src/librustc/traits/error_reporting.rs

   1 use super::{
   2     FulfillmentError,
   3     FulfillmentErrorCode,
   4     MismatchedProjectionTypes,
   5     Obligation,
   6     ObligationCause,
   7     ObligationCauseCode,
   8     OnUnimplementedDirective,
   9     OnUnimplementedNote,
  10     OutputTypeParameterMismatch,
  11     TraitNotObjectSafe,
  12     ConstEvalFailure,
  13     PredicateObligation,
  14     SelectionContext,
  15     SelectionError,
  16     ObjectSafetyViolation,
  17     Overflow,
  18 };
  19
  20 use errors::{Applicability, DiagnosticBuilder};
  21 use hir;
  22 use hir::Node;
  23 use hir::def_id::DefId;
  24 use infer::{self, InferCtxt};
  25 use infer::type_variable::TypeVariableOrigin;
  26 use std::fmt;
  27 use syntax::ast;
  28 use session::DiagnosticMessageId;
  29 use ty::{self, AdtKind, ToPredicate, ToPolyTraitRef, Ty, TyCtxt, TypeFoldable};
  30 use ty::GenericParamDefKind;
  31 use ty::error::ExpectedFound;
  32 use ty::fast_reject;
  33 use ty::fold::TypeFolder;
  34 use ty::subst::Subst;
  35 use ty::SubtypePredicate;
  36 use util::nodemap::{FxHashMap, FxHashSet};
  37
  38 use syntax_pos::{DUMMY_SP, Span, ExpnInfo, ExpnFormat};
  39
  40 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
  41     pub fn report_fulfillment_errors(&self,
  42                                      errors: &[FulfillmentError<'tcx>],
  43                                      body_id: Option<hir::BodyId>,
  44                                      fallback_has_occurred: bool) {
  45         #[derive(Debug)]
  46         struct ErrorDescriptor<'tcx> {
  47             predicate: ty::Predicate<'tcx>,
  48             index: Option<usize>, // None if this is an old error
  49         }
  50
  51         let mut error_map: FxHashMap<_, Vec<_>> =
  52             self.reported_trait_errors.borrow().iter().map(|(&span, predicates)| {
  53                 (span, predicates.iter().map(|predicate| ErrorDescriptor {
  54                     predicate: predicate.clone(),
  55                     index: None
  56                 }).collect())
  57             }).collect();
  58
  59         for (index, error) in errors.iter().enumerate() {
  60             // We want to ignore desugarings here: spans are equivalent even
  61             // if one is the result of a desugaring and the other is not.
  62             let mut span = error.obligation.cause.span;
  63             if let Some(ExpnInfo {
  64                 format: ExpnFormat::CompilerDesugaring(_),
  65                 def_site: Some(def_span),
  66                 ..
  67             }) = span.ctxt().outer().expn_info() {
  68                 span = def_span;
  69             }
  70
  71             error_map.entry(span).or_default().push(
  72                 ErrorDescriptor {
  73                     predicate: error.obligation.predicate.clone(),
  74                     index: Some(index)
  75                 }
  76             );
  77
  78             self.reported_trait_errors.borrow_mut()
  79                 .entry(span).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,
 124                      cond: &ty::Predicate<'tcx>,
 125                      error: &ty::Predicate<'tcx>)
 126                      -> bool
 127     {
 128         if cond == error {
 129             return true
 130         }
 131
 132         let (cond, error) = match (cond, error) {
 133             (&ty::Predicate::Trait(..), &ty::Predicate::Trait(ref error))
 134                 => (cond, error),
 135             _ => {
 136                 // FIXME: make this work in other cases too.
 137                 return false
 138             }
 139         };
 140
 141         for implication in super::elaborate_predicates(self.tcx, vec![cond.clone()]) {
 142             if let ty::Predicate::Trait(implication) = implication {
 143                 let error = error.to_poly_trait_ref();
 144                 let implication = implication.to_poly_trait_ref();
 145                 // FIXME: I'm just not taking associated types at all here.
 146                 // Eventually I'll need to implement param-env-aware
 147                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
 148                 let param_env = ty::ParamEnv::empty();
 149                 if self.can_sub(param_env, error, implication).is_ok() {
 150                     debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
 151                     return true
 152                 }
 153             }
 154         }
 155
 156         false
 157     }
 158
 159     fn report_fulfillment_error(&self, error: &FulfillmentError<'tcx>,
 160                                 body_id: Option<hir::BodyId>,
 161                                 fallback_has_occurred: bool) {
 162         debug!("report_fulfillment_errors({:?})", error);
 163         match error.code {
 164             FulfillmentErrorCode::CodeSelectionError(ref e) => {
 165                 self.report_selection_error(&error.obligation, e, fallback_has_occurred);
 166             }
 167             FulfillmentErrorCode::CodeProjectionError(ref e) => {
 168                 self.report_projection_error(&error.obligation, e);
 169             }
 170             FulfillmentErrorCode::CodeAmbiguity => {
 171                 self.maybe_report_ambiguity(&error.obligation, body_id);
 172             }
 173             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
 174                 self.report_mismatched_types(&error.obligation.cause,
 175                                              expected_found.expected,
 176                                              expected_found.found,
 177                                              err.clone())
 178                     .emit();
 179             }
 180         }
 181     }
 182
 183     fn report_projection_error(&self,
 184                                obligation: &PredicateObligation<'tcx>,
 185                                error: &MismatchedProjectionTypes<'tcx>)
 186     {
 187         let predicate =
 188             self.resolve_type_vars_if_possible(&obligation.predicate);
 189
 190         if predicate.references_error() {
 191             return
 192         }
 193
 194         self.probe(|_| {
 195             let err_buf;
 196             let mut err = &error.err;
 197             let mut values = None;
 198
 199             // try to find the mismatched types to report the error with.
 200             //
 201             // this can fail if the problem was higher-ranked, in which
 202             // cause I have no idea for a good error message.
 203             if let ty::Predicate::Projection(ref data) = predicate {
 204                 let mut selcx = SelectionContext::new(self);
 205                 let (data, _) = self.replace_bound_vars_with_fresh_vars(
 206                     obligation.cause.span,
 207                     infer::LateBoundRegionConversionTime::HigherRankedType,
 208                     data
 209                 );
 210                 let mut obligations = vec![];
 211                 let normalized_ty = super::normalize_projection_type(
 212                     &mut selcx,
 213                     obligation.param_env,
 214                     data.projection_ty,
 215                     obligation.cause.clone(),
 216                     0,
 217                     &mut obligations
 218                 );
 219                 if let Err(error) = self.at(&obligation.cause, obligation.param_env)
 220                                         .eq(normalized_ty, data.ty) {
 221                     values = Some(infer::ValuePairs::Types(ExpectedFound {
 222                         expected: normalized_ty,
 223                         found: data.ty,
 224                     }));
 225                     err_buf = error;
 226                     err = &err_buf;
 227                 }
 228             }
 229
 230             let msg = format!("type mismatch resolving `{}`", predicate);
 231             let error_id = (DiagnosticMessageId::ErrorId(271),
 232                             Some(obligation.cause.span), msg);
 233             let fresh = self.tcx.sess.one_time_diagnostics.borrow_mut().insert(error_id);
 234             if fresh {
 235                 let mut diag = struct_span_err!(
 236                     self.tcx.sess, obligation.cause.span, E0271,
 237                     "type mismatch resolving `{}`", predicate
 238                 );
 239                 self.note_type_err(&mut diag, &obligation.cause, None, values, err);
 240                 self.note_obligation_cause(&mut diag, obligation);
 241                 diag.emit();
 242             }
 243         });
 244     }
 245
 246     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
 247         /// returns the fuzzy category of a given type, or None
 248         /// if the type can be equated to any type.
