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

   1 mod ambiguity;
   2 pub mod method_chain;
   3 pub mod on_unimplemented;
   4 pub mod suggestions;
   5
   6 use super::{
   7     FulfillmentError, FulfillmentErrorCode, MismatchedProjectionTypes, Obligation, ObligationCause,
   8     ObligationCauseCode, ObligationCtxt, OutputTypeParameterMismatch, Overflow,
   9     PredicateObligation, SelectionContext, SelectionError, TraitNotObjectSafe,
  10 };
  11 use crate::infer::error_reporting::{TyCategory, TypeAnnotationNeeded as ErrorCode};
  12 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  13 use crate::infer::{self, InferCtxt};
  14 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
  15 use crate::traits::query::normalize::QueryNormalizeExt as _;
  16 use crate::traits::specialize::to_pretty_impl_header;
  17 use crate::traits::NormalizeExt;
  18 use on_unimplemented::OnUnimplementedNote;
  19 use on_unimplemented::TypeErrCtxtExt as _;
  20 use rustc_data_structures::fx::{FxHashMap, FxIndexMap};
  21 use rustc_errors::{
  22     pluralize, struct_span_err, Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed,
  23     MultiSpan, Style,
  24 };
  25 use rustc_hir as hir;
  26 use rustc_hir::def::Namespace;
  27 use rustc_hir::def_id::DefId;
  28 use rustc_hir::intravisit::Visitor;
  29 use rustc_hir::GenericParam;
  30 use rustc_hir::Item;
  31 use rustc_hir::Node;
  32 use rustc_infer::infer::error_reporting::TypeErrCtxt;
  33 use rustc_infer::infer::{InferOk, TypeTrace};
  34 use rustc_middle::traits::select::OverflowError;
  35 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
  36 use rustc_middle::ty::error::{ExpectedFound, TypeError};
  37 use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable};
  38 use rustc_middle::ty::print::{with_forced_trimmed_paths, FmtPrinter, Print};
  39 use rustc_middle::ty::{
  40     self, SubtypePredicate, ToPolyTraitRef, ToPredicate, TraitRef, Ty, TyCtxt, TypeFoldable,
  41     TypeVisitable,
  42 };
  43 use rustc_session::config::TraitSolver;
  44 use rustc_session::Limit;
  45 use rustc_span::def_id::LOCAL_CRATE;
  46 use rustc_span::symbol::sym;
  47 use rustc_span::{ExpnKind, Span, DUMMY_SP};
  48 use std::fmt;
  49 use std::iter;
  50 use std::ops::ControlFlow;
  51 use suggestions::TypeErrCtxtExt as _;
  52
  53 pub use rustc_infer::traits::error_reporting::*;
  54
  55 // When outputting impl candidates, prefer showing those that are more similar.
  56 //
  57 // We also compare candidates after skipping lifetimes, which has a lower
  58 // priority than exact matches.
  59 #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
  60 pub enum CandidateSimilarity {
  61     Exact { ignoring_lifetimes: bool },
  62     Fuzzy { ignoring_lifetimes: bool },
  63 }
  64
  65 #[derive(Debug, Clone, Copy)]
  66 pub struct ImplCandidate<'tcx> {
  67     pub trait_ref: ty::TraitRef<'tcx>,
  68     pub similarity: CandidateSimilarity,
  69 }
  70
  71 pub trait InferCtxtExt<'tcx> {
  72     /// Given some node representing a fn-like thing in the HIR map,
  73     /// returns a span and `ArgKind` information that describes the
  74     /// arguments it expects. This can be supplied to
  75     /// `report_arg_count_mismatch`.
  76     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Option<Span>, Vec<ArgKind>)>;
  77
  78     /// Reports an error when the number of arguments needed by a
  79     /// trait match doesn't match the number that the expression
  80     /// provides.
  81     fn report_arg_count_mismatch(
  82         &self,
  83         span: Span,
  84         found_span: Option<Span>,
  85         expected_args: Vec<ArgKind>,
  86         found_args: Vec<ArgKind>,
  87         is_closure: bool,
  88         closure_pipe_span: Option<Span>,
  89     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed>;
  90
  91     /// Checks if the type implements one of `Fn`, `FnMut`, or `FnOnce`
  92     /// in that order, and returns the generic type corresponding to the
  93     /// argument of that trait (corresponding to the closure arguments).
  94     fn type_implements_fn_trait(
  95         &self,
  96         param_env: ty::ParamEnv<'tcx>,
  97         ty: ty::Binder<'tcx, Ty<'tcx>>,
  98         constness: ty::BoundConstness,
  99         polarity: ty::ImplPolarity,
 100     ) -> Result<(ty::ClosureKind, ty::Binder<'tcx, Ty<'tcx>>), ()>;
 101 }
 102
 103 pub trait TypeErrCtxtExt<'tcx> {
 104     fn build_overflow_error<T>(
 105         &self,
 106         predicate: &T,
 107         span: Span,
 108         suggest_increasing_limit: bool,
 109     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed>
 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_overflow_error<T>(
 117         &self,
 118         predicate: &T,
 119         span: Span,
 120         suggest_increasing_limit: bool,
 121         mutate: impl FnOnce(&mut Diagnostic),
 122     ) -> !
 123     where
 124         T: fmt::Display
 125             + TypeFoldable<'tcx>
 126             + Print<'tcx, FmtPrinter<'tcx, 'tcx>, Output = FmtPrinter<'tcx, 'tcx>>,
 127         <T as Print<'tcx, FmtPrinter<'tcx, 'tcx>>>::Error: std::fmt::Debug;
 128
 129     fn report_fulfillment_errors(
 130         &self,
 131         errors: &[FulfillmentError<'tcx>],
 132         body_id: Option<hir::BodyId>,
 133     ) -> ErrorGuaranteed;
 134
 135     fn report_overflow_obligation<T>(
 136         &self,
 137         obligation: &Obligation<'tcx, T>,
 138         suggest_increasing_limit: bool,
 139     ) -> !
 140     where
 141         T: ToPredicate<'tcx> + Clone;
 142
 143     fn suggest_new_overflow_limit(&self, err: &mut Diagnostic);
 144
 145     fn report_overflow_obligation_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> !;
 146
 147     /// The `root_obligation` parameter should be the `root_obligation` field
 148     /// from a `FulfillmentError`. If no `FulfillmentError` is available,
 149     /// then it should be the same as `obligation`.
 150     fn report_selection_error(
 151         &self,
 152         obligation: PredicateObligation<'tcx>,
 153         root_obligation: &PredicateObligation<'tcx>,
 154         error: &SelectionError<'tcx>,
 155     );
 156 }
 157
 158 impl<'tcx> InferCtxtExt<'tcx> for InferCtxt<'tcx> {
 159     /// Given some node representing a fn-like thing in the HIR map,
 160     /// returns a span and `ArgKind` information that describes the
 161     /// arguments it expects. This can be supplied to
 162     /// `report_arg_count_mismatch`.
 163     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Option<Span>, Vec<ArgKind>)> {
 164         let sm = self.tcx.sess.source_map();
 165         let hir = self.tcx.hir();
 166         Some(match node {
 167             Node::Expr(&hir::Expr {
 168                 kind: hir::ExprKind::Closure(&hir::Closure { body, fn_decl_span, fn_arg_span, .. }),
 169                 ..
 170             }) => (
 171                 fn_decl_span,
 172                 fn_arg_span,
 173                 hir.body(body)
 174                     .params
 175                     .iter()
 176                     .map(|arg| {
 177                         if let hir::Pat { kind: hir::PatKind::Tuple(ref args, _), span, .. } =
 178                             *arg.pat
 179                         {
 180                             Some(ArgKind::Tuple(
 181                                 Some(span),
 182                                 args.iter()
 183                                     .map(|pat| {
 184                                         sm.span_to_snippet(pat.span)
 185                                             .ok()
 186                                             .map(|snippet| (snippet, "_".to_owned()))
 187                                     })
 188                                     .collect::<Option<Vec<_>>>()?,
 189                             ))
 190                         } else {
 191                             let name = sm.span_to_snippet(arg.pat.span).ok()?;
 192                             Some(ArgKind::Arg(name, "_".to_owned()))
 193                         }
 194                     })
 195                     .collect::<Option<Vec<ArgKind>>>()?,
 196             ),
 197             Node::Item(&hir::Item { kind: hir::ItemKind::Fn(ref sig, ..), .. })
 198             | Node::ImplItem(&hir::ImplItem { kind: hir::ImplItemKind::Fn(ref sig, _), .. })
 199             | Node::TraitItem(&hir::TraitItem {
 200                 kind: hir::TraitItemKind::Fn(ref sig, _), ..
 201             }) => (
 202                 sig.span,
 203                 None,
 204                 sig.decl
 205                     .inputs
 206                     .iter()
 207                     .map(|arg| match arg.kind {
 208                         hir::TyKind::Tup(ref tys) => ArgKind::Tuple(
 209                             Some(arg.span),
 210                             vec![("_".to_owned(), "_".to_owned()); tys.len()],
 211                         ),
 212                         _ => ArgKind::empty(),
 213                     })
 214                     .collect::<Vec<ArgKind>>(),
 215             ),
 216             Node::Ctor(ref variant_data) => {
 217                 let span = variant_data.ctor_hir_id().map_or(DUMMY_SP, |id| hir.span(id));
 218                 (span, None, vec![ArgKind::empty(); variant_data.fields().len()])
 219             }
 220             _ => panic!("non-FnLike node found: {:?}", node),
 221         })
 222     }
 223
 224     /// Reports an error when the number of arguments needed by a
 225     /// trait match doesn't match the number that the expression
 226     /// provides.
 227     fn report_arg_count_mismatch(
 228         &self,
 229         span: Span,
 230         found_span: Option<Span>,
 231         expected_args: Vec<ArgKind>,
 232         found_args: Vec<ArgKind>,
 233         is_closure: bool,
 234         closure_arg_span: Option<Span>,
 235     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
 236         let kind = if is_closure { "closure" } else { "function" };
 237
 238         let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
 239             let arg_length = arguments.len();
 240             let distinct = matches!(other, &[ArgKind::Tuple(..)]);
 241             match (arg_length, arguments.get(0)) {
 242                 (1, Some(ArgKind::Tuple(_, fields))) => {
 243                     format!("a single {}-tuple as argument", fields.len())
 244                 }
 245                 _ => format!(
 246                     "{} {}argument{}",
 247                     arg_length,
 248                     if distinct && arg_length > 1 { "distinct " } else { "" },
 249                     pluralize!(arg_length)
 250                 ),
 251             }
 252         };
 253
 254         let expected_str = args_str(&expected_args, &found_args);
 255         let found_str = args_str(&found_args, &expected_args);
 256
 257         let mut err = struct_span_err!(
 258             self.tcx.sess,
 259             span,
 260             E0593,
 261             "{} is expected to take {}, but it takes {}",
 262             kind,
 263             expected_str,
 264             found_str,
 265         );
 266
 267         err.span_label(span, format!("expected {} that takes {}", kind, expected_str));
 268
 269         if let Some(found_span) = found_span {
 270             err.span_label(found_span, format!("takes {}", found_str));
 271
 272             // Suggest to take and ignore the arguments with expected_args_length `_`s if
 273             // found arguments is empty (assume the user just wants to ignore args in this case).
 274             // For example, if `expected_args_length` is 2, suggest `|_, _|`.
 275             if found_args.is_empty() && is_closure {
 276                 let underscores = vec!["_"; expected_args.len()].join(", ");
 277                 err.span_suggestion_verbose(
 278                     closure_arg_span.unwrap_or(found_span),
 279                     &format!(
 280                         "consider changing the closure to take and ignore the expected argument{}",
 281                         pluralize!(expected_args.len())
 282                     ),
 283                     format!("|{}|", underscores),
 284                     Applicability::MachineApplicable,
 285                 );
 286             }
 287
 288             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
 289                 if fields.len() == expected_args.len() {
 290                     let sugg = fields
 291                         .iter()
 292                         .map(|(name, _)| name.to_owned())
 293                         .collect::<Vec<String>>()
 294                         .join(", ");
 295                     err.span_suggestion_verbose(
 296                         found_span,
 297                         "change the closure to take multiple arguments instead of a single tuple",
 298                         format!("|{}|", sugg),
 299                         Applicability::MachineApplicable,
 300                     );
 301                 }
 302             }
 303             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..]
 304                 && fields.len() == found_args.len()
 305                 && is_closure
 306             {
 307                 let sugg = format!(
 308                     "|({}){}|",
 309                     found_args
 310                         .iter()
 311                         .map(|arg| match arg {
 312                             ArgKind::Arg(name, _) => name.to_owned(),
 313                             _ => "_".to_owned(),
 314                         })
 315                         .collect::<Vec<String>>()
 316                         .join(", "),
 317                     // add type annotations if available
 318                     if found_args.iter().any(|arg| match arg {
 319                         ArgKind::Arg(_, ty) => ty != "_",
 320                         _ => false,
 321                     }) {
 322                         format!(
 323                             ": ({})",
 324                             fields
 325                                 .iter()
 326                                 .map(|(_, ty)| ty.to_owned())
 327                                 .collect::<Vec<String>>()
 328                                 .join(", ")
 329                         )
 330                     } else {
 331                         String::new()
 332                     },
 333                 );
 334                 err.span_suggestion_verbose(
 335                     found_span,
 336                     "change the closure to accept a tuple instead of individual arguments",
 337                     sugg,
 338                     Applicability::MachineApplicable,
 339                 );
 340             }
 341         }
 342
 343         err
 344     }
 345
 346     fn type_implements_fn_trait(
 347         &self,
 348         param_env: ty::ParamEnv<'tcx>,
 349         ty: ty::Binder<'tcx, Ty<'tcx>>,
 350         constness: ty::BoundConstness,
 351         polarity: ty::ImplPolarity,
 352     ) -> Result<(ty::ClosureKind, ty::Binder<'tcx, Ty<'tcx>>), ()> {
 353         self.commit_if_ok(|_| {
 354             for trait_def_id in [
 355                 self.tcx.lang_items().fn_trait(),
 356                 self.tcx.lang_items().fn_mut_trait(),
 357                 self.tcx.lang_items().fn_once_trait(),
 358             ] {
 359                 let Some(trait_def_id) = trait_def_id else { continue };
 360                 // Make a fresh inference variable so we can determine what the substitutions
 361                 // of the trait are.
