for a generator to be present.
1279         //
1280         // When a future does not implement a trait because of a captured type in one of the
1281         // generators somewhere in the call stack, then the result is a chain of obligations.
1282         //
1283         // Given a `async fn` A that calls a `async fn` B which captures a non-send type and that
1284         // future is passed as an argument to a function C which requires a `Send` type, then the
1285         // chain looks something like this:
1286         //
1287         // - `BuiltinDerivedObligation` with a generator witness (B)
1288         // - `BuiltinDerivedObligation` with a generator (B)
1289         // - `BuiltinDerivedObligation` with `std::future::GenFuture` (B)
1290         // - `BuiltinDerivedObligation` with `impl std::future::Future` (B)
1291         // - `BuiltinDerivedObligation` with `impl std::future::Future` (B)
1292         // - `BuiltinDerivedObligation` with a generator witness (A)
1293         // - `BuiltinDerivedObligation` with a generator (A)
1294         // - `BuiltinDerivedObligation` with `std::future::GenFuture` (A)
1295         // - `BuiltinDerivedObligation` with `impl std::future::Future` (A)
1296         // - `BuiltinDerivedObligation` with `impl std::future::Future` (A)
1297         // - `BindingObligation` with `impl_send (Send requirement)
1298         //
1299         // The first obligation in the chain is the most useful and has the generator that captured
1300         // the type. The last generator (`outer_generator` below) has information about where the
1301         // bound was introduced. At least one generator should be present for this diagnostic to be
1302         // modified.
1303         let (mut trait_ref, mut target_ty) = match obligation.predicate.skip_binders() {
1304             ty::PredicateAtom::Trait(p, _) => (Some(p.trait_ref), Some(p.self_ty())),
1305             _ => (None, None),
1306         };
1307         let mut generator = None;
1308         let mut outer_generator = None;
1309         let mut next_code = Some(&obligation.cause.code);
1310         while let Some(code) = next_code {
1311             debug!("maybe_note_obligation_cause_for_async_await: code={:?}", code);
1312             match code {
1313                 ObligationCauseCode::DerivedObligation(derived_obligation)
1314                 | ObligationCauseCode::BuiltinDerivedObligation(derived_obligation)
1315                 | ObligationCauseCode::ImplDerivedObligation(derived_obligation) => {
1316                     let ty = derived_obligation.parent_trait_ref.skip_binder().self_ty();
1317                     debug!(
1318                         "maybe_note_obligation_cause_for_async_await: \
1319                             parent_trait_ref={:?} self_ty.kind={:?}",
1320                         derived_obligation.parent_trait_ref, ty.kind
1321                     );
1322
1323                     match ty.kind {
1324                         ty::Generator(did, ..) => {
1325                             generator = generator.or(Some(did));
1326                             outer_generator = Some(did);
1327                         }
1328                         ty::GeneratorWitness(..) => {}
1329                         _ if generator.is_none() => {
1330                             trait_ref = Some(derived_obligation.parent_trait_ref.skip_binder());
1331                             target_ty = Some(ty);
1332                         }
1333                         _ => {}
1334                     }
1335
1336                     next_code = Some(derived_obligation.parent_code.as_ref());
1337                 }
1338                 _ => break,
1339             }
1340         }
1341
1342         // Only continue if a generator was found.
1343         debug!(
1344             "maybe_note_obligation_cause_for_async_await: generator={:?} trait_ref={:?} \
1345                 target_ty={:?}",
1346             generator, trait_ref, target_ty
1347         );
1348         let (generator_did, trait_ref, target_ty) = match (generator, trait_ref, target_ty) {
1349             (Some(generator_did), Some(trait_ref), Some(target_ty)) => {
1350                 (generator_did, trait_ref, target_ty)
1351             }
1352             _ => return false,
1353         };
1354
1355         let span = self.tcx.def_span(generator_did);
1356
1357         // Do not ICE on closure typeck (#66868).
1358         if !generator_did.is_local() {
1359             return false;
1360         }
1361
1362         // Get the typeck results from the infcx if the generator is the function we are
1363         // currently type-checking; otherwise, get them by performing a query.
1364         // This is needed to avoid cycles.
1365         let in_progress_typeck_results = self.in_progress_typeck_results.map(|t| t.borrow());
1366         let generator_did_root = self.tcx.closure_base_def_id(generator_did);
1367         debug!(
1368             "maybe_note_obligation_cause_for_async_await: generator_did={:?} \
1369              generator_did_root={:?} in_progress_typeck_results.hir_owner={:?} span={:?}",
1370             generator_did,
1371             generator_did_root,
1372             in_progress_typeck_results.as_ref().map(|t| t.hir_owner),
1373             span
1374         );
1375         let query_typeck_results;
1376         let typeck_results: &TypeckResults<'tcx> = match &in_progress_typeck_results {
1377             Some(t) if t.hir_owner.to_def_id() == generator_did_root => t,
1378             _ => {
1379                 query_typeck_results = self.tcx.typeck(generator_did.expect_local());
1380                 &query_typeck_results
1381             }
1382         };
1383
1384         let generator_body = generator_did
1385             .as_local()
1386             .map(|def_id| hir.as_local_hir_id(def_id))
1387             .and_then(|hir_id| hir.maybe_body_owned_by(hir_id))
1388             .map(|body_id| hir.body(body_id));
1389         let mut visitor = AwaitsVisitor::default();
1390         if let Some(body) = generator_body {
1391             visitor.visit_body(body);
1392         }
1393         debug!("maybe_note_obligation_cause_for_async_await: awaits = {:?}", visitor.awaits);
1394
1395         // Look for a type inside the generator interior that matches the target type to get
1396         // a span.
