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

   1 mod ambiguity;
   2 pub mod on_unimplemented;
   3 pub mod suggestions;
   4
   5 use super::{
   6     FulfillmentError, FulfillmentErrorCode, MismatchedProjectionTypes, Obligation, ObligationCause,
   7     ObligationCauseCode, ObligationCtxt, OutputTypeParameterMismatch, Overflow,
   8     PredicateObligation, SelectionContext, SelectionError, TraitNotObjectSafe,
   9 };
  10 use crate::infer::error_reporting::{TyCategory, TypeAnnotationNeeded as ErrorCode};
  11 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  12 use crate::infer::{self, InferCtxt};
  13 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
  14 use crate::traits::query::normalize::QueryNormalizeExt as _;
  15 use crate::traits::specialize::to_pretty_impl_header;
  16 use crate::traits::NormalizeExt;
  17 use on_unimplemented::OnUnimplementedNote;
  18 use on_unimplemented::TypeErrCtxtExt as _;
  19 use rustc_data_structures::fx::{FxHashMap, FxIndexMap};
  20 use rustc_errors::{
  21     pluralize, struct_span_err, Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed,
  22     MultiSpan, Style,
  23 };
  24 use rustc_hir as hir;
  25 use rustc_hir::def::Namespace;
  26 use rustc_hir::def_id::DefId;
  27 use rustc_hir::intravisit::Visitor;
  28 use rustc_hir::GenericParam;
  29 use rustc_hir::Item;
  30 use rustc_hir::Node;
  31 use rustc_infer::infer::error_reporting::TypeErrCtxt;
  32 use rustc_infer::infer::{InferOk, TypeTrace};
  33 use rustc_middle::traits::select::OverflowError;
  34 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
  35 use rustc_middle::ty::error::ExpectedFound;
  36 use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable};
  37 use rustc_middle::ty::print::{FmtPrinter, Print};
  38 use rustc_middle::ty::{
  39     self, SubtypePredicate, ToPolyTraitRef, ToPredicate, TraitRef, Ty, TyCtxt, TypeFoldable,
  40     TypeVisitable,
  41 };
  42 use rustc_session::Limit;
  43 use rustc_span::def_id::LOCAL_CRATE;
  44 use rustc_span::symbol::{kw, sym};
  45 use rustc_span::{ExpnKind, Span, DUMMY_SP};
  46 use std::fmt;
  47 use std::iter;
  48 use std::ops::ControlFlow;
  49 use suggestions::TypeErrCtxtExt as _;
  50
  51 pub use rustc_infer::traits::error_reporting::*;
  52
  53 // When outputting impl candidates, prefer showing those that are more similar.
  54 //
  55 // We also compare candidates after skipping lifetimes, which has a lower
  56 // priority than exact matches.
  57 #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
  58 pub enum CandidateSimilarity {
  59     Exact { ignoring_lifetimes: bool },
  60     Fuzzy { ignoring_lifetimes: bool },
  61 }
  62
  63 #[derive(Debug, Clone, Copy)]
  64 pub struct ImplCandidate<'tcx> {
  65     pub trait_ref: ty::TraitRef<'tcx>,
  66     pub similarity: CandidateSimilarity,
  67 }
  68
  69 pub trait InferCtxtExt<'tcx> {
  70     /// Given some node representing a fn-like thing in the HIR map,
  71     /// returns a span and `ArgKind` information that describes the
  72     /// arguments it expects. This can be supplied to
  73     /// `report_arg_count_mismatch`.
  74     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Option<Span>, Vec<ArgKind>)>;
  75
  76     /// Reports an error when the number of arguments needed by a
  77     /// trait match doesn't match the number that the expression
  78     /// provides.
  79     fn report_arg_count_mismatch(
  80         &self,
  81         span: Span,
  82         found_span: Option<Span>,
  83         expected_args: Vec<ArgKind>,
  84         found_args: Vec<ArgKind>,
  85         is_closure: bool,
  86         closure_pipe_span: Option<Span>,
  87     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed>;
  88
  89     /// Checks if the type implements one of `Fn`, `FnMut`, or `FnOnce`
  90     /// in that order, and returns the generic type corresponding to the
  91     /// argument of that trait (corresponding to the closure arguments).
  92     fn type_implements_fn_trait(
  93         &self,
  94         param_env: ty::ParamEnv<'tcx>,
  95         ty: ty::Binder<'tcx, Ty<'tcx>>,
  96         constness: ty::BoundConstness,
  97         polarity: ty::ImplPolarity,
  98     ) -> Result<(ty::ClosureKind, ty::Binder<'tcx, Ty<'tcx>>), ()>;
  99 }
 100
 101 pub trait TypeErrCtxtExt<'tcx> {
 102     fn report_overflow_error<T>(
 103         &self,
 104         predicate: &T,
 105         span: Span,
 106         suggest_increasing_limit: bool,
 107         mutate: impl FnOnce(&mut Diagnostic),
 108     ) -> !
 109     where
 110         T: fmt::Display
 111             + TypeFoldable<'tcx>
 112             + Print<'tcx, FmtPrinter<'tcx, 'tcx>, Output = FmtPrinter<'tcx, 'tcx>>,
 113         <T as Print<'tcx, FmtPrinter<'tcx, 'tcx>>>::Error: std::fmt::Debug;
 114
 115     fn report_fulfillment_errors(
 116         &self,
 117         errors: &[FulfillmentError<'tcx>],
 118         body_id: Option<hir::BodyId>,
 119     ) -> ErrorGuaranteed;
 120
 121     fn report_overflow_obligation<T>(
 122         &self,
 123         obligation: &Obligation<'tcx, T>,
 124         suggest_increasing_limit: bool,
 125     ) -> !
 126     where
 127         T: ToPredicate<'tcx> + Clone;
 128
 129     fn suggest_new_overflow_limit(&self, err: &mut Diagnostic);
 130
 131     fn report_overflow_obligation_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> !;
 132
 133     /// The `root_obligation` parameter should be the `root_obligation` field
 134     /// from a `FulfillmentError`. If no `FulfillmentError` is available,
 135     /// then it should be the same as `obligation`.
 136     fn report_selection_error(
 137         &self,
 138         obligation: PredicateObligation<'tcx>,
 139         root_obligation: &PredicateObligation<'tcx>,
 140         error: &SelectionError<'tcx>,
 141     );
 142 }
 143
 144 impl<'tcx> InferCtxtExt<'tcx> for InferCtxt<'tcx> {
 145     /// Given some node representing a fn-like thing in the HIR map,
 146     /// returns a span and `ArgKind` information that describes the
 147     /// arguments it expects. This can be supplied to
 148     /// `report_arg_count_mismatch`.
 149     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Option<Span>, Vec<ArgKind>)> {
 150         let sm = self.tcx.sess.source_map();
 151         let hir = self.tcx.hir();
 152         Some(match node {
 153             Node::Expr(&hir::Expr {
 154                 kind: hir::ExprKind::Closure(&hir::Closure { body, fn_decl_span, fn_arg_span, .. }),
 155                 ..
 156             }) => (
 157                 fn_decl_span,
 158                 fn_arg_span,
 159                 hir.body(body)
 160                     .params
 161                     .iter()
 162                     .map(|arg| {
 163                         if let hir::Pat { kind: hir::PatKind::Tuple(ref args, _), span, .. } =
 164                             *arg.pat
 165                         {
 166                             Some(ArgKind::Tuple(
 167                                 Some(span),
 168                                 args.iter()
 169                                     .map(|pat| {
 170                                         sm.span_to_snippet(pat.span)
 171                                             .ok()
 172                                             .map(|snippet| (snippet, "_".to_owned()))
 173                                     })
 174                                     .collect::<Option<Vec<_>>>()?,
 175                             ))
 176                         } else {
 177                             let name = sm.span_to_snippet(arg.pat.span).ok()?;
 178                             Some(ArgKind::Arg(name, "_".to_owned()))
 179                         }
 180                     })
 181                     .collect::<Option<Vec<ArgKind>>>()?,
 182             ),
 183             Node::Item(&hir::Item { kind: hir::ItemKind::Fn(ref sig, ..), .. })
 184             | Node::ImplItem(&hir::ImplItem { kind: hir::ImplItemKind::Fn(ref sig, _), .. })
 185             | Node::TraitItem(&hir::TraitItem {
 186                 kind: hir::TraitItemKind::Fn(ref sig, _), ..
 187             }) => (
 188                 sig.span,
 189                 None,
 190                 sig.decl
 191                     .inputs
 192                     .iter()
 193                     .map(|arg| match arg.kind {
 194                         hir::TyKind::Tup(ref tys) => ArgKind::Tuple(
 195                             Some(arg.span),
 196                             vec![("_".to_owned(), "_".to_owned()); tys.len()],
 197                         ),
 198                         _ => ArgKind::empty(),
 199                     })
 200                     .collect::<Vec<ArgKind>>(),
 201             ),
 202             Node::Ctor(ref variant_data) => {
 203                 let span = variant_data.ctor_hir_id().map_or(DUMMY_SP, |id| hir.span(id));
 204                 (span, None, vec![ArgKind::empty(); variant_data.fields().len()])
 205             }
 206             _ => panic!("non-FnLike node found: {:?}", node),
 207         })
 208     }
 209
 210     /// Reports an error when the number of arguments needed by a
 211     /// trait match doesn't match the number that the expression
 212     /// provides.
 213     fn report_arg_count_mismatch(
 214         &self,
 215         span: Span,
 216         found_span: Option<Span>,
 217         expected_args: Vec<ArgKind>,
 218         found_args: Vec<ArgKind>,
 219         is_closure: bool,
 220         closure_arg_span: Option<Span>,
 221     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
 222         let kind = if is_closure { "closure" } else { "function" };
 223
 224         let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
 225             let arg_length = arguments.len();
 226             let distinct = matches!(other, &[ArgKind::Tuple(..)]);
 227             match (arg_length, arguments.get(0)) {
 228                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
 229                     format!("a single {}-tuple as argument", fields.len())
 230                 }
 231                 _ => format!(
 232                     "{} {}argument{}",
 233                     arg_length,
 234                     if distinct && arg_length > 1 { "distinct " } else { "" },
 235                     pluralize!(arg_length)
 236                 ),
 237             }
 238         };
 239
 240         let expected_str = args_str(&expected_args, &found_args);
 241         let found_str = args_str(&found_args, &expected_args);
 242
 243         let mut err = struct_span_err!(
 244             self.tcx.sess,
 245             span,
 246             E0593,
 247             "{} is expected to take {}, but it takes {}",
 248             kind,
 249             expected_str,
 250             found_str,
 251         );
 252
 253         err.span_label(span, format!("expected {} that takes {}", kind, expected_str));
 254
 255         if let Some(found_span) = found_span {
 256             err.span_label(found_span, format!("takes {}", found_str));
 257
 258             // Suggest to take and ignore the arguments with expected_args_length `_`s if
 259             // found arguments is empty (assume the user just wants to ignore args in this case).
 260             // For example, if `expected_args_length` is 2, suggest `|_, _|`.
 261             if found_args.is_empty() && is_closure {
 262                 let underscores = vec!["_"; expected_args.len()].join(", ");
 263                 err.span_suggestion_verbose(
 264                     closure_arg_span.unwrap_or(found_span),
 265                     &format!(
 266                         "consider changing the closure to take and ignore the expected argument{}",
 267                         pluralize!(expected_args.len())
 268                     ),
 269                     format!("|{}|", underscores),
 270                     Applicability::MachineApplicable,
 271                 );
 272             }
 273
 274             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
 275                 if fields.len() == expected_args.len() {
 276                     let sugg = fields
 277                         .iter()
 278                         .map(|(name, _)| name.to_owned())
 279                         .collect::<Vec<String>>()
 280                         .join(", ");
 281                     err.span_suggestion_verbose(
 282                         found_span,
 283                         "change the closure to take multiple arguments instead of a single tuple",
 284                         format!("|{}|", sugg),
 285                         Applicability::MachineApplicable,
 286                     );
 287                 }
 288             }
 289             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..]
 290                 && fields.len() == found_args.len()
 291                 && is_closure
 292             {
 293                 let sugg = format!(
 294                     "|({}){}|",
 295                     found_args
 296                         .iter()
 297                         .map(|arg| match arg {
 298                             ArgKind::Arg(name, _) => name.to_owned(),
 299                             _ => "_".to_owned(),
 300                         })
 301                         .collect::<Vec<String>>()
 302                         .join(", "),
 303                     // add type annotations if available
 304                     if found_args.iter().any(|arg| match arg {
 305                         ArgKind::Arg(_, ty) => ty != "_",
 306                         _ => false,
 307                     }) {
 308                         format!(
 309                             ": ({})",
 310                             fields
 311                                 .iter()
 312                                 .map(|(_, ty)| ty.to_owned())
 313                                 .collect::<Vec<String>>()
 314                                 .join(", ")
 315                         )
 316                     } else {
 317                         String::new()
 318                     },
 319                 );
 320                 err.span_suggestion_verbose(
 321                     found_span,
 322                     "change the closure to accept a tuple instead of individual arguments",
 323                     sugg,
 324                     Applicability::MachineApplicable,
 325                 );
 326             }
 327         }
 328
 329         err
 330     }
 331
 332     fn type_implements_fn_trait(
 333         &self,
 334         param_env: ty::ParamEnv<'tcx>,
 335         ty: ty::Binder<'tcx, Ty<'tcx>>,
 336         constness: ty::BoundConstness,
 337         polarity: ty::ImplPolarity,
 338     ) -> Result<(ty::ClosureKind, ty::Binder<'tcx, Ty<'tcx>>), ()> {
 339         self.commit_if_ok(|_| {
 340             for trait_def_id in [
 341                 self.tcx.lang_items().fn_trait(),
 342                 self.tcx.lang_items().fn_mut_trait(),
 343                 self.tcx.lang_items().fn_once_trait(),
 344             ] {
 345                 let Some(trait_def_id) = trait_def_id else { continue };
 346                 // Make a fresh inference variable so we can determine what the substitutions
 347                 // of the trait are.
