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     OutputTypeParameterMismatch,
  19     TraitNotObjectSafe,
  20     PredicateObligation,
  21     SelectionContext,
  22     SelectionError,
  23     ObjectSafetyViolation,
  24 };
  25
  26 use errors::DiagnosticBuilder;
  27 use fmt_macros::{Parser, Piece, Position};
  28 use hir::{intravisit, Local, Pat};
  29 use hir::intravisit::{Visitor, NestedVisitorMap};
  30 use hir::map::NodeExpr;
  31 use hir::def_id::DefId;
  32 use infer::{self, InferCtxt};
  33 use infer::type_variable::TypeVariableOrigin;
  34 use rustc::lint::builtin::EXTRA_REQUIREMENT_IN_IMPL;
  35 use std::fmt;
  36 use syntax::ast;
  37 use ty::{self, AdtKind, ToPredicate, ToPolyTraitRef, Ty, TyCtxt, TypeFoldable};
  38 use ty::error::ExpectedFound;
  39 use ty::fast_reject;
  40 use ty::fold::TypeFolder;
  41 use ty::subst::Subst;
  42 use ty::SubtypePredicate;
  43 use util::nodemap::{FxHashMap, FxHashSet};
  44
  45 use syntax_pos::{DUMMY_SP, Span};
  46
  47
  48 #[derive(Debug, PartialEq, Eq, Hash)]
  49 pub struct TraitErrorKey<'tcx> {
  50     span: Span,
  51     predicate: ty::Predicate<'tcx>
  52 }
  53
  54 impl<'a, 'gcx, 'tcx> TraitErrorKey<'tcx> {
  55     fn from_error(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
  56                   e: &FulfillmentError<'tcx>) -> Self {
  57         let predicate =
  58             infcx.resolve_type_vars_if_possible(&e.obligation.predicate);
  59         TraitErrorKey {
  60             span: e.obligation.cause.span,
  61             predicate: infcx.tcx.erase_regions(&predicate)
  62         }
  63     }
  64 }
  65
  66 struct FindLocalByTypeVisitor<'a, 'gcx: 'a + 'tcx, 'tcx: 'a> {
  67     infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
  68     target_ty: &'a Ty<'tcx>,
  69     found_pattern: Option<&'a Pat>,
  70 }
  71
  72 impl<'a, 'gcx, 'tcx> FindLocalByTypeVisitor<'a, 'gcx, 'tcx> {
  73     fn is_match(&self, ty: Ty<'tcx>) -> bool {
  74         ty == *self.target_ty || match (&ty.sty, &self.target_ty.sty) {
  75             (&ty::TyInfer(ty::TyVar(a_vid)), &ty::TyInfer(ty::TyVar(b_vid))) =>
  76                 self.infcx.type_variables
  77                           .borrow_mut()
  78                           .sub_unified(a_vid, b_vid),
  79
  80             _ => false,
  81         }
  82     }
  83 }
  84
  85 impl<'a, 'gcx, 'tcx> Visitor<'a> for FindLocalByTypeVisitor<'a, 'gcx, 'tcx> {
  86     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'a> {
  87         NestedVisitorMap::None
  88     }
  89
  90     fn visit_local(&mut self, local: &'a Local) {
  91         if let Some(&ty) = self.infcx.tables.borrow().node_types.get(&local.id) {
  92             let ty = self.infcx.resolve_type_vars_if_possible(&ty);
  93             let is_match = ty.walk().any(|t| self.is_match(t));
  94
  95             if is_match && self.found_pattern.is_none() {
  96                 self.found_pattern = Some(&*local.pat);
  97             }
  98         }
  99         intravisit::walk_local(self, local);
 100     }
 101 }
 102
 103 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
 104     pub fn report_fulfillment_errors(&self, errors: &Vec<FulfillmentError<'tcx>>) {
 105         for error in errors {
 106             self.report_fulfillment_error(error);
 107         }
 108     }
 109
 110     fn report_fulfillment_error(&self,
 111                                 error: &FulfillmentError<'tcx>) {
 112         let error_key = TraitErrorKey::from_error(self, error);
 113         debug!("report_fulfillment_errors({:?}) - key={:?}",
 114                error, error_key);
 115         if !self.reported_trait_errors.borrow_mut().insert(error_key) {
 116             debug!("report_fulfillment_errors: skipping duplicate");
 117             return;
 118         }
 119         match error.code {
 120             FulfillmentErrorCode::CodeSelectionError(ref e) => {
 121                 self.report_selection_error(&error.obligation, e);
 122             }
 123             FulfillmentErrorCode::CodeProjectionError(ref e) => {
 124                 self.report_projection_error(&error.obligation, e);
 125             }
 126             FulfillmentErrorCode::CodeAmbiguity => {
 127                 self.maybe_report_ambiguity(&error.obligation);
 128             }
 129             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
 130                 self.report_mismatched_types(&error.obligation.cause,
 131                                              expected_found.expected,
 132                                              expected_found.found,
 133                                              err.clone())
 134                     .emit();
 135             }
 136         }
 137     }
 138
 139     fn report_projection_error(&self,
 140                                obligation: &PredicateObligation<'tcx>,
 141                                error: &MismatchedProjectionTypes<'tcx>)
 142     {
 143         let predicate =
 144             self.resolve_type_vars_if_possible(&obligation.predicate);
 145
 146         if predicate.references_error() {
 147             return
 148         }
 149
 150         self.probe(|_| {
 151             let err_buf;
 152             let mut err = &error.err;
 153             let mut values = None;
 154
 155             // try to find the mismatched types to report the error with.
