compiler/rustc_trait_selection/src/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::{
  24     self, fast_reject, AdtKind, SubtypePredicate, ToPolyTraitRef, ToPredicate, Ty, TyCtxt,
  25     TypeFoldable, WithConstness,
  26 };
  27 use rustc_session::DiagnosticMessageId;
  28 use rustc_span::symbol::{kw, sym};
  29 use rustc_span::{ExpnKind, MultiSpan, Span, DUMMY_SP};
  30 use std::fmt;
  31
  32 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
  33 use crate::traits::query::normalize::AtExt as _;
  34 use on_unimplemented::InferCtxtExt as _;
  35 use suggestions::InferCtxtExt as _;
  36
  37 pub use rustc_infer::traits::error_reporting::*;
  38
  39 pub trait InferCtxtExt<'tcx> {
  40     fn report_fulfillment_errors(
  41         &self,
  42         errors: &[FulfillmentError<'tcx>],
  43         body_id: Option<hir::BodyId>,
  44         fallback_has_occurred: bool,
  45     );
  46
  47     fn report_overflow_error<T>(
  48         &self,
  49         obligation: &Obligation<'tcx, T>,
  50         suggest_increasing_limit: bool,
  51     ) -> !
  52     where
  53         T: fmt::Display + TypeFoldable<'tcx>;
  54
  55     fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> !;
  56
  57     fn report_selection_error(
  58         &self,
  59         obligation: &PredicateObligation<'tcx>,
  60         error: &SelectionError<'tcx>,
  61         fallback_has_occurred: bool,
  62         points_at_arg: bool,
  63     );
  64
  65     /// Given some node representing a fn-like thing in the HIR map,
  66     /// returns a span and `ArgKind` information that describes the
  67     /// arguments it expects. This can be supplied to
  68     /// `report_arg_count_mismatch`.
  69     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)>;
  70
  71     /// Reports an error when the number of arguments needed by a
  72     /// trait match doesn't match the number that the expression
  73     /// provides.
  74     fn report_arg_count_mismatch(
  75         &self,
  76         span: Span,
  77         found_span: Option<Span>,
  78         expected_args: Vec<ArgKind>,
  79         found_args: Vec<ArgKind>,
  80         is_closure: bool,
  81     ) -> DiagnosticBuilder<'tcx>;
  82 }
  83
  84 impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
  85     fn report_fulfillment_errors(
  86         &self,
  87         errors: &[FulfillmentError<'tcx>],
  88         body_id: Option<hir::BodyId>,
  89         fallback_has_occurred: bool,
  90     ) {
  91         #[derive(Debug)]
  92         struct ErrorDescriptor<'tcx> {
  93             predicate: ty::Predicate<'tcx>,
  94             index: Option<usize>, // None if this is an old error
  95         }
  96
  97         let mut error_map: FxHashMap<_, Vec<_>> = self
  98             .reported_trait_errors
  99             .borrow()
 100             .iter()
 101             .map(|(&span, predicates)| {
 102                 (
 103                     span,
 104                     predicates
 105                         .iter()
 106                         .map(|&predicate| ErrorDescriptor { predicate, index: None })
 107                         .collect(),
 108                 )
 109             })
 110             .collect();
 111
 112         for (index, error) in errors.iter().enumerate() {
 113             // We want to ignore desugarings here: spans are equivalent even
 114             // if one is the result of a desugaring and the other is not.
 115             let mut span = error.obligation.cause.span;
 116             let expn_data = span.ctxt().outer_expn_data();
 117             if let ExpnKind::Desugaring(_) = expn_data.kind {
 118                 span = expn_data.call_site;
 119             }
 120
 121             error_map.entry(span).or_default().push(ErrorDescriptor {
 122                 predicate: error.obligation.predicate,
 123                 index: Some(index),
 124             });
 125
 126             self.reported_trait_errors
 127                 .borrow_mut()
 128                 .entry(span)
 129                 .or_default()
 130                 .push(error.obligation.predicate);
 131         }
 132
 133         // We do this in 2 passes because we want to display errors in order, though
 134         // maybe it *is* better to sort errors by span or something.
 135         let mut is_suppressed = vec![false; errors.len()];
 136         for (_, error_set) in error_map.iter() {
 137             // We want to suppress "duplicate" errors with the same span.
 138             for error in error_set {
 139                 if let Some(index) = error.index {
 140                     // Suppress errors that are either:
 141                     // 1) strictly implied by another error.
 142                     // 2) implied by an error with a smaller index.
 143                     for error2 in error_set {
 144                         if error2.index.map_or(false, |index2| is_suppressed[index2]) {
 145                             // Avoid errors being suppressed by already-suppressed
 146                             // errors, to prevent all errors from being suppressed
 147                             // at once.
 148                             continue;
 149                         }
 150
 151                         if self.error_implies(error2.predicate, error.predicate)
 152                             && !(error2.index >= error.index
 153                                 && self.error_implies(error.predicate, error2.predicate))
 154                         {
 155                             info!("skipping {:?} (implied by {:?})", error, error2);
 156                             is_suppressed[index] = true;
 157                             break;
 158                         }
 159                     }
 160                 }
 161             }
 162         }
 163
 164         for (error, suppressed) in errors.iter().zip(is_suppressed) {
 165             if !suppressed {
 166                 self.report_fulfillment_error(error, body_id, fallback_has_occurred);
 167             }
 168         }
 169     }
 170
 171     /// Reports that an overflow has occurred and halts compilation. We
 172     /// halt compilation unconditionally because it is important that
 173     /// overflows never be masked -- they basically represent computations
 174     /// whose result could not be truly determined and thus we can't say
 175     /// if the program type checks or not -- and they are unusual
 176     /// occurrences in any case.
 177     fn report_overflow_error<T>(
 178         &self,
 179         obligation: &Obligation<'tcx, T>,
 180         suggest_increasing_limit: bool,
 181     ) -> !
 182     where
 183         T: fmt::Display + TypeFoldable<'tcx>,
 184     {
 185         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
 186         let mut err = struct_span_err!(
 187             self.tcx.sess,
 188             obligation.cause.span,
 189             E0275,
 190             "overflow evaluating the requirement `{}`",
 191             predicate
 192         );
 193
 194         if suggest_increasing_limit {
 195             self.suggest_new_overflow_limit(&mut err);
 196         }
 197
 198         self.note_obligation_cause_code(
 199             &mut err,
 200             &obligation.predicate,
 201             &obligation.cause.code,
 202             &mut vec![],
 203         );
 204
 205         err.emit();
 206         self.tcx.sess.abort_if_errors();
 207         bug!();
 208     }
 209
 210     /// Reports that a cycle was detected which led to overflow and halts
 211     /// compilation. This is equivalent to `report_overflow_error` except
 212     /// that we can give a more helpful error message (and, in particular,
 213     /// we do not suggest increasing the overflow limit, which is not
 214     /// going to help).
 215     fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 216         let cycle = self.resolve_vars_if_possible(&cycle.to_owned());
 217         assert!(!cycle.is_empty());
 218
 219         debug!("report_overflow_error_cycle: cycle={:?}", cycle);
 220
 221         self.report_overflow_error(&cycle[0], false);
 222     }
 223
 224     fn report_selection_error(
 225         &self,
 226         obligation: &PredicateObligation<'tcx>,
 227         error: &SelectionError<'tcx>,
 228         fallback_has_occurred: bool,
 229         points_at_arg: bool,
 230     ) {
 231         let tcx = self.tcx;
 232         let span = obligation.cause.span;
 233
 234         let mut err = match *error {
 235             SelectionError::Unimplemented => {
 236                 if let ObligationCauseCode::CompareImplMethodObligation {
 237                     item_name,
 238                     impl_item_def_id,
 239                     trait_item_def_id,
 240                 }
 241                 | ObligationCauseCode::CompareImplTypeObligation {
 242                     item_name,
 243                     impl_item_def_id,
 244                     trait_item_def_id,
 245                 } = obligation.cause.code
 246                 {
 247                     self.report_extra_impl_obligation(
 248                         span,
 249                         item_name,
 250                         impl_item_def_id,
 251                         trait_item_def_id,
 252                         &format!("`{}`", obligation.predicate),
 253                     )
 254                     .emit();
 255                     return;
 256                 }
 257
 258                 match obligation.predicate.skip_binders() {
 259                     ty::PredicateAtom::Trait(trait_predicate, _) => {
 260                         let trait_predicate = ty::Binder::bind(trait_predicate);
 261                         let trait_predicate = self.resolve_vars_if_possible(&trait_predicate);
 262
 263                         if self.tcx.sess.has_errors() && trait_predicate.references_error() {
 264                             return;
 265                         }
 266                         let trait_ref = trait_predicate.to_poly_trait_ref();
 267                         let (post_message, pre_message, type_def) = self
 268                             .get_parent_trait_ref(&obligation.cause.code)
 269                             .map(|(t, s)| {
 270                                 (
 271                                     format!(" in `{}`", t),
 272                                     format!("within `{}`, ", t),
 273                                     s.map(|s| (format!("within this `{}`", t), s)),
 274                                 )
 275                             })
 276                             .unwrap_or_default();
 277
 278                         let OnUnimplementedNote { message, label, note, enclosing_scope } =
 279                             self.on_unimplemented_note(trait_ref, obligation);
 280                         let have_alt_message = message.is_some() || label.is_some();
 281                         let is_try = self
 282                             .tcx
 283                             .sess
 284                             .source_map()
 285                             .span_to_snippet(span)
 286                             .map(|s| &s == "?")