 348                 let var = self.next_ty_var(TypeVariableOrigin {
 349                     span: DUMMY_SP,
 350                     kind: TypeVariableOriginKind::MiscVariable,
 351                 });
 352                 let trait_ref = self.tcx.mk_trait_ref(trait_def_id, [ty.skip_binder(), var]);
 353                 let obligation = Obligation::new(
 354                     self.tcx,
 355                     ObligationCause::dummy(),
 356                     param_env,
 357                     ty.rebind(ty::TraitPredicate { trait_ref, constness, polarity }),
 358                 );
 359                 let ocx = ObligationCtxt::new_in_snapshot(self);
 360                 ocx.register_obligation(obligation);
 361                 if ocx.select_all_or_error().is_empty() {
 362                     return Ok((
 363                         self.tcx
 364                             .fn_trait_kind_from_def_id(trait_def_id)
 365                             .expect("expected to map DefId to ClosureKind"),
 366                         ty.rebind(self.resolve_vars_if_possible(var)),
 367                     ));
 368                 }
 369             }
 370
 371             Err(())
 372         })
 373     }
 374 }
 375 impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
 376     fn report_fulfillment_errors(
 377         &self,
 378         errors: &[FulfillmentError<'tcx>],
 379         body_id: Option<hir::BodyId>,
 380     ) -> ErrorGuaranteed {
 381         #[derive(Debug)]
 382         struct ErrorDescriptor<'tcx> {
 383             predicate: ty::Predicate<'tcx>,
 384             index: Option<usize>, // None if this is an old error
 385         }
 386
 387         let mut error_map: FxIndexMap<_, Vec<_>> = self
 388             .reported_trait_errors
 389             .borrow()
 390             .iter()
 391             .map(|(&span, predicates)| {
 392                 (
 393                     span,
 394                     predicates
 395                         .iter()
 396                         .map(|&predicate| ErrorDescriptor { predicate, index: None })
 397                         .collect(),
 398                 )
 399             })
 400             .collect();
 401
 402         for (index, error) in errors.iter().enumerate() {
 403             // We want to ignore desugarings here: spans are equivalent even
 404             // if one is the result of a desugaring and the other is not.
 405             let mut span = error.obligation.cause.span;
 406             let expn_data = span.ctxt().outer_expn_data();
 407             if let ExpnKind::Desugaring(_) = expn_data.kind {
 408                 span = expn_data.call_site;
 409             }
 410
 411             error_map.entry(span).or_default().push(ErrorDescriptor {
 412                 predicate: error.obligation.predicate,
 413                 index: Some(index),
 414             });
 415
 416             self.reported_trait_errors
 417                 .borrow_mut()
 418                 .entry(span)
 419                 .or_default()
 420                 .push(error.obligation.predicate);
 421         }
 422
 423         // We do this in 2 passes because we want to display errors in order, though
 424         // maybe it *is* better to sort errors by span or something.
 425         let mut is_suppressed = vec![false; errors.len()];
 426         for (_, error_set) in error_map.iter() {
 427             // We want to suppress "duplicate" errors with the same span.
 428             for error in error_set {
 429                 if let Some(index) = error.index {
 430                     // Suppress errors that are either:
 431                     // 1) strictly implied by another error.
 432                     // 2) implied by an error with a smaller index.
 433                     for error2 in error_set {
 434                         if error2.index.map_or(false, |index2| is_suppressed[index2]) {
 435                             // Avoid errors being suppressed by already-suppressed
 436                             // errors, to prevent all errors from being suppressed
 437                             // at once.
 438                             continue;
 439                         }
 440
 441                         if self.error_implies(error2.predicate, error.predicate)
 442                             && !(error2.index >= error.index
 443                                 && self.error_implies(error.predicate, error2.predicate))
 444                         {
 445                             info!("skipping {:?} (implied by {:?})", error, error2);
 446                             is_suppressed[index] = true;
 447                             break;
 448                         }
 449                     }
 450                 }
 451             }
 452         }
 453
 454         for (error, suppressed) in iter::zip(errors, is_suppressed) {
 455             if !suppressed {
 456                 self.report_fulfillment_error(error, body_id);
 457             }
 458         }
 459
 460         self.tcx.sess.delay_span_bug(DUMMY_SP, "expected fullfillment errors")
 461     }
 462
 463     /// Reports that an overflow has occurred and halts compilation. We
 464     /// halt compilation unconditionally because it is important that
 465     /// overflows never be masked -- they basically represent computations
 466     /// whose result could not be truly determined and thus we can't say
 467     /// if the program type checks or not -- and they are unusual
 468     /// occurrences in any case.
 469     fn report_overflow_error<T>(
 470         &self,
 471         predicate: &T,
 472         span: Span,
 473         suggest_increasing_limit: bool,
 474         mutate: impl FnOnce(&mut Diagnostic),
 475     ) -> !
 476     where
 477         T: fmt::Display
 478             + TypeFoldable<'tcx>
 479             + Print<'tcx, FmtPrinter<'tcx, 'tcx>, Output = FmtPrinter<'tcx, 'tcx>>,
 480         <T as Print<'tcx, FmtPrinter<'tcx, 'tcx>>>::Error: std::fmt::Debug,
 481     {
 482         let predicate = self.resolve_vars_if_possible(predicate.clone());
 483         let mut pred_str = predicate.to_string();
 484
 485         if pred_str.len() > 50 {
 486             // We don't need to save the type to a file, we will be talking about this type already
 487             // in a separate note when we explain the obligation, so it will be available that way.
 488             pred_str = predicate
 489                 .print(FmtPrinter::new_with_limit(
 490                     self.tcx,
 491                     Namespace::TypeNS,
 492                     rustc_session::Limit(6),
 493                 ))
 494                 .unwrap()
 495                 .into_buffer();
 496         }
 497         let mut err = struct_span_err!(
 498             self.tcx.sess,
 499             span,
 500             E0275,
 501             "overflow evaluating the requirement `{}`",
 502             pred_str,
 503         );
 504
 505         if suggest_increasing_limit {
 506             self.suggest_new_overflow_limit(&mut err);
 507         }
 508
 509         mutate(&mut err);
 510
 511         err.emit();
 512         self.tcx.sess.abort_if_errors();
 513         bug!();
 514     }
 515
 516     /// Reports that an overflow has occurred and halts compilation. We
 517     /// halt compilation unconditionally because it is important that
 518     /// overflows never be masked -- they basically represent computations
 519     /// whose result could not be truly determined and thus we can't say
 520     /// if the program type checks or not -- and they are unusual
 521     /// occurrences in any case.
 522     fn report_overflow_obligation<T>(
 523         &self,
 524         obligation: &Obligation<'tcx, T>,
 525         suggest_increasing_limit: bool,
 526     ) -> !
 527     where
 528         T: ToPredicate<'tcx> + Clone,
 529     {
 530         let predicate = obligation.predicate.clone().to_predicate(self.tcx);
 531         let predicate = self.resolve_vars_if_possible(predicate);
 532         self.report_overflow_error(
 533             &predicate,
 534             obligation.cause.span,
 535             suggest_increasing_limit,
 536             |err| {
 537                 self.note_obligation_cause_code(
 538                     err,
 539                     &predicate,
 540                     obligation.param_env,
 541                     obligation.cause.code(),
 542                     &mut vec![],
 543                     &mut Default::default(),
 544                 );
 545             },
 546         );
 547     }
 548
 549     fn suggest_new_overflow_limit(&self, err: &mut Diagnostic) {
 550         let suggested_limit = match self.tcx.recursion_limit() {
 551             Limit(0) => Limit(2),
 552             limit => limit * 2,
 553         };
 554         err.help(&format!(
 555             "consider increasing the recursion limit by adding a \
 556              `#![recursion_limit = \"{}\"]` attribute to your crate (`{}`)",
 557             suggested_limit,
 558             self.tcx.crate_name(LOCAL_CRATE),
 559         ));
 560     }
 561
 562     /// Reports that a cycle was detected which led to overflow and halts
 563     /// compilation. This is equivalent to `report_overflow_obligation` except
 564     /// that we can give a more helpful error message (and, in particular,
 565     /// we do not suggest increasing the overflow limit, which is not
 566     /// going to help).
 567     fn report_overflow_obligation_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 568         let cycle = self.resolve_vars_if_possible(cycle.to_owned());
 569         assert!(!cycle.is_empty());
 570
 571         debug!(?cycle, "report_overflow_error_cycle");
 572
 573         // The 'deepest' obligation is most likely to have a useful
 574         // cause 'backtrace'
 575         self.report_overflow_obligation(
 576             cycle.iter().max_by_key(|p| p.recursion_depth).unwrap(),
 577             false,
 578         );
 579     }
 580
 581     fn report_selection_error(
 582         &self,
 583         mut obligation: PredicateObligation<'tcx>,
 584         root_obligation: &PredicateObligation<'tcx>,
 585         error: &SelectionError<'tcx>,
 586     ) {
 587         let tcx = self.tcx;
 588         let mut span = obligation.cause.span;
 589         // FIXME: statically guarantee this by tainting after the diagnostic is emitted
 590         self.set_tainted_by_errors(
 591             tcx.sess.delay_span_bug(span, "`report_selection_error` did not emit an error"),
 592         );
 593
 594         let mut err = match *error {
 595             SelectionError::Unimplemented => {
 596                 // If this obligation was generated as a result of well-formedness checking, see if we
 597                 // can get a better error message by performing HIR-based well-formedness checking.
 598                 if let ObligationCauseCode::WellFormed(Some(wf_loc)) =
 599                     root_obligation.cause.code().peel_derives()
 600                 {
 601                     if let Some(cause) = self
 602                         .tcx
 603                         .diagnostic_hir_wf_check((tcx.erase_regions(obligation.predicate), *wf_loc))
 604                     {
 605                         obligation.cause = cause.clone();
 606                         span = obligation.cause.span;
 607                     }
 608                 }
 609                 if let ObligationCauseCode::CompareImplItemObligation {
 610                     impl_item_def_id,
 611                     trait_item_def_id,
 612                     kind: _,
 613                 } = *obligation.cause.code()
 614                 {
 615                     self.report_extra_impl_obligation(
 616                         span,
 617                         impl_item_def_id,
 618                         trait_item_def_id,
 619                         &format!("`{}`", obligation.predicate),
 620                     )
 621                     .emit();
 622                     return;
 623                 }
 624
 625                 let bound_predicate = obligation.predicate.kind();
 626                 match bound_predicate.skip_binder() {
 627                     ty::PredicateKind::Clause(ty::Clause::Trait(trait_predicate)) => {
 628                         let trait_predicate = bound_predicate.rebind(trait_predicate);
 629                         let mut trait_predicate = self.resolve_vars_if_possible(trait_predicate);
 630
 631                         trait_predicate.remap_constness_diag(obligation.param_env);
 632                         let predicate_is_const = ty::BoundConstness::ConstIfConst
 633                             == trait_predicate.skip_binder().constness;
 634
 635                         if self.tcx.sess.has_errors().is_some()
 636                             && trait_predicate.references_error()
 637                         {
 638                             return;
 639                         }
 640                         let trait_ref = trait_predicate.to_poly_trait_ref();
 641                         let (post_message, pre_message, type_def) = self
 642                             .get_parent_trait_ref(obligation.cause.code())
 643                             .map(|(t, s)| {
 644                                 (
 645                                     format!(" in `{}`", t),
 646                                     format!("within `{}`, ", t),
 647                                     s.map(|s| (format!("within this `{}`", t), s)),
 648                                 )
 649                             })
 650                             .unwrap_or_default();
 651
 652                         let OnUnimplementedNote {
 653                             message,
 654                             label,
 655                             note,
 656                             parent_label,
 657                             append_const_msg,
 658                         } = self.on_unimplemented_note(trait_ref, &obligation);
 659                         let have_alt_message = message.is_some() || label.is_some();
 660                         let is_try_conversion = self.is_try_conversion(span, trait_ref.def_id());
 661                         let is_unsize =
 662                             Some(trait_ref.def_id()) == self.tcx.lang_items().unsize_trait();
 663                         let (message, note, append_const_msg) = if is_try_conversion {
 664                             (
 665                                 Some(format!(
 666                                     "`?` couldn't convert the error to `{}`",
 667                                     trait_ref.skip_binder().self_ty(),
 668                                 )),
 669                                 Some(
 670                                     "the question mark operation (`?`) implicitly performs a \
 671                                      conversion on the error value using the `From` trait"
 672                                         .to_owned(),
 673                                 ),
 674                                 Some(None),
 675                             )
 676                         } else {
 677                             (message, note, append_const_msg)
 678                         };
 679
 680                         let mut err = struct_span_err!(
 681                             self.tcx.sess,
 682                             span,
 683                             E0277,
 684                             "{}",
 685                             message
 686                                 .and_then(|cannot_do_this| {
 687                                     match (predicate_is_const, append_const_msg) {
 688                                         // do nothing if predicate is not const
 689                                         (false, _) => Some(cannot_do_this),
 690                                         // suggested using default post message
 691                                         (true, Some(None)) => {
 692                                             Some(format!("{cannot_do_this} in const contexts"))
 693                                         }
 694                                         // overridden post message
 695                                         (true, Some(Some(post_message))) => {
 696                                             Some(format!("{cannot_do_this}{post_message}"))
 697                                         }
 698                                         // fallback to generic message
 699                                         (true, None) => None,
 700                                     }
 701                                 })
 702                                 .unwrap_or_else(|| format!(
 703                                     "the trait bound `{}` is not satisfied{}",
 704                                     trait_predicate, post_message,
 705                                 ))
 706                         );
 707
 708                         if is_try_conversion && let Some(ret_span) = self.return_type_span(&obligation) {
 709                             err.span_label(
 710                                 ret_span,
 711                                 &format!(
 712                                     "expected `{}` because of this",
 713                                     trait_ref.skip_binder().self_ty()
 714                                 ),
 715                             );
 716                         }
 717
 718                         if Some(trait_ref.def_id()) == tcx.lang_items().tuple_trait() {
 719                             match obligation.cause.code().peel_derives() {
 720                                 ObligationCauseCode::RustCall => {
 721                                     err.set_primary_message("functions with the \"rust-call\" ABI must take a single non-self tuple argument");
 722                                 }
 723                                 ObligationCauseCode::BindingObligation(def_id, _)
 724                                 | ObligationCauseCode::ItemObligation(def_id)
 725                                     if tcx.is_fn_trait(*def_id) =>
 726                                 {
 727                                     err.code(rustc_errors::error_code!(E0059));
 728                                     err.set_primary_message(format!(
 729                                         "type parameter to bare `{}` trait must be a tuple",
 730                                         tcx.def_path_str(*def_id)
 731                                     ));
 732                                 }
 733                                 _ => {}
 734                             }
 735                         }
 736
 737                         if Some(trait_ref.def_id()) == tcx.lang_items().drop_trait()
 738                             && predicate_is_const
 739                         {
 740                             err.note("`~const Drop` was renamed to `~const Destruct`");
 741                             err.note("See <https://github.com/rust-lang/rust/pull/94901> for more details");
 742                         }
 743
 744                         let explanation = if let ObligationCauseCode::MainFunctionType =
 745                             obligation.cause.code()
 746                         {
 747                             "consider using `()`, or a `Result`".to_owned()
 748                         } else {
 749                             let ty_desc = match trait_ref.skip_binder().self_ty().kind() {
 750                                 ty::FnDef(_, _) => Some("fn item"),
 751                                 ty::Closure(_, _) => Some("closure"),
 752                                 _ => None,
 753                             };
 754
 755                             match ty_desc {
 756                                 Some(desc) => format!(
 757                                     "{}the trait `{}` is not implemented for {} `{}`",
 758                                     pre_message,
 759                                     trait_predicate.print_modifiers_and_trait_path(),
 760                                     desc,
 761                                     trait_ref.skip_binder().self_ty(),
 762                                 ),
 763                                 None => format!(
 764                                     "{}the trait `{}` is not implemented for `{}`",
 765                                     pre_message,
 766                                     trait_predicate.print_modifiers_and_trait_path(),
 767                                     trait_ref.skip_binder().self_ty(),
 768                                 ),
 769                             }
 770                         };
 771
 772                         if self.suggest_add_reference_to_arg(
 773                             &obligation,
 774                             &mut err,
 775                             trait_predicate,
 776                             have_alt_message,
 777                         ) {
 778                             self.note_obligation_cause(&mut err, &obligation);
 779                             err.emit();
 780                             return;
 781                         }
 782                         if let Some(ref s) = label {
 783                             // If it has a custom `#[rustc_on_unimplemented]`
 784                             // error message, let's display it as the label!