 156             //
 157             // this can fail if the problem was higher-ranked, in which
 158             // cause I have no idea for a good error message.
 159             if let ty::Predicate::Projection(ref data) = predicate {
 160                 let mut selcx = SelectionContext::new(self);
 161                 let (data, _) = self.replace_late_bound_regions_with_fresh_var(
 162                     obligation.cause.span,
 163                     infer::LateBoundRegionConversionTime::HigherRankedType,
 164                     data);
 165                 let normalized = super::normalize_projection_type(
 166                     &mut selcx,
 167                     data.projection_ty,
 168                     obligation.cause.clone(),
 169                     0
 170                 );
 171                 if let Err(error) = self.eq_types(
 172                     false, &obligation.cause,
 173                     data.ty, normalized.value
 174                 ) {
 175                     values = Some(infer::ValuePairs::Types(ExpectedFound {
 176                         expected: normalized.value,
 177                         found: data.ty,
 178                     }));
 179                     err_buf = error;
 180                     err = &err_buf;
 181                 }
 182             }
 183
 184             let mut diag = struct_span_err!(
 185                 self.tcx.sess, obligation.cause.span, E0271,
 186                 "type mismatch resolving `{}`", predicate
 187             );
 188             self.note_type_err(&mut diag, &obligation.cause, None, values, err);
 189             self.note_obligation_cause(&mut diag, obligation);
 190             diag.emit();
 191         });
 192     }
 193
 194     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
 195         /// returns the fuzzy category of a given type, or None
 196         /// if the type can be equated to any type.
 197         fn type_category<'tcx>(t: Ty<'tcx>) -> Option<u32> {
 198             match t.sty {
 199                 ty::TyBool => Some(0),
 200                 ty::TyChar => Some(1),
 201                 ty::TyStr => Some(2),
 202                 ty::TyInt(..) | ty::TyUint(..) | ty::TyInfer(ty::IntVar(..)) => Some(3),
 203                 ty::TyFloat(..) | ty::TyInfer(ty::FloatVar(..)) => Some(4),
 204                 ty::TyRef(..) | ty::TyRawPtr(..) => Some(5),
 205                 ty::TyArray(..) | ty::TySlice(..) => Some(6),
 206                 ty::TyFnDef(..) | ty::TyFnPtr(..) => Some(7),
 207                 ty::TyDynamic(..) => Some(8),
 208                 ty::TyClosure(..) => Some(9),
 209                 ty::TyTuple(..) => Some(10),
 210                 ty::TyProjection(..) => Some(11),
 211                 ty::TyParam(..) => Some(12),
 212                 ty::TyAnon(..) => Some(13),
 213                 ty::TyNever => Some(14),
 214                 ty::TyAdt(adt, ..) => match adt.adt_kind() {
 215                     AdtKind::Struct => Some(15),
 216                     AdtKind::Union => Some(16),
 217                     AdtKind::Enum => Some(17),
 218                 },
 219                 ty::TyInfer(..) | ty::TyError => None
 220             }
 221         }
 222
 223         match (type_category(a), type_category(b)) {
 224             (Some(cat_a), Some(cat_b)) => match (&a.sty, &b.sty) {
 225                 (&ty::TyAdt(def_a, _), &ty::TyAdt(def_b, _)) => def_a == def_b,
 226                 _ => cat_a == cat_b
 227             },
 228             // infer and error can be equated to all types
 229             _ => true
 230         }
 231     }
 232
 233     fn impl_similar_to(&self,
 234                        trait_ref: ty::PolyTraitRef<'tcx>,
 235                        obligation: &PredicateObligation<'tcx>)
 236                        -> Option<DefId>
 237     {
 238         let tcx = self.tcx;
 239
 240         let trait_ref = tcx.erase_late_bound_regions(&trait_ref);
 241         let trait_self_ty = trait_ref.self_ty();
 242
 243         let mut self_match_impls = vec![];
 244         let mut fuzzy_match_impls = vec![];
 245
 246         self.tcx.lookup_trait_def(trait_ref.def_id)
 247             .for_each_relevant_impl(self.tcx, trait_self_ty, |def_id| {
 248                 let impl_substs = self.fresh_substs_for_item(obligation.cause.span, def_id);
 249                 let impl_trait_ref = tcx
 250                     .impl_trait_ref(def_id)
 251                     .unwrap()
 252                     .subst(tcx, impl_substs);
 253
 254                 let impl_self_ty = impl_trait_ref.self_ty();
 255
 256                 if let Ok(..) = self.can_equate(&trait_self_ty, &impl_self_ty) {
 257                     self_match_impls.push(def_id);
 258
 259                     if trait_ref.substs.types().skip(1)
 260                         .zip(impl_trait_ref.substs.types().skip(1))
 261                         .all(|(u,v)| self.fuzzy_match_tys(u, v))
 262                     {
 263                         fuzzy_match_impls.push(def_id);
 264                     }
 265                 }
 266             });
 267
 268         let impl_def_id = if self_match_impls.len() == 1 {
 269             self_match_impls[0]
 270         } else if fuzzy_match_impls.len() == 1 {
 271             fuzzy_match_impls[0]
 272         } else {
 273             return None
 274         };
 275
 276         if tcx.has_attr(impl_def_id, "rustc_on_unimplemented") {
 277             Some(impl_def_id)
 278         } else {
 279             None
 280         }
 281     }
 282
 283     fn on_unimplemented_note(&self,
 284                              trait_ref: ty::PolyTraitRef<'tcx>,
 285                              obligation: &PredicateObligation<'tcx>) -> Option<String> {
 286         let def_id = self.impl_similar_to(trait_ref, obligation)
 287             .unwrap_or(trait_ref.def_id());
 288         let trait_ref = trait_ref.skip_binder();
 289
 290         let span = obligation.cause.span;
 291         let mut report = None;
 292         if let Some(item) = self.tcx
 293             .get_attrs(def_id)
 294             .into_iter()
 295             .filter(|a| a.check_name("rustc_on_unimplemented"))
 296             .next()