 287                             .unwrap_or(false);
 288                         let is_from = self.tcx.get_diagnostic_item(sym::from_trait)
 289                             == Some(trait_ref.def_id());
 290                         let is_unsize =
 291                             { Some(trait_ref.def_id()) == self.tcx.lang_items().unsize_trait() };
 292                         let (message, note) = if is_try && is_from {
 293                             (
 294                                 Some(format!(
 295                                     "`?` couldn't convert the error to `{}`",
 296                                     trait_ref.skip_binder().self_ty(),
 297                                 )),
 298                                 Some(
 299                                     "the question mark operation (`?`) implicitly performs a \
 300                                         conversion on the error value using the `From` trait"
 301                                         .to_owned(),
 302                                 ),
 303                             )
 304                         } else {
 305                             (message, note)
 306                         };
 307
 308                         let mut err = struct_span_err!(
 309                             self.tcx.sess,
 310                             span,
 311                             E0277,
 312                             "{}",
 313                             message.unwrap_or_else(|| format!(
 314                                 "the trait bound `{}` is not satisfied{}",
 315                                 trait_ref.without_const().to_predicate(tcx),
 316                                 post_message,
 317                             ))
 318                         );
 319
 320                         if is_try && is_from {
 321                             let none_error = self
 322                                 .tcx
 323                                 .get_diagnostic_item(sym::none_error)
 324                                 .map(|def_id| tcx.type_of(def_id));
 325                             let should_convert_option_to_result =
 326                                 Some(trait_ref.skip_binder().substs.type_at(1)) == none_error;
 327                             let should_convert_result_to_option =
 328                                 Some(trait_ref.self_ty().skip_binder()) == none_error;
 329                             if should_convert_option_to_result {
 330                                 err.span_suggestion_verbose(
 331                                     span.shrink_to_lo(),
 332                                     "consider converting the `Option<T>` into a `Result<T, _>` \
 333                                      using `Option::ok_or` or `Option::ok_or_else`",
 334                                     ".ok_or_else(|| /* error value */)".to_string(),
 335                                     Applicability::HasPlaceholders,
 336                                 );
 337                             } else if should_convert_result_to_option {
 338                                 err.span_suggestion_verbose(
 339                                     span.shrink_to_lo(),
 340                                     "consider converting the `Result<T, _>` into an `Option<T>` \
 341                                      using `Result::ok`",
 342                                     ".ok()".to_string(),
 343                                     Applicability::MachineApplicable,
 344                                 );
 345                             }
 346                             if let Some(ret_span) = self.return_type_span(obligation) {
 347                                 err.span_label(
 348                                     ret_span,
 349                                     &format!(
 350                                         "expected `{}` because of this",
 351                                         trait_ref.skip_binder().self_ty()
 352                                     ),
 353                                 );
 354                             }
 355                         }
 356
 357                         let explanation =
 358                             if obligation.cause.code == ObligationCauseCode::MainFunctionType {
 359                                 "consider using `()`, or a `Result`".to_owned()
 360                             } else {
 361                                 format!(
 362                                     "{}the trait `{}` is not implemented for `{}`",
 363                                     pre_message,
 364                                     trait_ref.print_only_trait_path(),
 365                                     trait_ref.skip_binder().self_ty(),
 366                                 )
 367                             };
 368
 369                         if self.suggest_add_reference_to_arg(
 370                             &obligation,
 371                             &mut err,
 372                             &trait_ref,
 373                             points_at_arg,
 374                             have_alt_message,
 375                         ) {
 376                             self.note_obligation_cause(&mut err, obligation);
 377                             err.emit();
 378                             return;
 379                         }
 380                         if let Some(ref s) = label {
 381                             // If it has a custom `#[rustc_on_unimplemented]`
 382                             // error message, let's display it as the label!
 383                             err.span_label(span, s.as_str());
 384                             if !matches!(trait_ref.skip_binder().self_ty().kind(), ty::Param(_)) {
 385                                 // When the self type is a type param We don't need to "the trait
 386                                 // `std::marker::Sized` is not implemented for `T`" as we will point
 387                                 // at the type param with a label to suggest constraining it.
 388                                 err.help(&explanation);
 389                             }
 390                         } else {
 391                             err.span_label(span, explanation);
 392                         }
 393                         if let Some((msg, span)) = type_def {
 394                             err.span_label(span, &msg);
 395                         }
 396                         if let Some(ref s) = note {
 397                             // If it has a custom `#[rustc_on_unimplemented]` note, let's display it
 398                             err.note(s.as_str());
 399                         }
 400                         if let Some(ref s) = enclosing_scope {
 401                             let body = tcx
 402                                 .hir()
 403                                 .opt_local_def_id(obligation.cause.body_id)
 404                                 .unwrap_or_else(|| {
 405                                     tcx.hir().body_owner_def_id(hir::BodyId {
 406                                         hir_id: obligation.cause.body_id,
 407                                     })
 408                                 });
 409
 410                             let enclosing_scope_span =
 411                                 tcx.hir().span_with_body(tcx.hir().local_def_id_to_hir_id(body));
 412
 413                             err.span_label(enclosing_scope_span, s.as_str());
 414                         }
 415
 416                         self.suggest_dereferences(&obligation, &mut err, &trait_ref, points_at_arg);
 417                         self.suggest_fn_call(&obligation, &mut err, &trait_ref, points_at_arg);
 418                         self.suggest_remove_reference(&obligation, &mut err, &trait_ref);
 419                         self.suggest_semicolon_removal(&obligation, &mut err, span, &trait_ref);
 420                         self.note_version_mismatch(&mut err, &trait_ref);
 421
 422                         if Some(trait_ref.def_id()) == tcx.lang_items().try_trait() {
 423                             self.suggest_await_before_try(&mut err, &obligation, &trait_ref, span);
 424                         }
 425
 426                         if self.suggest_impl_trait(&mut err, span, &obligation, &trait_ref) {
 427                             err.emit();
 428                             return;
 429                         }
 430
 431                         if is_unsize {
 432                             // If the obligation failed due to a missing implementation of the
 433                             // `Unsize` trait, give a pointer to why that might be the case
 434                             err.note(
 435                                 "all implementations of `Unsize` are provided \
 436                                 automatically by the compiler, see \
 437                                 <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> \
 438                                 for more information",
 439                             );
 440                         }
 441
 442                         let is_fn_trait = [
 443                             self.tcx.lang_items().fn_trait(),
 444                             self.tcx.lang_items().fn_mut_trait(),
 445                             self.tcx.lang_items().fn_once_trait(),
 446                         ]
 447                         .contains(&Some(trait_ref.def_id()));
 448                         let is_target_feature_fn = if let ty::FnDef(def_id, _) =
 449                             *trait_ref.skip_binder().self_ty().kind()
 450                         {
 451                             !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
 452                         } else {
 453                             false
 454                         };
 455                         if is_fn_trait && is_target_feature_fn {
 456                             err.note(
 457                                 "`#[target_feature]` functions do not implement the `Fn` traits",
 458                             );
 459                         }
 460
 461                         // Try to report a help message
 462                         if !trait_ref.has_infer_types_or_consts()
 463                             && self.predicate_can_apply(obligation.param_env, trait_ref)
 464                         {
 465                             // If a where-clause may be useful, remind the
 466                             // user that they can add it.
 467                             //
 468                             // don't display an on-unimplemented note, as
 469                             // these notes will often be of the form
 470                             //     "the type `T` can't be frobnicated"
 471                             // which is somewhat confusing.
 472                             self.suggest_restricting_param_bound(
 473                                 &mut err,
 474                                 trait_ref,
 475                                 obligation.cause.body_id,
 476                             );
 477                         } else {
 478                             if !have_alt_message {
 479                                 // Can't show anything else useful, try to find similar impls.
 480                                 let impl_candidates = self.find_similar_impl_candidates(trait_ref);
 481                                 self.report_similar_impl_candidates(impl_candidates, &mut err);
 482                             }
 483                             // Changing mutability doesn't make a difference to whether we have
 484                             // an `Unsize` impl (Fixes ICE in #71036)
 485                             if !is_unsize {
 486                                 self.suggest_change_mut(
 487                                     &obligation,
 488                                     &mut err,
 489                                     &trait_ref,
 490                                     points_at_arg,
 491                                 );
 492                             }
 493                         }
 494
 495                         // If this error is due to `!: Trait` not implemented but `(): Trait` is
 496                         // implemented, and fallback has occurred, then it could be due to a
 497                         // variable that used to fallback to `()` now falling back to `!`. Issue a
 498                         // note informing about the change in behaviour.