 785                             err.span_label(span, s);
 786                             if !matches!(trait_ref.skip_binder().self_ty().kind(), ty::Param(_)) {
 787                                 // When the self type is a type param We don't need to "the trait
 788                                 // `std::marker::Sized` is not implemented for `T`" as we will point
 789                                 // at the type param with a label to suggest constraining it.
 790                                 err.help(&explanation);
 791                             }
 792                         } else {
 793                             err.span_label(span, explanation);
 794                         }
 795
 796                         if let ObligationCauseCode::ObjectCastObligation(concrete_ty, obj_ty) = obligation.cause.code().peel_derives() &&
 797                             Some(trait_ref.def_id()) == self.tcx.lang_items().sized_trait() {
 798                             self.suggest_borrowing_for_object_cast(&mut err, &root_obligation, *concrete_ty, *obj_ty);
 799                         }
 800
 801                         let mut unsatisfied_const = false;
 802                         if trait_predicate.is_const_if_const() && obligation.param_env.is_const() {
 803                             let non_const_predicate = trait_ref.without_const();
 804                             let non_const_obligation = Obligation {
 805                                 cause: obligation.cause.clone(),
 806                                 param_env: obligation.param_env.without_const(),
 807                                 predicate: non_const_predicate.to_predicate(tcx),
 808                                 recursion_depth: obligation.recursion_depth,
 809                             };
 810                             if self.predicate_may_hold(&non_const_obligation) {
 811                                 unsatisfied_const = true;
 812                                 err.span_note(
 813                                     span,
 814                                     &format!(
 815                                         "the trait `{}` is implemented for `{}`, \
 816                                         but that implementation is not `const`",
 817                                         non_const_predicate.print_modifiers_and_trait_path(),
 818                                         trait_ref.skip_binder().self_ty(),
 819                                     ),
 820                                 );
 821                             }
 822                         }
 823
 824                         if let Some((msg, span)) = type_def {
 825                             err.span_label(span, &msg);
 826                         }
 827                         if let Some(ref s) = note {
 828                             // If it has a custom `#[rustc_on_unimplemented]` note, let's display it
 829                             err.note(s.as_str());
 830                         }
 831                         if let Some(ref s) = parent_label {
 832                             let body = tcx
 833                                 .hir()
 834                                 .opt_local_def_id(obligation.cause.body_id)
 835                                 .unwrap_or_else(|| {
 836                                     tcx.hir().body_owner_def_id(hir::BodyId {
 837                                         hir_id: obligation.cause.body_id,
 838                                     })
 839                                 });
 840                             err.span_label(tcx.def_span(body), s);
 841                         }
 842
 843                         self.suggest_floating_point_literal(&obligation, &mut err, &trait_ref);
 844                         self.suggest_dereferencing_index(&obligation, &mut err, trait_predicate);
 845                         let mut suggested =
 846                             self.suggest_dereferences(&obligation, &mut err, trait_predicate);
 847                         suggested |= self.suggest_fn_call(&obligation, &mut err, trait_predicate);
 848                         suggested |=
 849                             self.suggest_remove_reference(&obligation, &mut err, trait_predicate);
 850                         suggested |= self.suggest_semicolon_removal(
 851                             &obligation,
 852                             &mut err,
 853                             span,
 854                             trait_predicate,
 855                         );
 856                         self.note_version_mismatch(&mut err, &trait_ref);
 857                         self.suggest_remove_await(&obligation, &mut err);
 858                         self.suggest_derive(&obligation, &mut err, trait_predicate);
 859
 860                         if Some(trait_ref.def_id()) == tcx.lang_items().try_trait() {
 861                             self.suggest_await_before_try(
 862                                 &mut err,
 863                                 &obligation,
 864                                 trait_predicate,
 865                                 span,
 866                             );
 867                         }
 868
 869                         if self.suggest_impl_trait(&mut err, span, &obligation, trait_predicate) {
 870                             err.emit();
 871                             return;
 872                         }
 873
 874                         if is_unsize {
 875                             // If the obligation failed due to a missing implementation of the
 876                             // `Unsize` trait, give a pointer to why that might be the case
 877                             err.note(
 878                                 "all implementations of `Unsize` are provided \
 879                                 automatically by the compiler, see \
 880                                 <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> \
 881                                 for more information",
 882                             );
 883                         }
 884
 885                         let is_fn_trait = tcx.is_fn_trait(trait_ref.def_id());
 886                         let is_target_feature_fn = if let ty::FnDef(def_id, _) =
 887                             *trait_ref.skip_binder().self_ty().kind()
 888                         {
 889                             !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
 890                         } else {
 891                             false
 892                         };
 893                         if is_fn_trait && is_target_feature_fn {
 894                             err.note(
 895                                 "`#[target_feature]` functions do not implement the `Fn` traits",
 896                             );
 897                         }
 898
 899                         // Try to report a help message
 900                         if is_fn_trait
 901                             && let Ok((implemented_kind, params)) = self.type_implements_fn_trait(
 902                             obligation.param_env,
 903                             trait_ref.self_ty(),
 904                             trait_predicate.skip_binder().constness,
 905                             trait_predicate.skip_binder().polarity,
 906                         )
 907                         {
 908                             // If the type implements `Fn`, `FnMut`, or `FnOnce`, suppress the following
 909                             // suggestion to add trait bounds for the type, since we only typically implement
 910                             // these traits once.
 911
 912                             // Note if the `FnMut` or `FnOnce` is less general than the trait we're trying
 913                             // to implement.
 914                             let selected_kind =
 915                                 self.tcx.fn_trait_kind_from_def_id(trait_ref.def_id())
 916                                     .expect("expected to map DefId to ClosureKind");
 917                             if !implemented_kind.extends(selected_kind) {
 918                                 err.note(
 919                                     &format!(
 920                                         "`{}` implements `{}`, but it must implement `{}`, which is more general",
 921                                         trait_ref.skip_binder().self_ty(),
 922                                         implemented_kind,
 923                                         selected_kind
 924                                     )
 925                                 );
 926                             }
 927
 928                             // Note any argument mismatches
 929                             let given_ty = params.skip_binder();
 930                             let expected_ty = trait_ref.skip_binder().substs.type_at(1);
 931                             if let ty::Tuple(given) = given_ty.kind()
 932                                 && let ty::Tuple(expected) = expected_ty.kind()
 933                             {
 934                                 if expected.len() != given.len() {
 935                                     // Note number of types that were expected and given
 936                                     err.note(
 937                                         &format!(
 938                                             "expected a closure taking {} argument{}, but one taking {} argument{} was given",
 939                                             given.len(),
 940                                             pluralize!(given.len()),
 941                                             expected.len(),
 942                                             pluralize!(expected.len()),
 943                                         )
 944                                     );
 945                                 } else if !self.same_type_modulo_infer(given_ty, expected_ty) {
 946                                     // Print type mismatch
 947                                     let (expected_args, given_args) =
 948                                         self.cmp(given_ty, expected_ty);
 949                                     err.note_expected_found(
 950                                         &"a closure with arguments",
 951                                         expected_args,
 952                                         &"a closure with arguments",
 953                                         given_args,
 954                                     );
 955                                 }
 956                             }
 957                         } else if !trait_ref.has_non_region_infer()
 958                             && self.predicate_can_apply(obligation.param_env, trait_predicate)
 959                         {
 960                             // If a where-clause may be useful, remind the
 961                             // user that they can add it.
 962                             //
 963                             // don't display an on-unimplemented note, as
 964                             // these notes will often be of the form
 965                             //     "the type `T` can't be frobnicated"
 966                             // which is somewhat confusing.
 967                             self.suggest_restricting_param_bound(
 968                                 &mut err,
 969                                 trait_predicate,
 970                                 None,
 971                                 obligation.cause.body_id,
 972                             );
 973                         } else if !suggested && !unsatisfied_const {
 974                             // Can't show anything else useful, try to find similar impls.
 975                             let impl_candidates = self.find_similar_impl_candidates(trait_predicate);
 976                             if !self.report_similar_impl_candidates(
 977                                 impl_candidates,
 978                                 trait_ref,
 979                                 obligation.cause.body_id,
 980                                 &mut err,
 981                             ) {
 982                                 // This is *almost* equivalent to
 983                                 // `obligation.cause.code().peel_derives()`, but it gives us the
 984                                 // trait predicate for that corresponding root obligation. This
 985                                 // lets us get a derived obligation from a type parameter, like
 986                                 // when calling `string.strip_suffix(p)` where `p` is *not* an
 987                                 // implementer of `Pattern<'_>`.
 988                                 let mut code = obligation.cause.code();
 989                                 let mut trait_pred = trait_predicate;
 990                                 let mut peeled = false;
 991                                 while let Some((parent_code, parent_trait_pred)) = code.parent() {
 992                                     code = parent_code;
 993                                     if let Some(parent_trait_pred) = parent_trait_pred {
 994                                         trait_pred = parent_trait_pred;
 995                                         peeled = true;
 996                                     }
 997                                 }
 998                                 let def_id = trait_pred.def_id();
 999                                 // Mention *all* the `impl`s for the *top most* obligation, the
1000                                 // user might have meant to use one of them, if any found. We skip
1001                                 // auto-traits or fundamental traits that might not be exactly what
1002                                 // the user might expect to be presented with. Instead this is
1003                                 // useful for less general traits.
1004                                 if peeled
1005                                     && !self.tcx.trait_is_auto(def_id)
1006                                     && !self.tcx.lang_items().iter().any(|(_, id)| id == def_id)
1007                                 {
1008                                     let trait_ref = trait_pred.to_poly_trait_ref();
1009                                     let impl_candidates =
1010                                         self.find_similar_impl_candidates(trait_pred);
1011                                     self.report_similar_impl_candidates(
1012                                         impl_candidates,
1013                                         trait_ref,
1014                                         obligation.cause.body_id,
1015                                         &mut err,
1016                                     );
1017                                 }
1018                             }
1019                         }
1020
1021                         // Changing mutability doesn't make a difference to whether we have
1022                         // an `Unsize` impl (Fixes ICE in #71036)
1023                         if !is_unsize {
1024                             self.suggest_change_mut(&obligation, &mut err, trait_predicate);
1025                         }
1026
1027                         // If this error is due to `!: Trait` not implemented but `(): Trait` is
1028                         // implemented, and fallback has occurred, then it could be due to a
1029                         // variable that used to fallback to `()` now falling back to `!`. Issue a
1030                         // note informing about the change in behaviour.
1031                         if trait_predicate.skip_binder().self_ty().is_never()
1032                             && self.fallback_has_occurred
1033                         {
1034                             let predicate = trait_predicate.map_bound(|trait_pred| {
1035                                 trait_pred.with_self_type(self.tcx, self.tcx.mk_unit())
1036                             });
1037                             let unit_obligation = obligation.with(tcx, predicate);
1038                             if self.predicate_may_hold(&unit_obligation) {
1039                                 err.note(
1040                                     "this error might have been caused by changes to \
1041                                     Rust's type-inference algorithm (see issue #48950 \
1042                                     <https://github.com/rust-lang/rust/issues/48950> \
1043                                     for more information)",
1044                                 );
1045                                 err.help("did you intend to use the type `()` here instead?");
1046                             }
1047                         }
1048
1049                         // Return early if the trait is Debug or Display and the invocation
1050                         // originates within a standard library macro, because the output
1051                         // is otherwise overwhelming and unhelpful (see #85844 for an
1052                         // example).
1053
1054                         let in_std_macro =
1055                             match obligation.cause.span.ctxt().outer_expn_data().macro_def_id {
1056                                 Some(macro_def_id) => {
1057                                     let crate_name = tcx.crate_name(macro_def_id.krate);
1058                                     crate_name == sym::std || crate_name == sym::core
1059                                 }
1060                                 None => false,
1061                             };
1062
1063                         if in_std_macro
1064                             && matches!(
1065                                 self.tcx.get_diagnostic_name(trait_ref.def_id()),
1066                                 Some(sym::Debug | sym::Display)
1067                             )
1068                         {
1069                             err.emit();
1070                             return;
1071                         }
1072
1073                         err
1074                     }
1075
1076                     ty::PredicateKind::Subtype(predicate) => {
1077                         // Errors for Subtype predicates show up as
1078                         // `FulfillmentErrorCode::CodeSubtypeError`,
1079                         // not selection error.
