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::intravisit::Visitor;
  19 use rustc_hir::Node;
  20 use rustc_middle::mir::interpret::ErrorHandled;
  21 use rustc_middle::ty::error::ExpectedFound;
  22 use rustc_middle::ty::fold::TypeFolder;
  23 use rustc_middle::ty::subst::GenericArgKind;
  24 use rustc_middle::ty::{
  25     self, fast_reject, AdtKind, SubtypePredicate, ToPolyTraitRef, ToPredicate, Ty, TyCtxt,
  26     TypeFoldable, WithConstness,
  27 };
  28 use rustc_session::DiagnosticMessageId;
  29 use rustc_span::symbol::{kw, sym};
  30 use rustc_span::{ExpnKind, MultiSpan, Span, DUMMY_SP};
  31 use std::fmt;
  32
  33 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
  34 use crate::traits::query::normalize::AtExt as _;
  35 use on_unimplemented::InferCtxtExt as _;
  36 use suggestions::InferCtxtExt as _;
  37
  38 pub use rustc_infer::traits::error_reporting::*;
  39
  40 pub trait InferCtxtExt<'tcx> {
  41     fn report_fulfillment_errors(
  42         &self,
  43         errors: &[FulfillmentError<'tcx>],
  44         body_id: Option<hir::BodyId>,
  45         fallback_has_occurred: bool,
  46     );
  47
  48     fn report_overflow_error<T>(
  49         &self,
  50         obligation: &Obligation<'tcx, T>,
  51         suggest_increasing_limit: bool,
  52     ) -> !
  53     where
  54         T: fmt::Display + TypeFoldable<'tcx>;
  55
  56     fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> !;
  57
  58     fn report_selection_error(
  59         &self,
  60         obligation: &PredicateObligation<'tcx>,
  61         error: &SelectionError<'tcx>,
  62         fallback_has_occurred: bool,
  63         points_at_arg: bool,
  64     );
  65
  66     /// Given some node representing a fn-like thing in the HIR map,
  67     /// returns a span and `ArgKind` information that describes the
  68     /// arguments it expects. This can be supplied to
  69     /// `report_arg_count_mismatch`.
  70     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)>;
  71
  72     /// Reports an error when the number of arguments needed by a
  73     /// trait match doesn't match the number that the expression
  74     /// provides.
  75     fn report_arg_count_mismatch(
  76         &self,
  77         span: Span,
  78         found_span: Option<Span>,
  79         expected_args: Vec<ArgKind>,
  80         found_args: Vec<ArgKind>,
  81         is_closure: bool,
  82     ) -> DiagnosticBuilder<'tcx>;
  83 }
  84
  85 impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
  86     fn report_fulfillment_errors(
  87         &self,
  88         errors: &[FulfillmentError<'tcx>],
  89         body_id: Option<hir::BodyId>,
  90         fallback_has_occurred: bool,
  91     ) {
  92         #[derive(Debug)]
  93         struct ErrorDescriptor<'tcx> {
  94             predicate: ty::Predicate<'tcx>,
  95             index: Option<usize>, // None if this is an old error
  96         }
  97
  98         let mut error_map: FxHashMap<_, Vec<_>> = self
  99             .reported_trait_errors
 100             .borrow()
 101             .iter()
 102             .map(|(&span, predicates)| {
 103                 (
 104                     span,
 105                     predicates
 106                         .iter()
 107                         .map(|&predicate| ErrorDescriptor { predicate, index: None })
 108                         .collect(),
 109                 )
 110             })
 111             .collect();
 112
 113         for (index, error) in errors.iter().enumerate() {
 114             // We want to ignore desugarings here: spans are equivalent even
 115             // if one is the result of a desugaring and the other is not.
 116             let mut span = error.obligation.cause.span;
 117             let expn_data = span.ctxt().outer_expn_data();
 118             if let ExpnKind::Desugaring(_) = expn_data.kind {
 119                 span = expn_data.call_site;
 120             }
 121
 122             error_map.entry(span).or_default().push(ErrorDescriptor {
 123                 predicate: error.obligation.predicate,
 124                 index: Some(index),
 125             });
 126
 127             self.reported_trait_errors
 128                 .borrow_mut()
 129                 .entry(span)
 130                 .or_default()
 131                 .push(error.obligation.predicate);
 132         }
 133
 134         // We do this in 2 passes because we want to display errors in order, though
 135         // maybe it *is* better to sort errors by span or something.
 136         let mut is_suppressed = vec![false; errors.len()];
 137         for (_, error_set) in error_map.iter() {
 138             // We want to suppress "duplicate" errors with the same span.
 139             for error in error_set {
 140                 if let Some(index) = error.index {
 141                     // Suppress errors that are either:
 142                     // 1) strictly implied by another error.
 143                     // 2) implied by an error with a smaller index.
 144                     for error2 in error_set {
 145                         if error2.index.map_or(false, |index2| is_suppressed[index2]) {
 146                             // Avoid errors being suppressed by already-suppressed
 147                             // errors, to prevent all errors from being suppressed
 148                             // at once.
 149                             continue;
 150                         }
 151
 152                         if self.error_implies(error2.predicate, error.predicate)
 153                             && !(error2.index >= error.index
 154                                 && self.error_implies(error.predicate, error2.predicate))
 155                         {
 156                             info!("skipping {:?} (implied by {:?})", error, error2);
 157                             is_suppressed[index] = true;
 158                             break;
 159                         }
 160                     }
 161                 }
 162             }
 163         }
 164
 165         for (error, suppressed) in errors.iter().zip(is_suppressed) {
 166             if !suppressed {
 167                 self.report_fulfillment_error(error, body_id, fallback_has_occurred);
 168             }
 169         }
 170     }
 171
 172     /// Reports that an overflow has occurred and halts compilation. We
 173     /// halt compilation unconditionally because it is important that
 174     /// overflows never be masked -- they basically represent computations
 175     /// whose result could not be truly determined and thus we can't say
 176     /// if the program type checks or not -- and they are unusual
 177     /// occurrences in any case.
 178     fn report_overflow_error<T>(
 179         &self,
 180         obligation: &Obligation<'tcx, T>,
 181         suggest_increasing_limit: bool,
 182     ) -> !
 183     where
 184         T: fmt::Display + TypeFoldable<'tcx>,
 185     {
 186         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
 187         let mut err = struct_span_err!(
 188             self.tcx.sess,
 189             obligation.cause.span,
 190             E0275,
 191             "overflow evaluating the requirement `{}`",
 192             predicate
 193         );
 194
 195         if suggest_increasing_limit {
 196             self.suggest_new_overflow_limit(&mut err);
 197         }
 198
 199         self.note_obligation_cause_code(
 200             &mut err,
 201             &obligation.predicate,
 202             &obligation.cause.code,
 203             &mut vec![],
 204         );
 205
 206         err.emit();
 207         self.tcx.sess.abort_if_errors();
 208         bug!();
 209     }
 210
 211     /// Reports that a cycle was detected which led to overflow and halts
 212     /// compilation. This is equivalent to `report_overflow_error` except
 213     /// that we can give a more helpful error message (and, in particular,
 214     /// we do not suggest increasing the overflow limit, which is not
 215     /// going to help).
 216     fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 217         let cycle = self.resolve_vars_if_possible(&cycle.to_owned());
 218         assert!(!cycle.is_empty());
 219
 220         debug!("report_overflow_error_cycle: cycle={:?}", cycle);
 221
 222         self.report_overflow_error(&cycle[0], false);
 223     }
 224
 225     fn report_selection_error(
 226         &self,
 227         obligation: &PredicateObligation<'tcx>,
 228         error: &SelectionError<'tcx>,
 229         fallback_has_occurred: bool,
 230         points_at_arg: bool,
 231     ) {
 232         let tcx = self.tcx;
 233         let span = obligation.cause.span;
 234
 235         let mut err = match *error {
 236             SelectionError::Unimplemented => {
 237                 if let ObligationCauseCode::CompareImplMethodObligation {
 238                     item_name,
 239                     impl_item_def_id,
 240                     trait_item_def_id,
 241                 }
 242                 | ObligationCauseCode::CompareImplTypeObligation {
 243                     item_name,
 244                     impl_item_def_id,
 245                     trait_item_def_id,
 246                 } = obligation.cause.code
 247                 {
 248                     self.report_extra_impl_obligation(
 249                         span,
 250                         item_name,
 251                         impl_item_def_id,
 252                         trait_item_def_id,
 253                         &format!("`{}`", obligation.predicate),
 254                     )
 255                     .emit();
 256                     return;
 257                 }
 258                 match obligation.predicate.kind() {
 259                     ty::PredicateKind::Trait(ref trait_predicate, _) => {
 260                         let trait_predicate = self.resolve_vars_if_possible(trait_predicate);
 261
 262                         if self.tcx.sess.has_errors() && trait_predicate.references_error() {
 263                             return;
 264                         }
 265                         let trait_ref = trait_predicate.to_poly_trait_ref();
 266                         let (post_message, pre_message, type_def) = self
 267                             .get_parent_trait_ref(&obligation.cause.code)
 268                             .map(|(t, s)| {
 269                                 (
 270                                     format!(" in `{}`", t),
 271                                     format!("within `{}`, ", t),
 272                                     s.map(|s| (format!("within this `{}`", t), s)),
 273                                 )
 274                             })
 275                             .unwrap_or_default();
 276
 277                         let OnUnimplementedNote { message, label, note, enclosing_scope } =
 278                             self.on_unimplemented_note(trait_ref, obligation);
 279                         let have_alt_message = message.is_some() || label.is_some();
 280                         let is_try = self
 281                             .tcx
 282                             .sess
 283                             .source_map()
 284                             .span_to_snippet(span)
 285                             .map(|s| &s == "?")
