src/librustc_infer/traits/error_reporting/mod.rs

   1 pub mod on_unimplemented;
   2 pub mod suggestions;
   3
   4 use super::{
   5     ConstEvalFailure, EvaluationResult, FulfillmentError, FulfillmentErrorCode,
   6     MismatchedProjectionTypes, ObjectSafetyViolation, Obligation, ObligationCause,
   7     ObligationCauseCode, OnUnimplementedDirective, OnUnimplementedNote,
   8     OutputTypeParameterMismatch, Overflow, PredicateObligation, SelectionContext, SelectionError,
   9     TraitNotObjectSafe,
  10 };
  11
  12 use crate::infer::error_reporting::{TyCategory, TypeAnnotationNeeded as ErrorCode};
  13 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  14 use crate::infer::{self, InferCtxt, TyCtxtInferExt};
  15 use rustc::mir::interpret::ErrorHandled;
  16 use rustc::session::DiagnosticMessageId;
  17 use rustc::ty::error::ExpectedFound;
  18 use rustc::ty::fast_reject;
  19 use rustc::ty::fold::TypeFolder;
  20 use rustc::ty::SubtypePredicate;
  21 use rustc::ty::{
  22     self, AdtKind, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness,
  23 };
  24 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
  25 use rustc_errors::{struct_span_err, Applicability, DiagnosticBuilder};
  26 use rustc_hir as hir;
  27 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
  28 use rustc_hir::{QPath, TyKind, WhereBoundPredicate, WherePredicate};
  29 use rustc_span::source_map::SourceMap;
  30 use rustc_span::{ExpnKind, Span, DUMMY_SP};
  31 use std::fmt;
  32 use syntax::ast;
  33
  34 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
  35     pub fn report_fulfillment_errors(
  36         &self,
  37         errors: &[FulfillmentError<'tcx>],
  38         body_id: Option<hir::BodyId>,
  39         fallback_has_occurred: bool,
  40     ) {
  41         #[derive(Debug)]
  42         struct ErrorDescriptor<'tcx> {
  43             predicate: ty::Predicate<'tcx>,
  44             index: Option<usize>, // None if this is an old error
  45         }
  46
  47         let mut error_map: FxHashMap<_, Vec<_>> = self
  48             .reported_trait_errors
  49             .borrow()
  50             .iter()
  51             .map(|(&span, predicates)| {
  52                 (
  53                     span,
  54                     predicates
  55                         .iter()
  56                         .map(|&predicate| ErrorDescriptor { predicate, index: None })
  57                         .collect(),
  58                 )
  59             })
  60             .collect();
  61
  62         for (index, error) in errors.iter().enumerate() {
  63             // We want to ignore desugarings here: spans are equivalent even
  64             // if one is the result of a desugaring and the other is not.
  65             let mut span = error.obligation.cause.span;
  66             let expn_data = span.ctxt().outer_expn_data();
  67             if let ExpnKind::Desugaring(_) = expn_data.kind {
  68                 span = expn_data.call_site;
  69             }
  70
  71             error_map.entry(span).or_default().push(ErrorDescriptor {
  72                 predicate: error.obligation.predicate,
  73                 index: Some(index),
  74             });
  75
  76             self.reported_trait_errors
  77                 .borrow_mut()
  78                 .entry(span)
  79                 .or_default()
  80                 .push(error.obligation.predicate.clone());
  81         }
  82
  83         // We do this in 2 passes because we want to display errors in order, though
  84         // maybe it *is* better to sort errors by span or something.
  85         let mut is_suppressed = vec![false; errors.len()];
  86         for (_, error_set) in error_map.iter() {
  87             // We want to suppress "duplicate" errors with the same span.
  88             for error in error_set {
  89                 if let Some(index) = error.index {
  90                     // Suppress errors that are either:
  91                     // 1) strictly implied by another error.
  92                     // 2) implied by an error with a smaller index.
  93                     for error2 in error_set {
  94                         if error2.index.map_or(false, |index2| is_suppressed[index2]) {
  95                             // Avoid errors being suppressed by already-suppressed
  96                             // errors, to prevent all errors from being suppressed
  97                             // at once.
  98                             continue;
  99                         }
 100
 101                         if self.error_implies(&error2.predicate, &error.predicate)
 102                             && !(error2.index >= error.index
 103                                 && self.error_implies(&error.predicate, &error2.predicate))
 104                         {
 105                             info!("skipping {:?} (implied by {:?})", error, error2);
 106                             is_suppressed[index] = true;
 107                             break;
 108                         }
 109                     }
 110                 }
 111             }
 112         }
 113
 114         for (error, suppressed) in errors.iter().zip(is_suppressed) {
 115             if !suppressed {
 116                 self.report_fulfillment_error(error, body_id, fallback_has_occurred);
 117             }
 118         }
 119     }
 120
 121     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
 122     // `error` occurring implies that `cond` occurs.
 123     fn error_implies(&self, cond: &ty::Predicate<'tcx>, error: &ty::Predicate<'tcx>) -> bool {
 124         if cond == error {
 125             return true;
 126         }
 127
 128         let (cond, error) = match (cond, error) {
 129             (&ty::Predicate::Trait(..), &ty::Predicate::Trait(ref error, _)) => (cond, error),
 130             _ => {
 131                 // FIXME: make this work in other cases too.
 132                 return false;
 133             }
 134         };
 135
 136         for implication in super::elaborate_predicates(self.tcx, vec![*cond]) {
 137             if let ty::Predicate::Trait(implication, _) = implication {
 138                 let error = error.to_poly_trait_ref();
 139                 let implication = implication.to_poly_trait_ref();
 140                 // FIXME: I'm just not taking associated types at all here.
 141                 // Eventually I'll need to implement param-env-aware
 142                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
 143                 let param_env = ty::ParamEnv::empty();
 144                 if self.can_sub(param_env, error, implication).is_ok() {
 145                     debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
 146                     return true;
 147                 }
 148             }
 149         }
 150
 151         false
 152     }
 153
 154     fn report_fulfillment_error(
 155         &self,
 156         error: &FulfillmentError<'tcx>,
 157         body_id: Option<hir::BodyId>,
 158         fallback_has_occurred: bool,
 159     ) {
 160         debug!("report_fulfillment_error({:?})", error);
 161         match error.code {
 162             FulfillmentErrorCode::CodeSelectionError(ref selection_error) => {
 163                 self.report_selection_error(
 164                     &error.obligation,
 165                     selection_error,
 166                     fallback_has_occurred,
 167                     error.points_at_arg_span,
 168                 );
 169             }
 170             FulfillmentErrorCode::CodeProjectionError(ref e) => {
 171                 self.report_projection_error(&error.obligation, e);
 172             }
 173             FulfillmentErrorCode::CodeAmbiguity => {
 174                 self.maybe_report_ambiguity(&error.obligation, body_id);
 175             }
 176             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
 177                 self.report_mismatched_types(
 178                     &error.obligation.cause,
 179                     expected_found.expected,
 180                     expected_found.found,
 181                     err.clone(),
 182                 )
 183                 .emit();
 184             }
 185         }
 186     }
 187
 188     fn report_projection_error(
 189         &self,
 190         obligation: &PredicateObligation<'tcx>,
 191         error: &MismatchedProjectionTypes<'tcx>,
 192     ) {
 193         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
 194
 195         if predicate.references_error() {
 196             return;
 197         }
 198
 199         self.probe(|_| {
 200             let err_buf;
 201             let mut err = &error.err;
 202             let mut values = None;
 203
 204             // try to find the mismatched types to report the error with.
 205             //
 206             // this can fail if the problem was higher-ranked, in which
 207             // cause I have no idea for a good error message.
