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         let tcx = self.tcx;
 557         let mut span = obligation.cause.span;
 558         // FIXME: statically guarantee this by tainting after the diagnostic is emitted
 559         self.set_tainted_by_errors(
 560             tcx.sess.delay_span_bug(span, "`report_selection_error` did not emit an error"),
 561         );
 562
 563         let mut err = match *error {
 564             SelectionError::Unimplemented => {
 565                 // If this obligation was generated as a result of well-formedness checking, see if we
 566                 // can get a better error message by performing HIR-based well-formedness checking.
 567                 if let ObligationCauseCode::WellFormed(Some(wf_loc)) =
 568                     root_obligation.cause.code().peel_derives()
 569                 {
 570                     if let Some(cause) = self
 571                         .tcx
 572                         .diagnostic_hir_wf_check((tcx.erase_regions(obligation.predicate), *wf_loc))
 573                     {
 574                         obligation.cause = cause.clone();
 575                         span = obligation.cause.span;
 576                     }
 577                 }
 578                 if let ObligationCauseCode::CompareImplItemObligation {
 579                     impl_item_def_id,
 580                     trait_item_def_id,
 581                     kind: _,
 582                 } = *obligation.cause.code()
 583                 {
 584                     self.report_extra_impl_obligation(
 585                         span,
 586                         impl_item_def_id,
 587                         trait_item_def_id,
 588                         &format!("`{}`", obligation.predicate),
 589                     )
 590                     .emit();
 591                     return;
 592                 }
 593
 594                 let bound_predicate = obligation.predicate.kind();
 595                 match bound_predicate.skip_binder() {
 596                     ty::PredicateKind::Trait(trait_predicate) => {
 597                         let trait_predicate = bound_predicate.rebind(trait_predicate);
 598                         let mut trait_predicate = self.resolve_vars_if_possible(trait_predicate);
 599
 600                         trait_predicate.remap_constness_diag(obligation.param_env);
 601                         let predicate_is_const = ty::BoundConstness::ConstIfConst
 602                             == trait_predicate.skip_binder().constness;
 603
 604                         if self.tcx.sess.has_errors().is_some()
 605                             && trait_predicate.references_error()
 606                         {
 607                             return;
 608                         }
 609                         let trait_ref = trait_predicate.to_poly_trait_ref();
 610                         let (post_message, pre_message, type_def) = self
 611                             .get_parent_trait_ref(obligation.cause.code())
 612                             .map(|(t, s)| {
 613                                 (
 614                                     format!(" in `{}`", t),
 615                                     format!("within `{}`, ", t),
 616                                     s.map(|s| (format!("within this `{}`", t), s)),
 617                                 )
 618                             })
 619                             .unwrap_or_default();
 620
 621                         let OnUnimplementedNote {
 622                             message,
 623                             label,
 624                             note,
 625                             parent_label,
 626                             append_const_msg,
 627                         } = self.on_unimplemented_note(trait_ref, &obligation);
 628                         let have_alt_message = message.is_some() || label.is_some();
 629                         let is_try_conversion = self.is_try_conversion(span, trait_ref.def_id());
 630                         let is_unsize =
 631                             Some(trait_ref.def_id()) == self.tcx.lang_items().unsize_trait();
 632                         let (message, note, append_const_msg) = if is_try_conversion {
 633                             (
 634                                 Some(format!(
 635                                     "`?` couldn't convert the error to `{}`",
 636                                     trait_ref.skip_binder().self_ty(),
 637                                 )),
 638                                 Some(
 639                                     "the question mark operation (`?`) implicitly performs a \
 640                                      conversion on the error value using the `From` trait"
 641                                         .to_owned(),
 642                                 ),
 643                                 Some(None),
 644                             )
 645                         } else {
 646                             (message, note, append_const_msg)
 647                         };
 648
 649                         let mut err = struct_span_err!(
 650                             self.tcx.sess,
 651                             span,
 652                             E0277,
 653                             "{}",
 654                             message
 655                                 .and_then(|cannot_do_this| {
 656                                     match (predicate_is_const, append_const_msg) {
 657                                         // do nothing if predicate is not const
 658                                         (false, _) => Some(cannot_do_this),
 659                                         // suggested using default post message
 660                                         (true, Some(None)) => {
 661                                             Some(format!("{cannot_do_this} in const contexts"))
 662                                         }
 663                                         // overridden post message
 664                                         (true, Some(Some(post_message))) => {
 665                                             Some(format!("{cannot_do_this}{post_message}"))
 666                                         }
 667                                         // fallback to generic message
 668                                         (true, None) => None,
 669                                     }
 670                                 })
 671                                 .unwrap_or_else(|| format!(
 672                                     "the trait bound `{}` is not satisfied{}",
 673                                     trait_predicate, post_message,
 674                                 ))
 675                         );
 676
 677                         if is_try_conversion && let Some(ret_span) = self.return_type_span(&obligation) {
 678                             err.span_label(
 679                                 ret_span,
 680                                 &format!(
 681                                     "expected `{}` because of this",
 682                                     trait_ref.skip_binder().self_ty()
 683                                 ),
 684                             );
 685                         }
 686
 687                         if Some(trait_ref.def_id()) == tcx.lang_items().tuple_trait() {
 688                             match obligation.cause.code().peel_derives() {
 689                                 ObligationCauseCode::RustCall => {
 690                                     err.set_primary_message("functions with the \"rust-call\" ABI must take a single non-self tuple argument");
 691                                 }
 692                                 ObligationCauseCode::BindingObligation(def_id, _)
 693                                 | ObligationCauseCode::ItemObligation(def_id)
 694                                     if ty::ClosureKind::from_def_id(tcx, *def_id).is_some() =>
 695                                 {
 696                                     err.code(rustc_errors::error_code!(E0059));
 697                                     err.set_primary_message(format!(
 698                                         "type parameter to bare `{}` trait must be a tuple",
 699                                         tcx.def_path_str(*def_id)
 700                                     ));
 701                                 }
 702                                 _ => {}
 703                             }
 704                         }
 705
 706                         if Some(trait_ref.def_id()) == tcx.lang_items().drop_trait()
 707                             && predicate_is_const
 708                         {
 709                             err.note("`~const Drop` was renamed to `~const Destruct`");
 710                             err.note("See <https://github.com/rust-lang/rust/pull/94901> for more details");
 711                         }
 712
 713                         let explanation = if let ObligationCauseCode::MainFunctionType =
 714                             obligation.cause.code()
 715                         {
 716                             "consider using `()`, or a `Result`".to_owned()
 717                         } else {
 718                             let ty_desc = match trait_ref.skip_binder().self_ty().kind() {
 719                                 ty::FnDef(_, _) => Some("fn item"),
 720                                 ty::Closure(_, _) => Some("closure"),
 721                                 _ => None,
 722                             };
 723
 724                             match ty_desc {
 725                                 Some(desc) => format!(
 726                                     "{}the trait `{}` is not implemented for {} `{}`",
 727                                     pre_message,
 728                                     trait_predicate.print_modifiers_and_trait_path(),
 729                                     desc,
 730                                     trait_ref.skip_binder().self_ty(),
 731                                 ),
 732                                 None => format!(
 733                                     "{}the trait `{}` is not implemented for `{}`",
 734                                     pre_message,
 735                                     trait_predicate.print_modifiers_and_trait_path(),
 736                                     trait_ref.skip_binder().self_ty(),
 737                                 ),
 738                             }
 739                         };
 740
 741                         if self.suggest_add_reference_to_arg(
 742                             &obligation,
 743                             &mut err,
 744                             trait_predicate,
 745                             have_alt_message,
 746                         ) {
 747                             self.note_obligation_cause(&mut err, &obligation);
 748                             err.emit();
 749                             return;
 750                         }
 751                         if let Some(ref s) = label {
 752                             // If it has a custom `#[rustc_on_unimplemented]`
 753                             // error message, let's display it as the label!
 754                             err.span_label(span, s);
 755                             if !matches!(trait_ref.skip_binder().self_ty().kind(), ty::Param(_)) {
 756                                 // When the self type is a type param We don't need to "the trait
 757                                 // `std::marker::Sized` is not implemented for `T`" as we will point
 758                                 // at the type param with a label to suggest constraining it.
 759                                 err.help(&explanation);
 760                             }
 761                         } else {
 762                             err.span_label(span, explanation);
 763                         }
 764
 765                         if let ObligationCauseCode::ObjectCastObligation(concrete_ty, obj_ty) = obligation.cause.code().peel_derives() &&
 766                             Some(trait_ref.def_id()) == self.tcx.lang_items().sized_trait() {
 767                             self.suggest_borrowing_for_object_cast(&mut err, &root_obligation, *concrete_ty, *obj_ty);
 768                         }
 769
 770                         let mut unsatisfied_const = false;
 771                         if trait_predicate.is_const_if_const() && obligation.param_env.is_const() {
 772                             let non_const_predicate = trait_ref.without_const();
 773                             let non_const_obligation = Obligation {
 774                                 cause: obligation.cause.clone(),
 775                                 param_env: obligation.param_env.without_const(),
 776                                 predicate: non_const_predicate.to_predicate(tcx),
 777                                 recursion_depth: obligation.recursion_depth,
 778                             };
 779                             if self.predicate_may_hold(&non_const_obligation) {
 780                                 unsatisfied_const = true;
 781                                 err.span_note(
 782                                     span,
 783                                     &format!(
 784                                         "the trait `{}` is implemented for `{}`, \
 785                                         but that implementation is not `const`",
 786                                         non_const_predicate.print_modifiers_and_trait_path(),
 787                                         trait_ref.skip_binder().self_ty(),
 788                                     ),
 789                                 );
 790                             }
 791                         }
 792
 793                         if let Some((msg, span)) = type_def {
 794                             err.span_label(span, &msg);
 795                         }
 796                         if let Some(ref s) = note {
 797                             // If it has a custom `#[rustc_on_unimplemented]` note, let's display it
 798                             err.note(s.as_str());
 799                         }
 800                         if let Some(ref s) = parent_label {
 801                             let body = tcx
 802                                 .hir()
 803                                 .opt_local_def_id(obligation.cause.body_id)
 804                                 .unwrap_or_else(|| {
 805                                     tcx.hir().body_owner_def_id(hir::BodyId {
 806                                         hir_id: obligation.cause.body_id,
 807                                     })
 808                                 });
 809                             err.span_label(tcx.def_span(body), s);
 810                         }
 811
 812                         self.suggest_floating_point_literal(&obligation, &mut err, &trait_ref);
 813                         self.suggest_dereferencing_index(&obligation, &mut err, trait_predicate);
 814                         let mut suggested =
 815                             self.suggest_dereferences(&obligation, &mut err, trait_predicate);
 816                         suggested |= self.suggest_fn_call(&obligation, &mut err, trait_predicate);
 817                         suggested |=
 818                             self.suggest_remove_reference(&obligation, &mut err, trait_predicate);
 819                         suggested |= self.suggest_semicolon_removal(
 820                             &obligation,
 821                             &mut err,
 822                             span,
 823                             trait_predicate,
 824                         );
 825                         self.note_version_mismatch(&mut err, &trait_ref);
 826                         self.suggest_remove_await(&obligation, &mut err);
 827                         self.suggest_derive(&obligation, &mut err, trait_predicate);
 828
 829                         if Some(trait_ref.def_id()) == tcx.lang_items().try_trait() {
 830                             self.suggest_await_before_try(
 831                                 &mut err,
 832                                 &obligation,
 833                                 trait_predicate,
 834                                 span,
 835                             );
 836                         }
 837
 838                         if self.suggest_impl_trait(&mut err, span, &obligation, trait_predicate) {
 839                             err.emit();
 840                             return;
 841                         }
 842
 843                         if is_unsize {
 844                             // If the obligation failed due to a missing implementation of the
 845                             // `Unsize` trait, give a pointer to why that might be the case
 846                             err.note(
 847                                 "all implementations of `Unsize` are provided \
 848                                 automatically by the compiler, see \
 849                                 <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> \
 850                                 for more information",
 851                             );
 852                         }
 853
 854                         let is_fn_trait =
 855                             ty::ClosureKind::from_def_id(tcx, trait_ref.def_id()).is_some();
 856                         let is_target_feature_fn = if let ty::FnDef(def_id, _) =
 857                             *trait_ref.skip_binder().self_ty().kind()
 858                         {
 859                             !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
 860                         } else {
 861                             false
 862                         };
 863                         if is_fn_trait && is_target_feature_fn {
 864                             err.note(
 865                                 "`#[target_feature]` functions do not implement the `Fn` traits",
 866                             );
 867                         }
 868
 869                         // Try to report a help message
 870                         if is_fn_trait
 871                             && let Ok((implemented_kind, params)) = self.type_implements_fn_trait(
 872                             obligation.param_env,
 873                             trait_ref.self_ty(),
 874                             trait_predicate.skip_binder().constness,
 875                             trait_predicate.skip_binder().polarity,
 876                         )
 877                         {
 878                             // If the type implements `Fn`, `FnMut`, or `FnOnce`, suppress the following
 879                             // suggestion to add trait bounds for the type, since we only typically implement
 880                             // these traits once.