 286                             .unwrap_or(false);
 287                         let is_from = self.tcx.get_diagnostic_item(sym::from_trait)
 288                             == Some(trait_ref.def_id());
 289                         let is_unsize =
 290                             { Some(trait_ref.def_id()) == self.tcx.lang_items().unsize_trait() };
 291                         let (message, note) = if is_try && is_from {
 292                             (
 293                                 Some(format!(
 294                                     "`?` couldn't convert the error to `{}`",
 295                                     trait_ref.skip_binder().self_ty(),
 296                                 )),
 297                                 Some(
 298                                     "the question mark operation (`?`) implicitly performs a \
 299                                         conversion on the error value using the `From` trait"
 300                                         .to_owned(),
 301                                 ),
 302                             )
 303                         } else {
 304                             (message, note)
 305                         };
 306
 307                         let mut err = struct_span_err!(
 308                             self.tcx.sess,
 309                             span,
 310                             E0277,
 311                             "{}",
 312                             message.unwrap_or_else(|| format!(
 313                                 "the trait bound `{}` is not satisfied{}",
 314                                 trait_ref.without_const().to_predicate(tcx),
 315                                 post_message,
 316                             ))
 317                         );
 318
 319                         if is_try && is_from {
 320                             let none_error = self
 321                                 .tcx
 322                                 .get_diagnostic_item(sym::none_error)
 323                                 .map(|def_id| tcx.type_of(def_id));
 324                             let should_convert_option_to_result =
 325                                 Some(trait_ref.skip_binder().substs.type_at(1)) == none_error;
 326                             let should_convert_result_to_option =
 327                                 Some(trait_ref.self_ty().skip_binder()) == none_error;
 328                             if should_convert_option_to_result {
 329                                 err.span_suggestion_verbose(
 330                                     span.shrink_to_lo(),
 331                                     "consider converting the `Option<T>` into a `Result<T, _>` \
 332                                      using `Option::ok_or` or `Option::ok_or_else`",
 333                                     ".ok_or_else(|| /* error value */)".to_string(),
 334                                     Applicability::HasPlaceholders,
 335                                 );
 336                             } else if should_convert_result_to_option {
 337                                 err.span_suggestion_verbose(
 338                                     span.shrink_to_lo(),
 339                                     "consider converting the `Result<T, _>` into an `Option<T>` \
 340                                      using `Result::ok`",
 341                                     ".ok()".to_string(),
 342                                     Applicability::MachineApplicable,
 343                                 );
 344                             }
 345                             if let Some(ret_span) = self.return_type_span(obligation) {
 346                                 err.span_label(
 347                                     ret_span,
 348                                     &format!(
 349                                         "expected `{}` because of this",
 350                                         trait_ref.skip_binder().self_ty()
 351                                     ),
 352                                 );
 353                             }
 354                         }
 355
 356                         let explanation =
 357                             if obligation.cause.code == ObligationCauseCode::MainFunctionType {
 358                                 "consider using `()`, or a `Result`".to_owned()
 359                             } else {
 360                                 format!(
 361                                     "{}the trait `{}` is not implemented for `{}`",
 362                                     pre_message,
 363                                     trait_ref.print_only_trait_path(),
 364                                     trait_ref.skip_binder().self_ty(),
 365                                 )
 366                             };
 367
 368                         if self.suggest_add_reference_to_arg(
 369                             &obligation,
 370                             &mut err,
 371                             &trait_ref,
 372                             points_at_arg,
 373                             have_alt_message,
 374                         ) {
 375                             self.note_obligation_cause(&mut err, obligation);
 376                             err.emit();
 377                             return;
 378                         }
 379                         if let Some(ref s) = label {
 380                             // If it has a custom `#[rustc_on_unimplemented]`
 381                             // error message, let's display it as the label!
 382                             err.span_label(span, s.as_str());
 383                             if !matches!(trait_ref.skip_binder().self_ty().kind, ty::Param(_)) {
 384                                 // When the self type is a type param We don't need to "the trait
 385                                 // `std::marker::Sized` is not implemented for `T`" as we will point
 386                                 // at the type param with a label to suggest constraining it.
 387                                 err.help(&explanation);
 388                             }
 389                         } else {
 390                             err.span_label(span, explanation);
 391                         }
 392                         if let Some((msg, span)) = type_def {
 393                             err.span_label(span, &msg);
 394                         }
 395                         if let Some(ref s) = note {
 396                             // If it has a custom `#[rustc_on_unimplemented]` note, let's display it
 397                             err.note(s.as_str());
 398                         }
 399                         if let Some(ref s) = enclosing_scope {
 400                             let enclosing_scope_span = tcx.def_span(
 401                                 tcx.hir()
 402                                     .opt_local_def_id(obligation.cause.body_id)
 403                                     .unwrap_or_else(|| {
 404                                         tcx.hir().body_owner_def_id(hir::BodyId {
 405                                             hir_id: obligation.cause.body_id,
 406                                         })
 407                                     })
 408                                     .to_def_id(),
 409                             );
 410
 411                             err.span_label(enclosing_scope_span, s.as_str());
 412                         }
 413
 414                         self.suggest_dereferences(&obligation, &mut err, &trait_ref, points_at_arg);
 415                         self.suggest_fn_call(&obligation, &mut err, &trait_ref, points_at_arg);
 416                         self.suggest_remove_reference(&obligation, &mut err, &trait_ref);
 417                         self.suggest_semicolon_removal(&obligation, &mut err, span, &trait_ref);
 418                         self.note_version_mismatch(&mut err, &trait_ref);
 419
 420                         if Some(trait_ref.def_id()) == tcx.lang_items().try_trait() {
 421                             self.suggest_await_before_try(&mut err, &obligation, &trait_ref, span);
 422                         }
 423
 424                         if self.suggest_impl_trait(&mut err, span, &obligation, &trait_ref) {
 425                             err.emit();
 426                             return;
 427                         }
 428
 429                         if is_unsize {
 430                             // If the obligation failed due to a missing implementation of the
 431                             // `Unsize` trait, give a pointer to why that might be the case
 432                             err.note(
 433                                 "all implementations of `Unsize` are provided \
 434                                 automatically by the compiler, see \
 435                                 <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> \
 436                                 for more information",
 437                             );
 438                         }
 439
 440                         let is_fn_trait = [
 441                             self.tcx.lang_items().fn_trait(),
 442                             self.tcx.lang_items().fn_mut_trait(),
 443                             self.tcx.lang_items().fn_once_trait(),
 444                         ]
 445                         .contains(&Some(trait_ref.def_id()));
 446                         let is_target_feature_fn =
 447                             if let ty::FnDef(def_id, _) = trait_ref.skip_binder().self_ty().kind {
 448                                 !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
 449                             } else {
 450                                 false
 451                             };
 452                         if is_fn_trait && is_target_feature_fn {
 453                             err.note(
 454                                 "`#[target_feature]` functions do not implement the `Fn` traits",
 455                             );
 456                         }
 457
 458                         // Try to report a help message
 459                         if !trait_ref.has_infer_types_or_consts()
 460                             && self.predicate_can_apply(obligation.param_env, trait_ref)
 461                         {
 462                             // If a where-clause may be useful, remind the
 463                             // user that they can add it.
 464                             //
 465                             // don't display an on-unimplemented note, as
 466                             // these notes will often be of the form
 467                             //     "the type `T` can't be frobnicated"
 468                             // which is somewhat confusing.
 469                             self.suggest_restricting_param_bound(
 470                                 &mut err,
 471                                 trait_ref,
 472                                 obligation.cause.body_id,
 473                             );
 474                         } else {
 475                             if !have_alt_message {
 476                                 // Can't show anything else useful, try to find similar impls.
 477                                 let impl_candidates = self.find_similar_impl_candidates(trait_ref);
 478                                 self.report_similar_impl_candidates(impl_candidates, &mut err);
 479                             }
 480                             // Changing mutability doesn't make a difference to whether we have
 481                             // an `Unsize` impl (Fixes ICE in #71036)
 482                             if !is_unsize {
 483                                 self.suggest_change_mut(
 484                                     &obligation,
 485                                     &mut err,
 486                                     &trait_ref,
 487                                     points_at_arg,
 488                                 );
 489                             }
 490                         }
 491
 492                         // If this error is due to `!: Trait` not implemented but `(): Trait` is
 493                         // implemented, and fallback has occurred, then it could be due to a
 494                         // variable that used to fallback to `()` now falling back to `!`. Issue a
 495                         // note informing about the change in behaviour.