 499                         if trait_predicate.skip_binder().self_ty().is_never()
 500                             && fallback_has_occurred
 501                         {
 502                             let predicate = trait_predicate.map_bound(|mut trait_pred| {
 503                                 trait_pred.trait_ref.substs = self.tcx.mk_substs_trait(
 504                                     self.tcx.mk_unit(),
 505                                     &trait_pred.trait_ref.substs[1..],
 506                                 );
 507                                 trait_pred
 508                             });
 509                             let unit_obligation =
 510                                 obligation.with(predicate.without_const().to_predicate(tcx));
 511                             if self.predicate_may_hold(&unit_obligation) {
 512                                 err.note(
 513                                     "the trait is implemented for `()`. \
 514                                      Possibly this error has been caused by changes to \
 515                                      Rust's type-inference algorithm (see issue #48950 \
 516                                      <https://github.com/rust-lang/rust/issues/48950> \
 517                                      for more information). Consider whether you meant to use \
 518                                      the type `()` here instead.",
 519                                 );
 520                             }
 521                         }
 522
 523                         err
 524                     }
 525
 526                     ty::PredicateAtom::Subtype(predicate) => {
 527                         // Errors for Subtype predicates show up as
 528                         // `FulfillmentErrorCode::CodeSubtypeError`,
 529                         // not selection error.
 530                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
 531                     }
 532
 533                     ty::PredicateAtom::RegionOutlives(predicate) => {
 534                         let predicate = ty::Binder::bind(predicate);
 535                         let predicate = self.resolve_vars_if_possible(&predicate);
 536                         let err = self
 537                             .region_outlives_predicate(&obligation.cause, predicate)
 538                             .err()
 539                             .unwrap();
 540                         struct_span_err!(
 541                             self.tcx.sess,
 542                             span,
 543                             E0279,
 544                             "the requirement `{}` is not satisfied (`{}`)",
 545                             predicate,
 546                             err,
 547                         )
 548                     }
 549
 550                     ty::PredicateAtom::Projection(..) | ty::PredicateAtom::TypeOutlives(..) => {
 551                         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
 552                         struct_span_err!(
 553                             self.tcx.sess,
 554                             span,
 555                             E0280,
 556                             "the requirement `{}` is not satisfied",
 557                             predicate
 558                         )
 559                     }
 560
 561                     ty::PredicateAtom::ObjectSafe(trait_def_id) => {
 562                         let violations = self.tcx.object_safety_violations(trait_def_id);
 563                         report_object_safety_error(self.tcx, span, trait_def_id, violations)
 564                     }
 565
 566                     ty::PredicateAtom::ClosureKind(closure_def_id, closure_substs, kind) => {
 567                         let found_kind = self.closure_kind(closure_substs).unwrap();
 568                         let closure_span =
 569                             self.tcx.sess.source_map().guess_head_span(
 570                                 self.tcx.hir().span_if_local(closure_def_id).unwrap(),
 571                             );
 572                         let hir_id =
 573                             self.tcx.hir().local_def_id_to_hir_id(closure_def_id.expect_local());
 574                         let mut err = struct_span_err!(
 575                             self.tcx.sess,
 576                             closure_span,
 577                             E0525,
 578                             "expected a closure that implements the `{}` trait, \
 579                              but this closure only implements `{}`",
 580                             kind,
 581                             found_kind
 582                         );
 583
 584                         err.span_label(
 585                             closure_span,
 586                             format!("this closure implements `{}`, not `{}`", found_kind, kind),
 587                         );
 588                         err.span_label(
 589                             obligation.cause.span,
 590                             format!("the requirement to implement `{}` derives from here", kind),
 591                         );
 592
 593                         // Additional context information explaining why the closure only implements
 594                         // a particular trait.
 595                         if let Some(typeck_results) = self.in_progress_typeck_results {
 596                             let typeck_results = typeck_results.borrow();
 597                             match (found_kind, typeck_results.closure_kind_origins().get(hir_id)) {
 598                                 (ty::ClosureKind::FnOnce, Some((span, name))) => {
 599                                     err.span_label(
 600                                         *span,
 601                                         format!(
 602                                             "closure is `FnOnce` because it moves the \
 603                                          variable `{}` out of its environment",
 604                                             name
 605                                         ),
 606                                     );
 607                                 }
 608                                 (ty::ClosureKind::FnMut, Some((span, name))) => {
 609                                     err.span_label(
 610                                         *span,
 611                                         format!(
 612                                             "closure is `FnMut` because it mutates the \
 613                                          variable `{}` here",
 614                                             name
 615                                         ),
 616                                     );
 617                                 }
 618                                 _ => {}
 619                             }
 620                         }
 621
 622                         err.emit();
 623                         return;
 624                     }
 625
 626                     ty::PredicateAtom::WellFormed(ty) => {
 627                         if !self.tcx.sess.opts.debugging_opts.chalk {
 628                             // WF predicates cannot themselves make
 629                             // errors. They can only block due to
 630                             // ambiguity; otherwise, they always
 631                             // degenerate into other obligations
 632                             // (which may fail).
 633                             span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 634                         } else {
 635                             // FIXME: we'll need a better message which takes into account
 636                             // which bounds actually failed to hold.
 637                             self.tcx.sess.struct_span_err(
 638                                 span,
 639                                 &format!("the type `{}` is not well-formed (chalk)", ty),
 640                             )
 641                         }
 642                     }
 643
 644                     ty::PredicateAtom::ConstEvaluatable(..) => {
 645                         // Errors for `ConstEvaluatable` predicates show up as
 646                         // `SelectionError::ConstEvalFailure`,
 647                         // not `Unimplemented`.
 648                         span_bug!(
 649                             span,
 650                             "const-evaluatable requirement gave wrong error: `{:?}`",
 651                             obligation
 652                         )
 653                     }
 654
 655                     ty::PredicateAtom::ConstEquate(..) => {
 656                         // Errors for `ConstEquate` predicates show up as
 657                         // `SelectionError::ConstEvalFailure`,
 658                         // not `Unimplemented`.
 659                         span_bug!(
 660                             span,
 661                             "const-equate requirement gave wrong error: `{:?}`",
 662                             obligation
 663                         )
 664                     }
 665
 666                     ty::PredicateAtom::TypeWellFormedFromEnv(..) => span_bug!(
 667                         span,
 668                         "TypeWellFormedFromEnv predicate should only exist in the environment"
 669                     ),
 670                 }
 671             }
 672
 673             OutputTypeParameterMismatch(ref found_trait_ref, ref expected_trait_ref, _) => {
 674                 let found_trait_ref = self.resolve_vars_if_possible(&*found_trait_ref);
 675                 let expected_trait_ref = self.resolve_vars_if_possible(&*expected_trait_ref);
 676
 677                 if expected_trait_ref.self_ty().references_error() {
 678                     return;
 679                 }
 680
 681                 let found_trait_ty = match found_trait_ref.self_ty().no_bound_vars() {
 682                     Some(ty) => ty,
 683                     None => return,
 684                 };
 685
 686                 let found_did = match *found_trait_ty.kind() {
 687                     ty::Closure(did, _) | ty::Foreign(did) | ty::FnDef(did, _) => Some(did),
 688                     ty::Adt(def, _) => Some(def.did),
 689                     _ => None,
 690                 };
 691
 692                 let found_span = found_did
 693                     .and_then(|did| self.tcx.hir().span_if_local(did))
 694                     .map(|sp| self.tcx.sess.source_map().guess_head_span(sp)); // the sp could be an fn def
 695
 696                 if self.reported_closure_mismatch.borrow().contains(&(span, found_span)) {
 697                     // We check closures twice, with obligations flowing in different directions,
 698                     // but we want to complain about them only once.
 699                     return;
 700                 }
 701
 702                 self.reported_closure_mismatch.borrow_mut().insert((span, found_span));
 703
 704                 let found = match found_trait_ref.skip_binder().substs.type_at(1).kind() {
 705                     ty::Tuple(ref tys) => vec![ArgKind::empty(); tys.len()],
 706                     _ => vec![ArgKind::empty()],
 707                 };
 708
 709                 let expected_ty = expected_trait_ref.skip_binder().substs.type_at(1);
 710                 let expected = match expected_ty.kind() {
 711                     ty::Tuple(ref tys) => tys
 712                         .iter()
 713                         .map(|t| ArgKind::from_expected_ty(t.expect_ty(), Some(span)))
 714                         .collect(),
 715                     _ => vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())],
 716                 };
 717
 718                 if found.len() == expected.len() {
 719                     self.report_closure_arg_mismatch(
 720                         span,
 721                         found_span,
 722                         found_trait_ref,
 723                         expected_trait_ref,
 724                     )
 725                 } else {
 726                     let (closure_span, found) = found_did
 727                         .and_then(|did| {
 728                             let node = self.tcx.hir().get_if_local(did)?;
 729                             let (found_span, found) = self.get_fn_like_arguments(node)?;
 730                             Some((Some(found_span), found))
 731                         })
 732                         .unwrap_or((found_span, found));
 733
 734                     self.report_arg_count_mismatch(
 735                         span,
 736                         closure_span,
 737                         expected,
 738                         found,
 739                         found_trait_ty.is_closure(),
 740                     )
 741                 }
 742             }
 743
 744             TraitNotObjectSafe(did) => {
 745                 let violations = self.tcx.object_safety_violations(did);
 746                 report_object_safety_error(self.tcx, span, did, violations)
 747             }
 748             ConstEvalFailure(ErrorHandled::TooGeneric) => {
 749                 bug!("too generic should have been handled in `is_const_evaluatable`");
 750             }
 751             // Already reported in the query.
