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

   1 mod ambiguity;
   2 pub mod on_unimplemented;
   3 pub mod suggestions;
   4
   5 use super::{
   6     FulfillmentError, FulfillmentErrorCode, MismatchedProjectionTypes, Obligation, ObligationCause,
   7     ObligationCauseCode, OutputTypeParameterMismatch, Overflow, PredicateObligation,
   8     SelectionContext, SelectionError, TraitNotObjectSafe,
   9 };
  10 use crate::infer::error_reporting::{TyCategory, TypeAnnotationNeeded as ErrorCode};
  11 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  12 use crate::infer::{self, InferCtxt, TyCtxtInferExt};
  13 use crate::traits::engine::TraitEngineExt as _;
  14 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
  15 use crate::traits::query::normalize::AtExt as _;
  16 use crate::traits::specialize::to_pretty_impl_header;
  17 use on_unimplemented::OnUnimplementedNote;
  18 use on_unimplemented::TypeErrCtxtExt as _;
  19 use rustc_data_structures::fx::{FxHashMap, FxIndexMap};
  20 use rustc_errors::{
  21     pluralize, struct_span_err, Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed,
  22     MultiSpan, Style,
  23 };
  24 use rustc_hir as hir;
  25 use rustc_hir::def::Namespace;
  26 use rustc_hir::def_id::DefId;
  27 use rustc_hir::intravisit::Visitor;
  28 use rustc_hir::GenericParam;
  29 use rustc_hir::Item;
  30 use rustc_hir::Node;
  31 use rustc_infer::infer::error_reporting::TypeErrCtxt;
  32 use rustc_infer::infer::TypeTrace;
  33 use rustc_infer::traits::TraitEngine;
  34 use rustc_middle::traits::select::OverflowError;
  35 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
  36 use rustc_middle::ty::error::ExpectedFound;
  37 use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable};
  38 use rustc_middle::ty::print::{FmtPrinter, Print};
  39 use rustc_middle::ty::{
  40     self, SubtypePredicate, ToPolyTraitRef, ToPredicate, TraitRef, Ty, TyCtxt, TypeFoldable,
  41     TypeVisitable,
  42 };
  43 use rustc_session::Limit;
  44 use rustc_span::def_id::LOCAL_CRATE;
  45 use rustc_span::symbol::{kw, sym};
  46 use rustc_span::{ExpnKind, Span, DUMMY_SP};
  47 use std::fmt;
  48 use std::iter;
  49 use std::ops::ControlFlow;
  50 use suggestions::TypeErrCtxtExt as _;
  51
  52 pub use rustc_infer::traits::error_reporting::*;
  53
  54 // When outputting impl candidates, prefer showing those that are more similar.
  55 //
  56 // We also compare candidates after skipping lifetimes, which has a lower
  57 // priority than exact matches.
  58 #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
  59 pub enum CandidateSimilarity {
  60     Exact { ignoring_lifetimes: bool },
  61     Fuzzy { ignoring_lifetimes: bool },
  62 }
  63
  64 #[derive(Debug, Clone, Copy)]
  65 pub struct ImplCandidate<'tcx> {
  66     pub trait_ref: ty::TraitRef<'tcx>,
  67     pub similarity: CandidateSimilarity,
  68 }
  69
  70 pub trait InferCtxtExt<'tcx> {
  71     /// Given some node representing a fn-like thing in the HIR map,
  72     /// returns a span and `ArgKind` information that describes the
  73     /// arguments it expects. This can be supplied to
  74     /// `report_arg_count_mismatch`.
  75     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)>;
  76
  77     /// Reports an error when the number of arguments needed by a
  78     /// trait match doesn't match the number that the expression
  79     /// provides.
  80     fn report_arg_count_mismatch(
  81         &self,
  82         span: Span,
  83         found_span: Option<Span>,
  84         expected_args: Vec<ArgKind>,
  85         found_args: Vec<ArgKind>,
  86         is_closure: bool,
  87     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed>;
  88
  89     /// Checks if the type implements one of `Fn`, `FnMut`, or `FnOnce`
  90     /// in that order, and returns the generic type corresponding to the
  91     /// argument of that trait (corresponding to the closure arguments).
  92     fn type_implements_fn_trait(
  93         &self,
  94         param_env: ty::ParamEnv<'tcx>,
  95         ty: ty::Binder<'tcx, Ty<'tcx>>,
  96         constness: ty::BoundConstness,
  97         polarity: ty::ImplPolarity,
  98     ) -> Result<(ty::ClosureKind, ty::Binder<'tcx, Ty<'tcx>>), ()>;
  99 }
 100
 101 pub trait TypeErrCtxtExt<'tcx> {
 102     fn report_fulfillment_errors(
 103         &self,
 104         errors: &[FulfillmentError<'tcx>],
 105         body_id: Option<hir::BodyId>,
 106     ) -> ErrorGuaranteed;
 107
 108     fn report_overflow_error<T>(
 109         &self,
 110         obligation: &Obligation<'tcx, T>,
 111         suggest_increasing_limit: bool,
 112     ) -> !
 113     where
 114         T: fmt::Display
 115             + TypeFoldable<'tcx>
 116             + Print<'tcx, FmtPrinter<'tcx, 'tcx>, Output = FmtPrinter<'tcx, 'tcx>>,
 117         <T as Print<'tcx, FmtPrinter<'tcx, 'tcx>>>::Error: std::fmt::Debug;
 118
 119     fn suggest_new_overflow_limit(&self, err: &mut Diagnostic);
 120
 121     fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> !;
 122
 123     /// The `root_obligation` parameter should be the `root_obligation` field
 124     /// from a `FulfillmentError`. If no `FulfillmentError` is available,
 125     /// then it should be the same as `obligation`.
 126     fn report_selection_error(
 127         &self,
 128         obligation: PredicateObligation<'tcx>,
 129         root_obligation: &PredicateObligation<'tcx>,
 130         error: &SelectionError<'tcx>,
 131     );
 132 }
 133
 134 impl<'tcx> InferCtxtExt<'tcx> for InferCtxt<'tcx> {
 135     /// Given some node representing a fn-like thing in the HIR map,
 136     /// returns a span and `ArgKind` information that describes the
 137     /// arguments it expects. This can be supplied to
 138     /// `report_arg_count_mismatch`.
 139     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)> {
 140         let sm = self.tcx.sess.source_map();
 141         let hir = self.tcx.hir();
 142         Some(match node {
 143             Node::Expr(&hir::Expr {
 144                 kind: hir::ExprKind::Closure(&hir::Closure { body, fn_decl_span, .. }),
 145                 ..
 146             }) => (
 147                 fn_decl_span,
 148                 hir.body(body)
 149                     .params
 150                     .iter()
 151                     .map(|arg| {
 152                         if let hir::Pat { kind: hir::PatKind::Tuple(ref args, _), span, .. } =
 153                             *arg.pat
 154                         {
 155                             Some(ArgKind::Tuple(
 156                                 Some(span),
 157                                 args.iter()
 158                                     .map(|pat| {
 159                                         sm.span_to_snippet(pat.span)
 160                                             .ok()
 161                                             .map(|snippet| (snippet, "_".to_owned()))
 162                                     })
 163                                     .collect::<Option<Vec<_>>>()?,
 164                             ))
 165                         } else {
 166                             let name = sm.span_to_snippet(arg.pat.span).ok()?;
 167                             Some(ArgKind::Arg(name, "_".to_owned()))
 168                         }
 169                     })
 170                     .collect::<Option<Vec<ArgKind>>>()?,
 171             ),
 172             Node::Item(&hir::Item { kind: hir::ItemKind::Fn(ref sig, ..), .. })
 173             | Node::ImplItem(&hir::ImplItem { kind: hir::ImplItemKind::Fn(ref sig, _), .. })
 174             | Node::TraitItem(&hir::TraitItem {
 175                 kind: hir::TraitItemKind::Fn(ref sig, _), ..
 176             }) => (
 177                 sig.span,
 178                 sig.decl
 179                     .inputs
 180                     .iter()
 181                     .map(|arg| match arg.kind {
 182                         hir::TyKind::Tup(ref tys) => ArgKind::Tuple(
 183                             Some(arg.span),
 184                             vec![("_".to_owned(), "_".to_owned()); tys.len()],
 185                         ),
 186                         _ => ArgKind::empty(),
 187                     })
 188                     .collect::<Vec<ArgKind>>(),
 189             ),
 190             Node::Ctor(ref variant_data) => {
 191                 let span = variant_data.ctor_hir_id().map_or(DUMMY_SP, |id| hir.span(id));
 192                 (span, vec![ArgKind::empty(); variant_data.fields().len()])
 193             }
 194             _ => panic!("non-FnLike node found: {:?}", node),
 195         })
 196     }
 197
 198     /// Reports an error when the number of arguments needed by a
 199     /// trait match doesn't match the number that the expression
 200     /// provides.
 201     fn report_arg_count_mismatch(
 202         &self,
 203         span: Span,
 204         found_span: Option<Span>,
 205         expected_args: Vec<ArgKind>,
 206         found_args: Vec<ArgKind>,
 207         is_closure: bool,
 208     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
 209         let kind = if is_closure { "closure" } else { "function" };
 210
 211         let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
 212             let arg_length = arguments.len();
 213             let distinct = matches!(other, &[ArgKind::Tuple(..)]);
 214             match (arg_length, arguments.get(0)) {
 215                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
 216                     format!("a single {}-tuple as argument", fields.len())
 217                 }
 218                 _ => format!(
 219                     "{} {}argument{}",
 220                     arg_length,
 221                     if distinct && arg_length > 1 { "distinct " } else { "" },
 222                     pluralize!(arg_length)
 223                 ),
 224             }
 225         };
 226
 227         let expected_str = args_str(&expected_args, &found_args);
 228         let found_str = args_str(&found_args, &expected_args);
 229
 230         let mut err = struct_span_err!(
 231             self.tcx.sess,
 232             span,
 233             E0593,
 234             "{} is expected to take {}, but it takes {}",
 235             kind,
 236             expected_str,
 237             found_str,
 238         );
 239
 240         err.span_label(span, format!("expected {} that takes {}", kind, expected_str));
 241
 242         if let Some(found_span) = found_span {
 243             err.span_label(found_span, format!("takes {}", found_str));
 244
 245             // move |_| { ... }
 246             // ^^^^^^^^-- def_span
 247             //
 248             // move |_| { ... }
 249             // ^^^^^-- prefix
 250             let prefix_span = self.tcx.sess.source_map().span_until_non_whitespace(found_span);
 251             // move |_| { ... }
 252             //      ^^^-- pipe_span
 253             let pipe_span =
 254                 if let Some(span) = found_span.trim_start(prefix_span) { span } else { found_span };
 255
 256             // Suggest to take and ignore the arguments with expected_args_length `_`s if
 257             // found arguments is empty (assume the user just wants to ignore args in this case).
 258             // For example, if `expected_args_length` is 2, suggest `|_, _|`.
 259             if found_args.is_empty() && is_closure {
 260                 let underscores = vec!["_"; expected_args.len()].join(", ");
 261                 err.span_suggestion_verbose(
 262                     pipe_span,
 263                     &format!(
 264                         "consider changing the closure to take and ignore the expected argument{}",
 265                         pluralize!(expected_args.len())
 266                     ),
 267                     format!("|{}|", underscores),
 268                     Applicability::MachineApplicable,
 269                 );
 270             }
 271
 272             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
 273                 if fields.len() == expected_args.len() {
 274                     let sugg = fields
 275                         .iter()
 276                         .map(|(name, _)| name.to_owned())
 277                         .collect::<Vec<String>>()
 278                         .join(", ");
 279                     err.span_suggestion_verbose(
 280                         found_span,
 281                         "change the closure to take multiple arguments instead of a single tuple",
 282                         format!("|{}|", sugg),
 283                         Applicability::MachineApplicable,
 284                     );
 285                 }
 286             }
 287             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..]
 288                 && fields.len() == found_args.len()
 289                 && is_closure
 290             {
 291                 let sugg = format!(
 292                     "|({}){}|",
 293                     found_args
 294                         .iter()
 295                         .map(|arg| match arg {
 296                             ArgKind::Arg(name, _) => name.to_owned(),
 297                             _ => "_".to_owned(),
 298                         })
 299                         .collect::<Vec<String>>()
 300                         .join(", "),
 301                     // add type annotations if available
 302                     if found_args.iter().any(|arg| match arg {
 303                         ArgKind::Arg(_, ty) => ty != "_",
 304                         _ => false,
 305                     }) {
 306                         format!(
 307                             ": ({})",
 308                             fields
 309                                 .iter()
 310                                 .map(|(_, ty)| ty.to_owned())
 311                                 .collect::<Vec<String>>()
 312                                 .join(", ")
 313                         )
 314                     } else {
 315                         String::new()
 316                     },
 317                 );
 318                 err.span_suggestion_verbose(
 319                     found_span,
 320                     "change the closure to accept a tuple instead of individual arguments",
 321                     sugg,
 322                     Applicability::MachineApplicable,
 323                 );
 324             }
 325         }
 326
 327         err
 328     }
 329
 330     fn type_implements_fn_trait(
 331         &self,
 332         param_env: ty::ParamEnv<'tcx>,
 333         ty: ty::Binder<'tcx, Ty<'tcx>>,
 334         constness: ty::BoundConstness,
 335         polarity: ty::ImplPolarity,
 336     ) -> Result<(ty::ClosureKind, ty::Binder<'tcx, Ty<'tcx>>), ()> {
 337         self.commit_if_ok(|_| {
 338             for trait_def_id in [
 339                 self.tcx.lang_items().fn_trait(),
 340                 self.tcx.lang_items().fn_mut_trait(),
 341                 self.tcx.lang_items().fn_once_trait(),
 342             ] {
 343                 let Some(trait_def_id) = trait_def_id else { continue };
 344                 // Make a fresh inference variable so we can determine what the substitutions
 345                 // of the trait are.