 362                 let var = self.next_ty_var(TypeVariableOrigin {
 363                     span: DUMMY_SP,
 364                     kind: TypeVariableOriginKind::MiscVariable,
 365                 });
 366                 let trait_ref = self.tcx.mk_trait_ref(trait_def_id, [ty.skip_binder(), var]);
 367                 let obligation = Obligation::new(
 368                     self.tcx,
 369                     ObligationCause::dummy(),
 370                     param_env,
 371                     ty.rebind(ty::TraitPredicate { trait_ref, constness, polarity }),
 372                 );
 373                 let ocx = ObligationCtxt::new_in_snapshot(self);
 374                 ocx.register_obligation(obligation);
 375                 if ocx.select_all_or_error().is_empty() {
 376                     return Ok((
 377                         self.tcx
 378                             .fn_trait_kind_from_def_id(trait_def_id)
 379                             .expect("expected to map DefId to ClosureKind"),
 380                         ty.rebind(self.resolve_vars_if_possible(var)),
 381                     ));
 382                 }
 383             }
 384
 385             Err(())
 386         })
 387     }
 388 }
 389
 390 impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
 391     fn report_fulfillment_errors(
 392         &self,
 393         errors: &[FulfillmentError<'tcx>],
 394         body_id: Option<hir::BodyId>,
 395     ) -> ErrorGuaranteed {
 396         #[derive(Debug)]
 397         struct ErrorDescriptor<'tcx> {
 398             predicate: ty::Predicate<'tcx>,
 399             index: Option<usize>, // None if this is an old error
 400         }
 401
 402         let mut error_map: FxIndexMap<_, Vec<_>> = self
 403             .reported_trait_errors
 404             .borrow()
 405             .iter()
 406             .map(|(&span, predicates)| {
 407                 (
 408                     span,
 409                     predicates
 410                         .iter()
 411                         .map(|&predicate| ErrorDescriptor { predicate, index: None })
 412                         .collect(),
 413                 )
 414             })
 415             .collect();
 416
 417         for (index, error) in errors.iter().enumerate() {
 418             // We want to ignore desugarings here: spans are equivalent even
 419             // if one is the result of a desugaring and the other is not.
 420             let mut span = error.obligation.cause.span;
 421             let expn_data = span.ctxt().outer_expn_data();
 422             if let ExpnKind::Desugaring(_) = expn_data.kind {
 423                 span = expn_data.call_site;
 424             }
 425
 426             error_map.entry(span).or_default().push(ErrorDescriptor {
 427                 predicate: error.obligation.predicate,
 428                 index: Some(index),
 429             });
 430
 431             self.reported_trait_errors
 432                 .borrow_mut()
 433                 .entry(span)
 434                 .or_default()
 435                 .push(error.obligation.predicate);
 436         }
 437
 438         // We do this in 2 passes because we want to display errors in order, though
 439         // maybe it *is* better to sort errors by span or something.
 440         let mut is_suppressed = vec![false; errors.len()];
 441         for (_, error_set) in error_map.iter() {
 442             // We want to suppress "duplicate" errors with the same span.
 443             for error in error_set {
 444                 if let Some(index) = error.index {
 445                     // Suppress errors that are either:
 446                     // 1) strictly implied by another error.
 447                     // 2) implied by an error with a smaller index.
 448                     for error2 in error_set {
 449                         if error2.index.map_or(false, |index2| is_suppressed[index2]) {
 450                             // Avoid errors being suppressed by already-suppressed
 451                             // errors, to prevent all errors from being suppressed
 452                             // at once.
 453                             continue;
 454                         }
 455
 456                         if self.error_implies(error2.predicate, error.predicate)
 457                             && !(error2.index >= error.index
 458                                 && self.error_implies(error.predicate, error2.predicate))
 459                         {
 460                             info!("skipping {:?} (implied by {:?})", error, error2);
 461                             is_suppressed[index] = true;
 462                             break;
 463                         }
 464                     }
 465                 }
 466             }
 467         }
 468
 469         for from_expansion in [false, true] {
 470             for (error, suppressed) in iter::zip(errors, &is_suppressed) {
 471                 if !suppressed && error.obligation.cause.span.from_expansion() == from_expansion {
 472                     self.report_fulfillment_error(error, body_id);
 473                 }
 474             }
 475         }
 476
 477         self.tcx.sess.delay_span_bug(DUMMY_SP, "expected fullfillment errors")
 478     }
 479
 480     /// Reports that an overflow has occurred and halts compilation. We
 481     /// halt compilation unconditionally because it is important that
 482     /// overflows never be masked -- they basically represent computations
 483     /// whose result could not be truly determined and thus we can't say
 484     /// if the program type checks or not -- and they are unusual
 485     /// occurrences in any case.
 486     fn report_overflow_error<T>(
 487         &self,
 488         predicate: &T,
 489         span: Span,
 490         suggest_increasing_limit: bool,
 491         mutate: impl FnOnce(&mut Diagnostic),
 492     ) -> !
 493     where
 494         T: fmt::Display
 495             + TypeFoldable<'tcx>
 496             + Print<'tcx, FmtPrinter<'tcx, 'tcx>, Output = FmtPrinter<'tcx, 'tcx>>,
 497         <T as Print<'tcx, FmtPrinter<'tcx, 'tcx>>>::Error: std::fmt::Debug,
 498     {
 499         let mut err = self.build_overflow_error(predicate, span, suggest_increasing_limit);
 500         mutate(&mut err);
 501         err.emit();
 502
 503         self.tcx.sess.abort_if_errors();
 504         bug!();
 505     }
 506
 507     fn build_overflow_error<T>(
 508         &self,
 509         predicate: &T,
 510         span: Span,
 511         suggest_increasing_limit: bool,
 512     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed>
 513     where
 514         T: fmt::Display
 515             + TypeFoldable<'tcx>
 516             + Print<'tcx, FmtPrinter<'tcx, 'tcx>, Output = FmtPrinter<'tcx, 'tcx>>,
 517         <T as Print<'tcx, FmtPrinter<'tcx, 'tcx>>>::Error: std::fmt::Debug,
 518     {
 519         let predicate = self.resolve_vars_if_possible(predicate.clone());
 520         let mut pred_str = predicate.to_string();
 521
 522         if pred_str.len() > 50 {
 523             // We don't need to save the type to a file, we will be talking about this type already
 524             // in a separate note when we explain the obligation, so it will be available that way.
 525             pred_str = predicate
 526                 .print(FmtPrinter::new_with_limit(
 527                     self.tcx,
 528                     Namespace::TypeNS,
 529                     rustc_session::Limit(6),
 530                 ))
 531                 .unwrap()
 532                 .into_buffer();
 533         }
 534         let mut err = struct_span_err!(
 535             self.tcx.sess,
 536             span,
 537             E0275,
 538             "overflow evaluating the requirement `{}`",
 539             pred_str,
 540         );
 541
 542         if suggest_increasing_limit {
 543             self.suggest_new_overflow_limit(&mut err);
 544         }
 545
 546         err
 547     }
 548
 549     /// Reports that an overflow has occurred and halts compilation. We
 550     /// halt compilation unconditionally because it is important that
 551     /// overflows never be masked -- they basically represent computations
 552     /// whose result could not be truly determined and thus we can't say
 553     /// if the program type checks or not -- and they are unusual
 554     /// occurrences in any case.
 555     fn report_overflow_obligation<T>(
 556         &self,
 557         obligation: &Obligation<'tcx, T>,
 558         suggest_increasing_limit: bool,
 559     ) -> !
 560     where
 561         T: ToPredicate<'tcx> + Clone,
 562     {
 563         let predicate = obligation.predicate.clone().to_predicate(self.tcx);
 564         let predicate = self.resolve_vars_if_possible(predicate);
 565         self.report_overflow_error(
 566             &predicate,
 567             obligation.cause.span,
 568             suggest_increasing_limit,
 569             |err| {
 570                 self.note_obligation_cause_code(
 571                     err,
 572                     predicate,
 573                     obligation.param_env,
 574                     obligation.cause.code(),
 575                     &mut vec![],
 576                     &mut Default::default(),
 577                 );
 578             },
 579         );
 580     }
 581
 582     fn suggest_new_overflow_limit(&self, err: &mut Diagnostic) {
 583         let suggested_limit = match self.tcx.recursion_limit() {
 584             Limit(0) => Limit(2),
 585             limit => limit * 2,
 586         };
 587         err.help(&format!(
 588             "consider increasing the recursion limit by adding a \
 589              `#![recursion_limit = \"{}\"]` attribute to your crate (`{}`)",
 590             suggested_limit,
 591             self.tcx.crate_name(LOCAL_CRATE),
 592         ));
 593     }
 594
 595     /// Reports that a cycle was detected which led to overflow and halts
 596     /// compilation. This is equivalent to `report_overflow_obligation` except
 597     /// that we can give a more helpful error message (and, in particular,
 598     /// we do not suggest increasing the overflow limit, which is not
 599     /// going to help).
 600     fn report_overflow_obligation_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 601         let cycle = self.resolve_vars_if_possible(cycle.to_owned());
 602         assert!(!cycle.is_empty());
 603
 604         debug!(?cycle, "report_overflow_error_cycle");
 605
 606         // The 'deepest' obligation is most likely to have a useful
 607         // cause 'backtrace'
 608         self.report_overflow_obligation(
 609             cycle.iter().max_by_key(|p| p.recursion_depth).unwrap(),
 610             false,
 611         );
 612     }
 613
 614     fn report_selection_error(
 615         &self,
 616         mut obligation: PredicateObligation<'tcx>,
 617         root_obligation: &PredicateObligation<'tcx>,
 618         error: &SelectionError<'tcx>,
 619     ) {
 620         let tcx = self.tcx;
 621         let mut span = obligation.cause.span;
 622         // FIXME: statically guarantee this by tainting after the diagnostic is emitted
 623         self.set_tainted_by_errors(
 624             tcx.sess.delay_span_bug(span, "`report_selection_error` did not emit an error"),
 625         );
 626
 627         let mut err = match *error {
 628             SelectionError::Unimplemented => {
 629                 // If this obligation was generated as a result of well-formedness checking, see if we
 630                 // can get a better error message by performing HIR-based well-formedness checking.
 631                 if let ObligationCauseCode::WellFormed(Some(wf_loc)) =
 632                     root_obligation.cause.code().peel_derives()
 633                     && !obligation.predicate.has_non_region_infer()
 634                 {
 635                     if let Some(cause) = self
 636                         .tcx
 637                         .diagnostic_hir_wf_check((tcx.erase_regions(obligation.predicate), *wf_loc))
 638                     {
 639                         obligation.cause = cause.clone();
 640                         span = obligation.cause.span;
 641                     }
 642                 }
 643                 if let ObligationCauseCode::CompareImplItemObligation {
 644                     impl_item_def_id,
 645                     trait_item_def_id,
 646                     kind: _,
 647                 } = *obligation.cause.code()
 648                 {
 649                     self.report_extra_impl_obligation(
 650                         span,
 651                         impl_item_def_id,
 652                         trait_item_def_id,
 653                         &format!("`{}`", obligation.predicate),
 654                     )
 655                     .emit();
 656                     return;
 657                 }
 658
 659                 let bound_predicate = obligation.predicate.kind();
 660                 match bound_predicate.skip_binder() {
 661                     ty::PredicateKind::Clause(ty::Clause::Trait(trait_predicate)) => {
 662                         let trait_predicate = bound_predicate.rebind(trait_predicate);
 663                         let mut trait_predicate = self.resolve_vars_if_possible(trait_predicate);
 664
 665                         trait_predicate.remap_constness_diag(obligation.param_env);
 666                         let predicate_is_const = ty::BoundConstness::ConstIfConst
 667                             == trait_predicate.skip_binder().constness;
 668
 669                         if self.tcx.sess.has_errors().is_some()
 670                             && trait_predicate.references_error()
 671                         {
 672                             return;
 673                         }
 674                         let trait_ref = trait_predicate.to_poly_trait_ref();
 675                         let (post_message, pre_message, type_def) = self
 676                             .get_parent_trait_ref(obligation.cause.code())
 677                             .map(|(t, s)| {
 678                                 (
 679                                     format!(" in `{}`", t),
 680                                     format!("within `{}`, ", t),
 681                                     s.map(|s| (format!("within this `{}`", t), s)),
 682                                 )
 683                             })
 684                             .unwrap_or_default();
 685
 686                         let OnUnimplementedNote {
 687                             message,
 688                             label,
 689                             note,
 690                             parent_label,
 691                             append_const_msg,
 692                         } = self.on_unimplemented_note(trait_ref, &obligation);
 693                         let have_alt_message = message.is_some() || label.is_some();
 694                         let is_try_conversion = self.is_try_conversion(span, trait_ref.def_id());
 695                         let is_unsize =
 696                             Some(trait_ref.def_id()) == self.tcx.lang_items().unsize_trait();
 697                         let (message, note, append_const_msg) = if is_try_conversion {
 698                             (
 699                                 Some(format!(
 700                                     "`?` couldn't convert the error to `{}`",
 701                                     trait_ref.skip_binder().self_ty(),
 702                                 )),
 703                                 Some(
 704                                     "the question mark operation (`?`) implicitly performs a \
 705                                      conversion on the error value using the `From` trait"
 706                                         .to_owned(),
 707                                 ),
 708                                 Some(None),
 709                             )
 710                         } else {
 711                             (message, note, append_const_msg)
 712                         };
 713
 714                         let mut err = struct_span_err!(
 715                             self.tcx.sess,
 716                             span,
 717                             E0277,
 718                             "{}",
 719                             message
 720                                 .and_then(|cannot_do_this| {
 721                                     match (predicate_is_const, append_const_msg) {
 722                                         // do nothing if predicate is not const
 723                                         (false, _) => Some(cannot_do_this),
 724                                         // suggested using default post message
 725                                         (true, Some(None)) => {
 726                                             Some(format!("{cannot_do_this} in const contexts"))
 727                                         }
 728                                         // overridden post message
 729                                         (true, Some(Some(post_message))) => {
 730                                             Some(format!("{cannot_do_this}{post_message}"))
 731                                         }
 732                                         // fallback to generic message
 733                                         (true, None) => None,
 734                                     }
 735                                 })
 736                                 .unwrap_or_else(|| format!(
 737                                     "the trait bound `{}` is not satisfied{}",
 738                                     trait_predicate, post_message,
 739                                 ))
 740                         );
 741
 742                         if is_try_conversion && let Some(ret_span) = self.return_type_span(&obligation) {
 743                             err.span_label(
 744                                 ret_span,
 745                                 &format!(
 746                                     "expected `{}` because of this",
 747                                     trait_ref.skip_binder().self_ty()
 748                                 ),
 749                             );
 750                         }
 751
 752                         if Some(trait_ref.def_id()) == tcx.lang_items().tuple_trait() {
 753                             match obligation.cause.code().peel_derives() {
 754                                 ObligationCauseCode::RustCall => {
 755                                     err.set_primary_message("functions with the \"rust-call\" ABI must take a single non-self tuple argument");
 756                                 }
 757                                 ObligationCauseCode::BindingObligation(def_id, _)
 758                                 | ObligationCauseCode::ItemObligation(def_id)
 759                                     if tcx.is_fn_trait(*def_id) =>
 760                                 {
 761                                     err.code(rustc_errors::error_code!(E0059));
 762                                     err.set_primary_message(format!(
 763                                         "type parameter to bare `{}` trait must be a tuple",
 764                                         tcx.def_path_str(*def_id)
 765                                     ));
 766                                 }
 767                                 _ => {}
 768                             }
 769                         }
 770
 771                         if Some(trait_ref.def_id()) == tcx.lang_items().drop_trait()
 772                             && predicate_is_const
 773                         {
 774                             err.note("`~const Drop` was renamed to `~const Destruct`");
 775                             err.note("See <https://github.com/rust-lang/rust/pull/94901> for more details");
 776                         }
 777
 778                         let explanation = if let ObligationCauseCode::MainFunctionType =
 779                             obligation.cause.code()
 780                         {
 781                             "consider using `()`, or a `Result`".to_owned()
 782                         } else {
 783                             let ty_desc = match trait_ref.skip_binder().self_ty().kind() {
 784                                 ty::FnDef(_, _) => Some("fn item"),
 785                                 ty::Closure(_, _) => Some("closure"),
 786                                 _ => None,
 787                             };
 788
 789                             match ty_desc {
 790                                 Some(desc) => format!(
 791                                     "{}the trait `{}` is not implemented for {} `{}`",
 792                                     pre_message,
 793                                     trait_predicate.print_modifiers_and_trait_path(),
 794                                     desc,
 795                                     trait_ref.skip_binder().self_ty(),
 796                                 ),
 797                                 None => format!(
 798                                     "{}the trait `{}` is not implemented for `{}`",
 799                                     pre_message,
 800                                     trait_predicate.print_modifiers_and_trait_path(),
 801                                     trait_ref.skip_binder().self_ty(),
 802                                 ),
 803                             }
 804                         };
 805                         self.check_for_binding_assigned_block_without_tail_expression(
 806                             &obligation,
 807                             &mut err,
 808                             trait_predicate,
 809                         );
 810                         if self.suggest_add_reference_to_arg(
 811                             &obligation,
 812                             &mut err,
 813                             trait_predicate,
 814                             have_alt_message,
 815                         ) {
 816                             self.note_obligation_cause(&mut err, &obligation);
 817                             err.emit();
 818                             return;
 819                         }
 820                         if let Some(ref s) = label {
 821                             // If it has a custom `#[rustc_on_unimplemented]`
 822                             // error message, let's display it as the label!
