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