1080                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
1081                     }
1082
1083                     ty::PredicateKind::Coerce(predicate) => {
1084                         // Errors for Coerce predicates show up as
1085                         // `FulfillmentErrorCode::CodeSubtypeError`,
1086                         // not selection error.
1087                         span_bug!(span, "coerce requirement gave wrong error: `{:?}`", predicate)
1088                     }
1089
1090                     ty::PredicateKind::Clause(ty::Clause::RegionOutlives(..))
1091                     | ty::PredicateKind::Clause(ty::Clause::Projection(..))
1092                     | ty::PredicateKind::Clause(ty::Clause::TypeOutlives(..)) => {
1093                         let predicate = self.resolve_vars_if_possible(obligation.predicate);
1094                         struct_span_err!(
1095                             self.tcx.sess,
1096                             span,
1097                             E0280,
1098                             "the requirement `{}` is not satisfied",
1099                             predicate
1100                         )
1101                     }
1102
1103                     ty::PredicateKind::ObjectSafe(trait_def_id) => {
1104                         let violations = self.tcx.object_safety_violations(trait_def_id);
1105                         report_object_safety_error(self.tcx, span, trait_def_id, violations)
1106                     }
1107
1108                     ty::PredicateKind::ClosureKind(closure_def_id, closure_substs, kind) => {
1109                         let found_kind = self.closure_kind(closure_substs).unwrap();
1110                         let closure_span = self.tcx.def_span(closure_def_id);
1111                         let mut err = struct_span_err!(
1112                             self.tcx.sess,
1113                             closure_span,
1114                             E0525,
1115                             "expected a closure that implements the `{}` trait, \
1116                              but this closure only implements `{}`",
1117                             kind,
1118                             found_kind
1119                         );
1120
1121                         err.span_label(
1122                             closure_span,
1123                             format!("this closure implements `{}`, not `{}`", found_kind, kind),
1124                         );
1125                         err.span_label(
1126                             obligation.cause.span,
1127                             format!("the requirement to implement `{}` derives from here", kind),
1128                         );
1129
1130                         // Additional context information explaining why the closure only implements
1131                         // a particular trait.
1132                         if let Some(typeck_results) = &self.typeck_results {
1133                             let hir_id = self
1134                                 .tcx
1135                                 .hir()
1136                                 .local_def_id_to_hir_id(closure_def_id.expect_local());
1137                             match (found_kind, typeck_results.closure_kind_origins().get(hir_id)) {
1138                                 (ty::ClosureKind::FnOnce, Some((span, place))) => {
1139                                     err.span_label(
1140                                         *span,
1141                                         format!(
1142                                             "closure is `FnOnce` because it moves the \
1143                                          variable `{}` out of its environment",
1144                                             ty::place_to_string_for_capture(tcx, place)
1145                                         ),
1146                                     );
1147                                 }
1148                                 (ty::ClosureKind::FnMut, Some((span, place))) => {
1149                                     err.span_label(
1150                                         *span,
1151                                         format!(
1152                                             "closure is `FnMut` because it mutates the \
1153                                          variable `{}` here",
1154                                             ty::place_to_string_for_capture(tcx, place)
1155                                         ),
1156                                     );
1157                                 }
1158                                 _ => {}
1159                             }
1160                         }
1161
1162                         err
1163                     }
1164
1165                     ty::PredicateKind::WellFormed(ty) => {
1166                         if !self.tcx.sess.opts.unstable_opts.chalk {
1167                             // WF predicates cannot themselves make
1168                             // errors. They can only block due to
1169                             // ambiguity; otherwise, they always
1170                             // degenerate into other obligations
1171                             // (which may fail).
1172                             span_bug!(span, "WF predicate not satisfied for {:?}", ty);
1173                         } else {
1174                             // FIXME: we'll need a better message which takes into account
1175                             // which bounds actually failed to hold.
1176                             self.tcx.sess.struct_span_err(
1177                                 span,
1178                                 &format!("the type `{}` is not well-formed (chalk)", ty),
1179                             )
1180                         }
1181                     }
1182
1183                     ty::PredicateKind::ConstEvaluatable(..) => {
1184                         // Errors for `ConstEvaluatable` predicates show up as
1185                         // `SelectionError::ConstEvalFailure`,
1186                         // not `Unimplemented`.
1187                         span_bug!(
1188                             span,
1189                             "const-evaluatable requirement gave wrong error: `{:?}`",
1190                             obligation
1191                         )
1192                     }
1193
1194                     ty::PredicateKind::ConstEquate(..) => {
1195                         // Errors for `ConstEquate` predicates show up as
1196                         // `SelectionError::ConstEvalFailure`,
1197                         // not `Unimplemented`.
1198                         span_bug!(
1199                             span,
1200                             "const-equate requirement gave wrong error: `{:?}`",
1201                             obligation
1202                         )
1203                     }
1204
1205                     ty::PredicateKind::Ambiguous => span_bug!(span, "ambiguous"),
1206
1207                     ty::PredicateKind::TypeWellFormedFromEnv(..) => span_bug!(
1208                         span,
1209                         "TypeWellFormedFromEnv predicate should only exist in the environment"
1210                     ),
1211                 }
1212             }
1213
1214             OutputTypeParameterMismatch(found_trait_ref, expected_trait_ref, _) => {
1215                 let found_trait_ref = self.resolve_vars_if_possible(found_trait_ref);
1216                 let expected_trait_ref = self.resolve_vars_if_possible(expected_trait_ref);
1217
1218                 if expected_trait_ref.self_ty().references_error() {
1219                     return;
1220                 }
1221
1222                 let Some(found_trait_ty) = found_trait_ref.self_ty().no_bound_vars() else {
1223                     return;
1224                 };
1225
1226                 let found_did = match *found_trait_ty.kind() {
1227                     ty::Closure(did, _)
1228                     | ty::Foreign(did)
1229                     | ty::FnDef(did, _)
1230                     | ty::Generator(did, ..) => Some(did),
1231                     ty::Adt(def, _) => Some(def.did()),
1232                     _ => None,
1233                 };
1234
1235                 let found_span = found_did.and_then(|did| self.tcx.hir().span_if_local(did));
1236
1237                 if self.reported_closure_mismatch.borrow().contains(&(span, found_span)) {
1238                     // We check closures twice, with obligations flowing in different directions,
1239                     // but we want to complain about them only once.
1240                     return;
1241                 }
1242
1243                 self.reported_closure_mismatch.borrow_mut().insert((span, found_span));
1244
1245                 let mut not_tupled = false;
1246
1247                 let found = match found_trait_ref.skip_binder().substs.type_at(1).kind() {
1248                     ty::Tuple(ref tys) => vec![ArgKind::empty(); tys.len()],
1249                     _ => {
1250                         not_tupled = true;
1251                         vec![ArgKind::empty()]
1252                     }
1253                 };
1254
1255                 let expected_ty = expected_trait_ref.skip_binder().substs.type_at(1);
1256                 let expected = match expected_ty.kind() {
1257                     ty::Tuple(ref tys) => {
1258                         tys.iter().map(|t| ArgKind::from_expected_ty(t, Some(span))).collect()
1259                     }
1260                     _ => {
1261                         not_tupled = true;
1262                         vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())]
1263                     }
1264                 };
1265
1266                 // If this is a `Fn` family trait and either the expected or found
1267                 // is not tupled, then fall back to just a regular mismatch error.
1268                 // This shouldn't be common unless manually implementing one of the
1269                 // traits manually, but don't make it more confusing when it does
1270                 // happen.
1271                 if Some(expected_trait_ref.def_id()) != tcx.lang_items().gen_trait() && not_tupled {
1272                     self.report_and_explain_type_error(
1273                         TypeTrace::poly_trait_refs(
1274                             &obligation.cause,
1275                             true,
1276                             expected_trait_ref,
1277                             found_trait_ref,
1278                         ),
1279                         ty::error::TypeError::Mismatch,
1280                     )
1281                 } else if found.len() == expected.len() {
1282                     self.report_closure_arg_mismatch(
1283                         span,
1284                         found_span,
1285                         found_trait_ref,
1286                         expected_trait_ref,
1287                         obligation.cause.code(),
1288                     )
1289                 } else {
1290                     let (closure_span, closure_arg_span, found) = found_did
1291                         .and_then(|did| {
1292                             let node = self.tcx.hir().get_if_local(did)?;
1293                             let (found_span, closure_arg_span, found) =
1294                                 self.get_fn_like_arguments(node)?;
1295                             Some((Some(found_span), closure_arg_span, found))
1296                         })
1297                         .unwrap_or((found_span, None, found));
1298
1299                     self.report_arg_count_mismatch(
1300                         span,
1301                         closure_span,
1302                         expected,
1303                         found,
1304                         found_trait_ty.is_closure(),
1305                         closure_arg_span,
1306                     )
1307                 }
1308             }
1309
1310             TraitNotObjectSafe(did) => {
1311                 let violations = self.tcx.object_safety_violations(did);
1312                 report_object_safety_error(self.tcx, span, did, violations)
1313             }
1314
1315             SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsInfer) => {
1316                 bug!(
1317                     "MentionsInfer should have been handled in `traits/fulfill.rs` or `traits/select/mod.rs`"
1318                 )
1319             }
1320             SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsParam) => {
1321                 if !self.tcx.features().generic_const_exprs {
1322                     let mut err = self.tcx.sess.struct_span_err(
1323                         span,
1324                         "constant expression depends on a generic parameter",
1325                     );
1326                     // FIXME(const_generics): we should suggest to the user how they can resolve this
1327                     // issue. However, this is currently not actually possible
1328                     // (see https://github.com/rust-lang/rust/issues/66962#issuecomment-575907083).
1329                     //
1330                     // Note that with `feature(generic_const_exprs)` this case should not
1331                     // be reachable.
1332                     err.note("this may fail depending on what value the parameter takes");
1333                     err.emit();
1334                     return;
1335                 }
1336
1337                 match obligation.predicate.kind().skip_binder() {
1338                     ty::PredicateKind::ConstEvaluatable(ct) => {
1339                         let ty::ConstKind::Unevaluated(uv) = ct.kind() else {
1340                             bug!("const evaluatable failed for non-unevaluated const `{ct:?}`");
1341                         };
1342                         let mut err =
1343                             self.tcx.sess.struct_span_err(span, "unconstrained generic constant");
1344                         let const_span = self.tcx.def_span(uv.def.did);
1345                         match self.tcx.sess.source_map().span_to_snippet(const_span) {
1346                             Ok(snippet) => err.help(&format!(
1347                                 "try adding a `where` bound using this expression: `where [(); {}]:`",
1348                                 snippet
1349                             )),
1350                             _ => err.help("consider adding a `where` bound using this expression"),
1351                         };
1352                         err
1353                     }
1354                     _ => {
1355                         span_bug!(
1356                             span,
1357                             "unexpected non-ConstEvaluatable predicate, this should not be reachable"
1358                         )
1359                     }
1360                 }
1361             }
1362
1363             // Already reported in the query.
1364             SelectionError::NotConstEvaluatable(NotConstEvaluatable::Error(_)) => {
1365                 // FIXME(eddyb) remove this once `ErrorGuaranteed` becomes a proof token.
1366                 self.tcx.sess.delay_span_bug(span, "`ErrorGuaranteed` without an error");
1367                 return;
1368             }
1369             // Already reported.
1370             Overflow(OverflowError::Error(_)) => {
1371                 self.tcx.sess.delay_span_bug(span, "`OverflowError` has been reported");
1372                 return;
1373             }
1374             Overflow(_) => {
1375                 bug!("overflow should be handled before the `report_selection_error` path");
1376             }
1377             SelectionError::ErrorReporting => {
1378                 bug!("ErrorReporting Overflow should not reach `report_selection_err` call")
1379             }
1380         };
1381
1382         self.note_obligation_cause(&mut err, &obligation);
1383         self.point_at_returns_when_relevant(&mut err, &obligation);
1384
1385         err.emit();
1386     }
1387 }
1388
1389 trait InferCtxtPrivExt<'tcx> {
1390     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1391     // `error` occurring implies that `cond` occurs.
1392     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool;
1393
1394     fn report_fulfillment_error(
1395         &self,
1396         error: &FulfillmentError<'tcx>,
1397         body_id: Option<hir::BodyId>,
1398     );
1399
1400     fn report_projection_error(
1401         &self,
1402         obligation: &PredicateObligation<'tcx>,
1403         error: &MismatchedProjectionTypes<'tcx>,
1404     );
1405
1406     fn maybe_detailed_projection_msg(
1407         &self,
1408         pred: ty::ProjectionPredicate<'tcx>,
1409         normalized_ty: ty::Term<'tcx>,
1410         expected_ty: ty::Term<'tcx>,
1411     ) -> Option<String>;
1412
1413     fn fuzzy_match_tys(
1414         &self,
1415         a: Ty<'tcx>,
1416         b: Ty<'tcx>,
1417         ignoring_lifetimes: bool,
1418     ) -> Option<CandidateSimilarity>;
1419
1420     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str>;
1421
1422     fn find_similar_impl_candidates(
1423         &self,
1424         trait_pred: ty::PolyTraitPredicate<'tcx>,
1425     ) -> Vec<ImplCandidate<'tcx>>;
1426
1427     fn report_similar_impl_candidates(
1428         &self,
1429         impl_candidates: Vec<ImplCandidate<'tcx>>,
1430         trait_ref: ty::PolyTraitRef<'tcx>,
1431         body_id: hir::HirId,
1432         err: &mut Diagnostic,
1433     ) -> bool;
1434
1435     /// Gets the parent trait chain start
1436     fn get_parent_trait_ref(
1437         &self,
1438         code: &ObligationCauseCode<'tcx>,
1439     ) -> Option<(String, Option<Span>)>;
1440
1441     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1442     /// with the same path as `trait_ref`, a help message about
1443     /// a probable version mismatch is added to `err`
1444     fn note_version_mismatch(
1445         &self,
1446         err: &mut Diagnostic,
1447         trait_ref: &ty::PolyTraitRef<'tcx>,
1448     ) -> bool;
1449
1450     /// Creates a `PredicateObligation` with `new_self_ty` replacing the existing type in the
1451     /// `trait_ref`.
