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.kind() {
 256                     ty::PredicateKind::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(tcx),
 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
 406                         if Some(trait_ref.def_id()) == tcx.lang_items().try_trait() {
 407                             self.suggest_await_before_try(&mut err, &obligation, &trait_ref, span);
 408                         }
 409
 410                         if self.suggest_impl_trait(&mut err, span, &obligation, &trait_ref) {
 411                             err.emit();
 412                             return;
 413                         }
 414
 415                         if is_unsize {
 416                             // If the obligation failed due to a missing implementation of the
 417                             // `Unsize` trait, give a pointer to why that might be the case
 418                             err.note(
 419                                 "all implementations of `Unsize` are provided \
 420                                 automatically by the compiler, see \
 421                                 <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> \
 422                                 for more information",
 423                             );
 424                         }
 425
 426                         // Try to report a help message
 427                         if !trait_ref.has_infer_types_or_consts()
 428                             && self.predicate_can_apply(obligation.param_env, trait_ref)
 429                         {
 430                             // If a where-clause may be useful, remind the
 431                             // user that they can add it.
 432                             //
 433                             // don't display an on-unimplemented note, as
 434                             // these notes will often be of the form
 435                             //     "the type `T` can't be frobnicated"
 436                             // which is somewhat confusing.
 437                             self.suggest_restricting_param_bound(
 438                                 &mut err,
 439                                 trait_ref,
 440                                 obligation.cause.body_id,
 441                             );
 442                         } else {
 443                             if !have_alt_message {
 444                                 // Can't show anything else useful, try to find similar impls.
 445                                 let impl_candidates = self.find_similar_impl_candidates(trait_ref);
 446                                 self.report_similar_impl_candidates(impl_candidates, &mut err);
 447                             }
 448                             // Changing mutability doesn't make a difference to whether we have
 449                             // an `Unsize` impl (Fixes ICE in #71036)
 450                             if !is_unsize {
 451                                 self.suggest_change_mut(
 452                                     &obligation,
 453                                     &mut err,
 454                                     &trait_ref,
 455                                     points_at_arg,
 456                                 );
 457                             }
 458                         }
 459
 460                         // If this error is due to `!: Trait` not implemented but `(): Trait` is
 461                         // implemented, and fallback has occurred, then it could be due to a
 462                         // variable that used to fallback to `()` now falling back to `!`. Issue a
 463                         // note informing about the change in behaviour.
 464                         if trait_predicate.skip_binder().self_ty().is_never()
 465                             && fallback_has_occurred
 466                         {
 467                             let predicate = trait_predicate.map_bound(|mut trait_pred| {
 468                                 trait_pred.trait_ref.substs = self.tcx.mk_substs_trait(
 469                                     self.tcx.mk_unit(),
 470                                     &trait_pred.trait_ref.substs[1..],
 471                                 );
 472                                 trait_pred
 473                             });
 474                             let unit_obligation = Obligation {
 475                                 predicate: ty::PredicateKind::Trait(
 476                                     predicate,
 477                                     hir::Constness::NotConst,
 478                                 )
 479                                 .to_predicate(self.tcx),
 480                                 ..obligation.clone()
 481                             };
 482                             if self.predicate_may_hold(&unit_obligation) {
 483                                 err.note(
 484                                     "the trait is implemented for `()`. \
 485                                      Possibly this error has been caused by changes to \
 486                                      Rust's type-inference algorithm (see issue #48950 \
 487                                      <https://github.com/rust-lang/rust/issues/48950> \
 488                                      for more information). Consider whether you meant to use \
 489                                      the type `()` here instead.",
 490                                 );
 491                             }
 492                         }
 493
 494                         err
 495                     }
 496
 497                     ty::PredicateKind::Subtype(ref predicate) => {
 498                         // Errors for Subtype predicates show up as
 499                         // `FulfillmentErrorCode::CodeSubtypeError`,
 500                         // not selection error.
 501                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
 502                     }
 503
 504                     ty::PredicateKind::RegionOutlives(ref predicate) => {
 505                         let predicate = self.resolve_vars_if_possible(predicate);
 506                         let err = self
 507                             .region_outlives_predicate(&obligation.cause, predicate)
 508                             .err()
 509                             .unwrap();
 510                         struct_span_err!(
 511                             self.tcx.sess,
 512                             span,
 513                             E0279,
 514                             "the requirement `{}` is not satisfied (`{}`)",
 515                             predicate,
 516                             err,
 517                         )
 518                     }
 519
 520                     ty::PredicateKind::Projection(..) | ty::PredicateKind::TypeOutlives(..) => {
 521                         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
 522                         struct_span_err!(
 523                             self.tcx.sess,
 524                             span,
 525                             E0280,
 526                             "the requirement `{}` is not satisfied",
 527                             predicate
 528                         )
 529                     }
 530
 531                     &ty::PredicateKind::ObjectSafe(trait_def_id) => {
 532                         let violations = self.tcx.object_safety_violations(trait_def_id);
 533                         report_object_safety_error(self.tcx, span, trait_def_id, violations)
 534                     }
 535
 536                     &ty::PredicateKind::ClosureKind(closure_def_id, closure_substs, kind) => {
 537                         let found_kind = self.closure_kind(closure_substs).unwrap();
 538                         let closure_span =
 539                             self.tcx.sess.source_map().guess_head_span(
 540                                 self.tcx.hir().span_if_local(closure_def_id).unwrap(),
 541                             );
 542                         let hir_id = self.tcx.hir().as_local_hir_id(closure_def_id.expect_local());
 543                         let mut err = struct_span_err!(
 544                             self.tcx.sess,
 545                             closure_span,
 546                             E0525,
 547                             "expected a closure that implements the `{}` trait, \
 548                              but this closure only implements `{}`",
 549                             kind,
 550                             found_kind
 551                         );
 552
 553                         err.span_label(
 554                             closure_span,
 555                             format!("this closure implements `{}`, not `{}`", found_kind, kind),
 556                         );
 557                         err.span_label(
 558                             obligation.cause.span,
 559                             format!("the requirement to implement `{}` derives from here", kind),
 560                         );
 561
 562                         // Additional context information explaining why the closure only implements
 563                         // a particular trait.