 752             ConstEvalFailure(ErrorHandled::Reported(ErrorReported)) => {
 753                 // FIXME(eddyb) remove this once `ErrorReported` becomes a proof token.
 754                 self.tcx.sess.delay_span_bug(span, "`ErrorReported` without an error");
 755                 return;
 756             }
 757
 758             // Already reported in the query, but only as a lint.
 759             // This shouldn't actually happen for constants used in types, modulo
 760             // bugs. The `delay_span_bug` here ensures it won't be ignored.
 761             ConstEvalFailure(ErrorHandled::Linted) => {
 762                 self.tcx.sess.delay_span_bug(span, "constant in type had error reported as lint");
 763                 return;
 764             }
 765
 766             Overflow => {
 767                 bug!("overflow should be handled before the `report_selection_error` path");
 768             }
 769         };
 770
 771         self.note_obligation_cause(&mut err, obligation);
 772         self.point_at_returns_when_relevant(&mut err, &obligation);
 773
 774         err.emit();
 775     }
 776
 777     /// Given some node representing a fn-like thing in the HIR map,
 778     /// returns a span and `ArgKind` information that describes the
 779     /// arguments it expects. This can be supplied to
 780     /// `report_arg_count_mismatch`.
 781     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)> {
 782         let sm = self.tcx.sess.source_map();
 783         let hir = self.tcx.hir();
 784         Some(match node {
 785             Node::Expr(&hir::Expr {
 786                 kind: hir::ExprKind::Closure(_, ref _decl, id, span, _),
 787                 ..
 788             }) => (
 789                 sm.guess_head_span(span),
 790                 hir.body(id)
 791                     .params
 792                     .iter()
 793                     .map(|arg| {
 794                         if let hir::Pat { kind: hir::PatKind::Tuple(ref args, _), span, .. } =
 795                             *arg.pat
 796                         {
 797                             Some(ArgKind::Tuple(
 798                                 Some(span),
 799                                 args.iter()
 800                                     .map(|pat| {
 801                                         sm.span_to_snippet(pat.span)
 802                                             .ok()
 803                                             .map(|snippet| (snippet, "_".to_owned()))
 804                                     })
 805                                     .collect::<Option<Vec<_>>>()?,
 806                             ))
 807                         } else {
 808                             let name = sm.span_to_snippet(arg.pat.span).ok()?;
 809                             Some(ArgKind::Arg(name, "_".to_owned()))
 810                         }
 811                     })
 812                     .collect::<Option<Vec<ArgKind>>>()?,
 813             ),
 814             Node::Item(&hir::Item { span, kind: hir::ItemKind::Fn(ref sig, ..), .. })
 815             | Node::ImplItem(&hir::ImplItem {
 816                 span,
 817                 kind: hir::ImplItemKind::Fn(ref sig, _),
 818                 ..
 819             })
 820             | Node::TraitItem(&hir::TraitItem {
 821                 span,
 822                 kind: hir::TraitItemKind::Fn(ref sig, _),
 823                 ..
 824             }) => (
 825                 sm.guess_head_span(span),
 826                 sig.decl
 827                     .inputs
 828                     .iter()
 829                     .map(|arg| match arg.clone().kind {
 830                         hir::TyKind::Tup(ref tys) => ArgKind::Tuple(
 831                             Some(arg.span),
 832                             vec![("_".to_owned(), "_".to_owned()); tys.len()],
 833                         ),
 834                         _ => ArgKind::empty(),
 835                     })
 836                     .collect::<Vec<ArgKind>>(),
 837             ),
 838             Node::Ctor(ref variant_data) => {
 839                 let span = variant_data.ctor_hir_id().map(|id| hir.span(id)).unwrap_or(DUMMY_SP);
 840                 let span = sm.guess_head_span(span);
 841                 (span, vec![ArgKind::empty(); variant_data.fields().len()])
 842             }
 843             _ => panic!("non-FnLike node found: {:?}", node),
 844         })
 845     }
 846
 847     /// Reports an error when the number of arguments needed by a
 848     /// trait match doesn't match the number that the expression
 849     /// provides.
 850     fn report_arg_count_mismatch(
 851         &self,
 852         span: Span,
 853         found_span: Option<Span>,
 854         expected_args: Vec<ArgKind>,
 855         found_args: Vec<ArgKind>,
 856         is_closure: bool,
 857     ) -> DiagnosticBuilder<'tcx> {
 858         let kind = if is_closure { "closure" } else { "function" };
 859
 860         let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
 861             let arg_length = arguments.len();
 862             let distinct = match &other[..] {
 863                 &[ArgKind::Tuple(..)] => true,
 864                 _ => false,
 865             };
 866             match (arg_length, arguments.get(0)) {
 867                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
 868                     format!("a single {}-tuple as argument", fields.len())
 869                 }
 870                 _ => format!(
 871                     "{} {}argument{}",
 872                     arg_length,
 873                     if distinct && arg_length > 1 { "distinct " } else { "" },
 874                     pluralize!(arg_length)
 875                 ),
 876             }
 877         };
 878
 879         let expected_str = args_str(&expected_args, &found_args);
 880         let found_str = args_str(&found_args, &expected_args);
 881
 882         let mut err = struct_span_err!(
 883             self.tcx.sess,
 884             span,
 885             E0593,
 886             "{} is expected to take {}, but it takes {}",
 887             kind,
 888             expected_str,
 889             found_str,
 890         );
 891
 892         err.span_label(span, format!("expected {} that takes {}", kind, expected_str));
 893
 894         if let Some(found_span) = found_span {
 895             err.span_label(found_span, format!("takes {}", found_str));
 896
 897             // move |_| { ... }
 898             // ^^^^^^^^-- def_span
 899             //
 900             // move |_| { ... }
 901             // ^^^^^-- prefix
 902             let prefix_span = self.tcx.sess.source_map().span_until_non_whitespace(found_span);
 903             // move |_| { ... }
 904             //      ^^^-- pipe_span
 905             let pipe_span =
 906                 if let Some(span) = found_span.trim_start(prefix_span) { span } else { found_span };
 907
 908             // Suggest to take and ignore the arguments with expected_args_length `_`s if
 909             // found arguments is empty (assume the user just wants to ignore args in this case).
 910             // For example, if `expected_args_length` is 2, suggest `|_, _|`.
 911             if found_args.is_empty() && is_closure {
 912                 let underscores = vec!["_"; expected_args.len()].join(", ");
 913                 err.span_suggestion_verbose(
 914                     pipe_span,
 915                     &format!(
 916                         "consider changing the closure to take and ignore the expected argument{}",
 917                         pluralize!(expected_args.len())
 918                     ),
 919                     format!("|{}|", underscores),
 920                     Applicability::MachineApplicable,
 921                 );
 922             }
 923
 924             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
 925                 if fields.len() == expected_args.len() {
 926                     let sugg = fields
 927                         .iter()
 928                         .map(|(name, _)| name.to_owned())
 929                         .collect::<Vec<String>>()
 930                         .join(", ");
 931                     err.span_suggestion_verbose(
 932                         found_span,
 933                         "change the closure to take multiple arguments instead of a single tuple",
 934                         format!("|{}|", sugg),
 935                         Applicability::MachineApplicable,
 936                     );
 937                 }
 938             }
 939             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..] {
 940                 if fields.len() == found_args.len() && is_closure {
 941                     let sugg = format!(
 942                         "|({}){}|",
 943                         found_args
 944                             .iter()
 945                             .map(|arg| match arg {
 946                                 ArgKind::Arg(name, _) => name.to_owned(),
 947                                 _ => "_".to_owned(),
 948                             })
 949                             .collect::<Vec<String>>()
 950                             .join(", "),
 951                         // add type annotations if available
 952                         if found_args.iter().any(|arg| match arg {
 953                             ArgKind::Arg(_, ty) => ty != "_",
 954                             _ => false,
 955                         }) {
 956                             format!(
 957                                 ": ({})",
 958                                 fields
 959                                     .iter()
 960                                     .map(|(_, ty)| ty.to_owned())
 961                                     .collect::<Vec<String>>()
 962                                     .join(", ")
 963                             )
 964                         } else {
 965                             String::new()
 966                         },
 967                     );
 968                     err.span_suggestion_verbose(
 969                         found_span,
 970                         "change the closure to accept a tuple instead of individual arguments",
 971                         sugg,
 972                         Applicability::MachineApplicable,
 973                     );
 974                 }
 975             }
 976         }
 977
 978         err
 979     }
 980 }
 981
 982 trait InferCtxtPrivExt<'tcx> {
 983     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
 984     // `error` occurring implies that `cond` occurs.
