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::{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                 match obligation.predicate.kind() {
 258                     ty::PredicateKind::Trait(ref trait_predicate, _) => {
 259                         let trait_predicate = self.resolve_vars_if_possible(trait_predicate);
 260
 261                         if self.tcx.sess.has_errors() && trait_predicate.references_error() {
 262                             return;
 263                         }
 264                         let trait_ref = trait_predicate.to_poly_trait_ref();
 265                         let (post_message, pre_message, type_def) = self
 266                             .get_parent_trait_ref(&obligation.cause.code)
 267                             .map(|(t, s)| {
 268                                 (
 269                                     format!(" in `{}`", t),
 270                                     format!("within `{}`, ", t),
 271                                     s.map(|s| (format!("within this `{}`", t), s)),
 272                                 )
 273                             })
 274                             .unwrap_or_default();
 275
 276                         let OnUnimplementedNote { message, label, note, enclosing_scope } =
 277                             self.on_unimplemented_note(trait_ref, obligation);
 278                         let have_alt_message = message.is_some() || label.is_some();
 279                         let is_try = self
 280                             .tcx
 281                             .sess
 282                             .source_map()
 283                             .span_to_snippet(span)
 284                             .map(|s| &s == "?")
 285                             .unwrap_or(false);
 286                         let is_from = format!("{}", trait_ref.print_only_trait_path())
 287                             .starts_with("std::convert::From<");
 288                         let is_unsize =
 289                             { Some(trait_ref.def_id()) == self.tcx.lang_items().unsize_trait() };
 290                         let (message, note) = if is_try && is_from {
 291                             (
 292                                 Some(format!(
 293                                     "`?` couldn't convert the error to `{}`",
 294                                     trait_ref.skip_binder().self_ty(),
 295                                 )),
 296                                 Some(
 297                                     "the question mark operation (`?`) implicitly performs a \
 298                                         conversion on the error value using the `From` trait"
 299                                         .to_owned(),
 300                                 ),
 301                             )
 302                         } else {
 303                             (message, note)
 304                         };
 305
 306                         let mut err = struct_span_err!(
 307                             self.tcx.sess,
 308                             span,
 309                             E0277,
 310                             "{}",
 311                             message.unwrap_or_else(|| format!(
 312                                 "the trait bound `{}` is not satisfied{}",
 313                                 trait_ref.without_const().to_predicate(tcx),
 314                                 post_message,
 315                             ))
 316                         );
 317
 318                         let should_convert_option_to_result =
 319                             format!("{}", trait_ref.print_only_trait_path())
 320                                 .starts_with("std::convert::From<std::option::NoneError");
 321                         let should_convert_result_to_option = format!("{}", trait_ref)
 322                             .starts_with("<std::option::NoneError as std::convert::From<");
 323                         if is_try && is_from {
 324                             if should_convert_option_to_result {
 325                                 err.span_suggestion_verbose(
 326                                     span.shrink_to_lo(),
 327                                     "consider converting the `Option<T>` into a `Result<T, _>` \
 328                                      using `Option::ok_or` or `Option::ok_or_else`",
 329                                     ".ok_or_else(|| /* error value */)".to_string(),
 330                                     Applicability::HasPlaceholders,
 331                                 );
 332                             } else if should_convert_result_to_option {
 333                                 err.span_suggestion_verbose(
 334                                     span.shrink_to_lo(),
 335                                     "consider converting the `Result<T, _>` into an `Option<T>` \
 336                                      using `Result::ok`",
 337                                     ".ok()".to_string(),
 338                                     Applicability::MachineApplicable,
 339                                 );
 340                             }
 341                             if let Some(ret_span) = self.return_type_span(obligation) {
 342                                 err.span_label(
 343                                     ret_span,
 344                                     &format!(
 345                                         "expected `{}` because of this",
 346                                         trait_ref.skip_binder().self_ty()
 347                                     ),
 348                                 );
 349                             }
 350                         }
 351
 352                         let explanation =
 353                             if obligation.cause.code == ObligationCauseCode::MainFunctionType {
 354                                 "consider using `()`, or a `Result`".to_owned()
 355                             } else {
 356                                 format!(
 357                                     "{}the trait `{}` is not implemented for `{}`",
 358                                     pre_message,
 359                                     trait_ref.print_only_trait_path(),
 360                                     trait_ref.skip_binder().self_ty(),
 361                                 )
 362                             };
 363
 364                         if self.suggest_add_reference_to_arg(
 365                             &obligation,
 366                             &mut err,
 367                             &trait_ref,
 368                             points_at_arg,
 369                             have_alt_message,
 370                         ) {
 371                             self.note_obligation_cause(&mut err, obligation);
 372                             err.emit();
 373                             return;
 374                         }
 375                         if let Some(ref s) = label {
 376                             // If it has a custom `#[rustc_on_unimplemented]`
 377                             // error message, let's display it as the label!
