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