compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

   1 pub mod on_unimplemented;
   2 pub mod suggestions;
   3
   4 use super::{
   5     EvaluationResult, FulfillmentError, FulfillmentErrorCode, MismatchedProjectionTypes,
   6     Obligation, ObligationCause, ObligationCauseCode, OnUnimplementedDirective,
   7     OnUnimplementedNote, OutputTypeParameterMismatch, Overflow, PredicateObligation,
   8     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;
  18 use rustc_hir::intravisit::Visitor;
  19 use rustc_hir::GenericParam;
  20 use rustc_hir::Item;
  21 use rustc_hir::Node;
  22 use rustc_middle::mir::abstract_const::NotConstEvaluatable;
  23 use rustc_middle::ty::error::ExpectedFound;
  24 use rustc_middle::ty::fold::TypeFolder;
  25 use rustc_middle::ty::{
  26     self, fast_reject, AdtKind, SubtypePredicate, ToPolyTraitRef, ToPredicate, Ty, TyCtxt,
  27     TypeFoldable, WithConstness,
  28 };
  29 use rustc_session::DiagnosticMessageId;
  30 use rustc_span::symbol::{kw, sym};
  31 use rustc_span::{ExpnKind, MultiSpan, Span, DUMMY_SP};
  32 use std::fmt;
  33 use std::iter;
  34
  35 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
  36 use crate::traits::query::normalize::AtExt as _;
  37 use on_unimplemented::InferCtxtExt as _;
  38 use suggestions::InferCtxtExt as _;
  39
  40 pub use rustc_infer::traits::error_reporting::*;
  41
  42 pub trait InferCtxtExt<'tcx> {
  43     fn report_fulfillment_errors(
  44         &self,
  45         errors: &[FulfillmentError<'tcx>],
  46         body_id: Option<hir::BodyId>,
  47         fallback_has_occurred: bool,
  48     );
  49
  50     fn report_overflow_error<T>(
  51         &self,
  52         obligation: &Obligation<'tcx, T>,
  53         suggest_increasing_limit: bool,
  54     ) -> !
  55     where
  56         T: fmt::Display + TypeFoldable<'tcx>;
  57
  58     fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> !;
  59
  60     /// The `root_obligation` parameter should be the `root_obligation` field
  61     /// from a `FulfillmentError`. If no `FulfillmentError` is available,
  62     /// then it should be the same as `obligation`.
  63     fn report_selection_error(
  64         &self,
  65         obligation: PredicateObligation<'tcx>,
  66         root_obligation: &PredicateObligation<'tcx>,
  67         error: &SelectionError<'tcx>,
  68         fallback_has_occurred: bool,
  69         points_at_arg: bool,
  70     );
  71
  72     /// Given some node representing a fn-like thing in the HIR map,
  73     /// returns a span and `ArgKind` information that describes the
  74     /// arguments it expects. This can be supplied to
  75     /// `report_arg_count_mismatch`.
  76     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)>;
  77
  78     /// Reports an error when the number of arguments needed by a
  79     /// trait match doesn't match the number that the expression
  80     /// provides.
  81     fn report_arg_count_mismatch(
  82         &self,
  83         span: Span,
  84         found_span: Option<Span>,
  85         expected_args: Vec<ArgKind>,
  86         found_args: Vec<ArgKind>,
  87         is_closure: bool,
  88     ) -> DiagnosticBuilder<'tcx>;
  89 }
  90
  91 impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
  92     fn report_fulfillment_errors(
  93         &self,
  94         errors: &[FulfillmentError<'tcx>],
  95         body_id: Option<hir::BodyId>,
  96         fallback_has_occurred: bool,
  97     ) {
  98         #[derive(Debug)]
  99         struct ErrorDescriptor<'tcx> {
 100             predicate: ty::Predicate<'tcx>,
 101             index: Option<usize>, // None if this is an old error
 102         }
 103
 104         let mut error_map: FxHashMap<_, Vec<_>> = self
 105             .reported_trait_errors
 106             .borrow()
 107             .iter()
 108             .map(|(&span, predicates)| {
 109                 (
 110                     span,
 111                     predicates
 112                         .iter()
 113                         .map(|&predicate| ErrorDescriptor { predicate, index: None })
 114                         .collect(),
 115                 )
 116             })
 117             .collect();
 118
 119         for (index, error) in errors.iter().enumerate() {
 120             // We want to ignore desugarings here: spans are equivalent even
 121             // if one is the result of a desugaring and the other is not.
 122             let mut span = error.obligation.cause.span;
 123             let expn_data = span.ctxt().outer_expn_data();
 124             if let ExpnKind::Desugaring(_) = expn_data.kind {
 125                 span = expn_data.call_site;
 126             }
 127
 128             error_map.entry(span).or_default().push(ErrorDescriptor {
 129                 predicate: error.obligation.predicate,
 130                 index: Some(index),
 131             });
 132
 133             self.reported_trait_errors
 134                 .borrow_mut()
 135                 .entry(span)
 136                 .or_default()
 137                 .push(error.obligation.predicate);
 138         }
 139
 140         // We do this in 2 passes because we want to display errors in order, though
 141         // maybe it *is* better to sort errors by span or something.
 142         let mut is_suppressed = vec![false; errors.len()];
 143         for (_, error_set) in error_map.iter() {
 144             // We want to suppress "duplicate" errors with the same span.
 145             for error in error_set {
 146                 if let Some(index) = error.index {
 147                     // Suppress errors that are either:
 148                     // 1) strictly implied by another error.
 149                     // 2) implied by an error with a smaller index.
 150                     for error2 in error_set {
 151                         if error2.index.map_or(false, |index2| is_suppressed[index2]) {
 152                             // Avoid errors being suppressed by already-suppressed
 153                             // errors, to prevent all errors from being suppressed
 154                             // at once.
 155                             continue;
 156                         }
 157
 158                         if self.error_implies(error2.predicate, error.predicate)
 159                             && !(error2.index >= error.index
 160                                 && self.error_implies(error.predicate, error2.predicate))
 161                         {
 162                             info!("skipping {:?} (implied by {:?})", error, error2);
 163                             is_suppressed[index] = true;
 164                             break;
 165                         }
 166                     }
 167                 }
 168             }
 169         }
 170
 171         for (error, suppressed) in iter::zip(errors, is_suppressed) {
 172             if !suppressed {
 173                 self.report_fulfillment_error(error, body_id, fallback_has_occurred);
 174             }
 175         }
 176     }
 177
 178     /// Reports that an overflow has occurred and halts compilation. We
 179     /// halt compilation unconditionally because it is important that
 180     /// overflows never be masked -- they basically represent computations
 181     /// whose result could not be truly determined and thus we can't say
 182     /// if the program type checks or not -- and they are unusual
 183     /// occurrences in any case.
 184     fn report_overflow_error<T>(
 185         &self,
 186         obligation: &Obligation<'tcx, T>,
 187         suggest_increasing_limit: bool,
 188     ) -> !
 189     where
 190         T: fmt::Display + TypeFoldable<'tcx>,
 191     {
 192         let predicate = self.resolve_vars_if_possible(obligation.predicate.clone());
 193         let mut err = struct_span_err!(
 194             self.tcx.sess,
 195             obligation.cause.span,
 196             E0275,
 197             "overflow evaluating the requirement `{}`",
 198             predicate
 199         );
 200
 201         if suggest_increasing_limit {
 202             self.suggest_new_overflow_limit(&mut err);
 203         }
 204
 205         self.note_obligation_cause_code(
 206             &mut err,
 207             &obligation.predicate,
 208             &obligation.cause.code,
 209             &mut vec![],
 210             &mut Default::default(),
 211         );
 212
 213         err.emit();
 214         self.tcx.sess.abort_if_errors();
 215         bug!();
 216     }
 217
 218     /// Reports that a cycle was detected which led to overflow and halts
 219     /// compilation. This is equivalent to `report_overflow_error` except
 220     /// that we can give a more helpful error message (and, in particular,
 221     /// we do not suggest increasing the overflow limit, which is not
 222     /// going to help).
 223     fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 224         let cycle = self.resolve_vars_if_possible(cycle.to_owned());
 225         assert!(!cycle.is_empty());
 226
 227         debug!("report_overflow_error_cycle: cycle={:?}", cycle);
 228
 229         // The 'deepest' obligation is most likely to have a useful
 230         // cause 'backtrace'
 231         self.report_overflow_error(cycle.iter().max_by_key(|p| p.recursion_depth).unwrap(), false);
 232     }
 233
 234     fn report_selection_error(
 235         &self,
 236         mut obligation: PredicateObligation<'tcx>,
 237         root_obligation: &PredicateObligation<'tcx>,
 238         error: &SelectionError<'tcx>,
 239         fallback_has_occurred: bool,
 240         points_at_arg: bool,
 241     ) {
 242         let tcx = self.tcx;
 243         let mut span = obligation.cause.span;
 244
 245         let mut err = match *error {
 246             SelectionError::Unimplemented => {
 247                 // If this obligation was generated as a result of well-formedness checking, see if we
 248                 // can get a better error message by performing HIR-based well-formedness checking.