 297         {
 298             let err_sp = item.span.substitute_dummy(span);
 299             let trait_str = self.tcx.item_path_str(trait_ref.def_id);
 300             if let Some(istring) = item.value_str() {
 301                 let istring = &*istring.as_str();
 302                 let generics = self.tcx.item_generics(trait_ref.def_id);
 303                 let generic_map = generics.types.iter().map(|param| {
 304                     (param.name.as_str().to_string(),
 305                         trait_ref.substs.type_for_def(param).to_string())
 306                 }).collect::<FxHashMap<String, String>>();
 307                 let parser = Parser::new(istring);
 308                 let mut errored = false;
 309                 let err: String = parser.filter_map(|p| {
 310                     match p {
 311                         Piece::String(s) => Some(s),
 312                         Piece::NextArgument(a) => match a.position {
 313                             Position::ArgumentNamed(s) => match generic_map.get(s) {
 314                                 Some(val) => Some(val),
 315                                 None => {
 316                                     span_err!(self.tcx.sess, err_sp, E0272,
 317                                                     "the #[rustc_on_unimplemented] \
 318                                                             attribute on \
 319                                                             trait definition for {} refers to \
 320                                                             non-existent type parameter {}",
 321                                                             trait_str, s);
 322                                     errored = true;
 323                                     None
 324                                 }
 325                             },
 326                             _ => {
 327                                 span_err!(self.tcx.sess, err_sp, E0273,
 328                                             "the #[rustc_on_unimplemented] attribute \
 329                                             on trait definition for {} must have \
 330                                             named format arguments, eg \
 331                                             `#[rustc_on_unimplemented = \
 332                                             \"foo {{T}}\"]`", trait_str);
 333                                 errored = true;
 334                                 None
 335                             }
 336                         }
 337                     }
 338                 }).collect();
 339                 // Report only if the format string checks out
 340                 if !errored {
 341                     report = Some(err);
 342                 }
 343             } else {
 344                 span_err!(self.tcx.sess, err_sp, E0274,
 345                                         "the #[rustc_on_unimplemented] attribute on \
 346                                                     trait definition for {} must have a value, \
 347                                                     eg `#[rustc_on_unimplemented = \"foo\"]`",
 348                                                     trait_str);
 349             }
 350         }
 351         report
 352     }
 353
 354     fn find_similar_impl_candidates(&self,
 355                                     trait_ref: ty::PolyTraitRef<'tcx>)
 356                                     -> Vec<ty::TraitRef<'tcx>>
 357     {
 358         let simp = fast_reject::simplify_type(self.tcx,
 359                                               trait_ref.skip_binder().self_ty(),
 360                                               true);
 361         let mut impl_candidates = Vec::new();
 362         let trait_def = self.tcx.lookup_trait_def(trait_ref.def_id());
 363
 364         match simp {
 365             Some(simp) => trait_def.for_each_impl(self.tcx, |def_id| {
 366                 let imp = self.tcx.impl_trait_ref(def_id).unwrap();
 367                 let imp_simp = fast_reject::simplify_type(self.tcx,
 368                                                           imp.self_ty(),
 369                                                           true);
 370                 if let Some(imp_simp) = imp_simp {
 371                     if simp != imp_simp {
 372                         return;
 373                     }
 374                 }
 375                 impl_candidates.push(imp);
 376             }),
 377             None => trait_def.for_each_impl(self.tcx, |def_id| {
 378                 impl_candidates.push(
 379                     self.tcx.impl_trait_ref(def_id).unwrap());
 380             })
 381         };
 382         impl_candidates
 383     }
 384
 385     fn report_similar_impl_candidates(&self,
 386                                       impl_candidates: Vec<ty::TraitRef<'tcx>>,
 387                                       err: &mut DiagnosticBuilder)
 388     {
 389         if impl_candidates.is_empty() {
 390             return;
 391         }
 392
 393         let end = if impl_candidates.len() <= 5 {
 394             impl_candidates.len()
 395         } else {
 396             4
 397         };
 398         err.help(&format!("the following implementations were found:{}{}",
 399                           &impl_candidates[0..end].iter().map(|candidate| {
 400                               format!("\n  {:?}", candidate)
 401                           }).collect::<String>(),
 402                           if impl_candidates.len() > 5 {
 403                               format!("\nand {} others", impl_candidates.len() - 4)
 404                           } else {
 405                               "".to_owned()
 406                           }
 407                           ));
 408     }
 409
 410     /// Reports that an overflow has occurred and halts compilation. We
 411     /// halt compilation unconditionally because it is important that
 412     /// overflows never be masked -- they basically represent computations
 413     /// whose result could not be truly determined and thus we can't say
 414     /// if the program type checks or not -- and they are unusual
 415     /// occurrences in any case.
