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