1397         let target_ty_erased = self.tcx.erase_regions(&target_ty);
1398         let ty_matches = |ty| -> bool {
1399             // Careful: the regions for types that appear in the
1400             // generator interior are not generally known, so we
1401             // want to erase them when comparing (and anyway,
1402             // `Send` and other bounds are generally unaffected by
1403             // the choice of region).  When erasing regions, we
1404             // also have to erase late-bound regions. This is
1405             // because the types that appear in the generator
1406             // interior generally contain "bound regions" to
1407             // represent regions that are part of the suspended
1408             // generator frame. Bound regions are preserved by
1409             // `erase_regions` and so we must also call
1410             // `erase_late_bound_regions`.
1411             let ty_erased = self.tcx.erase_late_bound_regions(&ty::Binder::bind(ty));
1412             let ty_erased = self.tcx.erase_regions(&ty_erased);
1413             let eq = ty::TyS::same_type(ty_erased, target_ty_erased);
1414             debug!(
1415                 "maybe_note_obligation_cause_for_async_await: ty_erased={:?} \
1416                     target_ty_erased={:?} eq={:?}",
1417                 ty_erased, target_ty_erased, eq
1418             );
1419             eq
1420         };
1421
1422         let mut interior_or_upvar_span = None;
1423         let mut interior_extra_info = None;
1424
1425         if let Some(upvars) = self.tcx.upvars_mentioned(generator_did) {
1426             interior_or_upvar_span = upvars.iter().find_map(|(upvar_id, upvar)| {
1427                 let upvar_ty = typeck_results.node_type(*upvar_id);
1428                 let upvar_ty = self.resolve_vars_if_possible(&upvar_ty);
1429                 if ty_matches(&upvar_ty) {
1430                     Some(GeneratorInteriorOrUpvar::Upvar(upvar.span))
1431                 } else {
1432                     None
1433                 }
1434             });
1435         };
1436
1437         typeck_results
1438             .generator_interior_types
1439             .iter()
1440             .find(|ty::GeneratorInteriorTypeCause { ty, .. }| ty_matches(ty))
1441             .map(|cause| {
1442                 // Check to see if any awaited expressions have the target type.
1443                 let from_awaited_ty = visitor
1444                     .awaits
1445                     .into_iter()
1446                     .map(|id| hir.expect_expr(id))
1447                     .find(|await_expr| {
1448                         let ty = typeck_results.expr_ty_adjusted(&await_expr);
1449                         debug!(
1450                             "maybe_note_obligation_cause_for_async_await: await_expr={:?}",
1451                             await_expr
1452                         );
1453                         ty_matches(ty)
1454                     })
1455                     .map(|expr| expr.span);
1456                 let ty::GeneratorInteriorTypeCause { span, scope_span, yield_span, expr, .. } =
1457                     cause;
1458
1459                 interior_or_upvar_span = Some(GeneratorInteriorOrUpvar::Interior(*span));
1460                 interior_extra_info = Some((*scope_span, *yield_span, *expr, from_awaited_ty));
1461             });
1462
1463         debug!(
1464             "maybe_note_obligation_cause_for_async_await: interior_or_upvar={:?} \
1465                 generator_interior_types={:?}",
1466             interior_or_upvar_span, typeck_results.generator_interior_types
1467         );
1468         if let Some(interior_or_upvar_span) = interior_or_upvar_span {
1469             self.note_obligation_cause_for_async_await(
1470                 err,
1471                 interior_or_upvar_span,
1472                 interior_extra_info,
1473                 generator_body,
1474                 outer_generator,
1475                 trait_ref,
1476                 target_ty,
1477                 typeck_results,
1478                 obligation,
1479                 next_code,
1480             );
1481             true
1482         } else {
1483             false
1484         }
1485     }
1486
1487     /// Unconditionally adds the diagnostic note described in
1488     /// `maybe_note_obligation_cause_for_async_await`'s documentation comment.
