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

   1 pub mod on_unimplemented;
   2 pub mod suggestions;
   3
   4 use super::{
   5     FulfillmentContext, FulfillmentError, FulfillmentErrorCode, MismatchedProjectionTypes,
   6     Obligation, ObligationCause, ObligationCauseCode, OutputTypeParameterMismatch, Overflow,
   7     PredicateObligation, SelectionContext, SelectionError, TraitNotObjectSafe,
   8 };
   9 use crate::infer::error_reporting::{TyCategory, TypeAnnotationNeeded as ErrorCode};
  10 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  11 use crate::infer::{self, InferCtxt, TyCtxtInferExt};
  12 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
  13 use crate::traits::query::normalize::AtExt as _;
  14 use crate::traits::specialize::to_pretty_impl_header;
  15 use on_unimplemented::OnUnimplementedNote;
  16 use on_unimplemented::TypeErrCtxtExt as _;
  17 use rustc_data_structures::fx::FxHashMap;
  18 use rustc_errors::{
  19     pluralize, struct_span_err, Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed,
  20     MultiSpan, Style,
  21 };
  22 use rustc_hir as hir;
  23 use rustc_hir::def_id::DefId;
  24 use rustc_hir::intravisit::Visitor;
  25 use rustc_hir::GenericParam;
  26 use rustc_hir::Item;
  27 use rustc_hir::Node;
  28 use rustc_infer::infer::error_reporting::TypeErrCtxt;
  29 use rustc_infer::infer::TypeTrace;
  30 use rustc_infer::traits::TraitEngine;
  31 use rustc_middle::traits::select::OverflowError;
  32 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
  33 use rustc_middle::ty::error::ExpectedFound;
  34 use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable};
  35 use rustc_middle::ty::{
  36     self, SubtypePredicate, ToPolyTraitRef, ToPredicate, TraitRef, Ty, TyCtxt, TypeFoldable,
  37     TypeVisitable,
  38 };
  39 use rustc_session::Limit;
  40 use rustc_span::def_id::LOCAL_CRATE;
  41 use rustc_span::symbol::{kw, sym};
  42 use rustc_span::{ExpnKind, Span, DUMMY_SP};
  43 use std::fmt;
  44 use std::iter;
  45 use std::ops::ControlFlow;
  46 use suggestions::TypeErrCtxtExt as _;
  47
  48 pub use rustc_infer::traits::error_reporting::*;
  49
  50 // When outputting impl candidates, prefer showing those that are more similar.
  51 //
  52 // We also compare candidates after skipping lifetimes, which has a lower
  53 // priority than exact matches.
  54 #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
  55 pub enum CandidateSimilarity {
  56     Exact { ignoring_lifetimes: bool },
  57     Fuzzy { ignoring_lifetimes: bool },
  58 }
  59
  60 #[derive(Debug, Clone, Copy)]
  61 pub struct ImplCandidate<'tcx> {
  62     pub trait_ref: ty::TraitRef<'tcx>,
  63     pub similarity: CandidateSimilarity,
  64 }
  65
  66 pub trait InferCtxtExt<'tcx> {
  67     /// Given some node representing a fn-like thing in the HIR map,
  68     /// returns a span and `ArgKind` information that describes the
  69     /// arguments it expects. This can be supplied to
  70     /// `report_arg_count_mismatch`.
  71     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)>;
  72
  73     /// Reports an error when the number of arguments needed by a
  74     /// trait match doesn't match the number that the expression
  75     /// provides.
  76     fn report_arg_count_mismatch(
  77         &self,
  78         span: Span,
  79         found_span: Option<Span>,
  80         expected_args: Vec<ArgKind>,
  81         found_args: Vec<ArgKind>,
  82         is_closure: bool,
  83     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed>;
  84
  85     /// Checks if the type implements one of `Fn`, `FnMut`, or `FnOnce`
  86     /// in that order, and returns the generic type corresponding to the
  87     /// argument of that trait (corresponding to the closure arguments).
  88     fn type_implements_fn_trait(
  89         &self,
  90         param_env: ty::ParamEnv<'tcx>,
  91         ty: ty::Binder<'tcx, Ty<'tcx>>,
  92         constness: ty::BoundConstness,
  93         polarity: ty::ImplPolarity,
  94     ) -> Result<(ty::ClosureKind, ty::Binder<'tcx, Ty<'tcx>>), ()>;
  95 }
  96
  97 pub trait TypeErrCtxtExt<'tcx> {
  98     fn report_fulfillment_errors(
  99         &self,
 100         errors: &[FulfillmentError<'tcx>],
 101         body_id: Option<hir::BodyId>,
 102     ) -> ErrorGuaranteed;
 103
 104     fn report_overflow_error<T>(
 105         &self,
 106         obligation: &Obligation<'tcx, T>,
 107         suggest_increasing_limit: bool,
 108     ) -> !
 109     where
 110         T: fmt::Display + TypeFoldable<'tcx>;
 111
 112     fn suggest_new_overflow_limit(&self, err: &mut Diagnostic);
 113
 114     fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> !;
 115
 116     /// The `root_obligation` parameter should be the `root_obligation` field
 117     /// from a `FulfillmentError`. If no `FulfillmentError` is available,
 118     /// then it should be the same as `obligation`.
 119     fn report_selection_error(
 120         &self,
 121         obligation: PredicateObligation<'tcx>,
 122         root_obligation: &PredicateObligation<'tcx>,
 123         error: &SelectionError<'tcx>,
 124     );
 125 }
 126
 127 impl<'tcx> InferCtxtExt<'tcx> for InferCtxt<'tcx> {
 128     /// Given some node representing a fn-like thing in the HIR map,
 129     /// returns a span and `ArgKind` information that describes the
 130     /// arguments it expects. This can be supplied to
 131     /// `report_arg_count_mismatch`.
 132     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)> {
 133         let sm = self.tcx.sess.source_map();
 134         let hir = self.tcx.hir();
 135         Some(match node {
 136             Node::Expr(&hir::Expr {
 137                 kind: hir::ExprKind::Closure(&hir::Closure { body, fn_decl_span, .. }),
 138                 ..
 139             }) => (
 140                 fn_decl_span,
 141                 hir.body(body)
 142                     .params
 143                     .iter()
 144                     .map(|arg| {
 145                         if let hir::Pat { kind: hir::PatKind::Tuple(ref args, _), span, .. } =
 146                             *arg.pat
 147                         {
 148                             Some(ArgKind::Tuple(
 149                                 Some(span),
 150                                 args.iter()
 151                                     .map(|pat| {
 152                                         sm.span_to_snippet(pat.span)
 153                                             .ok()
 154                                             .map(|snippet| (snippet, "_".to_owned()))
 155                                     })
 156                                     .collect::<Option<Vec<_>>>()?,
 157                             ))
 158                         } else {
 159                             let name = sm.span_to_snippet(arg.pat.span).ok()?;
 160                             Some(ArgKind::Arg(name, "_".to_owned()))
 161                         }
 162                     })
 163                     .collect::<Option<Vec<ArgKind>>>()?,
 164             ),
 165             Node::Item(&hir::Item { kind: hir::ItemKind::Fn(ref sig, ..), .. })
 166             | Node::ImplItem(&hir::ImplItem { kind: hir::ImplItemKind::Fn(ref sig, _), .. })
 167             | Node::TraitItem(&hir::TraitItem {
 168                 kind: hir::TraitItemKind::Fn(ref sig, _), ..
 169             }) => (
 170                 sig.span,
 171                 sig.decl
 172                     .inputs
 173                     .iter()
 174                     .map(|arg| match arg.kind {
 175                         hir::TyKind::Tup(ref tys) => ArgKind::Tuple(
 176                             Some(arg.span),
 177                             vec![("_".to_owned(), "_".to_owned()); tys.len()],
 178                         ),
 179                         _ => ArgKind::empty(),
 180                     })
 181                     .collect::<Vec<ArgKind>>(),
 182             ),
 183             Node::Ctor(ref variant_data) => {
 184                 let span = variant_data.ctor_hir_id().map_or(DUMMY_SP, |id| hir.span(id));
 185                 (span, vec![ArgKind::empty(); variant_data.fields().len()])
 186             }
 187             _ => panic!("non-FnLike node found: {:?}", node),
 188         })
 189     }
 190
 191     /// Reports an error when the number of arguments needed by a
 192     /// trait match doesn't match the number that the expression
 193     /// provides.
 194     fn report_arg_count_mismatch(
 195         &self,
 196         span: Span,
 197         found_span: Option<Span>,
 198         expected_args: Vec<ArgKind>,
 199         found_args: Vec<ArgKind>,
 200         is_closure: bool,
 201     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
 202         let kind = if is_closure { "closure" } else { "function" };
 203
 204         let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
 205             let arg_length = arguments.len();
 206             let distinct = matches!(other, &[ArgKind::Tuple(..)]);
 207             match (arg_length, arguments.get(0)) {
 208                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
 209                     format!("a single {}-tuple as argument", fields.len())
 210                 }
 211                 _ => format!(
 212                     "{} {}argument{}",
 213                     arg_length,
 214                     if distinct && arg_length > 1 { "distinct " } else { "" },
 215                     pluralize!(arg_length)
 216                 ),
 217             }
 218         };
 219
 220         let expected_str = args_str(&expected_args, &found_args);
 221         let found_str = args_str(&found_args, &expected_args);
 222
 223         let mut err = struct_span_err!(
 224             self.tcx.sess,
 225             span,
 226             E0593,
 227             "{} is expected to take {}, but it takes {}",
 228             kind,
 229             expected_str,
 230             found_str,
 231         );
 232
 233         err.span_label(span, format!("expected {} that takes {}", kind, expected_str));
 234
 235         if let Some(found_span) = found_span {
 236             err.span_label(found_span, format!("takes {}", found_str));
 237
 238             // move |_| { ... }
 239             // ^^^^^^^^-- def_span
 240             //
 241             // move |_| { ... }
 242             // ^^^^^-- prefix
 243             let prefix_span = self.tcx.sess.source_map().span_until_non_whitespace(found_span);
 244             // move |_| { ... }
 245             //      ^^^-- pipe_span
 246             let pipe_span =
 247                 if let Some(span) = found_span.trim_start(prefix_span) { span } else { found_span };
 248
 249             // Suggest to take and ignore the arguments with expected_args_length `_`s if
 250             // found arguments is empty (assume the user just wants to ignore args in this case).
 251             // For example, if `expected_args_length` is 2, suggest `|_, _|`.
 252             if found_args.is_empty() && is_closure {
 253                 let underscores = vec!["_"; expected_args.len()].join(", ");
 254                 err.span_suggestion_verbose(
 255                     pipe_span,
 256                     &format!(
 257                         "consider changing the closure to take and ignore the expected argument{}",
 258                         pluralize!(expected_args.len())
 259                     ),
 260                     format!("|{}|", underscores),
 261                     Applicability::MachineApplicable,
 262                 );
 263             }
 264
 265             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
 266                 if fields.len() == expected_args.len() {
 267                     let sugg = fields
 268                         .iter()
 269                         .map(|(name, _)| name.to_owned())
 270                         .collect::<Vec<String>>()
 271                         .join(", ");
 272                     err.span_suggestion_verbose(
 273                         found_span,
 274                         "change the closure to take multiple arguments instead of a single tuple",
 275                         format!("|{}|", sugg),
 276                         Applicability::MachineApplicable,
 277                     );
 278                 }
 279             }
 280             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..]
 281                 && fields.len() == found_args.len()
 282                 && is_closure
 283             {
 284                 let sugg = format!(
 285                     "|({}){}|",
 286                     found_args
 287                         .iter()
 288                         .map(|arg| match arg {
 289                             ArgKind::Arg(name, _) => name.to_owned(),
 290                             _ => "_".to_owned(),
 291                         })
 292                         .collect::<Vec<String>>()
 293                         .join(", "),
 294                     // add type annotations if available
 295                     if found_args.iter().any(|arg| match arg {
 296                         ArgKind::Arg(_, ty) => ty != "_",
 297                         _ => false,
 298                     }) {
 299                         format!(
 300                             ": ({})",
 301                             fields
 302                                 .iter()
 303                                 .map(|(_, ty)| ty.to_owned())
 304                                 .collect::<Vec<String>>()
 305                                 .join(", ")
 306                         )
 307                     } else {
 308                         String::new()
 309                     },
 310                 );
 311                 err.span_suggestion_verbose(
 312                     found_span,
 313                     "change the closure to accept a tuple instead of individual arguments",
 314                     sugg,
 315                     Applicability::MachineApplicable,
 316                 );
 317             }
 318         }
 319
 320         err
 321     }
 322
 323     fn type_implements_fn_trait(
 324         &self,
 325         param_env: ty::ParamEnv<'tcx>,
 326         ty: ty::Binder<'tcx, Ty<'tcx>>,
 327         constness: ty::BoundConstness,
 328         polarity: ty::ImplPolarity,
 329     ) -> Result<(ty::ClosureKind, ty::Binder<'tcx, Ty<'tcx>>), ()> {
 330         self.commit_if_ok(|_| {
 331             for trait_def_id in [
 332                 self.tcx.lang_items().fn_trait(),
 333                 self.tcx.lang_items().fn_mut_trait(),
 334                 self.tcx.lang_items().fn_once_trait(),
 335             ] {
 336                 let Some(trait_def_id) = trait_def_id else { continue };
 337                 // Make a fresh inference variable so we can determine what the substitutions
 338                 // of the trait are.