 378                             err.span_label(span, s.as_str());
 379                             err.help(&explanation);
 380                         } else {
 381                             err.span_label(span, explanation);
 382                         }
 383                         if let Some((msg, span)) = type_def {
 384                             err.span_label(span, &msg);
 385                         }
 386                         if let Some(ref s) = note {
 387                             // If it has a custom `#[rustc_on_unimplemented]` note, let's display it
 388                             err.note(s.as_str());
 389                         }
 390                         if let Some(ref s) = enclosing_scope {
 391                             let enclosing_scope_span = tcx.def_span(
 392                                 tcx.hir()
 393                                     .opt_local_def_id(obligation.cause.body_id)
 394                                     .unwrap_or_else(|| {
 395                                         tcx.hir().body_owner_def_id(hir::BodyId {
 396                                             hir_id: obligation.cause.body_id,
 397                                         })
 398                                     })
 399                                     .to_def_id(),
 400                             );
 401
 402                             err.span_label(enclosing_scope_span, s.as_str());
 403                         }
 404
 405                         self.suggest_dereferences(&obligation, &mut err, &trait_ref, points_at_arg);
 406                         self.suggest_fn_call(&obligation, &mut err, &trait_ref, points_at_arg);
 407                         self.suggest_remove_reference(&obligation, &mut err, &trait_ref);
 408                         self.suggest_semicolon_removal(&obligation, &mut err, span, &trait_ref);
 409                         self.note_version_mismatch(&mut err, &trait_ref);
 410
 411                         if Some(trait_ref.def_id()) == tcx.lang_items().try_trait() {
 412                             self.suggest_await_before_try(&mut err, &obligation, &trait_ref, span);
 413                         }
 414
 415                         if self.suggest_impl_trait(&mut err, span, &obligation, &trait_ref) {
 416                             err.emit();
 417                             return;
 418                         }
 419
 420                         if is_unsize {
 421                             // If the obligation failed due to a missing implementation of the
 422                             // `Unsize` trait, give a pointer to why that might be the case
 423                             err.note(
 424                                 "all implementations of `Unsize` are provided \
 425                                 automatically by the compiler, see \
 426                                 <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> \
 427                                 for more information",
 428                             );
 429                         }
 430
 431                         let is_fn_trait = [
 432                             self.tcx.lang_items().fn_trait(),
 433                             self.tcx.lang_items().fn_mut_trait(),
 434                             self.tcx.lang_items().fn_once_trait(),
 435                         ]
 436                         .contains(&Some(trait_ref.def_id()));
 437                         let is_target_feature_fn =
 438                             if let ty::FnDef(def_id, _) = trait_ref.skip_binder().self_ty().kind {
 439                                 !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
 440                             } else {
 441                                 false
 442                             };
 443                         if is_fn_trait && is_target_feature_fn {
 444                             err.note(
 445                                 "`#[target_feature]` functions do not implement the `Fn` traits",
 446                             );
 447                         }
 448
 449                         // Try to report a help message
 450                         if !trait_ref.has_infer_types_or_consts()
 451                             && self.predicate_can_apply(obligation.param_env, trait_ref)
 452                         {
 453                             // If a where-clause may be useful, remind the
 454                             // user that they can add it.
 455                             //
 456                             // don't display an on-unimplemented note, as
 457                             // these notes will often be of the form
 458                             //     "the type `T` can't be frobnicated"
 459                             // which is somewhat confusing.
 460                             self.suggest_restricting_param_bound(
 461                                 &mut err,
 462                                 trait_ref,
 463                                 obligation.cause.body_id,
 464                             );
 465                         } else {
 466                             if !have_alt_message {
 467                                 // Can't show anything else useful, try to find similar impls.
 468                                 let impl_candidates = self.find_similar_impl_candidates(trait_ref);
 469                                 self.report_similar_impl_candidates(impl_candidates, &mut err);
 470                             }
 471                             // Changing mutability doesn't make a difference to whether we have
 472                             // an `Unsize` impl (Fixes ICE in #71036)
 473                             if !is_unsize {
 474                                 self.suggest_change_mut(
 475                                     &obligation,
 476                                     &mut err,
 477                                     &trait_ref,
 478                                     points_at_arg,
 479                                 );
 480                             }
 481                         }
 482
 483                         // If this error is due to `!: Trait` not implemented but `(): Trait` is
 484                         // implemented, and fallback has occurred, then it could be due to a
 485                         // variable that used to fallback to `()` now falling back to `!`. Issue a
 486                         // note informing about the change in behaviour.
 487                         if trait_predicate.skip_binder().self_ty().is_never()
 488                             && fallback_has_occurred
 489                         {
 490                             let predicate = trait_predicate.map_bound(|mut trait_pred| {
 491                                 trait_pred.trait_ref.substs = self.tcx.mk_substs_trait(
 492                                     self.tcx.mk_unit(),
 493                                     &trait_pred.trait_ref.substs[1..],
 494                                 );
 495                                 trait_pred
 496                             });
 497                             let unit_obligation = Obligation {
 498                                 predicate: ty::PredicateKind::Trait(
 499                                     predicate,
 500                                     hir::Constness::NotConst,
 501                                 )
 502                                 .to_predicate(self.tcx),
 503                                 ..obligation.clone()
 504                             };
 505                             if self.predicate_may_hold(&unit_obligation) {
 506                                 err.note(
 507                                     "the trait is implemented for `()`. \
 508                                      Possibly this error has been caused by changes to \
 509                                      Rust's type-inference algorithm (see issue #48950 \
 510                                      <https://github.com/rust-lang/rust/issues/48950> \
 511                                      for more information). Consider whether you meant to use \
 512                                      the type `()` here instead.",
 513                                 );
 514                             }
 515                         }
 516
 517                         err
 518                     }
 519
 520                     ty::PredicateKind::Subtype(ref predicate) => {
 521                         // Errors for Subtype predicates show up as
 522                         // `FulfillmentErrorCode::CodeSubtypeError`,
 523                         // not selection error.
 524                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
 525                     }
 526
 527                     ty::PredicateKind::RegionOutlives(ref predicate) => {
 528                         let predicate = self.resolve_vars_if_possible(predicate);
 529                         let err = self
 530                             .region_outlives_predicate(&obligation.cause, predicate)
 531                             .err()
 532                             .unwrap();
 533                         struct_span_err!(
 534                             self.tcx.sess,
 535                             span,
 536                             E0279,
 537                             "the requirement `{}` is not satisfied (`{}`)",
 538                             predicate,
 539                             err,
 540                         )
 541                     }
 542
 543                     ty::PredicateKind::Projection(..) | ty::PredicateKind::TypeOutlives(..) => {
 544                         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
 545                         struct_span_err!(
 546                             self.tcx.sess,
 547                             span,
 548                             E0280,
 549                             "the requirement `{}` is not satisfied",
 550                             predicate
 551                         )
 552                     }
 553
 554                     &ty::PredicateKind::ObjectSafe(trait_def_id) => {
 555                         let violations = self.tcx.object_safety_violations(trait_def_id);
 556                         report_object_safety_error(self.tcx, span, trait_def_id, violations)
 557                     }
 558
 559                     &ty::PredicateKind::ClosureKind(closure_def_id, closure_substs, kind) => {
 560                         let found_kind = self.closure_kind(closure_substs).unwrap();
 561                         let closure_span =
 562                             self.tcx.sess.source_map().guess_head_span(
 563                                 self.tcx.hir().span_if_local(closure_def_id).unwrap(),
 564                             );
 565                         let hir_id = self.tcx.hir().as_local_hir_id(closure_def_id.expect_local());
 566                         let mut err = struct_span_err!(
 567                             self.tcx.sess,
 568                             closure_span,
 569                             E0525,
 570                             "expected a closure that implements the `{}` trait, \
 571                              but this closure only implements `{}`",
 572                             kind,
 573                             found_kind
 574                         );
 575
 576                         err.span_label(
 577                             closure_span,
 578                             format!("this closure implements `{}`, not `{}`", found_kind, kind),
 579                         );
 580                         err.span_label(
 581                             obligation.cause.span,
 582                             format!("the requirement to implement `{}` derives from here", kind),
 583                         );
 584
 585                         // Additional context information explaining why the closure only implements
 586                         // a particular trait.
