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     MethodViolationCode,
  25 };
  26
  27 use fmt_macros::{Parser, Piece, Position};
  28 use hir::def_id::DefId;
  29 use infer::{self, InferCtxt, TypeOrigin};
  30 use ty::{self, ToPredicate, ToPolyTraitRef, Ty, TyCtxt, TypeFoldable};
  31 use ty::error::ExpectedFound;
  32 use ty::fast_reject;
  33 use ty::fold::TypeFolder;
  34 use ty::subst::{self, Subst, TypeSpace};
  35 use util::nodemap::{FnvHashMap, FnvHashSet};
  36
  37 use std::cmp;
  38 use std::fmt;
  39 use syntax::ast;
  40 use syntax::attr::{AttributeMethods, AttrMetaMethods};
  41 use syntax_pos::Span;
  42 use errors::DiagnosticBuilder;
  43
  44 #[derive(Debug, PartialEq, Eq, Hash)]
  45 pub struct TraitErrorKey<'tcx> {
  46     span: Span,
  47     warning_node_id: Option<ast::NodeId>,
  48     predicate: ty::Predicate<'tcx>
  49 }
  50
  51 impl<'a, 'gcx, 'tcx> TraitErrorKey<'tcx> {
  52     fn from_error(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
  53                   e: &FulfillmentError<'tcx>,
  54                   warning_node_id: Option<ast::NodeId>) -> Self {
  55         let predicate =
  56             infcx.resolve_type_vars_if_possible(&e.obligation.predicate);
  57         TraitErrorKey {
  58             span: e.obligation.cause.span,
  59             predicate: infcx.tcx.erase_regions(&predicate),
  60             warning_node_id: warning_node_id
  61         }
  62     }
  63 }
  64
  65 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
  66     pub fn report_fulfillment_errors(&self, errors: &Vec<FulfillmentError<'tcx>>) {
  67         for error in errors {
  68             self.report_fulfillment_error(error, None);
  69         }
  70     }
  71
  72     pub fn report_fulfillment_errors_as_warnings(&self,
  73                                                  errors: &Vec<FulfillmentError<'tcx>>,
  74                                                  node_id: ast::NodeId) {
  75         for error in errors {
  76             self.report_fulfillment_error(error, Some(node_id));
  77         }
  78     }
  79
  80     fn report_fulfillment_error(&self,
  81                                 error: &FulfillmentError<'tcx>,
  82                                 warning_node_id: Option<ast::NodeId>) {
  83         let error_key = TraitErrorKey::from_error(self, error, warning_node_id);
  84         debug!("report_fulfillment_errors({:?}) - key={:?}",
  85                error, error_key);
  86         if !self.reported_trait_errors.borrow_mut().insert(error_key) {
  87             debug!("report_fulfillment_errors: skipping duplicate");
  88             return;
  89         }
  90         match error.code {
  91             FulfillmentErrorCode::CodeSelectionError(ref e) => {
  92                 self.report_selection_error(&error.obligation, e, warning_node_id);
  93             }
  94             FulfillmentErrorCode::CodeProjectionError(ref e) => {
  95                 self.report_projection_error(&error.obligation, e, warning_node_id);
  96             }
  97             FulfillmentErrorCode::CodeAmbiguity => {
  98                 self.maybe_report_ambiguity(&error.obligation);
  99             }
 100         }
 101     }
 102
 103     fn report_projection_error(&self,
 104                                obligation: &PredicateObligation<'tcx>,
 105                                error: &MismatchedProjectionTypes<'tcx>,
 106                                warning_node_id: Option<ast::NodeId>)
 107     {
 108         let predicate =
 109             self.resolve_type_vars_if_possible(&obligation.predicate);
 110
 111         if predicate.references_error() {
 112             return
 113         }
 114         if let Some(warning_node_id) = warning_node_id {
 115             self.tcx.sess.add_lint(
 116                 ::lint::builtin::UNSIZED_IN_TUPLE,
 117                 warning_node_id,
 118                 obligation.cause.span,
 119                 format!("type mismatch resolving `{}`: {}",
 120                         predicate,
 121                         error.err));
 122             return
 123         }
 124         self.probe(|_| {
 125             let origin = TypeOrigin::Misc(obligation.cause.span);
 126             let err_buf;
 127             let mut err = &error.err;
 128             let mut values = None;
 129
 130             // try to find the mismatched types to report the error with.
 131             //
 132             // this can fail if the problem was higher-ranked, in which
 133             // cause I have no idea for a good error message.