 496                         if trait_predicate.skip_binder().self_ty().is_never()
 497                             && fallback_has_occurred
 498                         {
 499                             let predicate = trait_predicate.map_bound(|mut trait_pred| {
 500                                 trait_pred.trait_ref.substs = self.tcx.mk_substs_trait(
 501                                     self.tcx.mk_unit(),
 502                                     &trait_pred.trait_ref.substs[1..],
 503                                 );
 504                                 trait_pred
 505                             });
 506                             let unit_obligation = Obligation {
 507                                 predicate: ty::PredicateKind::Trait(
 508                                     predicate,
 509                                     hir::Constness::NotConst,
 510                                 )
 511                                 .to_predicate(self.tcx),
 512                                 ..obligation.clone()
 513                             };
 514                             if self.predicate_may_hold(&unit_obligation) {
 515                                 err.note(
 516                                     "the trait is implemented for `()`. \
 517                                      Possibly this error has been caused by changes to \
 518                                      Rust's type-inference algorithm (see issue #48950 \
 519                                      <https://github.com/rust-lang/rust/issues/48950> \
 520                                      for more information). Consider whether you meant to use \
 521                                      the type `()` here instead.",
 522                                 );
 523                             }
 524                         }
 525
 526                         err
 527                     }
 528
 529                     ty::PredicateKind::Subtype(ref predicate) => {
 530                         // Errors for Subtype predicates show up as
 531                         // `FulfillmentErrorCode::CodeSubtypeError`,
 532                         // not selection error.
 533                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
 534                     }
 535
 536                     ty::PredicateKind::RegionOutlives(ref predicate) => {
 537                         let predicate = self.resolve_vars_if_possible(predicate);
 538                         let err = self
 539                             .region_outlives_predicate(&obligation.cause, predicate)
 540                             .err()
 541                             .unwrap();
 542                         struct_span_err!(
 543                             self.tcx.sess,
 544                             span,
 545                             E0279,
 546                             "the requirement `{}` is not satisfied (`{}`)",
 547                             predicate,
 548                             err,
 549                         )
 550                     }
 551
 552                     ty::PredicateKind::Projection(..) | ty::PredicateKind::TypeOutlives(..) => {
 553                         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
 554                         struct_span_err!(
 555                             self.tcx.sess,
 556                             span,
 557                             E0280,
 558                             "the requirement `{}` is not satisfied",
 559                             predicate
 560                         )
 561                     }
 562
 563                     &ty::PredicateKind::ObjectSafe(trait_def_id) => {
 564                         let violations = self.tcx.object_safety_violations(trait_def_id);
 565                         report_object_safety_error(self.tcx, span, trait_def_id, violations)
 566                     }
 567
 568                     &ty::PredicateKind::ClosureKind(closure_def_id, closure_substs, kind) => {
 569                         let found_kind = self.closure_kind(closure_substs).unwrap();
 570                         let closure_span =
 571                             self.tcx.sess.source_map().guess_head_span(
 572                                 self.tcx.hir().span_if_local(closure_def_id).unwrap(),
 573                             );
 574                         let hir_id = self.tcx.hir().as_local_hir_id(closure_def_id.expect_local());
 575                         let mut err = struct_span_err!(
 576                             self.tcx.sess,
 577                             closure_span,
 578                             E0525,
 579                             "expected a closure that implements the `{}` trait, \
 580                              but this closure only implements `{}`",
 581                             kind,
 582                             found_kind
 583                         );
 584
 585                         err.span_label(
 586                             closure_span,
 587                             format!("this closure implements `{}`, not `{}`", found_kind, kind),
 588                         );
 589                         err.span_label(
 590                             obligation.cause.span,
 591                             format!("the requirement to implement `{}` derives from here", kind),
 592                         );
 593
 594                         // Additional context information explaining why the closure only implements
 595                         // a particular trait.
 596                         if let Some(typeck_results) = self.in_progress_typeck_results {
 597                             let typeck_results = typeck_results.borrow();
 598                             match (found_kind, typeck_results.closure_kind_origins().get(hir_id)) {
 599                                 (ty::ClosureKind::FnOnce, Some((span, name))) => {
 600                                     err.span_label(
 601                                         *span,
 602                                         format!(
 603                                             "closure is `FnOnce` because it moves the \
 604                                          variable `{}` out of its environment",
 605                                             name
 606                                         ),
 607                                     );
 608                                 }
 609                                 (ty::ClosureKind::FnMut, Some((span, name))) => {
 610                                     err.span_label(
 611                                         *span,
 612                                         format!(
 613                                             "closure is `FnMut` because it mutates the \
 614                                          variable `{}` here",
 615                                             name
 616                                         ),
 617                                     );
 618                                 }
 619                                 _ => {}
 620                             }
 621                         }
 622
 623                         err.emit();
 624                         return;
 625                     }
 626
 627                     ty::PredicateKind::WellFormed(ty) => {
 628                         if !self.tcx.sess.opts.debugging_opts.chalk {
 629                             // WF predicates cannot themselves make
 630                             // errors. They can only block due to
 631                             // ambiguity; otherwise, they always
 632                             // degenerate into other obligations
 633                             // (which may fail).
 634                             span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 635                         } else {
 636                             // FIXME: we'll need a better message which takes into account
 637                             // which bounds actually failed to hold.
 638                             self.tcx.sess.struct_span_err(
 639                                 span,
 640                                 &format!("the type `{}` is not well-formed (chalk)", ty),
 641                             )
 642                         }
 643                     }
 644
 645                     ty::PredicateKind::ConstEvaluatable(..) => {
 646                         // Errors for `ConstEvaluatable` predicates show up as
 647                         // `SelectionError::ConstEvalFailure`,
 648                         // not `Unimplemented`.
 649                         span_bug!(
 650                             span,
 651                             "const-evaluatable requirement gave wrong error: `{:?}`",
 652                             obligation
 653                         )
 654                     }
 655
 656                     ty::PredicateKind::ConstEquate(..) => {
 657                         // Errors for `ConstEquate` predicates show up as
 658                         // `SelectionError::ConstEvalFailure`,
 659                         // not `Unimplemented`.
 660                         span_bug!(
 661                             span,
 662                             "const-equate requirement gave wrong error: `{:?}`",
 663                             obligation
 664                         )
 665                     }
 666                 }
 667             }
 668
 669             OutputTypeParameterMismatch(ref found_trait_ref, ref expected_trait_ref, _) => {
 670                 let found_trait_ref = self.resolve_vars_if_possible(&*found_trait_ref);
 671                 let expected_trait_ref = self.resolve_vars_if_possible(&*expected_trait_ref);
 672
 673                 if expected_trait_ref.self_ty().references_error() {
 674                     return;
 675                 }
 676
 677                 let found_trait_ty = match found_trait_ref.self_ty().no_bound_vars() {
 678                     Some(ty) => ty,
 679                     None => return,
 680                 };
 681
 682                 let found_did = match found_trait_ty.kind {
 683                     ty::Closure(did, _) | ty::Foreign(did) | ty::FnDef(did, _) => Some(did),
 684                     ty::Adt(def, _) => Some(def.did),
 685                     _ => None,
 686                 };
 687
 688                 let found_span = found_did
 689                     .and_then(|did| self.tcx.hir().span_if_local(did))
 690                     .map(|sp| self.tcx.sess.source_map().guess_head_span(sp)); // the sp could be an fn def
 691
 692                 if self.reported_closure_mismatch.borrow().contains(&(span, found_span)) {
 693                     // We check closures twice, with obligations flowing in different directions,
 694                     // but we want to complain about them only once.
 695                     return;
 696                 }
 697
 698                 self.reported_closure_mismatch.borrow_mut().insert((span, found_span));
 699
 700                 let found = match found_trait_ref.skip_binder().substs.type_at(1).kind {
 701                     ty::Tuple(ref tys) => vec![ArgKind::empty(); tys.len()],
 702                     _ => vec![ArgKind::empty()],
 703                 };
 704
 705                 let expected_ty = expected_trait_ref.skip_binder().substs.type_at(1);
 706                 let expected = match expected_ty.kind {
 707                     ty::Tuple(ref tys) => tys
 708                         .iter()
 709                         .map(|t| ArgKind::from_expected_ty(t.expect_ty(), Some(span)))
 710                         .collect(),
 711                     _ => vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())],
 712                 };
 713
 714                 if found.len() == expected.len() {
 715                     self.report_closure_arg_mismatch(
 716                         span,
 717                         found_span,
 718                         found_trait_ref,
 719                         expected_trait_ref,
 720                     )
 721                 } else {
 722                     let (closure_span, found) = found_did
 723                         .and_then(|did| {
 724                             let node = self.tcx.hir().get_if_local(did)?;
 725                             let (found_span, found) = self.get_fn_like_arguments(node)?;
 726                             Some((Some(found_span), found))
 727                         })
 728                         .unwrap_or((found_span, found));
 729
 730                     self.report_arg_count_mismatch(
 731                         span,
 732                         closure_span,
 733                         expected,
 734                         found,
 735                         found_trait_ty.is_closure(),
 736                     )
 737                 }
 738             }
 739
 740             TraitNotObjectSafe(did) => {
 741                 let violations = self.tcx.object_safety_violations(did);
 742                 report_object_safety_error(self.tcx, span, did, violations)
 743             }
 744
 745             ConstEvalFailure(ErrorHandled::TooGeneric) => {
 746                 // In this instance, we have a const expression containing an unevaluated
 747                 // generic parameter. We have no idea whether this expression is valid or
 748                 // not (e.g. it might result in an error), but we don't want to just assume
 749                 // that it's okay, because that might result in post-monomorphisation time
 750                 // errors. The onus is really on the caller to provide values that it can
 751                 // prove are well-formed.
