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