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

   1 mod ambiguity;
   2 pub mod method_chain;
   3 pub mod on_unimplemented;
   4 pub mod suggestions;
   5
   6 use super::{
   7     FulfillmentError, FulfillmentErrorCode, MismatchedProjectionTypes, Obligation, ObligationCause,
   8     ObligationCauseCode, ObligationCtxt, OutputTypeParameterMismatch, Overflow,
   9     PredicateObligation, SelectionContext, SelectionError, TraitNotObjectSafe,
  10 };
  11 use crate::infer::error_reporting::{TyCategory, TypeAnnotationNeeded as ErrorCode};
  12 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  13 use crate::infer::{self, InferCtxt};
  14 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
  15 use crate::traits::query::normalize::QueryNormalizeExt as _;
  16 use crate::traits::specialize::to_pretty_impl_header;
  17 use crate::traits::NormalizeExt;
  18 use on_unimplemented::OnUnimplementedNote;
  19 use on_unimplemented::TypeErrCtxtExt as _;
  20 use rustc_data_structures::fx::{FxHashMap, FxIndexMap};
  21 use rustc_errors::{
  22     pluralize, struct_span_err, Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed,
  23     MultiSpan, Style,
  24 };
  25 use rustc_hir as hir;
  26 use rustc_hir::def::Namespace;
  27 use rustc_hir::def_id::DefId;
  28 use rustc_hir::intravisit::Visitor;
  29 use rustc_hir::GenericParam;
  30 use rustc_hir::Item;
  31 use rustc_hir::Node;
  32 use rustc_infer::infer::error_reporting::TypeErrCtxt;
  33 use rustc_infer::infer::{InferOk, TypeTrace};
  34 use rustc_middle::traits::select::OverflowError;
  35 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
  36 use rustc_middle::ty::error::{ExpectedFound, TypeError};
  37 use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable};
  38 use rustc_middle::ty::print::{with_forced_trimmed_paths, FmtPrinter, Print};
  39 use rustc_middle::ty::{
  40     self, SubtypePredicate, ToPolyTraitRef, ToPredicate, TraitRef, Ty, TyCtxt, TypeFoldable,
  41     TypeVisitable,
  42 };
  43 use rustc_session::config::TraitSolver;
  44 use rustc_session::Limit;
  45 use rustc_span::def_id::LOCAL_CRATE;
  46 use rustc_span::symbol::sym;
  47 use rustc_span::{ExpnKind, Span, DUMMY_SP};
  48 use std::fmt;
  49 use std::iter;
  50 use std::ops::ControlFlow;
  51 use suggestions::TypeErrCtxtExt as _;
  52
  53 pub use rustc_infer::traits::error_reporting::*;
  54
  55 // When outputting impl candidates, prefer showing those that are more similar.
  56 //
  57 // We also compare candidates after skipping lifetimes, which has a lower
  58 // priority than exact matches.
  59 #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
  60 pub enum CandidateSimilarity {
  61     Exact { ignoring_lifetimes: bool },
  62     Fuzzy { ignoring_lifetimes: bool },
  63 }
  64
  65 #[derive(Debug, Clone, Copy)]
  66 pub struct ImplCandidate<'tcx> {
  67     pub trait_ref: ty::TraitRef<'tcx>,
  68     pub similarity: CandidateSimilarity,
  69 }
  70
  71 pub trait InferCtxtExt<'tcx> {
  72     /// Given some node representing a fn-like thing in the HIR map,
  73     /// returns a span and `ArgKind` information that describes the
  74     /// arguments it expects. This can be supplied to
  75     /// `report_arg_count_mismatch`.
  76     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Option<Span>, Vec<ArgKind>)>;
  77
  78     /// Reports an error when the number of arguments needed by a
  79     /// trait match doesn't match the number that the expression
  80     /// provides.
  81     fn report_arg_count_mismatch(
  82         &self,
  83         span: Span,
  84         found_span: Option<Span>,
  85         expected_args: Vec<ArgKind>,
  86         found_args: Vec<ArgKind>,
  87         is_closure: bool,
  88         closure_pipe_span: Option<Span>,
  89     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed>;
  90
  91     /// Checks if the type implements one of `Fn`, `FnMut`, or `FnOnce`
  92     /// in that order, and returns the generic type corresponding to the
  93     /// argument of that trait (corresponding to the closure arguments).
  94     fn type_implements_fn_trait(
  95         &self,
  96         param_env: ty::ParamEnv<'tcx>,
  97         ty: ty::Binder<'tcx, Ty<'tcx>>,
  98         constness: ty::BoundConstness,
  99         polarity: ty::ImplPolarity,
 100     ) -> Result<(ty::ClosureKind, ty::Binder<'tcx, Ty<'tcx>>), ()>;
 101 }
 102
 103 pub trait TypeErrCtxtExt<'tcx> {
 104     fn report_overflow_error<T>(
 105         &self,
 106         predicate: &T,
 107         span: Span,
 108         suggest_increasing_limit: bool,
 109         mutate: impl FnOnce(&mut Diagnostic),
 110     ) -> !
 111     where
 112         T: fmt::Display
 113             + TypeFoldable<'tcx>
 114             + Print<'tcx, FmtPrinter<'tcx, 'tcx>, Output = FmtPrinter<'tcx, 'tcx>>,
 115         <T as Print<'tcx, FmtPrinter<'tcx, 'tcx>>>::Error: std::fmt::Debug;
 116
 117     fn report_fulfillment_errors(
 118         &self,
 119         errors: &[FulfillmentError<'tcx>],
 120         body_id: Option<hir::BodyId>,
 121     ) -> ErrorGuaranteed;
 122
 123     fn report_overflow_obligation<T>(
 124         &self,
 125         obligation: &Obligation<'tcx, T>,
 126         suggest_increasing_limit: bool,
 127     ) -> !
 128     where
 129         T: ToPredicate<'tcx> + Clone;
 130
 131     fn suggest_new_overflow_limit(&self, err: &mut Diagnostic);
 132
 133     fn report_overflow_obligation_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> !;
 134
 135     /// The `root_obligation` parameter should be the `root_obligation` field
 136     /// from a `FulfillmentError`. If no `FulfillmentError` is available,
 137     /// then it should be the same as `obligation`.
 138     fn report_selection_error(
 139         &self,
 140         obligation: PredicateObligation<'tcx>,
 141         root_obligation: &PredicateObligation<'tcx>,
 142         error: &SelectionError<'tcx>,
 143     );
 144 }
 145
 146 impl<'tcx> InferCtxtExt<'tcx> for InferCtxt<'tcx> {
 147     /// Given some node representing a fn-like thing in the HIR map,
 148     /// returns a span and `ArgKind` information that describes the
 149     /// arguments it expects. This can be supplied to
 150     /// `report_arg_count_mismatch`.
 151     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Option<Span>, Vec<ArgKind>)> {
 152         let sm = self.tcx.sess.source_map();
 153         let hir = self.tcx.hir();
 154         Some(match node {
 155             Node::Expr(&hir::Expr {
 156                 kind: hir::ExprKind::Closure(&hir::Closure { body, fn_decl_span, fn_arg_span, .. }),
 157                 ..
 158             }) => (
 159                 fn_decl_span,
 160                 fn_arg_span,
 161                 hir.body(body)
 162                     .params
 163                     .iter()
 164                     .map(|arg| {
 165                         if let hir::Pat { kind: hir::PatKind::Tuple(ref args, _), span, .. } =
 166                             *arg.pat
 167                         {
 168                             Some(ArgKind::Tuple(
 169                                 Some(span),
 170                                 args.iter()
 171                                     .map(|pat| {
 172                                         sm.span_to_snippet(pat.span)
 173                                             .ok()
 174                                             .map(|snippet| (snippet, "_".to_owned()))
 175                                     })
 176                                     .collect::<Option<Vec<_>>>()?,
 177                             ))
 178                         } else {
 179                             let name = sm.span_to_snippet(arg.pat.span).ok()?;
 180                             Some(ArgKind::Arg(name, "_".to_owned()))
 181                         }
 182                     })
 183                     .collect::<Option<Vec<ArgKind>>>()?,
 184             ),
 185             Node::Item(&hir::Item { kind: hir::ItemKind::Fn(ref sig, ..), .. })
 186             | Node::ImplItem(&hir::ImplItem { kind: hir::ImplItemKind::Fn(ref sig, _), .. })
 187             | Node::TraitItem(&hir::TraitItem {
 188                 kind: hir::TraitItemKind::Fn(ref sig, _), ..
 189             }) => (
 190                 sig.span,
 191                 None,
 192                 sig.decl
 193                     .inputs
 194                     .iter()
 195                     .map(|arg| match arg.kind {
 196                         hir::TyKind::Tup(ref tys) => ArgKind::Tuple(
 197                             Some(arg.span),
 198                             vec![("_".to_owned(), "_".to_owned()); tys.len()],
 199                         ),
 200                         _ => ArgKind::empty(),
 201                     })
 202                     .collect::<Vec<ArgKind>>(),
 203             ),
 204             Node::Ctor(ref variant_data) => {
 205                 let span = variant_data.ctor_hir_id().map_or(DUMMY_SP, |id| hir.span(id));
 206                 (span, None, vec![ArgKind::empty(); variant_data.fields().len()])
 207             }
 208             _ => panic!("non-FnLike node found: {:?}", node),
 209         })
 210     }
 211
 212     /// Reports an error when the number of arguments needed by a
 213     /// trait match doesn't match the number that the expression
 214     /// provides.
 215     fn report_arg_count_mismatch(
 216         &self,
 217         span: Span,
 218         found_span: Option<Span>,
 219         expected_args: Vec<ArgKind>,
 220         found_args: Vec<ArgKind>,
 221         is_closure: bool,
 222         closure_arg_span: Option<Span>,
 223     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
 224         let kind = if is_closure { "closure" } else { "function" };
 225
 226         let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
 227             let arg_length = arguments.len();
 228             let distinct = matches!(other, &[ArgKind::Tuple(..)]);
 229             match (arg_length, arguments.get(0)) {
 230                 (1, Some(ArgKind::Tuple(_, fields))) => {
 231                     format!("a single {}-tuple as argument", fields.len())
 232                 }
 233                 _ => format!(
 234                     "{} {}argument{}",
 235                     arg_length,
 236                     if distinct && arg_length > 1 { "distinct " } else { "" },
 237                     pluralize!(arg_length)
 238                 ),
 239             }
 240         };
 241
 242         let expected_str = args_str(&expected_args, &found_args);
 243         let found_str = args_str(&found_args, &expected_args);
 244
 245         let mut err = struct_span_err!(
 246             self.tcx.sess,
 247             span,
 248             E0593,
 249             "{} is expected to take {}, but it takes {}",
 250             kind,
 251             expected_str,
 252             found_str,
 253         );
 254
 255         err.span_label(span, format!("expected {} that takes {}", kind, expected_str));
 256
 257         if let Some(found_span) = found_span {
 258             err.span_label(found_span, format!("takes {}", found_str));
 259
 260             // Suggest to take and ignore the arguments with expected_args_length `_`s if
 261             // found arguments is empty (assume the user just wants to ignore args in this case).
 262             // For example, if `expected_args_length` is 2, suggest `|_, _|`.
 263             if found_args.is_empty() && is_closure {
 264                 let underscores = vec!["_"; expected_args.len()].join(", ");
 265                 err.span_suggestion_verbose(
 266                     closure_arg_span.unwrap_or(found_span),
 267                     &format!(
 268                         "consider changing the closure to take and ignore the expected argument{}",
 269                         pluralize!(expected_args.len())
 270                     ),
 271                     format!("|{}|", underscores),
 272                     Applicability::MachineApplicable,
 273                 );
 274             }
 275
 276             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
 277                 if fields.len() == expected_args.len() {
 278                     let sugg = fields
 279                         .iter()
 280                         .map(|(name, _)| name.to_owned())
 281                         .collect::<Vec<String>>()
 282                         .join(", ");
 283                     err.span_suggestion_verbose(
 284                         found_span,
 285                         "change the closure to take multiple arguments instead of a single tuple",
 286                         format!("|{}|", sugg),
 287                         Applicability::MachineApplicable,
 288                     );
 289                 }
 290             }
 291             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..]
 292                 && fields.len() == found_args.len()
 293                 && is_closure
 294             {
 295                 let sugg = format!(
 296                     "|({}){}|",
 297                     found_args
 298                         .iter()
 299                         .map(|arg| match arg {
 300                             ArgKind::Arg(name, _) => name.to_owned(),
 301                             _ => "_".to_owned(),
 302                         })
 303                         .collect::<Vec<String>>()
 304                         .join(", "),
 305                     // add type annotations if available
 306                     if found_args.iter().any(|arg| match arg {
 307                         ArgKind::Arg(_, ty) => ty != "_",
 308                         _ => false,
 309                     }) {
 310                         format!(
 311                             ": ({})",
 312                             fields
 313                                 .iter()
 314                                 .map(|(_, ty)| ty.to_owned())
 315                                 .collect::<Vec<String>>()
 316                                 .join(", ")
 317                         )
 318                     } else {
 319                         String::new()
 320                     },
 321                 );
 322                 err.span_suggestion_verbose(
 323                     found_span,
 324                     "change the closure to accept a tuple instead of individual arguments",
 325                     sugg,
 326                     Applicability::MachineApplicable,
 327                 );
 328             }
 329         }
 330
 331         err
 332     }
 333
 334     fn type_implements_fn_trait(
 335         &self,
 336         param_env: ty::ParamEnv<'tcx>,
 337         ty: ty::Binder<'tcx, Ty<'tcx>>,
 338         constness: ty::BoundConstness,
 339         polarity: ty::ImplPolarity,
 340     ) -> Result<(ty::ClosureKind, ty::Binder<'tcx, Ty<'tcx>>), ()> {
 341         self.commit_if_ok(|_| {
 342             for trait_def_id in [
 343                 self.tcx.lang_items().fn_trait(),
 344                 self.tcx.lang_items().fn_mut_trait(),
 345                 self.tcx.lang_items().fn_once_trait(),
 346             ] {
 347                 let Some(trait_def_id) = trait_def_id else { continue };
 348                 // Make a fresh inference variable so we can determine what the substitutions
 349                 // of the trait are.