 346                 let var = self.next_ty_var(TypeVariableOrigin {
 347                     span: DUMMY_SP,
 348                     kind: TypeVariableOriginKind::MiscVariable,
 349                 });
 350                 let substs = self.tcx.mk_substs_trait(ty.skip_binder(), &[var.into()]);
 351                 let obligation = Obligation::new(
 352                     self.tcx,
 353                     ObligationCause::dummy(),
 354                     param_env,
 355                     ty.rebind(ty::TraitPredicate {
 356                         trait_ref: ty::TraitRef::new(trait_def_id, substs),
 357                         constness,
 358                         polarity,
 359                     }),
 360                 );
 361                 let mut fulfill_cx = <dyn TraitEngine<'tcx>>::new_in_snapshot(self.tcx);
 362                 fulfill_cx.register_predicate_obligation(self, obligation);
 363                 if fulfill_cx.select_all_or_error(self).is_empty() {
 364                     return Ok((
 365                         ty::ClosureKind::from_def_id(self.tcx, trait_def_id)
 366                             .expect("expected to map DefId to ClosureKind"),
 367                         ty.rebind(self.resolve_vars_if_possible(var)),
 368                     ));
 369                 }
 370             }
 371
 372             Err(())
 373         })
 374     }
 375 }
 376 impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
 377     fn report_fulfillment_errors(
 378         &self,
 379         errors: &[FulfillmentError<'tcx>],
 380         body_id: Option<hir::BodyId>,
 381     ) -> ErrorGuaranteed {
 382         #[derive(Debug)]
 383         struct ErrorDescriptor<'tcx> {
 384             predicate: ty::Predicate<'tcx>,
 385             index: Option<usize>, // None if this is an old error
 386         }
 387
 388         let mut error_map: FxIndexMap<_, Vec<_>> = self
 389             .reported_trait_errors
 390             .borrow()
 391             .iter()
 392             .map(|(&span, predicates)| {
 393                 (
 394                     span,
 395                     predicates
 396                         .iter()
 397                         .map(|&predicate| ErrorDescriptor { predicate, index: None })
 398                         .collect(),
 399                 )
 400             })
 401             .collect();
 402
 403         for (index, error) in errors.iter().enumerate() {
 404             // We want to ignore desugarings here: spans are equivalent even
 405             // if one is the result of a desugaring and the other is not.
 406             let mut span = error.obligation.cause.span;
 407             let expn_data = span.ctxt().outer_expn_data();
 408             if let ExpnKind::Desugaring(_) = expn_data.kind {
 409                 span = expn_data.call_site;
 410             }
 411
 412             error_map.entry(span).or_default().push(ErrorDescriptor {
 413                 predicate: error.obligation.predicate,
 414                 index: Some(index),
 415             });
 416
 417             self.reported_trait_errors
 418                 .borrow_mut()
 419                 .entry(span)
 420                 .or_default()
 421                 .push(error.obligation.predicate);
 422         }
 423
 424         // We do this in 2 passes because we want to display errors in order, though
 425         // maybe it *is* better to sort errors by span or something.
 426         let mut is_suppressed = vec![false; errors.len()];
 427         for (_, error_set) in error_map.iter() {
 428             // We want to suppress "duplicate" errors with the same span.
 429             for error in error_set {
 430                 if let Some(index) = error.index {
 431                     // Suppress errors that are either:
 432                     // 1) strictly implied by another error.
 433                     // 2) implied by an error with a smaller index.
 434                     for error2 in error_set {
 435                         if error2.index.map_or(false, |index2| is_suppressed[index2]) {
 436                             // Avoid errors being suppressed by already-suppressed
 437                             // errors, to prevent all errors from being suppressed
 438                             // at once.
 439                             continue;
 440                         }
 441
 442                         if self.error_implies(error2.predicate, error.predicate)
 443                             && !(error2.index >= error.index
 444                                 && self.error_implies(error.predicate, error2.predicate))
 445                         {
 446                             info!("skipping {:?} (implied by {:?})", error, error2);
 447                             is_suppressed[index] = true;
 448                             break;
 449                         }
 450                     }
 451                 }
 452             }
 453         }
 454
 455         for (error, suppressed) in iter::zip(errors, is_suppressed) {
 456             if !suppressed {
 457                 self.report_fulfillment_error(error, body_id);
 458             }
 459         }
 460
 461         self.tcx.sess.delay_span_bug(DUMMY_SP, "expected fullfillment errors")
 462     }
 463
 464     /// Reports that an overflow has occurred and halts compilation. We
 465     /// halt compilation unconditionally because it is important that
 466     /// overflows never be masked -- they basically represent computations
 467     /// whose result could not be truly determined and thus we can't say
 468     /// if the program type checks or not -- and they are unusual
 469     /// occurrences in any case.
 470     fn report_overflow_error<T>(
 471         &self,
 472         obligation: &Obligation<'tcx, T>,
 473         suggest_increasing_limit: bool,
 474     ) -> !
 475     where
 476         T: fmt::Display
 477             + TypeFoldable<'tcx>
 478             + Print<'tcx, FmtPrinter<'tcx, 'tcx>, Output = FmtPrinter<'tcx, 'tcx>>,
 479         <T as Print<'tcx, FmtPrinter<'tcx, 'tcx>>>::Error: std::fmt::Debug,
 480     {
 481         let predicate = self.resolve_vars_if_possible(obligation.predicate.clone());
 482         let mut pred_str = predicate.to_string();
 483         if pred_str.len() > 50 {
 484             // We don't need to save the type to a file, we will be talking about this type already
 485             // in a separate note when we explain the obligation, so it will be available that way.
 486             pred_str = predicate
 487                 .print(FmtPrinter::new_with_limit(
 488                     self.tcx,
 489                     Namespace::TypeNS,
 490                     rustc_session::Limit(6),
 491                 ))
 492                 .unwrap()
 493                 .into_buffer();
 494         }
 495         let mut err = struct_span_err!(
 496             self.tcx.sess,
 497             obligation.cause.span,
 498             E0275,
 499             "overflow evaluating the requirement `{}`",
 500             pred_str,
 501         );
 502
 503         if suggest_increasing_limit {
 504             self.suggest_new_overflow_limit(&mut err);
 505         }
 506
 507         self.note_obligation_cause_code(
 508             &mut err,
 509             &obligation.predicate,
 510             obligation.param_env,
 511             obligation.cause.code(),
 512             &mut vec![],
 513             &mut Default::default(),
 514         );
 515
 516         err.emit();
 517         self.tcx.sess.abort_if_errors();
 518         bug!();
 519     }
 520
 521     fn suggest_new_overflow_limit(&self, err: &mut Diagnostic) {
 522         let suggested_limit = match self.tcx.recursion_limit() {
 523             Limit(0) => Limit(2),
 524             limit => limit * 2,
 525         };
 526         err.help(&format!(
 527             "consider increasing the recursion limit by adding a \
 528              `#![recursion_limit = \"{}\"]` attribute to your crate (`{}`)",
 529             suggested_limit,
 530             self.tcx.crate_name(LOCAL_CRATE),
 531         ));
 532     }
 533
 534     /// Reports that a cycle was detected which led to overflow and halts
 535     /// compilation. This is equivalent to `report_overflow_error` except
 536     /// that we can give a more helpful error message (and, in particular,
 537     /// we do not suggest increasing the overflow limit, which is not
 538     /// going to help).
 539     fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 540         let cycle = self.resolve_vars_if_possible(cycle.to_owned());
 541         assert!(!cycle.is_empty());
 542
 543         debug!(?cycle, "report_overflow_error_cycle");
 544
 545         // The 'deepest' obligation is most likely to have a useful
 546         // cause 'backtrace'
 547         self.report_overflow_error(cycle.iter().max_by_key(|p| p.recursion_depth).unwrap(), false);
 548     }
 549
 550     fn report_selection_error(
 551         &self,
 552         mut obligation: PredicateObligation<'tcx>,
 553         root_obligation: &PredicateObligation<'tcx>,
 554         error: &SelectionError<'tcx>,
 555     ) {
 556         self.set_tainted_by_errors();
 557         let tcx = self.tcx;
 558         let mut span = obligation.cause.span;
 559
 560         let mut err = match *error {
 561             SelectionError::Unimplemented => {
 562                 // If this obligation was generated as a result of well-formedness checking, see if we
 563                 // can get a better error message by performing HIR-based well-formedness checking.
 564                 if let ObligationCauseCode::WellFormed(Some(wf_loc)) =
 565                     root_obligation.cause.code().peel_derives()
 566                 {
 567                     if let Some(cause) = self
 568                         .tcx
 569                         .diagnostic_hir_wf_check((tcx.erase_regions(obligation.predicate), *wf_loc))
 570                     {
 571                         obligation.cause = cause.clone();
 572                         span = obligation.cause.span;
 573                     }
 574                 }
 575                 if let ObligationCauseCode::CompareImplItemObligation {
 576                     impl_item_def_id,
 577                     trait_item_def_id,
 578                     kind: _,
 579                 } = *obligation.cause.code()
 580                 {
 581                     self.report_extra_impl_obligation(
 582                         span,
 583                         impl_item_def_id,
 584                         trait_item_def_id,
 585                         &format!("`{}`", obligation.predicate),
 586                     )
 587                     .emit();
 588                     return;
 589                 }
 590
 591                 let bound_predicate = obligation.predicate.kind();
 592                 match bound_predicate.skip_binder() {
 593                     ty::PredicateKind::Trait(trait_predicate) => {
 594                         let trait_predicate = bound_predicate.rebind(trait_predicate);
 595                         let mut trait_predicate = self.resolve_vars_if_possible(trait_predicate);
 596
 597                         trait_predicate.remap_constness_diag(obligation.param_env);
 598                         let predicate_is_const = ty::BoundConstness::ConstIfConst
 599                             == trait_predicate.skip_binder().constness;
 600
 601                         if self.tcx.sess.has_errors().is_some()
 602                             && trait_predicate.references_error()
 603                         {
 604                             return;
 605                         }
 606                         let trait_ref = trait_predicate.to_poly_trait_ref();
 607                         let (post_message, pre_message, type_def) = self
 608                             .get_parent_trait_ref(obligation.cause.code())
 609                             .map(|(t, s)| {
 610                                 (
 611                                     format!(" in `{}`", t),
 612                                     format!("within `{}`, ", t),
 613                                     s.map(|s| (format!("within this `{}`", t), s)),
 614                                 )
 615                             })
 616                             .unwrap_or_default();
 617
 618                         let OnUnimplementedNote {
 619                             message,
 620                             label,
 621                             note,
 622                             parent_label,
 623                             append_const_msg,
 624                         } = self.on_unimplemented_note(trait_ref, &obligation);
 625                         let have_alt_message = message.is_some() || label.is_some();
 626                         let is_try_conversion = self.is_try_conversion(span, trait_ref.def_id());
 627                         let is_unsize =
 628                             Some(trait_ref.def_id()) == self.tcx.lang_items().unsize_trait();
 629                         let (message, note, append_const_msg) = if is_try_conversion {
 630                             (
 631                                 Some(format!(
 632                                     "`?` couldn't convert the error to `{}`",
 633                                     trait_ref.skip_binder().self_ty(),
 634                                 )),
 635                                 Some(
 636                                     "the question mark operation (`?`) implicitly performs a \
 637                                      conversion on the error value using the `From` trait"
 638                                         .to_owned(),
 639                                 ),
 640                                 Some(None),
 641                             )
 642                         } else {
 643                             (message, note, append_const_msg)
 644                         };
 645
 646                         let mut err = struct_span_err!(
 647                             self.tcx.sess,
 648                             span,
 649                             E0277,
 650                             "{}",
 651                             message
 652                                 .and_then(|cannot_do_this| {
 653                                     match (predicate_is_const, append_const_msg) {
 654                                         // do nothing if predicate is not const
 655                                         (false, _) => Some(cannot_do_this),
 656                                         // suggested using default post message
 657                                         (true, Some(None)) => {
 658                                             Some(format!("{cannot_do_this} in const contexts"))
 659                                         }
 660                                         // overridden post message
 661                                         (true, Some(Some(post_message))) => {
 662                                             Some(format!("{cannot_do_this}{post_message}"))
 663                                         }
 664                                         // fallback to generic message
 665                                         (true, None) => None,
 666                                     }
 667                                 })
 668                                 .unwrap_or_else(|| format!(
 669                                     "the trait bound `{}` is not satisfied{}",
 670                                     trait_predicate, post_message,
 671                                 ))
 672                         );
 673
 674                         if is_try_conversion && let Some(ret_span) = self.return_type_span(&obligation) {
 675                             err.span_label(
 676                                 ret_span,
 677                                 &format!(
 678                                     "expected `{}` because of this",
 679                                     trait_ref.skip_binder().self_ty()
 680                                 ),
 681                             );
 682                         }
 683
 684                         if Some(trait_ref.def_id()) == tcx.lang_items().tuple_trait() {
 685                             match obligation.cause.code().peel_derives() {
 686                                 ObligationCauseCode::RustCall => {
 687                                     err.set_primary_message("functions with the \"rust-call\" ABI must take a single non-self tuple argument");
 688                                 }
 689                                 ObligationCauseCode::BindingObligation(def_id, _)
 690                                 | ObligationCauseCode::ItemObligation(def_id)
 691                                     if ty::ClosureKind::from_def_id(tcx, *def_id).is_some() =>
 692                                 {
 693                                     err.code(rustc_errors::error_code!(E0059));
 694                                     err.set_primary_message(format!(
 695                                         "type parameter to bare `{}` trait must be a tuple",
 696                                         tcx.def_path_str(*def_id)
 697                                     ));
 698                                 }
 699                                 _ => {}
 700                             }
 701                         }
 702
 703                         if Some(trait_ref.def_id()) == tcx.lang_items().drop_trait()
 704                             && predicate_is_const
 705                         {
 706                             err.note("`~const Drop` was renamed to `~const Destruct`");
 707                             err.note("See <https://github.com/rust-lang/rust/pull/94901> for more details");
 708                         }
 709
 710                         let explanation = if let ObligationCauseCode::MainFunctionType =
 711                             obligation.cause.code()
 712                         {
 713                             "consider using `()`, or a `Result`".to_owned()
 714                         } else {
 715                             let ty_desc = match trait_ref.skip_binder().self_ty().kind() {
 716                                 ty::FnDef(_, _) => Some("fn item"),
 717                                 ty::Closure(_, _) => Some("closure"),
 718                                 _ => None,
 719                             };
 720
 721                             match ty_desc {
 722                                 Some(desc) => format!(
 723                                     "{}the trait `{}` is not implemented for {} `{}`",
 724                                     pre_message,
 725                                     trait_predicate.print_modifiers_and_trait_path(),
 726                                     desc,
 727                                     trait_ref.skip_binder().self_ty(),
 728                                 ),
 729                                 None => format!(
 730                                     "{}the trait `{}` is not implemented for `{}`",
 731                                     pre_message,
 732                                     trait_predicate.print_modifiers_and_trait_path(),
 733                                     trait_ref.skip_binder().self_ty(),
 734                                 ),
 735                             }
 736                         };
 737
 738                         if self.suggest_add_reference_to_arg(
 739                             &obligation,
 740                             &mut err,
 741                             trait_predicate,
 742                             have_alt_message,
 743                         ) {
 744                             self.note_obligation_cause(&mut err, &obligation);
 745                             err.emit();
 746                             return;
 747                         }
 748                         if let Some(ref s) = label {
 749                             // If it has a custom `#[rustc_on_unimplemented]`
 750                             // error message, let's display it as the label!
