src/librustc_trait_selection/traits/error_reporting/mod.rs

   1 pub mod on_unimplemented;
   2 pub mod suggestions;
   3
   4 use super::{
   5     ConstEvalFailure, EvaluationResult, FulfillmentError, FulfillmentErrorCode,
   6     MismatchedProjectionTypes, Obligation, ObligationCause, ObligationCauseCode,
   7     OnUnimplementedDirective, OnUnimplementedNote, OutputTypeParameterMismatch, Overflow,
   8     PredicateObligation, SelectionContext, SelectionError, TraitNotObjectSafe,
   9 };
  10
  11 use crate::infer::error_reporting::{TyCategory, TypeAnnotationNeeded as ErrorCode};
  12 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  13 use crate::infer::{self, InferCtxt, TyCtxtInferExt};
  14 use rustc_data_structures::fx::FxHashMap;
  15 use rustc_errors::{pluralize, struct_span_err, Applicability, DiagnosticBuilder, ErrorReported};
  16 use rustc_hir as hir;
  17 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
  18 use rustc_hir::Node;
  19 use rustc_middle::mir::interpret::ErrorHandled;
  20 use rustc_middle::ty::error::ExpectedFound;
  21 use rustc_middle::ty::fold::TypeFolder;
  22 use rustc_middle::ty::subst::GenericArgKind;
  23 use rustc_middle::ty::{
  24     self, fast_reject, AdtKind, SubtypePredicate, ToPolyTraitRef, ToPredicate, Ty, TyCtxt,
  25     TypeFoldable, WithConstness,
  26 };
  27 use rustc_session::DiagnosticMessageId;
  28 use rustc_span::{ExpnKind, Span, DUMMY_SP};
  29 use std::fmt;
  30
  31 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
  32 use crate::traits::query::normalize::AtExt as _;
  33 use on_unimplemented::InferCtxtExt as _;
  34 use suggestions::InferCtxtExt as _;
  35
  36 pub use rustc_infer::traits::error_reporting::*;
  37
  38 pub trait InferCtxtExt<'tcx> {
  39     fn report_fulfillment_errors(
  40         &self,
  41         errors: &[FulfillmentError<'tcx>],
  42         body_id: Option<hir::BodyId>,
  43         fallback_has_occurred: bool,
  44     );
  45
  46     fn report_overflow_error<T>(
  47         &self,
  48         obligation: &Obligation<'tcx, T>,
  49         suggest_increasing_limit: bool,
  50     ) -> !
  51     where
  52         T: fmt::Display + TypeFoldable<'tcx>;
  53
  54     fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> !;
  55
  56     fn report_selection_error(
  57         &self,
  58         obligation: &PredicateObligation<'tcx>,
  59         error: &SelectionError<'tcx>,
  60         fallback_has_occurred: bool,
  61         points_at_arg: bool,
  62     );
  63
  64     /// Given some node representing a fn-like thing in the HIR map,
  65     /// returns a span and `ArgKind` information that describes the
  66     /// arguments it expects. This can be supplied to
  67     /// `report_arg_count_mismatch`.
  68     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)>;
  69
  70     /// Reports an error when the number of arguments needed by a
  71     /// trait match doesn't match the number that the expression
  72     /// provides.
  73     fn report_arg_count_mismatch(
  74         &self,
  75         span: Span,
  76         found_span: Option<Span>,
  77         expected_args: Vec<ArgKind>,
  78         found_args: Vec<ArgKind>,
  79         is_closure: bool,
  80     ) -> DiagnosticBuilder<'tcx>;
  81 }
  82
  83 impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
  84     fn report_fulfillment_errors(
  85         &self,
  86         errors: &[FulfillmentError<'tcx>],
  87         body_id: Option<hir::BodyId>,
  88         fallback_has_occurred: bool,
  89     ) {
  90         #[derive(Debug)]
  91         struct ErrorDescriptor<'tcx> {
  92             predicate: ty::Predicate<'tcx>,
  93             index: Option<usize>, // None if this is an old error
  94         }
  95
  96         let mut error_map: FxHashMap<_, Vec<_>> = self
  97             .reported_trait_errors
  98             .borrow()
  99             .iter()
 100             .map(|(&span, predicates)| {
 101                 (
 102                     span,
 103                     predicates
 104                         .iter()
 105                         .map(|&predicate| ErrorDescriptor { predicate, index: None })
 106                         .collect(),
 107                 )
 108             })
 109             .collect();
 110
 111         for (index, error) in errors.iter().enumerate() {
 112             // We want to ignore desugarings here: spans are equivalent even
 113             // if one is the result of a desugaring and the other is not.
 114             let mut span = error.obligation.cause.span;
 115             let expn_data = span.ctxt().outer_expn_data();
 116             if let ExpnKind::Desugaring(_) = expn_data.kind {
 117                 span = expn_data.call_site;
 118             }
 119
 120             error_map.entry(span).or_default().push(ErrorDescriptor {
 121                 predicate: error.obligation.predicate,
 122                 index: Some(index),
 123             });
 124
 125             self.reported_trait_errors
 126                 .borrow_mut()
 127                 .entry(span)
 128                 .or_default()
 129                 .push(error.obligation.predicate);
 130         }
 131
 132         // We do this in 2 passes because we want to display errors in order, though
 133         // maybe it *is* better to sort errors by span or something.
 134         let mut is_suppressed = vec![false; errors.len()];
 135         for (_, error_set) in error_map.iter() {
 136             // We want to suppress "duplicate" errors with the same span.
 137             for error in error_set {
 138                 if let Some(index) = error.index {
 139                     // Suppress errors that are either:
 140                     // 1) strictly implied by another error.
 141                     // 2) implied by an error with a smaller index.
 142                     for error2 in error_set {
 143                         if error2.index.map_or(false, |index2| is_suppressed[index2]) {
 144                             // Avoid errors being suppressed by already-suppressed
 145                             // errors, to prevent all errors from being suppressed
 146                             // at once.
 147                             continue;
 148                         }
 149
 150                         if self.error_implies(error2.predicate, error.predicate)
 151                             && !(error2.index >= error.index
 152                                 && self.error_implies(error.predicate, error2.predicate))
 153                         {
 154                             info!("skipping {:?} (implied by {:?})", error, error2);
 155                             is_suppressed[index] = true;
 156                             break;
 157                         }
 158                     }
 159                 }
 160             }
 161         }
 162
 163         for (error, suppressed) in errors.iter().zip(is_suppressed) {
 164             if !suppressed {
 165                 self.report_fulfillment_error(error, body_id, fallback_has_occurred);
 166             }
 167         }
 168     }
 169
 170     /// Reports that an overflow has occurred and halts compilation. We
 171     /// halt compilation unconditionally because it is important that
 172     /// overflows never be masked -- they basically represent computations
 173     /// whose result could not be truly determined and thus we can't say
 174     /// if the program type checks or not -- and they are unusual
 175     /// occurrences in any case.
 176     fn report_overflow_error<T>(
 177         &self,
 178         obligation: &Obligation<'tcx, T>,
 179         suggest_increasing_limit: bool,
 180     ) -> !
 181     where
 182         T: fmt::Display + TypeFoldable<'tcx>,
 183     {
 184         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
 185         let mut err = struct_span_err!(
 186             self.tcx.sess,
 187             obligation.cause.span,
 188             E0275,
 189             "overflow evaluating the requirement `{}`",
 190             predicate
 191         );
 192
 193         if suggest_increasing_limit {
 194             self.suggest_new_overflow_limit(&mut err);
 195         }
 196
 197         self.note_obligation_cause_code(
 198             &mut err,
 199             &obligation.predicate,
 200             &obligation.cause.code,
 201             &mut vec![],
 202         );
 203
 204         err.emit();
 205         self.tcx.sess.abort_if_errors();
 206         bug!();
 207     }
 208
 209     /// Reports that a cycle was detected which led to overflow and halts
 210     /// compilation. This is equivalent to `report_overflow_error` except
 211     /// that we can give a more helpful error message (and, in particular,
 212     /// we do not suggest increasing the overflow limit, which is not
 213     /// going to help).
