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

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