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

   1 mod ambiguity;
   2 pub mod method_chain;
   3 pub mod on_unimplemented;
   4 pub mod suggestions;
   5
   6 use super::{
   7     FulfillmentError, FulfillmentErrorCode, MismatchedProjectionTypes, Obligation, ObligationCause,
   8     ObligationCauseCode, ObligationCtxt, OutputTypeParameterMismatch, Overflow,
   9     PredicateObligation, SelectionContext, SelectionError, TraitNotObjectSafe,
  10 };
  11 use crate::infer::error_reporting::{TyCategory, TypeAnnotationNeeded as ErrorCode};
  12 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  13 use crate::infer::{self, InferCtxt};
  14 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
  15 use crate::traits::query::normalize::QueryNormalizeExt as _;
  16 use crate::traits::specialize::to_pretty_impl_header;
  17 use crate::traits::NormalizeExt;
  18 use on_unimplemented::OnUnimplementedNote;
  19 use on_unimplemented::TypeErrCtxtExt as _;
  20 use rustc_data_structures::fx::{FxHashMap, FxIndexMap};
  21 use rustc_errors::{
  22     pluralize, struct_span_err, Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed,
  23     MultiSpan, Style,
  24 };
  25 use rustc_hir as hir;
  26 use rustc_hir::def::Namespace;
  27 use rustc_hir::def_id::DefId;
  28 use rustc_hir::intravisit::Visitor;
  29 use rustc_hir::GenericParam;
  30 use rustc_hir::Item;
  31 use rustc_hir::Node;
  32 use rustc_infer::infer::error_reporting::TypeErrCtxt;
  33 use rustc_infer::infer::{InferOk, TypeTrace};
  34 use rustc_middle::traits::select::OverflowError;
  35 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
  36 use rustc_middle::ty::error::ExpectedFound;
  37 use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable};
  38 use rustc_middle::ty::print::{FmtPrinter, Print};
  39 use rustc_middle::ty::{
  40     self, SubtypePredicate, ToPolyTraitRef, ToPredicate, TraitRef, Ty, TyCtxt, TypeFoldable,
  41     TypeVisitable,
  42 };
  43 use rustc_session::Limit;
  44 use rustc_span::def_id::LOCAL_CRATE;
  45 use rustc_span::symbol::{kw, sym};
  46 use rustc_span::{ExpnKind, Span, DUMMY_SP};
  47 use std::fmt;
  48 use std::iter;
  49 use std::ops::ControlFlow;
  50 use suggestions::TypeErrCtxtExt as _;
  51
  52 pub use rustc_infer::traits::error_reporting::*;
  53
  54 // When outputting impl candidates, prefer showing those that are more similar.
  55 //
  56 // We also compare candidates after skipping lifetimes, which has a lower
  57 // priority than exact matches.
  58 #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
  59 pub enum CandidateSimilarity {
  60     Exact { ignoring_lifetimes: bool },
  61     Fuzzy { ignoring_lifetimes: bool },
  62 }
  63
  64 #[derive(Debug, Clone, Copy)]
  65 pub struct ImplCandidate<'tcx> {
  66     pub trait_ref: ty::TraitRef<'tcx>,
  67     pub similarity: CandidateSimilarity,
  68 }
  69
  70 pub trait InferCtxtExt<'tcx> {
  71     /// Given some node representing a fn-like thing in the HIR map,
  72     /// returns a span and `ArgKind` information that describes the
  73     /// arguments it expects. This can be supplied to
  74     /// `report_arg_count_mismatch`.
  75     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Option<Span>, Vec<ArgKind>)>;
  76
  77     /// Reports an error when the number of arguments needed by a
  78     /// trait match doesn't match the number that the expression
  79     /// provides.
  80     fn report_arg_count_mismatch(
  81         &self,
  82         span: Span,
  83         found_span: Option<Span>,
  84         expected_args: Vec<ArgKind>,
  85         found_args: Vec<ArgKind>,
  86         is_closure: bool,
  87         closure_pipe_span: Option<Span>,
  88     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed>;
  89
  90     /// Checks if the type implements one of `Fn`, `FnMut`, or `FnOnce`
  91     /// in that order, and returns the generic type corresponding to the
  92     /// argument of that trait (corresponding to the closure arguments).
  93     fn type_implements_fn_trait(
  94         &self,
  95         param_env: ty::ParamEnv<'tcx>,
  96         ty: ty::Binder<'tcx, Ty<'tcx>>,
  97         constness: ty::BoundConstness,
  98         polarity: ty::ImplPolarity,
  99     ) -> Result<(ty::ClosureKind, ty::Binder<'tcx, Ty<'tcx>>), ()>;
 100 }
 101
 102 pub trait TypeErrCtxtExt<'tcx> {
 103     fn report_overflow_error<T>(
 104         &self,
 105         predicate: &T,
 106         span: Span,
 107         suggest_increasing_limit: bool,
 108         mutate: impl FnOnce(&mut Diagnostic),
 109     ) -> !
 110     where
 111         T: fmt::Display
 112             + TypeFoldable<'tcx>
 113             + Print<'tcx, FmtPrinter<'tcx, 'tcx>, Output = FmtPrinter<'tcx, 'tcx>>,
 114         <T as Print<'tcx, FmtPrinter<'tcx, 'tcx>>>::Error: std::fmt::Debug;
 115
 116     fn report_fulfillment_errors(
 117         &self,
 118         errors: &[FulfillmentError<'tcx>],
 119         body_id: Option<hir::BodyId>,
 120     ) -> ErrorGuaranteed;
 121
 122     fn report_overflow_obligation<T>(
 123         &self,
 124         obligation: &Obligation<'tcx, T>,
 125         suggest_increasing_limit: bool,
 126     ) -> !
 127     where
 128         T: ToPredicate<'tcx> + Clone;
 129
 130     fn suggest_new_overflow_limit(&self, err: &mut Diagnostic);
 131
 132     fn report_overflow_obligation_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> !;
 133
 134     /// The `root_obligation` parameter should be the `root_obligation` field
 135     /// from a `FulfillmentError`. If no `FulfillmentError` is available,
 136     /// then it should be the same as `obligation`.
 137     fn report_selection_error(
 138         &self,
 139         obligation: PredicateObligation<'tcx>,
 140         root_obligation: &PredicateObligation<'tcx>,
 141         error: &SelectionError<'tcx>,
 142     );
 143 }
 144
 145 impl<'tcx> InferCtxtExt<'tcx> for InferCtxt<'tcx> {
 146     /// Given some node representing a fn-like thing in the HIR map,
 147     /// returns a span and `ArgKind` information that describes the
 148     /// arguments it expects. This can be supplied to
 149     /// `report_arg_count_mismatch`.
 150     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Option<Span>, Vec<ArgKind>)> {
 151         let sm = self.tcx.sess.source_map();
 152         let hir = self.tcx.hir();
 153         Some(match node {
 154             Node::Expr(&hir::Expr {
 155                 kind: hir::ExprKind::Closure(&hir::Closure { body, fn_decl_span, fn_arg_span, .. }),
 156                 ..
 157             }) => (
 158                 fn_decl_span,
 159                 fn_arg_span,
 160                 hir.body(body)
 161                     .params
 162                     .iter()
 163                     .map(|arg| {
 164                         if let hir::Pat { kind: hir::PatKind::Tuple(ref args, _), span, .. } =
 165                             *arg.pat
 166                         {
 167                             Some(ArgKind::Tuple(
 168                                 Some(span),
 169                                 args.iter()
 170                                     .map(|pat| {
 171                                         sm.span_to_snippet(pat.span)
 172                                             .ok()
 173                                             .map(|snippet| (snippet, "_".to_owned()))
 174                                     })
 175                                     .collect::<Option<Vec<_>>>()?,
 176                             ))
 177                         } else {
 178                             let name = sm.span_to_snippet(arg.pat.span).ok()?;
 179                             Some(ArgKind::Arg(name, "_".to_owned()))
 180                         }
 181                     })
 182                     .collect::<Option<Vec<ArgKind>>>()?,
 183             ),
 184             Node::Item(&hir::Item { kind: hir::ItemKind::Fn(ref sig, ..), .. })
 185             | Node::ImplItem(&hir::ImplItem { kind: hir::ImplItemKind::Fn(ref sig, _), .. })
 186             | Node::TraitItem(&hir::TraitItem {
 187                 kind: hir::TraitItemKind::Fn(ref sig, _), ..
 188             }) => (
 189                 sig.span,
 190                 None,
 191                 sig.decl
 192                     .inputs
 193                     .iter()
 194                     .map(|arg| match arg.kind {
 195                         hir::TyKind::Tup(ref tys) => ArgKind::Tuple(
 196                             Some(arg.span),
 197                             vec![("_".to_owned(), "_".to_owned()); tys.len()],
 198                         ),
 199                         _ => ArgKind::empty(),
 200                     })
 201                     .collect::<Vec<ArgKind>>(),
 202             ),
 203             Node::Ctor(ref variant_data) => {
 204                 let span = variant_data.ctor_hir_id().map_or(DUMMY_SP, |id| hir.span(id));
 205                 (span, None, vec![ArgKind::empty(); variant_data.fields().len()])
 206             }
 207             _ => panic!("non-FnLike node found: {:?}", node),
 208         })
 209     }
 210
 211     /// Reports an error when the number of arguments needed by a
 212     /// trait match doesn't match the number that the expression
 213     /// provides.
 214     fn report_arg_count_mismatch(
 215         &self,
 216         span: Span,
 217         found_span: Option<Span>,
 218         expected_args: Vec<ArgKind>,
 219         found_args: Vec<ArgKind>,
 220         is_closure: bool,
 221         closure_arg_span: Option<Span>,
 222     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
 223         let kind = if is_closure { "closure" } else { "function" };
 224
 225         let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
 226             let arg_length = arguments.len();
 227             let distinct = matches!(other, &[ArgKind::Tuple(..)]);
 228             match (arg_length, arguments.get(0)) {
 229                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
 230                     format!("a single {}-tuple as argument", fields.len())
 231                 }
 232                 _ => format!(
 233                     "{} {}argument{}",
 234                     arg_length,
 235                     if distinct && arg_length > 1 { "distinct " } else { "" },
 236                     pluralize!(arg_length)
 237                 ),
 238             }
 239         };
 240
 241         let expected_str = args_str(&expected_args, &found_args);
 242         let found_str = args_str(&found_args, &expected_args);
 243
 244         let mut err = struct_span_err!(
 245             self.tcx.sess,
 246             span,
 247             E0593,
 248             "{} is expected to take {}, but it takes {}",
 249             kind,
 250             expected_str,
 251             found_str,
 252         );
 253
 254         err.span_label(span, format!("expected {} that takes {}", kind, expected_str));
 255
 256         if let Some(found_span) = found_span {
 257             err.span_label(found_span, format!("takes {}", found_str));
 258
 259             // Suggest to take and ignore the arguments with expected_args_length `_`s if
 260             // found arguments is empty (assume the user just wants to ignore args in this case).
 261             // For example, if `expected_args_length` is 2, suggest `|_, _|`.
 262             if found_args.is_empty() && is_closure {
 263                 let underscores = vec!["_"; expected_args.len()].join(", ");
 264                 err.span_suggestion_verbose(
 265                     closure_arg_span.unwrap_or(found_span),
 266                     &format!(
 267                         "consider changing the closure to take and ignore the expected argument{}",
 268                         pluralize!(expected_args.len())
 269                     ),
 270                     format!("|{}|", underscores),
 271                     Applicability::MachineApplicable,
 272                 );
 273             }
 274
 275             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
 276                 if fields.len() == expected_args.len() {
 277                     let sugg = fields
 278                         .iter()
 279                         .map(|(name, _)| name.to_owned())
 280                         .collect::<Vec<String>>()
 281                         .join(", ");
 282                     err.span_suggestion_verbose(
 283                         found_span,
 284                         "change the closure to take multiple arguments instead of a single tuple",
 285                         format!("|{}|", sugg),
 286                         Applicability::MachineApplicable,
 287                     );
 288                 }
 289             }
 290             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..]
 291                 && fields.len() == found_args.len()
 292                 && is_closure
 293             {
 294                 let sugg = format!(
 295                     "|({}){}|",
 296                     found_args
 297                         .iter()
 298                         .map(|arg| match arg {
 299                             ArgKind::Arg(name, _) => name.to_owned(),
 300                             _ => "_".to_owned(),
 301                         })
 302                         .collect::<Vec<String>>()
 303                         .join(", "),
 304                     // add type annotations if available
 305                     if found_args.iter().any(|arg| match arg {
 306                         ArgKind::Arg(_, ty) => ty != "_",
 307                         _ => false,
 308                     }) {
 309                         format!(
 310                             ": ({})",
 311                             fields
 312                                 .iter()
 313                                 .map(|(_, ty)| ty.to_owned())
 314                                 .collect::<Vec<String>>()
 315                                 .join(", ")
 316                         )
 317                     } else {
 318                         String::new()
 319                     },
 320                 );
 321                 err.span_suggestion_verbose(
 322                     found_span,
 323                     "change the closure to accept a tuple instead of individual arguments",
 324                     sugg,
 325                     Applicability::MachineApplicable,
 326                 );
 327             }
 328         }
 329
 330         err
 331     }
 332
 333     fn type_implements_fn_trait(
 334         &self,
 335         param_env: ty::ParamEnv<'tcx>,
 336         ty: ty::Binder<'tcx, Ty<'tcx>>,
 337         constness: ty::BoundConstness,
 338         polarity: ty::ImplPolarity,
 339     ) -> Result<(ty::ClosureKind, ty::Binder<'tcx, Ty<'tcx>>), ()> {
 340         self.commit_if_ok(|_| {
 341             for trait_def_id in [
 342                 self.tcx.lang_items().fn_trait(),
 343                 self.tcx.lang_items().fn_mut_trait(),
 344                 self.tcx.lang_items().fn_once_trait(),
 345             ] {
 346                 let Some(trait_def_id) = trait_def_id else { continue };
 347                 // Make a fresh inference variable so we can determine what the substitutions
 348                 // of the trait are.