 214     fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 215         let cycle = self.resolve_vars_if_possible(&cycle.to_owned());
 216         assert!(!cycle.is_empty());
 217
 218         debug!("report_overflow_error_cycle: cycle={:?}", cycle);
 219
 220         self.report_overflow_error(&cycle[0], false);
 221     }
 222
 223     fn report_selection_error(
 224         &self,
 225         obligation: &PredicateObligation<'tcx>,
 226         error: &SelectionError<'tcx>,
 227         fallback_has_occurred: bool,
 228         points_at_arg: bool,
 229     ) {
 230         let tcx = self.tcx;
 231         let span = obligation.cause.span;
 232
 233         let mut err = match *error {
 234             SelectionError::Unimplemented => {
 235                 if let ObligationCauseCode::CompareImplMethodObligation {
 236                     item_name,
 237                     impl_item_def_id,
 238                     trait_item_def_id,
 239                 }
 240                 | ObligationCauseCode::CompareImplTypeObligation {
 241                     item_name,
 242                     impl_item_def_id,
 243                     trait_item_def_id,
 244                 } = obligation.cause.code
 245                 {
 246                     self.report_extra_impl_obligation(
 247                         span,
 248                         item_name,
 249                         impl_item_def_id,
 250                         trait_item_def_id,
 251                         &format!("`{}`", obligation.predicate),
 252                     )
 253                     .emit();
 254                     return;
 255                 }
 256                 match obligation.predicate.kind() {
 257                     ty::PredicateKind::Trait(ref trait_predicate, _) => {
 258                         let trait_predicate = self.resolve_vars_if_possible(trait_predicate);
 259
 260                         if self.tcx.sess.has_errors() && trait_predicate.references_error() {
 261                             return;
 262                         }
 263                         let trait_ref = trait_predicate.to_poly_trait_ref();
 264                         let (post_message, pre_message, type_def) = self
 265                             .get_parent_trait_ref(&obligation.cause.code)
 266                             .map(|(t, s)| {
 267                                 (
 268                                     format!(" in `{}`", t),
 269                                     format!("within `{}`, ", t),
 270                                     s.map(|s| (format!("within this `{}`", t), s)),
 271                                 )
 272                             })
 273                             .unwrap_or_default();
 274
 275                         let OnUnimplementedNote { message, label, note, enclosing_scope } =
 276                             self.on_unimplemented_note(trait_ref, obligation);
 277                         let have_alt_message = message.is_some() || label.is_some();
 278                         let is_try = self
 279                             .tcx
 280                             .sess
 281                             .source_map()
 282                             .span_to_snippet(span)
 283                             .map(|s| &s == "?")
 284                             .unwrap_or(false);
 285                         let is_from = format!("{}", trait_ref.print_only_trait_path())
 286                             .starts_with("std::convert::From<");
 287                         let is_unsize =
 288                             { Some(trait_ref.def_id()) == self.tcx.lang_items().unsize_trait() };
 289                         let (message, note) = if is_try && is_from {
 290                             (
 291                                 Some(format!(
 292                                     "`?` couldn't convert the error to `{}`",
 293                                     trait_ref.skip_binder().self_ty(),
 294                                 )),
 295                                 Some(
 296                                     "the question mark operation (`?`) implicitly performs a \
 297                                         conversion on the error value using the `From` trait"
 298                                         .to_owned(),
 299                                 ),
 300                             )
 301                         } else {
 302                             (message, note)
 303                         };
 304
 305                         let mut err = struct_span_err!(
 306                             self.tcx.sess,
 307                             span,
 308                             E0277,
 309                             "{}",
 310                             message.unwrap_or_else(|| format!(
 311                                 "the trait bound `{}` is not satisfied{}",
 312                                 trait_ref.without_const().to_predicate(tcx),
 313                                 post_message,
 314                             ))
 315                         );
 316
 317                         let should_convert_option_to_result =
 318                             format!("{}", trait_ref.print_only_trait_path())
 319                                 .starts_with("std::convert::From<std::option::NoneError");
 320                         let should_convert_result_to_option = format!("{}", trait_ref)
 321                             .starts_with("<std::option::NoneError as std::convert::From<");
 322                         if is_try && is_from {
 323                             if should_convert_option_to_result {
 324                                 err.span_suggestion_verbose(
 325                                     span.shrink_to_lo(),
 326                                     "consider converting the `Option<T>` into a `Result<T, _>` \
 327                                      using `Option::ok_or` or `Option::ok_or_else`",
 328                                     ".ok_or_else(|| /* error value */)".to_string(),
 329                                     Applicability::HasPlaceholders,
 330                                 );
 331                             } else if should_convert_result_to_option {
 332                                 err.span_suggestion_verbose(
 333                                     span.shrink_to_lo(),
 334                                     "consider converting the `Result<T, _>` into an `Option<T>` \
 335                                      using `Result::ok`",
 336                                     ".ok()".to_string(),
 337                                     Applicability::MachineApplicable,
 338                                 );
 339                             }
 340                             if let Some(ret_span) = self.return_type_span(obligation) {
 341                                 err.span_label(
 342                                     ret_span,
 343                                     &format!(
 344                                         "expected `{}` because of this",
 345                                         trait_ref.skip_binder().self_ty()
 346                                     ),
 347                                 );
 348                             }
 349                         }
 350
 351                         let explanation =
 352                             if obligation.cause.code == ObligationCauseCode::MainFunctionType {
 353                                 "consider using `()`, or a `Result`".to_owned()
 354                             } else {
 355                                 format!(
 356                                     "{}the trait `{}` is not implemented for `{}`",
 357                                     pre_message,
 358                                     trait_ref.print_only_trait_path(),
 359                                     trait_ref.skip_binder().self_ty(),
 360                                 )
 361                             };
 362
 363                         if self.suggest_add_reference_to_arg(
 364                             &obligation,
 365                             &mut err,
 366                             &trait_ref,
 367                             points_at_arg,
 368                             have_alt_message,
 369                         ) {
 370                             self.note_obligation_cause(&mut err, obligation);
 371                             err.emit();
 372                             return;
 373                         }
 374                         if let Some(ref s) = label {
 375                             // If it has a custom `#[rustc_on_unimplemented]`
 376                             // error message, let's display it as the label!
 377                             err.span_label(span, s.as_str());
 378                             err.help(&explanation);
 379                         } else {
 380                             err.span_label(span, explanation);
 381                         }
 382                         if let Some((msg, span)) = type_def {
 383                             err.span_label(span, &msg);
 384                         }
 385                         if let Some(ref s) = note {
 386                             // If it has a custom `#[rustc_on_unimplemented]` note, let's display it
 387                             err.note(s.as_str());
 388                         }
 389                         if let Some(ref s) = enclosing_scope {
 390                             let enclosing_scope_span = tcx.def_span(
 391                                 tcx.hir()
 392                                     .opt_local_def_id(obligation.cause.body_id)
 393                                     .unwrap_or_else(|| {
 394                                         tcx.hir().body_owner_def_id(hir::BodyId {
 395                                             hir_id: obligation.cause.body_id,
 396                                         })
 397                                     })
 398                                     .to_def_id(),
 399                             );
 400
 401                             err.span_label(enclosing_scope_span, s.as_str());
 402                         }
 403
 404                         self.suggest_borrow_on_unsized_slice(&obligation.cause.code, &mut err);
 405                         self.suggest_fn_call(&obligation, &mut err, &trait_ref, points_at_arg);
 406                         self.suggest_remove_reference(&obligation, &mut err, &trait_ref);
 407                         self.suggest_semicolon_removal(&obligation, &mut err, span, &trait_ref);
 408                         self.note_version_mismatch(&mut err, &trait_ref);
 409
 410                         if Some(trait_ref.def_id()) == tcx.lang_items().try_trait() {
 411                             self.suggest_await_before_try(&mut err, &obligation, &trait_ref, span);
 412                         }
 413
 414                         if self.suggest_impl_trait(&mut err, span, &obligation, &trait_ref) {
 415                             err.emit();
 416                             return;
 417                         }
 418
 419                         if is_unsize {
 420                             // If the obligation failed due to a missing implementation of the
 421                             // `Unsize` trait, give a pointer to why that might be the case
 422                             err.note(
 423                                 "all implementations of `Unsize` are provided \
 424                                 automatically by the compiler, see \
 425                                 <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> \
 426                                 for more information",
 427                             );
 428                         }
 429
 430                         // Try to report a help message
 431                         if !trait_ref.has_infer_types_or_consts()
 432                             && self.predicate_can_apply(obligation.param_env, trait_ref)
 433                         {
 434                             // If a where-clause may be useful, remind the
 435                             // user that they can add it.
 436                             //
 437                             // don't display an on-unimplemented note, as
 438                             // these notes will often be of the form
 439                             //     "the type `T` can't be frobnicated"
 440                             // which is somewhat confusing.
 441                             self.suggest_restricting_param_bound(
 442                                 &mut err,
 443                                 trait_ref,
 444                                 obligation.cause.body_id,
 445                             );
 446                         } else {
 447                             if !have_alt_message {
 448                                 // Can't show anything else useful, try to find similar impls.
