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, QPath, TyKind, WhereBoundPredicate, WherePredicate};
  19 use rustc_middle::mir::interpret::ErrorHandled;
  20 use rustc_middle::ty::error::ExpectedFound;
  21 use rustc_middle::ty::fast_reject;
  22 use rustc_middle::ty::fold::TypeFolder;
  23 use rustc_middle::ty::SubtypePredicate;
  24 use rustc_middle::ty::{
  25     self, AdtKind, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness,
  26 };
  27 use rustc_session::DiagnosticMessageId;
  28 use rustc_span::{BytePos, ExpnKind, Span, DUMMY_SP};
  29 use std::fmt;
  30
  31 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
  32 use crate::traits::query::normalize::AtExt as _;
  33 use on_unimplemented::InferCtxtExt as _;
  34 use suggestions::InferCtxtExt as _;
  35
  36 pub use rustc_infer::traits::error_reporting::*;
  37
  38 pub trait InferCtxtExt<'tcx> {
  39     fn report_fulfillment_errors(
  40         &self,
  41         errors: &[FulfillmentError<'tcx>],
  42         body_id: Option<hir::BodyId>,
  43         fallback_has_occurred: bool,
  44     );
  45
  46     fn report_overflow_error<T>(
  47         &self,
  48         obligation: &Obligation<'tcx, T>,
  49         suggest_increasing_limit: bool,
  50     ) -> !
  51     where
  52         T: fmt::Display + TypeFoldable<'tcx>;
  53
  54     fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> !;
  55
  56     fn report_selection_error(
  57         &self,
  58         obligation: &PredicateObligation<'tcx>,
  59         error: &SelectionError<'tcx>,
  60         fallback_has_occurred: bool,
  61         points_at_arg: bool,
  62     );
  63
  64     /// Given some node representing a fn-like thing in the HIR map,
  65     /// returns a span and `ArgKind` information that describes the
  66     /// arguments it expects. This can be supplied to
  67     /// `report_arg_count_mismatch`.
  68     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)>;
  69
  70     /// Reports an error when the number of arguments needed by a
  71     /// trait match doesn't match the number that the expression
  72     /// provides.
  73     fn report_arg_count_mismatch(
  74         &self,
  75         span: Span,
  76         found_span: Option<Span>,
  77         expected_args: Vec<ArgKind>,
  78         found_args: Vec<ArgKind>,
  79         is_closure: bool,
  80     ) -> DiagnosticBuilder<'tcx>;
  81 }
  82
  83 impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
  84     fn report_fulfillment_errors(
  85         &self,
  86         errors: &[FulfillmentError<'tcx>],
  87         body_id: Option<hir::BodyId>,
  88         fallback_has_occurred: bool,
  89     ) {
  90         #[derive(Debug)]
  91         struct ErrorDescriptor<'tcx> {
  92             predicate: ty::Predicate<'tcx>,
  93             index: Option<usize>, // None if this is an old error
  94         }
  95
  96         let mut error_map: FxHashMap<_, Vec<_>> = self
  97             .reported_trait_errors
  98             .borrow()
  99             .iter()
 100             .map(|(&span, predicates)| {
 101                 (
 102                     span,
 103                     predicates
 104                         .iter()
 105                         .map(|&predicate| ErrorDescriptor { predicate, index: None })
 106                         .collect(),
 107                 )
 108             })
 109             .collect();
 110
 111         for (index, error) in errors.iter().enumerate() {
 112             // We want to ignore desugarings here: spans are equivalent even
 113             // if one is the result of a desugaring and the other is not.
 114             let mut span = error.obligation.cause.span;
 115             let expn_data = span.ctxt().outer_expn_data();
 116             if let ExpnKind::Desugaring(_) = expn_data.kind {
 117                 span = expn_data.call_site;
 118             }
 119
 120             error_map.entry(span).or_default().push(ErrorDescriptor {
 121                 predicate: error.obligation.predicate,
 122                 index: Some(index),
 123             });
 124
 125             self.reported_trait_errors
 126                 .borrow_mut()
 127                 .entry(span)
 128                 .or_default()
 129                 .push(error.obligation.predicate);
 130         }
 131
 132         // We do this in 2 passes because we want to display errors in order, though
 133         // maybe it *is* better to sort errors by span or something.
 134         let mut is_suppressed = vec![false; errors.len()];
 135         for (_, error_set) in error_map.iter() {
 136             // We want to suppress "duplicate" errors with the same span.
 137             for error in error_set {
 138                 if let Some(index) = error.index {
 139                     // Suppress errors that are either:
 140                     // 1) strictly implied by another error.
 141                     // 2) implied by an error with a smaller index.
 142                     for error2 in error_set {
 143                         if error2.index.map_or(false, |index2| is_suppressed[index2]) {
 144                             // Avoid errors being suppressed by already-suppressed
 145                             // errors, to prevent all errors from being suppressed
 146                             // at once.
 147                             continue;
 148                         }
 149
 150                         if self.error_implies(&error2.predicate, &error.predicate)
 151                             && !(error2.index >= error.index
 152                                 && self.error_implies(&error.predicate, &error2.predicate))
 153                         {
 154                             info!("skipping {:?} (implied by {:?})", error, error2);
 155                             is_suppressed[index] = true;
 156                             break;
 157                         }
 158                     }
 159                 }
 160             }
 161         }
 162
 163         for (error, suppressed) in errors.iter().zip(is_suppressed) {
 164             if !suppressed {
 165                 self.report_fulfillment_error(error, body_id, fallback_has_occurred);
 166             }
 167         }
 168     }
 169
 170     /// Reports that an overflow has occurred and halts compilation. We
 171     /// halt compilation unconditionally because it is important that
 172     /// overflows never be masked -- they basically represent computations
 173     /// whose result could not be truly determined and thus we can't say
 174     /// if the program type checks or not -- and they are unusual
 175     /// occurrences in any case.
 176     fn report_overflow_error<T>(
 177         &self,
 178         obligation: &Obligation<'tcx, T>,
 179         suggest_increasing_limit: bool,
 180     ) -> !
 181     where
 182         T: fmt::Display + TypeFoldable<'tcx>,
 183     {
 184         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
 185         let mut err = struct_span_err!(
 186             self.tcx.sess,
 187             obligation.cause.span,
 188             E0275,
 189             "overflow evaluating the requirement `{}`",
 190             predicate
 191         );
 192
 193         if suggest_increasing_limit {
 194             self.suggest_new_overflow_limit(&mut err);
 195         }
 196
 197         self.note_obligation_cause_code(
 198             &mut err,
 199             &obligation.predicate,
 200             &obligation.cause.code,
 201             &mut vec![],
 202         );
 203
 204         err.emit();
 205         self.tcx.sess.abort_if_errors();
 206         bug!();
 207     }
 208
 209     /// Reports that a cycle was detected which led to overflow and halts
 210     /// compilation. This is equivalent to `report_overflow_error` except
 211     /// that we can give a more helpful error message (and, in particular,
 212     /// we do not suggest increasing the overflow limit, which is not
 213     /// going to help).
 214     fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 215         let cycle = self.resolve_vars_if_possible(&cycle.to_owned());
 216         assert!(!cycle.is_empty());
 217
 218         debug!("report_overflow_error_cycle: cycle={:?}", cycle);
 219
 220         self.report_overflow_error(&cycle[0], false);
 221     }
 222
 223     fn report_selection_error(
 224         &self,
 225         obligation: &PredicateObligation<'tcx>,
 226         error: &SelectionError<'tcx>,
 227         fallback_has_occurred: bool,
 228         points_at_arg: bool,
 229     ) {
 230         let tcx = self.tcx;
 231         let span = obligation.cause.span;
 232
 233         let mut err = match *error {
 234             SelectionError::Unimplemented => {
 235                 if let ObligationCauseCode::CompareImplMethodObligation {
 236                     item_name,
 237                     impl_item_def_id,
 238                     trait_item_def_id,
 239                 }
 240                 | ObligationCauseCode::CompareImplTypeObligation {
 241                     item_name,
 242                     impl_item_def_id,
 243                     trait_item_def_id,
 244                 } = obligation.cause.code
 245                 {
 246                     self.report_extra_impl_obligation(
 247                         span,
 248                         item_name,
 249                         impl_item_def_id,
 250                         trait_item_def_id,
 251                         &format!("`{}`", obligation.predicate),
 252                     )
 253                     .emit();
 254                     return;
 255                 }
 256                 match obligation.predicate {
 257                     ty::Predicate::Trait(ref trait_predicate, _) => {
 258                         let trait_predicate = self.resolve_vars_if_possible(trait_predicate);
 259
 260                         if self.tcx.sess.has_errors() && trait_predicate.references_error() {
 261                             return;
 262                         }
 263                         let trait_ref = trait_predicate.to_poly_trait_ref();
 264                         let (post_message, pre_message, type_def) = self
 265                             .get_parent_trait_ref(&obligation.cause.code)
 266                             .map(|(t, s)| {
 267                                 (
 268                                     format!(" in `{}`", t),
 269                                     format!("within `{}`, ", t),
 270                                     s.map(|s| (format!("within this `{}`", t), s)),
 271                                 )
 272                             })
 273                             .unwrap_or_default();
 274
 275                         let OnUnimplementedNote { message, label, note, enclosing_scope } =
 276                             self.on_unimplemented_note(trait_ref, obligation);
 277                         let have_alt_message = message.is_some() || label.is_some();
 278                         let is_try = self
 279                             .tcx
 280                             .sess
 281                             .source_map()
 282                             .span_to_snippet(span)
 283                             .map(|s| &s == "?")
