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

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