 208             if let ty::Predicate::Projection(ref data) = predicate {
 209                 let mut selcx = SelectionContext::new(self);
 210                 let (data, _) = self.replace_bound_vars_with_fresh_vars(
 211                     obligation.cause.span,
 212                     infer::LateBoundRegionConversionTime::HigherRankedType,
 213                     data,
 214                 );
 215                 let mut obligations = vec![];
 216                 let normalized_ty = super::normalize_projection_type(
 217                     &mut selcx,
 218                     obligation.param_env,
 219                     data.projection_ty,
 220                     obligation.cause.clone(),
 221                     0,
 222                     &mut obligations,
 223                 );
 224
 225                 debug!(
 226                     "report_projection_error obligation.cause={:?} obligation.param_env={:?}",
 227                     obligation.cause, obligation.param_env
 228                 );
 229
 230                 debug!(
 231                     "report_projection_error normalized_ty={:?} data.ty={:?}",
 232                     normalized_ty, data.ty
 233                 );
 234
 235                 let is_normalized_ty_expected = match &obligation.cause.code {
 236                     ObligationCauseCode::ItemObligation(_)
 237                     | ObligationCauseCode::BindingObligation(_, _)
 238                     | ObligationCauseCode::ObjectCastObligation(_) => false,
 239                     _ => true,
 240                 };
 241
 242                 if let Err(error) = self.at(&obligation.cause, obligation.param_env).eq_exp(
 243                     is_normalized_ty_expected,
 244                     normalized_ty,
 245                     data.ty,
 246                 ) {
 247                     values = Some(infer::ValuePairs::Types(ExpectedFound::new(
 248                         is_normalized_ty_expected,
 249                         normalized_ty,
 250                         data.ty,
 251                     )));
 252
 253                     err_buf = error;
 254                     err = &err_buf;
 255                 }
 256             }
 257
 258             let msg = format!("type mismatch resolving `{}`", predicate);
 259             let error_id = (DiagnosticMessageId::ErrorId(271), Some(obligation.cause.span), msg);
 260             let fresh = self.tcx.sess.one_time_diagnostics.borrow_mut().insert(error_id);
 261             if fresh {
 262                 let mut diag = struct_span_err!(
 263                     self.tcx.sess,
 264                     obligation.cause.span,
 265                     E0271,
 266                     "type mismatch resolving `{}`",
 267                     predicate
 268                 );
 269                 self.note_type_err(&mut diag, &obligation.cause, None, values, err);
 270                 self.note_obligation_cause(&mut diag, obligation);
 271                 diag.emit();
 272             }
 273         });
 274     }
 275
 276     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
 277         /// returns the fuzzy category of a given type, or None
 278         /// if the type can be equated to any type.
 279         fn type_category(t: Ty<'_>) -> Option<u32> {
 280             match t.kind {
 281                 ty::Bool => Some(0),
 282                 ty::Char => Some(1),
 283                 ty::Str => Some(2),
 284                 ty::Int(..) | ty::Uint(..) | ty::Infer(ty::IntVar(..)) => Some(3),
 285                 ty::Float(..) | ty::Infer(ty::FloatVar(..)) => Some(4),
 286                 ty::Ref(..) | ty::RawPtr(..) => Some(5),
 287                 ty::Array(..) | ty::Slice(..) => Some(6),
 288                 ty::FnDef(..) | ty::FnPtr(..) => Some(7),
 289                 ty::Dynamic(..) => Some(8),
 290                 ty::Closure(..) => Some(9),
 291                 ty::Tuple(..) => Some(10),
 292                 ty::Projection(..) => Some(11),
 293                 ty::Param(..) => Some(12),
 294                 ty::Opaque(..) => Some(13),
 295                 ty::Never => Some(14),
 296                 ty::Adt(adt, ..) => match adt.adt_kind() {
 297                     AdtKind::Struct => Some(15),
 298                     AdtKind::Union => Some(16),
 299                     AdtKind::Enum => Some(17),
 300                 },
 301                 ty::Generator(..) => Some(18),
 302                 ty::Foreign(..) => Some(19),
 303                 ty::GeneratorWitness(..) => Some(20),
 304                 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error => None,
 305                 ty::UnnormalizedProjection(..) => bug!("only used with chalk-engine"),
 306             }
 307         }
 308
 309         match (type_category(a), type_category(b)) {
 310             (Some(cat_a), Some(cat_b)) => match (&a.kind, &b.kind) {
 311                 (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => def_a == def_b,
 312                 _ => cat_a == cat_b,
 313             },
 314             // infer and error can be equated to all types
 315             _ => true,
 316         }
 317     }
 318
 319     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str> {
 320         self.tcx.hir().body(body_id).generator_kind.map(|gen_kind| match gen_kind {
 321             hir::GeneratorKind::Gen => "a generator",
 322             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block) => "an async block",
 323             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn) => "an async function",
 324             hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Closure) => "an async closure",
 325         })
 326     }
 327
 328     fn find_similar_impl_candidates(
 329         &self,
 330         trait_ref: ty::PolyTraitRef<'tcx>,
 331     ) -> Vec<ty::TraitRef<'tcx>> {
 332         let simp = fast_reject::simplify_type(self.tcx, trait_ref.skip_binder().self_ty(), true);
 333         let all_impls = self.tcx.all_impls(trait_ref.def_id());
 334
 335         match simp {
 336             Some(simp) => all_impls
 337                 .iter()
 338                 .filter_map(|&def_id| {
 339                     let imp = self.tcx.impl_trait_ref(def_id).unwrap();
 340                     let imp_simp = fast_reject::simplify_type(self.tcx, imp.self_ty(), true);
 341                     if let Some(imp_simp) = imp_simp {
 342                         if simp != imp_simp {
 343                             return None;
 344                         }
 345                     }
 346
 347                     Some(imp)
 348                 })
 349                 .collect(),
 350             None => {
 351                 all_impls.iter().map(|&def_id| self.tcx.impl_trait_ref(def_id).unwrap()).collect()
 352             }
 353         }
 354     }
 355
 356     fn report_similar_impl_candidates(
 357         &self,
 358         impl_candidates: Vec<ty::TraitRef<'tcx>>,
 359         err: &mut DiagnosticBuilder<'_>,
 360     ) {
 361         if impl_candidates.is_empty() {
 362             return;
 363         }
 364
 365         let len = impl_candidates.len();
 366         let end = if impl_candidates.len() <= 5 { impl_candidates.len() } else { 4 };
 367
 368         let normalize = |candidate| {
 369             self.tcx.infer_ctxt().enter(|ref infcx| {
 370                 let normalized = infcx
 371                     .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
 372                     .normalize(candidate)
 373                     .ok();
 374                 match normalized {
 375                     Some(normalized) => format!("\n  {:?}", normalized.value),
 376                     None => format!("\n  {:?}", candidate),
 377                 }
 378             })
 379         };
 380
 381         // Sort impl candidates so that ordering is consistent for UI tests.
 382         let mut normalized_impl_candidates =
 383             impl_candidates.iter().map(normalize).collect::<Vec<String>>();
 384
 385         // Sort before taking the `..end` range,
 386         // because the ordering of `impl_candidates` may not be deterministic:
 387         // https://github.com/rust-lang/rust/pull/57475#issuecomment-455519507
 388         normalized_impl_candidates.sort();
 389
 390         err.help(&format!(
 391             "the following implementations were found:{}{}",
 392             normalized_impl_candidates[..end].join(""),
 393             if len > 5 { format!("\nand {} others", len - 4) } else { String::new() }
 394         ));
 395     }
 396
 397     /// Reports that an overflow has occurred and halts compilation. We
 398     /// halt compilation unconditionally because it is important that
 399     /// overflows never be masked -- they basically represent computations
 400     /// whose result could not be truly determined and thus we can't say
 401     /// if the program type checks or not -- and they are unusual
 402     /// occurrences in any case.
 403     pub fn report_overflow_error<T>(
 404         &self,
 405         obligation: &Obligation<'tcx, T>,
 406         suggest_increasing_limit: bool,
 407     ) -> !