 284                             .unwrap_or(false);
 285                         let is_from = format!("{}", trait_ref.print_only_trait_path())
 286                             .starts_with("std::convert::From<");
 287                         let (message, note) = if is_try && is_from {
 288                             (
 289                                 Some(format!(
 290                                     "`?` couldn't convert the error to `{}`",
 291                                     trait_ref.self_ty(),
 292                                 )),
 293                                 Some(
 294                                     "the question mark operation (`?`) implicitly performs a \
 295                                         conversion on the error value using the `From` trait"
 296                                         .to_owned(),
 297                                 ),
 298                             )
 299                         } else {
 300                             (message, note)
 301                         };
 302
 303                         let mut err = struct_span_err!(
 304                             self.tcx.sess,
 305                             span,
 306                             E0277,
 307                             "{}",
 308                             message.unwrap_or_else(|| format!(
 309                                 "the trait bound `{}` is not satisfied{}",
 310                                 trait_ref.without_const().to_predicate(),
 311                                 post_message,
 312                             ))
 313                         );
 314
 315                         let should_convert_option_to_result =
 316                             format!("{}", trait_ref.print_only_trait_path())
 317                                 .starts_with("std::convert::From<std::option::NoneError");
 318                         let should_convert_result_to_option = format!("{}", trait_ref)
 319                             .starts_with("<std::option::NoneError as std::convert::From<");
 320                         if is_try && is_from && should_convert_option_to_result {
 321                             err.span_suggestion_verbose(
 322                                 span.shrink_to_lo(),
 323                                 "consider converting the `Option<T>` into a `Result<T, _>` using `Option::ok_or` or `Option::ok_or_else`",
 324                                 ".ok_or_else(|| /* error value */)".to_string(),
 325                                 Applicability::HasPlaceholders,
 326                             );
 327                         } else if is_try && is_from && should_convert_result_to_option {
 328                             err.span_suggestion_verbose(
 329                                 span.shrink_to_lo(),
 330                                 "consider converting the `Result<T, _>` into an `Option<T>` using `Result::ok`",
 331                                 ".ok()".to_string(),
 332                                 Applicability::MachineApplicable,
 333                             );
 334                         }
 335
 336                         let explanation =
 337                             if obligation.cause.code == ObligationCauseCode::MainFunctionType {
 338                                 "consider using `()`, or a `Result`".to_owned()
 339                             } else {
 340                                 format!(
 341                                     "{}the trait `{}` is not implemented for `{}`",
 342                                     pre_message,
 343                                     trait_ref.print_only_trait_path(),
 344                                     trait_ref.self_ty(),
 345                                 )
 346                             };
 347
 348                         if self.suggest_add_reference_to_arg(
 349                             &obligation,
 350                             &mut err,
 351                             &trait_ref,
 352                             points_at_arg,
 353                             have_alt_message,
 354                         ) {
 355                             self.note_obligation_cause(&mut err, obligation);
 356                             err.emit();
 357                             return;
 358                         }
 359                         if let Some(ref s) = label {
 360                             // If it has a custom `#[rustc_on_unimplemented]`
 361                             // error message, let's display it as the label!
 362                             err.span_label(span, s.as_str());
 363                             err.help(&explanation);
 364                         } else {
 365                             err.span_label(span, explanation);
 366                         }
 367                         if let Some((msg, span)) = type_def {
 368                             err.span_label(span, &msg);
 369                         }
 370                         if let Some(ref s) = note {
 371                             // If it has a custom `#[rustc_on_unimplemented]` note, let's display it
 372                             err.note(s.as_str());
 373                         }
 374                         if let Some(ref s) = enclosing_scope {
 375                             let enclosing_scope_span = tcx.def_span(
 376                                 tcx.hir()
 377                                     .opt_local_def_id(obligation.cause.body_id)
 378                                     .unwrap_or_else(|| {
 379                                         tcx.hir().body_owner_def_id(hir::BodyId {
 380                                             hir_id: obligation.cause.body_id,
 381                                         })
 382                                     })
 383                                     .to_def_id(),
 384                             );
 385
 386                             err.span_label(enclosing_scope_span, s.as_str());
 387                         }
 388
 389                         self.suggest_borrow_on_unsized_slice(&obligation.cause.code, &mut err);
 390                         self.suggest_fn_call(&obligation, &mut err, &trait_ref, points_at_arg);
 391                         self.suggest_remove_reference(&obligation, &mut err, &trait_ref);
 392                         self.suggest_semicolon_removal(&obligation, &mut err, span, &trait_ref);
 393                         self.note_version_mismatch(&mut err, &trait_ref);
 394                         if self.suggest_impl_trait(&mut err, span, &obligation, &trait_ref) {
 395                             err.emit();
 396                             return;
 397                         }
 398
 399                         // Try to report a help message
 400                         if !trait_ref.has_infer_types_or_consts()
 401                             && self.predicate_can_apply(obligation.param_env, trait_ref)
 402                         {
 403                             // If a where-clause may be useful, remind the
 404                             // user that they can add it.
 405                             //
 406                             // don't display an on-unimplemented note, as
 407                             // these notes will often be of the form
 408                             //     "the type `T` can't be frobnicated"
 409                             // which is somewhat confusing.
 410                             self.suggest_restricting_param_bound(
 411                                 &mut err,
 412                                 trait_ref,
 413                                 obligation.cause.body_id,
 414                             );
 415                         } else {
 416                             if !have_alt_message {
 417                                 // Can't show anything else useful, try to find similar impls.
 418                                 let impl_candidates = self.find_similar_impl_candidates(trait_ref);
 419                                 self.report_similar_impl_candidates(impl_candidates, &mut err);
 420                             }
 421                             self.suggest_change_mut(
 422                                 &obligation,
 423                                 &mut err,
 424                                 &trait_ref,
 425                                 points_at_arg,
 426                             );
 427                         }
 428
 429                         // If this error is due to `!: Trait` not implemented but `(): Trait` is
 430                         // implemented, and fallback has occurred, then it could be due to a
 431                         // variable that used to fallback to `()` now falling back to `!`. Issue a
 432                         // note informing about the change in behaviour.
 433                         if trait_predicate.skip_binder().self_ty().is_never()
 434                             && fallback_has_occurred
 435                         {
 436                             let predicate = trait_predicate.map_bound(|mut trait_pred| {
 437                                 trait_pred.trait_ref.substs = self.tcx.mk_substs_trait(
 438                                     self.tcx.mk_unit(),
 439                                     &trait_pred.trait_ref.substs[1..],
 440                                 );
 441                                 trait_pred
 442                             });
 443                             let unit_obligation = Obligation {
 444                                 predicate: ty::Predicate::Trait(
 445                                     predicate,
 446                                     hir::Constness::NotConst,
 447                                 ),
 448                                 ..obligation.clone()
 449                             };
 450                             if self.predicate_may_hold(&unit_obligation) {
 451                                 err.note(
 452                                     "the trait is implemented for `()`. \
 453                                      Possibly this error has been caused by changes to \
 454                                      Rust's type-inference algorithm (see issue #48950 \
 455                                      <https://github.com/rust-lang/rust/issues/48950> \
 456                                      for more information). Consider whether you meant to use \
 457                                      the type `()` here instead.",
 458                                 );
 459                             }
 460                         }
 461
 462                         err
 463                     }
 464
 465                     ty::Predicate::Subtype(ref predicate) => {
 466                         // Errors for Subtype predicates show up as
 467                         // `FulfillmentErrorCode::CodeSubtypeError`,
 468                         // not selection error.