 350                 let var = self.next_ty_var(TypeVariableOrigin {
 351                     span: DUMMY_SP,
 352                     kind: TypeVariableOriginKind::MiscVariable,
 353                 });
 354                 let trait_ref = self.tcx.mk_trait_ref(trait_def_id, [ty.skip_binder(), var]);
 355                 let obligation = Obligation::new(
 356                     self.tcx,
 357                     ObligationCause::dummy(),
 358                     param_env,
 359                     ty.rebind(ty::TraitPredicate { trait_ref, constness, polarity }),
 360                 );
 361                 let ocx = ObligationCtxt::new_in_snapshot(self);
 362                 ocx.register_obligation(obligation);
 363                 if ocx.select_all_or_error().is_empty() {
 364                     return Ok((
 365                         self.tcx
 366                             .fn_trait_kind_from_def_id(trait_def_id)
 367                             .expect("expected to map DefId to ClosureKind"),
 368                         ty.rebind(self.resolve_vars_if_possible(var)),
 369                     ));
 370                 }
 371             }
 372
 373             Err(())
 374         })
 375     }
 376 }
 377 impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
 378     fn report_fulfillment_errors(
 379         &self,
 380         errors: &[FulfillmentError<'tcx>],
 381         body_id: Option<hir::BodyId>,
 382     ) -> ErrorGuaranteed {
 383         #[derive(Debug)]
 384         struct ErrorDescriptor<'tcx> {
 385             predicate: ty::Predicate<'tcx>,
 386             index: Option<usize>, // None if this is an old error
 387         }
 388
 389         let mut error_map: FxIndexMap<_, Vec<_>> = self
 390             .reported_trait_errors
 391             .borrow()
 392             .iter()
 393             .map(|(&span, predicates)| {
 394                 (
 395                     span,
 396                     predicates
 397                         .iter()
 398                         .map(|&predicate| ErrorDescriptor { predicate, index: None })
 399                         .collect(),
 400                 )
 401             })
 402             .collect();
 403
 404         for (index, error) in errors.iter().enumerate() {
 405             // We want to ignore desugarings here: spans are equivalent even
 406             // if one is the result of a desugaring and the other is not.
 407             let mut span = error.obligation.cause.span;
 408             let expn_data = span.ctxt().outer_expn_data();
 409             if let ExpnKind::Desugaring(_) = expn_data.kind {
 410                 span = expn_data.call_site;
 411             }
 412
 413             error_map.entry(span).or_default().push(ErrorDescriptor {
 414                 predicate: error.obligation.predicate,
 415                 index: Some(index),
 416             });
 417
 418             self.reported_trait_errors
 419                 .borrow_mut()
 420                 .entry(span)
 421                 .or_default()
 422                 .push(error.obligation.predicate);
 423         }
 424
 425         // We do this in 2 passes because we want to display errors in order, though
 426         // maybe it *is* better to sort errors by span or something.
 427         let mut is_suppressed = vec![false; errors.len()];
 428         for (_, error_set) in error_map.iter() {
 429             // We want to suppress "duplicate" errors with the same span.
 430             for error in error_set {
 431                 if let Some(index) = error.index {
 432                     // Suppress errors that are either:
 433                     // 1) strictly implied by another error.
 434                     // 2) implied by an error with a smaller index.
 435                     for error2 in error_set {
 436                         if error2.index.map_or(false, |index2| is_suppressed[index2]) {
 437                             // Avoid errors being suppressed by already-suppressed
 438                             // errors, to prevent all errors from being suppressed
 439                             // at once.
 440                             continue;
 441                         }
 442
 443                         if self.error_implies(error2.predicate, error.predicate)
 444                             && !(error2.index >= error.index
 445                                 && self.error_implies(error.predicate, error2.predicate))
 446                         {
 447                             info!("skipping {:?} (implied by {:?})", error, error2);
 448                             is_suppressed[index] = true;
 449                             break;
 450                         }
 451                     }
 452                 }
 453             }
 454         }
 455
 456         for (error, suppressed) in iter::zip(errors, is_suppressed) {
 457             if !suppressed {
 458                 self.report_fulfillment_error(error, body_id);
 459             }
 460         }
 461
 462         self.tcx.sess.delay_span_bug(DUMMY_SP, "expected fullfillment errors")
 463     }
 464
 465     /// Reports that an overflow has occurred and halts compilation. We
 466     /// halt compilation unconditionally because it is important that
 467     /// overflows never be masked -- they basically represent computations
 468     /// whose result could not be truly determined and thus we can't say
 469     /// if the program type checks or not -- and they are unusual
 470     /// occurrences in any case.
 471     fn report_overflow_error<T>(
 472         &self,
 473         predicate: &T,
 474         span: Span,
 475         suggest_increasing_limit: bool,
 476         mutate: impl FnOnce(&mut Diagnostic),
 477     ) -> !
 478     where
 479         T: fmt::Display
 480             + TypeFoldable<'tcx>
 481             + Print<'tcx, FmtPrinter<'tcx, 'tcx>, Output = FmtPrinter<'tcx, 'tcx>>,
 482         <T as Print<'tcx, FmtPrinter<'tcx, 'tcx>>>::Error: std::fmt::Debug,
 483     {
 484         let predicate = self.resolve_vars_if_possible(predicate.clone());
 485         let mut pred_str = predicate.to_string();
 486
 487         if pred_str.len() > 50 {
 488             // We don't need to save the type to a file, we will be talking about this type already
 489             // in a separate note when we explain the obligation, so it will be available that way.
 490             pred_str = predicate
 491                 .print(FmtPrinter::new_with_limit(
 492                     self.tcx,
 493                     Namespace::TypeNS,
 494                     rustc_session::Limit(6),
 495                 ))
 496                 .unwrap()
 497                 .into_buffer();
 498         }
 499         let mut err = struct_span_err!(
 500             self.tcx.sess,
 501             span,
 502             E0275,
 503             "overflow evaluating the requirement `{}`",
 504             pred_str,
 505         );
 506
 507         if suggest_increasing_limit {
 508             self.suggest_new_overflow_limit(&mut err);
 509         }
 510
 511         mutate(&mut err);
 512
 513         err.emit();
 514         self.tcx.sess.abort_if_errors();
 515         bug!();
 516     }
 517
 518     /// Reports that an overflow has occurred and halts compilation. We
 519     /// halt compilation unconditionally because it is important that
 520     /// overflows never be masked -- they basically represent computations
 521     /// whose result could not be truly determined and thus we can't say
 522     /// if the program type checks or not -- and they are unusual
 523     /// occurrences in any case.
 524     fn report_overflow_obligation<T>(
 525         &self,
 526         obligation: &Obligation<'tcx, T>,
 527         suggest_increasing_limit: bool,
 528     ) -> !
 529     where
 530         T: ToPredicate<'tcx> + Clone,
 531     {
 532         let predicate = obligation.predicate.clone().to_predicate(self.tcx);
 533         let predicate = self.resolve_vars_if_possible(predicate);
 534         self.report_overflow_error(
 535             &predicate,
 536             obligation.cause.span,
 537             suggest_increasing_limit,
 538             |err| {
 539                 self.note_obligation_cause_code(
 540                     err,
 541                     predicate,
 542                     obligation.param_env,
 543                     obligation.cause.code(),
 544                     &mut vec![],
 545                     &mut Default::default(),
 546                 );
 547             },
 548         );
 549     }
 550
 551     fn suggest_new_overflow_limit(&self, err: &mut Diagnostic) {
 552         let suggested_limit = match self.tcx.recursion_limit() {
 553             Limit(0) => Limit(2),
 554             limit => limit * 2,
 555         };
 556         err.help(&format!(
 557             "consider increasing the recursion limit by adding a \
 558              `#![recursion_limit = \"{}\"]` attribute to your crate (`{}`)",
 559             suggested_limit,
 560             self.tcx.crate_name(LOCAL_CRATE),
 561         ));
 562     }
 563
 564     /// Reports that a cycle was detected which led to overflow and halts
 565     /// compilation. This is equivalent to `report_overflow_obligation` except
 566     /// that we can give a more helpful error message (and, in particular,
 567     /// we do not suggest increasing the overflow limit, which is not
 568     /// going to help).
 569     fn report_overflow_obligation_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 570         let cycle = self.resolve_vars_if_possible(cycle.to_owned());
 571         assert!(!cycle.is_empty());
 572
 573         debug!(?cycle, "report_overflow_error_cycle");
 574
 575         // The 'deepest' obligation is most likely to have a useful
 576         // cause 'backtrace'
 577         self.report_overflow_obligation(
 578             cycle.iter().max_by_key(|p| p.recursion_depth).unwrap(),
 579             false,
 580         );
 581     }
 582
 583     fn report_selection_error(
 584         &self,
 585         mut obligation: PredicateObligation<'tcx>,
 586         root_obligation: &PredicateObligation<'tcx>,
 587         error: &SelectionError<'tcx>,
 588     ) {
 589         let tcx = self.tcx;
 590         let mut span = obligation.cause.span;
 591         // FIXME: statically guarantee this by tainting after the diagnostic is emitted
 592         self.set_tainted_by_errors(
 593             tcx.sess.delay_span_bug(span, "`report_selection_error` did not emit an error"),
 594         );
 595
 596         let mut err = match *error {
 597             SelectionError::Unimplemented => {
 598                 // If this obligation was generated as a result of well-formedness checking, see if we
 599                 // can get a better error message by performing HIR-based well-formedness checking.
 600                 if let ObligationCauseCode::WellFormed(Some(wf_loc)) =
 601                     root_obligation.cause.code().peel_derives()
 602                     && !obligation.predicate.has_non_region_infer()
 603                 {
 604                     if let Some(cause) = self
 605                         .tcx
 606                         .diagnostic_hir_wf_check((tcx.erase_regions(obligation.predicate), *wf_loc))
 607                     {
 608                         obligation.cause = cause.clone();
 609                         span = obligation.cause.span;
 610                     }
 611                 }
 612                 if let ObligationCauseCode::CompareImplItemObligation {
 613                     impl_item_def_id,
 614                     trait_item_def_id,
 615                     kind: _,
 616                 } = *obligation.cause.code()
 617                 {
 618                     self.report_extra_impl_obligation(
 619                         span,
 620                         impl_item_def_id,
 621                         trait_item_def_id,
 622                         &format!("`{}`", obligation.predicate),
 623                     )
 624                     .emit();
 625                     return;
 626                 }
 627
 628                 let bound_predicate = obligation.predicate.kind();
 629                 match bound_predicate.skip_binder() {
 630                     ty::PredicateKind::Clause(ty::Clause::Trait(trait_predicate)) => {
 631                         let trait_predicate = bound_predicate.rebind(trait_predicate);
 632                         let mut trait_predicate = self.resolve_vars_if_possible(trait_predicate);
 633
 634                         trait_predicate.remap_constness_diag(obligation.param_env);
 635                         let predicate_is_const = ty::BoundConstness::ConstIfConst
 636                             == trait_predicate.skip_binder().constness;
 637
 638                         if self.tcx.sess.has_errors().is_some()
 639                             && trait_predicate.references_error()
 640                         {
 641                             return;
 642                         }
 643                         let trait_ref = trait_predicate.to_poly_trait_ref();
 644                         let (post_message, pre_message, type_def) = self
 645                             .get_parent_trait_ref(obligation.cause.code())
 646                             .map(|(t, s)| {
 647                                 (
 648                                     format!(" in `{}`", t),
 649                                     format!("within `{}`, ", t),
 650                                     s.map(|s| (format!("within this `{}`", t), s)),
 651                                 )
 652                             })
 653                             .unwrap_or_default();
 654
 655                         let OnUnimplementedNote {
 656                             message,
 657                             label,
 658                             note,
 659                             parent_label,
 660                             append_const_msg,
 661                         } = self.on_unimplemented_note(trait_ref, &obligation);
 662                         let have_alt_message = message.is_some() || label.is_some();
 663                         let is_try_conversion = self.is_try_conversion(span, trait_ref.def_id());
 664                         let is_unsize =
 665                             Some(trait_ref.def_id()) == self.tcx.lang_items().unsize_trait();
 666                         let (message, note, append_const_msg) = if is_try_conversion {
 667                             (
 668                                 Some(format!(
 669                                     "`?` couldn't convert the error to `{}`",
 670                                     trait_ref.skip_binder().self_ty(),
 671                                 )),
 672                                 Some(
 673                                     "the question mark operation (`?`) implicitly performs a \
 674                                      conversion on the error value using the `From` trait"
 675                                         .to_owned(),
 676                                 ),
 677                                 Some(None),
 678                             )
 679                         } else {
 680                             (message, note, append_const_msg)
 681                         };
 682
 683                         let mut err = struct_span_err!(
 684                             self.tcx.sess,
 685                             span,
 686                             E0277,
 687                             "{}",
 688                             message
 689                                 .and_then(|cannot_do_this| {
 690                                     match (predicate_is_const, append_const_msg) {
 691                                         // do nothing if predicate is not const
 692                                         (false, _) => Some(cannot_do_this),
 693                                         // suggested using default post message
 694                                         (true, Some(None)) => {
 695                                             Some(format!("{cannot_do_this} in const contexts"))
 696                                         }
 697                                         // overridden post message
 698                                         (true, Some(Some(post_message))) => {
 699                                             Some(format!("{cannot_do_this}{post_message}"))
 700                                         }
 701                                         // fallback to generic message
 702                                         (true, None) => None,
 703                                     }
 704                                 })
 705                                 .unwrap_or_else(|| format!(
 706                                     "the trait bound `{}` is not satisfied{}",
 707                                     trait_predicate, post_message,
 708                                 ))
 709                         );
 710
 711                         if is_try_conversion && let Some(ret_span) = self.return_type_span(&obligation) {
 712                             err.span_label(
 713                                 ret_span,
 714                                 &format!(
 715                                     "expected `{}` because of this",
 716                                     trait_ref.skip_binder().self_ty()
 717                                 ),
 718                             );
 719                         }
 720
 721                         if Some(trait_ref.def_id()) == tcx.lang_items().tuple_trait() {
 722                             match obligation.cause.code().peel_derives() {
 723                                 ObligationCauseCode::RustCall => {
 724                                     err.set_primary_message("functions with the \"rust-call\" ABI must take a single non-self tuple argument");
 725                                 }
 726                                 ObligationCauseCode::BindingObligation(def_id, _)
 727                                 | ObligationCauseCode::ItemObligation(def_id)
 728                                     if tcx.is_fn_trait(*def_id) =>
 729                                 {
 730                                     err.code(rustc_errors::error_code!(E0059));
 731                                     err.set_primary_message(format!(
 732                                         "type parameter to bare `{}` trait must be a tuple",
 733                                         tcx.def_path_str(*def_id)
 734                                     ));
 735                                 }
 736                                 _ => {}
 737                             }
 738                         }
 739
 740                         if Some(trait_ref.def_id()) == tcx.lang_items().drop_trait()
 741                             && predicate_is_const
 742                         {
 743                             err.note("`~const Drop` was renamed to `~const Destruct`");
 744                             err.note("See <https://github.com/rust-lang/rust/pull/94901> for more details");
 745                         }
 746
 747                         let explanation = if let ObligationCauseCode::MainFunctionType =
 748                             obligation.cause.code()
 749                         {
 750                             "consider using `()`, or a `Result`".to_owned()
 751                         } else {
 752                             let ty_desc = match trait_ref.skip_binder().self_ty().kind() {
 753                                 ty::FnDef(_, _) => Some("fn item"),
 754                                 ty::Closure(_, _) => Some("closure"),
 755                                 _ => None,
 756                             };
 757
 758                             match ty_desc {
 759                                 Some(desc) => format!(
 760                                     "{}the trait `{}` is not implemented for {} `{}`",
 761                                     pre_message,
 762                                     trait_predicate.print_modifiers_and_trait_path(),
 763                                     desc,
 764                                     trait_ref.skip_binder().self_ty(),
 765                                 ),
 766                                 None => format!(
 767                                     "{}the trait `{}` is not implemented for `{}`",
 768                                     pre_message,
 769                                     trait_predicate.print_modifiers_and_trait_path(),
 770                                     trait_ref.skip_binder().self_ty(),
 771                                 ),
 772                             }
 773                         };
 774                         self.check_for_binding_assigned_block_without_tail_expression(
 775                             &obligation,
 776                             &mut err,
 777                             trait_predicate,
 778                         );
 779                         if self.suggest_add_reference_to_arg(
 780                             &obligation,
 781                             &mut err,
 782                             trait_predicate,
 783                             have_alt_message,
 784                         ) {
 785                             self.note_obligation_cause(&mut err, &obligation);
 786                             err.emit();
 787                             return;
 788                         }
 789                         if let Some(ref s) = label {
 790                             // If it has a custom `#[rustc_on_unimplemented]`
 791                             // error message, let's display it as the label!