 416     pub fn report_overflow_error<T>(&self,
 417                                     obligation: &Obligation<'tcx, T>,
 418                                     suggest_increasing_limit: bool) -> !
 419         where T: fmt::Display + TypeFoldable<'tcx>
 420     {
 421         let predicate =
 422             self.resolve_type_vars_if_possible(&obligation.predicate);
 423         let mut err = struct_span_err!(self.tcx.sess, obligation.cause.span, E0275,
 424                                        "overflow evaluating the requirement `{}`",
 425                                        predicate);
 426
 427         if suggest_increasing_limit {
 428             self.suggest_new_overflow_limit(&mut err);
 429         }
 430
 431         self.note_obligation_cause(&mut err, obligation);
 432
 433         err.emit();
 434         self.tcx.sess.abort_if_errors();
 435         bug!();
 436     }
 437
 438     /// Reports that a cycle was detected which led to overflow and halts
 439     /// compilation. This is equivalent to `report_overflow_error` except
 440     /// that we can give a more helpful error message (and, in particular,
 441     /// we do not suggest increasing the overflow limit, which is not
 442     /// going to help).
 443     pub fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 444         let cycle = self.resolve_type_vars_if_possible(&cycle.to_owned());
 445         assert!(cycle.len() > 0);
 446
 447         debug!("report_overflow_error_cycle: cycle={:?}", cycle);
 448
 449         self.report_overflow_error(&cycle[0], false);
 450     }
 451
 452     pub fn report_extra_impl_obligation(&self,
 453                                         error_span: Span,
 454                                         item_name: ast::Name,
 455                                         _impl_item_def_id: DefId,
 456                                         trait_item_def_id: DefId,
 457                                         requirement: &fmt::Display,
 458                                         lint_id: Option<ast::NodeId>) // (*)
 459                                         -> DiagnosticBuilder<'tcx>
 460     {
 461         // (*) This parameter is temporary and used only for phasing
 462         // in the bug fix to #18937. If it is `Some`, it has a kind of
 463         // weird effect -- the diagnostic is reported as a lint, and
 464         // the builder which is returned is marked as canceled.
 465
 466         let mut err =
 467             struct_span_err!(self.tcx.sess,
 468                              error_span,
 469                              E0276,
 470                              "impl has stricter requirements than trait");
 471
 472         if let Some(trait_item_span) = self.tcx.hir.span_if_local(trait_item_def_id) {
 473             err.span_label(trait_item_span,
 474                            &format!("definition of `{}` from trait", item_name));
 475         }
 476
 477         err.span_label(
 478             error_span,
 479             &format!("impl has extra requirement {}", requirement));
 480
 481         if let Some(node_id) = lint_id {
 482             self.tcx.sess.add_lint_diagnostic(EXTRA_REQUIREMENT_IN_IMPL,
 483                                               node_id,
 484                                               (*err).clone());
 485             err.cancel();
 486         }
 487
 488         err
 489     }
 490
 491
 492     /// Get the parent trait chain start
 493     fn get_parent_trait_ref(&self, code: &ObligationCauseCode<'tcx>) -> Option<String> {
 494         match code {
 495             &ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
 496                 let parent_trait_ref = self.resolve_type_vars_if_possible(
 497                     &data.parent_trait_ref);
 498                 match self.get_parent_trait_ref(&data.parent_code) {
 499                     Some(t) => Some(t),
 500                     None => Some(format!("{}", parent_trait_ref.0.self_ty())),
 501                 }
 502             }
 503             _ => None,
 504         }
 505     }
 506
 507     pub fn report_selection_error(&self,
 508                                   obligation: &PredicateObligation<'tcx>,
 509                                   error: &SelectionError<'tcx>)
 510     {
 511         let span = obligation.cause.span;
 512
 513         let mut err = match *error {
 514             SelectionError::Unimplemented => {
 515                 if let ObligationCauseCode::CompareImplMethodObligation {
 516                     item_name, impl_item_def_id, trait_item_def_id, lint_id
 517                 } = obligation.cause.code {
 518                     self.report_extra_impl_obligation(
 519                         span,
 520                         item_name,
 521                         impl_item_def_id,
 522                         trait_item_def_id,
 523                         &format!("`{}`", obligation.predicate),
 524                         lint_id)
 525                         .emit();
 526                     return;
 527                 }
 528                 match obligation.predicate {
 529                     ty::Predicate::Trait(ref trait_predicate) => {
 530                         let trait_predicate =
 531                             self.resolve_type_vars_if_possible(trait_predicate);
 532
 533                         if self.tcx.sess.has_errors() && trait_predicate.references_error() {
 534                             return;
 535                         }
 536                         let trait_ref = trait_predicate.to_poly_trait_ref();
 537                         let (post_message, pre_message) =
 538                             self.get_parent_trait_ref(&obligation.cause.code)
 539                                 .map(|t| (format!(" in `{}`", t), format!("within `{}`, ", t)))
 540                                 .unwrap_or((String::new(), String::new()));
 541                         let mut err = struct_span_err!(
 542                             self.tcx.sess,
 543                             span,
 544                             E0277,
 545                             "the trait bound `{}` is not satisfied{}",
 546                             trait_ref.to_predicate(),
 547                             post_message);
 548
 549                         // Try to report a help message
 550                         if !trait_ref.has_infer_types() &&
 551                             self.predicate_can_apply(trait_ref) {
 552                             // If a where-clause may be useful, remind the
 553                             // user that they can add it.