 881
 882                             // Note if the `FnMut` or `FnOnce` is less general than the trait we're trying
 883                             // to implement.
 884                             let selected_kind =
 885                                 ty::ClosureKind::from_def_id(self.tcx, trait_ref.def_id())
 886                                     .expect("expected to map DefId to ClosureKind");
 887                             if !implemented_kind.extends(selected_kind) {
 888                                 err.note(
 889                                     &format!(
 890                                         "`{}` implements `{}`, but it must implement `{}`, which is more general",
 891                                         trait_ref.skip_binder().self_ty(),
 892                                         implemented_kind,
 893                                         selected_kind
 894                                     )
 895                                 );
 896                             }
 897
 898                             // Note any argument mismatches
 899                             let given_ty = params.skip_binder();
 900                             let expected_ty = trait_ref.skip_binder().substs.type_at(1);
 901                             if let ty::Tuple(given) = given_ty.kind()
 902                                 && let ty::Tuple(expected) = expected_ty.kind()
 903                             {
 904                                 if expected.len() != given.len() {
 905                                     // Note number of types that were expected and given
 906                                     err.note(
 907                                         &format!(
 908                                             "expected a closure taking {} argument{}, but one taking {} argument{} was given",
 909                                             given.len(),
 910                                             pluralize!(given.len()),
 911                                             expected.len(),
 912                                             pluralize!(expected.len()),
 913                                         )
 914                                     );
 915                                 } else if !self.same_type_modulo_infer(given_ty, expected_ty) {
 916                                     // Print type mismatch
 917                                     let (expected_args, given_args) =
 918                                         self.cmp(given_ty, expected_ty);
 919                                     err.note_expected_found(
 920                                         &"a closure with arguments",
 921                                         expected_args,
 922                                         &"a closure with arguments",
 923                                         given_args,
 924                                     );
 925                                 }
 926                             }
 927                         } else if !trait_ref.has_non_region_infer()
 928                             && self.predicate_can_apply(obligation.param_env, trait_predicate)
 929                         {
 930                             // If a where-clause may be useful, remind the
 931                             // user that they can add it.
 932                             //
 933                             // don't display an on-unimplemented note, as
 934                             // these notes will often be of the form
 935                             //     "the type `T` can't be frobnicated"
 936                             // which is somewhat confusing.
 937                             self.suggest_restricting_param_bound(
 938                                 &mut err,
 939                                 trait_predicate,
 940                                 None,
 941                                 obligation.cause.body_id,
 942                             );
 943                         } else if !suggested && !unsatisfied_const {
 944                             // Can't show anything else useful, try to find similar impls.
 945                             let impl_candidates = self.find_similar_impl_candidates(trait_predicate);
 946                             if !self.report_similar_impl_candidates(
 947                                 impl_candidates,
 948                                 trait_ref,
 949                                 obligation.cause.body_id,
 950                                 &mut err,
 951                             ) {
 952                                 // This is *almost* equivalent to
 953                                 // `obligation.cause.code().peel_derives()`, but it gives us the
 954                                 // trait predicate for that corresponding root obligation. This
 955                                 // lets us get a derived obligation from a type parameter, like
 956                                 // when calling `string.strip_suffix(p)` where `p` is *not* an
 957                                 // implementer of `Pattern<'_>`.
 958                                 let mut code = obligation.cause.code();
 959                                 let mut trait_pred = trait_predicate;
 960                                 let mut peeled = false;
 961                                 while let Some((parent_code, parent_trait_pred)) = code.parent() {
 962                                     code = parent_code;
 963                                     if let Some(parent_trait_pred) = parent_trait_pred {
 964                                         trait_pred = parent_trait_pred;
 965                                         peeled = true;
 966                                     }
 967                                 }
 968                                 let def_id = trait_pred.def_id();
 969                                 // Mention *all* the `impl`s for the *top most* obligation, the
 970                                 // user might have meant to use one of them, if any found. We skip
 971                                 // auto-traits or fundamental traits that might not be exactly what
 972                                 // the user might expect to be presented with. Instead this is
 973                                 // useful for less general traits.
 974                                 if peeled
 975                                     && !self.tcx.trait_is_auto(def_id)
 976                                     && !self.tcx.lang_items().iter().any(|(_, id)| id == def_id)
 977                                 {
 978                                     let trait_ref = trait_pred.to_poly_trait_ref();
 979                                     let impl_candidates =
 980                                         self.find_similar_impl_candidates(trait_pred);
 981                                     self.report_similar_impl_candidates(
 982                                         impl_candidates,
 983                                         trait_ref,
 984                                         obligation.cause.body_id,
 985                                         &mut err,
 986                                     );
 987                                 }
 988                             }
 989                         }
 990
 991                         // Changing mutability doesn't make a difference to whether we have
 992                         // an `Unsize` impl (Fixes ICE in #71036)
 993                         if !is_unsize {
 994                             self.suggest_change_mut(&obligation, &mut err, trait_predicate);
 995                         }
 996
 997                         // If this error is due to `!: Trait` not implemented but `(): Trait` is
 998                         // implemented, and fallback has occurred, then it could be due to a
 999                         // variable that used to fallback to `()` now falling back to `!`. Issue a
1000                         // note informing about the change in behaviour.
1001                         if trait_predicate.skip_binder().self_ty().is_never()
1002                             && self.fallback_has_occurred
1003                         {
1004                             let predicate = trait_predicate.map_bound(|mut trait_pred| {
1005                                 trait_pred.trait_ref.substs = self.tcx.mk_substs_trait(
1006                                     self.tcx.mk_unit(),
1007                                     &trait_pred.trait_ref.substs[1..],
1008                                 );
1009                                 trait_pred
1010                             });
1011                             let unit_obligation = obligation.with(tcx, predicate);
1012                             if self.predicate_may_hold(&unit_obligation) {
1013                                 err.note(
1014                                     "this error might have been caused by changes to \
1015                                     Rust's type-inference algorithm (see issue #48950 \
1016                                     <https://github.com/rust-lang/rust/issues/48950> \
1017                                     for more information)",
1018                                 );
1019                                 err.help("did you intend to use the type `()` here instead?");
1020                             }
1021                         }
1022
1023                         // Return early if the trait is Debug or Display and the invocation
1024                         // originates within a standard library macro, because the output
1025                         // is otherwise overwhelming and unhelpful (see #85844 for an
1026                         // example).
1027
1028                         let in_std_macro =
1029                             match obligation.cause.span.ctxt().outer_expn_data().macro_def_id {
1030                                 Some(macro_def_id) => {
1031                                     let crate_name = tcx.crate_name(macro_def_id.krate);
1032                                     crate_name == sym::std || crate_name == sym::core
1033                                 }
1034                                 None => false,
1035                             };
1036
1037                         if in_std_macro
1038                             && matches!(
1039                                 self.tcx.get_diagnostic_name(trait_ref.def_id()),
1040                                 Some(sym::Debug | sym::Display)
1041                             )
1042                         {
1043                             err.emit();
1044                             return;
1045                         }
1046
1047                         err
1048                     }
1049
1050                     ty::PredicateKind::Subtype(predicate) => {
1051                         // Errors for Subtype predicates show up as
1052                         // `FulfillmentErrorCode::CodeSubtypeError`,
1053                         // not selection error.
1054                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
1055                     }
1056
1057                     ty::PredicateKind::Coerce(predicate) => {
1058                         // Errors for Coerce predicates show up as
1059                         // `FulfillmentErrorCode::CodeSubtypeError`,
1060                         // not selection error.
1061                         span_bug!(span, "coerce requirement gave wrong error: `{:?}`", predicate)
1062                     }
1063
1064                     ty::PredicateKind::RegionOutlives(..)
1065                     | ty::PredicateKind::Projection(..)
1066                     | ty::PredicateKind::TypeOutlives(..) => {
1067                         let predicate = self.resolve_vars_if_possible(obligation.predicate);
1068                         struct_span_err!(
1069                             self.tcx.sess,
1070                             span,
1071                             E0280,
1072                             "the requirement `{}` is not satisfied",
1073                             predicate
1074                         )
1075                     }
1076
1077                     ty::PredicateKind::ObjectSafe(trait_def_id) => {
1078                         let violations = self.tcx.object_safety_violations(trait_def_id);
1079                         report_object_safety_error(self.tcx, span, trait_def_id, violations)
1080                     }
1081
1082                     ty::PredicateKind::ClosureKind(closure_def_id, closure_substs, kind) => {
1083                         let found_kind = self.closure_kind(closure_substs).unwrap();
1084                         let closure_span = self.tcx.def_span(closure_def_id);
1085                         let mut err = struct_span_err!(
1086                             self.tcx.sess,
1087                             closure_span,
1088                             E0525,
1089                             "expected a closure that implements the `{}` trait, \
1090                              but this closure only implements `{}`",
1091                             kind,
1092                             found_kind
1093                         );
1094
1095                         err.span_label(
1096                             closure_span,
1097                             format!("this closure implements `{}`, not `{}`", found_kind, kind),
1098                         );
1099                         err.span_label(
1100                             obligation.cause.span,
1101                             format!("the requirement to implement `{}` derives from here", kind),
1102                         );
1103
1104                         // Additional context information explaining why the closure only implements
1105                         // a particular trait.