 134             if let ty::Predicate::Projection(ref data) = predicate {
 135                 let mut selcx = SelectionContext::new(self);
 136                 let (data, _) = self.replace_late_bound_regions_with_fresh_var(
 137                     obligation.cause.span,
 138                     infer::LateBoundRegionConversionTime::HigherRankedType,
 139                     data);
 140                 let normalized = super::normalize_projection_type(
 141                     &mut selcx,
 142                     data.projection_ty,
 143                     obligation.cause.clone(),
 144                     0
 145                 );
 146                 let origin = TypeOrigin::Misc(obligation.cause.span);
 147                 if let Err(error) = self.eq_types(
 148                     false, origin,
 149                     data.ty, normalized.value
 150                 ) {
 151                     values = Some(infer::ValuePairs::Types(ExpectedFound {
 152                         expected: normalized.value,
 153                         found: data.ty,
 154                     }));
 155                     err_buf = error;
 156                     err = &err_buf;
 157                 }
 158             }
 159
 160             let mut diag = type_err!(
 161                 self,
 162                 origin,
 163                 values,
 164                 err,
 165                 E0271,
 166                 "type mismatch resolving `{}`",
 167                 predicate);
 168             self.note_obligation_cause(&mut diag, obligation);
 169             diag.emit();
 170         });
 171     }
 172
 173     fn impl_substs(&self,
 174                    did: DefId,
 175                    obligation: PredicateObligation<'tcx>)
 176                    -> subst::Substs<'tcx> {
 177         let tcx = self.tcx;
 178
 179         let ity = tcx.lookup_item_type(did);
 180         let (tps, rps, _) =
 181             (ity.generics.types.get_slice(TypeSpace),
 182              ity.generics.regions.get_slice(TypeSpace),
 183              ity.ty);
 184
 185         let rps = self.region_vars_for_defs(obligation.cause.span, rps);
 186         let mut substs = subst::Substs::new(
 187             subst::VecPerParamSpace::empty(),
 188             subst::VecPerParamSpace::new(rps, Vec::new(), Vec::new()));
 189         self.type_vars_for_defs(obligation.cause.span,
 190                                 TypeSpace,
 191                                 &mut substs,
 192                                 tps);
 193         substs
 194     }
 195
 196     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
 197         /// returns the fuzzy category of a given type, or None
 198         /// if the type can be equated to any type.
 199         fn type_category<'tcx>(t: Ty<'tcx>) -> Option<u32> {
 200             match t.sty {
 201                 ty::TyBool => Some(0),
 202                 ty::TyChar => Some(1),
 203                 ty::TyStr => Some(2),
 204                 ty::TyInt(..) | ty::TyUint(..) |
 205                 ty::TyInfer(ty::IntVar(..)) => Some(3),
 206                 ty::TyFloat(..) | ty::TyInfer(ty::FloatVar(..)) => Some(4),
 207                 ty::TyEnum(..) => Some(5),
 208                 ty::TyStruct(..) => Some(6),
 209                 ty::TyBox(..) | ty::TyRef(..) | ty::TyRawPtr(..) => Some(7),
 210                 ty::TyArray(..) | ty::TySlice(..) => Some(8),
 211                 ty::TyFnDef(..) | ty::TyFnPtr(..) => Some(9),
 212                 ty::TyTrait(..) => Some(10),
 213                 ty::TyClosure(..) => Some(11),
 214                 ty::TyTuple(..) => Some(12),
 215                 ty::TyProjection(..) => Some(13),
 216                 ty::TyParam(..) => Some(14),
 217                 ty::TyInfer(..) | ty::TyError => None
 218             }
 219         }
 220
 221         match (type_category(a), type_category(b)) {
 222             (Some(cat_a), Some(cat_b)) => match (&a.sty, &b.sty) {
 223                 (&ty::TyStruct(def_a, _), &ty::TyStruct(def_b, _)) |
 224                 (&ty::TyEnum(def_a, _), &ty::TyEnum(def_b, _)) =>
 225                     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_trait_ref = tcx
 249                     .impl_trait_ref(def_id)
 250                     .unwrap()
 251                     .subst(tcx, &self.impl_substs(def_id, obligation.clone()));
 252
 253                 let impl_self_ty = impl_trait_ref.self_ty();
 254
 255                 if let Ok(..) = self.can_equate(&trait_self_ty, &impl_self_ty) {
 256                     self_match_impls.push(def_id);
 257
 258                     if trait_ref.substs.types.get_slice(TypeSpace).iter()
 259                         .zip(impl_trait_ref.substs.types.get_slice(TypeSpace))
 260                         .all(|(u,v)| self.fuzzy_match_tys(u, v))
 261                     {
 262                         fuzzy_match_impls.push(def_id);
 263                     }
 264                 }
 265             });
 266
 267         let impl_def_id = if self_match_impls.len() == 1 {
 268             self_match_impls[0]
 269         } else if fuzzy_match_impls.len() == 1 {
 270             fuzzy_match_impls[0]
 271         } else {
 272             return None
 273         };
 274
 275         if tcx.has_attr(impl_def_id, "rustc_on_unimplemented") {
 276             Some(impl_def_id)
 277         } else {
 278             None
 279         }
 280     }
 281
 282     fn on_unimplemented_note(&self,