 792                             err.span_label(span, s);
 793                             if !matches!(trait_ref.skip_binder().self_ty().kind(), ty::Param(_)) {
 794                                 // When the self type is a type param We don't need to "the trait
 795                                 // `std::marker::Sized` is not implemented for `T`" as we will point
 796                                 // at the type param with a label to suggest constraining it.
 797                                 err.help(&explanation);
 798                             }
 799                         } else {
 800                             err.span_label(span, explanation);
 801                         }
 802
 803                         if let ObligationCauseCode::ObjectCastObligation(concrete_ty, obj_ty) = obligation.cause.code().peel_derives() &&
 804                             Some(trait_ref.def_id()) == self.tcx.lang_items().sized_trait() {
 805                             self.suggest_borrowing_for_object_cast(&mut err, &root_obligation, *concrete_ty, *obj_ty);
 806                         }
 807
 808                         let mut unsatisfied_const = false;
 809                         if trait_predicate.is_const_if_const() && obligation.param_env.is_const() {
 810                             let non_const_predicate = trait_ref.without_const();
 811                             let non_const_obligation = Obligation {
 812                                 cause: obligation.cause.clone(),
 813                                 param_env: obligation.param_env.without_const(),
 814                                 predicate: non_const_predicate.to_predicate(tcx),
 815                                 recursion_depth: obligation.recursion_depth,
 816                             };
 817                             if self.predicate_may_hold(&non_const_obligation) {
 818                                 unsatisfied_const = true;
 819                                 err.span_note(
 820                                     span,
 821                                     &format!(
 822                                         "the trait `{}` is implemented for `{}`, \
 823                                         but that implementation is not `const`",
 824                                         non_const_predicate.print_modifiers_and_trait_path(),
 825                                         trait_ref.skip_binder().self_ty(),
 826                                     ),
 827                                 );
 828                             }
 829                         }
 830
 831                         if let Some((msg, span)) = type_def {
 832                             err.span_label(span, &msg);
 833                         }
 834                         if let Some(ref s) = note {
 835                             // If it has a custom `#[rustc_on_unimplemented]` note, let's display it
 836                             err.note(s.as_str());
 837                         }
 838                         if let Some(ref s) = parent_label {
 839                             let body = tcx
 840                                 .hir()
 841                                 .opt_local_def_id(obligation.cause.body_id)
 842                                 .unwrap_or_else(|| {
 843                                     tcx.hir().body_owner_def_id(hir::BodyId {
 844                                         hir_id: obligation.cause.body_id,
 845                                     })
 846                                 });
 847                             err.span_label(tcx.def_span(body), s);
 848                         }
 849
 850                         self.suggest_floating_point_literal(&obligation, &mut err, &trait_ref);
 851                         self.suggest_dereferencing_index(&obligation, &mut err, trait_predicate);
 852                         let mut suggested =
 853                             self.suggest_dereferences(&obligation, &mut err, trait_predicate);
 854                         suggested |= self.suggest_fn_call(&obligation, &mut err, trait_predicate);
 855                         suggested |=
 856                             self.suggest_remove_reference(&obligation, &mut err, trait_predicate);
 857                         suggested |= self.suggest_semicolon_removal(
 858                             &obligation,
 859                             &mut err,
 860                             span,
 861                             trait_predicate,
 862                         );
 863                         self.note_version_mismatch(&mut err, &trait_ref);
 864                         self.suggest_remove_await(&obligation, &mut err);
 865                         self.suggest_derive(&obligation, &mut err, trait_predicate);
 866
 867                         if Some(trait_ref.def_id()) == tcx.lang_items().try_trait() {
 868                             self.suggest_await_before_try(
 869                                 &mut err,
 870                                 &obligation,
 871                                 trait_predicate,
 872                                 span,
 873                             );
 874                         }
 875
 876                         if self.suggest_add_clone_to_arg(&obligation, &mut err, trait_predicate) {
 877                             err.emit();
 878                             return;
 879                         }
 880
 881                         if self.suggest_impl_trait(&mut err, span, &obligation, trait_predicate) {
 882                             err.emit();
 883                             return;
 884                         }
 885
 886                         if is_unsize {
 887                             // If the obligation failed due to a missing implementation of the
 888                             // `Unsize` trait, give a pointer to why that might be the case
 889                             err.note(
 890                                 "all implementations of `Unsize` are provided \
 891                                 automatically by the compiler, see \
 892                                 <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> \
 893                                 for more information",
 894                             );
 895                         }
 896
 897                         let is_fn_trait = tcx.is_fn_trait(trait_ref.def_id());
 898                         let is_target_feature_fn = if let ty::FnDef(def_id, _) =
 899                             *trait_ref.skip_binder().self_ty().kind()
 900                         {
 901                             !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
 902                         } else {
 903                             false
 904                         };
 905                         if is_fn_trait && is_target_feature_fn {
 906                             err.note(
 907                                 "`#[target_feature]` functions do not implement the `Fn` traits",
 908                             );
 909                         }
 910
 911                         // Try to report a help message
 912                         if is_fn_trait
 913                             && let Ok((implemented_kind, params)) = self.type_implements_fn_trait(
 914                             obligation.param_env,
 915                             trait_ref.self_ty(),
 916                             trait_predicate.skip_binder().constness,
 917                             trait_predicate.skip_binder().polarity,
 918                         )
 919                         {
 920                             // If the type implements `Fn`, `FnMut`, or `FnOnce`, suppress the following
 921                             // suggestion to add trait bounds for the type, since we only typically implement
 922                             // these traits once.
 923
 924                             // Note if the `FnMut` or `FnOnce` is less general than the trait we're trying
 925                             // to implement.
 926                             let selected_kind =
 927                                 self.tcx.fn_trait_kind_from_def_id(trait_ref.def_id())
 928                                     .expect("expected to map DefId to ClosureKind");
 929                             if !implemented_kind.extends(selected_kind) {
 930                                 err.note(
 931                                     &format!(
 932                                         "`{}` implements `{}`, but it must implement `{}`, which is more general",
 933                                         trait_ref.skip_binder().self_ty(),
 934                                         implemented_kind,
 935                                         selected_kind
 936                                     )
 937                                 );
 938                             }
 939
 940                             // Note any argument mismatches
 941                             let given_ty = params.skip_binder();
 942                             let expected_ty = trait_ref.skip_binder().substs.type_at(1);
 943                             if let ty::Tuple(given) = given_ty.kind()
 944                                 && let ty::Tuple(expected) = expected_ty.kind()
 945                             {
 946                                 if expected.len() != given.len() {
 947                                     // Note number of types that were expected and given
 948                                     err.note(
 949                                         &format!(
 950                                             "expected a closure taking {} argument{}, but one taking {} argument{} was given",
 951                                             given.len(),
 952                                             pluralize!(given.len()),
 953                                             expected.len(),
 954                                             pluralize!(expected.len()),
 955                                         )
 956                                     );
 957                                 } else if !self.same_type_modulo_infer(given_ty, expected_ty) {
 958                                     // Print type mismatch
 959                                     let (expected_args, given_args) =
 960                                         self.cmp(given_ty, expected_ty);
 961                                     err.note_expected_found(
 962                                         &"a closure with arguments",
 963                                         expected_args,
 964                                         &"a closure with arguments",
 965                                         given_args,
 966                                     );
 967                                 }
 968                             }
 969                         } else if !trait_ref.has_non_region_infer()
 970                             && self.predicate_can_apply(obligation.param_env, trait_predicate)
 971                         {
 972                             // If a where-clause may be useful, remind the
 973                             // user that they can add it.
 974                             //
 975                             // don't display an on-unimplemented note, as
 976                             // these notes will often be of the form
 977                             //     "the type `T` can't be frobnicated"
 978                             // which is somewhat confusing.
 979                             self.suggest_restricting_param_bound(
 980                                 &mut err,
 981                                 trait_predicate,
 982                                 None,
 983                                 obligation.cause.body_id,
 984                             );
 985                         } else if !suggested && !unsatisfied_const {
 986                             // Can't show anything else useful, try to find similar impls.
 987                             let impl_candidates = self.find_similar_impl_candidates(trait_predicate);
 988                             if !self.report_similar_impl_candidates(
 989                                 impl_candidates,
 990                                 trait_ref,
 991                                 obligation.cause.body_id,
 992                                 &mut err,
 993                                 true,
 994                             ) {
 995                                 // This is *almost* equivalent to
 996                                 // `obligation.cause.code().peel_derives()`, but it gives us the
 997                                 // trait predicate for that corresponding root obligation. This
 998                                 // lets us get a derived obligation from a type parameter, like
 999                                 // when calling `string.strip_suffix(p)` where `p` is *not* an
1000                                 // implementer of `Pattern<'_>`.
1001                                 let mut code = obligation.cause.code();
1002                                 let mut trait_pred = trait_predicate;
1003                                 let mut peeled = false;
1004                                 while let Some((parent_code, parent_trait_pred)) = code.parent() {
1005                                     code = parent_code;
1006                                     if let Some(parent_trait_pred) = parent_trait_pred {
1007                                         trait_pred = parent_trait_pred;
1008                                         peeled = true;
1009                                     }
1010                                 }
1011                                 let def_id = trait_pred.def_id();
1012                                 // Mention *all* the `impl`s for the *top most* obligation, the
1013                                 // user might have meant to use one of them, if any found. We skip
1014                                 // auto-traits or fundamental traits that might not be exactly what
1015                                 // the user might expect to be presented with. Instead this is
1016                                 // useful for less general traits.
1017                                 if peeled
1018                                     && !self.tcx.trait_is_auto(def_id)
1019                                     && !self.tcx.lang_items().iter().any(|(_, id)| id == def_id)
1020                                 {
1021                                     let trait_ref = trait_pred.to_poly_trait_ref();
1022                                     let impl_candidates =
1023                                         self.find_similar_impl_candidates(trait_pred);
1024                                     self.report_similar_impl_candidates(
1025                                         impl_candidates,
1026                                         trait_ref,
1027                                         obligation.cause.body_id,
1028                                         &mut err,
1029                                         true,
1030                                     );
1031                                 }
1032                             }
1033                         }
1034
1035                         // Changing mutability doesn't make a difference to whether we have
1036                         // an `Unsize` impl (Fixes ICE in #71036)
1037                         if !is_unsize {
1038                             self.suggest_change_mut(&obligation, &mut err, trait_predicate);
1039                         }
1040
1041                         // If this error is due to `!: Trait` not implemented but `(): Trait` is
1042                         // implemented, and fallback has occurred, then it could be due to a
1043                         // variable that used to fallback to `()` now falling back to `!`. Issue a
1044                         // note informing about the change in behaviour.
1045                         if trait_predicate.skip_binder().self_ty().is_never()
1046                             && self.fallback_has_occurred
1047                         {
1048                             let predicate = trait_predicate.map_bound(|trait_pred| {
1049                                 trait_pred.with_self_ty(self.tcx, self.tcx.mk_unit())
1050                             });
1051                             let unit_obligation = obligation.with(tcx, predicate);
1052                             if self.predicate_may_hold(&unit_obligation) {
1053                                 err.note(
1054                                     "this error might have been caused by changes to \
1055                                     Rust's type-inference algorithm (see issue #48950 \
1056                                     <https://github.com/rust-lang/rust/issues/48950> \
1057                                     for more information)",
1058                                 );
1059                                 err.help("did you intend to use the type `()` here instead?");
1060                             }
1061                         }
1062
1063                         // Return early if the trait is Debug or Display and the invocation
1064                         // originates within a standard library macro, because the output
1065                         // is otherwise overwhelming and unhelpful (see #85844 for an
1066                         // example).
1067
1068                         let in_std_macro =
1069                             match obligation.cause.span.ctxt().outer_expn_data().macro_def_id {
1070                                 Some(macro_def_id) => {
1071                                     let crate_name = tcx.crate_name(macro_def_id.krate);
1072                                     crate_name == sym::std || crate_name == sym::core
1073                                 }
1074                                 None => false,
1075                             };
1076
1077                         if in_std_macro
1078                             && matches!(
1079                                 self.tcx.get_diagnostic_name(trait_ref.def_id()),
1080                                 Some(sym::Debug | sym::Display)
1081                             )
1082                         {
1083                             err.emit();
1084                             return;
1085                         }
1086
1087                         err
1088                     }
1089
1090                     ty::PredicateKind::Subtype(predicate) => {
1091                         // Errors for Subtype predicates show up as
1092                         // `FulfillmentErrorCode::CodeSubtypeError`,
1093                         // not selection error.