 249         fn type_category<'tcx>(t: Ty<'tcx>) -> Option<u32> {
 250             match t.sty {
 251                 ty::Bool => Some(0),
 252                 ty::Char => Some(1),
 253                 ty::Str => Some(2),
 254                 ty::Int(..) | ty::Uint(..) | ty::Infer(ty::IntVar(..)) => Some(3),
 255                 ty::Float(..) | ty::Infer(ty::FloatVar(..)) => Some(4),
 256                 ty::Ref(..) | ty::RawPtr(..) => Some(5),
 257                 ty::Array(..) | ty::Slice(..) => Some(6),
 258                 ty::FnDef(..) | ty::FnPtr(..) => Some(7),
 259                 ty::Dynamic(..) => Some(8),
 260                 ty::Closure(..) => Some(9),
 261                 ty::Tuple(..) => Some(10),
 262                 ty::Projection(..) => Some(11),
 263                 ty::Param(..) => Some(12),
 264                 ty::Opaque(..) => Some(13),
 265                 ty::Never => Some(14),
 266                 ty::Adt(adt, ..) => match adt.adt_kind() {
 267                     AdtKind::Struct => Some(15),
 268                     AdtKind::Union => Some(16),
 269                     AdtKind::Enum => Some(17),
 270                 },
 271                 ty::Generator(..) => Some(18),
 272                 ty::Foreign(..) => Some(19),
 273                 ty::GeneratorWitness(..) => Some(20),
 274                 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error => None,
 275                 ty::UnnormalizedProjection(..) => bug!("only used with chalk-engine"),
 276             }
 277         }
 278
 279         match (type_category(a), type_category(b)) {
 280             (Some(cat_a), Some(cat_b)) => match (&a.sty, &b.sty) {
 281                 (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => def_a == def_b,
 282                 _ => cat_a == cat_b
 283             },
 284             // infer and error can be equated to all types
 285             _ => true
 286         }
 287     }
 288
 289     fn impl_similar_to(&self,
 290                        trait_ref: ty::PolyTraitRef<'tcx>,
 291                        obligation: &PredicateObligation<'tcx>)
 292                        -> Option<DefId>
 293     {
 294         let tcx = self.tcx;
 295         let param_env = obligation.param_env;
 296         let trait_ref = tcx.erase_late_bound_regions(&trait_ref);
 297         let trait_self_ty = trait_ref.self_ty();
 298
 299         let mut self_match_impls = vec![];
 300         let mut fuzzy_match_impls = vec![];
 301
 302         self.tcx.for_each_relevant_impl(
 303             trait_ref.def_id, trait_self_ty, |def_id| {
 304                 let impl_substs = self.fresh_substs_for_item(obligation.cause.span, def_id);
 305                 let impl_trait_ref = tcx
 306                     .impl_trait_ref(def_id)
 307                     .unwrap()
 308                     .subst(tcx, impl_substs);
 309
 310                 let impl_self_ty = impl_trait_ref.self_ty();
 311
 312                 if let Ok(..) = self.can_eq(param_env, trait_self_ty, impl_self_ty) {
 313                     self_match_impls.push(def_id);
 314
 315                     if trait_ref.substs.types().skip(1)
 316                         .zip(impl_trait_ref.substs.types().skip(1))
 317                         .all(|(u,v)| self.fuzzy_match_tys(u, v))
 318                     {
 319                         fuzzy_match_impls.push(def_id);
 320                     }
 321                 }
 322             });
 323
 324         let impl_def_id = if self_match_impls.len() == 1 {
 325             self_match_impls[0]
 326         } else if fuzzy_match_impls.len() == 1 {
 327             fuzzy_match_impls[0]
 328         } else {
 329             return None
 330         };
 331
 332         if tcx.has_attr(impl_def_id, "rustc_on_unimplemented") {
 333             Some(impl_def_id)
 334         } else {
 335             None
 336         }
 337     }
 338
 339     fn on_unimplemented_note(
 340         &self,
 341         trait_ref: ty::PolyTraitRef<'tcx>,
 342         obligation: &PredicateObligation<'tcx>,
 343     ) -> OnUnimplementedNote {
 344         let def_id = self.impl_similar_to(trait_ref, obligation)
 345             .unwrap_or_else(|| trait_ref.def_id());
 346         let trait_ref = *trait_ref.skip_binder();
 347
 348         let mut flags = vec![];
 349         match obligation.cause.code {
 350             ObligationCauseCode::BuiltinDerivedObligation(..) |
 351             ObligationCauseCode::ImplDerivedObligation(..) => {}
 352             _ => {
 353                 // this is a "direct", user-specified, rather than derived,
 354                 // obligation.
 355                 flags.push(("direct".to_owned(), None));
 356             }
 357         }
 358
 359         if let ObligationCauseCode::ItemObligation(item) = obligation.cause.code {
 360             // FIXME: maybe also have some way of handling methods
 361             // from other traits? That would require name resolution,
 362             // which we might want to be some sort of hygienic.
 363             //
 364             // Currently I'm leaving it for what I need for `try`.
 365             if self.tcx.trait_of_item(item) == Some(trait_ref.def_id) {
 366                 let method = self.tcx.item_name(item);
 367                 flags.push(("from_method".to_owned(), None));
 368                 flags.push(("from_method".to_owned(), Some(method.to_string())));
 369             }
 370         }
 371         if let Some(t) = self.get_parent_trait_ref(&obligation.cause.code) {
 372             flags.push(("parent_trait".to_owned(), Some(t)));
 373         }
 374
 375         if let Some(k) = obligation.cause.span.compiler_desugaring_kind() {
 376             flags.push(("from_desugaring".to_owned(), None));
 377             flags.push(("from_desugaring".to_owned(), Some(k.name().to_string())));
 378         }
 379         let generics = self.tcx.generics_of(def_id);
 380         let self_ty = trait_ref.self_ty();
 381         // This is also included through the generics list as `Self`,
 382         // but the parser won't allow you to use it
 383         flags.push(("_Self".to_owned(), Some(self_ty.to_string())));
 384         if let Some(def) = self_ty.ty_adt_def() {
 385             // We also want to be able to select self's original
 386             // signature with no type arguments resolved
 387             flags.push(("_Self".to_owned(), Some(self.tcx.type_of(def.did).to_string())));
 388         }
 389
 390         for param in generics.params.iter() {
 391             let value = match param.kind {
 392                 GenericParamDefKind::Type {..} => {
 393                     trait_ref.substs[param.index as usize].to_string()
 394                 },
 395                 GenericParamDefKind::Lifetime => continue,
 396             };
 397             let name = param.name.to_string();
 398             flags.push((name, Some(value)));
 399         }
 400
 401         if let Some(true) = self_ty.ty_adt_def().map(|def| def.did.is_local()) {
 402             flags.push(("crate_local".to_owned(), None));
 403         }
 404
 405         // Allow targeting all integers using `{integral}`, even if the exact type was resolved
 406         if self_ty.is_integral() {
 407             flags.push(("_Self".to_owned(), Some("{integral}".to_owned())));
 408         }
 409
 410         if let ty::Array(aty, len) = self_ty.sty {
 411             flags.push(("_Self".to_owned(), Some("[]".to_owned())));
 412             flags.push(("_Self".to_owned(), Some(format!("[{}]", aty))));
 413             if let Some(def) = aty.ty_adt_def() {
 414                 // We also want to be able to select the array's type's original
 415                 // signature with no type arguments resolved
 416                 flags.push((
 417                     "_Self".to_owned(),
 418                     Some(format!("[{}]", self.tcx.type_of(def.did).to_string())),
 419                 ));
 420                 let tcx = self.tcx;
 421                 if let Some(len) = len.val.try_to_scalar().and_then(|scalar| {
 422                     scalar.to_usize(&tcx).ok()
 423                 }) {
 424                     flags.push((
 425                         "_Self".to_owned(),
 426                         Some(format!("[{}; {}]", self.tcx.type_of(def.did).to_string(), len)),
 427                     ));
 428                 } else {
 429                     flags.push((
 430                         "_Self".to_owned(),
 431                         Some(format!("[{}; _]", self.tcx.type_of(def.did).to_string())),
 432                     ));
 433                 }
 434             }
 435         }
 436
 437         if let Ok(Some(command)) = OnUnimplementedDirective::of_item(
 438             self.tcx, trait_ref.def_id, def_id
 439         ) {
 440             command.evaluate(self.tcx, trait_ref, &flags[..])