1489     fn note_obligation_cause_for_async_await(
1490         &self,
1491         err: &mut DiagnosticBuilder<'_>,
1492         interior_or_upvar_span: GeneratorInteriorOrUpvar,
1493         interior_extra_info: Option<(Option<Span>, Span, Option<hir::HirId>, Option<Span>)>,
1494         inner_generator_body: Option<&hir::Body<'tcx>>,
1495         outer_generator: Option<DefId>,
1496         trait_ref: ty::TraitRef<'tcx>,
1497         target_ty: Ty<'tcx>,
1498         typeck_results: &ty::TypeckResults<'tcx>,
1499         obligation: &PredicateObligation<'tcx>,
1500         next_code: Option<&ObligationCauseCode<'tcx>>,
1501     ) {
1502         let source_map = self.tcx.sess.source_map();
1503
1504         let is_async = inner_generator_body
1505             .and_then(|body| body.generator_kind())
1506             .map(|generator_kind| matches!(generator_kind, hir::GeneratorKind::Async(..)))
1507             .unwrap_or(false);
1508         let (await_or_yield, an_await_or_yield) =
1509             if is_async { ("await", "an await") } else { ("yield", "a yield") };
1510         let future_or_generator = if is_async { "future" } else { "generator" };
1511
1512         // Special case the primary error message when send or sync is the trait that was
1513         // not implemented.
1514         let is_send = self.tcx.is_diagnostic_item(sym::send_trait, trait_ref.def_id);
1515         let is_sync = self.tcx.is_diagnostic_item(sym::sync_trait, trait_ref.def_id);
1516         let hir = self.tcx.hir();
1517         let trait_explanation = if is_send || is_sync {
1518             let (trait_name, trait_verb) =
1519                 if is_send { ("`Send`", "sent") } else { ("`Sync`", "shared") };
1520
1521             err.clear_code();
1522             err.set_primary_message(format!(
1523                 "{} cannot be {} between threads safely",
1524                 future_or_generator, trait_verb
1525             ));
1526
1527             let original_span = err.span.primary_span().unwrap();
1528             let mut span = MultiSpan::from_span(original_span);
1529
1530             let message = outer_generator
1531                 .and_then(|generator_did| {
1532                     Some(match self.tcx.generator_kind(generator_did).unwrap() {
1533                         GeneratorKind::Gen => format!("generator is not {}", trait_name),
1534                         GeneratorKind::Async(AsyncGeneratorKind::Fn) => self
1535                             .tcx
1536                             .parent(generator_did)
1537                             .and_then(|parent_did| parent_did.as_local())
1538                             .map(|parent_did| hir.as_local_hir_id(parent_did))
1539                             .and_then(|parent_hir_id| hir.opt_name(parent_hir_id))
1540                             .map(|name| {
1541                                 format!("future returned by `{}` is not {}", name, trait_name)
1542                             })?,
1543                         GeneratorKind::Async(AsyncGeneratorKind::Block) => {
1544                             format!("future created by async block is not {}", trait_name)
1545                         }
1546                         GeneratorKind::Async(AsyncGeneratorKind::Closure) => {
1547                             format!("future created by async closure is not {}", trait_name)
1548                         }
1549                     })
1550                 })
1551                 .unwrap_or_else(|| format!("{} is not {}", future_or_generator, trait_name));
1552
1553             span.push_span_label(original_span, message);
1554             err.set_span(span);
1555
1556             format!("is not {}", trait_name)
1557         } else {
1558             format!("does not implement `{}`", trait_ref.print_only_trait_path())
1559         };
1560
1561         let mut explain_yield = |interior_span: Span,
1562                                  yield_span: Span,
1563                                  scope_span: Option<Span>| {
1564             let mut span = MultiSpan::from_span(yield_span);
1565             if let Ok(snippet) = source_map.span_to_snippet(interior_span) {
1566                 span.push_span_label(
1567                     yield_span,
1568                     format!("{} occurs here, with `{}` maybe used later", await_or_yield, snippet),
1569                 );
1570                 // If available, use the scope span to annotate the drop location.
1571                 if let Some(scope_span) = scope_span {
1572                     span.push_span_label(
1573                         source_map.end_point(scope_span),
1574                         format!("`{}` is later dropped here", snippet),
1575                     );
1576                 }
1577             }
1578             span.push_span_label(
1579                 interior_span,
1580                 format!("has type `{}` which {}", target_ty, trait_explanation),
1581             );
1582
1583             err.span_note(
1584                 span,
1585                 &format!(
1586                     "{} {} as this value is used across {}",
1587                     future_or_generator, trait_explanation, an_await_or_yield
1588                 ),
1589             );
1590         };
1591         match interior_or_upvar_span {
1592             GeneratorInteriorOrUpvar::Interior(interior_span) => {
1593                 if let Some((scope_span, yield_span, expr, from_awaited_ty)) = interior_extra_info {
1594                     if let Some(await_span) = from_awaited_ty {
1595                         // The type causing this obligation is one being awaited at await_span.
1596                         let mut span = MultiSpan::from_span(await_span);
1597                         span.push_span_label(
1598                             await_span,
1599                             format!(
1600                                 "await occurs here on type `{}`, which {}",
1601                                 target_ty, trait_explanation
1602                             ),
1603                         );
1604                         err.span_note(
1605                             span,
1606                             &format!(
1607                                 "future {not_trait} as it awaits another future which {not_trait}",
1608                                 not_trait = trait_explanation
1609                             ),
1610                         );
1611                     } else {
1612                         // Look at the last interior type to get a span for the `.await`.
