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                         // WF predicates cannot themselves make
 574                         // errors. They can only block due to
 575                         // ambiguity; otherwise, they always
 576                         // degenerate into other obligations
 577                         // (which may fail).
 578                         span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 579                     }
 580
 581                     ty::Predicate::ConstEvaluatable(..) => {
 582                         // Errors for `ConstEvaluatable` predicates show up as
 583                         // `SelectionError::ConstEvalFailure`,
 584                         // not `Unimplemented`.
 585                         span_bug!(
 586                             span,
 587                             "const-evaluatable requirement gave wrong error: `{:?}`",
 588                             obligation
 589                         )
 590                     }
 591                 }
 592             }
 593
 594             OutputTypeParameterMismatch(ref found_trait_ref, ref expected_trait_ref, _) => {
 595                 let found_trait_ref = self.resolve_vars_if_possible(&*found_trait_ref);
 596                 let expected_trait_ref = self.resolve_vars_if_possible(&*expected_trait_ref);
 597
 598                 if expected_trait_ref.self_ty().references_error() {
 599                     return;
 600                 }
 601
 602                 let found_trait_ty = found_trait_ref.self_ty();
 603
 604                 let found_did = match found_trait_ty.kind {
 605                     ty::Closure(did, _) | ty::Foreign(did) | ty::FnDef(did, _) => Some(did),
 606                     ty::Adt(def, _) => Some(def.did),
 607                     _ => None,
 608                 };
 609
 610                 let found_span = found_did
 611                     .and_then(|did| self.tcx.hir().span_if_local(did))
 612                     .map(|sp| self.tcx.sess.source_map().guess_head_span(sp)); // the sp could be an fn def
 613
 614                 if self.reported_closure_mismatch.borrow().contains(&(span, found_span)) {
 615                     // We check closures twice, with obligations flowing in different directions,
 616                     // but we want to complain about them only once.
 617                     return;
 618                 }
 619
 620                 self.reported_closure_mismatch.borrow_mut().insert((span, found_span));
 621
 622                 let found = match found_trait_ref.skip_binder().substs.type_at(1).kind {
 623                     ty::Tuple(ref tys) => vec![ArgKind::empty(); tys.len()],
 624                     _ => vec![ArgKind::empty()],
 625                 };
 626
 627                 let expected_ty = expected_trait_ref.skip_binder().substs.type_at(1);
 628                 let expected = match expected_ty.kind {
 629                     ty::Tuple(ref tys) => tys
 630                         .iter()
 631                         .map(|t| ArgKind::from_expected_ty(t.expect_ty(), Some(span)))
 632                         .collect(),
 633                     _ => vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())],
 634                 };
 635
 636                 if found.len() == expected.len() {
 637                     self.report_closure_arg_mismatch(
 638                         span,
 639                         found_span,
 640                         found_trait_ref,
 641                         expected_trait_ref,
 642                     )
 643                 } else {
 644                     let (closure_span, found) = found_did
 645                         .and_then(|did| {
 646                             let node = self.tcx.hir().get_if_local(did)?;
 647                             let (found_span, found) = self.get_fn_like_arguments(node)?;
 648                             Some((Some(found_span), found))
 649                         })
 650                         .unwrap_or((found_span, found));
 651
 652                     self.report_arg_count_mismatch(
 653                         span,
 654                         closure_span,
 655                         expected,
 656                         found,
 657                         found_trait_ty.is_closure(),
 658                     )
 659                 }
 660             }
 661
 662             TraitNotObjectSafe(did) => {
 663                 let violations = self.tcx.object_safety_violations(did);
 664                 report_object_safety_error(self.tcx, span, did, violations)
 665             }
 666
 667             ConstEvalFailure(ErrorHandled::TooGeneric) => {
 668                 // In this instance, we have a const expression containing an unevaluated
 669                 // generic parameter. We have no idea whether this expression is valid or
 670                 // not (e.g. it might result in an error), but we don't want to just assume
 671                 // that it's okay, because that might result in post-monomorphisation time
 672                 // errors. The onus is really on the caller to provide values that it can
 673                 // prove are well-formed.
 674                 let mut err = self
 675                     .tcx
 676                     .sess
 677                     .struct_span_err(span, "constant expression depends on a generic parameter");
 678                 // FIXME(const_generics): we should suggest to the user how they can resolve this
 679                 // issue. However, this is currently not actually possible
 680                 // (see https://github.com/rust-lang/rust/issues/66962#issuecomment-575907083).
 681                 err.note("this may fail depending on what value the parameter takes");
 682                 err
 683             }
 684
 685             // Already reported in the query.
 686             ConstEvalFailure(ErrorHandled::Reported(ErrorReported)) => {
 687                 // FIXME(eddyb) remove this once `ErrorReported` becomes a proof token.
 688                 self.tcx.sess.delay_span_bug(span, "`ErrorReported` without an error");
 689                 return;
 690             }
 691
 692             // Already reported in the query, but only as a lint.
 693             // This shouldn't actually happen for constants used in types, modulo
 694             // bugs. The `delay_span_bug` here ensures it won't be ignored.
 695             ConstEvalFailure(ErrorHandled::Linted) => {
 696                 self.tcx.sess.delay_span_bug(span, "constant in type had error reported as lint");
 697                 return;
 698             }
 699
 700             Overflow => {
 701                 bug!("overflow should be handled before the `report_selection_error` path");
 702             }
 703         };
 704
 705         self.note_obligation_cause(&mut err, obligation);
 706         self.point_at_returns_when_relevant(&mut err, &obligation);
 707
 708         err.emit();
 709     }
 710
 711     /// Given some node representing a fn-like thing in the HIR map,
 712     /// returns a span and `ArgKind` information that describes the
 713     /// arguments it expects. This can be supplied to
 714     /// `report_arg_count_mismatch`.
