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