1613                         debug!(
1614                             "note_obligation_cause_for_async_await generator_interior_types: {:#?}",
1615                             typeck_results.generator_interior_types
1616                         );
1617                         explain_yield(interior_span, yield_span, scope_span);
1618                     }
1619
1620                     if let Some(expr_id) = expr {
1621                         let expr = hir.expect_expr(expr_id);
1622                         debug!("target_ty evaluated from {:?}", expr);
1623
1624                         let parent = hir.get_parent_node(expr_id);
1625                         if let Some(hir::Node::Expr(e)) = hir.find(parent) {
1626                             let parent_span = hir.span(parent);
1627                             let parent_did = parent.owner.to_def_id();
1628                             // ```rust
1629                             // impl T {
1630                             //     fn foo(&self) -> i32 {}
1631                             // }
1632                             // T.foo();
1633                             // ^^^^^^^ a temporary `&T` created inside this method call due to `&self`
1634                             // ```
1635                             //
1636                             let is_region_borrow = typeck_results
1637                                 .expr_adjustments(expr)
1638                                 .iter()
1639                                 .any(|adj| adj.is_region_borrow());
1640
1641                             // ```rust
1642                             // struct Foo(*const u8);
1643                             // bar(Foo(std::ptr::null())).await;
1644                             //     ^^^^^^^^^^^^^^^^^^^^^ raw-ptr `*T` created inside this struct ctor.
1645                             // ```
1646                             debug!("parent_def_kind: {:?}", self.tcx.def_kind(parent_did));
1647                             let is_raw_borrow_inside_fn_like_call =
1648                                 match self.tcx.def_kind(parent_did) {
1649                                     DefKind::Fn | DefKind::Ctor(..) => target_ty.is_unsafe_ptr(),
1650                                     _ => false,
1651                                 };
1652
1653                             if (typeck_results.is_method_call(e) && is_region_borrow)
1654                                 || is_raw_borrow_inside_fn_like_call
1655                             {
1656                                 err.span_help(
1657                                     parent_span,
1658                                     "consider moving this into a `let` \
1659                         binding to create a shorter lived borrow",
1660                                 );
1661                             }
1662                         }
1663                     }
1664                 }
1665             }
1666             GeneratorInteriorOrUpvar::Upvar(upvar_span) => {
1667                 let mut span = MultiSpan::from_span(upvar_span);
1668                 span.push_span_label(
1669                     upvar_span,
1670                     format!("has type `{}` which {}", target_ty, trait_explanation),
1671                 );
1672                 err.span_note(span, &format!("captured value {}", trait_explanation));
1673             }
1674         }
1675
1676         // Add a note for the item obligation that remains - normally a note pointing to the
1677         // bound that introduced the obligation (e.g. `T: Send`).
1678         debug!("note_obligation_cause_for_async_await: next_code={:?}", next_code);
1679         self.note_obligation_cause_code(
1680             err,
1681             &obligation.predicate,
1682             next_code.unwrap(),
1683             &mut Vec::new(),
1684         );
1685     }
1686
1687     fn note_obligation_cause_code<T>(
1688         &self,
1689         err: &mut DiagnosticBuilder<'_>,
1690         predicate: &T,
1691         cause_code: &ObligationCauseCode<'tcx>,
1692         obligated_types: &mut Vec<&ty::TyS<'tcx>>,
1693     ) where
1694         T: fmt::Display,
1695     {
1696         let tcx = self.tcx;
1697         match *cause_code {
1698             ObligationCauseCode::ExprAssignable
1699             | ObligationCauseCode::MatchExpressionArm { .. }
1700             | ObligationCauseCode::Pattern { .. }
1701             | ObligationCauseCode::IfExpression { .. }
1702             | ObligationCauseCode::IfExpressionWithNoElse
1703             | ObligationCauseCode::MainFunctionType
1704             | ObligationCauseCode::StartFunctionType
1705             | ObligationCauseCode::IntrinsicType
1706             | ObligationCauseCode::MethodReceiver
1707             | ObligationCauseCode::ReturnNoExpression
1708             | ObligationCauseCode::UnifyReceiver(..)