 751                             err.span_label(span, s);
 752                             if !matches!(trait_ref.skip_binder().self_ty().kind(), ty::Param(_)) {
 753                                 // When the self type is a type param We don't need to "the trait
 754                                 // `std::marker::Sized` is not implemented for `T`" as we will point
 755                                 // at the type param with a label to suggest constraining it.
 756                                 err.help(&explanation);
 757                             }
 758                         } else {
 759                             err.span_label(span, explanation);
 760                         }
 761
 762                         if let ObligationCauseCode::ObjectCastObligation(concrete_ty, obj_ty) = obligation.cause.code().peel_derives() &&
 763                             Some(trait_ref.def_id()) == self.tcx.lang_items().sized_trait() {
 764                             self.suggest_borrowing_for_object_cast(&mut err, &root_obligation, *concrete_ty, *obj_ty);
 765                         }
 766
 767                         let mut unsatisfied_const = false;
 768                         if trait_predicate.is_const_if_const() && obligation.param_env.is_const() {
 769                             let non_const_predicate = trait_ref.without_const();
 770                             let non_const_obligation = Obligation {
 771                                 cause: obligation.cause.clone(),
 772                                 param_env: obligation.param_env.without_const(),
 773                                 predicate: non_const_predicate.to_predicate(tcx),
 774                                 recursion_depth: obligation.recursion_depth,
 775                             };
 776                             if self.predicate_may_hold(&non_const_obligation) {
 777                                 unsatisfied_const = true;
 778                                 err.span_note(
 779                                     span,
 780                                     &format!(
 781                                         "the trait `{}` is implemented for `{}`, \
 782                                         but that implementation is not `const`",
 783                                         non_const_predicate.print_modifiers_and_trait_path(),
 784                                         trait_ref.skip_binder().self_ty(),
 785                                     ),
 786                                 );
 787                             }
 788                         }
 789
 790                         if let Some((msg, span)) = type_def {
 791                             err.span_label(span, &msg);
 792                         }
 793                         if let Some(ref s) = note {
 794                             // If it has a custom `#[rustc_on_unimplemented]` note, let's display it
 795                             err.note(s.as_str());
 796                         }
 797                         if let Some(ref s) = parent_label {
 798                             let body = tcx
 799                                 .hir()
 800                                 .opt_local_def_id(obligation.cause.body_id)
 801                                 .unwrap_or_else(|| {
 802                                     tcx.hir().body_owner_def_id(hir::BodyId {
 803                                         hir_id: obligation.cause.body_id,
 804                                     })
 805                                 });
 806                             err.span_label(tcx.def_span(body), s);
 807                         }
 808
 809                         self.suggest_floating_point_literal(&obligation, &mut err, &trait_ref);
 810                         self.suggest_dereferencing_index(&obligation, &mut err, trait_predicate);
 811                         let mut suggested =
 812                             self.suggest_dereferences(&obligation, &mut err, trait_predicate);
 813                         suggested |= self.suggest_fn_call(&obligation, &mut err, trait_predicate);
 814                         suggested |=
 815                             self.suggest_remove_reference(&obligation, &mut err, trait_predicate);
 816                         suggested |= self.suggest_semicolon_removal(
 817                             &obligation,
 818                             &mut err,
 819                             span,
 820                             trait_predicate,
 821                         );
 822                         self.note_version_mismatch(&mut err, &trait_ref);
 823                         self.suggest_remove_await(&obligation, &mut err);
 824                         self.suggest_derive(&obligation, &mut err, trait_predicate);
 825
 826                         if Some(trait_ref.def_id()) == tcx.lang_items().try_trait() {
 827                             self.suggest_await_before_try(
 828                                 &mut err,
 829                                 &obligation,
 830                                 trait_predicate,
 831                                 span,
 832                             );
 833                         }
 834
 835                         if self.suggest_impl_trait(&mut err, span, &obligation, trait_predicate) {
 836                             err.emit();
 837                             return;
 838                         }
 839
 840                         if is_unsize {
 841                             // If the obligation failed due to a missing implementation of the
 842                             // `Unsize` trait, give a pointer to why that might be the case
 843                             err.note(
 844                                 "all implementations of `Unsize` are provided \
 845                                 automatically by the compiler, see \
 846                                 <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> \
 847                                 for more information",
 848                             );
 849                         }
 850
 851                         let is_fn_trait =
 852                             ty::ClosureKind::from_def_id(tcx, trait_ref.def_id()).is_some();
 853                         let is_target_feature_fn = if let ty::FnDef(def_id, _) =
 854                             *trait_ref.skip_binder().self_ty().kind()
 855                         {
 856                             !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
 857                         } else {
 858                             false
 859                         };
 860                         if is_fn_trait && is_target_feature_fn {
 861                             err.note(
 862                                 "`#[target_feature]` functions do not implement the `Fn` traits",
 863                             );
 864                         }
 865
 866                         // Try to report a help message
 867                         if is_fn_trait
 868                             && let Ok((implemented_kind, params)) = self.type_implements_fn_trait(
 869                             obligation.param_env,
 870                             trait_ref.self_ty(),
 871                             trait_predicate.skip_binder().constness,
 872                             trait_predicate.skip_binder().polarity,
 873                         )
 874                         {
 875                             // If the type implements `Fn`, `FnMut`, or `FnOnce`, suppress the following
 876                             // suggestion to add trait bounds for the type, since we only typically implement
 877                             // these traits once.
 878
 879                             // Note if the `FnMut` or `FnOnce` is less general than the trait we're trying
 880                             // to implement.
 881                             let selected_kind =
 882                                 ty::ClosureKind::from_def_id(self.tcx, trait_ref.def_id())
 883                                     .expect("expected to map DefId to ClosureKind");
 884                             if !implemented_kind.extends(selected_kind) {
 885                                 err.note(
 886                                     &format!(
 887                                         "`{}` implements `{}`, but it must implement `{}`, which is more general",
 888                                         trait_ref.skip_binder().self_ty(),
 889                                         implemented_kind,
 890                                         selected_kind
 891                                     )
 892                                 );
 893                             }
 894
 895                             // Note any argument mismatches
 896                             let given_ty = params.skip_binder();
 897                             let expected_ty = trait_ref.skip_binder().substs.type_at(1);
 898                             if let ty::Tuple(given) = given_ty.kind()
 899                                 && let ty::Tuple(expected) = expected_ty.kind()
 900                             {
 901                                 if expected.len() != given.len() {
 902                                     // Note number of types that were expected and given
 903                                     err.note(
 904                                         &format!(
 905                                             "expected a closure taking {} argument{}, but one taking {} argument{} was given",
 906                                             given.len(),
 907                                             pluralize!(given.len()),
 908                                             expected.len(),
 909                                             pluralize!(expected.len()),
 910                                         )
 911                                     );
 912                                 } else if !self.same_type_modulo_infer(given_ty, expected_ty) {
 913                                     // Print type mismatch
 914                                     let (expected_args, given_args) =
 915                                         self.cmp(given_ty, expected_ty);
 916                                     err.note_expected_found(
 917                                         &"a closure with arguments",
 918                                         expected_args,
 919                                         &"a closure with arguments",
 920                                         given_args,
 921                                     );
 922                                 }
 923                             }
 924                         } else if !trait_ref.has_non_region_infer()
 925                             && self.predicate_can_apply(obligation.param_env, trait_predicate)
 926                         {
 927                             // If a where-clause may be useful, remind the
 928                             // user that they can add it.
 929                             //
 930                             // don't display an on-unimplemented note, as
 931                             // these notes will often be of the form
 932                             //     "the type `T` can't be frobnicated"
 933                             // which is somewhat confusing.
 934                             self.suggest_restricting_param_bound(
 935                                 &mut err,
 936                                 trait_predicate,
 937                                 None,
 938                                 obligation.cause.body_id,
 939                             );
 940                         } else if !suggested && !unsatisfied_const {
 941                             // Can't show anything else useful, try to find similar impls.
 942                             let impl_candidates = self.find_similar_impl_candidates(trait_predicate);
 943                             if !self.report_similar_impl_candidates(
 944                                 impl_candidates,
 945                                 trait_ref,
 946                                 obligation.cause.body_id,
 947                                 &mut err,
 948                             ) {
 949                                 // This is *almost* equivalent to
 950                                 // `obligation.cause.code().peel_derives()`, but it gives us the
 951                                 // trait predicate for that corresponding root obligation. This
 952                                 // lets us get a derived obligation from a type parameter, like
 953                                 // when calling `string.strip_suffix(p)` where `p` is *not* an
 954                                 // implementer of `Pattern<'_>`.
 955                                 let mut code = obligation.cause.code();
 956                                 let mut trait_pred = trait_predicate;
 957                                 let mut peeled = false;
 958                                 while let Some((parent_code, parent_trait_pred)) = code.parent() {
 959                                     code = parent_code;
 960                                     if let Some(parent_trait_pred) = parent_trait_pred {
 961                                         trait_pred = parent_trait_pred;
 962                                         peeled = true;
 963                                     }
 964                                 }
 965                                 let def_id = trait_pred.def_id();
 966                                 // Mention *all* the `impl`s for the *top most* obligation, the
 967                                 // user might have meant to use one of them, if any found. We skip
 968                                 // auto-traits or fundamental traits that might not be exactly what
 969                                 // the user might expect to be presented with. Instead this is
 970                                 // useful for less general traits.
 971                                 if peeled
 972                                     && !self.tcx.trait_is_auto(def_id)
 973                                     && !self.tcx.lang_items().iter().any(|(_, id)| id == def_id)
 974                                 {
 975                                     let trait_ref = trait_pred.to_poly_trait_ref();
 976                                     let impl_candidates =
 977                                         self.find_similar_impl_candidates(trait_pred);
 978                                     self.report_similar_impl_candidates(
 979                                         impl_candidates,
 980                                         trait_ref,
 981                                         obligation.cause.body_id,
 982                                         &mut err,
 983                                     );
 984                                 }
 985                             }
 986                         }
 987
 988                         // Changing mutability doesn't make a difference to whether we have
 989                         // an `Unsize` impl (Fixes ICE in #71036)
 990                         if !is_unsize {
 991                             self.suggest_change_mut(&obligation, &mut err, trait_predicate);
 992                         }
 993
 994                         // If this error is due to `!: Trait` not implemented but `(): Trait` is
 995                         // implemented, and fallback has occurred, then it could be due to a
 996                         // variable that used to fallback to `()` now falling back to `!`. Issue a
 997                         // note informing about the change in behaviour.
 998                         if trait_predicate.skip_binder().self_ty().is_never()
 999                             && self.fallback_has_occurred
1000                         {
1001                             let predicate = trait_predicate.map_bound(|mut trait_pred| {
1002                                 trait_pred.trait_ref.substs = self.tcx.mk_substs_trait(
1003                                     self.tcx.mk_unit(),
1004                                     &trait_pred.trait_ref.substs[1..],
1005                                 );
1006                                 trait_pred
1007                             });
1008                             let unit_obligation = obligation.with(tcx, predicate);
1009                             if self.predicate_may_hold(&unit_obligation) {
1010                                 err.note(
1011                                     "this error might have been caused by changes to \
1012                                     Rust's type-inference algorithm (see issue #48950 \
1013                                     <https://github.com/rust-lang/rust/issues/48950> \
1014                                     for more information)",
1015                                 );
1016                                 err.help("did you intend to use the type `()` here instead?");
1017                             }
1018                         }
1019
1020                         // Return early if the trait is Debug or Display and the invocation
1021                         // originates within a standard library macro, because the output
1022                         // is otherwise overwhelming and unhelpful (see #85844 for an
1023                         // example).
1024
1025                         let in_std_macro =
1026                             match obligation.cause.span.ctxt().outer_expn_data().macro_def_id {
1027                                 Some(macro_def_id) => {
1028                                     let crate_name = tcx.crate_name(macro_def_id.krate);
1029                                     crate_name == sym::std || crate_name == sym::core
1030                                 }
1031                                 None => false,
1032                             };
1033
1034                         if in_std_macro
1035                             && matches!(
1036                                 self.tcx.get_diagnostic_name(trait_ref.def_id()),
1037                                 Some(sym::Debug | sym::Display)
1038                             )
1039                         {
1040                             err.emit();
1041                             return;
1042                         }
1043
1044                         err
1045                     }
1046
1047                     ty::PredicateKind::Subtype(predicate) => {
1048                         // Errors for Subtype predicates show up as
1049                         // `FulfillmentErrorCode::CodeSubtypeError`,
1050                         // not selection error.
