src/librustc/traits/error_reporting.rs

   1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 use super::{
  12     FulfillmentError,
  13     FulfillmentErrorCode,
  14     MismatchedProjectionTypes,
  15     Obligation,
  16     ObligationCause,
  17     ObligationCauseCode,
  18     OnUnimplementedDirective,
  19     OnUnimplementedNote,
  20     OutputTypeParameterMismatch,
  21     TraitNotObjectSafe,
  22     ConstEvalFailure,
  23     PredicateObligation,
  24     SelectionContext,
  25     SelectionError,
  26     ObjectSafetyViolation,
  27     Overflow,
  28 };
  29
  30 use errors::{Applicability, DiagnosticBuilder};
  31 use hir;
  32 use hir::def_id::DefId;
  33 use infer::{self, InferCtxt};
  34 use infer::type_variable::TypeVariableOrigin;
  35 use std::fmt;
  36 use syntax::ast;
  37 use session::DiagnosticMessageId;
  38 use ty::{self, AdtKind, ToPredicate, ToPolyTraitRef, Ty, TyCtxt, TypeFoldable};
  39 use ty::GenericParamDefKind;
  40 use ty::error::ExpectedFound;
  41 use ty::fast_reject;
  42 use ty::fold::TypeFolder;
  43 use ty::subst::Subst;
  44 use ty::SubtypePredicate;
  45 use util::nodemap::{FxHashMap, FxHashSet};
  46
  47 use syntax_pos::{DUMMY_SP, Span};
  48
  49 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
  50     pub fn report_fulfillment_errors(&self,
  51                                      errors: &[FulfillmentError<'tcx>],
  52                                      body_id: Option<hir::BodyId>,
  53                                      fallback_has_occurred: bool) {
  54         #[derive(Debug)]
  55         struct ErrorDescriptor<'tcx> {
  56             predicate: ty::Predicate<'tcx>,
  57             index: Option<usize>, // None if this is an old error
  58         }
  59
  60         let mut error_map : FxHashMap<_, Vec<_>> =
  61             self.reported_trait_errors.borrow().iter().map(|(&span, predicates)| {
  62                 (span, predicates.iter().map(|predicate| ErrorDescriptor {
  63                     predicate: predicate.clone(),
  64                     index: None
  65                 }).collect())
  66             }).collect();
  67
  68         for (index, error) in errors.iter().enumerate() {
  69             error_map.entry(error.obligation.cause.span).or_default().push(
  70                 ErrorDescriptor {
  71                     predicate: error.obligation.predicate.clone(),
  72                     index: Some(index)
  73                 });
  74
  75             self.reported_trait_errors.borrow_mut()
  76                 .entry(error.obligation.cause.span).or_default()
  77                 .push(error.obligation.predicate.clone());
  78         }
  79
  80         // We do this in 2 passes because we want to display errors in order, tho
  81         // maybe it *is* better to sort errors by span or something.
  82         let mut is_suppressed: Vec<bool> = errors.iter().map(|_| false).collect();
  83         for (_, error_set) in error_map.iter() {
  84             // We want to suppress "duplicate" errors with the same span.
  85             for error in error_set {
  86                 if let Some(index) = error.index {
  87                     // Suppress errors that are either:
  88                     // 1) strictly implied by another error.
  89                     // 2) implied by an error with a smaller index.
  90                     for error2 in error_set {
  91                         if error2.index.map_or(false, |index2| is_suppressed[index2]) {
  92                             // Avoid errors being suppressed by already-suppressed
  93                             // errors, to prevent all errors from being suppressed
  94                             // at once.
  95                             continue
  96                         }
  97
  98                         if self.error_implies(&error2.predicate, &error.predicate) &&
  99                             !(error2.index >= error.index &&
 100                               self.error_implies(&error.predicate, &error2.predicate))
 101                         {
 102                             info!("skipping {:?} (implied by {:?})", error, error2);
 103                             is_suppressed[index] = true;
 104                             break
 105                         }
 106                     }
 107                 }
 108             }
 109         }
 110
 111         for (error, suppressed) in errors.iter().zip(is_suppressed) {
 112             if !suppressed {
 113                 self.report_fulfillment_error(error, body_id, fallback_has_occurred);
 114             }
 115         }
 116     }
 117
 118     // returns if `cond` not occurring implies that `error` does not occur - i.e. that
 119     // `error` occurring implies that `cond` occurs.
 120     fn error_implies(&self,
 121                      cond: &ty::Predicate<'tcx>,
 122                      error: &ty::Predicate<'tcx>)
 123                      -> bool
 124     {
 125         if cond == error {
 126             return true
 127         }
 128
 129         let (cond, error) = match (cond, error) {
 130             (&ty::Predicate::Trait(..), &ty::Predicate::Trait(ref error))
 131                 => (cond, error),
 132             _ => {
 133                 // FIXME: make this work in other cases too.
 134                 return false
 135             }
 136         };
 137
 138         for implication in super::elaborate_predicates(self.tcx, vec![cond.clone()]) {
 139             if let ty::Predicate::Trait(implication) = implication {
 140                 let error = error.to_poly_trait_ref();
 141                 let implication = implication.to_poly_trait_ref();
 142                 // FIXME: I'm just not taking associated types at all here.
 143                 // Eventually I'll need to implement param-env-aware
 144                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
 145                 let param_env = ty::ParamEnv::empty();
 146                 if self.can_sub(param_env, error, implication).is_ok() {
 147                     debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
 148                     return true
 149                 }
 150             }
 151         }
 152
 153         false
 154     }
 155
 156     fn report_fulfillment_error(&self, error: &FulfillmentError<'tcx>,
 157                                 body_id: Option<hir::BodyId>,
 158                                 fallback_has_occurred: bool) {
 159         debug!("report_fulfillment_errors({:?})", error);
 160         match error.code {
 161             FulfillmentErrorCode::CodeSelectionError(ref e) => {
 162                 self.report_selection_error(&error.obligation, e, fallback_has_occurred);
 163             }
 164             FulfillmentErrorCode::CodeProjectionError(ref e) => {
 165                 self.report_projection_error(&error.obligation, e);
 166             }
 167             FulfillmentErrorCode::CodeAmbiguity => {
 168                 self.maybe_report_ambiguity(&error.obligation, body_id);
 169             }
 170             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
 171                 self.report_mismatched_types(&error.obligation.cause,
 172                                              expected_found.expected,
 173                                              expected_found.found,
 174                                              err.clone())
 175                     .emit();
 176             }
 177         }
 178     }
 179
 180     fn report_projection_error(&self,
 181                                obligation: &PredicateObligation<'tcx>,
 182                                error: &MismatchedProjectionTypes<'tcx>)
 183     {
 184         let predicate =
 185             self.resolve_type_vars_if_possible(&obligation.predicate);
 186
 187         if predicate.references_error() {
 188             return
 189         }
 190
 191         self.probe(|_| {
 192             let err_buf;
 193             let mut err = &error.err;
 194             let mut values = None;
 195
 196             // try to find the mismatched types to report the error with.
 197             //
 198             // this can fail if the problem was higher-ranked, in which
 199             // cause I have no idea for a good error message.
 200             if let ty::Predicate::Projection(ref data) = predicate {
 201                 let mut selcx = SelectionContext::new(self);
 202                 let (data, _) = self.replace_late_bound_regions_with_fresh_var(
 203                     obligation.cause.span,
 204                     infer::LateBoundRegionConversionTime::HigherRankedType,
 205                     data);
 206                 let mut obligations = vec![];
 207                 let normalized_ty = super::normalize_projection_type(
 208                     &mut selcx,
 209                     obligation.param_env,
 210                     data.projection_ty,
 211                     obligation.cause.clone(),
 212                     0,
 213                     &mut obligations
 214                 );
 215                 if let Err(error) = self.at(&obligation.cause, obligation.param_env)
 216                                         .eq(normalized_ty, data.ty) {
 217                     values = Some(infer::ValuePairs::Types(ExpectedFound {
 218                         expected: normalized_ty,
 219                         found: data.ty,
 220                     }));
 221                     err_buf = error;
 222                     err = &err_buf;
 223                 }
 224             }
 225
 226             let msg = format!("type mismatch resolving `{}`", predicate);
 227             let error_id = (DiagnosticMessageId::ErrorId(271),
 228                             Some(obligation.cause.span), msg.clone());
 229             let fresh = self.tcx.sess.one_time_diagnostics.borrow_mut().insert(error_id);
 230             if fresh {
 231                 let mut diag = struct_span_err!(
 232                     self.tcx.sess, obligation.cause.span, E0271,
 233                     "type mismatch resolving `{}`", predicate
 234                 );
 235                 self.note_type_err(&mut diag, &obligation.cause, None, values, err);
 236                 self.note_obligation_cause(&mut diag, obligation);
 237                 diag.emit();
 238             }
 239         });
 240     }
 241
 242     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
 243         /// returns the fuzzy category of a given type, or None
 244         /// if the type can be equated to any type.