 587                         if let Some(tables) = self.in_progress_tables {
 588                             let tables = tables.borrow();
 589                             match (found_kind, tables.closure_kind_origins().get(hir_id)) {
 590                                 (ty::ClosureKind::FnOnce, Some((span, name))) => {
 591                                     err.span_label(
 592                                         *span,
 593                                         format!(
 594                                             "closure is `FnOnce` because it moves the \
 595                                          variable `{}` out of its environment",
 596                                             name
 597                                         ),
 598                                     );
 599                                 }
 600                                 (ty::ClosureKind::FnMut, Some((span, name))) => {
 601                                     err.span_label(
 602                                         *span,
 603                                         format!(
 604                                             "closure is `FnMut` because it mutates the \
 605                                          variable `{}` here",
 606                                             name
 607                                         ),
 608                                     );
 609                                 }
 610                                 _ => {}
 611                             }
 612                         }
 613
 614                         err.emit();
 615                         return;
 616                     }
 617
 618                     ty::PredicateKind::WellFormed(ty) => {
 619                         if !self.tcx.sess.opts.debugging_opts.chalk {
 620                             // WF predicates cannot themselves make
 621                             // errors. They can only block due to
 622                             // ambiguity; otherwise, they always
 623                             // degenerate into other obligations
 624                             // (which may fail).
 625                             span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 626                         } else {
 627                             // FIXME: we'll need a better message which takes into account
 628                             // which bounds actually failed to hold.
 629                             self.tcx.sess.struct_span_err(
 630                                 span,
 631                                 &format!("the type `{}` is not well-formed (chalk)", ty),
 632                             )
 633                         }
 634                     }
 635
 636                     ty::PredicateKind::ConstEvaluatable(..) => {
 637                         // Errors for `ConstEvaluatable` predicates show up as
 638                         // `SelectionError::ConstEvalFailure`,
 639                         // not `Unimplemented`.
 640                         span_bug!(
 641                             span,
 642                             "const-evaluatable requirement gave wrong error: `{:?}`",
 643                             obligation
 644                         )
 645                     }
 646
 647                     ty::PredicateKind::ConstEquate(..) => {
 648                         // Errors for `ConstEquate` predicates show up as
 649                         // `SelectionError::ConstEvalFailure`,
 650                         // not `Unimplemented`.
 651                         span_bug!(
 652                             span,
 653                             "const-equate requirement gave wrong error: `{:?}`",
 654                             obligation
 655                         )
 656                     }
 657                 }
 658             }
 659
 660             OutputTypeParameterMismatch(ref found_trait_ref, ref expected_trait_ref, _) => {
 661                 let found_trait_ref = self.resolve_vars_if_possible(&*found_trait_ref);
 662                 let expected_trait_ref = self.resolve_vars_if_possible(&*expected_trait_ref);
 663
 664                 if expected_trait_ref.self_ty().references_error() {
 665                     return;
 666                 }
 667
 668                 let found_trait_ty = match found_trait_ref.self_ty().no_bound_vars() {
 669                     Some(ty) => ty,
 670                     None => return,
 671                 };
 672
 673                 let found_did = match found_trait_ty.kind {
 674                     ty::Closure(did, _) | ty::Foreign(did) | ty::FnDef(did, _) => Some(did),
 675                     ty::Adt(def, _) => Some(def.did),
 676                     _ => None,
 677                 };
 678
 679                 let found_span = found_did
 680                     .and_then(|did| self.tcx.hir().span_if_local(did))
 681                     .map(|sp| self.tcx.sess.source_map().guess_head_span(sp)); // the sp could be an fn def
 682
 683                 if self.reported_closure_mismatch.borrow().contains(&(span, found_span)) {
 684                     // We check closures twice, with obligations flowing in different directions,
 685                     // but we want to complain about them only once.
 686                     return;
 687                 }
 688
 689                 self.reported_closure_mismatch.borrow_mut().insert((span, found_span));
 690
 691                 let found = match found_trait_ref.skip_binder().substs.type_at(1).kind {
 692                     ty::Tuple(ref tys) => vec![ArgKind::empty(); tys.len()],
 693                     _ => vec![ArgKind::empty()],
 694                 };
 695
 696                 let expected_ty = expected_trait_ref.skip_binder().substs.type_at(1);
 697                 let expected = match expected_ty.kind {
 698                     ty::Tuple(ref tys) => tys
 699                         .iter()
 700                         .map(|t| ArgKind::from_expected_ty(t.expect_ty(), Some(span)))
 701                         .collect(),
 702                     _ => vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())],
 703                 };
 704
 705                 if found.len() == expected.len() {
 706                     self.report_closure_arg_mismatch(
 707                         span,
 708                         found_span,
 709                         found_trait_ref,
 710                         expected_trait_ref,
 711                     )
 712                 } else {
 713                     let (closure_span, found) = found_did
 714                         .and_then(|did| {
 715                             let node = self.tcx.hir().get_if_local(did)?;
 716                             let (found_span, found) = self.get_fn_like_arguments(node)?;
 717                             Some((Some(found_span), found))
 718                         })
 719                         .unwrap_or((found_span, found));
 720
 721                     self.report_arg_count_mismatch(
 722                         span,
 723                         closure_span,
 724                         expected,
 725                         found,
 726                         found_trait_ty.is_closure(),
 727                     )
 728                 }
 729             }
 730
 731             TraitNotObjectSafe(did) => {
 732                 let violations = self.tcx.object_safety_violations(did);
 733                 report_object_safety_error(self.tcx, span, did, violations)
 734             }
 735
 736             ConstEvalFailure(ErrorHandled::TooGeneric) => {
 737                 // In this instance, we have a const expression containing an unevaluated
 738                 // generic parameter. We have no idea whether this expression is valid or
 739                 // not (e.g. it might result in an error), but we don't want to just assume
 740                 // that it's okay, because that might result in post-monomorphisation time
 741                 // errors. The onus is really on the caller to provide values that it can
 742                 // prove are well-formed.