 823                             err.span_label(span, s);
 824                             if !matches!(trait_ref.skip_binder().self_ty().kind(), ty::Param(_)) {
 825                                 // When the self type is a type param We don't need to "the trait
 826                                 // `std::marker::Sized` is not implemented for `T`" as we will point
 827                                 // at the type param with a label to suggest constraining it.
 828                                 err.help(&explanation);
 829                             }
 830                         } else {
 831                             err.span_label(span, explanation);
 832                         }
 833
 834                         if let ObligationCauseCode::ObjectCastObligation(concrete_ty, obj_ty) = obligation.cause.code().peel_derives() &&
 835                             Some(trait_ref.def_id()) == self.tcx.lang_items().sized_trait() {
 836                             self.suggest_borrowing_for_object_cast(&mut err, &root_obligation, *concrete_ty, *obj_ty);
 837                         }
 838
 839                         let mut unsatisfied_const = false;
 840                         if trait_predicate.is_const_if_const() && obligation.param_env.is_const() {
 841                             let non_const_predicate = trait_ref.without_const();
 842                             let non_const_obligation = Obligation {
 843                                 cause: obligation.cause.clone(),
 844                                 param_env: obligation.param_env.without_const(),
 845                                 predicate: non_const_predicate.to_predicate(tcx),
 846                                 recursion_depth: obligation.recursion_depth,
 847                             };
 848                             if self.predicate_may_hold(&non_const_obligation) {
 849                                 unsatisfied_const = true;
 850                                 err.span_note(
 851                                     span,
 852                                     &format!(
 853                                         "the trait `{}` is implemented for `{}`, \
 854                                         but that implementation is not `const`",
 855                                         non_const_predicate.print_modifiers_and_trait_path(),
 856                                         trait_ref.skip_binder().self_ty(),
 857                                     ),
 858                                 );
 859                             }
 860                         }
 861
 862                         if let Some((msg, span)) = type_def {
 863                             err.span_label(span, &msg);
 864                         }
 865                         if let Some(ref s) = note {
 866                             // If it has a custom `#[rustc_on_unimplemented]` note, let's display it
 867                             err.note(s.as_str());
 868                         }
 869                         if let Some(ref s) = parent_label {
 870                             let body = obligation.cause.body_id;
 871                             err.span_label(tcx.def_span(body), s);
 872                         }
 873
 874                         self.suggest_floating_point_literal(&obligation, &mut err, &trait_ref);
 875                         self.suggest_dereferencing_index(&obligation, &mut err, trait_predicate);
 876                         let mut suggested =
 877                             self.suggest_dereferences(&obligation, &mut err, trait_predicate);
 878                         suggested |= self.suggest_fn_call(&obligation, &mut err, trait_predicate);
 879                         let impl_candidates = self.find_similar_impl_candidates(trait_predicate);
 880                         suggested = if let &[cand] = &impl_candidates[..] {
 881                             let cand = cand.trait_ref;
 882                             if let (ty::FnPtr(_), ty::FnDef(..)) =
 883                                 (cand.self_ty().kind(), trait_ref.self_ty().skip_binder().kind())
 884                             {
 885                                 err.span_suggestion(
 886                                     span.shrink_to_hi(),
 887                                     format!(
 888                                         "the trait `{}` is implemented for fn pointer `{}`, try casting using `as`",
 889                                         cand.print_only_trait_path(),
 890                                         cand.self_ty(),
 891                                     ),
 892                                     format!(" as {}", cand.self_ty()),
 893                                     Applicability::MaybeIncorrect,
 894                                 );
 895                                 true
 896                             } else {
 897                                 false
 898                             }
 899                         } else {
 900                             false
 901                         } || suggested;
 902                         suggested |=
 903                             self.suggest_remove_reference(&obligation, &mut err, trait_predicate);
 904                         suggested |= self.suggest_semicolon_removal(
 905                             &obligation,
 906                             &mut err,
 907                             span,
 908                             trait_predicate,
 909                         );
 910                         self.note_version_mismatch(&mut err, &trait_ref);
 911                         self.suggest_remove_await(&obligation, &mut err);
 912                         self.suggest_derive(&obligation, &mut err, trait_predicate);
 913
 914                         if Some(trait_ref.def_id()) == tcx.lang_items().try_trait() {
 915                             self.suggest_await_before_try(
 916                                 &mut err,
 917                                 &obligation,
 918                                 trait_predicate,
 919                                 span,
 920                             );
 921                         }
 922
 923                         if self.suggest_add_clone_to_arg(&obligation, &mut err, trait_predicate) {
 924                             err.emit();
 925                             return;
 926                         }
 927
 928                         if self.suggest_impl_trait(&mut err, span, &obligation, trait_predicate) {
 929                             err.emit();
 930                             return;
 931                         }
 932
 933                         if is_unsize {
 934                             // If the obligation failed due to a missing implementation of the
 935                             // `Unsize` trait, give a pointer to why that might be the case
 936                             err.note(
 937                                 "all implementations of `Unsize` are provided \
 938                                 automatically by the compiler, see \
 939                                 <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> \
 940                                 for more information",
 941                             );
 942                         }
 943
 944                         let is_fn_trait = tcx.is_fn_trait(trait_ref.def_id());
 945                         let is_target_feature_fn = if let ty::FnDef(def_id, _) =
 946                             *trait_ref.skip_binder().self_ty().kind()
 947                         {
 948                             !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
 949                         } else {
 950                             false
 951                         };
 952                         if is_fn_trait && is_target_feature_fn {
 953                             err.note(
 954                                 "`#[target_feature]` functions do not implement the `Fn` traits",
 955                             );
 956                         }
 957
 958                         let body_hir_id =
 959                             self.tcx.hir().local_def_id_to_hir_id(obligation.cause.body_id);
 960                         // Try to report a help message
 961                         if is_fn_trait
 962                             && let Ok((implemented_kind, params)) = self.type_implements_fn_trait(
 963                             obligation.param_env,
 964                             trait_ref.self_ty(),
 965                             trait_predicate.skip_binder().constness,
 966                             trait_predicate.skip_binder().polarity,
 967                         )
 968                         {
 969                             // If the type implements `Fn`, `FnMut`, or `FnOnce`, suppress the following
 970                             // suggestion to add trait bounds for the type, since we only typically implement
 971                             // these traits once.
 972
 973                             // Note if the `FnMut` or `FnOnce` is less general than the trait we're trying
 974                             // to implement.
 975                             let selected_kind =
 976                                 self.tcx.fn_trait_kind_from_def_id(trait_ref.def_id())
 977                                     .expect("expected to map DefId to ClosureKind");
 978                             if !implemented_kind.extends(selected_kind) {
 979                                 err.note(
 980                                     &format!(
 981                                         "`{}` implements `{}`, but it must implement `{}`, which is more general",
 982                                         trait_ref.skip_binder().self_ty(),
 983                                         implemented_kind,
 984                                         selected_kind
 985                                     )
 986                                 );
 987                             }
 988
 989                             // Note any argument mismatches
 990                             let given_ty = params.skip_binder();
 991                             let expected_ty = trait_ref.skip_binder().substs.type_at(1);
 992                             if let ty::Tuple(given) = given_ty.kind()
 993                                 && let ty::Tuple(expected) = expected_ty.kind()
 994                             {
 995                                 if expected.len() != given.len() {
 996                                     // Note number of types that were expected and given
 997                                     err.note(
 998                                         &format!(
 999                                             "expected a closure taking {} argument{}, but one taking {} argument{} was given",
1000                                             given.len(),
1001                                             pluralize!(given.len()),
1002                                             expected.len(),
1003                                             pluralize!(expected.len()),
1004                                         )
1005                                     );
1006                                 } else if !self.same_type_modulo_infer(given_ty, expected_ty) {
1007                                     // Print type mismatch
1008                                     let (expected_args, given_args) =
1009                                         self.cmp(given_ty, expected_ty);
1010                                     err.note_expected_found(
1011                                         &"a closure with arguments",
1012                                         expected_args,
1013                                         &"a closure with arguments",
1014                                         given_args,
1015                                     );
1016                                 }
1017                             }
1018                         } else if !trait_ref.has_non_region_infer()
1019                             && self.predicate_can_apply(obligation.param_env, trait_predicate)
1020                         {
1021                             // If a where-clause may be useful, remind the
1022                             // user that they can add it.
1023                             //
1024                             // don't display an on-unimplemented note, as
1025                             // these notes will often be of the form
1026                             //     "the type `T` can't be frobnicated"
1027                             // which is somewhat confusing.
1028                             self.suggest_restricting_param_bound(
1029                                 &mut err,
1030                                 trait_predicate,
1031                                 None,
1032                                 obligation.cause.body_id,
1033                             );
1034                         } else if !suggested && !unsatisfied_const {
1035                             // Can't show anything else useful, try to find similar impls.
1036                             let impl_candidates = self.find_similar_impl_candidates(trait_predicate);
1037                             if !self.report_similar_impl_candidates(
1038                                 impl_candidates,
1039                                 trait_ref,
1040                                 body_hir_id,
1041                                 &mut err,
1042                                 true,
1043                             ) {
1044                                 // This is *almost* equivalent to
1045                                 // `obligation.cause.code().peel_derives()`, but it gives us the
1046                                 // trait predicate for that corresponding root obligation. This
1047                                 // lets us get a derived obligation from a type parameter, like
1048                                 // when calling `string.strip_suffix(p)` where `p` is *not* an
1049                                 // implementer of `Pattern<'_>`.
1050                                 let mut code = obligation.cause.code();
1051                                 let mut trait_pred = trait_predicate;
1052                                 let mut peeled = false;
1053                                 while let Some((parent_code, parent_trait_pred)) = code.parent() {
1054                                     code = parent_code;
1055                                     if let Some(parent_trait_pred) = parent_trait_pred {
1056                                         trait_pred = parent_trait_pred;
1057                                         peeled = true;
1058                                     }
1059                                 }
1060                                 let def_id = trait_pred.def_id();
1061                                 // Mention *all* the `impl`s for the *top most* obligation, the
1062                                 // user might have meant to use one of them, if any found. We skip
1063                                 // auto-traits or fundamental traits that might not be exactly what
1064                                 // the user might expect to be presented with. Instead this is
1065                                 // useful for less general traits.
1066                                 if peeled
1067                                     && !self.tcx.trait_is_auto(def_id)
1068                                     && !self.tcx.lang_items().iter().any(|(_, id)| id == def_id)
1069                                 {
1070                                     let trait_ref = trait_pred.to_poly_trait_ref();
1071                                     let impl_candidates =
1072                                         self.find_similar_impl_candidates(trait_pred);
1073                                     self.report_similar_impl_candidates(
1074                                         impl_candidates,
1075                                         trait_ref,
1076                                         body_hir_id,
1077                                         &mut err,
1078                                         true,
1079                                     );
1080                                 }
1081                             }
1082                         }
1083
1084                         // Changing mutability doesn't make a difference to whether we have
1085                         // an `Unsize` impl (Fixes ICE in #71036)
1086                         if !is_unsize {
1087                             self.suggest_change_mut(&obligation, &mut err, trait_predicate);
1088                         }
1089
1090                         // If this error is due to `!: Trait` not implemented but `(): Trait` is
1091                         // implemented, and fallback has occurred, then it could be due to a
1092                         // variable that used to fallback to `()` now falling back to `!`. Issue a
1093                         // note informing about the change in behaviour.
1094                         if trait_predicate.skip_binder().self_ty().is_never()
1095                             && self.fallback_has_occurred
1096                         {
1097                             let predicate = trait_predicate.map_bound(|trait_pred| {
1098                                 trait_pred.with_self_ty(self.tcx, self.tcx.mk_unit())
1099                             });
1100                             let unit_obligation = obligation.with(tcx, predicate);
1101                             if self.predicate_may_hold(&unit_obligation) {
1102                                 err.note(
1103                                     "this error might have been caused by changes to \
1104                                     Rust's type-inference algorithm (see issue #48950 \
1105                                     <https://github.com/rust-lang/rust/issues/48950> \
1106                                     for more information)",
1107                                 );
1108                                 err.help("did you intend to use the type `()` here instead?");
1109                             }
1110                         }
1111
1112                         // Return early if the trait is Debug or Display and the invocation
1113                         // originates within a standard library macro, because the output
1114                         // is otherwise overwhelming and unhelpful (see #85844 for an
1115                         // example).
