src/librustc/traits/error_reporting.rs

   1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 use super::{
  12     FulfillmentError,
  13     FulfillmentErrorCode,
  14     MismatchedProjectionTypes,
  15     Obligation,
  16     ObligationCause,
  17     ObligationCauseCode,
  18     OnUnimplementedDirective,
  19     OnUnimplementedNote,
  20     OutputTypeParameterMismatch,
  21     TraitNotObjectSafe,
  22     PredicateObligation,
  23     Reveal,
  24     SelectionContext,
  25     SelectionError,
  26     ObjectSafetyViolation,
  27 };
  28
  29 use errors::DiagnosticBuilder;
  30 use hir;
  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, TypeError};
  39 use ty::fast_reject;
  40 use ty::fold::TypeFolder;
  41 use ty::subst::Subst;
  42 use ty::SubtypePredicate;
  43 use util::nodemap::{FxHashMap, FxHashSet};
  44
  45 use syntax_pos::{DUMMY_SP, Span};
  46
  47 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
  48     pub fn report_fulfillment_errors(&self,
  49                                      errors: &Vec<FulfillmentError<'tcx>>,
  50                                      body_id: Option<hir::BodyId>) {
  51         #[derive(Debug)]
  52         struct ErrorDescriptor<'tcx> {
  53             predicate: ty::Predicate<'tcx>,
  54             index: Option<usize>, // None if this is an old error
  55         }
  56
  57         let mut error_map : FxHashMap<_, _> =
  58             self.reported_trait_errors.borrow().iter().map(|(&span, predicates)| {
  59                 (span, predicates.iter().map(|predicate| ErrorDescriptor {
  60                     predicate: predicate.clone(),
  61                     index: None
  62                 }).collect())
  63             }).collect();
  64
  65         for (index, error) in errors.iter().enumerate() {
  66             error_map.entry(error.obligation.cause.span).or_insert(Vec::new()).push(
  67                 ErrorDescriptor {
  68                     predicate: error.obligation.predicate.clone(),
  69                     index: Some(index)
  70                 });
  71
  72             self.reported_trait_errors.borrow_mut()
  73                 .entry(error.obligation.cause.span).or_insert(Vec::new())
  74                 .push(error.obligation.predicate.clone());
  75         }
  76
  77         // We do this in 2 passes because we want to display errors in order, tho
  78         // maybe it *is* better to sort errors by span or something.
  79         let mut is_suppressed: Vec<bool> = errors.iter().map(|_| false).collect();
  80         for (_, error_set) in error_map.iter() {
  81             // We want to suppress "duplicate" errors with the same span.
  82             for error in error_set {
  83                 if let Some(index) = error.index {
  84                     // Suppress errors that are either:
  85                     // 1) strictly implied by another error.
  86                     // 2) implied by an error with a smaller index.
  87                     for error2 in error_set {
  88                         if error2.index.map_or(false, |index2| is_suppressed[index2]) {
  89                             // Avoid errors being suppressed by already-suppressed
  90                             // errors, to prevent all errors from being suppressed
  91                             // at once.
  92                             continue
  93                         }
  94
  95                         if self.error_implies(&error2.predicate, &error.predicate) &&
  96                             !(error2.index >= error.index &&
  97                               self.error_implies(&error.predicate, &error2.predicate))
  98                         {
  99                             info!("skipping {:?} (implied by {:?})", error, error2);
 100                             is_suppressed[index] = true;
 101                             break
 102                         }
 103                     }
 104                 }
 105             }
 106         }
 107
 108         for (error, suppressed) in errors.iter().zip(is_suppressed) {
 109             if !suppressed {
 110                 self.report_fulfillment_error(error, body_id);
 111             }
 112         }
 113     }
 114
 115     // returns if `cond` not occurring implies that `error` does not occur - i.e. that
 116     // `error` occurring implies that `cond` occurs.
 117     fn error_implies(&self,
 118                      cond: &ty::Predicate<'tcx>,
 119                      error: &ty::Predicate<'tcx>)
 120                      -> bool
 121     {
 122         if cond == error {
 123             return true
 124         }
 125
 126         let (cond, error) = match (cond, error) {
 127             (&ty::Predicate::Trait(..), &ty::Predicate::Trait(ref error))
 128                 => (cond, error),
 129             _ => {
 130                 // FIXME: make this work in other cases too.