 985     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool;
 986
 987     fn report_fulfillment_error(
 988         &self,
 989         error: &FulfillmentError<'tcx>,
 990         body_id: Option<hir::BodyId>,
 991         fallback_has_occurred: bool,
 992     );
 993
 994     fn report_projection_error(
 995         &self,
 996         obligation: &PredicateObligation<'tcx>,
 997         error: &MismatchedProjectionTypes<'tcx>,
 998     );
 999
1000     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool;
1001
1002     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str>;
1003
1004     fn find_similar_impl_candidates(
1005         &self,
1006         trait_ref: ty::PolyTraitRef<'tcx>,
1007     ) -> Vec<ty::TraitRef<'tcx>>;
1008
1009     fn report_similar_impl_candidates(
1010         &self,
1011         impl_candidates: Vec<ty::TraitRef<'tcx>>,
1012         err: &mut DiagnosticBuilder<'_>,
1013     );
1014
1015     /// Gets the parent trait chain start
1016     fn get_parent_trait_ref(
1017         &self,
1018         code: &ObligationCauseCode<'tcx>,
1019     ) -> Option<(String, Option<Span>)>;
1020
1021     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1022     /// with the same path as `trait_ref`, a help message about
1023     /// a probable version mismatch is added to `err`
1024     fn note_version_mismatch(
1025         &self,
1026         err: &mut DiagnosticBuilder<'_>,
1027         trait_ref: &ty::PolyTraitRef<'tcx>,
1028     );
1029
1030     /// Creates a `PredicateObligation` with `new_self_ty` replacing the existing type in the
1031     /// `trait_ref`.
1032     ///
1033     /// For this to work, `new_self_ty` must have no escaping bound variables.
1034     fn mk_trait_obligation_with_new_self_ty(
1035         &self,
1036         param_env: ty::ParamEnv<'tcx>,
1037         trait_ref: &ty::PolyTraitRef<'tcx>,
1038         new_self_ty: Ty<'tcx>,
1039     ) -> PredicateObligation<'tcx>;
1040
1041     fn maybe_report_ambiguity(
1042         &self,
1043         obligation: &PredicateObligation<'tcx>,
1044         body_id: Option<hir::BodyId>,
1045     );
1046
1047     fn predicate_can_apply(
1048         &self,
1049         param_env: ty::ParamEnv<'tcx>,
1050         pred: ty::PolyTraitRef<'tcx>,
1051     ) -> bool;
1052
1053     fn note_obligation_cause(
1054         &self,
1055         err: &mut DiagnosticBuilder<'tcx>,
1056         obligation: &PredicateObligation<'tcx>,
1057     );
1058
1059     fn suggest_unsized_bound_if_applicable(
1060         &self,
1061         err: &mut DiagnosticBuilder<'tcx>,
1062         obligation: &PredicateObligation<'tcx>,
1063     );
1064
1065     fn is_recursive_obligation(
1066         &self,
1067         obligated_types: &mut Vec<&ty::TyS<'tcx>>,
1068         cause_code: &ObligationCauseCode<'tcx>,
1069     ) -> bool;
1070 }
1071
1072 impl<'a, 'tcx> InferCtxtPrivExt<'tcx> for InferCtxt<'a, 'tcx> {
1073     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1074     // `error` occurring implies that `cond` occurs.
1075     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool {
1076         if cond == error {
1077             return true;
1078         }
1079
1080         // FIXME: It should be possible to deal with `ForAll` in a cleaner way.
1081         let (cond, error) = match (cond.skip_binders(), error.skip_binders()) {
1082             (ty::PredicateAtom::Trait(..), ty::PredicateAtom::Trait(error, _)) => {
1083                 (cond, ty::Binder::bind(error))
1084             }
1085             _ => {
1086                 // FIXME: make this work in other cases too.
1087                 return false;
1088             }
1089         };
1090
1091         for obligation in super::elaborate_predicates(self.tcx, std::iter::once(cond)) {
1092             if let ty::PredicateAtom::Trait(implication, _) = obligation.predicate.skip_binders() {
1093                 let error = error.to_poly_trait_ref();
1094                 let implication = ty::Binder::bind(implication.trait_ref);
1095                 // FIXME: I'm just not taking associated types at all here.
1096                 // Eventually I'll need to implement param-env-aware
1097                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
1098                 let param_env = ty::ParamEnv::empty();
1099                 if self.can_sub(param_env, error, implication).is_ok() {
1100                     debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
1101                     return true;
1102                 }
1103             }
1104         }
1105
1106         false
1107     }
1108
1109     fn report_fulfillment_error(
1110         &self,
1111         error: &FulfillmentError<'tcx>,
1112         body_id: Option<hir::BodyId>,
1113         fallback_has_occurred: bool,
1114     ) {
1115         debug!("report_fulfillment_error({:?})", error);
1116         match error.code {
1117             FulfillmentErrorCode::CodeSelectionError(ref selection_error) => {
1118                 self.report_selection_error(
1119                     &error.obligation,
1120                     selection_error,
1121                     fallback_has_occurred,
1122                     error.points_at_arg_span,
1123                 );
1124             }
1125             FulfillmentErrorCode::CodeProjectionError(ref e) => {
1126                 self.report_projection_error(&error.obligation, e);
1127             }
1128             FulfillmentErrorCode::CodeAmbiguity => {
1129                 self.maybe_report_ambiguity(&error.obligation, body_id);
1130             }
1131             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
1132                 self.report_mismatched_types(
1133                     &error.obligation.cause,
1134                     expected_found.expected,
1135                     expected_found.found,
1136                     err.clone(),
1137                 )
1138                 .emit();
1139             }
1140             FulfillmentErrorCode::CodeConstEquateError(ref expected_found, ref err) => {
1141                 self.report_mismatched_consts(
1142                     &error.obligation.cause,
1143                     expected_found.expected,
1144                     expected_found.found,
1145                     err.clone(),
1146                 )
1147                 .emit();
1148             }
1149         }
1150     }
1151
1152     fn report_projection_error(
1153         &self,
1154         obligation: &PredicateObligation<'tcx>,
1155         error: &MismatchedProjectionTypes<'tcx>,
1156     ) {
1157         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
1158
1159         if predicate.references_error() {
1160             return;
1161         }
1162
1163         self.probe(|_| {
1164             let err_buf;
1165             let mut err = &error.err;
1166             let mut values = None;
1167
1168             // try to find the mismatched types to report the error with.
1169             //
1170             // this can fail if the problem was higher-ranked, in which
1171             // cause I have no idea for a good error message.
1172             if let ty::PredicateAtom::Projection(data) = predicate.skip_binders() {
1173                 let mut selcx = SelectionContext::new(self);
1174                 let (data, _) = self.replace_bound_vars_with_fresh_vars(
1175                     obligation.cause.span,
1176                     infer::LateBoundRegionConversionTime::HigherRankedType,
1177                     &ty::Binder::bind(data),
1178                 );
1179                 let mut obligations = vec![];
1180                 let normalized_ty = super::normalize_projection_type(
1181                     &mut selcx,
1182                     obligation.param_env,
1183                     data.projection_ty,
1184                     obligation.cause.clone(),
1185                     0,
1186                     &mut obligations,
1187                 );
1188
1189                 debug!(
1190                     "report_projection_error obligation.cause={:?} obligation.param_env={:?}",
1191                     obligation.cause, obligation.param_env
1192                 );
1193
1194                 debug!(
1195                     "report_projection_error normalized_ty={:?} data.ty={:?}",
1196                     normalized_ty, data.ty
1197                 );
1198
1199                 let is_normalized_ty_expected = match &obligation.cause.code {
1200                     ObligationCauseCode::ItemObligation(_)
1201                     | ObligationCauseCode::BindingObligation(_, _)
1202                     | ObligationCauseCode::ObjectCastObligation(_) => false,
1203                     _ => true,
1204                 };
1205
1206                 if let Err(error) = self.at(&obligation.cause, obligation.param_env).eq_exp(
1207                     is_normalized_ty_expected,
1208                     normalized_ty,
1209                     data.ty,
1210                 ) {
1211                     values = Some(infer::ValuePairs::Types(ExpectedFound::new(
1212                         is_normalized_ty_expected,
1213                         normalized_ty,
1214                         data.ty,
1215                     )));
1216
1217                     err_buf = error;
1218                     err = &err_buf;
1219                 }
1220             }
1221
1222             let msg = format!("type mismatch resolving `{}`", predicate);
1223             let error_id = (DiagnosticMessageId::ErrorId(271), Some(obligation.cause.span), msg);
1224             let fresh = self.tcx.sess.one_time_diagnostics.borrow_mut().insert(error_id);
1225             if fresh {
1226                 let mut diag = struct_span_err!(
1227                     self.tcx.sess,
1228                     obligation.cause.span,
1229                     E0271,
1230                     "type mismatch resolving `{}`",
1231                     predicate
1232                 );
1233                 self.note_type_err(&mut diag, &obligation.cause, None, values, err);
1234                 self.note_obligation_cause(&mut diag, obligation);
1235                 diag.emit();
1236             }
1237         });
1238     }
1239
1240     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
1241         /// returns the fuzzy category of a given type, or None
1242         /// if the type can be equated to any type.