1116
1117                         let in_std_macro =
1118                             match obligation.cause.span.ctxt().outer_expn_data().macro_def_id {
1119                                 Some(macro_def_id) => {
1120                                     let crate_name = tcx.crate_name(macro_def_id.krate);
1121                                     crate_name == sym::std || crate_name == sym::core
1122                                 }
1123                                 None => false,
1124                             };
1125
1126                         if in_std_macro
1127                             && matches!(
1128                                 self.tcx.get_diagnostic_name(trait_ref.def_id()),
1129                                 Some(sym::Debug | sym::Display)
1130                             )
1131                         {
1132                             err.emit();
1133                             return;
1134                         }
1135
1136                         err
1137                     }
1138
1139                     ty::PredicateKind::Subtype(predicate) => {
1140                         // Errors for Subtype predicates show up as
1141                         // `FulfillmentErrorCode::CodeSubtypeError`,
1142                         // not selection error.
1143                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
1144                     }
1145
1146                     ty::PredicateKind::Coerce(predicate) => {
1147                         // Errors for Coerce predicates show up as
1148                         // `FulfillmentErrorCode::CodeSubtypeError`,
1149                         // not selection error.
1150                         span_bug!(span, "coerce requirement gave wrong error: `{:?}`", predicate)
1151                     }
1152
1153                     ty::PredicateKind::Clause(ty::Clause::RegionOutlives(..))
1154                     | ty::PredicateKind::Clause(ty::Clause::TypeOutlives(..)) => {
1155                         span_bug!(
1156                             span,
1157                             "outlives clauses should not error outside borrowck. obligation: `{:?}`",
1158                             obligation
1159                         )
1160                     }
1161
1162                     ty::PredicateKind::Clause(ty::Clause::Projection(..)) => {
1163                         span_bug!(
1164                             span,
1165                             "projection clauses should be implied from elsewhere. obligation: `{:?}`",
1166                             obligation
1167                         )
1168                     }
1169
1170                     ty::PredicateKind::ObjectSafe(trait_def_id) => {
1171                         let violations = self.tcx.object_safety_violations(trait_def_id);
1172                         report_object_safety_error(self.tcx, span, trait_def_id, violations)
1173                     }
1174
1175                     ty::PredicateKind::ClosureKind(closure_def_id, closure_substs, kind) => {
1176                         let found_kind = self.closure_kind(closure_substs).unwrap();
1177                         let closure_span = self.tcx.def_span(closure_def_id);
1178                         let mut err = struct_span_err!(
1179                             self.tcx.sess,
1180                             closure_span,
1181                             E0525,
1182                             "expected a closure that implements the `{}` trait, \
1183                              but this closure only implements `{}`",
1184                             kind,
1185                             found_kind
1186                         );
1187
1188                         err.span_label(
1189                             closure_span,
1190                             format!("this closure implements `{}`, not `{}`", found_kind, kind),
1191                         );
1192                         err.span_label(
1193                             obligation.cause.span,
1194                             format!("the requirement to implement `{}` derives from here", kind),
1195                         );
1196
1197                         // Additional context information explaining why the closure only implements
1198                         // a particular trait.
1199                         if let Some(typeck_results) = &self.typeck_results {
1200                             let hir_id = self
1201                                 .tcx
1202                                 .hir()
1203                                 .local_def_id_to_hir_id(closure_def_id.expect_local());
1204                             match (found_kind, typeck_results.closure_kind_origins().get(hir_id)) {
1205                                 (ty::ClosureKind::FnOnce, Some((span, place))) => {
1206                                     err.span_label(
1207                                         *span,
1208                                         format!(
1209                                             "closure is `FnOnce` because it moves the \
1210                                          variable `{}` out of its environment",
1211                                             ty::place_to_string_for_capture(tcx, place)
1212                                         ),
1213                                     );
1214                                 }
1215                                 (ty::ClosureKind::FnMut, Some((span, place))) => {
1216                                     err.span_label(
1217                                         *span,
1218                                         format!(
1219                                             "closure is `FnMut` because it mutates the \
1220                                          variable `{}` here",
1221                                             ty::place_to_string_for_capture(tcx, place)
1222                                         ),
1223                                     );
1224                                 }
1225                                 _ => {}
1226                             }
1227                         }
1228
1229                         err
1230                     }
1231
1232                     ty::PredicateKind::WellFormed(ty) => {
1233                         match self.tcx.sess.opts.unstable_opts.trait_solver {
1234                             TraitSolver::Classic => {
1235                                 // WF predicates cannot themselves make
1236                                 // errors. They can only block due to
1237                                 // ambiguity; otherwise, they always
1238                                 // degenerate into other obligations
1239                                 // (which may fail).
1240                                 span_bug!(span, "WF predicate not satisfied for {:?}", ty);
1241                             }
1242                             TraitSolver::Chalk | TraitSolver::Next => {
1243                                 // FIXME: we'll need a better message which takes into account
1244                                 // which bounds actually failed to hold.
1245                                 self.tcx.sess.struct_span_err(
1246                                     span,
1247                                     &format!("the type `{}` is not well-formed", ty),
1248                                 )
1249                             }
1250                         }
1251                     }
1252
1253                     ty::PredicateKind::ConstEvaluatable(..) => {
1254                         // Errors for `ConstEvaluatable` predicates show up as
1255                         // `SelectionError::ConstEvalFailure`,
1256                         // not `Unimplemented`.
1257                         span_bug!(
1258                             span,
1259                             "const-evaluatable requirement gave wrong error: `{:?}`",
1260                             obligation
1261                         )
1262                     }
1263
1264                     ty::PredicateKind::ConstEquate(..) => {
1265                         // Errors for `ConstEquate` predicates show up as
1266                         // `SelectionError::ConstEvalFailure`,
1267                         // not `Unimplemented`.
1268                         span_bug!(
1269                             span,
1270                             "const-equate requirement gave wrong error: `{:?}`",
1271                             obligation
1272                         )
1273                     }
1274
1275                     ty::PredicateKind::Ambiguous => span_bug!(span, "ambiguous"),
1276
1277                     ty::PredicateKind::TypeWellFormedFromEnv(..) => span_bug!(
1278                         span,
1279                         "TypeWellFormedFromEnv predicate should only exist in the environment"
1280                     ),
1281                 }
1282             }
1283
1284             OutputTypeParameterMismatch(
1285                 found_trait_ref,
1286                 expected_trait_ref,
1287                 terr @ TypeError::CyclicTy(_),
1288             ) => {
1289                 let self_ty = found_trait_ref.self_ty().skip_binder();
1290                 let (cause, terr) = if let ty::Closure(def_id, _) = self_ty.kind() {
1291                     (
1292                         ObligationCause::dummy_with_span(tcx.def_span(def_id)),
1293                         TypeError::CyclicTy(self_ty),
1294                     )
1295                 } else {
1296                     (obligation.cause.clone(), terr)
1297                 };
1298                 self.report_and_explain_type_error(
1299                     TypeTrace::poly_trait_refs(&cause, true, expected_trait_ref, found_trait_ref),
1300                     terr,
1301                 )
1302             }
1303             OutputTypeParameterMismatch(found_trait_ref, expected_trait_ref, _) => {
1304                 let found_trait_ref = self.resolve_vars_if_possible(found_trait_ref);
1305                 let expected_trait_ref = self.resolve_vars_if_possible(expected_trait_ref);
1306
1307                 if expected_trait_ref.self_ty().references_error() {
1308                     return;
1309                 }
1310
1311                 let Some(found_trait_ty) = found_trait_ref.self_ty().no_bound_vars() else {
1312                     return;
1313                 };
1314
1315                 let found_did = match *found_trait_ty.kind() {
1316                     ty::Closure(did, _)
1317                     | ty::Foreign(did)
1318                     | ty::FnDef(did, _)
1319                     | ty::Generator(did, ..) => Some(did),
1320                     ty::Adt(def, _) => Some(def.did()),
1321                     _ => None,
1322                 };
1323
1324                 let found_node = found_did.and_then(|did| self.tcx.hir().get_if_local(did));
1325                 let found_span = found_did.and_then(|did| self.tcx.hir().span_if_local(did));
1326
1327                 if self.reported_closure_mismatch.borrow().contains(&(span, found_span)) {
1328                     // We check closures twice, with obligations flowing in different directions,
1329                     // but we want to complain about them only once.
1330                     return;
1331                 }
1332
1333                 self.reported_closure_mismatch.borrow_mut().insert((span, found_span));
1334
1335                 let mut not_tupled = false;
1336
1337                 let found = match found_trait_ref.skip_binder().substs.type_at(1).kind() {
1338                     ty::Tuple(ref tys) => vec![ArgKind::empty(); tys.len()],
1339                     _ => {
1340                         not_tupled = true;
1341                         vec![ArgKind::empty()]
1342                     }
1343                 };
1344
1345                 let expected_ty = expected_trait_ref.skip_binder().substs.type_at(1);
1346                 let expected = match expected_ty.kind() {
1347                     ty::Tuple(ref tys) => {
1348                         tys.iter().map(|t| ArgKind::from_expected_ty(t, Some(span))).collect()
1349                     }
1350                     _ => {
1351                         not_tupled = true;
1352                         vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())]
1353                     }
1354                 };
1355
1356                 // If this is a `Fn` family trait and either the expected or found
1357                 // is not tupled, then fall back to just a regular mismatch error.
1358                 // This shouldn't be common unless manually implementing one of the
1359                 // traits manually, but don't make it more confusing when it does
1360                 // happen.
1361                 if Some(expected_trait_ref.def_id()) != tcx.lang_items().gen_trait() && not_tupled {
1362                     self.report_and_explain_type_error(
1363                         TypeTrace::poly_trait_refs(
1364                             &obligation.cause,
1365                             true,
1366                             expected_trait_ref,
1367                             found_trait_ref,
1368                         ),
1369                         ty::error::TypeError::Mismatch,
1370                     )
1371                 } else if found.len() == expected.len() {
1372                     self.report_closure_arg_mismatch(
1373                         span,
1374                         found_span,
1375                         found_trait_ref,
1376                         expected_trait_ref,
1377                         obligation.cause.code(),
1378                         found_node,
1379                         obligation.param_env,
1380                     )
1381                 } else {
1382                     let (closure_span, closure_arg_span, found) = found_did
1383                         .and_then(|did| {
1384                             let node = self.tcx.hir().get_if_local(did)?;
1385                             let (found_span, closure_arg_span, found) =
1386                                 self.get_fn_like_arguments(node)?;
1387                             Some((Some(found_span), closure_arg_span, found))
1388                         })
1389                         .unwrap_or((found_span, None, found));
1390
1391                     self.report_arg_count_mismatch(
1392                         span,
1393                         closure_span,
1394                         expected,
1395                         found,
1396                         found_trait_ty.is_closure(),
1397                         closure_arg_span,
1398                     )
1399                 }
1400             }
1401
1402             TraitNotObjectSafe(did) => {
1403                 let violations = self.tcx.object_safety_violations(did);
1404                 report_object_safety_error(self.tcx, span, did, violations)
1405             }
1406
1407             SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsInfer) => {
1408                 bug!(
1409                     "MentionsInfer should have been handled in `traits/fulfill.rs` or `traits/select/mod.rs`"
1410                 )
1411             }
1412             SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsParam) => {
1413                 if !self.tcx.features().generic_const_exprs {
1414                     let mut err = self.tcx.sess.struct_span_err(
1415                         span,
1416                         "constant expression depends on a generic parameter",
1417                     );
1418                     // FIXME(const_generics): we should suggest to the user how they can resolve this
1419                     // issue. However, this is currently not actually possible
1420                     // (see https://github.com/rust-lang/rust/issues/66962#issuecomment-575907083).
1421                     //
1422                     // Note that with `feature(generic_const_exprs)` this case should not
1423                     // be reachable.
1424                     err.note("this may fail depending on what value the parameter takes");
1425                     err.emit();
1426                     return;
1427                 }
1428
1429                 match obligation.predicate.kind().skip_binder() {
1430                     ty::PredicateKind::ConstEvaluatable(ct) => {
1431                         let ty::ConstKind::Unevaluated(uv) = ct.kind() else {
1432                             bug!("const evaluatable failed for non-unevaluated const `{ct:?}`");
1433                         };
1434                         let mut err =
1435                             self.tcx.sess.struct_span_err(span, "unconstrained generic constant");
1436                         let const_span = self.tcx.def_span(uv.def.did);
1437                         match self.tcx.sess.source_map().span_to_snippet(const_span) {
1438                             Ok(snippet) => err.help(&format!(
1439                                 "try adding a `where` bound using this expression: `where [(); {}]:`",
1440                                 snippet
1441                             )),
1442                             _ => err.help("consider adding a `where` bound using this expression"),
1443                         };
1444                         err
1445                     }
1446                     _ => {
1447                         span_bug!(
1448                             span,
1449                             "unexpected non-ConstEvaluatable predicate, this should not be reachable"
1450                         )
1451                     }
1452                 }
1453             }
1454
1455             // Already reported in the query.
1456             SelectionError::NotConstEvaluatable(NotConstEvaluatable::Error(_)) => {
1457                 // FIXME(eddyb) remove this once `ErrorGuaranteed` becomes a proof token.
1458                 self.tcx.sess.delay_span_bug(span, "`ErrorGuaranteed` without an error");
1459                 return;
1460             }
1461             // Already reported.
1462             Overflow(OverflowError::Error(_)) => {
1463                 self.tcx.sess.delay_span_bug(span, "`OverflowError` has been reported");
1464                 return;
1465             }
1466             Overflow(_) => {
1467                 bug!("overflow should be handled before the `report_selection_error` path");
1468             }
1469             SelectionError::ErrorReporting => {
1470                 bug!("ErrorReporting Overflow should not reach `report_selection_err` call")
1471             }
1472         };
1473
1474         self.note_obligation_cause(&mut err, &obligation);
1475         self.point_at_returns_when_relevant(&mut err, &obligation);
1476         err.emit();
1477     }
1478 }
1479
1480 trait InferCtxtPrivExt<'tcx> {
1481     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1482     // `error` occurring implies that `cond` occurs.