1094                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
1095                     }
1096
1097                     ty::PredicateKind::Coerce(predicate) => {
1098                         // Errors for Coerce predicates show up as
1099                         // `FulfillmentErrorCode::CodeSubtypeError`,
1100                         // not selection error.
1101                         span_bug!(span, "coerce requirement gave wrong error: `{:?}`", predicate)
1102                     }
1103
1104                     ty::PredicateKind::Clause(ty::Clause::RegionOutlives(..))
1105                     | ty::PredicateKind::Clause(ty::Clause::Projection(..))
1106                     | ty::PredicateKind::Clause(ty::Clause::TypeOutlives(..)) => {
1107                         let predicate = self.resolve_vars_if_possible(obligation.predicate);
1108                         struct_span_err!(
1109                             self.tcx.sess,
1110                             span,
1111                             E0280,
1112                             "the requirement `{}` is not satisfied",
1113                             predicate
1114                         )
1115                     }
1116
1117                     ty::PredicateKind::ObjectSafe(trait_def_id) => {
1118                         let violations = self.tcx.object_safety_violations(trait_def_id);
1119                         report_object_safety_error(self.tcx, span, trait_def_id, violations)
1120                     }
1121
1122                     ty::PredicateKind::ClosureKind(closure_def_id, closure_substs, kind) => {
1123                         let found_kind = self.closure_kind(closure_substs).unwrap();
1124                         let closure_span = self.tcx.def_span(closure_def_id);
1125                         let mut err = struct_span_err!(
1126                             self.tcx.sess,
1127                             closure_span,
1128                             E0525,
1129                             "expected a closure that implements the `{}` trait, \
1130                              but this closure only implements `{}`",
1131                             kind,
1132                             found_kind
1133                         );
1134
1135                         err.span_label(
1136                             closure_span,
1137                             format!("this closure implements `{}`, not `{}`", found_kind, kind),
1138                         );
1139                         err.span_label(
1140                             obligation.cause.span,
1141                             format!("the requirement to implement `{}` derives from here", kind),
1142                         );
1143
1144                         // Additional context information explaining why the closure only implements
1145                         // a particular trait.
1146                         if let Some(typeck_results) = &self.typeck_results {
1147                             let hir_id = self
1148                                 .tcx
1149                                 .hir()
1150                                 .local_def_id_to_hir_id(closure_def_id.expect_local());
1151                             match (found_kind, typeck_results.closure_kind_origins().get(hir_id)) {
1152                                 (ty::ClosureKind::FnOnce, Some((span, place))) => {
1153                                     err.span_label(
1154                                         *span,
1155                                         format!(
1156                                             "closure is `FnOnce` because it moves the \
1157                                          variable `{}` out of its environment",
1158                                             ty::place_to_string_for_capture(tcx, place)
1159                                         ),
1160                                     );
1161                                 }
1162                                 (ty::ClosureKind::FnMut, Some((span, place))) => {
1163                                     err.span_label(
1164                                         *span,
1165                                         format!(
1166                                             "closure is `FnMut` because it mutates the \
1167                                          variable `{}` here",
1168                                             ty::place_to_string_for_capture(tcx, place)
1169                                         ),
1170                                     );
1171                                 }
1172                                 _ => {}
1173                             }
1174                         }
1175
1176                         err
1177                     }
1178
1179                     ty::PredicateKind::WellFormed(ty) => {
1180                         if self.tcx.sess.opts.unstable_opts.trait_solver == TraitSolver::Classic {
1181                             // WF predicates cannot themselves make
1182                             // errors. They can only block due to
1183                             // ambiguity; otherwise, they always
1184                             // degenerate into other obligations
1185                             // (which may fail).
1186                             span_bug!(span, "WF predicate not satisfied for {:?}", ty);
1187                         } else {
1188                             // FIXME: we'll need a better message which takes into account
1189                             // which bounds actually failed to hold.
1190                             self.tcx.sess.struct_span_err(
1191                                 span,
1192                                 &format!("the type `{}` is not well-formed", ty),
1193                             )
1194                         }
1195                     }
1196
1197                     ty::PredicateKind::ConstEvaluatable(..) => {
1198                         // Errors for `ConstEvaluatable` predicates show up as
1199                         // `SelectionError::ConstEvalFailure`,
1200                         // not `Unimplemented`.
1201                         span_bug!(
1202                             span,
1203                             "const-evaluatable requirement gave wrong error: `{:?}`",
1204                             obligation
1205                         )
1206                     }
1207
1208                     ty::PredicateKind::ConstEquate(..) => {
1209                         // Errors for `ConstEquate` predicates show up as
1210                         // `SelectionError::ConstEvalFailure`,
1211                         // not `Unimplemented`.
1212                         span_bug!(
1213                             span,
1214                             "const-equate requirement gave wrong error: `{:?}`",
1215                             obligation
1216                         )
1217                     }
1218
1219                     ty::PredicateKind::Ambiguous => span_bug!(span, "ambiguous"),
1220
1221                     ty::PredicateKind::TypeWellFormedFromEnv(..) => span_bug!(
1222                         span,
1223                         "TypeWellFormedFromEnv predicate should only exist in the environment"
1224                     ),
1225                 }
1226             }
1227
1228             OutputTypeParameterMismatch(
1229                 found_trait_ref,
1230                 expected_trait_ref,
1231                 terr @ TypeError::CyclicTy(_),
1232             ) => {
1233                 let self_ty = found_trait_ref.self_ty().skip_binder();
1234                 let (cause, terr) = if let ty::Closure(def_id, _) = self_ty.kind() {
1235                     (
1236                         ObligationCause::dummy_with_span(tcx.def_span(def_id)),
1237                         TypeError::CyclicTy(self_ty),
1238                     )
1239                 } else {
1240                     (obligation.cause.clone(), terr)
1241                 };
1242                 self.report_and_explain_type_error(
1243                     TypeTrace::poly_trait_refs(&cause, true, expected_trait_ref, found_trait_ref),
1244                     terr,
1245                 )
1246             }
1247             OutputTypeParameterMismatch(found_trait_ref, expected_trait_ref, _) => {
1248                 let found_trait_ref = self.resolve_vars_if_possible(found_trait_ref);
1249                 let expected_trait_ref = self.resolve_vars_if_possible(expected_trait_ref);
1250
1251                 if expected_trait_ref.self_ty().references_error() {
1252                     return;
1253                 }
1254
1255                 let Some(found_trait_ty) = found_trait_ref.self_ty().no_bound_vars() else {
1256                     return;
1257                 };
1258
1259                 let found_did = match *found_trait_ty.kind() {
1260                     ty::Closure(did, _)
1261                     | ty::Foreign(did)
1262                     | ty::FnDef(did, _)
1263                     | ty::Generator(did, ..) => Some(did),
1264                     ty::Adt(def, _) => Some(def.did()),
1265                     _ => None,
1266                 };
1267
1268                 let found_node = found_did.and_then(|did| self.tcx.hir().get_if_local(did));
1269                 let found_span = found_did.and_then(|did| self.tcx.hir().span_if_local(did));
1270
1271                 if self.reported_closure_mismatch.borrow().contains(&(span, found_span)) {
1272                     // We check closures twice, with obligations flowing in different directions,
1273                     // but we want to complain about them only once.
1274                     return;
1275                 }
1276
1277                 self.reported_closure_mismatch.borrow_mut().insert((span, found_span));
1278
1279                 let mut not_tupled = false;
1280
1281                 let found = match found_trait_ref.skip_binder().substs.type_at(1).kind() {
1282                     ty::Tuple(ref tys) => vec![ArgKind::empty(); tys.len()],
1283                     _ => {
1284                         not_tupled = true;
1285                         vec![ArgKind::empty()]
1286                     }
1287                 };
1288
1289                 let expected_ty = expected_trait_ref.skip_binder().substs.type_at(1);
1290                 let expected = match expected_ty.kind() {
1291                     ty::Tuple(ref tys) => {
1292                         tys.iter().map(|t| ArgKind::from_expected_ty(t, Some(span))).collect()
1293                     }
1294                     _ => {
1295                         not_tupled = true;
1296                         vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())]
1297                     }
1298                 };
1299
1300                 // If this is a `Fn` family trait and either the expected or found
1301                 // is not tupled, then fall back to just a regular mismatch error.
1302                 // This shouldn't be common unless manually implementing one of the
1303                 // traits manually, but don't make it more confusing when it does
1304                 // happen.
1305                 if Some(expected_trait_ref.def_id()) != tcx.lang_items().gen_trait() && not_tupled {
1306                     self.report_and_explain_type_error(
1307                         TypeTrace::poly_trait_refs(
1308                             &obligation.cause,
1309                             true,
1310                             expected_trait_ref,
1311                             found_trait_ref,
1312                         ),
1313                         ty::error::TypeError::Mismatch,
1314                     )
1315                 } else if found.len() == expected.len() {
1316                     self.report_closure_arg_mismatch(
1317                         span,
1318                         found_span,
1319                         found_trait_ref,
1320                         expected_trait_ref,
1321                         obligation.cause.code(),
1322                         found_node,
1323                     )
1324                 } else {
1325                     let (closure_span, closure_arg_span, found) = found_did
1326                         .and_then(|did| {
1327                             let node = self.tcx.hir().get_if_local(did)?;
1328                             let (found_span, closure_arg_span, found) =
1329                                 self.get_fn_like_arguments(node)?;
1330                             Some((Some(found_span), closure_arg_span, found))
1331                         })
1332                         .unwrap_or((found_span, None, found));
1333
1334                     self.report_arg_count_mismatch(
1335                         span,
1336                         closure_span,
1337                         expected,
1338                         found,
1339                         found_trait_ty.is_closure(),
1340                         closure_arg_span,
1341                     )
1342                 }
1343             }
1344
1345             TraitNotObjectSafe(did) => {
1346                 let violations = self.tcx.object_safety_violations(did);
1347                 report_object_safety_error(self.tcx, span, did, violations)
1348             }
1349
1350             SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsInfer) => {
1351                 bug!(
1352                     "MentionsInfer should have been handled in `traits/fulfill.rs` or `traits/select/mod.rs`"
1353                 )
1354             }
1355             SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsParam) => {
1356                 if !self.tcx.features().generic_const_exprs {
1357                     let mut err = self.tcx.sess.struct_span_err(
1358                         span,
1359                         "constant expression depends on a generic parameter",
1360                     );
1361                     // FIXME(const_generics): we should suggest to the user how they can resolve this
1362                     // issue. However, this is currently not actually possible
1363                     // (see https://github.com/rust-lang/rust/issues/66962#issuecomment-575907083).
1364                     //
1365                     // Note that with `feature(generic_const_exprs)` this case should not
1366                     // be reachable.
1367                     err.note("this may fail depending on what value the parameter takes");
1368                     err.emit();
1369                     return;
1370                 }
1371
1372                 match obligation.predicate.kind().skip_binder() {
1373                     ty::PredicateKind::ConstEvaluatable(ct) => {
1374                         let ty::ConstKind::Unevaluated(uv) = ct.kind() else {
1375                             bug!("const evaluatable failed for non-unevaluated const `{ct:?}`");
1376                         };
1377                         let mut err =
1378                             self.tcx.sess.struct_span_err(span, "unconstrained generic constant");
1379                         let const_span = self.tcx.def_span(uv.def.did);
1380                         match self.tcx.sess.source_map().span_to_snippet(const_span) {
1381                             Ok(snippet) => err.help(&format!(
1382                                 "try adding a `where` bound using this expression: `where [(); {}]:`",
1383                                 snippet
1384                             )),
1385                             _ => err.help("consider adding a `where` bound using this expression"),
1386                         };
1387                         err
1388                     }
1389                     _ => {
1390                         span_bug!(
1391                             span,
1392                             "unexpected non-ConstEvaluatable predicate, this should not be reachable"
1393                         )
1394                     }
1395                 }
1396             }
1397
1398             // Already reported in the query.
1399             SelectionError::NotConstEvaluatable(NotConstEvaluatable::Error(_)) => {
1400                 // FIXME(eddyb) remove this once `ErrorGuaranteed` becomes a proof token.
1401                 self.tcx.sess.delay_span_bug(span, "`ErrorGuaranteed` without an error");
1402                 return;
1403             }
1404             // Already reported.
1405             Overflow(OverflowError::Error(_)) => {
1406                 self.tcx.sess.delay_span_bug(span, "`OverflowError` has been reported");
1407                 return;
1408             }
1409             Overflow(_) => {
1410                 bug!("overflow should be handled before the `report_selection_error` path");
1411             }
1412             SelectionError::ErrorReporting => {
1413                 bug!("ErrorReporting Overflow should not reach `report_selection_err` call")
1414             }
1415         };
1416
1417         self.note_obligation_cause(&mut err, &obligation);
1418         self.point_at_returns_when_relevant(&mut err, &obligation);
1419         err.emit();
1420     }
1421 }
1422
1423 trait InferCtxtPrivExt<'tcx> {
1424     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1425     // `error` occurring implies that `cond` occurs.
1426     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool;
1427
1428     fn report_fulfillment_error(
1429         &self,
1430         error: &FulfillmentError<'tcx>,
1431         body_id: Option<hir::BodyId>,
1432     );
1433
1434     fn report_projection_error(
1435         &self,
1436         obligation: &PredicateObligation<'tcx>,
1437         error: &MismatchedProjectionTypes<'tcx>,
1438     );
1439
1440     fn maybe_detailed_projection_msg(
1441         &self,
1442         pred: ty::ProjectionPredicate<'tcx>,
1443         normalized_ty: ty::Term<'tcx>,
1444         expected_ty: ty::Term<'tcx>,
1445     ) -> Option<String>;
1446
1447     fn fuzzy_match_tys(
1448         &self,
1449         a: Ty<'tcx>,
1450         b: Ty<'tcx>,
1451         ignoring_lifetimes: bool,
1452     ) -> Option<CandidateSimilarity>;
1453
1454     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str>;
1455
1456     fn find_similar_impl_candidates(
1457         &self,
1458         trait_pred: ty::PolyTraitPredicate<'tcx>,
1459     ) -> Vec<ImplCandidate<'tcx>>;
1460
1461     fn report_similar_impl_candidates(
1462         &self,
1463         impl_candidates: Vec<ImplCandidate<'tcx>>,
1464         trait_ref: ty::PolyTraitRef<'tcx>,
1465         body_id: hir::HirId,
1466         err: &mut Diagnostic,
1467         other: bool,
1468     ) -> bool;
1469
1470     /// Gets the parent trait chain start
1471     fn get_parent_trait_ref(
1472         &self,
1473         code: &ObligationCauseCode<'tcx>,
1474     ) -> Option<(String, Option<Span>)>;
1475
1476     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1477     /// with the same path as `trait_ref`, a help message about
1478     /// a probable version mismatch is added to `err`
1479     fn note_version_mismatch(
1480         &self,
1481         err: &mut Diagnostic,
1482         trait_ref: &ty::PolyTraitRef<'tcx>,
1483     ) -> bool;
1484
1485     /// Creates a `PredicateObligation` with `new_self_ty` replacing the existing type in the
1486     /// `trait_ref`.
