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