 245         fn type_category<'tcx>(t: Ty<'tcx>) -> Option<u32> {
 246             match t.sty {
 247                 ty::TyBool => Some(0),
 248                 ty::TyChar => Some(1),
 249                 ty::TyStr => Some(2),
 250                 ty::TyInt(..) | ty::TyUint(..) | ty::Infer(ty::IntVar(..)) => Some(3),
 251                 ty::TyFloat(..) | ty::Infer(ty::FloatVar(..)) => Some(4),
 252                 ty::Ref(..) | ty::RawPtr(..) => Some(5),
 253                 ty::Array(..) | ty::Slice(..) => Some(6),
 254                 ty::FnDef(..) | ty::FnPtr(..) => Some(7),
 255                 ty::Dynamic(..) => Some(8),
 256                 ty::Closure(..) => Some(9),
 257                 ty::Tuple(..) => Some(10),
 258                 ty::Projection(..) => Some(11),
 259                 ty::TyParam(..) => Some(12),
 260                 ty::Anon(..) => Some(13),
 261                 ty::Never => Some(14),
 262                 ty::Adt(adt, ..) => match adt.adt_kind() {
 263                     AdtKind::Struct => Some(15),
 264                     AdtKind::Union => Some(16),
 265                     AdtKind::Enum => Some(17),
 266                 },
 267                 ty::Generator(..) => Some(18),
 268                 ty::TyForeign(..) => Some(19),
 269                 ty::GeneratorWitness(..) => Some(20),
 270                 ty::Infer(..) | ty::Error => None
 271             }
 272         }
 273
 274         match (type_category(a), type_category(b)) {
 275             (Some(cat_a), Some(cat_b)) => match (&a.sty, &b.sty) {
 276                 (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => def_a == def_b,
 277                 _ => cat_a == cat_b
 278             },
 279             // infer and error can be equated to all types
 280             _ => true
 281         }
 282     }
 283
 284     fn impl_similar_to(&self,
 285                        trait_ref: ty::PolyTraitRef<'tcx>,
 286                        obligation: &PredicateObligation<'tcx>)
 287                        -> Option<DefId>
 288     {
 289         let tcx = self.tcx;
 290         let param_env = obligation.param_env;
 291         let trait_ref = tcx.erase_late_bound_regions(&trait_ref);
 292         let trait_self_ty = trait_ref.self_ty();
 293
 294         let mut self_match_impls = vec![];
 295         let mut fuzzy_match_impls = vec![];
 296
 297         self.tcx.for_each_relevant_impl(
 298             trait_ref.def_id, trait_self_ty, |def_id| {
 299                 let impl_substs = self.fresh_substs_for_item(obligation.cause.span, def_id);
 300                 let impl_trait_ref = tcx
 301                     .impl_trait_ref(def_id)
 302                     .unwrap()
 303                     .subst(tcx, impl_substs);
 304
 305                 let impl_self_ty = impl_trait_ref.self_ty();
 306
 307                 if let Ok(..) = self.can_eq(param_env, trait_self_ty, impl_self_ty) {
 308                     self_match_impls.push(def_id);
 309
 310                     if trait_ref.substs.types().skip(1)
 311                         .zip(impl_trait_ref.substs.types().skip(1))
 312                         .all(|(u,v)| self.fuzzy_match_tys(u, v))
 313                     {
 314                         fuzzy_match_impls.push(def_id);
 315                     }
 316                 }
 317             });
 318
 319         let impl_def_id = if self_match_impls.len() == 1 {
 320             self_match_impls[0]
 321         } else if fuzzy_match_impls.len() == 1 {
 322             fuzzy_match_impls[0]
 323         } else {
 324             return None
 325         };
 326
 327         if tcx.has_attr(impl_def_id, "rustc_on_unimplemented") {
 328             Some(impl_def_id)
 329         } else {
 330             None
 331         }
 332     }
 333
 334     fn on_unimplemented_note(
 335         &self,
 336         trait_ref: ty::PolyTraitRef<'tcx>,
 337         obligation: &PredicateObligation<'tcx>) ->
 338         OnUnimplementedNote
 339     {
 340         let def_id = self.impl_similar_to(trait_ref, obligation)
 341             .unwrap_or(trait_ref.def_id());
 342         let trait_ref = *trait_ref.skip_binder();
 343
 344         let mut flags = vec![];
 345         match obligation.cause.code {
 346             ObligationCauseCode::BuiltinDerivedObligation(..) |
 347             ObligationCauseCode::ImplDerivedObligation(..) => {}
 348             _ => {
 349                 // this is a "direct", user-specified, rather than derived,
 350                 // obligation.
 351                 flags.push(("direct".to_string(), None));
 352             }
 353         }
 354
 355         if let ObligationCauseCode::ItemObligation(item) = obligation.cause.code {
 356             // FIXME: maybe also have some way of handling methods
 357             // from other traits? That would require name resolution,
 358             // which we might want to be some sort of hygienic.
 359             //
 360             // Currently I'm leaving it for what I need for `try`.
 361             if self.tcx.trait_of_item(item) == Some(trait_ref.def_id) {
 362                 let method = self.tcx.item_name(item);
 363                 flags.push(("from_method".to_string(), None));
 364                 flags.push(("from_method".to_string(), Some(method.to_string())));
 365             }
 366         }
 367
 368         if let Some(k) = obligation.cause.span.compiler_desugaring_kind() {
 369             flags.push(("from_desugaring".to_string(), None));
 370             flags.push(("from_desugaring".to_string(), Some(k.name().to_string())));
 371         }
 372         let generics = self.tcx.generics_of(def_id);
 373         let self_ty = trait_ref.self_ty();
 374         // This is also included through the generics list as `Self`,
 375         // but the parser won't allow you to use it
 376         flags.push(("_Self".to_string(), Some(self_ty.to_string())));
 377         if let Some(def) = self_ty.ty_adt_def() {
 378             // We also want to be able to select self's original
 379             // signature with no type arguments resolved
 380             flags.push(("_Self".to_string(), Some(self.tcx.type_of(def.did).to_string())));
 381         }
 382
 383         for param in generics.params.iter() {
 384             let value = match param.kind {
 385                 GenericParamDefKind::Type {..} => {
 386                     trait_ref.substs[param.index as usize].to_string()
 387                 },
 388                 GenericParamDefKind::Lifetime => continue,
 389             };
 390             let name = param.name.to_string();
 391             flags.push((name, Some(value)));
 392         }
 393
 394         if let Some(true) = self_ty.ty_adt_def().map(|def| def.did.is_local()) {
 395             flags.push(("crate_local".to_string(), None));
 396         }
 397
 398         if let Ok(Some(command)) = OnUnimplementedDirective::of_item(
 399             self.tcx, trait_ref.def_id, def_id
 400         ) {
 401             command.evaluate(self.tcx, trait_ref, &flags[..])