 408     where
 409         T: fmt::Display + TypeFoldable<'tcx>,
 410     {
 411         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
 412         let mut err = struct_span_err!(
 413             self.tcx.sess,
 414             obligation.cause.span,
 415             E0275,
 416             "overflow evaluating the requirement `{}`",
 417             predicate
 418         );
 419
 420         if suggest_increasing_limit {
 421             self.suggest_new_overflow_limit(&mut err);
 422         }
 423
 424         self.note_obligation_cause_code(
 425             &mut err,
 426             &obligation.predicate,
 427             &obligation.cause.code,
 428             &mut vec![],
 429         );
 430
 431         err.emit();
 432         self.tcx.sess.abort_if_errors();
 433         bug!();
 434     }
 435
 436     /// Reports that a cycle was detected which led to overflow and halts
 437     /// compilation. This is equivalent to `report_overflow_error` except
 438     /// that we can give a more helpful error message (and, in particular,
 439     /// we do not suggest increasing the overflow limit, which is not
 440     /// going to help).
 441     pub fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 442         let cycle = self.resolve_vars_if_possible(&cycle.to_owned());
 443         assert!(cycle.len() > 0);
 444
 445         debug!("report_overflow_error_cycle: cycle={:?}", cycle);
 446
 447         self.report_overflow_error(&cycle[0], false);
 448     }
 449
 450     pub fn report_extra_impl_obligation(
 451         &self,
 452         error_span: Span,
 453         item_name: ast::Name,
 454         _impl_item_def_id: DefId,
 455         trait_item_def_id: DefId,
 456         requirement: &dyn fmt::Display,
 457     ) -> DiagnosticBuilder<'tcx> {
 458         let msg = "impl has stricter requirements than trait";
 459         let sp = self.tcx.sess.source_map().def_span(error_span);
 460
 461         let mut err = struct_span_err!(self.tcx.sess, sp, E0276, "{}", msg);
 462
 463         if let Some(trait_item_span) = self.tcx.hir().span_if_local(trait_item_def_id) {
 464             let span = self.tcx.sess.source_map().def_span(trait_item_span);
 465             err.span_label(span, format!("definition of `{}` from trait", item_name));
 466         }
 467
 468         err.span_label(sp, format!("impl has extra requirement {}", requirement));
 469
 470         err
 471     }
 472
 473     /// Gets the parent trait chain start
 474     fn get_parent_trait_ref(
 475         &self,
 476         code: &ObligationCauseCode<'tcx>,
 477     ) -> Option<(String, Option<Span>)> {
 478         match code {
 479             &ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
 480                 let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
 481                 match self.get_parent_trait_ref(&data.parent_code) {
 482                     Some(t) => Some(t),
 483                     None => {
 484                         let ty = parent_trait_ref.skip_binder().self_ty();
 485                         let span =
 486                             TyCategory::from_ty(ty).map(|(_, def_id)| self.tcx.def_span(def_id));
 487                         Some((ty.to_string(), span))
 488                     }
 489                 }
 490             }
 491             _ => None,
 492         }
 493     }
 494
 495     pub fn report_selection_error(
 496         &self,
 497         obligation: &PredicateObligation<'tcx>,
 498         error: &SelectionError<'tcx>,
 499         fallback_has_occurred: bool,
 500         points_at_arg: bool,
 501     ) {
 502         let tcx = self.tcx;
 503         let span = obligation.cause.span;
 504
 505         let mut err = match *error {
 506             SelectionError::Unimplemented => {
 507                 if let ObligationCauseCode::CompareImplMethodObligation {
 508                     item_name,
 509                     impl_item_def_id,
 510                     trait_item_def_id,
 511                 }
 512                 | ObligationCauseCode::CompareImplTypeObligation {
 513                     item_name,
 514                     impl_item_def_id,
 515                     trait_item_def_id,
 516                 } = obligation.cause.code
 517                 {
 518                     self.report_extra_impl_obligation(
 519                         span,
 520                         item_name,
 521                         impl_item_def_id,
 522                         trait_item_def_id,
 523                         &format!("`{}`", obligation.predicate),
 524                     )
 525                     .emit();
 526                     return;
 527                 }
 528                 match obligation.predicate {
 529                     ty::Predicate::Trait(ref trait_predicate, _) => {
 530                         let trait_predicate = self.resolve_vars_if_possible(trait_predicate);
 531
 532                         if self.tcx.sess.has_errors() && trait_predicate.references_error() {
 533                             return;
 534                         }
 535                         let trait_ref = trait_predicate.to_poly_trait_ref();
 536                         let (post_message, pre_message, type_def) = self
 537                             .get_parent_trait_ref(&obligation.cause.code)
 538                             .map(|(t, s)| {
 539                                 (
 540                                     format!(" in `{}`", t),
 541                                     format!("within `{}`, ", t),
 542                                     s.map(|s| (format!("within this `{}`", t), s)),
 543                                 )
 544                             })
 545                             .unwrap_or_default();
 546
 547                         let OnUnimplementedNote { message, label, note, enclosing_scope } =
 548                             self.on_unimplemented_note(trait_ref, obligation);
 549                         let have_alt_message = message.is_some() || label.is_some();
 550                         let is_try = self
 551                             .tcx
 552                             .sess
 553                             .source_map()
 554                             .span_to_snippet(span)
 555                             .map(|s| &s == "?")
 556                             .unwrap_or(false);
 557                         let is_from = format!("{}", trait_ref.print_only_trait_path())
 558                             .starts_with("std::convert::From<");
 559                         let (message, note) = if is_try && is_from {
 560                             (
 561                                 Some(format!(
 562                                     "`?` couldn't convert the error to `{}`",
 563                                     trait_ref.self_ty(),
 564                                 )),
 565                                 Some(
 566                                     "the question mark operation (`?`) implicitly performs a \
 567                                      conversion on the error value using the `From` trait"
 568                                         .to_owned(),
 569                                 ),
 570                             )
 571                         } else {
 572                             (message, note)
 573                         };
 574
 575                         let mut err = struct_span_err!(
 576                             self.tcx.sess,
 577                             span,
 578                             E0277,
 579                             "{}",
 580                             message.unwrap_or_else(|| format!(
 581                                 "the trait bound `{}` is not satisfied{}",
 582                                 trait_ref.without_const().to_predicate(),
 583                                 post_message,
 584                             ))
 585                         );
 586
 587                         let explanation =
 588                             if obligation.cause.code == ObligationCauseCode::MainFunctionType {
 589                                 "consider using `()`, or a `Result`".to_owned()
 590                             } else {
 591                                 format!(
 592                                     "{}the trait `{}` is not implemented for `{}`",
 593                                     pre_message,
 594                                     trait_ref.print_only_trait_path(),
 595                                     trait_ref.self_ty(),
 596                                 )
 597                             };
 598
 599                         if self.suggest_add_reference_to_arg(
 600                             &obligation,
 601                             &mut err,
 602                             &trait_ref,
 603                             points_at_arg,
 604                             have_alt_message,
 605                         ) {
 606                             self.note_obligation_cause(&mut err, obligation);
 607                             err.emit();
 608                             return;
 609                         }
 610                         if let Some(ref s) = label {
 611                             // If it has a custom `#[rustc_on_unimplemented]`
 612                             // error message, let's display it as the label!