1709             | ObligationCauseCode::MiscObligation => {}
1710             ObligationCauseCode::SliceOrArrayElem => {
1711                 err.note("slice and array elements must have `Sized` type");
1712             }
1713             ObligationCauseCode::TupleElem => {
1714                 err.note("only the last element of a tuple may have a dynamically sized type");
1715             }
1716             ObligationCauseCode::ProjectionWf(data) => {
1717                 err.note(&format!("required so that the projection `{}` is well-formed", data,));
1718             }
1719             ObligationCauseCode::ReferenceOutlivesReferent(ref_ty) => {
1720                 err.note(&format!(
1721                     "required so that reference `{}` does not outlive its referent",
1722                     ref_ty,
1723                 ));
1724             }
1725             ObligationCauseCode::ObjectTypeBound(object_ty, region) => {
1726                 err.note(&format!(
1727                     "required so that the lifetime bound of `{}` for `{}` is satisfied",
1728                     region, object_ty,
1729                 ));
1730             }
1731             ObligationCauseCode::ItemObligation(item_def_id) => {
1732                 let item_name = tcx.def_path_str(item_def_id);
1733                 let msg = format!("required by `{}`", item_name);
1734                 if let Some(sp) = tcx.hir().span_if_local(item_def_id) {
1735                     let sp = tcx.sess.source_map().guess_head_span(sp);
1736                     err.span_label(sp, &msg);
1737                 } else {
1738                     err.note(&msg);
1739                 }
1740             }
1741             ObligationCauseCode::BindingObligation(item_def_id, span) => {
1742                 let item_name = tcx.def_path_str(item_def_id);
1743                 let msg = format!("required by this bound in `{}`", item_name);
1744                 if let Some(ident) = tcx.opt_item_name(item_def_id) {
1745                     let sm = tcx.sess.source_map();
1746                     let same_line =
1747                         match (sm.lookup_line(ident.span.hi()), sm.lookup_line(span.lo())) {
1748                             (Ok(l), Ok(r)) => l.line == r.line,
1749                             _ => true,
1750                         };
1751                     if !ident.span.overlaps(span) && !same_line {
1752                         err.span_label(ident.span, "required by a bound in this");
1753                     }
1754                 }
1755                 if span != DUMMY_SP {
1756                     err.span_label(span, &msg);
1757                 } else {
1758                     err.note(&msg);
1759                 }
1760             }
1761             ObligationCauseCode::ObjectCastObligation(object_ty) => {
1762                 err.note(&format!(
1763                     "required for the cast to the object type `{}`",
1764                     self.ty_to_string(object_ty)
1765                 ));
1766             }
1767             ObligationCauseCode::Coercion { source: _, target } => {
1768                 err.note(&format!("required by cast to type `{}`", self.ty_to_string(target)));
1769             }
1770             ObligationCauseCode::RepeatVec(suggest_const_in_array_repeat_expressions) => {
1771                 err.note(
1772                     "the `Copy` trait is required because the repeated element will be copied",
1773                 );
1774                 if suggest_const_in_array_repeat_expressions {
1775                     err.note(
1776                         "this array initializer can be evaluated at compile-time, see issue \
1777                          #49147 <https://github.com/rust-lang/rust/issues/49147> \
1778                          for more information",
1779                     );
1780                     if tcx.sess.opts.unstable_features.is_nightly_build() {
1781                         err.help(
1782                             "add `#![feature(const_in_array_repeat_expressions)]` to the \
1783                              crate attributes to enable",
1784                         );
1785                     }
1786                 }
1787             }
1788             ObligationCauseCode::VariableType(hir_id) => {
1789                 let parent_node = self.tcx.hir().get_parent_node(hir_id);
1790                 match self.tcx.hir().find(parent_node) {
1791                     Some(Node::Local(hir::Local {
1792                         init: Some(hir::Expr { kind: hir::ExprKind::Index(_, _), span, .. }),
1793                         ..
1794                     })) => {
1795                         // When encountering an assignment of an unsized trait, like
1796                         // `let x = ""[..];`, provide a suggestion to borrow the initializer in
1797                         // order to use have a slice instead.
1798                         err.span_suggestion_verbose(
1799                             span.shrink_to_lo(),
1800                             "consider borrowing here",
1801                             "&".to_owned(),
1802                             Applicability::MachineApplicable,
1803                         );
1804                         err.note("all local variables must have a statically known size");
1805                     }
1806                     Some(Node::Param(param)) => {
1807                         err.span_suggestion_verbose(
1808                             param.ty_span.shrink_to_lo(),
1809                             "function arguments must have a statically known size, borrowed types \
1810                             always have a known size",
1811                             "&".to_owned(),
1812                             Applicability::MachineApplicable,
1813                         );
1814                     }
1815                     _ => {
1816                         err.note("all local variables must have a statically known size");
1817                     }
1818                 }
1819                 if !self.tcx.features().unsized_locals {
1820                     err.help("unsized locals are gated as an unstable feature");
1821                 }
1822             }
1823             ObligationCauseCode::SizedArgumentType(sp) => {
1824                 if let Some(span) = sp {
1825                     err.span_suggestion_verbose(
1826                         span.shrink_to_lo(),
1827                         "function arguments must have a statically known size, borrowed types \