1051                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
1052                     }
1053
1054                     ty::PredicateKind::Coerce(predicate) => {
1055                         // Errors for Coerce predicates show up as
1056                         // `FulfillmentErrorCode::CodeSubtypeError`,
1057                         // not selection error.
1058                         span_bug!(span, "coerce requirement gave wrong error: `{:?}`", predicate)
1059                     }
1060
1061                     ty::PredicateKind::RegionOutlives(..)
1062                     | ty::PredicateKind::Projection(..)
1063                     | ty::PredicateKind::TypeOutlives(..) => {
1064                         let predicate = self.resolve_vars_if_possible(obligation.predicate);
1065                         struct_span_err!(
1066                             self.tcx.sess,
1067                             span,
1068                             E0280,
1069                             "the requirement `{}` is not satisfied",
1070                             predicate
1071                         )
1072                     }
1073
1074                     ty::PredicateKind::ObjectSafe(trait_def_id) => {
1075                         let violations = self.tcx.object_safety_violations(trait_def_id);
1076                         report_object_safety_error(self.tcx, span, trait_def_id, violations)
1077                     }
1078
1079                     ty::PredicateKind::ClosureKind(closure_def_id, closure_substs, kind) => {
1080                         let found_kind = self.closure_kind(closure_substs).unwrap();
1081                         let closure_span = self.tcx.def_span(closure_def_id);
1082                         let mut err = struct_span_err!(
1083                             self.tcx.sess,
1084                             closure_span,
1085                             E0525,
1086                             "expected a closure that implements the `{}` trait, \
1087                              but this closure only implements `{}`",
1088                             kind,
1089                             found_kind
1090                         );
1091
1092                         err.span_label(
1093                             closure_span,
1094                             format!("this closure implements `{}`, not `{}`", found_kind, kind),
1095                         );
1096                         err.span_label(
1097                             obligation.cause.span,
1098                             format!("the requirement to implement `{}` derives from here", kind),
1099                         );
1100
1101                         // Additional context information explaining why the closure only implements
1102                         // a particular trait.
1103                         if let Some(typeck_results) = &self.typeck_results {
1104                             let hir_id = self
1105                                 .tcx
1106                                 .hir()
1107                                 .local_def_id_to_hir_id(closure_def_id.expect_local());
1108                             match (found_kind, typeck_results.closure_kind_origins().get(hir_id)) {
1109                                 (ty::ClosureKind::FnOnce, Some((span, place))) => {
1110                                     err.span_label(
1111                                         *span,
1112                                         format!(
1113                                             "closure is `FnOnce` because it moves the \
1114                                          variable `{}` out of its environment",
1115                                             ty::place_to_string_for_capture(tcx, place)
1116                                         ),
1117                                     );
1118                                 }
1119                                 (ty::ClosureKind::FnMut, Some((span, place))) => {
1120                                     err.span_label(
1121                                         *span,
1122                                         format!(
1123                                             "closure is `FnMut` because it mutates the \
1124                                          variable `{}` here",
1125                                             ty::place_to_string_for_capture(tcx, place)
1126                                         ),
1127                                     );
1128                                 }
1129                                 _ => {}
1130                             }
1131                         }
1132
1133                         err
1134                     }
1135
1136                     ty::PredicateKind::WellFormed(ty) => {
1137                         if !self.tcx.sess.opts.unstable_opts.chalk {
1138                             // WF predicates cannot themselves make
1139                             // errors. They can only block due to
1140                             // ambiguity; otherwise, they always
1141                             // degenerate into other obligations
1142                             // (which may fail).
1143                             span_bug!(span, "WF predicate not satisfied for {:?}", ty);
1144                         } else {
1145                             // FIXME: we'll need a better message which takes into account
1146                             // which bounds actually failed to hold.
1147                             self.tcx.sess.struct_span_err(
1148                                 span,
1149                                 &format!("the type `{}` is not well-formed (chalk)", ty),
1150                             )
1151                         }
1152                     }
1153
1154                     ty::PredicateKind::ConstEvaluatable(..) => {
1155                         // Errors for `ConstEvaluatable` predicates show up as
1156                         // `SelectionError::ConstEvalFailure`,
1157                         // not `Unimplemented`.
1158                         span_bug!(
1159                             span,
1160                             "const-evaluatable requirement gave wrong error: `{:?}`",
1161                             obligation
1162                         )
1163                     }
1164
1165                     ty::PredicateKind::ConstEquate(..) => {
1166                         // Errors for `ConstEquate` predicates show up as
1167                         // `SelectionError::ConstEvalFailure`,
1168                         // not `Unimplemented`.
1169                         span_bug!(
1170                             span,
1171                             "const-equate requirement gave wrong error: `{:?}`",
1172                             obligation
1173                         )
1174                     }
1175
1176                     ty::PredicateKind::TypeWellFormedFromEnv(..) => span_bug!(
1177                         span,
1178                         "TypeWellFormedFromEnv predicate should only exist in the environment"
1179                     ),
1180                 }
1181             }
1182
1183             OutputTypeParameterMismatch(found_trait_ref, expected_trait_ref, _) => {
1184                 let found_trait_ref = self.resolve_vars_if_possible(found_trait_ref);
1185                 let expected_trait_ref = self.resolve_vars_if_possible(expected_trait_ref);
1186
1187                 if expected_trait_ref.self_ty().references_error() {
1188                     return;
1189                 }
1190
1191                 let Some(found_trait_ty) = found_trait_ref.self_ty().no_bound_vars() else {
1192                     return;
1193                 };
1194
1195                 let found_did = match *found_trait_ty.kind() {
1196                     ty::Closure(did, _)
1197                     | ty::Foreign(did)
1198                     | ty::FnDef(did, _)
1199                     | ty::Generator(did, ..) => Some(did),
1200                     ty::Adt(def, _) => Some(def.did()),
1201                     _ => None,
1202                 };
1203
1204                 let found_span = found_did.and_then(|did| self.tcx.hir().span_if_local(did));
1205
1206                 if self.reported_closure_mismatch.borrow().contains(&(span, found_span)) {
1207                     // We check closures twice, with obligations flowing in different directions,
1208                     // but we want to complain about them only once.
1209                     return;
1210                 }
1211
1212                 self.reported_closure_mismatch.borrow_mut().insert((span, found_span));
1213
1214                 let mut not_tupled = false;
1215
1216                 let found = match found_trait_ref.skip_binder().substs.type_at(1).kind() {
1217                     ty::Tuple(ref tys) => vec![ArgKind::empty(); tys.len()],
1218                     _ => {
1219                         not_tupled = true;
1220                         vec![ArgKind::empty()]
1221                     }
1222                 };
1223
1224                 let expected_ty = expected_trait_ref.skip_binder().substs.type_at(1);
1225                 let expected = match expected_ty.kind() {
1226                     ty::Tuple(ref tys) => {
1227                         tys.iter().map(|t| ArgKind::from_expected_ty(t, Some(span))).collect()
1228                     }
1229                     _ => {
1230                         not_tupled = true;
1231                         vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())]
1232                     }
1233                 };
1234
1235                 // If this is a `Fn` family trait and either the expected or found
1236                 // is not tupled, then fall back to just a regular mismatch error.
1237                 // This shouldn't be common unless manually implementing one of the
1238                 // traits manually, but don't make it more confusing when it does
1239                 // happen.
1240                 if Some(expected_trait_ref.def_id()) != tcx.lang_items().gen_trait() && not_tupled {
1241                     self.report_and_explain_type_error(
1242                         TypeTrace::poly_trait_refs(
1243                             &obligation.cause,
1244                             true,
1245                             expected_trait_ref,
1246                             found_trait_ref,
1247                         ),
1248                         ty::error::TypeError::Mismatch,
1249                     )
1250                 } else if found.len() == expected.len() {
1251                     self.report_closure_arg_mismatch(
1252                         span,
1253                         found_span,
1254                         found_trait_ref,
1255                         expected_trait_ref,
1256                         obligation.cause.code(),
1257                     )
1258                 } else {
1259                     let (closure_span, found) = found_did
1260                         .and_then(|did| {
1261                             let node = self.tcx.hir().get_if_local(did)?;
1262                             let (found_span, found) = self.get_fn_like_arguments(node)?;
1263                             Some((Some(found_span), found))
1264                         })
1265                         .unwrap_or((found_span, found));
1266
1267                     self.report_arg_count_mismatch(
1268                         span,
1269                         closure_span,
1270                         expected,
1271                         found,
1272                         found_trait_ty.is_closure(),
1273                     )
1274                 }
1275             }
1276
1277             TraitNotObjectSafe(did) => {
1278                 let violations = self.tcx.object_safety_violations(did);
1279                 report_object_safety_error(self.tcx, span, did, violations)
1280             }
1281
1282             SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsInfer) => {
1283                 bug!(
1284                     "MentionsInfer should have been handled in `traits/fulfill.rs` or `traits/select/mod.rs`"
1285                 )
1286             }
1287             SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsParam) => {
1288                 if !self.tcx.features().generic_const_exprs {
1289                     let mut err = self.tcx.sess.struct_span_err(
1290                         span,
1291                         "constant expression depends on a generic parameter",
1292                     );
1293                     // FIXME(const_generics): we should suggest to the user how they can resolve this
1294                     // issue. However, this is currently not actually possible
1295                     // (see https://github.com/rust-lang/rust/issues/66962#issuecomment-575907083).
1296                     //
1297                     // Note that with `feature(generic_const_exprs)` this case should not
1298                     // be reachable.
1299                     err.note("this may fail depending on what value the parameter takes");
1300                     err.emit();
1301                     return;
1302                 }
1303
1304                 match obligation.predicate.kind().skip_binder() {
1305                     ty::PredicateKind::ConstEvaluatable(ct) => {
1306                         let ty::ConstKind::Unevaluated(uv) = ct.kind() else {
1307                             bug!("const evaluatable failed for non-unevaluated const `{ct:?}`");
1308                         };
1309                         let mut err =
1310                             self.tcx.sess.struct_span_err(span, "unconstrained generic constant");
1311                         let const_span = self.tcx.def_span(uv.def.did);
1312                         match self.tcx.sess.source_map().span_to_snippet(const_span) {
1313                             Ok(snippet) => err.help(&format!(
1314                                 "try adding a `where` bound using this expression: `where [(); {}]:`",
1315                                 snippet
1316                             )),
1317                             _ => err.help("consider adding a `where` bound using this expression"),
1318                         };
1319                         err
1320                     }
1321                     _ => {
1322                         span_bug!(
1323                             span,
1324                             "unexpected non-ConstEvaluatable predicate, this should not be reachable"
1325                         )
1326                     }
1327                 }
1328             }
1329
1330             // Already reported in the query.
1331             SelectionError::NotConstEvaluatable(NotConstEvaluatable::Error(_)) => {
1332                 // FIXME(eddyb) remove this once `ErrorGuaranteed` becomes a proof token.
1333                 self.tcx.sess.delay_span_bug(span, "`ErrorGuaranteed` without an error");
1334                 return;
1335             }
1336             // Already reported.
1337             Overflow(OverflowError::Error(_)) => {
1338                 self.tcx.sess.delay_span_bug(span, "`OverflowError` has been reported");
1339                 return;
1340             }
1341             Overflow(_) => {
1342                 bug!("overflow should be handled before the `report_selection_error` path");
1343             }
1344             SelectionError::ErrorReporting => {
1345                 bug!("ErrorReporting Overflow should not reach `report_selection_err` call")
1346             }
1347         };
1348
1349         self.note_obligation_cause(&mut err, &obligation);
1350         self.point_at_returns_when_relevant(&mut err, &obligation);
1351
1352         err.emit();
1353     }
1354 }
1355
1356 trait InferCtxtPrivExt<'tcx> {
1357     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1358     // `error` occurring implies that `cond` occurs.
1359     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool;
1360
1361     fn report_fulfillment_error(
1362         &self,
1363         error: &FulfillmentError<'tcx>,
1364         body_id: Option<hir::BodyId>,
1365     );
1366
1367     fn report_projection_error(
1368         &self,
1369         obligation: &PredicateObligation<'tcx>,
1370         error: &MismatchedProjectionTypes<'tcx>,
1371     );
1372
1373     fn maybe_detailed_projection_msg(
1374         &self,
1375         pred: ty::ProjectionPredicate<'tcx>,
1376         normalized_ty: ty::Term<'tcx>,
1377         expected_ty: ty::Term<'tcx>,
1378     ) -> Option<String>;
1379
1380     fn fuzzy_match_tys(
1381         &self,
1382         a: Ty<'tcx>,
1383         b: Ty<'tcx>,
1384         ignoring_lifetimes: bool,
1385     ) -> Option<CandidateSimilarity>;
1386
1387     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str>;
1388
1389     fn find_similar_impl_candidates(
1390         &self,
1391         trait_pred: ty::PolyTraitPredicate<'tcx>,
1392     ) -> Vec<ImplCandidate<'tcx>>;
1393
1394     fn report_similar_impl_candidates(
1395         &self,
1396         impl_candidates: Vec<ImplCandidate<'tcx>>,
1397         trait_ref: ty::PolyTraitRef<'tcx>,
1398         body_id: hir::HirId,
1399         err: &mut Diagnostic,
1400     ) -> bool;
1401
1402     /// Gets the parent trait chain start
1403     fn get_parent_trait_ref(
1404         &self,
1405         code: &ObligationCauseCode<'tcx>,
1406     ) -> Option<(String, Option<Span>)>;
1407
1408     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1409     /// with the same path as `trait_ref`, a help message about
1410     /// a probable version mismatch is added to `err`
1411     fn note_version_mismatch(
1412         &self,
1413         err: &mut Diagnostic,
1414         trait_ref: &ty::PolyTraitRef<'tcx>,
1415     ) -> bool;
1416
1417     /// Creates a `PredicateObligation` with `new_self_ty` replacing the existing type in the
1418     /// `trait_ref`.