 349                 let var = self.next_ty_var(TypeVariableOrigin {
 350                     span: DUMMY_SP,
 351                     kind: TypeVariableOriginKind::MiscVariable,
 352                 });
 353                 let trait_ref = self.tcx.mk_trait_ref(trait_def_id, [ty.skip_binder(), var]);
 354                 let obligation = Obligation::new(
 355                     self.tcx,
 356                     ObligationCause::dummy(),
 357                     param_env,
 358                     ty.rebind(ty::TraitPredicate { trait_ref, constness, polarity }),
 359                 );
 360                 let ocx = ObligationCtxt::new_in_snapshot(self);
 361                 ocx.register_obligation(obligation);
 362                 if ocx.select_all_or_error().is_empty() {
 363                     return Ok((
 364                         self.tcx
 365                             .fn_trait_kind_from_def_id(trait_def_id)
 366                             .expect("expected to map DefId to ClosureKind"),
 367                         ty.rebind(self.resolve_vars_if_possible(var)),
 368                     ));
 369                 }
 370             }
 371
 372             Err(())
 373         })
 374     }
 375 }
 376 impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
 377     fn report_fulfillment_errors(
 378         &self,
 379         errors: &[FulfillmentError<'tcx>],
 380         body_id: Option<hir::BodyId>,
 381     ) -> ErrorGuaranteed {
 382         #[derive(Debug)]
 383         struct ErrorDescriptor<'tcx> {
 384             predicate: ty::Predicate<'tcx>,
 385             index: Option<usize>, // None if this is an old error
 386         }
 387
 388         let mut error_map: FxIndexMap<_, Vec<_>> = self
 389             .reported_trait_errors
 390             .borrow()
 391             .iter()
 392             .map(|(&span, predicates)| {
 393                 (
 394                     span,
 395                     predicates
 396                         .iter()
 397                         .map(|&predicate| ErrorDescriptor { predicate, index: None })
 398                         .collect(),
 399                 )
 400             })
 401             .collect();
 402
 403         for (index, error) in errors.iter().enumerate() {
 404             // We want to ignore desugarings here: spans are equivalent even
 405             // if one is the result of a desugaring and the other is not.
 406             let mut span = error.obligation.cause.span;
 407             let expn_data = span.ctxt().outer_expn_data();
 408             if let ExpnKind::Desugaring(_) = expn_data.kind {
 409                 span = expn_data.call_site;
 410             }
 411
 412             error_map.entry(span).or_default().push(ErrorDescriptor {
 413                 predicate: error.obligation.predicate,
 414                 index: Some(index),
 415             });
 416
 417             self.reported_trait_errors
 418                 .borrow_mut()
 419                 .entry(span)
 420                 .or_default()
 421                 .push(error.obligation.predicate);
 422         }
 423
 424         // We do this in 2 passes because we want to display errors in order, though
 425         // maybe it *is* better to sort errors by span or something.
 426         let mut is_suppressed = vec![false; errors.len()];
 427         for (_, error_set) in error_map.iter() {
 428             // We want to suppress "duplicate" errors with the same span.
 429             for error in error_set {
 430                 if let Some(index) = error.index {
 431                     // Suppress errors that are either:
 432                     // 1) strictly implied by another error.
 433                     // 2) implied by an error with a smaller index.
 434                     for error2 in error_set {
 435                         if error2.index.map_or(false, |index2| is_suppressed[index2]) {
 436                             // Avoid errors being suppressed by already-suppressed
 437                             // errors, to prevent all errors from being suppressed
 438                             // at once.
 439                             continue;
 440                         }
 441
 442                         if self.error_implies(error2.predicate, error.predicate)
 443                             && !(error2.index >= error.index
 444                                 && self.error_implies(error.predicate, error2.predicate))
 445                         {
 446                             info!("skipping {:?} (implied by {:?})", error, error2);
 447                             is_suppressed[index] = true;
 448                             break;
 449                         }
 450                     }
 451                 }
 452             }
 453         }
 454
 455         for (error, suppressed) in iter::zip(errors, is_suppressed) {
 456             if !suppressed {
 457                 self.report_fulfillment_error(error, body_id);
 458             }
 459         }
 460
 461         self.tcx.sess.delay_span_bug(DUMMY_SP, "expected fullfillment errors")
 462     }
 463
 464     /// Reports that an overflow has occurred and halts compilation. We
 465     /// halt compilation unconditionally because it is important that
 466     /// overflows never be masked -- they basically represent computations
 467     /// whose result could not be truly determined and thus we can't say
 468     /// if the program type checks or not -- and they are unusual
 469     /// occurrences in any case.
 470     fn report_overflow_error<T>(
 471         &self,
 472         predicate: &T,
 473         span: Span,
 474         suggest_increasing_limit: bool,
 475         mutate: impl FnOnce(&mut Diagnostic),
 476     ) -> !
 477     where
 478         T: fmt::Display
 479             + TypeFoldable<'tcx>
 480             + Print<'tcx, FmtPrinter<'tcx, 'tcx>, Output = FmtPrinter<'tcx, 'tcx>>,
 481         <T as Print<'tcx, FmtPrinter<'tcx, 'tcx>>>::Error: std::fmt::Debug,
 482     {
 483         let predicate = self.resolve_vars_if_possible(predicate.clone());
 484         let mut pred_str = predicate.to_string();
 485
 486         if pred_str.len() > 50 {
 487             // We don't need to save the type to a file, we will be talking about this type already
 488             // in a separate note when we explain the obligation, so it will be available that way.
 489             pred_str = predicate
 490                 .print(FmtPrinter::new_with_limit(
 491                     self.tcx,
 492                     Namespace::TypeNS,
 493                     rustc_session::Limit(6),
 494                 ))
 495                 .unwrap()
 496                 .into_buffer();
 497         }
 498         let mut err = struct_span_err!(
 499             self.tcx.sess,
 500             span,
 501             E0275,
 502             "overflow evaluating the requirement `{}`",
 503             pred_str,
 504         );
 505
 506         if suggest_increasing_limit {
 507             self.suggest_new_overflow_limit(&mut err);
 508         }
 509
 510         mutate(&mut err);
 511
 512         err.emit();
 513         self.tcx.sess.abort_if_errors();
 514         bug!();
 515     }
 516
 517     /// Reports that an overflow has occurred and halts compilation. We
 518     /// halt compilation unconditionally because it is important that
 519     /// overflows never be masked -- they basically represent computations
 520     /// whose result could not be truly determined and thus we can't say
 521     /// if the program type checks or not -- and they are unusual
 522     /// occurrences in any case.
 523     fn report_overflow_obligation<T>(
 524         &self,
 525         obligation: &Obligation<'tcx, T>,
 526         suggest_increasing_limit: bool,
 527     ) -> !
 528     where
 529         T: ToPredicate<'tcx> + Clone,
 530     {
 531         let predicate = obligation.predicate.clone().to_predicate(self.tcx);
 532         let predicate = self.resolve_vars_if_possible(predicate);
 533         self.report_overflow_error(
 534             &predicate,
 535             obligation.cause.span,
 536             suggest_increasing_limit,
 537             |err| {
 538                 self.note_obligation_cause_code(
 539                     err,
 540                     predicate,
 541                     obligation.param_env,
 542                     obligation.cause.code(),
 543                     &mut vec![],
 544                     &mut Default::default(),
 545                 );
 546             },
 547         );
 548     }
 549
 550     fn suggest_new_overflow_limit(&self, err: &mut Diagnostic) {
 551         let suggested_limit = match self.tcx.recursion_limit() {
 552             Limit(0) => Limit(2),
 553             limit => limit * 2,
 554         };
 555         err.help(&format!(
 556             "consider increasing the recursion limit by adding a \
 557              `#![recursion_limit = \"{}\"]` attribute to your crate (`{}`)",
 558             suggested_limit,
 559             self.tcx.crate_name(LOCAL_CRATE),
 560         ));
 561     }
 562
 563     /// Reports that a cycle was detected which led to overflow and halts
 564     /// compilation. This is equivalent to `report_overflow_obligation` except
 565     /// that we can give a more helpful error message (and, in particular,
 566     /// we do not suggest increasing the overflow limit, which is not
 567     /// going to help).
 568     fn report_overflow_obligation_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 569         let cycle = self.resolve_vars_if_possible(cycle.to_owned());
 570         assert!(!cycle.is_empty());
 571
 572         debug!(?cycle, "report_overflow_error_cycle");
 573
 574         // The 'deepest' obligation is most likely to have a useful
 575         // cause 'backtrace'
 576         self.report_overflow_obligation(
 577             cycle.iter().max_by_key(|p| p.recursion_depth).unwrap(),
 578             false,
 579         );
 580     }
 581
 582     fn report_selection_error(
 583         &self,
 584         mut obligation: PredicateObligation<'tcx>,
 585         root_obligation: &PredicateObligation<'tcx>,
 586         error: &SelectionError<'tcx>,
 587     ) {
 588         let tcx = self.tcx;
 589         let mut span = obligation.cause.span;
 590         // FIXME: statically guarantee this by tainting after the diagnostic is emitted
 591         self.set_tainted_by_errors(
 592             tcx.sess.delay_span_bug(span, "`report_selection_error` did not emit an error"),
 593         );
 594
 595         let mut err = match *error {
 596             SelectionError::Unimplemented => {
 597                 // If this obligation was generated as a result of well-formedness checking, see if we
 598                 // can get a better error message by performing HIR-based well-formedness checking.
 599                 if let ObligationCauseCode::WellFormed(Some(wf_loc)) =
 600                     root_obligation.cause.code().peel_derives()
 601                     && !obligation.predicate.has_non_region_infer()
 602                 {
 603                     if let Some(cause) = self
 604                         .tcx
 605                         .diagnostic_hir_wf_check((tcx.erase_regions(obligation.predicate), *wf_loc))
 606                     {
 607                         obligation.cause = cause.clone();
 608                         span = obligation.cause.span;
 609                     }
 610                 }
 611                 if let ObligationCauseCode::CompareImplItemObligation {
 612                     impl_item_def_id,
 613                     trait_item_def_id,
 614                     kind: _,
 615                 } = *obligation.cause.code()
 616                 {
 617                     self.report_extra_impl_obligation(
 618                         span,
 619                         impl_item_def_id,
 620                         trait_item_def_id,
 621                         &format!("`{}`", obligation.predicate),
 622                     )
 623                     .emit();
 624                     return;
 625                 }
 626
 627                 let bound_predicate = obligation.predicate.kind();
 628                 match bound_predicate.skip_binder() {
 629                     ty::PredicateKind::Clause(ty::Clause::Trait(trait_predicate)) => {
 630                         let trait_predicate = bound_predicate.rebind(trait_predicate);
 631                         let mut trait_predicate = self.resolve_vars_if_possible(trait_predicate);
 632
 633                         trait_predicate.remap_constness_diag(obligation.param_env);
 634                         let predicate_is_const = ty::BoundConstness::ConstIfConst
 635                             == trait_predicate.skip_binder().constness;
 636
 637                         if self.tcx.sess.has_errors().is_some()
 638                             && trait_predicate.references_error()
 639                         {
 640                             return;
 641                         }
 642                         let trait_ref = trait_predicate.to_poly_trait_ref();
 643                         let (post_message, pre_message, type_def) = self
 644                             .get_parent_trait_ref(obligation.cause.code())
 645                             .map(|(t, s)| {
 646                                 (
 647                                     format!(" in `{}`", t),
 648                                     format!("within `{}`, ", t),
 649                                     s.map(|s| (format!("within this `{}`", t), s)),
 650                                 )
 651                             })
 652                             .unwrap_or_default();
 653
 654                         let OnUnimplementedNote {
 655                             message,
 656                             label,
 657                             note,
 658                             parent_label,
 659                             append_const_msg,
 660                         } = self.on_unimplemented_note(trait_ref, &obligation);
 661                         let have_alt_message = message.is_some() || label.is_some();
 662                         let is_try_conversion = self.is_try_conversion(span, trait_ref.def_id());
 663                         let is_unsize =
 664                             Some(trait_ref.def_id()) == self.tcx.lang_items().unsize_trait();
 665                         let (message, note, append_const_msg) = if is_try_conversion {
 666                             (
 667                                 Some(format!(
 668                                     "`?` couldn't convert the error to `{}`",
 669                                     trait_ref.skip_binder().self_ty(),
 670                                 )),
 671                                 Some(
 672                                     "the question mark operation (`?`) implicitly performs a \
 673                                      conversion on the error value using the `From` trait"
 674                                         .to_owned(),
 675                                 ),
 676                                 Some(None),
 677                             )
 678                         } else {
 679                             (message, note, append_const_msg)
 680                         };
 681
 682                         let mut err = struct_span_err!(
 683                             self.tcx.sess,
 684                             span,
 685                             E0277,
 686                             "{}",
 687                             message
 688                                 .and_then(|cannot_do_this| {
 689                                     match (predicate_is_const, append_const_msg) {
 690                                         // do nothing if predicate is not const
 691                                         (false, _) => Some(cannot_do_this),
 692                                         // suggested using default post message
 693                                         (true, Some(None)) => {
 694                                             Some(format!("{cannot_do_this} in const contexts"))
 695                                         }
 696                                         // overridden post message
 697                                         (true, Some(Some(post_message))) => {
 698                                             Some(format!("{cannot_do_this}{post_message}"))
 699                                         }
 700                                         // fallback to generic message
 701                                         (true, None) => None,
 702                                     }
 703                                 })
 704                                 .unwrap_or_else(|| format!(
 705                                     "the trait bound `{}` is not satisfied{}",
 706                                     trait_predicate, post_message,
 707                                 ))
 708                         );
 709
 710                         if is_try_conversion && let Some(ret_span) = self.return_type_span(&obligation) {
 711                             err.span_label(
 712                                 ret_span,
 713                                 &format!(
 714                                     "expected `{}` because of this",
 715                                     trait_ref.skip_binder().self_ty()
 716                                 ),
 717                             );
 718                         }
 719
 720                         if Some(trait_ref.def_id()) == tcx.lang_items().tuple_trait() {
 721                             match obligation.cause.code().peel_derives() {
 722                                 ObligationCauseCode::RustCall => {
 723                                     err.set_primary_message("functions with the \"rust-call\" ABI must take a single non-self tuple argument");
 724                                 }
 725                                 ObligationCauseCode::BindingObligation(def_id, _)
 726                                 | ObligationCauseCode::ItemObligation(def_id)
 727                                     if tcx.is_fn_trait(*def_id) =>
 728                                 {
 729                                     err.code(rustc_errors::error_code!(E0059));
 730                                     err.set_primary_message(format!(
 731                                         "type parameter to bare `{}` trait must be a tuple",
 732                                         tcx.def_path_str(*def_id)
 733                                     ));
 734                                 }
 735                                 _ => {}
 736                             }
 737                         }
 738
 739                         if Some(trait_ref.def_id()) == tcx.lang_items().drop_trait()
 740                             && predicate_is_const
 741                         {
 742                             err.note("`~const Drop` was renamed to `~const Destruct`");
 743                             err.note("See <https://github.com/rust-lang/rust/pull/94901> for more details");
 744                         }
 745
 746                         let explanation = if let ObligationCauseCode::MainFunctionType =
 747                             obligation.cause.code()
 748                         {
 749                             "consider using `()`, or a `Result`".to_owned()
 750                         } else {
 751                             let ty_desc = match trait_ref.skip_binder().self_ty().kind() {
 752                                 ty::FnDef(_, _) => Some("fn item"),
 753                                 ty::Closure(_, _) => Some("closure"),
 754                                 _ => None,
 755                             };
 756
 757                             match ty_desc {
 758                                 Some(desc) => format!(
 759                                     "{}the trait `{}` is not implemented for {} `{}`",
 760                                     pre_message,
 761                                     trait_predicate.print_modifiers_and_trait_path(),
 762                                     desc,
 763                                     trait_ref.skip_binder().self_ty(),
 764                                 ),
 765                                 None => format!(
 766                                     "{}the trait `{}` is not implemented for `{}`",
 767                                     pre_message,
 768                                     trait_predicate.print_modifiers_and_trait_path(),
 769                                     trait_ref.skip_binder().self_ty(),
 770                                 ),
 771                             }
 772                         };
 773
 774                         if self.suggest_add_reference_to_arg(
 775                             &obligation,
 776                             &mut err,
 777                             trait_predicate,
 778                             have_alt_message,
 779                         ) {
 780                             self.note_obligation_cause(&mut err, &obligation);
 781                             err.emit();
 782                             return;
 783                         }
 784                         if let Some(ref s) = label {
 785                             // If it has a custom `#[rustc_on_unimplemented]`
 786                             // error message, let's display it as the label!