 469                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
 470                     }
 471
 472                     ty::Predicate::RegionOutlives(ref predicate) => {
 473                         let predicate = self.resolve_vars_if_possible(predicate);
 474                         let err = self
 475                             .region_outlives_predicate(&obligation.cause, &predicate)
 476                             .err()
 477                             .unwrap();
 478                         struct_span_err!(
 479                             self.tcx.sess,
 480                             span,
 481                             E0279,
 482                             "the requirement `{}` is not satisfied (`{}`)",
 483                             predicate,
 484                             err,
 485                         )
 486                     }
 487
 488                     ty::Predicate::Projection(..) | ty::Predicate::TypeOutlives(..) => {
 489                         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
 490                         struct_span_err!(
 491                             self.tcx.sess,
 492                             span,
 493                             E0280,
 494                             "the requirement `{}` is not satisfied",
 495                             predicate
 496                         )
 497                     }
 498
 499                     ty::Predicate::ObjectSafe(trait_def_id) => {
 500                         let violations = self.tcx.object_safety_violations(trait_def_id);
 501                         report_object_safety_error(self.tcx, span, trait_def_id, violations)
 502                     }
 503
 504                     ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => {
 505                         let found_kind = self.closure_kind(closure_substs).unwrap();
 506                         let closure_span =
 507                             self.tcx.sess.source_map().guess_head_span(
 508                                 self.tcx.hir().span_if_local(closure_def_id).unwrap(),
 509                             );
 510                         let hir_id = self.tcx.hir().as_local_hir_id(closure_def_id).unwrap();
 511                         let mut err = struct_span_err!(
 512                             self.tcx.sess,
 513                             closure_span,
 514                             E0525,
 515                             "expected a closure that implements the `{}` trait, \
 516                              but this closure only implements `{}`",
 517                             kind,
 518                             found_kind
 519                         );
 520
 521                         err.span_label(
 522                             closure_span,
 523                             format!("this closure implements `{}`, not `{}`", found_kind, kind),
 524                         );
 525                         err.span_label(
 526                             obligation.cause.span,
 527                             format!("the requirement to implement `{}` derives from here", kind),
 528                         );
 529
 530                         // Additional context information explaining why the closure only implements
 531                         // a particular trait.
 532                         if let Some(tables) = self.in_progress_tables {
 533                             let tables = tables.borrow();
 534                             match (found_kind, tables.closure_kind_origins().get(hir_id)) {
 535                                 (ty::ClosureKind::FnOnce, Some((span, name))) => {
 536                                     err.span_label(
 537                                         *span,
 538                                         format!(
 539                                             "closure is `FnOnce` because it moves the \
 540                                          variable `{}` out of its environment",
 541                                             name
 542                                         ),
 543                                     );
 544                                 }
 545                                 (ty::ClosureKind::FnMut, Some((span, name))) => {
 546                                     err.span_label(
 547                                         *span,
 548                                         format!(
 549                                             "closure is `FnMut` because it mutates the \
 550                                          variable `{}` here",
 551                                             name
 552                                         ),
 553                                     );
 554                                 }
 555                                 _ => {}
 556                             }
 557                         }
 558
 559                         err.emit();
 560                         return;
 561                     }
 562
 563                     ty::Predicate::WellFormed(ty) => {
 564                         // WF predicates cannot themselves make
 565                         // errors. They can only block due to
 566                         // ambiguity; otherwise, they always
 567                         // degenerate into other obligations
 568                         // (which may fail).
 569                         span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 570                     }
 571
 572                     ty::Predicate::ConstEvaluatable(..) => {
 573                         // Errors for `ConstEvaluatable` predicates show up as
 574                         // `SelectionError::ConstEvalFailure`,
 575                         // not `Unimplemented`.
 576                         span_bug!(
 577                             span,
 578                             "const-evaluatable requirement gave wrong error: `{:?}`",
 579                             obligation
 580                         )
 581                     }
 582                 }
 583             }
 584
 585             OutputTypeParameterMismatch(ref found_trait_ref, ref expected_trait_ref, _) => {
 586                 let found_trait_ref = self.resolve_vars_if_possible(&*found_trait_ref);
 587                 let expected_trait_ref = self.resolve_vars_if_possible(&*expected_trait_ref);
 588
 589                 if expected_trait_ref.self_ty().references_error() {
 590                     return;
 591                 }
 592
 593                 let found_trait_ty = found_trait_ref.self_ty();
 594
 595                 let found_did = match found_trait_ty.kind {
 596                     ty::Closure(did, _) | ty::Foreign(did) | ty::FnDef(did, _) => Some(did),
 597                     ty::Adt(def, _) => Some(def.did),
 598                     _ => None,
 599                 };
 600
 601                 let found_span = found_did
 602                     .and_then(|did| self.tcx.hir().span_if_local(did))
 603                     .map(|sp| self.tcx.sess.source_map().guess_head_span(sp)); // the sp could be an fn def
 604
 605                 if self.reported_closure_mismatch.borrow().contains(&(span, found_span)) {
 606                     // We check closures twice, with obligations flowing in different directions,
 607                     // but we want to complain about them only once.
 608                     return;
 609                 }
 610
 611                 self.reported_closure_mismatch.borrow_mut().insert((span, found_span));
 612
 613                 let found = match found_trait_ref.skip_binder().substs.type_at(1).kind {
 614                     ty::Tuple(ref tys) => vec![ArgKind::empty(); tys.len()],
 615                     _ => vec![ArgKind::empty()],
 616                 };
 617
 618                 let expected_ty = expected_trait_ref.skip_binder().substs.type_at(1);
 619                 let expected = match expected_ty.kind {
 620                     ty::Tuple(ref tys) => tys
 621                         .iter()
 622                         .map(|t| ArgKind::from_expected_ty(t.expect_ty(), Some(span)))
 623                         .collect(),
 624                     _ => vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())],
 625                 };
 626
 627                 if found.len() == expected.len() {
 628                     self.report_closure_arg_mismatch(
 629                         span,
 630                         found_span,
 631                         found_trait_ref,
 632                         expected_trait_ref,
 633                     )
 634                 } else {
 635                     let (closure_span, found) = found_did
 636                         .and_then(|did| {
 637                             let node = self.tcx.hir().get_if_local(did)?;
 638                             let (found_span, found) = self.get_fn_like_arguments(node)?;
 639                             Some((Some(found_span), found))
 640                         })
 641                         .unwrap_or((found_span, found));
 642
 643                     self.report_arg_count_mismatch(
 644                         span,
 645                         closure_span,
 646                         expected,
 647                         found,
 648                         found_trait_ty.is_closure(),
 649                     )
 650                 }
 651             }
 652
 653             TraitNotObjectSafe(did) => {
 654                 let violations = self.tcx.object_safety_violations(did);
 655                 report_object_safety_error(self.tcx, span, did, violations)
 656             }
 657
 658             ConstEvalFailure(ErrorHandled::TooGeneric) => {
 659                 // In this instance, we have a const expression containing an unevaluated
 660                 // generic parameter. We have no idea whether this expression is valid or
 661                 // not (e.g. it might result in an error), but we don't want to just assume
 662                 // that it's okay, because that might result in post-monomorphisation time
 663                 // errors. The onus is really on the caller to provide values that it can
 664                 // prove are well-formed.
 665                 let mut err = self
 666                     .tcx
 667                     .sess
 668                     .struct_span_err(span, "constant expression depends on a generic parameter");
 669                 // FIXME(const_generics): we should suggest to the user how they can resolve this
 670                 // issue. However, this is currently not actually possible
 671                 // (see https://github.com/rust-lang/rust/issues/66962#issuecomment-575907083).
 672                 err.note("this may fail depending on what value the parameter takes");
 673                 err
 674             }
 675
 676             // Already reported in the query.
 677             ConstEvalFailure(ErrorHandled::Reported(ErrorReported)) => {
 678                 // FIXME(eddyb) remove this once `ErrorReported` becomes a proof token.
 679                 self.tcx.sess.delay_span_bug(span, "`ErrorReported` without an error");
 680                 return;
 681             }
 682
 683             // Already reported in the query, but only as a lint.
 684             // This shouldn't actually happen for constants used in types, modulo
 685             // bugs. The `delay_span_bug` here ensures it won't be ignored.
 686             ConstEvalFailure(ErrorHandled::Linted) => {
 687                 self.tcx.sess.delay_span_bug(span, "constant in type had error reported as lint");
 688                 return;
 689             }
 690
 691             Overflow => {
 692                 bug!("overflow should be handled before the `report_selection_error` path");
 693             }
 694         };
 695
 696         self.note_obligation_cause(&mut err, obligation);
 697         self.point_at_returns_when_relevant(&mut err, &obligation);
 698
 699         err.emit();
 700     }
 701
 702     /// Given some node representing a fn-like thing in the HIR map,
 703     /// returns a span and `ArgKind` information that describes the
 704     /// arguments it expects. This can be supplied to
 705     /// `report_arg_count_mismatch`.
 706     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)> {
 707         let sm = self.tcx.sess.source_map();
 708         let hir = self.tcx.hir();
 709         Some(match node {
 710             Node::Expr(&hir::Expr {
 711                 kind: hir::ExprKind::Closure(_, ref _decl, id, span, _),
 712                 ..
