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

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