 613                             err.span_label(span, s.as_str());
 614                             err.help(&explanation);
 615                         } else {
 616                             err.span_label(span, explanation);
 617                         }
 618                         if let Some((msg, span)) = type_def {
 619                             err.span_label(span, &msg);
 620                         }
 621                         if let Some(ref s) = note {
 622                             // If it has a custom `#[rustc_on_unimplemented]` note, let's display it
 623                             err.note(s.as_str());
 624                         }
 625                         if let Some(ref s) = enclosing_scope {
 626                             let enclosing_scope_span = tcx.def_span(
 627                                 tcx.hir()
 628                                     .opt_local_def_id(obligation.cause.body_id)
 629                                     .unwrap_or_else(|| {
 630                                         tcx.hir().body_owner_def_id(hir::BodyId {
 631                                             hir_id: obligation.cause.body_id,
 632                                         })
 633                                     }),
 634                             );
 635
 636                             err.span_label(enclosing_scope_span, s.as_str());
 637                         }
 638
 639                         self.suggest_borrow_on_unsized_slice(&obligation.cause.code, &mut err);
 640                         self.suggest_fn_call(&obligation, &mut err, &trait_ref, points_at_arg);
 641                         self.suggest_remove_reference(&obligation, &mut err, &trait_ref);
 642                         self.suggest_semicolon_removal(&obligation, &mut err, span, &trait_ref);
 643                         self.note_version_mismatch(&mut err, &trait_ref);
 644                         if self.suggest_impl_trait(&mut err, span, &obligation, &trait_ref) {
 645                             err.emit();
 646                             return;
 647                         }
 648
 649                         // Try to report a help message
 650                         if !trait_ref.has_infer_types()
 651                             && self.predicate_can_apply(obligation.param_env, trait_ref)
 652                         {
 653                             // If a where-clause may be useful, remind the
 654                             // user that they can add it.
 655                             //
 656                             // don't display an on-unimplemented note, as
 657                             // these notes will often be of the form
 658                             //     "the type `T` can't be frobnicated"
 659                             // which is somewhat confusing.
 660                             self.suggest_restricting_param_bound(
 661                                 &mut err,
 662                                 &trait_ref,
 663                                 obligation.cause.body_id,
 664                             );
 665                         } else {
 666                             if !have_alt_message {
 667                                 // Can't show anything else useful, try to find similar impls.
 668                                 let impl_candidates = self.find_similar_impl_candidates(trait_ref);
 669                                 self.report_similar_impl_candidates(impl_candidates, &mut err);
 670                             }
 671                             self.suggest_change_mut(
 672                                 &obligation,
 673                                 &mut err,
 674                                 &trait_ref,
 675                                 points_at_arg,
 676                             );
 677                         }
 678
 679                         // If this error is due to `!: Trait` not implemented but `(): Trait` is
 680                         // implemented, and fallback has occurred, then it could be due to a
 681                         // variable that used to fallback to `()` now falling back to `!`. Issue a
 682                         // note informing about the change in behaviour.
 683                         if trait_predicate.skip_binder().self_ty().is_never()
 684                             && fallback_has_occurred
 685                         {
 686                             let predicate = trait_predicate.map_bound(|mut trait_pred| {
 687                                 trait_pred.trait_ref.substs = self.tcx.mk_substs_trait(
 688                                     self.tcx.mk_unit(),
 689                                     &trait_pred.trait_ref.substs[1..],
 690                                 );
 691                                 trait_pred
 692                             });
 693                             let unit_obligation = Obligation {
 694                                 predicate: ty::Predicate::Trait(
 695                                     predicate,
 696                                     hir::Constness::NotConst,
 697                                 ),
 698                                 ..obligation.clone()
 699                             };
 700                             if self.predicate_may_hold(&unit_obligation) {
 701                                 err.note(
 702                                     "the trait is implemented for `()`. \
 703                                      Possibly this error has been caused by changes to \
 704                                      Rust's type-inference algorithm (see issue #48950 \
 705                                      <https://github.com/rust-lang/rust/issues/48950> \
 706                                      for more information). Consider whether you meant to use \
 707                                      the type `()` here instead.",
 708                                 );
 709                             }
 710                         }
 711
 712                         err
 713                     }
 714
 715                     ty::Predicate::Subtype(ref predicate) => {
 716                         // Errors for Subtype predicates show up as
 717                         // `FulfillmentErrorCode::CodeSubtypeError`,
 718                         // not selection error.
 719                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
 720                     }
 721
 722                     ty::Predicate::RegionOutlives(ref predicate) => {
 723                         let predicate = self.resolve_vars_if_possible(predicate);
 724                         let err = self
 725                             .region_outlives_predicate(&obligation.cause, &predicate)
 726                             .err()
 727                             .unwrap();
 728                         struct_span_err!(
 729                             self.tcx.sess,
 730                             span,
 731                             E0279,
 732                             "the requirement `{}` is not satisfied (`{}`)",
 733                             predicate,
 734                             err,
 735                         )
 736                     }
 737
 738                     ty::Predicate::Projection(..) | ty::Predicate::TypeOutlives(..) => {
 739                         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
 740                         struct_span_err!(
 741                             self.tcx.sess,
 742                             span,
 743                             E0280,
 744                             "the requirement `{}` is not satisfied",
 745                             predicate
 746                         )
 747                     }
 748
 749                     ty::Predicate::ObjectSafe(trait_def_id) => {
 750                         let violations = self.tcx.object_safety_violations(trait_def_id);
 751                         report_object_safety_error(self.tcx, span, trait_def_id, violations)
 752                     }
 753
 754                     ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => {
 755                         let found_kind = self.closure_kind(closure_def_id, closure_substs).unwrap();
 756                         let closure_span = self
 757                             .tcx
 758                             .sess
 759                             .source_map()
 760                             .def_span(self.tcx.hir().span_if_local(closure_def_id).unwrap());
 761                         let hir_id = self.tcx.hir().as_local_hir_id(closure_def_id).unwrap();
 762                         let mut err = struct_span_err!(
 763                             self.tcx.sess,
 764                             closure_span,
 765                             E0525,
 766                             "expected a closure that implements the `{}` trait, \
 767                              but this closure only implements `{}`",
 768                             kind,
 769                             found_kind
 770                         );
 771
 772                         err.span_label(
 773                             closure_span,
 774                             format!("this closure implements `{}`, not `{}`", found_kind, kind),
 775                         );
 776                         err.span_label(
 777                             obligation.cause.span,
 778                             format!("the requirement to implement `{}` derives from here", kind),
 779                         );
 780
 781                         // Additional context information explaining why the closure only implements
 782                         // a particular trait.
 783                         if let Some(tables) = self.in_progress_tables {
 784                             let tables = tables.borrow();
 785                             match (found_kind, tables.closure_kind_origins().get(hir_id)) {
 786                                 (ty::ClosureKind::FnOnce, Some((span, name))) => {
 787                                     err.span_label(
 788                                         *span,
 789                                         format!(
 790                                             "closure is `FnOnce` because it moves the \
 791                                          variable `{}` out of its environment",
 792                                             name
 793                                         ),
 794                                     );
 795                                 }
 796                                 (ty::ClosureKind::FnMut, Some((span, name))) => {
 797                                     err.span_label(
 798                                         *span,
 799                                         format!(
 800                                             "closure is `FnMut` because it mutates the \
 801                                          variable `{}` here",
 802                                             name
 803                                         ),
 804                                     );
 805                                 }
 806                                 _ => {}
 807                             }
 808                         }
 809
 810                         err.emit();
 811                         return;
 812                     }
 813
 814                     ty::Predicate::WellFormed(ty) => {
 815                         if !self.tcx.sess.opts.debugging_opts.chalk {
 816                             // WF predicates cannot themselves make
 817                             // errors. They can only block due to
 818                             // ambiguity; otherwise, they always
 819                             // degenerate into other obligations
 820                             // (which may fail).
 821                             span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 822                         } else {
 823                             // FIXME: we'll need a better message which takes into account
 824                             // which bounds actually failed to hold.
 825                             self.tcx.sess.struct_span_err(
 826                                 span,
 827                                 &format!("the type `{}` is not well-formed (chalk)", ty),
 828                             )
 829                         }
 830                     }
 831
 832                     ty::Predicate::ConstEvaluatable(..) => {
 833                         // Errors for `ConstEvaluatable` predicates show up as
 834                         // `SelectionError::ConstEvalFailure`,
 835                         // not `Unimplemented`.