1106                         if let Some(typeck_results) = &self.typeck_results {
1107                             let hir_id = self
1108                                 .tcx
1109                                 .hir()
1110                                 .local_def_id_to_hir_id(closure_def_id.expect_local());
1111                             match (found_kind, typeck_results.closure_kind_origins().get(hir_id)) {
1112                                 (ty::ClosureKind::FnOnce, Some((span, place))) => {
1113                                     err.span_label(
1114                                         *span,
1115                                         format!(
1116                                             "closure is `FnOnce` because it moves the \
1117                                          variable `{}` out of its environment",
1118                                             ty::place_to_string_for_capture(tcx, place)
1119                                         ),
1120                                     );
1121                                 }
1122                                 (ty::ClosureKind::FnMut, Some((span, place))) => {
1123                                     err.span_label(
1124                                         *span,
1125                                         format!(
1126                                             "closure is `FnMut` because it mutates the \
1127                                          variable `{}` here",
1128                                             ty::place_to_string_for_capture(tcx, place)
1129                                         ),
1130                                     );
1131                                 }
1132                                 _ => {}
1133                             }
1134                         }
1135
1136                         err
1137                     }
1138
1139                     ty::PredicateKind::WellFormed(ty) => {
1140                         if !self.tcx.sess.opts.unstable_opts.chalk {
1141                             // WF predicates cannot themselves make
1142                             // errors. They can only block due to
1143                             // ambiguity; otherwise, they always
1144                             // degenerate into other obligations
1145                             // (which may fail).
1146                             span_bug!(span, "WF predicate not satisfied for {:?}", ty);
1147                         } else {
1148                             // FIXME: we'll need a better message which takes into account
1149                             // which bounds actually failed to hold.
1150                             self.tcx.sess.struct_span_err(
1151                                 span,
1152                                 &format!("the type `{}` is not well-formed (chalk)", ty),
1153                             )
1154                         }
1155                     }
1156
1157                     ty::PredicateKind::ConstEvaluatable(..) => {
1158                         // Errors for `ConstEvaluatable` predicates show up as
1159                         // `SelectionError::ConstEvalFailure`,
1160                         // not `Unimplemented`.
1161                         span_bug!(
1162                             span,
1163                             "const-evaluatable requirement gave wrong error: `{:?}`",
1164                             obligation
1165                         )
1166                     }
1167
1168                     ty::PredicateKind::ConstEquate(..) => {
1169                         // Errors for `ConstEquate` predicates show up as
1170                         // `SelectionError::ConstEvalFailure`,
1171                         // not `Unimplemented`.
1172                         span_bug!(
1173                             span,
1174                             "const-equate requirement gave wrong error: `{:?}`",
1175                             obligation
1176                         )
1177                     }
1178
1179                     ty::PredicateKind::TypeWellFormedFromEnv(..) => span_bug!(
1180                         span,
1181                         "TypeWellFormedFromEnv predicate should only exist in the environment"
1182                     ),
1183                 }
1184             }
1185
1186             OutputTypeParameterMismatch(found_trait_ref, expected_trait_ref, _) => {
1187                 let found_trait_ref = self.resolve_vars_if_possible(found_trait_ref);
1188                 let expected_trait_ref = self.resolve_vars_if_possible(expected_trait_ref);
1189
1190                 if expected_trait_ref.self_ty().references_error() {
1191                     return;
1192                 }
1193
1194                 let Some(found_trait_ty) = found_trait_ref.self_ty().no_bound_vars() else {
1195                     return;
1196                 };
1197
1198                 let found_did = match *found_trait_ty.kind() {
1199                     ty::Closure(did, _)
1200                     | ty::Foreign(did)
1201                     | ty::FnDef(did, _)
1202                     | ty::Generator(did, ..) => Some(did),
1203                     ty::Adt(def, _) => Some(def.did()),
1204                     _ => None,
1205                 };
1206
1207                 let found_span = found_did.and_then(|did| self.tcx.hir().span_if_local(did));
1208
1209                 if self.reported_closure_mismatch.borrow().contains(&(span, found_span)) {
1210                     // We check closures twice, with obligations flowing in different directions,
1211                     // but we want to complain about them only once.
1212                     return;
1213                 }
1214
1215                 self.reported_closure_mismatch.borrow_mut().insert((span, found_span));
1216
1217                 let mut not_tupled = false;
1218
1219                 let found = match found_trait_ref.skip_binder().substs.type_at(1).kind() {
1220                     ty::Tuple(ref tys) => vec![ArgKind::empty(); tys.len()],
1221                     _ => {
1222                         not_tupled = true;
1223                         vec![ArgKind::empty()]
1224                     }
1225                 };
1226
1227                 let expected_ty = expected_trait_ref.skip_binder().substs.type_at(1);
1228                 let expected = match expected_ty.kind() {
1229                     ty::Tuple(ref tys) => {
1230                         tys.iter().map(|t| ArgKind::from_expected_ty(t, Some(span))).collect()
1231                     }
1232                     _ => {
1233                         not_tupled = true;
1234                         vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())]
1235                     }
1236                 };
1237
1238                 // If this is a `Fn` family trait and either the expected or found
1239                 // is not tupled, then fall back to just a regular mismatch error.
1240                 // This shouldn't be common unless manually implementing one of the
1241                 // traits manually, but don't make it more confusing when it does
1242                 // happen.
1243                 if Some(expected_trait_ref.def_id()) != tcx.lang_items().gen_trait() && not_tupled {
1244                     self.report_and_explain_type_error(
1245                         TypeTrace::poly_trait_refs(
1246                             &obligation.cause,
1247                             true,
1248                             expected_trait_ref,
1249                             found_trait_ref,
1250                         ),
1251                         ty::error::TypeError::Mismatch,
1252                     )
1253                 } else if found.len() == expected.len() {
1254                     self.report_closure_arg_mismatch(
1255                         span,
1256                         found_span,
1257                         found_trait_ref,
1258                         expected_trait_ref,
1259                         obligation.cause.code(),
1260                     )
1261                 } else {
1262                     let (closure_span, found) = found_did
1263                         .and_then(|did| {
1264                             let node = self.tcx.hir().get_if_local(did)?;
1265                             let (found_span, found) = self.get_fn_like_arguments(node)?;
1266                             Some((Some(found_span), found))
1267                         })
1268                         .unwrap_or((found_span, found));
1269
1270                     self.report_arg_count_mismatch(
1271                         span,
1272                         closure_span,
1273                         expected,
1274                         found,
1275                         found_trait_ty.is_closure(),
1276                     )
1277                 }
1278             }
1279
1280             TraitNotObjectSafe(did) => {
1281                 let violations = self.tcx.object_safety_violations(did);
1282                 report_object_safety_error(self.tcx, span, did, violations)
1283             }
1284
1285             SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsInfer) => {
1286                 bug!(
1287                     "MentionsInfer should have been handled in `traits/fulfill.rs` or `traits/select/mod.rs`"
1288                 )
1289             }
1290             SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsParam) => {
1291                 if !self.tcx.features().generic_const_exprs {
1292                     let mut err = self.tcx.sess.struct_span_err(
1293                         span,
1294                         "constant expression depends on a generic parameter",
1295                     );
1296                     // FIXME(const_generics): we should suggest to the user how they can resolve this
1297                     // issue. However, this is currently not actually possible
1298                     // (see https://github.com/rust-lang/rust/issues/66962#issuecomment-575907083).
1299                     //
1300                     // Note that with `feature(generic_const_exprs)` this case should not
1301                     // be reachable.
1302                     err.note("this may fail depending on what value the parameter takes");
1303                     err.emit();
1304                     return;
1305                 }
1306
1307                 match obligation.predicate.kind().skip_binder() {
1308                     ty::PredicateKind::ConstEvaluatable(ct) => {
1309                         let ty::ConstKind::Unevaluated(uv) = ct.kind() else {
1310                             bug!("const evaluatable failed for non-unevaluated const `{ct:?}`");
1311                         };
1312                         let mut err =
1313                             self.tcx.sess.struct_span_err(span, "unconstrained generic constant");
1314                         let const_span = self.tcx.def_span(uv.def.did);
1315                         match self.tcx.sess.source_map().span_to_snippet(const_span) {
1316                             Ok(snippet) => err.help(&format!(
1317                                 "try adding a `where` bound using this expression: `where [(); {}]:`",
1318                                 snippet
1319                             )),
1320                             _ => err.help("consider adding a `where` bound using this expression"),
1321                         };
1322                         err
1323                     }
1324                     _ => {
1325                         span_bug!(
1326                             span,
1327                             "unexpected non-ConstEvaluatable predicate, this should not be reachable"
1328                         )
1329                     }
1330                 }
1331             }
1332
1333             // Already reported in the query.
1334             SelectionError::NotConstEvaluatable(NotConstEvaluatable::Error(_)) => {
1335                 // FIXME(eddyb) remove this once `ErrorGuaranteed` becomes a proof token.
1336                 self.tcx.sess.delay_span_bug(span, "`ErrorGuaranteed` without an error");
1337                 return;
1338             }
1339             // Already reported.
1340             Overflow(OverflowError::Error(_)) => {
1341                 self.tcx.sess.delay_span_bug(span, "`OverflowError` has been reported");
1342                 return;
1343             }
1344             Overflow(_) => {
1345                 bug!("overflow should be handled before the `report_selection_error` path");
1346             }
1347             SelectionError::ErrorReporting => {
1348                 bug!("ErrorReporting Overflow should not reach `report_selection_err` call")
1349             }
1350         };
1351
1352         self.note_obligation_cause(&mut err, &obligation);
1353         self.point_at_returns_when_relevant(&mut err, &obligation);
1354
1355         err.emit();
1356     }
1357 }
1358
1359 trait InferCtxtPrivExt<'tcx> {
1360     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1361     // `error` occurring implies that `cond` occurs.
1362     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool;
1363
1364     fn report_fulfillment_error(
1365         &self,
1366         error: &FulfillmentError<'tcx>,
1367         body_id: Option<hir::BodyId>,
1368     );
1369
1370     fn report_projection_error(
1371         &self,
1372         obligation: &PredicateObligation<'tcx>,
1373         error: &MismatchedProjectionTypes<'tcx>,
1374     );
1375
1376     fn maybe_detailed_projection_msg(
1377         &self,
1378         pred: ty::ProjectionPredicate<'tcx>,
1379         normalized_ty: ty::Term<'tcx>,
1380         expected_ty: ty::Term<'tcx>,
1381     ) -> Option<String>;
1382
1383     fn fuzzy_match_tys(
1384         &self,
1385         a: Ty<'tcx>,
1386         b: Ty<'tcx>,
1387         ignoring_lifetimes: bool,
1388     ) -> Option<CandidateSimilarity>;
1389
1390     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str>;
1391
1392     fn find_similar_impl_candidates(
1393         &self,
1394         trait_pred: ty::PolyTraitPredicate<'tcx>,
1395     ) -> Vec<ImplCandidate<'tcx>>;
1396
1397     fn report_similar_impl_candidates(
1398         &self,
1399         impl_candidates: Vec<ImplCandidate<'tcx>>,
1400         trait_ref: ty::PolyTraitRef<'tcx>,
1401         body_id: hir::HirId,
1402         err: &mut Diagnostic,
1403     ) -> bool;
1404
1405     /// Gets the parent trait chain start
1406     fn get_parent_trait_ref(
1407         &self,
1408         code: &ObligationCauseCode<'tcx>,
1409     ) -> Option<(String, Option<Span>)>;
1410
1411     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1412     /// with the same path as `trait_ref`, a help message about
1413     /// a probable version mismatch is added to `err`
1414     fn note_version_mismatch(
1415         &self,
1416         err: &mut Diagnostic,
1417         trait_ref: &ty::PolyTraitRef<'tcx>,
1418     ) -> bool;
1419
1420     /// Creates a `PredicateObligation` with `new_self_ty` replacing the existing type in the
1421     /// `trait_ref`.