 564                         if let Some(tables) = self.in_progress_tables {
 565                             let tables = tables.borrow();
 566                             match (found_kind, tables.closure_kind_origins().get(hir_id)) {
 567                                 (ty::ClosureKind::FnOnce, Some((span, name))) => {
 568                                     err.span_label(
 569                                         *span,
 570                                         format!(
 571                                             "closure is `FnOnce` because it moves the \
 572                                          variable `{}` out of its environment",
 573                                             name
 574                                         ),
 575                                     );
 576                                 }
 577                                 (ty::ClosureKind::FnMut, Some((span, name))) => {
 578                                     err.span_label(
 579                                         *span,
 580                                         format!(
 581                                             "closure is `FnMut` because it mutates the \
 582                                          variable `{}` here",
 583                                             name
 584                                         ),
 585                                     );
 586                                 }
 587                                 _ => {}
 588                             }
 589                         }
 590
 591                         err.emit();
 592                         return;
 593                     }
 594
 595                     ty::PredicateKind::WellFormed(ty) => {
 596                         if !self.tcx.sess.opts.debugging_opts.chalk {
 597                             // WF predicates cannot themselves make
 598                             // errors. They can only block due to
 599                             // ambiguity; otherwise, they always
 600                             // degenerate into other obligations
 601                             // (which may fail).
 602                             span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 603                         } else {
 604                             // FIXME: we'll need a better message which takes into account
 605                             // which bounds actually failed to hold.
 606                             self.tcx.sess.struct_span_err(
 607                                 span,
 608                                 &format!("the type `{}` is not well-formed (chalk)", ty),
 609                             )
 610                         }
 611                     }
 612
 613                     ty::PredicateKind::ConstEvaluatable(..) => {
 614                         // Errors for `ConstEvaluatable` predicates show up as
 615                         // `SelectionError::ConstEvalFailure`,
 616                         // not `Unimplemented`.
 617                         span_bug!(
 618                             span,
 619                             "const-evaluatable requirement gave wrong error: `{:?}`",
 620                             obligation
 621                         )
 622                     }
 623
 624                     ty::PredicateKind::ConstEquate(..) => {
 625                         // Errors for `ConstEquate` predicates show up as
 626                         // `SelectionError::ConstEvalFailure`,
 627                         // not `Unimplemented`.
 628                         span_bug!(
 629                             span,
 630                             "const-equate requirement gave wrong error: `{:?}`",
 631                             obligation
 632                         )
 633                     }
 634                 }
 635             }
 636
 637             OutputTypeParameterMismatch(ref found_trait_ref, ref expected_trait_ref, _) => {
 638                 let found_trait_ref = self.resolve_vars_if_possible(&*found_trait_ref);
 639                 let expected_trait_ref = self.resolve_vars_if_possible(&*expected_trait_ref);
 640
 641                 if expected_trait_ref.self_ty().references_error() {
 642                     return;
 643                 }
 644
 645                 let found_trait_ty = found_trait_ref.self_ty();
 646
 647                 let found_did = match found_trait_ty.kind {
 648                     ty::Closure(did, _) | ty::Foreign(did) | ty::FnDef(did, _) => Some(did),
 649                     ty::Adt(def, _) => Some(def.did),
 650                     _ => None,
 651                 };
 652
 653                 let found_span = found_did
 654                     .and_then(|did| self.tcx.hir().span_if_local(did))
 655                     .map(|sp| self.tcx.sess.source_map().guess_head_span(sp)); // the sp could be an fn def
 656
 657                 if self.reported_closure_mismatch.borrow().contains(&(span, found_span)) {
 658                     // We check closures twice, with obligations flowing in different directions,
 659                     // but we want to complain about them only once.
 660                     return;
 661                 }
 662
 663                 self.reported_closure_mismatch.borrow_mut().insert((span, found_span));
 664
 665                 let found = match found_trait_ref.skip_binder().substs.type_at(1).kind {
 666                     ty::Tuple(ref tys) => vec![ArgKind::empty(); tys.len()],
 667                     _ => vec![ArgKind::empty()],
 668                 };
 669
 670                 let expected_ty = expected_trait_ref.skip_binder().substs.type_at(1);
 671                 let expected = match expected_ty.kind {
 672                     ty::Tuple(ref tys) => tys
 673                         .iter()
 674                         .map(|t| ArgKind::from_expected_ty(t.expect_ty(), Some(span)))
 675                         .collect(),
 676                     _ => vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())],
 677                 };
 678
 679                 if found.len() == expected.len() {
 680                     self.report_closure_arg_mismatch(
 681                         span,
 682                         found_span,
 683                         found_trait_ref,
 684                         expected_trait_ref,
 685                     )
 686                 } else {
 687                     let (closure_span, found) = found_did
 688                         .and_then(|did| {
 689                             let node = self.tcx.hir().get_if_local(did)?;
 690                             let (found_span, found) = self.get_fn_like_arguments(node)?;
 691                             Some((Some(found_span), found))
 692                         })
 693                         .unwrap_or((found_span, found));
 694
 695                     self.report_arg_count_mismatch(
 696                         span,
 697                         closure_span,
 698                         expected,
 699                         found,
 700                         found_trait_ty.is_closure(),
 701                     )
 702                 }
 703             }
 704
 705             TraitNotObjectSafe(did) => {
 706                 let violations = self.tcx.object_safety_violations(did);
 707                 report_object_safety_error(self.tcx, span, did, violations)
 708             }
 709
 710             ConstEvalFailure(ErrorHandled::TooGeneric) => {
 711                 // In this instance, we have a const expression containing an unevaluated
 712                 // generic parameter. We have no idea whether this expression is valid or
 713                 // not (e.g. it might result in an error), but we don't want to just assume
 714                 // that it's okay, because that might result in post-monomorphisation time
 715                 // errors. The onus is really on the caller to provide values that it can
 716                 // prove are well-formed.
 717                 let mut err = self
 718                     .tcx
 719                     .sess
 720                     .struct_span_err(span, "constant expression depends on a generic parameter");
 721                 // FIXME(const_generics): we should suggest to the user how they can resolve this
 722                 // issue. However, this is currently not actually possible
 723                 // (see https://github.com/rust-lang/rust/issues/66962#issuecomment-575907083).
 724                 err.note("this may fail depending on what value the parameter takes");
 725                 err
 726             }
 727
 728             // Already reported in the query.
 729             ConstEvalFailure(ErrorHandled::Reported(ErrorReported)) => {
 730                 // FIXME(eddyb) remove this once `ErrorReported` becomes a proof token.
 731                 self.tcx.sess.delay_span_bug(span, "`ErrorReported` without an error");
 732                 return;
 733             }
 734
 735             // Already reported in the query, but only as a lint.
 736             // This shouldn't actually happen for constants used in types, modulo
 737             // bugs. The `delay_span_bug` here ensures it won't be ignored.