 441         } else {
 442             OnUnimplementedNote::empty()
 443         }
 444     }
 445
 446     fn find_similar_impl_candidates(&self,
 447                                     trait_ref: ty::PolyTraitRef<'tcx>)
 448                                     -> Vec<ty::TraitRef<'tcx>>
 449     {
 450         let simp = fast_reject::simplify_type(self.tcx,
 451                                               trait_ref.skip_binder().self_ty(),
 452                                               true,);
 453         let all_impls = self.tcx.all_impls(trait_ref.def_id());
 454
 455         match simp {
 456             Some(simp) => all_impls.iter().filter_map(|&def_id| {
 457                 let imp = self.tcx.impl_trait_ref(def_id).unwrap();
 458                 let imp_simp = fast_reject::simplify_type(self.tcx,
 459                                                           imp.self_ty(),
 460                                                           true);
 461                 if let Some(imp_simp) = imp_simp {
 462                     if simp != imp_simp {
 463                         return None
 464                     }
 465                 }
 466
 467                 Some(imp)
 468             }).collect(),
 469             None => all_impls.iter().map(|&def_id|
 470                 self.tcx.impl_trait_ref(def_id).unwrap()
 471             ).collect()
 472         }
 473     }
 474
 475     fn report_similar_impl_candidates(&self,
 476                                       mut impl_candidates: Vec<ty::TraitRef<'tcx>>,
 477                                       err: &mut DiagnosticBuilder<'_>)
 478     {
 479         if impl_candidates.is_empty() {
 480             return;
 481         }
 482
 483         let len = impl_candidates.len();
 484         let end = if impl_candidates.len() <= 5 {
 485             impl_candidates.len()
 486         } else {
 487             4
 488         };
 489
 490         let normalize = |candidate| self.tcx.global_tcx().infer_ctxt().enter(|ref infcx| {
 491             let normalized = infcx
 492                 .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
 493                 .normalize(candidate)
 494                 .ok();
 495             match normalized {
 496                 Some(normalized) => format!("\n  {:?}", normalized.value),
 497                 None => format!("\n  {:?}", candidate),
 498             }
 499         });
 500
 501         // Sort impl candidates so that ordering is consistent for UI tests.
 502         let normalized_impl_candidates = &mut impl_candidates[0..end]
 503             .iter()
 504             .map(normalize)
 505             .collect::<Vec<String>>();
 506         normalized_impl_candidates.sort();
 507
 508         err.help(&format!("the following implementations were found:{}{}",
 509                           normalized_impl_candidates.join(""),
 510                           if len > 5 {
 511                               format!("\nand {} others", len - 4)
 512                           } else {
 513                               String::new()
 514                           }
 515                           ));
 516     }
 517
 518     /// Reports that an overflow has occurred and halts compilation. We
 519     /// halt compilation unconditionally because it is important that
 520     /// overflows never be masked -- they basically represent computations
 521     /// whose result could not be truly determined and thus we can't say
 522     /// if the program type checks or not -- and they are unusual
 523     /// occurrences in any case.
 524     pub fn report_overflow_error<T>(&self,
 525                                     obligation: &Obligation<'tcx, T>,
 526                                     suggest_increasing_limit: bool) -> !
 527         where T: fmt::Display + TypeFoldable<'tcx>
 528     {
 529         let predicate =
 530             self.resolve_type_vars_if_possible(&obligation.predicate);
 531         let mut err = struct_span_err!(self.tcx.sess, obligation.cause.span, E0275,
 532                                        "overflow evaluating the requirement `{}`",
 533                                        predicate);
 534
 535         if suggest_increasing_limit {
 536             self.suggest_new_overflow_limit(&mut err);
 537         }
 538
 539         self.note_obligation_cause(&mut err, obligation);
 540
 541         err.emit();
 542         self.tcx.sess.abort_if_errors();
 543         bug!();
 544     }
 545
 546     /// Reports that a cycle was detected which led to overflow and halts
 547     /// compilation. This is equivalent to `report_overflow_error` except
 548     /// that we can give a more helpful error message (and, in particular,
 549     /// we do not suggest increasing the overflow limit, which is not
 550     /// going to help).
 551     pub fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 552         let cycle = self.resolve_type_vars_if_possible(&cycle.to_owned());
 553         assert!(cycle.len() > 0);
 554
 555         debug!("report_overflow_error_cycle: cycle={:?}", cycle);
 556
 557         self.report_overflow_error(&cycle[0], false);
 558     }
 559
 560     pub fn report_extra_impl_obligation(&self,
 561                                         error_span: Span,
 562                                         item_name: ast::Name,
 563                                         _impl_item_def_id: DefId,
 564                                         trait_item_def_id: DefId,
 565                                         requirement: &dyn fmt::Display)
 566                                         -> DiagnosticBuilder<'tcx>
 567     {
 568         let msg = "impl has stricter requirements than trait";
 569         let sp = self.tcx.sess.source_map().def_span(error_span);
 570
 571         let mut err = struct_span_err!(self.tcx.sess, sp, E0276, "{}", msg);
 572
 573         if let Some(trait_item_span) = self.tcx.hir().span_if_local(trait_item_def_id) {
 574             let span = self.tcx.sess.source_map().def_span(trait_item_span);
 575             err.span_label(span, format!("definition of `{}` from trait", item_name));
 576         }
 577
 578         err.span_label(sp, format!("impl has extra requirement {}", requirement));
 579
 580         err
 581     }
 582
 583
 584     /// Get the parent trait chain start
 585     fn get_parent_trait_ref(&self, code: &ObligationCauseCode<'tcx>) -> Option<String> {
 586         match code {
 587             &ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
 588                 let parent_trait_ref = self.resolve_type_vars_if_possible(
 589                     &data.parent_trait_ref);
 590                 match self.get_parent_trait_ref(&data.parent_code) {
 591                     Some(t) => Some(t),
 592                     None => Some(parent_trait_ref.skip_binder().self_ty().to_string()),
 593                 }
 594             }
 595             _ => None,
 596         }
 597     }
 598
 599     pub fn report_selection_error(&self,
 600                                   obligation: &PredicateObligation<'tcx>,
 601                                   error: &SelectionError<'tcx>,
 602                                   fallback_has_occurred: bool)
 603     {
 604         let span = obligation.cause.span;
 605
 606         let mut err = match *error {
 607             SelectionError::Unimplemented => {
 608                 if let ObligationCauseCode::CompareImplMethodObligation {
 609                     item_name, impl_item_def_id, trait_item_def_id,
 610                 } = obligation.cause.code {
 611                     self.report_extra_impl_obligation(
 612                         span,
 613                         item_name,
 614                         impl_item_def_id,
 615                         trait_item_def_id,
 616                         &format!("`{}`", obligation.predicate))
 617                         .emit();
 618                     return;
 619                 }
 620                 match obligation.predicate {
 621                     ty::Predicate::Trait(ref trait_predicate) => {
 622                         let trait_predicate =
 623                             self.resolve_type_vars_if_possible(trait_predicate);
 624
 625                         if self.tcx.sess.has_errors() && trait_predicate.references_error() {
 626                             return;
 627                         }
 628                         let trait_ref = trait_predicate.to_poly_trait_ref();
 629                         let (post_message, pre_message) =
 630                             self.get_parent_trait_ref(&obligation.cause.code)
 631                                 .map(|t| (format!(" in `{}`", t), format!("within `{}`, ", t)))
 632                             .unwrap_or_default();
 633
 634                         let OnUnimplementedNote { message, label, note }
 635                             = self.on_unimplemented_note(trait_ref, obligation);
 636                         let have_alt_message = message.is_some() || label.is_some();
 637
 638                         let mut err = struct_span_err!(
 639                             self.tcx.sess,
 640                             span,
 641                             E0277,
 642                             "{}",
 643                             message.unwrap_or_else(||
 644                                 format!("the trait bound `{}` is not satisfied{}",
 645                                          trait_ref.to_predicate(), post_message)
 646                             ));
 647
 648                         let explanation =
 649                             if obligation.cause.code == ObligationCauseCode::MainFunctionType {
 650                                 "consider using `()`, or a `Result`".to_owned()
 651                             } else {
 652                                 format!("{}the trait `{}` is not implemented for `{}`",
 653                                         pre_message,
 654                                         trait_ref,
 655                                         trait_ref.self_ty())
 656                             };
 657
 658                         if let Some(ref s) = label {
 659                             // If it has a custom "#[rustc_on_unimplemented]"
 660                             // error message, let's display it as the label!