 339                 let var = self.next_ty_var(TypeVariableOrigin {
 340                     span: DUMMY_SP,
 341                     kind: TypeVariableOriginKind::MiscVariable,
 342                 });
 343                 let substs = self.tcx.mk_substs_trait(ty.skip_binder(), &[var.into()]);
 344                 let obligation = Obligation::new(
 345                     ObligationCause::dummy(),
 346                     param_env,
 347                     ty.rebind(ty::TraitPredicate {
 348                         trait_ref: ty::TraitRef::new(trait_def_id, substs),
 349                         constness,
 350                         polarity,
 351                     })
 352                     .to_predicate(self.tcx),
 353                 );
 354                 let mut fulfill_cx = FulfillmentContext::new_in_snapshot();
 355                 fulfill_cx.register_predicate_obligation(self, obligation);
 356                 if fulfill_cx.select_all_or_error(self).is_empty() {
 357                     return Ok((
 358                         ty::ClosureKind::from_def_id(self.tcx, trait_def_id)
 359                             .expect("expected to map DefId to ClosureKind"),
 360                         ty.rebind(self.resolve_vars_if_possible(var)),
 361                     ));
 362                 }
 363             }
 364
 365             Err(())
 366         })
 367     }
 368 }
 369 impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
 370     fn report_fulfillment_errors(
 371         &self,
 372         errors: &[FulfillmentError<'tcx>],
 373         body_id: Option<hir::BodyId>,
 374     ) -> ErrorGuaranteed {
 375         #[derive(Debug)]
 376         struct ErrorDescriptor<'tcx> {
 377             predicate: ty::Predicate<'tcx>,
 378             index: Option<usize>, // None if this is an old error
 379         }
 380
 381         let mut error_map: FxHashMap<_, Vec<_>> = self
 382             .reported_trait_errors
 383             .borrow()
 384             .iter()
 385             .map(|(&span, predicates)| {
 386                 (
 387                     span,
 388                     predicates
 389                         .iter()
 390                         .map(|&predicate| ErrorDescriptor { predicate, index: None })
 391                         .collect(),
 392                 )
 393             })
 394             .collect();
 395
 396         for (index, error) in errors.iter().enumerate() {
 397             // We want to ignore desugarings here: spans are equivalent even
 398             // if one is the result of a desugaring and the other is not.
 399             let mut span = error.obligation.cause.span;
 400             let expn_data = span.ctxt().outer_expn_data();
 401             if let ExpnKind::Desugaring(_) = expn_data.kind {
 402                 span = expn_data.call_site;
 403             }
 404
 405             error_map.entry(span).or_default().push(ErrorDescriptor {
 406                 predicate: error.obligation.predicate,
 407                 index: Some(index),
 408             });
 409
 410             self.reported_trait_errors
 411                 .borrow_mut()
 412                 .entry(span)
 413                 .or_default()
 414                 .push(error.obligation.predicate);
 415         }
 416
 417         // We do this in 2 passes because we want to display errors in order, though
 418         // maybe it *is* better to sort errors by span or something.
 419         let mut is_suppressed = vec![false; errors.len()];
 420         for (_, error_set) in error_map.iter() {
 421             // We want to suppress "duplicate" errors with the same span.
 422             for error in error_set {
 423                 if let Some(index) = error.index {
 424                     // Suppress errors that are either:
 425                     // 1) strictly implied by another error.
 426                     // 2) implied by an error with a smaller index.
 427                     for error2 in error_set {
 428                         if error2.index.map_or(false, |index2| is_suppressed[index2]) {
 429                             // Avoid errors being suppressed by already-suppressed
 430                             // errors, to prevent all errors from being suppressed
 431                             // at once.
 432                             continue;
 433                         }
 434
 435                         if self.error_implies(error2.predicate, error.predicate)
 436                             && !(error2.index >= error.index
 437                                 && self.error_implies(error.predicate, error2.predicate))
 438                         {
 439                             info!("skipping {:?} (implied by {:?})", error, error2);
 440                             is_suppressed[index] = true;
 441                             break;
 442                         }
 443                     }
 444                 }
 445             }
 446         }
 447
 448         for (error, suppressed) in iter::zip(errors, is_suppressed) {
 449             if !suppressed {
 450                 self.report_fulfillment_error(error, body_id);
 451             }
 452         }
 453
 454         self.tcx.sess.delay_span_bug(DUMMY_SP, "expected fullfillment errors")
 455     }
 456
 457     /// Reports that an overflow has occurred and halts compilation. We
 458     /// halt compilation unconditionally because it is important that
 459     /// overflows never be masked -- they basically represent computations
 460     /// whose result could not be truly determined and thus we can't say
 461     /// if the program type checks or not -- and they are unusual
 462     /// occurrences in any case.
 463     fn report_overflow_error<T>(
 464         &self,
 465         obligation: &Obligation<'tcx, T>,
 466         suggest_increasing_limit: bool,
 467     ) -> !
 468     where
 469         T: fmt::Display + TypeFoldable<'tcx>,
 470     {
 471         let predicate = self.resolve_vars_if_possible(obligation.predicate.clone());
 472         let mut err = struct_span_err!(
 473             self.tcx.sess,
 474             obligation.cause.span,
 475             E0275,
 476             "overflow evaluating the requirement `{}`",
 477             predicate
 478         );
 479
 480         if suggest_increasing_limit {
 481             self.suggest_new_overflow_limit(&mut err);
 482         }
 483
 484         self.note_obligation_cause_code(
 485             &mut err,
 486             &obligation.predicate,
 487             obligation.param_env,
 488             obligation.cause.code(),
 489             &mut vec![],
 490             &mut Default::default(),
 491         );
 492
 493         err.emit();
 494         self.tcx.sess.abort_if_errors();
 495         bug!();
 496     }
 497
 498     fn suggest_new_overflow_limit(&self, err: &mut Diagnostic) {
 499         let suggested_limit = match self.tcx.recursion_limit() {
 500             Limit(0) => Limit(2),
 501             limit => limit * 2,
 502         };
 503         err.help(&format!(
 504             "consider increasing the recursion limit by adding a \
 505              `#![recursion_limit = \"{}\"]` attribute to your crate (`{}`)",
 506             suggested_limit,
 507             self.tcx.crate_name(LOCAL_CRATE),
 508         ));
 509     }
 510
 511     /// Reports that a cycle was detected which led to overflow and halts
 512     /// compilation. This is equivalent to `report_overflow_error` except
 513     /// that we can give a more helpful error message (and, in particular,
 514     /// we do not suggest increasing the overflow limit, which is not
 515     /// going to help).
 516     fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 517         let cycle = self.resolve_vars_if_possible(cycle.to_owned());
 518         assert!(!cycle.is_empty());
 519
 520         debug!(?cycle, "report_overflow_error_cycle");
 521
 522         // The 'deepest' obligation is most likely to have a useful
 523         // cause 'backtrace'
 524         self.report_overflow_error(cycle.iter().max_by_key(|p| p.recursion_depth).unwrap(), false);
 525     }
 526
 527     fn report_selection_error(
 528         &self,
 529         mut obligation: PredicateObligation<'tcx>,
 530         root_obligation: &PredicateObligation<'tcx>,
 531         error: &SelectionError<'tcx>,
 532     ) {
 533         self.set_tainted_by_errors();
 534         let tcx = self.tcx;
 535         let mut span = obligation.cause.span;
 536
 537         let mut err = match *error {
 538             SelectionError::Ambiguous(ref impls) => {
 539                 let mut err = self.tcx.sess.struct_span_err(
 540                     obligation.cause.span,
 541                     &format!("multiple applicable `impl`s for `{}`", obligation.predicate),
 542                 );
 543                 self.annotate_source_of_ambiguity(&mut err, impls, obligation.predicate);
 544                 err.emit();
 545                 return;
 546             }
 547             SelectionError::Unimplemented => {
 548                 // If this obligation was generated as a result of well-formedness checking, see if we
 549                 // can get a better error message by performing HIR-based well-formedness checking.
 550                 if let ObligationCauseCode::WellFormed(Some(wf_loc)) =
 551                     root_obligation.cause.code().peel_derives()
 552                 {
 553                     if let Some(cause) = self
 554                         .tcx
 555                         .diagnostic_hir_wf_check((tcx.erase_regions(obligation.predicate), *wf_loc))
 556                     {
 557                         obligation.cause = cause.clone();
 558                         span = obligation.cause.span;
 559                     }
 560                 }
 561                 if let ObligationCauseCode::CompareImplItemObligation {
 562                     impl_item_def_id,
 563                     trait_item_def_id,
 564                     kind: _,
 565                 } = *obligation.cause.code()
 566                 {
 567                     self.report_extra_impl_obligation(
 568                         span,
 569                         impl_item_def_id,
 570                         trait_item_def_id,
 571                         &format!("`{}`", obligation.predicate),
 572                     )
 573                     .emit();
 574                     return;
 575                 }
 576
 577                 let bound_predicate = obligation.predicate.kind();
 578                 match bound_predicate.skip_binder() {
 579                     ty::PredicateKind::Trait(trait_predicate) => {
 580                         let trait_predicate = bound_predicate.rebind(trait_predicate);
 581                         let mut trait_predicate = self.resolve_vars_if_possible(trait_predicate);
 582
 583                         trait_predicate.remap_constness_diag(obligation.param_env);
 584                         let predicate_is_const = ty::BoundConstness::ConstIfConst
 585                             == trait_predicate.skip_binder().constness;
 586
 587                         if self.tcx.sess.has_errors().is_some()
 588                             && trait_predicate.references_error()
 589                         {
 590                             return;
 591                         }
 592                         let trait_ref = trait_predicate.to_poly_trait_ref();
 593                         let (post_message, pre_message, type_def) = self
 594                             .get_parent_trait_ref(obligation.cause.code())
 595                             .map(|(t, s)| {
 596                                 (
 597                                     format!(" in `{}`", t),
 598                                     format!("within `{}`, ", t),
 599                                     s.map(|s| (format!("within this `{}`", t), s)),
 600                                 )
 601                             })
 602                             .unwrap_or_default();
 603
 604                         let OnUnimplementedNote {
 605                             message,
 606                             label,
 607                             note,
 608                             parent_label,
 609                             append_const_msg,
 610                         } = self.on_unimplemented_note(trait_ref, &obligation);
 611                         let have_alt_message = message.is_some() || label.is_some();
 612                         let is_try_conversion = self.is_try_conversion(span, trait_ref.def_id());
 613                         let is_unsize =
 614                             Some(trait_ref.def_id()) == self.tcx.lang_items().unsize_trait();
 615                         let (message, note, append_const_msg) = if is_try_conversion {
 616                             (
 617                                 Some(format!(
 618                                     "`?` couldn't convert the error to `{}`",
 619                                     trait_ref.skip_binder().self_ty(),
 620                                 )),
 621                                 Some(
 622                                     "the question mark operation (`?`) implicitly performs a \
 623                                      conversion on the error value using the `From` trait"
 624                                         .to_owned(),
 625                                 ),
 626                                 Some(None),
 627                             )
 628                         } else {
 629                             (message, note, append_const_msg)
 630                         };
 631
 632                         let mut err = struct_span_err!(
 633                             self.tcx.sess,
 634                             span,
 635                             E0277,
 636                             "{}",
 637                             message
 638                                 .and_then(|cannot_do_this| {
 639                                     match (predicate_is_const, append_const_msg) {
 640                                         // do nothing if predicate is not const
 641                                         (false, _) => Some(cannot_do_this),
 642                                         // suggested using default post message
 643                                         (true, Some(None)) => {
 644                                             Some(format!("{cannot_do_this} in const contexts"))
 645                                         }
 646                                         // overridden post message
 647                                         (true, Some(Some(post_message))) => {
 648                                             Some(format!("{cannot_do_this}{post_message}"))
 649                                         }
 650                                         // fallback to generic message
 651                                         (true, None) => None,
 652                                     }
 653                                 })
 654                                 .unwrap_or_else(|| format!(
 655                                     "the trait bound `{}` is not satisfied{}",
 656                                     trait_predicate, post_message,
 657                                 ))
 658                         );
 659
 660                         if is_try_conversion {
 661                             let none_error = self
 662                                 .tcx
 663                                 .get_diagnostic_item(sym::none_error)
 664                                 .map(|def_id| tcx.type_of(def_id));
 665                             let should_convert_option_to_result =
 666                                 Some(trait_ref.skip_binder().substs.type_at(1)) == none_error;
 667                             let should_convert_result_to_option =
 668                                 Some(trait_ref.self_ty().skip_binder()) == none_error;
 669                             if should_convert_option_to_result {
 670                                 err.span_suggestion_verbose(
 671                                     span.shrink_to_lo(),
 672                                     "consider converting the `Option<T>` into a `Result<T, _>` \
 673                                      using `Option::ok_or` or `Option::ok_or_else`",
 674                                     ".ok_or_else(|| /* error value */)",
 675                                     Applicability::HasPlaceholders,
 676                                 );
 677                             } else if should_convert_result_to_option {
 678                                 err.span_suggestion_verbose(
 679                                     span.shrink_to_lo(),
 680                                     "consider converting the `Result<T, _>` into an `Option<T>` \
 681                                      using `Result::ok`",
 682                                     ".ok()",
 683                                     Applicability::MachineApplicable,
 684                                 );
 685                             }
 686                             if let Some(ret_span) = self.return_type_span(&obligation) {
 687                                 err.span_label(
 688                                     ret_span,
 689                                     &format!(
 690                                         "expected `{}` because of this",
 691                                         trait_ref.skip_binder().self_ty()
 692                                     ),
 693                                 );
 694                             }
 695                         }
 696
 697                         if Some(trait_ref.def_id()) == tcx.lang_items().tuple_trait() {
 698                             match obligation.cause.code().peel_derives() {
 699                                 ObligationCauseCode::RustCall => {
 700                                     err.set_primary_message("functions with the \"rust-call\" ABI must take a single non-self tuple argument");
 701                                 }
 702                                 ObligationCauseCode::BindingObligation(def_id, _)
 703                                 | ObligationCauseCode::ItemObligation(def_id)
 704                                     if ty::ClosureKind::from_def_id(tcx, *def_id).is_some() =>
 705                                 {
 706                                     err.code(rustc_errors::error_code!(E0059));
 707                                     err.set_primary_message(format!(
 708                                         "type parameter to bare `{}` trait must be a tuple",
 709                                         tcx.def_path_str(*def_id)
 710                                     ));
 711                                 }
 712                                 _ => {}
 713                             }
 714                         }
 715
 716                         if Some(trait_ref.def_id()) == tcx.lang_items().drop_trait()
 717                             && predicate_is_const
 718                         {
 719                             err.note("`~const Drop` was renamed to `~const Destruct`");
 720                             err.note("See <https://github.com/rust-lang/rust/pull/94901> for more details");
 721                         }
 722
 723                         let explanation = if let ObligationCauseCode::MainFunctionType =
 724                             obligation.cause.code()
 725                         {
 726                             "consider using `()`, or a `Result`".to_owned()
 727                         } else {
 728                             let ty_desc = match trait_ref.skip_binder().self_ty().kind() {
 729                                 ty::FnDef(_, _) => Some("fn item"),
 730                                 ty::Closure(_, _) => Some("closure"),
 731                                 _ => None,
 732                             };
 733
 734                             match ty_desc {
 735                                 Some(desc) => format!(
 736                                     "{}the trait `{}` is not implemented for {} `{}`",
 737                                     pre_message,
 738                                     trait_predicate.print_modifiers_and_trait_path(),
 739                                     desc,
 740                                     trait_ref.skip_binder().self_ty(),
 741                                 ),
 742                                 None => format!(
 743                                     "{}the trait `{}` is not implemented for `{}`",
 744                                     pre_message,
 745                                     trait_predicate.print_modifiers_and_trait_path(),
 746                                     trait_ref.skip_binder().self_ty(),
 747                                 ),
 748                             }
 749                         };
 750
 751                         if self.suggest_add_reference_to_arg(
 752                             &obligation,
 753                             &mut err,
 754                             trait_predicate,
 755                             have_alt_message,
 756                         ) {
 757                             self.note_obligation_cause(&mut err, &obligation);
 758                             err.emit();
 759                             return;
 760                         }
 761                         if let Some(ref s) = label {
 762                             // If it has a custom `#[rustc_on_unimplemented]`
 763                             // error message, let's display it as the label!