 131                 return false
 132             }
 133         };
 134
 135         for implication in super::elaborate_predicates(self.tcx, vec![cond.clone()]) {
 136             if let ty::Predicate::Trait(implication) = implication {
 137                 let error = error.to_poly_trait_ref();
 138                 let implication = implication.to_poly_trait_ref();
 139                 // FIXME: I'm just not taking associated types at all here.
 140                 // Eventually I'll need to implement param-env-aware
 141                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
 142                 let param_env = ty::ParamEnv::empty(Reveal::UserFacing);
 143                 if let Ok(_) = self.can_sub(param_env, error, implication) {
 144                     debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
 145                     return true
 146                 }
 147             }
 148         }
 149
 150         false
 151     }
 152
 153     fn report_fulfillment_error(&self, error: &FulfillmentError<'tcx>,
 154                                 body_id: Option<hir::BodyId>) {
 155         debug!("report_fulfillment_errors({:?})", error);
 156         match error.code {
 157             FulfillmentErrorCode::CodeSelectionError(ref e) => {
 158                 self.report_selection_error(&error.obligation, e);
 159             }
 160             FulfillmentErrorCode::CodeProjectionError(ref e) => {
 161                 self.report_projection_error(&error.obligation, e);
 162             }
 163             FulfillmentErrorCode::CodeAmbiguity => {
 164                 self.maybe_report_ambiguity(&error.obligation, body_id);
 165             }
 166             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
 167                 self.report_mismatched_types(&error.obligation.cause,
 168                                              expected_found.expected,
 169                                              expected_found.found,
 170                                              err.clone())
 171                     .emit();
 172             }
 173         }
 174     }
 175
 176     fn report_projection_error(&self,
 177                                obligation: &PredicateObligation<'tcx>,
 178                                error: &MismatchedProjectionTypes<'tcx>)
 179     {
 180         let predicate =
 181             self.resolve_type_vars_if_possible(&obligation.predicate);
 182
 183         if predicate.references_error() {
 184             return
 185         }
 186
 187         self.probe(|_| {
 188             let err_buf;
 189             let mut err = &error.err;
 190             let mut values = None;
 191
 192             // try to find the mismatched types to report the error with.
 193             //
 194             // this can fail if the problem was higher-ranked, in which
 195             // cause I have no idea for a good error message.
 196             if let ty::Predicate::Projection(ref data) = predicate {
 197                 let mut selcx = SelectionContext::new(self);
 198                 let (data, _) = self.replace_late_bound_regions_with_fresh_var(
 199                     obligation.cause.span,
 200                     infer::LateBoundRegionConversionTime::HigherRankedType,
 201                     data);
 202                 let normalized = super::normalize_projection_type(
 203                     &mut selcx,
 204                     obligation.param_env,
 205                     data.projection_ty,
 206                     obligation.cause.clone(),
 207                     0
 208                 );
 209                 if let Err(error) = self.at(&obligation.cause, obligation.param_env)
 210                                         .eq(normalized.value, data.ty) {
 211                     values = Some(infer::ValuePairs::Types(ExpectedFound {
 212                         expected: normalized.value,
 213                         found: data.ty,
 214                     }));
 215                     err_buf = error;
 216                     err = &err_buf;
 217                 }
 218             }
 219
 220             let mut diag = struct_span_err!(
 221                 self.tcx.sess, obligation.cause.span, E0271,
 222                 "type mismatch resolving `{}`", predicate
 223             );
 224             self.note_type_err(&mut diag, &obligation.cause, None, values, err);
 225             self.note_obligation_cause(&mut diag, obligation);
 226             diag.emit();
 227         });
 228     }
 229
 230     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
 231         /// returns the fuzzy category of a given type, or None
 232         /// if the type can be equated to any type.