1828                          always have a known size",
1829                         "&".to_string(),
1830                         Applicability::MachineApplicable,
1831                     );
1832                 } else {
1833                     err.note("all function arguments must have a statically known size");
1834                 }
1835                 if tcx.sess.opts.unstable_features.is_nightly_build()
1836                     && !self.tcx.features().unsized_locals
1837                 {
1838                     err.help("unsized locals are gated as an unstable feature");
1839                 }
1840             }
1841             ObligationCauseCode::SizedReturnType => {
1842                 err.note("the return type of a function must have a statically known size");
1843             }
1844             ObligationCauseCode::SizedYieldType => {
1845                 err.note("the yield type of a generator must have a statically known size");
1846             }
1847             ObligationCauseCode::AssignmentLhsSized => {
1848                 err.note("the left-hand-side of an assignment must have a statically known size");
1849             }
1850             ObligationCauseCode::TupleInitializerSized => {
1851                 err.note("tuples must have a statically known size to be initialized");
1852             }
1853             ObligationCauseCode::StructInitializerSized => {
1854                 err.note("structs must have a statically known size to be initialized");
1855             }
1856             ObligationCauseCode::FieldSized { adt_kind: ref item, last, span } => {
1857                 match *item {
1858                     AdtKind::Struct => {
1859                         if last {
1860                             err.note(
1861                                 "the last field of a packed struct may only have a \
1862                                 dynamically sized type if it does not need drop to be run",
1863                             );
1864                         } else {
1865                             err.note(
1866                                 "only the last field of a struct may have a dynamically sized type",
1867                             );
1868                         }
1869                     }
1870                     AdtKind::Union => {
1871                         err.note("no field of a union may have a dynamically sized type");
1872                     }
1873                     AdtKind::Enum => {
1874                         err.note("no field of an enum variant may have a dynamically sized type");
1875                     }
1876                 }
1877                 err.help("change the field's type to have a statically known size");
1878                 err.span_suggestion(
1879                     span.shrink_to_lo(),
1880                     "borrowed types always have a statically known size",
1881                     "&".to_string(),
1882                     Applicability::MachineApplicable,
1883                 );
1884                 err.multipart_suggestion(
1885                     "the `Box` type always has a statically known size and allocates its contents \
1886                      in the heap",
1887                     vec![
1888                         (span.shrink_to_lo(), "Box<".to_string()),
1889                         (span.shrink_to_hi(), ">".to_string()),
1890                     ],
1891                     Applicability::MachineApplicable,
1892                 );
1893             }
1894             ObligationCauseCode::ConstSized => {
1895                 err.note("constant expressions must have a statically known size");
1896             }
1897             ObligationCauseCode::InlineAsmSized => {
1898                 err.note("all inline asm arguments must have a statically known size");
1899             }
1900             ObligationCauseCode::ConstPatternStructural => {
1901                 err.note("constants used for pattern-matching must derive `PartialEq` and `Eq`");
1902             }
1903             ObligationCauseCode::SharedStatic => {
1904                 err.note("shared static variables must have a type that implements `Sync`");
1905             }
1906             ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
1907                 let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
1908                 let ty = parent_trait_ref.skip_binder().self_ty();
1909                 err.note(&format!("required because it appears within the type `{}`", ty));
1910                 obligated_types.push(ty);
1911
1912                 let parent_predicate = parent_trait_ref.without_const().to_predicate(tcx);
1913                 if !self.is_recursive_obligation(obligated_types, &data.parent_code) {
1914                     // #74711: avoid a stack overflow
1915                     ensure_sufficient_stack(|| {
1916                         self.note_obligation_cause_code(
1917                             err,
1918                             &parent_predicate,
1919                             &data.parent_code,
1920                             obligated_types,
1921                         )
1922                     });
1923                 }
1924             }
1925             ObligationCauseCode::ImplDerivedObligation(ref data) => {
1926                 let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
1927                 err.note(&format!(
1928                     "required because of the requirements on the impl of `{}` for `{}`",
1929                     parent_trait_ref.print_only_trait_path(),
1930                     parent_trait_ref.skip_binder().self_ty()
1931                 ));
1932                 let parent_predicate = parent_trait_ref.without_const().to_predicate(tcx);
1933                 // #74711: avoid a stack overflow
1934                 ensure_sufficient_stack(|| {
1935                     self.note_obligation_cause_code(
1936                         err,
1937                         &parent_predicate,
1938                         &data.parent_code,
1939                         obligated_types,
1940                     )
1941                 });
1942             }
1943             ObligationCauseCode::DerivedObligation(ref data) => {
1944                 let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
1945                 let parent_predicate = parent_trait_ref.without_const().to_predicate(tcx);
1946                 // #74711: avoid a stack overflow
1947                 ensure_sufficient_stack(|| {
1948                     self.note_obligation_cause_code(