 715     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)> {
 716         let sm = self.tcx.sess.source_map();
 717         let hir = self.tcx.hir();
 718         Some(match node {
 719             Node::Expr(&hir::Expr {
 720                 kind: hir::ExprKind::Closure(_, ref _decl, id, span, _),
 721                 ..
 722             }) => (
 723                 sm.guess_head_span(span),
 724                 hir.body(id)
 725                     .params
 726                     .iter()
 727                     .map(|arg| {
 728                         if let hir::Pat { kind: hir::PatKind::Tuple(ref args, _), span, .. } =
 729                             *arg.pat
 730                         {
 731                             Some(ArgKind::Tuple(
 732                                 Some(span),
 733                                 args.iter()
 734                                     .map(|pat| {
 735                                         sm.span_to_snippet(pat.span)
 736                                             .ok()
 737                                             .map(|snippet| (snippet, "_".to_owned()))
 738                                     })
 739                                     .collect::<Option<Vec<_>>>()?,
 740                             ))
 741                         } else {
 742                             let name = sm.span_to_snippet(arg.pat.span).ok()?;
 743                             Some(ArgKind::Arg(name, "_".to_owned()))
 744                         }
 745                     })
 746                     .collect::<Option<Vec<ArgKind>>>()?,
 747             ),
 748             Node::Item(&hir::Item { span, kind: hir::ItemKind::Fn(ref sig, ..), .. })
 749             | Node::ImplItem(&hir::ImplItem {
 750                 span,
 751                 kind: hir::ImplItemKind::Fn(ref sig, _),
 752                 ..
 753             })
 754             | Node::TraitItem(&hir::TraitItem {
 755                 span,
 756                 kind: hir::TraitItemKind::Fn(ref sig, _),
 757                 ..
 758             }) => (
 759                 sm.guess_head_span(span),
 760                 sig.decl
 761                     .inputs
 762                     .iter()
 763                     .map(|arg| match arg.clone().kind {
 764                         hir::TyKind::Tup(ref tys) => ArgKind::Tuple(
 765                             Some(arg.span),
 766                             vec![("_".to_owned(), "_".to_owned()); tys.len()],
 767                         ),
 768                         _ => ArgKind::empty(),
 769                     })
 770                     .collect::<Vec<ArgKind>>(),
 771             ),
 772             Node::Ctor(ref variant_data) => {
 773                 let span = variant_data.ctor_hir_id().map(|id| hir.span(id)).unwrap_or(DUMMY_SP);
 774                 let span = sm.guess_head_span(span);
 775                 (span, vec![ArgKind::empty(); variant_data.fields().len()])
 776             }
 777             _ => panic!("non-FnLike node found: {:?}", node),
 778         })
 779     }
 780
 781     /// Reports an error when the number of arguments needed by a
 782     /// trait match doesn't match the number that the expression
 783     /// provides.
 784     fn report_arg_count_mismatch(
 785         &self,
 786         span: Span,
 787         found_span: Option<Span>,
 788         expected_args: Vec<ArgKind>,
 789         found_args: Vec<ArgKind>,
 790         is_closure: bool,
 791     ) -> DiagnosticBuilder<'tcx> {
 792         let kind = if is_closure { "closure" } else { "function" };
 793
 794         let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
 795             let arg_length = arguments.len();
 796             let distinct = match &other[..] {
 797                 &[ArgKind::Tuple(..)] => true,
 798                 _ => false,
 799             };
 800             match (arg_length, arguments.get(0)) {
 801                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
 802                     format!("a single {}-tuple as argument", fields.len())
 803                 }
 804                 _ => format!(
 805                     "{} {}argument{}",
 806                     arg_length,
 807                     if distinct && arg_length > 1 { "distinct " } else { "" },
 808                     pluralize!(arg_length)
 809                 ),
 810             }
 811         };
 812
 813         let expected_str = args_str(&expected_args, &found_args);
 814         let found_str = args_str(&found_args, &expected_args);
 815
 816         let mut err = struct_span_err!(
 817             self.tcx.sess,
 818             span,
 819             E0593,
 820             "{} is expected to take {}, but it takes {}",
 821             kind,
 822             expected_str,
 823             found_str,
 824         );
 825
 826         err.span_label(span, format!("expected {} that takes {}", kind, expected_str));
 827
 828         if let Some(found_span) = found_span {
 829             err.span_label(found_span, format!("takes {}", found_str));
 830
 831             // move |_| { ... }
 832             // ^^^^^^^^-- def_span
 833             //
 834             // move |_| { ... }
 835             // ^^^^^-- prefix
 836             let prefix_span = self.tcx.sess.source_map().span_until_non_whitespace(found_span);
 837             // move |_| { ... }
 838             //      ^^^-- pipe_span
 839             let pipe_span =
 840                 if let Some(span) = found_span.trim_start(prefix_span) { span } else { found_span };
 841
 842             // Suggest to take and ignore the arguments with expected_args_length `_`s if
 843             // found arguments is empty (assume the user just wants to ignore args in this case).
 844             // For example, if `expected_args_length` is 2, suggest `|_, _|`.