1422     ///
1423     /// For this to work, `new_self_ty` must have no escaping bound variables.
1424     fn mk_trait_obligation_with_new_self_ty(
1425         &self,
1426         param_env: ty::ParamEnv<'tcx>,
1427         trait_ref_and_ty: ty::Binder<'tcx, (ty::TraitPredicate<'tcx>, Ty<'tcx>)>,
1428     ) -> PredicateObligation<'tcx>;
1429
1430     fn maybe_report_ambiguity(
1431         &self,
1432         obligation: &PredicateObligation<'tcx>,
1433         body_id: Option<hir::BodyId>,
1434     );
1435
1436     fn predicate_can_apply(
1437         &self,
1438         param_env: ty::ParamEnv<'tcx>,
1439         pred: ty::PolyTraitPredicate<'tcx>,
1440     ) -> bool;
1441
1442     fn note_obligation_cause(&self, err: &mut Diagnostic, obligation: &PredicateObligation<'tcx>);
1443
1444     fn suggest_unsized_bound_if_applicable(
1445         &self,
1446         err: &mut Diagnostic,
1447         obligation: &PredicateObligation<'tcx>,
1448     );
1449
1450     fn annotate_source_of_ambiguity(
1451         &self,
1452         err: &mut Diagnostic,
1453         impls: &[DefId],
1454         predicate: ty::Predicate<'tcx>,
1455     );
1456
1457     fn maybe_suggest_unsized_generics(&self, err: &mut Diagnostic, span: Span, node: Node<'tcx>);
1458
1459     fn maybe_indirection_for_unsized(
1460         &self,
1461         err: &mut Diagnostic,
1462         item: &'tcx Item<'tcx>,
1463         param: &'tcx GenericParam<'tcx>,
1464     ) -> bool;
1465
1466     fn is_recursive_obligation(
1467         &self,
1468         obligated_types: &mut Vec<Ty<'tcx>>,
1469         cause_code: &ObligationCauseCode<'tcx>,
1470     ) -> bool;
1471 }
1472
1473 impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
1474     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1475     // `error` occurring implies that `cond` occurs.
1476     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool {
1477         if cond == error {
1478             return true;
1479         }
1480
1481         // FIXME: It should be possible to deal with `ForAll` in a cleaner way.
1482         let bound_error = error.kind();
1483         let (cond, error) = match (cond.kind().skip_binder(), bound_error.skip_binder()) {
1484             (ty::PredicateKind::Trait(..), ty::PredicateKind::Trait(error)) => {
1485                 (cond, bound_error.rebind(error))
1486             }
1487             _ => {
1488                 // FIXME: make this work in other cases too.
1489                 return false;
1490             }
1491         };
1492
1493         for obligation in super::elaborate_predicates(self.tcx, std::iter::once(cond)) {
1494             let bound_predicate = obligation.predicate.kind();
1495             if let ty::PredicateKind::Trait(implication) = bound_predicate.skip_binder() {
1496                 let error = error.to_poly_trait_ref();
1497                 let implication = bound_predicate.rebind(implication.trait_ref);
1498                 // FIXME: I'm just not taking associated types at all here.
1499                 // Eventually I'll need to implement param-env-aware
1500                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
1501                 let param_env = ty::ParamEnv::empty();
1502                 if self.can_sub(param_env, error, implication).is_ok() {
1503                     debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
1504                     return true;
1505                 }
1506             }
1507         }
1508
1509         false
1510     }
1511
1512     #[instrument(skip(self), level = "debug")]
1513     fn report_fulfillment_error(
1514         &self,
1515         error: &FulfillmentError<'tcx>,
1516         body_id: Option<hir::BodyId>,
1517     ) {
1518         match error.code {
1519             FulfillmentErrorCode::CodeSelectionError(ref selection_error) => {
1520                 self.report_selection_error(
1521                     error.obligation.clone(),
1522                     &error.root_obligation,
1523                     selection_error,
1524                 );
1525             }
1526             FulfillmentErrorCode::CodeProjectionError(ref e) => {
1527                 self.report_projection_error(&error.obligation, e);
1528             }
1529             FulfillmentErrorCode::CodeAmbiguity => {
1530                 self.maybe_report_ambiguity(&error.obligation, body_id);
1531             }
1532             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
1533                 self.report_mismatched_types(
1534                     &error.obligation.cause,
1535                     expected_found.expected,
1536                     expected_found.found,
1537                     err.clone(),
1538                 )
1539                 .emit();
1540             }
1541             FulfillmentErrorCode::CodeConstEquateError(ref expected_found, ref err) => {
1542                 let mut diag = self.report_mismatched_consts(
1543                     &error.obligation.cause,
1544                     expected_found.expected,
1545                     expected_found.found,
1546                     err.clone(),
1547                 );
1548                 let code = error.obligation.cause.code().peel_derives().peel_match_impls();
1549                 if let ObligationCauseCode::BindingObligation(..)
1550                 | ObligationCauseCode::ItemObligation(..)
1551                 | ObligationCauseCode::ExprBindingObligation(..)
1552                 | ObligationCauseCode::ExprItemObligation(..) = code
1553                 {
1554                     self.note_obligation_cause_code(
1555                         &mut diag,
1556                         &error.obligation.predicate,
1557                         error.obligation.param_env,
1558                         code,
1559                         &mut vec![],
1560                         &mut Default::default(),
1561                     );
1562                 }
1563                 diag.emit();
1564             }
1565             FulfillmentErrorCode::CodeCycle(ref cycle) => {
1566                 self.report_overflow_error_cycle(cycle);
1567             }
1568         }
1569     }
1570
1571     #[instrument(level = "debug", skip_all)]
1572     fn report_projection_error(
1573         &self,
1574         obligation: &PredicateObligation<'tcx>,
1575         error: &MismatchedProjectionTypes<'tcx>,
1576     ) {
1577         let predicate = self.resolve_vars_if_possible(obligation.predicate);
1578
1579         if predicate.references_error() {
1580             return;
1581         }
1582
1583         self.probe(|_| {
1584             let mut err = error.err;
1585             let mut values = None;
1586
1587             // try to find the mismatched types to report the error with.
1588             //
1589             // this can fail if the problem was higher-ranked, in which
1590             // cause I have no idea for a good error message.
1591             let bound_predicate = predicate.kind();
1592             if let ty::PredicateKind::Projection(data) = bound_predicate.skip_binder() {
1593                 let mut selcx = SelectionContext::new(self);
1594                 let data = self.replace_bound_vars_with_fresh_vars(
1595                     obligation.cause.span,
1596                     infer::LateBoundRegionConversionTime::HigherRankedType,
1597                     bound_predicate.rebind(data),
1598                 );
1599                 let mut obligations = vec![];
1600                 let normalized_ty = super::normalize_projection_type(
1601                     &mut selcx,
1602                     obligation.param_env,
1603                     data.projection_ty,
1604                     obligation.cause.clone(),
1605                     0,
1606                     &mut obligations,
1607                 );
1608
1609                 debug!(?obligation.cause, ?obligation.param_env);
1610
1611                 debug!(?normalized_ty, data.ty = ?data.term);
1612
1613                 let is_normalized_ty_expected = !matches!(
1614                     obligation.cause.code().peel_derives(),
1615                     ObligationCauseCode::ItemObligation(_)
1616                         | ObligationCauseCode::BindingObligation(_, _)
1617                         | ObligationCauseCode::ExprItemObligation(..)
1618                         | ObligationCauseCode::ExprBindingObligation(..)
1619                         | ObligationCauseCode::ObjectCastObligation(..)
1620                         | ObligationCauseCode::OpaqueType
1621                 );
1622                 if let Err(new_err) = self.at(&obligation.cause, obligation.param_env).eq_exp(
1623                     is_normalized_ty_expected,
1624                     normalized_ty,
1625                     data.term,
1626                 ) {
1627                     values = Some((data, is_normalized_ty_expected, normalized_ty, data.term));
1628                     err = new_err;
1629                 }
1630             }
1631
1632             let msg = values
1633                 .and_then(|(predicate, _, normalized_ty, expected_ty)| {
1634                     self.maybe_detailed_projection_msg(predicate, normalized_ty, expected_ty)
1635                 })
1636                 .unwrap_or_else(|| format!("type mismatch resolving `{}`", predicate));
1637             let mut diag = struct_span_err!(self.tcx.sess, obligation.cause.span, E0271, "{msg}");
1638
1639             let secondary_span = match predicate.kind().skip_binder() {
1640                 ty::PredicateKind::Projection(proj) => self
1641                     .tcx
1642                     .opt_associated_item(proj.projection_ty.item_def_id)
1643                     .and_then(|trait_assoc_item| {
1644                         self.tcx
1645                             .trait_of_item(proj.projection_ty.item_def_id)
1646                             .map(|id| (trait_assoc_item, id))
1647                     })
1648                     .and_then(|(trait_assoc_item, id)| {
1649                         let trait_assoc_ident = trait_assoc_item.ident(self.tcx);
1650                         self.tcx.find_map_relevant_impl(id, proj.projection_ty.self_ty(), |did| {
1651                             self.tcx
1652                                 .associated_items(did)
1653                                 .in_definition_order()
1654                                 .find(|assoc| assoc.ident(self.tcx) == trait_assoc_ident)
1655                         })
1656                     })
1657                     .and_then(|item| match self.tcx.hir().get_if_local(item.def_id) {
1658                         Some(
1659                             hir::Node::TraitItem(hir::TraitItem {
1660                                 kind: hir::TraitItemKind::Type(_, Some(ty)),
1661                                 ..
1662                             })
1663                             | hir::Node::ImplItem(hir::ImplItem {
1664                                 kind: hir::ImplItemKind::Type(ty),
1665                                 ..