 764                             err.span_label(span, s);
 765                             if !matches!(trait_ref.skip_binder().self_ty().kind(), ty::Param(_)) {
 766                                 // When the self type is a type param We don't need to "the trait
 767                                 // `std::marker::Sized` is not implemented for `T`" as we will point
 768                                 // at the type param with a label to suggest constraining it.
 769                                 err.help(&explanation);
 770                             }
 771                         } else {
 772                             err.span_label(span, explanation);
 773                         }
 774
 775                         if let ObligationCauseCode::ObjectCastObligation(concrete_ty, obj_ty) = obligation.cause.code().peel_derives() &&
 776                             Some(trait_ref.def_id()) == self.tcx.lang_items().sized_trait() {
 777                             self.suggest_borrowing_for_object_cast(&mut err, &root_obligation, *concrete_ty, *obj_ty);
 778                         }
 779
 780                         let mut unsatisfied_const = false;
 781                         if trait_predicate.is_const_if_const() && obligation.param_env.is_const() {
 782                             let non_const_predicate = trait_ref.without_const();
 783                             let non_const_obligation = Obligation {
 784                                 cause: obligation.cause.clone(),
 785                                 param_env: obligation.param_env.without_const(),
 786                                 predicate: non_const_predicate.to_predicate(tcx),
 787                                 recursion_depth: obligation.recursion_depth,
 788                             };
 789                             if self.predicate_may_hold(&non_const_obligation) {
 790                                 unsatisfied_const = true;
 791                                 err.span_note(
 792                                     span,
 793                                     &format!(
 794                                         "the trait `{}` is implemented for `{}`, \
 795                                         but that implementation is not `const`",
 796                                         non_const_predicate.print_modifiers_and_trait_path(),
 797                                         trait_ref.skip_binder().self_ty(),
 798                                     ),
 799                                 );
 800                             }
 801                         }
 802
 803                         if let Some((msg, span)) = type_def {
 804                             err.span_label(span, &msg);
 805                         }
 806                         if let Some(ref s) = note {
 807                             // If it has a custom `#[rustc_on_unimplemented]` note, let's display it
 808                             err.note(s.as_str());
 809                         }
 810                         if let Some(ref s) = parent_label {
 811                             let body = tcx
 812                                 .hir()
 813                                 .opt_local_def_id(obligation.cause.body_id)
 814                                 .unwrap_or_else(|| {
 815                                     tcx.hir().body_owner_def_id(hir::BodyId {
 816                                         hir_id: obligation.cause.body_id,
 817                                     })
 818                                 });
 819                             err.span_label(tcx.def_span(body), s);
 820                         }
 821
 822                         self.suggest_floating_point_literal(&obligation, &mut err, &trait_ref);
 823                         self.suggest_dereferencing_index(&obligation, &mut err, trait_predicate);
 824                         let mut suggested =
 825                             self.suggest_dereferences(&obligation, &mut err, trait_predicate);
 826                         suggested |= self.suggest_fn_call(&obligation, &mut err, trait_predicate);
 827                         suggested |=
 828                             self.suggest_remove_reference(&obligation, &mut err, trait_predicate);
 829                         suggested |= self.suggest_semicolon_removal(
 830                             &obligation,
 831                             &mut err,
 832                             span,
 833                             trait_predicate,
 834                         );
 835                         self.note_version_mismatch(&mut err, &trait_ref);
 836                         self.suggest_remove_await(&obligation, &mut err);
 837                         self.suggest_derive(&obligation, &mut err, trait_predicate);
 838
 839                         if Some(trait_ref.def_id()) == tcx.lang_items().try_trait() {
 840                             self.suggest_await_before_try(
 841                                 &mut err,
 842                                 &obligation,
 843                                 trait_predicate,
 844                                 span,
 845                             );
 846                         }
 847
 848                         if self.suggest_impl_trait(&mut err, span, &obligation, trait_predicate) {
 849                             err.emit();
 850                             return;
 851                         }
 852
 853                         if is_unsize {
 854                             // If the obligation failed due to a missing implementation of the
 855                             // `Unsize` trait, give a pointer to why that might be the case
 856                             err.note(
 857                                 "all implementations of `Unsize` are provided \
 858                                 automatically by the compiler, see \
 859                                 <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> \
 860                                 for more information",
 861                             );
 862                         }
 863
 864                         let is_fn_trait =
 865                             ty::ClosureKind::from_def_id(tcx, trait_ref.def_id()).is_some();
 866                         let is_target_feature_fn = if let ty::FnDef(def_id, _) =
 867                             *trait_ref.skip_binder().self_ty().kind()
 868                         {
 869                             !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
 870                         } else {
 871                             false
 872                         };
 873                         if is_fn_trait && is_target_feature_fn {
 874                             err.note(
 875                                 "`#[target_feature]` functions do not implement the `Fn` traits",
 876                             );
 877                         }
 878
 879                         // Try to report a help message
 880                         if is_fn_trait
 881                             && let Ok((implemented_kind, params)) = self.type_implements_fn_trait(
 882                             obligation.param_env,
 883                             trait_ref.self_ty(),
 884                             trait_predicate.skip_binder().constness,
 885                             trait_predicate.skip_binder().polarity,
 886                         )
 887                         {
 888                             // If the type implements `Fn`, `FnMut`, or `FnOnce`, suppress the following
 889                             // suggestion to add trait bounds for the type, since we only typically implement
 890                             // these traits once.
 891
 892                             // Note if the `FnMut` or `FnOnce` is less general than the trait we're trying
 893                             // to implement.
 894                             let selected_kind =
 895                                 ty::ClosureKind::from_def_id(self.tcx, trait_ref.def_id())
 896                                     .expect("expected to map DefId to ClosureKind");
 897                             if !implemented_kind.extends(selected_kind) {
 898                                 err.note(
 899                                     &format!(
 900                                         "`{}` implements `{}`, but it must implement `{}`, which is more general",
 901                                         trait_ref.skip_binder().self_ty(),
 902                                         implemented_kind,
 903                                         selected_kind
 904                                     )
 905                                 );
 906                             }
 907
 908                             // Note any argument mismatches
 909                             let given_ty = params.skip_binder();
 910                             let expected_ty = trait_ref.skip_binder().substs.type_at(1);
 911                             if let ty::Tuple(given) = given_ty.kind()
 912                                 && let ty::Tuple(expected) = expected_ty.kind()
 913                             {
 914                                 if expected.len() != given.len() {
 915                                     // Note number of types that were expected and given
 916                                     err.note(
 917                                         &format!(
 918                                             "expected a closure taking {} argument{}, but one taking {} argument{} was given",
 919                                             given.len(),
 920                                             pluralize!(given.len()),
 921                                             expected.len(),
 922                                             pluralize!(expected.len()),
 923                                         )
 924                                     );
 925                                 } else if !self.same_type_modulo_infer(given_ty, expected_ty) {
 926                                     // Print type mismatch
 927                                     let (expected_args, given_args) =
 928                                         self.cmp(given_ty, expected_ty);
 929                                     err.note_expected_found(
 930                                         &"a closure with arguments",
 931                                         expected_args,
 932                                         &"a closure with arguments",
 933                                         given_args,
 934                                     );
 935                                 }
 936                             }
 937                         } else if !trait_ref.has_non_region_infer()
 938                             && self.predicate_can_apply(obligation.param_env, trait_predicate)
 939                         {
 940                             // If a where-clause may be useful, remind the
 941                             // user that they can add it.
 942                             //
 943                             // don't display an on-unimplemented note, as
 944                             // these notes will often be of the form
 945                             //     "the type `T` can't be frobnicated"
 946                             // which is somewhat confusing.
 947                             self.suggest_restricting_param_bound(
 948                                 &mut err,
 949                                 trait_predicate,
 950                                 None,
 951                                 obligation.cause.body_id,
 952                             );
 953                         } else if !suggested && !unsatisfied_const {
 954                             // Can't show anything else useful, try to find similar impls.
 955                             let impl_candidates = self.find_similar_impl_candidates(trait_predicate);
 956                             if !self.report_similar_impl_candidates(
 957                                 impl_candidates,
 958                                 trait_ref,
 959                                 obligation.cause.body_id,
 960                                 &mut err,
 961                             ) {
 962                                 // This is *almost* equivalent to
 963                                 // `obligation.cause.code().peel_derives()`, but it gives us the
 964                                 // trait predicate for that corresponding root obligation. This
 965                                 // lets us get a derived obligation from a type parameter, like
 966                                 // when calling `string.strip_suffix(p)` where `p` is *not* an
 967                                 // implementer of `Pattern<'_>`.
 968                                 let mut code = obligation.cause.code();
 969                                 let mut trait_pred = trait_predicate;
 970                                 let mut peeled = false;
 971                                 while let Some((parent_code, parent_trait_pred)) = code.parent() {
 972                                     code = parent_code;
 973                                     if let Some(parent_trait_pred) = parent_trait_pred {
 974                                         trait_pred = parent_trait_pred;
 975                                         peeled = true;
 976                                     }
 977                                 }
 978                                 let def_id = trait_pred.def_id();
 979                                 // Mention *all* the `impl`s for the *top most* obligation, the
 980                                 // user might have meant to use one of them, if any found. We skip
 981                                 // auto-traits or fundamental traits that might not be exactly what
 982                                 // the user might expect to be presented with. Instead this is
 983                                 // useful for less general traits.
 984                                 if peeled
 985                                     && !self.tcx.trait_is_auto(def_id)
 986                                     && !self.tcx.lang_items().iter().any(|(_, id)| id == def_id)
 987                                 {
 988                                     let trait_ref = trait_pred.to_poly_trait_ref();
 989                                     let impl_candidates =
 990                                         self.find_similar_impl_candidates(trait_pred);
 991                                     self.report_similar_impl_candidates(
 992                                         impl_candidates,
 993                                         trait_ref,
 994                                         obligation.cause.body_id,
 995                                         &mut err,
 996                                     );
 997                                 }
 998                             }
 999                         }
1000
1001                         // Changing mutability doesn't make a difference to whether we have
1002                         // an `Unsize` impl (Fixes ICE in #71036)
1003                         if !is_unsize {
1004                             self.suggest_change_mut(&obligation, &mut err, trait_predicate);
1005                         }
1006
1007                         // If this error is due to `!: Trait` not implemented but `(): Trait` is
1008                         // implemented, and fallback has occurred, then it could be due to a
1009                         // variable that used to fallback to `()` now falling back to `!`. Issue a
1010                         // note informing about the change in behaviour.
1011                         if trait_predicate.skip_binder().self_ty().is_never()
1012                             && self.fallback_has_occurred
1013                         {
1014                             let predicate = trait_predicate.map_bound(|mut trait_pred| {
1015                                 trait_pred.trait_ref.substs = self.tcx.mk_substs_trait(
1016                                     self.tcx.mk_unit(),
1017                                     &trait_pred.trait_ref.substs[1..],
1018                                 );
1019                                 trait_pred
1020                             });
1021                             let unit_obligation = obligation.with(predicate.to_predicate(tcx));
1022                             if self.predicate_may_hold(&unit_obligation) {
1023                                 err.note(
1024                                     "this error might have been caused by changes to \
1025                                     Rust's type-inference algorithm (see issue #48950 \
1026                                     <https://github.com/rust-lang/rust/issues/48950> \
1027                                     for more information)",
1028                                 );
1029                                 err.help("did you intend to use the type `()` here instead?");
1030                             }
1031                         }
1032
1033                         // Return early if the trait is Debug or Display and the invocation
1034                         // originates within a standard library macro, because the output
1035                         // is otherwise overwhelming and unhelpful (see #85844 for an
1036                         // example).
1037
1038                         let in_std_macro =
1039                             match obligation.cause.span.ctxt().outer_expn_data().macro_def_id {
1040                                 Some(macro_def_id) => {
1041                                     let crate_name = tcx.crate_name(macro_def_id.krate);
1042                                     crate_name == sym::std || crate_name == sym::core
1043                                 }
1044                                 None => false,
1045                             };
1046
1047                         if in_std_macro
1048                             && matches!(
1049                                 self.tcx.get_diagnostic_name(trait_ref.def_id()),
1050                                 Some(sym::Debug | sym::Display)
1051                             )
1052                         {
1053                             err.emit();
1054                             return;
1055                         }
1056
1057                         err
1058                     }
1059
1060                     ty::PredicateKind::Subtype(predicate) => {
1061                         // Errors for Subtype predicates show up as
1062                         // `FulfillmentErrorCode::CodeSubtypeError`,
1063                         // not selection error.