 249                 if let ObligationCauseCode::WellFormed(Some(wf_loc)) =
 250                     root_obligation.cause.code.peel_derives()
 251                 {
 252                     if let Some(cause) = self.tcx.diagnostic_hir_wf_check((
 253                         tcx.erase_regions(obligation.predicate),
 254                         wf_loc.clone(),
 255                     )) {
 256                         obligation.cause = cause;
 257                         span = obligation.cause.span;
 258                     }
 259                 }
 260                 if let ObligationCauseCode::CompareImplMethodObligation {
 261                     item_name,
 262                     impl_item_def_id,
 263                     trait_item_def_id,
 264                 }
 265                 | ObligationCauseCode::CompareImplTypeObligation {
 266                     item_name,
 267                     impl_item_def_id,
 268                     trait_item_def_id,
 269                 } = obligation.cause.code
 270                 {
 271                     self.report_extra_impl_obligation(
 272                         span,
 273                         item_name,
 274                         impl_item_def_id,
 275                         trait_item_def_id,
 276                         &format!("`{}`", obligation.predicate),
 277                     )
 278                     .emit();
 279                     return;
 280                 }
 281
 282                 let bound_predicate = obligation.predicate.kind();
 283                 match bound_predicate.skip_binder() {
 284                     ty::PredicateKind::Trait(trait_predicate) => {
 285                         let trait_predicate = bound_predicate.rebind(trait_predicate);
 286                         let trait_predicate = self.resolve_vars_if_possible(trait_predicate);
 287
 288                         if self.tcx.sess.has_errors() && trait_predicate.references_error() {
 289                             return;
 290                         }
 291                         let trait_ref = trait_predicate.to_poly_trait_ref();
 292                         let (post_message, pre_message, type_def) = self
 293                             .get_parent_trait_ref(&obligation.cause.code)
 294                             .map(|(t, s)| {
 295                                 (
 296                                     format!(" in `{}`", t),
 297                                     format!("within `{}`, ", t),
 298                                     s.map(|s| (format!("within this `{}`", t), s)),
 299                                 )
 300                             })
 301                             .unwrap_or_default();
 302
 303                         let OnUnimplementedNote { message, label, note, enclosing_scope } =
 304                             self.on_unimplemented_note(trait_ref, &obligation);
 305                         let have_alt_message = message.is_some() || label.is_some();
 306                         let is_try_conversion = self.is_try_conversion(span, trait_ref.def_id());
 307                         let is_unsize =
 308                             { Some(trait_ref.def_id()) == self.tcx.lang_items().unsize_trait() };
 309                         let (message, note) = if is_try_conversion {
 310                             (
 311                                 Some(format!(
 312                                     "`?` couldn't convert the error to `{}`",
 313                                     trait_ref.skip_binder().self_ty(),
 314                                 )),
 315                                 Some(
 316                                     "the question mark operation (`?`) implicitly performs a \
 317                                         conversion on the error value using the `From` trait"
 318                                         .to_owned(),
 319                                 ),
 320                             )
 321                         } else {
 322                             (message, note)
 323                         };
 324
 325                         let mut err = struct_span_err!(
 326                             self.tcx.sess,
 327                             span,
 328                             E0277,
 329                             "{}",
 330                             message.unwrap_or_else(|| format!(
 331                                 "the trait bound `{}` is not satisfied{}",
 332                                 trait_ref.without_const().to_predicate(tcx),
 333                                 post_message,
 334                             ))
 335                         );
 336
 337                         if is_try_conversion {
 338                             let none_error = self
 339                                 .tcx
 340                                 .get_diagnostic_item(sym::none_error)
 341                                 .map(|def_id| tcx.type_of(def_id));
 342                             let should_convert_option_to_result =
 343                                 Some(trait_ref.skip_binder().substs.type_at(1)) == none_error;
 344                             let should_convert_result_to_option =
 345                                 Some(trait_ref.self_ty().skip_binder()) == none_error;
 346                             if should_convert_option_to_result {
 347                                 err.span_suggestion_verbose(
 348                                     span.shrink_to_lo(),
 349                                     "consider converting the `Option<T>` into a `Result<T, _>` \
 350                                      using `Option::ok_or` or `Option::ok_or_else`",
 351                                     ".ok_or_else(|| /* error value */)".to_string(),
 352                                     Applicability::HasPlaceholders,
 353                                 );
 354                             } else if should_convert_result_to_option {
 355                                 err.span_suggestion_verbose(
 356                                     span.shrink_to_lo(),
 357                                     "consider converting the `Result<T, _>` into an `Option<T>` \
 358                                      using `Result::ok`",
 359                                     ".ok()".to_string(),
 360                                     Applicability::MachineApplicable,
 361                                 );
 362                             }
 363                             if let Some(ret_span) = self.return_type_span(&obligation) {
 364                                 err.span_label(
 365                                     ret_span,
 366                                     &format!(
 367                                         "expected `{}` because of this",
 368                                         trait_ref.skip_binder().self_ty()
 369                                     ),
 370                                 );
 371                             }
 372                         }
 373
 374                         let explanation =
 375                             if obligation.cause.code == ObligationCauseCode::MainFunctionType {
 376                                 "consider using `()`, or a `Result`".to_owned()
 377                             } else {
 378                                 format!(
 379                                     "{}the trait `{}` is not implemented for `{}`",
 380                                     pre_message,
 381                                     trait_ref.print_only_trait_path(),
 382                                     trait_ref.skip_binder().self_ty(),
 383                                 )
 384                             };
 385
 386                         if self.suggest_add_reference_to_arg(
 387                             &obligation,
 388                             &mut err,
 389                             &trait_ref,
 390                             points_at_arg,
 391                             have_alt_message,
 392                         ) {
 393                             self.note_obligation_cause(&mut err, &obligation);
 394                             err.emit();
 395                             return;
 396                         }
 397                         if let Some(ref s) = label {
 398                             // If it has a custom `#[rustc_on_unimplemented]`
 399                             // error message, let's display it as the label!
 400                             err.span_label(span, s.as_str());
 401                             if !matches!(trait_ref.skip_binder().self_ty().kind(), ty::Param(_)) {
 402                                 // When the self type is a type param We don't need to "the trait
 403                                 // `std::marker::Sized` is not implemented for `T`" as we will point
 404                                 // at the type param with a label to suggest constraining it.
 405                                 err.help(&explanation);
 406                             }
 407                         } else {
 408                             err.span_label(span, explanation);
 409                         }
 410                         if let Some((msg, span)) = type_def {
 411                             err.span_label(span, &msg);
 412                         }
 413                         if let Some(ref s) = note {
 414                             // If it has a custom `#[rustc_on_unimplemented]` note, let's display it
 415                             err.note(s.as_str());
 416                         }
 417                         if let Some(ref s) = enclosing_scope {
 418                             let body = tcx
 419                                 .hir()
 420                                 .opt_local_def_id(obligation.cause.body_id)
 421                                 .unwrap_or_else(|| {
 422                                     tcx.hir().body_owner_def_id(hir::BodyId {
 423                                         hir_id: obligation.cause.body_id,
 424                                     })
 425                                 });
 426
 427                             let enclosing_scope_span =
 428                                 tcx.hir().span_with_body(tcx.hir().local_def_id_to_hir_id(body));
 429
 430                             err.span_label(enclosing_scope_span, s.as_str());
 431                         }
 432
 433                         self.suggest_dereferences(&obligation, &mut err, trait_ref, points_at_arg);
 434                         self.suggest_fn_call(&obligation, &mut err, trait_ref, points_at_arg);
 435                         self.suggest_remove_reference(&obligation, &mut err, trait_ref);
 436                         self.suggest_semicolon_removal(&obligation, &mut err, span, trait_ref);
 437                         self.note_version_mismatch(&mut err, &trait_ref);
 438
 439                         if Some(trait_ref.def_id()) == tcx.lang_items().try_trait() {
 440                             self.suggest_await_before_try(&mut err, &obligation, trait_ref, span);
 441                         }
 442
 443                         if self.suggest_impl_trait(&mut err, span, &obligation, trait_ref) {
 444                             err.emit();
 445                             return;
 446                         }
 447
 448                         if is_unsize {
 449                             // If the obligation failed due to a missing implementation of the
 450                             // `Unsize` trait, give a pointer to why that might be the case
 451                             err.note(
 452                                 "all implementations of `Unsize` are provided \
 453                                 automatically by the compiler, see \
 454                                 <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> \
 455                                 for more information",
 456                             );
 457                         }
 458
 459                         let is_fn_trait = [
 460                             self.tcx.lang_items().fn_trait(),
 461                             self.tcx.lang_items().fn_mut_trait(),
 462                             self.tcx.lang_items().fn_once_trait(),
 463                         ]
 464                         .contains(&Some(trait_ref.def_id()));
 465                         let is_target_feature_fn = if let ty::FnDef(def_id, _) =
 466                             *trait_ref.skip_binder().self_ty().kind()
 467                         {
 468                             !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
 469                         } else {
 470                             false
 471                         };
 472                         if is_fn_trait && is_target_feature_fn {
 473                             err.note(
 474                                 "`#[target_feature]` functions do not implement the `Fn` traits",
 475                             );
 476                         }
 477
 478                         // Try to report a help message
 479                         if !trait_ref.has_infer_types_or_consts()
 480                             && self.predicate_can_apply(obligation.param_env, trait_ref)
 481                         {
 482                             // If a where-clause may be useful, remind the
 483                             // user that they can add it.
 484                             //
 485                             // don't display an on-unimplemented note, as
 486                             // these notes will often be of the form
 487                             //     "the type `T` can't be frobnicated"
 488                             // which is somewhat confusing.
 489                             self.suggest_restricting_param_bound(
 490                                 &mut err,
 491                                 trait_ref,
 492                                 obligation.cause.body_id,
 493                             );
 494                         } else if !have_alt_message {
 495                             // Can't show anything else useful, try to find similar impls.
 496                             let impl_candidates = self.find_similar_impl_candidates(trait_ref);
 497                             self.report_similar_impl_candidates(impl_candidates, &mut err);
 498                         }
 499
 500                         // Changing mutability doesn't make a difference to whether we have
 501                         // an `Unsize` impl (Fixes ICE in #71036)
 502                         if !is_unsize {
 503                             self.suggest_change_mut(
 504                                 &obligation,
 505                                 &mut err,
 506                                 trait_ref,
 507                                 points_at_arg,
 508                             );
 509                         }
 510
 511                         // If this error is due to `!: Trait` not implemented but `(): Trait` is
 512                         // implemented, and fallback has occurred, then it could be due to a
 513                         // variable that used to fallback to `()` now falling back to `!`. Issue a
 514                         // note informing about the change in behaviour.