1666                             }),
1667                         ) => Some((ty.span, format!("type mismatch resolving `{}`", predicate))),
1668                         _ => None,
1669                     }),
1670                 _ => None,
1671             };
1672             self.note_type_err(
1673                 &mut diag,
1674                 &obligation.cause,
1675                 secondary_span,
1676                 values.map(|(_, is_normalized_ty_expected, normalized_ty, term)| {
1677                     infer::ValuePairs::Terms(ExpectedFound::new(
1678                         is_normalized_ty_expected,
1679                         normalized_ty,
1680                         term,
1681                     ))
1682                 }),
1683                 err,
1684                 true,
1685                 false,
1686             );
1687             self.note_obligation_cause(&mut diag, obligation);
1688             diag.emit();
1689         });
1690     }
1691
1692     fn maybe_detailed_projection_msg(
1693         &self,
1694         pred: ty::ProjectionPredicate<'tcx>,
1695         normalized_ty: ty::Term<'tcx>,
1696         expected_ty: ty::Term<'tcx>,
1697     ) -> Option<String> {
1698         let trait_def_id = pred.projection_ty.trait_def_id(self.tcx);
1699         let self_ty = pred.projection_ty.self_ty();
1700
1701         if Some(pred.projection_ty.item_def_id) == self.tcx.lang_items().fn_once_output() {
1702             Some(format!(
1703                 "expected `{self_ty}` to be a {fn_kind} that returns `{expected_ty}`, but it returns `{normalized_ty}`",
1704                 fn_kind = self_ty.prefix_string(self.tcx)
1705             ))
1706         } else if Some(trait_def_id) == self.tcx.lang_items().future_trait() {
1707             Some(format!(
1708                 "expected `{self_ty}` to be a future that resolves to `{expected_ty}`, but it resolves to `{normalized_ty}`"
1709             ))
1710         } else if Some(trait_def_id) == self.tcx.get_diagnostic_item(sym::Iterator) {
1711             Some(format!(
1712                 "expected `{self_ty}` to be an iterator that yields `{expected_ty}`, but it yields `{normalized_ty}`"
1713             ))
1714         } else {
1715             None
1716         }
1717     }
1718
1719     fn fuzzy_match_tys(
1720         &self,
1721         mut a: Ty<'tcx>,
1722         mut b: Ty<'tcx>,
1723         ignoring_lifetimes: bool,
1724     ) -> Option<CandidateSimilarity> {
1725         /// returns the fuzzy category of a given type, or None
1726         /// if the type can be equated to any type.
1727         fn type_category(tcx: TyCtxt<'_>, t: Ty<'_>) -> Option<u32> {
1728             match t.kind() {
1729                 ty::Bool => Some(0),
1730                 ty::Char => Some(1),
1731                 ty::Str => Some(2),
1732                 ty::Adt(def, _) if Some(def.did()) == tcx.lang_items().string() => Some(2),
1733                 ty::Int(..)
1734                 | ty::Uint(..)
1735                 | ty::Float(..)
1736                 | ty::Infer(ty::IntVar(..) | ty::FloatVar(..)) => Some(4),
1737                 ty::Ref(..) | ty::RawPtr(..) => Some(5),
1738                 ty::Array(..) | ty::Slice(..) => Some(6),
1739                 ty::FnDef(..) | ty::FnPtr(..) => Some(7),
1740                 ty::Dynamic(..) => Some(8),
1741                 ty::Closure(..) => Some(9),
1742                 ty::Tuple(..) => Some(10),
1743                 ty::Param(..) => Some(11),
1744                 ty::Projection(..) => Some(12),
1745                 ty::Opaque(..) => Some(13),
1746                 ty::Never => Some(14),
1747                 ty::Adt(..) => Some(15),
1748                 ty::Generator(..) => Some(16),
1749                 ty::Foreign(..) => Some(17),
1750                 ty::GeneratorWitness(..) => Some(18),
1751                 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error(_) => None,
1752             }
1753         }
1754
1755         let strip_references = |mut t: Ty<'tcx>| -> Ty<'tcx> {
1756             loop {
1757                 match t.kind() {
1758                     ty::Ref(_, inner, _) | ty::RawPtr(ty::TypeAndMut { ty: inner, .. }) => {
1759                         t = *inner
1760                     }
1761                     _ => break t,
1762                 }
1763             }
1764         };
1765
1766         if !ignoring_lifetimes {
1767             a = strip_references(a);
1768             b = strip_references(b);
1769         }
1770
1771         let cat_a = type_category(self.tcx, a)?;
1772         let cat_b = type_category(self.tcx, b)?;
1773         if a == b {
1774             Some(CandidateSimilarity::Exact { ignoring_lifetimes })
1775         } else if cat_a == cat_b {
1776             match (a.kind(), b.kind()) {
1777                 (ty::Adt(def_a, _), ty::Adt(def_b, _)) => def_a == def_b,
1778                 (ty::Foreign(def_a), ty::Foreign(def_b)) => def_a == def_b,
1779                 // Matching on references results in a lot of unhelpful
1780                 // suggestions, so let's just not do that for now.
1781                 //
1782                 // We still upgrade successful matches to `ignoring_lifetimes: true`
1783                 // to prioritize that impl.
1784                 (ty::Ref(..) | ty::RawPtr(..), ty::Ref(..) | ty::RawPtr(..)) => {
1785                     self.fuzzy_match_tys(a, b, true).is_some()
1786                 }
1787                 _ => true,
1788             }
1789             .then_some(CandidateSimilarity::Fuzzy { ignoring_lifetimes })
1790         } else if ignoring_lifetimes {
1791             None
1792         } else {
1793             self.fuzzy_match_tys(a, b, true)
1794         }
1795     }
1796
1797     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str> {
1798         self.tcx.hir().body(body_id).generator_kind.map(|gen_kind| match gen_kind {
1799             hir::GeneratorKind::Gen => "a generator",
1800             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block) => "an async block",
1801             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn) => "an async function",
1802             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Closure) => "an async closure",
1803         })
1804     }
1805
1806     fn find_similar_impl_candidates(
1807         &self,
1808         trait_pred: ty::PolyTraitPredicate<'tcx>,
1809     ) -> Vec<ImplCandidate<'tcx>> {
1810         self.tcx
1811             .all_impls(trait_pred.def_id())
1812             .filter_map(|def_id| {
1813                 if self.tcx.impl_polarity(def_id) == ty::ImplPolarity::Negative
1814                     || !trait_pred
1815                         .skip_binder()
1816                         .is_constness_satisfied_by(self.tcx.constness(def_id))
1817                 {
1818                     return None;
1819                 }
1820
1821                 let imp = self.tcx.impl_trait_ref(def_id).unwrap();
1822
1823                 self.fuzzy_match_tys(trait_pred.skip_binder().self_ty(), imp.self_ty(), false)
1824                     .map(|similarity| ImplCandidate { trait_ref: imp, similarity })
1825             })
1826             .collect()
1827     }
1828
1829     fn report_similar_impl_candidates(
1830         &self,
1831         impl_candidates: Vec<ImplCandidate<'tcx>>,
1832         trait_ref: ty::PolyTraitRef<'tcx>,
1833         body_id: hir::HirId,
1834         err: &mut Diagnostic,
1835     ) -> bool {
1836         let report = |mut candidates: Vec<TraitRef<'tcx>>, err: &mut Diagnostic| {
1837             candidates.sort();
1838             candidates.dedup();
1839             let len = candidates.len();
1840             if candidates.len() == 0 {
1841                 return false;
1842             }
1843             if candidates.len() == 1 {
1844                 let ty_desc = match candidates[0].self_ty().kind() {
1845                     ty::FnPtr(_) => Some("fn pointer"),
1846                     _ => None,
1847                 };
1848                 let the_desc = match ty_desc {
1849                     Some(desc) => format!(" implemented for {} `", desc),
1850                     None => " implemented for `".to_string(),
1851                 };
1852                 err.highlighted_help(vec![
1853                     (
1854                         format!("the trait `{}` ", candidates[0].print_only_trait_path()),
1855                         Style::NoStyle,
1856                     ),
1857                     ("is".to_string(), Style::Highlight),
1858                     (the_desc, Style::NoStyle),
1859                     (candidates[0].self_ty().to_string(), Style::Highlight),
1860                     ("`".to_string(), Style::NoStyle),
1861                 ]);
1862                 return true;
1863             }
1864             let trait_ref = TraitRef::identity(self.tcx, candidates[0].def_id);
1865             // Check if the trait is the same in all cases. If so, we'll only show the type.
1866             let mut traits: Vec<_> =
1867                 candidates.iter().map(|c| c.print_only_trait_path().to_string()).collect();
1868             traits.sort();
1869             traits.dedup();
1870
1871             let mut candidates: Vec<String> = candidates
1872                 .into_iter()
1873                 .map(|c| {
1874                     if traits.len() == 1 {
1875                         format!("\n  {}", c.self_ty())
1876                     } else {
1877                         format!("\n  {}", c)
1878                     }
1879                 })
1880                 .collect();
1881
1882             candidates.sort();
1883             candidates.dedup();
1884             let end = if candidates.len() <= 9 { candidates.len() } else { 8 };
1885             err.help(&format!(
1886                 "the following other types implement trait `{}`:{}{}",
1887                 trait_ref.print_only_trait_path(),
1888                 candidates[..end].join(""),
1889                 if len > 9 { format!("\nand {} others", len - 8) } else { String::new() }
1890             ));
1891             true
1892         };
1893
1894         let def_id = trait_ref.def_id();
1895         if impl_candidates.is_empty() {
1896             if self.tcx.trait_is_auto(def_id)
1897                 || self.tcx.lang_items().iter().any(|(_, id)| id == def_id)
1898                 || self.tcx.get_diagnostic_name(def_id).is_some()
1899             {
1900                 // Mentioning implementers of `Copy`, `Debug` and friends is not useful.
1901                 return false;
1902             }
1903             let normalized_impl_candidates: Vec<_> = self
1904                 .tcx
1905                 .all_impls(def_id)
1906                 // Ignore automatically derived impls and `!Trait` impls.
1907                 .filter(|&def_id| {
1908                     self.tcx.impl_polarity(def_id) != ty::ImplPolarity::Negative
1909                         || self.tcx.is_builtin_derive(def_id)
1910                 })
1911                 .filter_map(|def_id| self.tcx.impl_trait_ref(def_id))
1912                 .filter(|trait_ref| {
1913                     let self_ty = trait_ref.self_ty();
1914                     // Avoid mentioning type parameters.
1915                     if let ty::Param(_) = self_ty.kind() {
1916                         false
1917                     }
1918                     // Avoid mentioning types that are private to another crate
1919                     else if let ty::Adt(def, _) = self_ty.peel_refs().kind() {
1920                         // FIXME(compiler-errors): This could be generalized, both to
1921                         // be more granular, and probably look past other `#[fundamental]`
1922                         // types, too.
1923                         self.tcx
1924                             .visibility(def.did())
1925                             .is_accessible_from(body_id.owner.def_id, self.tcx)
1926                     } else {
1927                         true
1928                     }
1929                 })
1930                 .collect();
1931             return report(normalized_impl_candidates, err);
1932         }
1933
1934         let normalize = |candidate| {
1935             let infcx = self.tcx.infer_ctxt().build();
1936             infcx
1937                 .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
1938                 .normalize(candidate)
1939                 .map_or(candidate, |normalized| normalized.value)
1940         };
1941
1942         // Sort impl candidates so that ordering is consistent for UI tests.
1943         // because the ordering of `impl_candidates` may not be deterministic:
1944         // https://github.com/rust-lang/rust/pull/57475#issuecomment-455519507
1945         //
1946         // Prefer more similar candidates first, then sort lexicographically
1947         // by their normalized string representation.