 738             ConstEvalFailure(ErrorHandled::Linted) => {
 739                 self.tcx.sess.delay_span_bug(span, "constant in type had error reported as lint");
 740                 return;
 741             }
 742
 743             Overflow => {
 744                 bug!("overflow should be handled before the `report_selection_error` path");
 745             }
 746         };
 747
 748         self.note_obligation_cause(&mut err, obligation);
 749         self.point_at_returns_when_relevant(&mut err, &obligation);
 750
 751         err.emit();
 752     }
 753
 754     /// Given some node representing a fn-like thing in the HIR map,
 755     /// returns a span and `ArgKind` information that describes the
 756     /// arguments it expects. This can be supplied to
 757     /// `report_arg_count_mismatch`.
 758     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)> {
 759         let sm = self.tcx.sess.source_map();
 760         let hir = self.tcx.hir();
 761         Some(match node {
 762             Node::Expr(&hir::Expr {
 763                 kind: hir::ExprKind::Closure(_, ref _decl, id, span, _),
 764                 ..
 765             }) => (
 766                 sm.guess_head_span(span),
 767                 hir.body(id)
 768                     .params
 769                     .iter()
 770                     .map(|arg| {
 771                         if let hir::Pat { kind: hir::PatKind::Tuple(ref args, _), span, .. } =
 772                             *arg.pat
 773                         {
 774                             Some(ArgKind::Tuple(
 775                                 Some(span),
 776                                 args.iter()
 777                                     .map(|pat| {
 778                                         sm.span_to_snippet(pat.span)
 779                                             .ok()
 780                                             .map(|snippet| (snippet, "_".to_owned()))
 781                                     })
 782                                     .collect::<Option<Vec<_>>>()?,
 783                             ))
 784                         } else {
 785                             let name = sm.span_to_snippet(arg.pat.span).ok()?;
 786                             Some(ArgKind::Arg(name, "_".to_owned()))
 787                         }
 788                     })
 789                     .collect::<Option<Vec<ArgKind>>>()?,
 790             ),
 791             Node::Item(&hir::Item { span, kind: hir::ItemKind::Fn(ref sig, ..), .. })
 792             | Node::ImplItem(&hir::ImplItem {
 793                 span,
 794                 kind: hir::ImplItemKind::Fn(ref sig, _),
 795                 ..
 796             })
 797             | Node::TraitItem(&hir::TraitItem {
 798                 span,
 799                 kind: hir::TraitItemKind::Fn(ref sig, _),
 800                 ..
 801             }) => (
 802                 sm.guess_head_span(span),
 803                 sig.decl
 804                     .inputs
 805                     .iter()
 806                     .map(|arg| match arg.clone().kind {
 807                         hir::TyKind::Tup(ref tys) => ArgKind::Tuple(
 808                             Some(arg.span),
 809                             vec![("_".to_owned(), "_".to_owned()); tys.len()],
 810                         ),
 811                         _ => ArgKind::empty(),
 812                     })
 813                     .collect::<Vec<ArgKind>>(),
 814             ),
 815             Node::Ctor(ref variant_data) => {
 816                 let span = variant_data.ctor_hir_id().map(|id| hir.span(id)).unwrap_or(DUMMY_SP);
 817                 let span = sm.guess_head_span(span);
 818                 (span, vec![ArgKind::empty(); variant_data.fields().len()])
 819             }
 820             _ => panic!("non-FnLike node found: {:?}", node),
 821         })
 822     }
 823
 824     /// Reports an error when the number of arguments needed by a
 825     /// trait match doesn't match the number that the expression
 826     /// provides.
 827     fn report_arg_count_mismatch(
 828         &self,
 829         span: Span,
 830         found_span: Option<Span>,
 831         expected_args: Vec<ArgKind>,
 832         found_args: Vec<ArgKind>,
 833         is_closure: bool,
 834     ) -> DiagnosticBuilder<'tcx> {
 835         let kind = if is_closure { "closure" } else { "function" };
 836
 837         let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
 838             let arg_length = arguments.len();
 839             let distinct = match &other[..] {
 840                 &[ArgKind::Tuple(..)] => true,
 841                 _ => false,
 842             };
 843             match (arg_length, arguments.get(0)) {
 844                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
 845                     format!("a single {}-tuple as argument", fields.len())
 846                 }
 847                 _ => format!(
 848                     "{} {}argument{}",
 849                     arg_length,
 850                     if distinct && arg_length > 1 { "distinct " } else { "" },
 851                     pluralize!(arg_length)
 852                 ),
 853             }
 854         };
 855
 856         let expected_str = args_str(&expected_args, &found_args);
 857         let found_str = args_str(&found_args, &expected_args);
 858
 859         let mut err = struct_span_err!(
 860             self.tcx.sess,
 861             span,
 862             E0593,
 863             "{} is expected to take {}, but it takes {}",
 864             kind,
 865             expected_str,
 866             found_str,
 867         );
 868
 869         err.span_label(span, format!("expected {} that takes {}", kind, expected_str));
 870
 871         if let Some(found_span) = found_span {
 872             err.span_label(found_span, format!("takes {}", found_str));
 873
 874             // move |_| { ... }
 875             // ^^^^^^^^-- def_span
 876             //
 877             // move |_| { ... }
 878             // ^^^^^-- prefix
 879             let prefix_span = self.tcx.sess.source_map().span_until_non_whitespace(found_span);
 880             // move |_| { ... }
 881             //      ^^^-- pipe_span
 882             let pipe_span =
 883                 if let Some(span) = found_span.trim_start(prefix_span) { span } else { found_span };
 884
 885             // Suggest to take and ignore the arguments with expected_args_length `_`s if
 886             // found arguments is empty (assume the user just wants to ignore args in this case).
 887             // For example, if `expected_args_length` is 2, suggest `|_, _|`.