1419     ///
1420     /// For this to work, `new_self_ty` must have no escaping bound variables.
1421     fn mk_trait_obligation_with_new_self_ty(
1422         &self,
1423         param_env: ty::ParamEnv<'tcx>,
1424         trait_ref_and_ty: ty::Binder<'tcx, (ty::TraitPredicate<'tcx>, Ty<'tcx>)>,
1425     ) -> PredicateObligation<'tcx>;
1426
1427     fn maybe_report_ambiguity(
1428         &self,
1429         obligation: &PredicateObligation<'tcx>,
1430         body_id: Option<hir::BodyId>,
1431     );
1432
1433     fn predicate_can_apply(
1434         &self,
1435         param_env: ty::ParamEnv<'tcx>,
1436         pred: ty::PolyTraitPredicate<'tcx>,
1437     ) -> bool;
1438
1439     fn note_obligation_cause(&self, err: &mut Diagnostic, obligation: &PredicateObligation<'tcx>);
1440
1441     fn suggest_unsized_bound_if_applicable(
1442         &self,
1443         err: &mut Diagnostic,
1444         obligation: &PredicateObligation<'tcx>,
1445     );
1446
1447     fn annotate_source_of_ambiguity(
1448         &self,
1449         err: &mut Diagnostic,
1450         impls: &[DefId],
1451         predicate: ty::Predicate<'tcx>,
1452     );
1453
1454     fn maybe_suggest_unsized_generics(&self, err: &mut Diagnostic, span: Span, node: Node<'tcx>);
1455
1456     fn maybe_indirection_for_unsized(
1457         &self,
1458         err: &mut Diagnostic,
1459         item: &'tcx Item<'tcx>,
1460         param: &'tcx GenericParam<'tcx>,
1461     ) -> bool;
1462
1463     fn is_recursive_obligation(
1464         &self,
1465         obligated_types: &mut Vec<Ty<'tcx>>,
1466         cause_code: &ObligationCauseCode<'tcx>,
1467     ) -> bool;
1468 }
1469
1470 impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
1471     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1472     // `error` occurring implies that `cond` occurs.
1473     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool {
1474         if cond == error {
1475             return true;
1476         }
1477
1478         // FIXME: It should be possible to deal with `ForAll` in a cleaner way.
1479         let bound_error = error.kind();
1480         let (cond, error) = match (cond.kind().skip_binder(), bound_error.skip_binder()) {
1481             (ty::PredicateKind::Trait(..), ty::PredicateKind::Trait(error)) => {
1482                 (cond, bound_error.rebind(error))
1483             }
1484             _ => {
1485                 // FIXME: make this work in other cases too.
1486                 return false;
1487             }
1488         };
1489
1490         for obligation in super::elaborate_predicates(self.tcx, std::iter::once(cond)) {
1491             let bound_predicate = obligation.predicate.kind();
1492             if let ty::PredicateKind::Trait(implication) = bound_predicate.skip_binder() {
1493                 let error = error.to_poly_trait_ref();
1494                 let implication = bound_predicate.rebind(implication.trait_ref);
1495                 // FIXME: I'm just not taking associated types at all here.
1496                 // Eventually I'll need to implement param-env-aware
1497                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
1498                 let param_env = ty::ParamEnv::empty();
1499                 if self.can_sub(param_env, error, implication).is_ok() {
1500                     debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
1501                     return true;
1502                 }
1503             }
1504         }
1505
1506         false
1507     }
1508
1509     #[instrument(skip(self), level = "debug")]
1510     fn report_fulfillment_error(
1511         &self,
1512         error: &FulfillmentError<'tcx>,
1513         body_id: Option<hir::BodyId>,
1514     ) {
1515         match error.code {
1516             FulfillmentErrorCode::CodeSelectionError(ref selection_error) => {
1517                 self.report_selection_error(
1518                     error.obligation.clone(),
1519                     &error.root_obligation,
1520                     selection_error,
1521                 );
1522             }
1523             FulfillmentErrorCode::CodeProjectionError(ref e) => {
1524                 self.report_projection_error(&error.obligation, e);
1525             }
1526             FulfillmentErrorCode::CodeAmbiguity => {
1527                 self.maybe_report_ambiguity(&error.obligation, body_id);
1528             }
1529             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
1530                 self.report_mismatched_types(
1531                     &error.obligation.cause,
1532                     expected_found.expected,
1533                     expected_found.found,
1534                     err.clone(),
1535                 )
1536                 .emit();
1537             }
1538             FulfillmentErrorCode::CodeConstEquateError(ref expected_found, ref err) => {
1539                 let mut diag = self.report_mismatched_consts(
1540                     &error.obligation.cause,
1541                     expected_found.expected,
1542                     expected_found.found,
1543                     err.clone(),
1544                 );
1545                 let code = error.obligation.cause.code().peel_derives().peel_match_impls();
1546                 if let ObligationCauseCode::BindingObligation(..)
1547                 | ObligationCauseCode::ItemObligation(..)
1548                 | ObligationCauseCode::ExprBindingObligation(..)
1549                 | ObligationCauseCode::ExprItemObligation(..) = code
1550                 {
1551                     self.note_obligation_cause_code(
1552                         &mut diag,
1553                         &error.obligation.predicate,
1554                         error.obligation.param_env,
1555                         code,
1556                         &mut vec![],
1557                         &mut Default::default(),
1558                     );
1559                 }
1560                 diag.emit();
1561             }
1562             FulfillmentErrorCode::CodeCycle(ref cycle) => {
1563                 self.report_overflow_error_cycle(cycle);
1564             }
1565         }
1566     }
1567
1568     #[instrument(level = "debug", skip_all)]
1569     fn report_projection_error(
1570         &self,
1571         obligation: &PredicateObligation<'tcx>,
1572         error: &MismatchedProjectionTypes<'tcx>,
1573     ) {
1574         let predicate = self.resolve_vars_if_possible(obligation.predicate);
1575
1576         if predicate.references_error() {
1577             return;
1578         }
1579
1580         self.probe(|_| {
1581             let mut err = error.err;
1582             let mut values = None;
1583
1584             // try to find the mismatched types to report the error with.
1585             //
1586             // this can fail if the problem was higher-ranked, in which
1587             // cause I have no idea for a good error message.
1588             let bound_predicate = predicate.kind();
1589             if let ty::PredicateKind::Projection(data) = bound_predicate.skip_binder() {
1590                 let mut selcx = SelectionContext::new(self);
1591                 let data = self.replace_bound_vars_with_fresh_vars(
1592                     obligation.cause.span,
1593                     infer::LateBoundRegionConversionTime::HigherRankedType,
1594                     bound_predicate.rebind(data),
1595                 );
1596                 let mut obligations = vec![];
1597                 let normalized_ty = super::normalize_projection_type(
1598                     &mut selcx,
1599                     obligation.param_env,
1600                     data.projection_ty,
1601                     obligation.cause.clone(),
1602                     0,
1603                     &mut obligations,
1604                 );
1605
1606                 debug!(?obligation.cause, ?obligation.param_env);
1607
1608                 debug!(?normalized_ty, data.ty = ?data.term);
1609
1610                 let is_normalized_ty_expected = !matches!(
1611                     obligation.cause.code().peel_derives(),
1612                     ObligationCauseCode::ItemObligation(_)
1613                         | ObligationCauseCode::BindingObligation(_, _)
1614                         | ObligationCauseCode::ExprItemObligation(..)
1615                         | ObligationCauseCode::ExprBindingObligation(..)
1616                         | ObligationCauseCode::ObjectCastObligation(..)
1617                         | ObligationCauseCode::OpaqueType
1618                 );
1619                 if let Err(new_err) = self.at(&obligation.cause, obligation.param_env).eq_exp(
1620                     is_normalized_ty_expected,
1621                     normalized_ty,
1622                     data.term,
1623                 ) {
1624                     values = Some((data, is_normalized_ty_expected, normalized_ty, data.term));
1625                     err = new_err;
1626                 }
1627             }
1628
1629             let msg = values
1630                 .and_then(|(predicate, _, normalized_ty, expected_ty)| {
1631                     self.maybe_detailed_projection_msg(predicate, normalized_ty, expected_ty)
1632                 })
1633                 .unwrap_or_else(|| format!("type mismatch resolving `{}`", predicate));
1634             let mut diag = struct_span_err!(self.tcx.sess, obligation.cause.span, E0271, "{msg}");
1635
1636             let secondary_span = match predicate.kind().skip_binder() {
1637                 ty::PredicateKind::Projection(proj) => self
1638                     .tcx
1639                     .opt_associated_item(proj.projection_ty.item_def_id)
1640                     .and_then(|trait_assoc_item| {
1641                         self.tcx
1642                             .trait_of_item(proj.projection_ty.item_def_id)
1643                             .map(|id| (trait_assoc_item, id))
1644                     })
1645                     .and_then(|(trait_assoc_item, id)| {
1646                         let trait_assoc_ident = trait_assoc_item.ident(self.tcx);
1647                         self.tcx.find_map_relevant_impl(id, proj.projection_ty.self_ty(), |did| {
1648                             self.tcx
1649                                 .associated_items(did)
1650                                 .in_definition_order()
1651                                 .find(|assoc| assoc.ident(self.tcx) == trait_assoc_ident)
1652                         })
1653                     })
1654                     .and_then(|item| match self.tcx.hir().get_if_local(item.def_id) {
1655                         Some(
1656                             hir::Node::TraitItem(hir::TraitItem {
1657                                 kind: hir::TraitItemKind::Type(_, Some(ty)),
1658                                 ..
1659                             })
1660                             | hir::Node::ImplItem(hir::ImplItem {
1661                                 kind: hir::ImplItemKind::Type(ty),
1662                                 ..
1663                             }),
1664                         ) => Some((ty.span, format!("type mismatch resolving `{}`", predicate))),
1665                         _ => None,
1666                     }),
1667                 _ => None,
1668             };
1669             self.note_type_err(
1670                 &mut diag,
1671                 &obligation.cause,
1672                 secondary_span,
1673                 values.map(|(_, is_normalized_ty_expected, normalized_ty, term)| {
1674                     infer::ValuePairs::Terms(ExpectedFound::new(
1675                         is_normalized_ty_expected,
1676                         normalized_ty,
1677                         term,
1678                     ))
1679                 }),
1680                 err,
1681                 true,
1682                 false,
1683             );
1684             self.note_obligation_cause(&mut diag, obligation);
1685             diag.emit();
1686         });
1687     }
1688
1689     fn maybe_detailed_projection_msg(
1690         &self,
1691         pred: ty::ProjectionPredicate<'tcx>,
1692         normalized_ty: ty::Term<'tcx>,
1693         expected_ty: ty::Term<'tcx>,
1694     ) -> Option<String> {
1695         let trait_def_id = pred.projection_ty.trait_def_id(self.tcx);
1696         let self_ty = pred.projection_ty.self_ty();
1697
1698         if Some(pred.projection_ty.item_def_id) == self.tcx.lang_items().fn_once_output() {
1699             Some(format!(
1700                 "expected `{self_ty}` to be a {fn_kind} that returns `{expected_ty}`, but it returns `{normalized_ty}`",
1701                 fn_kind = self_ty.prefix_string(self.tcx)
1702             ))
1703         } else if Some(trait_def_id) == self.tcx.lang_items().future_trait() {
1704             Some(format!(
1705                 "expected `{self_ty}` to be a future that resolves to `{expected_ty}`, but it resolves to `{normalized_ty}`"
1706             ))
1707         } else if Some(trait_def_id) == self.tcx.get_diagnostic_item(sym::Iterator) {
1708             Some(format!(
1709                 "expected `{self_ty}` to be an iterator that yields `{expected_ty}`, but it yields `{normalized_ty}`"
1710             ))
1711         } else {
1712             None
1713         }
1714     }
1715
1716     fn fuzzy_match_tys(
1717         &self,
1718         mut a: Ty<'tcx>,
1719         mut b: Ty<'tcx>,
1720         ignoring_lifetimes: bool,
1721     ) -> Option<CandidateSimilarity> {
1722         /// returns the fuzzy category of a given type, or None
1723         /// if the type can be equated to any type.
1724         fn type_category(tcx: TyCtxt<'_>, t: Ty<'_>) -> Option<u32> {
1725             match t.kind() {
1726                 ty::Bool => Some(0),
1727                 ty::Char => Some(1),
1728                 ty::Str => Some(2),
1729                 ty::Adt(def, _) if tcx.is_diagnostic_item(sym::String, def.did()) => Some(2),
1730                 ty::Int(..)
1731                 | ty::Uint(..)
1732                 | ty::Float(..)
1733                 | ty::Infer(ty::IntVar(..) | ty::FloatVar(..)) => Some(4),
1734                 ty::Ref(..) | ty::RawPtr(..) => Some(5),
1735                 ty::Array(..) | ty::Slice(..) => Some(6),
1736                 ty::FnDef(..) | ty::FnPtr(..) => Some(7),
1737                 ty::Dynamic(..) => Some(8),
1738                 ty::Closure(..) => Some(9),
1739                 ty::Tuple(..) => Some(10),
1740                 ty::Param(..) => Some(11),
1741                 ty::Projection(..) => Some(12),
1742                 ty::Opaque(..) => Some(13),
1743                 ty::Never => Some(14),
1744                 ty::Adt(..) => Some(15),
1745                 ty::Generator(..) => Some(16),
1746                 ty::Foreign(..) => Some(17),
1747                 ty::GeneratorWitness(..) => Some(18),
1748                 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error(_) => None,
1749             }
1750         }
1751
1752         let strip_references = |mut t: Ty<'tcx>| -> Ty<'tcx> {
1753             loop {
1754                 match t.kind() {
1755                     ty::Ref(_, inner, _) | ty::RawPtr(ty::TypeAndMut { ty: inner, .. }) => {
1756                         t = *inner
1757                     }
1758                     _ => break t,
1759                 }
1760             }
1761         };
1762
1763         if !ignoring_lifetimes {
1764             a = strip_references(a);
1765             b = strip_references(b);
1766         }
1767
1768         let cat_a = type_category(self.tcx, a)?;
1769         let cat_b = type_category(self.tcx, b)?;
1770         if a == b {
1771             Some(CandidateSimilarity::Exact { ignoring_lifetimes })
1772         } else if cat_a == cat_b {
1773             match (a.kind(), b.kind()) {
1774                 (ty::Adt(def_a, _), ty::Adt(def_b, _)) => def_a == def_b,
1775                 (ty::Foreign(def_a), ty::Foreign(def_b)) => def_a == def_b,
1776                 // Matching on references results in a lot of unhelpful
1777                 // suggestions, so let's just not do that for now.