1483     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool;
1484
1485     fn report_fulfillment_error(
1486         &self,
1487         error: &FulfillmentError<'tcx>,
1488         body_id: Option<hir::BodyId>,
1489     );
1490
1491     fn report_projection_error(
1492         &self,
1493         obligation: &PredicateObligation<'tcx>,
1494         error: &MismatchedProjectionTypes<'tcx>,
1495     );
1496
1497     fn maybe_detailed_projection_msg(
1498         &self,
1499         pred: ty::ProjectionPredicate<'tcx>,
1500         normalized_ty: ty::Term<'tcx>,
1501         expected_ty: ty::Term<'tcx>,
1502     ) -> Option<String>;
1503
1504     fn fuzzy_match_tys(
1505         &self,
1506         a: Ty<'tcx>,
1507         b: Ty<'tcx>,
1508         ignoring_lifetimes: bool,
1509     ) -> Option<CandidateSimilarity>;
1510
1511     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str>;
1512
1513     fn find_similar_impl_candidates(
1514         &self,
1515         trait_pred: ty::PolyTraitPredicate<'tcx>,
1516     ) -> Vec<ImplCandidate<'tcx>>;
1517
1518     fn report_similar_impl_candidates(
1519         &self,
1520         impl_candidates: Vec<ImplCandidate<'tcx>>,
1521         trait_ref: ty::PolyTraitRef<'tcx>,
1522         body_id: hir::HirId,
1523         err: &mut Diagnostic,
1524         other: bool,
1525     ) -> bool;
1526
1527     /// Gets the parent trait chain start
1528     fn get_parent_trait_ref(
1529         &self,
1530         code: &ObligationCauseCode<'tcx>,
1531     ) -> Option<(String, Option<Span>)>;
1532
1533     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1534     /// with the same path as `trait_ref`, a help message about
1535     /// a probable version mismatch is added to `err`
1536     fn note_version_mismatch(
1537         &self,
1538         err: &mut Diagnostic,
1539         trait_ref: &ty::PolyTraitRef<'tcx>,
1540     ) -> bool;
1541
1542     /// Creates a `PredicateObligation` with `new_self_ty` replacing the existing type in the
1543     /// `trait_ref`.
1544     ///
1545     /// For this to work, `new_self_ty` must have no escaping bound variables.
1546     fn mk_trait_obligation_with_new_self_ty(
1547         &self,
1548         param_env: ty::ParamEnv<'tcx>,
1549         trait_ref_and_ty: ty::Binder<'tcx, (ty::TraitPredicate<'tcx>, Ty<'tcx>)>,
1550     ) -> PredicateObligation<'tcx>;
1551
1552     fn maybe_report_ambiguity(
1553         &self,
1554         obligation: &PredicateObligation<'tcx>,
1555         body_id: Option<hir::BodyId>,
1556     );
1557
1558     fn predicate_can_apply(
1559         &self,
1560         param_env: ty::ParamEnv<'tcx>,
1561         pred: ty::PolyTraitPredicate<'tcx>,
1562     ) -> bool;
1563
1564     fn note_obligation_cause(&self, err: &mut Diagnostic, obligation: &PredicateObligation<'tcx>);
1565
1566     fn suggest_unsized_bound_if_applicable(
1567         &self,
1568         err: &mut Diagnostic,
1569         obligation: &PredicateObligation<'tcx>,
1570     );
1571
1572     fn annotate_source_of_ambiguity(
1573         &self,
1574         err: &mut Diagnostic,
1575         impls: &[ambiguity::Ambiguity],
1576         predicate: ty::Predicate<'tcx>,
1577     );
1578
1579     fn maybe_suggest_unsized_generics(&self, err: &mut Diagnostic, span: Span, node: Node<'tcx>);
1580
1581     fn maybe_indirection_for_unsized(
1582         &self,
1583         err: &mut Diagnostic,
1584         item: &'tcx Item<'tcx>,
1585         param: &'tcx GenericParam<'tcx>,
1586     ) -> bool;
1587
1588     fn is_recursive_obligation(
1589         &self,
1590         obligated_types: &mut Vec<Ty<'tcx>>,
1591         cause_code: &ObligationCauseCode<'tcx>,
1592     ) -> bool;
1593 }
1594
1595 impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
1596     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1597     // `error` occurring implies that `cond` occurs.
1598     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool {
1599         if cond == error {
1600             return true;
1601         }
1602
1603         // FIXME: It should be possible to deal with `ForAll` in a cleaner way.
1604         let bound_error = error.kind();
1605         let (cond, error) = match (cond.kind().skip_binder(), bound_error.skip_binder()) {
1606             (
1607                 ty::PredicateKind::Clause(ty::Clause::Trait(..)),
1608                 ty::PredicateKind::Clause(ty::Clause::Trait(error)),
1609             ) => (cond, bound_error.rebind(error)),
1610             _ => {
1611                 // FIXME: make this work in other cases too.
1612                 return false;
1613             }
1614         };
1615
1616         for obligation in super::elaborate_predicates(self.tcx, std::iter::once(cond)) {
1617             let bound_predicate = obligation.predicate.kind();
1618             if let ty::PredicateKind::Clause(ty::Clause::Trait(implication)) =
1619                 bound_predicate.skip_binder()
1620             {
1621                 let error = error.to_poly_trait_ref();
1622                 let implication = bound_predicate.rebind(implication.trait_ref);
1623                 // FIXME: I'm just not taking associated types at all here.
1624                 // Eventually I'll need to implement param-env-aware
1625                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
1626                 let param_env = ty::ParamEnv::empty();
1627                 if self.can_sub(param_env, error, implication).is_ok() {
1628                     debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
1629                     return true;
1630                 }
1631             }
1632         }
1633
1634         false
1635     }
1636
1637     #[instrument(skip(self), level = "debug")]
1638     fn report_fulfillment_error(
1639         &self,
1640         error: &FulfillmentError<'tcx>,
1641         body_id: Option<hir::BodyId>,
1642     ) {
1643         match error.code {
1644             FulfillmentErrorCode::CodeSelectionError(ref selection_error) => {
1645                 self.report_selection_error(
1646                     error.obligation.clone(),
1647                     &error.root_obligation,
1648                     selection_error,
1649                 );
1650             }
1651             FulfillmentErrorCode::CodeProjectionError(ref e) => {
1652                 self.report_projection_error(&error.obligation, e);
1653             }
1654             FulfillmentErrorCode::CodeAmbiguity => {
1655                 self.maybe_report_ambiguity(&error.obligation, body_id);
1656             }
1657             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
1658                 self.report_mismatched_types(
1659                     &error.obligation.cause,
1660                     expected_found.expected,
1661                     expected_found.found,
1662                     *err,
1663                 )
1664                 .emit();
1665             }
1666             FulfillmentErrorCode::CodeConstEquateError(ref expected_found, ref err) => {
1667                 let mut diag = self.report_mismatched_consts(
1668                     &error.obligation.cause,
1669                     expected_found.expected,
1670                     expected_found.found,
1671                     *err,
1672                 );
1673                 let code = error.obligation.cause.code().peel_derives().peel_match_impls();
1674                 if let ObligationCauseCode::BindingObligation(..)
1675                 | ObligationCauseCode::ItemObligation(..)
1676                 | ObligationCauseCode::ExprBindingObligation(..)
1677                 | ObligationCauseCode::ExprItemObligation(..) = code
1678                 {
1679                     self.note_obligation_cause_code(
1680                         &mut diag,
1681                         error.obligation.predicate,
1682                         error.obligation.param_env,
1683                         code,
1684                         &mut vec![],
1685                         &mut Default::default(),
1686                     );
1687                 }
1688                 diag.emit();
1689             }
1690             FulfillmentErrorCode::CodeCycle(ref cycle) => {
1691                 self.report_overflow_obligation_cycle(cycle);
1692             }
1693         }
1694     }
1695
1696     #[instrument(level = "debug", skip_all)]
1697     fn report_projection_error(
1698         &self,
1699         obligation: &PredicateObligation<'tcx>,
1700         error: &MismatchedProjectionTypes<'tcx>,
1701     ) {
1702         let predicate = self.resolve_vars_if_possible(obligation.predicate);
1703
1704         if predicate.references_error() {
1705             return;
1706         }
1707
1708         self.probe(|_| {
1709             let ocx = ObligationCtxt::new_in_snapshot(self);
1710
1711             // try to find the mismatched types to report the error with.
1712             //
1713             // this can fail if the problem was higher-ranked, in which
1714             // cause I have no idea for a good error message.
1715             let bound_predicate = predicate.kind();
1716             let (values, err) = if let ty::PredicateKind::Clause(ty::Clause::Projection(data)) =
1717                 bound_predicate.skip_binder()
1718             {
1719                 let data = self.replace_bound_vars_with_fresh_vars(
1720                     obligation.cause.span,
1721                     infer::LateBoundRegionConversionTime::HigherRankedType,
1722                     bound_predicate.rebind(data),
1723                 );
1724                 let unnormalized_term = match data.term.unpack() {
1725                     ty::TermKind::Ty(_) => self
1726                         .tcx
1727                         .mk_projection(data.projection_ty.def_id, data.projection_ty.substs)
1728                         .into(),
1729                     ty::TermKind::Const(ct) => self
1730                         .tcx
1731                         .mk_const(
1732                             ty::UnevaluatedConst {
1733                                 def: ty::WithOptConstParam::unknown(data.projection_ty.def_id),
1734                                 substs: data.projection_ty.substs,
1735                             },
1736                             ct.ty(),
1737                         )
1738                         .into(),
1739                 };
1740                 let normalized_term =
1741                     ocx.normalize(&obligation.cause, obligation.param_env, unnormalized_term);
1742
1743                 debug!(?obligation.cause, ?obligation.param_env);
1744
1745                 debug!(?normalized_term, data.ty = ?data.term);
1746
1747                 let is_normalized_term_expected = !matches!(
1748                     obligation.cause.code().peel_derives(),
1749                     ObligationCauseCode::ItemObligation(_)
1750                         | ObligationCauseCode::BindingObligation(_, _)
1751                         | ObligationCauseCode::ExprItemObligation(..)
1752                         | ObligationCauseCode::ExprBindingObligation(..)
1753                         | ObligationCauseCode::ObjectCastObligation(..)
1754                         | ObligationCauseCode::OpaqueType
1755                 );
1756
1757                 // constrain inference variables a bit more to nested obligations from normalize so
1758                 // we can have more helpful errors.
1759                 ocx.select_where_possible();
1760
1761                 if let Err(new_err) = ocx.eq_exp(
1762                     &obligation.cause,
1763                     obligation.param_env,
1764                     is_normalized_term_expected,
1765                     normalized_term,
1766                     data.term,
1767                 ) {
1768                     (Some((data, is_normalized_term_expected, normalized_term, data.term)), new_err)
1769                 } else {
1770                     (None, error.err)
1771                 }
1772             } else {
1773                 (None, error.err)
1774             };
1775
1776             let msg = values
1777                 .and_then(|(predicate, _, normalized_term, expected_term)| {
1778                     self.maybe_detailed_projection_msg(predicate, normalized_term, expected_term)
1779                 })
1780                 .unwrap_or_else(|| {
1781                     with_forced_trimmed_paths!(format!(
1782                         "type mismatch resolving `{}`",
1783                         self.resolve_vars_if_possible(predicate)
1784                             .print(FmtPrinter::new_with_limit(
1785                                 self.tcx,
1786                                 Namespace::TypeNS,
1787                                 rustc_session::Limit(10),
1788                             ))
1789                             .unwrap()
1790                             .into_buffer()
1791                     ))
1792                 });
1793             let mut diag = struct_span_err!(self.tcx.sess, obligation.cause.span, E0271, "{msg}");
1794
1795             let secondary_span = match predicate.kind().skip_binder() {
1796                 ty::PredicateKind::Clause(ty::Clause::Projection(proj)) => self
1797                     .tcx
1798                     .opt_associated_item(proj.projection_ty.def_id)
1799                     .and_then(|trait_assoc_item| {
1800                         self.tcx
1801                             .trait_of_item(proj.projection_ty.def_id)
1802                             .map(|id| (trait_assoc_item, id))
1803                     })
1804                     .and_then(|(trait_assoc_item, id)| {
1805                         let trait_assoc_ident = trait_assoc_item.ident(self.tcx);
1806                         self.tcx.find_map_relevant_impl(id, proj.projection_ty.self_ty(), |did| {
1807                             self.tcx
1808                                 .associated_items(did)
1809                                 .in_definition_order()
1810                                 .find(|assoc| assoc.ident(self.tcx) == trait_assoc_ident)
1811                         })
1812                     })
1813                     .and_then(|item| match self.tcx.hir().get_if_local(item.def_id) {
1814                         Some(
1815                             hir::Node::TraitItem(hir::TraitItem {
1816                                 kind: hir::TraitItemKind::Type(_, Some(ty)),
1817                                 ..
1818                             })
1819                             | hir::Node::ImplItem(hir::ImplItem {
1820                                 kind: hir::ImplItemKind::Type(ty),
1821                                 ..
1822                             }),
1823                         ) => Some((
1824                             ty.span,
1825                             with_forced_trimmed_paths!(format!(
1826                                 "type mismatch resolving `{}`",
1827                                 self.resolve_vars_if_possible(predicate)
1828                                     .print(FmtPrinter::new_with_limit(
1829                                         self.tcx,
1830                                         Namespace::TypeNS,
1831                                         rustc_session::Limit(5),
1832                                     ))
1833                                     .unwrap()
1834                                     .into_buffer()
1835                             )),
1836                         )),
1837                         _ => None,
1838                     }),
1839                 _ => None,
1840             };
1841             self.note_type_err(
1842                 &mut diag,
1843                 &obligation.cause,
1844                 secondary_span,
1845                 values.map(|(_, is_normalized_ty_expected, normalized_ty, expected_ty)| {
1846                     infer::ValuePairs::Terms(ExpectedFound::new(
1847                         is_normalized_ty_expected,
1848                         normalized_ty,
1849                         expected_ty,
1850                     ))
1851                 }),
1852                 err,
1853                 true,
1854                 false,
1855             );
1856             self.note_obligation_cause(&mut diag, obligation);
1857             diag.emit();
1858         });
1859     }
1860
1861     fn maybe_detailed_projection_msg(
1862         &self,
1863         pred: ty::ProjectionPredicate<'tcx>,
1864         normalized_ty: ty::Term<'tcx>,
1865         expected_ty: ty::Term<'tcx>,
1866     ) -> Option<String> {
1867         let trait_def_id = pred.projection_ty.trait_def_id(self.tcx);
1868         let self_ty = pred.projection_ty.self_ty();
1869
1870         with_forced_trimmed_paths! {
1871             if Some(pred.projection_ty.def_id) == self.tcx.lang_items().fn_once_output() {
1872                 let fn_kind = self_ty.prefix_string(self.tcx);
1873                 let item = match self_ty.kind() {
1874                     ty::FnDef(def, _) => self.tcx.item_name(*def).to_string(),
1875                     _ => self_ty.to_string(),
1876                 };
1877                 Some(format!(
1878                     "expected `{item}` to be a {fn_kind} that returns `{expected_ty}`, but it \
1879                      returns `{normalized_ty}`",
1880                 ))
1881             } else if Some(trait_def_id) == self.tcx.lang_items().future_trait() {
1882                 Some(format!(
1883                     "expected `{self_ty}` to be a future that resolves to `{expected_ty}`, but it \
1884                      resolves to `{normalized_ty}`"
1885                 ))
1886             } else if Some(trait_def_id) == self.tcx.get_diagnostic_item(sym::Iterator) {
1887                 Some(format!(
1888                     "expected `{self_ty}` to be an iterator that yields `{expected_ty}`, but it \
1889                      yields `{normalized_ty}`"
1890                 ))
1891             } else {
1892                 None
1893             }
1894         }
1895     }
1896
1897     fn fuzzy_match_tys(
1898         &self,
1899         mut a: Ty<'tcx>,
1900         mut b: Ty<'tcx>,
1901         ignoring_lifetimes: bool,
1902     ) -> Option<CandidateSimilarity> {
1903         /// returns the fuzzy category of a given type, or None
1904         /// if the type can be equated to any type.