 554                             //
 555                             // don't display an on-unimplemented note, as
 556                             // these notes will often be of the form
 557                             //     "the type `T` can't be frobnicated"
 558                             // which is somewhat confusing.
 559                             err.help(&format!("consider adding a `where {}` bound",
 560                                                 trait_ref.to_predicate()));
 561                         } else if let Some(s) = self.on_unimplemented_note(trait_ref, obligation) {
 562                             // If it has a custom "#[rustc_on_unimplemented]"
 563                             // error message, let's display it!
 564                             err.note(&s);
 565                         } else {
 566                             // Can't show anything else useful, try to find similar impls.
 567                             let impl_candidates = self.find_similar_impl_candidates(trait_ref);
 568                             self.report_similar_impl_candidates(impl_candidates, &mut err);
 569                         }
 570
 571                         err.span_label(span,
 572                                        &format!("{}the trait `{}` is not implemented for `{}`",
 573                                                 pre_message,
 574                                                 trait_ref,
 575                                                 trait_ref.self_ty()));
 576                         err
 577                     }
 578
 579                     ty::Predicate::Subtype(ref predicate) => {
 580                         // Errors for Subtype predicates show up as
 581                         // `FulfillmentErrorCode::CodeSubtypeError`,
 582                         // not selection error.
 583                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
 584                     }
 585
 586                     ty::Predicate::Equate(ref predicate) => {
 587                         let predicate = self.resolve_type_vars_if_possible(predicate);
 588                         let err = self.equality_predicate(&obligation.cause,
 589                                                             &predicate).err().unwrap();
 590                         struct_span_err!(self.tcx.sess, span, E0278,
 591                             "the requirement `{}` is not satisfied (`{}`)",
 592                             predicate, err)
 593                     }
 594
 595                     ty::Predicate::RegionOutlives(ref predicate) => {
 596                         let predicate = self.resolve_type_vars_if_possible(predicate);
 597                         let err = self.region_outlives_predicate(&obligation.cause,
 598                                                                     &predicate).err().unwrap();
 599                         struct_span_err!(self.tcx.sess, span, E0279,
 600                             "the requirement `{}` is not satisfied (`{}`)",
 601                             predicate, err)
 602                     }
 603
 604                     ty::Predicate::Projection(..) | ty::Predicate::TypeOutlives(..) => {
 605                         let predicate =
 606                             self.resolve_type_vars_if_possible(&obligation.predicate);
 607                         struct_span_err!(self.tcx.sess, span, E0280,
 608                             "the requirement `{}` is not satisfied",
 609                             predicate)
 610                     }
 611
 612                     ty::Predicate::ObjectSafe(trait_def_id) => {
 613                         let violations = self.tcx.object_safety_violations(trait_def_id);
 614                         self.tcx.report_object_safety_error(span,
 615                                                             trait_def_id,
 616                                                             violations)
 617                     }
 618
 619                     ty::Predicate::ClosureKind(closure_def_id, kind) => {
 620                         let found_kind = self.closure_kind(closure_def_id).unwrap();
 621                         let closure_span = self.tcx.hir.span_if_local(closure_def_id).unwrap();
 622                         let mut err = struct_span_err!(
 623                             self.tcx.sess, closure_span, E0525,
 624                             "expected a closure that implements the `{}` trait, \
 625                                 but this closure only implements `{}`",
 626                             kind,
 627                             found_kind);
 628                         err.span_note(
 629                             obligation.cause.span,
 630                             &format!("the requirement to implement \
 631                                         `{}` derives from here", kind));
 632                         err.emit();
 633                         return;
 634                     }
 635
 636                     ty::Predicate::WellFormed(ty) => {
 637                         // WF predicates cannot themselves make
 638                         // errors. They can only block due to
 639                         // ambiguity; otherwise, they always
 640                         // degenerate into other obligations
 641                         // (which may fail).