 845             if found_args.is_empty() && is_closure {
 846                 let underscores = vec!["_"; expected_args.len()].join(", ");
 847                 err.span_suggestion_verbose(
 848                     pipe_span,
 849                     &format!(
 850                         "consider changing the closure to take and ignore the expected argument{}",
 851                         pluralize!(expected_args.len())
 852                     ),
 853                     format!("|{}|", underscores),
 854                     Applicability::MachineApplicable,
 855                 );
 856             }
 857
 858             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
 859                 if fields.len() == expected_args.len() {
 860                     let sugg = fields
 861                         .iter()
 862                         .map(|(name, _)| name.to_owned())
 863                         .collect::<Vec<String>>()
 864                         .join(", ");
 865                     err.span_suggestion_verbose(
 866                         found_span,
 867                         "change the closure to take multiple arguments instead of a single tuple",
 868                         format!("|{}|", sugg),
 869                         Applicability::MachineApplicable,
 870                     );
 871                 }
 872             }
 873             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..] {
 874                 if fields.len() == found_args.len() && is_closure {
 875                     let sugg = format!(
 876                         "|({}){}|",
 877                         found_args
 878                             .iter()
 879                             .map(|arg| match arg {
 880                                 ArgKind::Arg(name, _) => name.to_owned(),
 881                                 _ => "_".to_owned(),
 882                             })
 883                             .collect::<Vec<String>>()
 884                             .join(", "),
 885                         // add type annotations if available
 886                         if found_args.iter().any(|arg| match arg {
 887                             ArgKind::Arg(_, ty) => ty != "_",
 888                             _ => false,
 889                         }) {
 890                             format!(
 891                                 ": ({})",
 892                                 fields
 893                                     .iter()
 894                                     .map(|(_, ty)| ty.to_owned())
 895                                     .collect::<Vec<String>>()
 896                                     .join(", ")
 897                             )
 898                         } else {
 899                             String::new()
 900                         },
 901                     );
 902                     err.span_suggestion_verbose(
 903                         found_span,
 904                         "change the closure to accept a tuple instead of individual arguments",
 905                         sugg,
 906                         Applicability::MachineApplicable,
 907                     );
 908                 }
 909             }
 910         }
 911
 912         err
 913     }
 914 }
 915
 916 trait InferCtxtPrivExt<'tcx> {
 917     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
 918     // `error` occurring implies that `cond` occurs.
 919     fn error_implies(&self, cond: &ty::Predicate<'tcx>, error: &ty::Predicate<'tcx>) -> bool;
 920
 921     fn report_fulfillment_error(
 922         &self,
 923         error: &FulfillmentError<'tcx>,
 924         body_id: Option<hir::BodyId>,
 925         fallback_has_occurred: bool,
 926     );
 927
 928     fn report_projection_error(
 929         &self,
 930         obligation: &PredicateObligation<'tcx>,
 931         error: &MismatchedProjectionTypes<'tcx>,
 932     );
 933
 934     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool;
 935
 936     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str>;
 937
 938     fn find_similar_impl_candidates(
 939         &self,
 940         trait_ref: ty::PolyTraitRef<'tcx>,
 941     ) -> Vec<ty::TraitRef<'tcx>>;
 942
 943     fn report_similar_impl_candidates(
 944         &self,
 945         impl_candidates: Vec<ty::TraitRef<'tcx>>,
 946         err: &mut DiagnosticBuilder<'_>,
 947     );
 948
 949     /// Gets the parent trait chain start
 950     fn get_parent_trait_ref(
 951         &self,
 952         code: &ObligationCauseCode<'tcx>,
 953     ) -> Option<(String, Option<Span>)>;
 954
 955     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
 956     /// with the same path as `trait_ref`, a help message about
 957     /// a probable version mismatch is added to `err`
 958     fn note_version_mismatch(
 959         &self,
 960         err: &mut DiagnosticBuilder<'_>,
 961         trait_ref: &ty::PolyTraitRef<'tcx>,
 962     );
 963
 964     fn mk_obligation_for_def_id(
 965         &self,
 966         def_id: DefId,
 967         output_ty: Ty<'tcx>,
 968         cause: ObligationCause<'tcx>,
 969         param_env: ty::ParamEnv<'tcx>,
 970     ) -> PredicateObligation<'tcx>;
 971
 972     fn maybe_report_ambiguity(
 973         &self,
 974         obligation: &PredicateObligation<'tcx>,
 975         body_id: Option<hir::BodyId>,
 976     );
 977
 978     fn predicate_can_apply(
 979         &self,
 980         param_env: ty::ParamEnv<'tcx>,
 981         pred: ty::PolyTraitRef<'tcx>,
 982     ) -> bool;
 983
 984     fn note_obligation_cause(
 985         &self,
 986         err: &mut DiagnosticBuilder<'_>,
 987         obligation: &PredicateObligation<'tcx>,
 988     );
 989
 990     fn suggest_unsized_bound_if_applicable(
 991         &self,
 992         err: &mut DiagnosticBuilder<'_>,
 993         obligation: &PredicateObligation<'tcx>,
 994     );
 995
 996     fn is_recursive_obligation(
 997         &self,
 998         obligated_types: &mut Vec<&ty::TyS<'tcx>>,
 999         cause_code: &ObligationCauseCode<'tcx>,
1000     ) -> bool;
1001 }
1002
1003 impl<'a, 'tcx> InferCtxtPrivExt<'tcx> for InferCtxt<'a, 'tcx> {
1004     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1005     // `error` occurring implies that `cond` occurs.
1006     fn error_implies(&self, cond: &ty::Predicate<'tcx>, error: &ty::Predicate<'tcx>) -> bool {
1007         if cond == error {
1008             return true;
1009         }
1010
1011         let (cond, error) = match (cond, error) {
1012             (&ty::Predicate::Trait(..), &ty::Predicate::Trait(ref error, _)) => (cond, error),
1013             _ => {
1014                 // FIXME: make this work in other cases too.