1905         fn type_category(tcx: TyCtxt<'_>, t: Ty<'_>) -> Option<u32> {
1906             match t.kind() {
1907                 ty::Bool => Some(0),
1908                 ty::Char => Some(1),
1909                 ty::Str => Some(2),
1910                 ty::Adt(def, _) if Some(def.did()) == tcx.lang_items().string() => Some(2),
1911                 ty::Int(..)
1912                 | ty::Uint(..)
1913                 | ty::Float(..)
1914                 | ty::Infer(ty::IntVar(..) | ty::FloatVar(..)) => Some(4),
1915                 ty::Ref(..) | ty::RawPtr(..) => Some(5),
1916                 ty::Array(..) | ty::Slice(..) => Some(6),
1917                 ty::FnDef(..) | ty::FnPtr(..) => Some(7),
1918                 ty::Dynamic(..) => Some(8),
1919                 ty::Closure(..) => Some(9),
1920                 ty::Tuple(..) => Some(10),
1921                 ty::Param(..) => Some(11),
1922                 ty::Alias(ty::Projection, ..) => Some(12),
1923                 ty::Alias(ty::Opaque, ..) => Some(13),
1924                 ty::Never => Some(14),
1925                 ty::Adt(..) => Some(15),
1926                 ty::Generator(..) => Some(16),
1927                 ty::Foreign(..) => Some(17),
1928                 ty::GeneratorWitness(..) => Some(18),
1929                 ty::GeneratorWitnessMIR(..) => Some(19),
1930                 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error(_) => None,
1931             }
1932         }
1933
1934         let strip_references = |mut t: Ty<'tcx>| -> Ty<'tcx> {
1935             loop {
1936                 match t.kind() {
1937                     ty::Ref(_, inner, _) | ty::RawPtr(ty::TypeAndMut { ty: inner, .. }) => {
1938                         t = *inner
1939                     }
1940                     _ => break t,
1941                 }
1942             }
1943         };
1944
1945         if !ignoring_lifetimes {
1946             a = strip_references(a);
1947             b = strip_references(b);
1948         }
1949
1950         let cat_a = type_category(self.tcx, a)?;
1951         let cat_b = type_category(self.tcx, b)?;
1952         if a == b {
1953             Some(CandidateSimilarity::Exact { ignoring_lifetimes })
1954         } else if cat_a == cat_b {
1955             match (a.kind(), b.kind()) {
1956                 (ty::Adt(def_a, _), ty::Adt(def_b, _)) => def_a == def_b,
1957                 (ty::Foreign(def_a), ty::Foreign(def_b)) => def_a == def_b,
1958                 // Matching on references results in a lot of unhelpful
1959                 // suggestions, so let's just not do that for now.
1960                 //
1961                 // We still upgrade successful matches to `ignoring_lifetimes: true`
1962                 // to prioritize that impl.
1963                 (ty::Ref(..) | ty::RawPtr(..), ty::Ref(..) | ty::RawPtr(..)) => {
1964                     self.fuzzy_match_tys(a, b, true).is_some()
1965                 }
1966                 _ => true,
1967             }
1968             .then_some(CandidateSimilarity::Fuzzy { ignoring_lifetimes })
1969         } else if ignoring_lifetimes {
1970             None
1971         } else {
1972             self.fuzzy_match_tys(a, b, true)
1973         }
1974     }
1975
1976     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str> {
1977         self.tcx.hir().body(body_id).generator_kind.map(|gen_kind| match gen_kind {
1978             hir::GeneratorKind::Gen => "a generator",
1979             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block) => "an async block",
1980             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn) => "an async function",
1981             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Closure) => "an async closure",
1982         })
1983     }
1984
1985     fn find_similar_impl_candidates(
1986         &self,
1987         trait_pred: ty::PolyTraitPredicate<'tcx>,
1988     ) -> Vec<ImplCandidate<'tcx>> {
1989         let mut candidates: Vec<_> = self
1990             .tcx
1991             .all_impls(trait_pred.def_id())
1992             .filter_map(|def_id| {
1993                 if self.tcx.impl_polarity(def_id) == ty::ImplPolarity::Negative
1994                     || !trait_pred
1995                         .skip_binder()
1996                         .is_constness_satisfied_by(self.tcx.constness(def_id))
1997                 {
1998                     return None;
1999                 }
2000
2001                 let imp = self.tcx.impl_trait_ref(def_id).unwrap().skip_binder();
2002
2003                 self.fuzzy_match_tys(trait_pred.skip_binder().self_ty(), imp.self_ty(), false)
2004                     .map(|similarity| ImplCandidate { trait_ref: imp, similarity })
2005             })
2006             .collect();
2007         if candidates.iter().any(|c| matches!(c.similarity, CandidateSimilarity::Exact { .. })) {
2008             // If any of the candidates is a perfect match, we don't want to show all of them.
2009             // This is particularly relevant for the case of numeric types (as they all have the
2010             // same cathegory).
2011             candidates.retain(|c| matches!(c.similarity, CandidateSimilarity::Exact { .. }));
2012         }
2013         candidates
2014     }
2015
2016     fn report_similar_impl_candidates(
2017         &self,
2018         impl_candidates: Vec<ImplCandidate<'tcx>>,
2019         trait_ref: ty::PolyTraitRef<'tcx>,
2020         body_id: hir::HirId,
2021         err: &mut Diagnostic,
2022         other: bool,
2023     ) -> bool {
2024         let other = if other { "other " } else { "" };
2025         let report = |mut candidates: Vec<TraitRef<'tcx>>, err: &mut Diagnostic| {
2026             candidates.sort();
2027             candidates.dedup();
2028             let len = candidates.len();
2029             if candidates.is_empty() {
2030                 return false;
2031             }
2032             if let &[cand] = &candidates[..] {
2033                 let (desc, mention_castable) =
2034                     match (cand.self_ty().kind(), trait_ref.self_ty().skip_binder().kind()) {
2035                         (ty::FnPtr(_), ty::FnDef(..)) => {
2036                             (" implemented for fn pointer `", ", cast using `as`")
2037                         }
2038                         (ty::FnPtr(_), _) => (" implemented for fn pointer `", ""),
2039                         _ => (" implemented for `", ""),
2040                     };
2041                 err.highlighted_help(vec![
2042                     (format!("the trait `{}` ", cand.print_only_trait_path()), Style::NoStyle),
2043                     ("is".to_string(), Style::Highlight),
2044                     (desc.to_string(), Style::NoStyle),
2045                     (cand.self_ty().to_string(), Style::Highlight),
2046                     ("`".to_string(), Style::NoStyle),
2047                     (mention_castable.to_string(), Style::NoStyle),
2048                 ]);
2049                 return true;
2050             }
2051             let trait_ref = TraitRef::identity(self.tcx, candidates[0].def_id);
2052             // Check if the trait is the same in all cases. If so, we'll only show the type.
2053             let mut traits: Vec<_> =
2054                 candidates.iter().map(|c| c.print_only_trait_path().to_string()).collect();
2055             traits.sort();
2056             traits.dedup();
2057
2058             let mut candidates: Vec<String> = candidates
2059                 .into_iter()
2060                 .map(|c| {
2061                     if traits.len() == 1 {
2062                         format!("\n  {}", c.self_ty())
2063                     } else {
2064                         format!("\n  {}", c)
2065                     }
2066                 })
2067                 .collect();
2068
2069             candidates.sort();
2070             candidates.dedup();
2071             let end = if candidates.len() <= 9 { candidates.len() } else { 8 };
2072             err.help(&format!(
2073                 "the following {other}types implement trait `{}`:{}{}",
2074                 trait_ref.print_only_trait_path(),
2075                 candidates[..end].join(""),
2076                 if len > 9 { format!("\nand {} others", len - 8) } else { String::new() }
2077             ));
2078             true
2079         };
2080
2081         let def_id = trait_ref.def_id();
2082         if impl_candidates.is_empty() {
2083             if self.tcx.trait_is_auto(def_id)
2084                 || self.tcx.lang_items().iter().any(|(_, id)| id == def_id)
2085                 || self.tcx.get_diagnostic_name(def_id).is_some()
2086             {
2087                 // Mentioning implementers of `Copy`, `Debug` and friends is not useful.
2088                 return false;
2089             }
2090             let normalized_impl_candidates: Vec<_> = self
2091                 .tcx
2092                 .all_impls(def_id)
2093                 // Ignore automatically derived impls and `!Trait` impls.
2094                 .filter(|&def_id| {
2095                     self.tcx.impl_polarity(def_id) != ty::ImplPolarity::Negative
2096                         || self.tcx.is_builtin_derive(def_id)
2097                 })
2098                 .filter_map(|def_id| self.tcx.impl_trait_ref(def_id))
2099                 .map(ty::EarlyBinder::subst_identity)
2100                 .filter(|trait_ref| {
2101                     let self_ty = trait_ref.self_ty();
2102                     // Avoid mentioning type parameters.
2103                     if let ty::Param(_) = self_ty.kind() {
2104                         false
2105                     }
2106                     // Avoid mentioning types that are private to another crate
2107                     else if let ty::Adt(def, _) = self_ty.peel_refs().kind() {
2108                         // FIXME(compiler-errors): This could be generalized, both to
2109                         // be more granular, and probably look past other `#[fundamental]`
2110                         // types, too.
2111                         self.tcx
2112                             .visibility(def.did())
2113                             .is_accessible_from(body_id.owner.def_id, self.tcx)
2114                     } else {
2115                         true
2116                     }
2117                 })
2118                 .collect();
2119             return report(normalized_impl_candidates, err);
2120         }
2121
2122         // Sort impl candidates so that ordering is consistent for UI tests.
2123         // because the ordering of `impl_candidates` may not be deterministic:
2124         // https://github.com/rust-lang/rust/pull/57475#issuecomment-455519507
2125         //
2126         // Prefer more similar candidates first, then sort lexicographically
2127         // by their normalized string representation.
2128         let mut normalized_impl_candidates_and_similarities = impl_candidates
2129             .into_iter()
2130             .map(|ImplCandidate { trait_ref, similarity }| {
2131                 // FIXME(compiler-errors): This should be using `NormalizeExt::normalize`
2132                 let normalized = self
2133                     .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
2134                     .query_normalize(trait_ref)
2135                     .map_or(trait_ref, |normalized| normalized.value);
2136                 (similarity, normalized)
2137             })
2138             .collect::<Vec<_>>();
2139         normalized_impl_candidates_and_similarities.sort();
2140         normalized_impl_candidates_and_similarities.dedup();
2141
2142         let normalized_impl_candidates = normalized_impl_candidates_and_similarities
2143             .into_iter()
2144             .map(|(_, normalized)| normalized)
2145             .collect::<Vec<_>>();
2146
2147         report(normalized_impl_candidates, err)
2148     }
2149
2150     /// Gets the parent trait chain start
2151     fn get_parent_trait_ref(
2152         &self,
2153         code: &ObligationCauseCode<'tcx>,
2154     ) -> Option<(String, Option<Span>)> {
2155         match code {
2156             ObligationCauseCode::BuiltinDerivedObligation(data) => {
2157                 let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
2158                 match self.get_parent_trait_ref(&data.parent_code) {
2159                     Some(t) => Some(t),
2160                     None => {
2161                         let ty = parent_trait_ref.skip_binder().self_ty();
2162                         let span = TyCategory::from_ty(self.tcx, ty)
2163                             .map(|(_, def_id)| self.tcx.def_span(def_id));
2164                         Some((ty.to_string(), span))
2165                     }
2166                 }
2167             }
2168             ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } => {
2169                 self.get_parent_trait_ref(&parent_code)
2170             }
2171             _ => None,
2172         }
2173     }
2174
2175     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
2176     /// with the same path as `trait_ref`, a help message about
2177     /// a probable version mismatch is added to `err`
2178     fn note_version_mismatch(
2179         &self,
2180         err: &mut Diagnostic,
2181         trait_ref: &ty::PolyTraitRef<'tcx>,
2182     ) -> bool {
2183         let get_trait_impl = |trait_def_id| {
2184             self.tcx.find_map_relevant_impl(trait_def_id, trait_ref.skip_binder().self_ty(), Some)
2185         };
2186         let required_trait_path = self.tcx.def_path_str(trait_ref.def_id());
2187         let traits_with_same_path: std::collections::BTreeSet<_> = self
2188             .tcx
2189             .all_traits()
2190             .filter(|trait_def_id| *trait_def_id != trait_ref.def_id())
2191             .filter(|trait_def_id| self.tcx.def_path_str(*trait_def_id) == required_trait_path)
2192             .collect();
2193         let mut suggested = false;
2194         for trait_with_same_path in traits_with_same_path {
2195             if let Some(impl_def_id) = get_trait_impl(trait_with_same_path) {
2196                 let impl_span = self.tcx.def_span(impl_def_id);
2197                 err.span_help(impl_span, "trait impl with same name found");
2198                 let trait_crate = self.tcx.crate_name(trait_with_same_path.krate);
2199                 let crate_msg = format!(
2200                     "perhaps two different versions of crate `{}` are being used?",
2201                     trait_crate
2202                 );
2203                 err.note(&crate_msg);
2204                 suggested = true;
2205             }
2206         }
2207         suggested
2208     }
2209
2210     fn mk_trait_obligation_with_new_self_ty(
2211         &self,
2212         param_env: ty::ParamEnv<'tcx>,
2213         trait_ref_and_ty: ty::Binder<'tcx, (ty::TraitPredicate<'tcx>, Ty<'tcx>)>,
2214     ) -> PredicateObligation<'tcx> {
2215         let trait_pred =
2216             trait_ref_and_ty.map_bound(|(tr, new_self_ty)| tr.with_self_ty(self.tcx, new_self_ty));
2217
2218         Obligation::new(self.tcx, ObligationCause::dummy(), param_env, trait_pred)
2219     }
2220
2221     #[instrument(skip(self), level = "debug")]
2222     fn maybe_report_ambiguity(
2223         &self,
2224         obligation: &PredicateObligation<'tcx>,
2225         body_id: Option<hir::BodyId>,
2226     ) {
2227         // Unable to successfully determine, probably means
2228         // insufficient type information, but could mean
2229         // ambiguous impls. The latter *ought* to be a
2230         // coherence violation, so we don't report it here.