1487     ///
1488     /// For this to work, `new_self_ty` must have no escaping bound variables.
1489     fn mk_trait_obligation_with_new_self_ty(
1490         &self,
1491         param_env: ty::ParamEnv<'tcx>,
1492         trait_ref_and_ty: ty::Binder<'tcx, (ty::TraitPredicate<'tcx>, Ty<'tcx>)>,
1493     ) -> PredicateObligation<'tcx>;
1494
1495     fn maybe_report_ambiguity(
1496         &self,
1497         obligation: &PredicateObligation<'tcx>,
1498         body_id: Option<hir::BodyId>,
1499     );
1500
1501     fn predicate_can_apply(
1502         &self,
1503         param_env: ty::ParamEnv<'tcx>,
1504         pred: ty::PolyTraitPredicate<'tcx>,
1505     ) -> bool;
1506
1507     fn note_obligation_cause(&self, err: &mut Diagnostic, obligation: &PredicateObligation<'tcx>);
1508
1509     fn suggest_unsized_bound_if_applicable(
1510         &self,
1511         err: &mut Diagnostic,
1512         obligation: &PredicateObligation<'tcx>,
1513     );
1514
1515     fn annotate_source_of_ambiguity(
1516         &self,
1517         err: &mut Diagnostic,
1518         impls: &[ambiguity::Ambiguity],
1519         predicate: ty::Predicate<'tcx>,
1520     );
1521
1522     fn maybe_suggest_unsized_generics(&self, err: &mut Diagnostic, span: Span, node: Node<'tcx>);
1523
1524     fn maybe_indirection_for_unsized(
1525         &self,
1526         err: &mut Diagnostic,
1527         item: &'tcx Item<'tcx>,
1528         param: &'tcx GenericParam<'tcx>,
1529     ) -> bool;
1530
1531     fn is_recursive_obligation(
1532         &self,
1533         obligated_types: &mut Vec<Ty<'tcx>>,
1534         cause_code: &ObligationCauseCode<'tcx>,
1535     ) -> bool;
1536 }
1537
1538 impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
1539     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1540     // `error` occurring implies that `cond` occurs.
1541     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool {
1542         if cond == error {
1543             return true;
1544         }
1545
1546         // FIXME: It should be possible to deal with `ForAll` in a cleaner way.
1547         let bound_error = error.kind();
1548         let (cond, error) = match (cond.kind().skip_binder(), bound_error.skip_binder()) {
1549             (
1550                 ty::PredicateKind::Clause(ty::Clause::Trait(..)),
1551                 ty::PredicateKind::Clause(ty::Clause::Trait(error)),
1552             ) => (cond, bound_error.rebind(error)),
1553             _ => {
1554                 // FIXME: make this work in other cases too.
1555                 return false;
1556             }
1557         };
1558
1559         for obligation in super::elaborate_predicates(self.tcx, std::iter::once(cond)) {
1560             let bound_predicate = obligation.predicate.kind();
1561             if let ty::PredicateKind::Clause(ty::Clause::Trait(implication)) =
1562                 bound_predicate.skip_binder()
1563             {
1564                 let error = error.to_poly_trait_ref();
1565                 let implication = bound_predicate.rebind(implication.trait_ref);
1566                 // FIXME: I'm just not taking associated types at all here.
1567                 // Eventually I'll need to implement param-env-aware
1568                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
1569                 let param_env = ty::ParamEnv::empty();
1570                 if self.can_sub(param_env, error, implication).is_ok() {
1571                     debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
1572                     return true;
1573                 }
1574             }
1575         }
1576
1577         false
1578     }
1579
1580     #[instrument(skip(self), level = "debug")]
1581     fn report_fulfillment_error(
1582         &self,
1583         error: &FulfillmentError<'tcx>,
1584         body_id: Option<hir::BodyId>,
1585     ) {
1586         match error.code {
1587             FulfillmentErrorCode::CodeSelectionError(ref selection_error) => {
1588                 self.report_selection_error(
1589                     error.obligation.clone(),
1590                     &error.root_obligation,
1591                     selection_error,
1592                 );
1593             }
1594             FulfillmentErrorCode::CodeProjectionError(ref e) => {
1595                 self.report_projection_error(&error.obligation, e);
1596             }
1597             FulfillmentErrorCode::CodeAmbiguity => {
1598                 self.maybe_report_ambiguity(&error.obligation, body_id);
1599             }
1600             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
1601                 self.report_mismatched_types(
1602                     &error.obligation.cause,
1603                     expected_found.expected,
1604                     expected_found.found,
1605                     *err,
1606                 )
1607                 .emit();
1608             }
1609             FulfillmentErrorCode::CodeConstEquateError(ref expected_found, ref err) => {
1610                 let mut diag = self.report_mismatched_consts(
1611                     &error.obligation.cause,
1612                     expected_found.expected,
1613                     expected_found.found,
1614                     *err,
1615                 );
1616                 let code = error.obligation.cause.code().peel_derives().peel_match_impls();
1617                 if let ObligationCauseCode::BindingObligation(..)
1618                 | ObligationCauseCode::ItemObligation(..)
1619                 | ObligationCauseCode::ExprBindingObligation(..)
1620                 | ObligationCauseCode::ExprItemObligation(..) = code
1621                 {
1622                     self.note_obligation_cause_code(
1623                         &mut diag,
1624                         error.obligation.predicate,
1625                         error.obligation.param_env,
1626                         code,
1627                         &mut vec![],
1628                         &mut Default::default(),
1629                     );
1630                 }
1631                 diag.emit();
1632             }
1633             FulfillmentErrorCode::CodeCycle(ref cycle) => {
1634                 self.report_overflow_obligation_cycle(cycle);
1635             }
1636         }
1637     }
1638
1639     #[instrument(level = "debug", skip_all)]
1640     fn report_projection_error(
1641         &self,
1642         obligation: &PredicateObligation<'tcx>,
1643         error: &MismatchedProjectionTypes<'tcx>,
1644     ) {
1645         let predicate = self.resolve_vars_if_possible(obligation.predicate);
1646
1647         if predicate.references_error() {
1648             return;
1649         }
1650
1651         self.probe(|_| {
1652             let ocx = ObligationCtxt::new_in_snapshot(self);
1653
1654             // try to find the mismatched types to report the error with.
1655             //
1656             // this can fail if the problem was higher-ranked, in which
1657             // cause I have no idea for a good error message.
1658             let bound_predicate = predicate.kind();
1659             let (values, err) = if let ty::PredicateKind::Clause(ty::Clause::Projection(data)) =
1660                 bound_predicate.skip_binder()
1661             {
1662                 let data = self.replace_bound_vars_with_fresh_vars(
1663                     obligation.cause.span,
1664                     infer::LateBoundRegionConversionTime::HigherRankedType,
1665                     bound_predicate.rebind(data),
1666                 );
1667                 let unnormalized_term = match data.term.unpack() {
1668                     ty::TermKind::Ty(_) => self
1669                         .tcx
1670                         .mk_projection(data.projection_ty.def_id, data.projection_ty.substs)
1671                         .into(),
1672                     ty::TermKind::Const(ct) => self
1673                         .tcx
1674                         .mk_const(
1675                             ty::UnevaluatedConst {
1676                                 def: ty::WithOptConstParam::unknown(data.projection_ty.def_id),
1677                                 substs: data.projection_ty.substs,
1678                             },
1679                             ct.ty(),
1680                         )
1681                         .into(),
1682                 };
1683                 let normalized_term =
1684                     ocx.normalize(&obligation.cause, obligation.param_env, unnormalized_term);
1685
1686                 debug!(?obligation.cause, ?obligation.param_env);
1687
1688                 debug!(?normalized_term, data.ty = ?data.term);
1689
1690                 let is_normalized_term_expected = !matches!(
1691                     obligation.cause.code().peel_derives(),
1692                     ObligationCauseCode::ItemObligation(_)
1693                         | ObligationCauseCode::BindingObligation(_, _)
1694                         | ObligationCauseCode::ExprItemObligation(..)
1695                         | ObligationCauseCode::ExprBindingObligation(..)
1696                         | ObligationCauseCode::ObjectCastObligation(..)
1697                         | ObligationCauseCode::OpaqueType
1698                 );
1699
1700                 // constrain inference variables a bit more to nested obligations from normalize so
1701                 // we can have more helpful errors.
1702                 ocx.select_where_possible();
1703
1704                 if let Err(new_err) = ocx.eq_exp(
1705                     &obligation.cause,
1706                     obligation.param_env,
1707                     is_normalized_term_expected,
1708                     normalized_term,
1709                     data.term,
1710                 ) {
1711                     (Some((data, is_normalized_term_expected, normalized_term, data.term)), new_err)
1712                 } else {
1713                     (None, error.err)
1714                 }
1715             } else {
1716                 (None, error.err)
1717             };
1718
1719             let msg = values
1720                 .and_then(|(predicate, _, normalized_term, expected_term)| {
1721                     self.maybe_detailed_projection_msg(predicate, normalized_term, expected_term)
1722                 })
1723                 .unwrap_or_else(|| format!("type mismatch resolving `{}`", predicate));
1724             let mut diag = struct_span_err!(self.tcx.sess, obligation.cause.span, E0271, "{msg}");
1725
1726             let secondary_span = match predicate.kind().skip_binder() {
1727                 ty::PredicateKind::Clause(ty::Clause::Projection(proj)) => self
1728                     .tcx
1729                     .opt_associated_item(proj.projection_ty.def_id)
1730                     .and_then(|trait_assoc_item| {
1731                         self.tcx
1732                             .trait_of_item(proj.projection_ty.def_id)
1733                             .map(|id| (trait_assoc_item, id))
1734                     })
1735                     .and_then(|(trait_assoc_item, id)| {
1736                         let trait_assoc_ident = trait_assoc_item.ident(self.tcx);
1737                         self.tcx.find_map_relevant_impl(id, proj.projection_ty.self_ty(), |did| {
1738                             self.tcx
1739                                 .associated_items(did)
1740                                 .in_definition_order()
1741                                 .find(|assoc| assoc.ident(self.tcx) == trait_assoc_ident)
1742                         })
1743                     })
1744                     .and_then(|item| match self.tcx.hir().get_if_local(item.def_id) {
1745                         Some(
1746                             hir::Node::TraitItem(hir::TraitItem {
1747                                 kind: hir::TraitItemKind::Type(_, Some(ty)),
1748                                 ..
1749                             })
1750                             | hir::Node::ImplItem(hir::ImplItem {
1751                                 kind: hir::ImplItemKind::Type(ty),
1752                                 ..
1753                             }),
1754                         ) => Some((ty.span, format!("type mismatch resolving `{}`", predicate))),
1755                         _ => None,
1756                     }),
1757                 _ => None,
1758             };
1759             self.note_type_err(
1760                 &mut diag,
1761                 &obligation.cause,
1762                 secondary_span,
1763                 values.map(|(_, is_normalized_ty_expected, normalized_ty, expected_ty)| {
1764                     infer::ValuePairs::Terms(ExpectedFound::new(
1765                         is_normalized_ty_expected,
1766                         normalized_ty,
1767                         expected_ty,
1768                     ))
1769                 }),
1770                 err,
1771                 true,
1772                 false,
1773             );
1774             self.note_obligation_cause(&mut diag, obligation);
1775             diag.emit();
1776         });
1777     }
1778
1779     fn maybe_detailed_projection_msg(
1780         &self,
1781         pred: ty::ProjectionPredicate<'tcx>,
1782         normalized_ty: ty::Term<'tcx>,
1783         expected_ty: ty::Term<'tcx>,
1784     ) -> Option<String> {
1785         let trait_def_id = pred.projection_ty.trait_def_id(self.tcx);
1786         let self_ty = pred.projection_ty.self_ty();
1787
1788         with_forced_trimmed_paths! {
1789             if Some(pred.projection_ty.def_id) == self.tcx.lang_items().fn_once_output() {
1790                 Some(format!(
1791                     "expected `{self_ty}` to be a {fn_kind} that returns `{expected_ty}`, but it \
1792                      returns `{normalized_ty}`",
1793                     fn_kind = self_ty.prefix_string(self.tcx)
1794                 ))
1795             } else if Some(trait_def_id) == self.tcx.lang_items().future_trait() {
1796                 Some(format!(
1797                     "expected `{self_ty}` to be a future that resolves to `{expected_ty}`, but it \
1798                      resolves to `{normalized_ty}`"
1799                 ))
1800             } else if Some(trait_def_id) == self.tcx.get_diagnostic_item(sym::Iterator) {
1801                 Some(format!(
1802                     "expected `{self_ty}` to be an iterator that yields `{expected_ty}`, but it \
1803                      yields `{normalized_ty}`"
1804                 ))
1805             } else {
1806                 None
1807             }
1808         }
1809     }
1810
1811     fn fuzzy_match_tys(
1812         &self,
1813         mut a: Ty<'tcx>,
1814         mut b: Ty<'tcx>,
1815         ignoring_lifetimes: bool,
1816     ) -> Option<CandidateSimilarity> {
1817         /// returns the fuzzy category of a given type, or None
1818         /// if the type can be equated to any type.
1819         fn type_category(tcx: TyCtxt<'_>, t: Ty<'_>) -> Option<u32> {
1820             match t.kind() {
1821                 ty::Bool => Some(0),
1822                 ty::Char => Some(1),
1823                 ty::Str => Some(2),
1824                 ty::Adt(def, _) if Some(def.did()) == tcx.lang_items().string() => Some(2),
1825                 ty::Int(..)
1826                 | ty::Uint(..)