 402         } else {
 403             OnUnimplementedNote::empty()
 404         }
 405     }
 406
 407     fn find_similar_impl_candidates(&self,
 408                                     trait_ref: ty::PolyTraitRef<'tcx>)
 409                                     -> Vec<ty::TraitRef<'tcx>>
 410     {
 411         let simp = fast_reject::simplify_type(self.tcx,
 412                                               trait_ref.skip_binder().self_ty(),
 413                                               true);
 414         let mut impl_candidates = Vec::new();
 415
 416         match simp {
 417             Some(simp) => self.tcx.for_each_impl(trait_ref.def_id(), |def_id| {
 418                 let imp = self.tcx.impl_trait_ref(def_id).unwrap();
 419                 let imp_simp = fast_reject::simplify_type(self.tcx,
 420                                                           imp.self_ty(),
 421                                                           true);
 422                 if let Some(imp_simp) = imp_simp {
 423                     if simp != imp_simp {
 424                         return;
 425                     }
 426                 }
 427                 impl_candidates.push(imp);
 428             }),
 429             None => self.tcx.for_each_impl(trait_ref.def_id(), |def_id| {
 430                 impl_candidates.push(
 431                     self.tcx.impl_trait_ref(def_id).unwrap());
 432             })
 433         };
 434         impl_candidates
 435     }
 436
 437     fn report_similar_impl_candidates(&self,
 438                                       mut impl_candidates: Vec<ty::TraitRef<'tcx>>,
 439                                       err: &mut DiagnosticBuilder)
 440     {
 441         if impl_candidates.is_empty() {
 442             return;
 443         }
 444
 445         let len = impl_candidates.len();
 446         let end = if impl_candidates.len() <= 5 {
 447             impl_candidates.len()
 448         } else {
 449             4
 450         };
 451
 452         let normalize = |candidate| self.tcx.global_tcx().infer_ctxt().enter(|ref infcx| {
 453             let normalized = infcx
 454                 .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
 455                 .normalize(candidate)
 456                 .ok();
 457             match normalized {
 458                 Some(normalized) => format!("\n  {:?}", normalized.value),
 459                 None => format!("\n  {:?}", candidate),
 460             }
 461         });
 462
 463         // Sort impl candidates so that ordering is consistent for UI tests.
 464         let normalized_impl_candidates = &mut impl_candidates[0..end]
 465             .iter()
 466             .map(normalize)
 467             .collect::<Vec<String>>();
 468         normalized_impl_candidates.sort();
 469
 470         err.help(&format!("the following implementations were found:{}{}",
 471                           normalized_impl_candidates.join(""),
 472                           if len > 5 {
 473                               format!("\nand {} others", len - 4)
 474                           } else {
 475                               "".to_owned()
 476                           }
 477                           ));
 478     }
 479
 480     /// Reports that an overflow has occurred and halts compilation. We
 481     /// halt compilation unconditionally because it is important that
 482     /// overflows never be masked -- they basically represent computations
 483     /// whose result could not be truly determined and thus we can't say
 484     /// if the program type checks or not -- and they are unusual
 485     /// occurrences in any case.
 486     pub fn report_overflow_error<T>(&self,
 487                                     obligation: &Obligation<'tcx, T>,
 488                                     suggest_increasing_limit: bool) -> !
 489         where T: fmt::Display + TypeFoldable<'tcx>
 490     {
 491         let predicate =
 492             self.resolve_type_vars_if_possible(&obligation.predicate);
 493         let mut err = struct_span_err!(self.tcx.sess, obligation.cause.span, E0275,
 494                                        "overflow evaluating the requirement `{}`",
 495                                        predicate);
 496
 497         if suggest_increasing_limit {
 498             self.suggest_new_overflow_limit(&mut err);
 499         }
 500
 501         self.note_obligation_cause(&mut err, obligation);
 502
 503         err.emit();
 504         self.tcx.sess.abort_if_errors();
 505         bug!();
 506     }
 507
 508     /// Reports that a cycle was detected which led to overflow and halts
 509     /// compilation. This is equivalent to `report_overflow_error` except
 510     /// that we can give a more helpful error message (and, in particular,
 511     /// we do not suggest increasing the overflow limit, which is not
 512     /// going to help).
 513     pub fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 514         let cycle = self.resolve_type_vars_if_possible(&cycle.to_owned());
 515         assert!(cycle.len() > 0);
 516
 517         debug!("report_overflow_error_cycle: cycle={:?}", cycle);
 518
 519         self.report_overflow_error(&cycle[0], false);
 520     }
 521
 522     pub fn report_extra_impl_obligation(&self,
 523                                         error_span: Span,
 524                                         item_name: ast::Name,
 525                                         _impl_item_def_id: DefId,
 526                                         trait_item_def_id: DefId,
 527                                         requirement: &dyn fmt::Display)
 528                                         -> DiagnosticBuilder<'tcx>
 529     {
 530         let msg = "impl has stricter requirements than trait";
 531         let sp = self.tcx.sess.source_map().def_span(error_span);
 532
 533         let mut err = struct_span_err!(self.tcx.sess, sp, E0276, "{}", msg);
 534
 535         if let Some(trait_item_span) = self.tcx.hir.span_if_local(trait_item_def_id) {
 536             let span = self.tcx.sess.source_map().def_span(trait_item_span);
 537             err.span_label(span, format!("definition of `{}` from trait", item_name));
 538         }
 539
 540         err.span_label(sp, format!("impl has extra requirement {}", requirement));
 541
 542         err
 543     }
 544
 545
 546     /// Get the parent trait chain start
 547     fn get_parent_trait_ref(&self, code: &ObligationCauseCode<'tcx>) -> Option<String> {
 548         match code {
 549             &ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
 550                 let parent_trait_ref = self.resolve_type_vars_if_possible(
 551                     &data.parent_trait_ref);
 552                 match self.get_parent_trait_ref(&data.parent_code) {
 553                     Some(t) => Some(t),
 554                     None => Some(parent_trait_ref.skip_binder().self_ty().to_string()),
 555                 }
 556             }
 557             _ => None,
 558         }
 559     }
 560
 561     pub fn report_selection_error(&self,
 562                                   obligation: &PredicateObligation<'tcx>,
 563                                   error: &SelectionError<'tcx>,
 564                                   fallback_has_occurred: bool)
 565     {
 566         let span = obligation.cause.span;
 567
 568         let mut err = match *error {
 569             SelectionError::Unimplemented => {
 570                 if let ObligationCauseCode::CompareImplMethodObligation {
 571                     item_name, impl_item_def_id, trait_item_def_id,
 572                 } = obligation.cause.code {
 573                     self.report_extra_impl_obligation(
 574                         span,
 575                         item_name,
 576                         impl_item_def_id,
 577                         trait_item_def_id,
 578                         &format!("`{}`", obligation.predicate))
 579                         .emit();
 580                     return;
 581                 }
 582                 match obligation.predicate {
 583                     ty::Predicate::Trait(ref trait_predicate) => {
 584                         let trait_predicate =
 585                             self.resolve_type_vars_if_possible(trait_predicate);
 586
 587                         if self.tcx.sess.has_errors() && trait_predicate.references_error() {
 588                             return;
 589                         }
 590                         let trait_ref = trait_predicate.to_poly_trait_ref();
 591                         let (post_message, pre_message) =
 592                             self.get_parent_trait_ref(&obligation.cause.code)
 593                                 .map(|t| (format!(" in `{}`", t), format!("within `{}`, ", t)))
 594                             .unwrap_or((String::new(), String::new()));
 595
 596                         let OnUnimplementedNote { message, label, note }
 597                             = self.on_unimplemented_note(trait_ref, obligation);
 598                         let have_alt_message = message.is_some() || label.is_some();
 599
 600                         let mut err = struct_span_err!(
 601                             self.tcx.sess,
 602                             span,
 603                             E0277,
 604                             "{}",
 605                             message.unwrap_or_else(|| {
 606                                 format!("the trait bound `{}` is not satisfied{}",
 607                                          trait_ref.to_predicate(), post_message)
 608                             }));
 609
 610                         let explanation =
 611                             if obligation.cause.code == ObligationCauseCode::MainFunctionType {
 612                                 "consider using `()`, or a `Result`".to_owned()
 613                             } else {
 614                                 format!("{}the trait `{}` is not implemented for `{}`",
 615                                         pre_message,
 616                                         trait_ref,
 617                                         trait_ref.self_ty())
 618                             };
 619
 620                         if let Some(ref s) = label {
 621                             // If it has a custom "#[rustc_on_unimplemented]"
 622                             // error message, let's display it as the label!