 752                 let mut err = self
 753                     .tcx
 754                     .sess
 755                     .struct_span_err(span, "constant expression depends on a generic parameter");
 756                 // FIXME(const_generics): we should suggest to the user how they can resolve this
 757                 // issue. However, this is currently not actually possible
 758                 // (see https://github.com/rust-lang/rust/issues/66962#issuecomment-575907083).
 759                 err.note("this may fail depending on what value the parameter takes");
 760                 err
 761             }
 762
 763             // Already reported in the query.
 764             ConstEvalFailure(ErrorHandled::Reported(ErrorReported)) => {
 765                 // FIXME(eddyb) remove this once `ErrorReported` becomes a proof token.
 766                 self.tcx.sess.delay_span_bug(span, "`ErrorReported` without an error");
 767                 return;
 768             }
 769
 770             // Already reported in the query, but only as a lint.
 771             // This shouldn't actually happen for constants used in types, modulo
 772             // bugs. The `delay_span_bug` here ensures it won't be ignored.
 773             ConstEvalFailure(ErrorHandled::Linted) => {
 774                 self.tcx.sess.delay_span_bug(span, "constant in type had error reported as lint");
 775                 return;
 776             }
 777
 778             Overflow => {
 779                 bug!("overflow should be handled before the `report_selection_error` path");
 780             }
 781         };
 782
 783         self.note_obligation_cause(&mut err, obligation);
 784         self.point_at_returns_when_relevant(&mut err, &obligation);
 785
 786         err.emit();
 787     }
 788
 789     /// Given some node representing a fn-like thing in the HIR map,
 790     /// returns a span and `ArgKind` information that describes the
 791     /// arguments it expects. This can be supplied to
 792     /// `report_arg_count_mismatch`.
 793     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)> {
 794         let sm = self.tcx.sess.source_map();
 795         let hir = self.tcx.hir();
 796         Some(match node {
 797             Node::Expr(&hir::Expr {
 798                 kind: hir::ExprKind::Closure(_, ref _decl, id, span, _),
 799                 ..
 800             }) => (
 801                 sm.guess_head_span(span),
 802                 hir.body(id)
 803                     .params
 804                     .iter()
 805                     .map(|arg| {
 806                         if let hir::Pat { kind: hir::PatKind::Tuple(ref args, _), span, .. } =
 807                             *arg.pat
 808                         {
 809                             Some(ArgKind::Tuple(
 810                                 Some(span),
 811                                 args.iter()
 812                                     .map(|pat| {
 813                                         sm.span_to_snippet(pat.span)
 814                                             .ok()
 815                                             .map(|snippet| (snippet, "_".to_owned()))
 816                                     })
 817                                     .collect::<Option<Vec<_>>>()?,
 818                             ))
 819                         } else {
 820                             let name = sm.span_to_snippet(arg.pat.span).ok()?;
 821                             Some(ArgKind::Arg(name, "_".to_owned()))
 822                         }
 823                     })
 824                     .collect::<Option<Vec<ArgKind>>>()?,
 825             ),
 826             Node::Item(&hir::Item { span, kind: hir::ItemKind::Fn(ref sig, ..), .. })
 827             | Node::ImplItem(&hir::ImplItem {
 828                 span,
 829                 kind: hir::ImplItemKind::Fn(ref sig, _),
 830                 ..
 831             })
 832             | Node::TraitItem(&hir::TraitItem {
 833                 span,
 834                 kind: hir::TraitItemKind::Fn(ref sig, _),
 835                 ..
 836             }) => (
 837                 sm.guess_head_span(span),
 838                 sig.decl
 839                     .inputs
 840                     .iter()
 841                     .map(|arg| match arg.clone().kind {
 842                         hir::TyKind::Tup(ref tys) => ArgKind::Tuple(
 843                             Some(arg.span),
 844                             vec![("_".to_owned(), "_".to_owned()); tys.len()],
 845                         ),
 846                         _ => ArgKind::empty(),
 847                     })
 848                     .collect::<Vec<ArgKind>>(),
 849             ),
 850             Node::Ctor(ref variant_data) => {
 851                 let span = variant_data.ctor_hir_id().map(|id| hir.span(id)).unwrap_or(DUMMY_SP);
 852                 let span = sm.guess_head_span(span);
 853                 (span, vec![ArgKind::empty(); variant_data.fields().len()])
 854             }
 855             _ => panic!("non-FnLike node found: {:?}", node),
 856         })
 857     }
 858
 859     /// Reports an error when the number of arguments needed by a
 860     /// trait match doesn't match the number that the expression
 861     /// provides.
 862     fn report_arg_count_mismatch(
 863         &self,
 864         span: Span,
 865         found_span: Option<Span>,
 866         expected_args: Vec<ArgKind>,
 867         found_args: Vec<ArgKind>,
 868         is_closure: bool,
 869     ) -> DiagnosticBuilder<'tcx> {
 870         let kind = if is_closure { "closure" } else { "function" };
 871
 872         let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
 873             let arg_length = arguments.len();
 874             let distinct = match &other[..] {
 875                 &[ArgKind::Tuple(..)] => true,
 876                 _ => false,
 877             };
 878             match (arg_length, arguments.get(0)) {
 879                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
 880                     format!("a single {}-tuple as argument", fields.len())
 881                 }
 882                 _ => format!(
 883                     "{} {}argument{}",
 884                     arg_length,
 885                     if distinct && arg_length > 1 { "distinct " } else { "" },
 886                     pluralize!(arg_length)
 887                 ),
 888             }
 889         };
 890
 891         let expected_str = args_str(&expected_args, &found_args);
 892         let found_str = args_str(&found_args, &expected_args);
 893
 894         let mut err = struct_span_err!(
 895             self.tcx.sess,
 896             span,
 897             E0593,
 898             "{} is expected to take {}, but it takes {}",
 899             kind,
 900             expected_str,
 901             found_str,
 902         );
 903
 904         err.span_label(span, format!("expected {} that takes {}", kind, expected_str));
 905
 906         if let Some(found_span) = found_span {
 907             err.span_label(found_span, format!("takes {}", found_str));
 908
 909             // move |_| { ... }
 910             // ^^^^^^^^-- def_span
 911             //
 912             // move |_| { ... }
 913             // ^^^^^-- prefix
 914             let prefix_span = self.tcx.sess.source_map().span_until_non_whitespace(found_span);
 915             // move |_| { ... }
 916             //      ^^^-- pipe_span
 917             let pipe_span =
 918                 if let Some(span) = found_span.trim_start(prefix_span) { span } else { found_span };
 919
 920             // Suggest to take and ignore the arguments with expected_args_length `_`s if
 921             // found arguments is empty (assume the user just wants to ignore args in this case).
 922             // For example, if `expected_args_length` is 2, suggest `|_, _|`.
 923             if found_args.is_empty() && is_closure {
 924                 let underscores = vec!["_"; expected_args.len()].join(", ");
 925                 err.span_suggestion_verbose(
 926                     pipe_span,
 927                     &format!(
 928                         "consider changing the closure to take and ignore the expected argument{}",
 929                         pluralize!(expected_args.len())
 930                     ),
 931                     format!("|{}|", underscores),
 932                     Applicability::MachineApplicable,
 933                 );
 934             }
 935
 936             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
 937                 if fields.len() == expected_args.len() {
 938                     let sugg = fields
 939                         .iter()
 940                         .map(|(name, _)| name.to_owned())
 941                         .collect::<Vec<String>>()
 942                         .join(", ");
 943                     err.span_suggestion_verbose(
 944                         found_span,
 945                         "change the closure to take multiple arguments instead of a single tuple",
 946                         format!("|{}|", sugg),
 947                         Applicability::MachineApplicable,
 948                     );
 949                 }
 950             }
 951             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..] {
 952                 if fields.len() == found_args.len() && is_closure {
 953                     let sugg = format!(
 954                         "|({}){}|",
 955                         found_args
 956                             .iter()
 957                             .map(|arg| match arg {
 958                                 ArgKind::Arg(name, _) => name.to_owned(),
 959                                 _ => "_".to_owned(),
 960                             })
 961                             .collect::<Vec<String>>()
 962                             .join(", "),
 963                         // add type annotations if available
 964                         if found_args.iter().any(|arg| match arg {
 965                             ArgKind::Arg(_, ty) => ty != "_",
 966                             _ => false,
 967                         }) {
 968                             format!(
 969                                 ": ({})",
 970                                 fields
 971                                     .iter()
 972                                     .map(|(_, ty)| ty.to_owned())
 973                                     .collect::<Vec<String>>()
 974                                     .join(", ")
 975                             )
 976                         } else {
 977                             String::new()
 978                         },
 979                     );
 980                     err.span_suggestion_verbose(
 981                         found_span,
 982                         "change the closure to accept a tuple instead of individual arguments",
 983                         sugg,
 984                         Applicability::MachineApplicable,
 985                     );
 986                 }
 987             }
 988         }
 989
 990         err
 991     }
 992 }
 993
 994 trait InferCtxtPrivExt<'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
 999     fn report_fulfillment_error(
1000         &self,
1001         error: &FulfillmentError<'tcx>,
1002         body_id: Option<hir::BodyId>,
1003         fallback_has_occurred: bool,
1004     );
1005
1006     fn report_projection_error(
1007         &self,
1008         obligation: &PredicateObligation<'tcx>,
1009         error: &MismatchedProjectionTypes<'tcx>,
1010     );
1011
1012     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool;
1013
1014     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str>;
1015
1016     fn find_similar_impl_candidates(
1017         &self,
1018         trait_ref: ty::PolyTraitRef<'tcx>,
1019     ) -> Vec<ty::TraitRef<'tcx>>;
1020
1021     fn report_similar_impl_candidates(
1022         &self,
1023         impl_candidates: Vec<ty::TraitRef<'tcx>>,
1024         err: &mut DiagnosticBuilder<'_>,
1025     );
1026
1027     /// Gets the parent trait chain start
1028     fn get_parent_trait_ref(
1029         &self,
1030         code: &ObligationCauseCode<'tcx>,
1031     ) -> Option<(String, Option<Span>)>;
1032
1033     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1034     /// with the same path as `trait_ref`, a help message about
1035     /// a probable version mismatch is added to `err`
1036     fn note_version_mismatch(
1037         &self,
1038         err: &mut DiagnosticBuilder<'_>,
1039         trait_ref: &ty::PolyTraitRef<'tcx>,
1040     );
1041
1042     /// Creates a `PredicateObligation` with `new_self_ty` replacing the existing type in the
1043     /// `trait_ref`.