1827                 | ty::Float(..)
1828                 | ty::Infer(ty::IntVar(..) | ty::FloatVar(..)) => Some(4),
1829                 ty::Ref(..) | ty::RawPtr(..) => Some(5),
1830                 ty::Array(..) | ty::Slice(..) => Some(6),
1831                 ty::FnDef(..) | ty::FnPtr(..) => Some(7),
1832                 ty::Dynamic(..) => Some(8),
1833                 ty::Closure(..) => Some(9),
1834                 ty::Tuple(..) => Some(10),
1835                 ty::Param(..) => Some(11),
1836                 ty::Alias(ty::Projection, ..) => Some(12),
1837                 ty::Alias(ty::Opaque, ..) => Some(13),
1838                 ty::Never => Some(14),
1839                 ty::Adt(..) => Some(15),
1840                 ty::Generator(..) => Some(16),
1841                 ty::Foreign(..) => Some(17),
1842                 ty::GeneratorWitness(..) => Some(18),
1843                 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error(_) => None,
1844             }
1845         }
1846
1847         let strip_references = |mut t: Ty<'tcx>| -> Ty<'tcx> {
1848             loop {
1849                 match t.kind() {
1850                     ty::Ref(_, inner, _) | ty::RawPtr(ty::TypeAndMut { ty: inner, .. }) => {
1851                         t = *inner
1852                     }
1853                     _ => break t,
1854                 }
1855             }
1856         };
1857
1858         if !ignoring_lifetimes {
1859             a = strip_references(a);
1860             b = strip_references(b);
1861         }
1862
1863         let cat_a = type_category(self.tcx, a)?;
1864         let cat_b = type_category(self.tcx, b)?;
1865         if a == b {
1866             Some(CandidateSimilarity::Exact { ignoring_lifetimes })
1867         } else if cat_a == cat_b {
1868             match (a.kind(), b.kind()) {
1869                 (ty::Adt(def_a, _), ty::Adt(def_b, _)) => def_a == def_b,
1870                 (ty::Foreign(def_a), ty::Foreign(def_b)) => def_a == def_b,
1871                 // Matching on references results in a lot of unhelpful
1872                 // suggestions, so let's just not do that for now.
1873                 //
1874                 // We still upgrade successful matches to `ignoring_lifetimes: true`
1875                 // to prioritize that impl.
1876                 (ty::Ref(..) | ty::RawPtr(..), ty::Ref(..) | ty::RawPtr(..)) => {
1877                     self.fuzzy_match_tys(a, b, true).is_some()
1878                 }
1879                 _ => true,
1880             }
1881             .then_some(CandidateSimilarity::Fuzzy { ignoring_lifetimes })
1882         } else if ignoring_lifetimes {
1883             None
1884         } else {
1885             self.fuzzy_match_tys(a, b, true)
1886         }
1887     }
1888
1889     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str> {
1890         self.tcx.hir().body(body_id).generator_kind.map(|gen_kind| match gen_kind {
1891             hir::GeneratorKind::Gen => "a generator",
1892             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block) => "an async block",
1893             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn) => "an async function",
1894             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Closure) => "an async closure",
1895         })
1896     }
1897
1898     fn find_similar_impl_candidates(
1899         &self,
1900         trait_pred: ty::PolyTraitPredicate<'tcx>,
1901     ) -> Vec<ImplCandidate<'tcx>> {
1902         let mut candidates: Vec<_> = self
1903             .tcx
1904             .all_impls(trait_pred.def_id())
1905             .filter_map(|def_id| {
1906                 if self.tcx.impl_polarity(def_id) == ty::ImplPolarity::Negative
1907                     || !trait_pred
1908                         .skip_binder()
1909                         .is_constness_satisfied_by(self.tcx.constness(def_id))
1910                 {
1911                     return None;
1912                 }
1913
1914                 let imp = self.tcx.impl_trait_ref(def_id).unwrap();
1915
1916                 self.fuzzy_match_tys(trait_pred.skip_binder().self_ty(), imp.self_ty(), false)
1917                     .map(|similarity| ImplCandidate { trait_ref: imp, similarity })
1918             })
1919             .collect();
1920         if candidates.iter().any(|c| matches!(c.similarity, CandidateSimilarity::Exact { .. })) {
1921             // If any of the candidates is a perfect match, we don't want to show all of them.
1922             // This is particularly relevant for the case of numeric types (as they all have the
1923             // same cathegory).
1924             candidates.retain(|c| matches!(c.similarity, CandidateSimilarity::Exact { .. }));
1925         }
1926         candidates
1927     }
1928
1929     fn report_similar_impl_candidates(
1930         &self,
1931         impl_candidates: Vec<ImplCandidate<'tcx>>,
1932         trait_ref: ty::PolyTraitRef<'tcx>,
1933         body_id: hir::HirId,
1934         err: &mut Diagnostic,
1935         other: bool,
1936     ) -> bool {
1937         let other = if other { "other " } else { "" };
1938         let report = |mut candidates: Vec<TraitRef<'tcx>>, err: &mut Diagnostic| {
1939             candidates.sort();
1940             candidates.dedup();
1941             let len = candidates.len();
1942             if candidates.len() == 0 {
1943                 return false;
1944             }
1945             if candidates.len() == 1 {
1946                 let ty_desc = match candidates[0].self_ty().kind() {
1947                     ty::FnPtr(_) => Some("fn pointer"),
1948                     _ => None,
1949                 };
1950                 let the_desc = match ty_desc {
1951                     Some(desc) => format!(" implemented for {} `", desc),
1952                     None => " implemented for `".to_string(),
1953                 };
1954                 err.highlighted_help(vec![
1955                     (
1956                         format!("the trait `{}` ", candidates[0].print_only_trait_path()),
1957                         Style::NoStyle,
1958                     ),
1959                     ("is".to_string(), Style::Highlight),
1960                     (the_desc, Style::NoStyle),
1961                     (candidates[0].self_ty().to_string(), Style::Highlight),
1962                     ("`".to_string(), Style::NoStyle),
1963                 ]);
1964                 return true;
1965             }
1966             let trait_ref = TraitRef::identity(self.tcx, candidates[0].def_id);
1967             // Check if the trait is the same in all cases. If so, we'll only show the type.
1968             let mut traits: Vec<_> =
1969                 candidates.iter().map(|c| c.print_only_trait_path().to_string()).collect();
1970             traits.sort();
1971             traits.dedup();
1972
1973             let mut candidates: Vec<String> = candidates
1974                 .into_iter()
1975                 .map(|c| {
1976                     if traits.len() == 1 {
1977                         format!("\n  {}", c.self_ty())
1978                     } else {
1979                         format!("\n  {}", c)
1980                     }
1981                 })
1982                 .collect();
1983
1984             candidates.sort();
1985             candidates.dedup();
1986             let end = if candidates.len() <= 9 { candidates.len() } else { 8 };
1987             err.help(&format!(
1988                 "the following {other}types implement trait `{}`:{}{}",
1989                 trait_ref.print_only_trait_path(),
1990                 candidates[..end].join(""),
1991                 if len > 9 { format!("\nand {} others", len - 8) } else { String::new() }
1992             ));
1993             true
1994         };
1995
1996         let def_id = trait_ref.def_id();
1997         if impl_candidates.is_empty() {
1998             if self.tcx.trait_is_auto(def_id)
1999                 || self.tcx.lang_items().iter().any(|(_, id)| id == def_id)
2000                 || self.tcx.get_diagnostic_name(def_id).is_some()
2001             {
2002                 // Mentioning implementers of `Copy`, `Debug` and friends is not useful.
2003                 return false;
2004             }
2005             let normalized_impl_candidates: Vec<_> = self
2006                 .tcx
2007                 .all_impls(def_id)
2008                 // Ignore automatically derived impls and `!Trait` impls.
2009                 .filter(|&def_id| {
2010                     self.tcx.impl_polarity(def_id) != ty::ImplPolarity::Negative
2011                         || self.tcx.is_builtin_derive(def_id)
2012                 })
2013                 .filter_map(|def_id| self.tcx.impl_trait_ref(def_id))
2014                 .filter(|trait_ref| {
2015                     let self_ty = trait_ref.self_ty();
2016                     // Avoid mentioning type parameters.
2017                     if let ty::Param(_) = self_ty.kind() {
2018                         false
2019                     }
2020                     // Avoid mentioning types that are private to another crate
2021                     else if let ty::Adt(def, _) = self_ty.peel_refs().kind() {
2022                         // FIXME(compiler-errors): This could be generalized, both to
2023                         // be more granular, and probably look past other `#[fundamental]`
2024                         // types, too.
2025                         self.tcx
2026                             .visibility(def.did())
2027                             .is_accessible_from(body_id.owner.def_id, self.tcx)
2028                     } else {
2029                         true
2030                     }
2031                 })
2032                 .collect();
2033             return report(normalized_impl_candidates, err);
2034         }
2035
2036         // Sort impl candidates so that ordering is consistent for UI tests.
2037         // because the ordering of `impl_candidates` may not be deterministic:
2038         // https://github.com/rust-lang/rust/pull/57475#issuecomment-455519507
2039         //
2040         // Prefer more similar candidates first, then sort lexicographically
2041         // by their normalized string representation.
2042         let mut normalized_impl_candidates_and_similarities = impl_candidates
2043             .into_iter()
2044             .map(|ImplCandidate { trait_ref, similarity }| {
2045                 // FIXME(compiler-errors): This should be using `NormalizeExt::normalize`
2046                 let normalized = self
2047                     .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
2048                     .query_normalize(trait_ref)
2049                     .map_or(trait_ref, |normalized| normalized.value);
2050                 (similarity, normalized)
2051             })
2052             .collect::<Vec<_>>();
2053         normalized_impl_candidates_and_similarities.sort();
2054         normalized_impl_candidates_and_similarities.dedup();
2055
2056         let normalized_impl_candidates = normalized_impl_candidates_and_similarities
2057             .into_iter()
2058             .map(|(_, normalized)| normalized)
2059             .collect::<Vec<_>>();
2060
2061         report(normalized_impl_candidates, err)
2062     }
2063
2064     /// Gets the parent trait chain start
2065     fn get_parent_trait_ref(
2066         &self,
2067         code: &ObligationCauseCode<'tcx>,
2068     ) -> Option<(String, Option<Span>)> {
2069         match code {
2070             ObligationCauseCode::BuiltinDerivedObligation(data) => {
2071                 let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
2072                 match self.get_parent_trait_ref(&data.parent_code) {
2073                     Some(t) => Some(t),
2074                     None => {
2075                         let ty = parent_trait_ref.skip_binder().self_ty();
2076                         let span = TyCategory::from_ty(self.tcx, ty)
2077                             .map(|(_, def_id)| self.tcx.def_span(def_id));
2078                         Some((ty.to_string(), span))
2079                     }
2080                 }
2081             }
2082             ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } => {
2083                 self.get_parent_trait_ref(&parent_code)
2084             }
2085             _ => None,
2086         }
2087     }
2088
2089     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
2090     /// with the same path as `trait_ref`, a help message about
2091     /// a probable version mismatch is added to `err`
2092     fn note_version_mismatch(
2093         &self,
2094         err: &mut Diagnostic,
2095         trait_ref: &ty::PolyTraitRef<'tcx>,
2096     ) -> bool {
2097         let get_trait_impl = |trait_def_id| {
2098             self.tcx.find_map_relevant_impl(trait_def_id, trait_ref.skip_binder().self_ty(), Some)
2099         };
2100         let required_trait_path = self.tcx.def_path_str(trait_ref.def_id());
2101         let traits_with_same_path: std::collections::BTreeSet<_> = self
2102             .tcx
2103             .all_traits()
2104             .filter(|trait_def_id| *trait_def_id != trait_ref.def_id())
2105             .filter(|trait_def_id| self.tcx.def_path_str(*trait_def_id) == required_trait_path)
2106             .collect();
2107         let mut suggested = false;
2108         for trait_with_same_path in traits_with_same_path {
2109             if let Some(impl_def_id) = get_trait_impl(trait_with_same_path) {
2110                 let impl_span = self.tcx.def_span(impl_def_id);
2111                 err.span_help(impl_span, "trait impl with same name found");
2112                 let trait_crate = self.tcx.crate_name(trait_with_same_path.krate);
2113                 let crate_msg = format!(
2114                     "perhaps two different versions of crate `{}` are being used?",
2115                     trait_crate
2116                 );
2117                 err.note(&crate_msg);
2118                 suggested = true;
2119             }
2120         }
2121         suggested
2122     }
2123
2124     fn mk_trait_obligation_with_new_self_ty(
2125         &self,
2126         param_env: ty::ParamEnv<'tcx>,
2127         trait_ref_and_ty: ty::Binder<'tcx, (ty::TraitPredicate<'tcx>, Ty<'tcx>)>,
2128     ) -> PredicateObligation<'tcx> {
2129         let trait_pred =
2130             trait_ref_and_ty.map_bound(|(tr, new_self_ty)| tr.with_self_ty(self.tcx, new_self_ty));
2131
2132         Obligation::new(self.tcx, ObligationCause::dummy(), param_env, trait_pred)
2133     }
2134
2135     #[instrument(skip(self), level = "debug")]
2136     fn maybe_report_ambiguity(
2137         &self,
2138         obligation: &PredicateObligation<'tcx>,
2139         body_id: Option<hir::BodyId>,
2140     ) {
2141         // Unable to successfully determine, probably means
2142         // insufficient type information, but could mean
2143         // ambiguous impls. The latter *ought* to be a
2144         // coherence violation, so we don't report it here.
2145
2146         let predicate = self.resolve_vars_if_possible(obligation.predicate);
2147         let span = obligation.cause.span;
2148
2149         debug!(?predicate, obligation.cause.code = ?obligation.cause.code());
2150
2151         // Ambiguity errors are often caused as fallout from earlier errors.
2152         // We ignore them if this `infcx` is tainted in some cases below.