1948         let mut normalized_impl_candidates_and_similarities = impl_candidates
1949             .into_iter()
1950             .map(|ImplCandidate { trait_ref, similarity }| {
1951                 let normalized = normalize(trait_ref);
1952                 (similarity, normalized)
1953             })
1954             .collect::<Vec<_>>();
1955         normalized_impl_candidates_and_similarities.sort();
1956         normalized_impl_candidates_and_similarities.dedup();
1957
1958         let normalized_impl_candidates = normalized_impl_candidates_and_similarities
1959             .into_iter()
1960             .map(|(_, normalized)| normalized)
1961             .collect::<Vec<_>>();
1962
1963         report(normalized_impl_candidates, err)
1964     }
1965
1966     /// Gets the parent trait chain start
1967     fn get_parent_trait_ref(
1968         &self,
1969         code: &ObligationCauseCode<'tcx>,
1970     ) -> Option<(String, Option<Span>)> {
1971         match code {
1972             ObligationCauseCode::BuiltinDerivedObligation(data) => {
1973                 let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
1974                 match self.get_parent_trait_ref(&data.parent_code) {
1975                     Some(t) => Some(t),
1976                     None => {
1977                         let ty = parent_trait_ref.skip_binder().self_ty();
1978                         let span = TyCategory::from_ty(self.tcx, ty)
1979                             .map(|(_, def_id)| self.tcx.def_span(def_id));
1980                         Some((ty.to_string(), span))
1981                     }
1982                 }
1983             }
1984             ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } => {
1985                 self.get_parent_trait_ref(&parent_code)
1986             }
1987             _ => None,
1988         }
1989     }
1990
1991     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1992     /// with the same path as `trait_ref`, a help message about
1993     /// a probable version mismatch is added to `err`
1994     fn note_version_mismatch(
1995         &self,
1996         err: &mut Diagnostic,
1997         trait_ref: &ty::PolyTraitRef<'tcx>,
1998     ) -> bool {
1999         let get_trait_impl = |trait_def_id| {
2000             self.tcx.find_map_relevant_impl(trait_def_id, trait_ref.skip_binder().self_ty(), Some)
2001         };
2002         let required_trait_path = self.tcx.def_path_str(trait_ref.def_id());
2003         let traits_with_same_path: std::collections::BTreeSet<_> = self
2004             .tcx
2005             .all_traits()
2006             .filter(|trait_def_id| *trait_def_id != trait_ref.def_id())
2007             .filter(|trait_def_id| self.tcx.def_path_str(*trait_def_id) == required_trait_path)
2008             .collect();
2009         let mut suggested = false;
2010         for trait_with_same_path in traits_with_same_path {
2011             if let Some(impl_def_id) = get_trait_impl(trait_with_same_path) {
2012                 let impl_span = self.tcx.def_span(impl_def_id);
2013                 err.span_help(impl_span, "trait impl with same name found");
2014                 let trait_crate = self.tcx.crate_name(trait_with_same_path.krate);
2015                 let crate_msg = format!(
2016                     "perhaps two different versions of crate `{}` are being used?",
2017                     trait_crate
2018                 );
2019                 err.note(&crate_msg);
2020                 suggested = true;
2021             }
2022         }
2023         suggested
2024     }
2025
2026     fn mk_trait_obligation_with_new_self_ty(
2027         &self,
2028         param_env: ty::ParamEnv<'tcx>,
2029         trait_ref_and_ty: ty::Binder<'tcx, (ty::TraitPredicate<'tcx>, Ty<'tcx>)>,
2030     ) -> PredicateObligation<'tcx> {
2031         let trait_pred = trait_ref_and_ty.map_bound_ref(|(tr, new_self_ty)| ty::TraitPredicate {
2032             trait_ref: ty::TraitRef {
2033                 substs: self.tcx.mk_substs_trait(*new_self_ty, &tr.trait_ref.substs[1..]),
2034                 ..tr.trait_ref
2035             },
2036             ..*tr
2037         });
2038
2039         Obligation::new(self.tcx, ObligationCause::dummy(), param_env, trait_pred)
2040     }
2041
2042     #[instrument(skip(self), level = "debug")]
2043     fn maybe_report_ambiguity(
2044         &self,
2045         obligation: &PredicateObligation<'tcx>,
2046         body_id: Option<hir::BodyId>,
2047     ) {
2048         // Unable to successfully determine, probably means
2049         // insufficient type information, but could mean
2050         // ambiguous impls. The latter *ought* to be a
2051         // coherence violation, so we don't report it here.
2052
2053         let predicate = self.resolve_vars_if_possible(obligation.predicate);
2054         let span = obligation.cause.span;
2055
2056         debug!(?predicate, obligation.cause.code = ?obligation.cause.code());
2057
2058         // Ambiguity errors are often caused as fallout from earlier errors.
2059         // We ignore them if this `infcx` is tainted in some cases below.
2060
2061         let bound_predicate = predicate.kind();
2062         let mut err = match bound_predicate.skip_binder() {
2063             ty::PredicateKind::Trait(data) => {
2064                 let trait_ref = bound_predicate.rebind(data.trait_ref);
2065                 debug!(?trait_ref);
2066
2067                 if predicate.references_error() {
2068                     return;
2069                 }
2070
2071                 // This is kind of a hack: it frequently happens that some earlier
2072                 // error prevents types from being fully inferred, and then we get
2073                 // a bunch of uninteresting errors saying something like "<generic
2074                 // #0> doesn't implement Sized".  It may even be true that we
2075                 // could just skip over all checks where the self-ty is an
2076                 // inference variable, but I was afraid that there might be an
2077                 // inference variable created, registered as an obligation, and
2078                 // then never forced by writeback, and hence by skipping here we'd
2079                 // be ignoring the fact that we don't KNOW the type works
2080                 // out. Though even that would probably be harmless, given that
2081                 // we're only talking about builtin traits, which are known to be
2082                 // inhabited. We used to check for `self.tcx.sess.has_errors()` to
2083                 // avoid inundating the user with unnecessary errors, but we now
2084                 // check upstream for type errors and don't add the obligations to
2085                 // begin with in those cases.
2086                 if self.tcx.lang_items().sized_trait() == Some(trait_ref.def_id()) {
2087                     if let None = self.tainted_by_errors() {
2088                         self.emit_inference_failure_err(
2089                             body_id,
2090                             span,
2091                             trait_ref.self_ty().skip_binder().into(),
2092                             ErrorCode::E0282,
2093                             false,
2094                         )
2095                         .emit();
2096                     }
2097                     return;
2098                 }
2099
2100                 // Typically, this ambiguity should only happen if
2101                 // there are unresolved type inference variables
2102                 // (otherwise it would suggest a coherence
2103                 // failure). But given #21974 that is not necessarily
2104                 // the case -- we can have multiple where clauses that
2105                 // are only distinguished by a region, which results
2106                 // in an ambiguity even when all types are fully
2107                 // known, since we don't dispatch based on region
2108                 // relationships.
2109
2110                 // Pick the first substitution that still contains inference variables as the one
2111                 // we're going to emit an error for. If there are none (see above), fall back to
2112                 // a more general error.
2113                 let subst = data.trait_ref.substs.iter().find(|s| s.has_non_region_infer());
2114
2115                 let mut err = if let Some(subst) = subst {
2116                     self.emit_inference_failure_err(body_id, span, subst, ErrorCode::E0283, true)
2117                 } else {
2118                     struct_span_err!(
2119                         self.tcx.sess,
2120                         span,
2121                         E0283,
2122                         "type annotations needed: cannot satisfy `{}`",
2123                         predicate,
2124                     )
2125                 };
2126
2127                 let obligation = obligation.with(self.tcx, trait_ref.to_poly_trait_predicate());
2128                 let mut selcx = SelectionContext::with_query_mode(
2129                     &self,
2130                     crate::traits::TraitQueryMode::Standard,
2131                 );
2132                 match selcx.select_from_obligation(&obligation) {
2133                     Ok(None) => {
2134                         let impls = ambiguity::recompute_applicable_impls(self.infcx, &obligation);
2135                         let has_non_region_infer =
2136                             trait_ref.skip_binder().substs.types().any(|t| !t.is_ty_infer());
2137                         // It doesn't make sense to talk about applicable impls if there are more
2138                         // than a handful of them.
2139                         if impls.len() > 1 && impls.len() < 5 && has_non_region_infer {
2140                             self.annotate_source_of_ambiguity(&mut err, &impls, predicate);
2141                         } else {
2142                             if self.tainted_by_errors().is_some() {
2143                                 err.cancel();
2144                                 return;
2145                             }
2146                             err.note(&format!("cannot satisfy `{}`", predicate));
2147                         }
2148                     }
2149                     _ => {
2150                         if self.tainted_by_errors().is_some() {
2151                             err.cancel();
2152                             return;
2153                         }
2154                         err.note(&format!("cannot satisfy `{}`", predicate));
2155                     }
2156                 }
2157
2158                 if let ObligationCauseCode::ItemObligation(def_id) | ObligationCauseCode::ExprItemObligation(def_id, ..) = *obligation.cause.code() {
2159                     self.suggest_fully_qualified_path(&mut err, def_id, span, trait_ref.def_id());
2160                 } else if let Ok(snippet) = &self.tcx.sess.source_map().span_to_snippet(span)
2161                     && let ObligationCauseCode::BindingObligation(def_id, _) | ObligationCauseCode::ExprBindingObligation(def_id, ..)
2162                         = *obligation.cause.code()
2163                 {
2164                     let generics = self.tcx.generics_of(def_id);
2165                     if generics.params.iter().any(|p| p.name != kw::SelfUpper)
2166                         && !snippet.ends_with('>')
2167                         && !generics.has_impl_trait()
2168                         && !self.tcx.fn_trait_kind_from_lang_item(def_id).is_some()
2169                     {
2170                         // FIXME: To avoid spurious suggestions in functions where type arguments
2171                         // where already supplied, we check the snippet to make sure it doesn't
2172                         // end with a turbofish. Ideally we would have access to a `PathSegment`
2173                         // instead. Otherwise we would produce the following output:
2174                         //
2175                         // error[E0283]: type annotations needed
2176                         //   --> $DIR/issue-54954.rs:3:24
2177                         //    |
2178                         // LL | const ARR_LEN: usize = Tt::const_val::<[i8; 123]>();
2179                         //    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^
2180                         //    |                        |
2181                         //    |                        cannot infer type
2182                         //    |                        help: consider specifying the type argument
2183                         //    |                        in the function call:
2184                         //    |                        `Tt::const_val::<[i8; 123]>::<T>`
2185                         // ...
2186                         // LL |     const fn const_val<T: Sized>() -> usize {
2187                         //    |                        - required by this bound in `Tt::const_val`
2188                         //    |
2189                         //    = note: cannot satisfy `_: Tt`
2190
2191                         // Clear any more general suggestions in favor of our specific one
2192                         err.clear_suggestions();
2193
2194                         err.span_suggestion_verbose(
2195                             span.shrink_to_hi(),
2196                             &format!(
2197                                 "consider specifying the type argument{} in the function call",
2198                                 pluralize!(generics.params.len()),
2199                             ),
2200                             format!(
2201                                 "::<{}>",
2202                                 generics
2203                                     .params
2204                                     .iter()
2205                                     .map(|p| p.name.to_string())
2206                                     .collect::<Vec<String>>()
2207                                     .join(", ")
2208                             ),
2209                             Applicability::HasPlaceholders,
2210                         );
2211                     }
2212                 }
2213
2214                 if let (Some(body_id), Some(ty::subst::GenericArgKind::Type(_))) =
2215                     (body_id, subst.map(|subst| subst.unpack()))
2216                 {
2217                     struct FindExprBySpan<'hir> {
2218                         span: Span,
2219                         result: Option<&'hir hir::Expr<'hir>>,
2220                     }
2221
2222                     impl<'v> hir::intravisit::Visitor<'v> for FindExprBySpan<'v> {
2223                         fn visit_expr(&mut self, ex: &'v hir::Expr<'v>) {
2224                             if self.span == ex.span {
2225                                 self.result = Some(ex);
2226                             } else {
2227                                 hir::intravisit::walk_expr(self, ex);
2228                             }
2229                         }
2230                     }
2231
2232                     let mut expr_finder = FindExprBySpan { span, result: None };
2233
2234                     expr_finder.visit_expr(&self.tcx.hir().body(body_id).value);
2235
2236                     if let Some(hir::Expr {
2237                         kind: hir::ExprKind::Path(hir::QPath::Resolved(None, path)), .. }
2238                     ) = expr_finder.result
2239                         && let [
2240                             ..,
2241                             trait_path_segment @ hir::PathSegment {
2242                                 res: rustc_hir::def::Res::Def(rustc_hir::def::DefKind::Trait, trait_id),
2243                                 ..