1452     ///
1453     /// For this to work, `new_self_ty` must have no escaping bound variables.
1454     fn mk_trait_obligation_with_new_self_ty(
1455         &self,
1456         param_env: ty::ParamEnv<'tcx>,
1457         trait_ref_and_ty: ty::Binder<'tcx, (ty::TraitPredicate<'tcx>, Ty<'tcx>)>,
1458     ) -> PredicateObligation<'tcx>;
1459
1460     fn maybe_report_ambiguity(
1461         &self,
1462         obligation: &PredicateObligation<'tcx>,
1463         body_id: Option<hir::BodyId>,
1464     );
1465
1466     fn predicate_can_apply(
1467         &self,
1468         param_env: ty::ParamEnv<'tcx>,
1469         pred: ty::PolyTraitPredicate<'tcx>,
1470     ) -> bool;
1471
1472     fn note_obligation_cause(&self, err: &mut Diagnostic, obligation: &PredicateObligation<'tcx>);
1473
1474     fn suggest_unsized_bound_if_applicable(
1475         &self,
1476         err: &mut Diagnostic,
1477         obligation: &PredicateObligation<'tcx>,
1478     );
1479
1480     fn annotate_source_of_ambiguity(
1481         &self,
1482         err: &mut Diagnostic,
1483         impls: &[DefId],
1484         predicate: ty::Predicate<'tcx>,
1485     );
1486
1487     fn maybe_suggest_unsized_generics(&self, err: &mut Diagnostic, span: Span, node: Node<'tcx>);
1488
1489     fn maybe_indirection_for_unsized(
1490         &self,
1491         err: &mut Diagnostic,
1492         item: &'tcx Item<'tcx>,
1493         param: &'tcx GenericParam<'tcx>,
1494     ) -> bool;
1495
1496     fn is_recursive_obligation(
1497         &self,
1498         obligated_types: &mut Vec<Ty<'tcx>>,
1499         cause_code: &ObligationCauseCode<'tcx>,
1500     ) -> bool;
1501 }
1502
1503 impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
1504     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1505     // `error` occurring implies that `cond` occurs.
1506     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool {
1507         if cond == error {
1508             return true;
1509         }
1510
1511         // FIXME: It should be possible to deal with `ForAll` in a cleaner way.
1512         let bound_error = error.kind();
1513         let (cond, error) = match (cond.kind().skip_binder(), bound_error.skip_binder()) {
1514             (
1515                 ty::PredicateKind::Clause(ty::Clause::Trait(..)),
1516                 ty::PredicateKind::Clause(ty::Clause::Trait(error)),
1517             ) => (cond, bound_error.rebind(error)),
1518             _ => {
1519                 // FIXME: make this work in other cases too.
1520                 return false;
1521             }
1522         };
1523
1524         for obligation in super::elaborate_predicates(self.tcx, std::iter::once(cond)) {
1525             let bound_predicate = obligation.predicate.kind();
1526             if let ty::PredicateKind::Clause(ty::Clause::Trait(implication)) =
1527                 bound_predicate.skip_binder()
1528             {
1529                 let error = error.to_poly_trait_ref();
1530                 let implication = bound_predicate.rebind(implication.trait_ref);
1531                 // FIXME: I'm just not taking associated types at all here.
1532                 // Eventually I'll need to implement param-env-aware
1533                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
1534                 let param_env = ty::ParamEnv::empty();
1535                 if self.can_sub(param_env, error, implication).is_ok() {
1536                     debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
1537                     return true;
1538                 }
1539             }
1540         }
1541
1542         false
1543     }
1544
1545     #[instrument(skip(self), level = "debug")]
1546     fn report_fulfillment_error(
1547         &self,
1548         error: &FulfillmentError<'tcx>,
1549         body_id: Option<hir::BodyId>,
1550     ) {
1551         match error.code {
1552             FulfillmentErrorCode::CodeSelectionError(ref selection_error) => {
1553                 self.report_selection_error(
1554                     error.obligation.clone(),
1555                     &error.root_obligation,
1556                     selection_error,
1557                 );
1558             }
1559             FulfillmentErrorCode::CodeProjectionError(ref e) => {
1560                 self.report_projection_error(&error.obligation, e);
1561             }
1562             FulfillmentErrorCode::CodeAmbiguity => {
1563                 self.maybe_report_ambiguity(&error.obligation, body_id);
1564             }
1565             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
1566                 self.report_mismatched_types(
1567                     &error.obligation.cause,
1568                     expected_found.expected,
1569                     expected_found.found,
1570                     err.clone(),
1571                 )
1572                 .emit();
1573             }
1574             FulfillmentErrorCode::CodeConstEquateError(ref expected_found, ref err) => {
1575                 let mut diag = self.report_mismatched_consts(
1576                     &error.obligation.cause,
1577                     expected_found.expected,
1578                     expected_found.found,
1579                     err.clone(),
1580                 );
1581                 let code = error.obligation.cause.code().peel_derives().peel_match_impls();
1582                 if let ObligationCauseCode::BindingObligation(..)
1583                 | ObligationCauseCode::ItemObligation(..)
1584                 | ObligationCauseCode::ExprBindingObligation(..)
1585                 | ObligationCauseCode::ExprItemObligation(..) = code
1586                 {
1587                     self.note_obligation_cause_code(
1588                         &mut diag,
1589                         &error.obligation.predicate,
1590                         error.obligation.param_env,
1591                         code,
1592                         &mut vec![],
1593                         &mut Default::default(),
1594                     );
1595                 }
1596                 diag.emit();
1597             }
1598             FulfillmentErrorCode::CodeCycle(ref cycle) => {
1599                 self.report_overflow_obligation_cycle(cycle);
1600             }
1601         }
1602     }
1603
1604     #[instrument(level = "debug", skip_all)]
1605     fn report_projection_error(
1606         &self,
1607         obligation: &PredicateObligation<'tcx>,
1608         error: &MismatchedProjectionTypes<'tcx>,
1609     ) {
1610         let predicate = self.resolve_vars_if_possible(obligation.predicate);
1611
1612         if predicate.references_error() {
1613             return;
1614         }
1615
1616         self.probe(|_| {
1617             let ocx = ObligationCtxt::new_in_snapshot(self);
1618
1619             // try to find the mismatched types to report the error with.
1620             //
1621             // this can fail if the problem was higher-ranked, in which
1622             // cause I have no idea for a good error message.
1623             let bound_predicate = predicate.kind();
1624             let (values, err) = if let ty::PredicateKind::Clause(ty::Clause::Projection(data)) =
1625                 bound_predicate.skip_binder()
1626             {
1627                 let data = self.replace_bound_vars_with_fresh_vars(
1628                     obligation.cause.span,
1629                     infer::LateBoundRegionConversionTime::HigherRankedType,
1630                     bound_predicate.rebind(data),
1631                 );
1632                 let normalized_ty = ocx.normalize(
1633                     &obligation.cause,
1634                     obligation.param_env,
1635                     self.tcx
1636                         .mk_projection(data.projection_ty.item_def_id, data.projection_ty.substs),
1637                 );
1638
1639                 debug!(?obligation.cause, ?obligation.param_env);
1640
1641                 debug!(?normalized_ty, data.ty = ?data.term);
1642
1643                 let is_normalized_ty_expected = !matches!(
1644                     obligation.cause.code().peel_derives(),
1645                     ObligationCauseCode::ItemObligation(_)
1646                         | ObligationCauseCode::BindingObligation(_, _)
1647                         | ObligationCauseCode::ExprItemObligation(..)
1648                         | ObligationCauseCode::ExprBindingObligation(..)
1649                         | ObligationCauseCode::ObjectCastObligation(..)
1650                         | ObligationCauseCode::OpaqueType
1651                 );
1652                 let expected_ty = data.term.ty().unwrap_or_else(|| self.tcx.ty_error());
1653
1654                 // constrain inference variables a bit more to nested obligations from normalize so
1655                 // we can have more helpful errors.
1656                 ocx.select_where_possible();
1657
1658                 if let Err(new_err) = ocx.eq_exp(
1659                     &obligation.cause,
1660                     obligation.param_env,
1661                     is_normalized_ty_expected,
1662                     normalized_ty,
1663                     expected_ty,
1664                 ) {
1665                     (Some((data, is_normalized_ty_expected, normalized_ty, expected_ty)), new_err)
1666                 } else {
1667                     (None, error.err)
1668                 }
1669             } else {
1670                 (None, error.err)
1671             };
1672
1673             let msg = values
1674                 .and_then(|(predicate, _, normalized_ty, expected_ty)| {
1675                     self.maybe_detailed_projection_msg(
1676                         predicate,
1677                         normalized_ty.into(),
1678                         expected_ty.into(),
1679                     )
1680                 })
1681                 .unwrap_or_else(|| format!("type mismatch resolving `{}`", predicate));
1682             let mut diag = struct_span_err!(self.tcx.sess, obligation.cause.span, E0271, "{msg}");
1683
1684             let secondary_span = match predicate.kind().skip_binder() {
1685                 ty::PredicateKind::Clause(ty::Clause::Projection(proj)) => self
1686                     .tcx
1687                     .opt_associated_item(proj.projection_ty.item_def_id)
1688                     .and_then(|trait_assoc_item| {
1689                         self.tcx
1690                             .trait_of_item(proj.projection_ty.item_def_id)
1691                             .map(|id| (trait_assoc_item, id))
1692                     })
1693                     .and_then(|(trait_assoc_item, id)| {
1694                         let trait_assoc_ident = trait_assoc_item.ident(self.tcx);
1695                         self.tcx.find_map_relevant_impl(id, proj.projection_ty.self_ty(), |did| {
1696                             self.tcx
1697                                 .associated_items(did)
1698                                 .in_definition_order()
1699                                 .find(|assoc| assoc.ident(self.tcx) == trait_assoc_ident)
1700                         })
1701                     })
1702                     .and_then(|item| match self.tcx.hir().get_if_local(item.def_id) {
1703                         Some(
1704                             hir::Node::TraitItem(hir::TraitItem {
1705                                 kind: hir::TraitItemKind::Type(_, Some(ty)),
1706                                 ..
1707                             })
1708                             | hir::Node::ImplItem(hir::ImplItem {
1709                                 kind: hir::ImplItemKind::Type(ty),
1710                                 ..
1711                             }),
1712                         ) => Some((ty.span, format!("type mismatch resolving `{}`", predicate))),
1713                         _ => None,
1714                     }),
1715                 _ => None,
1716             };
1717             self.note_type_err(
1718                 &mut diag,
1719                 &obligation.cause,
1720                 secondary_span,
1721                 values.map(|(_, is_normalized_ty_expected, normalized_ty, expected_ty)| {
1722                     infer::ValuePairs::Terms(ExpectedFound::new(
1723                         is_normalized_ty_expected,
1724                         normalized_ty.into(),
1725                         expected_ty.into(),
1726                     ))
1727                 }),
1728                 err,
1729                 true,
1730                 false,
1731             );
1732             self.note_obligation_cause(&mut diag, obligation);
1733             diag.emit();
1734         });
1735     }
1736
1737     fn maybe_detailed_projection_msg(
1738         &self,
1739         pred: ty::ProjectionPredicate<'tcx>,
1740         normalized_ty: ty::Term<'tcx>,
1741         expected_ty: ty::Term<'tcx>,
1742     ) -> Option<String> {
1743         let trait_def_id = pred.projection_ty.trait_def_id(self.tcx);
1744         let self_ty = pred.projection_ty.self_ty();
1745
1746         if Some(pred.projection_ty.item_def_id) == self.tcx.lang_items().fn_once_output() {
1747             Some(format!(
1748                 "expected `{self_ty}` to be a {fn_kind} that returns `{expected_ty}`, but it returns `{normalized_ty}`",
1749                 fn_kind = self_ty.prefix_string(self.tcx)
1750             ))
1751         } else if Some(trait_def_id) == self.tcx.lang_items().future_trait() {
1752             Some(format!(
1753                 "expected `{self_ty}` to be a future that resolves to `{expected_ty}`, but it resolves to `{normalized_ty}`"
1754             ))
1755         } else if Some(trait_def_id) == self.tcx.get_diagnostic_item(sym::Iterator) {
1756             Some(format!(
1757                 "expected `{self_ty}` to be an iterator that yields `{expected_ty}`, but it yields `{normalized_ty}`"
1758             ))
1759         } else {
1760             None
1761         }
1762     }
1763
1764     fn fuzzy_match_tys(
1765         &self,
1766         mut a: Ty<'tcx>,
1767         mut b: Ty<'tcx>,
1768         ignoring_lifetimes: bool,
1769     ) -> Option<CandidateSimilarity> {
1770         /// returns the fuzzy category of a given type, or None
1771         /// if the type can be equated to any type.
1772         fn type_category(tcx: TyCtxt<'_>, t: Ty<'_>) -> Option<u32> {
1773             match t.kind() {
1774                 ty::Bool => Some(0),
1775                 ty::Char => Some(1),
1776                 ty::Str => Some(2),
1777                 ty::Adt(def, _) if Some(def.did()) == tcx.lang_items().string() => Some(2),
1778                 ty::Int(..)
1779                 | ty::Uint(..)
1780                 | ty::Float(..)