 283                              trait_ref: ty::PolyTraitRef<'tcx>,
 284                              obligation: &PredicateObligation<'tcx>) -> Option<String> {
 285         let def_id = self.impl_similar_to(trait_ref, obligation)
 286             .unwrap_or(trait_ref.def_id());
 287         let trait_ref = trait_ref.skip_binder();
 288
 289         let span = obligation.cause.span;
 290         let mut report = None;
 291         for item in self.tcx.get_attrs(def_id).iter() {
 292             if item.check_name("rustc_on_unimplemented") {
 293                 let err_sp = item.meta().span.substitute_dummy(span);
 294                 let def = self.tcx.lookup_trait_def(trait_ref.def_id);
 295                 let trait_str = def.trait_ref.to_string();
 296                 if let Some(ref istring) = item.value_str() {
 297                     let mut generic_map = def.generics.types.iter_enumerated()
 298                                              .map(|(param, i, gen)| {
 299                                                    (gen.name.as_str().to_string(),
 300                                                     trait_ref.substs.types.get(param, i)
 301                                                              .to_string())
 302                                                   }).collect::<FnvHashMap<String, String>>();
 303                     generic_map.insert("Self".to_string(),
 304                                        trait_ref.self_ty().to_string());
 305                     let parser = Parser::new(&istring);
 306                     let mut errored = false;
 307                     let err: String = parser.filter_map(|p| {
 308                         match p {
 309                             Piece::String(s) => Some(s),
 310                             Piece::NextArgument(a) => match a.position {
 311                                 Position::ArgumentNamed(s) => match generic_map.get(s) {
 312                                     Some(val) => Some(val),
 313                                     None => {
 314                                         span_err!(self.tcx.sess, err_sp, E0272,
 315                                                        "the #[rustc_on_unimplemented] \
 316                                                                 attribute on \
 317                                                                 trait definition for {} refers to \
 318                                                                 non-existent type parameter {}",
 319                                                                trait_str, s);
 320                                         errored = true;
 321                                         None
 322                                     }
 323                                 },
 324                                 _ => {
 325                                     span_err!(self.tcx.sess, err_sp, E0273,
 326                                               "the #[rustc_on_unimplemented] attribute \
 327                                                on trait definition for {} must have \
 328                                                named format arguments, eg \
 329                                                `#[rustc_on_unimplemented = \
 330                                                 \"foo {{T}}\"]`", trait_str);
 331                                     errored = true;
 332                                     None
 333                                 }
 334                             }
 335                         }
 336                     }).collect();
 337                     // Report only if the format string checks out
 338                     if !errored {
 339                         report = Some(err);
 340                     }
 341                 } else {
 342                     span_err!(self.tcx.sess, err_sp, E0274,
 343                                             "the #[rustc_on_unimplemented] attribute on \
 344                                                      trait definition for {} must have a value, \
 345                                                      eg `#[rustc_on_unimplemented = \"foo\"]`",
 346                                                      trait_str);
 347                 }
 348                 break;
 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                                       trait_ref: ty::PolyTraitRef<'tcx>,
 387                                       err: &mut DiagnosticBuilder)
 388     {
 389         let simp = fast_reject::simplify_type(self.tcx,
 390                                               trait_ref.skip_binder().self_ty(),
 391                                               true);
 392         let mut impl_candidates = Vec::new();
 393         let trait_def = self.tcx.lookup_trait_def(trait_ref.def_id());
 394
 395         match simp {
 396             Some(simp) => trait_def.for_each_impl(self.tcx, |def_id| {
 397                 let imp = self.tcx.impl_trait_ref(def_id).unwrap();
 398                 let imp_simp = fast_reject::simplify_type(self.tcx,
 399                                                           imp.self_ty(),
 400                                                           true);
 401                 if let Some(imp_simp) = imp_simp {
 402                     if simp != imp_simp {
 403                         return;
 404                     }
 405                 }
 406                 impl_candidates.push(imp);
 407             }),