 449                                 let impl_candidates = self.find_similar_impl_candidates(trait_ref);
 450                                 self.report_similar_impl_candidates(impl_candidates, &mut err);
 451                             }
 452                             // Changing mutability doesn't make a difference to whether we have
 453                             // an `Unsize` impl (Fixes ICE in #71036)
 454                             if !is_unsize {
 455                                 self.suggest_change_mut(
 456                                     &obligation,
 457                                     &mut err,
 458                                     &trait_ref,
 459                                     points_at_arg,
 460                                 );
 461                             }
 462                         }
 463
 464                         // If this error is due to `!: Trait` not implemented but `(): Trait` is
 465                         // implemented, and fallback has occurred, then it could be due to a
 466                         // variable that used to fallback to `()` now falling back to `!`. Issue a
 467                         // note informing about the change in behaviour.
 468                         if trait_predicate.skip_binder().self_ty().is_never()
 469                             && fallback_has_occurred
 470                         {
 471                             let predicate = trait_predicate.map_bound(|mut trait_pred| {
 472                                 trait_pred.trait_ref.substs = self.tcx.mk_substs_trait(
 473                                     self.tcx.mk_unit(),
 474                                     &trait_pred.trait_ref.substs[1..],
 475                                 );
 476                                 trait_pred
 477                             });
 478                             let unit_obligation = Obligation {
 479                                 predicate: ty::PredicateKind::Trait(
 480                                     predicate,
 481                                     hir::Constness::NotConst,
 482                                 )
 483                                 .to_predicate(self.tcx),
 484                                 ..obligation.clone()
 485                             };
 486                             if self.predicate_may_hold(&unit_obligation) {
 487                                 err.note(
 488                                     "the trait is implemented for `()`. \
 489                                      Possibly this error has been caused by changes to \
 490                                      Rust's type-inference algorithm (see issue #48950 \
 491                                      <https://github.com/rust-lang/rust/issues/48950> \
 492                                      for more information). Consider whether you meant to use \
 493                                      the type `()` here instead.",
 494                                 );
 495                             }
 496                         }
 497
 498                         err
 499                     }
 500
 501                     ty::PredicateKind::Subtype(ref predicate) => {
 502                         // Errors for Subtype predicates show up as
 503                         // `FulfillmentErrorCode::CodeSubtypeError`,
 504                         // not selection error.
 505                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
 506                     }
 507
 508                     ty::PredicateKind::RegionOutlives(ref predicate) => {
 509                         let predicate = self.resolve_vars_if_possible(predicate);
 510                         let err = self
 511                             .region_outlives_predicate(&obligation.cause, predicate)
 512                             .err()
 513                             .unwrap();
 514                         struct_span_err!(
 515                             self.tcx.sess,
 516                             span,
 517                             E0279,
 518                             "the requirement `{}` is not satisfied (`{}`)",
 519                             predicate,
 520                             err,
 521                         )
 522                     }
 523
 524                     ty::PredicateKind::Projection(..) | ty::PredicateKind::TypeOutlives(..) => {
 525                         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
 526                         struct_span_err!(
 527                             self.tcx.sess,
 528                             span,
 529                             E0280,
 530                             "the requirement `{}` is not satisfied",
 531                             predicate
 532                         )
 533                     }
 534
 535                     &ty::PredicateKind::ObjectSafe(trait_def_id) => {
 536                         let violations = self.tcx.object_safety_violations(trait_def_id);
 537                         report_object_safety_error(self.tcx, span, trait_def_id, violations)
 538                     }
 539
 540                     &ty::PredicateKind::ClosureKind(closure_def_id, closure_substs, kind) => {
 541                         let found_kind = self.closure_kind(closure_substs).unwrap();
 542                         let closure_span =
 543                             self.tcx.sess.source_map().guess_head_span(
 544                                 self.tcx.hir().span_if_local(closure_def_id).unwrap(),
 545                             );
 546                         let hir_id = self.tcx.hir().as_local_hir_id(closure_def_id.expect_local());
 547                         let mut err = struct_span_err!(
 548                             self.tcx.sess,
 549                             closure_span,
 550                             E0525,
 551                             "expected a closure that implements the `{}` trait, \
 552                              but this closure only implements `{}`",
 553                             kind,
 554                             found_kind
 555                         );
 556
 557                         err.span_label(
 558                             closure_span,
 559                             format!("this closure implements `{}`, not `{}`", found_kind, kind),
 560                         );
 561                         err.span_label(
 562                             obligation.cause.span,
 563                             format!("the requirement to implement `{}` derives from here", kind),
 564                         );
 565
 566                         // Additional context information explaining why the closure only implements
 567                         // a particular trait.
 568                         if let Some(tables) = self.in_progress_tables {
 569                             let tables = tables.borrow();
 570                             match (found_kind, tables.closure_kind_origins().get(hir_id)) {
 571                                 (ty::ClosureKind::FnOnce, Some((span, name))) => {
 572                                     err.span_label(
 573                                         *span,
 574                                         format!(
 575                                             "closure is `FnOnce` because it moves the \
 576                                          variable `{}` out of its environment",
 577                                             name
 578                                         ),
 579                                     );
 580                                 }
 581                                 (ty::ClosureKind::FnMut, Some((span, name))) => {
 582                                     err.span_label(
 583                                         *span,
 584                                         format!(
 585                                             "closure is `FnMut` because it mutates the \
 586                                          variable `{}` here",
 587                                             name
 588                                         ),
 589                                     );
 590                                 }
 591                                 _ => {}
 592                             }
 593                         }
 594
 595                         err.emit();
 596                         return;
 597                     }
 598
 599                     ty::PredicateKind::WellFormed(ty) => {
 600                         if !self.tcx.sess.opts.debugging_opts.chalk {
 601                             // WF predicates cannot themselves make
 602                             // errors. They can only block due to
 603                             // ambiguity; otherwise, they always
 604                             // degenerate into other obligations
 605                             // (which may fail).
 606                             span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 607                         } else {
 608                             // FIXME: we'll need a better message which takes into account
 609                             // which bounds actually failed to hold.
 610                             self.tcx.sess.struct_span_err(
 611                                 span,
 612                                 &format!("the type `{}` is not well-formed (chalk)", ty),
 613                             )
 614                         }
 615                     }
 616
 617                     ty::PredicateKind::ConstEvaluatable(..) => {
 618                         // Errors for `ConstEvaluatable` predicates show up as
 619                         // `SelectionError::ConstEvalFailure`,
 620                         // not `Unimplemented`.
 621                         span_bug!(
 622                             span,
 623                             "const-evaluatable requirement gave wrong error: `{:?}`",
 624                             obligation
 625                         )
 626                     }
 627
 628                     ty::PredicateKind::ConstEquate(..) => {
 629                         // Errors for `ConstEquate` predicates show up as
 630                         // `SelectionError::ConstEvalFailure`,
 631                         // not `Unimplemented`.
 632                         span_bug!(
 633                             span,
 634                             "const-equate requirement gave wrong error: `{:?}`",
 635                             obligation
 636                         )
 637                     }
 638                 }
 639             }
 640
 641             OutputTypeParameterMismatch(ref found_trait_ref, ref expected_trait_ref, _) => {
 642                 let found_trait_ref = self.resolve_vars_if_possible(&*found_trait_ref);
 643                 let expected_trait_ref = self.resolve_vars_if_possible(&*expected_trait_ref);
 644
 645                 if expected_trait_ref.self_ty().references_error() {
 646                     return;
 647                 }
 648
 649                 let found_trait_ty = match found_trait_ref.self_ty().no_bound_vars() {
 650                     Some(ty) => ty,
 651                     None => return,
 652                 };
 653
 654                 let found_did = match found_trait_ty.kind {
 655                     ty::Closure(did, _) | ty::Foreign(did) | ty::FnDef(did, _) => Some(did),
 656                     ty::Adt(def, _) => Some(def.did),
 657                     _ => None,
 658                 };
 659
 660                 let found_span = found_did
 661                     .and_then(|did| self.tcx.hir().span_if_local(did))
 662                     .map(|sp| self.tcx.sess.source_map().guess_head_span(sp)); // the sp could be an fn def
 663
 664                 if self.reported_closure_mismatch.borrow().contains(&(span, found_span)) {
 665                     // We check closures twice, with obligations flowing in different directions,
 666                     // but we want to complain about them only once.