1064                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
1065                     }
1066
1067                     ty::PredicateKind::Coerce(predicate) => {
1068                         // Errors for Coerce predicates show up as
1069                         // `FulfillmentErrorCode::CodeSubtypeError`,
1070                         // not selection error.
1071                         span_bug!(span, "coerce requirement gave wrong error: `{:?}`", predicate)
1072                     }
1073
1074                     ty::PredicateKind::RegionOutlives(..)
1075                     | ty::PredicateKind::Projection(..)
1076                     | ty::PredicateKind::TypeOutlives(..) => {
1077                         let predicate = self.resolve_vars_if_possible(obligation.predicate);
1078                         struct_span_err!(
1079                             self.tcx.sess,
1080                             span,
1081                             E0280,
1082                             "the requirement `{}` is not satisfied",
1083                             predicate
1084                         )
1085                     }
1086
1087                     ty::PredicateKind::ObjectSafe(trait_def_id) => {
1088                         let violations = self.tcx.object_safety_violations(trait_def_id);
1089                         report_object_safety_error(self.tcx, span, trait_def_id, violations)
1090                     }
1091
1092                     ty::PredicateKind::ClosureKind(closure_def_id, closure_substs, kind) => {
1093                         let found_kind = self.closure_kind(closure_substs).unwrap();
1094                         let closure_span = self.tcx.def_span(closure_def_id);
1095                         let mut err = struct_span_err!(
1096                             self.tcx.sess,
1097                             closure_span,
1098                             E0525,
1099                             "expected a closure that implements the `{}` trait, \
1100                              but this closure only implements `{}`",
1101                             kind,
1102                             found_kind
1103                         );
1104
1105                         err.span_label(
1106                             closure_span,
1107                             format!("this closure implements `{}`, not `{}`", found_kind, kind),
1108                         );
1109                         err.span_label(
1110                             obligation.cause.span,
1111                             format!("the requirement to implement `{}` derives from here", kind),
1112                         );
1113
1114                         // Additional context information explaining why the closure only implements
1115                         // a particular trait.
1116                         if let Some(typeck_results) = &self.typeck_results {
1117                             let hir_id = self
1118                                 .tcx
1119                                 .hir()
1120                                 .local_def_id_to_hir_id(closure_def_id.expect_local());
1121                             match (found_kind, typeck_results.closure_kind_origins().get(hir_id)) {
1122                                 (ty::ClosureKind::FnOnce, Some((span, place))) => {
1123                                     err.span_label(
1124                                         *span,
1125                                         format!(
1126                                             "closure is `FnOnce` because it moves the \
1127                                          variable `{}` out of its environment",
1128                                             ty::place_to_string_for_capture(tcx, place)
1129                                         ),
1130                                     );
1131                                 }
1132                                 (ty::ClosureKind::FnMut, Some((span, place))) => {
1133                                     err.span_label(
1134                                         *span,
1135                                         format!(
1136                                             "closure is `FnMut` because it mutates the \
1137                                          variable `{}` here",
1138                                             ty::place_to_string_for_capture(tcx, place)
1139                                         ),
1140                                     );
1141                                 }
1142                                 _ => {}
1143                             }
1144                         }
1145
1146                         err
1147                     }
1148
1149                     ty::PredicateKind::WellFormed(ty) => {
1150                         if !self.tcx.sess.opts.unstable_opts.chalk {
1151                             // WF predicates cannot themselves make
1152                             // errors. They can only block due to
1153                             // ambiguity; otherwise, they always
1154                             // degenerate into other obligations
1155                             // (which may fail).
1156                             span_bug!(span, "WF predicate not satisfied for {:?}", ty);
1157                         } else {
1158                             // FIXME: we'll need a better message which takes into account
1159                             // which bounds actually failed to hold.
1160                             self.tcx.sess.struct_span_err(
1161                                 span,
1162                                 &format!("the type `{}` is not well-formed (chalk)", ty),
1163                             )
1164                         }
1165                     }
1166
1167                     ty::PredicateKind::ConstEvaluatable(..) => {
1168                         // Errors for `ConstEvaluatable` predicates show up as
1169                         // `SelectionError::ConstEvalFailure`,
1170                         // not `Unimplemented`.
1171                         span_bug!(
1172                             span,
1173                             "const-evaluatable requirement gave wrong error: `{:?}`",
1174                             obligation
1175                         )
1176                     }
1177
1178                     ty::PredicateKind::ConstEquate(..) => {
1179                         // Errors for `ConstEquate` predicates show up as
1180                         // `SelectionError::ConstEvalFailure`,
1181                         // not `Unimplemented`.
1182                         span_bug!(
1183                             span,
1184                             "const-equate requirement gave wrong error: `{:?}`",
1185                             obligation
1186                         )
1187                     }
1188
1189                     ty::PredicateKind::TypeWellFormedFromEnv(..) => span_bug!(
1190                         span,
1191                         "TypeWellFormedFromEnv predicate should only exist in the environment"
1192                     ),
1193                 }
1194             }
1195
1196             OutputTypeParameterMismatch(found_trait_ref, expected_trait_ref, _) => {
1197                 let found_trait_ref = self.resolve_vars_if_possible(found_trait_ref);
1198                 let expected_trait_ref = self.resolve_vars_if_possible(expected_trait_ref);
1199
1200                 if expected_trait_ref.self_ty().references_error() {
1201                     return;
1202                 }
1203
1204                 let Some(found_trait_ty) = found_trait_ref.self_ty().no_bound_vars() else {
1205                     return;
1206                 };
1207
1208                 let found_did = match *found_trait_ty.kind() {
1209                     ty::Closure(did, _)
1210                     | ty::Foreign(did)
1211                     | ty::FnDef(did, _)
1212                     | ty::Generator(did, ..) => Some(did),
1213                     ty::Adt(def, _) => Some(def.did()),
1214                     _ => None,
1215                 };
1216
1217                 let found_span = found_did.and_then(|did| self.tcx.hir().span_if_local(did));
1218
1219                 if self.reported_closure_mismatch.borrow().contains(&(span, found_span)) {
1220                     // We check closures twice, with obligations flowing in different directions,
1221                     // but we want to complain about them only once.
1222                     return;
1223                 }
1224
1225                 self.reported_closure_mismatch.borrow_mut().insert((span, found_span));
1226
1227                 let mut not_tupled = false;
1228
1229                 let found = match found_trait_ref.skip_binder().substs.type_at(1).kind() {
1230                     ty::Tuple(ref tys) => vec![ArgKind::empty(); tys.len()],
1231                     _ => {
1232                         not_tupled = true;
1233                         vec![ArgKind::empty()]
1234                     }
1235                 };
1236
1237                 let expected_ty = expected_trait_ref.skip_binder().substs.type_at(1);
1238                 let expected = match expected_ty.kind() {
1239                     ty::Tuple(ref tys) => {
1240                         tys.iter().map(|t| ArgKind::from_expected_ty(t, Some(span))).collect()
1241                     }
1242                     _ => {
1243                         not_tupled = true;
1244                         vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())]
1245                     }
1246                 };
1247
1248                 // If this is a `Fn` family trait and either the expected or found
1249                 // is not tupled, then fall back to just a regular mismatch error.
1250                 // This shouldn't be common unless manually implementing one of the
1251                 // traits manually, but don't make it more confusing when it does
1252                 // happen.
1253                 if Some(expected_trait_ref.def_id()) != tcx.lang_items().gen_trait() && not_tupled {
1254                     self.report_and_explain_type_error(
1255                         TypeTrace::poly_trait_refs(
1256                             &obligation.cause,
1257                             true,
1258                             expected_trait_ref,
1259                             found_trait_ref,
1260                         ),
1261                         ty::error::TypeError::Mismatch,
1262                     )
1263                 } else if found.len() == expected.len() {
1264                     self.report_closure_arg_mismatch(
1265                         span,
1266                         found_span,
1267                         found_trait_ref,
1268                         expected_trait_ref,
1269                         obligation.cause.code(),
1270                     )
1271                 } else {
1272                     let (closure_span, found) = found_did
1273                         .and_then(|did| {
1274                             let node = self.tcx.hir().get_if_local(did)?;
1275                             let (found_span, found) = self.get_fn_like_arguments(node)?;
1276                             Some((Some(found_span), found))
1277                         })
1278                         .unwrap_or((found_span, found));
1279
1280                     self.report_arg_count_mismatch(
1281                         span,
1282                         closure_span,
1283                         expected,
1284                         found,
1285                         found_trait_ty.is_closure(),
1286                     )
1287                 }
1288             }
1289
1290             TraitNotObjectSafe(did) => {
1291                 let violations = self.tcx.object_safety_violations(did);
1292                 report_object_safety_error(self.tcx, span, did, violations)
1293             }
1294
1295             SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsInfer) => {
1296                 bug!(
1297                     "MentionsInfer should have been handled in `traits/fulfill.rs` or `traits/select/mod.rs`"
1298                 )
1299             }
1300             SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsParam) => {
1301                 if !self.tcx.features().generic_const_exprs {
1302                     let mut err = self.tcx.sess.struct_span_err(
1303                         span,
1304                         "constant expression depends on a generic parameter",
1305                     );
1306                     // FIXME(const_generics): we should suggest to the user how they can resolve this
1307                     // issue. However, this is currently not actually possible
1308                     // (see https://github.com/rust-lang/rust/issues/66962#issuecomment-575907083).
1309                     //
1310                     // Note that with `feature(generic_const_exprs)` this case should not
1311                     // be reachable.
1312                     err.note("this may fail depending on what value the parameter takes");
1313                     err.emit();
1314                     return;
1315                 }
1316
1317                 match obligation.predicate.kind().skip_binder() {
1318                     ty::PredicateKind::ConstEvaluatable(ct) => {
1319                         let ty::ConstKind::Unevaluated(uv) = ct.kind() else {
1320                             bug!("const evaluatable failed for non-unevaluated const `{ct:?}`");
1321                         };
1322                         let mut err =
1323                             self.tcx.sess.struct_span_err(span, "unconstrained generic constant");
1324                         let const_span = self.tcx.def_span(uv.def.did);
1325                         match self.tcx.sess.source_map().span_to_snippet(const_span) {
1326                             Ok(snippet) => err.help(&format!(
1327                                 "try adding a `where` bound using this expression: `where [(); {}]:`",
1328                                 snippet
1329                             )),
1330                             _ => err.help("consider adding a `where` bound using this expression"),
1331                         };
1332                         err
1333                     }
1334                     _ => {
1335                         span_bug!(
1336                             span,
1337                             "unexpected non-ConstEvaluatable predicate, this should not be reachable"
1338                         )
1339                     }
1340                 }
1341             }
1342
1343             // Already reported in the query.
1344             SelectionError::NotConstEvaluatable(NotConstEvaluatable::Error(_)) => {
1345                 // FIXME(eddyb) remove this once `ErrorGuaranteed` becomes a proof token.
1346                 self.tcx.sess.delay_span_bug(span, "`ErrorGuaranteed` without an error");
1347                 return;
1348             }
1349             // Already reported.
1350             Overflow(OverflowError::Error(_)) => {
1351                 self.tcx.sess.delay_span_bug(span, "`OverflowError` has been reported");
1352                 return;
1353             }
1354             Overflow(_) => {
1355                 bug!("overflow should be handled before the `report_selection_error` path");
1356             }
1357             SelectionError::ErrorReporting => {
1358                 bug!("ErrorReporting Overflow should not reach `report_selection_err` call")
1359             }
1360         };
1361
1362         self.note_obligation_cause(&mut err, &obligation);
1363         self.point_at_returns_when_relevant(&mut err, &obligation);
1364
1365         err.emit();
1366     }
1367 }
1368
1369 trait InferCtxtPrivExt<'tcx> {
1370     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1371     // `error` occurring implies that `cond` occurs.
1372     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool;
1373
1374     fn report_fulfillment_error(
1375         &self,
1376         error: &FulfillmentError<'tcx>,
1377         body_id: Option<hir::BodyId>,
1378     );
1379
1380     fn report_projection_error(
1381         &self,
1382         obligation: &PredicateObligation<'tcx>,
1383         error: &MismatchedProjectionTypes<'tcx>,
1384     );
1385
1386     fn maybe_detailed_projection_msg(
1387         &self,
1388         pred: ty::ProjectionPredicate<'tcx>,
1389         normalized_ty: ty::Term<'tcx>,
1390         expected_ty: ty::Term<'tcx>,
1391     ) -> Option<String>;
1392
1393     fn fuzzy_match_tys(
1394         &self,
1395         a: Ty<'tcx>,
1396         b: Ty<'tcx>,
1397         ignoring_lifetimes: bool,
1398     ) -> Option<CandidateSimilarity>;
1399
1400     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str>;
1401
1402     fn find_similar_impl_candidates(
1403         &self,
1404         trait_pred: ty::PolyTraitPredicate<'tcx>,
1405     ) -> Vec<ImplCandidate<'tcx>>;
1406
1407     fn report_similar_impl_candidates(
1408         &self,
1409         impl_candidates: Vec<ImplCandidate<'tcx>>,
1410         trait_ref: ty::PolyTraitRef<'tcx>,
1411         body_id: hir::HirId,
1412         err: &mut Diagnostic,
1413     ) -> bool;
1414
1415     /// Gets the parent trait chain start
1416     fn get_parent_trait_ref(
1417         &self,
1418         code: &ObligationCauseCode<'tcx>,
1419     ) -> Option<(String, Option<Span>)>;
1420
1421     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1422     /// with the same path as `trait_ref`, a help message about
1423     /// a probable version mismatch is added to `err`
1424     fn note_version_mismatch(
1425         &self,
1426         err: &mut Diagnostic,
1427         trait_ref: &ty::PolyTraitRef<'tcx>,
1428     ) -> bool;
1429
1430     /// Creates a `PredicateObligation` with `new_self_ty` replacing the existing type in the
1431     /// `trait_ref`.