 408             None => trait_def.for_each_impl(self.tcx, |def_id| {
 409                 impl_candidates.push(
 410                     self.tcx.impl_trait_ref(def_id).unwrap());
 411             })
 412         };
 413
 414         if impl_candidates.is_empty() {
 415             return;
 416         }
 417
 418         err.help(&format!("the following implementations were found:"));
 419
 420         let end = cmp::min(4, impl_candidates.len());
 421         for candidate in &impl_candidates[0..end] {
 422             err.help(&format!("  {:?}", candidate));
 423         }
 424         if impl_candidates.len() > 4 {
 425             err.help(&format!("and {} others", impl_candidates.len()-4));
 426         }
 427     }
 428
 429     /// Reports that an overflow has occurred and halts compilation. We
 430     /// halt compilation unconditionally because it is important that
 431     /// overflows never be masked -- they basically represent computations
 432     /// whose result could not be truly determined and thus we can't say
 433     /// if the program type checks or not -- and they are unusual
 434     /// occurrences in any case.
 435     pub fn report_overflow_error<T>(&self,
 436                                     obligation: &Obligation<'tcx, T>,
 437                                     suggest_increasing_limit: bool) -> !
 438         where T: fmt::Display + TypeFoldable<'tcx>
 439     {
 440         let predicate =
 441             self.resolve_type_vars_if_possible(&obligation.predicate);
 442         let mut err = struct_span_err!(self.tcx.sess, obligation.cause.span, E0275,
 443                                        "overflow evaluating the requirement `{}`",
 444                                        predicate);
 445
 446         if suggest_increasing_limit {
 447             self.suggest_new_overflow_limit(&mut err);
 448         }
 449
 450         self.note_obligation_cause(&mut err, obligation);
 451
 452         err.emit();
 453         self.tcx.sess.abort_if_errors();
 454         bug!();
 455     }
 456
 457     /// Reports that a cycle was detected which led to overflow and halts
 458     /// compilation. This is equivalent to `report_overflow_error` except
 459     /// that we can give a more helpful error message (and, in particular,
 460     /// we do not suggest increasing the overflow limit, which is not
 461     /// going to help).
 462     pub fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 463         let cycle = self.resolve_type_vars_if_possible(&cycle.to_owned());
 464         assert!(cycle.len() > 0);
 465
 466         debug!("report_overflow_error_cycle: cycle={:?}", cycle);
 467
 468         self.report_overflow_error(&cycle[0], false);
 469     }
 470
 471     pub fn report_selection_error(&self,
 472                                   obligation: &PredicateObligation<'tcx>,
 473                                   error: &SelectionError<'tcx>,
 474                                   warning_node_id: Option<ast::NodeId>)
 475     {
 476         let span = obligation.cause.span;
 477         let mut err = match *error {
 478             SelectionError::Unimplemented => {
 479                 if let ObligationCauseCode::CompareImplMethodObligation = obligation.cause.code {
 480                     span_err!(
 481                         self.tcx.sess, span, E0276,
 482                         "the requirement `{}` appears on the impl \
 483                          method but not on the corresponding trait method",
 484                         obligation.predicate);
 485                     return;
 486                 } else {
 487                     match obligation.predicate {
 488                         ty::Predicate::Trait(ref trait_predicate) => {
 489                             let trait_predicate =
 490                                 self.resolve_type_vars_if_possible(trait_predicate);
 491
 492                             if self.tcx.sess.has_errors() && trait_predicate.references_error() {
 493                                 return;
 494                             } else {
 495                                 let trait_ref = trait_predicate.to_poly_trait_ref();
 496
 497                                 if let Some(warning_node_id) = warning_node_id {
 498                                     self.tcx.sess.add_lint(
 499                                         ::lint::builtin::UNSIZED_IN_TUPLE,
 500                                         warning_node_id,
 501                                         obligation.cause.span,
 502                                         format!("the trait bound `{}` is not satisfied",
 503                                                 trait_ref.to_predicate()));
 504                                     return;
 505                                 }
 506
 507                                 let mut err = struct_span_err!(
 508                                     self.tcx.sess, span, E0277,
 509                                     "the trait bound `{}` is not satisfied",
 510                                     trait_ref.to_predicate());
 511
 512                                 // Try to report a help message
 513
 514                                 if !trait_ref.has_infer_types() &&
 515                                     self.predicate_can_apply(trait_ref) {
 516                                     // If a where-clause may be useful, remind the
 517                                     // user that they can add it.