 888             if found_args.is_empty() && is_closure {
 889                 let underscores = vec!["_"; expected_args.len()].join(", ");
 890                 err.span_suggestion_verbose(
 891                     pipe_span,
 892                     &format!(
 893                         "consider changing the closure to take and ignore the expected argument{}",
 894                         pluralize!(expected_args.len())
 895                     ),
 896                     format!("|{}|", underscores),
 897                     Applicability::MachineApplicable,
 898                 );
 899             }
 900
 901             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
 902                 if fields.len() == expected_args.len() {
 903                     let sugg = fields
 904                         .iter()
 905                         .map(|(name, _)| name.to_owned())
 906                         .collect::<Vec<String>>()
 907                         .join(", ");
 908                     err.span_suggestion_verbose(
 909                         found_span,
 910                         "change the closure to take multiple arguments instead of a single tuple",
 911                         format!("|{}|", sugg),
 912                         Applicability::MachineApplicable,
 913                     );
 914                 }
 915             }
 916             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..] {
 917                 if fields.len() == found_args.len() && is_closure {
 918                     let sugg = format!(
 919                         "|({}){}|",
 920                         found_args
 921                             .iter()
 922                             .map(|arg| match arg {
 923                                 ArgKind::Arg(name, _) => name.to_owned(),
 924                                 _ => "_".to_owned(),
 925                             })
 926                             .collect::<Vec<String>>()
 927                             .join(", "),
 928                         // add type annotations if available
 929                         if found_args.iter().any(|arg| match arg {
 930                             ArgKind::Arg(_, ty) => ty != "_",
 931                             _ => false,
 932                         }) {
 933                             format!(
 934                                 ": ({})",
 935                                 fields
 936                                     .iter()
 937                                     .map(|(_, ty)| ty.to_owned())
 938                                     .collect::<Vec<String>>()
 939                                     .join(", ")
 940                             )
 941                         } else {
 942                             String::new()
 943                         },
 944                     );
 945                     err.span_suggestion_verbose(
 946                         found_span,
 947                         "change the closure to accept a tuple instead of individual arguments",
 948                         sugg,
 949                         Applicability::MachineApplicable,
 950                     );
 951                 }
 952             }
 953         }
 954
 955         err
 956     }
 957 }
 958
 959 trait InferCtxtPrivExt<'tcx> {
 960     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
 961     // `error` occurring implies that `cond` occurs.
 962     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool;
 963
 964     fn report_fulfillment_error(
 965         &self,
 966         error: &FulfillmentError<'tcx>,
 967         body_id: Option<hir::BodyId>,
 968         fallback_has_occurred: bool,
 969     );
 970
 971     fn report_projection_error(
 972         &self,
 973         obligation: &PredicateObligation<'tcx>,
 974         error: &MismatchedProjectionTypes<'tcx>,
 975     );
 976
 977     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool;
 978
 979     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str>;
 980
 981     fn find_similar_impl_candidates(
 982         &self,
 983         trait_ref: ty::PolyTraitRef<'tcx>,
 984     ) -> Vec<ty::TraitRef<'tcx>>;
 985
 986     fn report_similar_impl_candidates(
 987         &self,
 988         impl_candidates: Vec<ty::TraitRef<'tcx>>,
 989         err: &mut DiagnosticBuilder<'_>,
 990     );
 991
 992     /// Gets the parent trait chain start
 993     fn get_parent_trait_ref(
 994         &self,
 995         code: &ObligationCauseCode<'tcx>,
 996     ) -> Option<(String, Option<Span>)>;
 997
 998     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
 999     /// with the same path as `trait_ref`, a help message about
1000     /// a probable version mismatch is added to `err`
1001     fn note_version_mismatch(
1002         &self,
1003         err: &mut DiagnosticBuilder<'_>,
1004         trait_ref: &ty::PolyTraitRef<'tcx>,
1005     );
1006
1007     /// Creates a `PredicateObligation` with `new_self_ty` replacing the existing type in the
1008     /// `trait_ref`.
1009     ///
1010     /// For this to work, `new_self_ty` must have no escaping bound variables.
1011     fn mk_trait_obligation_with_new_self_ty(
1012         &self,
1013         param_env: ty::ParamEnv<'tcx>,
1014         trait_ref: &ty::PolyTraitRef<'tcx>,
1015         new_self_ty: Ty<'tcx>,
1016     ) -> PredicateObligation<'tcx>;
1017
1018     fn maybe_report_ambiguity(
1019         &self,
1020         obligation: &PredicateObligation<'tcx>,
1021         body_id: Option<hir::BodyId>,
1022     );
1023
1024     fn predicate_can_apply(
1025         &self,
1026         param_env: ty::ParamEnv<'tcx>,
1027         pred: ty::PolyTraitRef<'tcx>,
1028     ) -> bool;
1029
1030     fn note_obligation_cause(
1031         &self,
1032         err: &mut DiagnosticBuilder<'tcx>,
1033         obligation: &PredicateObligation<'tcx>,
1034     );
1035
1036     fn suggest_unsized_bound_if_applicable(
1037         &self,
1038         err: &mut DiagnosticBuilder<'tcx>,
1039         obligation: &PredicateObligation<'tcx>,
1040     );
1041
1042     fn is_recursive_obligation(
1043         &self,
1044         obligated_types: &mut Vec<&ty::TyS<'tcx>>,
1045         cause_code: &ObligationCauseCode<'tcx>,
1046     ) -> bool;
1047 }
1048
1049 impl<'a, 'tcx> InferCtxtPrivExt<'tcx> for InferCtxt<'a, 'tcx> {
1050     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1051     // `error` occurring implies that `cond` occurs.
1052     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool {
1053         if cond == error {
1054             return true;
1055         }
1056
1057         let (cond, error) = match (cond.kind(), error.kind()) {
1058             (ty::PredicateKind::Trait(..), ty::PredicateKind::Trait(error, _)) => (cond, error),
1059             _ => {
1060                 // FIXME: make this work in other cases too.
1061                 return false;
1062             }
1063         };
1064
1065         for obligation in super::elaborate_predicates(self.tcx, std::iter::once(cond)) {
1066             if let ty::PredicateKind::Trait(implication, _) = obligation.predicate.kind() {
1067                 let error = error.to_poly_trait_ref();
1068                 let implication = implication.to_poly_trait_ref();
1069                 // FIXME: I'm just not taking associated types at all here.
1070                 // Eventually I'll need to implement param-env-aware
1071                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
1072                 let param_env = ty::ParamEnv::empty();
1073                 if self.can_sub(param_env, error, implication).is_ok() {
1074                     debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
1075                     return true;
1076                 }
1077             }
1078         }
1079
1080         false
1081     }
1082
1083     fn report_fulfillment_error(
1084         &self,
1085         error: &FulfillmentError<'tcx>,
1086         body_id: Option<hir::BodyId>,
1087         fallback_has_occurred: bool,
1088     ) {
1089         debug!("report_fulfillment_error({:?})", error);
1090         match error.code {
1091             FulfillmentErrorCode::CodeSelectionError(ref selection_error) => {
1092                 self.report_selection_error(
1093                     &error.obligation,
1094                     selection_error,
1095                     fallback_has_occurred,
1096                     error.points_at_arg_span,
1097                 );
1098             }
1099             FulfillmentErrorCode::CodeProjectionError(ref e) => {
1100                 self.report_projection_error(&error.obligation, e);
1101             }
1102             FulfillmentErrorCode::CodeAmbiguity => {
1103                 self.maybe_report_ambiguity(&error.obligation, body_id);
1104             }
1105             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
1106                 self.report_mismatched_types(
1107                     &error.obligation.cause,
1108                     expected_found.expected,
1109                     expected_found.found,
1110                     err.clone(),
1111                 )
1112                 .emit();
1113             }
1114             FulfillmentErrorCode::CodeConstEquateError(ref expected_found, ref err) => {
1115                 self.report_mismatched_consts(
1116                     &error.obligation.cause,
1117                     expected_found.expected,
1118                     expected_found.found,
1119                     err.clone(),
1120                 )
1121                 .emit();
1122             }
1123         }
1124     }
1125
1126     fn report_projection_error(
1127         &self,
1128         obligation: &PredicateObligation<'tcx>,
1129         error: &MismatchedProjectionTypes<'tcx>,
1130     ) {
1131         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
1132
1133         if predicate.references_error() {
1134             return;
1135         }
1136
1137         self.probe(|_| {
1138             let err_buf;
1139             let mut err = &error.err;
1140             let mut values = None;
1141
1142             // try to find the mismatched types to report the error with.