 836                         span_bug!(
 837                             span,
 838                             "const-evaluatable requirement gave wrong error: `{:?}`",
 839                             obligation
 840                         )
 841                     }
 842                 }
 843             }
 844
 845             OutputTypeParameterMismatch(ref found_trait_ref, ref expected_trait_ref, _) => {
 846                 let found_trait_ref = self.resolve_vars_if_possible(&*found_trait_ref);
 847                 let expected_trait_ref = self.resolve_vars_if_possible(&*expected_trait_ref);
 848
 849                 if expected_trait_ref.self_ty().references_error() {
 850                     return;
 851                 }
 852
 853                 let found_trait_ty = found_trait_ref.self_ty();
 854
 855                 let found_did = match found_trait_ty.kind {
 856                     ty::Closure(did, _) | ty::Foreign(did) | ty::FnDef(did, _) => Some(did),
 857                     ty::Adt(def, _) => Some(def.did),
 858                     _ => None,
 859                 };
 860
 861                 let found_span = found_did
 862                     .and_then(|did| self.tcx.hir().span_if_local(did))
 863                     .map(|sp| self.tcx.sess.source_map().def_span(sp)); // the sp could be an fn def
 864
 865                 if self.reported_closure_mismatch.borrow().contains(&(span, found_span)) {
 866                     // We check closures twice, with obligations flowing in different directions,
 867                     // but we want to complain about them only once.
 868                     return;
 869                 }
 870
 871                 self.reported_closure_mismatch.borrow_mut().insert((span, found_span));
 872
 873                 let found = match found_trait_ref.skip_binder().substs.type_at(1).kind {
 874                     ty::Tuple(ref tys) => vec![ArgKind::empty(); tys.len()],
 875                     _ => vec![ArgKind::empty()],
 876                 };
 877
 878                 let expected_ty = expected_trait_ref.skip_binder().substs.type_at(1);
 879                 let expected = match expected_ty.kind {
 880                     ty::Tuple(ref tys) => tys
 881                         .iter()
 882                         .map(|t| ArgKind::from_expected_ty(t.expect_ty(), Some(span)))
 883                         .collect(),
 884                     _ => vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())],
 885                 };
 886
 887                 if found.len() == expected.len() {
 888                     self.report_closure_arg_mismatch(
 889                         span,
 890                         found_span,
 891                         found_trait_ref,
 892                         expected_trait_ref,
 893                     )
 894                 } else {
 895                     let (closure_span, found) = found_did
 896                         .and_then(|did| self.tcx.hir().get_if_local(did))
 897                         .map(|node| {
 898                             let (found_span, found) = self.get_fn_like_arguments(node);
 899                             (Some(found_span), found)
 900                         })
 901                         .unwrap_or((found_span, found));
 902
 903                     self.report_arg_count_mismatch(
 904                         span,
 905                         closure_span,
 906                         expected,
 907                         found,
 908                         found_trait_ty.is_closure(),
 909                     )
 910                 }
 911             }
 912
 913             TraitNotObjectSafe(did) => {
 914                 let violations = self.tcx.object_safety_violations(did);
 915                 report_object_safety_error(self.tcx, span, did, violations)
 916             }
 917
 918             ConstEvalFailure(ErrorHandled::TooGeneric) => {
 919                 // In this instance, we have a const expression containing an unevaluated
 920                 // generic parameter. We have no idea whether this expression is valid or
 921                 // not (e.g. it might result in an error), but we don't want to just assume
 922                 // that it's okay, because that might result in post-monomorphisation time
 923                 // errors. The onus is really on the caller to provide values that it can
 924                 // prove are well-formed.
 925                 let mut err = self
 926                     .tcx
 927                     .sess
 928                     .struct_span_err(span, "constant expression depends on a generic parameter");
 929                 // FIXME(const_generics): we should suggest to the user how they can resolve this
 930                 // issue. However, this is currently not actually possible
 931                 // (see https://github.com/rust-lang/rust/issues/66962#issuecomment-575907083).
 932                 err.note("this may fail depending on what value the parameter takes");
 933                 err
 934             }
 935
 936             // Already reported in the query.
 937             ConstEvalFailure(ErrorHandled::Reported) => {
 938                 self.tcx
 939                     .sess
 940                     .delay_span_bug(span, &format!("constant in type had an ignored error"));
 941                 return;
 942             }
 943
 944             Overflow => {
 945                 bug!("overflow should be handled before the `report_selection_error` path");
 946             }
 947         };
 948
 949         self.note_obligation_cause(&mut err, obligation);
 950         self.point_at_returns_when_relevant(&mut err, &obligation);
 951
 952         err.emit();
 953     }
 954
 955     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
 956     /// with the same path as `trait_ref`, a help message about
 957     /// a probable version mismatch is added to `err`
 958     fn note_version_mismatch(
 959         &self,
 960         err: &mut DiagnosticBuilder<'_>,
 961         trait_ref: &ty::PolyTraitRef<'tcx>,
 962     ) {
 963         let get_trait_impl = |trait_def_id| {
 964             let mut trait_impl = None;
 965             self.tcx.for_each_relevant_impl(trait_def_id, trait_ref.self_ty(), |impl_def_id| {
 966                 if trait_impl.is_none() {
 967                     trait_impl = Some(impl_def_id);
 968                 }
 969             });
 970             trait_impl
 971         };
 972         let required_trait_path = self.tcx.def_path_str(trait_ref.def_id());
 973         let all_traits = self.tcx.all_traits(LOCAL_CRATE);
 974         let traits_with_same_path: std::collections::BTreeSet<_> = all_traits
 975             .iter()
 976             .filter(|trait_def_id| **trait_def_id != trait_ref.def_id())
 977             .filter(|trait_def_id| self.tcx.def_path_str(**trait_def_id) == required_trait_path)
 978             .collect();
 979         for trait_with_same_path in traits_with_same_path {
 980             if let Some(impl_def_id) = get_trait_impl(*trait_with_same_path) {
 981                 let impl_span = self.tcx.def_span(impl_def_id);
 982                 err.span_help(impl_span, "trait impl with same name found");
 983                 let trait_crate = self.tcx.crate_name(trait_with_same_path.krate);
 984                 let crate_msg = format!(
 985                     "perhaps two different versions of crate `{}` are being used?",
 986                     trait_crate
 987                 );
 988                 err.note(&crate_msg);
 989             }
 990         }
 991     }
 992
 993     fn mk_obligation_for_def_id(
 994         &self,
 995         def_id: DefId,
 996         output_ty: Ty<'tcx>,
 997         cause: ObligationCause<'tcx>,
 998         param_env: ty::ParamEnv<'tcx>,
 999     ) -> PredicateObligation<'tcx> {
1000         let new_trait_ref =
1001             ty::TraitRef { def_id, substs: self.tcx.mk_substs_trait(output_ty, &[]) };
1002         Obligation::new(cause, param_env, new_trait_ref.without_const().to_predicate())
1003     }
1004 }
1005
1006 pub fn recursive_type_with_infinite_size_error(
1007     tcx: TyCtxt<'tcx>,
1008     type_def_id: DefId,
1009 ) -> DiagnosticBuilder<'tcx> {
1010     assert!(type_def_id.is_local());
1011     let span = tcx.hir().span_if_local(type_def_id).unwrap();
1012     let span = tcx.sess.source_map().def_span(span);
1013     let mut err = struct_span_err!(
1014         tcx.sess,
1015         span,
1016         E0072,
1017         "recursive type `{}` has infinite size",
1018         tcx.def_path_str(type_def_id)
1019     );
1020     err.span_label(span, "recursive type has infinite size");
1021     err.help(&format!(
1022         "insert indirection (e.g., a `Box`, `Rc`, or `&`) \
1023                            at some point to make `{}` representable",
1024         tcx.def_path_str(type_def_id)
1025     ));
1026     err
1027 }
1028
1029 pub fn report_object_safety_error(
1030     tcx: TyCtxt<'tcx>,
1031     span: Span,
1032     trait_def_id: DefId,
1033     violations: Vec<ObjectSafetyViolation>,
1034 ) -> DiagnosticBuilder<'tcx> {
1035     let trait_str = tcx.def_path_str(trait_def_id);
1036     let trait_span = tcx.hir().get_if_local(trait_def_id).and_then(|node| match node {
1037         hir::Node::Item(item) => Some(item.ident.span),
1038         _ => None,
1039     });
1040     let span = tcx.sess.source_map().def_span(span);
1041     let mut err = struct_span_err!(
1042         tcx.sess,
1043         span,
1044         E0038,
1045         "the trait `{}` cannot be made into an object",
1046         trait_str
1047     );
1048     err.span_label(span, format!("the trait `{}` cannot be made into an object", trait_str));
1049
1050     let mut reported_violations = FxHashSet::default();
1051     let mut had_span_label = false;
1052     for violation in violations {
1053         if let ObjectSafetyViolation::SizedSelf(sp) = &violation {
1054             if !sp.is_empty() {
1055                 // Do not report `SizedSelf` without spans pointing at `SizedSelf` obligations
1056                 // with a `Span`.