1949                         err,
1950                         &parent_predicate,
1951                         &data.parent_code,
1952                         obligated_types,
1953                     )
1954                 });
1955             }
1956             ObligationCauseCode::CompareImplMethodObligation { .. } => {
1957                 err.note(&format!(
1958                     "the requirement `{}` appears on the impl method \
1959                      but not on the corresponding trait method",
1960                     predicate
1961                 ));
1962             }
1963             ObligationCauseCode::CompareImplTypeObligation { .. } => {
1964                 err.note(&format!(
1965                     "the requirement `{}` appears on the associated impl type \
1966                      but not on the corresponding associated trait type",
1967                     predicate
1968                 ));
1969             }
1970             ObligationCauseCode::CompareImplConstObligation => {
1971                 err.note(&format!(
1972                     "the requirement `{}` appears on the associated impl constant \
1973                      but not on the corresponding associated trait constant",
1974                     predicate
1975                 ));
1976             }
1977             ObligationCauseCode::ReturnType
1978             | ObligationCauseCode::ReturnValue(_)
1979             | ObligationCauseCode::BlockTailExpression(_) => (),
1980             ObligationCauseCode::TrivialBound => {
1981                 err.help("see issue #48214");
1982                 if tcx.sess.opts.unstable_features.is_nightly_build() {
1983                     err.help("add `#![feature(trivial_bounds)]` to the crate attributes to enable");
1984                 }
1985             }
1986         }
1987     }
1988
1989     fn suggest_new_overflow_limit(&self, err: &mut DiagnosticBuilder<'_>) {
1990         let current_limit = self.tcx.sess.recursion_limit();
1991         let suggested_limit = current_limit * 2;
1992         err.help(&format!(
1993             "consider adding a `#![recursion_limit=\"{}\"]` attribute to your crate (`{}`)",
1994             suggested_limit, self.tcx.crate_name,
1995         ));
1996     }
1997
1998     fn suggest_await_before_try(
1999         &self,
2000         err: &mut DiagnosticBuilder<'_>,
2001         obligation: &PredicateObligation<'tcx>,
2002         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
2003         span: Span,
2004     ) {
2005         debug!(
2006             "suggest_await_before_try: obligation={:?}, span={:?}, trait_ref={:?}, trait_ref_self_ty={:?}",
2007             obligation,
2008             span,
2009             trait_ref,
2010             trait_ref.self_ty()
2011         );
2012         let body_hir_id = obligation.cause.body_id;
2013         let item_id = self.tcx.hir().get_parent_node(body_hir_id);
2014
2015         if let Some(body_id) = self.tcx.hir().maybe_body_owned_by(item_id) {
2016             let body = self.tcx.hir().body(body_id);
2017             if let Some(hir::GeneratorKind::Async(_)) = body.generator_kind {
2018                 let future_trait =
2019                     self.tcx.require_lang_item(lang_items::FutureTraitLangItem, None);
2020
2021                 let self_ty = self.resolve_vars_if_possible(&trait_ref.self_ty());
2022
2023                 // Do not check on infer_types to avoid panic in evaluate_obligation.
2024                 if self_ty.has_infer_types() {
2025                     return;
2026                 }
2027                 let self_ty = self.tcx.erase_regions(&self_ty);
2028
2029                 let impls_future = self.tcx.type_implements_trait((
2030                     future_trait,
2031                     self_ty.skip_binder(),
2032                     ty::List::empty(),
2033                     obligation.param_env,
2034                 ));
2035
2036                 let item_def_id = self
2037                     .tcx
2038                     .associated_items(future_trait)
2039                     .in_definition_order()
2040                     .next()
2041                     .unwrap()
2042                     .def_id;
2043                 // `<T as Future>::Output`
2044                 let projection_ty = ty::ProjectionTy {
2045                     // `T`
2046                     substs: self.tcx.mk_substs_trait(
2047                         trait_ref.self_ty().skip_binder(),
2048                         self.fresh_substs_for_item(span, item_def_id),
2049                     ),
2050                     // `Future::Output`
2051                     item_def_id,
2052                 };
2053
2054                 let mut selcx = SelectionContext::new(self);
2055
2056                 let mut obligations = vec![];
2057                 let normalized_ty = normalize_projection_type(
2058                     &mut selcx,
2059                     obligation.param_env,
2060                     projection_ty,
2061                     obligation.cause.clone(),
2062                     0,
2063                     &mut obligations,
2064                 );
2065
2066                 debug!(
2067                     "suggest_await_before_try: normalized_projection_type {:?}",
2068                     self.resolve_vars_if_possible(&normalized_ty)
2069                 );
2070                 let try_obligation = self.mk_trait_obligation_with_new_self_ty(
2071                     obligation.param_env,
2072                     trait_ref,
2073                     normalized_ty,
2074                 );
2075                 debug!("suggest_await_before_try: try_trait_obligation {:?}", try_obligation);
2076                 if self.predicate_may_hold(&try_obligation) && impls_future {
2077                     if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
2078                         if snippet.ends_with('?') {
2079                             err.span_suggestion(
2080                                 span,
2081                                 "consider using `.await` here",
2082                                 format!("{}.await?", snippet.trim_end_matches('?')),
2083                                 Applicability::MaybeIncorrect,
2084                             );
2085                         }
2086                     }
2087                 }
2088             }
2089         }
2090     }
2091 }
2092
2093 /// Collect all the returned expressions within the input expression.
2094 /// Used to point at the return spans when we want to suggest some change to them.