1143             //
1144             // this can fail if the problem was higher-ranked, in which
1145             // cause I have no idea for a good error message.
1146             if let ty::PredicateKind::Projection(ref data) = predicate.kind() {
1147                 let mut selcx = SelectionContext::new(self);
1148                 let (data, _) = self.replace_bound_vars_with_fresh_vars(
1149                     obligation.cause.span,
1150                     infer::LateBoundRegionConversionTime::HigherRankedType,
1151                     data,
1152                 );
1153                 let mut obligations = vec![];
1154                 let normalized_ty = super::normalize_projection_type(
1155                     &mut selcx,
1156                     obligation.param_env,
1157                     data.projection_ty,
1158                     obligation.cause.clone(),
1159                     0,
1160                     &mut obligations,
1161                 );
1162
1163                 debug!(
1164                     "report_projection_error obligation.cause={:?} obligation.param_env={:?}",
1165                     obligation.cause, obligation.param_env
1166                 );
1167
1168                 debug!(
1169                     "report_projection_error normalized_ty={:?} data.ty={:?}",
1170                     normalized_ty, data.ty
1171                 );
1172
1173                 let is_normalized_ty_expected = match &obligation.cause.code {
1174                     ObligationCauseCode::ItemObligation(_)
1175                     | ObligationCauseCode::BindingObligation(_, _)
1176                     | ObligationCauseCode::ObjectCastObligation(_) => false,
1177                     _ => true,
1178                 };
1179
1180                 if let Err(error) = self.at(&obligation.cause, obligation.param_env).eq_exp(
1181                     is_normalized_ty_expected,
1182                     normalized_ty,
1183                     data.ty,
1184                 ) {
1185                     values = Some(infer::ValuePairs::Types(ExpectedFound::new(
1186                         is_normalized_ty_expected,
1187                         normalized_ty,
1188                         data.ty,
1189                     )));
1190
1191                     err_buf = error;
1192                     err = &err_buf;
1193                 }
1194             }
1195
1196             let msg = format!("type mismatch resolving `{}`", predicate);
1197             let error_id = (DiagnosticMessageId::ErrorId(271), Some(obligation.cause.span), msg);
1198             let fresh = self.tcx.sess.one_time_diagnostics.borrow_mut().insert(error_id);
1199             if fresh {
1200                 let mut diag = struct_span_err!(
1201                     self.tcx.sess,
1202                     obligation.cause.span,
1203                     E0271,
1204                     "type mismatch resolving `{}`",
1205                     predicate
1206                 );
1207                 self.note_type_err(&mut diag, &obligation.cause, None, values, err);
1208                 self.note_obligation_cause(&mut diag, obligation);
1209                 diag.emit();
1210             }
1211         });
1212     }
1213
1214     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
1215         /// returns the fuzzy category of a given type, or None
1216         /// if the type can be equated to any type.
1217         fn type_category(t: Ty<'_>) -> Option<u32> {
1218             match t.kind {
1219                 ty::Bool => Some(0),
1220                 ty::Char => Some(1),
1221                 ty::Str => Some(2),
1222                 ty::Int(..) | ty::Uint(..) | ty::Infer(ty::IntVar(..)) => Some(3),
1223                 ty::Float(..) | ty::Infer(ty::FloatVar(..)) => Some(4),
1224                 ty::Ref(..) | ty::RawPtr(..) => Some(5),
1225                 ty::Array(..) | ty::Slice(..) => Some(6),
1226                 ty::FnDef(..) | ty::FnPtr(..) => Some(7),
1227                 ty::Dynamic(..) => Some(8),
1228                 ty::Closure(..) => Some(9),
1229                 ty::Tuple(..) => Some(10),
1230                 ty::Projection(..) => Some(11),
1231                 ty::Param(..) => Some(12),
1232                 ty::Opaque(..) => Some(13),
1233                 ty::Never => Some(14),
1234                 ty::Adt(adt, ..) => match adt.adt_kind() {
1235                     AdtKind::Struct => Some(15),
1236                     AdtKind::Union => Some(16),
1237                     AdtKind::Enum => Some(17),
1238                 },
1239                 ty::Generator(..) => Some(18),
1240                 ty::Foreign(..) => Some(19),
1241                 ty::GeneratorWitness(..) => Some(20),
1242                 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error => None,
1243             }
1244         }
1245
1246         match (type_category(a), type_category(b)) {
1247             (Some(cat_a), Some(cat_b)) => match (&a.kind, &b.kind) {
1248                 (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => def_a == def_b,
1249                 _ => cat_a == cat_b,
1250             },
1251             // infer and error can be equated to all types
1252             _ => true,
1253         }
1254     }
1255
1256     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str> {
1257         self.tcx.hir().body(body_id).generator_kind.map(|gen_kind| match gen_kind {
1258             hir::GeneratorKind::Gen => "a generator",
1259             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block) => "an async block",
1260             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn) => "an async function",
1261             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Closure) => "an async closure",
1262         })
1263     }
1264
1265     fn find_similar_impl_candidates(
1266         &self,
1267         trait_ref: ty::PolyTraitRef<'tcx>,
1268     ) -> Vec<ty::TraitRef<'tcx>> {
1269         let simp = fast_reject::simplify_type(self.tcx, trait_ref.skip_binder().self_ty(), true);
1270         let all_impls = self.tcx.all_impls(trait_ref.def_id());
1271
1272         match simp {
1273             Some(simp) => all_impls
1274                 .filter_map(|def_id| {
1275                     let imp = self.tcx.impl_trait_ref(def_id).unwrap();
1276                     let imp_simp = fast_reject::simplify_type(self.tcx, imp.self_ty(), true);
1277                     if let Some(imp_simp) = imp_simp {
1278                         if simp != imp_simp {
1279                             return None;
1280                         }
1281                     }
1282                     Some(imp)
1283                 })
1284                 .collect(),
1285             None => all_impls.map(|def_id| self.tcx.impl_trait_ref(def_id).unwrap()).collect(),
1286         }
1287     }
1288
1289     fn report_similar_impl_candidates(
1290         &self,
1291         impl_candidates: Vec<ty::TraitRef<'tcx>>,
1292         err: &mut DiagnosticBuilder<'_>,
1293     ) {
1294         if impl_candidates.is_empty() {
1295             return;
1296         }
1297
1298         let len = impl_candidates.len();
1299         let end = if impl_candidates.len() <= 5 { impl_candidates.len() } else { 4 };
1300
1301         let normalize = |candidate| {
1302             self.tcx.infer_ctxt().enter(|ref infcx| {
1303                 let normalized = infcx
1304                     .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
1305                     .normalize(candidate)
1306                     .ok();
1307                 match normalized {
1308                     Some(normalized) => format!("\n  {:?}", normalized.value),
1309                     None => format!("\n  {:?}", candidate),
1310                 }
1311             })
1312         };
1313
1314         // Sort impl candidates so that ordering is consistent for UI tests.