1044     ///
1045     /// For this to work, `new_self_ty` must have no escaping bound variables.
1046     fn mk_trait_obligation_with_new_self_ty(
1047         &self,
1048         param_env: ty::ParamEnv<'tcx>,
1049         trait_ref: &ty::PolyTraitRef<'tcx>,
1050         new_self_ty: Ty<'tcx>,
1051     ) -> PredicateObligation<'tcx>;
1052
1053     fn maybe_report_ambiguity(
1054         &self,
1055         obligation: &PredicateObligation<'tcx>,
1056         body_id: Option<hir::BodyId>,
1057     );
1058
1059     fn predicate_can_apply(
1060         &self,
1061         param_env: ty::ParamEnv<'tcx>,
1062         pred: ty::PolyTraitRef<'tcx>,
1063     ) -> bool;
1064
1065     fn note_obligation_cause(
1066         &self,
1067         err: &mut DiagnosticBuilder<'tcx>,
1068         obligation: &PredicateObligation<'tcx>,
1069     );
1070
1071     fn suggest_unsized_bound_if_applicable(
1072         &self,
1073         err: &mut DiagnosticBuilder<'tcx>,
1074         obligation: &PredicateObligation<'tcx>,
1075     );
1076
1077     fn is_recursive_obligation(
1078         &self,
1079         obligated_types: &mut Vec<&ty::TyS<'tcx>>,
1080         cause_code: &ObligationCauseCode<'tcx>,
1081     ) -> bool;
1082 }
1083
1084 impl<'a, 'tcx> InferCtxtPrivExt<'tcx> for InferCtxt<'a, 'tcx> {
1085     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1086     // `error` occurring implies that `cond` occurs.
1087     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool {
1088         if cond == error {
1089             return true;
1090         }
1091
1092         let (cond, error) = match (cond.kind(), error.kind()) {
1093             (ty::PredicateKind::Trait(..), ty::PredicateKind::Trait(error, _)) => (cond, error),
1094             _ => {
1095                 // FIXME: make this work in other cases too.
1096                 return false;
1097             }
1098         };
1099
1100         for obligation in super::elaborate_predicates(self.tcx, std::iter::once(cond)) {
1101             if let ty::PredicateKind::Trait(implication, _) = obligation.predicate.kind() {
1102                 let error = error.to_poly_trait_ref();
1103                 let implication = implication.to_poly_trait_ref();
1104                 // FIXME: I'm just not taking associated types at all here.
1105                 // Eventually I'll need to implement param-env-aware
1106                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
1107                 let param_env = ty::ParamEnv::empty();
1108                 if self.can_sub(param_env, error, implication).is_ok() {
1109                     debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
1110                     return true;
1111                 }
1112             }
1113         }
1114
1115         false
1116     }
1117
1118     fn report_fulfillment_error(
1119         &self,
1120         error: &FulfillmentError<'tcx>,
1121         body_id: Option<hir::BodyId>,
1122         fallback_has_occurred: bool,
1123     ) {
1124         debug!("report_fulfillment_error({:?})", error);
1125         match error.code {
1126             FulfillmentErrorCode::CodeSelectionError(ref selection_error) => {
1127                 self.report_selection_error(
1128                     &error.obligation,
1129                     selection_error,
1130                     fallback_has_occurred,
1131                     error.points_at_arg_span,
1132                 );
1133             }
1134             FulfillmentErrorCode::CodeProjectionError(ref e) => {
1135                 self.report_projection_error(&error.obligation, e);
1136             }
1137             FulfillmentErrorCode::CodeAmbiguity => {
1138                 self.maybe_report_ambiguity(&error.obligation, body_id);
1139             }
1140             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
1141                 self.report_mismatched_types(
1142                     &error.obligation.cause,
1143                     expected_found.expected,
1144                     expected_found.found,
1145                     err.clone(),
1146                 )
1147                 .emit();
1148             }
1149             FulfillmentErrorCode::CodeConstEquateError(ref expected_found, ref err) => {
1150                 self.report_mismatched_consts(
1151                     &error.obligation.cause,
1152                     expected_found.expected,
1153                     expected_found.found,
1154                     err.clone(),
1155                 )
1156                 .emit();
1157             }
1158         }
1159     }
1160
1161     fn report_projection_error(
1162         &self,
1163         obligation: &PredicateObligation<'tcx>,
1164         error: &MismatchedProjectionTypes<'tcx>,
1165     ) {
1166         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
1167
1168         if predicate.references_error() {
1169             return;
1170         }
1171
1172         self.probe(|_| {
1173             let err_buf;
1174             let mut err = &error.err;
1175             let mut values = None;
1176
1177             // try to find the mismatched types to report the error with.
1178             //
1179             // this can fail if the problem was higher-ranked, in which
1180             // cause I have no idea for a good error message.
1181             if let ty::PredicateKind::Projection(ref data) = predicate.kind() {
1182                 let mut selcx = SelectionContext::new(self);
1183                 let (data, _) = self.replace_bound_vars_with_fresh_vars(
1184                     obligation.cause.span,
1185                     infer::LateBoundRegionConversionTime::HigherRankedType,
1186                     data,
1187                 );
1188                 let mut obligations = vec![];
1189                 let normalized_ty = super::normalize_projection_type(
1190                     &mut selcx,
1191                     obligation.param_env,
1192                     data.projection_ty,
1193                     obligation.cause.clone(),
1194                     0,
1195                     &mut obligations,
1196                 );
1197
1198                 debug!(
1199                     "report_projection_error obligation.cause={:?} obligation.param_env={:?}",
1200                     obligation.cause, obligation.param_env
1201                 );
1202
1203                 debug!(
1204                     "report_projection_error normalized_ty={:?} data.ty={:?}",
1205                     normalized_ty, data.ty
1206                 );
1207
1208                 let is_normalized_ty_expected = match &obligation.cause.code {
1209                     ObligationCauseCode::ItemObligation(_)
1210                     | ObligationCauseCode::BindingObligation(_, _)
1211                     | ObligationCauseCode::ObjectCastObligation(_) => false,
1212                     _ => true,
1213                 };
1214
1215                 if let Err(error) = self.at(&obligation.cause, obligation.param_env).eq_exp(
1216                     is_normalized_ty_expected,
1217                     normalized_ty,
1218                     data.ty,
1219                 ) {
1220                     values = Some(infer::ValuePairs::Types(ExpectedFound::new(
1221                         is_normalized_ty_expected,
1222                         normalized_ty,
1223                         data.ty,
1224                     )));
1225
1226                     err_buf = error;
1227                     err = &err_buf;
1228                 }
1229             }
1230
1231             let msg = format!("type mismatch resolving `{}`", predicate);
1232             let error_id = (DiagnosticMessageId::ErrorId(271), Some(obligation.cause.span), msg);
1233             let fresh = self.tcx.sess.one_time_diagnostics.borrow_mut().insert(error_id);
1234             if fresh {
1235                 let mut diag = struct_span_err!(
1236                     self.tcx.sess,
1237                     obligation.cause.span,
1238                     E0271,
1239                     "type mismatch resolving `{}`",
1240                     predicate
1241                 );
1242                 self.note_type_err(&mut diag, &obligation.cause, None, values, err);
1243                 self.note_obligation_cause(&mut diag, obligation);
1244                 diag.emit();
1245             }
1246         });
1247     }
1248
1249     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
1250         /// returns the fuzzy category of a given type, or None
1251         /// if the type can be equated to any type.