 661                             err.span_label(span, s.as_str());
 662                             err.help(&explanation);
 663                         } else {
 664                             err.span_label(span, explanation);
 665                         }
 666                         if let Some(ref s) = note {
 667                             // If it has a custom "#[rustc_on_unimplemented]" note, let's display it
 668                             err.note(s.as_str());
 669                         }
 670
 671                         self.suggest_borrow_on_unsized_slice(&obligation.cause.code, &mut err);
 672                         self.suggest_remove_reference(&obligation, &mut err, &trait_ref);
 673
 674                         // Try to report a help message
 675                         if !trait_ref.has_infer_types() &&
 676                             self.predicate_can_apply(obligation.param_env, trait_ref) {
 677                             // If a where-clause may be useful, remind the
 678                             // user that they can add it.
 679                             //
 680                             // don't display an on-unimplemented note, as
 681                             // these notes will often be of the form
 682                             //     "the type `T` can't be frobnicated"
 683                             // which is somewhat confusing.
 684                             err.help(&format!("consider adding a `where {}` bound",
 685                                               trait_ref.to_predicate()));
 686                         } else if !have_alt_message {
 687                             // Can't show anything else useful, try to find similar impls.
 688                             let impl_candidates = self.find_similar_impl_candidates(trait_ref);
 689                             self.report_similar_impl_candidates(impl_candidates, &mut err);
 690                         }
 691
 692                         // If this error is due to `!: Trait` not implemented but `(): Trait` is
 693                         // implemented, and fallback has occurred, then it could be due to a
 694                         // variable that used to fallback to `()` now falling back to `!`. Issue a
 695                         // note informing about the change in behaviour.
 696                         if trait_predicate.skip_binder().self_ty().is_never()
 697                             && fallback_has_occurred
 698                         {
 699                             let predicate = trait_predicate.map_bound(|mut trait_pred| {
 700                                 trait_pred.trait_ref.substs = self.tcx.mk_substs_trait(
 701                                     self.tcx.mk_unit(),
 702                                     &trait_pred.trait_ref.substs[1..],
 703                                 );
 704                                 trait_pred
 705                             });
 706                             let unit_obligation = Obligation {
 707                                 predicate: ty::Predicate::Trait(predicate),
 708                                 .. obligation.clone()
 709                             };
 710                             if self.predicate_may_hold(&unit_obligation) {
 711                                 err.note("the trait is implemented for `()`. \
 712                                          Possibly this error has been caused by changes to \
 713                                          Rust's type-inference algorithm \
 714                                          (see: https://github.com/rust-lang/rust/issues/48950 \
 715                                          for more info). Consider whether you meant to use the \
 716                                          type `()` here instead.");
 717                             }
 718                         }
 719
 720                         err
 721                     }
 722
 723                     ty::Predicate::Subtype(ref predicate) => {
 724                         // Errors for Subtype predicates show up as
 725                         // `FulfillmentErrorCode::CodeSubtypeError`,
 726                         // not selection error.
 727                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
 728                     }
 729
 730                     ty::Predicate::RegionOutlives(ref predicate) => {
 731                         let predicate = self.resolve_type_vars_if_possible(predicate);
 732                         let err = self.region_outlives_predicate(&obligation.cause,
 733                                                                  &predicate).err().unwrap();
 734                         struct_span_err!(self.tcx.sess, span, E0279,
 735                             "the requirement `{}` is not satisfied (`{}`)",
 736                             predicate, err)
 737                     }
 738
 739                     ty::Predicate::Projection(..) | ty::Predicate::TypeOutlives(..) => {
 740                         let predicate =
 741                             self.resolve_type_vars_if_possible(&obligation.predicate);
 742                         struct_span_err!(self.tcx.sess, span, E0280,
 743                             "the requirement `{}` is not satisfied",
 744                             predicate)
 745                     }
 746
 747                     ty::Predicate::ObjectSafe(trait_def_id) => {
 748                         let violations = self.tcx.global_tcx()
 749                             .object_safety_violations(trait_def_id);
 750                         self.tcx.report_object_safety_error(span,
 751                                                             trait_def_id,
 752                                                             violations)
 753                     }
 754
 755                     ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => {
 756                         let found_kind = self.closure_kind(closure_def_id, closure_substs).unwrap();
 757                         let closure_span = self.tcx.sess.source_map()
 758                             .def_span(self.tcx.hir().span_if_local(closure_def_id).unwrap());
 759                         let node_id = self.tcx.hir().as_local_node_id(closure_def_id).unwrap();
 760                         let mut err = struct_span_err!(
 761                             self.tcx.sess, closure_span, E0525,
 762                             "expected a closure that implements the `{}` trait, \
 763                              but this closure only implements `{}`",
 764                             kind,
 765                             found_kind);
 766
 767                         err.span_label(
 768                             closure_span,
 769                             format!("this closure implements `{}`, not `{}`", found_kind, kind));
 770                         err.span_label(
 771                             obligation.cause.span,
 772                             format!("the requirement to implement `{}` derives from here", kind));
 773
 774                         // Additional context information explaining why the closure only implements
 775                         // a particular trait.
 776                         if let Some(tables) = self.in_progress_tables {
 777                             let tables = tables.borrow();
 778                             let closure_hir_id = self.tcx.hir().node_to_hir_id(node_id);
 779                             match (found_kind, tables.closure_kind_origins().get(closure_hir_id)) {
 780                                 (ty::ClosureKind::FnOnce, Some((span, name))) => {
 781                                     err.span_label(*span, format!(
 782                                         "closure is `FnOnce` because it moves the \
 783                                          variable `{}` out of its environment", name));
 784                                 },
 785                                 (ty::ClosureKind::FnMut, Some((span, name))) => {
 786                                     err.span_label(*span, format!(
 787                                         "closure is `FnMut` because it mutates the \
 788                                          variable `{}` here", name));
 789                                 },
 790                                 _ => {}
 791                             }
 792                         }
 793
 794                         err.emit();
 795                         return;
 796                     }
 797
 798                     ty::Predicate::WellFormed(ty) => {
 799                         if !self.tcx.sess.opts.debugging_opts.chalk {
 800                             // WF predicates cannot themselves make
 801                             // errors. They can only block due to
 802                             // ambiguity; otherwise, they always
 803                             // degenerate into other obligations
 804                             // (which may fail).
 805                             span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 806                         } else {
 807                             // FIXME: we'll need a better message which takes into account
 808                             // which bounds actually failed to hold.
 809                             self.tcx.sess.struct_span_err(
 810                                 span,
 811                                 &format!("the type `{}` is not well-formed (chalk)", ty)
 812                             )
 813                         }
 814                     }
 815
 816                     ty::Predicate::ConstEvaluatable(..) => {
 817                         // Errors for `ConstEvaluatable` predicates show up as
 818                         // `SelectionError::ConstEvalFailure`,
 819                         // not `Unimplemented`.