1015                 return false;
1016             }
1017         };
1018
1019         for obligation in super::elaborate_predicates(self.tcx, std::iter::once(*cond)) {
1020             if let ty::Predicate::Trait(implication, _) = obligation.predicate {
1021                 let error = error.to_poly_trait_ref();
1022                 let implication = implication.to_poly_trait_ref();
1023                 // FIXME: I'm just not taking associated types at all here.
1024                 // Eventually I'll need to implement param-env-aware
1025                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
1026                 let param_env = ty::ParamEnv::empty();
1027                 if self.can_sub(param_env, error, implication).is_ok() {
1028                     debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
1029                     return true;
1030                 }
1031             }
1032         }
1033
1034         false
1035     }
1036
1037     fn report_fulfillment_error(
1038         &self,
1039         error: &FulfillmentError<'tcx>,
1040         body_id: Option<hir::BodyId>,
1041         fallback_has_occurred: bool,
1042     ) {
1043         debug!("report_fulfillment_error({:?})", error);
1044         match error.code {
1045             FulfillmentErrorCode::CodeSelectionError(ref selection_error) => {
1046                 self.report_selection_error(
1047                     &error.obligation,
1048                     selection_error,
1049                     fallback_has_occurred,
1050                     error.points_at_arg_span,
1051                 );
1052             }
1053             FulfillmentErrorCode::CodeProjectionError(ref e) => {
1054                 self.report_projection_error(&error.obligation, e);
1055             }
1056             FulfillmentErrorCode::CodeAmbiguity => {
1057                 self.maybe_report_ambiguity(&error.obligation, body_id);
1058             }
1059             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
1060                 self.report_mismatched_types(
1061                     &error.obligation.cause,
1062                     expected_found.expected,
1063                     expected_found.found,
1064                     err.clone(),
1065                 )
1066                 .emit();
1067             }
1068         }
1069     }
1070
1071     fn report_projection_error(
1072         &self,
1073         obligation: &PredicateObligation<'tcx>,
1074         error: &MismatchedProjectionTypes<'tcx>,
1075     ) {
1076         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
1077
1078         if predicate.references_error() {
1079             return;
1080         }
1081
1082         self.probe(|_| {
1083             let err_buf;
1084             let mut err = &error.err;
1085             let mut values = None;
1086
1087             // try to find the mismatched types to report the error with.
1088             //
1089             // this can fail if the problem was higher-ranked, in which
1090             // cause I have no idea for a good error message.
1091             if let ty::Predicate::Projection(ref data) = predicate {
1092                 let mut selcx = SelectionContext::new(self);
1093                 let (data, _) = self.replace_bound_vars_with_fresh_vars(
1094                     obligation.cause.span,
1095                     infer::LateBoundRegionConversionTime::HigherRankedType,
1096                     data,
1097                 );
1098                 let mut obligations = vec![];
1099                 let normalized_ty = super::normalize_projection_type(
1100                     &mut selcx,
1101                     obligation.param_env,
1102                     data.projection_ty,
1103                     obligation.cause.clone(),
1104                     0,
1105                     &mut obligations,
1106                 );
1107
1108                 debug!(
1109                     "report_projection_error obligation.cause={:?} obligation.param_env={:?}",
1110                     obligation.cause, obligation.param_env
1111                 );
1112
1113                 debug!(
1114                     "report_projection_error normalized_ty={:?} data.ty={:?}",
1115                     normalized_ty, data.ty
1116                 );
1117
1118                 let is_normalized_ty_expected = match &obligation.cause.code {
1119                     ObligationCauseCode::ItemObligation(_)
1120                     | ObligationCauseCode::BindingObligation(_, _)
1121                     | ObligationCauseCode::ObjectCastObligation(_) => false,
1122                     _ => true,
1123                 };
1124
1125                 if let Err(error) = self.at(&obligation.cause, obligation.param_env).eq_exp(
1126                     is_normalized_ty_expected,
1127                     normalized_ty,
1128                     data.ty,
1129                 ) {
1130                     values = Some(infer::ValuePairs::Types(ExpectedFound::new(
1131                         is_normalized_ty_expected,
1132                         normalized_ty,
1133                         data.ty,
1134                     )));
1135
1136                     err_buf = error;
1137                     err = &err_buf;
1138                 }
1139             }
1140
1141             let msg = format!("type mismatch resolving `{}`", predicate);
1142             let error_id = (DiagnosticMessageId::ErrorId(271), Some(obligation.cause.span), msg);
1143             let fresh = self.tcx.sess.one_time_diagnostics.borrow_mut().insert(error_id);
1144             if fresh {
1145                 let mut diag = struct_span_err!(
1146                     self.tcx.sess,
1147                     obligation.cause.span,
1148                     E0271,
1149                     "type mismatch resolving `{}`",
1150                     predicate
1151                 );
1152                 self.note_type_err(&mut diag, &obligation.cause, None, values, err);
1153                 self.note_obligation_cause(&mut diag, obligation);
1154                 diag.emit();
1155             }
1156         });
1157     }
1158
1159     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
1160         /// returns the fuzzy category of a given type, or None
1161         /// if the type can be equated to any type.