 713             }) => (
 714                 sm.guess_head_span(span),
 715                 hir.body(id)
 716                     .params
 717                     .iter()
 718                     .map(|arg| {
 719                         if let hir::Pat { kind: hir::PatKind::Tuple(ref args, _), span, .. } =
 720                             *arg.pat
 721                         {
 722                             Some(ArgKind::Tuple(
 723                                 Some(span),
 724                                 args.iter()
 725                                     .map(|pat| {
 726                                         sm.span_to_snippet(pat.span)
 727                                             .ok()
 728                                             .map(|snippet| (snippet, "_".to_owned()))
 729                                     })
 730                                     .collect::<Option<Vec<_>>>()?,
 731                             ))
 732                         } else {
 733                             let name = sm.span_to_snippet(arg.pat.span).ok()?;
 734                             Some(ArgKind::Arg(name, "_".to_owned()))
 735                         }
 736                     })
 737                     .collect::<Option<Vec<ArgKind>>>()?,
 738             ),
 739             Node::Item(&hir::Item { span, kind: hir::ItemKind::Fn(ref sig, ..), .. })
 740             | Node::ImplItem(&hir::ImplItem {
 741                 span,
 742                 kind: hir::ImplItemKind::Fn(ref sig, _),
 743                 ..
 744             })
 745             | Node::TraitItem(&hir::TraitItem {
 746                 span,
 747                 kind: hir::TraitItemKind::Fn(ref sig, _),
 748                 ..
 749             }) => (
 750                 sm.guess_head_span(span),
 751                 sig.decl
 752                     .inputs
 753                     .iter()
 754                     .map(|arg| match arg.clone().kind {
 755                         hir::TyKind::Tup(ref tys) => ArgKind::Tuple(
 756                             Some(arg.span),
 757                             vec![("_".to_owned(), "_".to_owned()); tys.len()],
 758                         ),
 759                         _ => ArgKind::empty(),
 760                     })
 761                     .collect::<Vec<ArgKind>>(),
 762             ),
 763             Node::Ctor(ref variant_data) => {
 764                 let span = variant_data.ctor_hir_id().map(|id| hir.span(id)).unwrap_or(DUMMY_SP);
 765                 let span = sm.guess_head_span(span);
 766                 (span, vec![ArgKind::empty(); variant_data.fields().len()])
 767             }
 768             _ => panic!("non-FnLike node found: {:?}", node),
 769         })
 770     }
 771
 772     /// Reports an error when the number of arguments needed by a
 773     /// trait match doesn't match the number that the expression
 774     /// provides.
 775     fn report_arg_count_mismatch(
 776         &self,
 777         span: Span,
 778         found_span: Option<Span>,
 779         expected_args: Vec<ArgKind>,
 780         found_args: Vec<ArgKind>,
 781         is_closure: bool,
 782     ) -> DiagnosticBuilder<'tcx> {
 783         let kind = if is_closure { "closure" } else { "function" };
 784
 785         let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
 786             let arg_length = arguments.len();
 787             let distinct = match &other[..] {
 788                 &[ArgKind::Tuple(..)] => true,
 789                 _ => false,
 790             };
 791             match (arg_length, arguments.get(0)) {
 792                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
 793                     format!("a single {}-tuple as argument", fields.len())
 794                 }
 795                 _ => format!(
 796                     "{} {}argument{}",
 797                     arg_length,
 798                     if distinct && arg_length > 1 { "distinct " } else { "" },
 799                     pluralize!(arg_length)
 800                 ),
 801             }
 802         };
 803
 804         let expected_str = args_str(&expected_args, &found_args);
 805         let found_str = args_str(&found_args, &expected_args);
 806
 807         let mut err = struct_span_err!(
 808             self.tcx.sess,
 809             span,
 810             E0593,
 811             "{} is expected to take {}, but it takes {}",
 812             kind,
 813             expected_str,
 814             found_str,
 815         );
 816
 817         err.span_label(span, format!("expected {} that takes {}", kind, expected_str));
 818
 819         if let Some(found_span) = found_span {
 820             err.span_label(found_span, format!("takes {}", found_str));
 821
 822             // move |_| { ... }
 823             // ^^^^^^^^-- def_span
 824             //
 825             // move |_| { ... }
 826             // ^^^^^-- prefix
 827             let prefix_span = self.tcx.sess.source_map().span_until_non_whitespace(found_span);
 828             // move |_| { ... }
 829             //      ^^^-- pipe_span
 830             let pipe_span =
 831                 if let Some(span) = found_span.trim_start(prefix_span) { span } else { found_span };
 832
 833             // Suggest to take and ignore the arguments with expected_args_length `_`s if
 834             // found arguments is empty (assume the user just wants to ignore args in this case).
 835             // For example, if `expected_args_length` is 2, suggest `|_, _|`.
 836             if found_args.is_empty() && is_closure {
 837                 let underscores = vec!["_"; expected_args.len()].join(", ");
 838                 err.span_suggestion_verbose(
 839                     pipe_span,
 840                     &format!(
 841                         "consider changing the closure to take and ignore the expected argument{}",
 842                         pluralize!(expected_args.len())
 843                     ),
 844                     format!("|{}|", underscores),
 845                     Applicability::MachineApplicable,
 846                 );
 847             }
 848
 849             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
 850                 if fields.len() == expected_args.len() {
 851                     let sugg = fields
 852                         .iter()
 853                         .map(|(name, _)| name.to_owned())
 854                         .collect::<Vec<String>>()
 855                         .join(", ");
 856                     err.span_suggestion_verbose(
 857                         found_span,
 858                         "change the closure to take multiple arguments instead of a single tuple",
 859                         format!("|{}|", sugg),
 860                         Applicability::MachineApplicable,
 861                     );
 862                 }
 863             }
 864             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..] {
 865                 if fields.len() == found_args.len() && is_closure {
 866                     let sugg = format!(
 867                         "|({}){}|",
 868                         found_args
 869                             .iter()
 870                             .map(|arg| match arg {
 871                                 ArgKind::Arg(name, _) => name.to_owned(),
 872                                 _ => "_".to_owned(),
 873                             })
 874                             .collect::<Vec<String>>()
 875                             .join(", "),
 876                         // add type annotations if available
 877                         if found_args.iter().any(|arg| match arg {
 878                             ArgKind::Arg(_, ty) => ty != "_",
 879                             _ => false,
 880                         }) {
 881                             format!(
 882                                 ": ({})",
 883                                 fields
 884                                     .iter()
 885                                     .map(|(_, ty)| ty.to_owned())
 886                                     .collect::<Vec<String>>()
 887                                     .join(", ")
 888                             )
 889                         } else {
 890                             String::new()
 891                         },
 892                     );
 893                     err.span_suggestion_verbose(
 894                         found_span,
 895                         "change the closure to accept a tuple instead of individual arguments",
 896                         sugg,
 897                         Applicability::MachineApplicable,
 898                     );
 899                 }
 900             }
 901         }
 902
 903         err
 904     }
 905 }
 906
 907 trait InferCtxtPrivExt<'tcx> {
 908     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
 909     // `error` occurring implies that `cond` occurs.
 910     fn error_implies(&self, cond: &ty::Predicate<'tcx>, error: &ty::Predicate<'tcx>) -> bool;
 911
 912     fn report_fulfillment_error(
 913         &self,
 914         error: &FulfillmentError<'tcx>,
 915         body_id: Option<hir::BodyId>,
 916         fallback_has_occurred: bool,
 917     );
 918
 919     fn report_projection_error(
 920         &self,
 921         obligation: &PredicateObligation<'tcx>,
 922         error: &MismatchedProjectionTypes<'tcx>,
 923     );
 924
 925     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool;
 926
 927     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str>;
 928
 929     fn find_similar_impl_candidates(
 930         &self,
 931         trait_ref: ty::PolyTraitRef<'tcx>,
 932     ) -> Vec<ty::TraitRef<'tcx>>;
 933
 934     fn report_similar_impl_candidates(
 935         &self,
 936         impl_candidates: Vec<ty::TraitRef<'tcx>>,
 937         err: &mut DiagnosticBuilder<'_>,
 938     );
 939
 940     /// Gets the parent trait chain start
 941     fn get_parent_trait_ref(
 942         &self,
 943         code: &ObligationCauseCode<'tcx>,
 944     ) -> Option<(String, Option<Span>)>;
 945
 946     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
 947     /// with the same path as `trait_ref`, a help message about
 948     /// a probable version mismatch is added to `err`
 949     fn note_version_mismatch(
 950         &self,
 951         err: &mut DiagnosticBuilder<'_>,
 952         trait_ref: &ty::PolyTraitRef<'tcx>,
 953     );
 954
 955     fn mk_obligation_for_def_id(
 956         &self,
 957         def_id: DefId,
 958         output_ty: Ty<'tcx>,
 959         cause: ObligationCause<'tcx>,
 960         param_env: ty::ParamEnv<'tcx>,
 961     ) -> PredicateObligation<'tcx>;
 962
 963     fn maybe_report_ambiguity(
 964         &self,
 965         obligation: &PredicateObligation<'tcx>,
 966         body_id: Option<hir::BodyId>,
 967     );
 968
 969     fn predicate_can_apply(
 970         &self,
 971         param_env: ty::ParamEnv<'tcx>,
 972         pred: ty::PolyTraitRef<'tcx>,
 973     ) -> bool;
 974
 975     fn note_obligation_cause(
 976         &self,
 977         err: &mut DiagnosticBuilder<'_>,
 978         obligation: &PredicateObligation<'tcx>,
 979     );
 980
 981     fn suggest_unsized_bound_if_applicable(
 982         &self,
 983         err: &mut DiagnosticBuilder<'_>,
 984         obligation: &PredicateObligation<'tcx>,
 985     );
 986
 987     fn is_recursive_obligation(
 988         &self,
 989         obligated_types: &mut Vec<&ty::TyS<'tcx>>,
 990         cause_code: &ObligationCauseCode<'tcx>,
 991     ) -> bool;
 992 }
 993
 994 impl<'a, 'tcx> InferCtxtPrivExt<'tcx> for InferCtxt<'a, 'tcx> {
 995     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
 996     // `error` occurring implies that `cond` occurs.