 820                         span_bug!(span,
 821                             "const-evaluatable requirement gave wrong error: `{:?}`", obligation)
 822                     }
 823                 }
 824             }
 825
 826             OutputTypeParameterMismatch(ref found_trait_ref, ref expected_trait_ref, _) => {
 827                 let found_trait_ref = self.resolve_type_vars_if_possible(&*found_trait_ref);
 828                 let expected_trait_ref = self.resolve_type_vars_if_possible(&*expected_trait_ref);
 829
 830                 if expected_trait_ref.self_ty().references_error() {
 831                     return;
 832                 }
 833
 834                 let found_trait_ty = found_trait_ref.self_ty();
 835
 836                 let found_did = match found_trait_ty.sty {
 837                     ty::Closure(did, _) | ty::Foreign(did) | ty::FnDef(did, _) => Some(did),
 838                     ty::Adt(def, _) => Some(def.did),
 839                     _ => None,
 840                 };
 841
 842                 let found_span = found_did.and_then(|did|
 843                     self.tcx.hir().span_if_local(did)
 844                 ).map(|sp| self.tcx.sess.source_map().def_span(sp)); // the sp could be an fn def
 845
 846                 let found = match found_trait_ref.skip_binder().substs.type_at(1).sty {
 847                     ty::Tuple(ref tys) => vec![ArgKind::empty(); tys.len()],
 848                     _ => vec![ArgKind::empty()],
 849                 };
 850
 851                 let expected = match expected_trait_ref.skip_binder().substs.type_at(1).sty {
 852                     ty::Tuple(ref tys) => tys.iter()
 853                         .map(|t| ArgKind::from_expected_ty(t, Some(span))).collect(),
 854                     ref sty => vec![ArgKind::Arg("_".to_owned(), sty.to_string())],
 855                 };
 856
 857                 if found.len() == expected.len() {
 858                     self.report_closure_arg_mismatch(span,
 859                                                      found_span,
 860                                                      found_trait_ref,
 861                                                      expected_trait_ref)
 862                 } else {
 863                     let (closure_span, found) = found_did
 864                         .and_then(|did| self.tcx.hir().get_if_local(did))
 865                         .map(|node| {
 866                             let (found_span, found) = self.get_fn_like_arguments(node);
 867                             (Some(found_span), found)
 868                         }).unwrap_or((found_span, found));
 869
 870                     self.report_arg_count_mismatch(span,
 871                                                    closure_span,
 872                                                    expected,
 873                                                    found,
 874                                                    found_trait_ty.is_closure())
 875                 }
 876             }
 877
 878             TraitNotObjectSafe(did) => {
 879                 let violations = self.tcx.global_tcx().object_safety_violations(did);
 880                 self.tcx.report_object_safety_error(span, did, violations)
 881             }
 882
 883             // already reported in the query
 884             ConstEvalFailure(_) => {
 885                 self.tcx.sess.delay_span_bug(span, "constant in type had an ignored error");
 886                 return;
 887             }
 888
 889             Overflow => {
 890                 bug!("overflow should be handled before the `report_selection_error` path");
 891             }
 892         };
 893         self.note_obligation_cause(&mut err, obligation);
 894         err.emit();
 895     }
 896
 897     /// When encountering an assignment of an unsized trait, like `let x = ""[..];`, provide a
 898     /// suggestion to borrow the initializer in order to use have a slice instead.
 899     fn suggest_borrow_on_unsized_slice(&self,
 900                                        code: &ObligationCauseCode<'tcx>,
 901                                        err: &mut DiagnosticBuilder<'tcx>) {
 902         if let &ObligationCauseCode::VariableType(node_id) = code {
 903             let parent_node = self.tcx.hir().get_parent_node(node_id);
 904             if let Some(Node::Local(ref local)) = self.tcx.hir().find(parent_node) {
 905                 if let Some(ref expr) = local.init {
 906                     if let hir::ExprKind::Index(_, _) = expr.node {
 907                         if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(expr.span) {
 908                             err.span_suggestion_with_applicability(
 909                                 expr.span,
 910                                 "consider borrowing here",
 911                                 format!("&{}", snippet),
 912                                 Applicability::MachineApplicable
 913                             );
 914                         }
 915                     }
 916                 }
 917             }
 918         }
 919     }
 920
 921     /// Whenever references are used by mistake, like `for (i, e) in &vec.iter().enumerate()`,
 922     /// suggest removing these references until we reach a type that implements the trait.
 923     fn suggest_remove_reference(&self,
 924                                 obligation: &PredicateObligation<'tcx>,
 925                                 err: &mut DiagnosticBuilder<'tcx>,
 926                                 trait_ref: &ty::Binder<ty::TraitRef<'tcx>>) {
 927         let trait_ref = trait_ref.skip_binder();
 928         let span = obligation.cause.span;
 929
 930         if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
 931             let refs_number = snippet.chars()
 932                 .filter(|c| !c.is_whitespace())
 933                 .take_while(|c| *c == '&')
 934                 .count();
 935
 936             let mut trait_type = trait_ref.self_ty();
 937
 938             for refs_remaining in 0..refs_number {
 939                 if let ty::Ref(_, t_type, _) = trait_type.sty {
 940                     trait_type = t_type;
 941
 942                     let substs = self.tcx.mk_substs_trait(trait_type, &[]);
 943                     let new_trait_ref = ty::TraitRef::new(trait_ref.def_id, substs);
 944                     let new_obligation = Obligation::new(ObligationCause::dummy(),
 945                                                          obligation.param_env,
 946                                                          new_trait_ref.to_predicate());
 947
 948                     if self.predicate_may_hold(&new_obligation) {
 949                         let sp = self.tcx.sess.source_map()
 950                             .span_take_while(span, |c| c.is_whitespace() || *c == '&');
 951
 952                         let remove_refs = refs_remaining + 1;
 953                         let format_str = format!("consider removing {} leading `&`-references",
 954                                                  remove_refs);
 955
 956                         err.span_suggestion_short_with_applicability(
 957                             sp, &format_str, String::new(), Applicability::MachineApplicable
 958                         );
 959                         break;
 960                     }
 961                 } else {
 962                     break;
 963                 }
 964             }
 965         }
 966     }
 967
 968     /// Given some node representing a fn-like thing in the HIR map,
 969     /// returns a span and `ArgKind` information that describes the
 970     /// arguments it expects. This can be supplied to
 971     /// `report_arg_count_mismatch`.
 972     pub fn get_fn_like_arguments(&self, node: Node<'_>) -> (Span, Vec<ArgKind>) {
 973         match node {
 974             Node::Expr(&hir::Expr {
 975                 node: hir::ExprKind::Closure(_, ref _decl, id, span, _),
 976                 ..
 977             }) => {
 978                 (self.tcx.sess.source_map().def_span(span), self.tcx.hir().body(id).arguments.iter()
 979                     .map(|arg| {
 980                         if let hir::Pat {
 981                             node: hir::PatKind::Tuple(args, _),
 982                             span,
 983                             ..
 984                         } = arg.pat.clone().into_inner() {
 985                             ArgKind::Tuple(
 986                                 Some(span),
 987                                 args.iter().map(|pat| {
 988                                     let snippet = self.tcx.sess.source_map()
 989                                         .span_to_snippet(pat.span).unwrap();
 990                                     (snippet, "_".to_owned())
 991                                 }).collect::<Vec<_>>(),
 992                             )
 993                         } else {
 994                             let name = self.tcx.sess.source_map()
 995                                 .span_to_snippet(arg.pat.span).unwrap();
 996                             ArgKind::Arg(name, "_".to_owned())
 997                         }
 998                     })
 999                     .collect::<Vec<ArgKind>>())
1000             }
1001             Node::Item(&hir::Item {
1002                 span,
1003                 node: hir::ItemKind::Fn(ref decl, ..),
1004                 ..
1005             }) |
1006             Node::ImplItem(&hir::ImplItem {
1007                 span,
1008                 node: hir::ImplItemKind::Method(hir::MethodSig { ref decl, .. }, _),
1009                 ..
1010             }) |
1011             Node::TraitItem(&hir::TraitItem {
1012                 span,
1013                 node: hir::TraitItemKind::Method(hir::MethodSig { ref decl, .. }, _),
1014                 ..
1015             }) => {
1016                 (self.tcx.sess.source_map().def_span(span), decl.inputs.iter()
1017                         .map(|arg| match arg.clone().node {
1018                     hir::TyKind::Tup(ref tys) => ArgKind::Tuple(
1019                         Some(arg.span),
1020                         vec![("_".to_owned(), "_".to_owned()); tys.len()]
1021                     ),
1022                     _ => ArgKind::empty()
1023                 }).collect::<Vec<ArgKind>>())
1024             }
1025             Node::Variant(&hir::Variant {
1026                 span,
1027                 node: hir::VariantKind {
1028                     data: hir::VariantData::Tuple(ref fields, _),
1029                     ..
1030                 },
1031                 ..