 787                             err.span_label(span, s);
 788                             if !matches!(trait_ref.skip_binder().self_ty().kind(), ty::Param(_)) {
 789                                 // When the self type is a type param We don't need to "the trait
 790                                 // `std::marker::Sized` is not implemented for `T`" as we will point
 791                                 // at the type param with a label to suggest constraining it.
 792                                 err.help(&explanation);
 793                             }
 794                         } else {
 795                             err.span_label(span, explanation);
 796                         }
 797
 798                         if let ObligationCauseCode::ObjectCastObligation(concrete_ty, obj_ty) = obligation.cause.code().peel_derives() &&
 799                             Some(trait_ref.def_id()) == self.tcx.lang_items().sized_trait() {
 800                             self.suggest_borrowing_for_object_cast(&mut err, &root_obligation, *concrete_ty, *obj_ty);
 801                         }
 802
 803                         let mut unsatisfied_const = false;
 804                         if trait_predicate.is_const_if_const() && obligation.param_env.is_const() {
 805                             let non_const_predicate = trait_ref.without_const();
 806                             let non_const_obligation = Obligation {
 807                                 cause: obligation.cause.clone(),
 808                                 param_env: obligation.param_env.without_const(),
 809                                 predicate: non_const_predicate.to_predicate(tcx),
 810                                 recursion_depth: obligation.recursion_depth,
 811                             };
 812                             if self.predicate_may_hold(&non_const_obligation) {
 813                                 unsatisfied_const = true;
 814                                 err.span_note(
 815                                     span,
 816                                     &format!(
 817                                         "the trait `{}` is implemented for `{}`, \
 818                                         but that implementation is not `const`",
 819                                         non_const_predicate.print_modifiers_and_trait_path(),
 820                                         trait_ref.skip_binder().self_ty(),
 821                                     ),
 822                                 );
 823                             }
 824                         }
 825
 826                         if let Some((msg, span)) = type_def {
 827                             err.span_label(span, &msg);
 828                         }
 829                         if let Some(ref s) = note {
 830                             // If it has a custom `#[rustc_on_unimplemented]` note, let's display it
 831                             err.note(s.as_str());
 832                         }
 833                         if let Some(ref s) = parent_label {
 834                             let body = tcx
 835                                 .hir()
 836                                 .opt_local_def_id(obligation.cause.body_id)
 837                                 .unwrap_or_else(|| {
 838                                     tcx.hir().body_owner_def_id(hir::BodyId {
 839                                         hir_id: obligation.cause.body_id,
 840                                     })
 841                                 });
 842                             err.span_label(tcx.def_span(body), s);
 843                         }
 844
 845                         self.suggest_floating_point_literal(&obligation, &mut err, &trait_ref);
 846                         self.suggest_dereferencing_index(&obligation, &mut err, trait_predicate);
 847                         let mut suggested =
 848                             self.suggest_dereferences(&obligation, &mut err, trait_predicate);
 849                         suggested |= self.suggest_fn_call(&obligation, &mut err, trait_predicate);
 850                         suggested |=
 851                             self.suggest_remove_reference(&obligation, &mut err, trait_predicate);
 852                         suggested |= self.suggest_semicolon_removal(
 853                             &obligation,
 854                             &mut err,
 855                             span,
 856                             trait_predicate,
 857                         );
 858                         self.note_version_mismatch(&mut err, &trait_ref);
 859                         self.suggest_remove_await(&obligation, &mut err);
 860                         self.suggest_derive(&obligation, &mut err, trait_predicate);
 861
 862                         if Some(trait_ref.def_id()) == tcx.lang_items().try_trait() {
 863                             self.suggest_await_before_try(
 864                                 &mut err,
 865                                 &obligation,
 866                                 trait_predicate,
 867                                 span,
 868                             );
 869                         }
 870
 871                         if self.suggest_impl_trait(&mut err, span, &obligation, trait_predicate) {
 872                             err.emit();
 873                             return;
 874                         }
 875
 876                         if is_unsize {
 877                             // If the obligation failed due to a missing implementation of the
 878                             // `Unsize` trait, give a pointer to why that might be the case
 879                             err.note(
 880                                 "all implementations of `Unsize` are provided \
 881                                 automatically by the compiler, see \
 882                                 <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> \
 883                                 for more information",
 884                             );
 885                         }
 886
 887                         let is_fn_trait = tcx.is_fn_trait(trait_ref.def_id());
 888                         let is_target_feature_fn = if let ty::FnDef(def_id, _) =
 889                             *trait_ref.skip_binder().self_ty().kind()
 890                         {
 891                             !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
 892                         } else {
 893                             false
 894                         };
 895                         if is_fn_trait && is_target_feature_fn {
 896                             err.note(
 897                                 "`#[target_feature]` functions do not implement the `Fn` traits",
 898                             );
 899                         }
 900
 901                         // Try to report a help message
 902                         if is_fn_trait
 903                             && let Ok((implemented_kind, params)) = self.type_implements_fn_trait(
 904                             obligation.param_env,
 905                             trait_ref.self_ty(),
 906                             trait_predicate.skip_binder().constness,
 907                             trait_predicate.skip_binder().polarity,
 908                         )
 909                         {
 910                             // If the type implements `Fn`, `FnMut`, or `FnOnce`, suppress the following
 911                             // suggestion to add trait bounds for the type, since we only typically implement
 912                             // these traits once.
 913
 914                             // Note if the `FnMut` or `FnOnce` is less general than the trait we're trying
 915                             // to implement.
 916                             let selected_kind =
 917                                 self.tcx.fn_trait_kind_from_def_id(trait_ref.def_id())
 918                                     .expect("expected to map DefId to ClosureKind");
 919                             if !implemented_kind.extends(selected_kind) {
 920                                 err.note(
 921                                     &format!(
 922                                         "`{}` implements `{}`, but it must implement `{}`, which is more general",
 923                                         trait_ref.skip_binder().self_ty(),
 924                                         implemented_kind,
 925                                         selected_kind
 926                                     )
 927                                 );
 928                             }
 929
 930                             // Note any argument mismatches
 931                             let given_ty = params.skip_binder();
 932                             let expected_ty = trait_ref.skip_binder().substs.type_at(1);
 933                             if let ty::Tuple(given) = given_ty.kind()
 934                                 && let ty::Tuple(expected) = expected_ty.kind()
 935                             {
 936                                 if expected.len() != given.len() {
 937                                     // Note number of types that were expected and given
 938                                     err.note(
 939                                         &format!(
 940                                             "expected a closure taking {} argument{}, but one taking {} argument{} was given",
 941                                             given.len(),
 942                                             pluralize!(given.len()),
 943                                             expected.len(),
 944                                             pluralize!(expected.len()),
 945                                         )
 946                                     );
 947                                 } else if !self.same_type_modulo_infer(given_ty, expected_ty) {
 948                                     // Print type mismatch
 949                                     let (expected_args, given_args) =
 950                                         self.cmp(given_ty, expected_ty);
 951                                     err.note_expected_found(
 952                                         &"a closure with arguments",
 953                                         expected_args,
 954                                         &"a closure with arguments",
 955                                         given_args,
 956                                     );
 957                                 }
 958                             }
 959                         } else if !trait_ref.has_non_region_infer()
 960                             && self.predicate_can_apply(obligation.param_env, trait_predicate)
 961                         {
 962                             // If a where-clause may be useful, remind the
 963                             // user that they can add it.
 964                             //
 965                             // don't display an on-unimplemented note, as
 966                             // these notes will often be of the form
 967                             //     "the type `T` can't be frobnicated"
 968                             // which is somewhat confusing.
 969                             self.suggest_restricting_param_bound(
 970                                 &mut err,
 971                                 trait_predicate,
 972                                 None,
 973                                 obligation.cause.body_id,
 974                             );
 975                         } else if !suggested && !unsatisfied_const {
 976                             // Can't show anything else useful, try to find similar impls.
 977                             let impl_candidates = self.find_similar_impl_candidates(trait_predicate);
 978                             if !self.report_similar_impl_candidates(
 979                                 impl_candidates,
 980                                 trait_ref,
 981                                 obligation.cause.body_id,
 982                                 &mut err,
 983                             ) {
 984                                 // This is *almost* equivalent to
 985                                 // `obligation.cause.code().peel_derives()`, but it gives us the
 986                                 // trait predicate for that corresponding root obligation. This
 987                                 // lets us get a derived obligation from a type parameter, like
 988                                 // when calling `string.strip_suffix(p)` where `p` is *not* an
 989                                 // implementer of `Pattern<'_>`.
 990                                 let mut code = obligation.cause.code();
 991                                 let mut trait_pred = trait_predicate;
 992                                 let mut peeled = false;
 993                                 while let Some((parent_code, parent_trait_pred)) = code.parent() {
 994                                     code = parent_code;
 995                                     if let Some(parent_trait_pred) = parent_trait_pred {
 996                                         trait_pred = parent_trait_pred;
 997                                         peeled = true;
 998                                     }
 999                                 }
1000                                 let def_id = trait_pred.def_id();
1001                                 // Mention *all* the `impl`s for the *top most* obligation, the
1002                                 // user might have meant to use one of them, if any found. We skip
1003                                 // auto-traits or fundamental traits that might not be exactly what
1004                                 // the user might expect to be presented with. Instead this is
1005                                 // useful for less general traits.
1006                                 if peeled
1007                                     && !self.tcx.trait_is_auto(def_id)
1008                                     && !self.tcx.lang_items().iter().any(|(_, id)| id == def_id)
1009                                 {
1010                                     let trait_ref = trait_pred.to_poly_trait_ref();
1011                                     let impl_candidates =
1012                                         self.find_similar_impl_candidates(trait_pred);
1013                                     self.report_similar_impl_candidates(
1014                                         impl_candidates,
1015                                         trait_ref,
1016                                         obligation.cause.body_id,
1017                                         &mut err,
1018                                     );
1019                                 }
1020                             }
1021                         }
1022
1023                         // Changing mutability doesn't make a difference to whether we have
1024                         // an `Unsize` impl (Fixes ICE in #71036)
1025                         if !is_unsize {
1026                             self.suggest_change_mut(&obligation, &mut err, trait_predicate);
1027                         }
1028
1029                         // If this error is due to `!: Trait` not implemented but `(): Trait` is
1030                         // implemented, and fallback has occurred, then it could be due to a
1031                         // variable that used to fallback to `()` now falling back to `!`. Issue a
1032                         // note informing about the change in behaviour.
1033                         if trait_predicate.skip_binder().self_ty().is_never()
1034                             && self.fallback_has_occurred
1035                         {
1036                             let predicate = trait_predicate.map_bound(|trait_pred| {
1037                                 trait_pred.with_self_type(self.tcx, self.tcx.mk_unit())
1038                             });
1039                             let unit_obligation = obligation.with(tcx, predicate);
1040                             if self.predicate_may_hold(&unit_obligation) {
1041                                 err.note(
1042                                     "this error might have been caused by changes to \
1043                                     Rust's type-inference algorithm (see issue #48950 \
1044                                     <https://github.com/rust-lang/rust/issues/48950> \
1045                                     for more information)",
1046                                 );
1047                                 err.help("did you intend to use the type `()` here instead?");
1048                             }
1049                         }
1050
1051                         // Return early if the trait is Debug or Display and the invocation
1052                         // originates within a standard library macro, because the output
1053                         // is otherwise overwhelming and unhelpful (see #85844 for an
1054                         // example).
1055
1056                         let in_std_macro =
1057                             match obligation.cause.span.ctxt().outer_expn_data().macro_def_id {
1058                                 Some(macro_def_id) => {
1059                                     let crate_name = tcx.crate_name(macro_def_id.krate);
1060                                     crate_name == sym::std || crate_name == sym::core
1061                                 }
1062                                 None => false,
1063                             };
1064
1065                         if in_std_macro
1066                             && matches!(
1067                                 self.tcx.get_diagnostic_name(trait_ref.def_id()),
1068                                 Some(sym::Debug | sym::Display)
1069                             )
1070                         {
1071                             err.emit();
1072                             return;
1073                         }
1074
1075                         err
1076                     }
1077
1078                     ty::PredicateKind::Subtype(predicate) => {
1079                         // Errors for Subtype predicates show up as
1080                         // `FulfillmentErrorCode::CodeSubtypeError`,
1081                         // not selection error.