 997     fn error_implies(&self, cond: &ty::Predicate<'tcx>, error: &ty::Predicate<'tcx>) -> bool {
 998         if cond == error {
 999             return true;
1000         }
1001
1002         let (cond, error) = match (cond, error) {
1003             (&ty::Predicate::Trait(..), &ty::Predicate::Trait(ref error, _)) => (cond, error),
1004             _ => {
1005                 // FIXME: make this work in other cases too.
1006                 return false;
1007             }
1008         };
1009
1010         for obligation in super::elaborate_predicates(self.tcx, std::iter::once(*cond)) {
1011             if let ty::Predicate::Trait(implication, _) = obligation.predicate {
1012                 let error = error.to_poly_trait_ref();
1013                 let implication = implication.to_poly_trait_ref();
1014                 // FIXME: I'm just not taking associated types at all here.
1015                 // Eventually I'll need to implement param-env-aware
1016                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
1017                 let param_env = ty::ParamEnv::empty();
1018                 if self.can_sub(param_env, error, implication).is_ok() {
1019                     debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
1020                     return true;
1021                 }
1022             }
1023         }
1024
1025         false
1026     }
1027
1028     fn report_fulfillment_error(
1029         &self,
1030         error: &FulfillmentError<'tcx>,
1031         body_id: Option<hir::BodyId>,
1032         fallback_has_occurred: bool,
1033     ) {
1034         debug!("report_fulfillment_error({:?})", error);
1035         match error.code {
1036             FulfillmentErrorCode::CodeSelectionError(ref selection_error) => {
1037                 self.report_selection_error(
1038                     &error.obligation,
1039                     selection_error,
1040                     fallback_has_occurred,
1041                     error.points_at_arg_span,
1042                 );
1043             }
1044             FulfillmentErrorCode::CodeProjectionError(ref e) => {
1045                 self.report_projection_error(&error.obligation, e);
1046             }
1047             FulfillmentErrorCode::CodeAmbiguity => {
1048                 self.maybe_report_ambiguity(&error.obligation, body_id);
1049             }
1050             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
1051                 self.report_mismatched_types(
1052                     &error.obligation.cause,
1053                     expected_found.expected,
1054                     expected_found.found,
1055                     err.clone(),
1056                 )
1057                 .emit();
1058             }
1059         }
1060     }
1061
1062     fn report_projection_error(
1063         &self,
1064         obligation: &PredicateObligation<'tcx>,
1065         error: &MismatchedProjectionTypes<'tcx>,
1066     ) {
1067         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
1068
1069         if predicate.references_error() {
1070             return;
1071         }
1072
1073         self.probe(|_| {
1074             let err_buf;
1075             let mut err = &error.err;
1076             let mut values = None;
1077
1078             // try to find the mismatched types to report the error with.
1079             //
1080             // this can fail if the problem was higher-ranked, in which
1081             // cause I have no idea for a good error message.
1082             if let ty::Predicate::Projection(ref data) = predicate {
1083                 let mut selcx = SelectionContext::new(self);
1084                 let (data, _) = self.replace_bound_vars_with_fresh_vars(
1085                     obligation.cause.span,
1086                     infer::LateBoundRegionConversionTime::HigherRankedType,
1087                     data,
1088                 );
1089                 let mut obligations = vec![];
1090                 let normalized_ty = super::normalize_projection_type(
1091                     &mut selcx,
1092                     obligation.param_env,
1093                     data.projection_ty,
1094                     obligation.cause.clone(),
1095                     0,
1096                     &mut obligations,
1097                 );
1098
1099                 debug!(
1100                     "report_projection_error obligation.cause={:?} obligation.param_env={:?}",
1101                     obligation.cause, obligation.param_env
1102                 );
1103
1104                 debug!(
1105                     "report_projection_error normalized_ty={:?} data.ty={:?}",
1106                     normalized_ty, data.ty
1107                 );
1108
1109                 let is_normalized_ty_expected = match &obligation.cause.code {
1110                     ObligationCauseCode::ItemObligation(_)
1111                     | ObligationCauseCode::BindingObligation(_, _)
1112                     | ObligationCauseCode::ObjectCastObligation(_) => false,
1113                     _ => true,
1114                 };
1115
1116                 if let Err(error) = self.at(&obligation.cause, obligation.param_env).eq_exp(
1117                     is_normalized_ty_expected,
1118                     normalized_ty,
1119                     data.ty,
1120                 ) {
1121                     values = Some(infer::ValuePairs::Types(ExpectedFound::new(
1122                         is_normalized_ty_expected,
1123                         normalized_ty,
1124                         data.ty,
1125                     )));
1126
1127                     err_buf = error;
1128                     err = &err_buf;
1129                 }
1130             }
1131
1132             let msg = format!("type mismatch resolving `{}`", predicate);
1133             let error_id = (DiagnosticMessageId::ErrorId(271), Some(obligation.cause.span), msg);
1134             let fresh = self.tcx.sess.one_time_diagnostics.borrow_mut().insert(error_id);
1135             if fresh {
1136                 let mut diag = struct_span_err!(
1137                     self.tcx.sess,
1138                     obligation.cause.span,
1139                     E0271,
1140                     "type mismatch resolving `{}`",
1141                     predicate
1142                 );
1143                 self.note_type_err(&mut diag, &obligation.cause, None, values, err);
1144                 self.note_obligation_cause(&mut diag, obligation);
1145                 diag.emit();
1146             }
1147         });
1148     }
1149
1150     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
1151         /// returns the fuzzy category of a given type, or None
1152         /// if the type can be equated to any type.
1153         fn type_category(t: Ty<'_>) -> Option<u32> {
1154             match t.kind {
1155                 ty::Bool => Some(0),
1156                 ty::Char => Some(1),
1157                 ty::Str => Some(2),
1158                 ty::Int(..) | ty::Uint(..) | ty::Infer(ty::IntVar(..)) => Some(3),
1159                 ty::Float(..) | ty::Infer(ty::FloatVar(..)) => Some(4),
1160                 ty::Ref(..) | ty::RawPtr(..) => Some(5),
1161                 ty::Array(..) | ty::Slice(..) => Some(6),
1162                 ty::FnDef(..) | ty::FnPtr(..) => Some(7),
1163                 ty::Dynamic(..) => Some(8),
1164                 ty::Closure(..) => Some(9),
1165                 ty::Tuple(..) => Some(10),
1166                 ty::Projection(..) => Some(11),
1167                 ty::Param(..) => Some(12),
1168                 ty::Opaque(..) => Some(13),
1169                 ty::Never => Some(14),
1170                 ty::Adt(adt, ..) => match adt.adt_kind() {
1171                     AdtKind::Struct => Some(15),
1172                     AdtKind::Union => Some(16),
1173                     AdtKind::Enum => Some(17),
1174                 },
1175                 ty::Generator(..) => Some(18),
1176                 ty::Foreign(..) => Some(19),
1177                 ty::GeneratorWitness(..) => Some(20),
1178                 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error => None,
1179                 ty::UnnormalizedProjection(..) => bug!("only used with chalk-engine"),
1180             }
1181         }
1182
1183         match (type_category(a), type_category(b)) {
1184             (Some(cat_a), Some(cat_b)) => match (&a.kind, &b.kind) {
1185                 (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => def_a == def_b,
1186                 _ => cat_a == cat_b,
1187             },
1188             // infer and error can be equated to all types
1189             _ => true,
1190         }
1191     }
1192
1193     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str> {
1194         self.tcx.hir().body(body_id).generator_kind.map(|gen_kind| match gen_kind {
1195             hir::GeneratorKind::Gen => "a generator",
1196             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block) => "an async block",
1197             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn) => "an async function",
1198             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Closure) => "an async closure",
1199         })
1200     }
1201
1202     fn find_similar_impl_candidates(
1203         &self,
1204         trait_ref: ty::PolyTraitRef<'tcx>,
1205     ) -> Vec<ty::TraitRef<'tcx>> {
1206         let simp = fast_reject::simplify_type(self.tcx, trait_ref.skip_binder().self_ty(), true);
1207         let all_impls = self.tcx.all_impls(trait_ref.def_id());
1208
1209         match simp {
1210             Some(simp) => all_impls
1211                 .filter_map(|def_id| {
1212                     let imp = self.tcx.impl_trait_ref(def_id).unwrap();
1213                     let imp_simp = fast_reject::simplify_type(self.tcx, imp.self_ty(), true);
1214                     if let Some(imp_simp) = imp_simp {
1215                         if simp != imp_simp {
1216                             return None;
1217                         }
1218                     }
1219                     Some(imp)
1220                 })
1221                 .collect(),
1222             None => all_impls.map(|def_id| self.tcx.impl_trait_ref(def_id).unwrap()).collect(),
1223         }
1224     }
1225
1226     fn report_similar_impl_candidates(
1227         &self,
1228         impl_candidates: Vec<ty::TraitRef<'tcx>>,
1229         err: &mut DiagnosticBuilder<'_>,
1230     ) {
1231         if impl_candidates.is_empty() {
1232             return;
1233         }
1234
1235         let len = impl_candidates.len();
1236         let end = if impl_candidates.len() <= 5 { impl_candidates.len() } else { 4 };
1237
1238         let normalize = |candidate| {
1239             self.tcx.infer_ctxt().enter(|ref infcx| {
1240                 let normalized = infcx
1241                     .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
1242                     .normalize(candidate)
1243                     .ok();
1244                 match normalized {
1245                     Some(normalized) => format!("\n  {:?}", normalized.value),
1246                     None => format!("\n  {:?}", candidate),
1247                 }
1248             })
1249         };
1250
1251         // Sort impl candidates so that ordering is consistent for UI tests.