1032             }) => {
1033                 (self.tcx.sess.source_map().def_span(span),
1034                  fields.iter().map(|field|
1035                      ArgKind::Arg(field.ident.to_string(), "_".to_string())
1036                  ).collect::<Vec<_>>())
1037             }
1038             Node::StructCtor(ref variant_data) => {
1039                 (self.tcx.sess.source_map().def_span(self.tcx.hir().span(variant_data.id())),
1040                  vec![ArgKind::empty(); variant_data.fields().len()])
1041             }
1042             _ => panic!("non-FnLike node found: {:?}", node),
1043         }
1044     }
1045
1046     /// Reports an error when the number of arguments needed by a
1047     /// trait match doesn't match the number that the expression
1048     /// provides.
1049     pub fn report_arg_count_mismatch(
1050         &self,
1051         span: Span,
1052         found_span: Option<Span>,
1053         expected_args: Vec<ArgKind>,
1054         found_args: Vec<ArgKind>,
1055         is_closure: bool,
1056     ) -> DiagnosticBuilder<'tcx> {
1057         let kind = if is_closure { "closure" } else { "function" };
1058
1059         let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
1060             let arg_length = arguments.len();
1061             let distinct = match &other[..] {
1062                 &[ArgKind::Tuple(..)] => true,
1063                 _ => false,
1064             };
1065             match (arg_length, arguments.get(0)) {
1066                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
1067                     format!("a single {}-tuple as argument", fields.len())
1068                 }
1069                 _ => format!("{} {}argument{}",
1070                              arg_length,
1071                              if distinct && arg_length > 1 { "distinct " } else { "" },
1072                              if arg_length == 1 { "" } else { "s" }),
1073             }
1074         };
1075
1076         let expected_str = args_str(&expected_args, &found_args);
1077         let found_str = args_str(&found_args, &expected_args);
1078
1079         let mut err = struct_span_err!(
1080             self.tcx.sess,
1081             span,
1082             E0593,
1083             "{} is expected to take {}, but it takes {}",
1084             kind,
1085             expected_str,
1086             found_str,
1087         );
1088
1089         err.span_label(span, format!("expected {} that takes {}", kind, expected_str));
1090
1091         if let Some(found_span) = found_span {
1092             err.span_label(found_span, format!("takes {}", found_str));
1093
1094             // move |_| { ... }
1095             // ^^^^^^^^-- def_span
1096             //
1097             // move |_| { ... }
1098             // ^^^^^-- prefix
1099             let prefix_span = self.tcx.sess.source_map().span_until_non_whitespace(found_span);
1100             // move |_| { ... }
1101             //      ^^^-- pipe_span
1102             let pipe_span = if let Some(span) = found_span.trim_start(prefix_span) {
1103                 span
1104             } else {
1105                 found_span
1106             };
1107
1108             // Suggest to take and ignore the arguments with expected_args_length `_`s if
1109             // found arguments is empty (assume the user just wants to ignore args in this case).
1110             // For example, if `expected_args_length` is 2, suggest `|_, _|`.
1111             if found_args.is_empty() && is_closure {
1112                 let underscores = vec!["_"; expected_args.len()].join(", ");
1113                 err.span_suggestion_with_applicability(
1114                     pipe_span,
1115                     &format!(
1116                         "consider changing the closure to take and ignore the expected argument{}",
1117                         if expected_args.len() < 2 {
1118                             ""
1119                         } else {
1120                             "s"
1121                         }
1122                     ),
1123                     format!("|{}|", underscores),
1124                     Applicability::MachineApplicable,
1125                 );
1126             }
1127
1128             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
1129                 if fields.len() == expected_args.len() {
1130                     let sugg = fields.iter()
1131                         .map(|(name, _)| name.to_owned())
1132                         .collect::<Vec<String>>()
1133                         .join(", ");
1134                     err.span_suggestion_with_applicability(found_span,
1135                                                            "change the closure to take multiple \
1136                                                             arguments instead of a single tuple",
1137                                                            format!("|{}|", sugg),
1138                                                            Applicability::MachineApplicable);
1139                 }
1140             }
1141             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..] {
1142                 if fields.len() == found_args.len() && is_closure {
1143                     let sugg = format!(
1144                         "|({}){}|",
1145                         found_args.iter()
1146                             .map(|arg| match arg {
1147                                 ArgKind::Arg(name, _) => name.to_owned(),
1148                                 _ => "_".to_owned(),
1149                             })
1150                             .collect::<Vec<String>>()
1151                             .join(", "),
1152                         // add type annotations if available
1153                         if found_args.iter().any(|arg| match arg {
1154                             ArgKind::Arg(_, ty) => ty != "_",
1155                             _ => false,
1156                         }) {
1157                             format!(": ({})",
1158                                     fields.iter()
1159                                         .map(|(_, ty)| ty.to_owned())
1160                                         .collect::<Vec<String>>()
1161                                         .join(", "))
1162                         } else {
1163                             String::new()
1164                         },
1165                     );
1166                     err.span_suggestion_with_applicability(
1167                         found_span,
1168                         "change the closure to accept a tuple instead of \
1169                          individual arguments",
1170                         sugg,
1171                         Applicability::MachineApplicable
1172                     );
1173                 }
1174             }
1175         }
1176
1177         err
1178     }
1179
1180     fn report_closure_arg_mismatch(&self,
1181                            span: Span,
1182                            found_span: Option<Span>,
1183                            expected_ref: ty::PolyTraitRef<'tcx>,
1184                            found: ty::PolyTraitRef<'tcx>)
1185         -> DiagnosticBuilder<'tcx>
1186     {
1187         fn build_fn_sig_string<'a, 'gcx, 'tcx>(tcx: ty::TyCtxt<'a, 'gcx, 'tcx>,
1188                                                trait_ref: &ty::TraitRef<'tcx>) -> String {
1189             let inputs = trait_ref.substs.type_at(1);
1190             let sig = if let ty::Tuple(inputs) = inputs.sty {
1191                 tcx.mk_fn_sig(
1192                     inputs.iter().cloned(),
1193                     tcx.mk_infer(ty::TyVar(ty::TyVid { index: 0 })),
1194                     false,
1195                     hir::Unsafety::Normal,
1196                     ::rustc_target::spec::abi::Abi::Rust
1197                 )
1198             } else {
1199                 tcx.mk_fn_sig(
1200                     ::std::iter::once(inputs),
1201                     tcx.mk_infer(ty::TyVar(ty::TyVid { index: 0 })),
1202                     false,
1203                     hir::Unsafety::Normal,
1204                     ::rustc_target::spec::abi::Abi::Rust
1205                 )
1206             };
1207             ty::Binder::bind(sig).to_string()
1208         }
1209
1210         let argument_is_closure = expected_ref.skip_binder().substs.type_at(0).is_closure();
1211         let mut err = struct_span_err!(self.tcx.sess, span, E0631,
1212                                        "type mismatch in {} arguments",
1213                                        if argument_is_closure { "closure" } else { "function" });
1214
1215         let found_str = format!(
1216             "expected signature of `{}`",
1217             build_fn_sig_string(self.tcx, found.skip_binder())
1218         );
1219         err.span_label(span, found_str);
1220
1221         let found_span = found_span.unwrap_or(span);
1222         let expected_str = format!(
1223             "found signature of `{}`",
1224             build_fn_sig_string(self.tcx, expected_ref.skip_binder())
1225         );
1226         err.span_label(found_span, expected_str);
1227
1228         err
1229     }
1230 }
1231
1232 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
1233     pub fn recursive_type_with_infinite_size_error(self,
1234                                                    type_def_id: DefId)
1235                                                    -> DiagnosticBuilder<'tcx>
1236     {
1237         assert!(type_def_id.is_local());
1238         let span = self.hir().span_if_local(type_def_id).unwrap();
1239         let span = self.sess.source_map().def_span(span);
1240         let mut err = struct_span_err!(self.sess, span, E0072,
1241                                        "recursive type `{}` has infinite size",
1242                                        self.item_path_str(type_def_id));
1243         err.span_label(span, "recursive type has infinite size");
1244         err.help(&format!("insert indirection (e.g., a `Box`, `Rc`, or `&`) \
1245                            at some point to make `{}` representable",
1246                           self.item_path_str(type_def_id)));
1247         err
1248     }
1249
1250     pub fn report_object_safety_error(self,
1251                                       span: Span,
1252                                       trait_def_id: DefId,
1253                                       violations: Vec<ObjectSafetyViolation>)
1254                                       -> DiagnosticBuilder<'tcx>
1255     {
1256         let trait_str = self.item_path_str(trait_def_id);
1257         let span = self.sess.source_map().def_span(span);
1258         let mut err = struct_span_err!(
1259             self.sess, span, E0038,
1260             "the trait `{}` cannot be made into an object",
1261             trait_str);
1262         err.span_label(span, format!("the trait `{}` cannot be made into an object", trait_str));
1263
1264         let mut reported_violations = FxHashSet::default();
1265         for violation in violations {
1266             if reported_violations.insert(violation.clone()) {
1267                 err.note(&violation.error_msg());
1268             }
1269         }
1270         err
1271     }
1272 }
1273
1274 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
1275     fn maybe_report_ambiguity(&self, obligation: &PredicateObligation<'tcx>,
1276                               body_id: Option<hir::BodyId>) {
1277         // Unable to successfully determine, probably means
1278         // insufficient type information, but could mean
1279         // ambiguous impls. The latter *ought* to be a
1280         // coherence violation, so we don't report it here.