1781                 | ty::Infer(ty::IntVar(..) | ty::FloatVar(..)) => Some(4),
1782                 ty::Ref(..) | ty::RawPtr(..) => Some(5),
1783                 ty::Array(..) | ty::Slice(..) => Some(6),
1784                 ty::FnDef(..) | ty::FnPtr(..) => Some(7),
1785                 ty::Dynamic(..) => Some(8),
1786                 ty::Closure(..) => Some(9),
1787                 ty::Tuple(..) => Some(10),
1788                 ty::Param(..) => Some(11),
1789                 ty::Projection(..) => Some(12),
1790                 ty::Opaque(..) => Some(13),
1791                 ty::Never => Some(14),
1792                 ty::Adt(..) => Some(15),
1793                 ty::Generator(..) => Some(16),
1794                 ty::Foreign(..) => Some(17),
1795                 ty::GeneratorWitness(..) => Some(18),
1796                 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error(_) => None,
1797             }
1798         }
1799
1800         let strip_references = |mut t: Ty<'tcx>| -> Ty<'tcx> {
1801             loop {
1802                 match t.kind() {
1803                     ty::Ref(_, inner, _) | ty::RawPtr(ty::TypeAndMut { ty: inner, .. }) => {
1804                         t = *inner
1805                     }
1806                     _ => break t,
1807                 }
1808             }
1809         };
1810
1811         if !ignoring_lifetimes {
1812             a = strip_references(a);
1813             b = strip_references(b);
1814         }
1815
1816         let cat_a = type_category(self.tcx, a)?;
1817         let cat_b = type_category(self.tcx, b)?;
1818         if a == b {
1819             Some(CandidateSimilarity::Exact { ignoring_lifetimes })
1820         } else if cat_a == cat_b {
1821             match (a.kind(), b.kind()) {
1822                 (ty::Adt(def_a, _), ty::Adt(def_b, _)) => def_a == def_b,
1823                 (ty::Foreign(def_a), ty::Foreign(def_b)) => def_a == def_b,
1824                 // Matching on references results in a lot of unhelpful
1825                 // suggestions, so let's just not do that for now.
1826                 //
1827                 // We still upgrade successful matches to `ignoring_lifetimes: true`
1828                 // to prioritize that impl.
1829                 (ty::Ref(..) | ty::RawPtr(..), ty::Ref(..) | ty::RawPtr(..)) => {
1830                     self.fuzzy_match_tys(a, b, true).is_some()
1831                 }
1832                 _ => true,
1833             }
1834             .then_some(CandidateSimilarity::Fuzzy { ignoring_lifetimes })
1835         } else if ignoring_lifetimes {
1836             None
1837         } else {
1838             self.fuzzy_match_tys(a, b, true)
1839         }
1840     }
1841
1842     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str> {
1843         self.tcx.hir().body(body_id).generator_kind.map(|gen_kind| match gen_kind {
1844             hir::GeneratorKind::Gen => "a generator",
1845             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block) => "an async block",
1846             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn) => "an async function",
1847             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Closure) => "an async closure",
1848         })
1849     }
1850
1851     fn find_similar_impl_candidates(
1852         &self,
1853         trait_pred: ty::PolyTraitPredicate<'tcx>,
1854     ) -> Vec<ImplCandidate<'tcx>> {
1855         let mut candidates: Vec<_> = self
1856             .tcx
1857             .all_impls(trait_pred.def_id())
1858             .filter_map(|def_id| {
1859                 if self.tcx.impl_polarity(def_id) == ty::ImplPolarity::Negative
1860                     || !trait_pred
1861                         .skip_binder()
1862                         .is_constness_satisfied_by(self.tcx.constness(def_id))
1863                 {
1864                     return None;
1865                 }
1866
1867                 let imp = self.tcx.impl_trait_ref(def_id).unwrap();
1868
1869                 self.fuzzy_match_tys(trait_pred.skip_binder().self_ty(), imp.self_ty(), false)
1870                     .map(|similarity| ImplCandidate { trait_ref: imp, similarity })
1871             })
1872             .collect();
1873         if candidates.iter().any(|c| matches!(c.similarity, CandidateSimilarity::Exact { .. })) {
1874             // If any of the candidates is a perfect match, we don't want to show all of them.
1875             // This is particularly relevant for the case of numeric types (as they all have the
1876             // same cathegory).
1877             candidates.retain(|c| matches!(c.similarity, CandidateSimilarity::Exact { .. }));
1878         }
1879         candidates
1880     }
1881
1882     fn report_similar_impl_candidates(
1883         &self,
1884         impl_candidates: Vec<ImplCandidate<'tcx>>,
1885         trait_ref: ty::PolyTraitRef<'tcx>,
1886         body_id: hir::HirId,
1887         err: &mut Diagnostic,
1888     ) -> bool {
1889         let report = |mut candidates: Vec<TraitRef<'tcx>>, err: &mut Diagnostic| {
1890             candidates.sort();
1891             candidates.dedup();
1892             let len = candidates.len();
1893             if candidates.len() == 0 {
1894                 return false;
1895             }
1896             if candidates.len() == 1 {
1897                 let ty_desc = match candidates[0].self_ty().kind() {
1898                     ty::FnPtr(_) => Some("fn pointer"),
1899                     _ => None,
1900                 };
1901                 let the_desc = match ty_desc {
1902                     Some(desc) => format!(" implemented for {} `", desc),
1903                     None => " implemented for `".to_string(),
1904                 };
1905                 err.highlighted_help(vec![
1906                     (
1907                         format!("the trait `{}` ", candidates[0].print_only_trait_path()),
1908                         Style::NoStyle,
1909                     ),
1910                     ("is".to_string(), Style::Highlight),
1911                     (the_desc, Style::NoStyle),
1912                     (candidates[0].self_ty().to_string(), Style::Highlight),
1913                     ("`".to_string(), Style::NoStyle),
1914                 ]);
1915                 return true;
1916             }
1917             let trait_ref = TraitRef::identity(self.tcx, candidates[0].def_id);
1918             // Check if the trait is the same in all cases. If so, we'll only show the type.
1919             let mut traits: Vec<_> =
1920                 candidates.iter().map(|c| c.print_only_trait_path().to_string()).collect();
1921             traits.sort();
1922             traits.dedup();
1923
1924             let mut candidates: Vec<String> = candidates
1925                 .into_iter()
1926                 .map(|c| {
1927                     if traits.len() == 1 {
1928                         format!("\n  {}", c.self_ty())
1929                     } else {
1930                         format!("\n  {}", c)
1931                     }
1932                 })
1933                 .collect();
1934
1935             candidates.sort();
1936             candidates.dedup();
1937             let end = if candidates.len() <= 9 { candidates.len() } else { 8 };
1938             err.help(&format!(
1939                 "the following other types implement trait `{}`:{}{}",
1940                 trait_ref.print_only_trait_path(),
1941                 candidates[..end].join(""),
1942                 if len > 9 { format!("\nand {} others", len - 8) } else { String::new() }
1943             ));
1944             true
1945         };
1946
1947         let def_id = trait_ref.def_id();
1948         if impl_candidates.is_empty() {
1949             if self.tcx.trait_is_auto(def_id)
1950                 || self.tcx.lang_items().iter().any(|(_, id)| id == def_id)
1951                 || self.tcx.get_diagnostic_name(def_id).is_some()
1952             {
1953                 // Mentioning implementers of `Copy`, `Debug` and friends is not useful.
1954                 return false;
1955             }
1956             let normalized_impl_candidates: Vec<_> = self
1957                 .tcx
1958                 .all_impls(def_id)
1959                 // Ignore automatically derived impls and `!Trait` impls.
1960                 .filter(|&def_id| {
1961                     self.tcx.impl_polarity(def_id) != ty::ImplPolarity::Negative
1962                         || self.tcx.is_builtin_derive(def_id)
1963                 })
1964                 .filter_map(|def_id| self.tcx.impl_trait_ref(def_id))
1965                 .filter(|trait_ref| {
1966                     let self_ty = trait_ref.self_ty();
1967                     // Avoid mentioning type parameters.
1968                     if let ty::Param(_) = self_ty.kind() {
1969                         false
1970                     }
1971                     // Avoid mentioning types that are private to another crate
1972                     else if let ty::Adt(def, _) = self_ty.peel_refs().kind() {
1973                         // FIXME(compiler-errors): This could be generalized, both to
1974                         // be more granular, and probably look past other `#[fundamental]`
1975                         // types, too.
1976                         self.tcx
1977                             .visibility(def.did())
1978                             .is_accessible_from(body_id.owner.def_id, self.tcx)
1979                     } else {
1980                         true
1981                     }
1982                 })
1983                 .collect();
1984             return report(normalized_impl_candidates, err);
1985         }
1986
1987         // Sort impl candidates so that ordering is consistent for UI tests.
1988         // because the ordering of `impl_candidates` may not be deterministic:
1989         // https://github.com/rust-lang/rust/pull/57475#issuecomment-455519507
1990         //
1991         // Prefer more similar candidates first, then sort lexicographically
1992         // by their normalized string representation.
1993         let mut normalized_impl_candidates_and_similarities = impl_candidates
1994             .into_iter()
1995             .map(|ImplCandidate { trait_ref, similarity }| {
1996                 // FIXME(compiler-errors): This should be using `NormalizeExt::normalize`
1997                 let normalized = self
1998                     .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
1999                     .query_normalize(trait_ref)
2000                     .map_or(trait_ref, |normalized| normalized.value);
2001                 (similarity, normalized)
2002             })
2003             .collect::<Vec<_>>();
2004         normalized_impl_candidates_and_similarities.sort();
2005         normalized_impl_candidates_and_similarities.dedup();
2006
2007         let normalized_impl_candidates = normalized_impl_candidates_and_similarities
2008             .into_iter()
2009             .map(|(_, normalized)| normalized)
2010             .collect::<Vec<_>>();
2011
2012         report(normalized_impl_candidates, err)
2013     }
2014
2015     /// Gets the parent trait chain start
2016     fn get_parent_trait_ref(
2017         &self,
2018         code: &ObligationCauseCode<'tcx>,
2019     ) -> Option<(String, Option<Span>)> {
2020         match code {
2021             ObligationCauseCode::BuiltinDerivedObligation(data) => {
2022                 let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
2023                 match self.get_parent_trait_ref(&data.parent_code) {
2024                     Some(t) => Some(t),
2025                     None => {
2026                         let ty = parent_trait_ref.skip_binder().self_ty();
2027                         let span = TyCategory::from_ty(self.tcx, ty)
2028                             .map(|(_, def_id)| self.tcx.def_span(def_id));
2029                         Some((ty.to_string(), span))
2030                     }
2031                 }
2032             }
2033             ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } => {
2034                 self.get_parent_trait_ref(&parent_code)
2035             }
2036             _ => None,
2037         }
2038     }
2039
2040     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
2041     /// with the same path as `trait_ref`, a help message about
2042     /// a probable version mismatch is added to `err`
2043     fn note_version_mismatch(
2044         &self,
2045         err: &mut Diagnostic,
2046         trait_ref: &ty::PolyTraitRef<'tcx>,
2047     ) -> bool {
2048         let get_trait_impl = |trait_def_id| {
2049             self.tcx.find_map_relevant_impl(trait_def_id, trait_ref.skip_binder().self_ty(), Some)
2050         };
2051         let required_trait_path = self.tcx.def_path_str(trait_ref.def_id());
2052         let traits_with_same_path: std::collections::BTreeSet<_> = self
2053             .tcx
2054             .all_traits()
2055             .filter(|trait_def_id| *trait_def_id != trait_ref.def_id())
2056             .filter(|trait_def_id| self.tcx.def_path_str(*trait_def_id) == required_trait_path)
2057             .collect();
2058         let mut suggested = false;
2059         for trait_with_same_path in traits_with_same_path {
2060             if let Some(impl_def_id) = get_trait_impl(trait_with_same_path) {
2061                 let impl_span = self.tcx.def_span(impl_def_id);
2062                 err.span_help(impl_span, "trait impl with same name found");
2063                 let trait_crate = self.tcx.crate_name(trait_with_same_path.krate);
2064                 let crate_msg = format!(
2065                     "perhaps two different versions of crate `{}` are being used?",
2066                     trait_crate
2067                 );
2068                 err.note(&crate_msg);
2069                 suggested = true;
2070             }
2071         }
2072         suggested
2073     }
2074
2075     fn mk_trait_obligation_with_new_self_ty(
2076         &self,
2077         param_env: ty::ParamEnv<'tcx>,
2078         trait_ref_and_ty: ty::Binder<'tcx, (ty::TraitPredicate<'tcx>, Ty<'tcx>)>,
2079     ) -> PredicateObligation<'tcx> {
2080         let trait_pred = trait_ref_and_ty
2081             .map_bound(|(tr, new_self_ty)| tr.with_self_type(self.tcx, new_self_ty));
2082
2083         Obligation::new(self.tcx, ObligationCause::dummy(), param_env, trait_pred)
2084     }
2085
2086     #[instrument(skip(self), level = "debug")]
2087     fn maybe_report_ambiguity(
2088         &self,
2089         obligation: &PredicateObligation<'tcx>,
2090         body_id: Option<hir::BodyId>,
2091     ) {
2092         // Unable to successfully determine, probably means
2093         // insufficient type information, but could mean
2094         // ambiguous impls. The latter *ought* to be a
2095         // coherence violation, so we don't report it here.
2096
2097         let predicate = self.resolve_vars_if_possible(obligation.predicate);
2098         let span = obligation.cause.span;
2099
2100         debug!(?predicate, obligation.cause.code = ?obligation.cause.code());
2101
2102         // Ambiguity errors are often caused as fallout from earlier errors.
2103         // We ignore them if this `infcx` is tainted in some cases below.
2104
2105         let bound_predicate = predicate.kind();
2106         let mut err = match bound_predicate.skip_binder() {
2107             ty::PredicateKind::Clause(ty::Clause::Trait(data)) => {
2108                 let trait_ref = bound_predicate.rebind(data.trait_ref);
2109                 debug!(?trait_ref);
2110
2111                 if predicate.references_error() {
2112                     return;
2113                 }
2114
2115                 // This is kind of a hack: it frequently happens that some earlier
2116                 // error prevents types from being fully inferred, and then we get
2117                 // a bunch of uninteresting errors saying something like "<generic
2118                 // #0> doesn't implement Sized".  It may even be true that we
2119                 // could just skip over all checks where the self-ty is an
2120                 // inference variable, but I was afraid that there might be an
2121                 // inference variable created, registered as an obligation, and
2122                 // then never forced by writeback, and hence by skipping here we'd
2123                 // be ignoring the fact that we don't KNOW the type works
2124                 // out. Though even that would probably be harmless, given that
2125                 // we're only talking about builtin traits, which are known to be
2126                 // inhabited. We used to check for `self.tcx.sess.has_errors()` to
2127                 // avoid inundating the user with unnecessary errors, but we now
2128                 // check upstream for type errors and don't add the obligations to
2129                 // begin with in those cases.