1082                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
1083                     }
1084
1085                     ty::PredicateKind::Coerce(predicate) => {
1086                         // Errors for Coerce predicates show up as
1087                         // `FulfillmentErrorCode::CodeSubtypeError`,
1088                         // not selection error.
1089                         span_bug!(span, "coerce requirement gave wrong error: `{:?}`", predicate)
1090                     }
1091
1092                     ty::PredicateKind::Clause(ty::Clause::RegionOutlives(..))
1093                     | ty::PredicateKind::Clause(ty::Clause::Projection(..))
1094                     | ty::PredicateKind::Clause(ty::Clause::TypeOutlives(..)) => {
1095                         let predicate = self.resolve_vars_if_possible(obligation.predicate);
1096                         struct_span_err!(
1097                             self.tcx.sess,
1098                             span,
1099                             E0280,
1100                             "the requirement `{}` is not satisfied",
1101                             predicate
1102                         )
1103                     }
1104
1105                     ty::PredicateKind::ObjectSafe(trait_def_id) => {
1106                         let violations = self.tcx.object_safety_violations(trait_def_id);
1107                         report_object_safety_error(self.tcx, span, trait_def_id, violations)
1108                     }
1109
1110                     ty::PredicateKind::ClosureKind(closure_def_id, closure_substs, kind) => {
1111                         let found_kind = self.closure_kind(closure_substs).unwrap();
1112                         let closure_span = self.tcx.def_span(closure_def_id);
1113                         let mut err = struct_span_err!(
1114                             self.tcx.sess,
1115                             closure_span,
1116                             E0525,
1117                             "expected a closure that implements the `{}` trait, \
1118                              but this closure only implements `{}`",
1119                             kind,
1120                             found_kind
1121                         );
1122
1123                         err.span_label(
1124                             closure_span,
1125                             format!("this closure implements `{}`, not `{}`", found_kind, kind),
1126                         );
1127                         err.span_label(
1128                             obligation.cause.span,
1129                             format!("the requirement to implement `{}` derives from here", kind),
1130                         );
1131
1132                         // Additional context information explaining why the closure only implements
1133                         // a particular trait.
1134                         if let Some(typeck_results) = &self.typeck_results {
1135                             let hir_id = self
1136                                 .tcx
1137                                 .hir()
1138                                 .local_def_id_to_hir_id(closure_def_id.expect_local());
1139                             match (found_kind, typeck_results.closure_kind_origins().get(hir_id)) {
1140                                 (ty::ClosureKind::FnOnce, Some((span, place))) => {
1141                                     err.span_label(
1142                                         *span,
1143                                         format!(
1144                                             "closure is `FnOnce` because it moves the \
1145                                          variable `{}` out of its environment",
1146                                             ty::place_to_string_for_capture(tcx, place)
1147                                         ),
1148                                     );
1149                                 }
1150                                 (ty::ClosureKind::FnMut, Some((span, place))) => {
1151                                     err.span_label(
1152                                         *span,
1153                                         format!(
1154                                             "closure is `FnMut` because it mutates the \
1155                                          variable `{}` here",
1156                                             ty::place_to_string_for_capture(tcx, place)
1157                                         ),
1158                                     );
1159                                 }
1160                                 _ => {}
1161                             }
1162                         }
1163
1164                         err
1165                     }
1166
1167                     ty::PredicateKind::WellFormed(ty) => {
1168                         if !self.tcx.sess.opts.unstable_opts.chalk {
1169                             // WF predicates cannot themselves make
1170                             // errors. They can only block due to
1171                             // ambiguity; otherwise, they always
1172                             // degenerate into other obligations
1173                             // (which may fail).
1174                             span_bug!(span, "WF predicate not satisfied for {:?}", ty);
1175                         } else {
1176                             // FIXME: we'll need a better message which takes into account
1177                             // which bounds actually failed to hold.
1178                             self.tcx.sess.struct_span_err(
1179                                 span,
1180                                 &format!("the type `{}` is not well-formed (chalk)", ty),
1181                             )
1182                         }
1183                     }
1184
1185                     ty::PredicateKind::ConstEvaluatable(..) => {
1186                         // Errors for `ConstEvaluatable` predicates show up as
1187                         // `SelectionError::ConstEvalFailure`,
1188                         // not `Unimplemented`.
1189                         span_bug!(
1190                             span,
1191                             "const-evaluatable requirement gave wrong error: `{:?}`",
1192                             obligation
1193                         )
1194                     }
1195
1196                     ty::PredicateKind::ConstEquate(..) => {
1197                         // Errors for `ConstEquate` predicates show up as
1198                         // `SelectionError::ConstEvalFailure`,
1199                         // not `Unimplemented`.
1200                         span_bug!(
1201                             span,
1202                             "const-equate requirement gave wrong error: `{:?}`",
1203                             obligation
1204                         )
1205                     }
1206
1207                     ty::PredicateKind::Ambiguous => span_bug!(span, "ambiguous"),
1208
1209                     ty::PredicateKind::TypeWellFormedFromEnv(..) => span_bug!(
1210                         span,
1211                         "TypeWellFormedFromEnv predicate should only exist in the environment"
1212                     ),
1213                 }
1214             }
1215
1216             OutputTypeParameterMismatch(found_trait_ref, expected_trait_ref, _) => {
1217                 let found_trait_ref = self.resolve_vars_if_possible(found_trait_ref);
1218                 let expected_trait_ref = self.resolve_vars_if_possible(expected_trait_ref);
1219
1220                 if expected_trait_ref.self_ty().references_error() {
1221                     return;
1222                 }
1223
1224                 let Some(found_trait_ty) = found_trait_ref.self_ty().no_bound_vars() else {
1225                     return;
1226                 };
1227
1228                 let found_did = match *found_trait_ty.kind() {
1229                     ty::Closure(did, _)
1230                     | ty::Foreign(did)
1231                     | ty::FnDef(did, _)
1232                     | ty::Generator(did, ..) => Some(did),
1233                     ty::Adt(def, _) => Some(def.did()),
1234                     _ => None,
1235                 };
1236
1237                 let found_span = found_did.and_then(|did| self.tcx.hir().span_if_local(did));
1238
1239                 if self.reported_closure_mismatch.borrow().contains(&(span, found_span)) {
1240                     // We check closures twice, with obligations flowing in different directions,
1241                     // but we want to complain about them only once.
1242                     return;
1243                 }
1244
1245                 self.reported_closure_mismatch.borrow_mut().insert((span, found_span));
1246
1247                 let mut not_tupled = false;
1248
1249                 let found = match found_trait_ref.skip_binder().substs.type_at(1).kind() {
1250                     ty::Tuple(ref tys) => vec![ArgKind::empty(); tys.len()],
1251                     _ => {
1252                         not_tupled = true;
1253                         vec![ArgKind::empty()]
1254                     }
1255                 };
1256
1257                 let expected_ty = expected_trait_ref.skip_binder().substs.type_at(1);
1258                 let expected = match expected_ty.kind() {
1259                     ty::Tuple(ref tys) => {
1260                         tys.iter().map(|t| ArgKind::from_expected_ty(t, Some(span))).collect()
1261                     }
1262                     _ => {
1263                         not_tupled = true;
1264                         vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())]
1265                     }
1266                 };
1267
1268                 // If this is a `Fn` family trait and either the expected or found
1269                 // is not tupled, then fall back to just a regular mismatch error.
1270                 // This shouldn't be common unless manually implementing one of the
1271                 // traits manually, but don't make it more confusing when it does
1272                 // happen.
1273                 if Some(expected_trait_ref.def_id()) != tcx.lang_items().gen_trait() && not_tupled {
1274                     self.report_and_explain_type_error(
1275                         TypeTrace::poly_trait_refs(
1276                             &obligation.cause,
1277                             true,
1278                             expected_trait_ref,
1279                             found_trait_ref,
1280                         ),
1281                         ty::error::TypeError::Mismatch,
1282                     )
1283                 } else if found.len() == expected.len() {
1284                     self.report_closure_arg_mismatch(
1285                         span,
1286                         found_span,
1287                         found_trait_ref,
1288                         expected_trait_ref,
1289                         obligation.cause.code(),
1290                     )
1291                 } else {
1292                     let (closure_span, closure_arg_span, found) = found_did
1293                         .and_then(|did| {
1294                             let node = self.tcx.hir().get_if_local(did)?;
1295                             let (found_span, closure_arg_span, found) =
1296                                 self.get_fn_like_arguments(node)?;
1297                             Some((Some(found_span), closure_arg_span, found))
1298                         })
1299                         .unwrap_or((found_span, None, found));
1300
1301                     self.report_arg_count_mismatch(
1302                         span,
1303                         closure_span,
1304                         expected,
1305                         found,
1306                         found_trait_ty.is_closure(),
1307                         closure_arg_span,
1308                     )
1309                 }
1310             }
1311
1312             TraitNotObjectSafe(did) => {
1313                 let violations = self.tcx.object_safety_violations(did);
1314                 report_object_safety_error(self.tcx, span, did, violations)
1315             }
1316
1317             SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsInfer) => {
1318                 bug!(
1319                     "MentionsInfer should have been handled in `traits/fulfill.rs` or `traits/select/mod.rs`"
1320                 )
1321             }
1322             SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsParam) => {
1323                 if !self.tcx.features().generic_const_exprs {
1324                     let mut err = self.tcx.sess.struct_span_err(
1325                         span,
1326                         "constant expression depends on a generic parameter",
1327                     );
1328                     // FIXME(const_generics): we should suggest to the user how they can resolve this
1329                     // issue. However, this is currently not actually possible
1330                     // (see https://github.com/rust-lang/rust/issues/66962#issuecomment-575907083).
1331                     //
1332                     // Note that with `feature(generic_const_exprs)` this case should not
1333                     // be reachable.
1334                     err.note("this may fail depending on what value the parameter takes");
1335                     err.emit();
1336                     return;
1337                 }
1338
1339                 match obligation.predicate.kind().skip_binder() {
1340                     ty::PredicateKind::ConstEvaluatable(ct) => {
1341                         let ty::ConstKind::Unevaluated(uv) = ct.kind() else {
1342                             bug!("const evaluatable failed for non-unevaluated const `{ct:?}`");
1343                         };
1344                         let mut err =
1345                             self.tcx.sess.struct_span_err(span, "unconstrained generic constant");
1346                         let const_span = self.tcx.def_span(uv.def.did);
1347                         match self.tcx.sess.source_map().span_to_snippet(const_span) {
1348                             Ok(snippet) => err.help(&format!(
1349                                 "try adding a `where` bound using this expression: `where [(); {}]:`",
1350                                 snippet
1351                             )),
1352                             _ => err.help("consider adding a `where` bound using this expression"),
1353                         };
1354                         err
1355                     }
1356                     _ => {
1357                         span_bug!(
1358                             span,
1359                             "unexpected non-ConstEvaluatable predicate, this should not be reachable"
1360                         )
1361                     }
1362                 }
1363             }
1364
1365             // Already reported in the query.
1366             SelectionError::NotConstEvaluatable(NotConstEvaluatable::Error(_)) => {
1367                 // FIXME(eddyb) remove this once `ErrorGuaranteed` becomes a proof token.
1368                 self.tcx.sess.delay_span_bug(span, "`ErrorGuaranteed` without an error");
1369                 return;
1370             }
1371             // Already reported.
1372             Overflow(OverflowError::Error(_)) => {
1373                 self.tcx.sess.delay_span_bug(span, "`OverflowError` has been reported");
1374                 return;
1375             }
1376             Overflow(_) => {
1377                 bug!("overflow should be handled before the `report_selection_error` path");
1378             }
1379             SelectionError::ErrorReporting => {
1380                 bug!("ErrorReporting Overflow should not reach `report_selection_err` call")
1381             }
1382         };
1383
1384         self.note_obligation_cause(&mut err, &obligation);
1385         self.point_at_returns_when_relevant(&mut err, &obligation);
1386
1387         err.emit();
1388     }
1389 }
1390
1391 trait InferCtxtPrivExt<'tcx> {
1392     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1393     // `error` occurring implies that `cond` occurs.
1394     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool;
1395
1396     fn report_fulfillment_error(
1397         &self,
1398         error: &FulfillmentError<'tcx>,
1399         body_id: Option<hir::BodyId>,
1400     );
1401
1402     fn report_projection_error(
1403         &self,
1404         obligation: &PredicateObligation<'tcx>,
1405         error: &MismatchedProjectionTypes<'tcx>,
1406     );
1407
1408     fn maybe_detailed_projection_msg(
1409         &self,
1410         pred: ty::ProjectionPredicate<'tcx>,
1411         normalized_ty: ty::Term<'tcx>,
1412         expected_ty: ty::Term<'tcx>,
1413     ) -> Option<String>;
1414
1415     fn fuzzy_match_tys(
1416         &self,
1417         a: Ty<'tcx>,
1418         b: Ty<'tcx>,
1419         ignoring_lifetimes: bool,
1420     ) -> Option<CandidateSimilarity>;
1421
1422     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str>;
1423
1424     fn find_similar_impl_candidates(
1425         &self,
1426         trait_pred: ty::PolyTraitPredicate<'tcx>,
1427     ) -> Vec<ImplCandidate<'tcx>>;
1428
1429     fn report_similar_impl_candidates(
1430         &self,
1431         impl_candidates: Vec<ImplCandidate<'tcx>>,
1432         trait_ref: ty::PolyTraitRef<'tcx>,
1433         body_id: hir::HirId,
1434         err: &mut Diagnostic,
1435     ) -> bool;
1436
1437     /// Gets the parent trait chain start
1438     fn get_parent_trait_ref(
1439         &self,
1440         code: &ObligationCauseCode<'tcx>,
1441     ) -> Option<(String, Option<Span>)>;
1442
1443     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1444     /// with the same path as `trait_ref`, a help message about
1445     /// a probable version mismatch is added to `err`
1446     fn note_version_mismatch(
1447         &self,
1448         err: &mut Diagnostic,
1449         trait_ref: &ty::PolyTraitRef<'tcx>,
1450     ) -> bool;
1451
1452     /// Creates a `PredicateObligation` with `new_self_ty` replacing the existing type in the
1453     /// `trait_ref`.