 667                     return;
 668                 }
 669
 670                 self.reported_closure_mismatch.borrow_mut().insert((span, found_span));
 671
 672                 let found = match found_trait_ref.skip_binder().substs.type_at(1).kind {
 673                     ty::Tuple(ref tys) => vec![ArgKind::empty(); tys.len()],
 674                     _ => vec![ArgKind::empty()],
 675                 };
 676
 677                 let expected_ty = expected_trait_ref.skip_binder().substs.type_at(1);
 678                 let expected = match expected_ty.kind {
 679                     ty::Tuple(ref tys) => tys
 680                         .iter()
 681                         .map(|t| ArgKind::from_expected_ty(t.expect_ty(), Some(span)))
 682                         .collect(),
 683                     _ => vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())],
 684                 };
 685
 686                 if found.len() == expected.len() {
 687                     self.report_closure_arg_mismatch(
 688                         span,
 689                         found_span,
 690                         found_trait_ref,
 691                         expected_trait_ref,
 692                     )
 693                 } else {
 694                     let (closure_span, found) = found_did
 695                         .and_then(|did| {
 696                             let node = self.tcx.hir().get_if_local(did)?;
 697                             let (found_span, found) = self.get_fn_like_arguments(node)?;
 698                             Some((Some(found_span), found))
 699                         })
 700                         .unwrap_or((found_span, found));
 701
 702                     self.report_arg_count_mismatch(
 703                         span,
 704                         closure_span,
 705                         expected,
 706                         found,
 707                         found_trait_ty.is_closure(),
 708                     )
 709                 }
 710             }
 711
 712             TraitNotObjectSafe(did) => {
 713                 let violations = self.tcx.object_safety_violations(did);
 714                 report_object_safety_error(self.tcx, span, did, violations)
 715             }
 716
 717             ConstEvalFailure(ErrorHandled::TooGeneric) => {
 718                 // In this instance, we have a const expression containing an unevaluated
 719                 // generic parameter. We have no idea whether this expression is valid or
 720                 // not (e.g. it might result in an error), but we don't want to just assume
 721                 // that it's okay, because that might result in post-monomorphisation time
 722                 // errors. The onus is really on the caller to provide values that it can
 723                 // prove are well-formed.
 724                 let mut err = self
 725                     .tcx
 726                     .sess
 727                     .struct_span_err(span, "constant expression depends on a generic parameter");
 728                 // FIXME(const_generics): we should suggest to the user how they can resolve this
 729                 // issue. However, this is currently not actually possible
 730                 // (see https://github.com/rust-lang/rust/issues/66962#issuecomment-575907083).
 731                 err.note("this may fail depending on what value the parameter takes");
 732                 err
 733             }
 734
 735             // Already reported in the query.
 736             ConstEvalFailure(ErrorHandled::Reported(ErrorReported)) => {
 737                 // FIXME(eddyb) remove this once `ErrorReported` becomes a proof token.
 738                 self.tcx.sess.delay_span_bug(span, "`ErrorReported` without an error");
 739                 return;
 740             }
 741
 742             // Already reported in the query, but only as a lint.
 743             // This shouldn't actually happen for constants used in types, modulo
 744             // bugs. The `delay_span_bug` here ensures it won't be ignored.
 745             ConstEvalFailure(ErrorHandled::Linted) => {
 746                 self.tcx.sess.delay_span_bug(span, "constant in type had error reported as lint");
 747                 return;
 748             }
 749
 750             Overflow => {
 751                 bug!("overflow should be handled before the `report_selection_error` path");
 752             }
 753         };
 754
 755         self.note_obligation_cause(&mut err, obligation);
 756         self.point_at_returns_when_relevant(&mut err, &obligation);
 757
 758         err.emit();
 759     }
 760
 761     /// Given some node representing a fn-like thing in the HIR map,
 762     /// returns a span and `ArgKind` information that describes the
 763     /// arguments it expects. This can be supplied to
 764     /// `report_arg_count_mismatch`.
 765     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)> {
 766         let sm = self.tcx.sess.source_map();
 767         let hir = self.tcx.hir();
 768         Some(match node {
 769             Node::Expr(&hir::Expr {
 770                 kind: hir::ExprKind::Closure(_, ref _decl, id, span, _),
 771                 ..
 772             }) => (
 773                 sm.guess_head_span(span),
 774                 hir.body(id)
 775                     .params
 776                     .iter()
 777                     .map(|arg| {
 778                         if let hir::Pat { kind: hir::PatKind::Tuple(ref args, _), span, .. } =
 779                             *arg.pat
 780                         {
 781                             Some(ArgKind::Tuple(
 782                                 Some(span),
 783                                 args.iter()
 784                                     .map(|pat| {
 785                                         sm.span_to_snippet(pat.span)
 786                                             .ok()
 787                                             .map(|snippet| (snippet, "_".to_owned()))
 788                                     })
 789                                     .collect::<Option<Vec<_>>>()?,
 790                             ))
 791                         } else {
 792                             let name = sm.span_to_snippet(arg.pat.span).ok()?;
 793                             Some(ArgKind::Arg(name, "_".to_owned()))
 794                         }
 795                     })
 796                     .collect::<Option<Vec<ArgKind>>>()?,
 797             ),
 798             Node::Item(&hir::Item { span, kind: hir::ItemKind::Fn(ref sig, ..), .. })
 799             | Node::ImplItem(&hir::ImplItem {
 800                 span,
 801                 kind: hir::ImplItemKind::Fn(ref sig, _),
 802                 ..
 803             })
 804             | Node::TraitItem(&hir::TraitItem {
 805                 span,
 806                 kind: hir::TraitItemKind::Fn(ref sig, _),
 807                 ..
 808             }) => (
 809                 sm.guess_head_span(span),
 810                 sig.decl
 811                     .inputs
 812                     .iter()
 813                     .map(|arg| match arg.clone().kind {
 814                         hir::TyKind::Tup(ref tys) => ArgKind::Tuple(
 815                             Some(arg.span),
 816                             vec![("_".to_owned(), "_".to_owned()); tys.len()],
 817                         ),
 818                         _ => ArgKind::empty(),
 819                     })
 820                     .collect::<Vec<ArgKind>>(),
 821             ),
 822             Node::Ctor(ref variant_data) => {
 823                 let span = variant_data.ctor_hir_id().map(|id| hir.span(id)).unwrap_or(DUMMY_SP);
 824                 let span = sm.guess_head_span(span);
 825                 (span, vec![ArgKind::empty(); variant_data.fields().len()])
 826             }
 827             _ => panic!("non-FnLike node found: {:?}", node),
 828         })
 829     }
 830
 831     /// Reports an error when the number of arguments needed by a
 832     /// trait match doesn't match the number that the expression
 833     /// provides.
 834     fn report_arg_count_mismatch(
 835         &self,
 836         span: Span,
 837         found_span: Option<Span>,
 838         expected_args: Vec<ArgKind>,
 839         found_args: Vec<ArgKind>,
 840         is_closure: bool,
 841     ) -> DiagnosticBuilder<'tcx> {
 842         let kind = if is_closure { "closure" } else { "function" };
 843
 844         let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
 845             let arg_length = arguments.len();
 846             let distinct = match &other[..] {
 847                 &[ArgKind::Tuple(..)] => true,
 848                 _ => false,
 849             };
 850             match (arg_length, arguments.get(0)) {
 851                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
 852                     format!("a single {}-tuple as argument", fields.len())
 853                 }
 854                 _ => format!(
 855                     "{} {}argument{}",
 856                     arg_length,
 857                     if distinct && arg_length > 1 { "distinct " } else { "" },
 858                     pluralize!(arg_length)
 859                 ),
 860             }
 861         };
 862
 863         let expected_str = args_str(&expected_args, &found_args);
 864         let found_str = args_str(&found_args, &expected_args);
 865
 866         let mut err = struct_span_err!(
 867             self.tcx.sess,
 868             span,
 869             E0593,
 870             "{} is expected to take {}, but it takes {}",
 871             kind,
 872             expected_str,
 873             found_str,
 874         );
 875
 876         err.span_label(span, format!("expected {} that takes {}", kind, expected_str));
 877
 878         if let Some(found_span) = found_span {
 879             err.span_label(found_span, format!("takes {}", found_str));
 880
 881             // move |_| { ... }
 882             // ^^^^^^^^-- def_span
 883             //
 884             // move |_| { ... }
 885             // ^^^^^-- prefix
 886             let prefix_span = self.tcx.sess.source_map().span_until_non_whitespace(found_span);
 887             // move |_| { ... }
 888             //      ^^^-- pipe_span
 889             let pipe_span =
 890                 if let Some(span) = found_span.trim_start(prefix_span) { span } else { found_span };
 891
 892             // Suggest to take and ignore the arguments with expected_args_length `_`s if
 893             // found arguments is empty (assume the user just wants to ignore args in this case).
 894             // For example, if `expected_args_length` is 2, suggest `|_, _|`.