 642                         span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 643                     }
 644                 }
 645             }
 646
 647             OutputTypeParameterMismatch(ref expected_trait_ref, ref actual_trait_ref, ref e) => {
 648                 let expected_trait_ref = self.resolve_type_vars_if_possible(&*expected_trait_ref);
 649                 let actual_trait_ref = self.resolve_type_vars_if_possible(&*actual_trait_ref);
 650                 if actual_trait_ref.self_ty().references_error() {
 651                     return;
 652                 }
 653                 struct_span_err!(self.tcx.sess, span, E0281,
 654                     "type mismatch: the type `{}` implements the trait `{}`, \
 655                      but the trait `{}` is required ({})",
 656                     expected_trait_ref.self_ty(),
 657                     expected_trait_ref,
 658                     actual_trait_ref,
 659                     e)
 660             }
 661
 662             TraitNotObjectSafe(did) => {
 663                 let violations = self.tcx.object_safety_violations(did);
 664                 self.tcx.report_object_safety_error(span, did,
 665                                                     violations)
 666             }
 667         };
 668         self.note_obligation_cause(&mut err, obligation);
 669         err.emit();
 670     }
 671 }
 672
 673 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
 674     pub fn recursive_type_with_infinite_size_error(self,
 675                                                    type_def_id: DefId)
 676                                                    -> DiagnosticBuilder<'tcx>
 677     {
 678         assert!(type_def_id.is_local());
 679         let span = self.hir.span_if_local(type_def_id).unwrap();
 680         let mut err = struct_span_err!(self.sess, span, E0072,
 681                                        "recursive type `{}` has infinite size",
 682                                        self.item_path_str(type_def_id));
 683         err.span_label(span, &format!("recursive type has infinite size"));
 684         err.help(&format!("insert indirection (e.g., a `Box`, `Rc`, or `&`) \
 685                            at some point to make `{}` representable",
 686                           self.item_path_str(type_def_id)));
 687         err
 688     }
 689
 690     pub fn report_object_safety_error(self,
 691                                       span: Span,
 692                                       trait_def_id: DefId,
 693                                       violations: Vec<ObjectSafetyViolation>)
 694                                       -> DiagnosticBuilder<'tcx>
 695     {
 696         let trait_str = self.item_path_str(trait_def_id);
 697         let mut err = struct_span_err!(
 698             self.sess, span, E0038,
 699             "the trait `{}` cannot be made into an object",
 700             trait_str);
 701         err.span_label(span, &format!(
 702             "the trait `{}` cannot be made into an object", trait_str
 703         ));
 704
 705         let mut reported_violations = FxHashSet();
 706         for violation in violations {
 707             if !reported_violations.insert(violation.clone()) {
 708                 continue;
 709             }
 710             err.note(&violation.error_msg());
 711         }
 712         err
 713     }
 714 }
 715
 716 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
 717     fn maybe_report_ambiguity(&self, obligation: &PredicateObligation<'tcx>) {
 718         // Unable to successfully determine, probably means
 719         // insufficient type information, but could mean
 720         // ambiguous impls. The latter *ought* to be a
 721         // coherence violation, so we don't report it here.
 722
 723         let predicate = self.resolve_type_vars_if_possible(&obligation.predicate);
 724
 725         debug!("maybe_report_ambiguity(predicate={:?}, obligation={:?})",
 726                predicate,
 727                obligation);
 728
 729         // Ambiguity errors are often caused as fallout from earlier
 730         // errors. So just ignore them if this infcx is tainted.
 731         if self.is_tainted_by_errors() {
 732             return;
 733         }
 734
 735         match predicate {
 736             ty::Predicate::Trait(ref data) => {
 737                 let trait_ref = data.to_poly_trait_ref();
 738                 let self_ty = trait_ref.self_ty();
 739                 if predicate.references_error() {
 740                     return;
 741                 }
 742                 // Typically, this ambiguity should only happen if
 743                 // there are unresolved type inference variables
 744                 // (otherwise it would suggest a coherence
 745                 // failure). But given #21974 that is not necessarily
 746                 // the case -- we can have multiple where clauses that
 747                 // are only distinguished by a region, which results
 748                 // in an ambiguity even when all types are fully
 749                 // known, since we don't dispatch based on region
 750                 // relationships.
 751
 752                 // This is kind of a hack: it frequently happens that some earlier
 753                 // error prevents types from being fully inferred, and then we get
 754                 // a bunch of uninteresting errors saying something like "<generic
 755                 // #0> doesn't implement Sized".  It may even be true that we
 756                 // could just skip over all checks where the self-ty is an
 757                 // inference variable, but I was afraid that there might be an
 758                 // inference variable created, registered as an obligation, and
 759                 // then never forced by writeback, and hence by skipping here we'd
 760                 // be ignoring the fact that we don't KNOW the type works
 761                 // out. Though even that would probably be harmless, given that
 762                 // we're only talking about builtin traits, which are known to be
 763                 // inhabited. But in any case I just threw in this check for
 764                 // has_errors() to be sure that compilation isn't happening
 765                 // anyway. In that case, why inundate the user.
 766                 if !self.tcx.sess.has_errors() {
 767                     if
 768                         self.tcx.lang_items.sized_trait()
 769                         .map_or(false, |sized_id| sized_id == trait_ref.def_id())
 770                     {
 771                         self.need_type_info(obligation, self_ty);
 772                     } else {
 773                         let mut err = struct_span_err!(self.tcx.sess,
 774                                                         obligation.cause.span, E0283,
 775                                                         "type annotations required: \
 776                                                         cannot resolve `{}`",
 777                                                         predicate);
 778                         self.note_obligation_cause(&mut err, obligation);
 779                         err.emit();
 780                     }
 781                 }
 782             }
 783
 784             ty::Predicate::WellFormed(ty) => {
 785                 // Same hacky approach as above to avoid deluging user
 786                 // with error messages.