1454     ///
1455     /// For this to work, `new_self_ty` must have no escaping bound variables.
1456     fn mk_trait_obligation_with_new_self_ty(
1457         &self,
1458         param_env: ty::ParamEnv<'tcx>,
1459         trait_ref_and_ty: ty::Binder<'tcx, (ty::TraitPredicate<'tcx>, Ty<'tcx>)>,
1460     ) -> PredicateObligation<'tcx>;
1461
1462     fn maybe_report_ambiguity(
1463         &self,
1464         obligation: &PredicateObligation<'tcx>,
1465         body_id: Option<hir::BodyId>,
1466     );
1467
1468     fn predicate_can_apply(
1469         &self,
1470         param_env: ty::ParamEnv<'tcx>,
1471         pred: ty::PolyTraitPredicate<'tcx>,
1472     ) -> bool;
1473
1474     fn note_obligation_cause(&self, err: &mut Diagnostic, obligation: &PredicateObligation<'tcx>);
1475
1476     fn suggest_unsized_bound_if_applicable(
1477         &self,
1478         err: &mut Diagnostic,
1479         obligation: &PredicateObligation<'tcx>,
1480     );
1481
1482     fn annotate_source_of_ambiguity(
1483         &self,
1484         err: &mut Diagnostic,
1485         impls: &[DefId],
1486         predicate: ty::Predicate<'tcx>,
1487     );
1488
1489     fn maybe_suggest_unsized_generics(&self, err: &mut Diagnostic, span: Span, node: Node<'tcx>);
1490
1491     fn maybe_indirection_for_unsized(
1492         &self,
1493         err: &mut Diagnostic,
1494         item: &'tcx Item<'tcx>,
1495         param: &'tcx GenericParam<'tcx>,
1496     ) -> bool;
1497
1498     fn is_recursive_obligation(
1499         &self,
1500         obligated_types: &mut Vec<Ty<'tcx>>,
1501         cause_code: &ObligationCauseCode<'tcx>,
1502     ) -> bool;
1503 }
1504
1505 impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
1506     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1507     // `error` occurring implies that `cond` occurs.
1508     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool {
1509         if cond == error {
1510             return true;
1511         }
1512
1513         // FIXME: It should be possible to deal with `ForAll` in a cleaner way.
1514         let bound_error = error.kind();
1515         let (cond, error) = match (cond.kind().skip_binder(), bound_error.skip_binder()) {
1516             (
1517                 ty::PredicateKind::Clause(ty::Clause::Trait(..)),
1518                 ty::PredicateKind::Clause(ty::Clause::Trait(error)),
1519             ) => (cond, bound_error.rebind(error)),
1520             _ => {
1521                 // FIXME: make this work in other cases too.
1522                 return false;
1523             }
1524         };
1525
1526         for obligation in super::elaborate_predicates(self.tcx, std::iter::once(cond)) {
1527             let bound_predicate = obligation.predicate.kind();
1528             if let ty::PredicateKind::Clause(ty::Clause::Trait(implication)) =
1529                 bound_predicate.skip_binder()
1530             {
1531                 let error = error.to_poly_trait_ref();
1532                 let implication = bound_predicate.rebind(implication.trait_ref);
1533                 // FIXME: I'm just not taking associated types at all here.
1534                 // Eventually I'll need to implement param-env-aware
1535                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
1536                 let param_env = ty::ParamEnv::empty();
1537                 if self.can_sub(param_env, error, implication).is_ok() {
1538                     debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
1539                     return true;
1540                 }
1541             }
1542         }
1543
1544         false
1545     }
1546
1547     #[instrument(skip(self), level = "debug")]
1548     fn report_fulfillment_error(
1549         &self,
1550         error: &FulfillmentError<'tcx>,
1551         body_id: Option<hir::BodyId>,
1552     ) {
1553         match error.code {
1554             FulfillmentErrorCode::CodeSelectionError(ref selection_error) => {
1555                 self.report_selection_error(
1556                     error.obligation.clone(),
1557                     &error.root_obligation,
1558                     selection_error,
1559                 );
1560             }
1561             FulfillmentErrorCode::CodeProjectionError(ref e) => {
1562                 self.report_projection_error(&error.obligation, e);
1563             }
1564             FulfillmentErrorCode::CodeAmbiguity => {
1565                 self.maybe_report_ambiguity(&error.obligation, body_id);
1566             }
1567             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
1568                 self.report_mismatched_types(
1569                     &error.obligation.cause,
1570                     expected_found.expected,
1571                     expected_found.found,
1572                     err.clone(),
1573                 )
1574                 .emit();
1575             }
1576             FulfillmentErrorCode::CodeConstEquateError(ref expected_found, ref err) => {
1577                 let mut diag = self.report_mismatched_consts(
1578                     &error.obligation.cause,
1579                     expected_found.expected,
1580                     expected_found.found,
1581                     err.clone(),
1582                 );
1583                 let code = error.obligation.cause.code().peel_derives().peel_match_impls();
1584                 if let ObligationCauseCode::BindingObligation(..)
1585                 | ObligationCauseCode::ItemObligation(..)
1586                 | ObligationCauseCode::ExprBindingObligation(..)
1587                 | ObligationCauseCode::ExprItemObligation(..) = code
1588                 {
1589                     self.note_obligation_cause_code(
1590                         &mut diag,
1591                         error.obligation.predicate,
1592                         error.obligation.param_env,
1593                         code,
1594                         &mut vec![],
1595                         &mut Default::default(),
1596                     );
1597                 }
1598                 diag.emit();
1599             }
1600             FulfillmentErrorCode::CodeCycle(ref cycle) => {
1601                 self.report_overflow_obligation_cycle(cycle);
1602             }
1603         }
1604     }
1605
1606     #[instrument(level = "debug", skip_all)]
1607     fn report_projection_error(
1608         &self,
1609         obligation: &PredicateObligation<'tcx>,
1610         error: &MismatchedProjectionTypes<'tcx>,
1611     ) {
1612         let predicate = self.resolve_vars_if_possible(obligation.predicate);
1613
1614         if predicate.references_error() {
1615             return;
1616         }
1617
1618         self.probe(|_| {
1619             let ocx = ObligationCtxt::new_in_snapshot(self);
1620
1621             // try to find the mismatched types to report the error with.
1622             //
1623             // this can fail if the problem was higher-ranked, in which
1624             // cause I have no idea for a good error message.
1625             let bound_predicate = predicate.kind();
1626             let (values, err) = if let ty::PredicateKind::Clause(ty::Clause::Projection(data)) =
1627                 bound_predicate.skip_binder()
1628             {
1629                 let data = self.replace_bound_vars_with_fresh_vars(
1630                     obligation.cause.span,
1631                     infer::LateBoundRegionConversionTime::HigherRankedType,
1632                     bound_predicate.rebind(data),
1633                 );
1634                 let normalized_ty = ocx.normalize(
1635                     &obligation.cause,
1636                     obligation.param_env,
1637                     self.tcx
1638                         .mk_projection(data.projection_ty.item_def_id, data.projection_ty.substs),
1639                 );
1640
1641                 debug!(?obligation.cause, ?obligation.param_env);
1642
1643                 debug!(?normalized_ty, data.ty = ?data.term);
1644
1645                 let is_normalized_ty_expected = !matches!(
1646                     obligation.cause.code().peel_derives(),
1647                     ObligationCauseCode::ItemObligation(_)
1648                         | ObligationCauseCode::BindingObligation(_, _)
1649                         | ObligationCauseCode::ExprItemObligation(..)
1650                         | ObligationCauseCode::ExprBindingObligation(..)
1651                         | ObligationCauseCode::ObjectCastObligation(..)
1652                         | ObligationCauseCode::OpaqueType
1653                 );
1654                 let expected_ty = data.term.ty().unwrap_or_else(|| self.tcx.ty_error());
1655
1656                 // constrain inference variables a bit more to nested obligations from normalize so
1657                 // we can have more helpful errors.
1658                 ocx.select_where_possible();
1659
1660                 if let Err(new_err) = ocx.eq_exp(
1661                     &obligation.cause,
1662                     obligation.param_env,
1663                     is_normalized_ty_expected,
1664                     normalized_ty,
1665                     expected_ty,
1666                 ) {
1667                     (Some((data, is_normalized_ty_expected, normalized_ty, expected_ty)), new_err)
1668                 } else {
1669                     (None, error.err)
1670                 }
1671             } else {
1672                 (None, error.err)
1673             };
1674
1675             let msg = values
1676                 .and_then(|(predicate, _, normalized_ty, expected_ty)| {
1677                     self.maybe_detailed_projection_msg(
1678                         predicate,
1679                         normalized_ty.into(),
1680                         expected_ty.into(),
1681                     )
1682                 })
1683                 .unwrap_or_else(|| format!("type mismatch resolving `{}`", predicate));
1684             let mut diag = struct_span_err!(self.tcx.sess, obligation.cause.span, E0271, "{msg}");
1685
1686             let secondary_span = match predicate.kind().skip_binder() {
1687                 ty::PredicateKind::Clause(ty::Clause::Projection(proj)) => self
1688                     .tcx
1689                     .opt_associated_item(proj.projection_ty.item_def_id)
1690                     .and_then(|trait_assoc_item| {
1691                         self.tcx
1692                             .trait_of_item(proj.projection_ty.item_def_id)
1693                             .map(|id| (trait_assoc_item, id))
1694                     })
1695                     .and_then(|(trait_assoc_item, id)| {
1696                         let trait_assoc_ident = trait_assoc_item.ident(self.tcx);
1697                         self.tcx.find_map_relevant_impl(id, proj.projection_ty.self_ty(), |did| {
1698                             self.tcx
1699                                 .associated_items(did)
1700                                 .in_definition_order()
1701                                 .find(|assoc| assoc.ident(self.tcx) == trait_assoc_ident)
1702                         })
1703                     })
1704                     .and_then(|item| match self.tcx.hir().get_if_local(item.def_id) {
1705                         Some(
1706                             hir::Node::TraitItem(hir::TraitItem {
1707                                 kind: hir::TraitItemKind::Type(_, Some(ty)),
1708                                 ..
1709                             })
1710                             | hir::Node::ImplItem(hir::ImplItem {
1711                                 kind: hir::ImplItemKind::Type(ty),
1712                                 ..
1713                             }),
1714                         ) => Some((ty.span, format!("type mismatch resolving `{}`", predicate))),
1715                         _ => None,
1716                     }),
1717                 _ => None,
1718             };
1719             self.note_type_err(
1720                 &mut diag,
1721                 &obligation.cause,
1722                 secondary_span,
1723                 values.map(|(_, is_normalized_ty_expected, normalized_ty, expected_ty)| {
1724                     infer::ValuePairs::Terms(ExpectedFound::new(
1725                         is_normalized_ty_expected,
1726                         normalized_ty.into(),
1727                         expected_ty.into(),
1728                     ))
1729                 }),
1730                 err,
1731                 true,
1732                 false,
1733             );
1734             self.note_obligation_cause(&mut diag, obligation);
1735             diag.emit();
1736         });
1737     }
1738
1739     fn maybe_detailed_projection_msg(
1740         &self,
1741         pred: ty::ProjectionPredicate<'tcx>,
1742         normalized_ty: ty::Term<'tcx>,
1743         expected_ty: ty::Term<'tcx>,
1744     ) -> Option<String> {
1745         let trait_def_id = pred.projection_ty.trait_def_id(self.tcx);
1746         let self_ty = pred.projection_ty.self_ty();
1747
1748         if Some(pred.projection_ty.item_def_id) == self.tcx.lang_items().fn_once_output() {
1749             Some(format!(
1750                 "expected `{self_ty}` to be a {fn_kind} that returns `{expected_ty}`, but it returns `{normalized_ty}`",
1751                 fn_kind = self_ty.prefix_string(self.tcx)
1752             ))
1753         } else if Some(trait_def_id) == self.tcx.lang_items().future_trait() {
1754             Some(format!(
1755                 "expected `{self_ty}` to be a future that resolves to `{expected_ty}`, but it resolves to `{normalized_ty}`"
1756             ))
1757         } else if Some(trait_def_id) == self.tcx.get_diagnostic_item(sym::Iterator) {
1758             Some(format!(
1759                 "expected `{self_ty}` to be an iterator that yields `{expected_ty}`, but it yields `{normalized_ty}`"
1760             ))
1761         } else {
1762             None
1763         }
1764     }
1765
1766     fn fuzzy_match_tys(
1767         &self,
1768         mut a: Ty<'tcx>,
1769         mut b: Ty<'tcx>,
1770         ignoring_lifetimes: bool,
1771     ) -> Option<CandidateSimilarity> {
1772         /// returns the fuzzy category of a given type, or None
1773         /// if the type can be equated to any type.
1774         fn type_category(tcx: TyCtxt<'_>, t: Ty<'_>) -> Option<u32> {
1775             match t.kind() {
1776                 ty::Bool => Some(0),
1777                 ty::Char => Some(1),
1778                 ty::Str => Some(2),
1779                 ty::Adt(def, _) if Some(def.did()) == tcx.lang_items().string() => Some(2),
1780                 ty::Int(..)
1781                 | ty::Uint(..)
1782                 | ty::Float(..)