1778                 //
1779                 // We still upgrade successful matches to `ignoring_lifetimes: true`
1780                 // to prioritize that impl.
1781                 (ty::Ref(..) | ty::RawPtr(..), ty::Ref(..) | ty::RawPtr(..)) => {
1782                     self.fuzzy_match_tys(a, b, true).is_some()
1783                 }
1784                 _ => true,
1785             }
1786             .then_some(CandidateSimilarity::Fuzzy { ignoring_lifetimes })
1787         } else if ignoring_lifetimes {
1788             None
1789         } else {
1790             self.fuzzy_match_tys(a, b, true)
1791         }
1792     }
1793
1794     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str> {
1795         self.tcx.hir().body(body_id).generator_kind.map(|gen_kind| match gen_kind {
1796             hir::GeneratorKind::Gen => "a generator",
1797             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block) => "an async block",
1798             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn) => "an async function",
1799             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Closure) => "an async closure",
1800         })
1801     }
1802
1803     fn find_similar_impl_candidates(
1804         &self,
1805         trait_pred: ty::PolyTraitPredicate<'tcx>,
1806     ) -> Vec<ImplCandidate<'tcx>> {
1807         self.tcx
1808             .all_impls(trait_pred.def_id())
1809             .filter_map(|def_id| {
1810                 if self.tcx.impl_polarity(def_id) == ty::ImplPolarity::Negative
1811                     || !trait_pred
1812                         .skip_binder()
1813                         .is_constness_satisfied_by(self.tcx.constness(def_id))
1814                 {
1815                     return None;
1816                 }
1817
1818                 let imp = self.tcx.impl_trait_ref(def_id).unwrap();
1819
1820                 self.fuzzy_match_tys(trait_pred.skip_binder().self_ty(), imp.self_ty(), false)
1821                     .map(|similarity| ImplCandidate { trait_ref: imp, similarity })
1822             })
1823             .collect()
1824     }
1825
1826     fn report_similar_impl_candidates(
1827         &self,
1828         impl_candidates: Vec<ImplCandidate<'tcx>>,
1829         trait_ref: ty::PolyTraitRef<'tcx>,
1830         body_id: hir::HirId,
1831         err: &mut Diagnostic,
1832     ) -> bool {
1833         let report = |mut candidates: Vec<TraitRef<'tcx>>, err: &mut Diagnostic| {
1834             candidates.sort();
1835             candidates.dedup();
1836             let len = candidates.len();
1837             if candidates.len() == 0 {
1838                 return false;
1839             }
1840             if candidates.len() == 1 {
1841                 let ty_desc = match candidates[0].self_ty().kind() {
1842                     ty::FnPtr(_) => Some("fn pointer"),
1843                     _ => None,
1844                 };
1845                 let the_desc = match ty_desc {
1846                     Some(desc) => format!(" implemented for {} `", desc),
1847                     None => " implemented for `".to_string(),
1848                 };
1849                 err.highlighted_help(vec![
1850                     (
1851                         format!("the trait `{}` ", candidates[0].print_only_trait_path()),
1852                         Style::NoStyle,
1853                     ),
1854                     ("is".to_string(), Style::Highlight),
1855                     (the_desc, Style::NoStyle),
1856                     (candidates[0].self_ty().to_string(), Style::Highlight),
1857                     ("`".to_string(), Style::NoStyle),
1858                 ]);
1859                 return true;
1860             }
1861             let trait_ref = TraitRef::identity(self.tcx, candidates[0].def_id);
1862             // Check if the trait is the same in all cases. If so, we'll only show the type.
1863             let mut traits: Vec<_> =
1864                 candidates.iter().map(|c| c.print_only_trait_path().to_string()).collect();
1865             traits.sort();
1866             traits.dedup();
1867
1868             let mut candidates: Vec<String> = candidates
1869                 .into_iter()
1870                 .map(|c| {
1871                     if traits.len() == 1 {
1872                         format!("\n  {}", c.self_ty())
1873                     } else {
1874                         format!("\n  {}", c)
1875                     }
1876                 })
1877                 .collect();
1878
1879             candidates.sort();
1880             candidates.dedup();
1881             let end = if candidates.len() <= 9 { candidates.len() } else { 8 };
1882             err.help(&format!(
1883                 "the following other types implement trait `{}`:{}{}",
1884                 trait_ref.print_only_trait_path(),
1885                 candidates[..end].join(""),
1886                 if len > 9 { format!("\nand {} others", len - 8) } else { String::new() }
1887             ));
1888             true
1889         };
1890
1891         let def_id = trait_ref.def_id();
1892         if impl_candidates.is_empty() {
1893             if self.tcx.trait_is_auto(def_id)
1894                 || self.tcx.lang_items().iter().any(|(_, id)| id == def_id)
1895                 || self.tcx.get_diagnostic_name(def_id).is_some()
1896             {
1897                 // Mentioning implementers of `Copy`, `Debug` and friends is not useful.
1898                 return false;
1899             }
1900             let normalized_impl_candidates: Vec<_> = self
1901                 .tcx
1902                 .all_impls(def_id)
1903                 // Ignore automatically derived impls and `!Trait` impls.
1904                 .filter(|&def_id| {
1905                     self.tcx.impl_polarity(def_id) != ty::ImplPolarity::Negative
1906                         || self.tcx.is_builtin_derive(def_id)
1907                 })
1908                 .filter_map(|def_id| self.tcx.impl_trait_ref(def_id))
1909                 .filter(|trait_ref| {
1910                     let self_ty = trait_ref.self_ty();
1911                     // Avoid mentioning type parameters.
1912                     if let ty::Param(_) = self_ty.kind() {
1913                         false
1914                     }
1915                     // Avoid mentioning types that are private to another crate
1916                     else if let ty::Adt(def, _) = self_ty.peel_refs().kind() {
1917                         // FIXME(compiler-errors): This could be generalized, both to
1918                         // be more granular, and probably look past other `#[fundamental]`
1919                         // types, too.
1920                         self.tcx
1921                             .visibility(def.did())
1922                             .is_accessible_from(body_id.owner.def_id, self.tcx)
1923                     } else {
1924                         true
1925                     }
1926                 })
1927                 .collect();
1928             return report(normalized_impl_candidates, err);
1929         }
1930
1931         let normalize = |candidate| {
1932             let infcx = self.tcx.infer_ctxt().build();
1933             infcx
1934                 .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
1935                 .normalize(candidate)
1936                 .map_or(candidate, |normalized| normalized.value)
1937         };
1938
1939         // Sort impl candidates so that ordering is consistent for UI tests.
1940         // because the ordering of `impl_candidates` may not be deterministic:
1941         // https://github.com/rust-lang/rust/pull/57475#issuecomment-455519507
1942         //
1943         // Prefer more similar candidates first, then sort lexicographically
1944         // by their normalized string representation.
1945         let mut normalized_impl_candidates_and_similarities = impl_candidates
1946             .into_iter()
1947             .map(|ImplCandidate { trait_ref, similarity }| {
1948                 let normalized = normalize(trait_ref);
1949                 (similarity, normalized)
1950             })
1951             .collect::<Vec<_>>();
1952         normalized_impl_candidates_and_similarities.sort();
1953         normalized_impl_candidates_and_similarities.dedup();
1954
1955         let normalized_impl_candidates = normalized_impl_candidates_and_similarities
1956             .into_iter()
1957             .map(|(_, normalized)| normalized)
1958             .collect::<Vec<_>>();
1959
1960         report(normalized_impl_candidates, err)
1961     }
1962
1963     /// Gets the parent trait chain start
1964     fn get_parent_trait_ref(
1965         &self,
1966         code: &ObligationCauseCode<'tcx>,
1967     ) -> Option<(String, Option<Span>)> {
1968         match code {
1969             ObligationCauseCode::BuiltinDerivedObligation(data) => {
1970                 let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
1971                 match self.get_parent_trait_ref(&data.parent_code) {
1972                     Some(t) => Some(t),
1973                     None => {
1974                         let ty = parent_trait_ref.skip_binder().self_ty();
1975                         let span = TyCategory::from_ty(self.tcx, ty)
1976                             .map(|(_, def_id)| self.tcx.def_span(def_id));
1977                         Some((ty.to_string(), span))
1978                     }
1979                 }
1980             }
1981             ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } => {
1982                 self.get_parent_trait_ref(&parent_code)
1983             }
1984             _ => None,
1985         }
1986     }
1987
1988     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1989     /// with the same path as `trait_ref`, a help message about
1990     /// a probable version mismatch is added to `err`
1991     fn note_version_mismatch(
1992         &self,
1993         err: &mut Diagnostic,
1994         trait_ref: &ty::PolyTraitRef<'tcx>,
1995     ) -> bool {
1996         let get_trait_impl = |trait_def_id| {
1997             self.tcx.find_map_relevant_impl(trait_def_id, trait_ref.skip_binder().self_ty(), Some)
1998         };
1999         let required_trait_path = self.tcx.def_path_str(trait_ref.def_id());
2000         let traits_with_same_path: std::collections::BTreeSet<_> = self
2001             .tcx
2002             .all_traits()
2003             .filter(|trait_def_id| *trait_def_id != trait_ref.def_id())
2004             .filter(|trait_def_id| self.tcx.def_path_str(*trait_def_id) == required_trait_path)
2005             .collect();
2006         let mut suggested = false;
2007         for trait_with_same_path in traits_with_same_path {
2008             if let Some(impl_def_id) = get_trait_impl(trait_with_same_path) {
2009                 let impl_span = self.tcx.def_span(impl_def_id);
2010                 err.span_help(impl_span, "trait impl with same name found");
2011                 let trait_crate = self.tcx.crate_name(trait_with_same_path.krate);
2012                 let crate_msg = format!(
2013                     "perhaps two different versions of crate `{}` are being used?",
2014                     trait_crate
2015                 );
2016                 err.note(&crate_msg);
2017                 suggested = true;
2018             }
2019         }
2020         suggested
2021     }
2022
2023     fn mk_trait_obligation_with_new_self_ty(
2024         &self,
2025         param_env: ty::ParamEnv<'tcx>,
2026         trait_ref_and_ty: ty::Binder<'tcx, (ty::TraitPredicate<'tcx>, Ty<'tcx>)>,
2027     ) -> PredicateObligation<'tcx> {
2028         let trait_pred = trait_ref_and_ty.map_bound_ref(|(tr, new_self_ty)| ty::TraitPredicate {
2029             trait_ref: ty::TraitRef {
2030                 substs: self.tcx.mk_substs_trait(*new_self_ty, &tr.trait_ref.substs[1..]),
2031                 ..tr.trait_ref
2032             },
2033             ..*tr
2034         });
2035
2036         Obligation::new(self.tcx, ObligationCause::dummy(), param_env, trait_pred)
2037     }
2038
2039     #[instrument(skip(self), level = "debug")]
2040     fn maybe_report_ambiguity(
2041         &self,
2042         obligation: &PredicateObligation<'tcx>,
2043         body_id: Option<hir::BodyId>,
2044     ) {
2045         // Unable to successfully determine, probably means
2046         // insufficient type information, but could mean
2047         // ambiguous impls. The latter *ought* to be a
2048         // coherence violation, so we don't report it here.
2049
2050         let predicate = self.resolve_vars_if_possible(obligation.predicate);
2051         let span = obligation.cause.span;
2052
2053         debug!(?predicate, obligation.cause.code = ?obligation.cause.code());
2054
2055         // Ambiguity errors are often caused as fallout from earlier errors.
2056         // We ignore them if this `infcx` is tainted in some cases below.
2057
2058         let bound_predicate = predicate.kind();
2059         let mut err = match bound_predicate.skip_binder() {
2060             ty::PredicateKind::Trait(data) => {
2061                 let trait_ref = bound_predicate.rebind(data.trait_ref);
2062                 debug!(?trait_ref);
2063
2064                 if predicate.references_error() {
2065                     return;
2066                 }
2067
2068                 // This is kind of a hack: it frequently happens that some earlier
2069                 // error prevents types from being fully inferred, and then we get
2070                 // a bunch of uninteresting errors saying something like "<generic
2071                 // #0> doesn't implement Sized".  It may even be true that we
2072                 // could just skip over all checks where the self-ty is an
2073                 // inference variable, but I was afraid that there might be an
2074                 // inference variable created, registered as an obligation, and
2075                 // then never forced by writeback, and hence by skipping here we'd
2076                 // be ignoring the fact that we don't KNOW the type works
2077                 // out. Though even that would probably be harmless, given that
2078                 // we're only talking about builtin traits, which are known to be
2079                 // inhabited. We used to check for `self.tcx.sess.has_errors()` to
2080                 // avoid inundating the user with unnecessary errors, but we now
2081                 // check upstream for type errors and don't add the obligations to
2082                 // begin with in those cases.