1315         let mut normalized_impl_candidates =
1316             impl_candidates.iter().map(normalize).collect::<Vec<String>>();
1317
1318         // Sort before taking the `..end` range,
1319         // because the ordering of `impl_candidates` may not be deterministic:
1320         // https://github.com/rust-lang/rust/pull/57475#issuecomment-455519507
1321         normalized_impl_candidates.sort();
1322
1323         err.help(&format!(
1324             "the following implementations were found:{}{}",
1325             normalized_impl_candidates[..end].join(""),
1326             if len > 5 { format!("\nand {} others", len - 4) } else { String::new() }
1327         ));
1328     }
1329
1330     /// Gets the parent trait chain start
1331     fn get_parent_trait_ref(
1332         &self,
1333         code: &ObligationCauseCode<'tcx>,
1334     ) -> Option<(String, Option<Span>)> {
1335         match code {
1336             &ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
1337                 let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
1338                 match self.get_parent_trait_ref(&data.parent_code) {
1339                     Some(t) => Some(t),
1340                     None => {
1341                         let ty = parent_trait_ref.skip_binder().self_ty();
1342                         let span =
1343                             TyCategory::from_ty(ty).map(|(_, def_id)| self.tcx.def_span(def_id));
1344                         Some((ty.to_string(), span))
1345                     }
1346                 }
1347             }
1348             _ => None,
1349         }
1350     }
1351
1352     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1353     /// with the same path as `trait_ref`, a help message about
1354     /// a probable version mismatch is added to `err`
1355     fn note_version_mismatch(
1356         &self,
1357         err: &mut DiagnosticBuilder<'_>,
1358         trait_ref: &ty::PolyTraitRef<'tcx>,
1359     ) {
1360         let get_trait_impl = |trait_def_id| {
1361             let mut trait_impl = None;
1362             self.tcx.for_each_relevant_impl(trait_def_id, trait_ref.self_ty(), |impl_def_id| {
1363                 if trait_impl.is_none() {
1364                     trait_impl = Some(impl_def_id);
1365                 }
1366             });
1367             trait_impl
1368         };
1369         let required_trait_path = self.tcx.def_path_str(trait_ref.def_id());
1370         let all_traits = self.tcx.all_traits(LOCAL_CRATE);
1371         let traits_with_same_path: std::collections::BTreeSet<_> = all_traits
1372             .iter()
1373             .filter(|trait_def_id| **trait_def_id != trait_ref.def_id())
1374             .filter(|trait_def_id| self.tcx.def_path_str(**trait_def_id) == required_trait_path)
1375             .collect();
1376         for trait_with_same_path in traits_with_same_path {
1377             if let Some(impl_def_id) = get_trait_impl(*trait_with_same_path) {
1378                 let impl_span = self.tcx.def_span(impl_def_id);
1379                 err.span_help(impl_span, "trait impl with same name found");
1380                 let trait_crate = self.tcx.crate_name(trait_with_same_path.krate);
1381                 let crate_msg = format!(
1382                     "perhaps two different versions of crate `{}` are being used?",
1383                     trait_crate
1384                 );
1385                 err.note(&crate_msg);
1386             }
1387         }
1388     }
1389
1390     fn mk_trait_obligation_with_new_self_ty(
1391         &self,
1392         param_env: ty::ParamEnv<'tcx>,
1393         trait_ref: &ty::PolyTraitRef<'tcx>,
1394         new_self_ty: Ty<'tcx>,
1395     ) -> PredicateObligation<'tcx> {
1396         assert!(!new_self_ty.has_escaping_bound_vars());
1397
1398         let trait_ref = trait_ref.map_bound_ref(|tr| ty::TraitRef {
1399             substs: self.tcx.mk_substs_trait(new_self_ty, &tr.substs[1..]),
1400             ..*tr
1401         });
1402
1403         Obligation::new(
1404             ObligationCause::dummy(),
1405             param_env,
1406             trait_ref.without_const().to_predicate(self.tcx),
1407         )
1408     }
1409
1410     fn maybe_report_ambiguity(
1411         &self,
1412         obligation: &PredicateObligation<'tcx>,
1413         body_id: Option<hir::BodyId>,
1414     ) {
1415         // Unable to successfully determine, probably means
1416         // insufficient type information, but could mean
1417         // ambiguous impls. The latter *ought* to be a
1418         // coherence violation, so we don't report it here.
1419
1420         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
1421         let span = obligation.cause.span;
1422
1423         debug!(
1424             "maybe_report_ambiguity(predicate={:?}, obligation={:?} body_id={:?}, code={:?})",
1425             predicate, obligation, body_id, obligation.cause.code,
1426         );
1427
1428         // Ambiguity errors are often caused as fallout from earlier
1429         // errors. So just ignore them if this infcx is tainted.
1430         if self.is_tainted_by_errors() {
1431             return;
1432         }
1433
1434         let mut err = match predicate.kind() {
1435             ty::PredicateKind::Trait(ref data, _) => {
1436                 let trait_ref = data.to_poly_trait_ref();
1437                 let self_ty = trait_ref.self_ty();
1438                 debug!("self_ty {:?} {:?} trait_ref {:?}", self_ty, self_ty.kind, trait_ref);
1439
1440                 if predicate.references_error() {
1441                     return;
1442                 }
1443                 // Typically, this ambiguity should only happen if
1444                 // there are unresolved type inference variables
1445                 // (otherwise it would suggest a coherence
1446                 // failure). But given #21974 that is not necessarily
1447                 // the case -- we can have multiple where clauses that
1448                 // are only distinguished by a region, which results
1449                 // in an ambiguity even when all types are fully
1450                 // known, since we don't dispatch based on region
1451                 // relationships.