 233         fn type_category<'tcx>(t: Ty<'tcx>) -> Option<u32> {
 234             match t.sty {
 235                 ty::TyBool => Some(0),
 236                 ty::TyChar => Some(1),
 237                 ty::TyStr => Some(2),
 238                 ty::TyInt(..) | ty::TyUint(..) | ty::TyInfer(ty::IntVar(..)) => Some(3),
 239                 ty::TyFloat(..) | ty::TyInfer(ty::FloatVar(..)) => Some(4),
 240                 ty::TyRef(..) | ty::TyRawPtr(..) => Some(5),
 241                 ty::TyArray(..) | ty::TySlice(..) => Some(6),
 242                 ty::TyFnDef(..) | ty::TyFnPtr(..) => Some(7),
 243                 ty::TyDynamic(..) => Some(8),
 244                 ty::TyClosure(..) => Some(9),
 245                 ty::TyTuple(..) => Some(10),
 246                 ty::TyProjection(..) => Some(11),
 247                 ty::TyParam(..) => Some(12),
 248                 ty::TyAnon(..) => Some(13),
 249                 ty::TyNever => Some(14),
 250                 ty::TyAdt(adt, ..) => match adt.adt_kind() {
 251                     AdtKind::Struct => Some(15),
 252                     AdtKind::Union => Some(16),
 253                     AdtKind::Enum => Some(17),
 254                 },
 255                 ty::TyGenerator(..) => Some(18),
 256                 ty::TyInfer(..) | ty::TyError => None
 257             }
 258         }
 259
 260         match (type_category(a), type_category(b)) {
 261             (Some(cat_a), Some(cat_b)) => match (&a.sty, &b.sty) {
 262                 (&ty::TyAdt(def_a, _), &ty::TyAdt(def_b, _)) => def_a == def_b,
 263                 _ => cat_a == cat_b
 264             },
 265             // infer and error can be equated to all types
 266             _ => true
 267         }
 268     }
 269
 270     fn impl_similar_to(&self,
 271                        trait_ref: ty::PolyTraitRef<'tcx>,
 272                        obligation: &PredicateObligation<'tcx>)
 273                        -> Option<DefId>
 274     {
 275         let tcx = self.tcx;
 276         let param_env = obligation.param_env;
 277         let trait_ref = tcx.erase_late_bound_regions(&trait_ref);
 278         let trait_self_ty = trait_ref.self_ty();
 279
 280         let mut self_match_impls = vec![];
 281         let mut fuzzy_match_impls = vec![];
 282
 283         self.tcx.for_each_relevant_impl(
 284             trait_ref.def_id, trait_self_ty, |def_id| {
 285                 let impl_substs = self.fresh_substs_for_item(obligation.cause.span, def_id);
 286                 let impl_trait_ref = tcx
 287                     .impl_trait_ref(def_id)
 288                     .unwrap()
 289                     .subst(tcx, impl_substs);
 290
 291                 let impl_self_ty = impl_trait_ref.self_ty();
 292
 293                 if let Ok(..) = self.can_eq(param_env, trait_self_ty, impl_self_ty) {
 294                     self_match_impls.push(def_id);
 295
 296                     if trait_ref.substs.types().skip(1)
 297                         .zip(impl_trait_ref.substs.types().skip(1))
 298                         .all(|(u,v)| self.fuzzy_match_tys(u, v))
 299                     {
 300                         fuzzy_match_impls.push(def_id);
 301                     }
 302                 }
 303             });
 304
 305         let impl_def_id = if self_match_impls.len() == 1 {
 306             self_match_impls[0]
 307         } else if fuzzy_match_impls.len() == 1 {
 308             fuzzy_match_impls[0]
 309         } else {
 310             return None
 311         };
 312
 313         if tcx.has_attr(impl_def_id, "rustc_on_unimplemented") {
 314             Some(impl_def_id)
 315         } else {
 316             None
 317         }
 318     }
 319
 320     fn on_unimplemented_note(
 321         &self,
 322         trait_ref: ty::PolyTraitRef<'tcx>,
 323         obligation: &PredicateObligation<'tcx>) ->
 324         OnUnimplementedNote
 325     {
 326         let def_id = self.impl_similar_to(trait_ref, obligation)
 327             .unwrap_or(trait_ref.def_id());
 328         let trait_ref = *trait_ref.skip_binder();
 329
 330         let desugaring;
 331         let method;
 332         let mut flags = vec![];
 333         let direct = match obligation.cause.code {
 334             ObligationCauseCode::BuiltinDerivedObligation(..) |
 335             ObligationCauseCode::ImplDerivedObligation(..) => false,
 336             _ => true
 337         };
 338         if direct {
 339             // this is a "direct", user-specified, rather than derived,
 340             // obligation.