1281
1282         let predicate = self.resolve_type_vars_if_possible(&obligation.predicate);
1283         let span = obligation.cause.span;
1284
1285         debug!("maybe_report_ambiguity(predicate={:?}, obligation={:?})",
1286                predicate,
1287                obligation);
1288
1289         // Ambiguity errors are often caused as fallout from earlier
1290         // errors. So just ignore them if this infcx is tainted.
1291         if self.is_tainted_by_errors() {
1292             return;
1293         }
1294
1295         match predicate {
1296             ty::Predicate::Trait(ref data) => {
1297                 let trait_ref = data.to_poly_trait_ref();
1298                 let self_ty = trait_ref.self_ty();
1299                 if predicate.references_error() {
1300                     return;
1301                 }
1302                 // Typically, this ambiguity should only happen if
1303                 // there are unresolved type inference variables
1304                 // (otherwise it would suggest a coherence
1305                 // failure). But given #21974 that is not necessarily
1306                 // the case -- we can have multiple where clauses that
1307                 // are only distinguished by a region, which results
1308                 // in an ambiguity even when all types are fully
1309                 // known, since we don't dispatch based on region
1310                 // relationships.
1311
1312                 // This is kind of a hack: it frequently happens that some earlier
1313                 // error prevents types from being fully inferred, and then we get
1314                 // a bunch of uninteresting errors saying something like "<generic
1315                 // #0> doesn't implement Sized".  It may even be true that we
1316                 // could just skip over all checks where the self-ty is an
1317                 // inference variable, but I was afraid that there might be an
1318                 // inference variable created, registered as an obligation, and
1319                 // then never forced by writeback, and hence by skipping here we'd
1320                 // be ignoring the fact that we don't KNOW the type works
1321                 // out. Though even that would probably be harmless, given that
1322                 // we're only talking about builtin traits, which are known to be
1323                 // inhabited. But in any case I just threw in this check for
1324                 // has_errors() to be sure that compilation isn't happening
1325                 // anyway. In that case, why inundate the user.
1326                 if !self.tcx.sess.has_errors() {
1327                     if
1328                         self.tcx.lang_items().sized_trait()
1329                         .map_or(false, |sized_id| sized_id == trait_ref.def_id())
1330                     {
1331                         self.need_type_info_err(body_id, span, self_ty).emit();
1332                     } else {
1333                         let mut err = struct_span_err!(self.tcx.sess,
1334                                                        span, E0283,
1335                                                        "type annotations required: \
1336                                                         cannot resolve `{}`",
1337                                                        predicate);
1338                         self.note_obligation_cause(&mut err, obligation);
1339                         err.emit();
1340                     }
1341                 }
1342             }
1343
1344             ty::Predicate::WellFormed(ty) => {
1345                 // Same hacky approach as above to avoid deluging user
1346                 // with error messages.
1347                 if !ty.references_error() && !self.tcx.sess.has_errors() {
1348                     self.need_type_info_err(body_id, span, ty).emit();
1349                 }
1350             }
1351
1352             ty::Predicate::Subtype(ref data) => {
1353                 if data.references_error() || self.tcx.sess.has_errors() {
1354                     // no need to overload user in such cases
1355                 } else {
1356                     let &SubtypePredicate { a_is_expected: _, a, b } = data.skip_binder();
1357                     // both must be type variables, or the other would've been instantiated
1358                     assert!(a.is_ty_var() && b.is_ty_var());
1359                     self.need_type_info_err(body_id,
1360                                             obligation.cause.span,
1361                                             a).emit();
1362                 }
1363             }
1364
1365             _ => {
1366                 if !self.tcx.sess.has_errors() {
1367                     let mut err = struct_span_err!(self.tcx.sess,
1368                                                    obligation.cause.span, E0284,
1369                                                    "type annotations required: \
1370                                                     cannot resolve `{}`",
1371                                                    predicate);
1372                     self.note_obligation_cause(&mut err, obligation);
1373                     err.emit();
1374                 }
1375             }
1376         }
1377     }
1378
1379     /// Returns whether the trait predicate may apply for *some* assignment
1380     /// to the type parameters.
1381     fn predicate_can_apply(&self,
1382                            param_env: ty::ParamEnv<'tcx>,
1383                            pred: ty::PolyTraitRef<'tcx>)
1384                            -> bool {
1385         struct ParamToVarFolder<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
1386             infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
1387             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>
1388         }
1389
1390         impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for ParamToVarFolder<'a, 'gcx, 'tcx> {
1391             fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> { self.infcx.tcx }
1392
1393             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
1394                 if let ty::Param(ty::ParamTy {name, ..}) = ty.sty {
1395                     let infcx = self.infcx;
1396                     self.var_map.entry(ty).or_insert_with(||
1397                         infcx.next_ty_var(
1398                             TypeVariableOrigin::TypeParameterDefinition(DUMMY_SP, name)))
1399                 } else {
1400                     ty.super_fold_with(self)
1401                 }
1402             }
1403         }
1404
1405         self.probe(|_| {
1406             let mut selcx = SelectionContext::new(self);
1407
1408             let cleaned_pred = pred.fold_with(&mut ParamToVarFolder {
1409                 infcx: self,
1410                 var_map: Default::default()
1411             });
1412
1413             let cleaned_pred = super::project::normalize(
1414                 &mut selcx,
1415                 param_env,
1416                 ObligationCause::dummy(),
1417                 &cleaned_pred
1418             ).value;
1419
1420             let obligation = Obligation::new(
1421                 ObligationCause::dummy(),
1422                 param_env,
1423                 cleaned_pred.to_predicate()
1424             );
1425
1426             self.predicate_may_hold(&obligation)
1427         })
1428     }
1429
1430     fn note_obligation_cause<T>(&self,
1431                                 err: &mut DiagnosticBuilder<'_>,
1432                                 obligation: &Obligation<'tcx, T>)
1433         where T: fmt::Display
1434     {
1435         self.note_obligation_cause_code(err,
1436                                         &obligation.predicate,
1437                                         &obligation.cause.code,
1438                                         &mut vec![]);
1439     }
1440
1441     fn note_obligation_cause_code<T>(&self,
1442                                      err: &mut DiagnosticBuilder<'_>,
1443                                      predicate: &T,
1444                                      cause_code: &ObligationCauseCode<'tcx>,
1445                                      obligated_types: &mut Vec<&ty::TyS<'tcx>>)
1446         where T: fmt::Display
1447     {
1448         let tcx = self.tcx;
1449         match *cause_code {
1450             ObligationCauseCode::ExprAssignable |
1451             ObligationCauseCode::MatchExpressionArm { .. } |
1452             ObligationCauseCode::IfExpression |
1453             ObligationCauseCode::IfExpressionWithNoElse |
1454             ObligationCauseCode::MainFunctionType |
1455             ObligationCauseCode::StartFunctionType |
1456             ObligationCauseCode::IntrinsicType |
1457             ObligationCauseCode::MethodReceiver |
1458             ObligationCauseCode::ReturnNoExpression |
1459             ObligationCauseCode::MiscObligation => {
1460             }
1461             ObligationCauseCode::SliceOrArrayElem => {
1462                 err.note("slice and array elements must have `Sized` type");
1463             }
1464             ObligationCauseCode::TupleElem => {
1465                 err.note("only the last element of a tuple may have a dynamically sized type");
1466             }
1467             ObligationCauseCode::ProjectionWf(data) => {