 895             if found_args.is_empty() && is_closure {
 896                 let underscores = vec!["_"; expected_args.len()].join(", ");
 897                 err.span_suggestion_verbose(
 898                     pipe_span,
 899                     &format!(
 900                         "consider changing the closure to take and ignore the expected argument{}",
 901                         pluralize!(expected_args.len())
 902                     ),
 903                     format!("|{}|", underscores),
 904                     Applicability::MachineApplicable,
 905                 );
 906             }
 907
 908             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
 909                 if fields.len() == expected_args.len() {
 910                     let sugg = fields
 911                         .iter()
 912                         .map(|(name, _)| name.to_owned())
 913                         .collect::<Vec<String>>()
 914                         .join(", ");
 915                     err.span_suggestion_verbose(
 916                         found_span,
 917                         "change the closure to take multiple arguments instead of a single tuple",
 918                         format!("|{}|", sugg),
 919                         Applicability::MachineApplicable,
 920                     );
 921                 }
 922             }
 923             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..] {
 924                 if fields.len() == found_args.len() && is_closure {
 925                     let sugg = format!(
 926                         "|({}){}|",
 927                         found_args
 928                             .iter()
 929                             .map(|arg| match arg {
 930                                 ArgKind::Arg(name, _) => name.to_owned(),
 931                                 _ => "_".to_owned(),
 932                             })
 933                             .collect::<Vec<String>>()
 934                             .join(", "),
 935                         // add type annotations if available
 936                         if found_args.iter().any(|arg| match arg {
 937                             ArgKind::Arg(_, ty) => ty != "_",
 938                             _ => false,
 939                         }) {
 940                             format!(
 941                                 ": ({})",
 942                                 fields
 943                                     .iter()
 944                                     .map(|(_, ty)| ty.to_owned())
 945                                     .collect::<Vec<String>>()
 946                                     .join(", ")
 947                             )
 948                         } else {
 949                             String::new()
 950                         },
 951                     );
 952                     err.span_suggestion_verbose(
 953                         found_span,
 954                         "change the closure to accept a tuple instead of individual arguments",
 955                         sugg,
 956                         Applicability::MachineApplicable,
 957                     );
 958                 }
 959             }
 960         }
 961
 962         err
 963     }
 964 }
 965
 966 trait InferCtxtPrivExt<'tcx> {
 967     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
 968     // `error` occurring implies that `cond` occurs.
 969     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool;
 970
 971     fn report_fulfillment_error(
 972         &self,
 973         error: &FulfillmentError<'tcx>,
 974         body_id: Option<hir::BodyId>,
 975         fallback_has_occurred: bool,
 976     );
 977
 978     fn report_projection_error(
 979         &self,
 980         obligation: &PredicateObligation<'tcx>,
 981         error: &MismatchedProjectionTypes<'tcx>,
 982     );
 983
 984     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool;
 985
 986     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str>;
 987
 988     fn find_similar_impl_candidates(
 989         &self,
 990         trait_ref: ty::PolyTraitRef<'tcx>,
 991     ) -> Vec<ty::TraitRef<'tcx>>;
 992
 993     fn report_similar_impl_candidates(
 994         &self,
 995         impl_candidates: Vec<ty::TraitRef<'tcx>>,
 996         err: &mut DiagnosticBuilder<'_>,
 997     );
 998
 999     /// Gets the parent trait chain start
1000     fn get_parent_trait_ref(
1001         &self,
1002         code: &ObligationCauseCode<'tcx>,
1003     ) -> Option<(String, Option<Span>)>;
1004
1005     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1006     /// with the same path as `trait_ref`, a help message about
1007     /// a probable version mismatch is added to `err`
1008     fn note_version_mismatch(
1009         &self,
1010         err: &mut DiagnosticBuilder<'_>,
1011         trait_ref: &ty::PolyTraitRef<'tcx>,
1012     );
1013
1014     /// Creates a `PredicateObligation` with `new_self_ty` replacing the existing type in the
1015     /// `trait_ref`.
1016     ///
1017     /// For this to work, `new_self_ty` must have no escaping bound variables.
1018     fn mk_trait_obligation_with_new_self_ty(
1019         &self,
1020         param_env: ty::ParamEnv<'tcx>,
1021         trait_ref: &ty::PolyTraitRef<'tcx>,
1022         new_self_ty: Ty<'tcx>,
1023     ) -> PredicateObligation<'tcx>;
1024
1025     fn maybe_report_ambiguity(
1026         &self,
1027         obligation: &PredicateObligation<'tcx>,
1028         body_id: Option<hir::BodyId>,
1029     );
1030
1031     fn predicate_can_apply(
1032         &self,
1033         param_env: ty::ParamEnv<'tcx>,
1034         pred: ty::PolyTraitRef<'tcx>,
1035     ) -> bool;
1036
1037     fn note_obligation_cause(
1038         &self,
1039         err: &mut DiagnosticBuilder<'tcx>,
1040         obligation: &PredicateObligation<'tcx>,
1041     );
1042
1043     fn suggest_unsized_bound_if_applicable(
1044         &self,
1045         err: &mut DiagnosticBuilder<'tcx>,
1046         obligation: &PredicateObligation<'tcx>,
1047     );
1048
1049     fn is_recursive_obligation(
1050         &self,
1051         obligated_types: &mut Vec<&ty::TyS<'tcx>>,
1052         cause_code: &ObligationCauseCode<'tcx>,
1053     ) -> bool;
1054 }
1055
1056 impl<'a, 'tcx> InferCtxtPrivExt<'tcx> for InferCtxt<'a, 'tcx> {
1057     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1058     // `error` occurring implies that `cond` occurs.
1059     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool {
1060         if cond == error {
1061             return true;
1062         }
1063
1064         let (cond, error) = match (cond.kind(), error.kind()) {
1065             (ty::PredicateKind::Trait(..), ty::PredicateKind::Trait(error, _)) => (cond, error),
1066             _ => {
1067                 // FIXME: make this work in other cases too.
1068                 return false;
1069             }
1070         };
1071
1072         for obligation in super::elaborate_predicates(self.tcx, std::iter::once(cond)) {
1073             if let ty::PredicateKind::Trait(implication, _) = obligation.predicate.kind() {
1074                 let error = error.to_poly_trait_ref();
1075                 let implication = implication.to_poly_trait_ref();
1076                 // FIXME: I'm just not taking associated types at all here.
1077                 // Eventually I'll need to implement param-env-aware
1078                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
1079                 let param_env = ty::ParamEnv::empty();
1080                 if self.can_sub(param_env, error, implication).is_ok() {
1081                     debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
1082                     return true;
1083                 }
1084             }
1085         }
1086
1087         false
1088     }
1089
1090     fn report_fulfillment_error(
1091         &self,
1092         error: &FulfillmentError<'tcx>,
1093         body_id: Option<hir::BodyId>,
1094         fallback_has_occurred: bool,
1095     ) {
1096         debug!("report_fulfillment_error({:?})", error);
1097         match error.code {
1098             FulfillmentErrorCode::CodeSelectionError(ref selection_error) => {
1099                 self.report_selection_error(
1100                     &error.obligation,
1101                     selection_error,
1102                     fallback_has_occurred,
1103                     error.points_at_arg_span,
1104                 );
1105             }
1106             FulfillmentErrorCode::CodeProjectionError(ref e) => {
1107                 self.report_projection_error(&error.obligation, e);
1108             }
1109             FulfillmentErrorCode::CodeAmbiguity => {
1110                 self.maybe_report_ambiguity(&error.obligation, body_id);
1111             }
1112             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
1113                 self.report_mismatched_types(
1114                     &error.obligation.cause,
1115                     expected_found.expected,
1116                     expected_found.found,
1117                     err.clone(),
1118                 )
1119                 .emit();
1120             }
1121             FulfillmentErrorCode::CodeConstEquateError(ref expected_found, ref err) => {
1122                 self.report_mismatched_consts(
1123                     &error.obligation.cause,
1124                     expected_found.expected,
1125                     expected_found.found,
1126                     err.clone(),
1127                 )
1128                 .emit();
1129             }
1130         }
1131     }
1132
1133     fn report_projection_error(
1134         &self,
1135         obligation: &PredicateObligation<'tcx>,
1136         error: &MismatchedProjectionTypes<'tcx>,
1137     ) {
1138         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
1139
1140         if predicate.references_error() {
1141             return;
1142         }
1143
1144         self.probe(|_| {
1145             let err_buf;
1146             let mut err = &error.err;
1147             let mut values = None;
1148
1149             // try to find the mismatched types to report the error with.
1150             //
1151             // this can fail if the problem was higher-ranked, in which
1152             // cause I have no idea for a good error message.