2095 #[derive(Default)]
2096 pub struct ReturnsVisitor<'v> {
2097     pub returns: Vec<&'v hir::Expr<'v>>,
2098     in_block_tail: bool,
2099 }
2100
2101 impl<'v> Visitor<'v> for ReturnsVisitor<'v> {
2102     type Map = hir::intravisit::ErasedMap<'v>;
2103
2104     fn nested_visit_map(&mut self) -> hir::intravisit::NestedVisitorMap<Self::Map> {
2105         hir::intravisit::NestedVisitorMap::None
2106     }
2107
2108     fn visit_expr(&mut self, ex: &'v hir::Expr<'v>) {
2109         // Visit every expression to detect `return` paths, either through the function's tail
2110         // expression or `return` statements. We walk all nodes to find `return` statements, but
2111         // we only care about tail expressions when `in_block_tail` is `true`, which means that
2112         // they're in the return path of the function body.
2113         match ex.kind {
2114             hir::ExprKind::Ret(Some(ex)) => {
2115                 self.returns.push(ex);
2116             }
2117             hir::ExprKind::Block(block, _) if self.in_block_tail => {
2118                 self.in_block_tail = false;
2119                 for stmt in block.stmts {
2120                     hir::intravisit::walk_stmt(self, stmt);
2121                 }
2122                 self.in_block_tail = true;
2123                 if let Some(expr) = block.expr {
2124                     self.visit_expr(expr);
2125                 }
2126             }
2127             hir::ExprKind::Match(_, arms, _) if self.in_block_tail => {
2128                 for arm in arms {
2129                     self.visit_expr(arm.body);
2130                 }
2131             }
2132             // We need to walk to find `return`s in the entire body.
2133             _ if !self.in_block_tail => hir::intravisit::walk_expr(self, ex),
2134             _ => self.returns.push(ex),
2135         }
2136     }
2137
2138     fn visit_body(&mut self, body: &'v hir::Body<'v>) {
2139         assert!(!self.in_block_tail);
2140         if body.generator_kind().is_none() {
2141             if let hir::ExprKind::Block(block, None) = body.value.kind {
2142                 if block.expr.is_some() {
2143                     self.in_block_tail = true;
2144                 }
2145             }
2146         }
2147         hir::intravisit::walk_body(self, body);
2148     }
2149 }
2150
2151 /// Collect all the awaited expressions within the input expression.
2152 #[derive(Default)]
2153 struct AwaitsVisitor {
2154     awaits: Vec<hir::HirId>,
2155 }
2156
2157 impl<'v> Visitor<'v> for AwaitsVisitor {
2158     type Map = hir::intravisit::ErasedMap<'v>;
2159
2160     fn nested_visit_map(&mut self) -> hir::intravisit::NestedVisitorMap<Self::Map> {
2161         hir::intravisit::NestedVisitorMap::None
2162     }
2163
2164     fn visit_expr(&mut self, ex: &'v hir::Expr<'v>) {
2165         if let hir::ExprKind::Yield(_, hir::YieldSource::Await { expr: Some(id) }) = ex.kind {
2166             self.awaits.push(id)
2167         }
2168         hir::intravisit::walk_expr(self, ex)
2169     }
2170 }
2171
2172 pub trait NextTypeParamName {
2173     fn next_type_param_name(&self, name: Option<&str>) -> String;
2174 }
2175
2176 impl NextTypeParamName for &[hir::GenericParam<'_>] {
2177     fn next_type_param_name(&self, name: Option<&str>) -> String {
2178         // This is the list of possible parameter names that we might suggest.
2179         let name = name.and_then(|n| n.chars().next()).map(|c| c.to_string().to_uppercase());
2180         let name = name.as_deref();
2181         let possible_names = [name.unwrap_or("T"), "T", "U", "V", "X", "Y", "Z", "A", "B", "C"];
2182         let used_names = self
2183             .iter()
2184             .filter_map(|p| match p.name {
2185                 hir::ParamName::Plain(ident) => Some(ident.name),
2186                 _ => None,
2187             })
2188             .collect::<Vec<_>>();
2189
2190         possible_names
2191             .iter()
2192             .find(|n| !used_names.contains(&Symbol::intern(n)))
2193             .unwrap_or(&"ParamName")
2194             .to_string()
2195     }
2196 }
2197
2198 fn suggest_trait_object_return_type_alternatives(
2199     err: &mut DiagnosticBuilder<'_>,
2200     ret_ty: Span,
2201     trait_obj: &str,
2202     is_object_safe: bool,
2203 ) {
2204     err.span_suggestion(
2205         ret_ty,
2206         "use some type `T` that is `T: Sized` as the return type if all return paths have the \
2207             same type",
2208         "T".to_string(),
2209         Applicability::MaybeIncorrect,
2210     );
2211     err.span_suggestion(
2212         ret_ty,
2213         &format!(
2214             "use `impl {}` as the return type if all return paths have the same type but you \
2215                 want to expose only the trait in the signature",
2216             trait_obj,
2217         ),
2218         format!("impl {}", trait_obj),
2219         Applicability::MaybeIncorrect,
2220     );
2221     if is_object_safe {
2222         err.span_suggestion(
2223             ret_ty,
2224             &format!(
2225                 "use a boxed trait object if all return paths implement trait `{}`",
2226                 trait_obj,
2227             ),
2228             format!("Box<dyn {}>", trait_obj),
2229             Applicability::MaybeIncorrect,
2230         );
2231     }
2232 }