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

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