2231
2232         let predicate = self.resolve_vars_if_possible(obligation.predicate);
2233         let span = obligation.cause.span;
2234
2235         debug!(?predicate, obligation.cause.code = ?obligation.cause.code());
2236
2237         // Ambiguity errors are often caused as fallout from earlier errors.
2238         // We ignore them if this `infcx` is tainted in some cases below.
2239
2240         let bound_predicate = predicate.kind();
2241         let mut err = match bound_predicate.skip_binder() {
2242             ty::PredicateKind::Clause(ty::Clause::Trait(data)) => {
2243                 let trait_ref = bound_predicate.rebind(data.trait_ref);
2244                 debug!(?trait_ref);
2245
2246                 if predicate.references_error() {
2247                     return;
2248                 }
2249
2250                 // This is kind of a hack: it frequently happens that some earlier
2251                 // error prevents types from being fully inferred, and then we get
2252                 // a bunch of uninteresting errors saying something like "<generic
2253                 // #0> doesn't implement Sized". It may even be true that we
2254                 // could just skip over all checks where the self-ty is an
2255                 // inference variable, but I was afraid that there might be an
2256                 // inference variable created, registered as an obligation, and
2257                 // then never forced by writeback, and hence by skipping here we'd
2258                 // be ignoring the fact that we don't KNOW the type works
2259                 // out. Though even that would probably be harmless, given that
2260                 // we're only talking about builtin traits, which are known to be
2261                 // inhabited. We used to check for `self.tcx.sess.has_errors()` to
2262                 // avoid inundating the user with unnecessary errors, but we now
2263                 // check upstream for type errors and don't add the obligations to
2264                 // begin with in those cases.
2265                 if self.tcx.lang_items().sized_trait() == Some(trait_ref.def_id()) {
2266                     if let None = self.tainted_by_errors() {
2267                         self.emit_inference_failure_err(
2268                             body_id,
2269                             span,
2270                             trait_ref.self_ty().skip_binder().into(),
2271                             ErrorCode::E0282,
2272                             false,
2273                         )
2274                         .emit();
2275                     }
2276                     return;
2277                 }
2278
2279                 // Typically, this ambiguity should only happen if
2280                 // there are unresolved type inference variables
2281                 // (otherwise it would suggest a coherence
2282                 // failure). But given #21974 that is not necessarily
2283                 // the case -- we can have multiple where clauses that
2284                 // are only distinguished by a region, which results
2285                 // in an ambiguity even when all types are fully
2286                 // known, since we don't dispatch based on region
2287                 // relationships.
2288
2289                 // Pick the first substitution that still contains inference variables as the one
2290                 // we're going to emit an error for. If there are none (see above), fall back to
2291                 // a more general error.
2292                 let subst = data.trait_ref.substs.iter().find(|s| s.has_non_region_infer());
2293
2294                 let mut err = if let Some(subst) = subst {
2295                     self.emit_inference_failure_err(body_id, span, subst, ErrorCode::E0283, true)
2296                 } else {
2297                     struct_span_err!(
2298                         self.tcx.sess,
2299                         span,
2300                         E0283,
2301                         "type annotations needed: cannot satisfy `{}`",
2302                         predicate,
2303                     )
2304                 };
2305
2306                 let obligation = obligation.with(self.tcx, trait_ref);
2307                 let mut selcx = SelectionContext::new(&self);
2308                 match selcx.select_from_obligation(&obligation) {
2309                     Ok(None) => {
2310                         let ambiguities =
2311                             ambiguity::recompute_applicable_impls(self.infcx, &obligation);
2312                         let has_non_region_infer = trait_ref
2313                             .skip_binder()
2314                             .substs
2315                             .types()
2316                             .any(|t| !t.is_ty_or_numeric_infer());
2317                         // It doesn't make sense to talk about applicable impls if there are more
2318                         // than a handful of them.
2319                         if ambiguities.len() > 1 && ambiguities.len() < 10 && has_non_region_infer {
2320                             if self.tainted_by_errors().is_some() && subst.is_none() {
2321                                 // If `subst.is_none()`, then this is probably two param-env
2322                                 // candidates or impl candidates that are equal modulo lifetimes.
2323                                 // Therefore, if we've already emitted an error, just skip this
2324                                 // one, since it's not particularly actionable.
2325                                 err.cancel();
2326                                 return;
2327                             }
2328                             self.annotate_source_of_ambiguity(&mut err, &ambiguities, predicate);
2329                         } else {
2330                             if self.tainted_by_errors().is_some() {
2331                                 err.cancel();
2332                                 return;
2333                             }
2334                             err.note(&format!("cannot satisfy `{}`", predicate));
2335                             let impl_candidates = self.find_similar_impl_candidates(
2336                                 predicate.to_opt_poly_trait_pred().unwrap(),
2337                             );
2338                             if impl_candidates.len() < 10 {
2339                                 let hir =
2340                                     self.tcx.hir().local_def_id_to_hir_id(obligation.cause.body_id);
2341                                 self.report_similar_impl_candidates(
2342                                     impl_candidates,
2343                                     trait_ref,
2344                                     body_id.map(|id| id.hir_id).unwrap_or(hir),
2345                                     &mut err,
2346                                     false,
2347                                 );
2348                             }
2349                         }
2350                     }
2351                     _ => {
2352                         if self.tainted_by_errors().is_some() {
2353                             err.cancel();
2354                             return;
2355                         }
2356                         err.note(&format!("cannot satisfy `{}`", predicate));
2357                     }
2358                 }
2359
2360                 if let ObligationCauseCode::ItemObligation(def_id)
2361                 | ObligationCauseCode::ExprItemObligation(def_id, ..) = *obligation.cause.code()
2362                 {
2363                     self.suggest_fully_qualified_path(&mut err, def_id, span, trait_ref.def_id());
2364                 }
2365
2366                 if let (Some(body_id), Some(ty::subst::GenericArgKind::Type(_))) =
2367                     (body_id, subst.map(|subst| subst.unpack()))
2368                 {
2369                     let mut expr_finder = FindExprBySpan::new(span);
2370                     expr_finder.visit_expr(&self.tcx.hir().body(body_id).value);
2371
2372                     if let Some(hir::Expr {
2373                         kind: hir::ExprKind::Path(hir::QPath::Resolved(None, path)), .. }
2374                     ) = expr_finder.result
2375                         && let [
2376                             ..,
2377                             trait_path_segment @ hir::PathSegment {
2378                                 res: rustc_hir::def::Res::Def(rustc_hir::def::DefKind::Trait, trait_id),
2379                                 ..
2380                             },
2381                             hir::PathSegment {
2382                                 ident: assoc_item_name,
2383                                 res: rustc_hir::def::Res::Def(_, item_id),
2384                                 ..
2385                             }
2386                         ] = path.segments
2387                         && data.trait_ref.def_id == *trait_id
2388                         && self.tcx.trait_of_item(*item_id) == Some(*trait_id)
2389                         && let None = self.tainted_by_errors()
2390                     {
2391                         let (verb, noun) = match self.tcx.associated_item(item_id).kind {
2392                             ty::AssocKind::Const => ("refer to the", "constant"),
2393                             ty::AssocKind::Fn => ("call", "function"),
2394                             ty::AssocKind::Type => ("refer to the", "type"), // this is already covered by E0223, but this single match arm doesn't hurt here
2395                         };
2396
2397                         // Replace the more general E0283 with a more specific error
2398                         err.cancel();
2399                         err = self.tcx.sess.struct_span_err_with_code(
2400                             span,
2401                             &format!(
2402                                 "cannot {verb} associated {noun} on trait without specifying the corresponding `impl` type",
2403                              ),
2404                             rustc_errors::error_code!(E0790),
2405                         );
2406
2407                         if let Some(local_def_id) = data.trait_ref.def_id.as_local()
2408                             && 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)
2409                             && let Some(method_ref) = trait_item_refs.iter().find(|item_ref| item_ref.ident == *assoc_item_name) {
2410                             err.span_label(method_ref.span, format!("`{}::{}` defined here", trait_name, assoc_item_name));
2411                         }
2412
2413                         err.span_label(span, format!("cannot {verb} associated {noun} of trait"));
2414
2415                         let trait_impls = self.tcx.trait_impls_of(data.trait_ref.def_id);
2416
2417                         if trait_impls.blanket_impls().is_empty()
2418                             && let Some(impl_def_id) = trait_impls.non_blanket_impls().values().flatten().next()
2419                         {
2420                             let non_blanket_impl_count = trait_impls.non_blanket_impls().values().flatten().count();
2421                             let message = if non_blanket_impl_count == 1 {
2422                                 "use the fully-qualified path to the only available implementation".to_string()
2423                             } else {
2424                                 format!(
2425                                     "use a fully-qualified path to a specific available implementation ({} found)",
2426                                     non_blanket_impl_count
2427                                 )
2428                             };
2429                             let mut suggestions = vec![(
2430                                 path.span.shrink_to_lo(),
2431                                 format!("<{} as ", self.tcx.type_of(impl_def_id))
2432                             )];
2433                             if let Some(generic_arg) = trait_path_segment.args {
2434                                 let between_span = trait_path_segment.ident.span.between(generic_arg.span_ext);
2435                                 // get rid of :: between Trait and <type>
2436                                 // must be '::' between them, otherwise the parser won't accept the code
2437                                 suggestions.push((between_span, "".to_string(),));
2438                                 suggestions.push((generic_arg.span_ext.shrink_to_hi(), ">".to_string()));
2439                             } else {
2440                                 suggestions.push((trait_path_segment.ident.span.shrink_to_hi(), ">".to_string()));
2441                             }
2442                             err.multipart_suggestion(
2443                                 message,
2444                                 suggestions,
2445                                 Applicability::MaybeIncorrect
2446                             );
2447                         }
2448                     }
2449                 };
2450
2451                 err
2452             }
2453
2454             ty::PredicateKind::WellFormed(arg) => {
2455                 // Same hacky approach as above to avoid deluging user
2456                 // with error messages.
2457                 if arg.references_error()
2458                     || self.tcx.sess.has_errors().is_some()
2459                     || self.tainted_by_errors().is_some()
2460                 {
2461                     return;
2462                 }
2463
2464                 self.emit_inference_failure_err(body_id, span, arg, ErrorCode::E0282, false)
2465             }
2466
2467             ty::PredicateKind::Subtype(data) => {
2468                 if data.references_error()
2469                     || self.tcx.sess.has_errors().is_some()
2470                     || self.tainted_by_errors().is_some()
2471                 {
2472                     // no need to overload user in such cases
2473                     return;
2474                 }
2475                 let SubtypePredicate { a_is_expected: _, a, b } = data;
2476                 // both must be type variables, or the other would've been instantiated
2477                 assert!(a.is_ty_var() && b.is_ty_var());
2478                 self.emit_inference_failure_err(body_id, span, a.into(), ErrorCode::E0282, true)
2479             }
2480             ty::PredicateKind::Clause(ty::Clause::Projection(data)) => {
2481                 if predicate.references_error() || self.tainted_by_errors().is_some() {
2482                     return;
2483                 }
2484                 let subst = data
2485                     .projection_ty
2486                     .substs
2487                     .iter()
2488                     .chain(Some(data.term.into_arg()))
2489                     .find(|g| g.has_non_region_infer());
2490                 if let Some(subst) = subst {
2491                     let mut err = self.emit_inference_failure_err(
2492                         body_id,
2493                         span,
2494                         subst,
2495                         ErrorCode::E0284,
2496                         true,
2497                     );
2498                     err.note(&format!("cannot satisfy `{}`", predicate));
2499                     err
2500                 } else {
2501                     // If we can't find a substitution, just print a generic error
2502                     let mut err = struct_span_err!(
2503                         self.tcx.sess,
2504                         span,
2505                         E0284,
2506                         "type annotations needed: cannot satisfy `{}`",
2507                         predicate,
2508                     );
2509                     err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2510                     err
2511                 }
2512             }
2513
2514             ty::PredicateKind::ConstEvaluatable(data) => {
2515                 if predicate.references_error() || self.tainted_by_errors().is_some() {
2516                     return;
2517                 }
2518                 let subst = data.walk().find(|g| g.is_non_region_infer());
2519                 if let Some(subst) = subst {
2520                     let err = self.emit_inference_failure_err(
2521                         body_id,
2522                         span,
2523                         subst,
2524                         ErrorCode::E0284,
2525                         true,
2526                     );
2527                     err
2528                 } else {
2529                     // If we can't find a substitution, just print a generic error
2530                     let mut err = struct_span_err!(
2531                         self.tcx.sess,
2532                         span,
2533                         E0284,
2534                         "type annotations needed: cannot satisfy `{}`",
2535                         predicate,
2536                     );
2537                     err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2538                     err
2539                 }
2540             }
2541             _ => {
2542                 if self.tcx.sess.has_errors().is_some() || self.tainted_by_errors().is_some() {
2543                     return;
2544                 }
2545                 let mut err = struct_span_err!(
2546                     self.tcx.sess,
2547                     span,
2548                     E0284,
2549                     "type annotations needed: cannot satisfy `{}`",
2550                     predicate,
2551                 );
2552                 err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2553                 err
2554             }
2555         };
2556         self.note_obligation_cause(&mut err, obligation);
2557         err.emit();
2558     }
2559
2560     fn annotate_source_of_ambiguity(
2561         &self,
2562         err: &mut Diagnostic,
2563         ambiguities: &[ambiguity::Ambiguity],
2564         predicate: ty::Predicate<'tcx>,
2565     ) {
2566         let mut spans = vec![];
2567         let mut crates = vec![];
2568         let mut post = vec![];
2569         let mut has_param_env = false;
2570         for ambiguity in ambiguities {
2571             match ambiguity {
2572                 ambiguity::Ambiguity::DefId(impl_def_id) => {
2573                     match self.tcx.span_of_impl(*impl_def_id) {
2574                         Ok(span) => spans.push(span),
2575                         Err(name) => {
2576                             crates.push(name);
2577                             if let Some(header) = to_pretty_impl_header(self.tcx, *impl_def_id) {
2578                                 post.push(header);
2579                             }
2580                         }
2581                     }
2582                 }
2583                 ambiguity::Ambiguity::ParamEnv(span) => {
2584                     has_param_env = true;
2585                     spans.push(*span);
2586                 }
2587             }
2588         }
2589         let mut crate_names: Vec<_> = crates.iter().map(|n| format!("`{}`", n)).collect();
2590         crate_names.sort();
2591         crate_names.dedup();
2592         post.sort();
2593         post.dedup();
2594
2595         if self.tainted_by_errors().is_some()
2596             && (crate_names.len() == 1
2597                 && spans.len() == 0
2598                 && ["`core`", "`alloc`", "`std`"].contains(&crate_names[0].as_str())
2599                 || predicate.visit_with(&mut HasNumericInferVisitor).is_break())
2600         {
2601             // Avoid complaining about other inference issues for expressions like
2602             // `42 >> 1`, where the types are still `{integer}`, but we want to
2603             // Do we need `trait_ref.skip_binder().self_ty().is_numeric() &&` too?