1252         let mut normalized_impl_candidates =
1253             impl_candidates.iter().map(normalize).collect::<Vec<String>>();
1254
1255         // Sort before taking the `..end` range,
1256         // because the ordering of `impl_candidates` may not be deterministic:
1257         // https://github.com/rust-lang/rust/pull/57475#issuecomment-455519507
1258         normalized_impl_candidates.sort();
1259
1260         err.help(&format!(
1261             "the following implementations were found:{}{}",
1262             normalized_impl_candidates[..end].join(""),
1263             if len > 5 { format!("\nand {} others", len - 4) } else { String::new() }
1264         ));
1265     }
1266
1267     /// Gets the parent trait chain start
1268     fn get_parent_trait_ref(
1269         &self,
1270         code: &ObligationCauseCode<'tcx>,
1271     ) -> Option<(String, Option<Span>)> {
1272         match code {
1273             &ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
1274                 let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
1275                 match self.get_parent_trait_ref(&data.parent_code) {
1276                     Some(t) => Some(t),
1277                     None => {
1278                         let ty = parent_trait_ref.skip_binder().self_ty();
1279                         let span =
1280                             TyCategory::from_ty(ty).map(|(_, def_id)| self.tcx.def_span(def_id));
1281                         Some((ty.to_string(), span))
1282                     }
1283                 }
1284             }
1285             _ => None,
1286         }
1287     }
1288
1289     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1290     /// with the same path as `trait_ref`, a help message about
1291     /// a probable version mismatch is added to `err`
1292     fn note_version_mismatch(
1293         &self,
1294         err: &mut DiagnosticBuilder<'_>,
1295         trait_ref: &ty::PolyTraitRef<'tcx>,
1296     ) {
1297         let get_trait_impl = |trait_def_id| {
1298             let mut trait_impl = None;
1299             self.tcx.for_each_relevant_impl(trait_def_id, trait_ref.self_ty(), |impl_def_id| {
1300                 if trait_impl.is_none() {
1301                     trait_impl = Some(impl_def_id);
1302                 }
1303             });
1304             trait_impl
1305         };
1306         let required_trait_path = self.tcx.def_path_str(trait_ref.def_id());
1307         let all_traits = self.tcx.all_traits(LOCAL_CRATE);
1308         let traits_with_same_path: std::collections::BTreeSet<_> = all_traits
1309             .iter()
1310             .filter(|trait_def_id| **trait_def_id != trait_ref.def_id())
1311             .filter(|trait_def_id| self.tcx.def_path_str(**trait_def_id) == required_trait_path)
1312             .collect();
1313         for trait_with_same_path in traits_with_same_path {
1314             if let Some(impl_def_id) = get_trait_impl(*trait_with_same_path) {
1315                 let impl_span = self.tcx.def_span(impl_def_id);
1316                 err.span_help(impl_span, "trait impl with same name found");
1317                 let trait_crate = self.tcx.crate_name(trait_with_same_path.krate);
1318                 let crate_msg = format!(
1319                     "perhaps two different versions of crate `{}` are being used?",
1320                     trait_crate
1321                 );
1322                 err.note(&crate_msg);
1323             }
1324         }
1325     }
1326
1327     fn mk_obligation_for_def_id(
1328         &self,
1329         def_id: DefId,
1330         output_ty: Ty<'tcx>,
1331         cause: ObligationCause<'tcx>,
1332         param_env: ty::ParamEnv<'tcx>,
1333     ) -> PredicateObligation<'tcx> {
1334         let new_trait_ref =
1335             ty::TraitRef { def_id, substs: self.tcx.mk_substs_trait(output_ty, &[]) };
1336         Obligation::new(cause, param_env, new_trait_ref.without_const().to_predicate())
1337     }
1338
1339     fn maybe_report_ambiguity(
1340         &self,
1341         obligation: &PredicateObligation<'tcx>,
1342         body_id: Option<hir::BodyId>,
1343     ) {
1344         // Unable to successfully determine, probably means
1345         // insufficient type information, but could mean
1346         // ambiguous impls. The latter *ought* to be a
1347         // coherence violation, so we don't report it here.
1348
1349         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
1350         let span = obligation.cause.span;
1351
1352         debug!(
1353             "maybe_report_ambiguity(predicate={:?}, obligation={:?} body_id={:?}, code={:?})",
1354             predicate, obligation, body_id, obligation.cause.code,
1355         );
1356
1357         // Ambiguity errors are often caused as fallout from earlier
1358         // errors. So just ignore them if this infcx is tainted.
1359         if self.is_tainted_by_errors() {
1360             return;
1361         }
1362
1363         let mut err = match predicate {
1364             ty::Predicate::Trait(ref data, _) => {
1365                 let trait_ref = data.to_poly_trait_ref();
1366                 let self_ty = trait_ref.self_ty();
1367                 debug!("self_ty {:?} {:?} trait_ref {:?}", self_ty, self_ty.kind, trait_ref);
1368
1369                 if predicate.references_error() {
1370                     return;
1371                 }
1372                 // Typically, this ambiguity should only happen if
1373                 // there are unresolved type inference variables
1374                 // (otherwise it would suggest a coherence
1375                 // failure). But given #21974 that is not necessarily
1376                 // the case -- we can have multiple where clauses that
1377                 // are only distinguished by a region, which results
1378                 // in an ambiguity even when all types are fully
1379                 // known, since we don't dispatch based on region
1380                 // relationships.
1381
1382                 // This is kind of a hack: it frequently happens that some earlier
1383                 // error prevents types from being fully inferred, and then we get
1384                 // a bunch of uninteresting errors saying something like "<generic
1385                 // #0> doesn't implement Sized".  It may even be true that we
1386                 // could just skip over all checks where the self-ty is an
1387                 // inference variable, but I was afraid that there might be an
1388                 // inference variable created, registered as an obligation, and
1389                 // then never forced by writeback, and hence by skipping here we'd
1390                 // be ignoring the fact that we don't KNOW the type works
1391                 // out. Though even that would probably be harmless, given that
1392                 // we're only talking about builtin traits, which are known to be
1393                 // inhabited. We used to check for `self.tcx.sess.has_errors()` to
1394                 // avoid inundating the user with unnecessary errors, but we now
1395                 // check upstream for type errors and don't add the obligations to
1396                 // begin with in those cases.