 743                 let mut err = self
 744                     .tcx
 745                     .sess
 746                     .struct_span_err(span, "constant expression depends on a generic parameter");
 747                 // FIXME(const_generics): we should suggest to the user how they can resolve this
 748                 // issue. However, this is currently not actually possible
 749                 // (see https://github.com/rust-lang/rust/issues/66962#issuecomment-575907083).
 750                 err.note("this may fail depending on what value the parameter takes");
 751                 err
 752             }
 753
 754             // Already reported in the query.
 755             ConstEvalFailure(ErrorHandled::Reported(ErrorReported)) => {
 756                 // FIXME(eddyb) remove this once `ErrorReported` becomes a proof token.
 757                 self.tcx.sess.delay_span_bug(span, "`ErrorReported` without an error");
 758                 return;
 759             }
 760
 761             // Already reported in the query, but only as a lint.
 762             // This shouldn't actually happen for constants used in types, modulo
 763             // bugs. The `delay_span_bug` here ensures it won't be ignored.
 764             ConstEvalFailure(ErrorHandled::Linted) => {
 765                 self.tcx.sess.delay_span_bug(span, "constant in type had error reported as lint");
 766                 return;
 767             }
 768
 769             Overflow => {
 770                 bug!("overflow should be handled before the `report_selection_error` path");
 771             }
 772         };
 773
 774         self.note_obligation_cause(&mut err, obligation);
 775         self.point_at_returns_when_relevant(&mut err, &obligation);
 776
 777         err.emit();
 778     }
 779
 780     /// Given some node representing a fn-like thing in the HIR map,
 781     /// returns a span and `ArgKind` information that describes the
 782     /// arguments it expects. This can be supplied to
 783     /// `report_arg_count_mismatch`.
 784     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)> {
 785         let sm = self.tcx.sess.source_map();
 786         let hir = self.tcx.hir();
 787         Some(match node {
 788             Node::Expr(&hir::Expr {
 789                 kind: hir::ExprKind::Closure(_, ref _decl, id, span, _),
 790                 ..
 791             }) => (
 792                 sm.guess_head_span(span),
 793                 hir.body(id)
 794                     .params
 795                     .iter()
 796                     .map(|arg| {
 797                         if let hir::Pat { kind: hir::PatKind::Tuple(ref args, _), span, .. } =
 798                             *arg.pat
 799                         {
 800                             Some(ArgKind::Tuple(
 801                                 Some(span),
 802                                 args.iter()
 803                                     .map(|pat| {
 804                                         sm.span_to_snippet(pat.span)
 805                                             .ok()
 806                                             .map(|snippet| (snippet, "_".to_owned()))
 807                                     })
 808                                     .collect::<Option<Vec<_>>>()?,
 809                             ))
 810                         } else {
 811                             let name = sm.span_to_snippet(arg.pat.span).ok()?;
 812                             Some(ArgKind::Arg(name, "_".to_owned()))
 813                         }
 814                     })
 815                     .collect::<Option<Vec<ArgKind>>>()?,
 816             ),
 817             Node::Item(&hir::Item { span, kind: hir::ItemKind::Fn(ref sig, ..), .. })
 818             | Node::ImplItem(&hir::ImplItem {
 819                 span,
 820                 kind: hir::ImplItemKind::Fn(ref sig, _),
 821                 ..
 822             })
 823             | Node::TraitItem(&hir::TraitItem {
 824                 span,
 825                 kind: hir::TraitItemKind::Fn(ref sig, _),
 826                 ..
 827             }) => (
 828                 sm.guess_head_span(span),
 829                 sig.decl
 830                     .inputs
 831                     .iter()
 832                     .map(|arg| match arg.clone().kind {
 833                         hir::TyKind::Tup(ref tys) => ArgKind::Tuple(
 834                             Some(arg.span),
 835                             vec![("_".to_owned(), "_".to_owned()); tys.len()],
 836                         ),
 837                         _ => ArgKind::empty(),
 838                     })
 839                     .collect::<Vec<ArgKind>>(),
 840             ),
 841             Node::Ctor(ref variant_data) => {
 842                 let span = variant_data.ctor_hir_id().map(|id| hir.span(id)).unwrap_or(DUMMY_SP);
 843                 let span = sm.guess_head_span(span);
 844                 (span, vec![ArgKind::empty(); variant_data.fields().len()])
 845             }
 846             _ => panic!("non-FnLike node found: {:?}", node),
 847         })
 848     }
 849
 850     /// Reports an error when the number of arguments needed by a
 851     /// trait match doesn't match the number that the expression
 852     /// provides.