2244                             },
2245                             hir::PathSegment {
2246                                 ident: assoc_item_name,
2247                                 res: rustc_hir::def::Res::Def(_, item_id),
2248                                 ..
2249                             }
2250                         ] = path.segments
2251                         && data.trait_ref.def_id == *trait_id
2252                         && self.tcx.trait_of_item(*item_id) == Some(*trait_id)
2253                         && let None = self.tainted_by_errors()
2254                     {
2255                         let (verb, noun) = match self.tcx.associated_item(item_id).kind {
2256                             ty::AssocKind::Const => ("refer to the", "constant"),
2257                             ty::AssocKind::Fn => ("call", "function"),
2258                             ty::AssocKind::Type => ("refer to the", "type"), // this is already covered by E0223, but this single match arm doesn't hurt here
2259                         };
2260
2261                         // Replace the more general E0283 with a more specific error
2262                         err.cancel();
2263                         err = self.tcx.sess.struct_span_err_with_code(
2264                             span,
2265                             &format!(
2266                                 "cannot {verb} associated {noun} on trait without specifying the corresponding `impl` type",
2267                              ),
2268                             rustc_errors::error_code!(E0790),
2269                         );
2270
2271                         if let Some(local_def_id) = data.trait_ref.def_id.as_local()
2272                             && 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)
2273                             && let Some(method_ref) = trait_item_refs.iter().find(|item_ref| item_ref.ident == *assoc_item_name) {
2274                             err.span_label(method_ref.span, format!("`{}::{}` defined here", trait_name, assoc_item_name));
2275                         }
2276
2277                         err.span_label(span, format!("cannot {verb} associated {noun} of trait"));
2278
2279                         let trait_impls = self.tcx.trait_impls_of(data.trait_ref.def_id);
2280
2281                         if trait_impls.blanket_impls().is_empty()
2282                             && let Some((impl_ty, _)) = trait_impls.non_blanket_impls().iter().next()
2283                             && let Some(impl_def_id) = impl_ty.def() {
2284                             let message = if trait_impls.non_blanket_impls().len() == 1 {
2285                                 "use the fully-qualified path to the only available implementation".to_string()
2286                             } else {
2287                                 format!(
2288                                     "use a fully-qualified path to a specific available implementation ({} found)",
2289                                     trait_impls.non_blanket_impls().len()
2290                                 )
2291                             };
2292                             let mut suggestions = vec![(
2293                                 trait_path_segment.ident.span.shrink_to_lo(),
2294                                 format!("<{} as ", self.tcx.type_of(impl_def_id))
2295                             )];
2296                             if let Some(generic_arg) = trait_path_segment.args {
2297                                 let between_span = trait_path_segment.ident.span.between(generic_arg.span_ext);
2298                                 // get rid of :: between Trait and <type>
2299                                 // must be '::' between them, otherwise the parser won't accept the code
2300                                 suggestions.push((between_span, "".to_string(),));
2301                                 suggestions.push((generic_arg.span_ext.shrink_to_hi(), format!(">")));
2302                             } else {
2303                                 suggestions.push((trait_path_segment.ident.span.shrink_to_hi(), format!(">")));
2304                             }
2305                             err.multipart_suggestion(
2306                                 message,
2307                                 suggestions,
2308                                 Applicability::MaybeIncorrect
2309                             );
2310                         }
2311                     }
2312                 };
2313
2314                 err
2315             }
2316
2317             ty::PredicateKind::WellFormed(arg) => {
2318                 // Same hacky approach as above to avoid deluging user
2319                 // with error messages.
2320                 if arg.references_error()
2321                     || self.tcx.sess.has_errors().is_some()
2322                     || self.tainted_by_errors().is_some()
2323                 {
2324                     return;
2325                 }
2326
2327                 self.emit_inference_failure_err(body_id, span, arg, ErrorCode::E0282, false)
2328             }
2329
2330             ty::PredicateKind::Subtype(data) => {
2331                 if data.references_error()
2332                     || self.tcx.sess.has_errors().is_some()
2333                     || self.tainted_by_errors().is_some()
2334                 {
2335                     // no need to overload user in such cases
2336                     return;
2337                 }
2338                 let SubtypePredicate { a_is_expected: _, a, b } = data;
2339                 // both must be type variables, or the other would've been instantiated
2340                 assert!(a.is_ty_var() && b.is_ty_var());
2341                 self.emit_inference_failure_err(body_id, span, a.into(), ErrorCode::E0282, true)
2342             }
2343             ty::PredicateKind::Projection(data) => {
2344                 if predicate.references_error() || self.tainted_by_errors().is_some() {
2345                     return;
2346                 }
2347                 let subst = data
2348                     .projection_ty
2349                     .substs
2350                     .iter()
2351                     .chain(Some(data.term.into_arg()))
2352                     .find(|g| g.has_non_region_infer());
2353                 if let Some(subst) = subst {
2354                     let mut err = self.emit_inference_failure_err(
2355                         body_id,
2356                         span,
2357                         subst,
2358                         ErrorCode::E0284,
2359                         true,
2360                     );
2361                     err.note(&format!("cannot satisfy `{}`", predicate));
2362                     err
2363                 } else {
2364                     // If we can't find a substitution, just print a generic error
2365                     let mut err = struct_span_err!(
2366                         self.tcx.sess,
2367                         span,
2368                         E0284,
2369                         "type annotations needed: cannot satisfy `{}`",
2370                         predicate,
2371                     );
2372                     err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2373                     err
2374                 }
2375             }
2376
2377             ty::PredicateKind::ConstEvaluatable(data) => {
2378                 if predicate.references_error() || self.tainted_by_errors().is_some() {
2379                     return;
2380                 }
2381                 let subst = data.walk().find(|g| g.is_non_region_infer());
2382                 if let Some(subst) = subst {
2383                     let err = self.emit_inference_failure_err(
2384                         body_id,
2385                         span,
2386                         subst,
2387                         ErrorCode::E0284,
2388                         true,
2389                     );
2390                     err
2391                 } else {
2392                     // If we can't find a substitution, just print a generic error
2393                     let mut err = struct_span_err!(
2394                         self.tcx.sess,
2395                         span,
2396                         E0284,
2397                         "type annotations needed: cannot satisfy `{}`",
2398                         predicate,
2399                     );
2400                     err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2401                     err
2402                 }
2403             }
2404             _ => {
2405                 if self.tcx.sess.has_errors().is_some() || self.tainted_by_errors().is_some() {
2406                     return;
2407                 }
2408                 let mut err = struct_span_err!(
2409                     self.tcx.sess,
2410                     span,
2411                     E0284,
2412                     "type annotations needed: cannot satisfy `{}`",
2413                     predicate,
2414                 );
2415                 err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2416                 err
2417             }
2418         };
2419         self.note_obligation_cause(&mut err, obligation);
2420         err.emit();
2421     }
2422
2423     fn annotate_source_of_ambiguity(
2424         &self,
2425         err: &mut Diagnostic,
2426         impls: &[DefId],
2427         predicate: ty::Predicate<'tcx>,
2428     ) {
2429         let mut spans = vec![];
2430         let mut crates = vec![];
2431         let mut post = vec![];
2432         for def_id in impls {
2433             match self.tcx.span_of_impl(*def_id) {
2434                 Ok(span) => spans.push(span),
2435                 Err(name) => {
2436                     crates.push(name);
2437                     if let Some(header) = to_pretty_impl_header(self.tcx, *def_id) {
2438                         post.push(header);
2439                     }
2440                 }
2441             }
2442         }
2443         let mut crate_names: Vec<_> = crates.iter().map(|n| format!("`{}`", n)).collect();
2444         crate_names.sort();
2445         crate_names.dedup();
2446         post.sort();
2447         post.dedup();
2448
2449         if self.tainted_by_errors().is_some()
2450             && (crate_names.len() == 1
2451                 && spans.len() == 0
2452                 && ["`core`", "`alloc`", "`std`"].contains(&crate_names[0].as_str())
2453                 || predicate.visit_with(&mut HasNumericInferVisitor).is_break())
2454         {
2455             // Avoid complaining about other inference issues for expressions like
2456             // `42 >> 1`, where the types are still `{integer}`, but we want to
2457             // Do we need `trait_ref.skip_binder().self_ty().is_numeric() &&` too?
2458             // NOTE(eddyb) this was `.cancel()`, but `err`
2459             // is borrowed, so we can't fully defuse it.
2460             err.downgrade_to_delayed_bug();
2461             return;
2462         }
2463
2464         let msg = format!("multiple `impl`s satisfying `{}` found", predicate);
2465         let post = if post.len() > 1 || (post.len() == 1 && post[0].contains('\n')) {
2466             format!(":\n{}", post.iter().map(|p| format!("- {}", p)).collect::<Vec<_>>().join("\n"),)
2467         } else if post.len() == 1 {
2468             format!(": `{}`", post[0])
2469         } else {
2470             String::new()
2471         };
2472
2473         match (spans.len(), crates.len(), crate_names.len()) {
2474             (0, 0, 0) => {
2475                 err.note(&format!("cannot satisfy `{}`", predicate));
2476             }
2477             (0, _, 1) => {
2478                 err.note(&format!("{} in the `{}` crate{}", msg, crates[0], post,));
2479             }
2480             (0, _, _) => {
2481                 err.note(&format!(
2482                     "{} in the following crates: {}{}",
2483                     msg,
2484                     crate_names.join(", "),
2485                     post,
2486                 ));
2487             }
2488             (_, 0, 0) => {
2489                 let span: MultiSpan = spans.into();
2490                 err.span_note(span, &msg);
2491             }
2492             (_, 1, 1) => {
2493                 let span: MultiSpan = spans.into();
2494                 err.span_note(span, &msg);
2495                 err.note(
2496                     &format!("and another `impl` found in the `{}` crate{}", crates[0], post,),
2497                 );
2498             }
2499             _ => {
2500                 let span: MultiSpan = spans.into();
2501                 err.span_note(span, &msg);
2502                 err.note(&format!(
2503                     "and more `impl`s found in the following crates: {}{}",
2504                     crate_names.join(", "),
2505                     post,
2506                 ));
2507             }
2508         }
2509     }
2510
2511     /// Returns `true` if the trait predicate may apply for *some* assignment
2512     /// to the type parameters.