1432     ///
1433     /// For this to work, `new_self_ty` must have no escaping bound variables.
1434     fn mk_trait_obligation_with_new_self_ty(
1435         &self,
1436         param_env: ty::ParamEnv<'tcx>,
1437         trait_ref_and_ty: ty::Binder<'tcx, (ty::TraitPredicate<'tcx>, Ty<'tcx>)>,
1438     ) -> PredicateObligation<'tcx>;
1439
1440     fn maybe_report_ambiguity(
1441         &self,
1442         obligation: &PredicateObligation<'tcx>,
1443         body_id: Option<hir::BodyId>,
1444     );
1445
1446     fn predicate_can_apply(
1447         &self,
1448         param_env: ty::ParamEnv<'tcx>,
1449         pred: ty::PolyTraitPredicate<'tcx>,
1450     ) -> bool;
1451
1452     fn note_obligation_cause(&self, err: &mut Diagnostic, obligation: &PredicateObligation<'tcx>);
1453
1454     fn suggest_unsized_bound_if_applicable(
1455         &self,
1456         err: &mut Diagnostic,
1457         obligation: &PredicateObligation<'tcx>,
1458     );
1459
1460     fn annotate_source_of_ambiguity(
1461         &self,
1462         err: &mut Diagnostic,
1463         impls: &[DefId],
1464         predicate: ty::Predicate<'tcx>,
1465     );
1466
1467     fn maybe_suggest_unsized_generics(&self, err: &mut Diagnostic, span: Span, node: Node<'tcx>);
1468
1469     fn maybe_indirection_for_unsized(
1470         &self,
1471         err: &mut Diagnostic,
1472         item: &'tcx Item<'tcx>,
1473         param: &'tcx GenericParam<'tcx>,
1474     ) -> bool;
1475
1476     fn is_recursive_obligation(
1477         &self,
1478         obligated_types: &mut Vec<Ty<'tcx>>,
1479         cause_code: &ObligationCauseCode<'tcx>,
1480     ) -> bool;
1481 }
1482
1483 impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
1484     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1485     // `error` occurring implies that `cond` occurs.
1486     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool {
1487         if cond == error {
1488             return true;
1489         }
1490
1491         // FIXME: It should be possible to deal with `ForAll` in a cleaner way.
1492         let bound_error = error.kind();
1493         let (cond, error) = match (cond.kind().skip_binder(), bound_error.skip_binder()) {
1494             (ty::PredicateKind::Trait(..), ty::PredicateKind::Trait(error)) => {
1495                 (cond, bound_error.rebind(error))
1496             }
1497             _ => {
1498                 // FIXME: make this work in other cases too.
1499                 return false;
1500             }
1501         };
1502
1503         for obligation in super::elaborate_predicates(self.tcx, std::iter::once(cond)) {
1504             let bound_predicate = obligation.predicate.kind();
1505             if let ty::PredicateKind::Trait(implication) = bound_predicate.skip_binder() {
1506                 let error = error.to_poly_trait_ref();
1507                 let implication = bound_predicate.rebind(implication.trait_ref);
1508                 // FIXME: I'm just not taking associated types at all here.
1509                 // Eventually I'll need to implement param-env-aware
1510                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
1511                 let param_env = ty::ParamEnv::empty();
1512                 if self.can_sub(param_env, error, implication).is_ok() {
1513                     debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
1514                     return true;
1515                 }
1516             }
1517         }
1518
1519         false
1520     }
1521
1522     #[instrument(skip(self), level = "debug")]
1523     fn report_fulfillment_error(
1524         &self,
1525         error: &FulfillmentError<'tcx>,
1526         body_id: Option<hir::BodyId>,
1527     ) {
1528         match error.code {
1529             FulfillmentErrorCode::CodeSelectionError(ref selection_error) => {
1530                 self.report_selection_error(
1531                     error.obligation.clone(),
1532                     &error.root_obligation,
1533                     selection_error,
1534                 );
1535             }
1536             FulfillmentErrorCode::CodeProjectionError(ref e) => {
1537                 self.report_projection_error(&error.obligation, e);
1538             }
1539             FulfillmentErrorCode::CodeAmbiguity => {
1540                 self.maybe_report_ambiguity(&error.obligation, body_id);
1541             }
1542             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
1543                 self.report_mismatched_types(
1544                     &error.obligation.cause,
1545                     expected_found.expected,
1546                     expected_found.found,
1547                     err.clone(),
1548                 )
1549                 .emit();
1550             }
1551             FulfillmentErrorCode::CodeConstEquateError(ref expected_found, ref err) => {
1552                 let mut diag = self.report_mismatched_consts(
1553                     &error.obligation.cause,
1554                     expected_found.expected,
1555                     expected_found.found,
1556                     err.clone(),
1557                 );
1558                 let code = error.obligation.cause.code().peel_derives().peel_match_impls();
1559                 if let ObligationCauseCode::BindingObligation(..)
1560                 | ObligationCauseCode::ItemObligation(..)
1561                 | ObligationCauseCode::ExprBindingObligation(..)
1562                 | ObligationCauseCode::ExprItemObligation(..) = code
1563                 {
1564                     self.note_obligation_cause_code(
1565                         &mut diag,
1566                         &error.obligation.predicate,
1567                         error.obligation.param_env,
1568                         code,
1569                         &mut vec![],
1570                         &mut Default::default(),
1571                     );
1572                 }
1573                 diag.emit();
1574             }
1575             FulfillmentErrorCode::CodeCycle(ref cycle) => {
1576                 self.report_overflow_error_cycle(cycle);
1577             }
1578         }
1579     }
1580
1581     #[instrument(level = "debug", skip_all)]
1582     fn report_projection_error(
1583         &self,
1584         obligation: &PredicateObligation<'tcx>,
1585         error: &MismatchedProjectionTypes<'tcx>,
1586     ) {
1587         let predicate = self.resolve_vars_if_possible(obligation.predicate);
1588
1589         if predicate.references_error() {
1590             return;
1591         }
1592
1593         self.probe(|_| {
1594             let mut err = error.err;
1595             let mut values = None;
1596
1597             // try to find the mismatched types to report the error with.
1598             //
1599             // this can fail if the problem was higher-ranked, in which
1600             // cause I have no idea for a good error message.
1601             let bound_predicate = predicate.kind();
1602             if let ty::PredicateKind::Projection(data) = bound_predicate.skip_binder() {
1603                 let mut selcx = SelectionContext::new(self);
1604                 let data = self.replace_bound_vars_with_fresh_vars(
1605                     obligation.cause.span,
1606                     infer::LateBoundRegionConversionTime::HigherRankedType,
1607                     bound_predicate.rebind(data),
1608                 );
1609                 let mut obligations = vec![];
1610                 let normalized_ty = super::normalize_projection_type(
1611                     &mut selcx,
1612                     obligation.param_env,
1613                     data.projection_ty,
1614                     obligation.cause.clone(),
1615                     0,
1616                     &mut obligations,
1617                 );
1618
1619                 debug!(?obligation.cause, ?obligation.param_env);
1620
1621                 debug!(?normalized_ty, data.ty = ?data.term);
1622
1623                 let is_normalized_ty_expected = !matches!(
1624                     obligation.cause.code().peel_derives(),
1625                     ObligationCauseCode::ItemObligation(_)
1626                         | ObligationCauseCode::BindingObligation(_, _)
1627                         | ObligationCauseCode::ExprItemObligation(..)
1628                         | ObligationCauseCode::ExprBindingObligation(..)
1629                         | ObligationCauseCode::ObjectCastObligation(..)
1630                         | ObligationCauseCode::OpaqueType
1631                 );
1632                 if let Err(new_err) = self.at(&obligation.cause, obligation.param_env).eq_exp(
1633                     is_normalized_ty_expected,
1634                     normalized_ty,
1635                     data.term,
1636                 ) {
1637                     values = Some((data, is_normalized_ty_expected, normalized_ty, data.term));
1638                     err = new_err;
1639                 }
1640             }
1641
1642             let msg = values
1643                 .and_then(|(predicate, _, normalized_ty, expected_ty)| {
1644                     self.maybe_detailed_projection_msg(predicate, normalized_ty, expected_ty)
1645                 })
1646                 .unwrap_or_else(|| format!("type mismatch resolving `{}`", predicate));
1647             let mut diag = struct_span_err!(self.tcx.sess, obligation.cause.span, E0271, "{msg}");
1648
1649             let secondary_span = match predicate.kind().skip_binder() {
1650                 ty::PredicateKind::Projection(proj) => self
1651                     .tcx
1652                     .opt_associated_item(proj.projection_ty.item_def_id)
1653                     .and_then(|trait_assoc_item| {
1654                         self.tcx
1655                             .trait_of_item(proj.projection_ty.item_def_id)
1656                             .map(|id| (trait_assoc_item, id))
1657                     })
1658                     .and_then(|(trait_assoc_item, id)| {
1659                         let trait_assoc_ident = trait_assoc_item.ident(self.tcx);
1660                         self.tcx.find_map_relevant_impl(id, proj.projection_ty.self_ty(), |did| {
1661                             self.tcx
1662                                 .associated_items(did)
1663                                 .in_definition_order()
1664                                 .find(|assoc| assoc.ident(self.tcx) == trait_assoc_ident)
1665                         })
1666                     })
1667                     .and_then(|item| match self.tcx.hir().get_if_local(item.def_id) {
1668                         Some(
1669                             hir::Node::TraitItem(hir::TraitItem {
1670                                 kind: hir::TraitItemKind::Type(_, Some(ty)),
1671                                 ..
1672                             })
1673                             | hir::Node::ImplItem(hir::ImplItem {
1674                                 kind: hir::ImplItemKind::Type(ty),
1675                                 ..
1676                             }),
1677                         ) => Some((ty.span, format!("type mismatch resolving `{}`", predicate))),
1678                         _ => None,
1679                     }),
1680                 _ => None,
1681             };
1682             self.note_type_err(
1683                 &mut diag,
1684                 &obligation.cause,
1685                 secondary_span,
1686                 values.map(|(_, is_normalized_ty_expected, normalized_ty, term)| {
1687                     infer::ValuePairs::Terms(ExpectedFound::new(
1688                         is_normalized_ty_expected,
1689                         normalized_ty,
1690                         term,
1691                     ))
1692                 }),
1693                 err,
1694                 true,
1695                 false,
1696             );
1697             self.note_obligation_cause(&mut diag, obligation);
1698             diag.emit();
1699         });
1700     }
1701
1702     fn maybe_detailed_projection_msg(
1703         &self,
1704         pred: ty::ProjectionPredicate<'tcx>,
1705         normalized_ty: ty::Term<'tcx>,
1706         expected_ty: ty::Term<'tcx>,
1707     ) -> Option<String> {
1708         let trait_def_id = pred.projection_ty.trait_def_id(self.tcx);
1709         let self_ty = pred.projection_ty.self_ty();
1710
1711         if Some(pred.projection_ty.item_def_id) == self.tcx.lang_items().fn_once_output() {
1712             Some(format!(
1713                 "expected `{self_ty}` to be a {fn_kind} that returns `{expected_ty}`, but it returns `{normalized_ty}`",
1714                 fn_kind = self_ty.prefix_string(self.tcx)
1715             ))
1716         } else if Some(trait_def_id) == self.tcx.lang_items().future_trait() {
1717             Some(format!(
1718                 "expected `{self_ty}` to be a future that resolves to `{expected_ty}`, but it resolves to `{normalized_ty}`"
1719             ))
1720         } else if Some(trait_def_id) == self.tcx.get_diagnostic_item(sym::Iterator) {
1721             Some(format!(
1722                 "expected `{self_ty}` to be an iterator that yields `{expected_ty}`, but it yields `{normalized_ty}`"
1723             ))
1724         } else {
1725             None
1726         }
1727     }
1728
1729     fn fuzzy_match_tys(
1730         &self,
1731         mut a: Ty<'tcx>,
1732         mut b: Ty<'tcx>,
1733         ignoring_lifetimes: bool,
1734     ) -> Option<CandidateSimilarity> {
1735         /// returns the fuzzy category of a given type, or None
1736         /// if the type can be equated to any type.
1737         fn type_category(tcx: TyCtxt<'_>, t: Ty<'_>) -> Option<u32> {
1738             match t.kind() {
1739                 ty::Bool => Some(0),
1740                 ty::Char => Some(1),
1741                 ty::Str => Some(2),
1742                 ty::Adt(def, _) if tcx.is_diagnostic_item(sym::String, def.did()) => Some(2),
1743                 ty::Int(..)
1744                 | ty::Uint(..)
1745                 | ty::Float(..)
1746                 | ty::Infer(ty::IntVar(..) | ty::FloatVar(..)) => Some(4),
1747                 ty::Ref(..) | ty::RawPtr(..) => Some(5),
1748                 ty::Array(..) | ty::Slice(..) => Some(6),
1749                 ty::FnDef(..) | ty::FnPtr(..) => Some(7),
1750                 ty::Dynamic(..) => Some(8),
1751                 ty::Closure(..) => Some(9),
1752                 ty::Tuple(..) => Some(10),
1753                 ty::Param(..) => Some(11),
1754                 ty::Projection(..) => Some(12),
1755                 ty::Opaque(..) => Some(13),
1756                 ty::Never => Some(14),
1757                 ty::Adt(..) => Some(15),
1758                 ty::Generator(..) => Some(16),
1759                 ty::Foreign(..) => Some(17),
1760                 ty::GeneratorWitness(..) => Some(18),
1761                 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error(_) => None,
1762             }
1763         }
1764
1765         let strip_references = |mut t: Ty<'tcx>| -> Ty<'tcx> {
1766             loop {
1767                 match t.kind() {
1768                     ty::Ref(_, inner, _) | ty::RawPtr(ty::TypeAndMut { ty: inner, .. }) => {
1769                         t = *inner
1770                     }
1771                     _ => break t,
1772                 }
1773             }
1774         };
1775
1776         if !ignoring_lifetimes {
1777             a = strip_references(a);
1778             b = strip_references(b);
1779         }
1780
1781         let cat_a = type_category(self.tcx, a)?;
1782         let cat_b = type_category(self.tcx, b)?;
1783         if a == b {
1784             Some(CandidateSimilarity::Exact { ignoring_lifetimes })
1785         } else if cat_a == cat_b {
1786             match (a.kind(), b.kind()) {
1787                 (ty::Adt(def_a, _), ty::Adt(def_b, _)) => def_a == def_b,
1788                 (ty::Foreign(def_a), ty::Foreign(def_b)) => def_a == def_b,
1789                 // Matching on references results in a lot of unhelpful
1790                 // suggestions, so let's just not do that for now.