1252         fn type_category(t: Ty<'_>) -> Option<u32> {
1253             match t.kind {
1254                 ty::Bool => Some(0),
1255                 ty::Char => Some(1),
1256                 ty::Str => Some(2),
1257                 ty::Int(..) | ty::Uint(..) | ty::Infer(ty::IntVar(..)) => Some(3),
1258                 ty::Float(..) | ty::Infer(ty::FloatVar(..)) => Some(4),
1259                 ty::Ref(..) | ty::RawPtr(..) => Some(5),
1260                 ty::Array(..) | ty::Slice(..) => Some(6),
1261                 ty::FnDef(..) | ty::FnPtr(..) => Some(7),
1262                 ty::Dynamic(..) => Some(8),
1263                 ty::Closure(..) => Some(9),
1264                 ty::Tuple(..) => Some(10),
1265                 ty::Projection(..) => Some(11),
1266                 ty::Param(..) => Some(12),
1267                 ty::Opaque(..) => Some(13),
1268                 ty::Never => Some(14),
1269                 ty::Adt(adt, ..) => match adt.adt_kind() {
1270                     AdtKind::Struct => Some(15),
1271                     AdtKind::Union => Some(16),
1272                     AdtKind::Enum => Some(17),
1273                 },
1274                 ty::Generator(..) => Some(18),
1275                 ty::Foreign(..) => Some(19),
1276                 ty::GeneratorWitness(..) => Some(20),
1277                 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error(_) => None,
1278             }
1279         }
1280
1281         match (type_category(a), type_category(b)) {
1282             (Some(cat_a), Some(cat_b)) => match (&a.kind, &b.kind) {
1283                 (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => def_a == def_b,
1284                 _ => cat_a == cat_b,
1285             },
1286             // infer and error can be equated to all types
1287             _ => true,
1288         }
1289     }
1290
1291     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str> {
1292         self.tcx.hir().body(body_id).generator_kind.map(|gen_kind| match gen_kind {
1293             hir::GeneratorKind::Gen => "a generator",
1294             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block) => "an async block",
1295             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn) => "an async function",
1296             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Closure) => "an async closure",
1297         })
1298     }
1299
1300     fn find_similar_impl_candidates(
1301         &self,
1302         trait_ref: ty::PolyTraitRef<'tcx>,
1303     ) -> Vec<ty::TraitRef<'tcx>> {
1304         let simp = fast_reject::simplify_type(self.tcx, trait_ref.skip_binder().self_ty(), true);
1305         let all_impls = self.tcx.all_impls(trait_ref.def_id());
1306
1307         match simp {
1308             Some(simp) => all_impls
1309                 .filter_map(|def_id| {
1310                     let imp = self.tcx.impl_trait_ref(def_id).unwrap();
1311                     let imp_simp = fast_reject::simplify_type(self.tcx, imp.self_ty(), true);
1312                     if let Some(imp_simp) = imp_simp {
1313                         if simp != imp_simp {
1314                             return None;
1315                         }
1316                     }
1317                     Some(imp)
1318                 })
1319                 .collect(),
1320             None => all_impls.map(|def_id| self.tcx.impl_trait_ref(def_id).unwrap()).collect(),
1321         }
1322     }
1323
1324     fn report_similar_impl_candidates(
1325         &self,
1326         impl_candidates: Vec<ty::TraitRef<'tcx>>,
1327         err: &mut DiagnosticBuilder<'_>,
1328     ) {
1329         if impl_candidates.is_empty() {
1330             return;
1331         }
1332
1333         let len = impl_candidates.len();
1334         let end = if impl_candidates.len() <= 5 { impl_candidates.len() } else { 4 };
1335
1336         let normalize = |candidate| {
1337             self.tcx.infer_ctxt().enter(|ref infcx| {
1338                 let normalized = infcx
1339                     .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
1340                     .normalize(candidate)
1341                     .ok();
1342                 match normalized {
1343                     Some(normalized) => format!("\n  {:?}", normalized.value),
1344                     None => format!("\n  {:?}", candidate),
1345                 }
1346             })
1347         };
1348
1349         // Sort impl candidates so that ordering is consistent for UI tests.
1350         let mut normalized_impl_candidates =
1351             impl_candidates.iter().map(normalize).collect::<Vec<String>>();
1352
1353         // Sort before taking the `..end` range,
1354         // because the ordering of `impl_candidates` may not be deterministic:
1355         // https://github.com/rust-lang/rust/pull/57475#issuecomment-455519507
1356         normalized_impl_candidates.sort();
1357
1358         err.help(&format!(
1359             "the following implementations were found:{}{}",
1360             normalized_impl_candidates[..end].join(""),
1361             if len > 5 { format!("\nand {} others", len - 4) } else { String::new() }
1362         ));
1363     }
1364
1365     /// Gets the parent trait chain start
1366     fn get_parent_trait_ref(
1367         &self,
1368         code: &ObligationCauseCode<'tcx>,
1369     ) -> Option<(String, Option<Span>)> {
1370         match code {
1371             &ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
1372                 let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
1373                 match self.get_parent_trait_ref(&data.parent_code) {
1374                     Some(t) => Some(t),
1375                     None => {
1376                         let ty = parent_trait_ref.skip_binder().self_ty();
1377                         let span =
1378                             TyCategory::from_ty(ty).map(|(_, def_id)| self.tcx.def_span(def_id));
1379                         Some((ty.to_string(), span))
1380                     }
1381                 }
1382             }
1383             _ => None,
1384         }
1385     }
1386
1387     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1388     /// with the same path as `trait_ref`, a help message about
1389     /// a probable version mismatch is added to `err`
1390     fn note_version_mismatch(
1391         &self,
1392         err: &mut DiagnosticBuilder<'_>,
1393         trait_ref: &ty::PolyTraitRef<'tcx>,
1394     ) {
1395         let get_trait_impl = |trait_def_id| {
1396             let mut trait_impl = None;
1397             self.tcx.for_each_relevant_impl(
1398                 trait_def_id,
1399                 trait_ref.skip_binder().self_ty(),
1400                 |impl_def_id| {
1401                     if trait_impl.is_none() {
1402                         trait_impl = Some(impl_def_id);
1403                     }
1404                 },
1405             );
1406             trait_impl
1407         };
1408         let required_trait_path = self.tcx.def_path_str(trait_ref.def_id());
1409         let all_traits = self.tcx.all_traits(LOCAL_CRATE);
1410         let traits_with_same_path: std::collections::BTreeSet<_> = all_traits
1411             .iter()
1412             .filter(|trait_def_id| **trait_def_id != trait_ref.def_id())
1413             .filter(|trait_def_id| self.tcx.def_path_str(**trait_def_id) == required_trait_path)
1414             .collect();
1415         for trait_with_same_path in traits_with_same_path {
1416             if let Some(impl_def_id) = get_trait_impl(*trait_with_same_path) {
1417                 let impl_span = self.tcx.def_span(impl_def_id);
1418                 err.span_help(impl_span, "trait impl with same name found");
1419                 let trait_crate = self.tcx.crate_name(trait_with_same_path.krate);
1420                 let crate_msg = format!(
1421                     "perhaps two different versions of crate `{}` are being used?",
1422                     trait_crate
1423                 );
1424                 err.note(&crate_msg);
1425             }
1426         }
1427     }
1428
1429     fn mk_trait_obligation_with_new_self_ty(
1430         &self,
1431         param_env: ty::ParamEnv<'tcx>,
1432         trait_ref: &ty::PolyTraitRef<'tcx>,
1433         new_self_ty: Ty<'tcx>,
1434     ) -> PredicateObligation<'tcx> {
1435         assert!(!new_self_ty.has_escaping_bound_vars());
1436
1437         let trait_ref = trait_ref.map_bound_ref(|tr| ty::TraitRef {
1438             substs: self.tcx.mk_substs_trait(new_self_ty, &tr.substs[1..]),
1439             ..*tr
1440         });
1441
1442         Obligation::new(
1443             ObligationCause::dummy(),
1444             param_env,
1445             trait_ref.without_const().to_predicate(self.tcx),
1446         )
1447     }
1448
1449     fn maybe_report_ambiguity(
1450         &self,
1451         obligation: &PredicateObligation<'tcx>,
1452         body_id: Option<hir::BodyId>,
1453     ) {
1454         // Unable to successfully determine, probably means
1455         // insufficient type information, but could mean
1456         // ambiguous impls. The latter *ought* to be a
1457         // coherence violation, so we don't report it here.
1458
1459         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
1460         let span = obligation.cause.span;
1461
1462         debug!(
1463             "maybe_report_ambiguity(predicate={:?}, obligation={:?} body_id={:?}, code={:?})",
1464             predicate, obligation, body_id, obligation.cause.code,
1465         );
1466
1467         // Ambiguity errors are often caused as fallout from earlier
1468         // errors. So just ignore them if this infcx is tainted.