1162         fn type_category(t: Ty<'_>) -> Option<u32> {
1163             match t.kind {
1164                 ty::Bool => Some(0),
1165                 ty::Char => Some(1),
1166                 ty::Str => Some(2),
1167                 ty::Int(..) | ty::Uint(..) | ty::Infer(ty::IntVar(..)) => Some(3),
1168                 ty::Float(..) | ty::Infer(ty::FloatVar(..)) => Some(4),
1169                 ty::Ref(..) | ty::RawPtr(..) => Some(5),
1170                 ty::Array(..) | ty::Slice(..) => Some(6),
1171                 ty::FnDef(..) | ty::FnPtr(..) => Some(7),
1172                 ty::Dynamic(..) => Some(8),
1173                 ty::Closure(..) => Some(9),
1174                 ty::Tuple(..) => Some(10),
1175                 ty::Projection(..) => Some(11),
1176                 ty::Param(..) => Some(12),
1177                 ty::Opaque(..) => Some(13),
1178                 ty::Never => Some(14),
1179                 ty::Adt(adt, ..) => match adt.adt_kind() {
1180                     AdtKind::Struct => Some(15),
1181                     AdtKind::Union => Some(16),
1182                     AdtKind::Enum => Some(17),
1183                 },
1184                 ty::Generator(..) => Some(18),
1185                 ty::Foreign(..) => Some(19),
1186                 ty::GeneratorWitness(..) => Some(20),
1187                 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error => None,
1188                 ty::UnnormalizedProjection(..) => bug!("only used with chalk-engine"),
1189             }
1190         }
1191
1192         match (type_category(a), type_category(b)) {
1193             (Some(cat_a), Some(cat_b)) => match (&a.kind, &b.kind) {
1194                 (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => def_a == def_b,
1195                 _ => cat_a == cat_b,
1196             },
1197             // infer and error can be equated to all types
1198             _ => true,
1199         }
1200     }
1201
1202     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str> {
1203         self.tcx.hir().body(body_id).generator_kind.map(|gen_kind| match gen_kind {
1204             hir::GeneratorKind::Gen => "a generator",
1205             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block) => "an async block",
1206             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn) => "an async function",
1207             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Closure) => "an async closure",
1208         })
1209     }
1210
1211     fn find_similar_impl_candidates(
1212         &self,
1213         trait_ref: ty::PolyTraitRef<'tcx>,
1214     ) -> Vec<ty::TraitRef<'tcx>> {
1215         let simp = fast_reject::simplify_type(self.tcx, trait_ref.skip_binder().self_ty(), true);
1216         let all_impls = self.tcx.all_impls(trait_ref.def_id());
1217
1218         match simp {
1219             Some(simp) => all_impls
1220                 .filter_map(|def_id| {
1221                     let imp = self.tcx.impl_trait_ref(def_id).unwrap();
1222                     let imp_simp = fast_reject::simplify_type(self.tcx, imp.self_ty(), true);
1223                     if let Some(imp_simp) = imp_simp {
1224                         if simp != imp_simp {
1225                             return None;
1226                         }
1227                     }
1228                     Some(imp)
1229                 })
1230                 .collect(),
1231             None => all_impls.map(|def_id| self.tcx.impl_trait_ref(def_id).unwrap()).collect(),
1232         }
1233     }
1234
1235     fn report_similar_impl_candidates(
1236         &self,
1237         impl_candidates: Vec<ty::TraitRef<'tcx>>,
1238         err: &mut DiagnosticBuilder<'_>,
1239     ) {
1240         if impl_candidates.is_empty() {
1241             return;
1242         }
1243
1244         let len = impl_candidates.len();
1245         let end = if impl_candidates.len() <= 5 { impl_candidates.len() } else { 4 };
1246
1247         let normalize = |candidate| {
1248             self.tcx.infer_ctxt().enter(|ref infcx| {
1249                 let normalized = infcx
1250                     .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
1251                     .normalize(candidate)
1252                     .ok();
1253                 match normalized {
1254                     Some(normalized) => format!("\n  {:?}", normalized.value),
1255                     None => format!("\n  {:?}", candidate),
1256                 }
1257             })
1258         };
1259
1260         // Sort impl candidates so that ordering is consistent for UI tests.