2153
2154         let bound_predicate = predicate.kind();
2155         let mut err = match bound_predicate.skip_binder() {
2156             ty::PredicateKind::Clause(ty::Clause::Trait(data)) => {
2157                 let trait_ref = bound_predicate.rebind(data.trait_ref);
2158                 debug!(?trait_ref);
2159
2160                 if predicate.references_error() {
2161                     return;
2162                 }
2163
2164                 // This is kind of a hack: it frequently happens that some earlier
2165                 // error prevents types from being fully inferred, and then we get
2166                 // a bunch of uninteresting errors saying something like "<generic
2167                 // #0> doesn't implement Sized".  It may even be true that we
2168                 // could just skip over all checks where the self-ty is an
2169                 // inference variable, but I was afraid that there might be an
2170                 // inference variable created, registered as an obligation, and
2171                 // then never forced by writeback, and hence by skipping here we'd
2172                 // be ignoring the fact that we don't KNOW the type works
2173                 // out. Though even that would probably be harmless, given that
2174                 // we're only talking about builtin traits, which are known to be
2175                 // inhabited. We used to check for `self.tcx.sess.has_errors()` to
2176                 // avoid inundating the user with unnecessary errors, but we now
2177                 // check upstream for type errors and don't add the obligations to
2178                 // begin with in those cases.
2179                 if self.tcx.lang_items().sized_trait() == Some(trait_ref.def_id()) {
2180                     if let None = self.tainted_by_errors() {
2181                         self.emit_inference_failure_err(
2182                             body_id,
2183                             span,
2184                             trait_ref.self_ty().skip_binder().into(),
2185                             ErrorCode::E0282,
2186                             false,
2187                         )
2188                         .emit();
2189                     }
2190                     return;
2191                 }
2192
2193                 // Typically, this ambiguity should only happen if
2194                 // there are unresolved type inference variables
2195                 // (otherwise it would suggest a coherence
2196                 // failure). But given #21974 that is not necessarily
2197                 // the case -- we can have multiple where clauses that
2198                 // are only distinguished by a region, which results
2199                 // in an ambiguity even when all types are fully
2200                 // known, since we don't dispatch based on region
2201                 // relationships.
2202
2203                 // Pick the first substitution that still contains inference variables as the one
2204                 // we're going to emit an error for. If there are none (see above), fall back to
2205                 // a more general error.
2206                 let subst = data.trait_ref.substs.iter().find(|s| s.has_non_region_infer());
2207
2208                 let mut err = if let Some(subst) = subst {
2209                     self.emit_inference_failure_err(body_id, span, subst, ErrorCode::E0283, true)
2210                 } else {
2211                     struct_span_err!(
2212                         self.tcx.sess,
2213                         span,
2214                         E0283,
2215                         "type annotations needed: cannot satisfy `{}`",
2216                         predicate,
2217                     )
2218                 };
2219
2220                 let obligation = obligation.with(self.tcx, trait_ref);
2221                 let mut selcx = SelectionContext::new(&self);
2222                 match selcx.select_from_obligation(&obligation) {
2223                     Ok(None) => {
2224                         let ambiguities =
2225                             ambiguity::recompute_applicable_impls(self.infcx, &obligation);
2226                         let has_non_region_infer =
2227                             trait_ref.skip_binder().substs.types().any(|t| !t.is_ty_infer());
2228                         // It doesn't make sense to talk about applicable impls if there are more
2229                         // than a handful of them.
2230                         if ambiguities.len() > 1 && ambiguities.len() < 10 && has_non_region_infer {
2231                             if self.tainted_by_errors().is_some() && subst.is_none() {
2232                                 // If `subst.is_none()`, then this is probably two param-env
2233                                 // candidates or impl candidates that are equal modulo lifetimes.
2234                                 // Therefore, if we've already emitted an error, just skip this
2235                                 // one, since it's not particularly actionable.
2236                                 err.cancel();
2237                                 return;
2238                             }
2239                             self.annotate_source_of_ambiguity(&mut err, &ambiguities, predicate);
2240                         } else {
2241                             if self.tainted_by_errors().is_some() {
2242                                 err.cancel();
2243                                 return;
2244                             }
2245                             err.note(&format!("cannot satisfy `{}`", predicate));
2246                             let impl_candidates = self.find_similar_impl_candidates(
2247                                 predicate.to_opt_poly_trait_pred().unwrap(),
2248                             );
2249                             if impl_candidates.len() < 10 {
2250                                 self.report_similar_impl_candidates(
2251                                     impl_candidates,
2252                                     trait_ref,
2253                                     body_id.map(|id| id.hir_id).unwrap_or(obligation.cause.body_id),
2254                                     &mut err,
2255                                     false,
2256                                 );
2257                             }
2258                         }
2259                     }
2260                     _ => {
2261                         if self.tainted_by_errors().is_some() {
2262                             err.cancel();
2263                             return;
2264                         }
2265                         err.note(&format!("cannot satisfy `{}`", predicate));
2266                     }
2267                 }
2268
2269                 if let ObligationCauseCode::ItemObligation(def_id)
2270                 | ObligationCauseCode::ExprItemObligation(def_id, ..) = *obligation.cause.code()
2271                 {
2272                     self.suggest_fully_qualified_path(&mut err, def_id, span, trait_ref.def_id());
2273                 }
2274
2275                 if let (Some(body_id), Some(ty::subst::GenericArgKind::Type(_))) =
2276                     (body_id, subst.map(|subst| subst.unpack()))
2277                 {
2278                     let mut expr_finder = FindExprBySpan::new(span);
2279                     expr_finder.visit_expr(&self.tcx.hir().body(body_id).value);
2280
2281                     if let Some(hir::Expr {
2282                         kind: hir::ExprKind::Path(hir::QPath::Resolved(None, path)), .. }
2283                     ) = expr_finder.result
2284                         && let [
2285                             ..,
2286                             trait_path_segment @ hir::PathSegment {
2287                                 res: rustc_hir::def::Res::Def(rustc_hir::def::DefKind::Trait, trait_id),
2288                                 ..
2289                             },
2290                             hir::PathSegment {
2291                                 ident: assoc_item_name,
2292                                 res: rustc_hir::def::Res::Def(_, item_id),
2293                                 ..
2294                             }
2295                         ] = path.segments
2296                         && data.trait_ref.def_id == *trait_id
2297                         && self.tcx.trait_of_item(*item_id) == Some(*trait_id)
2298                         && let None = self.tainted_by_errors()
2299                     {
2300                         let (verb, noun) = match self.tcx.associated_item(item_id).kind {
2301                             ty::AssocKind::Const => ("refer to the", "constant"),
2302                             ty::AssocKind::Fn => ("call", "function"),
2303                             ty::AssocKind::Type => ("refer to the", "type"), // this is already covered by E0223, but this single match arm doesn't hurt here
2304                         };
2305
2306                         // Replace the more general E0283 with a more specific error
2307                         err.cancel();
2308                         err = self.tcx.sess.struct_span_err_with_code(
2309                             span,
2310                             &format!(
2311                                 "cannot {verb} associated {noun} on trait without specifying the corresponding `impl` type",
2312                              ),
2313                             rustc_errors::error_code!(E0790),
2314                         );
2315
2316                         if let Some(local_def_id) = data.trait_ref.def_id.as_local()
2317                             && let Some(hir::Node::Item(hir::Item { ident: trait_name, kind: hir::ItemKind::Trait(_, _, _, _, trait_item_refs), .. })) = self.tcx.hir().find_by_def_id(local_def_id)
2318                             && let Some(method_ref) = trait_item_refs.iter().find(|item_ref| item_ref.ident == *assoc_item_name) {
2319                             err.span_label(method_ref.span, format!("`{}::{}` defined here", trait_name, assoc_item_name));
2320                         }
2321
2322                         err.span_label(span, format!("cannot {verb} associated {noun} of trait"));
2323
2324                         let trait_impls = self.tcx.trait_impls_of(data.trait_ref.def_id);
2325
2326                         if trait_impls.blanket_impls().is_empty()
2327                             && let Some(impl_def_id) = trait_impls.non_blanket_impls().values().flatten().next()
2328                         {
2329                             let non_blanket_impl_count = trait_impls.non_blanket_impls().values().flatten().count();
2330                             let message = if non_blanket_impl_count == 1 {
2331                                 "use the fully-qualified path to the only available implementation".to_string()
2332                             } else {
2333                                 format!(
2334                                     "use a fully-qualified path to a specific available implementation ({} found)",
2335                                     non_blanket_impl_count
2336                                 )
2337                             };
2338                             let mut suggestions = vec![(
2339                                 path.span.shrink_to_lo(),
2340                                 format!("<{} as ", self.tcx.type_of(impl_def_id))
2341                             )];
2342                             if let Some(generic_arg) = trait_path_segment.args {
2343                                 let between_span = trait_path_segment.ident.span.between(generic_arg.span_ext);
2344                                 // get rid of :: between Trait and <type>
2345                                 // must be '::' between them, otherwise the parser won't accept the code
2346                                 suggestions.push((between_span, "".to_string(),));
2347                                 suggestions.push((generic_arg.span_ext.shrink_to_hi(), ">".to_string()));
2348                             } else {
2349                                 suggestions.push((trait_path_segment.ident.span.shrink_to_hi(), ">".to_string()));
2350                             }
2351                             err.multipart_suggestion(
2352                                 message,
2353                                 suggestions,
2354                                 Applicability::MaybeIncorrect
2355                             );
2356                         }
2357                     }
2358                 };
2359
2360                 err
2361             }
2362
2363             ty::PredicateKind::WellFormed(arg) => {
2364                 // Same hacky approach as above to avoid deluging user
2365                 // with error messages.
2366                 if arg.references_error()
2367                     || self.tcx.sess.has_errors().is_some()
2368                     || self.tainted_by_errors().is_some()
2369                 {
2370                     return;
2371                 }
2372
2373                 self.emit_inference_failure_err(body_id, span, arg, ErrorCode::E0282, false)
2374             }
2375
2376             ty::PredicateKind::Subtype(data) => {
2377                 if data.references_error()
2378                     || self.tcx.sess.has_errors().is_some()
2379                     || self.tainted_by_errors().is_some()
2380                 {
2381                     // no need to overload user in such cases
2382                     return;
2383                 }
2384                 let SubtypePredicate { a_is_expected: _, a, b } = data;
2385                 // both must be type variables, or the other would've been instantiated
2386                 assert!(a.is_ty_var() && b.is_ty_var());
2387                 self.emit_inference_failure_err(body_id, span, a.into(), ErrorCode::E0282, true)
2388             }
2389             ty::PredicateKind::Clause(ty::Clause::Projection(data)) => {
2390                 if predicate.references_error() || self.tainted_by_errors().is_some() {
2391                     return;
2392                 }
2393                 let subst = data
2394                     .projection_ty
2395                     .substs
2396                     .iter()
2397                     .chain(Some(data.term.into_arg()))
2398                     .find(|g| g.has_non_region_infer());
2399                 if let Some(subst) = subst {
2400                     let mut err = self.emit_inference_failure_err(
2401                         body_id,
2402                         span,
2403                         subst,
2404                         ErrorCode::E0284,
2405                         true,
2406                     );
2407                     err.note(&format!("cannot satisfy `{}`", predicate));
2408                     err
2409                 } else {
2410                     // If we can't find a substitution, just print a generic error
2411                     let mut err = struct_span_err!(
2412                         self.tcx.sess,
2413                         span,
2414                         E0284,
2415                         "type annotations needed: cannot satisfy `{}`",
2416                         predicate,
2417                     );
2418                     err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2419                     err
2420                 }
2421             }
2422
2423             ty::PredicateKind::ConstEvaluatable(data) => {
2424                 if predicate.references_error() || self.tainted_by_errors().is_some() {
2425                     return;
2426                 }
2427                 let subst = data.walk().find(|g| g.is_non_region_infer());
2428                 if let Some(subst) = subst {
2429                     let err = self.emit_inference_failure_err(
2430                         body_id,
2431                         span,
2432                         subst,
2433                         ErrorCode::E0284,
2434                         true,
2435                     );
2436                     err
2437                 } else {
2438                     // If we can't find a substitution, just print a generic error
2439                     let mut err = struct_span_err!(
2440                         self.tcx.sess,
2441                         span,
2442                         E0284,
2443                         "type annotations needed: cannot satisfy `{}`",
2444                         predicate,
2445                     );
2446                     err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2447                     err
2448                 }
2449             }
2450             _ => {
2451                 if self.tcx.sess.has_errors().is_some() || self.tainted_by_errors().is_some() {
2452                     return;
2453                 }
2454                 let mut err = struct_span_err!(
2455                     self.tcx.sess,
2456                     span,
2457                     E0284,
2458                     "type annotations needed: cannot satisfy `{}`",
2459                     predicate,
2460                 );
2461                 err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2462                 err
2463             }
2464         };
2465         self.note_obligation_cause(&mut err, obligation);
2466         err.emit();
2467     }
2468
2469     fn annotate_source_of_ambiguity(
2470         &self,
2471         err: &mut Diagnostic,
2472         ambiguities: &[ambiguity::Ambiguity],
2473         predicate: ty::Predicate<'tcx>,
2474     ) {
2475         let mut spans = vec![];
2476         let mut crates = vec![];
2477         let mut post = vec![];
2478         let mut has_param_env = false;
2479         for ambiguity in ambiguities {
2480             match ambiguity {
2481                 ambiguity::Ambiguity::DefId(impl_def_id) => {
2482                     match self.tcx.span_of_impl(*impl_def_id) {
2483                         Ok(span) => spans.push(span),
2484                         Err(name) => {
2485                             crates.push(name);
2486                             if let Some(header) = to_pretty_impl_header(self.tcx, *impl_def_id) {
2487                                 post.push(header);
2488                             }
2489                         }
2490                     }
2491                 }
2492                 ambiguity::Ambiguity::ParamEnv(span) => {
2493                     has_param_env = true;
2494                     spans.push(*span);
2495                 }
2496             }
2497         }
2498         let mut crate_names: Vec<_> = crates.iter().map(|n| format!("`{}`", n)).collect();
2499         crate_names.sort();
2500         crate_names.dedup();
2501         post.sort();
2502         post.dedup();
2503
2504         if self.tainted_by_errors().is_some()
2505             && (crate_names.len() == 1
2506                 && spans.len() == 0
2507                 && ["`core`", "`alloc`", "`std`"].contains(&crate_names[0].as_str())
2508                 || predicate.visit_with(&mut HasNumericInferVisitor).is_break())
2509         {
2510             // Avoid complaining about other inference issues for expressions like
2511             // `42 >> 1`, where the types are still `{integer}`, but we want to
2512             // Do we need `trait_ref.skip_binder().self_ty().is_numeric() &&` too?
2513             // NOTE(eddyb) this was `.cancel()`, but `err`
2514             // is borrowed, so we can't fully defuse it.