1469         if self.is_tainted_by_errors() {
1470             return;
1471         }
1472
1473         let mut err = match predicate.kind() {
1474             ty::PredicateKind::Trait(ref data, _) => {
1475                 let trait_ref = data.to_poly_trait_ref();
1476                 let self_ty = trait_ref.skip_binder().self_ty();
1477                 debug!("self_ty {:?} {:?} trait_ref {:?}", self_ty, self_ty.kind, trait_ref);
1478
1479                 if predicate.references_error() {
1480                     return;
1481                 }
1482                 // Typically, this ambiguity should only happen if
1483                 // there are unresolved type inference variables
1484                 // (otherwise it would suggest a coherence
1485                 // failure). But given #21974 that is not necessarily
1486                 // the case -- we can have multiple where clauses that
1487                 // are only distinguished by a region, which results
1488                 // in an ambiguity even when all types are fully
1489                 // known, since we don't dispatch based on region
1490                 // relationships.
1491
1492                 // This is kind of a hack: it frequently happens that some earlier
1493                 // error prevents types from being fully inferred, and then we get
1494                 // a bunch of uninteresting errors saying something like "<generic
1495                 // #0> doesn't implement Sized".  It may even be true that we
1496                 // could just skip over all checks where the self-ty is an
1497                 // inference variable, but I was afraid that there might be an
1498                 // inference variable created, registered as an obligation, and
1499                 // then never forced by writeback, and hence by skipping here we'd
1500                 // be ignoring the fact that we don't KNOW the type works
1501                 // out. Though even that would probably be harmless, given that
1502                 // we're only talking about builtin traits, which are known to be
1503                 // inhabited. We used to check for `self.tcx.sess.has_errors()` to
1504                 // avoid inundating the user with unnecessary errors, but we now
1505                 // check upstream for type errors and don't add the obligations to
1506                 // begin with in those cases.
1507                 if self
1508                     .tcx
1509                     .lang_items()
1510                     .sized_trait()
1511                     .map_or(false, |sized_id| sized_id == trait_ref.def_id())
1512                 {
1513                     self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0282).emit();
1514                     return;
1515                 }
1516                 let mut err = self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0283);
1517                 err.note(&format!("cannot satisfy `{}`", predicate));
1518                 if let ObligationCauseCode::ItemObligation(def_id) = obligation.cause.code {
1519                     self.suggest_fully_qualified_path(&mut err, def_id, span, trait_ref.def_id());
1520                 } else if let (
1521                     Ok(ref snippet),
1522                     ObligationCauseCode::BindingObligation(ref def_id, _),
1523                 ) =
1524                     (self.tcx.sess.source_map().span_to_snippet(span), &obligation.cause.code)
1525                 {
1526                     let generics = self.tcx.generics_of(*def_id);
1527                     if generics.params.iter().any(|p| p.name != kw::SelfUpper)
1528                         && !snippet.ends_with('>')
1529                     {
1530                         // FIXME: To avoid spurious suggestions in functions where type arguments
1531                         // where already supplied, we check the snippet to make sure it doesn't
1532                         // end with a turbofish. Ideally we would have access to a `PathSegment`
1533                         // instead. Otherwise we would produce the following output:
1534                         //
1535                         // error[E0283]: type annotations needed
1536                         //   --> $DIR/issue-54954.rs:3:24
1537                         //    |
1538                         // LL | const ARR_LEN: usize = Tt::const_val::<[i8; 123]>();
1539                         //    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^
1540                         //    |                        |
1541                         //    |                        cannot infer type
1542                         //    |                        help: consider specifying the type argument
1543                         //    |                        in the function call:
1544                         //    |                        `Tt::const_val::<[i8; 123]>::<T>`
1545                         // ...
1546                         // LL |     const fn const_val<T: Sized>() -> usize {
1547                         //    |                        - required by this bound in `Tt::const_val`
1548                         //    |
1549                         //    = note: cannot satisfy `_: Tt`
1550
1551                         err.span_suggestion_verbose(
1552                             span.shrink_to_hi(),
1553                             &format!(
1554                                 "consider specifying the type argument{} in the function call",
1555                                 pluralize!(generics.params.len()),
1556                             ),
1557                             format!(
1558                                 "::<{}>",
1559                                 generics
1560                                     .params
1561                                     .iter()
1562                                     .map(|p| p.name.to_string())
1563                                     .collect::<Vec<String>>()
1564                                     .join(", ")
1565                             ),
1566                             Applicability::HasPlaceholders,
1567                         );
1568                     }
1569                 }
1570                 err
1571             }
1572
1573             ty::PredicateKind::WellFormed(arg) => {
1574                 // Same hacky approach as above to avoid deluging user
1575                 // with error messages.
1576                 if arg.references_error() || self.tcx.sess.has_errors() {
1577                     return;
1578                 }
1579
1580                 match arg.unpack() {
1581                     GenericArgKind::Lifetime(lt) => {
1582                         span_bug!(span, "unexpected well formed predicate: {:?}", lt)
1583                     }
1584                     GenericArgKind::Type(ty) => {
1585                         self.need_type_info_err(body_id, span, ty, ErrorCode::E0282)
1586                     }
1587                     GenericArgKind::Const(ct) => {
1588                         self.need_type_info_err_const(body_id, span, ct, ErrorCode::E0282)
1589                     }
1590                 }
1591             }
1592
1593             ty::PredicateKind::Subtype(ref data) => {
1594                 if data.references_error() || self.tcx.sess.has_errors() {
1595                     // no need to overload user in such cases
1596                     return;
1597                 }
1598                 let SubtypePredicate { a_is_expected: _, a, b } = data.skip_binder();
1599                 // both must be type variables, or the other would've been instantiated
1600                 assert!(a.is_ty_var() && b.is_ty_var());
1601                 self.need_type_info_err(body_id, span, a, ErrorCode::E0282)
1602             }
1603             ty::PredicateKind::Projection(ref data) => {
1604                 let trait_ref = data.to_poly_trait_ref(self.tcx);
1605                 let self_ty = trait_ref.skip_binder().self_ty();
1606                 let ty = data.skip_binder().ty;
1607                 if predicate.references_error() {
1608                     return;
1609                 }
1610                 if self_ty.needs_infer() && ty.needs_infer() {
1611                     // We do this for the `foo.collect()?` case to produce a suggestion.
1612                     let mut err = self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0284);
1613                     err.note(&format!("cannot satisfy `{}`", predicate));
1614                     err
1615                 } else {
1616                     let mut err = struct_span_err!(
1617                         self.tcx.sess,
1618                         span,
1619                         E0284,
1620                         "type annotations needed: cannot satisfy `{}`",
1621                         predicate,
1622                     );
1623                     err.span_label(span, &format!("cannot satisfy `{}`", predicate));
1624                     err
1625                 }
1626             }
1627
1628             _ => {
1629                 if self.tcx.sess.has_errors() {
1630                     return;
1631                 }
1632                 let mut err = struct_span_err!(
1633                     self.tcx.sess,
1634                     span,
1635                     E0284,
1636                     "type annotations needed: cannot satisfy `{}`",
1637                     predicate,
1638                 );
1639                 err.span_label(span, &format!("cannot satisfy `{}`", predicate));
1640                 err
1641             }
1642         };
1643         self.note_obligation_cause(&mut err, obligation);
1644         err.emit();
1645     }
1646
1647     /// Returns `true` if the trait predicate may apply for *some* assignment
1648     /// to the type parameters.
1649     fn predicate_can_apply(
1650         &self,
1651         param_env: ty::ParamEnv<'tcx>,
1652         pred: ty::PolyTraitRef<'tcx>,
1653     ) -> bool {
1654         struct ParamToVarFolder<'a, 'tcx> {
1655             infcx: &'a InferCtxt<'a, 'tcx>,
1656             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
1657         }
1658
1659         impl<'a, 'tcx> TypeFolder<'tcx> for ParamToVarFolder<'a, 'tcx> {
1660             fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
1661                 self.infcx.tcx
1662             }
1663
1664             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
1665                 if let ty::Param(ty::ParamTy { name, .. }) = ty.kind {
1666                     let infcx = self.infcx;
1667                     self.var_map.entry(ty).or_insert_with(|| {
1668                         infcx.next_ty_var(TypeVariableOrigin {
1669                             kind: TypeVariableOriginKind::TypeParameterDefinition(name, None),
1670                             span: DUMMY_SP,
1671                         })
1672                     })
1673                 } else {
1674                     ty.super_fold_with(self)
1675                 }
1676             }
1677         }
1678
1679         self.probe(|_| {
1680             let mut selcx = SelectionContext::new(self);
1681
1682             let cleaned_pred =
1683                 pred.fold_with(&mut ParamToVarFolder { infcx: self, var_map: Default::default() });
1684
1685             let cleaned_pred = super::project::normalize(
1686                 &mut selcx,
1687                 param_env,
1688                 ObligationCause::dummy(),
1689                 &cleaned_pred,
1690             )
1691             .value;
1692
1693             let obligation = Obligation::new(
1694                 ObligationCause::dummy(),
1695                 param_env,
1696                 cleaned_pred.without_const().to_predicate(selcx.tcx()),
1697             );
1698
1699             self.predicate_may_hold(&obligation)
1700         })
1701     }
1702
1703     fn note_obligation_cause(
1704         &self,
1705         err: &mut DiagnosticBuilder<'tcx>,
1706         obligation: &PredicateObligation<'tcx>,
1707     ) {
1708         // First, attempt to add note to this error with an async-await-specific
1709         // message, and fall back to regular note otherwise.