 515                         if trait_predicate.skip_binder().self_ty().is_never()
 516                             && fallback_has_occurred
 517                         {
 518                             let predicate = trait_predicate.map_bound(|mut trait_pred| {
 519                                 trait_pred.trait_ref.substs = self.tcx.mk_substs_trait(
 520                                     self.tcx.mk_unit(),
 521                                     &trait_pred.trait_ref.substs[1..],
 522                                 );
 523                                 trait_pred
 524                             });
 525                             let unit_obligation = obligation.with(predicate.to_predicate(tcx));
 526                             if self.predicate_may_hold(&unit_obligation) {
 527                                 err.note("this trait is implemented for `()`.");
 528                                 err.note(
 529                                     "this error might have been caused by changes to \
 530                                     Rust's type-inference algorithm (see issue #48950 \
 531                                     <https://github.com/rust-lang/rust/issues/48950> \
 532                                     for more information).",
 533                                 );
 534                                 err.help("did you intend to use the type `()` here instead?");
 535                             }
 536                         }
 537
 538                         // Return early if the trait is Debug or Display and the invocation
 539                         // originates within a standard library macro, because the output
 540                         // is otherwise overwhelming and unhelpful (see #85844 for an
 541                         // example).
 542
 543                         let trait_is_debug =
 544                             self.tcx.is_diagnostic_item(sym::debug_trait, trait_ref.def_id());
 545                         let trait_is_display =
 546                             self.tcx.is_diagnostic_item(sym::display_trait, trait_ref.def_id());
 547
 548                         let in_std_macro =
 549                             match obligation.cause.span.ctxt().outer_expn_data().macro_def_id {
 550                                 Some(macro_def_id) => {
 551                                     let crate_name = tcx.crate_name(macro_def_id.krate);
 552                                     crate_name == sym::std || crate_name == sym::core
 553                                 }
 554                                 None => false,
 555                             };
 556
 557                         if in_std_macro && (trait_is_debug || trait_is_display) {
 558                             err.emit();
 559                             return;
 560                         }
 561
 562                         err
 563                     }
 564
 565                     ty::PredicateKind::Subtype(predicate) => {
 566                         // Errors for Subtype predicates show up as
 567                         // `FulfillmentErrorCode::CodeSubtypeError`,
 568                         // not selection error.
 569                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
 570                     }
 571
 572                     ty::PredicateKind::Coerce(predicate) => {
 573                         // Errors for Coerce predicates show up as
 574                         // `FulfillmentErrorCode::CodeSubtypeError`,
 575                         // not selection error.
 576                         span_bug!(span, "coerce requirement gave wrong error: `{:?}`", predicate)
 577                     }
 578
 579                     ty::PredicateKind::RegionOutlives(predicate) => {
 580                         let predicate = bound_predicate.rebind(predicate);
 581                         let predicate = self.resolve_vars_if_possible(predicate);
 582                         let err = self
 583                             .region_outlives_predicate(&obligation.cause, predicate)
 584                             .err()
 585                             .unwrap();
 586                         struct_span_err!(
 587                             self.tcx.sess,
 588                             span,
 589                             E0279,
 590                             "the requirement `{}` is not satisfied (`{}`)",
 591                             predicate,
 592                             err,
 593                         )
 594                     }
 595
 596                     ty::PredicateKind::Projection(..) | ty::PredicateKind::TypeOutlives(..) => {
 597                         let predicate = self.resolve_vars_if_possible(obligation.predicate);
 598                         struct_span_err!(
 599                             self.tcx.sess,
 600                             span,
 601                             E0280,
 602                             "the requirement `{}` is not satisfied",
 603                             predicate
 604                         )
 605                     }
 606
 607                     ty::PredicateKind::ObjectSafe(trait_def_id) => {
 608                         let violations = self.tcx.object_safety_violations(trait_def_id);
 609                         report_object_safety_error(self.tcx, span, trait_def_id, violations)
 610                     }
 611
 612                     ty::PredicateKind::ClosureKind(closure_def_id, closure_substs, kind) => {
 613                         let found_kind = self.closure_kind(closure_substs).unwrap();
 614                         let closure_span =
 615                             self.tcx.sess.source_map().guess_head_span(
 616                                 self.tcx.hir().span_if_local(closure_def_id).unwrap(),
 617                             );
 618                         let hir_id =
 619                             self.tcx.hir().local_def_id_to_hir_id(closure_def_id.expect_local());
 620                         let mut err = struct_span_err!(
 621                             self.tcx.sess,
 622                             closure_span,
 623                             E0525,
 624                             "expected a closure that implements the `{}` trait, \
 625                              but this closure only implements `{}`",
 626                             kind,
 627                             found_kind
 628                         );
 629
 630                         err.span_label(
 631                             closure_span,
 632                             format!("this closure implements `{}`, not `{}`", found_kind, kind),
 633                         );
 634                         err.span_label(
 635                             obligation.cause.span,
 636                             format!("the requirement to implement `{}` derives from here", kind),
 637                         );
 638
 639                         // Additional context information explaining why the closure only implements
 640                         // a particular trait.
 641                         if let Some(typeck_results) = self.in_progress_typeck_results {
 642                             let typeck_results = typeck_results.borrow();
 643                             match (found_kind, typeck_results.closure_kind_origins().get(hir_id)) {
 644                                 (ty::ClosureKind::FnOnce, Some((span, place))) => {
 645                                     err.span_label(
 646                                         *span,
 647                                         format!(
 648                                             "closure is `FnOnce` because it moves the \
 649                                          variable `{}` out of its environment",
 650                                             ty::place_to_string_for_capture(tcx, place)
 651                                         ),
 652                                     );
 653                                 }
 654                                 (ty::ClosureKind::FnMut, Some((span, place))) => {
 655                                     err.span_label(
 656                                         *span,
 657                                         format!(
 658                                             "closure is `FnMut` because it mutates the \
 659                                          variable `{}` here",
 660                                             ty::place_to_string_for_capture(tcx, place)
 661                                         ),
 662                                     );
 663                                 }
 664                                 _ => {}
 665                             }
 666                         }
 667
 668                         err.emit();
 669                         return;
 670                     }
 671
 672                     ty::PredicateKind::WellFormed(ty) => {
 673                         if !self.tcx.sess.opts.debugging_opts.chalk {
 674                             // WF predicates cannot themselves make
 675                             // errors. They can only block due to
 676                             // ambiguity; otherwise, they always
 677                             // degenerate into other obligations
 678                             // (which may fail).
 679                             span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 680                         } else {
 681                             // FIXME: we'll need a better message which takes into account
 682                             // which bounds actually failed to hold.
 683                             self.tcx.sess.struct_span_err(
 684                                 span,
 685                                 &format!("the type `{}` is not well-formed (chalk)", ty),
 686                             )
 687                         }
 688                     }
 689
 690                     ty::PredicateKind::ConstEvaluatable(..) => {
 691                         // Errors for `ConstEvaluatable` predicates show up as
 692                         // `SelectionError::ConstEvalFailure`,
 693                         // not `Unimplemented`.
 694                         span_bug!(
 695                             span,
 696                             "const-evaluatable requirement gave wrong error: `{:?}`",
 697                             obligation
 698                         )
 699                     }
 700
 701                     ty::PredicateKind::ConstEquate(..) => {
 702                         // Errors for `ConstEquate` predicates show up as
 703                         // `SelectionError::ConstEvalFailure`,
 704                         // not `Unimplemented`.
 705                         span_bug!(
 706                             span,
 707                             "const-equate requirement gave wrong error: `{:?}`",
 708                             obligation
 709                         )
 710                     }
 711
 712                     ty::PredicateKind::TypeWellFormedFromEnv(..) => span_bug!(
 713                         span,
 714                         "TypeWellFormedFromEnv predicate should only exist in the environment"
 715                     ),
 716                 }
 717             }
 718
 719             OutputTypeParameterMismatch(found_trait_ref, expected_trait_ref, _) => {
 720                 let found_trait_ref = self.resolve_vars_if_possible(found_trait_ref);
 721                 let expected_trait_ref = self.resolve_vars_if_possible(expected_trait_ref);
 722
 723                 if expected_trait_ref.self_ty().references_error() {
 724                     return;
 725                 }
 726
 727                 let found_trait_ty = match found_trait_ref.self_ty().no_bound_vars() {
 728                     Some(ty) => ty,
 729                     None => return,
 730                 };
 731
 732                 let found_did = match *found_trait_ty.kind() {
 733                     ty::Closure(did, _) | ty::Foreign(did) | ty::FnDef(did, _) => Some(did),
 734                     ty::Adt(def, _) => Some(def.did),
 735                     _ => None,
 736                 };
 737
 738                 let found_span = found_did
 739                     .and_then(|did| self.tcx.hir().span_if_local(did))
 740                     .map(|sp| self.tcx.sess.source_map().guess_head_span(sp)); // the sp could be an fn def
 741
 742                 if self.reported_closure_mismatch.borrow().contains(&(span, found_span)) {
 743                     // We check closures twice, with obligations flowing in different directions,
 744                     // but we want to complain about them only once.
 745                     return;
 746                 }
 747
 748                 self.reported_closure_mismatch.borrow_mut().insert((span, found_span));
 749
 750                 let found = match found_trait_ref.skip_binder().substs.type_at(1).kind() {
 751                     ty::Tuple(ref tys) => vec![ArgKind::empty(); tys.len()],
 752                     _ => vec![ArgKind::empty()],
 753                 };
 754
 755                 let expected_ty = expected_trait_ref.skip_binder().substs.type_at(1);
 756                 let expected = match expected_ty.kind() {
 757                     ty::Tuple(ref tys) => tys
 758                         .iter()
 759                         .map(|t| ArgKind::from_expected_ty(t.expect_ty(), Some(span)))
 760                         .collect(),
 761                     _ => vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())],
 762                 };
 763
 764                 if found.len() == expected.len() {
 765                     self.report_closure_arg_mismatch(
 766                         span,
 767                         found_span,
 768                         found_trait_ref,
 769                         expected_trait_ref,
 770                     )
 771                 } else {
 772                     let (closure_span, found) = found_did
 773                         .and_then(|did| {
 774                             let node = self.tcx.hir().get_if_local(did)?;
 775                             let (found_span, found) = self.get_fn_like_arguments(node)?;
 776                             Some((Some(found_span), found))
 777                         })
 778                         .unwrap_or((found_span, found));
 779
 780                     self.report_arg_count_mismatch(
 781                         span,
 782                         closure_span,
 783                         expected,
 784                         found,
 785                         found_trait_ty.is_closure(),
 786                     )
 787                 }
 788             }
 789
 790             TraitNotObjectSafe(did) => {
 791                 let violations = self.tcx.object_safety_violations(did);
 792                 report_object_safety_error(self.tcx, span, did, violations)
 793             }
 794
 795             SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsInfer) => {
 796                 bug!(
 797                     "MentionsInfer should have been handled in `traits/fulfill.rs` or `traits/select/mod.rs`"
 798                 )
 799             }
 800             SelectionError::NotConstEvaluatable(NotConstEvaluatable::MentionsParam) => {
 801                 if !self.tcx.features().generic_const_exprs {
 802                     let mut err = self.tcx.sess.struct_span_err(
 803                         span,
 804                         "constant expression depends on a generic parameter",
 805                     );
 806                     // FIXME(const_generics): we should suggest to the user how they can resolve this
 807                     // issue. However, this is currently not actually possible
 808                     // (see https://github.com/rust-lang/rust/issues/66962#issuecomment-575907083).