1153             if let ty::PredicateKind::Projection(ref data) = predicate.kind() {
1154                 let mut selcx = SelectionContext::new(self);
1155                 let (data, _) = self.replace_bound_vars_with_fresh_vars(
1156                     obligation.cause.span,
1157                     infer::LateBoundRegionConversionTime::HigherRankedType,
1158                     data,
1159                 );
1160                 let mut obligations = vec![];
1161                 let normalized_ty = super::normalize_projection_type(
1162                     &mut selcx,
1163                     obligation.param_env,
1164                     data.projection_ty,
1165                     obligation.cause.clone(),
1166                     0,
1167                     &mut obligations,
1168                 );
1169
1170                 debug!(
1171                     "report_projection_error obligation.cause={:?} obligation.param_env={:?}",
1172                     obligation.cause, obligation.param_env
1173                 );
1174
1175                 debug!(
1176                     "report_projection_error normalized_ty={:?} data.ty={:?}",
1177                     normalized_ty, data.ty
1178                 );
1179
1180                 let is_normalized_ty_expected = match &obligation.cause.code {
1181                     ObligationCauseCode::ItemObligation(_)
1182                     | ObligationCauseCode::BindingObligation(_, _)
1183                     | ObligationCauseCode::ObjectCastObligation(_) => false,
1184                     _ => true,
1185                 };
1186
1187                 if let Err(error) = self.at(&obligation.cause, obligation.param_env).eq_exp(
1188                     is_normalized_ty_expected,
1189                     normalized_ty,
1190                     data.ty,
1191                 ) {
1192                     values = Some(infer::ValuePairs::Types(ExpectedFound::new(
1193                         is_normalized_ty_expected,
1194                         normalized_ty,
1195                         data.ty,
1196                     )));
1197
1198                     err_buf = error;
1199                     err = &err_buf;
1200                 }
1201             }
1202
1203             let msg = format!("type mismatch resolving `{}`", predicate);
1204             let error_id = (DiagnosticMessageId::ErrorId(271), Some(obligation.cause.span), msg);
1205             let fresh = self.tcx.sess.one_time_diagnostics.borrow_mut().insert(error_id);
1206             if fresh {
1207                 let mut diag = struct_span_err!(
1208                     self.tcx.sess,
1209                     obligation.cause.span,
1210                     E0271,
1211                     "type mismatch resolving `{}`",
1212                     predicate
1213                 );
1214                 self.note_type_err(&mut diag, &obligation.cause, None, values, err);
1215                 self.note_obligation_cause(&mut diag, obligation);
1216                 diag.emit();
1217             }
1218         });
1219     }
1220
1221     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
1222         /// returns the fuzzy category of a given type, or None
1223         /// if the type can be equated to any type.
1224         fn type_category(t: Ty<'_>) -> Option<u32> {
1225             match t.kind {
1226                 ty::Bool => Some(0),
1227                 ty::Char => Some(1),
1228                 ty::Str => Some(2),
1229                 ty::Int(..) | ty::Uint(..) | ty::Infer(ty::IntVar(..)) => Some(3),
1230                 ty::Float(..) | ty::Infer(ty::FloatVar(..)) => Some(4),
1231                 ty::Ref(..) | ty::RawPtr(..) => Some(5),
1232                 ty::Array(..) | ty::Slice(..) => Some(6),
1233                 ty::FnDef(..) | ty::FnPtr(..) => Some(7),
1234                 ty::Dynamic(..) => Some(8),
1235                 ty::Closure(..) => Some(9),
1236                 ty::Tuple(..) => Some(10),
1237                 ty::Projection(..) => Some(11),
1238                 ty::Param(..) => Some(12),
1239                 ty::Opaque(..) => Some(13),
1240                 ty::Never => Some(14),
1241                 ty::Adt(adt, ..) => match adt.adt_kind() {
1242                     AdtKind::Struct => Some(15),
1243                     AdtKind::Union => Some(16),
1244                     AdtKind::Enum => Some(17),
1245                 },
1246                 ty::Generator(..) => Some(18),
1247                 ty::Foreign(..) => Some(19),
1248                 ty::GeneratorWitness(..) => Some(20),
1249                 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error => None,
1250             }
1251         }
1252
1253         match (type_category(a), type_category(b)) {
1254             (Some(cat_a), Some(cat_b)) => match (&a.kind, &b.kind) {
1255                 (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => def_a == def_b,
1256                 _ => cat_a == cat_b,
1257             },
1258             // infer and error can be equated to all types
1259             _ => true,
1260         }
1261     }
1262
1263     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str> {
1264         self.tcx.hir().body(body_id).generator_kind.map(|gen_kind| match gen_kind {
1265             hir::GeneratorKind::Gen => "a generator",
1266             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block) => "an async block",
1267             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn) => "an async function",
1268             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Closure) => "an async closure",
1269         })
1270     }
1271
1272     fn find_similar_impl_candidates(
1273         &self,
1274         trait_ref: ty::PolyTraitRef<'tcx>,
1275     ) -> Vec<ty::TraitRef<'tcx>> {
1276         let simp = fast_reject::simplify_type(self.tcx, trait_ref.skip_binder().self_ty(), true);
1277         let all_impls = self.tcx.all_impls(trait_ref.def_id());
1278
1279         match simp {
1280             Some(simp) => all_impls
1281                 .filter_map(|def_id| {
1282                     let imp = self.tcx.impl_trait_ref(def_id).unwrap();
1283                     let imp_simp = fast_reject::simplify_type(self.tcx, imp.self_ty(), true);
1284                     if let Some(imp_simp) = imp_simp {
1285                         if simp != imp_simp {
1286                             return None;
1287                         }
1288                     }
1289                     Some(imp)
1290                 })
1291                 .collect(),
1292             None => all_impls.map(|def_id| self.tcx.impl_trait_ref(def_id).unwrap()).collect(),
1293         }
1294     }
1295
1296     fn report_similar_impl_candidates(
1297         &self,
1298         impl_candidates: Vec<ty::TraitRef<'tcx>>,
1299         err: &mut DiagnosticBuilder<'_>,
1300     ) {
1301         if impl_candidates.is_empty() {
1302             return;
1303         }
1304
1305         let len = impl_candidates.len();
1306         let end = if impl_candidates.len() <= 5 { impl_candidates.len() } else { 4 };
1307
1308         let normalize = |candidate| {
1309             self.tcx.infer_ctxt().enter(|ref infcx| {
1310                 let normalized = infcx
1311                     .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
1312                     .normalize(candidate)
1313                     .ok();
1314                 match normalized {
1315                     Some(normalized) => format!("\n  {:?}", normalized.value),
1316                     None => format!("\n  {:?}", candidate),
1317                 }
1318             })
1319         };
1320
1321         // Sort impl candidates so that ordering is consistent for UI tests.
1322         let mut normalized_impl_candidates =
1323             impl_candidates.iter().map(normalize).collect::<Vec<String>>();
1324
1325         // Sort before taking the `..end` range,
1326         // because the ordering of `impl_candidates` may not be deterministic:
1327         // https://github.com/rust-lang/rust/pull/57475#issuecomment-455519507
1328         normalized_impl_candidates.sort();
1329
1330         err.help(&format!(
1331             "the following implementations were found:{}{}",
1332             normalized_impl_candidates[..end].join(""),
1333             if len > 5 { format!("\nand {} others", len - 4) } else { String::new() }
1334         ));
1335     }
1336
1337     /// Gets the parent trait chain start
1338     fn get_parent_trait_ref(
1339         &self,
1340         code: &ObligationCauseCode<'tcx>,
1341     ) -> Option<(String, Option<Span>)> {
1342         match code {
1343             &ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
1344                 let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
1345                 match self.get_parent_trait_ref(&data.parent_code) {
1346                     Some(t) => Some(t),
1347                     None => {
1348                         let ty = parent_trait_ref.skip_binder().self_ty();
1349                         let span =
1350                             TyCategory::from_ty(ty).map(|(_, def_id)| self.tcx.def_span(def_id));
1351                         Some((ty.to_string(), span))
1352                     }
1353                 }
1354             }
1355             _ => None,
1356         }
1357     }
1358
1359     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1360     /// with the same path as `trait_ref`, a help message about
1361     /// a probable version mismatch is added to `err`
1362     fn note_version_mismatch(
1363         &self,
1364         err: &mut DiagnosticBuilder<'_>,
1365         trait_ref: &ty::PolyTraitRef<'tcx>,
1366     ) {
1367         let get_trait_impl = |trait_def_id| {
1368             let mut trait_impl = None;
1369             self.tcx.for_each_relevant_impl(
1370                 trait_def_id,
1371                 trait_ref.skip_binder().self_ty(),
1372                 |impl_def_id| {
1373                     if trait_impl.is_none() {
1374                         trait_impl = Some(impl_def_id);
1375                     }
1376                 },
1377             );
1378             trait_impl
1379         };
1380         let required_trait_path = self.tcx.def_path_str(trait_ref.def_id());
1381         let all_traits = self.tcx.all_traits(LOCAL_CRATE);
1382         let traits_with_same_path: std::collections::BTreeSet<_> = all_traits
1383             .iter()
1384             .filter(|trait_def_id| **trait_def_id != trait_ref.def_id())
1385             .filter(|trait_def_id| self.tcx.def_path_str(**trait_def_id) == required_trait_path)
1386             .collect();
1387         for trait_with_same_path in traits_with_same_path {
1388             if let Some(impl_def_id) = get_trait_impl(*trait_with_same_path) {
1389                 let impl_span = self.tcx.def_span(impl_def_id);
1390                 err.span_help(impl_span, "trait impl with same name found");
1391                 let trait_crate = self.tcx.crate_name(trait_with_same_path.krate);
1392                 let crate_msg = format!(
1393                     "perhaps two different versions of crate `{}` are being used?",
1394                     trait_crate
1395                 );
1396                 err.note(&crate_msg);
1397             }
1398         }
1399     }
1400
1401     fn mk_trait_obligation_with_new_self_ty(
1402         &self,
1403         param_env: ty::ParamEnv<'tcx>,
1404         trait_ref: &ty::PolyTraitRef<'tcx>,
1405         new_self_ty: Ty<'tcx>,
1406     ) -> PredicateObligation<'tcx> {
1407         assert!(!new_self_ty.has_escaping_bound_vars());
1408
1409         let trait_ref = trait_ref.map_bound_ref(|tr| ty::TraitRef {
1410             substs: self.tcx.mk_substs_trait(new_self_ty, &tr.substs[1..]),
1411             ..*tr
1412         });
1413
1414         Obligation::new(
1415             ObligationCause::dummy(),
1416             param_env,
1417             trait_ref.without_const().to_predicate(self.tcx),
1418         )
1419     }
1420
1421     fn maybe_report_ambiguity(
1422         &self,
1423         obligation: &PredicateObligation<'tcx>,
1424         body_id: Option<hir::BodyId>,
1425     ) {
1426         // Unable to successfully determine, probably means
1427         // insufficient type information, but could mean
1428         // ambiguous impls. The latter *ought* to be a
1429         // coherence violation, so we don't report it here.