1057                 reported_violations.insert(ObjectSafetyViolation::SizedSelf(vec![].into()));
1058             }
1059         }
1060         if reported_violations.insert(violation.clone()) {
1061             let spans = violation.spans();
1062             let msg = if trait_span.is_none() || spans.is_empty() {
1063                 format!("the trait cannot be made into an object because {}", violation.error_msg())
1064             } else {
1065                 had_span_label = true;
1066                 format!("...because {}", violation.error_msg())
1067             };
1068             if spans.is_empty() {
1069                 err.note(&msg);
1070             } else {
1071                 for span in spans {
1072                     err.span_label(span, &msg);
1073                 }
1074             }
1075             match (trait_span, violation.solution()) {
1076                 (Some(_), Some((note, None))) => {
1077                     err.help(&note);
1078                 }
1079                 (Some(_), Some((note, Some((sugg, span))))) => {
1080                     err.span_suggestion(span, &note, sugg, Applicability::MachineApplicable);
1081                 }
1082                 // Only provide the help if its a local trait, otherwise it's not actionable.
1083                 _ => {}
1084             }
1085         }
1086     }
1087     if let (Some(trait_span), true) = (trait_span, had_span_label) {
1088         err.span_label(trait_span, "this trait cannot be made into an object...");
1089     }
1090
1091     if tcx.sess.trait_methods_not_found.borrow().contains(&span) {
1092         // Avoid emitting error caused by non-existing method (#58734)
1093         err.cancel();
1094     }
1095
1096     err
1097 }
1098
1099 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
1100     fn maybe_report_ambiguity(
1101         &self,
1102         obligation: &PredicateObligation<'tcx>,
1103         body_id: Option<hir::BodyId>,
1104     ) {
1105         // Unable to successfully determine, probably means
1106         // insufficient type information, but could mean
1107         // ambiguous impls. The latter *ought* to be a
1108         // coherence violation, so we don't report it here.
1109
1110         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
1111         let span = obligation.cause.span;
1112
1113         debug!(
1114             "maybe_report_ambiguity(predicate={:?}, obligation={:?} body_id={:?}, code={:?})",
1115             predicate, obligation, body_id, obligation.cause.code,
1116         );
1117
1118         // Ambiguity errors are often caused as fallout from earlier
1119         // errors. So just ignore them if this infcx is tainted.
1120         if self.is_tainted_by_errors() {
1121             return;
1122         }
1123
1124         let mut err = match predicate {
1125             ty::Predicate::Trait(ref data, _) => {
1126                 let trait_ref = data.to_poly_trait_ref();
1127                 let self_ty = trait_ref.self_ty();
1128                 debug!("self_ty {:?} {:?} trait_ref {:?}", self_ty, self_ty.kind, trait_ref);
1129
1130                 if predicate.references_error() {
1131                     return;
1132                 }
1133                 // Typically, this ambiguity should only happen if
1134                 // there are unresolved type inference variables
1135                 // (otherwise it would suggest a coherence
1136                 // failure). But given #21974 that is not necessarily
1137                 // the case -- we can have multiple where clauses that
1138                 // are only distinguished by a region, which results
1139                 // in an ambiguity even when all types are fully
1140                 // known, since we don't dispatch based on region
1141                 // relationships.
1142
1143                 // This is kind of a hack: it frequently happens that some earlier
1144                 // error prevents types from being fully inferred, and then we get
1145                 // a bunch of uninteresting errors saying something like "<generic
1146                 // #0> doesn't implement Sized".  It may even be true that we
1147                 // could just skip over all checks where the self-ty is an
1148                 // inference variable, but I was afraid that there might be an
1149                 // inference variable created, registered as an obligation, and
1150                 // then never forced by writeback, and hence by skipping here we'd
1151                 // be ignoring the fact that we don't KNOW the type works
1152                 // out. Though even that would probably be harmless, given that
1153                 // we're only talking about builtin traits, which are known to be
1154                 // inhabited. We used to check for `self.tcx.sess.has_errors()` to
1155                 // avoid inundating the user with unnecessary errors, but we now
1156                 // check upstream for type errors and dont add the obligations to
1157                 // begin with in those cases.
1158                 if self
1159                     .tcx
1160                     .lang_items()
1161                     .sized_trait()
1162                     .map_or(false, |sized_id| sized_id == trait_ref.def_id())
1163                 {
1164                     self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0282).emit();
1165                     return;
1166                 }
1167                 let mut err = self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0283);
1168                 err.note(&format!("cannot resolve `{}`", predicate));
1169                 if let ObligationCauseCode::ItemObligation(def_id) = obligation.cause.code {
1170                     self.suggest_fully_qualified_path(&mut err, def_id, span, trait_ref.def_id());
1171                 } else if let (
1172                     Ok(ref snippet),
1173                     ObligationCauseCode::BindingObligation(ref def_id, _),
1174                 ) =
1175                     (self.tcx.sess.source_map().span_to_snippet(span), &obligation.cause.code)
1176                 {
1177                     let generics = self.tcx.generics_of(*def_id);
1178                     if !generics.params.is_empty() && !snippet.ends_with('>') {
1179                         // FIXME: To avoid spurious suggestions in functions where type arguments
1180                         // where already supplied, we check the snippet to make sure it doesn't
1181                         // end with a turbofish. Ideally we would have access to a `PathSegment`
1182                         // instead. Otherwise we would produce the following output:
1183                         //
1184                         // error[E0283]: type annotations needed
1185                         //   --> $DIR/issue-54954.rs:3:24
1186                         //    |
1187                         // LL | const ARR_LEN: usize = Tt::const_val::<[i8; 123]>();
1188                         //    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^
1189                         //    |                        |
1190                         //    |                        cannot infer type
1191                         //    |                        help: consider specifying the type argument
1192                         //    |                        in the function call:
1193                         //    |                        `Tt::const_val::<[i8; 123]>::<T>`
1194                         // ...
1195                         // LL |     const fn const_val<T: Sized>() -> usize {
1196                         //    |              --------- - required by this bound in `Tt::const_val`
1197                         //    |
1198                         //    = note: cannot resolve `_: Tt`
1199
1200                         err.span_suggestion(
1201                             span,
1202                             &format!(
1203                                 "consider specifying the type argument{} in the function call",
1204                                 if generics.params.len() > 1 { "s" } else { "" },
1205                             ),
1206                             format!(
1207                                 "{}::<{}>",
1208                                 snippet,
1209                                 generics
1210                                     .params
1211                                     .iter()
1212                                     .map(|p| p.name.to_string())
1213                                     .collect::<Vec<String>>()
1214                                     .join(", ")
1215                             ),
1216                             Applicability::HasPlaceholders,
1217                         );
1218                     }
1219                 }
1220                 err
1221             }
1222
1223             ty::Predicate::WellFormed(ty) => {
1224                 // Same hacky approach as above to avoid deluging user
1225                 // with error messages.