1791                 //
1792                 // We still upgrade successful matches to `ignoring_lifetimes: true`
1793                 // to prioritize that impl.
1794                 (ty::Ref(..) | ty::RawPtr(..), ty::Ref(..) | ty::RawPtr(..)) => {
1795                     self.fuzzy_match_tys(a, b, true).is_some()
1796                 }
1797                 _ => true,
1798             }
1799             .then_some(CandidateSimilarity::Fuzzy { ignoring_lifetimes })
1800         } else if ignoring_lifetimes {
1801             None
1802         } else {
1803             self.fuzzy_match_tys(a, b, true)
1804         }
1805     }
1806
1807     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str> {
1808         self.tcx.hir().body(body_id).generator_kind.map(|gen_kind| match gen_kind {
1809             hir::GeneratorKind::Gen => "a generator",
1810             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block) => "an async block",
1811             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn) => "an async function",
1812             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Closure) => "an async closure",
1813         })
1814     }
1815
1816     fn find_similar_impl_candidates(
1817         &self,
1818         trait_pred: ty::PolyTraitPredicate<'tcx>,
1819     ) -> Vec<ImplCandidate<'tcx>> {
1820         self.tcx
1821             .all_impls(trait_pred.def_id())
1822             .filter_map(|def_id| {
1823                 if self.tcx.impl_polarity(def_id) == ty::ImplPolarity::Negative
1824                     || !trait_pred
1825                         .skip_binder()
1826                         .is_constness_satisfied_by(self.tcx.constness(def_id))
1827                 {
1828                     return None;
1829                 }
1830
1831                 let imp = self.tcx.impl_trait_ref(def_id).unwrap();
1832
1833                 self.fuzzy_match_tys(trait_pred.skip_binder().self_ty(), imp.self_ty(), false)
1834                     .map(|similarity| ImplCandidate { trait_ref: imp, similarity })
1835             })
1836             .collect()
1837     }
1838
1839     fn report_similar_impl_candidates(
1840         &self,
1841         impl_candidates: Vec<ImplCandidate<'tcx>>,
1842         trait_ref: ty::PolyTraitRef<'tcx>,
1843         body_id: hir::HirId,
1844         err: &mut Diagnostic,
1845     ) -> bool {
1846         let report = |mut candidates: Vec<TraitRef<'tcx>>, err: &mut Diagnostic| {
1847             candidates.sort();
1848             candidates.dedup();
1849             let len = candidates.len();
1850             if candidates.len() == 0 {
1851                 return false;
1852             }
1853             if candidates.len() == 1 {
1854                 let ty_desc = match candidates[0].self_ty().kind() {
1855                     ty::FnPtr(_) => Some("fn pointer"),
1856                     _ => None,
1857                 };
1858                 let the_desc = match ty_desc {
1859                     Some(desc) => format!(" implemented for {} `", desc),
1860                     None => " implemented for `".to_string(),
1861                 };
1862                 err.highlighted_help(vec![
1863                     (
1864                         format!("the trait `{}` ", candidates[0].print_only_trait_path()),
1865                         Style::NoStyle,
1866                     ),
1867                     ("is".to_string(), Style::Highlight),
1868                     (the_desc, Style::NoStyle),
1869                     (candidates[0].self_ty().to_string(), Style::Highlight),
1870                     ("`".to_string(), Style::NoStyle),
1871                 ]);
1872                 return true;
1873             }
1874             let trait_ref = TraitRef::identity(self.tcx, candidates[0].def_id);
1875             // Check if the trait is the same in all cases. If so, we'll only show the type.
1876             let mut traits: Vec<_> =
1877                 candidates.iter().map(|c| c.print_only_trait_path().to_string()).collect();
1878             traits.sort();
1879             traits.dedup();
1880
1881             let mut candidates: Vec<String> = candidates
1882                 .into_iter()
1883                 .map(|c| {
1884                     if traits.len() == 1 {
1885                         format!("\n  {}", c.self_ty())
1886                     } else {
1887                         format!("\n  {}", c)
1888                     }
1889                 })
1890                 .collect();
1891
1892             candidates.sort();
1893             candidates.dedup();
1894             let end = if candidates.len() <= 9 { candidates.len() } else { 8 };
1895             err.help(&format!(
1896                 "the following other types implement trait `{}`:{}{}",
1897                 trait_ref.print_only_trait_path(),
1898                 candidates[..end].join(""),
1899                 if len > 9 { format!("\nand {} others", len - 8) } else { String::new() }
1900             ));
1901             true
1902         };
1903
1904         let def_id = trait_ref.def_id();
1905         if impl_candidates.is_empty() {
1906             if self.tcx.trait_is_auto(def_id)
1907                 || self.tcx.lang_items().iter().any(|(_, id)| id == def_id)
1908                 || self.tcx.get_diagnostic_name(def_id).is_some()
1909             {
1910                 // Mentioning implementers of `Copy`, `Debug` and friends is not useful.
1911                 return false;
1912             }
1913             let normalized_impl_candidates: Vec<_> = self
1914                 .tcx
1915                 .all_impls(def_id)
1916                 // Ignore automatically derived impls and `!Trait` impls.
1917                 .filter(|&def_id| {
1918                     self.tcx.impl_polarity(def_id) != ty::ImplPolarity::Negative
1919                         || self.tcx.is_builtin_derive(def_id)
1920                 })
1921                 .filter_map(|def_id| self.tcx.impl_trait_ref(def_id))
1922                 .filter(|trait_ref| {
1923                     let self_ty = trait_ref.self_ty();
1924                     // Avoid mentioning type parameters.
1925                     if let ty::Param(_) = self_ty.kind() {
1926                         false
1927                     }
1928                     // Avoid mentioning types that are private to another crate
1929                     else if let ty::Adt(def, _) = self_ty.peel_refs().kind() {
1930                         // FIXME(compiler-errors): This could be generalized, both to
1931                         // be more granular, and probably look past other `#[fundamental]`
1932                         // types, too.
1933                         self.tcx
1934                             .visibility(def.did())
1935                             .is_accessible_from(body_id.owner.def_id, self.tcx)
1936                     } else {
1937                         true
1938                     }
1939                 })
1940                 .collect();
1941             return report(normalized_impl_candidates, err);
1942         }
1943
1944         let normalize = |candidate| {
1945             let infcx = self.tcx.infer_ctxt().build();
1946             infcx
1947                 .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
1948                 .normalize(candidate)
1949                 .map_or(candidate, |normalized| normalized.value)
1950         };
1951
1952         // Sort impl candidates so that ordering is consistent for UI tests.
1953         // because the ordering of `impl_candidates` may not be deterministic:
1954         // https://github.com/rust-lang/rust/pull/57475#issuecomment-455519507
1955         //
1956         // Prefer more similar candidates first, then sort lexicographically
1957         // by their normalized string representation.
1958         let mut normalized_impl_candidates_and_similarities = impl_candidates
1959             .into_iter()
1960             .map(|ImplCandidate { trait_ref, similarity }| {
1961                 let normalized = normalize(trait_ref);
1962                 (similarity, normalized)
1963             })
1964             .collect::<Vec<_>>();
1965         normalized_impl_candidates_and_similarities.sort();
1966         normalized_impl_candidates_and_similarities.dedup();
1967
1968         let normalized_impl_candidates = normalized_impl_candidates_and_similarities
1969             .into_iter()
1970             .map(|(_, normalized)| normalized)
1971             .collect::<Vec<_>>();
1972
1973         report(normalized_impl_candidates, err)
1974     }
1975
1976     /// Gets the parent trait chain start
1977     fn get_parent_trait_ref(
1978         &self,
1979         code: &ObligationCauseCode<'tcx>,
1980     ) -> Option<(String, Option<Span>)> {
1981         match code {
1982             ObligationCauseCode::BuiltinDerivedObligation(data) => {
1983                 let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
1984                 match self.get_parent_trait_ref(&data.parent_code) {
1985                     Some(t) => Some(t),
1986                     None => {
1987                         let ty = parent_trait_ref.skip_binder().self_ty();
1988                         let span = TyCategory::from_ty(self.tcx, ty)
1989                             .map(|(_, def_id)| self.tcx.def_span(def_id));
1990                         Some((ty.to_string(), span))
1991                     }
1992                 }
1993             }
1994             ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } => {
1995                 self.get_parent_trait_ref(&parent_code)
1996             }
1997             _ => None,
1998         }
1999     }
2000
2001     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
2002     /// with the same path as `trait_ref`, a help message about
2003     /// a probable version mismatch is added to `err`
2004     fn note_version_mismatch(
2005         &self,
2006         err: &mut Diagnostic,
2007         trait_ref: &ty::PolyTraitRef<'tcx>,
2008     ) -> bool {
2009         let get_trait_impl = |trait_def_id| {
2010             self.tcx.find_map_relevant_impl(trait_def_id, trait_ref.skip_binder().self_ty(), Some)
2011         };
2012         let required_trait_path = self.tcx.def_path_str(trait_ref.def_id());
2013         let traits_with_same_path: std::collections::BTreeSet<_> = self
2014             .tcx
2015             .all_traits()
2016             .filter(|trait_def_id| *trait_def_id != trait_ref.def_id())
2017             .filter(|trait_def_id| self.tcx.def_path_str(*trait_def_id) == required_trait_path)
2018             .collect();
2019         let mut suggested = false;
2020         for trait_with_same_path in traits_with_same_path {
2021             if let Some(impl_def_id) = get_trait_impl(trait_with_same_path) {
2022                 let impl_span = self.tcx.def_span(impl_def_id);
2023                 err.span_help(impl_span, "trait impl with same name found");
2024                 let trait_crate = self.tcx.crate_name(trait_with_same_path.krate);
2025                 let crate_msg = format!(
2026                     "perhaps two different versions of crate `{}` are being used?",
2027                     trait_crate
2028                 );
2029                 err.note(&crate_msg);
2030                 suggested = true;
2031             }
2032         }
2033         suggested
2034     }
2035
2036     fn mk_trait_obligation_with_new_self_ty(
2037         &self,
2038         param_env: ty::ParamEnv<'tcx>,
2039         trait_ref_and_ty: ty::Binder<'tcx, (ty::TraitPredicate<'tcx>, Ty<'tcx>)>,
2040     ) -> PredicateObligation<'tcx> {
2041         let trait_pred = trait_ref_and_ty.map_bound_ref(|(tr, new_self_ty)| ty::TraitPredicate {
2042             trait_ref: ty::TraitRef {
2043                 substs: self.tcx.mk_substs_trait(*new_self_ty, &tr.trait_ref.substs[1..]),
2044                 ..tr.trait_ref
2045             },
2046             ..*tr
2047         });
2048
2049         Obligation::new(ObligationCause::dummy(), param_env, trait_pred.to_predicate(self.tcx))
2050     }
2051
2052     #[instrument(skip(self), level = "debug")]
2053     fn maybe_report_ambiguity(
2054         &self,
2055         obligation: &PredicateObligation<'tcx>,
2056         body_id: Option<hir::BodyId>,
2057     ) {
2058         // Unable to successfully determine, probably means
2059         // insufficient type information, but could mean
2060         // ambiguous impls. The latter *ought* to be a
2061         // coherence violation, so we don't report it here.
2062
2063         let predicate = self.resolve_vars_if_possible(obligation.predicate);
2064         let span = obligation.cause.span;
2065
2066         debug!(?predicate, obligation.cause.code = ?obligation.cause.code());
2067
2068         // Ambiguity errors are often caused as fallout from earlier errors.
2069         // We ignore them if this `infcx` is tainted in some cases below.
2070
2071         let bound_predicate = predicate.kind();
2072         let mut err = match bound_predicate.skip_binder() {
2073             ty::PredicateKind::Trait(data) => {
2074                 let trait_ref = bound_predicate.rebind(data.trait_ref);
2075                 debug!(?trait_ref);
2076
2077                 if predicate.references_error() {
2078                     return;
2079                 }
2080
2081                 // This is kind of a hack: it frequently happens that some earlier
2082                 // error prevents types from being fully inferred, and then we get
2083                 // a bunch of uninteresting errors saying something like "<generic
2084                 // #0> doesn't implement Sized".  It may even be true that we
2085                 // could just skip over all checks where the self-ty is an
2086                 // inference variable, but I was afraid that there might be an
2087                 // inference variable created, registered as an obligation, and
2088                 // then never forced by writeback, and hence by skipping here we'd
2089                 // be ignoring the fact that we don't KNOW the type works
2090                 // out. Though even that would probably be harmless, given that
2091                 // we're only talking about builtin traits, which are known to be
2092                 // inhabited. We used to check for `self.tcx.sess.has_errors()` to
2093                 // avoid inundating the user with unnecessary errors, but we now
2094                 // check upstream for type errors and don't add the obligations to
2095                 // begin with in those cases.