1430
1431         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
1432         let span = obligation.cause.span;
1433
1434         debug!(
1435             "maybe_report_ambiguity(predicate={:?}, obligation={:?} body_id={:?}, code={:?})",
1436             predicate, obligation, body_id, obligation.cause.code,
1437         );
1438
1439         // Ambiguity errors are often caused as fallout from earlier
1440         // errors. So just ignore them if this infcx is tainted.
1441         if self.is_tainted_by_errors() {
1442             return;
1443         }
1444
1445         let mut err = match predicate.kind() {
1446             ty::PredicateKind::Trait(ref data, _) => {
1447                 let trait_ref = data.to_poly_trait_ref();
1448                 let self_ty = trait_ref.skip_binder().self_ty();
1449                 debug!("self_ty {:?} {:?} trait_ref {:?}", self_ty, self_ty.kind, trait_ref);
1450
1451                 if predicate.references_error() {
1452                     return;
1453                 }
1454                 // Typically, this ambiguity should only happen if
1455                 // there are unresolved type inference variables
1456                 // (otherwise it would suggest a coherence
1457                 // failure). But given #21974 that is not necessarily
1458                 // the case -- we can have multiple where clauses that
1459                 // are only distinguished by a region, which results
1460                 // in an ambiguity even when all types are fully
1461                 // known, since we don't dispatch based on region
1462                 // relationships.
1463
1464                 // This is kind of a hack: it frequently happens that some earlier
1465                 // error prevents types from being fully inferred, and then we get
1466                 // a bunch of uninteresting errors saying something like "<generic
1467                 // #0> doesn't implement Sized".  It may even be true that we
1468                 // could just skip over all checks where the self-ty is an
1469                 // inference variable, but I was afraid that there might be an
1470                 // inference variable created, registered as an obligation, and
1471                 // then never forced by writeback, and hence by skipping here we'd
1472                 // be ignoring the fact that we don't KNOW the type works
1473                 // out. Though even that would probably be harmless, given that
1474                 // we're only talking about builtin traits, which are known to be
1475                 // inhabited. We used to check for `self.tcx.sess.has_errors()` to
1476                 // avoid inundating the user with unnecessary errors, but we now
1477                 // check upstream for type errors and don't add the obligations to
1478                 // begin with in those cases.
1479                 if self
1480                     .tcx
1481                     .lang_items()
1482                     .sized_trait()
1483                     .map_or(false, |sized_id| sized_id == trait_ref.def_id())
1484                 {
1485                     self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0282).emit();
1486                     return;
1487                 }
1488                 let mut err = self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0283);
1489                 err.note(&format!("cannot satisfy `{}`", predicate));
1490                 if let ObligationCauseCode::ItemObligation(def_id) = obligation.cause.code {
1491                     self.suggest_fully_qualified_path(&mut err, def_id, span, trait_ref.def_id());
1492                 } else if let (
1493                     Ok(ref snippet),
1494                     ObligationCauseCode::BindingObligation(ref def_id, _),
1495                 ) =
1496                     (self.tcx.sess.source_map().span_to_snippet(span), &obligation.cause.code)
1497                 {
1498                     let generics = self.tcx.generics_of(*def_id);
1499                     if generics.params.iter().any(|p| p.name.as_str() != "Self")
1500                         && !snippet.ends_with('>')
1501                     {
1502                         // FIXME: To avoid spurious suggestions in functions where type arguments
1503                         // where already supplied, we check the snippet to make sure it doesn't
1504                         // end with a turbofish. Ideally we would have access to a `PathSegment`
1505                         // instead. Otherwise we would produce the following output:
1506                         //
1507                         // error[E0283]: type annotations needed
1508                         //   --> $DIR/issue-54954.rs:3:24
1509                         //    |
1510                         // LL | const ARR_LEN: usize = Tt::const_val::<[i8; 123]>();
1511                         //    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^
1512                         //    |                        |
1513                         //    |                        cannot infer type
1514                         //    |                        help: consider specifying the type argument
1515                         //    |                        in the function call:
1516                         //    |                        `Tt::const_val::<[i8; 123]>::<T>`
1517                         // ...
1518                         // LL |     const fn const_val<T: Sized>() -> usize {
1519                         //    |                        - required by this bound in `Tt::const_val`
1520                         //    |
1521                         //    = note: cannot satisfy `_: Tt`
1522
1523                         err.span_suggestion_verbose(
1524                             span.shrink_to_hi(),
1525                             &format!(
1526                                 "consider specifying the type argument{} in the function call",
1527                                 pluralize!(generics.params.len()),
1528                             ),
1529                             format!(
1530                                 "::<{}>",
1531                                 generics
1532                                     .params
1533                                     .iter()
1534                                     .map(|p| p.name.to_string())
1535                                     .collect::<Vec<String>>()
1536                                     .join(", ")
1537                             ),
1538                             Applicability::HasPlaceholders,
1539                         );
1540                     }
1541                 }
1542                 err
1543             }
1544
1545             ty::PredicateKind::WellFormed(arg) => {
1546                 // Same hacky approach as above to avoid deluging user
1547                 // with error messages.
1548                 if arg.references_error() || self.tcx.sess.has_errors() {
1549                     return;
1550                 }
1551
1552                 match arg.unpack() {
1553                     GenericArgKind::Lifetime(lt) => {
1554                         span_bug!(span, "unexpected well formed predicate: {:?}", lt)
1555                     }
1556                     GenericArgKind::Type(ty) => {
1557                         self.need_type_info_err(body_id, span, ty, ErrorCode::E0282)
1558                     }
1559                     GenericArgKind::Const(ct) => {
1560                         self.need_type_info_err_const(body_id, span, ct, ErrorCode::E0282)
1561                     }
1562                 }
1563             }
1564
1565             ty::PredicateKind::Subtype(ref data) => {
1566                 if data.references_error() || self.tcx.sess.has_errors() {
1567                     // no need to overload user in such cases
1568                     return;
1569                 }
1570                 let &SubtypePredicate { a_is_expected: _, a, b } = data.skip_binder();
1571                 // both must be type variables, or the other would've been instantiated
1572                 assert!(a.is_ty_var() && b.is_ty_var());
1573                 self.need_type_info_err(body_id, span, a, ErrorCode::E0282)
1574             }
1575             ty::PredicateKind::Projection(ref data) => {
1576                 let trait_ref = data.to_poly_trait_ref(self.tcx);
1577                 let self_ty = trait_ref.skip_binder().self_ty();
1578                 let ty = data.skip_binder().ty;
1579                 if predicate.references_error() {
1580                     return;
1581                 }
1582                 if self_ty.needs_infer() && ty.needs_infer() {
1583                     // We do this for the `foo.collect()?` case to produce a suggestion.