 623                             err.span_label(span, s.as_str());
 624                             err.help(&explanation);
 625                         } else {
 626                             err.span_label(span, explanation);
 627                         }
 628                         if let Some(ref s) = note {
 629                             // If it has a custom "#[rustc_on_unimplemented]" note, let's display it
 630                             err.note(s.as_str());
 631                         }
 632
 633                         self.suggest_borrow_on_unsized_slice(&obligation.cause.code, &mut err);
 634                         self.suggest_remove_reference(&obligation, &mut err, &trait_ref);
 635
 636                         // Try to report a help message
 637                         if !trait_ref.has_infer_types() &&
 638                             self.predicate_can_apply(obligation.param_env, trait_ref) {
 639                             // If a where-clause may be useful, remind the
 640                             // user that they can add it.
 641                             //
 642                             // don't display an on-unimplemented note, as
 643                             // these notes will often be of the form
 644                             //     "the type `T` can't be frobnicated"
 645                             // which is somewhat confusing.
 646                             err.help(&format!("consider adding a `where {}` bound",
 647                                                 trait_ref.to_predicate()));
 648                         } else if !have_alt_message {
 649                             // Can't show anything else useful, try to find similar impls.
 650                             let impl_candidates = self.find_similar_impl_candidates(trait_ref);
 651                             self.report_similar_impl_candidates(impl_candidates, &mut err);
 652                         }
 653
 654                         // If this error is due to `!: Trait` not implemented but `(): Trait` is
 655                         // implemented, and fallback has occurred, then it could be due to a
 656                         // variable that used to fallback to `()` now falling back to `!`. Issue a
 657                         // note informing about the change in behaviour.
 658                         if trait_predicate.skip_binder().self_ty().is_never()
 659                             && fallback_has_occurred
 660                         {
 661                             let predicate = trait_predicate.map_bound(|mut trait_pred| {
 662                                 trait_pred.trait_ref.substs = self.tcx.mk_substs_trait(
 663                                     self.tcx.mk_nil(),
 664                                     &trait_pred.trait_ref.substs[1..],
 665                                 );
 666                                 trait_pred
 667                             });
 668                             let unit_obligation = Obligation {
 669                                 predicate: ty::Predicate::Trait(predicate),
 670                                 .. obligation.clone()
 671                             };
 672                             if self.predicate_may_hold(&unit_obligation) {
 673                                 err.note("the trait is implemented for `()`. \
 674                                          Possibly this error has been caused by changes to \
 675                                          Rust's type-inference algorithm \
 676                                          (see: https://github.com/rust-lang/rust/issues/48950 \
 677                                          for more info). Consider whether you meant to use the \
 678                                          type `()` here instead.");
 679                             }
 680                         }
 681
 682                         err
 683                     }
 684
 685                     ty::Predicate::Subtype(ref predicate) => {
 686                         // Errors for Subtype predicates show up as
 687                         // `FulfillmentErrorCode::CodeSubtypeError`,
 688                         // not selection error.
 689                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
 690                     }
 691
 692                     ty::Predicate::RegionOutlives(ref predicate) => {
 693                         let predicate = self.resolve_type_vars_if_possible(predicate);
 694                         let err = self.region_outlives_predicate(&obligation.cause,
 695                                                                     &predicate).err().unwrap();
 696                         struct_span_err!(self.tcx.sess, span, E0279,
 697                             "the requirement `{}` is not satisfied (`{}`)",
 698                             predicate, err)
 699                     }
 700
 701                     ty::Predicate::Projection(..) | ty::Predicate::TypeOutlives(..) => {
 702                         let predicate =
 703                             self.resolve_type_vars_if_possible(&obligation.predicate);
 704                         struct_span_err!(self.tcx.sess, span, E0280,
 705                             "the requirement `{}` is not satisfied",
 706                             predicate)
 707                     }
 708
 709                     ty::Predicate::ObjectSafe(trait_def_id) => {
 710                         let violations = self.tcx.object_safety_violations(trait_def_id);
 711                         self.tcx.report_object_safety_error(span,
 712                                                             trait_def_id,
 713                                                             violations)
 714                     }
 715
 716                     ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => {
 717                         let found_kind = self.closure_kind(closure_def_id, closure_substs).unwrap();
 718                         let closure_span = self.tcx.sess.source_map()
 719                             .def_span(self.tcx.hir.span_if_local(closure_def_id).unwrap());
 720                         let node_id = self.tcx.hir.as_local_node_id(closure_def_id).unwrap();
 721                         let mut err = struct_span_err!(
 722                             self.tcx.sess, closure_span, E0525,
 723                             "expected a closure that implements the `{}` trait, \
 724                                 but this closure only implements `{}`",
 725                             kind,
 726                             found_kind);
 727
 728                         err.span_label(
 729                             closure_span,
 730                             format!("this closure implements `{}`, not `{}`", found_kind, kind));
 731                         err.span_label(
 732                             obligation.cause.span,
 733                             format!("the requirement to implement `{}` derives from here", kind));
 734
 735                         // Additional context information explaining why the closure only implements
 736                         // a particular trait.
 737                         if let Some(tables) = self.in_progress_tables {
 738                             let tables = tables.borrow();
 739                             let closure_hir_id = self.tcx.hir.node_to_hir_id(node_id);
 740                             match (found_kind, tables.closure_kind_origins().get(closure_hir_id)) {
 741                                 (ty::ClosureKind::FnOnce, Some((span, name))) => {
 742                                     err.span_label(*span, format!(
 743                                         "closure is `FnOnce` because it moves the \
 744                                          variable `{}` out of its environment", name));
 745                                 },
 746                                 (ty::ClosureKind::FnMut, Some((span, name))) => {
 747                                     err.span_label(*span, format!(
 748                                         "closure is `FnMut` because it mutates the \
 749                                          variable `{}` here", name));
 750                                 },
 751                                 _ => {}
 752                             }
 753                         }
 754
 755                         err.emit();
 756                         return;
 757                     }
 758
 759                     ty::Predicate::WellFormed(ty) => {
 760                         // WF predicates cannot themselves make
 761                         // errors. They can only block due to
 762                         // ambiguity; otherwise, they always
 763                         // degenerate into other obligations
 764                         // (which may fail).
 765                         span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 766                     }
 767
 768                     ty::Predicate::ConstEvaluatable(..) => {
 769                         // Errors for `ConstEvaluatable` predicates show up as
 770                         // `SelectionError::ConstEvalFailure`,
 771                         // not `Unimplemented`.