1783                 | ty::Infer(ty::IntVar(..) | ty::FloatVar(..)) => Some(4),
1784                 ty::Ref(..) | ty::RawPtr(..) => Some(5),
1785                 ty::Array(..) | ty::Slice(..) => Some(6),
1786                 ty::FnDef(..) | ty::FnPtr(..) => Some(7),
1787                 ty::Dynamic(..) => Some(8),
1788                 ty::Closure(..) => Some(9),
1789                 ty::Tuple(..) => Some(10),
1790                 ty::Param(..) => Some(11),
1791                 ty::Projection(..) => Some(12),
1792                 ty::Opaque(..) => Some(13),
1793                 ty::Never => Some(14),
1794                 ty::Adt(..) => Some(15),
1795                 ty::Generator(..) => Some(16),
1796                 ty::Foreign(..) => Some(17),
1797                 ty::GeneratorWitness(..) => Some(18),
1798                 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error(_) => None,
1799             }
1800         }
1801
1802         let strip_references = |mut t: Ty<'tcx>| -> Ty<'tcx> {
1803             loop {
1804                 match t.kind() {
1805                     ty::Ref(_, inner, _) | ty::RawPtr(ty::TypeAndMut { ty: inner, .. }) => {
1806                         t = *inner
1807                     }
1808                     _ => break t,
1809                 }
1810             }
1811         };
1812
1813         if !ignoring_lifetimes {
1814             a = strip_references(a);
1815             b = strip_references(b);
1816         }
1817
1818         let cat_a = type_category(self.tcx, a)?;
1819         let cat_b = type_category(self.tcx, b)?;
1820         if a == b {
1821             Some(CandidateSimilarity::Exact { ignoring_lifetimes })
1822         } else if cat_a == cat_b {
1823             match (a.kind(), b.kind()) {
1824                 (ty::Adt(def_a, _), ty::Adt(def_b, _)) => def_a == def_b,
1825                 (ty::Foreign(def_a), ty::Foreign(def_b)) => def_a == def_b,
1826                 // Matching on references results in a lot of unhelpful
1827                 // suggestions, so let's just not do that for now.
1828                 //
1829                 // We still upgrade successful matches to `ignoring_lifetimes: true`
1830                 // to prioritize that impl.
1831                 (ty::Ref(..) | ty::RawPtr(..), ty::Ref(..) | ty::RawPtr(..)) => {
1832                     self.fuzzy_match_tys(a, b, true).is_some()
1833                 }
1834                 _ => true,
1835             }
1836             .then_some(CandidateSimilarity::Fuzzy { ignoring_lifetimes })
1837         } else if ignoring_lifetimes {
1838             None
1839         } else {
1840             self.fuzzy_match_tys(a, b, true)
1841         }
1842     }
1843
1844     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str> {
1845         self.tcx.hir().body(body_id).generator_kind.map(|gen_kind| match gen_kind {
1846             hir::GeneratorKind::Gen => "a generator",
1847             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block) => "an async block",
1848             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn) => "an async function",
1849             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Closure) => "an async closure",
1850         })
1851     }
1852
1853     fn find_similar_impl_candidates(
1854         &self,
1855         trait_pred: ty::PolyTraitPredicate<'tcx>,
1856     ) -> Vec<ImplCandidate<'tcx>> {
1857         let mut candidates: Vec<_> = self
1858             .tcx
1859             .all_impls(trait_pred.def_id())
1860             .filter_map(|def_id| {
1861                 if self.tcx.impl_polarity(def_id) == ty::ImplPolarity::Negative
1862                     || !trait_pred
1863                         .skip_binder()
1864                         .is_constness_satisfied_by(self.tcx.constness(def_id))
1865                 {
1866                     return None;
1867                 }
1868
1869                 let imp = self.tcx.impl_trait_ref(def_id).unwrap();
1870
1871                 self.fuzzy_match_tys(trait_pred.skip_binder().self_ty(), imp.self_ty(), false)
1872                     .map(|similarity| ImplCandidate { trait_ref: imp, similarity })
1873             })
1874             .collect();
1875         if candidates.iter().any(|c| matches!(c.similarity, CandidateSimilarity::Exact { .. })) {
1876             // If any of the candidates is a perfect match, we don't want to show all of them.
1877             // This is particularly relevant for the case of numeric types (as they all have the
1878             // same cathegory).
1879             candidates.retain(|c| matches!(c.similarity, CandidateSimilarity::Exact { .. }));
1880         }
1881         candidates
1882     }
1883
1884     fn report_similar_impl_candidates(
1885         &self,
1886         impl_candidates: Vec<ImplCandidate<'tcx>>,
1887         trait_ref: ty::PolyTraitRef<'tcx>,
1888         body_id: hir::HirId,
1889         err: &mut Diagnostic,
1890     ) -> bool {
1891         let report = |mut candidates: Vec<TraitRef<'tcx>>, err: &mut Diagnostic| {
1892             candidates.sort();
1893             candidates.dedup();
1894             let len = candidates.len();
1895             if candidates.len() == 0 {
1896                 return false;
1897             }
1898             if candidates.len() == 1 {
1899                 let ty_desc = match candidates[0].self_ty().kind() {
1900                     ty::FnPtr(_) => Some("fn pointer"),
1901                     _ => None,
1902                 };
1903                 let the_desc = match ty_desc {
1904                     Some(desc) => format!(" implemented for {} `", desc),
1905                     None => " implemented for `".to_string(),
1906                 };
1907                 err.highlighted_help(vec![
1908                     (
1909                         format!("the trait `{}` ", candidates[0].print_only_trait_path()),
1910                         Style::NoStyle,
1911                     ),
1912                     ("is".to_string(), Style::Highlight),
1913                     (the_desc, Style::NoStyle),
1914                     (candidates[0].self_ty().to_string(), Style::Highlight),
1915                     ("`".to_string(), Style::NoStyle),
1916                 ]);
1917                 return true;
1918             }
1919             let trait_ref = TraitRef::identity(self.tcx, candidates[0].def_id);
1920             // Check if the trait is the same in all cases. If so, we'll only show the type.
1921             let mut traits: Vec<_> =
1922                 candidates.iter().map(|c| c.print_only_trait_path().to_string()).collect();
1923             traits.sort();
1924             traits.dedup();
1925
1926             let mut candidates: Vec<String> = candidates
1927                 .into_iter()
1928                 .map(|c| {
1929                     if traits.len() == 1 {
1930                         format!("\n  {}", c.self_ty())
1931                     } else {
1932                         format!("\n  {}", c)
1933                     }
1934                 })
1935                 .collect();
1936
1937             candidates.sort();
1938             candidates.dedup();
1939             let end = if candidates.len() <= 9 { candidates.len() } else { 8 };
1940             err.help(&format!(
1941                 "the following other types implement trait `{}`:{}{}",
1942                 trait_ref.print_only_trait_path(),
1943                 candidates[..end].join(""),
1944                 if len > 9 { format!("\nand {} others", len - 8) } else { String::new() }
1945             ));
1946             true
1947         };
1948
1949         let def_id = trait_ref.def_id();
1950         if impl_candidates.is_empty() {
1951             if self.tcx.trait_is_auto(def_id)
1952                 || self.tcx.lang_items().iter().any(|(_, id)| id == def_id)
1953                 || self.tcx.get_diagnostic_name(def_id).is_some()
1954             {
1955                 // Mentioning implementers of `Copy`, `Debug` and friends is not useful.
1956                 return false;
1957             }
1958             let normalized_impl_candidates: Vec<_> = self
1959                 .tcx
1960                 .all_impls(def_id)
1961                 // Ignore automatically derived impls and `!Trait` impls.
1962                 .filter(|&def_id| {
1963                     self.tcx.impl_polarity(def_id) != ty::ImplPolarity::Negative
1964                         || self.tcx.is_builtin_derive(def_id)
1965                 })
1966                 .filter_map(|def_id| self.tcx.impl_trait_ref(def_id))
1967                 .filter(|trait_ref| {
1968                     let self_ty = trait_ref.self_ty();
1969                     // Avoid mentioning type parameters.
1970                     if let ty::Param(_) = self_ty.kind() {
1971                         false
1972                     }
1973                     // Avoid mentioning types that are private to another crate
1974                     else if let ty::Adt(def, _) = self_ty.peel_refs().kind() {
1975                         // FIXME(compiler-errors): This could be generalized, both to
1976                         // be more granular, and probably look past other `#[fundamental]`
1977                         // types, too.
1978                         self.tcx
1979                             .visibility(def.did())
1980                             .is_accessible_from(body_id.owner.def_id, self.tcx)
1981                     } else {
1982                         true
1983                     }
1984                 })
1985                 .collect();
1986             return report(normalized_impl_candidates, err);
1987         }
1988
1989         // Sort impl candidates so that ordering is consistent for UI tests.
1990         // because the ordering of `impl_candidates` may not be deterministic:
1991         // https://github.com/rust-lang/rust/pull/57475#issuecomment-455519507
1992         //
1993         // Prefer more similar candidates first, then sort lexicographically
1994         // by their normalized string representation.
1995         let mut normalized_impl_candidates_and_similarities = impl_candidates
1996             .into_iter()
1997             .map(|ImplCandidate { trait_ref, similarity }| {
1998                 // FIXME(compiler-errors): This should be using `NormalizeExt::normalize`
1999                 let normalized = self
2000                     .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
2001                     .query_normalize(trait_ref)
2002                     .map_or(trait_ref, |normalized| normalized.value);
2003                 (similarity, normalized)
2004             })
2005             .collect::<Vec<_>>();
2006         normalized_impl_candidates_and_similarities.sort();
2007         normalized_impl_candidates_and_similarities.dedup();
2008
2009         let normalized_impl_candidates = normalized_impl_candidates_and_similarities
2010             .into_iter()
2011             .map(|(_, normalized)| normalized)
2012             .collect::<Vec<_>>();
2013
2014         report(normalized_impl_candidates, err)
2015     }
2016
2017     /// Gets the parent trait chain start
2018     fn get_parent_trait_ref(
2019         &self,
2020         code: &ObligationCauseCode<'tcx>,
2021     ) -> Option<(String, Option<Span>)> {
2022         match code {
2023             ObligationCauseCode::BuiltinDerivedObligation(data) => {
2024                 let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
2025                 match self.get_parent_trait_ref(&data.parent_code) {
2026                     Some(t) => Some(t),
2027                     None => {
2028                         let ty = parent_trait_ref.skip_binder().self_ty();
2029                         let span = TyCategory::from_ty(self.tcx, ty)
2030                             .map(|(_, def_id)| self.tcx.def_span(def_id));
2031                         Some((ty.to_string(), span))
2032                     }
2033                 }
2034             }
2035             ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } => {
2036                 self.get_parent_trait_ref(&parent_code)
2037             }
2038             _ => None,
2039         }
2040     }
2041
2042     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
2043     /// with the same path as `trait_ref`, a help message about
2044     /// a probable version mismatch is added to `err`
2045     fn note_version_mismatch(
2046         &self,
2047         err: &mut Diagnostic,
2048         trait_ref: &ty::PolyTraitRef<'tcx>,
2049     ) -> bool {
2050         let get_trait_impl = |trait_def_id| {
2051             self.tcx.find_map_relevant_impl(trait_def_id, trait_ref.skip_binder().self_ty(), Some)
2052         };
2053         let required_trait_path = self.tcx.def_path_str(trait_ref.def_id());
2054         let traits_with_same_path: std::collections::BTreeSet<_> = self
2055             .tcx
2056             .all_traits()
2057             .filter(|trait_def_id| *trait_def_id != trait_ref.def_id())
2058             .filter(|trait_def_id| self.tcx.def_path_str(*trait_def_id) == required_trait_path)
2059             .collect();
2060         let mut suggested = false;
2061         for trait_with_same_path in traits_with_same_path {
2062             if let Some(impl_def_id) = get_trait_impl(trait_with_same_path) {
2063                 let impl_span = self.tcx.def_span(impl_def_id);
2064                 err.span_help(impl_span, "trait impl with same name found");
2065                 let trait_crate = self.tcx.crate_name(trait_with_same_path.krate);
2066                 let crate_msg = format!(
2067                     "perhaps two different versions of crate `{}` are being used?",
2068                     trait_crate
2069                 );
2070                 err.note(&crate_msg);
2071                 suggested = true;
2072             }
2073         }
2074         suggested
2075     }
2076
2077     fn mk_trait_obligation_with_new_self_ty(
2078         &self,
2079         param_env: ty::ParamEnv<'tcx>,
2080         trait_ref_and_ty: ty::Binder<'tcx, (ty::TraitPredicate<'tcx>, Ty<'tcx>)>,
2081     ) -> PredicateObligation<'tcx> {
2082         let trait_pred = trait_ref_and_ty
2083             .map_bound(|(tr, new_self_ty)| tr.with_self_type(self.tcx, new_self_ty));
2084
2085         Obligation::new(self.tcx, ObligationCause::dummy(), param_env, trait_pred)
2086     }
2087
2088     #[instrument(skip(self), level = "debug")]
2089     fn maybe_report_ambiguity(
2090         &self,
2091         obligation: &PredicateObligation<'tcx>,
2092         body_id: Option<hir::BodyId>,
2093     ) {
2094         // Unable to successfully determine, probably means
2095         // insufficient type information, but could mean
2096         // ambiguous impls. The latter *ought* to be a
2097         // coherence violation, so we don't report it here.
2098
2099         let predicate = self.resolve_vars_if_possible(obligation.predicate);
2100         let span = obligation.cause.span;
2101
2102         debug!(?predicate, obligation.cause.code = ?obligation.cause.code());
2103
2104         // Ambiguity errors are often caused as fallout from earlier errors.
2105         // We ignore them if this `infcx` is tainted in some cases below.
2106
2107         let bound_predicate = predicate.kind();
2108         let mut err = match bound_predicate.skip_binder() {
2109             ty::PredicateKind::Clause(ty::Clause::Trait(data)) => {
2110                 let trait_ref = bound_predicate.rebind(data.trait_ref);
2111                 debug!(?trait_ref);
2112
2113                 if predicate.references_error() {
2114                     return;
2115                 }
2116
2117                 // This is kind of a hack: it frequently happens that some earlier
2118                 // error prevents types from being fully inferred, and then we get
2119                 // a bunch of uninteresting errors saying something like "<generic
2120                 // #0> doesn't implement Sized".  It may even be true that we
2121                 // could just skip over all checks where the self-ty is an
2122                 // inference variable, but I was afraid that there might be an
2123                 // inference variable created, registered as an obligation, and
2124                 // then never forced by writeback, and hence by skipping here we'd
2125                 // be ignoring the fact that we don't KNOW the type works
2126                 // out. Though even that would probably be harmless, given that
2127                 // we're only talking about builtin traits, which are known to be
2128                 // inhabited. We used to check for `self.tcx.sess.has_errors()` to
2129                 // avoid inundating the user with unnecessary errors, but we now
2130                 // check upstream for type errors and don't add the obligations to
2131                 // begin with in those cases.