1584                     let mut err = self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0284);
1585                     err.note(&format!("cannot satisfy `{}`", predicate));
1586                     err
1587                 } else {
1588                     let mut err = struct_span_err!(
1589                         self.tcx.sess,
1590                         span,
1591                         E0284,
1592                         "type annotations needed: cannot satisfy `{}`",
1593                         predicate,
1594                     );
1595                     err.span_label(span, &format!("cannot satisfy `{}`", predicate));
1596                     err
1597                 }
1598             }
1599
1600             _ => {
1601                 if self.tcx.sess.has_errors() {
1602                     return;
1603                 }
1604                 let mut err = struct_span_err!(
1605                     self.tcx.sess,
1606                     span,
1607                     E0284,
1608                     "type annotations needed: cannot satisfy `{}`",
1609                     predicate,
1610                 );
1611                 err.span_label(span, &format!("cannot satisfy `{}`", predicate));
1612                 err
1613             }
1614         };
1615         self.note_obligation_cause(&mut err, obligation);
1616         err.emit();
1617     }
1618
1619     /// Returns `true` if the trait predicate may apply for *some* assignment
1620     /// to the type parameters.
1621     fn predicate_can_apply(
1622         &self,
1623         param_env: ty::ParamEnv<'tcx>,
1624         pred: ty::PolyTraitRef<'tcx>,
1625     ) -> bool {
1626         struct ParamToVarFolder<'a, 'tcx> {
1627             infcx: &'a InferCtxt<'a, 'tcx>,
1628             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
1629         }
1630
1631         impl<'a, 'tcx> TypeFolder<'tcx> for ParamToVarFolder<'a, 'tcx> {
1632             fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
1633                 self.infcx.tcx
1634             }
1635
1636             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
1637                 if let ty::Param(ty::ParamTy { name, .. }) = ty.kind {
1638                     let infcx = self.infcx;
1639                     self.var_map.entry(ty).or_insert_with(|| {
1640                         infcx.next_ty_var(TypeVariableOrigin {
1641                             kind: TypeVariableOriginKind::TypeParameterDefinition(name, None),
1642                             span: DUMMY_SP,
1643                         })
1644                     })
1645                 } else {
1646                     ty.super_fold_with(self)
1647                 }
1648             }
1649         }
1650
1651         self.probe(|_| {
1652             let mut selcx = SelectionContext::new(self);
1653
1654             let cleaned_pred =
1655                 pred.fold_with(&mut ParamToVarFolder { infcx: self, var_map: Default::default() });
1656
1657             let cleaned_pred = super::project::normalize(
1658                 &mut selcx,
1659                 param_env,
1660                 ObligationCause::dummy(),
1661                 &cleaned_pred,
1662             )
1663             .value;
1664
1665             let obligation = Obligation::new(
1666                 ObligationCause::dummy(),
1667                 param_env,
1668                 cleaned_pred.without_const().to_predicate(selcx.tcx()),
1669             );
1670
1671             self.predicate_may_hold(&obligation)
1672         })
1673     }
1674
1675     fn note_obligation_cause(
1676         &self,
1677         err: &mut DiagnosticBuilder<'tcx>,
1678         obligation: &PredicateObligation<'tcx>,
1679     ) {
1680         // First, attempt to add note to this error with an async-await-specific
1681         // message, and fall back to regular note otherwise.
1682         if !self.maybe_note_obligation_cause_for_async_await(err, obligation) {
1683             self.note_obligation_cause_code(
1684                 err,
1685                 &obligation.predicate,
1686                 &obligation.cause.code,
1687                 &mut vec![],
1688             );
1689             self.suggest_unsized_bound_if_applicable(err, obligation);
1690         }
1691     }
1692
1693     fn suggest_unsized_bound_if_applicable(
1694         &self,
1695         err: &mut DiagnosticBuilder<'tcx>,
1696         obligation: &PredicateObligation<'tcx>,
1697     ) {
1698         if let (
1699             ty::PredicateKind::Trait(pred, _),
1700             ObligationCauseCode::BindingObligation(item_def_id, span),
1701         ) = (obligation.predicate.kind(), &obligation.cause.code)
1702         {
1703             if let (Some(generics), true) = (
1704                 self.tcx.hir().get_if_local(*item_def_id).as_ref().and_then(|n| n.generics()),
1705                 Some(pred.def_id()) == self.tcx.lang_items().sized_trait(),
1706             ) {
1707                 for param in generics.params {
1708                     if param.span == *span
1709                         && !param.bounds.iter().any(|bound| {
1710                             bound.trait_ref().and_then(|trait_ref| trait_ref.trait_def_id())
1711                                 == self.tcx.lang_items().sized_trait()
1712                         })
1713                     {
1714                         let (span, separator) = match param.bounds {
1715                             [] => (span.shrink_to_hi(), ":"),
1716                             [.., bound] => (bound.span().shrink_to_hi(), " +"),
1717                         };
1718                         err.span_suggestion_verbose(
1719                             span,
1720                             "consider relaxing the implicit `Sized` restriction",
1721                             format!("{} ?Sized", separator),
1722                             Applicability::MachineApplicable,
1723                         );
1724                         return;
1725                     }
1726                 }
1727             }
1728         }
1729     }
1730
1731     fn is_recursive_obligation(
1732         &self,
1733         obligated_types: &mut Vec<&ty::TyS<'tcx>>,
1734         cause_code: &ObligationCauseCode<'tcx>,
1735     ) -> bool {
1736         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
1737             let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
1738
1739             if obligated_types.iter().any(|ot| ot == &parent_trait_ref.skip_binder().self_ty()) {
1740                 return true;
1741             }
1742         }
1743         false
1744     }
1745 }
1746
1747 pub fn recursive_type_with_infinite_size_error(
1748     tcx: TyCtxt<'tcx>,
1749     type_def_id: DefId,
1750     spans: Vec<Span>,
1751 ) {
1752     assert!(type_def_id.is_local());
1753     let span = tcx.hir().span_if_local(type_def_id).unwrap();
1754     let span = tcx.sess.source_map().guess_head_span(span);
1755     let path = tcx.def_path_str(type_def_id);
1756     let mut err =
1757         struct_span_err!(tcx.sess, span, E0072, "recursive type `{}` has infinite size", path);
1758     err.span_label(span, "recursive type has infinite size");
1759     for &span in &spans {
1760         err.span_label(span, "recursive without indirection");
1761     }
1762     let short_msg = format!("insert some indirection to make `{}` representable", path);
1763     let msg = format!(
1764         "insert some indirection (e.g., a `Box`, `Rc`, or `&`) to make `{}` representable",
1765         path,
1766     );
1767     match &spans[..] {
1768         [span] => {
1769             err.multipart_suggestions(
1770                 &short_msg,
1771                 vec![
1772                     vec![
1773                         (span.shrink_to_lo(), "Box<".to_string()),
1774                         (span.shrink_to_hi(), ">".to_string()),
1775                     ],
1776                     vec![
1777                         (span.shrink_to_lo(), "Rc<".to_string()),
1778                         (span.shrink_to_hi(), ">".to_string()),
1779                     ],
1780                     vec![(span.shrink_to_lo(), "&".to_string())],
1781                 ],
1782                 Applicability::HasPlaceholders,
1783             );
1784         }
1785         _ if spans.len() <= 4 => {
1786             err.multipart_suggestion(
1787                 &msg,
1788                 spans
1789                     .iter()
1790                     .flat_map(|&span| {
1791                         vec![
1792                             (span.shrink_to_lo(), "Box<".to_string()),
1793                             (span.shrink_to_hi(), ">".to_string()),
1794                         ]
1795                         .into_iter()
1796                     })
1797                     .collect(),
1798                 Applicability::HasPlaceholders,
1799             );
1800         }
1801         _ => {
1802             err.help(&msg);
1803         }
1804     }
1805     err.emit();
1806 }
1807
1808 /// Summarizes information
1809 #[derive(Clone)]
1810 pub enum ArgKind {
1811     /// An argument of non-tuple type. Parameters are (name, ty)
1812     Arg(String, String),
1813
1814     /// An argument of tuple type. For a "found" argument, the span is
1815     /// the locationo in the source of the pattern. For a "expected"
1816     /// argument, it will be None. The vector is a list of (name, ty)
1817     /// strings for the components of the tuple.
1818     Tuple(Option<Span>, Vec<(String, String)>),
1819 }
1820
1821 impl ArgKind {
1822     fn empty() -> ArgKind {
1823         ArgKind::Arg("_".to_owned(), "_".to_owned())
1824     }
1825
1826     /// Creates an `ArgKind` from the expected type of an
1827     /// argument. It has no name (`_`) and an optional source span.
1828     pub fn from_expected_ty(t: Ty<'_>, span: Option<Span>) -> ArgKind {
1829         match t.kind {
1830             ty::Tuple(ref tys) => ArgKind::Tuple(
1831                 span,
1832                 tys.iter().map(|ty| ("_".to_owned(), ty.to_string())).collect::<Vec<_>>(),
1833             ),
1834             _ => ArgKind::Arg("_".to_owned(), t.to_string()),
1835         }
1836     }
1837 }