 518                                     //
 519                                     // don't display an on-unimplemented note, as
 520                                     // these notes will often be of the form
 521                                     //     "the type `T` can't be frobnicated"
 522                                     // which is somewhat confusing.
 523                                     err.help(&format!("consider adding a `where {}` bound",
 524                                                       trait_ref.to_predicate()));
 525                                 } else if let Some(s) = self.on_unimplemented_note(trait_ref,
 526                                                                                    obligation) {
 527                                     // If it has a custom "#[rustc_on_unimplemented]"
 528                                     // error message, let's display it!
 529                                     err.note(&s);
 530                                 } else {
 531                                     // If we can't show anything useful, try to find
 532                                     // similar impls.
 533                                     let impl_candidates =
 534                                         self.find_similar_impl_candidates(trait_ref);
 535                                     if impl_candidates.len() > 0 {
 536                                         self.report_similar_impl_candidates(trait_ref, &mut err);
 537                                     }
 538                                 }
 539                                 err
 540                             }
 541                         }
 542
 543                         ty::Predicate::Equate(ref predicate) => {
 544                             let predicate = self.resolve_type_vars_if_possible(predicate);
 545                             let err = self.equality_predicate(span,
 546                                                               &predicate).err().unwrap();
 547                             struct_span_err!(self.tcx.sess, span, E0278,
 548                                 "the requirement `{}` is not satisfied (`{}`)",
 549                                 predicate, err)
 550                         }
 551
 552                         ty::Predicate::RegionOutlives(ref predicate) => {
 553                             let predicate = self.resolve_type_vars_if_possible(predicate);
 554                             let err = self.region_outlives_predicate(span,
 555                                                                      &predicate).err().unwrap();
 556                             struct_span_err!(self.tcx.sess, span, E0279,
 557                                 "the requirement `{}` is not satisfied (`{}`)",
 558                                 predicate, err)
 559                         }
 560
 561                         ty::Predicate::Projection(..) | ty::Predicate::TypeOutlives(..) => {
 562                             let predicate =
 563                                 self.resolve_type_vars_if_possible(&obligation.predicate);
 564                             struct_span_err!(self.tcx.sess, span, E0280,
 565                                 "the requirement `{}` is not satisfied",
 566                                 predicate)
 567                         }
 568
 569                         ty::Predicate::ObjectSafe(trait_def_id) => {
 570                             let violations = self.tcx.object_safety_violations(trait_def_id);
 571                             let err = self.tcx.report_object_safety_error(span,
 572                                                                           trait_def_id,
 573                                                                           warning_node_id,
 574                                                                           violations);
 575                             if let Some(err) = err {
 576                                 err
 577                             } else {
 578                                 return;
 579                             }
 580                         }
 581
 582                         ty::Predicate::ClosureKind(closure_def_id, kind) => {
 583                             let found_kind = self.closure_kind(closure_def_id).unwrap();
 584                             let closure_span = self.tcx.map.span_if_local(closure_def_id).unwrap();
 585                             let mut err = struct_span_err!(
 586                                 self.tcx.sess, closure_span, E0525,
 587                                 "expected a closure that implements the `{}` trait, \
 588                                  but this closure only implements `{}`",
 589                                 kind,
 590                                 found_kind);
 591                             err.span_note(
 592                                 obligation.cause.span,
 593                                 &format!("the requirement to implement \
 594                                           `{}` derives from here", kind));
 595                             err.emit();
 596                             return;
 597                         }
 598
 599                         ty::Predicate::WellFormed(ty) => {
 600                             // WF predicates cannot themselves make
 601                             // errors. They can only block due to
 602                             // ambiguity; otherwise, they always
 603                             // degenerate into other obligations
 604                             // (which may fail).