2132                 if self.tcx.lang_items().sized_trait() == Some(trait_ref.def_id()) {
2133                     if let None = self.tainted_by_errors() {
2134                         self.emit_inference_failure_err(
2135                             body_id,
2136                             span,
2137                             trait_ref.self_ty().skip_binder().into(),
2138                             ErrorCode::E0282,
2139                             false,
2140                         )
2141                         .emit();
2142                     }
2143                     return;
2144                 }
2145
2146                 // Typically, this ambiguity should only happen if
2147                 // there are unresolved type inference variables
2148                 // (otherwise it would suggest a coherence
2149                 // failure). But given #21974 that is not necessarily
2150                 // the case -- we can have multiple where clauses that
2151                 // are only distinguished by a region, which results
2152                 // in an ambiguity even when all types are fully
2153                 // known, since we don't dispatch based on region
2154                 // relationships.
2155
2156                 // Pick the first substitution that still contains inference variables as the one
2157                 // we're going to emit an error for. If there are none (see above), fall back to
2158                 // a more general error.
2159                 let subst = data.trait_ref.substs.iter().find(|s| s.has_non_region_infer());
2160
2161                 let mut err = if let Some(subst) = subst {
2162                     self.emit_inference_failure_err(body_id, span, subst, ErrorCode::E0283, true)
2163                 } else {
2164                     struct_span_err!(
2165                         self.tcx.sess,
2166                         span,
2167                         E0283,
2168                         "type annotations needed: cannot satisfy `{}`",
2169                         predicate,
2170                     )
2171                 };
2172
2173                 let obligation = obligation.with(self.tcx, trait_ref);
2174                 let mut selcx = SelectionContext::new(&self);
2175                 match selcx.select_from_obligation(&obligation) {
2176                     Ok(None) => {
2177                         let impls = ambiguity::recompute_applicable_impls(self.infcx, &obligation);
2178                         let has_non_region_infer =
2179                             trait_ref.skip_binder().substs.types().any(|t| !t.is_ty_infer());
2180                         // It doesn't make sense to talk about applicable impls if there are more
2181                         // than a handful of them.
2182                         if impls.len() > 1 && impls.len() < 5 && has_non_region_infer {
2183                             self.annotate_source_of_ambiguity(&mut err, &impls, predicate);
2184                         } else {
2185                             if self.tainted_by_errors().is_some() {
2186                                 err.cancel();
2187                                 return;
2188                             }
2189                             err.note(&format!("cannot satisfy `{}`", predicate));
2190                         }
2191                     }
2192                     _ => {
2193                         if self.tainted_by_errors().is_some() {
2194                             err.cancel();
2195                             return;
2196                         }
2197                         err.note(&format!("cannot satisfy `{}`", predicate));
2198                     }
2199                 }
2200
2201                 if let ObligationCauseCode::ItemObligation(def_id) | ObligationCauseCode::ExprItemObligation(def_id, ..) = *obligation.cause.code() {
2202                     self.suggest_fully_qualified_path(&mut err, def_id, span, trait_ref.def_id());
2203                 } else if let Ok(snippet) = &self.tcx.sess.source_map().span_to_snippet(span)
2204                     && let ObligationCauseCode::BindingObligation(def_id, _) | ObligationCauseCode::ExprBindingObligation(def_id, ..)
2205                         = *obligation.cause.code()
2206                 {
2207                     let generics = self.tcx.generics_of(def_id);
2208                     if generics.params.iter().any(|p| p.name != kw::SelfUpper)
2209                         && !snippet.ends_with('>')
2210                         && !generics.has_impl_trait()
2211                         && !self.tcx.is_fn_trait(def_id)
2212                     {
2213                         // FIXME: To avoid spurious suggestions in functions where type arguments
2214                         // where already supplied, we check the snippet to make sure it doesn't
2215                         // end with a turbofish. Ideally we would have access to a `PathSegment`
2216                         // instead. Otherwise we would produce the following output:
2217                         //
2218                         // error[E0283]: type annotations needed
2219                         //   --> $DIR/issue-54954.rs:3:24
2220                         //    |
2221                         // LL | const ARR_LEN: usize = Tt::const_val::<[i8; 123]>();
2222                         //    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^
2223                         //    |                        |
2224                         //    |                        cannot infer type
2225                         //    |                        help: consider specifying the type argument
2226                         //    |                        in the function call:
2227                         //    |                        `Tt::const_val::<[i8; 123]>::<T>`
2228                         // ...
2229                         // LL |     const fn const_val<T: Sized>() -> usize {
2230                         //    |                        - required by this bound in `Tt::const_val`
2231                         //    |
2232                         //    = note: cannot satisfy `_: Tt`
2233
2234                         // Clear any more general suggestions in favor of our specific one
2235                         err.clear_suggestions();
2236
2237                         err.span_suggestion_verbose(
2238                             span.shrink_to_hi(),
2239                             &format!(
2240                                 "consider specifying the type argument{} in the function call",
2241                                 pluralize!(generics.params.len()),
2242                             ),
2243                             format!(
2244                                 "::<{}>",
2245                                 generics
2246                                     .params
2247                                     .iter()
2248                                     .map(|p| p.name.to_string())
2249                                     .collect::<Vec<String>>()
2250                                     .join(", ")
2251                             ),
2252                             Applicability::HasPlaceholders,
2253                         );
2254                     }
2255                 }
2256
2257                 if let (Some(body_id), Some(ty::subst::GenericArgKind::Type(_))) =
2258                     (body_id, subst.map(|subst| subst.unpack()))
2259                 {
2260                     struct FindExprBySpan<'hir> {
2261                         span: Span,
2262                         result: Option<&'hir hir::Expr<'hir>>,
2263                     }
2264
2265                     impl<'v> hir::intravisit::Visitor<'v> for FindExprBySpan<'v> {
2266                         fn visit_expr(&mut self, ex: &'v hir::Expr<'v>) {
2267                             if self.span == ex.span {
2268                                 self.result = Some(ex);
2269                             } else {
2270                                 hir::intravisit::walk_expr(self, ex);
2271                             }
2272                         }
2273                     }
2274
2275                     let mut expr_finder = FindExprBySpan { span, result: None };
2276
2277                     expr_finder.visit_expr(&self.tcx.hir().body(body_id).value);
2278
2279                     if let Some(hir::Expr {
2280                         kind: hir::ExprKind::Path(hir::QPath::Resolved(None, path)), .. }
2281                     ) = expr_finder.result
2282                         && let [
2283                             ..,
2284                             trait_path_segment @ hir::PathSegment {
2285                                 res: rustc_hir::def::Res::Def(rustc_hir::def::DefKind::Trait, trait_id),
2286                                 ..
2287                             },
2288                             hir::PathSegment {
2289                                 ident: assoc_item_name,
2290                                 res: rustc_hir::def::Res::Def(_, item_id),
2291                                 ..
2292                             }
2293                         ] = path.segments
2294                         && data.trait_ref.def_id == *trait_id
2295                         && self.tcx.trait_of_item(*item_id) == Some(*trait_id)
2296                         && let None = self.tainted_by_errors()
2297                     {
2298                         let (verb, noun) = match self.tcx.associated_item(item_id).kind {
2299                             ty::AssocKind::Const => ("refer to the", "constant"),
2300                             ty::AssocKind::Fn => ("call", "function"),
2301                             ty::AssocKind::Type => ("refer to the", "type"), // this is already covered by E0223, but this single match arm doesn't hurt here
2302                         };
2303
2304                         // Replace the more general E0283 with a more specific error
2305                         err.cancel();
2306                         err = self.tcx.sess.struct_span_err_with_code(
2307                             span,
2308                             &format!(
2309                                 "cannot {verb} associated {noun} on trait without specifying the corresponding `impl` type",
2310                              ),
2311                             rustc_errors::error_code!(E0790),
2312                         );
2313
2314                         if let Some(local_def_id) = data.trait_ref.def_id.as_local()
2315                             && 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)
2316                             && let Some(method_ref) = trait_item_refs.iter().find(|item_ref| item_ref.ident == *assoc_item_name) {
2317                             err.span_label(method_ref.span, format!("`{}::{}` defined here", trait_name, assoc_item_name));
2318                         }
2319
2320                         err.span_label(span, format!("cannot {verb} associated {noun} of trait"));
2321
2322                         let trait_impls = self.tcx.trait_impls_of(data.trait_ref.def_id);
2323
2324                         if trait_impls.blanket_impls().is_empty()
2325                             && let Some((impl_ty, _)) = trait_impls.non_blanket_impls().iter().next()
2326                             && let Some(impl_def_id) = impl_ty.def() {
2327                             let message = if trait_impls.non_blanket_impls().len() == 1 {
2328                                 "use the fully-qualified path to the only available implementation".to_string()
2329                             } else {
2330                                 format!(
2331                                     "use a fully-qualified path to a specific available implementation ({} found)",
2332                                     trait_impls.non_blanket_impls().len()
2333                                 )
2334                             };
2335                             let mut suggestions = vec![(
2336                                 trait_path_segment.ident.span.shrink_to_lo(),
2337                                 format!("<{} as ", self.tcx.type_of(impl_def_id))
2338                             )];
2339                             if let Some(generic_arg) = trait_path_segment.args {
2340                                 let between_span = trait_path_segment.ident.span.between(generic_arg.span_ext);
2341                                 // get rid of :: between Trait and <type>
2342                                 // must be '::' between them, otherwise the parser won't accept the code
2343                                 suggestions.push((between_span, "".to_string(),));
2344                                 suggestions.push((generic_arg.span_ext.shrink_to_hi(), format!(">")));
2345                             } else {
2346                                 suggestions.push((trait_path_segment.ident.span.shrink_to_hi(), format!(">")));
2347                             }
2348                             err.multipart_suggestion(
2349                                 message,
2350                                 suggestions,
2351                                 Applicability::MaybeIncorrect
2352                             );
2353                         }
2354                     }
2355                 };
2356
2357                 err
2358             }
2359
2360             ty::PredicateKind::WellFormed(arg) => {
2361                 // Same hacky approach as above to avoid deluging user
2362                 // with error messages.
2363                 if arg.references_error()
2364                     || self.tcx.sess.has_errors().is_some()
2365                     || self.tainted_by_errors().is_some()
2366                 {
2367                     return;
2368                 }
2369
2370                 self.emit_inference_failure_err(body_id, span, arg, ErrorCode::E0282, false)
2371             }
2372
2373             ty::PredicateKind::Subtype(data) => {
2374                 if data.references_error()
2375                     || self.tcx.sess.has_errors().is_some()
2376                     || self.tainted_by_errors().is_some()
2377                 {
2378                     // no need to overload user in such cases
2379                     return;
2380                 }
2381                 let SubtypePredicate { a_is_expected: _, a, b } = data;
2382                 // both must be type variables, or the other would've been instantiated
2383                 assert!(a.is_ty_var() && b.is_ty_var());
2384                 self.emit_inference_failure_err(body_id, span, a.into(), ErrorCode::E0282, true)
2385             }
2386             ty::PredicateKind::Clause(ty::Clause::Projection(data)) => {
2387                 if predicate.references_error() || self.tainted_by_errors().is_some() {
2388                     return;
2389                 }
2390                 let subst = data
2391                     .projection_ty
2392                     .substs
2393                     .iter()
2394                     .chain(Some(data.term.into_arg()))
2395                     .find(|g| g.has_non_region_infer());
2396                 if let Some(subst) = subst {
2397                     let mut err = self.emit_inference_failure_err(
2398                         body_id,
2399                         span,
2400                         subst,
2401                         ErrorCode::E0284,
2402                         true,
2403                     );
2404                     err.note(&format!("cannot satisfy `{}`", predicate));
2405                     err
2406                 } else {
2407                     // If we can't find a substitution, just print a generic error
2408                     let mut err = struct_span_err!(
2409                         self.tcx.sess,
2410                         span,
2411                         E0284,
2412                         "type annotations needed: cannot satisfy `{}`",
2413                         predicate,
2414                     );
2415                     err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2416                     err
2417                 }
2418             }
2419
2420             ty::PredicateKind::ConstEvaluatable(data) => {
2421                 if predicate.references_error() || self.tainted_by_errors().is_some() {
2422                     return;
2423                 }
2424                 let subst = data.walk().find(|g| g.is_non_region_infer());
2425                 if let Some(subst) = subst {
2426                     let err = self.emit_inference_failure_err(
2427                         body_id,
2428                         span,
2429                         subst,
2430                         ErrorCode::E0284,
2431                         true,
2432                     );
2433                     err
2434                 } else {
2435                     // If we can't find a substitution, just print a generic error
2436                     let mut err = struct_span_err!(
2437                         self.tcx.sess,
2438                         span,
2439                         E0284,
2440                         "type annotations needed: cannot satisfy `{}`",
2441                         predicate,
2442                     );
2443                     err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2444                     err
2445                 }
2446             }
2447             _ => {
2448                 if self.tcx.sess.has_errors().is_some() || self.tainted_by_errors().is_some() {
2449                     return;
2450                 }
2451                 let mut err = struct_span_err!(
2452                     self.tcx.sess,
2453                     span,
2454                     E0284,
2455                     "type annotations needed: cannot satisfy `{}`",
2456                     predicate,
2457                 );
2458                 err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2459                 err
2460             }
2461         };
2462         self.note_obligation_cause(&mut err, obligation);
2463         err.emit();
2464     }
2465
2466     fn annotate_source_of_ambiguity(
2467         &self,
2468         err: &mut Diagnostic,
2469         impls: &[DefId],
2470         predicate: ty::Predicate<'tcx>,
2471     ) {
2472         let mut spans = vec![];
2473         let mut crates = vec![];
2474         let mut post = vec![];
2475         for def_id in impls {
2476             match self.tcx.span_of_impl(*def_id) {
2477                 Ok(span) => spans.push(span),
2478                 Err(name) => {
2479                     crates.push(name);
2480                     if let Some(header) = to_pretty_impl_header(self.tcx, *def_id) {
2481                         post.push(header);
2482                     }
2483                 }
2484             }
2485         }
2486         let mut crate_names: Vec<_> = crates.iter().map(|n| format!("`{}`", n)).collect();
2487         crate_names.sort();
2488         crate_names.dedup();
2489         post.sort();
2490         post.dedup();
2491
2492         if self.tainted_by_errors().is_some()
2493             && (crate_names.len() == 1
2494                 && spans.len() == 0
2495                 && ["`core`", "`alloc`", "`std`"].contains(&crate_names[0].as_str())
2496                 || predicate.visit_with(&mut HasNumericInferVisitor).is_break())
2497         {
2498             // Avoid complaining about other inference issues for expressions like
2499             // `42 >> 1`, where the types are still `{integer}`, but we want to
2500             // Do we need `trait_ref.skip_binder().self_ty().is_numeric() &&` too?