1468                 err.note(&format!("required so that the projection `{}` is well-formed",
1469                                   data));
1470             }
1471             ObligationCauseCode::ReferenceOutlivesReferent(ref_ty) => {
1472                 err.note(&format!("required so that reference `{}` does not outlive its referent",
1473                                   ref_ty));
1474             }
1475             ObligationCauseCode::ObjectTypeBound(object_ty, region) => {
1476                 err.note(&format!("required so that the lifetime bound of `{}` for `{}` \
1477                                    is satisfied",
1478                                   region, object_ty));
1479             }
1480             ObligationCauseCode::ItemObligation(item_def_id) => {
1481                 let item_name = tcx.item_path_str(item_def_id);
1482                 let msg = format!("required by `{}`", item_name);
1483
1484                 if let Some(sp) = tcx.hir().span_if_local(item_def_id) {
1485                     let sp = tcx.sess.source_map().def_span(sp);
1486                     err.span_note(sp, &msg);
1487                 } else {
1488                     err.note(&msg);
1489                 }
1490             }
1491             ObligationCauseCode::ObjectCastObligation(object_ty) => {
1492                 err.note(&format!("required for the cast to the object type `{}`",
1493                                   self.ty_to_string(object_ty)));
1494             }
1495             ObligationCauseCode::RepeatVec => {
1496                 err.note("the `Copy` trait is required because the \
1497                           repeated element will be copied");
1498             }
1499             ObligationCauseCode::VariableType(_) => {
1500                 err.note("all local variables must have a statically known size");
1501                 if !self.tcx.features().unsized_locals {
1502                     err.help("unsized locals are gated as an unstable feature");
1503                 }
1504             }
1505             ObligationCauseCode::SizedArgumentType => {
1506                 err.note("all function arguments must have a statically known size");
1507                 if !self.tcx.features().unsized_locals {
1508                     err.help("unsized locals are gated as an unstable feature");
1509                 }
1510             }
1511             ObligationCauseCode::SizedReturnType => {
1512                 err.note("the return type of a function must have a \
1513                           statically known size");
1514             }
1515             ObligationCauseCode::SizedYieldType => {
1516                 err.note("the yield type of a generator must have a \
1517                           statically known size");
1518             }
1519             ObligationCauseCode::AssignmentLhsSized => {
1520                 err.note("the left-hand-side of an assignment must have a statically known size");
1521             }
1522             ObligationCauseCode::TupleInitializerSized => {
1523                 err.note("tuples must have a statically known size to be initialized");
1524             }
1525             ObligationCauseCode::StructInitializerSized => {
1526                 err.note("structs must have a statically known size to be initialized");
1527             }
1528             ObligationCauseCode::FieldSized { adt_kind: ref item, last } => {
1529                 match *item {
1530                     AdtKind::Struct => {
1531                         if last {
1532                             err.note("the last field of a packed struct may only have a \
1533                                       dynamically sized type if it does not need drop to be run");
1534                         } else {
1535                             err.note("only the last field of a struct may have a dynamically \
1536                                       sized type");
1537                         }
1538                     }
1539                     AdtKind::Union => {
1540                         err.note("no field of a union may have a dynamically sized type");
1541                     }
1542                     AdtKind::Enum => {
1543                         err.note("no field of an enum variant may have a dynamically sized type");
1544                     }
1545                 }
1546             }
1547             ObligationCauseCode::ConstSized => {
1548                 err.note("constant expressions must have a statically known size");
1549             }
1550             ObligationCauseCode::SharedStatic => {
1551                 err.note("shared static variables must have a type that implements `Sync`");
1552             }
1553             ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
1554                 let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1555                 let ty = parent_trait_ref.skip_binder().self_ty();
1556                 err.note(&format!("required because it appears within the type `{}`", ty));
1557                 obligated_types.push(ty);
1558
1559                 let parent_predicate = parent_trait_ref.to_predicate();
1560                 if !self.is_recursive_obligation(obligated_types, &data.parent_code) {
1561                     self.note_obligation_cause_code(err,
1562                                                     &parent_predicate,
1563                                                     &data.parent_code,
1564                                                     obligated_types);
1565                 }
1566             }
1567             ObligationCauseCode::ImplDerivedObligation(ref data) => {
1568                 let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1569                 err.note(
1570                     &format!("required because of the requirements on the impl of `{}` for `{}`",
1571                              parent_trait_ref,
1572                              parent_trait_ref.skip_binder().self_ty()));
1573                 let parent_predicate = parent_trait_ref.to_predicate();
1574                 self.note_obligation_cause_code(err,
1575                                                 &parent_predicate,
1576                                                 &data.parent_code,
1577                                                 obligated_types);
1578             }
1579             ObligationCauseCode::CompareImplMethodObligation { .. } => {
1580                 err.note(
1581                     &format!("the requirement `{}` appears on the impl method \
1582                               but not on the corresponding trait method",
1583                              predicate));
1584             }
1585             ObligationCauseCode::ReturnType(_) |
1586             ObligationCauseCode::BlockTailExpression(_) => (),
1587             ObligationCauseCode::TrivialBound => {
1588                 err.help("see issue #48214");
1589                 if tcx.sess.opts.unstable_features.is_nightly_build() {
1590                     err.help("add #![feature(trivial_bounds)] to the \
1591                               crate attributes to enable",
1592                     );
1593                 }
1594             }
1595         }
1596     }
1597
1598     fn suggest_new_overflow_limit(&self, err: &mut DiagnosticBuilder<'_>) {
1599         let current_limit = self.tcx.sess.recursion_limit.get();
1600         let suggested_limit = current_limit * 2;
1601         err.help(&format!("consider adding a `#![recursion_limit=\"{}\"]` attribute to your crate",
1602                           suggested_limit));
1603     }
1604
1605     fn is_recursive_obligation(&self,
1606                                obligated_types: &mut Vec<&ty::TyS<'tcx>>,
1607                                cause_code: &ObligationCauseCode<'tcx>) -> bool {
1608         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
1609             let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1610
1611             if obligated_types.iter().any(|ot| ot == &parent_trait_ref.skip_binder().self_ty()) {
1612                 return true;
1613             }
1614         }
1615         false
1616     }
1617 }
1618
1619 /// Summarizes information
1620 #[derive(Clone)]
1621 pub enum ArgKind {
1622     /// An argument of non-tuple type. Parameters are (name, ty)
1623     Arg(String, String),
1624
1625     /// An argument of tuple type. For a "found" argument, the span is
1626     /// the locationo in the source of the pattern. For a "expected"
1627     /// argument, it will be None. The vector is a list of (name, ty)
1628     /// strings for the components of the tuple.
1629     Tuple(Option<Span>, Vec<(String, String)>),
1630 }
1631
1632 impl ArgKind {
1633     fn empty() -> ArgKind {
1634         ArgKind::Arg("_".to_owned(), "_".to_owned())
1635     }
1636
1637     /// Creates an `ArgKind` from the expected type of an
1638     /// argument. It has no name (`_`) and an optional source span.
1639     pub fn from_expected_ty(t: Ty<'_>, span: Option<Span>) -> ArgKind {
1640         match t.sty {
1641             ty::Tuple(ref tys) => ArgKind::Tuple(
1642                 span,
1643                 tys.iter()
1644                    .map(|ty| ("_".to_owned(), ty.sty.to_string()))
1645                    .collect::<Vec<_>>()
1646             ),
1647             _ => ArgKind::Arg("_".to_owned(), t.sty.to_string()),
1648         }
1649     }
1650 }