2130                 if self.tcx.lang_items().sized_trait() == Some(trait_ref.def_id()) {
2131                     if let None = self.tainted_by_errors() {
2132                         self.emit_inference_failure_err(
2133                             body_id,
2134                             span,
2135                             trait_ref.self_ty().skip_binder().into(),
2136                             ErrorCode::E0282,
2137                             false,
2138                         )
2139                         .emit();
2140                     }
2141                     return;
2142                 }
2143
2144                 // Typically, this ambiguity should only happen if
2145                 // there are unresolved type inference variables
2146                 // (otherwise it would suggest a coherence
2147                 // failure). But given #21974 that is not necessarily
2148                 // the case -- we can have multiple where clauses that
2149                 // are only distinguished by a region, which results
2150                 // in an ambiguity even when all types are fully
2151                 // known, since we don't dispatch based on region
2152                 // relationships.
2153
2154                 // Pick the first substitution that still contains inference variables as the one
2155                 // we're going to emit an error for. If there are none (see above), fall back to
2156                 // a more general error.
2157                 let subst = data.trait_ref.substs.iter().find(|s| s.has_non_region_infer());
2158
2159                 let mut err = if let Some(subst) = subst {
2160                     self.emit_inference_failure_err(body_id, span, subst, ErrorCode::E0283, true)
2161                 } else {
2162                     struct_span_err!(
2163                         self.tcx.sess,
2164                         span,
2165                         E0283,
2166                         "type annotations needed: cannot satisfy `{}`",
2167                         predicate,
2168                     )
2169                 };
2170
2171                 let obligation = obligation.with(self.tcx, trait_ref);
2172                 let mut selcx = SelectionContext::new(&self);
2173                 match selcx.select_from_obligation(&obligation) {
2174                     Ok(None) => {
2175                         let impls = ambiguity::recompute_applicable_impls(self.infcx, &obligation);
2176                         let has_non_region_infer =
2177                             trait_ref.skip_binder().substs.types().any(|t| !t.is_ty_infer());
2178                         // It doesn't make sense to talk about applicable impls if there are more
2179                         // than a handful of them.
2180                         if impls.len() > 1 && impls.len() < 5 && has_non_region_infer {
2181                             self.annotate_source_of_ambiguity(&mut err, &impls, predicate);
2182                         } else {
2183                             if self.tainted_by_errors().is_some() {
2184                                 err.cancel();
2185                                 return;
2186                             }
2187                             err.note(&format!("cannot satisfy `{}`", predicate));
2188                         }
2189                     }
2190                     _ => {
2191                         if self.tainted_by_errors().is_some() {
2192                             err.cancel();
2193                             return;
2194                         }
2195                         err.note(&format!("cannot satisfy `{}`", predicate));
2196                     }
2197                 }
2198
2199                 if let ObligationCauseCode::ItemObligation(def_id) | ObligationCauseCode::ExprItemObligation(def_id, ..) = *obligation.cause.code() {
2200                     self.suggest_fully_qualified_path(&mut err, def_id, span, trait_ref.def_id());
2201                 } else if let Ok(snippet) = &self.tcx.sess.source_map().span_to_snippet(span)
2202                     && let ObligationCauseCode::BindingObligation(def_id, _) | ObligationCauseCode::ExprBindingObligation(def_id, ..)
2203                         = *obligation.cause.code()
2204                 {
2205                     let generics = self.tcx.generics_of(def_id);
2206                     if generics.params.iter().any(|p| p.name != kw::SelfUpper)
2207                         && !snippet.ends_with('>')
2208                         && !generics.has_impl_trait()
2209                         && !self.tcx.is_fn_trait(def_id)
2210                     {
2211                         // FIXME: To avoid spurious suggestions in functions where type arguments
2212                         // where already supplied, we check the snippet to make sure it doesn't
2213                         // end with a turbofish. Ideally we would have access to a `PathSegment`
2214                         // instead. Otherwise we would produce the following output:
2215                         //
2216                         // error[E0283]: type annotations needed
2217                         //   --> $DIR/issue-54954.rs:3:24
2218                         //    |
2219                         // LL | const ARR_LEN: usize = Tt::const_val::<[i8; 123]>();
2220                         //    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^
2221                         //    |                        |
2222                         //    |                        cannot infer type
2223                         //    |                        help: consider specifying the type argument
2224                         //    |                        in the function call:
2225                         //    |                        `Tt::const_val::<[i8; 123]>::<T>`
2226                         // ...
2227                         // LL |     const fn const_val<T: Sized>() -> usize {
2228                         //    |                        - required by this bound in `Tt::const_val`
2229                         //    |
2230                         //    = note: cannot satisfy `_: Tt`
2231
2232                         // Clear any more general suggestions in favor of our specific one
2233                         err.clear_suggestions();
2234
2235                         err.span_suggestion_verbose(
2236                             span.shrink_to_hi(),
2237                             &format!(
2238                                 "consider specifying the type argument{} in the function call",
2239                                 pluralize!(generics.params.len()),
2240                             ),
2241                             format!(
2242                                 "::<{}>",
2243                                 generics
2244                                     .params
2245                                     .iter()
2246                                     .map(|p| p.name.to_string())
2247                                     .collect::<Vec<String>>()
2248                                     .join(", ")
2249                             ),
2250                             Applicability::HasPlaceholders,
2251                         );
2252                     }
2253                 }
2254
2255                 if let (Some(body_id), Some(ty::subst::GenericArgKind::Type(_))) =
2256                     (body_id, subst.map(|subst| subst.unpack()))
2257                 {
2258                     struct FindExprBySpan<'hir> {
2259                         span: Span,
2260                         result: Option<&'hir hir::Expr<'hir>>,
2261                     }
2262
2263                     impl<'v> hir::intravisit::Visitor<'v> for FindExprBySpan<'v> {
2264                         fn visit_expr(&mut self, ex: &'v hir::Expr<'v>) {
2265                             if self.span == ex.span {
2266                                 self.result = Some(ex);
2267                             } else {
2268                                 hir::intravisit::walk_expr(self, ex);
2269                             }
2270                         }
2271                     }
2272
2273                     let mut expr_finder = FindExprBySpan { span, result: None };
2274
2275                     expr_finder.visit_expr(&self.tcx.hir().body(body_id).value);
2276
2277                     if let Some(hir::Expr {
2278                         kind: hir::ExprKind::Path(hir::QPath::Resolved(None, path)), .. }
2279                     ) = expr_finder.result
2280                         && let [
2281                             ..,
2282                             trait_path_segment @ hir::PathSegment {
2283                                 res: rustc_hir::def::Res::Def(rustc_hir::def::DefKind::Trait, trait_id),
2284                                 ..
2285                             },
2286                             hir::PathSegment {
2287                                 ident: assoc_item_name,
2288                                 res: rustc_hir::def::Res::Def(_, item_id),
2289                                 ..
2290                             }
2291                         ] = path.segments
2292                         && data.trait_ref.def_id == *trait_id
2293                         && self.tcx.trait_of_item(*item_id) == Some(*trait_id)
2294                         && let None = self.tainted_by_errors()
2295                     {
2296                         let (verb, noun) = match self.tcx.associated_item(item_id).kind {
2297                             ty::AssocKind::Const => ("refer to the", "constant"),
2298                             ty::AssocKind::Fn => ("call", "function"),
2299                             ty::AssocKind::Type => ("refer to the", "type"), // this is already covered by E0223, but this single match arm doesn't hurt here
2300                         };
2301
2302                         // Replace the more general E0283 with a more specific error
2303                         err.cancel();
2304                         err = self.tcx.sess.struct_span_err_with_code(
2305                             span,
2306                             &format!(
2307                                 "cannot {verb} associated {noun} on trait without specifying the corresponding `impl` type",
2308                              ),
2309                             rustc_errors::error_code!(E0790),
2310                         );
2311
2312                         if let Some(local_def_id) = data.trait_ref.def_id.as_local()
2313                             && 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)
2314                             && let Some(method_ref) = trait_item_refs.iter().find(|item_ref| item_ref.ident == *assoc_item_name) {
2315                             err.span_label(method_ref.span, format!("`{}::{}` defined here", trait_name, assoc_item_name));
2316                         }
2317
2318                         err.span_label(span, format!("cannot {verb} associated {noun} of trait"));
2319
2320                         let trait_impls = self.tcx.trait_impls_of(data.trait_ref.def_id);
2321
2322                         if trait_impls.blanket_impls().is_empty()
2323                             && let Some((impl_ty, _)) = trait_impls.non_blanket_impls().iter().next()
2324                             && let Some(impl_def_id) = impl_ty.def() {
2325                             let message = if trait_impls.non_blanket_impls().len() == 1 {
2326                                 "use the fully-qualified path to the only available implementation".to_string()
2327                             } else {
2328                                 format!(
2329                                     "use a fully-qualified path to a specific available implementation ({} found)",
2330                                     trait_impls.non_blanket_impls().len()
2331                                 )
2332                             };
2333                             let mut suggestions = vec![(
2334                                 trait_path_segment.ident.span.shrink_to_lo(),
2335                                 format!("<{} as ", self.tcx.type_of(impl_def_id))
2336                             )];
2337                             if let Some(generic_arg) = trait_path_segment.args {
2338                                 let between_span = trait_path_segment.ident.span.between(generic_arg.span_ext);
2339                                 // get rid of :: between Trait and <type>
2340                                 // must be '::' between them, otherwise the parser won't accept the code
2341                                 suggestions.push((between_span, "".to_string(),));
2342                                 suggestions.push((generic_arg.span_ext.shrink_to_hi(), format!(">")));
2343                             } else {
2344                                 suggestions.push((trait_path_segment.ident.span.shrink_to_hi(), format!(">")));
2345                             }
2346                             err.multipart_suggestion(
2347                                 message,
2348                                 suggestions,
2349                                 Applicability::MaybeIncorrect
2350                             );
2351                         }
2352                     }
2353                 };
2354
2355                 err
2356             }
2357
2358             ty::PredicateKind::WellFormed(arg) => {
2359                 // Same hacky approach as above to avoid deluging user
2360                 // with error messages.
2361                 if arg.references_error()
2362                     || self.tcx.sess.has_errors().is_some()
2363                     || self.tainted_by_errors().is_some()
2364                 {
2365                     return;
2366                 }
2367
2368                 self.emit_inference_failure_err(body_id, span, arg, ErrorCode::E0282, false)
2369             }
2370
2371             ty::PredicateKind::Subtype(data) => {
2372                 if data.references_error()
2373                     || self.tcx.sess.has_errors().is_some()
2374                     || self.tainted_by_errors().is_some()
2375                 {
2376                     // no need to overload user in such cases
2377                     return;
2378                 }
2379                 let SubtypePredicate { a_is_expected: _, a, b } = data;
2380                 // both must be type variables, or the other would've been instantiated
2381                 assert!(a.is_ty_var() && b.is_ty_var());
2382                 self.emit_inference_failure_err(body_id, span, a.into(), ErrorCode::E0282, true)
2383             }
2384             ty::PredicateKind::Clause(ty::Clause::Projection(data)) => {
2385                 if predicate.references_error() || self.tainted_by_errors().is_some() {
2386                     return;
2387                 }
2388                 let subst = data
2389                     .projection_ty
2390                     .substs
2391                     .iter()
2392                     .chain(Some(data.term.into_arg()))
2393                     .find(|g| g.has_non_region_infer());
2394                 if let Some(subst) = subst {
2395                     let mut err = self.emit_inference_failure_err(
2396                         body_id,
2397                         span,
2398                         subst,
2399                         ErrorCode::E0284,
2400                         true,
2401                     );
2402                     err.note(&format!("cannot satisfy `{}`", predicate));
2403                     err
2404                 } else {
2405                     // If we can't find a substitution, just print a generic error
2406                     let mut err = struct_span_err!(
2407                         self.tcx.sess,
2408                         span,
2409                         E0284,
2410                         "type annotations needed: cannot satisfy `{}`",
2411                         predicate,
2412                     );
2413                     err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2414                     err
2415                 }
2416             }
2417
2418             ty::PredicateKind::ConstEvaluatable(data) => {
2419                 if predicate.references_error() || self.tainted_by_errors().is_some() {
2420                     return;
2421                 }
2422                 let subst = data.walk().find(|g| g.is_non_region_infer());
2423                 if let Some(subst) = subst {
2424                     let err = self.emit_inference_failure_err(
2425                         body_id,
2426                         span,
2427                         subst,
2428                         ErrorCode::E0284,
2429                         true,
2430                     );
2431                     err
2432                 } else {
2433                     // If we can't find a substitution, just print a generic error
2434                     let mut err = struct_span_err!(
2435                         self.tcx.sess,
2436                         span,
2437                         E0284,
2438                         "type annotations needed: cannot satisfy `{}`",
2439                         predicate,
2440                     );
2441                     err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2442                     err
2443                 }
2444             }
2445             _ => {
2446                 if self.tcx.sess.has_errors().is_some() || self.tainted_by_errors().is_some() {
2447                     return;
2448                 }
2449                 let mut err = struct_span_err!(
2450                     self.tcx.sess,
2451                     span,
2452                     E0284,
2453                     "type annotations needed: cannot satisfy `{}`",
2454                     predicate,
2455                 );
2456                 err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2457                 err
2458             }
2459         };
2460         self.note_obligation_cause(&mut err, obligation);
2461         err.emit();
2462     }
2463
2464     fn annotate_source_of_ambiguity(
2465         &self,
2466         err: &mut Diagnostic,
2467         impls: &[DefId],
2468         predicate: ty::Predicate<'tcx>,
2469     ) {
2470         let mut spans = vec![];
2471         let mut crates = vec![];
2472         let mut post = vec![];
2473         for def_id in impls {
2474             match self.tcx.span_of_impl(*def_id) {
2475                 Ok(span) => spans.push(span),
2476                 Err(name) => {
2477                     crates.push(name);
2478                     if let Some(header) = to_pretty_impl_header(self.tcx, *def_id) {
2479                         post.push(header);
2480                     }
2481                 }
2482             }
2483         }
2484         let mut crate_names: Vec<_> = crates.iter().map(|n| format!("`{}`", n)).collect();
2485         crate_names.sort();
2486         crate_names.dedup();
2487         post.sort();
2488         post.dedup();
2489
2490         if self.tainted_by_errors().is_some()
2491             && (crate_names.len() == 1
2492                 && spans.len() == 0
2493                 && ["`core`", "`alloc`", "`std`"].contains(&crate_names[0].as_str())
2494                 || predicate.visit_with(&mut HasNumericInferVisitor).is_break())
2495         {
2496             // Avoid complaining about other inference issues for expressions like
2497             // `42 >> 1`, where the types are still `{integer}`, but we want to
2498             // Do we need `trait_ref.skip_binder().self_ty().is_numeric() &&` too?