 853     fn report_arg_count_mismatch(
 854         &self,
 855         span: Span,
 856         found_span: Option<Span>,
 857         expected_args: Vec<ArgKind>,
 858         found_args: Vec<ArgKind>,
 859         is_closure: bool,
 860     ) -> DiagnosticBuilder<'tcx> {
 861         let kind = if is_closure { "closure" } else { "function" };
 862
 863         let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
 864             let arg_length = arguments.len();
 865             let distinct = match &other[..] {
 866                 &[ArgKind::Tuple(..)] => true,
 867                 _ => false,
 868             };
 869             match (arg_length, arguments.get(0)) {
 870                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
 871                     format!("a single {}-tuple as argument", fields.len())
 872                 }
 873                 _ => format!(
 874                     "{} {}argument{}",
 875                     arg_length,
 876                     if distinct && arg_length > 1 { "distinct " } else { "" },
 877                     pluralize!(arg_length)
 878                 ),
 879             }
 880         };
 881
 882         let expected_str = args_str(&expected_args, &found_args);
 883         let found_str = args_str(&found_args, &expected_args);
 884
 885         let mut err = struct_span_err!(
 886             self.tcx.sess,
 887             span,
 888             E0593,
 889             "{} is expected to take {}, but it takes {}",
 890             kind,
 891             expected_str,
 892             found_str,
 893         );
 894
 895         err.span_label(span, format!("expected {} that takes {}", kind, expected_str));
 896
 897         if let Some(found_span) = found_span {
 898             err.span_label(found_span, format!("takes {}", found_str));
 899
 900             // move |_| { ... }
 901             // ^^^^^^^^-- def_span
 902             //
 903             // move |_| { ... }
 904             // ^^^^^-- prefix
 905             let prefix_span = self.tcx.sess.source_map().span_until_non_whitespace(found_span);
 906             // move |_| { ... }
 907             //      ^^^-- pipe_span
 908             let pipe_span =
 909                 if let Some(span) = found_span.trim_start(prefix_span) { span } else { found_span };
 910
 911             // Suggest to take and ignore the arguments with expected_args_length `_`s if
 912             // found arguments is empty (assume the user just wants to ignore args in this case).
 913             // For example, if `expected_args_length` is 2, suggest `|_, _|`.
 914             if found_args.is_empty() && is_closure {
 915                 let underscores = vec!["_"; expected_args.len()].join(", ");
 916                 err.span_suggestion_verbose(
 917                     pipe_span,
 918                     &format!(
 919                         "consider changing the closure to take and ignore the expected argument{}",
 920                         pluralize!(expected_args.len())
 921                     ),
 922                     format!("|{}|", underscores),
 923                     Applicability::MachineApplicable,
 924                 );
 925             }
 926
 927             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
 928                 if fields.len() == expected_args.len() {
 929                     let sugg = fields
 930                         .iter()
 931                         .map(|(name, _)| name.to_owned())
 932                         .collect::<Vec<String>>()
 933                         .join(", ");
 934                     err.span_suggestion_verbose(
 935                         found_span,
 936                         "change the closure to take multiple arguments instead of a single tuple",
 937                         format!("|{}|", sugg),
 938                         Applicability::MachineApplicable,
 939                     );
 940                 }
 941             }
 942             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..] {
 943                 if fields.len() == found_args.len() && is_closure {
 944                     let sugg = format!(
 945                         "|({}){}|",
 946                         found_args
 947                             .iter()
 948                             .map(|arg| match arg {
 949                                 ArgKind::Arg(name, _) => name.to_owned(),
 950                                 _ => "_".to_owned(),
 951                             })
 952                             .collect::<Vec<String>>()
 953                             .join(", "),
 954                         // add type annotations if available
 955                         if found_args.iter().any(|arg| match arg {
 956                             ArgKind::Arg(_, ty) => ty != "_",
 957                             _ => false,
 958                         }) {
 959                             format!(
 960                                 ": ({})",
 961                                 fields
 962                                     .iter()
 963                                     .map(|(_, ty)| ty.to_owned())
 964                                     .collect::<Vec<String>>()
 965                                     .join(", ")
 966                             )
 967                         } else {
 968                             String::new()
 969                         },
 970                     );
 971                     err.span_suggestion_verbose(
 972                         found_span,
 973                         "change the closure to accept a tuple instead of individual arguments",
 974                         sugg,
 975                         Applicability::MachineApplicable,
 976                     );
 977                 }
 978             }
 979         }
 980
 981         err
 982     }
 983 }
 984
 985 trait InferCtxtPrivExt<'tcx> {
 986     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
 987     // `error` occurring implies that `cond` occurs.
 988     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool;
 989
 990     fn report_fulfillment_error(
 991         &self,
 992         error: &FulfillmentError<'tcx>,
 993         body_id: Option<hir::BodyId>,
 994         fallback_has_occurred: bool,
 995     );
 996
 997     fn report_projection_error(
 998         &self,
 999         obligation: &PredicateObligation<'tcx>,
1000         error: &MismatchedProjectionTypes<'tcx>,
1001     );
1002
1003     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool;
1004
1005     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str>;
1006
1007     fn find_similar_impl_candidates(
1008         &self,
1009         trait_ref: ty::PolyTraitRef<'tcx>,
1010     ) -> Vec<ty::TraitRef<'tcx>>;
1011
1012     fn report_similar_impl_candidates(
1013         &self,
1014         impl_candidates: Vec<ty::TraitRef<'tcx>>,
1015         err: &mut DiagnosticBuilder<'_>,
1016     );
1017
1018     /// Gets the parent trait chain start
1019     fn get_parent_trait_ref(
1020         &self,
1021         code: &ObligationCauseCode<'tcx>,
1022     ) -> Option<(String, Option<Span>)>;
1023
1024     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1025     /// with the same path as `trait_ref`, a help message about
1026     /// a probable version mismatch is added to `err`
1027     fn note_version_mismatch(
1028         &self,
1029         err: &mut DiagnosticBuilder<'_>,
1030         trait_ref: &ty::PolyTraitRef<'tcx>,
1031     );
1032
1033     /// Creates a `PredicateObligation` with `new_self_ty` replacing the existing type in the
1034     /// `trait_ref`.
1035     ///
1036     /// For this to work, `new_self_ty` must have no escaping bound variables.