 809                     //
 810                     // Note that with `feature(generic_const_exprs)` this case should not
 811                     // be reachable.
 812                     err.note("this may fail depending on what value the parameter takes");
 813                     err.emit();
 814                     return;
 815                 }
 816
 817                 match obligation.predicate.kind().skip_binder() {
 818                     ty::PredicateKind::ConstEvaluatable(uv) => {
 819                         let mut err =
 820                             self.tcx.sess.struct_span_err(span, "unconstrained generic constant");
 821                         let const_span = self.tcx.def_span(uv.def.did);
 822                         match self.tcx.sess.source_map().span_to_snippet(const_span) {
 823                             Ok(snippet) => err.help(&format!(
 824                                 "try adding a `where` bound using this expression: `where [(); {}]:`",
 825                                 snippet
 826                             )),
 827                             _ => err.help("consider adding a `where` bound using this expression"),
 828                         };
 829                         err
 830                     }
 831                     _ => {
 832                         span_bug!(
 833                             span,
 834                             "unexpected non-ConstEvaluatable predicate, this should not be reachable"
 835                         )
 836                     }
 837                 }
 838             }
 839
 840             // Already reported in the query.
 841             SelectionError::NotConstEvaluatable(NotConstEvaluatable::Error(ErrorReported)) => {
 842                 // FIXME(eddyb) remove this once `ErrorReported` becomes a proof token.
 843                 self.tcx.sess.delay_span_bug(span, "`ErrorReported` without an error");
 844                 return;
 845             }
 846
 847             Overflow => {
 848                 bug!("overflow should be handled before the `report_selection_error` path");
 849             }
 850         };
 851
 852         self.note_obligation_cause(&mut err, &obligation);
 853         self.point_at_returns_when_relevant(&mut err, &obligation);
 854
 855         err.emit();
 856     }
 857
 858     /// Given some node representing a fn-like thing in the HIR map,
 859     /// returns a span and `ArgKind` information that describes the
 860     /// arguments it expects. This can be supplied to
 861     /// `report_arg_count_mismatch`.
 862     fn get_fn_like_arguments(&self, node: Node<'_>) -> Option<(Span, Vec<ArgKind>)> {
 863         let sm = self.tcx.sess.source_map();
 864         let hir = self.tcx.hir();
 865         Some(match node {
 866             Node::Expr(&hir::Expr {
 867                 kind: hir::ExprKind::Closure(_, ref _decl, id, span, _),
 868                 ..
 869             }) => (
 870                 sm.guess_head_span(span),
 871                 hir.body(id)
 872                     .params
 873                     .iter()
 874                     .map(|arg| {
 875                         if let hir::Pat { kind: hir::PatKind::Tuple(ref args, _), span, .. } =
 876                             *arg.pat
 877                         {
 878                             Some(ArgKind::Tuple(
 879                                 Some(span),
 880                                 args.iter()
 881                                     .map(|pat| {
 882                                         sm.span_to_snippet(pat.span)
 883                                             .ok()
 884                                             .map(|snippet| (snippet, "_".to_owned()))
 885                                     })
 886                                     .collect::<Option<Vec<_>>>()?,
 887                             ))
 888                         } else {
 889                             let name = sm.span_to_snippet(arg.pat.span).ok()?;
 890                             Some(ArgKind::Arg(name, "_".to_owned()))
 891                         }
 892                     })
 893                     .collect::<Option<Vec<ArgKind>>>()?,
 894             ),
 895             Node::Item(&hir::Item { span, kind: hir::ItemKind::Fn(ref sig, ..), .. })
 896             | Node::ImplItem(&hir::ImplItem {
 897                 span,
 898                 kind: hir::ImplItemKind::Fn(ref sig, _),
 899                 ..
 900             })
 901             | Node::TraitItem(&hir::TraitItem {
 902                 span,
 903                 kind: hir::TraitItemKind::Fn(ref sig, _),
 904                 ..
 905             }) => (
 906                 sm.guess_head_span(span),
 907                 sig.decl
 908                     .inputs
 909                     .iter()
 910                     .map(|arg| match arg.kind {
 911                         hir::TyKind::Tup(ref tys) => ArgKind::Tuple(
 912                             Some(arg.span),
 913                             vec![("_".to_owned(), "_".to_owned()); tys.len()],
 914                         ),
 915                         _ => ArgKind::empty(),
 916                     })
 917                     .collect::<Vec<ArgKind>>(),
 918             ),
 919             Node::Ctor(ref variant_data) => {
 920                 let span = variant_data.ctor_hir_id().map_or(DUMMY_SP, |id| hir.span(id));
 921                 let span = sm.guess_head_span(span);
 922                 (span, vec![ArgKind::empty(); variant_data.fields().len()])
 923             }
 924             _ => panic!("non-FnLike node found: {:?}", node),
 925         })
 926     }
 927
 928     /// Reports an error when the number of arguments needed by a
 929     /// trait match doesn't match the number that the expression
 930     /// provides.
 931     fn report_arg_count_mismatch(
 932         &self,
 933         span: Span,
 934         found_span: Option<Span>,
 935         expected_args: Vec<ArgKind>,
 936         found_args: Vec<ArgKind>,
 937         is_closure: bool,
 938     ) -> DiagnosticBuilder<'tcx> {
 939         let kind = if is_closure { "closure" } else { "function" };
 940
 941         let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
 942             let arg_length = arguments.len();
 943             let distinct = matches!(other, &[ArgKind::Tuple(..)]);
 944             match (arg_length, arguments.get(0)) {
 945                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
 946                     format!("a single {}-tuple as argument", fields.len())
 947                 }
 948                 _ => format!(
 949                     "{} {}argument{}",
 950                     arg_length,
 951                     if distinct && arg_length > 1 { "distinct " } else { "" },
 952                     pluralize!(arg_length)
 953                 ),
 954             }
 955         };
 956
 957         let expected_str = args_str(&expected_args, &found_args);
 958         let found_str = args_str(&found_args, &expected_args);
 959
 960         let mut err = struct_span_err!(
 961             self.tcx.sess,
 962             span,
 963             E0593,
 964             "{} is expected to take {}, but it takes {}",
 965             kind,
 966             expected_str,
 967             found_str,
 968         );
 969
 970         err.span_label(span, format!("expected {} that takes {}", kind, expected_str));
 971
 972         if let Some(found_span) = found_span {
 973             err.span_label(found_span, format!("takes {}", found_str));
 974
 975             // move |_| { ... }
 976             // ^^^^^^^^-- def_span
 977             //
 978             // move |_| { ... }
 979             // ^^^^^-- prefix
 980             let prefix_span = self.tcx.sess.source_map().span_until_non_whitespace(found_span);
 981             // move |_| { ... }
 982             //      ^^^-- pipe_span
 983             let pipe_span =
 984                 if let Some(span) = found_span.trim_start(prefix_span) { span } else { found_span };
 985
 986             // Suggest to take and ignore the arguments with expected_args_length `_`s if
 987             // found arguments is empty (assume the user just wants to ignore args in this case).
 988             // For example, if `expected_args_length` is 2, suggest `|_, _|`.
 989             if found_args.is_empty() && is_closure {
 990                 let underscores = vec!["_"; expected_args.len()].join(", ");
 991                 err.span_suggestion_verbose(
 992                     pipe_span,
 993                     &format!(
 994                         "consider changing the closure to take and ignore the expected argument{}",
 995                         pluralize!(expected_args.len())
 996                     ),
 997                     format!("|{}|", underscores),
 998                     Applicability::MachineApplicable,
 999                 );
1000             }
1001
1002             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
1003                 if fields.len() == expected_args.len() {
1004                     let sugg = fields
1005                         .iter()
1006                         .map(|(name, _)| name.to_owned())
1007                         .collect::<Vec<String>>()
1008                         .join(", ");
1009                     err.span_suggestion_verbose(
1010                         found_span,
1011                         "change the closure to take multiple arguments instead of a single tuple",
1012                         format!("|{}|", sugg),
1013                         Applicability::MachineApplicable,
1014                     );
1015                 }
1016             }
1017             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..] {
1018                 if fields.len() == found_args.len() && is_closure {
1019                     let sugg = format!(
1020                         "|({}){}|",
1021                         found_args
1022                             .iter()
1023                             .map(|arg| match arg {
1024                                 ArgKind::Arg(name, _) => name.to_owned(),
1025                                 _ => "_".to_owned(),
1026                             })
1027                             .collect::<Vec<String>>()
1028                             .join(", "),
1029                         // add type annotations if available
1030                         if found_args.iter().any(|arg| match arg {
1031                             ArgKind::Arg(_, ty) => ty != "_",
1032                             _ => false,
1033                         }) {
1034                             format!(
1035                                 ": ({})",
1036                                 fields
1037                                     .iter()
1038                                     .map(|(_, ty)| ty.to_owned())
1039                                     .collect::<Vec<String>>()
1040                                     .join(", ")
1041                             )
1042                         } else {
1043                             String::new()
1044                         },
1045                     );
1046                     err.span_suggestion_verbose(
1047                         found_span,
1048                         "change the closure to accept a tuple instead of individual arguments",
1049                         sugg,
1050                         Applicability::MachineApplicable,
1051                     );
1052                 }
1053             }
1054         }
1055
1056         err
1057     }
1058 }
1059
1060 trait InferCtxtPrivExt<'tcx> {
1061     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1062     // `error` occurring implies that `cond` occurs.
