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