 772                         span_bug!(span,
 773                             "const-evaluatable requirement gave wrong error: `{:?}`", obligation)
 774                     }
 775                 }
 776             }
 777
 778             OutputTypeParameterMismatch(ref found_trait_ref, ref expected_trait_ref, _) => {
 779                 let found_trait_ref = self.resolve_type_vars_if_possible(&*found_trait_ref);
 780                 let expected_trait_ref = self.resolve_type_vars_if_possible(&*expected_trait_ref);
 781                 if expected_trait_ref.self_ty().references_error() {
 782                     return;
 783                 }
 784                 let found_trait_ty = found_trait_ref.self_ty();
 785
 786                 let found_did = match found_trait_ty.sty {
 787                     ty::Closure(did, _) |
 788                     ty::TyForeign(did) |
 789                     ty::FnDef(did, _) => Some(did),
 790                     ty::Adt(def, _) => Some(def.did),
 791                     _ => None,
 792                 };
 793                 let found_span = found_did.and_then(|did| {
 794                     self.tcx.hir.span_if_local(did)
 795                 }).map(|sp| self.tcx.sess.source_map().def_span(sp)); // the sp could be an fn def
 796
 797                 let found = match found_trait_ref.skip_binder().substs.type_at(1).sty {
 798                     ty::Tuple(ref tys) => tys.iter()
 799                         .map(|_| ArgKind::empty()).collect::<Vec<_>>(),
 800                     _ => vec![ArgKind::empty()],
 801                 };
 802                 let expected = match expected_trait_ref.skip_binder().substs.type_at(1).sty {
 803                     ty::Tuple(ref tys) => tys.iter()
 804                         .map(|t| match t.sty {
 805                             ty::TyKind::Tuple(ref tys) => ArgKind::Tuple(
 806                                 Some(span),
 807                                 tys.iter()
 808                                     .map(|ty| ("_".to_owned(), ty.sty.to_string()))
 809                                     .collect::<Vec<_>>()
 810                             ),
 811                             _ => ArgKind::Arg("_".to_owned(), t.sty.to_string()),
 812                         }).collect(),
 813                     ref sty => vec![ArgKind::Arg("_".to_owned(), sty.to_string())],
 814                 };
 815                 if found.len() == expected.len() {
 816                     self.report_closure_arg_mismatch(span,
 817                                                      found_span,
 818                                                      found_trait_ref,
 819                                                      expected_trait_ref)
 820                 } else {
 821                     let (closure_span, found) = found_did
 822                         .and_then(|did| self.tcx.hir.get_if_local(did))
 823                         .map(|node| {
 824                             let (found_span, found) = self.get_fn_like_arguments(node);
 825                             (Some(found_span), found)
 826                         }).unwrap_or((found_span, found));
 827
 828                     self.report_arg_count_mismatch(span,
 829                                                    closure_span,
 830                                                    expected,
 831                                                    found,
 832                                                    found_trait_ty.is_closure())
 833                 }
 834             }
 835
 836             TraitNotObjectSafe(did) => {
 837                 let violations = self.tcx.object_safety_violations(did);
 838                 self.tcx.report_object_safety_error(span, did,
 839                                                     violations)
 840             }
 841
 842             ConstEvalFailure(ref err) => {
 843                 match err.struct_error(
 844                     self.tcx.at(span),
 845                     "could not evaluate constant expression",
 846                 ) {
 847                     Some(err) => err,
 848                     None => return,
 849                 }
 850             }
 851
 852             Overflow => {
 853                 bug!("overflow should be handled before the `report_selection_error` path");
 854             }
 855         };
 856         self.note_obligation_cause(&mut err, obligation);
 857         err.emit();
 858     }
 859
 860     /// When encountering an assignment of an unsized trait, like `let x = ""[..];`, provide a
 861     /// suggestion to borrow the initializer in order to use have a slice instead.
 862     fn suggest_borrow_on_unsized_slice(&self,
 863                                        code: &ObligationCauseCode<'tcx>,
 864                                        err: &mut DiagnosticBuilder<'tcx>) {
 865         if let &ObligationCauseCode::VariableType(node_id) = code {
 866             let parent_node = self.tcx.hir.get_parent_node(node_id);
 867             if let Some(hir::map::NodeLocal(ref local)) = self.tcx.hir.find(parent_node) {
 868                 if let Some(ref expr) = local.init {
 869                     if let hir::ExprKind::Index(_, _) = expr.node {
 870                         if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(expr.span) {
 871                             err.span_suggestion_with_applicability(
 872                                 expr.span,
 873                                 "consider borrowing here",
 874                                 format!("&{}", snippet),
 875                                 Applicability::MachineApplicable
 876                             );
 877                         }
 878                     }
 879                 }
 880             }
 881         }
 882     }
 883
 884     /// Whenever references are used by mistake, like `for (i, e) in &vec.iter().enumerate()`,
 885     /// suggest removing these references until we reach a type that implements the trait.
 886     fn suggest_remove_reference(&self,
 887                                 obligation: &PredicateObligation<'tcx>,
 888                                 err: &mut DiagnosticBuilder<'tcx>,
 889                                 trait_ref: &ty::Binder<ty::TraitRef<'tcx>>) {
 890         let trait_ref = trait_ref.skip_binder();
 891         let span = obligation.cause.span;
 892
 893         if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
 894             let refs_number = snippet.chars()
 895                 .filter(|c| !c.is_whitespace())
 896                 .take_while(|c| *c == '&')
 897                 .count();
 898
 899             let mut trait_type = trait_ref.self_ty();
 900
 901             for refs_remaining in 0..refs_number {
 902                 if let ty::TyKind::Ref(_, t_type, _) = trait_type.sty {
 903                     trait_type = t_type;
 904
 905                     let substs = self.tcx.mk_substs_trait(trait_type, &[]);
 906                     let new_trait_ref = ty::TraitRef::new(trait_ref.def_id, substs);
 907                     let new_obligation = Obligation::new(ObligationCause::dummy(),
 908                                                          obligation.param_env,
 909                                                          new_trait_ref.to_predicate());
 910
 911                     if self.predicate_may_hold(&new_obligation) {
 912                         let sp = self.tcx.sess.source_map()
 913                             .span_take_while(span, |c| c.is_whitespace() || *c == '&');
 914
 915                         let remove_refs = refs_remaining + 1;
 916                         let format_str = format!("consider removing {} leading `&`-references",
 917                                                  remove_refs);
 918
 919                         err.span_suggestion_short_with_applicability(
 920                             sp, &format_str, String::from(""), Applicability::MachineApplicable
 921                         );
 922                         break;
 923                     }
 924                 } else {
 925                     break;
 926                 }
 927             }
 928         }
 929     }
 930
 931     /// Given some node representing a fn-like thing in the HIR map,
 932     /// returns a span and `ArgKind` information that describes the
 933     /// arguments it expects. This can be supplied to
 934     /// `report_arg_count_mismatch`.
 935     pub fn get_fn_like_arguments(&self, node: hir::map::Node) -> (Span, Vec<ArgKind>) {
 936         match node {
 937             hir::map::NodeExpr(&hir::Expr {
 938                 node: hir::ExprKind::Closure(_, ref _decl, id, span, _),
 939                 ..
 940             }) => {
 941                 (self.tcx.sess.source_map().def_span(span), self.tcx.hir.body(id).arguments.iter()
 942                     .map(|arg| {
 943                         if let hir::Pat {
 944                             node: hir::PatKind::Tuple(args, _),
 945                             span,
 946                             ..
 947                         } = arg.pat.clone().into_inner() {
 948                             ArgKind::Tuple(
 949                                 Some(span),
 950                                 args.iter().map(|pat| {
 951                                     let snippet = self.tcx.sess.source_map()
 952                                         .span_to_snippet(pat.span).unwrap();
 953                                     (snippet, "_".to_owned())
 954                                 }).collect::<Vec<_>>(),
 955                             )
 956                         } else {
 957                             let name = self.tcx.sess.source_map()
 958                                 .span_to_snippet(arg.pat.span).unwrap();
 959                             ArgKind::Arg(name, "_".to_owned())
 960                         }
 961                     })
 962                     .collect::<Vec<ArgKind>>())
 963             }
 964             hir::map::NodeItem(&hir::Item {
 965                 span,
 966                 node: hir::ItemKind::Fn(ref decl, ..),
 967                 ..
 968             }) |
 969             hir::map::NodeImplItem(&hir::ImplItem {
 970                 span,
 971                 node: hir::ImplItemKind::Method(hir::MethodSig { ref decl, .. }, _),
 972                 ..
 973             }) |
 974             hir::map::NodeTraitItem(&hir::TraitItem {
 975                 span,
 976                 node: hir::TraitItemKind::Method(hir::MethodSig { ref decl, .. }, _),
 977                 ..
 978             }) => {
 979                 (self.tcx.sess.source_map().def_span(span), decl.inputs.iter()
 980                         .map(|arg| match arg.clone().node {
 981                     hir::TyKind::Tup(ref tys) => ArgKind::Tuple(
 982                         Some(arg.span),
 983                         tys.iter()
 984                             .map(|_| ("_".to_owned(), "_".to_owned()))
 985                             .collect::<Vec<_>>(),
 986                     ),
 987                     _ => ArgKind::Arg("_".to_owned(), "_".to_owned())
 988                 }).collect::<Vec<ArgKind>>())
 989             }
 990             hir::map::NodeVariant(&hir::Variant {
 991                 span,
 992                 node: hir::VariantKind {
 993                     data: hir::VariantData::Tuple(ref fields, _),
 994                     ..