1261         let mut normalized_impl_candidates =
1262             impl_candidates.iter().map(normalize).collect::<Vec<String>>();
1263
1264         // Sort before taking the `..end` range,
1265         // because the ordering of `impl_candidates` may not be deterministic:
1266         // https://github.com/rust-lang/rust/pull/57475#issuecomment-455519507
1267         normalized_impl_candidates.sort();
1268
1269         err.help(&format!(
1270             "the following implementations were found:{}{}",
1271             normalized_impl_candidates[..end].join(""),
1272             if len > 5 { format!("\nand {} others", len - 4) } else { String::new() }
1273         ));
1274     }
1275
1276     /// Gets the parent trait chain start
1277     fn get_parent_trait_ref(
1278         &self,
1279         code: &ObligationCauseCode<'tcx>,
1280     ) -> Option<(String, Option<Span>)> {
1281         match code {
1282             &ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
1283                 let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
1284                 match self.get_parent_trait_ref(&data.parent_code) {
1285                     Some(t) => Some(t),
1286                     None => {
1287                         let ty = parent_trait_ref.skip_binder().self_ty();
1288                         let span =
1289                             TyCategory::from_ty(ty).map(|(_, def_id)| self.tcx.def_span(def_id));
1290                         Some((ty.to_string(), span))
1291                     }
1292                 }
1293             }
1294             _ => None,
1295         }
1296     }
1297
1298     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1299     /// with the same path as `trait_ref`, a help message about
1300     /// a probable version mismatch is added to `err`
1301     fn note_version_mismatch(
1302         &self,
1303         err: &mut DiagnosticBuilder<'_>,
1304         trait_ref: &ty::PolyTraitRef<'tcx>,
1305     ) {
1306         let get_trait_impl = |trait_def_id| {
1307             let mut trait_impl = None;
1308             self.tcx.for_each_relevant_impl(trait_def_id, trait_ref.self_ty(), |impl_def_id| {
1309                 if trait_impl.is_none() {
1310                     trait_impl = Some(impl_def_id);
1311                 }
1312             });
1313             trait_impl
1314         };
1315         let required_trait_path = self.tcx.def_path_str(trait_ref.def_id());
1316         let all_traits = self.tcx.all_traits(LOCAL_CRATE);
1317         let traits_with_same_path: std::collections::BTreeSet<_> = all_traits
1318             .iter()
1319             .filter(|trait_def_id| **trait_def_id != trait_ref.def_id())
1320             .filter(|trait_def_id| self.tcx.def_path_str(**trait_def_id) == required_trait_path)
1321             .collect();
1322         for trait_with_same_path in traits_with_same_path {
1323             if let Some(impl_def_id) = get_trait_impl(*trait_with_same_path) {
1324                 let impl_span = self.tcx.def_span(impl_def_id);
1325                 err.span_help(impl_span, "trait impl with same name found");
1326                 let trait_crate = self.tcx.crate_name(trait_with_same_path.krate);
1327                 let crate_msg = format!(
1328                     "perhaps two different versions of crate `{}` are being used?",
1329                     trait_crate
1330                 );
1331                 err.note(&crate_msg);
1332             }
1333         }
1334     }
1335
1336     fn mk_obligation_for_def_id(
1337         &self,
1338         def_id: DefId,
1339         output_ty: Ty<'tcx>,
1340         cause: ObligationCause<'tcx>,
1341         param_env: ty::ParamEnv<'tcx>,
1342     ) -> PredicateObligation<'tcx> {
1343         let new_trait_ref =
1344             ty::TraitRef { def_id, substs: self.tcx.mk_substs_trait(output_ty, &[]) };
1345         Obligation::new(cause, param_env, new_trait_ref.without_const().to_predicate())
1346     }
1347
1348     fn maybe_report_ambiguity(
1349         &self,
1350         obligation: &PredicateObligation<'tcx>,
1351         body_id: Option<hir::BodyId>,
1352     ) {
1353         // Unable to successfully determine, probably means
1354         // insufficient type information, but could mean
1355         // ambiguous impls. The latter *ought* to be a
1356         // coherence violation, so we don't report it here.
1357
1358         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
1359         let span = obligation.cause.span;
1360
1361         debug!(
1362             "maybe_report_ambiguity(predicate={:?}, obligation={:?} body_id={:?}, code={:?})",
1363             predicate, obligation, body_id, obligation.cause.code,
1364         );
1365
1366         // Ambiguity errors are often caused as fallout from earlier
1367         // errors. So just ignore them if this infcx is tainted.
1368         if self.is_tainted_by_errors() {
1369             return;
1370         }
1371
1372         let mut err = match predicate {
1373             ty::Predicate::Trait(ref data, _) => {
1374                 let trait_ref = data.to_poly_trait_ref();
1375                 let self_ty = trait_ref.self_ty();
1376                 debug!("self_ty {:?} {:?} trait_ref {:?}", self_ty, self_ty.kind, trait_ref);
1377
1378                 if predicate.references_error() {
1379                     return;
1380                 }
1381                 // Typically, this ambiguity should only happen if
1382                 // there are unresolved type inference variables
1383                 // (otherwise it would suggest a coherence
1384                 // failure). But given #21974 that is not necessarily
1385                 // the case -- we can have multiple where clauses that
1386                 // are only distinguished by a region, which results
1387                 // in an ambiguity even when all types are fully
1388                 // known, since we don't dispatch based on region
1389                 // relationships.
1390
1391                 // This is kind of a hack: it frequently happens that some earlier
1392                 // error prevents types from being fully inferred, and then we get
1393                 // a bunch of uninteresting errors saying something like "<generic
1394                 // #0> doesn't implement Sized".  It may even be true that we
1395                 // could just skip over all checks where the self-ty is an
1396                 // inference variable, but I was afraid that there might be an
1397                 // inference variable created, registered as an obligation, and
1398                 // then never forced by writeback, and hence by skipping here we'd
1399                 // be ignoring the fact that we don't KNOW the type works
1400                 // out. Though even that would probably be harmless, given that
1401                 // we're only talking about builtin traits, which are known to be
1402                 // inhabited. We used to check for `self.tcx.sess.has_errors()` to
1403                 // avoid inundating the user with unnecessary errors, but we now
1404                 // check upstream for type errors and don't add the obligations to
1405                 // begin with in those cases.