1452
1453                 // This is kind of a hack: it frequently happens that some earlier
1454                 // error prevents types from being fully inferred, and then we get
1455                 // a bunch of uninteresting errors saying something like "<generic
1456                 // #0> doesn't implement Sized".  It may even be true that we
1457                 // could just skip over all checks where the self-ty is an
1458                 // inference variable, but I was afraid that there might be an
1459                 // inference variable created, registered as an obligation, and
1460                 // then never forced by writeback, and hence by skipping here we'd
1461                 // be ignoring the fact that we don't KNOW the type works
1462                 // out. Though even that would probably be harmless, given that
1463                 // we're only talking about builtin traits, which are known to be
1464                 // inhabited. We used to check for `self.tcx.sess.has_errors()` to
1465                 // avoid inundating the user with unnecessary errors, but we now
1466                 // check upstream for type errors and don't add the obligations to
1467                 // begin with in those cases.
1468                 if self
1469                     .tcx
1470                     .lang_items()
1471                     .sized_trait()
1472                     .map_or(false, |sized_id| sized_id == trait_ref.def_id())
1473                 {
1474                     self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0282).emit();
1475                     return;
1476                 }
1477                 let mut err = self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0283);
1478                 err.note(&format!("cannot satisfy `{}`", predicate));
1479                 if let ObligationCauseCode::ItemObligation(def_id) = obligation.cause.code {
1480                     self.suggest_fully_qualified_path(&mut err, def_id, span, trait_ref.def_id());
1481                 } else if let (
1482                     Ok(ref snippet),
1483                     ObligationCauseCode::BindingObligation(ref def_id, _),
1484                 ) =
1485                     (self.tcx.sess.source_map().span_to_snippet(span), &obligation.cause.code)
1486                 {
1487                     let generics = self.tcx.generics_of(*def_id);
1488                     if generics.params.iter().any(|p| p.name.as_str() != "Self")
1489                         && !snippet.ends_with('>')
1490                     {
1491                         // FIXME: To avoid spurious suggestions in functions where type arguments
1492                         // where already supplied, we check the snippet to make sure it doesn't
1493                         // end with a turbofish. Ideally we would have access to a `PathSegment`
1494                         // instead. Otherwise we would produce the following output:
1495                         //
1496                         // error[E0283]: type annotations needed
1497                         //   --> $DIR/issue-54954.rs:3:24
1498                         //    |
1499                         // LL | const ARR_LEN: usize = Tt::const_val::<[i8; 123]>();
1500                         //    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^
1501                         //    |                        |
1502                         //    |                        cannot infer type
1503                         //    |                        help: consider specifying the type argument
1504                         //    |                        in the function call:
1505                         //    |                        `Tt::const_val::<[i8; 123]>::<T>`
1506                         // ...
1507                         // LL |     const fn const_val<T: Sized>() -> usize {
1508                         //    |                        - required by this bound in `Tt::const_val`
1509                         //    |
1510                         //    = note: cannot satisfy `_: Tt`
1511
1512                         err.span_suggestion_verbose(
1513                             span.shrink_to_hi(),
1514                             &format!(
1515                                 "consider specifying the type argument{} in the function call",
1516                                 pluralize!(generics.params.len()),
1517                             ),
1518                             format!(
1519                                 "::<{}>",
1520                                 generics
1521                                     .params
1522                                     .iter()
1523                                     .map(|p| p.name.to_string())
1524                                     .collect::<Vec<String>>()
1525                                     .join(", ")
1526                             ),
1527                             Applicability::HasPlaceholders,
1528                         );
1529                     }
1530                 }
1531                 err
1532             }
1533
1534             ty::PredicateKind::WellFormed(ty) => {
1535                 // Same hacky approach as above to avoid deluging user
1536                 // with error messages.
1537                 if ty.references_error() || self.tcx.sess.has_errors() {
1538                     return;
1539                 }
1540                 self.need_type_info_err(body_id, span, ty, ErrorCode::E0282)
1541             }
1542
1543             ty::PredicateKind::Subtype(ref data) => {
1544                 if data.references_error() || self.tcx.sess.has_errors() {
1545                     // no need to overload user in such cases
1546                     return;
1547                 }
1548                 let &SubtypePredicate { a_is_expected: _, a, b } = data.skip_binder();
1549                 // both must be type variables, or the other would've been instantiated
1550                 assert!(a.is_ty_var() && b.is_ty_var());
1551                 self.need_type_info_err(body_id, span, a, ErrorCode::E0282)
1552             }
1553             ty::PredicateKind::Projection(ref data) => {
1554                 let trait_ref = data.to_poly_trait_ref(self.tcx);
1555                 let self_ty = trait_ref.self_ty();
1556                 let ty = data.skip_binder().ty;
1557                 if predicate.references_error() {
1558                     return;
1559                 }
1560                 if self_ty.needs_infer() && ty.needs_infer() {
1561                     // We do this for the `foo.collect()?` case to produce a suggestion.
1562                     let mut err = self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0284);
1563                     err.note(&format!("cannot satisfy `{}`", predicate));
1564                     err
1565                 } else {
1566                     let mut err = struct_span_err!(
1567                         self.tcx.sess,
1568                         span,
1569                         E0284,
1570                         "type annotations needed: cannot satisfy `{}`",
1571                         predicate,
1572                     );
1573                     err.span_label(span, &format!("cannot satisfy `{}`", predicate));
1574                     err
1575                 }
1576             }
1577
1578             _ => {
1579                 if self.tcx.sess.has_errors() {
1580                     return;
1581                 }
1582                 let mut err = struct_span_err!(
1583                     self.tcx.sess,
1584                     span,
1585                     E0284,
1586                     "type annotations needed: cannot satisfy `{}`",
1587                     predicate,
1588                 );
1589                 err.span_label(span, &format!("cannot satisfy `{}`", predicate));
1590                 err
1591             }
1592         };
1593         self.note_obligation_cause(&mut err, obligation);
1594         err.emit();
1595     }
1596
1597     /// Returns `true` if the trait predicate may apply for *some* assignment
1598     /// to the type parameters.