 605                             span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 606                         }
 607
 608                         ty::Predicate::Rfc1592(ref data) => {
 609                             span_bug!(
 610                                 obligation.cause.span,
 611                                 "RFC1592 predicate not satisfied for {:?}",
 612                                 data);
 613                         }
 614                     }
 615                 }
 616             }
 617
 618             OutputTypeParameterMismatch(ref expected_trait_ref, ref actual_trait_ref, ref e) => {
 619                 let expected_trait_ref = self.resolve_type_vars_if_possible(&*expected_trait_ref);
 620                 let actual_trait_ref = self.resolve_type_vars_if_possible(&*actual_trait_ref);
 621                 if actual_trait_ref.self_ty().references_error() {
 622                     return;
 623                 }
 624                 struct_span_err!(self.tcx.sess, span, E0281,
 625                     "type mismatch: the type `{}` implements the trait `{}`, \
 626                      but the trait `{}` is required ({})",
 627                     expected_trait_ref.self_ty(),
 628                     expected_trait_ref,
 629                     actual_trait_ref,
 630                     e)
 631             }
 632
 633             TraitNotObjectSafe(did) => {
 634                 let violations = self.tcx.object_safety_violations(did);
 635                 let err = self.tcx.report_object_safety_error(span, did,
 636                                                               warning_node_id,
 637                                                               violations);
 638                 if let Some(err) = err {
 639                     err
 640                 } else {
 641                     return;
 642                 }
 643             }
 644         };
 645         self.note_obligation_cause(&mut err, obligation);
 646         err.emit();
 647     }
 648 }
 649
 650 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
 651     pub fn recursive_type_with_infinite_size_error(self,
 652                                                    type_def_id: DefId)
 653                                                    -> DiagnosticBuilder<'tcx>
 654     {
 655         assert!(type_def_id.is_local());
 656         let span = self.map.span_if_local(type_def_id).unwrap();
 657         let mut err = struct_span_err!(self.sess, span, E0072,
 658                                        "recursive type `{}` has infinite size",
 659                                        self.item_path_str(type_def_id));
 660         err.help(&format!("insert indirection (e.g., a `Box`, `Rc`, or `&`) \
 661                            at some point to make `{}` representable",
 662                           self.item_path_str(type_def_id)));
 663         err
 664     }
 665
 666     pub fn report_object_safety_error(self,
 667                                       span: Span,
 668                                       trait_def_id: DefId,
 669                                       warning_node_id: Option<ast::NodeId>,
 670                                       violations: Vec<ObjectSafetyViolation>)
 671                                       -> Option<DiagnosticBuilder<'tcx>>
 672     {
 673         let mut err = match warning_node_id {
 674             Some(_) => None,
 675             None => {
 676                 Some(struct_span_err!(
 677                     self.sess, span, E0038,
 678                     "the trait `{}` cannot be made into an object",
 679                     self.item_path_str(trait_def_id)))
 680             }
 681         };
 682
 683         let mut reported_violations = FnvHashSet();
 684         for violation in violations {
 685             if !reported_violations.insert(violation.clone()) {
 686                 continue;
 687             }
 688             let buf;
 689             let note = match violation {
 690                 ObjectSafetyViolation::SizedSelf => {
 691                     "the trait cannot require that `Self : Sized`"
 692                 }
 693
 694                 ObjectSafetyViolation::SupertraitSelf => {
 695                     "the trait cannot use `Self` as a type parameter \
 696                          in the supertrait listing"
 697                 }
 698
 699                 ObjectSafetyViolation::Method(method,
 700                                               MethodViolationCode::StaticMethod) => {
 701                     buf = format!("method `{}` has no receiver",
 702                                   method.name);
 703                     &buf
 704                 }
 705
 706                 ObjectSafetyViolation::Method(method,
 707                                               MethodViolationCode::ReferencesSelf) => {
 708                     buf = format!("method `{}` references the `Self` type \
 709                                        in its arguments or return type",
 710                                   method.name);
 711                     &buf
 712                 }
 713
 714                 ObjectSafetyViolation::Method(method,
 715                                               MethodViolationCode::Generic) => {
 716                     buf = format!("method `{}` has generic type parameters",
 717                                   method.name);
 718                     &buf
 719                 }
 720             };
 721             match (warning_node_id, &mut err) {
 722                 (Some(node_id), &mut None) => {
 723                     self.sess.add_lint(
 724                         ::lint::builtin::OBJECT_UNSAFE_FRAGMENT,
 725                         node_id,
 726                         span,
 727                         note.to_string());
 728                 }
 729                 (None, &mut Some(ref mut err)) => {
 730                     err.note(note);
 731                 }
 732                 _ => unreachable!()
 733             }
 734         }
 735         err
 736     }
 737 }
 738
 739 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
 740     fn maybe_report_ambiguity(&self, obligation: &PredicateObligation<'tcx>) {
 741         // Unable to successfully determine, probably means
 742         // insufficient type information, but could mean
 743         // ambiguous impls. The latter *ought* to be a
 744         // coherence violation, so we don't report it here.