2604             // NOTE(eddyb) this was `.cancel()`, but `err`
2605             // is borrowed, so we can't fully defuse it.
2606             err.downgrade_to_delayed_bug();
2607             return;
2608         }
2609
2610         let msg = format!(
2611             "multiple `impl`s{} satisfying `{}` found",
2612             if has_param_env { " or `where` clauses" } else { "" },
2613             predicate
2614         );
2615         let post = if post.len() > 1 || (post.len() == 1 && post[0].contains('\n')) {
2616             format!(":\n{}", post.iter().map(|p| format!("- {}", p)).collect::<Vec<_>>().join("\n"),)
2617         } else if post.len() == 1 {
2618             format!(": `{}`", post[0])
2619         } else {
2620             String::new()
2621         };
2622
2623         match (spans.len(), crates.len(), crate_names.len()) {
2624             (0, 0, 0) => {
2625                 err.note(&format!("cannot satisfy `{}`", predicate));
2626             }
2627             (0, _, 1) => {
2628                 err.note(&format!("{} in the `{}` crate{}", msg, crates[0], post,));
2629             }
2630             (0, _, _) => {
2631                 err.note(&format!(
2632                     "{} in the following crates: {}{}",
2633                     msg,
2634                     crate_names.join(", "),
2635                     post,
2636                 ));
2637             }
2638             (_, 0, 0) => {
2639                 let span: MultiSpan = spans.into();
2640                 err.span_note(span, &msg);
2641             }
2642             (_, 1, 1) => {
2643                 let span: MultiSpan = spans.into();
2644                 err.span_note(span, &msg);
2645                 err.note(
2646                     &format!("and another `impl` found in the `{}` crate{}", crates[0], post,),
2647                 );
2648             }
2649             _ => {
2650                 let span: MultiSpan = spans.into();
2651                 err.span_note(span, &msg);
2652                 err.note(&format!(
2653                     "and more `impl`s found in the following crates: {}{}",
2654                     crate_names.join(", "),
2655                     post,
2656                 ));
2657             }
2658         }
2659     }
2660
2661     /// Returns `true` if the trait predicate may apply for *some* assignment
2662     /// to the type parameters.
2663     fn predicate_can_apply(
2664         &self,
2665         param_env: ty::ParamEnv<'tcx>,
2666         pred: ty::PolyTraitPredicate<'tcx>,
2667     ) -> bool {
2668         struct ParamToVarFolder<'a, 'tcx> {
2669             infcx: &'a InferCtxt<'tcx>,
2670             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
2671         }
2672
2673         impl<'a, 'tcx> TypeFolder<'tcx> for ParamToVarFolder<'a, 'tcx> {
2674             fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
2675                 self.infcx.tcx
2676             }
2677
2678             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
2679                 if let ty::Param(ty::ParamTy { name, .. }) = *ty.kind() {
2680                     let infcx = self.infcx;
2681                     *self.var_map.entry(ty).or_insert_with(|| {
2682                         infcx.next_ty_var(TypeVariableOrigin {
2683                             kind: TypeVariableOriginKind::TypeParameterDefinition(name, None),
2684                             span: DUMMY_SP,
2685                         })
2686                     })
2687                 } else {
2688                     ty.super_fold_with(self)
2689                 }
2690             }
2691         }
2692
2693         self.probe(|_| {
2694             let cleaned_pred =
2695                 pred.fold_with(&mut ParamToVarFolder { infcx: self, var_map: Default::default() });
2696
2697             let InferOk { value: cleaned_pred, .. } =
2698                 self.infcx.at(&ObligationCause::dummy(), param_env).normalize(cleaned_pred);
2699
2700             let obligation =
2701                 Obligation::new(self.tcx, ObligationCause::dummy(), param_env, cleaned_pred);
2702
2703             self.predicate_may_hold(&obligation)
2704         })
2705     }
2706
2707     fn note_obligation_cause(&self, err: &mut Diagnostic, obligation: &PredicateObligation<'tcx>) {
2708         // First, attempt to add note to this error with an async-await-specific
2709         // message, and fall back to regular note otherwise.
2710         if !self.maybe_note_obligation_cause_for_async_await(err, obligation) {
2711             self.note_obligation_cause_code(
2712                 err,
2713                 obligation.predicate,
2714                 obligation.param_env,
2715                 obligation.cause.code(),
2716                 &mut vec![],
2717                 &mut Default::default(),
2718             );
2719             self.suggest_unsized_bound_if_applicable(err, obligation);
2720         }
2721     }
2722
2723     #[instrument(level = "debug", skip_all)]
2724     fn suggest_unsized_bound_if_applicable(
2725         &self,
2726         err: &mut Diagnostic,
2727         obligation: &PredicateObligation<'tcx>,
2728     ) {
2729         let ty::PredicateKind::Clause(ty::Clause::Trait(pred)) = obligation.predicate.kind().skip_binder() else { return; };
2730         let (ObligationCauseCode::BindingObligation(item_def_id, span)
2731         | ObligationCauseCode::ExprBindingObligation(item_def_id, span, ..))
2732             = *obligation.cause.code().peel_derives() else { return; };
2733         debug!(?pred, ?item_def_id, ?span);
2734
2735         let (Some(node), true) = (
2736             self.tcx.hir().get_if_local(item_def_id),
2737             Some(pred.def_id()) == self.tcx.lang_items().sized_trait(),
2738         ) else {
2739             return;
2740         };
2741         self.maybe_suggest_unsized_generics(err, span, node);
2742     }
2743
2744     #[instrument(level = "debug", skip_all)]
2745     fn maybe_suggest_unsized_generics(&self, err: &mut Diagnostic, span: Span, node: Node<'tcx>) {
2746         let Some(generics) = node.generics() else {
2747             return;
2748         };
2749         let sized_trait = self.tcx.lang_items().sized_trait();
2750         debug!(?generics.params);
2751         debug!(?generics.predicates);
2752         let Some(param) = generics.params.iter().find(|param| param.span == span) else {
2753             return;
2754         };
2755         // Check that none of the explicit trait bounds is `Sized`. Assume that an explicit
2756         // `Sized` bound is there intentionally and we don't need to suggest relaxing it.
2757         let explicitly_sized = generics
2758             .bounds_for_param(param.def_id)
2759             .flat_map(|bp| bp.bounds)
2760             .any(|bound| bound.trait_ref().and_then(|tr| tr.trait_def_id()) == sized_trait);
2761         if explicitly_sized {
2762             return;
2763         }
2764         debug!(?param);
2765         match node {
2766             hir::Node::Item(
2767                 item @ hir::Item {
2768                     // Only suggest indirection for uses of type parameters in ADTs.
2769                     kind:
2770                         hir::ItemKind::Enum(..) | hir::ItemKind::Struct(..) | hir::ItemKind::Union(..),
2771                     ..
2772                 },
2773             ) => {
2774                 if self.maybe_indirection_for_unsized(err, item, param) {
2775                     return;
2776                 }
2777             }
2778             _ => {}
2779         };
2780         // Didn't add an indirection suggestion, so add a general suggestion to relax `Sized`.
2781         let (span, separator) = if let Some(s) = generics.bounds_span_for_suggestions(param.def_id)
2782         {
2783             (s, " +")
2784         } else {
2785             (span.shrink_to_hi(), ":")
2786         };
2787         err.span_suggestion_verbose(
2788             span,
2789             "consider relaxing the implicit `Sized` restriction",
2790             format!("{} ?Sized", separator),
2791             Applicability::MachineApplicable,
2792         );
2793     }
2794
2795     fn maybe_indirection_for_unsized(
2796         &self,
2797         err: &mut Diagnostic,
2798         item: &Item<'tcx>,
2799         param: &GenericParam<'tcx>,
2800     ) -> bool {
2801         // Suggesting `T: ?Sized` is only valid in an ADT if `T` is only used in a
2802         // borrow. `struct S<'a, T: ?Sized>(&'a T);` is valid, `struct S<T: ?Sized>(T);`
2803         // is not. Look for invalid "bare" parameter uses, and suggest using indirection.
2804         let mut visitor =
2805             FindTypeParam { param: param.name.ident().name, invalid_spans: vec![], nested: false };
2806         visitor.visit_item(item);
2807         if visitor.invalid_spans.is_empty() {
2808             return false;
2809         }
2810         let mut multispan: MultiSpan = param.span.into();
2811         multispan.push_span_label(
2812             param.span,
2813             format!("this could be changed to `{}: ?Sized`...", param.name.ident()),
2814         );
2815         for sp in visitor.invalid_spans {
2816             multispan.push_span_label(
2817                 sp,
2818                 format!("...if indirection were used here: `Box<{}>`", param.name.ident()),
2819             );
2820         }
2821         err.span_help(
2822             multispan,
2823             &format!(
2824                 "you could relax the implicit `Sized` bound on `{T}` if it were \
2825                 used through indirection like `&{T}` or `Box<{T}>`",
2826                 T = param.name.ident(),
2827             ),
2828         );
2829         true
2830     }
2831
2832     fn is_recursive_obligation(
2833         &self,
2834         obligated_types: &mut Vec<Ty<'tcx>>,
2835         cause_code: &ObligationCauseCode<'tcx>,
2836     ) -> bool {
2837         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
2838             let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
2839             let self_ty = parent_trait_ref.skip_binder().self_ty();
2840             if obligated_types.iter().any(|ot| ot == &self_ty) {
2841                 return true;
2842             }
2843             if let ty::Adt(def, substs) = self_ty.kind()
2844                 && let [arg] = &substs[..]
2845                 && let ty::subst::GenericArgKind::Type(ty) = arg.unpack()
2846                 && let ty::Adt(inner_def, _) = ty.kind()
2847                 && inner_def == def
2848             {
2849                 return true;
2850             }
2851         }
2852         false
2853     }
2854 }
2855
2856 /// Crude way of getting back an `Expr` from a `Span`.
2857 pub struct FindExprBySpan<'hir> {
2858     pub span: Span,
2859     pub result: Option<&'hir hir::Expr<'hir>>,
2860     pub ty_result: Option<&'hir hir::Ty<'hir>>,
2861 }
2862
2863 impl<'hir> FindExprBySpan<'hir> {
2864     pub fn new(span: Span) -> Self {
2865         Self { span, result: None, ty_result: None }
2866     }
2867 }
2868
2869 impl<'v> Visitor<'v> for FindExprBySpan<'v> {
2870     fn visit_expr(&mut self, ex: &'v hir::Expr<'v>) {
2871         if self.span == ex.span {
2872             self.result = Some(ex);
2873         } else {
2874             hir::intravisit::walk_expr(self, ex);
2875         }
2876     }
2877     fn visit_ty(&mut self, ty: &'v hir::Ty<'v>) {
2878         if self.span == ty.span {
2879             self.ty_result = Some(ty);
2880         } else {
2881             hir::intravisit::walk_ty(self, ty);
2882         }
2883     }
2884 }
2885
2886 /// Look for type `param` in an ADT being used only through a reference to confirm that suggesting
2887 /// `param: ?Sized` would be a valid constraint.
2888 struct FindTypeParam {
2889     param: rustc_span::Symbol,
2890     invalid_spans: Vec<Span>,
2891     nested: bool,
2892 }
2893
2894 impl<'v> Visitor<'v> for FindTypeParam {
2895     fn visit_where_predicate(&mut self, _: &'v hir::WherePredicate<'v>) {
2896         // Skip where-clauses, to avoid suggesting indirection for type parameters found there.
2897     }
2898
2899     fn visit_ty(&mut self, ty: &hir::Ty<'_>) {
2900         // We collect the spans of all uses of the "bare" type param, like in `field: T` or
2901         // `field: (T, T)` where we could make `T: ?Sized` while skipping cases that are known to be
2902         // valid like `field: &'a T` or `field: *mut T` and cases that *might* have further `Sized`
2903         // obligations like `Box<T>` and `Vec<T>`, but we perform no extra analysis for those cases
2904         // and suggest `T: ?Sized` regardless of their obligations. This is fine because the errors
2905         // in that case should make what happened clear enough.
2906         match ty.kind {
2907             hir::TyKind::Ptr(_) | hir::TyKind::Ref(..) | hir::TyKind::TraitObject(..) => {}
2908             hir::TyKind::Path(hir::QPath::Resolved(None, path))
2909                 if path.segments.len() == 1 && path.segments[0].ident.name == self.param =>
2910             {
2911                 if !self.nested {
2912                     debug!(?ty, "FindTypeParam::visit_ty");
2913                     self.invalid_spans.push(ty.span);
2914                 }
2915             }
2916             hir::TyKind::Path(_) => {
2917                 let prev = self.nested;
2918                 self.nested = true;
2919                 hir::intravisit::walk_ty(self, ty);
2920                 self.nested = prev;
2921             }
2922             _ => {
2923                 hir::intravisit::walk_ty(self, ty);
2924             }
2925         }
2926     }
2927 }
2928
2929 /// Summarizes information
2930 #[derive(Clone)]
2931 pub enum ArgKind {
2932     /// An argument of non-tuple type. Parameters are (name, ty)
2933     Arg(String, String),
2934
2935     /// An argument of tuple type. For a "found" argument, the span is
2936     /// the location in the source of the pattern. For an "expected"
2937     /// argument, it will be None. The vector is a list of (name, ty)
2938     /// strings for the components of the tuple.
2939     Tuple(Option<Span>, Vec<(String, String)>),
2940 }
2941
2942 impl ArgKind {
2943     fn empty() -> ArgKind {
2944         ArgKind::Arg("_".to_owned(), "_".to_owned())
2945     }
2946
2947     /// Creates an `ArgKind` from the expected type of an
2948     /// argument. It has no name (`_`) and an optional source span.
2949     pub fn from_expected_ty(t: Ty<'_>, span: Option<Span>) -> ArgKind {
2950         match t.kind() {
2951             ty::Tuple(tys) => ArgKind::Tuple(
2952                 span,
2953                 tys.iter().map(|ty| ("_".to_owned(), ty.to_string())).collect::<Vec<_>>(),
2954             ),
2955             _ => ArgKind::Arg("_".to_owned(), t.to_string()),
2956         }
2957     }
2958 }
2959
2960 struct HasNumericInferVisitor;
2961
2962 impl<'tcx> ty::TypeVisitor<'tcx> for HasNumericInferVisitor {
2963     type BreakTy = ();
2964
2965     fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
2966         if matches!(ty.kind(), ty::Infer(ty::FloatVar(_) | ty::IntVar(_))) {
2967             ControlFlow::Break(())
2968         } else {
2969             ControlFlow::Continue(())
2970         }
2971     }
2972 }
2973
2974 #[derive(Copy, Clone)]
2975 pub enum DefIdOrName {
2976     DefId(DefId),
2977     Name(&'static str),
2978 }