2499             // NOTE(eddyb) this was `.cancel()`, but `err`
2500             // is borrowed, so we can't fully defuse it.
2501             err.downgrade_to_delayed_bug();
2502             return;
2503         }
2504
2505         let msg = format!("multiple `impl`s satisfying `{}` found", predicate);
2506         let post = if post.len() > 1 || (post.len() == 1 && post[0].contains('\n')) {
2507             format!(":\n{}", post.iter().map(|p| format!("- {}", p)).collect::<Vec<_>>().join("\n"),)
2508         } else if post.len() == 1 {
2509             format!(": `{}`", post[0])
2510         } else {
2511             String::new()
2512         };
2513
2514         match (spans.len(), crates.len(), crate_names.len()) {
2515             (0, 0, 0) => {
2516                 err.note(&format!("cannot satisfy `{}`", predicate));
2517             }
2518             (0, _, 1) => {
2519                 err.note(&format!("{} in the `{}` crate{}", msg, crates[0], post,));
2520             }
2521             (0, _, _) => {
2522                 err.note(&format!(
2523                     "{} in the following crates: {}{}",
2524                     msg,
2525                     crate_names.join(", "),
2526                     post,
2527                 ));
2528             }
2529             (_, 0, 0) => {
2530                 let span: MultiSpan = spans.into();
2531                 err.span_note(span, &msg);
2532             }
2533             (_, 1, 1) => {
2534                 let span: MultiSpan = spans.into();
2535                 err.span_note(span, &msg);
2536                 err.note(
2537                     &format!("and another `impl` found in the `{}` crate{}", crates[0], post,),
2538                 );
2539             }
2540             _ => {
2541                 let span: MultiSpan = spans.into();
2542                 err.span_note(span, &msg);
2543                 err.note(&format!(
2544                     "and more `impl`s found in the following crates: {}{}",
2545                     crate_names.join(", "),
2546                     post,
2547                 ));
2548             }
2549         }
2550     }
2551
2552     /// Returns `true` if the trait predicate may apply for *some* assignment
2553     /// to the type parameters.
2554     fn predicate_can_apply(
2555         &self,
2556         param_env: ty::ParamEnv<'tcx>,
2557         pred: ty::PolyTraitPredicate<'tcx>,
2558     ) -> bool {
2559         struct ParamToVarFolder<'a, 'tcx> {
2560             infcx: &'a InferCtxt<'tcx>,
2561             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
2562         }
2563
2564         impl<'a, 'tcx> TypeFolder<'tcx> for ParamToVarFolder<'a, 'tcx> {
2565             fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
2566                 self.infcx.tcx
2567             }
2568
2569             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
2570                 if let ty::Param(ty::ParamTy { name, .. }) = *ty.kind() {
2571                     let infcx = self.infcx;
2572                     *self.var_map.entry(ty).or_insert_with(|| {
2573                         infcx.next_ty_var(TypeVariableOrigin {
2574                             kind: TypeVariableOriginKind::TypeParameterDefinition(name, None),
2575                             span: DUMMY_SP,
2576                         })
2577                     })
2578                 } else {
2579                     ty.super_fold_with(self)
2580                 }
2581             }
2582         }
2583
2584         self.probe(|_| {
2585             let cleaned_pred =
2586                 pred.fold_with(&mut ParamToVarFolder { infcx: self, var_map: Default::default() });
2587
2588             let InferOk { value: cleaned_pred, .. } =
2589                 self.infcx.at(&ObligationCause::dummy(), param_env).normalize(cleaned_pred);
2590
2591             let obligation =
2592                 Obligation::new(self.tcx, ObligationCause::dummy(), param_env, cleaned_pred);
2593
2594             self.predicate_may_hold(&obligation)
2595         })
2596     }
2597
2598     fn note_obligation_cause(&self, err: &mut Diagnostic, obligation: &PredicateObligation<'tcx>) {
2599         // First, attempt to add note to this error with an async-await-specific
2600         // message, and fall back to regular note otherwise.
2601         if !self.maybe_note_obligation_cause_for_async_await(err, obligation) {
2602             self.note_obligation_cause_code(
2603                 err,
2604                 &obligation.predicate,
2605                 obligation.param_env,
2606                 obligation.cause.code(),
2607                 &mut vec![],
2608                 &mut Default::default(),
2609             );
2610             self.suggest_unsized_bound_if_applicable(err, obligation);
2611         }
2612     }
2613
2614     #[instrument(level = "debug", skip_all)]
2615     fn suggest_unsized_bound_if_applicable(
2616         &self,
2617         err: &mut Diagnostic,
2618         obligation: &PredicateObligation<'tcx>,
2619     ) {
2620         let ty::PredicateKind::Clause(ty::Clause::Trait(pred)) = obligation.predicate.kind().skip_binder() else { return; };
2621         let (ObligationCauseCode::BindingObligation(item_def_id, span)
2622         | ObligationCauseCode::ExprBindingObligation(item_def_id, span, ..))
2623             = *obligation.cause.code().peel_derives() else { return; };
2624         debug!(?pred, ?item_def_id, ?span);
2625
2626         let (Some(node), true) = (
2627             self.tcx.hir().get_if_local(item_def_id),
2628             Some(pred.def_id()) == self.tcx.lang_items().sized_trait(),
2629         ) else {
2630             return;
2631         };
2632         self.maybe_suggest_unsized_generics(err, span, node);
2633     }
2634
2635     #[instrument(level = "debug", skip_all)]
2636     fn maybe_suggest_unsized_generics(&self, err: &mut Diagnostic, span: Span, node: Node<'tcx>) {
2637         let Some(generics) = node.generics() else {
2638             return;
2639         };
2640         let sized_trait = self.tcx.lang_items().sized_trait();
2641         debug!(?generics.params);
2642         debug!(?generics.predicates);
2643         let Some(param) = generics.params.iter().find(|param| param.span == span) else {
2644             return;
2645         };
2646         // Check that none of the explicit trait bounds is `Sized`. Assume that an explicit
2647         // `Sized` bound is there intentionally and we don't need to suggest relaxing it.
2648         let explicitly_sized = generics
2649             .bounds_for_param(param.def_id)
2650             .flat_map(|bp| bp.bounds)
2651             .any(|bound| bound.trait_ref().and_then(|tr| tr.trait_def_id()) == sized_trait);
2652         if explicitly_sized {
2653             return;
2654         }
2655         debug!(?param);
2656         match node {
2657             hir::Node::Item(
2658                 item @ hir::Item {
2659                     // Only suggest indirection for uses of type parameters in ADTs.
2660                     kind:
2661                         hir::ItemKind::Enum(..) | hir::ItemKind::Struct(..) | hir::ItemKind::Union(..),
2662                     ..
2663                 },
2664             ) => {
2665                 if self.maybe_indirection_for_unsized(err, item, param) {
2666                     return;
2667                 }
2668             }
2669             _ => {}
2670         };
2671         // Didn't add an indirection suggestion, so add a general suggestion to relax `Sized`.
2672         let (span, separator) = if let Some(s) = generics.bounds_span_for_suggestions(param.def_id)
2673         {
2674             (s, " +")
2675         } else {
2676             (span.shrink_to_hi(), ":")
2677         };
2678         err.span_suggestion_verbose(
2679             span,
2680             "consider relaxing the implicit `Sized` restriction",
2681             format!("{} ?Sized", separator),
2682             Applicability::MachineApplicable,
2683         );
2684     }
2685
2686     fn maybe_indirection_for_unsized(
2687         &self,
2688         err: &mut Diagnostic,
2689         item: &Item<'tcx>,
2690         param: &GenericParam<'tcx>,
2691     ) -> bool {
2692         // Suggesting `T: ?Sized` is only valid in an ADT if `T` is only used in a
2693         // borrow. `struct S<'a, T: ?Sized>(&'a T);` is valid, `struct S<T: ?Sized>(T);`
2694         // is not. Look for invalid "bare" parameter uses, and suggest using indirection.
2695         let mut visitor =
2696             FindTypeParam { param: param.name.ident().name, invalid_spans: vec![], nested: false };
2697         visitor.visit_item(item);
2698         if visitor.invalid_spans.is_empty() {
2699             return false;
2700         }
2701         let mut multispan: MultiSpan = param.span.into();
2702         multispan.push_span_label(
2703             param.span,
2704             format!("this could be changed to `{}: ?Sized`...", param.name.ident()),
2705         );
2706         for sp in visitor.invalid_spans {
2707             multispan.push_span_label(
2708                 sp,
2709                 format!("...if indirection were used here: `Box<{}>`", param.name.ident()),
2710             );
2711         }
2712         err.span_help(
2713             multispan,
2714             &format!(
2715                 "you could relax the implicit `Sized` bound on `{T}` if it were \
2716                 used through indirection like `&{T}` or `Box<{T}>`",
2717                 T = param.name.ident(),
2718             ),
2719         );
2720         true
2721     }
2722
2723     fn is_recursive_obligation(
2724         &self,
2725         obligated_types: &mut Vec<Ty<'tcx>>,
2726         cause_code: &ObligationCauseCode<'tcx>,
2727     ) -> bool {
2728         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
2729             let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
2730             let self_ty = parent_trait_ref.skip_binder().self_ty();
2731             if obligated_types.iter().any(|ot| ot == &self_ty) {
2732                 return true;
2733             }
2734             if let ty::Adt(def, substs) = self_ty.kind()
2735                 && let [arg] = &substs[..]
2736                 && let ty::subst::GenericArgKind::Type(ty) = arg.unpack()
2737                 && let ty::Adt(inner_def, _) = ty.kind()
2738                 && inner_def == def
2739             {
2740                 return true;
2741             }
2742         }
2743         false
2744     }
2745 }
2746
2747 /// Look for type `param` in an ADT being used only through a reference to confirm that suggesting
2748 /// `param: ?Sized` would be a valid constraint.
2749 struct FindTypeParam {
2750     param: rustc_span::Symbol,
2751     invalid_spans: Vec<Span>,
2752     nested: bool,
2753 }
2754
2755 impl<'v> Visitor<'v> for FindTypeParam {
2756     fn visit_where_predicate(&mut self, _: &'v hir::WherePredicate<'v>) {
2757         // Skip where-clauses, to avoid suggesting indirection for type parameters found there.
2758     }
2759
2760     fn visit_ty(&mut self, ty: &hir::Ty<'_>) {
2761         // We collect the spans of all uses of the "bare" type param, like in `field: T` or
2762         // `field: (T, T)` where we could make `T: ?Sized` while skipping cases that are known to be
2763         // valid like `field: &'a T` or `field: *mut T` and cases that *might* have further `Sized`
2764         // obligations like `Box<T>` and `Vec<T>`, but we perform no extra analysis for those cases
2765         // and suggest `T: ?Sized` regardless of their obligations. This is fine because the errors
2766         // in that case should make what happened clear enough.
2767         match ty.kind {
2768             hir::TyKind::Ptr(_) | hir::TyKind::Rptr(..) | hir::TyKind::TraitObject(..) => {}
2769             hir::TyKind::Path(hir::QPath::Resolved(None, path))
2770                 if path.segments.len() == 1 && path.segments[0].ident.name == self.param =>
2771             {
2772                 if !self.nested {
2773                     debug!(?ty, "FindTypeParam::visit_ty");
2774                     self.invalid_spans.push(ty.span);
2775                 }
2776             }
2777             hir::TyKind::Path(_) => {
2778                 let prev = self.nested;
2779                 self.nested = true;
2780                 hir::intravisit::walk_ty(self, ty);
2781                 self.nested = prev;
2782             }
2783             _ => {
2784                 hir::intravisit::walk_ty(self, ty);
2785             }
2786         }
2787     }
2788 }
2789
2790 /// Summarizes information
2791 #[derive(Clone)]
2792 pub enum ArgKind {
2793     /// An argument of non-tuple type. Parameters are (name, ty)
2794     Arg(String, String),
2795
2796     /// An argument of tuple type. For a "found" argument, the span is
2797     /// the location in the source of the pattern. For an "expected"
2798     /// argument, it will be None. The vector is a list of (name, ty)
2799     /// strings for the components of the tuple.
2800     Tuple(Option<Span>, Vec<(String, String)>),
2801 }
2802
2803 impl ArgKind {
2804     fn empty() -> ArgKind {
2805         ArgKind::Arg("_".to_owned(), "_".to_owned())
2806     }
2807
2808     /// Creates an `ArgKind` from the expected type of an
2809     /// argument. It has no name (`_`) and an optional source span.
2810     pub fn from_expected_ty(t: Ty<'_>, span: Option<Span>) -> ArgKind {
2811         match t.kind() {
2812             ty::Tuple(tys) => ArgKind::Tuple(
2813                 span,
2814                 tys.iter().map(|ty| ("_".to_owned(), ty.to_string())).collect::<Vec<_>>(),
2815             ),
2816             _ => ArgKind::Arg("_".to_owned(), t.to_string()),
2817         }
2818     }
2819 }
2820
2821 struct HasNumericInferVisitor;
2822
2823 impl<'tcx> ty::TypeVisitor<'tcx> for HasNumericInferVisitor {
2824     type BreakTy = ();
2825
2826     fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
2827         if matches!(ty.kind(), ty::Infer(ty::FloatVar(_) | ty::IntVar(_))) {
2828             ControlFlow::Break(())
2829         } else {
2830             ControlFlow::CONTINUE
2831         }
2832     }
2833 }
2834
2835 pub enum DefIdOrName {
2836     DefId(DefId),
2837     Name(&'static str),
2838 }