 995                 },
 996                 ..
 997             }) => {
 998                 (self.tcx.sess.source_map().def_span(span),
 999                  fields.iter().map(|field| {
1000                      ArgKind::Arg(field.ident.to_string(), "_".to_string())
1001                  }).collect::<Vec<_>>())
1002             }
1003             hir::map::NodeStructCtor(ref variant_data) => {
1004                 (self.tcx.sess.source_map().def_span(self.tcx.hir.span(variant_data.id())),
1005                  variant_data.fields()
1006                     .iter().map(|_| ArgKind::Arg("_".to_owned(), "_".to_owned()))
1007                     .collect())
1008             }
1009             _ => panic!("non-FnLike node found: {:?}", node),
1010         }
1011     }
1012
1013     /// Reports an error when the number of arguments needed by a
1014     /// trait match doesn't match the number that the expression
1015     /// provides.
1016     pub fn report_arg_count_mismatch(
1017         &self,
1018         span: Span,
1019         found_span: Option<Span>,
1020         expected_args: Vec<ArgKind>,
1021         found_args: Vec<ArgKind>,
1022         is_closure: bool,
1023     ) -> DiagnosticBuilder<'tcx> {
1024         let kind = if is_closure { "closure" } else { "function" };
1025
1026         let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
1027             let arg_length = arguments.len();
1028             let distinct = match &other[..] {
1029                 &[ArgKind::Tuple(..)] => true,
1030                 _ => false,
1031             };
1032             match (arg_length, arguments.get(0)) {
1033                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
1034                     format!("a single {}-tuple as argument", fields.len())
1035                 }
1036                 _ => format!("{} {}argument{}",
1037                              arg_length,
1038                              if distinct && arg_length > 1 { "distinct " } else { "" },
1039                              if arg_length == 1 { "" } else { "s" }),
1040             }
1041         };
1042
1043         let expected_str = args_str(&expected_args, &found_args);
1044         let found_str = args_str(&found_args, &expected_args);
1045
1046         let mut err = struct_span_err!(
1047             self.tcx.sess,
1048             span,
1049             E0593,
1050             "{} is expected to take {}, but it takes {}",
1051             kind,
1052             expected_str,
1053             found_str,
1054         );
1055
1056         err.span_label(span, format!( "expected {} that takes {}", kind, expected_str));
1057
1058         if let Some(found_span) = found_span {
1059             err.span_label(found_span, format!("takes {}", found_str));
1060
1061             // Suggest to take and ignore the arguments with expected_args_length `_`s if
1062             // found arguments is empty (assume the user just wants to ignore args in this case).
1063             // For example, if `expected_args_length` is 2, suggest `|_, _|`.
1064             if found_args.is_empty() && is_closure {
1065                 let underscores = "_".repeat(expected_args.len())
1066                                       .split("")
1067                                       .filter(|s| !s.is_empty())
1068                                       .collect::<Vec<_>>()
1069                                       .join(", ");
1070                 err.span_suggestion_with_applicability(
1071                     found_span,
1072                     &format!(
1073                         "consider changing the closure to take and ignore the expected argument{}",
1074                         if expected_args.len() < 2 {
1075                             ""
1076                         } else {
1077                             "s"
1078                         }
1079                     ),
1080                     format!("|{}|", underscores),
1081                     Applicability::MachineApplicable,
1082                 );
1083             }
1084
1085             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
1086                 if fields.len() == expected_args.len() {
1087                     let sugg = fields.iter()
1088                         .map(|(name, _)| name.to_owned())
1089                         .collect::<Vec<String>>().join(", ");
1090                     err.span_suggestion_with_applicability(found_span,
1091                                                            "change the closure to take multiple \
1092                                                             arguments instead of a single tuple",
1093                                                            format!("|{}|", sugg),
1094                                                            Applicability::MachineApplicable);
1095                 }
1096             }
1097             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..] {
1098                 if fields.len() == found_args.len() && is_closure {
1099                     let sugg = format!(
1100                         "|({}){}|",
1101                         found_args.iter()
1102                             .map(|arg| match arg {
1103                                 ArgKind::Arg(name, _) => name.to_owned(),
1104                                 _ => "_".to_owned(),
1105                             })
1106                             .collect::<Vec<String>>()
1107                             .join(", "),
1108                         // add type annotations if available
1109                         if found_args.iter().any(|arg| match arg {
1110                             ArgKind::Arg(_, ty) => ty != "_",
1111                             _ => false,
1112                         }) {
1113                             format!(": ({})",
1114                                     fields.iter()
1115                                         .map(|(_, ty)| ty.to_owned())
1116                                         .collect::<Vec<String>>()
1117                                         .join(", "))
1118                         } else {
1119                             "".to_owned()
1120                         },
1121                     );
1122                     err.span_suggestion_with_applicability(
1123                         found_span,
1124                         "change the closure to accept a tuple instead of \
1125                          individual arguments",
1126                         sugg,
1127                         Applicability::MachineApplicable
1128                     );
1129                 }
1130             }
1131         }
1132
1133         err
1134     }
1135
1136     fn report_closure_arg_mismatch(&self,
1137                            span: Span,
1138                            found_span: Option<Span>,
1139                            expected_ref: ty::PolyTraitRef<'tcx>,
1140                            found: ty::PolyTraitRef<'tcx>)
1141         -> DiagnosticBuilder<'tcx>
1142     {
1143         fn build_fn_sig_string<'a, 'gcx, 'tcx>(tcx: ty::TyCtxt<'a, 'gcx, 'tcx>,
1144                                                trait_ref: &ty::TraitRef<'tcx>) -> String {
1145             let inputs = trait_ref.substs.type_at(1);
1146             let sig = if let ty::Tuple(inputs) = inputs.sty {
1147                 tcx.mk_fn_sig(
1148                     inputs.iter().map(|&x| x),
1149                     tcx.mk_infer(ty::TyVar(ty::TyVid { index: 0 })),
1150                     false,
1151                     hir::Unsafety::Normal,
1152                     ::rustc_target::spec::abi::Abi::Rust
1153                 )
1154             } else {
1155                 tcx.mk_fn_sig(
1156                     ::std::iter::once(inputs),
1157                     tcx.mk_infer(ty::TyVar(ty::TyVid { index: 0 })),
1158                     false,
1159                     hir::Unsafety::Normal,
1160                     ::rustc_target::spec::abi::Abi::Rust
1161                 )
1162             };
1163             ty::Binder::bind(sig).to_string()
1164         }
1165
1166         let argument_is_closure = expected_ref.skip_binder().substs.type_at(0).is_closure();
1167         let mut err = struct_span_err!(self.tcx.sess, span, E0631,
1168                                        "type mismatch in {} arguments",
1169                                        if argument_is_closure { "closure" } else { "function" });
1170
1171         let found_str = format!(
1172             "expected signature of `{}`",
1173             build_fn_sig_string(self.tcx, found.skip_binder())
1174         );
1175         err.span_label(span, found_str);
1176
1177         let found_span = found_span.unwrap_or(span);
1178         let expected_str = format!(
1179             "found signature of `{}`",
1180             build_fn_sig_string(self.tcx, expected_ref.skip_binder())
1181         );
1182         err.span_label(found_span, expected_str);
1183
1184         err
1185     }
1186 }
1187
1188 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
1189     pub fn recursive_type_with_infinite_size_error(self,
1190                                                    type_def_id: DefId)
1191                                                    -> DiagnosticBuilder<'tcx>
1192     {
1193         assert!(type_def_id.is_local());
1194         let span = self.hir.span_if_local(type_def_id).unwrap();
1195         let span = self.sess.source_map().def_span(span);
1196         let mut err = struct_span_err!(self.sess, span, E0072,
1197                                        "recursive type `{}` has infinite size",
1198                                        self.item_path_str(type_def_id));
1199         err.span_label(span, "recursive type has infinite size");
1200         err.help(&format!("insert indirection (e.g., a `Box`, `Rc`, or `&`) \
1201                            at some point to make `{}` representable",
1202                           self.item_path_str(type_def_id)));
1203         err
1204     }
1205
1206     pub fn report_object_safety_error(self,
1207                                       span: Span,
1208                                       trait_def_id: DefId,
1209                                       violations: Vec<ObjectSafetyViolation>)
1210                                       -> DiagnosticBuilder<'tcx>
1211     {
1212         let trait_str = self.item_path_str(trait_def_id);
1213         let span = self.sess.source_map().def_span(span);
1214         let mut err = struct_span_err!(
1215             self.sess, span, E0038,
1216             "the trait `{}` cannot be made into an object",
1217             trait_str);
1218         err.span_label(span, format!("the trait `{}` cannot be made into an object", trait_str));
1219
1220         let mut reported_violations = FxHashSet();
1221         for violation in violations {
1222             if !reported_violations.insert(violation.clone()) {
1223                 continue;
1224             }
1225             err.note(&violation.error_msg());
1226         }
1227         err
1228     }
1229 }
1230
1231 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
1232     fn maybe_report_ambiguity(&self, obligation: &PredicateObligation<'tcx>,
1233                               body_id: Option<hir::BodyId>) {
1234         // Unable to successfully determine, probably means
1235         // insufficient type information, but could mean
1236         // ambiguous impls. The latter *ought* to be a
1237         // coherence violation, so we don't report it here.