2096                 if self.tcx.lang_items().sized_trait() == Some(trait_ref.def_id()) {
2097                     if !self.is_tainted_by_errors() {
2098                         self.emit_inference_failure_err(
2099                             body_id,
2100                             span,
2101                             trait_ref.self_ty().skip_binder().into(),
2102                             ErrorCode::E0282,
2103                             false,
2104                         )
2105                         .emit();
2106                     }
2107                     return;
2108                 }
2109
2110                 // Typically, this ambiguity should only happen if
2111                 // there are unresolved type inference variables
2112                 // (otherwise it would suggest a coherence
2113                 // failure). But given #21974 that is not necessarily
2114                 // the case -- we can have multiple where clauses that
2115                 // are only distinguished by a region, which results
2116                 // in an ambiguity even when all types are fully
2117                 // known, since we don't dispatch based on region
2118                 // relationships.
2119
2120                 // Pick the first substitution that still contains inference variables as the one
2121                 // we're going to emit an error for. If there are none (see above), fall back to
2122                 // a more general error.
2123                 let subst = data.trait_ref.substs.iter().find(|s| s.has_non_region_infer());
2124
2125                 let mut err = if let Some(subst) = subst {
2126                     self.emit_inference_failure_err(body_id, span, subst, ErrorCode::E0283, true)
2127                 } else {
2128                     struct_span_err!(
2129                         self.tcx.sess,
2130                         span,
2131                         E0283,
2132                         "type annotations needed: cannot satisfy `{}`",
2133                         predicate,
2134                     )
2135                 };
2136
2137                 let obligation = Obligation::new(
2138                     obligation.cause.clone(),
2139                     obligation.param_env,
2140                     trait_ref.to_poly_trait_predicate(),
2141                 );
2142                 let mut selcx = SelectionContext::with_query_mode(
2143                     &self,
2144                     crate::traits::TraitQueryMode::Standard,
2145                 );
2146                 match selcx.select_from_obligation(&obligation) {
2147                     Err(SelectionError::Ambiguous(impls)) if impls.len() > 1 => {
2148                         self.annotate_source_of_ambiguity(&mut err, &impls, predicate);
2149                     }
2150                     _ => {
2151                         if self.is_tainted_by_errors() {
2152                             err.cancel();
2153                             return;
2154                         }
2155                         err.note(&format!("cannot satisfy `{}`", predicate));
2156                     }
2157                 }
2158
2159                 if let ObligationCauseCode::ItemObligation(def_id) | ObligationCauseCode::ExprItemObligation(def_id, ..) = *obligation.cause.code() {
2160                     self.suggest_fully_qualified_path(&mut err, def_id, span, trait_ref.def_id());
2161                 } else if let Ok(snippet) = &self.tcx.sess.source_map().span_to_snippet(span)
2162                     && let ObligationCauseCode::BindingObligation(def_id, _) | ObligationCauseCode::ExprBindingObligation(def_id, ..)
2163                         = *obligation.cause.code()
2164                 {
2165                     let generics = self.tcx.generics_of(def_id);
2166                     if generics.params.iter().any(|p| p.name != kw::SelfUpper)
2167                         && !snippet.ends_with('>')
2168                         && !generics.has_impl_trait()
2169                         && !self.tcx.fn_trait_kind_from_lang_item(def_id).is_some()
2170                     {
2171                         // FIXME: To avoid spurious suggestions in functions where type arguments
2172                         // where already supplied, we check the snippet to make sure it doesn't
2173                         // end with a turbofish. Ideally we would have access to a `PathSegment`
2174                         // instead. Otherwise we would produce the following output:
2175                         //
2176                         // error[E0283]: type annotations needed
2177                         //   --> $DIR/issue-54954.rs:3:24
2178                         //    |
2179                         // LL | const ARR_LEN: usize = Tt::const_val::<[i8; 123]>();
2180                         //    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^
2181                         //    |                        |
2182                         //    |                        cannot infer type
2183                         //    |                        help: consider specifying the type argument
2184                         //    |                        in the function call:
2185                         //    |                        `Tt::const_val::<[i8; 123]>::<T>`
2186                         // ...
2187                         // LL |     const fn const_val<T: Sized>() -> usize {
2188                         //    |                        - required by this bound in `Tt::const_val`
2189                         //    |
2190                         //    = note: cannot satisfy `_: Tt`
2191
2192                         // Clear any more general suggestions in favor of our specific one
2193                         err.clear_suggestions();
2194
2195                         err.span_suggestion_verbose(
2196                             span.shrink_to_hi(),
2197                             &format!(
2198                                 "consider specifying the type argument{} in the function call",
2199                                 pluralize!(generics.params.len()),
2200                             ),
2201                             format!(
2202                                 "::<{}>",
2203                                 generics
2204                                     .params
2205                                     .iter()
2206                                     .map(|p| p.name.to_string())
2207                                     .collect::<Vec<String>>()
2208                                     .join(", ")
2209                             ),
2210                             Applicability::HasPlaceholders,
2211                         );
2212                     }
2213                 }
2214
2215                 if let (Some(body_id), Some(ty::subst::GenericArgKind::Type(_))) =
2216                     (body_id, subst.map(|subst| subst.unpack()))
2217                 {
2218                     struct FindExprBySpan<'hir> {
2219                         span: Span,
2220                         result: Option<&'hir hir::Expr<'hir>>,
2221                     }
2222
2223                     impl<'v> hir::intravisit::Visitor<'v> for FindExprBySpan<'v> {
2224                         fn visit_expr(&mut self, ex: &'v hir::Expr<'v>) {
2225                             if self.span == ex.span {
2226                                 self.result = Some(ex);
2227                             } else {
2228                                 hir::intravisit::walk_expr(self, ex);
2229                             }
2230                         }
2231                     }
2232
2233                     let mut expr_finder = FindExprBySpan { span, result: None };
2234
2235                     expr_finder.visit_expr(&self.tcx.hir().body(body_id).value);
2236
2237                     if let Some(hir::Expr {
2238                         kind: hir::ExprKind::Path(hir::QPath::Resolved(None, path)), .. }
2239                     ) = expr_finder.result
2240                         && let [
2241                             ..,
2242                             trait_path_segment @ hir::PathSegment {
2243                                 res: rustc_hir::def::Res::Def(rustc_hir::def::DefKind::Trait, trait_id),
2244                                 ..
2245                             },
2246                             hir::PathSegment {
2247                                 ident: assoc_item_name,
2248                                 res: rustc_hir::def::Res::Def(_, item_id),
2249                                 ..
2250                             }
2251                         ] = path.segments
2252                         && data.trait_ref.def_id == *trait_id
2253                         && self.tcx.trait_of_item(*item_id) == Some(*trait_id)
2254                         && !self.is_tainted_by_errors()
2255                     {
2256                         let (verb, noun) = match self.tcx.associated_item(item_id).kind {
2257                             ty::AssocKind::Const => ("refer to the", "constant"),
2258                             ty::AssocKind::Fn => ("call", "function"),
2259                             ty::AssocKind::Type => ("refer to the", "type"), // this is already covered by E0223, but this single match arm doesn't hurt here
2260                         };
2261
2262                         // Replace the more general E0283 with a more specific error
2263                         err.cancel();
2264                         err = self.tcx.sess.struct_span_err_with_code(
2265                             span,
2266                             &format!(
2267                                 "cannot {verb} associated {noun} on trait without specifying the corresponding `impl` type",
2268                              ),
2269                             rustc_errors::error_code!(E0790),
2270                         );
2271
2272                         if let Some(local_def_id) = data.trait_ref.def_id.as_local()
2273                             && 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)
2274                             && let Some(method_ref) = trait_item_refs.iter().find(|item_ref| item_ref.ident == *assoc_item_name) {
2275                             err.span_label(method_ref.span, format!("`{}::{}` defined here", trait_name, assoc_item_name));
2276                         }
2277
2278                         err.span_label(span, format!("cannot {verb} associated {noun} of trait"));
2279
2280                         let trait_impls = self.tcx.trait_impls_of(data.trait_ref.def_id);
2281
2282                         if trait_impls.blanket_impls().is_empty()
2283                             && let Some((impl_ty, _)) = trait_impls.non_blanket_impls().iter().next()
2284                             && let Some(impl_def_id) = impl_ty.def() {
2285                             let message = if trait_impls.non_blanket_impls().len() == 1 {
2286                                 "use the fully-qualified path to the only available implementation".to_string()
2287                             } else {
2288                                 format!(
2289                                     "use a fully-qualified path to a specific available implementation ({} found)",
2290                                     trait_impls.non_blanket_impls().len()
2291                                 )
2292                             };
2293                             let mut suggestions = vec![(
2294                                 trait_path_segment.ident.span.shrink_to_lo(),
2295                                 format!("<{} as ", self.tcx.type_of(impl_def_id))
2296                             )];
2297                             if let Some(generic_arg) = trait_path_segment.args {
2298                                 let between_span = trait_path_segment.ident.span.between(generic_arg.span_ext);
2299                                 // get rid of :: between Trait and <type>
2300                                 // must be '::' between them, otherwise the parser won't accept the code
2301                                 suggestions.push((between_span, "".to_string(),));
2302                                 suggestions.push((generic_arg.span_ext.shrink_to_hi(), format!(">")));
2303                             } else {
2304                                 suggestions.push((trait_path_segment.ident.span.shrink_to_hi(), format!(">")));
2305                             }
2306                             err.multipart_suggestion(
2307                                 message,
2308                                 suggestions,
2309                                 Applicability::MaybeIncorrect
2310                             );
2311                         }
2312                     }
2313                 };
2314
2315                 err
2316             }
2317
2318             ty::PredicateKind::WellFormed(arg) => {
2319                 // Same hacky approach as above to avoid deluging user
2320                 // with error messages.
2321                 if arg.references_error()
2322                     || self.tcx.sess.has_errors().is_some()
2323                     || self.is_tainted_by_errors()
2324                 {
2325                     return;
2326                 }
2327
2328                 self.emit_inference_failure_err(body_id, span, arg, ErrorCode::E0282, false)
2329             }
2330
2331             ty::PredicateKind::Subtype(data) => {
2332                 if data.references_error()
2333                     || self.tcx.sess.has_errors().is_some()
2334                     || self.is_tainted_by_errors()
2335                 {
2336                     // no need to overload user in such cases
2337                     return;
2338                 }
2339                 let SubtypePredicate { a_is_expected: _, a, b } = data;
2340                 // both must be type variables, or the other would've been instantiated
2341                 assert!(a.is_ty_var() && b.is_ty_var());
2342                 self.emit_inference_failure_err(body_id, span, a.into(), ErrorCode::E0282, true)
2343             }
2344             ty::PredicateKind::Projection(data) => {
2345                 if predicate.references_error() || self.is_tainted_by_errors() {
2346                     return;
2347                 }
2348                 let subst = data
2349                     .projection_ty
2350                     .substs
2351                     .iter()
2352                     .chain(Some(data.term.into_arg()))
2353                     .find(|g| g.has_non_region_infer());
2354                 if let Some(subst) = subst {
2355                     let mut err = self.emit_inference_failure_err(
2356                         body_id,
2357                         span,
2358                         subst,
2359                         ErrorCode::E0284,
2360                         true,
2361                     );
2362                     err.note(&format!("cannot satisfy `{}`", predicate));
2363                     err
2364                 } else {
2365                     // If we can't find a substitution, just print a generic error
2366                     let mut err = struct_span_err!(
2367                         self.tcx.sess,
2368                         span,
2369                         E0284,
2370                         "type annotations needed: cannot satisfy `{}`",
2371                         predicate,
2372                     );
2373                     err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2374                     err
2375                 }
2376             }
2377
2378             ty::PredicateKind::ConstEvaluatable(data) => {
2379                 if predicate.references_error() || self.is_tainted_by_errors() {
2380                     return;
2381                 }
2382                 let subst = data.walk().find(|g| g.is_non_region_infer());
2383                 if let Some(subst) = subst {
2384                     let err = self.emit_inference_failure_err(
2385                         body_id,
2386                         span,
2387                         subst,
2388                         ErrorCode::E0284,
2389                         true,
2390                     );
2391                     err
2392                 } else {
2393                     // If we can't find a substitution, just print a generic error
2394                     let mut err = struct_span_err!(
2395                         self.tcx.sess,
2396                         span,
2397                         E0284,
2398                         "type annotations needed: cannot satisfy `{}`",
2399                         predicate,
2400                     );
2401                     err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2402                     err
2403                 }
2404             }
2405             _ => {
2406                 if self.tcx.sess.has_errors().is_some() || self.is_tainted_by_errors() {
2407                     return;
2408                 }
2409                 let mut err = struct_span_err!(
2410                     self.tcx.sess,
2411                     span,
2412                     E0284,
2413                     "type annotations needed: cannot satisfy `{}`",
2414                     predicate,
2415                 );
2416                 err.span_label(span, &format!("cannot satisfy `{}`", predicate));
2417                 err
2418             }
2419         };
2420         self.note_obligation_cause(&mut err, obligation);
2421         err.emit();
2422     }
2423
2424     fn annotate_source_of_ambiguity(
2425         &self,
2426         err: &mut Diagnostic,
2427         impls: &[DefId],
2428         predicate: ty::Predicate<'tcx>,
2429     ) {
2430         let mut spans = vec![];
2431         let mut crates = vec![];
2432         let mut post = vec![];
2433         for def_id in impls {
2434             match self.tcx.span_of_impl(*def_id) {
2435                 Ok(span) => spans.push(span),
2436                 Err(name) => {
2437                     crates.push(name);
2438                     if let Some(header) = to_pretty_impl_header(self.tcx, *def_id) {
2439                         post.push(header);
2440                     }
2441                 }
2442             }
2443         }
2444         let msg = format!("multiple `impl`s satisfying `{}` found", predicate);
2445         let mut crate_names: Vec<_> = crates.iter().map(|n| format!("`{}`", n)).collect();
2446         crate_names.sort();
2447         crate_names.dedup();
2448         post.sort();
2449         post.dedup();
2450
2451         if self.is_tainted_by_errors()
2452             && (crate_names.len() == 1
2453                 && spans.len() == 0
2454                 && ["`core`", "`alloc`", "`std`"].contains(&crate_names[0].as_str())
2455                 || predicate.visit_with(&mut HasNumericInferVisitor).is_break())
2456         {
2457             // Avoid complaining about other inference issues for expressions like
2458             // `42 >> 1`, where the types are still `{integer}`, but we want to
2459             // Do we need `trait_ref.skip_binder().self_ty().is_numeric() &&` too?