1037     fn mk_trait_obligation_with_new_self_ty(
1038         &self,
1039         param_env: ty::ParamEnv<'tcx>,
1040         trait_ref: &ty::PolyTraitRef<'tcx>,
1041         new_self_ty: Ty<'tcx>,
1042     ) -> PredicateObligation<'tcx>;
1043
1044     fn maybe_report_ambiguity(
1045         &self,
1046         obligation: &PredicateObligation<'tcx>,
1047         body_id: Option<hir::BodyId>,
1048     );
1049
1050     fn predicate_can_apply(
1051         &self,
1052         param_env: ty::ParamEnv<'tcx>,
1053         pred: ty::PolyTraitRef<'tcx>,
1054     ) -> bool;
1055
1056     fn note_obligation_cause(
1057         &self,
1058         err: &mut DiagnosticBuilder<'tcx>,
1059         obligation: &PredicateObligation<'tcx>,
1060     );
1061
1062     fn suggest_unsized_bound_if_applicable(
1063         &self,
1064         err: &mut DiagnosticBuilder<'tcx>,
1065         obligation: &PredicateObligation<'tcx>,
1066     );
1067
1068     fn is_recursive_obligation(
1069         &self,
1070         obligated_types: &mut Vec<&ty::TyS<'tcx>>,
1071         cause_code: &ObligationCauseCode<'tcx>,
1072     ) -> bool;
1073 }
1074
1075 impl<'a, 'tcx> InferCtxtPrivExt<'tcx> for InferCtxt<'a, 'tcx> {
1076     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1077     // `error` occurring implies that `cond` occurs.
1078     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool {
1079         if cond == error {
1080             return true;
1081         }
1082
1083         let (cond, error) = match (cond.kind(), error.kind()) {
1084             (ty::PredicateKind::Trait(..), ty::PredicateKind::Trait(error, _)) => (cond, error),
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::PredicateKind::Trait(implication, _) = obligation.predicate.kind() {
1093                 let error = error.to_poly_trait_ref();
1094                 let implication = implication.to_poly_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::PredicateKind::Projection(ref data) = predicate.kind() {
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                     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.kind() {
1465             ty::PredicateKind::Trait(ref data, _) => {
1466                 let trait_ref = data.to_poly_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
1499                     .tcx
1500                     .lang_items()
1501                     .sized_trait()
1502                     .map_or(false, |sized_id| sized_id == trait_ref.def_id())
1503                 {
1504                     self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0282).emit();
1505                     return;
1506                 }
1507                 let mut err = self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0283);
1508                 err.note(&format!("cannot satisfy `{}`", predicate));
1509                 if let ObligationCauseCode::ItemObligation(def_id) = obligation.cause.code {
1510                     self.suggest_fully_qualified_path(&mut err, def_id, span, trait_ref.def_id());
1511                 } else if let (
1512                     Ok(ref snippet),
1513                     ObligationCauseCode::BindingObligation(ref def_id, _),
1514                 ) =
1515                     (self.tcx.sess.source_map().span_to_snippet(span), &obligation.cause.code)
1516                 {
1517                     let generics = self.tcx.generics_of(*def_id);
1518                     if generics.params.iter().any(|p| p.name.as_str() != "Self")
1519                         && !snippet.ends_with('>')
1520                     {
1521                         // FIXME: To avoid spurious suggestions in functions where type arguments
1522                         // where already supplied, we check the snippet to make sure it doesn't
1523                         // end with a turbofish. Ideally we would have access to a `PathSegment`
1524                         // instead. Otherwise we would produce the following output:
1525                         //
1526                         // error[E0283]: type annotations needed
1527                         //   --> $DIR/issue-54954.rs:3:24
1528                         //    |
1529                         // LL | const ARR_LEN: usize = Tt::const_val::<[i8; 123]>();
1530                         //    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^
1531                         //    |                        |
1532                         //    |                        cannot infer type
1533                         //    |                        help: consider specifying the type argument
1534                         //    |                        in the function call:
1535                         //    |                        `Tt::const_val::<[i8; 123]>::<T>`
1536                         // ...
1537                         // LL |     const fn const_val<T: Sized>() -> usize {
1538                         //    |                        - required by this bound in `Tt::const_val`
1539                         //    |
1540                         //    = note: cannot satisfy `_: Tt`
1541
1542                         err.span_suggestion_verbose(
1543                             span.shrink_to_hi(),
1544                             &format!(
1545                                 "consider specifying the type argument{} in the function call",
1546                                 pluralize!(generics.params.len()),
1547                             ),
1548                             format!(
1549                                 "::<{}>",
1550                                 generics
1551                                     .params
1552                                     .iter()
1553                                     .map(|p| p.name.to_string())
1554                                     .collect::<Vec<String>>()
1555                                     .join(", ")
1556                             ),
1557                             Applicability::HasPlaceholders,
1558                         );
1559                     }
1560                 }
1561                 err
1562             }
1563
1564             ty::PredicateKind::WellFormed(arg) => {
1565                 // Same hacky approach as above to avoid deluging user
1566                 // with error messages.
1567                 if arg.references_error() || self.tcx.sess.has_errors() {
1568                     return;
1569                 }
1570
1571                 match arg.unpack() {
1572                     GenericArgKind::Lifetime(lt) => {
1573                         span_bug!(span, "unexpected well formed predicate: {:?}", lt)
1574                     }
1575                     GenericArgKind::Type(ty) => {
1576                         self.need_type_info_err(body_id, span, ty, ErrorCode::E0282)
1577                     }
1578                     GenericArgKind::Const(ct) => {
1579                         self.need_type_info_err_const(body_id, span, ct, ErrorCode::E0282)
1580                     }
1581                 }
1582             }
1583
1584             ty::PredicateKind::Subtype(ref data) => {
1585                 if data.references_error() || self.tcx.sess.has_errors() {
1586                     // no need to overload user in such cases
1587                     return;
1588                 }
1589                 let SubtypePredicate { a_is_expected: _, a, b } = data.skip_binder();
1590                 // both must be type variables, or the other would've been instantiated
1591                 assert!(a.is_ty_var() && b.is_ty_var());
1592                 self.need_type_info_err(body_id, span, a, ErrorCode::E0282)
1593             }
1594             ty::PredicateKind::Projection(ref data) => {
1595                 let trait_ref = data.to_poly_trait_ref(self.tcx);
1596                 let self_ty = trait_ref.skip_binder().self_ty();
1597                 let ty = data.skip_binder().ty;
1598                 if predicate.references_error() {
1599                     return;
1600                 }
1601                 if self_ty.needs_infer() && ty.needs_infer() {
1602                     // We do this for the `foo.collect()?` case to produce a suggestion.