1243         fn type_category(t: Ty<'_>) -> Option<u32> {
1244             match t.kind() {
1245                 ty::Bool => Some(0),
1246                 ty::Char => Some(1),
1247                 ty::Str => Some(2),
1248                 ty::Int(..) | ty::Uint(..) | ty::Infer(ty::IntVar(..)) => Some(3),
1249                 ty::Float(..) | ty::Infer(ty::FloatVar(..)) => Some(4),
1250                 ty::Ref(..) | ty::RawPtr(..) => Some(5),
1251                 ty::Array(..) | ty::Slice(..) => Some(6),
1252                 ty::FnDef(..) | ty::FnPtr(..) => Some(7),
1253                 ty::Dynamic(..) => Some(8),
1254                 ty::Closure(..) => Some(9),
1255                 ty::Tuple(..) => Some(10),
1256                 ty::Projection(..) => Some(11),
1257                 ty::Param(..) => Some(12),
1258                 ty::Opaque(..) => Some(13),
1259                 ty::Never => Some(14),
1260                 ty::Adt(adt, ..) => match adt.adt_kind() {
1261                     AdtKind::Struct => Some(15),
1262                     AdtKind::Union => Some(16),
1263                     AdtKind::Enum => Some(17),
1264                 },
1265                 ty::Generator(..) => Some(18),
1266                 ty::Foreign(..) => Some(19),
1267                 ty::GeneratorWitness(..) => Some(20),
1268                 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error(_) => None,
1269             }
1270         }
1271
1272         match (type_category(a), type_category(b)) {
1273             (Some(cat_a), Some(cat_b)) => match (a.kind(), b.kind()) {
1274                 (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => def_a == def_b,
1275                 _ => cat_a == cat_b,
1276             },
1277             // infer and error can be equated to all types
1278             _ => true,
1279         }
1280     }
1281
1282     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str> {
1283         self.tcx.hir().body(body_id).generator_kind.map(|gen_kind| match gen_kind {
1284             hir::GeneratorKind::Gen => "a generator",
1285             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block) => "an async block",
1286             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn) => "an async function",
1287             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Closure) => "an async closure",
1288         })
1289     }
1290
1291     fn find_similar_impl_candidates(
1292         &self,
1293         trait_ref: ty::PolyTraitRef<'tcx>,
1294     ) -> Vec<ty::TraitRef<'tcx>> {
1295         let simp = fast_reject::simplify_type(self.tcx, trait_ref.skip_binder().self_ty(), true);
1296         let all_impls = self.tcx.all_impls(trait_ref.def_id());
1297
1298         match simp {
1299             Some(simp) => all_impls
1300                 .filter_map(|def_id| {
1301                     let imp = self.tcx.impl_trait_ref(def_id).unwrap();
1302                     let imp_simp = fast_reject::simplify_type(self.tcx, imp.self_ty(), true);
1303                     if let Some(imp_simp) = imp_simp {
1304                         if simp != imp_simp {
1305                             return None;
1306                         }
1307                     }
1308                     Some(imp)
1309                 })
1310                 .collect(),
1311             None => all_impls.map(|def_id| self.tcx.impl_trait_ref(def_id).unwrap()).collect(),
1312         }
1313     }
1314
1315     fn report_similar_impl_candidates(
1316         &self,
1317         impl_candidates: Vec<ty::TraitRef<'tcx>>,
1318         err: &mut DiagnosticBuilder<'_>,
1319     ) {
1320         if impl_candidates.is_empty() {
1321             return;
1322         }
1323
1324         let len = impl_candidates.len();
1325         let end = if impl_candidates.len() <= 5 { impl_candidates.len() } else { 4 };
1326
1327         let normalize = |candidate| {
1328             self.tcx.infer_ctxt().enter(|ref infcx| {
1329                 let normalized = infcx
1330                     .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
1331                     .normalize(candidate)
1332                     .ok();
1333                 match normalized {
1334                     Some(normalized) => format!("\n  {}", normalized.value),
1335                     None => format!("\n  {}", candidate),
1336                 }
1337             })
1338         };
1339
1340         // Sort impl candidates so that ordering is consistent for UI tests.
1341         let mut normalized_impl_candidates =
1342             impl_candidates.iter().map(normalize).collect::<Vec<String>>();
1343
1344         // Sort before taking the `..end` range,
1345         // because the ordering of `impl_candidates` may not be deterministic:
1346         // https://github.com/rust-lang/rust/pull/57475#issuecomment-455519507
1347         normalized_impl_candidates.sort();
1348
1349         err.help(&format!(
1350             "the following implementations were found:{}{}",
1351             normalized_impl_candidates[..end].join(""),
1352             if len > 5 { format!("\nand {} others", len - 4) } else { String::new() }
1353         ));
1354     }
1355
1356     /// Gets the parent trait chain start
1357     fn get_parent_trait_ref(
1358         &self,
1359         code: &ObligationCauseCode<'tcx>,
1360     ) -> Option<(String, Option<Span>)> {
1361         match code {
1362             &ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
1363                 let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
1364                 match self.get_parent_trait_ref(&data.parent_code) {
1365                     Some(t) => Some(t),
1366                     None => {
1367                         let ty = parent_trait_ref.skip_binder().self_ty();
1368                         let span =
1369                             TyCategory::from_ty(ty).map(|(_, def_id)| self.tcx.def_span(def_id));
1370                         Some((ty.to_string(), span))
1371                     }
1372                 }
1373             }
1374             _ => None,
1375         }
1376     }
1377
1378     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1379     /// with the same path as `trait_ref`, a help message about
1380     /// a probable version mismatch is added to `err`
1381     fn note_version_mismatch(
1382         &self,
1383         err: &mut DiagnosticBuilder<'_>,
1384         trait_ref: &ty::PolyTraitRef<'tcx>,
1385     ) {
1386         let get_trait_impl = |trait_def_id| {
1387             let mut trait_impl = None;
1388             self.tcx.for_each_relevant_impl(
1389                 trait_def_id,
1390                 trait_ref.skip_binder().self_ty(),
1391                 |impl_def_id| {
1392                     if trait_impl.is_none() {
1393                         trait_impl = Some(impl_def_id);
1394                     }
1395                 },
1396             );
1397             trait_impl
1398         };
1399         let required_trait_path = self.tcx.def_path_str(trait_ref.def_id());
1400         let all_traits = self.tcx.all_traits(LOCAL_CRATE);
1401         let traits_with_same_path: std::collections::BTreeSet<_> = all_traits
1402             .iter()
1403             .filter(|trait_def_id| **trait_def_id != trait_ref.def_id())
1404             .filter(|trait_def_id| self.tcx.def_path_str(**trait_def_id) == required_trait_path)
1405             .collect();
1406         for trait_with_same_path in traits_with_same_path {
1407             if let Some(impl_def_id) = get_trait_impl(*trait_with_same_path) {
1408                 let impl_span = self.tcx.def_span(impl_def_id);
1409                 err.span_help(impl_span, "trait impl with same name found");
1410                 let trait_crate = self.tcx.crate_name(trait_with_same_path.krate);
1411                 let crate_msg = format!(
1412                     "perhaps two different versions of crate `{}` are being used?",
1413                     trait_crate
1414                 );
1415                 err.note(&crate_msg);
1416             }
1417         }
1418     }
1419
1420     fn mk_trait_obligation_with_new_self_ty(
1421         &self,
1422         param_env: ty::ParamEnv<'tcx>,
1423         trait_ref: &ty::PolyTraitRef<'tcx>,
1424         new_self_ty: Ty<'tcx>,
1425     ) -> PredicateObligation<'tcx> {
1426         assert!(!new_self_ty.has_escaping_bound_vars());
1427
1428         let trait_ref = trait_ref.map_bound_ref(|tr| ty::TraitRef {
1429             substs: self.tcx.mk_substs_trait(new_self_ty, &tr.substs[1..]),
1430             ..*tr
1431         });
1432
1433         Obligation::new(
1434             ObligationCause::dummy(),
1435             param_env,
1436             trait_ref.without_const().to_predicate(self.tcx),
1437         )
1438     }
1439
1440     fn maybe_report_ambiguity(
1441         &self,
1442         obligation: &PredicateObligation<'tcx>,
1443         body_id: Option<hir::BodyId>,
1444     ) {
1445         // Unable to successfully determine, probably means
1446         // insufficient type information, but could mean
1447         // ambiguous impls. The latter *ought* to be a
1448         // coherence violation, so we don't report it here.
1449
1450         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
1451         let span = obligation.cause.span;
1452
1453         debug!(
1454             "maybe_report_ambiguity(predicate={:?}, obligation={:?} body_id={:?}, code={:?})",
1455             predicate, obligation, body_id, obligation.cause.code,
1456         );
1457
1458         // Ambiguity errors are often caused as fallout from earlier
1459         // errors. So just ignore them if this infcx is tainted.