2083                 if self.tcx.lang_items().sized_trait() == Some(trait_ref.def_id()) {
2084                     if !self.is_tainted_by_errors() {
2085                         self.emit_inference_failure_err(
2086                             body_id,
2087                             span,
2088                             trait_ref.self_ty().skip_binder().into(),
2089                             ErrorCode::E0282,
2090                             false,
2091                         )
2092                         .emit();
2093                     }
2094                     return;
2095                 }
2096
2097                 // Typically, this ambiguity should only happen if
2098                 // there are unresolved type inference variables
2099                 // (otherwise it would suggest a coherence
2100                 // failure). But given #21974 that is not necessarily
2101                 // the case -- we can have multiple where clauses that
2102                 // are only distinguished by a region, which results
2103                 // in an ambiguity even when all types are fully
2104                 // known, since we don't dispatch based on region
2105                 // relationships.
2106
2107                 // Pick the first substitution that still contains inference variables as the one
2108                 // we're going to emit an error for. If there are none (see above), fall back to
2109                 // a more general error.
2110                 let subst = data.trait_ref.substs.iter().find(|s| s.has_non_region_infer());
2111
2112                 let mut err = if let Some(subst) = subst {
2113                     self.emit_inference_failure_err(body_id, span, subst, ErrorCode::E0283, true)
2114                 } else {
2115                     struct_span_err!(
2116                         self.tcx.sess,
2117                         span,
2118                         E0283,
2119                         "type annotations needed: cannot satisfy `{}`",
2120                         predicate,
2121                     )
2122                 };
2123
2124                 let obligation = obligation.with(self.tcx, trait_ref.to_poly_trait_predicate());
2125                 let mut selcx = SelectionContext::with_query_mode(
2126                     &self,
2127                     crate::traits::TraitQueryMode::Standard,
2128                 );
2129                 match selcx.select_from_obligation(&obligation) {
2130                     Ok(None) => {
2131                         let impls = ambiguity::recompute_applicable_impls(self.infcx, &obligation);
2132                         let has_non_region_infer =
2133                             trait_ref.skip_binder().substs.types().any(|t| !t.is_ty_infer());
2134                         // It doesn't make sense to talk about applicable impls if there are more
2135                         // than a handful of them.
2136                         if impls.len() > 1 && impls.len() < 5 && has_non_region_infer {
2137                             self.annotate_source_of_ambiguity(&mut err, &impls, predicate);
2138                         } else {
2139                             if self.is_tainted_by_errors() {
2140                                 err.delay_as_bug();
2141                                 return;
2142                             }
2143                             err.note(&format!("cannot satisfy `{}`", predicate));
2144                         }
2145                     }
2146                     _ => {
2147                         if self.is_tainted_by_errors() {
2148                             err.delay_as_bug();
2149                             return;
2150                         }
2151                         err.note(&format!("cannot satisfy `{}`", predicate));
2152                     }
2153                 }
2154
2155                 if let ObligationCauseCode::ItemObligation(def_id) | ObligationCauseCode::ExprItemObligation(def_id, ..) = *obligation.cause.code() {
2156                     self.suggest_fully_qualified_path(&mut err, def_id, span, trait_ref.def_id());
2157                 } else if let Ok(snippet) = &self.tcx.sess.source_map().span_to_snippet(span)
2158                     && let ObligationCauseCode::BindingObligation(def_id, _) | ObligationCauseCode::ExprBindingObligation(def_id, ..)
2159                         = *obligation.cause.code()
2160                 {
2161                     let generics = self.tcx.generics_of(def_id);
2162                     if generics.params.iter().any(|p| p.name != kw::SelfUpper)
2163                         && !snippet.ends_with('>')
2164                         && !generics.has_impl_trait()
2165                         && !self.tcx.fn_trait_kind_from_lang_item(def_id).is_some()
2166                     {
2167                         // FIXME: To avoid spurious suggestions in functions where type arguments
2168                         // where already supplied, we check the snippet to make sure it doesn't
2169                         // end with a turbofish. Ideally we would have access to a `PathSegment`
2170                         // instead. Otherwise we would produce the following output:
2171                         //
2172                         // error[E0283]: type annotations needed
2173                         //   --> $DIR/issue-54954.rs:3:24
2174                         //    |
2175                         // LL | const ARR_LEN: usize = Tt::const_val::<[i8; 123]>();
2176                         //    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^
2177                         //    |                        |
2178                         //    |                        cannot infer type
2179                         //    |                        help: consider specifying the type argument
2180                         //    |                        in the function call:
2181                         //    |                        `Tt::const_val::<[i8; 123]>::<T>`
2182                         // ...
2183                         // LL |     const fn const_val<T: Sized>() -> usize {
2184                         //    |                        - required by this bound in `Tt::const_val`
2185                         //    |
2186                         //    = note: cannot satisfy `_: Tt`
2187
2188                         // Clear any more general suggestions in favor of our specific one
2189                         err.clear_suggestions();
2190
2191                         err.span_suggestion_verbose(
2192                             span.shrink_to_hi(),
2193                             &format!(
2194                                 "consider specifying the type argument{} in the function call",
2195                                 pluralize!(generics.params.len()),
2196                             ),
2197                             format!(
2198                                 "::<{}>",
2199                                 generics
2200                                     .params
2201                                     .iter()
2202                                     .map(|p| p.name.to_string())
2203                                     .collect::<Vec<String>>()
2204                                     .join(", ")
2205                             ),
2206                             Applicability::HasPlaceholders,
2207                         );
2208                     }
2209                 }
2210
2211                 if let (Some(body_id), Some(ty::subst::GenericArgKind::Type(_))) =
2212                     (body_id, subst.map(|subst| subst.unpack()))
2213                 {
2214                     struct FindExprBySpan<'hir> {
2215                         span: Span,
2216                         result: Option<&'hir hir::Expr<'hir>>,
2217                     }
2218
2219                     impl<'v> hir::intravisit::Visitor<'v> for FindExprBySpan<'v> {
2220                         fn visit_expr(&mut self, ex: &'v hir::Expr<'v>) {
2221                             if self.span == ex.span {
2222                                 self.result = Some(ex);
2223                             } else {
2224                                 hir::intravisit::walk_expr(self, ex);
2225                             }
2226                         }
2227                     }
2228
2229                     let mut expr_finder = FindExprBySpan { span, result: None };
2230
2231                     expr_finder.visit_expr(&self.tcx.hir().body(body_id).value);
2232
2233                     if let Some(hir::Expr {
2234                         kind: hir::ExprKind::Path(hir::QPath::Resolved(None, path)), .. }
2235                     ) = expr_finder.result
2236                         && let [
2237                             ..,
2238                             trait_path_segment @ hir::PathSegment {
2239                                 res: rustc_hir::def::Res::Def(rustc_hir::def::DefKind::Trait, trait_id),
2240                                 ..
2241                             },
2242                             hir::PathSegment {
2243                                 ident: assoc_item_name,
2244                                 res: rustc_hir::def::Res::Def(_, item_id),
2245                                 ..
2246                             }
2247                         ] = path.segments
2248                         && data.trait_ref.def_id == *trait_id
2249                         && self.tcx.trait_of_item(*item_id) == Some(*trait_id)
2250                         && !self.is_tainted_by_errors()
2251                     {
2252                         let (verb, noun) = match self.tcx.associated_item(item_id).kind {
2253                             ty::AssocKind::Const => ("refer to the", "constant"),
2254                             ty::AssocKind::Fn => ("call", "function"),
2255                             ty::AssocKind::Type => ("refer to the", "type"), // this is already covered by E0223, but this single match arm doesn't hurt here
2256                         };
2257
2258                         // Replace the more general E0283 with a more specific error
2259                         err.cancel();
2260                         err = self.tcx.sess.struct_span_err_with_code(
2261                             span,
2262                             &format!(
2263                                 "cannot {verb} associated {noun} on trait without specifying the corresponding `impl` type",
2264                              ),
2265                             rustc_errors::error_code!(E0790),
2266                         );
2267
2268                         if let Some(local_def_id) = data.trait_ref.def_id.as_local()
2269                             && 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)
2270                             && let Some(method_ref) = trait_item_refs.iter().find(|item_ref| item_ref.ident == *assoc_item_name) {
2271                             err.span_label(method_ref.span, format!("`{}::{}` defined here", trait_name, assoc_item_name));
2272                         }
2273
2274                         err.span_label(span, format!("cannot {verb} associated {noun} of trait"));
2275
2276                         let trait_impls = self.tcx.trait_impls_of(data.trait_ref.def_id);
2277
2278                         if trait_impls.blanket_impls().is_empty()
2279                             && let Some((impl_ty, _)) = trait_impls.non_blanket_impls().iter().next()
2280                             && let Some(impl_def_id) = impl_ty.def() {
2281                             let message = if trait_impls.non_blanket_impls().len() == 1 {
2282                                 "use the fully-qualified path to the only available implementation".to_string()
2283                             } else {
2284                                 format!(
2285                                     "use a fully-qualified path to a specific available implementation ({} found)",
2286                                     trait_impls.non_blanket_impls().len()
2287                                 )
2288                             };
2289                             let mut suggestions = vec![(
2290                                 trait_path_segment.ident.span.shrink_to_lo(),
2291                                 format!("<{} as ", self.tcx.type_of(impl_def_id))
2292                             )];
2293                             if let Some(generic_arg) = trait_path_segment.args {
2294                                 let between_span = trait_path_segment.ident.span.between(generic_arg.span_ext);
2295                                 // get rid of :: between Trait and <type>
2296                                 // must be '::' between them, otherwise the parser won't accept the code
2297                                 suggestions.push((between_span, "".to_string(),));
2298                                 suggestions.push((generic_arg.span_ext.shrink_to_hi(), format!(">")));
2299                             } else {
2300                                 suggestions.push((trait_path_segment.ident.span.shrink_to_hi(), format!(">")));
2301                             }
2302                             err.multipart_suggestion(
2303                                 message,
2304                                 suggestions,
2305                                 Applicability::MaybeIncorrect
2306                             );
2307                         }
2308                     }
2309                 };
2310
2311                 err
2312             }
2313
2314             ty::PredicateKind::WellFormed(arg) => {
2315                 // Same hacky approach as above to avoid deluging user
2316                 // with error messages.
2317                 if arg.references_error()
2318                     || self.tcx.sess.has_errors().is_some()
2319                     || self.is_tainted_by_errors()
2320                 {
2321                     return;
2322                 }
2323
2324                 self.emit_inference_failure_err(body_id, span, arg, ErrorCode::E0282, false)
2325             }
2326
2327             ty::PredicateKind::Subtype(data) => {
2328                 if data.references_error()
2329                     || self.tcx.sess.has_errors().is_some()
2330                     || self.is_tainted_by_errors()
2331                 {
2332                     // no need to overload user in such cases
2333                     return;
2334                 }
2335                 let SubtypePredicate { a_is_expected: _, a, b } = data;
2336                 // both must be type variables, or the other would've been instantiated
2337                 assert!(a.is_ty_var() && b.is_ty_var());
2338                 self.emit_inference_failure_err(body_id, span, a.into(), ErrorCode::E0282, true)
2339             }
2340             ty::PredicateKind::Projection(data) => {
2341                 if predicate.references_error() || self.is_tainted_by_errors() {
2342                     return;
2343                 }
2344                 let subst = data
2345                     .projection_ty
2346                     .substs
2347                     .iter()
2348                     .chain(Some(data.term.into_arg()))
2349                     .find(|g| g.has_non_region_infer());
2350                 if let Some(subst) = subst {
2351                     let mut err = self.emit_inference_failure_err(
2352                         body_id,
2353                         span,
2354                         subst,
2355                         ErrorCode::E0284,
2356                         true,
2357                     );
2358                     err.note(&format!("cannot satisfy `{}`", predicate));
2359                     err
2360                 } else {
2361                     // If we can't find a substitution, just print a generic error
2362                     let mut err = struct_span_err!(
2363                         self.tcx.sess,
2364                         span,
2365                         E0284,
2366                         "type annotations needed: cannot satisfy `{}`",
2367                         predicate,
2368                     );
2369                     err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2370                     err
2371                 }
2372             }
2373
2374             ty::PredicateKind::ConstEvaluatable(data) => {
2375                 if predicate.references_error() || self.is_tainted_by_errors() {
2376                     return;
2377                 }
2378                 let subst = data.walk().find(|g| g.is_non_region_infer());
2379                 if let Some(subst) = subst {
2380                     let err = self.emit_inference_failure_err(
2381                         body_id,
2382                         span,
2383                         subst,
2384                         ErrorCode::E0284,
2385                         true,
2386                     );
2387                     err
2388                 } else {
2389                     // If we can't find a substitution, just print a generic error
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             _ => {
2402                 if self.tcx.sess.has_errors().is_some() || self.is_tainted_by_errors() {
2403                     return;
2404                 }
2405                 let mut err = struct_span_err!(
2406                     self.tcx.sess,
2407                     span,
2408                     E0284,
2409                     "type annotations needed: cannot satisfy `{}`",
2410                     predicate,
2411                 );
2412                 err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2413                 err
2414             }
2415         };
2416         self.note_obligation_cause(&mut err, obligation);
2417         err.emit();
2418     }
2419
2420     fn annotate_source_of_ambiguity(
2421         &self,
2422         err: &mut Diagnostic,
2423         impls: &[DefId],
2424         predicate: ty::Predicate<'tcx>,
2425     ) {
2426         let mut spans = vec![];
2427         let mut crates = vec![];
2428         let mut post = vec![];
2429         for def_id in impls {
2430             match self.tcx.span_of_impl(*def_id) {
2431                 Ok(span) => spans.push(span),
2432                 Err(name) => {
2433                     crates.push(name);
2434                     if let Some(header) = to_pretty_impl_header(self.tcx, *def_id) {
2435                         post.push(header);
2436                     }
2437                 }
2438             }
2439         }
2440         let mut crate_names: Vec<_> = crates.iter().map(|n| format!("`{}`", n)).collect();
2441         crate_names.sort();
2442         crate_names.dedup();
2443         post.sort();
2444         post.dedup();
2445
2446         if self.is_tainted_by_errors()
2447             && (crate_names.len() == 1
2448                 && spans.len() == 0
2449                 && ["`core`", "`alloc`", "`std`"].contains(&crate_names[0].as_str())
2450                 || predicate.visit_with(&mut HasNumericInferVisitor).is_break())
2451         {
2452             // Avoid complaining about other inference issues for expressions like
2453             // `42 >> 1`, where the types are still `{integer}`, but we want to
2454             // Do we need `trait_ref.skip_binder().self_ty().is_numeric() &&` too?