1406                 if self
1407                     .tcx
1408                     .lang_items()
1409                     .sized_trait()
1410                     .map_or(false, |sized_id| sized_id == trait_ref.def_id())
1411                 {
1412                     self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0282).emit();
1413                     return;
1414                 }
1415                 let mut err = self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0283);
1416                 err.note(&format!("cannot satisfy `{}`", predicate));
1417                 if let ObligationCauseCode::ItemObligation(def_id) = obligation.cause.code {
1418                     self.suggest_fully_qualified_path(&mut err, def_id, span, trait_ref.def_id());
1419                 } else if let (
1420                     Ok(ref snippet),
1421                     ObligationCauseCode::BindingObligation(ref def_id, _),
1422                 ) =
1423                     (self.tcx.sess.source_map().span_to_snippet(span), &obligation.cause.code)
1424                 {
1425                     let generics = self.tcx.generics_of(*def_id);
1426                     if generics.params.iter().any(|p| p.name.as_str() != "Self")
1427                         && !snippet.ends_with('>')
1428                     {
1429                         // FIXME: To avoid spurious suggestions in functions where type arguments
1430                         // where already supplied, we check the snippet to make sure it doesn't
1431                         // end with a turbofish. Ideally we would have access to a `PathSegment`
1432                         // instead. Otherwise we would produce the following output:
1433                         //
1434                         // error[E0283]: type annotations needed
1435                         //   --> $DIR/issue-54954.rs:3:24
1436                         //    |
1437                         // LL | const ARR_LEN: usize = Tt::const_val::<[i8; 123]>();
1438                         //    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^
1439                         //    |                        |
1440                         //    |                        cannot infer type
1441                         //    |                        help: consider specifying the type argument
1442                         //    |                        in the function call:
1443                         //    |                        `Tt::const_val::<[i8; 123]>::<T>`
1444                         // ...
1445                         // LL |     const fn const_val<T: Sized>() -> usize {
1446                         //    |                        - required by this bound in `Tt::const_val`
1447                         //    |
1448                         //    = note: cannot satisfy `_: Tt`
1449
1450                         err.span_suggestion_verbose(
1451                             span.shrink_to_hi(),
1452                             &format!(
1453                                 "consider specifying the type argument{} in the function call",
1454                                 pluralize!(generics.params.len()),
1455                             ),
1456                             format!(
1457                                 "::<{}>",
1458                                 generics
1459                                     .params
1460                                     .iter()
1461                                     .map(|p| p.name.to_string())
1462                                     .collect::<Vec<String>>()
1463                                     .join(", ")
1464                             ),
1465                             Applicability::HasPlaceholders,
1466                         );
1467                     }
1468                 }
1469                 err
1470             }
1471
1472             ty::Predicate::WellFormed(ty) => {
1473                 // Same hacky approach as above to avoid deluging user
1474                 // with error messages.
1475                 if ty.references_error() || self.tcx.sess.has_errors() {
1476                     return;
1477                 }
1478                 self.need_type_info_err(body_id, span, ty, ErrorCode::E0282)
1479             }
1480
1481             ty::Predicate::Subtype(ref data) => {
1482                 if data.references_error() || self.tcx.sess.has_errors() {
1483                     // no need to overload user in such cases
1484                     return;
1485                 }
1486                 let &SubtypePredicate { a_is_expected: _, a, b } = data.skip_binder();
1487                 // both must be type variables, or the other would've been instantiated
1488                 assert!(a.is_ty_var() && b.is_ty_var());
1489                 self.need_type_info_err(body_id, span, a, ErrorCode::E0282)
1490             }
1491             ty::Predicate::Projection(ref data) => {
1492                 let trait_ref = data.to_poly_trait_ref(self.tcx);
1493                 let self_ty = trait_ref.self_ty();
1494                 let ty = data.skip_binder().ty;
1495                 if predicate.references_error() {
1496                     return;
1497                 }
1498                 if self_ty.needs_infer() && ty.needs_infer() {
1499                     // We do this for the `foo.collect()?` case to produce a suggestion.
1500                     let mut err = self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0284);
1501                     err.note(&format!("cannot satisfy `{}`", predicate));
1502                     err
1503                 } else {
1504                     let mut err = struct_span_err!(
1505                         self.tcx.sess,
1506                         span,
1507                         E0284,
1508                         "type annotations needed: cannot satisfy `{}`",
1509                         predicate,
1510                     );
1511                     err.span_label(span, &format!("cannot satisfy `{}`", predicate));
1512                     err
1513                 }
1514             }
1515
1516             _ => {
1517                 if self.tcx.sess.has_errors() {
1518                     return;
1519                 }
1520                 let mut err = struct_span_err!(
1521                     self.tcx.sess,
1522                     span,
1523                     E0284,
1524                     "type annotations needed: cannot satisfy `{}`",
1525                     predicate,
1526                 );
1527                 err.span_label(span, &format!("cannot satisfy `{}`", predicate));
1528                 err
1529             }
1530         };
1531         self.note_obligation_cause(&mut err, obligation);
1532         err.emit();
1533     }
1534
1535     /// Returns `true` if the trait predicate may apply for *some* assignment
1536     /// to the type parameters.
1537     fn predicate_can_apply(
1538         &self,
1539         param_env: ty::ParamEnv<'tcx>,
1540         pred: ty::PolyTraitRef<'tcx>,
1541     ) -> bool {
1542         struct ParamToVarFolder<'a, 'tcx> {
1543             infcx: &'a InferCtxt<'a, 'tcx>,
1544             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
1545         }
1546
1547         impl<'a, 'tcx> TypeFolder<'tcx> for ParamToVarFolder<'a, 'tcx> {
1548             fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
1549                 self.infcx.tcx
1550             }
1551
1552             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
1553                 if let ty::Param(ty::ParamTy { name, .. }) = ty.kind {
1554                     let infcx = self.infcx;
1555                     self.var_map.entry(ty).or_insert_with(|| {
1556                         infcx.next_ty_var(TypeVariableOrigin {
1557                             kind: TypeVariableOriginKind::TypeParameterDefinition(name, None),
1558                             span: DUMMY_SP,
1559                         })
1560                     })
1561                 } else {
1562                     ty.super_fold_with(self)
1563                 }
1564             }
1565         }
1566
1567         self.probe(|_| {
1568             let mut selcx = SelectionContext::new(self);
1569
1570             let cleaned_pred =
1571                 pred.fold_with(&mut ParamToVarFolder { infcx: self, var_map: Default::default() });
1572
1573             let cleaned_pred = super::project::normalize(
1574                 &mut selcx,
1575                 param_env,
1576                 ObligationCause::dummy(),
1577                 &cleaned_pred,
1578             )
1579             .value;
1580
1581             let obligation = Obligation::new(
1582                 ObligationCause::dummy(),
1583                 param_env,
1584                 cleaned_pred.without_const().to_predicate(),
1585             );
1586
1587             self.predicate_may_hold(&obligation)
1588         })
1589     }
1590
1591     fn note_obligation_cause(
1592         &self,
1593         err: &mut DiagnosticBuilder<'_>,
1594         obligation: &PredicateObligation<'tcx>,
1595     ) {
1596         // First, attempt to add note to this error with an async-await-specific
1597         // message, and fall back to regular note otherwise.