1460         if self.is_tainted_by_errors() {
1461             return;
1462         }
1463
1464         let mut err = match predicate.skip_binders() {
1465             ty::PredicateAtom::Trait(data, _) => {
1466                 let trait_ref = ty::Binder::bind(data.trait_ref);
1467                 let self_ty = trait_ref.skip_binder().self_ty();
1468                 debug!("self_ty {:?} {:?} trait_ref {:?}", self_ty, self_ty.kind(), trait_ref);
1469
1470                 if predicate.references_error() {
1471                     return;
1472                 }
1473                 // Typically, this ambiguity should only happen if
1474                 // there are unresolved type inference variables
1475                 // (otherwise it would suggest a coherence
1476                 // failure). But given #21974 that is not necessarily
1477                 // the case -- we can have multiple where clauses that
1478                 // are only distinguished by a region, which results
1479                 // in an ambiguity even when all types are fully
1480                 // known, since we don't dispatch based on region
1481                 // relationships.
1482
1483                 // This is kind of a hack: it frequently happens that some earlier
1484                 // error prevents types from being fully inferred, and then we get
1485                 // a bunch of uninteresting errors saying something like "<generic
1486                 // #0> doesn't implement Sized".  It may even be true that we
1487                 // could just skip over all checks where the self-ty is an
1488                 // inference variable, but I was afraid that there might be an
1489                 // inference variable created, registered as an obligation, and
1490                 // then never forced by writeback, and hence by skipping here we'd
1491                 // be ignoring the fact that we don't KNOW the type works
1492                 // out. Though even that would probably be harmless, given that
1493                 // we're only talking about builtin traits, which are known to be
1494                 // inhabited. We used to check for `self.tcx.sess.has_errors()` to
1495                 // avoid inundating the user with unnecessary errors, but we now
1496                 // check upstream for type errors and don't add the obligations to
1497                 // begin with in those cases.
1498                 if self.tcx.lang_items().sized_trait() == Some(trait_ref.def_id()) {
1499                     self.emit_inference_failure_err(
1500                         body_id,
1501                         span,
1502                         self_ty.into(),
1503                         ErrorCode::E0282,
1504                     )
1505                     .emit();
1506                     return;
1507                 }
1508                 let mut err = self.emit_inference_failure_err(
1509                     body_id,
1510                     span,
1511                     self_ty.into(),
1512                     ErrorCode::E0283,
1513                 );
1514                 err.note(&format!("cannot satisfy `{}`", predicate));
1515                 if let ObligationCauseCode::ItemObligation(def_id) = obligation.cause.code {
1516                     self.suggest_fully_qualified_path(&mut err, def_id, span, trait_ref.def_id());
1517                 } else if let (
1518                     Ok(ref snippet),
1519                     ObligationCauseCode::BindingObligation(ref def_id, _),
1520                 ) =
1521                     (self.tcx.sess.source_map().span_to_snippet(span), &obligation.cause.code)
1522                 {
1523                     let generics = self.tcx.generics_of(*def_id);
1524                     if generics.params.iter().any(|p| p.name != kw::SelfUpper)
1525                         && !snippet.ends_with('>')
1526                     {
1527                         // FIXME: To avoid spurious suggestions in functions where type arguments
1528                         // where already supplied, we check the snippet to make sure it doesn't
1529                         // end with a turbofish. Ideally we would have access to a `PathSegment`
1530                         // instead. Otherwise we would produce the following output:
1531                         //
1532                         // error[E0283]: type annotations needed
1533                         //   --> $DIR/issue-54954.rs:3:24
1534                         //    |
1535                         // LL | const ARR_LEN: usize = Tt::const_val::<[i8; 123]>();
1536                         //    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^
1537                         //    |                        |
1538                         //    |                        cannot infer type
1539                         //    |                        help: consider specifying the type argument
1540                         //    |                        in the function call:
1541                         //    |                        `Tt::const_val::<[i8; 123]>::<T>`
1542                         // ...
1543                         // LL |     const fn const_val<T: Sized>() -> usize {
1544                         //    |                        - required by this bound in `Tt::const_val`
1545                         //    |
1546                         //    = note: cannot satisfy `_: Tt`
1547
1548                         err.span_suggestion_verbose(
1549                             span.shrink_to_hi(),
1550                             &format!(
1551                                 "consider specifying the type argument{} in the function call",
1552                                 pluralize!(generics.params.len()),
1553                             ),
1554                             format!(
1555                                 "::<{}>",
1556                                 generics
1557                                     .params
1558                                     .iter()
1559                                     .map(|p| p.name.to_string())
1560                                     .collect::<Vec<String>>()
1561                                     .join(", ")
1562                             ),
1563                             Applicability::HasPlaceholders,
1564                         );
1565                     }
1566                 }
1567                 err
1568             }
1569
1570             ty::PredicateAtom::WellFormed(arg) => {
1571                 // Same hacky approach as above to avoid deluging user
1572                 // with error messages.
1573                 if arg.references_error() || self.tcx.sess.has_errors() {
1574                     return;
1575                 }
1576
1577                 self.emit_inference_failure_err(body_id, span, arg, ErrorCode::E0282)
1578             }
1579
1580             ty::PredicateAtom::Subtype(data) => {
1581                 if data.references_error() || self.tcx.sess.has_errors() {
1582                     // no need to overload user in such cases
1583                     return;
1584                 }
1585                 let SubtypePredicate { a_is_expected: _, a, b } = data;
1586                 // both must be type variables, or the other would've been instantiated
1587                 assert!(a.is_ty_var() && b.is_ty_var());
1588                 self.emit_inference_failure_err(body_id, span, a.into(), ErrorCode::E0282)
1589             }
1590             ty::PredicateAtom::Projection(data) => {
1591                 let trait_ref = ty::Binder::bind(data).to_poly_trait_ref(self.tcx);
1592                 let self_ty = trait_ref.skip_binder().self_ty();
1593                 let ty = data.ty;
1594                 if predicate.references_error() {
1595                     return;
1596                 }
1597                 if self_ty.needs_infer() && ty.needs_infer() {
1598                     // We do this for the `foo.collect()?` case to produce a suggestion.
1599                     let mut err = self.emit_inference_failure_err(
1600                         body_id,
1601                         span,
1602                         self_ty.into(),
1603                         ErrorCode::E0284,
1604                     );
1605                     err.note(&format!("cannot satisfy `{}`", predicate));
1606                     err
1607                 } else {
1608                     let mut err = struct_span_err!(
1609                         self.tcx.sess,
1610                         span,
1611                         E0284,
1612                         "type annotations needed: cannot satisfy `{}`",
1613                         predicate,
1614                     );
1615                     err.span_label(span, &format!("cannot satisfy `{}`", predicate));
1616                     err
1617                 }
1618             }
1619
1620             _ => {
1621                 if self.tcx.sess.has_errors() {
1622                     return;
1623                 }
1624                 let mut err = struct_span_err!(
1625                     self.tcx.sess,
1626                     span,
1627                     E0284,
1628                     "type annotations needed: cannot satisfy `{}`",
1629                     predicate,
1630                 );
1631                 err.span_label(span, &format!("cannot satisfy `{}`", predicate));
1632                 err
1633             }
1634         };
1635         self.note_obligation_cause(&mut err, obligation);
1636         err.emit();
1637     }
1638
1639     /// Returns `true` if the trait predicate may apply for *some* assignment
1640     /// to the type parameters.
1641     fn predicate_can_apply(
1642         &self,
1643         param_env: ty::ParamEnv<'tcx>,
1644         pred: ty::PolyTraitRef<'tcx>,
1645     ) -> bool {
1646         struct ParamToVarFolder<'a, 'tcx> {
1647             infcx: &'a InferCtxt<'a, 'tcx>,
1648             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
1649         }
1650
1651         impl<'a, 'tcx> TypeFolder<'tcx> for ParamToVarFolder<'a, 'tcx> {
1652             fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
1653                 self.infcx.tcx
1654             }
1655
1656             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
1657                 if let ty::Param(ty::ParamTy { name, .. }) = *ty.kind() {
1658                     let infcx = self.infcx;
1659                     self.var_map.entry(ty).or_insert_with(|| {
1660                         infcx.next_ty_var(TypeVariableOrigin {
1661                             kind: TypeVariableOriginKind::TypeParameterDefinition(name, None),
1662                             span: DUMMY_SP,
1663                         })
1664                     })
1665                 } else {
1666                     ty.super_fold_with(self)
1667                 }
1668             }
1669         }
1670
1671         self.probe(|_| {
1672             let mut selcx = SelectionContext::new(self);
1673
1674             let cleaned_pred =
1675                 pred.fold_with(&mut ParamToVarFolder { infcx: self, var_map: Default::default() });
1676
1677             let cleaned_pred = super::project::normalize(
1678                 &mut selcx,
1679                 param_env,
1680                 ObligationCause::dummy(),
1681                 &cleaned_pred,
1682             )
1683             .value;
1684
1685             let obligation = Obligation::new(
1686                 ObligationCause::dummy(),
1687                 param_env,
1688                 cleaned_pred.without_const().to_predicate(selcx.tcx()),
1689             );
1690
1691             self.predicate_may_hold(&obligation)
1692         })
1693     }
1694
1695     fn note_obligation_cause(
1696         &self,
1697         err: &mut DiagnosticBuilder<'tcx>,
1698         obligation: &PredicateObligation<'tcx>,
1699     ) {
1700         // First, attempt to add note to this error with an async-await-specific
1701         // message, and fall back to regular note otherwise.