 745
 746         let predicate = self.resolve_type_vars_if_possible(&obligation.predicate);
 747
 748         debug!("maybe_report_ambiguity(predicate={:?}, obligation={:?})",
 749                predicate,
 750                obligation);
 751
 752         // Ambiguity errors are often caused as fallout from earlier
 753         // errors. So just ignore them if this infcx is tainted.
 754         if self.is_tainted_by_errors() {
 755             return;
 756         }
 757
 758         match predicate {
 759             ty::Predicate::Trait(ref data) => {
 760                 let trait_ref = data.to_poly_trait_ref();
 761                 let self_ty = trait_ref.self_ty();
 762                 let all_types = &trait_ref.substs().types;
 763                 if all_types.references_error() {
 764                 } else {
 765                     // Typically, this ambiguity should only happen if
 766                     // there are unresolved type inference variables
 767                     // (otherwise it would suggest a coherence
 768                     // failure). But given #21974 that is not necessarily
 769                     // the case -- we can have multiple where clauses that
 770                     // are only distinguished by a region, which results
 771                     // in an ambiguity even when all types are fully
 772                     // known, since we don't dispatch based on region
 773                     // relationships.
 774
 775                     // This is kind of a hack: it frequently happens that some earlier
 776                     // error prevents types from being fully inferred, and then we get
 777                     // a bunch of uninteresting errors saying something like "<generic
 778                     // #0> doesn't implement Sized".  It may even be true that we
 779                     // could just skip over all checks where the self-ty is an
 780                     // inference variable, but I was afraid that there might be an
 781                     // inference variable created, registered as an obligation, and
 782                     // then never forced by writeback, and hence by skipping here we'd
 783                     // be ignoring the fact that we don't KNOW the type works
 784                     // out. Though even that would probably be harmless, given that
 785                     // we're only talking about builtin traits, which are known to be
 786                     // inhabited. But in any case I just threw in this check for
 787                     // has_errors() to be sure that compilation isn't happening
 788                     // anyway. In that case, why inundate the user.
 789                     if !self.tcx.sess.has_errors() {
 790                         if
 791                             self.tcx.lang_items.sized_trait()
 792                             .map_or(false, |sized_id| sized_id == trait_ref.def_id())
 793                         {
 794                             self.need_type_info(obligation.cause.span, self_ty);
 795                         } else {
 796                             let mut err = struct_span_err!(self.tcx.sess,
 797                                                            obligation.cause.span, E0283,
 798                                                            "type annotations required: \
 799                                                             cannot resolve `{}`",
 800                                                            predicate);
 801                             self.note_obligation_cause(&mut err, obligation);
 802                             err.emit();
 803                         }
 804                     }
 805                 }
 806             }
 807
 808             ty::Predicate::WellFormed(ty) => {
 809                 // Same hacky approach as above to avoid deluging user
 810                 // with error messages.
 811                 if !ty.references_error() && !self.tcx.sess.has_errors() {
 812                     self.need_type_info(obligation.cause.span, ty);
 813                 }
 814             }
 815
 816             _ => {
 817                 if !self.tcx.sess.has_errors() {
 818                     let mut err = struct_span_err!(self.tcx.sess,
 819                                                    obligation.cause.span, E0284,
 820                                                    "type annotations required: \
 821                                                     cannot resolve `{}`",
 822                                                    predicate);
 823                     self.note_obligation_cause(&mut err, obligation);
 824                     err.emit();
 825                 }
 826             }
 827         }
 828     }
 829
 830     /// Returns whether the trait predicate may apply for *some* assignment
 831     /// to the type parameters.