1063     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool;
1064
1065     fn report_fulfillment_error(
1066         &self,
1067         error: &FulfillmentError<'tcx>,
1068         body_id: Option<hir::BodyId>,
1069         fallback_has_occurred: bool,
1070     );
1071
1072     fn report_projection_error(
1073         &self,
1074         obligation: &PredicateObligation<'tcx>,
1075         error: &MismatchedProjectionTypes<'tcx>,
1076     );
1077
1078     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool;
1079
1080     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str>;
1081
1082     fn find_similar_impl_candidates(
1083         &self,
1084         trait_ref: ty::PolyTraitRef<'tcx>,
1085     ) -> Vec<ty::TraitRef<'tcx>>;
1086
1087     fn report_similar_impl_candidates(
1088         &self,
1089         impl_candidates: Vec<ty::TraitRef<'tcx>>,
1090         err: &mut DiagnosticBuilder<'_>,
1091     );
1092
1093     /// Gets the parent trait chain start
1094     fn get_parent_trait_ref(
1095         &self,
1096         code: &ObligationCauseCode<'tcx>,
1097     ) -> Option<(String, Option<Span>)>;
1098
1099     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1100     /// with the same path as `trait_ref`, a help message about
1101     /// a probable version mismatch is added to `err`
1102     fn note_version_mismatch(
1103         &self,
1104         err: &mut DiagnosticBuilder<'_>,
1105         trait_ref: &ty::PolyTraitRef<'tcx>,
1106     );
1107
1108     /// Creates a `PredicateObligation` with `new_self_ty` replacing the existing type in the
1109     /// `trait_ref`.
1110     ///
1111     /// For this to work, `new_self_ty` must have no escaping bound variables.
1112     fn mk_trait_obligation_with_new_self_ty(
1113         &self,
1114         param_env: ty::ParamEnv<'tcx>,
1115         trait_ref: ty::PolyTraitRef<'tcx>,
1116         new_self_ty: Ty<'tcx>,
1117     ) -> PredicateObligation<'tcx>;
1118
1119     fn maybe_report_ambiguity(
1120         &self,
1121         obligation: &PredicateObligation<'tcx>,
1122         body_id: Option<hir::BodyId>,
1123     );
1124
1125     fn predicate_can_apply(
1126         &self,
1127         param_env: ty::ParamEnv<'tcx>,
1128         pred: ty::PolyTraitRef<'tcx>,
1129     ) -> bool;
1130
1131     fn note_obligation_cause(
1132         &self,
1133         err: &mut DiagnosticBuilder<'tcx>,
1134         obligation: &PredicateObligation<'tcx>,
1135     );
1136
1137     fn suggest_unsized_bound_if_applicable(
1138         &self,
1139         err: &mut DiagnosticBuilder<'tcx>,
1140         obligation: &PredicateObligation<'tcx>,
1141     );
1142
1143     fn maybe_suggest_unsized_generics(
1144         &self,
1145         err: &mut DiagnosticBuilder<'tcx>,
1146         span: Span,
1147         node: Node<'hir>,
1148     );
1149
1150     fn maybe_indirection_for_unsized(
1151         &self,
1152         err: &mut DiagnosticBuilder<'tcx>,
1153         item: &'hir Item<'hir>,
1154         param: &'hir GenericParam<'hir>,
1155     ) -> bool;
1156
1157     fn is_recursive_obligation(
1158         &self,
1159         obligated_types: &mut Vec<&ty::TyS<'tcx>>,
1160         cause_code: &ObligationCauseCode<'tcx>,
1161     ) -> bool;
1162 }
1163
1164 impl<'a, 'tcx> InferCtxtPrivExt<'tcx> for InferCtxt<'a, 'tcx> {
1165     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
1166     // `error` occurring implies that `cond` occurs.
1167     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool {
1168         if cond == error {
1169             return true;
1170         }
1171
1172         // FIXME: It should be possible to deal with `ForAll` in a cleaner way.
1173         let bound_error = error.kind();
1174         let (cond, error) = match (cond.kind().skip_binder(), bound_error.skip_binder()) {
1175             (ty::PredicateKind::Trait(..), ty::PredicateKind::Trait(error)) => {
1176                 (cond, bound_error.rebind(error))
1177             }
1178             _ => {
1179                 // FIXME: make this work in other cases too.
1180                 return false;
1181             }
1182         };
1183
1184         for obligation in super::elaborate_predicates(self.tcx, std::iter::once(cond)) {
1185             let bound_predicate = obligation.predicate.kind();
1186             if let ty::PredicateKind::Trait(implication) = bound_predicate.skip_binder() {
1187                 let error = error.to_poly_trait_ref();
1188                 let implication = bound_predicate.rebind(implication.trait_ref);
1189                 // FIXME: I'm just not taking associated types at all here.
1190                 // Eventually I'll need to implement param-env-aware
1191                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
1192                 let param_env = ty::ParamEnv::empty();
1193                 if self.can_sub(param_env, error, implication).is_ok() {
1194                     debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
1195                     return true;
1196                 }
1197             }
1198         }
1199
1200         false
1201     }
1202
1203     fn report_fulfillment_error(
1204         &self,
1205         error: &FulfillmentError<'tcx>,
1206         body_id: Option<hir::BodyId>,
1207         fallback_has_occurred: bool,
1208     ) {
1209         debug!("report_fulfillment_error({:?})", error);
1210         match error.code {
1211             FulfillmentErrorCode::CodeSelectionError(ref selection_error) => {
1212                 self.report_selection_error(
1213                     error.obligation.clone(),
1214                     &error.root_obligation,
1215                     selection_error,
1216                     fallback_has_occurred,
1217                     error.points_at_arg_span,
1218                 );
1219             }
1220             FulfillmentErrorCode::CodeProjectionError(ref e) => {
1221                 self.report_projection_error(&error.obligation, e);
1222             }
1223             FulfillmentErrorCode::CodeAmbiguity => {
1224                 self.maybe_report_ambiguity(&error.obligation, body_id);
1225             }
1226             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
1227                 self.report_mismatched_types(
1228                     &error.obligation.cause,
1229                     expected_found.expected,
1230                     expected_found.found,
1231                     err.clone(),
1232                 )
1233                 .emit();
1234             }
1235             FulfillmentErrorCode::CodeConstEquateError(ref expected_found, ref err) => {
1236                 self.report_mismatched_consts(
1237                     &error.obligation.cause,
1238                     expected_found.expected,
1239                     expected_found.found,
1240                     err.clone(),
1241                 )
1242                 .emit();
1243             }
1244         }
1245     }
1246
1247     fn report_projection_error(
1248         &self,
1249         obligation: &PredicateObligation<'tcx>,
1250         error: &MismatchedProjectionTypes<'tcx>,
1251     ) {
1252         let predicate = self.resolve_vars_if_possible(obligation.predicate);
1253
1254         if predicate.references_error() {
1255             return;
1256         }
1257
1258         self.probe(|_| {
1259             let err_buf;
1260             let mut err = &error.err;
1261             let mut values = None;
1262
1263             // try to find the mismatched types to report the error with.
1264             //
1265             // this can fail if the problem was higher-ranked, in which
1266             // cause I have no idea for a good error message.
1267             let bound_predicate = predicate.kind();
1268             if let ty::PredicateKind::Projection(data) = bound_predicate.skip_binder() {
1269                 let mut selcx = SelectionContext::new(self);
1270                 let (data, _) = self.replace_bound_vars_with_fresh_vars(
1271                     obligation.cause.span,
1272                     infer::LateBoundRegionConversionTime::HigherRankedType,
1273                     bound_predicate.rebind(data),
1274                 );
1275                 let mut obligations = vec![];
1276                 let normalized_ty = super::normalize_projection_type(
1277                     &mut selcx,
1278                     obligation.param_env,
1279                     data.projection_ty,
1280                     obligation.cause.clone(),
1281                     0,
1282                     &mut obligations,
1283                 );
1284
1285                 debug!(
1286                     "report_projection_error obligation.cause={:?} obligation.param_env={:?}",
1287                     obligation.cause, obligation.param_env
1288                 );
1289
1290                 debug!(
1291                     "report_projection_error normalized_ty={:?} data.ty={:?}",
1292                     normalized_ty, data.ty
1293                 );
1294
1295                 let is_normalized_ty_expected = !matches!(
1296                     obligation.cause.code.peel_derives(),
1297                     ObligationCauseCode::ItemObligation(_)
1298                         | ObligationCauseCode::BindingObligation(_, _)
1299                         | ObligationCauseCode::ObjectCastObligation(_)
1300                         | ObligationCauseCode::OpaqueType
1301                 );
1302
1303                 if let Err(error) = self.at(&obligation.cause, obligation.param_env).eq_exp(
1304                     is_normalized_ty_expected,
1305                     normalized_ty,
1306                     data.ty,
1307                 ) {
1308                     values = Some(infer::ValuePairs::Types(ExpectedFound::new(
1309                         is_normalized_ty_expected,
1310                         normalized_ty,
1311                         data.ty,
1312                     )));
1313
1314                     err_buf = error;
1315                     err = &err_buf;
1316                 }
1317             }
1318
1319             let msg = format!("type mismatch resolving `{}`", predicate);
1320             let error_id = (DiagnosticMessageId::ErrorId(271), Some(obligation.cause.span), msg);
1321             let fresh = self.tcx.sess.one_time_diagnostics.borrow_mut().insert(error_id);
1322             if fresh {
1323                 let mut diag = struct_span_err!(
1324                     self.tcx.sess,
1325                     obligation.cause.span,
1326                     E0271,
1327                     "type mismatch resolving `{}`",
1328                     predicate
1329                 );
1330                 self.note_type_err(&mut diag, &obligation.cause, None, values, err);
1331                 self.note_obligation_cause(&mut diag, obligation);
1332                 diag.emit();
1333             }
1334         });
1335     }
1336
1337     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
1338         /// returns the fuzzy category of a given type, or None
1339         /// if the type can be equated to any type.