1598         if !self.maybe_note_obligation_cause_for_async_await(err, obligation) {
1599             self.note_obligation_cause_code(
1600                 err,
1601                 &obligation.predicate,
1602                 &obligation.cause.code,
1603                 &mut vec![],
1604             );
1605             self.suggest_unsized_bound_if_applicable(err, obligation);
1606         }
1607     }
1608
1609     fn suggest_unsized_bound_if_applicable(
1610         &self,
1611         err: &mut DiagnosticBuilder<'_>,
1612         obligation: &PredicateObligation<'tcx>,
1613     ) {
1614         if let (
1615             ty::Predicate::Trait(pred, _),
1616             ObligationCauseCode::BindingObligation(item_def_id, span),
1617         ) = (&obligation.predicate, &obligation.cause.code)
1618         {
1619             if let (Some(generics), true) = (
1620                 self.tcx.hir().get_if_local(*item_def_id).as_ref().and_then(|n| n.generics()),
1621                 Some(pred.def_id()) == self.tcx.lang_items().sized_trait(),
1622             ) {
1623                 for param in generics.params {
1624                     if param.span == *span
1625                         && !param.bounds.iter().any(|bound| {
1626                             bound.trait_ref().and_then(|trait_ref| trait_ref.trait_def_id())
1627                                 == self.tcx.lang_items().sized_trait()
1628                         })
1629                     {
1630                         let (span, separator) = match param.bounds {
1631                             [] => (span.shrink_to_hi(), ":"),
1632                             [.., bound] => (bound.span().shrink_to_hi(), " + "),
1633                         };
1634                         err.span_suggestion_verbose(
1635                             span,
1636                             "consider relaxing the implicit `Sized` restriction",
1637                             format!("{} ?Sized", separator),
1638                             Applicability::MachineApplicable,
1639                         );
1640                         return;
1641                     }
1642                 }
1643             }
1644         }
1645     }
1646
1647     fn is_recursive_obligation(
1648         &self,
1649         obligated_types: &mut Vec<&ty::TyS<'tcx>>,
1650         cause_code: &ObligationCauseCode<'tcx>,
1651     ) -> bool {
1652         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
1653             let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
1654
1655             if obligated_types.iter().any(|ot| ot == &parent_trait_ref.skip_binder().self_ty()) {
1656                 return true;
1657             }
1658         }
1659         false
1660     }
1661 }
1662
1663 pub fn recursive_type_with_infinite_size_error(
1664     tcx: TyCtxt<'tcx>,
1665     type_def_id: DefId,
1666 ) -> DiagnosticBuilder<'tcx> {
1667     assert!(type_def_id.is_local());
1668     let span = tcx.hir().span_if_local(type_def_id).unwrap();
1669     let span = tcx.sess.source_map().guess_head_span(span);
1670     let mut err = struct_span_err!(
1671         tcx.sess,
1672         span,
1673         E0072,
1674         "recursive type `{}` has infinite size",
1675         tcx.def_path_str(type_def_id)
1676     );
1677     err.span_label(span, "recursive type has infinite size");
1678     err.help(&format!(
1679         "insert indirection (e.g., a `Box`, `Rc`, or `&`) \
1680                            at some point to make `{}` representable",
1681         tcx.def_path_str(type_def_id)
1682     ));
1683     err
1684 }
1685
1686 /// Summarizes information
1687 #[derive(Clone)]
1688 pub enum ArgKind {
1689     /// An argument of non-tuple type. Parameters are (name, ty)
1690     Arg(String, String),
1691
1692     /// An argument of tuple type. For a "found" argument, the span is
1693     /// the locationo in the source of the pattern. For a "expected"
1694     /// argument, it will be None. The vector is a list of (name, ty)
1695     /// strings for the components of the tuple.
1696     Tuple(Option<Span>, Vec<(String, String)>),
1697 }
1698
1699 impl ArgKind {
1700     fn empty() -> ArgKind {
1701         ArgKind::Arg("_".to_owned(), "_".to_owned())
1702     }
1703
1704     /// Creates an `ArgKind` from the expected type of an
1705     /// argument. It has no name (`_`) and an optional source span.
1706     pub fn from_expected_ty(t: Ty<'_>, span: Option<Span>) -> ArgKind {
1707         match t.kind {
1708             ty::Tuple(ref tys) => ArgKind::Tuple(
1709                 span,
1710                 tys.iter().map(|ty| ("_".to_owned(), ty.to_string())).collect::<Vec<_>>(),
1711             ),
1712             _ => ArgKind::Arg("_".to_owned(), t.to_string()),
1713         }
1714     }
1715 }