1226                 if ty.references_error() || self.tcx.sess.has_errors() {
1227                     return;
1228                 }
1229                 self.need_type_info_err(body_id, span, ty, ErrorCode::E0282)
1230             }
1231
1232             ty::Predicate::Subtype(ref data) => {
1233                 if data.references_error() || self.tcx.sess.has_errors() {
1234                     // no need to overload user in such cases
1235                     return;
1236                 }
1237                 let &SubtypePredicate { a_is_expected: _, a, b } = data.skip_binder();
1238                 // both must be type variables, or the other would've been instantiated
1239                 assert!(a.is_ty_var() && b.is_ty_var());
1240                 self.need_type_info_err(body_id, span, a, ErrorCode::E0282)
1241             }
1242             ty::Predicate::Projection(ref data) => {
1243                 let trait_ref = data.to_poly_trait_ref(self.tcx);
1244                 let self_ty = trait_ref.self_ty();
1245                 if predicate.references_error() {
1246                     return;
1247                 }
1248                 let mut err = self.need_type_info_err(body_id, span, self_ty, ErrorCode::E0284);
1249                 err.note(&format!("cannot resolve `{}`", predicate));
1250                 err
1251             }
1252
1253             _ => {
1254                 if self.tcx.sess.has_errors() {
1255                     return;
1256                 }
1257                 let mut err = struct_span_err!(
1258                     self.tcx.sess,
1259                     span,
1260                     E0284,
1261                     "type annotations needed: cannot resolve `{}`",
1262                     predicate,
1263                 );
1264                 err.span_label(span, &format!("cannot resolve `{}`", predicate));
1265                 err
1266             }
1267         };
1268         self.note_obligation_cause(&mut err, obligation);
1269         err.emit();
1270     }
1271
1272     /// Returns `true` if the trait predicate may apply for *some* assignment
1273     /// to the type parameters.
1274     fn predicate_can_apply(
1275         &self,
1276         param_env: ty::ParamEnv<'tcx>,
1277         pred: ty::PolyTraitRef<'tcx>,
1278     ) -> bool {
1279         struct ParamToVarFolder<'a, 'tcx> {
1280             infcx: &'a InferCtxt<'a, 'tcx>,
1281             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
1282         }
1283
1284         impl<'a, 'tcx> TypeFolder<'tcx> for ParamToVarFolder<'a, 'tcx> {
1285             fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
1286                 self.infcx.tcx
1287             }
1288
1289             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
1290                 if let ty::Param(ty::ParamTy { name, .. }) = ty.kind {
1291                     let infcx = self.infcx;
1292                     self.var_map.entry(ty).or_insert_with(|| {
1293                         infcx.next_ty_var(TypeVariableOrigin {
1294                             kind: TypeVariableOriginKind::TypeParameterDefinition(name, None),
1295                             span: DUMMY_SP,
1296                         })
1297                     })
1298                 } else {
1299                     ty.super_fold_with(self)
1300                 }
1301             }
1302         }
1303
1304         self.probe(|_| {
1305             let mut selcx = SelectionContext::new(self);
1306
1307             let cleaned_pred =
1308                 pred.fold_with(&mut ParamToVarFolder { infcx: self, var_map: Default::default() });
1309
1310             let cleaned_pred = super::project::normalize(
1311                 &mut selcx,
1312                 param_env,
1313                 ObligationCause::dummy(),
1314                 &cleaned_pred,
1315             )
1316             .value;
1317
1318             let obligation = Obligation::new(
1319                 ObligationCause::dummy(),
1320                 param_env,
1321                 cleaned_pred.without_const().to_predicate(),
1322             );
1323
1324             self.predicate_may_hold(&obligation)
1325         })
1326     }
1327
1328     fn note_obligation_cause(
1329         &self,
1330         err: &mut DiagnosticBuilder<'_>,
1331         obligation: &PredicateObligation<'tcx>,
1332     ) {
1333         // First, attempt to add note to this error with an async-await-specific
1334         // message, and fall back to regular note otherwise.
1335         if !self.maybe_note_obligation_cause_for_async_await(err, obligation) {
1336             self.note_obligation_cause_code(
1337                 err,
1338                 &obligation.predicate,
1339                 &obligation.cause.code,
1340                 &mut vec![],
1341             );
1342             self.suggest_unsized_bound_if_applicable(err, obligation);
1343         }
1344     }
1345
1346     fn suggest_unsized_bound_if_applicable(
1347         &self,
1348         err: &mut DiagnosticBuilder<'_>,
1349         obligation: &PredicateObligation<'tcx>,
1350     ) {
1351         if let (
1352             ty::Predicate::Trait(pred, _),
1353             ObligationCauseCode::BindingObligation(item_def_id, span),
1354         ) = (&obligation.predicate, &obligation.cause.code)
1355         {
1356             if let (Some(generics), true) = (
1357                 self.tcx.hir().get_if_local(*item_def_id).as_ref().and_then(|n| n.generics()),
1358                 Some(pred.def_id()) == self.tcx.lang_items().sized_trait(),
1359             ) {
1360                 for param in generics.params {
1361                     if param.span == *span
1362                         && !param.bounds.iter().any(|bound| {
1363                             bound.trait_def_id() == self.tcx.lang_items().sized_trait()
1364                         })
1365                     {
1366                         let (span, separator) = match param.bounds {
1367                             [] => (span.shrink_to_hi(), ":"),
1368                             [.., bound] => (bound.span().shrink_to_hi(), " + "),
1369                         };
1370                         err.span_suggestion(
1371                             span,
1372                             "consider relaxing the implicit `Sized` restriction",
1373                             format!("{} ?Sized", separator),
1374                             Applicability::MachineApplicable,
1375                         );
1376                         return;
1377                     }
1378                 }
1379             }
1380         }
1381     }
1382
1383     fn is_recursive_obligation(
1384         &self,
1385         obligated_types: &mut Vec<&ty::TyS<'tcx>>,
1386         cause_code: &ObligationCauseCode<'tcx>,
1387     ) -> bool {
1388         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
1389             let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
1390
1391             if obligated_types.iter().any(|ot| ot == &parent_trait_ref.skip_binder().self_ty()) {
1392                 return true;
1393             }
1394         }
1395         false
1396     }
1397 }
1398
1399 /// Summarizes information
1400 #[derive(Clone)]
1401 pub enum ArgKind {
1402     /// An argument of non-tuple type. Parameters are (name, ty)
1403     Arg(String, String),
1404
1405     /// An argument of tuple type. For a "found" argument, the span is
1406     /// the locationo in the source of the pattern. For a "expected"
1407     /// argument, it will be None. The vector is a list of (name, ty)
1408     /// strings for the components of the tuple.
1409     Tuple(Option<Span>, Vec<(String, String)>),
1410 }
1411
1412 impl ArgKind {
1413     fn empty() -> ArgKind {
1414         ArgKind::Arg("_".to_owned(), "_".to_owned())
1415     }
1416
1417     /// Creates an `ArgKind` from the expected type of an
1418     /// argument. It has no name (`_`) and an optional source span.
1419     pub fn from_expected_ty(t: Ty<'_>, span: Option<Span>) -> ArgKind {
1420         match t.kind {
1421             ty::Tuple(ref tys) => ArgKind::Tuple(
1422                 span,
1423                 tys.iter().map(|ty| ("_".to_owned(), ty.to_string())).collect::<Vec<_>>(),
1424             ),
1425             _ => ArgKind::Arg("_".to_owned(), t.to_string()),
1426         }
1427     }
1428 }
1429
1430 /// Suggest restricting a type param with a new bound.