1340         fn type_category(t: Ty<'_>) -> Option<u32> {
1341             match t.kind() {
1342                 ty::Bool => Some(0),
1343                 ty::Char => Some(1),
1344                 ty::Str => Some(2),
1345                 ty::Int(..) | ty::Uint(..) | ty::Infer(ty::IntVar(..)) => Some(3),
1346                 ty::Float(..) | ty::Infer(ty::FloatVar(..)) => Some(4),
1347                 ty::Ref(..) | ty::RawPtr(..) => Some(5),
1348                 ty::Array(..) | ty::Slice(..) => Some(6),
1349                 ty::FnDef(..) | ty::FnPtr(..) => Some(7),
1350                 ty::Dynamic(..) => Some(8),
1351                 ty::Closure(..) => Some(9),
1352                 ty::Tuple(..) => Some(10),
1353                 ty::Projection(..) => Some(11),
1354                 ty::Param(..) => Some(12),
1355                 ty::Opaque(..) => Some(13),
1356                 ty::Never => Some(14),
1357                 ty::Adt(adt, ..) => match adt.adt_kind() {
1358                     AdtKind::Struct => Some(15),
1359                     AdtKind::Union => Some(16),
1360                     AdtKind::Enum => Some(17),
1361                 },
1362                 ty::Generator(..) => Some(18),
1363                 ty::Foreign(..) => Some(19),
1364                 ty::GeneratorWitness(..) => Some(20),
1365                 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error(_) => None,
1366             }
1367         }
1368
1369         match (type_category(a), type_category(b)) {
1370             (Some(cat_a), Some(cat_b)) => match (a.kind(), b.kind()) {
1371                 (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => def_a == def_b,
1372                 _ => cat_a == cat_b,
1373             },
1374             // infer and error can be equated to all types
1375             _ => true,
1376         }
1377     }
1378
1379     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str> {
1380         self.tcx.hir().body(body_id).generator_kind.map(|gen_kind| match gen_kind {
1381             hir::GeneratorKind::Gen => "a generator",
1382             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block) => "an async block",
1383             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn) => "an async function",
1384             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Closure) => "an async closure",
1385         })
1386     }
1387
1388     fn find_similar_impl_candidates(
1389         &self,
1390         trait_ref: ty::PolyTraitRef<'tcx>,
1391     ) -> Vec<ty::TraitRef<'tcx>> {
1392         let simp = fast_reject::simplify_type(self.tcx, trait_ref.skip_binder().self_ty(), true);
1393         let all_impls = self.tcx.all_impls(trait_ref.def_id());
1394
1395         match simp {
1396             Some(simp) => all_impls
1397                 .filter_map(|def_id| {
1398                     let imp = self.tcx.impl_trait_ref(def_id).unwrap();
1399                     let imp_simp = fast_reject::simplify_type(self.tcx, imp.self_ty(), true);
1400                     if let Some(imp_simp) = imp_simp {
1401                         if simp != imp_simp {
1402                             return None;
1403                         }
1404                     }
1405                     if self.tcx.impl_polarity(def_id) == ty::ImplPolarity::Negative {
1406                         return None;
1407                     }
1408                     Some(imp)
1409                 })
1410                 .collect(),
1411             None => all_impls
1412                 .filter_map(|def_id| {
1413                     if self.tcx.impl_polarity(def_id) == ty::ImplPolarity::Negative {
1414                         return None;
1415                     }
1416                     self.tcx.impl_trait_ref(def_id)
1417                 })
1418                 .collect(),
1419         }
1420     }
1421
1422     fn report_similar_impl_candidates(
1423         &self,
1424         impl_candidates: Vec<ty::TraitRef<'tcx>>,
1425         err: &mut DiagnosticBuilder<'_>,
1426     ) {
1427         if impl_candidates.is_empty() {
1428             return;
1429         }
1430
1431         let len = impl_candidates.len();
1432         let end = if impl_candidates.len() <= 5 { impl_candidates.len() } else { 4 };
1433
1434         let normalize = |candidate| {
1435             self.tcx.infer_ctxt().enter(|ref infcx| {
1436                 let normalized = infcx
1437                     .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
1438                     .normalize(candidate)
1439                     .ok();
1440                 match normalized {
1441                     Some(normalized) => format!("\n  {}", normalized.value),
1442                     None => format!("\n  {}", candidate),
1443                 }
1444             })
1445         };
1446
1447         // Sort impl candidates so that ordering is consistent for UI tests.
1448         let mut normalized_impl_candidates =
1449             impl_candidates.iter().copied().map(normalize).collect::<Vec<String>>();
1450
1451         // Sort before taking the `..end` range,
1452         // because the ordering of `impl_candidates` may not be deterministic:
1453         // https://github.com/rust-lang/rust/pull/57475#issuecomment-455519507
1454         normalized_impl_candidates.sort();
1455
1456         err.help(&format!(
1457             "the following implementations were found:{}{}",
1458             normalized_impl_candidates[..end].join(""),
1459             if len > 5 { format!("\nand {} others", len - 4) } else { String::new() }
1460         ));
1461     }
1462
1463     /// Gets the parent trait chain start
1464     fn get_parent_trait_ref(
1465         &self,
1466         code: &ObligationCauseCode<'tcx>,
1467     ) -> Option<(String, Option<Span>)> {
1468         match code {
1469             ObligationCauseCode::BuiltinDerivedObligation(data) => {
1470                 let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_ref);
1471                 match self.get_parent_trait_ref(&data.parent_code) {
1472                     Some(t) => Some(t),
1473                     None => {
1474                         let ty = parent_trait_ref.skip_binder().self_ty();
1475                         let span = TyCategory::from_ty(self.tcx, ty)
1476                             .map(|(_, def_id)| self.tcx.def_span(def_id));
1477                         Some((ty.to_string(), span))
1478                     }
1479                 }
1480             }
1481             _ => None,
1482         }
1483     }
1484
1485     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1486     /// with the same path as `trait_ref`, a help message about
1487     /// a probable version mismatch is added to `err`
1488     fn note_version_mismatch(
1489         &self,
1490         err: &mut DiagnosticBuilder<'_>,
1491         trait_ref: &ty::PolyTraitRef<'tcx>,
1492     ) {
1493         let get_trait_impl = |trait_def_id| {
1494             self.tcx.find_map_relevant_impl(trait_def_id, trait_ref.skip_binder().self_ty(), Some)
1495         };
1496         let required_trait_path = self.tcx.def_path_str(trait_ref.def_id());
1497         let all_traits = self.tcx.all_traits(());
1498         let traits_with_same_path: std::collections::BTreeSet<_> = all_traits
1499             .iter()
1500             .filter(|trait_def_id| **trait_def_id != trait_ref.def_id())
1501             .filter(|trait_def_id| self.tcx.def_path_str(**trait_def_id) == required_trait_path)
1502             .collect();
1503         for trait_with_same_path in traits_with_same_path {
1504             if let Some(impl_def_id) = get_trait_impl(*trait_with_same_path) {
1505                 let impl_span = self.tcx.def_span(impl_def_id);
1506                 err.span_help(impl_span, "trait impl with same name found");
1507                 let trait_crate = self.tcx.crate_name(trait_with_same_path.krate);
1508                 let crate_msg = format!(
1509                     "perhaps two different versions of crate `{}` are being used?",
1510                     trait_crate
1511                 );
1512                 err.note(&crate_msg);
1513             }
1514         }
1515     }
1516
1517     fn mk_trait_obligation_with_new_self_ty(
1518         &self,
1519         param_env: ty::ParamEnv<'tcx>,
1520         trait_ref: ty::PolyTraitRef<'tcx>,
1521         new_self_ty: Ty<'tcx>,
1522     ) -> PredicateObligation<'tcx> {
1523         assert!(!new_self_ty.has_escaping_bound_vars());
1524
1525         let trait_ref = trait_ref.map_bound_ref(|tr| ty::TraitRef {
1526             substs: self.tcx.mk_substs_trait(new_self_ty, &tr.substs[1..]),
1527             ..*tr
1528         });
1529
1530         Obligation::new(
1531             ObligationCause::dummy(),
1532             param_env,
1533             trait_ref.without_const().to_predicate(self.tcx),
1534         )
1535     }
1536
1537     fn maybe_report_ambiguity(
1538         &self,
1539         obligation: &PredicateObligation<'tcx>,
1540         body_id: Option<hir::BodyId>,
1541     ) {
1542         // Unable to successfully determine, probably means
1543         // insufficient type information, but could mean
1544         // ambiguous impls. The latter *ought* to be a
1545         // coherence violation, so we don't report it here.
1546
1547         let predicate = self.resolve_vars_if_possible(obligation.predicate);
1548         let span = obligation.cause.span;
1549
1550         debug!(
1551             "maybe_report_ambiguity(predicate={:?}, obligation={:?} body_id={:?}, code={:?})",
1552             predicate, obligation, body_id, obligation.cause.code,
1553         );
1554
1555         // Ambiguity errors are often caused as fallout from earlier
1556         // errors. So just ignore them if this infcx is tainted.
1557         if self.is_tainted_by_errors() {
1558             return;
1559         }
1560
1561         let bound_predicate = predicate.kind();
1562         let mut err = match bound_predicate.skip_binder() {
1563             ty::PredicateKind::Trait(data) => {
1564                 let trait_ref = bound_predicate.rebind(data.trait_ref);
1565                 debug!("trait_ref {:?}", trait_ref);
1566
1567                 if predicate.references_error() {
1568                     return;
1569                 }
1570                 // Typically, this ambiguity should only happen if
1571                 // there are unresolved type inference variables
1572                 // (otherwise it would suggest a coherence
1573                 // failure). But given #21974 that is not necessarily
1574                 // the case -- we can have multiple where clauses that
1575                 // are only distinguished by a region, which results
1576                 // in an ambiguity even when all types are fully
1577                 // known, since we don't dispatch based on region
1578                 // relationships.