 787                 if !ty.references_error() && !self.tcx.sess.has_errors() {
 788                     self.need_type_info(obligation, ty);
 789                 }
 790             }
 791
 792             ty::Predicate::Subtype(ref data) => {
 793                 if data.references_error() || self.tcx.sess.has_errors() {
 794                     // no need to overload user in such cases
 795                 } else {
 796                     let &SubtypePredicate { a_is_expected: _, a, b } = data.skip_binder();
 797                     // both must be type variables, or the other would've been instantiated
 798                     assert!(a.is_ty_var() && b.is_ty_var());
 799                     self.need_type_info(obligation, a);
 800                 }
 801             }
 802
 803             _ => {
 804                 if !self.tcx.sess.has_errors() {
 805                     let mut err = struct_span_err!(self.tcx.sess,
 806                                                    obligation.cause.span, E0284,
 807                                                    "type annotations required: \
 808                                                     cannot resolve `{}`",
 809                                                    predicate);
 810                     self.note_obligation_cause(&mut err, obligation);
 811                     err.emit();
 812                 }
 813             }
 814         }
 815     }
 816
 817     /// Returns whether the trait predicate may apply for *some* assignment
 818     /// to the type parameters.
 819     fn predicate_can_apply(&self, pred: ty::PolyTraitRef<'tcx>) -> bool {
 820         struct ParamToVarFolder<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
 821             infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
 822             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>
 823         }
 824
 825         impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for ParamToVarFolder<'a, 'gcx, 'tcx> {
 826             fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> { self.infcx.tcx }
 827
 828             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
 829                 if let ty::TyParam(ty::ParamTy {name, ..}) = ty.sty {
 830                     let infcx = self.infcx;
 831                     self.var_map.entry(ty).or_insert_with(||
 832                         infcx.next_ty_var(
 833                             TypeVariableOrigin::TypeParameterDefinition(DUMMY_SP, name)))
 834                 } else {
 835                     ty.super_fold_with(self)
 836                 }
 837             }
 838         }
 839
 840         self.probe(|_| {
 841             let mut selcx = SelectionContext::new(self);
 842
 843             let cleaned_pred = pred.fold_with(&mut ParamToVarFolder {
 844                 infcx: self,
 845                 var_map: FxHashMap()
 846             });
 847
 848             let cleaned_pred = super::project::normalize(
 849                 &mut selcx,
 850                 ObligationCause::dummy(),
 851                 &cleaned_pred
 852             ).value;
 853
 854             let obligation = Obligation::new(
 855                 ObligationCause::dummy(),
 856                 cleaned_pred.to_predicate()
 857             );
 858
 859             selcx.evaluate_obligation(&obligation)
 860         })
 861     }
 862
 863     fn extract_type_name(&self, ty: &'a Ty<'tcx>) -> String {
 864         if let ty::TyInfer(ty::TyVar(ty_vid)) = (*ty).sty {
 865             let ty_vars = self.type_variables.borrow();
 866             if let TypeVariableOrigin::TypeParameterDefinition(_, name) =
 867                 *ty_vars.var_origin(ty_vid) {
 868                 name.to_string()
 869             } else {
 870                 ty.to_string()
 871             }
 872         } else {
 873             ty.to_string()
 874         }
 875     }
 876
 877     fn need_type_info(&self, obligation: &PredicateObligation<'tcx>, ty: Ty<'tcx>) {
 878         let ty = self.resolve_type_vars_if_possible(&ty);
 879         let name = self.extract_type_name(&ty);
 880         let ref cause = obligation.cause;
 881
 882         let mut err = struct_span_err!(self.tcx.sess,
 883                                        cause.span,
 884                                        E0282,
 885                                        "type annotations needed");
 886
 887         err.span_label(cause.span, &format!("cannot infer type for `{}`", name));
 888
 889         let mut local_visitor = FindLocalByTypeVisitor {
 890             infcx: &self,
 891             target_ty: &ty,
 892             found_pattern: None,
 893         };
 894
 895         // #40294: cause.body_id can also be a fn declaration.