1238
1239         let predicate = self.resolve_type_vars_if_possible(&obligation.predicate);
1240         let span = obligation.cause.span;
1241
1242         debug!("maybe_report_ambiguity(predicate={:?}, obligation={:?})",
1243                predicate,
1244                obligation);
1245
1246         // Ambiguity errors are often caused as fallout from earlier
1247         // errors. So just ignore them if this infcx is tainted.
1248         if self.is_tainted_by_errors() {
1249             return;
1250         }
1251
1252         match predicate {
1253             ty::Predicate::Trait(ref data) => {
1254                 let trait_ref = data.to_poly_trait_ref();
1255                 let self_ty = trait_ref.self_ty();
1256                 if predicate.references_error() {
1257                     return;
1258                 }
1259                 // Typically, this ambiguity should only happen if
1260                 // there are unresolved type inference variables
1261                 // (otherwise it would suggest a coherence
1262                 // failure). But given #21974 that is not necessarily
1263                 // the case -- we can have multiple where clauses that
1264                 // are only distinguished by a region, which results
1265                 // in an ambiguity even when all types are fully
1266                 // known, since we don't dispatch based on region
1267                 // relationships.
1268
1269                 // This is kind of a hack: it frequently happens that some earlier
1270                 // error prevents types from being fully inferred, and then we get
1271                 // a bunch of uninteresting errors saying something like "<generic
1272                 // #0> doesn't implement Sized".  It may even be true that we
1273                 // could just skip over all checks where the self-ty is an
1274                 // inference variable, but I was afraid that there might be an
1275                 // inference variable created, registered as an obligation, and
1276                 // then never forced by writeback, and hence by skipping here we'd
1277                 // be ignoring the fact that we don't KNOW the type works
1278                 // out. Though even that would probably be harmless, given that
1279                 // we're only talking about builtin traits, which are known to be
1280                 // inhabited. But in any case I just threw in this check for
1281                 // has_errors() to be sure that compilation isn't happening
1282                 // anyway. In that case, why inundate the user.
1283                 if !self.tcx.sess.has_errors() {
1284                     if
1285                         self.tcx.lang_items().sized_trait()
1286                         .map_or(false, |sized_id| sized_id == trait_ref.def_id())
1287                     {
1288                         self.need_type_info_err(body_id, span, self_ty).emit();
1289                     } else {
1290                         let mut err = struct_span_err!(self.tcx.sess,
1291                                                         span, E0283,
1292                                                         "type annotations required: \
1293                                                         cannot resolve `{}`",
1294                                                         predicate);
1295                         self.note_obligation_cause(&mut err, obligation);
1296                         err.emit();
1297                     }
1298                 }
1299             }
1300
1301             ty::Predicate::WellFormed(ty) => {
1302                 // Same hacky approach as above to avoid deluging user
1303                 // with error messages.
1304                 if !ty.references_error() && !self.tcx.sess.has_errors() {
1305                     self.need_type_info_err(body_id, span, ty).emit();
1306                 }
1307             }
1308
1309             ty::Predicate::Subtype(ref data) => {
1310                 if data.references_error() || self.tcx.sess.has_errors() {
1311                     // no need to overload user in such cases
1312                 } else {
1313                     let &SubtypePredicate { a_is_expected: _, a, b } = data.skip_binder();
1314                     // both must be type variables, or the other would've been instantiated
1315                     assert!(a.is_ty_var() && b.is_ty_var());
1316                     self.need_type_info_err(body_id,
1317                                             obligation.cause.span,
1318                                             a).emit();
1319                 }
1320             }
1321
1322             _ => {
1323                 if !self.tcx.sess.has_errors() {
1324                     let mut err = struct_span_err!(self.tcx.sess,
1325                                                    obligation.cause.span, E0284,
1326                                                    "type annotations required: \
1327                                                     cannot resolve `{}`",
1328                                                    predicate);
1329                     self.note_obligation_cause(&mut err, obligation);
1330                     err.emit();
1331                 }
1332             }
1333         }
1334     }
1335
1336     /// Returns whether the trait predicate may apply for *some* assignment
1337     /// to the type parameters.
1338     fn predicate_can_apply(&self,
1339                            param_env: ty::ParamEnv<'tcx>,
1340                            pred: ty::PolyTraitRef<'tcx>)
1341                            -> bool {
1342         struct ParamToVarFolder<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
1343             infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
1344             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>
1345         }
1346
1347         impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for ParamToVarFolder<'a, 'gcx, 'tcx> {
1348             fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> { self.infcx.tcx }
1349
1350             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
1351                 if let ty::TyParam(ty::ParamTy {name, ..}) = ty.sty {
1352                     let infcx = self.infcx;
1353                     self.var_map.entry(ty).or_insert_with(||
1354                         infcx.next_ty_var(
1355                             TypeVariableOrigin::TypeParameterDefinition(DUMMY_SP, name)))
1356                 } else {
1357                     ty.super_fold_with(self)
1358                 }
1359             }
1360         }
1361
1362         self.probe(|_| {
1363             let mut selcx = SelectionContext::new(self);
1364
1365             let cleaned_pred = pred.fold_with(&mut ParamToVarFolder {
1366                 infcx: self,
1367                 var_map: FxHashMap()
1368             });
1369
1370             let cleaned_pred = super::project::normalize(
1371                 &mut selcx,
1372                 param_env,
1373                 ObligationCause::dummy(),
1374                 &cleaned_pred
1375             ).value;
1376
1377             let obligation = Obligation::new(
1378                 ObligationCause::dummy(),
1379                 param_env,
1380                 cleaned_pred.to_predicate()
1381             );
1382
1383             self.predicate_may_hold(&obligation)
1384         })
1385     }
1386
1387     fn note_obligation_cause<T>(&self,
1388                                 err: &mut DiagnosticBuilder,
1389                                 obligation: &Obligation<'tcx, T>)
1390         where T: fmt::Display
1391     {
1392         self.note_obligation_cause_code(err,
1393                                         &obligation.predicate,
1394                                         &obligation.cause.code,
1395                                         &mut vec![]);
1396     }
1397
1398     fn note_obligation_cause_code<T>(&self,
1399                                      err: &mut DiagnosticBuilder,
1400                                      predicate: &T,
1401                                      cause_code: &ObligationCauseCode<'tcx>,
1402                                      obligated_types: &mut Vec<&ty::TyS<'tcx>>)
1403         where T: fmt::Display
1404     {
1405         let tcx = self.tcx;
1406         match *cause_code {
1407             ObligationCauseCode::ExprAssignable |
1408             ObligationCauseCode::MatchExpressionArm { .. } |
1409             ObligationCauseCode::IfExpression |
1410             ObligationCauseCode::IfExpressionWithNoElse |
1411             ObligationCauseCode::MainFunctionType |
1412             ObligationCauseCode::StartFunctionType |
1413             ObligationCauseCode::IntrinsicType |
1414             ObligationCauseCode::MethodReceiver |
1415             ObligationCauseCode::ReturnNoExpression |
1416             ObligationCauseCode::MiscObligation => {
1417             }
1418             ObligationCauseCode::SliceOrArrayElem => {
1419                 err.note("slice and array elements must have `Sized` type");
1420             }
1421             ObligationCauseCode::TupleElem => {
1422                 err.note("only the last element of a tuple may have a dynamically sized type");
1423             }
1424             ObligationCauseCode::ProjectionWf(data) => {