2455             // NOTE(eddyb) this was `.cancel()`, but `err`
2456             // is borrowed, so we can't fully defuse it.
2457             err.downgrade_to_delayed_bug();
2458             return;
2459         }
2460
2461         let msg = format!("multiple `impl`s satisfying `{}` found", predicate);
2462         let post = if post.len() > 1 || (post.len() == 1 && post[0].contains('\n')) {
2463             format!(":\n{}", post.iter().map(|p| format!("- {}", p)).collect::<Vec<_>>().join("\n"),)
2464         } else if post.len() == 1 {
2465             format!(": `{}`", post[0])
2466         } else {
2467             String::new()
2468         };
2469
2470         match (spans.len(), crates.len(), crate_names.len()) {
2471             (0, 0, 0) => {
2472                 err.note(&format!("cannot satisfy `{}`", predicate));
2473             }
2474             (0, _, 1) => {
2475                 err.note(&format!("{} in the `{}` crate{}", msg, crates[0], post,));
2476             }
2477             (0, _, _) => {
2478                 err.note(&format!(
2479                     "{} in the following crates: {}{}",
2480                     msg,
2481                     crate_names.join(", "),
2482                     post,
2483                 ));
2484             }
2485             (_, 0, 0) => {
2486                 let span: MultiSpan = spans.into();
2487                 err.span_note(span, &msg);
2488             }
2489             (_, 1, 1) => {
2490                 let span: MultiSpan = spans.into();
2491                 err.span_note(span, &msg);
2492                 err.note(
2493                     &format!("and another `impl` found in the `{}` crate{}", crates[0], post,),
2494                 );
2495             }
2496             _ => {
2497                 let span: MultiSpan = spans.into();
2498                 err.span_note(span, &msg);
2499                 err.note(&format!(
2500                     "and more `impl`s found in the following crates: {}{}",
2501                     crate_names.join(", "),
2502                     post,
2503                 ));
2504             }
2505         }
2506     }
2507
2508     /// Returns `true` if the trait predicate may apply for *some* assignment
2509     /// to the type parameters.
2510     fn predicate_can_apply(
2511         &self,
2512         param_env: ty::ParamEnv<'tcx>,
2513         pred: ty::PolyTraitPredicate<'tcx>,
2514     ) -> bool {
2515         struct ParamToVarFolder<'a, 'tcx> {
2516             infcx: &'a InferCtxt<'tcx>,
2517             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
2518         }
2519
2520         impl<'a, 'tcx> TypeFolder<'tcx> for ParamToVarFolder<'a, 'tcx> {
2521             fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
2522                 self.infcx.tcx
2523             }
2524
2525             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
2526                 if let ty::Param(ty::ParamTy { name, .. }) = *ty.kind() {
2527                     let infcx = self.infcx;
2528                     *self.var_map.entry(ty).or_insert_with(|| {
2529                         infcx.next_ty_var(TypeVariableOrigin {
2530                             kind: TypeVariableOriginKind::TypeParameterDefinition(name, None),
2531                             span: DUMMY_SP,
2532                         })
2533                     })
2534                 } else {
2535                     ty.super_fold_with(self)
2536                 }
2537             }
2538         }
2539
2540         self.probe(|_| {
2541             let mut selcx = SelectionContext::new(self);
2542
2543             let cleaned_pred =
2544                 pred.fold_with(&mut ParamToVarFolder { infcx: self, var_map: Default::default() });
2545
2546             let cleaned_pred = super::project::normalize(
2547                 &mut selcx,
2548                 param_env,
2549                 ObligationCause::dummy(),
2550                 cleaned_pred,
2551             )
2552             .value;
2553
2554             let obligation =
2555                 Obligation::new(self.tcx, ObligationCause::dummy(), param_env, cleaned_pred);
2556
2557             self.predicate_may_hold(&obligation)
2558         })
2559     }
2560
2561     fn note_obligation_cause(&self, err: &mut Diagnostic, obligation: &PredicateObligation<'tcx>) {
2562         // First, attempt to add note to this error with an async-await-specific
2563         // message, and fall back to regular note otherwise.
2564         if !self.maybe_note_obligation_cause_for_async_await(err, obligation) {
2565             self.note_obligation_cause_code(
2566                 err,
2567                 &obligation.predicate,
2568                 obligation.param_env,
2569                 obligation.cause.code(),
2570                 &mut vec![],
2571                 &mut Default::default(),
2572             );
2573             self.suggest_unsized_bound_if_applicable(err, obligation);
2574         }
2575     }
2576
2577     #[instrument(level = "debug", skip_all)]
2578     fn suggest_unsized_bound_if_applicable(
2579         &self,
2580         err: &mut Diagnostic,
2581         obligation: &PredicateObligation<'tcx>,
2582     ) {
2583         let ty::PredicateKind::Trait(pred) = obligation.predicate.kind().skip_binder() else { return; };
2584         let (ObligationCauseCode::BindingObligation(item_def_id, span)
2585         | ObligationCauseCode::ExprBindingObligation(item_def_id, span, ..))
2586             = *obligation.cause.code().peel_derives() else { return; };
2587         debug!(?pred, ?item_def_id, ?span);
2588
2589         let (Some(node), true) = (
2590             self.tcx.hir().get_if_local(item_def_id),
2591             Some(pred.def_id()) == self.tcx.lang_items().sized_trait(),
2592         ) else {
2593             return;
2594         };
2595         self.maybe_suggest_unsized_generics(err, span, node);
2596     }
2597
2598     #[instrument(level = "debug", skip_all)]
2599     fn maybe_suggest_unsized_generics(&self, err: &mut Diagnostic, span: Span, node: Node<'tcx>) {
2600         let Some(generics) = node.generics() else {
2601             return;
2602         };
2603         let sized_trait = self.tcx.lang_items().sized_trait();
2604         debug!(?generics.params);
2605         debug!(?generics.predicates);
2606         let Some(param) = generics.params.iter().find(|param| param.span == span) else {
2607             return;
2608         };
2609         // Check that none of the explicit trait bounds is `Sized`. Assume that an explicit
2610         // `Sized` bound is there intentionally and we don't need to suggest relaxing it.
2611         let explicitly_sized = generics
2612             .bounds_for_param(param.def_id)
2613             .flat_map(|bp| bp.bounds)
2614             .any(|bound| bound.trait_ref().and_then(|tr| tr.trait_def_id()) == sized_trait);
2615         if explicitly_sized {
2616             return;
2617         }
2618         debug!(?param);
2619         match node {
2620             hir::Node::Item(
2621                 item @ hir::Item {
2622                     // Only suggest indirection for uses of type parameters in ADTs.
2623                     kind:
2624                         hir::ItemKind::Enum(..) | hir::ItemKind::Struct(..) | hir::ItemKind::Union(..),
2625                     ..
2626                 },
2627             ) => {
2628                 if self.maybe_indirection_for_unsized(err, item, param) {
2629                     return;
2630                 }
2631             }
2632             _ => {}
2633         };
2634         // Didn't add an indirection suggestion, so add a general suggestion to relax `Sized`.
2635         let (span, separator) = if let Some(s) = generics.bounds_span_for_suggestions(param.def_id)
2636         {
2637             (s, " +")
2638         } else {
2639             (span.shrink_to_hi(), ":")
2640         };
2641         err.span_suggestion_verbose(
2642             span,
2643             "consider relaxing the implicit `Sized` restriction",
2644             format!("{} ?Sized", separator),
2645             Applicability::MachineApplicable,
2646         );
2647     }
2648
2649     fn maybe_indirection_for_unsized(
2650         &self,
2651         err: &mut Diagnostic,
2652         item: &Item<'tcx>,
2653         param: &GenericParam<'tcx>,
2654     ) -> bool {
2655         // Suggesting `T: ?Sized` is only valid in an ADT if `T` is only used in a
2656         // borrow. `struct S<'a, T: ?Sized>(&'a T);` is valid, `struct S<T: ?Sized>(T);`
2657         // is not. Look for invalid "bare" parameter uses, and suggest using indirection.
2658         let mut visitor =
2659             FindTypeParam { param: param.name.ident().name, invalid_spans: vec![], nested: false };
2660         visitor.visit_item(item);
2661         if visitor.invalid_spans.is_empty() {
2662             return false;
2663         }
2664         let mut multispan: MultiSpan = param.span.into();
2665         multispan.push_span_label(
2666             param.span,
2667             format!("this could be changed to `{}: ?Sized`...", param.name.ident()),
2668         );
2669         for sp in visitor.invalid_spans {
2670             multispan.push_span_label(
2671                 sp,
2672                 format!("...if indirection were used here: `Box<{}>`", param.name.ident()),
2673             );
2674         }
2675         err.span_help(
2676             multispan,
2677             &format!(
2678                 "you could relax the implicit `Sized` bound on `{T}` if it were \
2679                 used through indirection like `&{T}` or `Box<{T}>`",
2680                 T = param.name.ident(),
2681             ),
2682         );
2683         true
2684     }
2685
2686     fn is_recursive_obligation(
2687         &self,
2688         obligated_types: &mut Vec<Ty<'tcx>>,
2689         cause_code: &ObligationCauseCode<'tcx>,
2690     ) -> bool {
2691         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
2692             let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
2693             let self_ty = parent_trait_ref.skip_binder().self_ty();
2694             if obligated_types.iter().any(|ot| ot == &self_ty) {
2695                 return true;
2696             }
2697             if let ty::Adt(def, substs) = self_ty.kind()
2698                 && let [arg] = &substs[..]
2699                 && let ty::subst::GenericArgKind::Type(ty) = arg.unpack()
2700                 && let ty::Adt(inner_def, _) = ty.kind()
2701                 && inner_def == def
2702             {
2703                 return true;
2704             }
2705         }
2706         false
2707     }
2708 }
2709
2710 /// Look for type `param` in an ADT being used only through a reference to confirm that suggesting
2711 /// `param: ?Sized` would be a valid constraint.
2712 struct FindTypeParam {
2713     param: rustc_span::Symbol,
2714     invalid_spans: Vec<Span>,
2715     nested: bool,
2716 }
2717
2718 impl<'v> Visitor<'v> for FindTypeParam {
2719     fn visit_where_predicate(&mut self, _: &'v hir::WherePredicate<'v>) {
2720         // Skip where-clauses, to avoid suggesting indirection for type parameters found there.
2721     }
2722
2723     fn visit_ty(&mut self, ty: &hir::Ty<'_>) {
2724         // We collect the spans of all uses of the "bare" type param, like in `field: T` or
2725         // `field: (T, T)` where we could make `T: ?Sized` while skipping cases that are known to be
2726         // valid like `field: &'a T` or `field: *mut T` and cases that *might* have further `Sized`
2727         // obligations like `Box<T>` and `Vec<T>`, but we perform no extra analysis for those cases
2728         // and suggest `T: ?Sized` regardless of their obligations. This is fine because the errors
2729         // in that case should make what happened clear enough.
2730         match ty.kind {
2731             hir::TyKind::Ptr(_) | hir::TyKind::Rptr(..) | hir::TyKind::TraitObject(..) => {}
2732             hir::TyKind::Path(hir::QPath::Resolved(None, path))
2733                 if path.segments.len() == 1 && path.segments[0].ident.name == self.param =>
2734             {
2735                 if !self.nested {
2736                     debug!(?ty, "FindTypeParam::visit_ty");
2737                     self.invalid_spans.push(ty.span);
2738                 }
2739             }
2740             hir::TyKind::Path(_) => {
2741                 let prev = self.nested;
2742                 self.nested = true;
2743                 hir::intravisit::walk_ty(self, ty);
2744                 self.nested = prev;
2745             }
2746             _ => {
2747                 hir::intravisit::walk_ty(self, ty);
2748             }
2749         }
2750     }
2751 }
2752
2753 /// Summarizes information
2754 #[derive(Clone)]
2755 pub enum ArgKind {
2756     /// An argument of non-tuple type. Parameters are (name, ty)
2757     Arg(String, String),
2758
2759     /// An argument of tuple type. For a "found" argument, the span is
2760     /// the location in the source of the pattern. For an "expected"
2761     /// argument, it will be None. The vector is a list of (name, ty)
2762     /// strings for the components of the tuple.
2763     Tuple(Option<Span>, Vec<(String, String)>),
2764 }
2765
2766 impl ArgKind {
2767     fn empty() -> ArgKind {
2768         ArgKind::Arg("_".to_owned(), "_".to_owned())
2769     }
2770
2771     /// Creates an `ArgKind` from the expected type of an
2772     /// argument. It has no name (`_`) and an optional source span.
2773     pub fn from_expected_ty(t: Ty<'_>, span: Option<Span>) -> ArgKind {
2774         match t.kind() {
2775             ty::Tuple(tys) => ArgKind::Tuple(
2776                 span,
2777                 tys.iter().map(|ty| ("_".to_owned(), ty.to_string())).collect::<Vec<_>>(),
2778             ),
2779             _ => ArgKind::Arg("_".to_owned(), t.to_string()),
2780         }
2781     }
2782 }
2783
2784 struct HasNumericInferVisitor;
2785
2786 impl<'tcx> ty::TypeVisitor<'tcx> for HasNumericInferVisitor {
2787     type BreakTy = ();
2788
2789     fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
2790         if matches!(ty.kind(), ty::Infer(ty::FloatVar(_) | ty::IntVar(_))) {
2791             ControlFlow::Break(())
2792         } else {
2793             ControlFlow::CONTINUE
2794         }
2795     }
2796 }
2797
2798 pub enum DefIdOrName {
2799     DefId(DefId),
2800     Name(&'static str),
2801 }