1702         if !self.maybe_note_obligation_cause_for_async_await(err, obligation) {
1703             self.note_obligation_cause_code(
1704                 err,
1705                 &obligation.predicate,
1706                 &obligation.cause.code,
1707                 &mut vec![],
1708             );
1709             self.suggest_unsized_bound_if_applicable(err, obligation);
1710         }
1711     }
1712
1713     fn suggest_unsized_bound_if_applicable(
1714         &self,
1715         err: &mut DiagnosticBuilder<'tcx>,
1716         obligation: &PredicateObligation<'tcx>,
1717     ) {
1718         let (pred, item_def_id, span) =
1719             match (obligation.predicate.skip_binders(), obligation.cause.code.peel_derives()) {
1720                 (
1721                     ty::PredicateAtom::Trait(pred, _),
1722                     &ObligationCauseCode::BindingObligation(item_def_id, span),
1723                 ) => (pred, item_def_id, span),
1724                 _ => return,
1725             };
1726
1727         let node = match (
1728             self.tcx.hir().get_if_local(item_def_id),
1729             Some(pred.def_id()) == self.tcx.lang_items().sized_trait(),
1730         ) {
1731             (Some(node), true) => node,
1732             _ => return,
1733         };
1734         let generics = match node.generics() {
1735             Some(generics) => generics,
1736             None => return,
1737         };
1738         for param in generics.params {
1739             if param.span != span
1740                 || param.bounds.iter().any(|bound| {
1741                     bound.trait_ref().and_then(|trait_ref| trait_ref.trait_def_id())
1742                         == self.tcx.lang_items().sized_trait()
1743                 })
1744             {
1745                 continue;
1746             }
1747             match node {
1748                 hir::Node::Item(
1749                     item
1750                     @
1751                     hir::Item {
1752                         kind:
1753                             hir::ItemKind::Enum(..)
1754                             | hir::ItemKind::Struct(..)
1755                             | hir::ItemKind::Union(..),
1756                         ..
1757                     },
1758                 ) => {
1759                     // Suggesting `T: ?Sized` is only valid in an ADT if `T` is only used in a
1760                     // borrow. `struct S<'a, T: ?Sized>(&'a T);` is valid, `struct S<T: ?Sized>(T);`
1761                     // is not.
1762                     let mut visitor = FindTypeParam {
1763                         param: param.name.ident().name,
1764                         invalid_spans: vec![],
1765                         nested: false,
1766                     };
1767                     visitor.visit_item(item);
1768                     if !visitor.invalid_spans.is_empty() {
1769                         let mut multispan: MultiSpan = param.span.into();
1770                         multispan.push_span_label(
1771                             param.span,
1772                             format!("this could be changed to `{}: ?Sized`...", param.name.ident()),
1773                         );
1774                         for sp in visitor.invalid_spans {
1775                             multispan.push_span_label(
1776                                 sp,
1777                                 format!(
1778                                     "...if indirection was used here: `Box<{}>`",
1779                                     param.name.ident(),
1780                                 ),
1781                             );
1782                         }
1783                         err.span_help(
1784                             multispan,
1785                             &format!(
1786                                 "you could relax the implicit `Sized` bound on `{T}` if it were \
1787                                  used through indirection like `&{T}` or `Box<{T}>`",
1788                                 T = param.name.ident(),
1789                             ),
1790                         );
1791                         return;
1792                     }
1793                 }
1794                 _ => {}
1795             }
1796             let (span, separator) = match param.bounds {
1797                 [] => (span.shrink_to_hi(), ":"),
1798                 [.., bound] => (bound.span().shrink_to_hi(), " +"),
1799             };
1800             err.span_suggestion_verbose(
1801                 span,
1802                 "consider relaxing the implicit `Sized` restriction",
1803                 format!("{} ?Sized", separator),
1804                 Applicability::MachineApplicable,
1805             );
1806             return;
1807         }
1808     }
1809
1810     fn is_recursive_obligation(
1811         &self,
1812         obligated_types: &mut Vec<&ty::TyS<'tcx>>,
1813         cause_code: &ObligationCauseCode<'tcx>,
1814     ) -> bool {
1815         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
1816             let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
1817
1818             if obligated_types.iter().any(|ot| ot == &parent_trait_ref.skip_binder().self_ty()) {
1819                 return true;
1820             }
1821         }
1822         false
1823     }
1824 }
1825
1826 /// Look for type `param` in an ADT being used only through a reference to confirm that suggesting
1827 /// `param: ?Sized` would be a valid constraint.
1828 struct FindTypeParam {
1829     param: rustc_span::Symbol,
1830     invalid_spans: Vec<Span>,
1831     nested: bool,
1832 }
1833
1834 impl<'v> Visitor<'v> for FindTypeParam {
1835     type Map = rustc_hir::intravisit::ErasedMap<'v>;
1836
1837     fn nested_visit_map(&mut self) -> hir::intravisit::NestedVisitorMap<Self::Map> {
1838         hir::intravisit::NestedVisitorMap::None
1839     }
1840
1841     fn visit_ty(&mut self, ty: &hir::Ty<'_>) {
1842         // We collect the spans of all uses of the "bare" type param, like in `field: T` or
1843         // `field: (T, T)` where we could make `T: ?Sized` while skipping cases that are known to be
1844         // valid like `field: &'a T` or `field: *mut T` and cases that *might* have further `Sized`
1845         // obligations like `Box<T>` and `Vec<T>`, but we perform no extra analysis for those cases
1846         // and suggest `T: ?Sized` regardless of their obligations. This is fine because the errors
1847         // in that case should make what happened clear enough.
1848         match ty.kind {
1849             hir::TyKind::Ptr(_) | hir::TyKind::Rptr(..) | hir::TyKind::TraitObject(..) => {}
1850             hir::TyKind::Path(hir::QPath::Resolved(None, path))
1851                 if path.segments.len() == 1 && path.segments[0].ident.name == self.param =>
1852             {
1853                 if !self.nested {
1854                     self.invalid_spans.push(ty.span);
1855                 }
1856             }
1857             hir::TyKind::Path(_) => {
1858                 let prev = self.nested;
1859                 self.nested = true;
1860                 hir::intravisit::walk_ty(self, ty);
1861                 self.nested = prev;
1862             }
1863             _ => {
1864                 hir::intravisit::walk_ty(self, ty);
1865             }
1866         }
1867     }
1868 }
1869
1870 pub fn recursive_type_with_infinite_size_error(
1871     tcx: TyCtxt<'tcx>,
1872     type_def_id: DefId,
1873     spans: Vec<Span>,
1874 ) {
1875     assert!(type_def_id.is_local());
1876     let span = tcx.hir().span_if_local(type_def_id).unwrap();
1877     let span = tcx.sess.source_map().guess_head_span(span);
1878     let path = tcx.def_path_str(type_def_id);
1879     let mut err =
1880         struct_span_err!(tcx.sess, span, E0072, "recursive type `{}` has infinite size", path);
1881     err.span_label(span, "recursive type has infinite size");
1882     for &span in &spans {
1883         err.span_label(span, "recursive without indirection");
1884     }
1885     let msg = format!(
1886         "insert some indirection (e.g., a `Box`, `Rc`, or `&`) to make `{}` representable",
1887         path,
1888     );
1889     if spans.len() <= 4 {
1890         err.multipart_suggestion(
1891             &msg,
1892             spans
1893                 .iter()
1894                 .flat_map(|&span| {
1895                     vec![
1896                         (span.shrink_to_lo(), "Box<".to_string()),
1897                         (span.shrink_to_hi(), ">".to_string()),
1898                     ]
1899                     .into_iter()
1900                 })
1901                 .collect(),
1902             Applicability::HasPlaceholders,
1903         );
1904     } else {
1905         err.help(&msg);
1906     }
1907     err.emit();
1908 }
1909
1910 /// Summarizes information
1911 #[derive(Clone)]
1912 pub enum ArgKind {
1913     /// An argument of non-tuple type. Parameters are (name, ty)
1914     Arg(String, String),
1915
1916     /// An argument of tuple type. For a "found" argument, the span is
1917     /// the locationo in the source of the pattern. For a "expected"
1918     /// argument, it will be None. The vector is a list of (name, ty)
1919     /// strings for the components of the tuple.
1920     Tuple(Option<Span>, Vec<(String, String)>),
1921 }
1922
1923 impl ArgKind {
1924     fn empty() -> ArgKind {
1925         ArgKind::Arg("_".to_owned(), "_".to_owned())
1926     }
1927
1928     /// Creates an `ArgKind` from the expected type of an
1929     /// argument. It has no name (`_`) and an optional source span.
1930     pub fn from_expected_ty(t: Ty<'_>, span: Option<Span>) -> ArgKind {
1931         match t.kind() {
1932             ty::Tuple(tys) => ArgKind::Tuple(
1933                 span,
1934                 tys.iter().map(|ty| ("_".to_owned(), ty.to_string())).collect::<Vec<_>>(),
1935             ),
1936             _ => ArgKind::Arg("_".to_owned(), t.to_string()),
1937         }
1938     }
1939 }