1579
1580                 // Pick the first substitution that still contains inference variables as the one
1581                 // we're going to emit an error for. If there are none (see above), fall back to
1582                 // the substitution for `Self`.
1583                 let subst = {
1584                     let substs = data.trait_ref.substs;
1585                     substs
1586                         .iter()
1587                         .find(|s| s.has_infer_types_or_consts())
1588                         .unwrap_or_else(|| substs[0])
1589                 };
1590
1591                 // This is kind of a hack: it frequently happens that some earlier
1592                 // error prevents types from being fully inferred, and then we get
1593                 // a bunch of uninteresting errors saying something like "<generic
1594                 // #0> doesn't implement Sized".  It may even be true that we
1595                 // could just skip over all checks where the self-ty is an
1596                 // inference variable, but I was afraid that there might be an
1597                 // inference variable created, registered as an obligation, and
1598                 // then never forced by writeback, and hence by skipping here we'd
1599                 // be ignoring the fact that we don't KNOW the type works
1600                 // out. Though even that would probably be harmless, given that
1601                 // we're only talking about builtin traits, which are known to be
1602                 // inhabited. We used to check for `self.tcx.sess.has_errors()` to
1603                 // avoid inundating the user with unnecessary errors, but we now
1604                 // check upstream for type errors and don't add the obligations to
1605                 // begin with in those cases.
1606                 if self.tcx.lang_items().sized_trait() == Some(trait_ref.def_id()) {
1607                     self.emit_inference_failure_err(body_id, span, subst, vec![], ErrorCode::E0282)
1608                         .emit();
1609                     return;
1610                 }
1611                 let impl_candidates = self.find_similar_impl_candidates(trait_ref);
1612                 let mut err = self.emit_inference_failure_err(
1613                     body_id,
1614                     span,
1615                     subst,
1616                     impl_candidates,
1617                     ErrorCode::E0283,
1618                 );
1619                 err.note(&format!("cannot satisfy `{}`", predicate));
1620                 if let ObligationCauseCode::ItemObligation(def_id) = obligation.cause.code {
1621                     self.suggest_fully_qualified_path(&mut err, def_id, span, trait_ref.def_id());
1622                 } else if let (
1623                     Ok(ref snippet),
1624                     ObligationCauseCode::BindingObligation(ref def_id, _),
1625                 ) =
1626                     (self.tcx.sess.source_map().span_to_snippet(span), &obligation.cause.code)
1627                 {
1628                     let generics = self.tcx.generics_of(*def_id);
1629                     if generics.params.iter().any(|p| p.name != kw::SelfUpper)
1630                         && !snippet.ends_with('>')
1631                         && !generics.has_impl_trait()
1632                         && !self.tcx.fn_trait_kind_from_lang_item(*def_id).is_some()
1633                     {
1634                         // FIXME: To avoid spurious suggestions in functions where type arguments
1635                         // where already supplied, we check the snippet to make sure it doesn't
1636                         // end with a turbofish. Ideally we would have access to a `PathSegment`
1637                         // instead. Otherwise we would produce the following output:
1638                         //
1639                         // error[E0283]: type annotations needed
1640                         //   --> $DIR/issue-54954.rs:3:24
1641                         //    |
1642                         // LL | const ARR_LEN: usize = Tt::const_val::<[i8; 123]>();
1643                         //    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^
1644                         //    |                        |
1645                         //    |                        cannot infer type
1646                         //    |                        help: consider specifying the type argument
1647                         //    |                        in the function call:
1648                         //    |                        `Tt::const_val::<[i8; 123]>::<T>`
1649                         // ...
1650                         // LL |     const fn const_val<T: Sized>() -> usize {
1651                         //    |                        - required by this bound in `Tt::const_val`
1652                         //    |
1653                         //    = note: cannot satisfy `_: Tt`
1654
1655                         err.span_suggestion_verbose(
1656                             span.shrink_to_hi(),
1657                             &format!(
1658                                 "consider specifying the type argument{} in the function call",
1659                                 pluralize!(generics.params.len()),
1660                             ),
1661                             format!(
1662                                 "::<{}>",
1663                                 generics
1664                                     .params
1665                                     .iter()
1666                                     .map(|p| p.name.to_string())
1667                                     .collect::<Vec<String>>()
1668                                     .join(", ")
1669                             ),
1670                             Applicability::HasPlaceholders,
1671                         );
1672                     }
1673                 }
1674                 err
1675             }
1676
1677             ty::PredicateKind::WellFormed(arg) => {
1678                 // Same hacky approach as above to avoid deluging user
1679                 // with error messages.
1680                 if arg.references_error() || self.tcx.sess.has_errors() {
1681                     return;
1682                 }
1683
1684                 self.emit_inference_failure_err(body_id, span, arg, vec![], ErrorCode::E0282)
1685             }
1686
1687             ty::PredicateKind::Subtype(data) => {
1688                 if data.references_error() || self.tcx.sess.has_errors() {
1689                     // no need to overload user in such cases
1690                     return;
1691                 }
1692                 let SubtypePredicate { a_is_expected: _, a, b } = data;
1693                 // both must be type variables, or the other would've been instantiated
1694                 assert!(a.is_ty_var() && b.is_ty_var());
1695                 self.emit_inference_failure_err(body_id, span, a.into(), vec![], ErrorCode::E0282)
1696             }
1697             ty::PredicateKind::Projection(data) => {
1698                 let self_ty = data.projection_ty.self_ty();
1699                 let ty = data.ty;
1700                 if predicate.references_error() {
1701                     return;
1702                 }
1703                 if self_ty.needs_infer() && ty.needs_infer() {
1704                     // We do this for the `foo.collect()?` case to produce a suggestion.
1705                     let mut err = self.emit_inference_failure_err(
1706                         body_id,
1707                         span,
1708                         self_ty.into(),
1709                         vec![],
1710                         ErrorCode::E0284,
1711                     );
1712                     err.note(&format!("cannot satisfy `{}`", predicate));
1713                     err
1714                 } else {
1715                     let mut err = struct_span_err!(
1716                         self.tcx.sess,
1717                         span,
1718                         E0284,
1719                         "type annotations needed: cannot satisfy `{}`",
1720                         predicate,
1721                     );
1722                     err.span_label(span, &format!("cannot satisfy `{}`", predicate));
1723                     err
1724                 }
1725             }
1726
1727             _ => {
1728                 if self.tcx.sess.has_errors() {
1729                     return;
1730                 }
1731                 let mut err = struct_span_err!(
1732                     self.tcx.sess,
1733                     span,
1734                     E0284,
1735                     "type annotations needed: cannot satisfy `{}`",
1736                     predicate,
1737                 );
1738                 err.span_label(span, &format!("cannot satisfy `{}`", predicate));
1739                 err
1740             }
1741         };
1742         self.note_obligation_cause(&mut err, obligation);
1743         err.emit();
1744     }
1745
1746     /// Returns `true` if the trait predicate may apply for *some* assignment
1747     /// to the type parameters.
1748     fn predicate_can_apply(
1749         &self,
1750         param_env: ty::ParamEnv<'tcx>,
1751         pred: ty::PolyTraitRef<'tcx>,
1752     ) -> bool {
1753         struct ParamToVarFolder<'a, 'tcx> {
1754             infcx: &'a InferCtxt<'a, 'tcx>,
1755             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
1756         }
1757
1758         impl<'a, 'tcx> TypeFolder<'tcx> for ParamToVarFolder<'a, 'tcx> {
1759             fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
1760                 self.infcx.tcx
1761             }
1762
1763             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
1764                 if let ty::Param(ty::ParamTy { name, .. }) = *ty.kind() {
1765                     let infcx = self.infcx;
1766                     self.var_map.entry(ty).or_insert_with(|| {
1767                         infcx.next_ty_var(TypeVariableOrigin {
1768                             kind: TypeVariableOriginKind::TypeParameterDefinition(name, None),
1769                             span: DUMMY_SP,
1770                         })
1771                     })
1772                 } else {
1773                     ty.super_fold_with(self)
1774                 }
1775             }
1776         }
1777
1778         self.probe(|_| {
1779             let mut selcx = SelectionContext::new(self);
1780
1781             let cleaned_pred =
1782                 pred.fold_with(&mut ParamToVarFolder { infcx: self, var_map: Default::default() });
1783
1784             let cleaned_pred = super::project::normalize(
1785                 &mut selcx,
1786                 param_env,
1787                 ObligationCause::dummy(),
1788                 cleaned_pred,
1789             )
1790             .value;
1791
1792             let obligation = Obligation::new(
1793                 ObligationCause::dummy(),
1794                 param_env,
1795                 cleaned_pred.without_const().to_predicate(selcx.tcx()),
1796             );
1797
1798             self.predicate_may_hold(&obligation)
1799         })
1800     }
1801
1802     fn note_obligation_cause(
1803         &self,
1804         err: &mut DiagnosticBuilder<'tcx>,
1805         obligation: &PredicateObligation<'tcx>,
1806     ) {
1807         // First, attempt to add note to this error with an async-await-specific
1808         // message, and fall back to regular note otherwise.