2515             err.downgrade_to_delayed_bug();
2516             return;
2517         }
2518
2519         let msg = format!(
2520             "multiple `impl`s{} satisfying `{}` found",
2521             if has_param_env { " or `where` clauses" } else { "" },
2522             predicate
2523         );
2524         let post = if post.len() > 1 || (post.len() == 1 && post[0].contains('\n')) {
2525             format!(":\n{}", post.iter().map(|p| format!("- {}", p)).collect::<Vec<_>>().join("\n"),)
2526         } else if post.len() == 1 {
2527             format!(": `{}`", post[0])
2528         } else {
2529             String::new()
2530         };
2531
2532         match (spans.len(), crates.len(), crate_names.len()) {
2533             (0, 0, 0) => {
2534                 err.note(&format!("cannot satisfy `{}`", predicate));
2535             }
2536             (0, _, 1) => {
2537                 err.note(&format!("{} in the `{}` crate{}", msg, crates[0], post,));
2538             }
2539             (0, _, _) => {
2540                 err.note(&format!(
2541                     "{} in the following crates: {}{}",
2542                     msg,
2543                     crate_names.join(", "),
2544                     post,
2545                 ));
2546             }
2547             (_, 0, 0) => {
2548                 let span: MultiSpan = spans.into();
2549                 err.span_note(span, &msg);
2550             }
2551             (_, 1, 1) => {
2552                 let span: MultiSpan = spans.into();
2553                 err.span_note(span, &msg);
2554                 err.note(
2555                     &format!("and another `impl` found in the `{}` crate{}", crates[0], post,),
2556                 );
2557             }
2558             _ => {
2559                 let span: MultiSpan = spans.into();
2560                 err.span_note(span, &msg);
2561                 err.note(&format!(
2562                     "and more `impl`s found in the following crates: {}{}",
2563                     crate_names.join(", "),
2564                     post,
2565                 ));
2566             }
2567         }
2568     }
2569
2570     /// Returns `true` if the trait predicate may apply for *some* assignment
2571     /// to the type parameters.
2572     fn predicate_can_apply(
2573         &self,
2574         param_env: ty::ParamEnv<'tcx>,
2575         pred: ty::PolyTraitPredicate<'tcx>,
2576     ) -> bool {
2577         struct ParamToVarFolder<'a, 'tcx> {
2578             infcx: &'a InferCtxt<'tcx>,
2579             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
2580         }
2581
2582         impl<'a, 'tcx> TypeFolder<'tcx> for ParamToVarFolder<'a, 'tcx> {
2583             fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
2584                 self.infcx.tcx
2585             }
2586
2587             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
2588                 if let ty::Param(ty::ParamTy { name, .. }) = *ty.kind() {
2589                     let infcx = self.infcx;
2590                     *self.var_map.entry(ty).or_insert_with(|| {
2591                         infcx.next_ty_var(TypeVariableOrigin {
2592                             kind: TypeVariableOriginKind::TypeParameterDefinition(name, None),
2593                             span: DUMMY_SP,
2594                         })
2595                     })
2596                 } else {
2597                     ty.super_fold_with(self)
2598                 }
2599             }
2600         }
2601
2602         self.probe(|_| {
2603             let cleaned_pred =
2604                 pred.fold_with(&mut ParamToVarFolder { infcx: self, var_map: Default::default() });
2605
2606             let InferOk { value: cleaned_pred, .. } =
2607                 self.infcx.at(&ObligationCause::dummy(), param_env).normalize(cleaned_pred);
2608
2609             let obligation =
2610                 Obligation::new(self.tcx, ObligationCause::dummy(), param_env, cleaned_pred);
2611
2612             self.predicate_may_hold(&obligation)
2613         })
2614     }
2615
2616     fn note_obligation_cause(&self, err: &mut Diagnostic, obligation: &PredicateObligation<'tcx>) {
2617         // First, attempt to add note to this error with an async-await-specific
2618         // message, and fall back to regular note otherwise.
2619         if !self.maybe_note_obligation_cause_for_async_await(err, obligation) {
2620             self.note_obligation_cause_code(
2621                 err,
2622                 obligation.predicate,
2623                 obligation.param_env,
2624                 obligation.cause.code(),
2625                 &mut vec![],
2626                 &mut Default::default(),
2627             );
2628             self.suggest_unsized_bound_if_applicable(err, obligation);
2629         }
2630     }
2631
2632     #[instrument(level = "debug", skip_all)]
2633     fn suggest_unsized_bound_if_applicable(
2634         &self,
2635         err: &mut Diagnostic,
2636         obligation: &PredicateObligation<'tcx>,
2637     ) {
2638         let ty::PredicateKind::Clause(ty::Clause::Trait(pred)) = obligation.predicate.kind().skip_binder() else { return; };
2639         let (ObligationCauseCode::BindingObligation(item_def_id, span)
2640         | ObligationCauseCode::ExprBindingObligation(item_def_id, span, ..))
2641             = *obligation.cause.code().peel_derives() else { return; };
2642         debug!(?pred, ?item_def_id, ?span);
2643
2644         let (Some(node), true) = (
2645             self.tcx.hir().get_if_local(item_def_id),
2646             Some(pred.def_id()) == self.tcx.lang_items().sized_trait(),
2647         ) else {
2648             return;
2649         };
2650         self.maybe_suggest_unsized_generics(err, span, node);
2651     }
2652
2653     #[instrument(level = "debug", skip_all)]
2654     fn maybe_suggest_unsized_generics(&self, err: &mut Diagnostic, span: Span, node: Node<'tcx>) {
2655         let Some(generics) = node.generics() else {
2656             return;
2657         };
2658         let sized_trait = self.tcx.lang_items().sized_trait();
2659         debug!(?generics.params);
2660         debug!(?generics.predicates);
2661         let Some(param) = generics.params.iter().find(|param| param.span == span) else {
2662             return;
2663         };
2664         // Check that none of the explicit trait bounds is `Sized`. Assume that an explicit
2665         // `Sized` bound is there intentionally and we don't need to suggest relaxing it.
2666         let explicitly_sized = generics
2667             .bounds_for_param(param.def_id)
2668             .flat_map(|bp| bp.bounds)
2669             .any(|bound| bound.trait_ref().and_then(|tr| tr.trait_def_id()) == sized_trait);
2670         if explicitly_sized {
2671             return;
2672         }
2673         debug!(?param);
2674         match node {
2675             hir::Node::Item(
2676                 item @ hir::Item {
2677                     // Only suggest indirection for uses of type parameters in ADTs.
2678                     kind:
2679                         hir::ItemKind::Enum(..) | hir::ItemKind::Struct(..) | hir::ItemKind::Union(..),
2680                     ..
2681                 },
2682             ) => {
2683                 if self.maybe_indirection_for_unsized(err, item, param) {
2684                     return;
2685                 }
2686             }
2687             _ => {}
2688         };
2689         // Didn't add an indirection suggestion, so add a general suggestion to relax `Sized`.
2690         let (span, separator) = if let Some(s) = generics.bounds_span_for_suggestions(param.def_id)
2691         {
2692             (s, " +")
2693         } else {
2694             (span.shrink_to_hi(), ":")
2695         };
2696         err.span_suggestion_verbose(
2697             span,
2698             "consider relaxing the implicit `Sized` restriction",
2699             format!("{} ?Sized", separator),
2700             Applicability::MachineApplicable,
2701         );
2702     }
2703
2704     fn maybe_indirection_for_unsized(
2705         &self,
2706         err: &mut Diagnostic,
2707         item: &Item<'tcx>,
2708         param: &GenericParam<'tcx>,
2709     ) -> bool {
2710         // Suggesting `T: ?Sized` is only valid in an ADT if `T` is only used in a
2711         // borrow. `struct S<'a, T: ?Sized>(&'a T);` is valid, `struct S<T: ?Sized>(T);`
2712         // is not. Look for invalid "bare" parameter uses, and suggest using indirection.
2713         let mut visitor =
2714             FindTypeParam { param: param.name.ident().name, invalid_spans: vec![], nested: false };
2715         visitor.visit_item(item);
2716         if visitor.invalid_spans.is_empty() {
2717             return false;
2718         }
2719         let mut multispan: MultiSpan = param.span.into();
2720         multispan.push_span_label(
2721             param.span,
2722             format!("this could be changed to `{}: ?Sized`...", param.name.ident()),
2723         );
2724         for sp in visitor.invalid_spans {
2725             multispan.push_span_label(
2726                 sp,
2727                 format!("...if indirection were used here: `Box<{}>`", param.name.ident()),
2728             );
2729         }
2730         err.span_help(
2731             multispan,
2732             &format!(
2733                 "you could relax the implicit `Sized` bound on `{T}` if it were \
2734                 used through indirection like `&{T}` or `Box<{T}>`",
2735                 T = param.name.ident(),
2736             ),
2737         );
2738         true
2739     }
2740
2741     fn is_recursive_obligation(
2742         &self,
2743         obligated_types: &mut Vec<Ty<'tcx>>,
2744         cause_code: &ObligationCauseCode<'tcx>,
2745     ) -> bool {
2746         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
2747             let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
2748             let self_ty = parent_trait_ref.skip_binder().self_ty();
2749             if obligated_types.iter().any(|ot| ot == &self_ty) {
2750                 return true;
2751             }
2752             if let ty::Adt(def, substs) = self_ty.kind()
2753                 && let [arg] = &substs[..]
2754                 && let ty::subst::GenericArgKind::Type(ty) = arg.unpack()
2755                 && let ty::Adt(inner_def, _) = ty.kind()
2756                 && inner_def == def
2757             {
2758                 return true;
2759             }
2760         }
2761         false
2762     }
2763 }
2764
2765 /// Crude way of getting back an `Expr` from a `Span`.
2766 pub struct FindExprBySpan<'hir> {
2767     pub span: Span,
2768     pub result: Option<&'hir hir::Expr<'hir>>,
2769     pub ty_result: Option<&'hir hir::Ty<'hir>>,
2770 }
2771
2772 impl<'hir> FindExprBySpan<'hir> {
2773     fn new(span: Span) -> Self {
2774         Self { span, result: None, ty_result: None }
2775     }
2776 }
2777
2778 impl<'v> Visitor<'v> for FindExprBySpan<'v> {
2779     fn visit_expr(&mut self, ex: &'v hir::Expr<'v>) {
2780         if self.span == ex.span {
2781             self.result = Some(ex);
2782         } else {
2783             hir::intravisit::walk_expr(self, ex);
2784         }
2785     }
2786     fn visit_ty(&mut self, ty: &'v hir::Ty<'v>) {
2787         if self.span == ty.span {
2788             self.ty_result = Some(ty);
2789         } else {
2790             hir::intravisit::walk_ty(self, ty);
2791         }
2792     }
2793 }
2794
2795 /// Look for type `param` in an ADT being used only through a reference to confirm that suggesting
2796 /// `param: ?Sized` would be a valid constraint.
2797 struct FindTypeParam {
2798     param: rustc_span::Symbol,
2799     invalid_spans: Vec<Span>,
2800     nested: bool,
2801 }
2802
2803 impl<'v> Visitor<'v> for FindTypeParam {
2804     fn visit_where_predicate(&mut self, _: &'v hir::WherePredicate<'v>) {
2805         // Skip where-clauses, to avoid suggesting indirection for type parameters found there.
2806     }
2807
2808     fn visit_ty(&mut self, ty: &hir::Ty<'_>) {
2809         // We collect the spans of all uses of the "bare" type param, like in `field: T` or
2810         // `field: (T, T)` where we could make `T: ?Sized` while skipping cases that are known to be
2811         // valid like `field: &'a T` or `field: *mut T` and cases that *might* have further `Sized`
2812         // obligations like `Box<T>` and `Vec<T>`, but we perform no extra analysis for those cases
2813         // and suggest `T: ?Sized` regardless of their obligations. This is fine because the errors
2814         // in that case should make what happened clear enough.
2815         match ty.kind {
2816             hir::TyKind::Ptr(_) | hir::TyKind::Ref(..) | hir::TyKind::TraitObject(..) => {}
2817             hir::TyKind::Path(hir::QPath::Resolved(None, path))
2818                 if path.segments.len() == 1 && path.segments[0].ident.name == self.param =>
2819             {
2820                 if !self.nested {
2821                     debug!(?ty, "FindTypeParam::visit_ty");
2822                     self.invalid_spans.push(ty.span);
2823                 }
2824             }
2825             hir::TyKind::Path(_) => {
2826                 let prev = self.nested;
2827                 self.nested = true;
2828                 hir::intravisit::walk_ty(self, ty);
2829                 self.nested = prev;
2830             }
2831             _ => {
2832                 hir::intravisit::walk_ty(self, ty);
2833             }
2834         }
2835     }
2836 }
2837
2838 /// Summarizes information
2839 #[derive(Clone)]
2840 pub enum ArgKind {
2841     /// An argument of non-tuple type. Parameters are (name, ty)
2842     Arg(String, String),
2843
2844     /// An argument of tuple type. For a "found" argument, the span is
2845     /// the location in the source of the pattern. For an "expected"
2846     /// argument, it will be None. The vector is a list of (name, ty)
2847     /// strings for the components of the tuple.
2848     Tuple(Option<Span>, Vec<(String, String)>),
2849 }
2850
2851 impl ArgKind {
2852     fn empty() -> ArgKind {
2853         ArgKind::Arg("_".to_owned(), "_".to_owned())
2854     }
2855
2856     /// Creates an `ArgKind` from the expected type of an
2857     /// argument. It has no name (`_`) and an optional source span.
2858     pub fn from_expected_ty(t: Ty<'_>, span: Option<Span>) -> ArgKind {
2859         match t.kind() {
2860             ty::Tuple(tys) => ArgKind::Tuple(
2861                 span,
2862                 tys.iter().map(|ty| ("_".to_owned(), ty.to_string())).collect::<Vec<_>>(),
2863             ),
2864             _ => ArgKind::Arg("_".to_owned(), t.to_string()),
2865         }
2866     }
2867 }
2868
2869 struct HasNumericInferVisitor;
2870
2871 impl<'tcx> ty::TypeVisitor<'tcx> for HasNumericInferVisitor {
2872     type BreakTy = ();
2873
2874     fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
2875         if matches!(ty.kind(), ty::Infer(ty::FloatVar(_) | ty::IntVar(_))) {
2876             ControlFlow::Break(())
2877         } else {
2878             ControlFlow::CONTINUE
2879         }
2880     }
2881 }
2882
2883 pub enum DefIdOrName {
2884     DefId(DefId),
2885     Name(&'static str),
2886 }