 896         // Currently, if it's anything other than NodeExpr, we just ignore it
 897         match self.tcx.hir.find(cause.body_id) {
 898             Some(NodeExpr(expr)) => local_visitor.visit_expr(expr),
 899             _ => ()
 900         }
 901
 902         if let Some(pattern) = local_visitor.found_pattern {
 903             let pattern_span = pattern.span;
 904             if let Some(simple_name) = pattern.simple_name() {
 905                 err.span_label(pattern_span,
 906                                &format!("consider giving `{}` a type",
 907                                         simple_name));
 908             } else {
 909                 err.span_label(pattern_span, &format!("consider giving a type to pattern"));
 910             }
 911         }
 912
 913         err.emit();
 914     }
 915
 916     fn note_obligation_cause<T>(&self,
 917                                 err: &mut DiagnosticBuilder,
 918                                 obligation: &Obligation<'tcx, T>)
 919         where T: fmt::Display
 920     {
 921         self.note_obligation_cause_code(err,
 922                                         &obligation.predicate,
 923                                         &obligation.cause.code);
 924     }
 925
 926     fn note_obligation_cause_code<T>(&self,
 927                                      err: &mut DiagnosticBuilder,
 928                                      predicate: &T,
 929                                      cause_code: &ObligationCauseCode<'tcx>)
 930         where T: fmt::Display
 931     {
 932         let tcx = self.tcx;
 933         match *cause_code {
 934             ObligationCauseCode::ExprAssignable |
 935             ObligationCauseCode::MatchExpressionArm { .. } |
 936             ObligationCauseCode::IfExpression |
 937             ObligationCauseCode::IfExpressionWithNoElse |
 938             ObligationCauseCode::EquatePredicate |
 939             ObligationCauseCode::MainFunctionType |
 940             ObligationCauseCode::StartFunctionType |
 941             ObligationCauseCode::IntrinsicType |
 942             ObligationCauseCode::MethodReceiver |
 943             ObligationCauseCode::ReturnNoExpression |
 944             ObligationCauseCode::MiscObligation => {
 945             }
 946             ObligationCauseCode::SliceOrArrayElem => {
 947                 err.note("slice and array elements must have `Sized` type");
 948             }
 949             ObligationCauseCode::TupleElem => {
 950                 err.note("tuple elements must have `Sized` type");
 951             }
 952             ObligationCauseCode::ProjectionWf(data) => {
 953                 err.note(&format!("required so that the projection `{}` is well-formed",
 954                                   data));
 955             }
 956             ObligationCauseCode::ReferenceOutlivesReferent(ref_ty) => {
 957                 err.note(&format!("required so that reference `{}` does not outlive its referent",
 958                                   ref_ty));
 959             }
 960             ObligationCauseCode::ObjectTypeBound(object_ty, region) => {
 961                 err.note(&format!("required so that the lifetime bound of `{}` for `{}` \
 962                                    is satisfied",
 963                                   region, object_ty));
 964             }
 965             ObligationCauseCode::ItemObligation(item_def_id) => {
 966                 let item_name = tcx.item_path_str(item_def_id);
 967                 err.note(&format!("required by `{}`", item_name));
 968             }
 969             ObligationCauseCode::ObjectCastObligation(object_ty) => {
 970                 err.note(&format!("required for the cast to the object type `{}`",
 971                                   self.ty_to_string(object_ty)));
 972             }
 973             ObligationCauseCode::RepeatVec => {
 974                 err.note("the `Copy` trait is required because the \
 975                           repeated element will be copied");
 976             }
 977             ObligationCauseCode::VariableType(_) => {
 978                 err.note("all local variables must have a statically known size");
 979             }
 980             ObligationCauseCode::ReturnType => {
 981                 err.note("the return type of a function must have a \
 982                           statically known size");
 983             }
 984             ObligationCauseCode::AssignmentLhsSized => {
 985                 err.note("the left-hand-side of an assignment must have a statically known size");
 986             }
 987             ObligationCauseCode::StructInitializerSized => {
 988                 err.note("structs must have a statically known size to be initialized");
 989             }
 990             ObligationCauseCode::FieldSized => {
 991                 err.note("only the last field of a struct may have a dynamically sized type");
 992             }
 993             ObligationCauseCode::ConstSized => {
 994                 err.note("constant expressions must have a statically known size");
 995             }
 996             ObligationCauseCode::SharedStatic => {
 997                 err.note("shared static variables must have a type that implements `Sync`");
 998             }
 999             ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
1000                 let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1001                 err.note(&format!("required because it appears within the type `{}`",
1002                                   parent_trait_ref.0.self_ty()));
1003                 let parent_predicate = parent_trait_ref.to_predicate();
1004                 self.note_obligation_cause_code(err,
1005                                                 &parent_predicate,
1006                                                 &data.parent_code);
1007             }
1008             ObligationCauseCode::ImplDerivedObligation(ref data) => {
1009                 let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1010                 err.note(
1011                     &format!("required because of the requirements on the impl of `{}` for `{}`",
1012                              parent_trait_ref,
1013                              parent_trait_ref.0.self_ty()));
1014                 let parent_predicate = parent_trait_ref.to_predicate();
1015                 self.note_obligation_cause_code(err,
1016                                                 &parent_predicate,
1017                                                 &data.parent_code);
1018             }
1019             ObligationCauseCode::CompareImplMethodObligation { .. } => {
1020                 err.note(
1021                     &format!("the requirement `{}` appears on the impl method \
1022                               but not on the corresponding trait method",
1023                              predicate));
1024             }
1025         }
1026     }
1027
1028     fn suggest_new_overflow_limit(&self, err: &mut DiagnosticBuilder) {
1029         let current_limit = self.tcx.sess.recursion_limit.get();
1030         let suggested_limit = current_limit * 2;
1031         err.help(&format!(
1032                           "consider adding a `#![recursion_limit=\"{}\"]` attribute to your crate",
1033                           suggested_limit));
1034     }
1035 }
1036