2513     fn predicate_can_apply(
2514         &self,
2515         param_env: ty::ParamEnv<'tcx>,
2516         pred: ty::PolyTraitPredicate<'tcx>,
2517     ) -> bool {
2518         struct ParamToVarFolder<'a, 'tcx> {
2519             infcx: &'a InferCtxt<'tcx>,
2520             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
2521         }
2522
2523         impl<'a, 'tcx> TypeFolder<'tcx> for ParamToVarFolder<'a, 'tcx> {
2524             fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
2525                 self.infcx.tcx
2526             }
2527
2528             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
2529                 if let ty::Param(ty::ParamTy { name, .. }) = *ty.kind() {
2530                     let infcx = self.infcx;
2531                     *self.var_map.entry(ty).or_insert_with(|| {
2532                         infcx.next_ty_var(TypeVariableOrigin {
2533                             kind: TypeVariableOriginKind::TypeParameterDefinition(name, None),
2534                             span: DUMMY_SP,
2535                         })
2536                     })
2537                 } else {
2538                     ty.super_fold_with(self)
2539                 }
2540             }
2541         }
2542
2543         self.probe(|_| {
2544             let mut selcx = SelectionContext::new(self);
2545
2546             let cleaned_pred =
2547                 pred.fold_with(&mut ParamToVarFolder { infcx: self, var_map: Default::default() });
2548
2549             let cleaned_pred = super::project::normalize(
2550                 &mut selcx,
2551                 param_env,
2552                 ObligationCause::dummy(),
2553                 cleaned_pred,
2554             )
2555             .value;
2556
2557             let obligation =
2558                 Obligation::new(self.tcx, ObligationCause::dummy(), param_env, cleaned_pred);
2559
2560             self.predicate_may_hold(&obligation)
2561         })
2562     }
2563
2564     fn note_obligation_cause(&self, err: &mut Diagnostic, obligation: &PredicateObligation<'tcx>) {
2565         // First, attempt to add note to this error with an async-await-specific
2566         // message, and fall back to regular note otherwise.
2567         if !self.maybe_note_obligation_cause_for_async_await(err, obligation) {
2568             self.note_obligation_cause_code(
2569                 err,
2570                 &obligation.predicate,
2571                 obligation.param_env,
2572                 obligation.cause.code(),
2573                 &mut vec![],
2574                 &mut Default::default(),
2575             );
2576             self.suggest_unsized_bound_if_applicable(err, obligation);
2577         }
2578     }
2579
2580     #[instrument(level = "debug", skip_all)]
2581     fn suggest_unsized_bound_if_applicable(
2582         &self,
2583         err: &mut Diagnostic,
2584         obligation: &PredicateObligation<'tcx>,
2585     ) {
2586         let ty::PredicateKind::Trait(pred) = obligation.predicate.kind().skip_binder() else { return; };
2587         let (ObligationCauseCode::BindingObligation(item_def_id, span)
2588         | ObligationCauseCode::ExprBindingObligation(item_def_id, span, ..))
2589             = *obligation.cause.code().peel_derives() else { return; };
2590         debug!(?pred, ?item_def_id, ?span);
2591
2592         let (Some(node), true) = (
2593             self.tcx.hir().get_if_local(item_def_id),
2594             Some(pred.def_id()) == self.tcx.lang_items().sized_trait(),
2595         ) else {
2596             return;
2597         };
2598         self.maybe_suggest_unsized_generics(err, span, node);
2599     }
2600
2601     #[instrument(level = "debug", skip_all)]
2602     fn maybe_suggest_unsized_generics(&self, err: &mut Diagnostic, span: Span, node: Node<'tcx>) {
2603         let Some(generics) = node.generics() else {
2604             return;
2605         };
2606         let sized_trait = self.tcx.lang_items().sized_trait();
2607         debug!(?generics.params);
2608         debug!(?generics.predicates);
2609         let Some(param) = generics.params.iter().find(|param| param.span == span) else {
2610             return;
2611         };
2612         // Check that none of the explicit trait bounds is `Sized`. Assume that an explicit
2613         // `Sized` bound is there intentionally and we don't need to suggest relaxing it.
2614         let explicitly_sized = generics
2615             .bounds_for_param(param.def_id)
2616             .flat_map(|bp| bp.bounds)
2617             .any(|bound| bound.trait_ref().and_then(|tr| tr.trait_def_id()) == sized_trait);
2618         if explicitly_sized {
2619             return;
2620         }
2621         debug!(?param);
2622         match node {
2623             hir::Node::Item(
2624                 item @ hir::Item {
2625                     // Only suggest indirection for uses of type parameters in ADTs.
2626                     kind:
2627                         hir::ItemKind::Enum(..) | hir::ItemKind::Struct(..) | hir::ItemKind::Union(..),
2628                     ..
2629                 },
2630             ) => {
2631                 if self.maybe_indirection_for_unsized(err, item, param) {
2632                     return;
2633                 }
2634             }
2635             _ => {}
2636         };
2637         // Didn't add an indirection suggestion, so add a general suggestion to relax `Sized`.
2638         let (span, separator) = if let Some(s) = generics.bounds_span_for_suggestions(param.def_id)
2639         {
2640             (s, " +")
2641         } else {
2642             (span.shrink_to_hi(), ":")
2643         };
2644         err.span_suggestion_verbose(
2645             span,
2646             "consider relaxing the implicit `Sized` restriction",
2647             format!("{} ?Sized", separator),
2648             Applicability::MachineApplicable,
2649         );
2650     }
2651
2652     fn maybe_indirection_for_unsized(
2653         &self,
2654         err: &mut Diagnostic,
2655         item: &Item<'tcx>,
2656         param: &GenericParam<'tcx>,
2657     ) -> bool {
2658         // Suggesting `T: ?Sized` is only valid in an ADT if `T` is only used in a
2659         // borrow. `struct S<'a, T: ?Sized>(&'a T);` is valid, `struct S<T: ?Sized>(T);`
2660         // is not. Look for invalid "bare" parameter uses, and suggest using indirection.
2661         let mut visitor =
2662             FindTypeParam { param: param.name.ident().name, invalid_spans: vec![], nested: false };
2663         visitor.visit_item(item);
2664         if visitor.invalid_spans.is_empty() {
2665             return false;
2666         }
2667         let mut multispan: MultiSpan = param.span.into();
2668         multispan.push_span_label(
2669             param.span,
2670             format!("this could be changed to `{}: ?Sized`...", param.name.ident()),
2671         );
2672         for sp in visitor.invalid_spans {
2673             multispan.push_span_label(
2674                 sp,
2675                 format!("...if indirection were used here: `Box<{}>`", param.name.ident()),
2676             );
2677         }
2678         err.span_help(
2679             multispan,
2680             &format!(
2681                 "you could relax the implicit `Sized` bound on `{T}` if it were \
2682                 used through indirection like `&{T}` or `Box<{T}>`",
2683                 T = param.name.ident(),
2684             ),
2685         );
2686         true
2687     }
2688
2689     fn is_recursive_obligation(
2690         &self,
2691         obligated_types: &mut Vec<Ty<'tcx>>,
2692         cause_code: &ObligationCauseCode<'tcx>,
2693     ) -> bool {
2694         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
2695             let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
2696             let self_ty = parent_trait_ref.skip_binder().self_ty();
2697             if obligated_types.iter().any(|ot| ot == &self_ty) {
2698                 return true;
2699             }
2700             if let ty::Adt(def, substs) = self_ty.kind()
2701                 && let [arg] = &substs[..]
2702                 && let ty::subst::GenericArgKind::Type(ty) = arg.unpack()
2703                 && let ty::Adt(inner_def, _) = ty.kind()
2704                 && inner_def == def
2705             {
2706                 return true;
2707             }
2708         }
2709         false
2710     }
2711 }
2712
2713 /// Look for type `param` in an ADT being used only through a reference to confirm that suggesting
2714 /// `param: ?Sized` would be a valid constraint.
2715 struct FindTypeParam {
2716     param: rustc_span::Symbol,
2717     invalid_spans: Vec<Span>,
2718     nested: bool,
2719 }
2720
2721 impl<'v> Visitor<'v> for FindTypeParam {
2722     fn visit_where_predicate(&mut self, _: &'v hir::WherePredicate<'v>) {
2723         // Skip where-clauses, to avoid suggesting indirection for type parameters found there.
2724     }
2725
2726     fn visit_ty(&mut self, ty: &hir::Ty<'_>) {
2727         // We collect the spans of all uses of the "bare" type param, like in `field: T` or
2728         // `field: (T, T)` where we could make `T: ?Sized` while skipping cases that are known to be
2729         // valid like `field: &'a T` or `field: *mut T` and cases that *might* have further `Sized`
2730         // obligations like `Box<T>` and `Vec<T>`, but we perform no extra analysis for those cases
2731         // and suggest `T: ?Sized` regardless of their obligations. This is fine because the errors
2732         // in that case should make what happened clear enough.
2733         match ty.kind {
2734             hir::TyKind::Ptr(_) | hir::TyKind::Rptr(..) | hir::TyKind::TraitObject(..) => {}
2735             hir::TyKind::Path(hir::QPath::Resolved(None, path))
2736                 if path.segments.len() == 1 && path.segments[0].ident.name == self.param =>
2737             {
2738                 if !self.nested {
2739                     debug!(?ty, "FindTypeParam::visit_ty");
2740                     self.invalid_spans.push(ty.span);
2741                 }
2742             }
2743             hir::TyKind::Path(_) => {
2744                 let prev = self.nested;
2745                 self.nested = true;
2746                 hir::intravisit::walk_ty(self, ty);
2747                 self.nested = prev;
2748             }
2749             _ => {
2750                 hir::intravisit::walk_ty(self, ty);
2751             }
2752         }
2753     }
2754 }
2755
2756 /// Summarizes information
2757 #[derive(Clone)]
2758 pub enum ArgKind {
2759     /// An argument of non-tuple type. Parameters are (name, ty)
2760     Arg(String, String),
2761
2762     /// An argument of tuple type. For a "found" argument, the span is
2763     /// the location in the source of the pattern. For an "expected"
2764     /// argument, it will be None. The vector is a list of (name, ty)
2765     /// strings for the components of the tuple.
2766     Tuple(Option<Span>, Vec<(String, String)>),
2767 }
2768
2769 impl ArgKind {
2770     fn empty() -> ArgKind {
2771         ArgKind::Arg("_".to_owned(), "_".to_owned())
2772     }
2773
2774     /// Creates an `ArgKind` from the expected type of an
2775     /// argument. It has no name (`_`) and an optional source span.
2776     pub fn from_expected_ty(t: Ty<'_>, span: Option<Span>) -> ArgKind {
2777         match t.kind() {
2778             ty::Tuple(tys) => ArgKind::Tuple(
2779                 span,
2780                 tys.iter().map(|ty| ("_".to_owned(), ty.to_string())).collect::<Vec<_>>(),
2781             ),
2782             _ => ArgKind::Arg("_".to_owned(), t.to_string()),
2783         }
2784     }
2785 }
2786
2787 struct HasNumericInferVisitor;
2788
2789 impl<'tcx> ty::TypeVisitor<'tcx> for HasNumericInferVisitor {
2790     type BreakTy = ();
2791
2792     fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
2793         if matches!(ty.kind(), ty::Infer(ty::FloatVar(_) | ty::IntVar(_))) {
2794             ControlFlow::Break(())
2795         } else {
2796             ControlFlow::CONTINUE
2797         }
2798     }
2799 }
2800
2801 pub enum DefIdOrName {
2802     DefId(DefId),
2803     Name(&'static str),
2804 }