1603                     let mut err = self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0284);
1604                     err.note(&format!("cannot satisfy `{}`", predicate));
1605                     err
1606                 } else {
1607                     let mut err = struct_span_err!(
1608                         self.tcx.sess,
1609                         span,
1610                         E0284,
1611                         "type annotations needed: cannot satisfy `{}`",
1612                         predicate,
1613                     );
1614                     err.span_label(span, &format!("cannot satisfy `{}`", predicate));
1615                     err
1616                 }
1617             }
1618
1619             _ => {
1620                 if self.tcx.sess.has_errors() {
1621                     return;
1622                 }
1623                 let mut err = struct_span_err!(
1624                     self.tcx.sess,
1625                     span,
1626                     E0284,
1627                     "type annotations needed: cannot satisfy `{}`",
1628                     predicate,
1629                 );
1630                 err.span_label(span, &format!("cannot satisfy `{}`", predicate));
1631                 err
1632             }
1633         };
1634         self.note_obligation_cause(&mut err, obligation);
1635         err.emit();
1636     }
1637
1638     /// Returns `true` if the trait predicate may apply for *some* assignment
1639     /// to the type parameters.
1640     fn predicate_can_apply(
1641         &self,
1642         param_env: ty::ParamEnv<'tcx>,
1643         pred: ty::PolyTraitRef<'tcx>,
1644     ) -> bool {
1645         struct ParamToVarFolder<'a, 'tcx> {
1646             infcx: &'a InferCtxt<'a, 'tcx>,
1647             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
1648         }
1649
1650         impl<'a, 'tcx> TypeFolder<'tcx> for ParamToVarFolder<'a, 'tcx> {
1651             fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
1652                 self.infcx.tcx
1653             }
1654
1655             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
1656                 if let ty::Param(ty::ParamTy { name, .. }) = ty.kind {
1657                     let infcx = self.infcx;
1658                     self.var_map.entry(ty).or_insert_with(|| {
1659                         infcx.next_ty_var(TypeVariableOrigin {
1660                             kind: TypeVariableOriginKind::TypeParameterDefinition(name, None),
1661                             span: DUMMY_SP,
1662                         })
1663                     })
1664                 } else {
1665                     ty.super_fold_with(self)
1666                 }
1667             }
1668         }
1669
1670         self.probe(|_| {
1671             let mut selcx = SelectionContext::new(self);
1672
1673             let cleaned_pred =
1674                 pred.fold_with(&mut ParamToVarFolder { infcx: self, var_map: Default::default() });
1675
1676             let cleaned_pred = super::project::normalize(
1677                 &mut selcx,
1678                 param_env,
1679                 ObligationCause::dummy(),
1680                 &cleaned_pred,
1681             )
1682             .value;
1683
1684             let obligation = Obligation::new(
1685                 ObligationCause::dummy(),
1686                 param_env,
1687                 cleaned_pred.without_const().to_predicate(selcx.tcx()),
1688             );
1689
1690             self.predicate_may_hold(&obligation)
1691         })
1692     }
1693
1694     fn note_obligation_cause(
1695         &self,
1696         err: &mut DiagnosticBuilder<'tcx>,
1697         obligation: &PredicateObligation<'tcx>,
1698     ) {
1699         // First, attempt to add note to this error with an async-await-specific
1700         // message, and fall back to regular note otherwise.
1701         if !self.maybe_note_obligation_cause_for_async_await(err, obligation) {
1702             self.note_obligation_cause_code(
1703                 err,
1704                 &obligation.predicate,
1705                 &obligation.cause.code,
1706                 &mut vec![],
1707             );
1708             self.suggest_unsized_bound_if_applicable(err, obligation);
1709         }
1710     }
1711
1712     fn suggest_unsized_bound_if_applicable(
1713         &self,
1714         err: &mut DiagnosticBuilder<'tcx>,
1715         obligation: &PredicateObligation<'tcx>,
1716     ) {
1717         let (pred, item_def_id, span) =
1718             match (obligation.predicate.kind(), &obligation.cause.code.peel_derives()) {
1719                 (
1720                     ty::PredicateKind::Trait(pred, _),
1721                     ObligationCauseCode::BindingObligation(item_def_id, span),
1722                 ) => (pred, item_def_id, span),
1723                 _ => return,
1724             };
1725
1726         let node = match (
1727             self.tcx.hir().get_if_local(*item_def_id),
1728             Some(pred.def_id()) == self.tcx.lang_items().sized_trait(),
1729         ) {
1730             (Some(node), true) => node,
1731             _ => return,
1732         };
1733         let generics = match node.generics() {
1734             Some(generics) => generics,
1735             None => return,
1736         };
1737         for param in generics.params {
1738             if param.span != *span
1739                 || param.bounds.iter().any(|bound| {
1740                     bound.trait_ref().and_then(|trait_ref| trait_ref.trait_def_id())
1741                         == self.tcx.lang_items().sized_trait()
1742                 })
1743             {
1744                 continue;
1745             }
1746             match node {
1747                 hir::Node::Item(
1748                     item
1749                     @
1750                     hir::Item {
1751                         kind:
1752                             hir::ItemKind::Enum(..)
1753                             | hir::ItemKind::Struct(..)
1754                             | hir::ItemKind::Union(..),
1755                         ..
1756                     },
1757                 ) => {
1758                     // Suggesting `T: ?Sized` is only valid in an ADT if `T` is only used in a
1759                     // borrow. `struct S<'a, T: ?Sized>(&'a T);` is valid, `struct S<T: ?Sized>(T);`
1760                     // is not.