1431 pub fn suggest_constraining_type_param(
1432     tcx: TyCtxt<'_>,
1433     generics: &hir::Generics<'_>,
1434     err: &mut DiagnosticBuilder<'_>,
1435     param_name: &str,
1436     constraint: &str,
1437     source_map: &SourceMap,
1438     span: Span,
1439     def_id: Option<DefId>,
1440 ) -> bool {
1441     const MSG_RESTRICT_BOUND_FURTHER: &str = "consider further restricting this bound with";
1442     const MSG_RESTRICT_TYPE: &str = "consider restricting this type parameter with";
1443     const MSG_RESTRICT_TYPE_FURTHER: &str = "consider further restricting this type parameter with";
1444
1445     let param = generics.params.iter().find(|p| p.name.ident().as_str() == param_name);
1446
1447     let param = if let Some(param) = param {
1448         param
1449     } else {
1450         return false;
1451     };
1452
1453     if def_id == tcx.lang_items().sized_trait() {
1454         // Type parameters are already `Sized` by default.
1455         err.span_label(param.span, &format!("this type parameter needs to be `{}`", constraint));
1456         return true;
1457     }
1458
1459     if param_name.starts_with("impl ") {
1460         // If there's an `impl Trait` used in argument position, suggest
1461         // restricting it:
1462         //
1463         //   fn foo(t: impl Foo) { ... }
1464         //             --------
1465         //             |
1466         //             help: consider further restricting this bound with `+ Bar`
1467         //
1468         // Suggestion for tools in this case is:
1469         //
1470         //   fn foo(t: impl Foo) { ... }
1471         //             --------
1472         //             |
1473         //             replace with: `impl Foo + Bar`
1474
1475         err.span_help(param.span, &format!("{} `+ {}`", MSG_RESTRICT_BOUND_FURTHER, constraint));
1476
1477         err.tool_only_span_suggestion(
1478             param.span,
1479             MSG_RESTRICT_BOUND_FURTHER,
1480             format!("{} + {}", param_name, constraint),
1481             Applicability::MachineApplicable,
1482         );
1483
1484         return true;
1485     }
1486
1487     if generics.where_clause.predicates.is_empty() {
1488         if let Some(bounds_span) = param.bounds_span() {
1489             // If user has provided some bounds, suggest restricting them:
1490             //
1491             //   fn foo<T: Foo>(t: T) { ... }
1492             //             ---
1493             //             |
1494             //             help: consider further restricting this bound with `+ Bar`
1495             //
1496             // Suggestion for tools in this case is:
1497             //
1498             //   fn foo<T: Foo>(t: T) { ... }
1499             //          --
1500             //          |
1501             //          replace with: `T: Bar +`
1502
1503             err.span_help(
1504                 bounds_span,
1505                 &format!("{} `+ {}`", MSG_RESTRICT_BOUND_FURTHER, constraint),
1506             );
1507
1508             let span_hi = param.span.with_hi(span.hi());
1509             let span_with_colon = source_map.span_through_char(span_hi, ':');
1510
1511             if span_hi != param.span && span_with_colon != span_hi {
1512                 err.tool_only_span_suggestion(
1513                     span_with_colon,
1514                     MSG_RESTRICT_BOUND_FURTHER,
1515                     format!("{}: {} + ", param_name, constraint),
1516                     Applicability::MachineApplicable,
1517                 );
1518             }
1519         } else {
1520             // If user hasn't provided any bounds, suggest adding a new one:
1521             //
1522             //   fn foo<T>(t: T) { ... }
1523             //          - help: consider restricting this type parameter with `T: Foo`
1524
1525             err.span_help(
1526                 param.span,
1527                 &format!("{} `{}: {}`", MSG_RESTRICT_TYPE, param_name, constraint),
1528             );
1529
1530             err.tool_only_span_suggestion(
1531                 param.span,
1532                 MSG_RESTRICT_TYPE,
1533                 format!("{}: {}", param_name, constraint),
1534                 Applicability::MachineApplicable,
1535             );
1536         }
1537
1538         true
1539     } else {
1540         // This part is a bit tricky, because using the `where` clause user can
1541         // provide zero, one or many bounds for the same type parameter, so we
1542         // have following cases to consider:
1543         //
1544         // 1) When the type parameter has been provided zero bounds
1545         //
1546         //    Message:
1547         //      fn foo<X, Y>(x: X, y: Y) where Y: Foo { ... }
1548         //             - help: consider restricting this type parameter with `where X: Bar`
1549         //
1550         //    Suggestion:
1551         //      fn foo<X, Y>(x: X, y: Y) where Y: Foo { ... }
1552         //                                           - insert: `, X: Bar`
1553         //
1554         //
1555         // 2) When the type parameter has been provided one bound
1556         //
1557         //    Message:
1558         //      fn foo<T>(t: T) where T: Foo { ... }
1559         //                            ^^^^^^
1560         //                            |
1561         //                            help: consider further restricting this bound with `+ Bar`
1562         //
1563         //    Suggestion:
1564         //      fn foo<T>(t: T) where T: Foo { ... }
1565         //                            ^^
1566         //                            |
1567         //                            replace with: `T: Bar +`
1568         //
1569         //
1570         // 3) When the type parameter has been provided many bounds
1571         //
1572         //    Message:
1573         //      fn foo<T>(t: T) where T: Foo, T: Bar {... }
1574         //             - help: consider further restricting this type parameter with `where T: Zar`
1575         //
1576         //    Suggestion:
1577         //      fn foo<T>(t: T) where T: Foo, T: Bar {... }
1578         //                                          - insert: `, T: Zar`
1579
1580         let mut param_spans = Vec::new();
1581
1582         for predicate in generics.where_clause.predicates {
1583             if let WherePredicate::BoundPredicate(WhereBoundPredicate {
1584                 span, bounded_ty, ..
1585             }) = predicate
1586             {
1587                 if let TyKind::Path(QPath::Resolved(_, path)) = &bounded_ty.kind {
1588                     if let Some(segment) = path.segments.first() {
1589                         if segment.ident.to_string() == param_name {
1590                             param_spans.push(span);
1591                         }
1592                     }
1593                 }
1594             }
1595         }
1596
1597         let where_clause_span =
1598             generics.where_clause.span_for_predicates_or_empty_place().shrink_to_hi();
1599
1600         match &param_spans[..] {
1601             &[] => {
1602                 err.span_help(
1603                     param.span,
1604                     &format!("{} `where {}: {}`", MSG_RESTRICT_TYPE, param_name, constraint),
1605                 );
1606
1607                 err.tool_only_span_suggestion(
1608                     where_clause_span,
1609                     MSG_RESTRICT_TYPE,
1610                     format!(", {}: {}", param_name, constraint),
1611                     Applicability::MachineApplicable,
1612                 );
1613             }
1614
1615             &[&param_span] => {
1616                 err.span_help(
1617                     param_span,
1618                     &format!("{} `+ {}`", MSG_RESTRICT_BOUND_FURTHER, constraint),
1619                 );
1620
1621                 let span_hi = param_span.with_hi(span.hi());
1622                 let span_with_colon = source_map.span_through_char(span_hi, ':');
1623
1624                 if span_hi != param_span && span_with_colon != span_hi {
1625                     err.tool_only_span_suggestion(
1626                         span_with_colon,
1627                         MSG_RESTRICT_BOUND_FURTHER,
1628                         format!("{}: {} +", param_name, constraint),
1629                         Applicability::MachineApplicable,
1630                     );
1631                 }
1632             }
1633
1634             _ => {
1635                 err.span_help(
1636                     param.span,
1637                     &format!(
1638                         "{} `where {}: {}`",
1639                         MSG_RESTRICT_TYPE_FURTHER, param_name, constraint,
1640                     ),
1641                 );
1642
1643                 err.tool_only_span_suggestion(
1644                     where_clause_span,
1645                     MSG_RESTRICT_BOUND_FURTHER,
1646                     format!(", {}: {}", param_name, constraint),
1647                     Applicability::MachineApplicable,
1648                 );
1649             }
1650         }
1651
1652         true
1653     }
1654 }