1599     fn predicate_can_apply(
1600         &self,
1601         param_env: ty::ParamEnv<'tcx>,
1602         pred: ty::PolyTraitRef<'tcx>,
1603     ) -> bool {
1604         struct ParamToVarFolder<'a, 'tcx> {
1605             infcx: &'a InferCtxt<'a, 'tcx>,
1606             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
1607         }
1608
1609         impl<'a, 'tcx> TypeFolder<'tcx> for ParamToVarFolder<'a, 'tcx> {
1610             fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
1611                 self.infcx.tcx
1612             }
1613
1614             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
1615                 if let ty::Param(ty::ParamTy { name, .. }) = ty.kind {
1616                     let infcx = self.infcx;
1617                     self.var_map.entry(ty).or_insert_with(|| {
1618                         infcx.next_ty_var(TypeVariableOrigin {
1619                             kind: TypeVariableOriginKind::TypeParameterDefinition(name, None),
1620                             span: DUMMY_SP,
1621                         })
1622                     })
1623                 } else {
1624                     ty.super_fold_with(self)
1625                 }
1626             }
1627         }
1628
1629         self.probe(|_| {
1630             let mut selcx = SelectionContext::new(self);
1631
1632             let cleaned_pred =
1633                 pred.fold_with(&mut ParamToVarFolder { infcx: self, var_map: Default::default() });
1634
1635             let cleaned_pred = super::project::normalize(
1636                 &mut selcx,
1637                 param_env,
1638                 ObligationCause::dummy(),
1639                 &cleaned_pred,
1640             )
1641             .value;
1642
1643             let obligation = Obligation::new(
1644                 ObligationCause::dummy(),
1645                 param_env,
1646                 cleaned_pred.without_const().to_predicate(selcx.tcx()),
1647             );
1648
1649             self.predicate_may_hold(&obligation)
1650         })
1651     }
1652
1653     fn note_obligation_cause(
1654         &self,
1655         err: &mut DiagnosticBuilder<'tcx>,
1656         obligation: &PredicateObligation<'tcx>,
1657     ) {
1658         // First, attempt to add note to this error with an async-await-specific
1659         // message, and fall back to regular note otherwise.
1660         if !self.maybe_note_obligation_cause_for_async_await(err, obligation) {
1661             self.note_obligation_cause_code(
1662                 err,
1663                 &obligation.predicate,
1664                 &obligation.cause.code,
1665                 &mut vec![],
1666             );
1667             self.suggest_unsized_bound_if_applicable(err, obligation);
1668         }
1669     }
1670
1671     fn suggest_unsized_bound_if_applicable(
1672         &self,
1673         err: &mut DiagnosticBuilder<'tcx>,
1674         obligation: &PredicateObligation<'tcx>,
1675     ) {
1676         if let (
1677             ty::PredicateKind::Trait(pred, _),
1678             ObligationCauseCode::BindingObligation(item_def_id, span),
1679         ) = (obligation.predicate.kind(), &obligation.cause.code)
1680         {
1681             if let (Some(generics), true) = (
1682                 self.tcx.hir().get_if_local(*item_def_id).as_ref().and_then(|n| n.generics()),
1683                 Some(pred.def_id()) == self.tcx.lang_items().sized_trait(),
1684             ) {
1685                 for param in generics.params {
1686                     if param.span == *span
1687                         && !param.bounds.iter().any(|bound| {
1688                             bound.trait_ref().and_then(|trait_ref| trait_ref.trait_def_id())
1689                                 == self.tcx.lang_items().sized_trait()
1690                         })
1691                     {
1692                         let (span, separator) = match param.bounds {
1693                             [] => (span.shrink_to_hi(), ":"),
1694                             [.., bound] => (bound.span().shrink_to_hi(), " +"),
1695                         };
1696                         err.span_suggestion_verbose(
1697                             span,
1698                             "consider relaxing the implicit `Sized` restriction",
1699                             format!("{} ?Sized", separator),
1700                             Applicability::MachineApplicable,
1701                         );
1702                         return;
1703                     }
1704                 }
1705             }
1706         }
1707     }
1708
1709     fn is_recursive_obligation(
1710         &self,
1711         obligated_types: &mut Vec<&ty::TyS<'tcx>>,
1712         cause_code: &ObligationCauseCode<'tcx>,
1713     ) -> bool {
1714         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
1715             let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
1716
1717             if obligated_types.iter().any(|ot| ot == &parent_trait_ref.skip_binder().self_ty()) {
1718                 return true;
1719             }
1720         }
1721         false
1722     }
1723 }
1724
1725 pub fn recursive_type_with_infinite_size_error(
1726     tcx: TyCtxt<'tcx>,
1727     type_def_id: DefId,
1728 ) -> DiagnosticBuilder<'tcx> {
1729     assert!(type_def_id.is_local());
1730     let span = tcx.hir().span_if_local(type_def_id).unwrap();
1731     let span = tcx.sess.source_map().guess_head_span(span);
1732     let mut err = struct_span_err!(
1733         tcx.sess,
1734         span,
1735         E0072,
1736         "recursive type `{}` has infinite size",
1737         tcx.def_path_str(type_def_id)
1738     );
1739     err.span_label(span, "recursive type has infinite size");
1740     err.help(&format!(
1741         "insert indirection (e.g., a `Box`, `Rc`, or `&`) \
1742                            at some point to make `{}` representable",
1743         tcx.def_path_str(type_def_id)
1744     ));
1745     err
1746 }
1747
1748 /// Summarizes information
1749 #[derive(Clone)]
1750 pub enum ArgKind {
1751     /// An argument of non-tuple type. Parameters are (name, ty)
1752     Arg(String, String),
1753
1754     /// An argument of tuple type. For a "found" argument, the span is
1755     /// the locationo in the source of the pattern. For a "expected"
1756     /// argument, it will be None. The vector is a list of (name, ty)
1757     /// strings for the components of the tuple.
1758     Tuple(Option<Span>, Vec<(String, String)>),
1759 }
1760
1761 impl ArgKind {
1762     fn empty() -> ArgKind {
1763         ArgKind::Arg("_".to_owned(), "_".to_owned())
1764     }
1765
1766     /// Creates an `ArgKind` from the expected type of an
1767     /// argument. It has no name (`_`) and an optional source span.
1768     pub fn from_expected_ty(t: Ty<'_>, span: Option<Span>) -> ArgKind {
1769         match t.kind {
1770             ty::Tuple(ref tys) => ArgKind::Tuple(
1771                 span,
1772                 tys.iter().map(|ty| ("_".to_owned(), ty.to_string())).collect::<Vec<_>>(),
1773             ),
1774             _ => ArgKind::Arg("_".to_owned(), t.to_string()),
1775         }
1776     }
1777 }