1397                 if self
1398                     .tcx
1399                     .lang_items()
1400                     .sized_trait()
1401                     .map_or(false, |sized_id| sized_id == trait_ref.def_id())
1402                 {
1403                     self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0282).emit();
1404                     return;
1405                 }
1406                 let mut err = self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0283);
1407                 err.note(&format!("cannot satisfy `{}`", predicate));
1408                 if let ObligationCauseCode::ItemObligation(def_id) = obligation.cause.code {
1409                     self.suggest_fully_qualified_path(&mut err, def_id, span, trait_ref.def_id());
1410                 } else if let (
1411                     Ok(ref snippet),
1412                     ObligationCauseCode::BindingObligation(ref def_id, _),
1413                 ) =
1414                     (self.tcx.sess.source_map().span_to_snippet(span), &obligation.cause.code)
1415                 {
1416                     let generics = self.tcx.generics_of(*def_id);
1417                     if generics.params.iter().any(|p| p.name.as_str() != "Self")
1418                         && !snippet.ends_with('>')
1419                     {
1420                         // FIXME: To avoid spurious suggestions in functions where type arguments
1421                         // where already supplied, we check the snippet to make sure it doesn't
1422                         // end with a turbofish. Ideally we would have access to a `PathSegment`
1423                         // instead. Otherwise we would produce the following output:
1424                         //
1425                         // error[E0283]: type annotations needed
1426                         //   --> $DIR/issue-54954.rs:3:24
1427                         //    |
1428                         // LL | const ARR_LEN: usize = Tt::const_val::<[i8; 123]>();
1429                         //    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^
1430                         //    |                        |
1431                         //    |                        cannot infer type
1432                         //    |                        help: consider specifying the type argument
1433                         //    |                        in the function call:
1434                         //    |                        `Tt::const_val::<[i8; 123]>::<T>`
1435                         // ...
1436                         // LL |     const fn const_val<T: Sized>() -> usize {
1437                         //    |                        - required by this bound in `Tt::const_val`
1438                         //    |
1439                         //    = note: cannot satisfy `_: Tt`
1440
1441                         err.span_suggestion_verbose(
1442                             span.shrink_to_hi(),
1443                             &format!(
1444                                 "consider specifying the type argument{} in the function call",
1445                                 pluralize!(generics.params.len()),
1446                             ),
1447                             format!(
1448                                 "::<{}>",
1449                                 generics
1450                                     .params
1451                                     .iter()
1452                                     .map(|p| p.name.to_string())
1453                                     .collect::<Vec<String>>()
1454                                     .join(", ")
1455                             ),
1456                             Applicability::HasPlaceholders,
1457                         );
1458                     }
1459                 }
1460                 err
1461             }
1462
1463             ty::Predicate::WellFormed(ty) => {
1464                 // Same hacky approach as above to avoid deluging user
1465                 // with error messages.
1466                 if ty.references_error() || self.tcx.sess.has_errors() {
1467                     return;
1468                 }
1469                 self.need_type_info_err(body_id, span, ty, ErrorCode::E0282)
1470             }
1471
1472             ty::Predicate::Subtype(ref data) => {
1473                 if data.references_error() || self.tcx.sess.has_errors() {
1474                     // no need to overload user in such cases
1475                     return;
1476                 }
1477                 let &SubtypePredicate { a_is_expected: _, a, b } = data.skip_binder();
1478                 // both must be type variables, or the other would've been instantiated
1479                 assert!(a.is_ty_var() && b.is_ty_var());
1480                 self.need_type_info_err(body_id, span, a, ErrorCode::E0282)
1481             }
1482             ty::Predicate::Projection(ref data) => {
1483                 let trait_ref = data.to_poly_trait_ref(self.tcx);
1484                 let self_ty = trait_ref.self_ty();
1485                 let ty = data.skip_binder().ty;
1486                 if predicate.references_error() {
1487                     return;
1488                 }
1489                 if self_ty.needs_infer() && ty.needs_infer() {
1490                     // We do this for the `foo.collect()?` case to produce a suggestion.
1491                     let mut err = self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0284);
1492                     err.note(&format!("cannot satisfy `{}`", predicate));
1493                     err
1494                 } else {
1495                     let mut err = struct_span_err!(
1496                         self.tcx.sess,
1497                         span,
1498                         E0284,
1499                         "type annotations needed: cannot satisfy `{}`",
1500                         predicate,
1501                     );
1502                     err.span_label(span, &format!("cannot satisfy `{}`", predicate));
1503                     err
1504                 }
1505             }
1506
1507             _ => {
1508                 if self.tcx.sess.has_errors() {
1509                     return;
1510                 }
1511                 let mut err = struct_span_err!(
1512                     self.tcx.sess,
1513                     span,
1514                     E0284,
1515                     "type annotations needed: cannot satisfy `{}`",
1516                     predicate,
1517                 );
1518                 err.span_label(span, &format!("cannot satisfy `{}`", predicate));
1519                 err
1520             }
1521         };
1522         self.note_obligation_cause(&mut err, obligation);
1523         err.emit();
1524     }
1525
1526     /// Returns `true` if the trait predicate may apply for *some* assignment
1527     /// to the type parameters.
1528     fn predicate_can_apply(
1529         &self,
1530         param_env: ty::ParamEnv<'tcx>,
1531         pred: ty::PolyTraitRef<'tcx>,
1532     ) -> bool {
1533         struct ParamToVarFolder<'a, 'tcx> {
1534             infcx: &'a InferCtxt<'a, 'tcx>,
1535             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
1536         }
1537
1538         impl<'a, 'tcx> TypeFolder<'tcx> for ParamToVarFolder<'a, 'tcx> {
1539             fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
1540                 self.infcx.tcx
1541             }
1542
1543             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
1544                 if let ty::Param(ty::ParamTy { name, .. }) = ty.kind {
1545                     let infcx = self.infcx;
1546                     self.var_map.entry(ty).or_insert_with(|| {
1547                         infcx.next_ty_var(TypeVariableOrigin {
1548                             kind: TypeVariableOriginKind::TypeParameterDefinition(name, None),
1549                             span: DUMMY_SP,
1550                         })
1551                     })
1552                 } else {
1553                     ty.super_fold_with(self)
1554                 }
1555             }
1556         }
1557
1558         self.probe(|_| {
1559             let mut selcx = SelectionContext::new(self);
1560
1561             let cleaned_pred =
1562                 pred.fold_with(&mut ParamToVarFolder { infcx: self, var_map: Default::default() });
1563
1564             let cleaned_pred = super::project::normalize(
1565                 &mut selcx,
1566                 param_env,
1567                 ObligationCause::dummy(),
1568                 &cleaned_pred,
1569             )
1570             .value;
1571
1572             let obligation = Obligation::new(
1573                 ObligationCause::dummy(),
1574                 param_env,
1575                 cleaned_pred.without_const().to_predicate(),
1576             );
1577
1578             self.predicate_may_hold(&obligation)
1579         })
1580     }
1581
1582     fn note_obligation_cause(
1583         &self,
1584         err: &mut DiagnosticBuilder<'_>,
1585         obligation: &PredicateObligation<'tcx>,
1586     ) {
1587         // First, attempt to add note to this error with an async-await-specific
1588         // message, and fall back to regular note otherwise.
1589         if !self.maybe_note_obligation_cause_for_async_await(err, obligation) {
1590             self.note_obligation_cause_code(
1591                 err,
1592                 &obligation.predicate,
1593                 &obligation.cause.code,
1594                 &mut vec![],
1595             );
1596             self.suggest_unsized_bound_if_applicable(err, obligation);
1597         }
1598     }
1599
1600     fn suggest_unsized_bound_if_applicable(
1601         &self,
1602         err: &mut DiagnosticBuilder<'_>,
1603         obligation: &PredicateObligation<'tcx>,
1604     ) {
1605         if let (
1606             ty::Predicate::Trait(pred, _),
1607             ObligationCauseCode::BindingObligation(item_def_id, span),
1608         ) = (&obligation.predicate, &obligation.cause.code)
1609         {
1610             if let (Some(generics), true) = (
1611                 self.tcx.hir().get_if_local(*item_def_id).as_ref().and_then(|n| n.generics()),
1612                 Some(pred.def_id()) == self.tcx.lang_items().sized_trait(),
1613             ) {
1614                 for param in generics.params {
1615                     if param.span == *span
1616                         && !param.bounds.iter().any(|bound| {
1617                             bound.trait_ref().and_then(|trait_ref| trait_ref.trait_def_id())
1618                                 == self.tcx.lang_items().sized_trait()
1619                         })
1620                     {
1621                         let (span, separator) = match param.bounds {
1622                             [] => (span.shrink_to_hi(), ":"),
1623                             [.., bound] => (bound.span().shrink_to_hi(), " + "),
1624                         };
1625                         err.span_suggestion_verbose(
1626                             span,
1627                             "consider relaxing the implicit `Sized` restriction",
1628                             format!("{} ?Sized", separator),
1629                             Applicability::MachineApplicable,
1630                         );
1631                         return;
1632                     }
1633                 }
1634             }
1635         }
1636     }
1637
1638     fn is_recursive_obligation(
1639         &self,
1640         obligated_types: &mut Vec<&ty::TyS<'tcx>>,
1641         cause_code: &ObligationCauseCode<'tcx>,
1642     ) -> bool {
1643         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
1644             let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
1645
1646             if obligated_types.iter().any(|ot| ot == &parent_trait_ref.skip_binder().self_ty()) {
1647                 return true;
1648             }
1649         }
1650         false
1651     }
1652 }
1653
1654 pub fn recursive_type_with_infinite_size_error(
1655     tcx: TyCtxt<'tcx>,
1656     type_def_id: DefId,
1657 ) -> DiagnosticBuilder<'tcx> {
1658     assert!(type_def_id.is_local());
1659     let span = tcx.hir().span_if_local(type_def_id).unwrap();
1660     let span = tcx.sess.source_map().guess_head_span(span);
1661     let mut err = struct_span_err!(
1662         tcx.sess,
1663         span,
1664         E0072,
1665         "recursive type `{}` has infinite size",
1666         tcx.def_path_str(type_def_id)
1667     );
1668     err.span_label(span, "recursive type has infinite size");
1669     err.help(&format!(
1670         "insert indirection (e.g., a `Box`, `Rc`, or `&`) \
1671                            at some point to make `{}` representable",
1672         tcx.def_path_str(type_def_id)
1673     ));
1674     err
1675 }
1676
1677 /// Summarizes information
1678 #[derive(Clone)]
1679 pub enum ArgKind {
1680     /// An argument of non-tuple type. Parameters are (name, ty)
1681     Arg(String, String),
1682
1683     /// An argument of tuple type. For a "found" argument, the span is
1684     /// the locationo in the source of the pattern. For a "expected"
1685     /// argument, it will be None. The vector is a list of (name, ty)
1686     /// strings for the components of the tuple.