1425                 err.note(&format!("required so that the projection `{}` is well-formed",
1426                                   data));
1427             }
1428             ObligationCauseCode::ReferenceOutlivesReferent(ref_ty) => {
1429                 err.note(&format!("required so that reference `{}` does not outlive its referent",
1430                                   ref_ty));
1431             }
1432             ObligationCauseCode::ObjectTypeBound(object_ty, region) => {
1433                 err.note(&format!("required so that the lifetime bound of `{}` for `{}` \
1434                                    is satisfied",
1435                                   region, object_ty));
1436             }
1437             ObligationCauseCode::ItemObligation(item_def_id) => {
1438                 let item_name = tcx.item_path_str(item_def_id);
1439                 let msg = format!("required by `{}`", item_name);
1440                 if let Some(sp) = tcx.hir.span_if_local(item_def_id) {
1441                     let sp = tcx.sess.source_map().def_span(sp);
1442                     err.span_note(sp, &msg);
1443                 } else {
1444                     err.note(&msg);
1445                 }
1446             }
1447             ObligationCauseCode::ObjectCastObligation(object_ty) => {
1448                 err.note(&format!("required for the cast to the object type `{}`",
1449                                   self.ty_to_string(object_ty)));
1450             }
1451             ObligationCauseCode::RepeatVec => {
1452                 err.note("the `Copy` trait is required because the \
1453                           repeated element will be copied");
1454             }
1455             ObligationCauseCode::VariableType(_) => {
1456                 err.note("all local variables must have a statically known size");
1457                 if !self.tcx.features().unsized_locals {
1458                     err.help("unsized locals are gated as an unstable feature");
1459                 }
1460             }
1461             ObligationCauseCode::SizedArgumentType => {
1462                 err.note("all function arguments must have a statically known size");
1463                 if !self.tcx.features().unsized_locals {
1464                     err.help("unsized locals are gated as an unstable feature");
1465                 }
1466             }
1467             ObligationCauseCode::SizedReturnType => {
1468                 err.note("the return type of a function must have a \
1469                           statically known size");
1470             }
1471             ObligationCauseCode::SizedYieldType => {
1472                 err.note("the yield type of a generator must have a \
1473                           statically known size");
1474             }
1475             ObligationCauseCode::AssignmentLhsSized => {
1476                 err.note("the left-hand-side of an assignment must have a statically known size");
1477             }
1478             ObligationCauseCode::TupleInitializerSized => {
1479                 err.note("tuples must have a statically known size to be initialized");
1480             }
1481             ObligationCauseCode::StructInitializerSized => {
1482                 err.note("structs must have a statically known size to be initialized");
1483             }
1484             ObligationCauseCode::FieldSized { adt_kind: ref item, last } => {
1485                 match *item {
1486                     AdtKind::Struct => {
1487                         if last {
1488                             err.note("the last field of a packed struct may only have a \
1489                                       dynamically sized type if it does not need drop to be run");
1490                         } else {
1491                             err.note("only the last field of a struct may have a dynamically \
1492                                       sized type");
1493                         }
1494                     }
1495                     AdtKind::Union => {
1496                         err.note("no field of a union may have a dynamically sized type");
1497                     }
1498                     AdtKind::Enum => {
1499                         err.note("no field of an enum variant may have a dynamically sized type");
1500                     }
1501                 }
1502             }
1503             ObligationCauseCode::ConstSized => {
1504                 err.note("constant expressions must have a statically known size");
1505             }
1506             ObligationCauseCode::SharedStatic => {
1507                 err.note("shared static variables must have a type that implements `Sync`");
1508             }
1509             ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
1510                 let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1511                 let ty = parent_trait_ref.skip_binder().self_ty();
1512                 err.note(&format!("required because it appears within the type `{}`", ty));
1513                 obligated_types.push(ty);
1514
1515                 let parent_predicate = parent_trait_ref.to_predicate();
1516                 if !self.is_recursive_obligation(obligated_types, &data.parent_code) {
1517                     self.note_obligation_cause_code(err,
1518                                                     &parent_predicate,
1519                                                     &data.parent_code,
1520                                                     obligated_types);
1521                 }
1522             }
1523             ObligationCauseCode::ImplDerivedObligation(ref data) => {
1524                 let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1525                 err.note(
1526                     &format!("required because of the requirements on the impl of `{}` for `{}`",
1527                              parent_trait_ref,
1528                              parent_trait_ref.skip_binder().self_ty()));
1529                 let parent_predicate = parent_trait_ref.to_predicate();
1530                 self.note_obligation_cause_code(err,
1531                                             &parent_predicate,
1532                                             &data.parent_code,
1533                                             obligated_types);
1534             }
1535             ObligationCauseCode::CompareImplMethodObligation { .. } => {
1536                 err.note(
1537                     &format!("the requirement `{}` appears on the impl method \
1538                               but not on the corresponding trait method",
1539                              predicate));
1540             }
1541             ObligationCauseCode::ReturnType(_) |
1542             ObligationCauseCode::BlockTailExpression(_) => (),
1543             ObligationCauseCode::TrivialBound => {
1544                 err.help("see issue #48214");
1545                 if tcx.sess.opts.unstable_features.is_nightly_build() {
1546                     err.help("add #![feature(trivial_bounds)] to the \
1547                               crate attributes to enable",
1548                     );
1549                 }
1550             }
1551         }
1552     }
1553
1554     fn suggest_new_overflow_limit(&self, err: &mut DiagnosticBuilder) {
1555         let current_limit = self.tcx.sess.recursion_limit.get();
1556         let suggested_limit = current_limit * 2;
1557         err.help(&format!("consider adding a `#![recursion_limit=\"{}\"]` attribute to your crate",
1558                           suggested_limit));
1559     }
1560
1561     fn is_recursive_obligation(&self,
1562                                    obligated_types: &mut Vec<&ty::TyS<'tcx>>,
1563                                    cause_code: &ObligationCauseCode<'tcx>) -> bool {
1564         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
1565             let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1566             for obligated_type in obligated_types {
1567                 if obligated_type == &parent_trait_ref.skip_binder().self_ty() {
1568                     return true;
1569                 }
1570             }
1571         }
1572         return false;
1573     }
1574 }
1575
1576 /// Summarizes information
1577 pub enum ArgKind {
1578     /// An argument of non-tuple type. Parameters are (name, ty)
1579     Arg(String, String),
1580
1581     /// An argument of tuple type. For a "found" argument, the span is
1582     /// the locationo in the source of the pattern. For a "expected"
1583     /// argument, it will be None. The vector is a list of (name, ty)
1584     /// strings for the components of the tuple.
1585     Tuple(Option<Span>, Vec<(String, String)>),
1586 }
1587
1588 impl ArgKind {
1589     fn empty() -> ArgKind {
1590         ArgKind::Arg("_".to_owned(), "_".to_owned())
1591     }
1592
1593     /// Creates an `ArgKind` from the expected type of an
1594     /// argument. This has no name (`_`) and no source spans..
1595     pub fn from_expected_ty(t: Ty<'_>) -> ArgKind {
1596         match t.sty {
1597             ty::Tuple(ref tys) => ArgKind::Tuple(
1598                 None,
1599                 tys.iter()
1600                    .map(|ty| ("_".to_owned(), ty.sty.to_string()))
1601                    .collect::<Vec<_>>()
1602             ),
1603             _ => ArgKind::Arg("_".to_owned(), t.sty.to_string()),
1604         }
1605     }
1606 }