 832     fn predicate_can_apply(&self, pred: ty::PolyTraitRef<'tcx>) -> bool {
 833         struct ParamToVarFolder<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
 834             infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
 835             var_map: FnvHashMap<Ty<'tcx>, Ty<'tcx>>
 836         }
 837
 838         impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for ParamToVarFolder<'a, 'gcx, 'tcx> {
 839             fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> { self.infcx.tcx }
 840
 841             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
 842                 if let ty::TyParam(..) = ty.sty {
 843                     let infcx = self.infcx;
 844                     self.var_map.entry(ty).or_insert_with(|| infcx.next_ty_var())
 845                 } else {
 846                     ty.super_fold_with(self)
 847                 }
 848             }
 849         }
 850
 851         self.probe(|_| {
 852             let mut selcx = SelectionContext::new(self);
 853
 854             let cleaned_pred = pred.fold_with(&mut ParamToVarFolder {
 855                 infcx: self,
 856                 var_map: FnvHashMap()
 857             });
 858
 859             let cleaned_pred = super::project::normalize(
 860                 &mut selcx,
 861                 ObligationCause::dummy(),
 862                 &cleaned_pred
 863             ).value;
 864
 865             let obligation = Obligation::new(
 866                 ObligationCause::dummy(),
 867                 cleaned_pred.to_predicate()
 868             );
 869
 870             selcx.evaluate_obligation(&obligation)
 871         })
 872     }
 873
 874
 875     fn need_type_info(&self, span: Span, ty: Ty<'tcx>) {
 876         span_err!(self.tcx.sess, span, E0282,
 877                   "unable to infer enough type information about `{}`; \
 878                    type annotations or generic parameter binding required",
 879                   ty);
 880     }
 881
 882     fn note_obligation_cause<T>(&self,
 883                                 err: &mut DiagnosticBuilder,
 884                                 obligation: &Obligation<'tcx, T>)
 885         where T: fmt::Display
 886     {
 887         self.note_obligation_cause_code(err,
 888                                         &obligation.predicate,
 889                                         &obligation.cause.code);
 890     }
 891
 892     fn note_obligation_cause_code<T>(&self,
 893                                      err: &mut DiagnosticBuilder,
 894                                      predicate: &T,
 895                                      cause_code: &ObligationCauseCode<'tcx>)
 896         where T: fmt::Display
 897     {
 898         let tcx = self.tcx;
 899         match *cause_code {
 900             ObligationCauseCode::MiscObligation => { }
 901             ObligationCauseCode::SliceOrArrayElem => {
 902                 err.note("slice and array elements must have `Sized` type");
 903             }
 904             ObligationCauseCode::TupleElem => {
 905                 err.note("tuple elements must have `Sized` type");
 906             }
 907             ObligationCauseCode::ProjectionWf(data) => {
 908                 err.note(&format!("required so that the projection `{}` is well-formed",
 909                                   data));
 910             }
 911             ObligationCauseCode::ReferenceOutlivesReferent(ref_ty) => {
 912                 err.note(&format!("required so that reference `{}` does not outlive its referent",
 913                                   ref_ty));
 914             }
 915             ObligationCauseCode::ItemObligation(item_def_id) => {
 916                 let item_name = tcx.item_path_str(item_def_id);
 917                 err.note(&format!("required by `{}`", item_name));
 918             }
 919             ObligationCauseCode::ObjectCastObligation(object_ty) => {
 920                 err.note(&format!("required for the cast to the object type `{}`",
 921                                   self.ty_to_string(object_ty)));
 922             }
 923             ObligationCauseCode::RepeatVec => {
 924                 err.note("the `Copy` trait is required because the \
 925                           repeated element will be copied");
 926             }
 927             ObligationCauseCode::VariableType(_) => {
 928                 err.note("all local variables must have a statically known size");
 929             }
 930             ObligationCauseCode::ReturnType => {
 931                 err.note("the return type of a function must have a \
 932                           statically known size");
 933             }
 934             ObligationCauseCode::AssignmentLhsSized => {
 935                 err.note("the left-hand-side of an assignment must have a statically known size");
 936             }
 937             ObligationCauseCode::StructInitializerSized => {
 938                 err.note("structs must have a statically known size to be initialized");
 939             }
 940             ObligationCauseCode::ClosureCapture(var_id, _, builtin_bound) => {
 941                 let def_id = tcx.lang_items.from_builtin_kind(builtin_bound).unwrap();
 942                 let trait_name = tcx.item_path_str(def_id);
 943                 let name = tcx.local_var_name_str(var_id);
 944                 err.note(
 945                     &format!("the closure that captures `{}` requires that all captured variables \
 946                               implement the trait `{}`",
 947                              name,
 948                              trait_name));
 949             }
 950             ObligationCauseCode::FieldSized => {
 951                 err.note("only the last field of a struct or enum variant \
 952                           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.note(&format!(
 993                           "consider adding a `#![recursion_limit=\"{}\"]` attribute to your crate",
 994                           suggested_limit));
 995     }
 996 }