1710         if !self.maybe_note_obligation_cause_for_async_await(err, obligation) {
1711             self.note_obligation_cause_code(
1712                 err,
1713                 &obligation.predicate,
1714                 &obligation.cause.code,
1715                 &mut vec![],
1716             );
1717             self.suggest_unsized_bound_if_applicable(err, obligation);
1718         }
1719     }
1720
1721     fn suggest_unsized_bound_if_applicable(
1722         &self,
1723         err: &mut DiagnosticBuilder<'tcx>,
1724         obligation: &PredicateObligation<'tcx>,
1725     ) {
1726         let (pred, item_def_id, span) =
1727             match (obligation.predicate.kind(), &obligation.cause.code.peel_derives()) {
1728                 (
1729                     ty::PredicateKind::Trait(pred, _),
1730                     ObligationCauseCode::BindingObligation(item_def_id, span),
1731                 ) => (pred, item_def_id, span),
1732                 _ => return,
1733             };
1734
1735         let node = match (
1736             self.tcx.hir().get_if_local(*item_def_id),
1737             Some(pred.def_id()) == self.tcx.lang_items().sized_trait(),
1738         ) {
1739             (Some(node), true) => node,
1740             _ => return,
1741         };
1742         let generics = match node.generics() {
1743             Some(generics) => generics,
1744             None => return,
1745         };
1746         for param in generics.params {
1747             if param.span != *span
1748                 || param.bounds.iter().any(|bound| {
1749                     bound.trait_ref().and_then(|trait_ref| trait_ref.trait_def_id())
1750                         == self.tcx.lang_items().sized_trait()
1751                 })
1752             {
1753                 continue;
1754             }
1755             match node {
1756                 hir::Node::Item(
1757                     item
1758                     @
1759                     hir::Item {
1760                         kind:
1761                             hir::ItemKind::Enum(..)
1762                             | hir::ItemKind::Struct(..)
1763                             | hir::ItemKind::Union(..),
1764                         ..
1765                     },
1766                 ) => {
1767                     // Suggesting `T: ?Sized` is only valid in an ADT if `T` is only used in a
1768                     // borrow. `struct S<'a, T: ?Sized>(&'a T);` is valid, `struct S<T: ?Sized>(T);`
1769                     // is not.
1770                     let mut visitor = FindTypeParam {
1771                         param: param.name.ident().name,
1772                         invalid_spans: vec![],
1773                         nested: false,
1774                     };
1775                     visitor.visit_item(item);
1776                     if !visitor.invalid_spans.is_empty() {
1777                         let mut multispan: MultiSpan = param.span.into();
1778                         multispan.push_span_label(
1779                             param.span,
1780                             format!("this could be changed to `{}: ?Sized`...", param.name.ident()),
1781                         );
1782                         for sp in visitor.invalid_spans {
1783                             multispan.push_span_label(
1784                                 sp,
1785                                 format!(
1786                                     "...if indirection was used here: `Box<{}>`",
1787                                     param.name.ident(),
1788                                 ),
1789                             );
1790                         }
1791                         err.span_help(
1792                             multispan,
1793                             &format!(
1794                                 "you could relax the implicit `Sized` bound on `{T}` if it were \
1795                                  used through indirection like `&{T}` or `Box<{T}>`",
1796                                 T = param.name.ident(),
1797                             ),
1798                         );
1799                         return;
1800                     }
1801                 }
1802                 _ => {}
1803             }
1804             let (span, separator) = match param.bounds {
1805                 [] => (span.shrink_to_hi(), ":"),
1806                 [.., bound] => (bound.span().shrink_to_hi(), " +"),
1807             };
1808             err.span_suggestion_verbose(
1809                 span,
1810                 "consider relaxing the implicit `Sized` restriction",
1811                 format!("{} ?Sized", separator),
1812                 Applicability::MachineApplicable,
1813             );
1814             return;
1815         }
1816     }
1817
1818     fn is_recursive_obligation(
1819         &self,
1820         obligated_types: &mut Vec<&ty::TyS<'tcx>>,
1821         cause_code: &ObligationCauseCode<'tcx>,
1822     ) -> bool {
1823         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
1824             let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
1825
1826             if obligated_types.iter().any(|ot| ot == &parent_trait_ref.skip_binder().self_ty()) {
1827                 return true;
1828             }
1829         }
1830         false
1831     }
1832 }
1833
1834 /// Look for type `param` in an ADT being used only through a reference to confirm that suggesting
1835 /// `param: ?Sized` would be a valid constraint.
1836 struct FindTypeParam {
1837     param: rustc_span::Symbol,
1838     invalid_spans: Vec<Span>,
1839     nested: bool,
1840 }
1841
1842 impl<'v> Visitor<'v> for FindTypeParam {
1843     type Map = rustc_hir::intravisit::ErasedMap<'v>;
1844
1845     fn nested_visit_map(&mut self) -> hir::intravisit::NestedVisitorMap<Self::Map> {
1846         hir::intravisit::NestedVisitorMap::None
1847     }
1848
1849     fn visit_ty(&mut self, ty: &hir::Ty<'_>) {
1850         // We collect the spans of all uses of the "bare" type param, like in `field: T` or
1851         // `field: (T, T)` where we could make `T: ?Sized` while skipping cases that are known to be
1852         // valid like `field: &'a T` or `field: *mut T` and cases that *might* have further `Sized`
1853         // obligations like `Box<T>` and `Vec<T>`, but we perform no extra analysis for those cases
1854         // and suggest `T: ?Sized` regardless of their obligations. This is fine because the errors
1855         // in that case should make what happened clear enough.
1856         match ty.kind {
1857             hir::TyKind::Ptr(_) | hir::TyKind::Rptr(..) | hir::TyKind::TraitObject(..) => {}
1858             hir::TyKind::Path(hir::QPath::Resolved(None, path))
1859                 if path.segments.len() == 1 && path.segments[0].ident.name == self.param =>
1860             {
1861                 if !self.nested {
1862                     self.invalid_spans.push(ty.span);
1863                 }
1864             }
1865             hir::TyKind::Path(_) => {
1866                 let prev = self.nested;
1867                 self.nested = true;
1868                 hir::intravisit::walk_ty(self, ty);
1869                 self.nested = prev;
1870             }
1871             _ => {
1872                 hir::intravisit::walk_ty(self, ty);
1873             }
1874         }
1875     }
1876 }
1877
1878 pub fn recursive_type_with_infinite_size_error(
1879     tcx: TyCtxt<'tcx>,
1880     type_def_id: DefId,
1881     spans: Vec<Span>,
1882 ) {
1883     assert!(type_def_id.is_local());
1884     let span = tcx.hir().span_if_local(type_def_id).unwrap();
1885     let span = tcx.sess.source_map().guess_head_span(span);
1886     let path = tcx.def_path_str(type_def_id);
1887     let mut err =
1888         struct_span_err!(tcx.sess, span, E0072, "recursive type `{}` has infinite size", path);
1889     err.span_label(span, "recursive type has infinite size");
1890     for &span in &spans {
1891         err.span_label(span, "recursive without indirection");
1892     }
1893     let msg = format!(
1894         "insert some indirection (e.g., a `Box`, `Rc`, or `&`) to make `{}` representable",
1895         path,
1896     );
1897     if spans.len() <= 4 {
1898         err.multipart_suggestion(
1899             &msg,
1900             spans
1901                 .iter()
1902                 .flat_map(|&span| {
1903                     vec![
1904                         (span.shrink_to_lo(), "Box<".to_string()),
1905                         (span.shrink_to_hi(), ">".to_string()),
1906                     ]
1907                     .into_iter()
1908                 })
1909                 .collect(),
1910             Applicability::HasPlaceholders,
1911         );
1912     } else {
1913         err.help(&msg);
1914     }
1915     err.emit();
1916 }
1917
1918 /// Summarizes information
1919 #[derive(Clone)]
1920 pub enum ArgKind {
1921     /// An argument of non-tuple type. Parameters are (name, ty)
1922     Arg(String, String),
1923
1924     /// An argument of tuple type. For a "found" argument, the span is
1925     /// the locationo in the source of the pattern. For a "expected"
1926     /// argument, it will be None. The vector is a list of (name, ty)
1927     /// strings for the components of the tuple.
1928     Tuple(Option<Span>, Vec<(String, String)>),
1929 }
1930
1931 impl ArgKind {
1932     fn empty() -> ArgKind {
1933         ArgKind::Arg("_".to_owned(), "_".to_owned())
1934     }
1935
1936     /// Creates an `ArgKind` from the expected type of an
1937     /// argument. It has no name (`_`) and an optional source span.
1938     pub fn from_expected_ty(t: Ty<'_>, span: Option<Span>) -> ArgKind {
1939         match t.kind {
1940             ty::Tuple(ref tys) => ArgKind::Tuple(
1941                 span,
1942                 tys.iter().map(|ty| ("_".to_owned(), ty.to_string())).collect::<Vec<_>>(),
1943             ),
1944             _ => ArgKind::Arg("_".to_owned(), t.to_string()),
1945         }
1946     }
1947 }