1761                     let mut visitor = FindTypeParam {
1762                         param: param.name.ident().name,
1763                         invalid_spans: vec![],
1764                         nested: false,
1765                     };
1766                     visitor.visit_item(item);
1767                     if !visitor.invalid_spans.is_empty() {
1768                         let mut multispan: MultiSpan = param.span.into();
1769                         multispan.push_span_label(
1770                             param.span,
1771                             format!("this could be changed to `{}: ?Sized`...", param.name.ident()),
1772                         );
1773                         for sp in visitor.invalid_spans {
1774                             multispan.push_span_label(
1775                                 sp,
1776                                 format!(
1777                                     "...if indirection was used here: `Box<{}>`",
1778                                     param.name.ident(),
1779                                 ),
1780                             );
1781                         }
1782                         err.span_help(
1783                             multispan,
1784                             &format!(
1785                                 "you could relax the implicit `Sized` bound on `{T}` if it were \
1786                                  used through indirection like `&{T}` or `Box<{T}>`",
1787                                 T = param.name.ident(),
1788                             ),
1789                         );
1790                         return;
1791                     }
1792                 }
1793                 _ => {}
1794             }
1795             let (span, separator) = match param.bounds {
1796                 [] => (span.shrink_to_hi(), ":"),
1797                 [.., bound] => (bound.span().shrink_to_hi(), " +"),
1798             };
1799             err.span_suggestion_verbose(
1800                 span,
1801                 "consider relaxing the implicit `Sized` restriction",
1802                 format!("{} ?Sized", separator),
1803                 Applicability::MachineApplicable,
1804             );
1805             return;
1806         }
1807     }
1808
1809     fn is_recursive_obligation(
1810         &self,
1811         obligated_types: &mut Vec<&ty::TyS<'tcx>>,
1812         cause_code: &ObligationCauseCode<'tcx>,
1813     ) -> bool {
1814         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
1815             let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
1816
1817             if obligated_types.iter().any(|ot| ot == &parent_trait_ref.skip_binder().self_ty()) {
1818                 return true;
1819             }
1820         }
1821         false
1822     }
1823 }
1824
1825 /// Look for type `param` in an ADT being used only through a reference to confirm that suggesting
1826 /// `param: ?Sized` would be a valid constraint.
1827 struct FindTypeParam {
1828     param: rustc_span::Symbol,
1829     invalid_spans: Vec<Span>,
1830     nested: bool,
1831 }
1832
1833 impl<'v> Visitor<'v> for FindTypeParam {
1834     type Map = rustc_hir::intravisit::ErasedMap<'v>;
1835
1836     fn nested_visit_map(&mut self) -> hir::intravisit::NestedVisitorMap<Self::Map> {
1837         hir::intravisit::NestedVisitorMap::None
1838     }
1839
1840     fn visit_ty(&mut self, ty: &hir::Ty<'_>) {
1841         // We collect the spans of all uses of the "bare" type param, like in `field: T` or
1842         // `field: (T, T)` where we could make `T: ?Sized` while skipping cases that are known to be
1843         // valid like `field: &'a T` or `field: *mut T` and cases that *might* have further `Sized`
1844         // obligations like `Box<T>` and `Vec<T>`, but we perform no extra analysis for those cases
1845         // and suggest `T: ?Sized` regardless of their obligations. This is fine because the errors
1846         // in that case should make what happened clear enough.
1847         match ty.kind {
1848             hir::TyKind::Ptr(_) | hir::TyKind::Rptr(..) | hir::TyKind::TraitObject(..) => {}
1849             hir::TyKind::Path(hir::QPath::Resolved(None, path))
1850                 if path.segments.len() == 1 && path.segments[0].ident.name == self.param =>
1851             {
1852                 if !self.nested {
1853                     self.invalid_spans.push(ty.span);
1854                 }
1855             }
1856             hir::TyKind::Path(_) => {
1857                 let prev = self.nested;
1858                 self.nested = true;
1859                 hir::intravisit::walk_ty(self, ty);
1860                 self.nested = prev;
1861             }
1862             _ => {
1863                 hir::intravisit::walk_ty(self, ty);
1864             }
1865         }
1866     }
1867 }
1868
1869 pub fn recursive_type_with_infinite_size_error(
1870     tcx: TyCtxt<'tcx>,
1871     type_def_id: DefId,
1872     spans: Vec<Span>,
1873 ) {
1874     assert!(type_def_id.is_local());
1875     let span = tcx.hir().span_if_local(type_def_id).unwrap();
1876     let span = tcx.sess.source_map().guess_head_span(span);
1877     let path = tcx.def_path_str(type_def_id);
1878     let mut err =
1879         struct_span_err!(tcx.sess, span, E0072, "recursive type `{}` has infinite size", path);
1880     err.span_label(span, "recursive type has infinite size");
1881     for &span in &spans {
1882         err.span_label(span, "recursive without indirection");
1883     }
1884     let msg = format!(
1885         "insert some indirection (e.g., a `Box`, `Rc`, or `&`) to make `{}` representable",
1886         path,
1887     );
1888     if spans.len() <= 4 {
1889         err.multipart_suggestion(
1890             &msg,
1891             spans
1892                 .iter()
1893                 .flat_map(|&span| {
1894                     vec![
1895                         (span.shrink_to_lo(), "Box<".to_string()),
1896                         (span.shrink_to_hi(), ">".to_string()),
1897                     ]
1898                     .into_iter()
1899                 })
1900                 .collect(),
1901             Applicability::HasPlaceholders,
1902         );
1903     } else {
1904         err.help(&msg);
1905     }
1906     err.emit();
1907 }
1908
1909 /// Summarizes information
1910 #[derive(Clone)]
1911 pub enum ArgKind {
1912     /// An argument of non-tuple type. Parameters are (name, ty)
1913     Arg(String, String),
1914
1915     /// An argument of tuple type. For a "found" argument, the span is
1916     /// the locationo in the source of the pattern. For a "expected"
1917     /// argument, it will be None. The vector is a list of (name, ty)
1918     /// strings for the components of the tuple.
1919     Tuple(Option<Span>, Vec<(String, String)>),
1920 }
1921
1922 impl ArgKind {
1923     fn empty() -> ArgKind {
1924         ArgKind::Arg("_".to_owned(), "_".to_owned())
1925     }
1926
1927     /// Creates an `ArgKind` from the expected type of an
1928     /// argument. It has no name (`_`) and an optional source span.
1929     pub fn from_expected_ty(t: Ty<'_>, span: Option<Span>) -> ArgKind {
1930         match t.kind {
1931             ty::Tuple(ref tys) => ArgKind::Tuple(
1932                 span,
1933                 tys.iter().map(|ty| ("_".to_owned(), ty.to_string())).collect::<Vec<_>>(),
1934             ),
1935             _ => ArgKind::Arg("_".to_owned(), t.to_string()),
1936         }
1937     }
1938 }