1687     Tuple(Option<Span>, Vec<(String, String)>),
1688 }
1689
1690 impl ArgKind {
1691     fn empty() -> ArgKind {
1692         ArgKind::Arg("_".to_owned(), "_".to_owned())
1693     }
1694
1695     /// Creates an `ArgKind` from the expected type of an
1696     /// argument. It has no name (`_`) and an optional source span.
1697     pub fn from_expected_ty(t: Ty<'_>, span: Option<Span>) -> ArgKind {
1698         match t.kind {
1699             ty::Tuple(ref tys) => ArgKind::Tuple(
1700                 span,
1701                 tys.iter().map(|ty| ("_".to_owned(), ty.to_string())).collect::<Vec<_>>(),
1702             ),
1703             _ => ArgKind::Arg("_".to_owned(), t.to_string()),
1704         }
1705     }
1706 }
1707
1708 /// Suggest restricting a type param with a new bound.
1709 pub fn suggest_constraining_type_param(
1710     tcx: TyCtxt<'_>,
1711     generics: &hir::Generics<'_>,
1712     err: &mut DiagnosticBuilder<'_>,
1713     param_name: &str,
1714     constraint: &str,
1715     def_id: Option<DefId>,
1716 ) -> bool {
1717     let param = generics.params.iter().find(|p| p.name.ident().as_str() == param_name);
1718
1719     let param = if let Some(param) = param {
1720         param
1721     } else {
1722         return false;
1723     };
1724
1725     const MSG_RESTRICT_BOUND_FURTHER: &str = "consider further restricting this bound";
1726     let msg_restrict_type = format!("consider restricting type parameter `{}`", param_name);
1727     let msg_restrict_type_further =
1728         format!("consider further restricting type parameter `{}`", param_name);
1729
1730     if def_id == tcx.lang_items().sized_trait() {
1731         // Type parameters are already `Sized` by default.
1732         err.span_label(param.span, &format!("this type parameter needs to be `{}`", constraint));
1733         return true;
1734     }
1735     let mut suggest_restrict = |span| {
1736         err.span_suggestion_verbose(
1737             span,
1738             MSG_RESTRICT_BOUND_FURTHER,
1739             format!(" + {}", constraint),
1740             Applicability::MachineApplicable,
1741         );
1742     };
1743
1744     if param_name.starts_with("impl ") {
1745         // If there's an `impl Trait` used in argument position, suggest
1746         // restricting it:
1747         //
1748         //   fn foo(t: impl Foo) { ... }
1749         //             --------
1750         //             |
1751         //             help: consider further restricting this bound with `+ Bar`
1752         //
1753         // Suggestion for tools in this case is:
1754         //
1755         //   fn foo(t: impl Foo) { ... }
1756         //             --------
1757         //             |
1758         //             replace with: `impl Foo + Bar`
1759
1760         suggest_restrict(param.span.shrink_to_hi());
1761         return true;
1762     }
1763
1764     if generics.where_clause.predicates.is_empty()
1765         // Given `trait Base<T = String>: Super<T>` where `T: Copy`, suggest restricting in the
1766         // `where` clause instead of `trait Base<T: Copy = String>: Super<T>`.
1767         && !matches!(param.kind, hir::GenericParamKind::Type { default: Some(_), .. })
1768     {
1769         if let Some(bounds_span) = param.bounds_span() {
1770             // If user has provided some bounds, suggest restricting them:
1771             //
1772             //   fn foo<T: Foo>(t: T) { ... }
1773             //             ---
1774             //             |
1775             //             help: consider further restricting this bound with `+ Bar`
1776             //
1777             // Suggestion for tools in this case is:
1778             //
1779             //   fn foo<T: Foo>(t: T) { ... }
1780             //          --
1781             //          |
1782             //          replace with: `T: Bar +`
1783             suggest_restrict(bounds_span.shrink_to_hi());
1784         } else {
1785             // If user hasn't provided any bounds, suggest adding a new one:
1786             //
1787             //   fn foo<T>(t: T) { ... }
1788             //          - help: consider restricting this type parameter with `T: Foo`
1789             err.span_suggestion_verbose(
1790                 param.span.shrink_to_hi(),
1791                 &msg_restrict_type,
1792                 format!(": {}", constraint),
1793                 Applicability::MachineApplicable,
1794             );
1795         }
1796
1797         true
1798     } else {
1799         // This part is a bit tricky, because using the `where` clause user can
1800         // provide zero, one or many bounds for the same type parameter, so we
1801         // have following cases to consider:
1802         //
1803         // 1) When the type parameter has been provided zero bounds
1804         //
1805         //    Message:
1806         //      fn foo<X, Y>(x: X, y: Y) where Y: Foo { ... }
1807         //             - help: consider restricting this type parameter with `where X: Bar`
1808         //
1809         //    Suggestion:
1810         //      fn foo<X, Y>(x: X, y: Y) where Y: Foo { ... }
1811         //                                           - insert: `, X: Bar`
1812         //
1813         //
1814         // 2) When the type parameter has been provided one bound
1815         //
1816         //    Message:
1817         //      fn foo<T>(t: T) where T: Foo { ... }
1818         //                            ^^^^^^
1819         //                            |
1820         //                            help: consider further restricting this bound with `+ Bar`
1821         //
1822         //    Suggestion:
1823         //      fn foo<T>(t: T) where T: Foo { ... }
1824         //                            ^^
1825         //                            |
1826         //                            replace with: `T: Bar +`
1827         //
1828         //
1829         // 3) When the type parameter has been provided many bounds
1830         //
1831         //    Message:
1832         //      fn foo<T>(t: T) where T: Foo, T: Bar {... }
1833         //             - help: consider further restricting this type parameter with `where T: Zar`
1834         //
1835         //    Suggestion:
1836         //      fn foo<T>(t: T) where T: Foo, T: Bar {... }
1837         //                                          - insert: `, T: Zar`
1838
1839         let mut param_spans = Vec::new();
1840
1841         for predicate in generics.where_clause.predicates {
1842             if let WherePredicate::BoundPredicate(WhereBoundPredicate {
1843                 span, bounded_ty, ..
1844             }) = predicate
1845             {
1846                 if let TyKind::Path(QPath::Resolved(_, path)) = &bounded_ty.kind {
1847                     if let Some(segment) = path.segments.first() {
1848                         if segment.ident.to_string() == param_name {
1849                             param_spans.push(span);
1850                         }
1851                     }
1852                 }
1853             }
1854         }
1855
1856         let where_clause_span = generics.where_clause.span_for_predicates_or_empty_place();
1857         // Account for `fn foo<T>(t: T) where T: Foo,` so we don't suggest two trailing commas.
1858         let mut trailing_comma = false;
1859         if let Ok(snippet) = tcx.sess.source_map().span_to_snippet(where_clause_span) {
1860             trailing_comma = snippet.ends_with(',');
1861         }
1862         let where_clause_span = if trailing_comma {
1863             let hi = where_clause_span.hi();
1864             Span::new(hi - BytePos(1), hi, where_clause_span.ctxt())
1865         } else {
1866             where_clause_span.shrink_to_hi()
1867         };
1868
1869         match &param_spans[..] {
1870             &[&param_span] => suggest_restrict(param_span.shrink_to_hi()),
1871             _ => {
1872                 err.span_suggestion_verbose(
1873                     where_clause_span,
1874                     &msg_restrict_type_further,
1875                     format!(", {}: {}", param_name, constraint),
1876                     Applicability::MachineApplicable,
1877                 );
1878             }
1879         }
1880
1881         true
1882     }
1883 }