2501             // NOTE(eddyb) this was `.cancel()`, but `err`
2502             // is borrowed, so we can't fully defuse it.
2503             err.downgrade_to_delayed_bug();
2504             return;
2505         }
2506
2507         let msg = format!("multiple `impl`s satisfying `{}` found", predicate);
2508         let post = if post.len() > 1 || (post.len() == 1 && post[0].contains('\n')) {
2509             format!(":\n{}", post.iter().map(|p| format!("- {}", p)).collect::<Vec<_>>().join("\n"),)
2510         } else if post.len() == 1 {
2511             format!(": `{}`", post[0])
2512         } else {
2513             String::new()
2514         };
2515
2516         match (spans.len(), crates.len(), crate_names.len()) {
2517             (0, 0, 0) => {
2518                 err.note(&format!("cannot satisfy `{}`", predicate));
2519             }
2520             (0, _, 1) => {
2521                 err.note(&format!("{} in the `{}` crate{}", msg, crates[0], post,));
2522             }
2523             (0, _, _) => {
2524                 err.note(&format!(
2525                     "{} in the following crates: {}{}",
2526                     msg,
2527                     crate_names.join(", "),
2528                     post,
2529                 ));
2530             }
2531             (_, 0, 0) => {
2532                 let span: MultiSpan = spans.into();
2533                 err.span_note(span, &msg);
2534             }
2535             (_, 1, 1) => {
2536                 let span: MultiSpan = spans.into();
2537                 err.span_note(span, &msg);
2538                 err.note(
2539                     &format!("and another `impl` found in the `{}` crate{}", crates[0], post,),
2540                 );
2541             }
2542             _ => {
2543                 let span: MultiSpan = spans.into();
2544                 err.span_note(span, &msg);
2545                 err.note(&format!(
2546                     "and more `impl`s found in the following crates: {}{}",
2547                     crate_names.join(", "),
2548                     post,
2549                 ));
2550             }
2551         }
2552     }
2553
2554     /// Returns `true` if the trait predicate may apply for *some* assignment
2555     /// to the type parameters.
2556     fn predicate_can_apply(
2557         &self,
2558         param_env: ty::ParamEnv<'tcx>,
2559         pred: ty::PolyTraitPredicate<'tcx>,
2560     ) -> bool {
2561         struct ParamToVarFolder<'a, 'tcx> {
2562             infcx: &'a InferCtxt<'tcx>,
2563             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
2564         }
2565
2566         impl<'a, 'tcx> TypeFolder<'tcx> for ParamToVarFolder<'a, 'tcx> {
2567             fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
2568                 self.infcx.tcx
2569             }
2570
2571             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
2572                 if let ty::Param(ty::ParamTy { name, .. }) = *ty.kind() {
2573                     let infcx = self.infcx;
2574                     *self.var_map.entry(ty).or_insert_with(|| {
2575                         infcx.next_ty_var(TypeVariableOrigin {
2576                             kind: TypeVariableOriginKind::TypeParameterDefinition(name, None),
2577                             span: DUMMY_SP,
2578                         })
2579                     })
2580                 } else {
2581                     ty.super_fold_with(self)
2582                 }
2583             }
2584         }
2585
2586         self.probe(|_| {
2587             let cleaned_pred =
2588                 pred.fold_with(&mut ParamToVarFolder { infcx: self, var_map: Default::default() });
2589
2590             let InferOk { value: cleaned_pred, .. } =
2591                 self.infcx.at(&ObligationCause::dummy(), param_env).normalize(cleaned_pred);
2592
2593             let obligation =
2594                 Obligation::new(self.tcx, ObligationCause::dummy(), param_env, cleaned_pred);
2595
2596             self.predicate_may_hold(&obligation)
2597         })
2598     }
2599
2600     fn note_obligation_cause(&self, err: &mut Diagnostic, obligation: &PredicateObligation<'tcx>) {
2601         // First, attempt to add note to this error with an async-await-specific
2602         // message, and fall back to regular note otherwise.
2603         if !self.maybe_note_obligation_cause_for_async_await(err, obligation) {
2604             self.note_obligation_cause_code(
2605                 err,
2606                 obligation.predicate,
2607                 obligation.param_env,
2608                 obligation.cause.code(),
2609                 &mut vec![],
2610                 &mut Default::default(),
2611             );
2612             self.suggest_unsized_bound_if_applicable(err, obligation);
2613         }
2614     }
2615
2616     #[instrument(level = "debug", skip_all)]
2617     fn suggest_unsized_bound_if_applicable(
2618         &self,
2619         err: &mut Diagnostic,
2620         obligation: &PredicateObligation<'tcx>,
2621     ) {
2622         let ty::PredicateKind::Clause(ty::Clause::Trait(pred)) = obligation.predicate.kind().skip_binder() else { return; };
2623         let (ObligationCauseCode::BindingObligation(item_def_id, span)
2624         | ObligationCauseCode::ExprBindingObligation(item_def_id, span, ..))
2625             = *obligation.cause.code().peel_derives() else { return; };
2626         debug!(?pred, ?item_def_id, ?span);
2627
2628         let (Some(node), true) = (
2629             self.tcx.hir().get_if_local(item_def_id),
2630             Some(pred.def_id()) == self.tcx.lang_items().sized_trait(),
2631         ) else {
2632             return;
2633         };
2634         self.maybe_suggest_unsized_generics(err, span, node);
2635     }
2636
2637     #[instrument(level = "debug", skip_all)]
2638     fn maybe_suggest_unsized_generics(&self, err: &mut Diagnostic, span: Span, node: Node<'tcx>) {
2639         let Some(generics) = node.generics() else {
2640             return;
2641         };
2642         let sized_trait = self.tcx.lang_items().sized_trait();
2643         debug!(?generics.params);
2644         debug!(?generics.predicates);
2645         let Some(param) = generics.params.iter().find(|param| param.span == span) else {
2646             return;
2647         };
2648         // Check that none of the explicit trait bounds is `Sized`. Assume that an explicit
2649         // `Sized` bound is there intentionally and we don't need to suggest relaxing it.
2650         let explicitly_sized = generics
2651             .bounds_for_param(param.def_id)
2652             .flat_map(|bp| bp.bounds)
2653             .any(|bound| bound.trait_ref().and_then(|tr| tr.trait_def_id()) == sized_trait);
2654         if explicitly_sized {
2655             return;
2656         }
2657         debug!(?param);
2658         match node {
2659             hir::Node::Item(
2660                 item @ hir::Item {
2661                     // Only suggest indirection for uses of type parameters in ADTs.
2662                     kind:
2663                         hir::ItemKind::Enum(..) | hir::ItemKind::Struct(..) | hir::ItemKind::Union(..),
2664                     ..
2665                 },
2666             ) => {
2667                 if self.maybe_indirection_for_unsized(err, item, param) {
2668                     return;
2669                 }
2670             }
2671             _ => {}
2672         };
2673         // Didn't add an indirection suggestion, so add a general suggestion to relax `Sized`.
2674         let (span, separator) = if let Some(s) = generics.bounds_span_for_suggestions(param.def_id)
2675         {
2676             (s, " +")
2677         } else {
2678             (span.shrink_to_hi(), ":")
2679         };
2680         err.span_suggestion_verbose(
2681             span,
2682             "consider relaxing the implicit `Sized` restriction",
2683             format!("{} ?Sized", separator),
2684             Applicability::MachineApplicable,
2685         );
2686     }
2687
2688     fn maybe_indirection_for_unsized(
2689         &self,
2690         err: &mut Diagnostic,
2691         item: &Item<'tcx>,
2692         param: &GenericParam<'tcx>,
2693     ) -> bool {
2694         // Suggesting `T: ?Sized` is only valid in an ADT if `T` is only used in a
2695         // borrow. `struct S<'a, T: ?Sized>(&'a T);` is valid, `struct S<T: ?Sized>(T);`
2696         // is not. Look for invalid "bare" parameter uses, and suggest using indirection.
2697         let mut visitor =
2698             FindTypeParam { param: param.name.ident().name, invalid_spans: vec![], nested: false };
2699         visitor.visit_item(item);
2700         if visitor.invalid_spans.is_empty() {
2701             return false;
2702         }
2703         let mut multispan: MultiSpan = param.span.into();
2704         multispan.push_span_label(
2705             param.span,
2706             format!("this could be changed to `{}: ?Sized`...", param.name.ident()),
2707         );
2708         for sp in visitor.invalid_spans {
2709             multispan.push_span_label(
2710                 sp,
2711                 format!("...if indirection were used here: `Box<{}>`", param.name.ident()),
2712             );
2713         }
2714         err.span_help(
2715             multispan,
2716             &format!(
2717                 "you could relax the implicit `Sized` bound on `{T}` if it were \
2718                 used through indirection like `&{T}` or `Box<{T}>`",
2719                 T = param.name.ident(),
2720             ),
2721         );
2722         true
2723     }
2724
2725     fn is_recursive_obligation(
2726         &self,
2727         obligated_types: &mut Vec<Ty<'tcx>>,
2728         cause_code: &ObligationCauseCode<'tcx>,
2729     ) -> bool {
2730         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
2731             let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
2732             let self_ty = parent_trait_ref.skip_binder().self_ty();
2733             if obligated_types.iter().any(|ot| ot == &self_ty) {
2734                 return true;
2735             }
2736             if let ty::Adt(def, substs) = self_ty.kind()
2737                 && let [arg] = &substs[..]
2738                 && let ty::subst::GenericArgKind::Type(ty) = arg.unpack()
2739                 && let ty::Adt(inner_def, _) = ty.kind()
2740                 && inner_def == def
2741             {
2742                 return true;
2743             }
2744         }
2745         false
2746     }
2747 }
2748
2749 /// Look for type `param` in an ADT being used only through a reference to confirm that suggesting
2750 /// `param: ?Sized` would be a valid constraint.
2751 struct FindTypeParam {
2752     param: rustc_span::Symbol,
2753     invalid_spans: Vec<Span>,
2754     nested: bool,
2755 }
2756
2757 impl<'v> Visitor<'v> for FindTypeParam {
2758     fn visit_where_predicate(&mut self, _: &'v hir::WherePredicate<'v>) {
2759         // Skip where-clauses, to avoid suggesting indirection for type parameters found there.
2760     }
2761
2762     fn visit_ty(&mut self, ty: &hir::Ty<'_>) {
2763         // We collect the spans of all uses of the "bare" type param, like in `field: T` or
2764         // `field: (T, T)` where we could make `T: ?Sized` while skipping cases that are known to be
2765         // valid like `field: &'a T` or `field: *mut T` and cases that *might* have further `Sized`
2766         // obligations like `Box<T>` and `Vec<T>`, but we perform no extra analysis for those cases
2767         // and suggest `T: ?Sized` regardless of their obligations. This is fine because the errors
2768         // in that case should make what happened clear enough.
2769         match ty.kind {
2770             hir::TyKind::Ptr(_) | hir::TyKind::Rptr(..) | hir::TyKind::TraitObject(..) => {}
2771             hir::TyKind::Path(hir::QPath::Resolved(None, path))
2772                 if path.segments.len() == 1 && path.segments[0].ident.name == self.param =>
2773             {
2774                 if !self.nested {
2775                     debug!(?ty, "FindTypeParam::visit_ty");
2776                     self.invalid_spans.push(ty.span);
2777                 }
2778             }
2779             hir::TyKind::Path(_) => {
2780                 let prev = self.nested;
2781                 self.nested = true;
2782                 hir::intravisit::walk_ty(self, ty);
2783                 self.nested = prev;
2784             }
2785             _ => {
2786                 hir::intravisit::walk_ty(self, ty);
2787             }
2788         }
2789     }
2790 }
2791
2792 /// Summarizes information
2793 #[derive(Clone)]
2794 pub enum ArgKind {
2795     /// An argument of non-tuple type. Parameters are (name, ty)
2796     Arg(String, String),
2797
2798     /// An argument of tuple type. For a "found" argument, the span is
2799     /// the location in the source of the pattern. For an "expected"
2800     /// argument, it will be None. The vector is a list of (name, ty)
2801     /// strings for the components of the tuple.
2802     Tuple(Option<Span>, Vec<(String, String)>),
2803 }
2804
2805 impl ArgKind {
2806     fn empty() -> ArgKind {
2807         ArgKind::Arg("_".to_owned(), "_".to_owned())
2808     }
2809
2810     /// Creates an `ArgKind` from the expected type of an
2811     /// argument. It has no name (`_`) and an optional source span.
2812     pub fn from_expected_ty(t: Ty<'_>, span: Option<Span>) -> ArgKind {
2813         match t.kind() {
2814             ty::Tuple(tys) => ArgKind::Tuple(
2815                 span,
2816                 tys.iter().map(|ty| ("_".to_owned(), ty.to_string())).collect::<Vec<_>>(),
2817             ),
2818             _ => ArgKind::Arg("_".to_owned(), t.to_string()),
2819         }
2820     }
2821 }
2822
2823 struct HasNumericInferVisitor;
2824
2825 impl<'tcx> ty::TypeVisitor<'tcx> for HasNumericInferVisitor {
2826     type BreakTy = ();
2827
2828     fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
2829         if matches!(ty.kind(), ty::Infer(ty::FloatVar(_) | ty::IntVar(_))) {
2830             ControlFlow::Break(())
2831         } else {
2832             ControlFlow::CONTINUE
2833         }
2834     }
2835 }
2836
2837 pub enum DefIdOrName {
2838     DefId(DefId),
2839     Name(&'static str),
2840 }