2460             // NOTE(eddyb) this was `.cancel()`, but `err`
2461             // is borrowed, so we can't fully defuse it.
2462             err.downgrade_to_delayed_bug();
2463             return;
2464         }
2465         let post = if post.len() > 4 {
2466             format!(
2467                 ":\n{}\nand {} more",
2468                 post.iter().map(|p| format!("- {}", p)).take(4).collect::<Vec<_>>().join("\n"),
2469                 post.len() - 4,
2470             )
2471         } else if post.len() > 1 || (post.len() == 1 && post[0].contains('\n')) {
2472             format!(":\n{}", post.iter().map(|p| format!("- {}", p)).collect::<Vec<_>>().join("\n"),)
2473         } else if post.len() == 1 {
2474             format!(": `{}`", post[0])
2475         } else {
2476             String::new()
2477         };
2478
2479         match (spans.len(), crates.len(), crate_names.len()) {
2480             (0, 0, 0) => {
2481                 err.note(&format!("cannot satisfy `{}`", predicate));
2482             }
2483             (0, _, 1) => {
2484                 err.note(&format!("{} in the `{}` crate{}", msg, crates[0], post,));
2485             }
2486             (0, _, _) => {
2487                 err.note(&format!(
2488                     "{} in the following crates: {}{}",
2489                     msg,
2490                     crate_names.join(", "),
2491                     post,
2492                 ));
2493             }
2494             (_, 0, 0) => {
2495                 let span: MultiSpan = spans.into();
2496                 err.span_note(span, &msg);
2497             }
2498             (_, 1, 1) => {
2499                 let span: MultiSpan = spans.into();
2500                 err.span_note(span, &msg);
2501                 err.note(
2502                     &format!("and another `impl` found in the `{}` crate{}", crates[0], post,),
2503                 );
2504             }
2505             _ => {
2506                 let span: MultiSpan = spans.into();
2507                 err.span_note(span, &msg);
2508                 err.note(&format!(
2509                     "and more `impl`s found in the following crates: {}{}",
2510                     crate_names.join(", "),
2511                     post,
2512                 ));
2513             }
2514         }
2515     }
2516
2517     /// Returns `true` if the trait predicate may apply for *some* assignment
2518     /// to the type parameters.
2519     fn predicate_can_apply(
2520         &self,
2521         param_env: ty::ParamEnv<'tcx>,
2522         pred: ty::PolyTraitPredicate<'tcx>,
2523     ) -> bool {
2524         struct ParamToVarFolder<'a, 'tcx> {
2525             infcx: &'a InferCtxt<'tcx>,
2526             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
2527         }
2528
2529         impl<'a, 'tcx> TypeFolder<'tcx> for ParamToVarFolder<'a, 'tcx> {
2530             fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
2531                 self.infcx.tcx
2532             }
2533
2534             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
2535                 if let ty::Param(ty::ParamTy { name, .. }) = *ty.kind() {
2536                     let infcx = self.infcx;
2537                     *self.var_map.entry(ty).or_insert_with(|| {
2538                         infcx.next_ty_var(TypeVariableOrigin {
2539                             kind: TypeVariableOriginKind::TypeParameterDefinition(name, None),
2540                             span: DUMMY_SP,
2541                         })
2542                     })
2543                 } else {
2544                     ty.super_fold_with(self)
2545                 }
2546             }
2547         }
2548
2549         self.probe(|_| {
2550             let mut selcx = SelectionContext::new(self);
2551
2552             let cleaned_pred =
2553                 pred.fold_with(&mut ParamToVarFolder { infcx: self, var_map: Default::default() });
2554
2555             let cleaned_pred = super::project::normalize(
2556                 &mut selcx,
2557                 param_env,
2558                 ObligationCause::dummy(),
2559                 cleaned_pred,
2560             )
2561             .value;
2562
2563             let obligation = Obligation::new(
2564                 ObligationCause::dummy(),
2565                 param_env,
2566                 cleaned_pred.to_predicate(selcx.tcx()),
2567             );
2568
2569             self.predicate_may_hold(&obligation)
2570         })
2571     }
2572
2573     fn note_obligation_cause(&self, err: &mut Diagnostic, obligation: &PredicateObligation<'tcx>) {
2574         // First, attempt to add note to this error with an async-await-specific
2575         // message, and fall back to regular note otherwise.
2576         if !self.maybe_note_obligation_cause_for_async_await(err, obligation) {
2577             self.note_obligation_cause_code(
2578                 err,
2579                 &obligation.predicate,
2580                 obligation.param_env,
2581                 obligation.cause.code(),
2582                 &mut vec![],
2583                 &mut Default::default(),
2584             );
2585             self.suggest_unsized_bound_if_applicable(err, obligation);
2586         }
2587     }
2588
2589     #[instrument(level = "debug", skip_all)]
2590     fn suggest_unsized_bound_if_applicable(
2591         &self,
2592         err: &mut Diagnostic,
2593         obligation: &PredicateObligation<'tcx>,
2594     ) {
2595         let ty::PredicateKind::Trait(pred) = obligation.predicate.kind().skip_binder() else { return; };
2596         let (ObligationCauseCode::BindingObligation(item_def_id, span)
2597         | ObligationCauseCode::ExprBindingObligation(item_def_id, span, ..))
2598             = *obligation.cause.code().peel_derives() else { return; };
2599         debug!(?pred, ?item_def_id, ?span);
2600
2601         let (Some(node), true) = (
2602             self.tcx.hir().get_if_local(item_def_id),
2603             Some(pred.def_id()) == self.tcx.lang_items().sized_trait(),
2604         ) else {
2605             return;
2606         };
2607         self.maybe_suggest_unsized_generics(err, span, node);
2608     }
2609
2610     #[instrument(level = "debug", skip_all)]
2611     fn maybe_suggest_unsized_generics(&self, err: &mut Diagnostic, span: Span, node: Node<'tcx>) {
2612         let Some(generics) = node.generics() else {
2613             return;
2614         };
2615         let sized_trait = self.tcx.lang_items().sized_trait();
2616         debug!(?generics.params);
2617         debug!(?generics.predicates);
2618         let Some(param) = generics.params.iter().find(|param| param.span == span) else {
2619             return;
2620         };
2621         let param_def_id = self.tcx.hir().local_def_id(param.hir_id);
2622         // Check that none of the explicit trait bounds is `Sized`. Assume that an explicit
2623         // `Sized` bound is there intentionally and we don't need to suggest relaxing it.
2624         let explicitly_sized = generics
2625             .bounds_for_param(param_def_id)
2626             .flat_map(|bp| bp.bounds)
2627             .any(|bound| bound.trait_ref().and_then(|tr| tr.trait_def_id()) == sized_trait);
2628         if explicitly_sized {
2629             return;
2630         }
2631         debug!(?param);
2632         match node {
2633             hir::Node::Item(
2634                 item @ hir::Item {
2635                     // Only suggest indirection for uses of type parameters in ADTs.
2636                     kind:
2637                         hir::ItemKind::Enum(..) | hir::ItemKind::Struct(..) | hir::ItemKind::Union(..),
2638                     ..
2639                 },
2640             ) => {
2641                 if self.maybe_indirection_for_unsized(err, item, param) {
2642                     return;
2643                 }
2644             }
2645             _ => {}
2646         };
2647         // Didn't add an indirection suggestion, so add a general suggestion to relax `Sized`.
2648         let (span, separator) = if let Some(s) = generics.bounds_span_for_suggestions(param_def_id)
2649         {
2650             (s, " +")
2651         } else {
2652             (span.shrink_to_hi(), ":")
2653         };
2654         err.span_suggestion_verbose(
2655             span,
2656             "consider relaxing the implicit `Sized` restriction",
2657             format!("{} ?Sized", separator),
2658             Applicability::MachineApplicable,
2659         );
2660     }
2661
2662     fn maybe_indirection_for_unsized(
2663         &self,
2664         err: &mut Diagnostic,
2665         item: &Item<'tcx>,
2666         param: &GenericParam<'tcx>,
2667     ) -> bool {
2668         // Suggesting `T: ?Sized` is only valid in an ADT if `T` is only used in a
2669         // borrow. `struct S<'a, T: ?Sized>(&'a T);` is valid, `struct S<T: ?Sized>(T);`
2670         // is not. Look for invalid "bare" parameter uses, and suggest using indirection.
2671         let mut visitor =
2672             FindTypeParam { param: param.name.ident().name, invalid_spans: vec![], nested: false };
2673         visitor.visit_item(item);
2674         if visitor.invalid_spans.is_empty() {
2675             return false;
2676         }
2677         let mut multispan: MultiSpan = param.span.into();
2678         multispan.push_span_label(
2679             param.span,
2680             format!("this could be changed to `{}: ?Sized`...", param.name.ident()),
2681         );
2682         for sp in visitor.invalid_spans {
2683             multispan.push_span_label(
2684                 sp,
2685                 format!("...if indirection were used here: `Box<{}>`", param.name.ident()),
2686             );
2687         }
2688         err.span_help(
2689             multispan,
2690             &format!(
2691                 "you could relax the implicit `Sized` bound on `{T}` if it were \
2692                 used through indirection like `&{T}` or `Box<{T}>`",
2693                 T = param.name.ident(),
2694             ),
2695         );
2696         true
2697     }
2698
2699     fn is_recursive_obligation(
2700         &self,
2701         obligated_types: &mut Vec<Ty<'tcx>>,
2702         cause_code: &ObligationCauseCode<'tcx>,
2703     ) -> bool {
2704         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
2705             let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_pred);
2706             let self_ty = parent_trait_ref.skip_binder().self_ty();
2707             if obligated_types.iter().any(|ot| ot == &self_ty) {
2708                 return true;
2709             }
2710             if let ty::Adt(def, substs) = self_ty.kind()
2711                 && let [arg] = &substs[..]
2712                 && let ty::subst::GenericArgKind::Type(ty) = arg.unpack()
2713                 && let ty::Adt(inner_def, _) = ty.kind()
2714                 && inner_def == def
2715             {
2716                 return true;
2717             }
2718         }
2719         false
2720     }
2721 }
2722
2723 /// Look for type `param` in an ADT being used only through a reference to confirm that suggesting
2724 /// `param: ?Sized` would be a valid constraint.
2725 struct FindTypeParam {
2726     param: rustc_span::Symbol,
2727     invalid_spans: Vec<Span>,
2728     nested: bool,
2729 }
2730
2731 impl<'v> Visitor<'v> for FindTypeParam {
2732     fn visit_where_predicate(&mut self, _: &'v hir::WherePredicate<'v>) {
2733         // Skip where-clauses, to avoid suggesting indirection for type parameters found there.
2734     }
2735
2736     fn visit_ty(&mut self, ty: &hir::Ty<'_>) {
2737         // We collect the spans of all uses of the "bare" type param, like in `field: T` or
2738         // `field: (T, T)` where we could make `T: ?Sized` while skipping cases that are known to be
2739         // valid like `field: &'a T` or `field: *mut T` and cases that *might* have further `Sized`
2740         // obligations like `Box<T>` and `Vec<T>`, but we perform no extra analysis for those cases
2741         // and suggest `T: ?Sized` regardless of their obligations. This is fine because the errors
2742         // in that case should make what happened clear enough.
2743         match ty.kind {
2744             hir::TyKind::Ptr(_) | hir::TyKind::Rptr(..) | hir::TyKind::TraitObject(..) => {}
2745             hir::TyKind::Path(hir::QPath::Resolved(None, path))
2746                 if path.segments.len() == 1 && path.segments[0].ident.name == self.param =>
2747             {
2748                 if !self.nested {
2749                     debug!(?ty, "FindTypeParam::visit_ty");
2750                     self.invalid_spans.push(ty.span);
2751                 }
2752             }
2753             hir::TyKind::Path(_) => {
2754                 let prev = self.nested;
2755                 self.nested = true;
2756                 hir::intravisit::walk_ty(self, ty);
2757                 self.nested = prev;
2758             }
2759             _ => {
2760                 hir::intravisit::walk_ty(self, ty);
2761             }
2762         }
2763     }
2764 }
2765
2766 /// Summarizes information
2767 #[derive(Clone)]
2768 pub enum ArgKind {
2769     /// An argument of non-tuple type. Parameters are (name, ty)
2770     Arg(String, String),
2771
2772     /// An argument of tuple type. For a "found" argument, the span is
2773     /// the location in the source of the pattern. For an "expected"
2774     /// argument, it will be None. The vector is a list of (name, ty)
2775     /// strings for the components of the tuple.
2776     Tuple(Option<Span>, Vec<(String, String)>),
2777 }
2778
2779 impl ArgKind {
2780     fn empty() -> ArgKind {
2781         ArgKind::Arg("_".to_owned(), "_".to_owned())
2782     }
2783
2784     /// Creates an `ArgKind` from the expected type of an
2785     /// argument. It has no name (`_`) and an optional source span.
2786     pub fn from_expected_ty(t: Ty<'_>, span: Option<Span>) -> ArgKind {
2787         match t.kind() {
2788             ty::Tuple(tys) => ArgKind::Tuple(
2789                 span,
2790                 tys.iter().map(|ty| ("_".to_owned(), ty.to_string())).collect::<Vec<_>>(),
2791             ),
2792             _ => ArgKind::Arg("_".to_owned(), t.to_string()),
2793         }
2794     }
2795 }
2796
2797 struct HasNumericInferVisitor;
2798
2799 impl<'tcx> ty::TypeVisitor<'tcx> for HasNumericInferVisitor {
2800     type BreakTy = ();
2801
2802     fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
2803         if matches!(ty.kind(), ty::Infer(ty::FloatVar(_) | ty::IntVar(_))) {
2804             ControlFlow::Break(())
2805         } else {
2806             ControlFlow::CONTINUE
2807         }
2808     }
2809 }
2810
2811 pub enum DefIdOrName {
2812     DefId(DefId),
2813     Name(&'static str),
2814 }