1809         if !self.maybe_note_obligation_cause_for_async_await(err, obligation) {
1810             self.note_obligation_cause_code(
1811                 err,
1812                 &obligation.predicate,
1813                 &obligation.cause.code,
1814                 &mut vec![],
1815                 &mut Default::default(),
1816             );
1817             self.suggest_unsized_bound_if_applicable(err, obligation);
1818         }
1819     }
1820
1821     fn suggest_unsized_bound_if_applicable(
1822         &self,
1823         err: &mut DiagnosticBuilder<'tcx>,
1824         obligation: &PredicateObligation<'tcx>,
1825     ) {
1826         let (pred, item_def_id, span) =
1827             match (obligation.predicate.kind().skip_binder(), obligation.cause.code.peel_derives())
1828             {
1829                 (
1830                     ty::PredicateKind::Trait(pred),
1831                     &ObligationCauseCode::BindingObligation(item_def_id, span),
1832                 ) => (pred, item_def_id, span),
1833                 _ => return,
1834             };
1835         debug!(
1836             "suggest_unsized_bound_if_applicable: pred={:?} item_def_id={:?} span={:?}",
1837             pred, item_def_id, span
1838         );
1839         let node = match (
1840             self.tcx.hir().get_if_local(item_def_id),
1841             Some(pred.def_id()) == self.tcx.lang_items().sized_trait(),
1842         ) {
1843             (Some(node), true) => node,
1844             _ => return,
1845         };
1846         self.maybe_suggest_unsized_generics(err, span, node);
1847     }
1848
1849     fn maybe_suggest_unsized_generics(
1850         &self,
1851         err: &mut DiagnosticBuilder<'tcx>,
1852         span: Span,
1853         node: Node<'hir>,
1854     ) {
1855         let generics = match node.generics() {
1856             Some(generics) => generics,
1857             None => return,
1858         };
1859         let sized_trait = self.tcx.lang_items().sized_trait();
1860         debug!("maybe_suggest_unsized_generics: generics.params={:?}", generics.params);
1861         debug!("maybe_suggest_unsized_generics: generics.where_clause={:?}", generics.where_clause);
1862         let param = generics
1863             .params
1864             .iter()
1865             .filter(|param| param.span == span)
1866             .filter(|param| {
1867                 // Check that none of the explicit trait bounds is `Sized`. Assume that an explicit
1868                 // `Sized` bound is there intentionally and we don't need to suggest relaxing it.
1869                 param
1870                     .bounds
1871                     .iter()
1872                     .all(|bound| bound.trait_ref().and_then(|tr| tr.trait_def_id()) != sized_trait)
1873             })
1874             .next();
1875         let param = match param {
1876             Some(param) => param,
1877             _ => return,
1878         };
1879         debug!("maybe_suggest_unsized_generics: param={:?}", param);
1880         match node {
1881             hir::Node::Item(
1882                 item
1883                 @
1884                 hir::Item {
1885                     // Only suggest indirection for uses of type parameters in ADTs.
1886                     kind:
1887                         hir::ItemKind::Enum(..) | hir::ItemKind::Struct(..) | hir::ItemKind::Union(..),
1888                     ..
1889                 },
1890             ) => {
1891                 if self.maybe_indirection_for_unsized(err, item, param) {
1892                     return;
1893                 }
1894             }
1895             _ => {}
1896         };
1897         // Didn't add an indirection suggestion, so add a general suggestion to relax `Sized`.
1898         let (span, separator) = match param.bounds {
1899             [] => (span.shrink_to_hi(), ":"),
1900             [.., bound] => (bound.span().shrink_to_hi(), " +"),
1901         };
1902         err.span_suggestion_verbose(
1903             span,
1904             "consider relaxing the implicit `Sized` restriction",
1905             format!("{} ?Sized", separator),
1906             Applicability::MachineApplicable,
1907         );
1908     }
1909
1910     fn maybe_indirection_for_unsized(
1911         &self,
1912         err: &mut DiagnosticBuilder<'tcx>,
1913         item: &'hir Item<'hir>,
1914         param: &'hir GenericParam<'hir>,
1915     ) -> bool {
1916         // Suggesting `T: ?Sized` is only valid in an ADT if `T` is only used in a
1917         // borrow. `struct S<'a, T: ?Sized>(&'a T);` is valid, `struct S<T: ?Sized>(T);`
1918         // is not. Look for invalid "bare" parameter uses, and suggest using indirection.
1919         let mut visitor =
1920             FindTypeParam { param: param.name.ident().name, invalid_spans: vec![], nested: false };
1921         visitor.visit_item(item);
1922         if visitor.invalid_spans.is_empty() {
1923             return false;
1924         }
1925         let mut multispan: MultiSpan = param.span.into();
1926         multispan.push_span_label(
1927             param.span,
1928             format!("this could be changed to `{}: ?Sized`...", param.name.ident()),
1929         );
1930         for sp in visitor.invalid_spans {
1931             multispan.push_span_label(
1932                 sp,
1933                 format!("...if indirection were used here: `Box<{}>`", param.name.ident()),
1934             );
1935         }
1936         err.span_help(
1937             multispan,
1938             &format!(
1939                 "you could relax the implicit `Sized` bound on `{T}` if it were \
1940                 used through indirection like `&{T}` or `Box<{T}>`",
1941                 T = param.name.ident(),
1942             ),
1943         );
1944         true
1945     }
1946
1947     fn is_recursive_obligation(
1948         &self,
1949         obligated_types: &mut Vec<&ty::TyS<'tcx>>,
1950         cause_code: &ObligationCauseCode<'tcx>,
1951     ) -> bool {
1952         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
1953             let parent_trait_ref = self.resolve_vars_if_possible(data.parent_trait_ref);
1954
1955             if obligated_types.iter().any(|ot| ot == &parent_trait_ref.skip_binder().self_ty()) {
1956                 return true;
1957             }
1958         }
1959         false
1960     }
1961 }
1962
1963 /// Look for type `param` in an ADT being used only through a reference to confirm that suggesting
1964 /// `param: ?Sized` would be a valid constraint.
1965 struct FindTypeParam {
1966     param: rustc_span::Symbol,
1967     invalid_spans: Vec<Span>,
1968     nested: bool,
1969 }
1970
1971 impl<'v> Visitor<'v> for FindTypeParam {
1972     type Map = rustc_hir::intravisit::ErasedMap<'v>;
1973
1974     fn nested_visit_map(&mut self) -> hir::intravisit::NestedVisitorMap<Self::Map> {
1975         hir::intravisit::NestedVisitorMap::None
1976     }
1977
1978     fn visit_where_predicate(&mut self, _: &'v hir::WherePredicate<'v>) {
1979         // Skip where-clauses, to avoid suggesting indirection for type parameters found there.
1980     }
1981
1982     fn visit_ty(&mut self, ty: &hir::Ty<'_>) {
1983         // We collect the spans of all uses of the "bare" type param, like in `field: T` or
1984         // `field: (T, T)` where we could make `T: ?Sized` while skipping cases that are known to be
1985         // valid like `field: &'a T` or `field: *mut T` and cases that *might* have further `Sized`
1986         // obligations like `Box<T>` and `Vec<T>`, but we perform no extra analysis for those cases
1987         // and suggest `T: ?Sized` regardless of their obligations. This is fine because the errors
1988         // in that case should make what happened clear enough.
1989         match ty.kind {
1990             hir::TyKind::Ptr(_) | hir::TyKind::Rptr(..) | hir::TyKind::TraitObject(..) => {}
1991             hir::TyKind::Path(hir::QPath::Resolved(None, path))
1992                 if path.segments.len() == 1 && path.segments[0].ident.name == self.param =>
1993             {
1994                 if !self.nested {
1995                     debug!("FindTypeParam::visit_ty: ty={:?}", ty);
1996                     self.invalid_spans.push(ty.span);
1997                 }
1998             }
1999             hir::TyKind::Path(_) => {
2000                 let prev = self.nested;
2001                 self.nested = true;
2002                 hir::intravisit::walk_ty(self, ty);
2003                 self.nested = prev;
2004             }
2005             _ => {
2006                 hir::intravisit::walk_ty(self, ty);
2007             }
2008         }
2009     }
2010 }
2011
2012 pub fn recursive_type_with_infinite_size_error(
2013     tcx: TyCtxt<'tcx>,
2014     type_def_id: DefId,
2015     spans: Vec<Span>,
2016 ) {
2017     assert!(type_def_id.is_local());
2018     let span = tcx.hir().span_if_local(type_def_id).unwrap();
2019     let span = tcx.sess.source_map().guess_head_span(span);
2020     let path = tcx.def_path_str(type_def_id);
2021     let mut err =
2022         struct_span_err!(tcx.sess, span, E0072, "recursive type `{}` has infinite size", path);
2023     err.span_label(span, "recursive type has infinite size");
2024     for &span in &spans {
2025         err.span_label(span, "recursive without indirection");
2026     }
2027     let msg = format!(
2028         "insert some indirection (e.g., a `Box`, `Rc`, or `&`) to make `{}` representable",
2029         path,
2030     );
2031     if spans.len() <= 4 {
2032         err.multipart_suggestion(
2033             &msg,
2034             spans
2035                 .iter()
2036                 .flat_map(|&span| {
2037                     vec![
2038                         (span.shrink_to_lo(), "Box<".to_string()),
2039                         (span.shrink_to_hi(), ">".to_string()),
2040                     ]
2041                     .into_iter()
2042                 })
2043                 .collect(),
2044             Applicability::HasPlaceholders,
2045         );
2046     } else {
2047         err.help(&msg);
2048     }
2049     err.emit();
2050 }
2051
2052 /// Summarizes information
2053 #[derive(Clone)]
2054 pub enum ArgKind {
2055     /// An argument of non-tuple type. Parameters are (name, ty)
2056     Arg(String, String),
2057
2058     /// An argument of tuple type. For a "found" argument, the span is
2059     /// the location in the source of the pattern. For an "expected"
2060     /// argument, it will be None. The vector is a list of (name, ty)
2061     /// strings for the components of the tuple.
2062     Tuple(Option<Span>, Vec<(String, String)>),
2063 }
2064
2065 impl ArgKind {
2066     fn empty() -> ArgKind {
2067         ArgKind::Arg("_".to_owned(), "_".to_owned())
2068     }
2069
2070     /// Creates an `ArgKind` from the expected type of an
2071     /// argument. It has no name (`_`) and an optional source span.
2072     pub fn from_expected_ty(t: Ty<'_>, span: Option<Span>) -> ArgKind {
2073         match t.kind() {
2074             ty::Tuple(tys) => ArgKind::Tuple(
2075                 span,
2076                 tys.iter().map(|ty| ("_".to_owned(), ty.to_string())).collect::<Vec<_>>(),
2077             ),
2078             _ => ArgKind::Arg("_".to_owned(), t.to_string()),
2079         }
2080     }
2081 }