 341             flags.push(("direct", None));
 342         }
 343
 344         if let ObligationCauseCode::ItemObligation(item) = obligation.cause.code {
 345             // FIXME: maybe also have some way of handling methods
 346             // from other traits? That would require name resolution,
 347             // which we might want to be some sort of hygienic.
 348             //
 349             // Currently I'm leaving it for what I need for `try`.
 350             if self.tcx.trait_of_item(item) == Some(trait_ref.def_id) {
 351                 method = self.tcx.item_name(item).as_str();
 352                 flags.push(("from_method", None));
 353                 flags.push(("from_method", Some(&*method)));
 354             }
 355         }
 356
 357         if let Some(k) = obligation.cause.span.compiler_desugaring_kind() {
 358             desugaring = k.as_symbol().as_str();
 359             flags.push(("from_desugaring", None));
 360             flags.push(("from_desugaring", Some(&*desugaring)));
 361         }
 362
 363         if let Ok(Some(command)) = OnUnimplementedDirective::of_item(
 364             self.tcx, trait_ref.def_id, def_id
 365         ) {
 366             command.evaluate(self.tcx, trait_ref, &flags)
 367         } else {
 368             OnUnimplementedNote::empty()
 369         }
 370     }
 371
 372     fn find_similar_impl_candidates(&self,
 373                                     trait_ref: ty::PolyTraitRef<'tcx>)
 374                                     -> Vec<ty::TraitRef<'tcx>>
 375     {
 376         let simp = fast_reject::simplify_type(self.tcx,
 377                                               trait_ref.skip_binder().self_ty(),
 378                                               true);
 379         let mut impl_candidates = Vec::new();
 380
 381         match simp {
 382             Some(simp) => self.tcx.for_each_impl(trait_ref.def_id(), |def_id| {
 383                 let imp = self.tcx.impl_trait_ref(def_id).unwrap();
 384                 let imp_simp = fast_reject::simplify_type(self.tcx,
 385                                                           imp.self_ty(),
 386                                                           true);
 387                 if let Some(imp_simp) = imp_simp {
 388                     if simp != imp_simp {
 389                         return;
 390                     }
 391                 }
 392                 impl_candidates.push(imp);
 393             }),
 394             None => self.tcx.for_each_impl(trait_ref.def_id(), |def_id| {
 395                 impl_candidates.push(
 396                     self.tcx.impl_trait_ref(def_id).unwrap());
 397             })
 398         };
 399         impl_candidates
 400     }
 401
 402     fn report_similar_impl_candidates(&self,
 403                                       impl_candidates: Vec<ty::TraitRef<'tcx>>,
 404                                       err: &mut DiagnosticBuilder)
 405     {
 406         if impl_candidates.is_empty() {
 407             return;
 408         }
 409
 410         let end = if impl_candidates.len() <= 5 {
 411             impl_candidates.len()
 412         } else {
 413             4
 414         };
 415         err.help(&format!("the following implementations were found:{}{}",
 416                           &impl_candidates[0..end].iter().map(|candidate| {
 417                               format!("\n  {:?}", candidate)
 418                           }).collect::<String>(),
 419                           if impl_candidates.len() > 5 {
 420                               format!("\nand {} others", impl_candidates.len() - 4)
 421                           } else {
 422                               "".to_owned()
 423                           }
 424                           ));
 425     }
 426
 427     /// Reports that an overflow has occurred and halts compilation. We
 428     /// halt compilation unconditionally because it is important that
 429     /// overflows never be masked -- they basically represent computations
 430     /// whose result could not be truly determined and thus we can't say
 431     /// if the program type checks or not -- and they are unusual
 432     /// occurrences in any case.
 433     pub fn report_overflow_error<T>(&self,
 434                                     obligation: &Obligation<'tcx, T>,
 435                                     suggest_increasing_limit: bool) -> !
 436         where T: fmt::Display + TypeFoldable<'tcx>
 437     {
 438         let predicate =
 439             self.resolve_type_vars_if_possible(&obligation.predicate);
 440         let mut err = struct_span_err!(self.tcx.sess, obligation.cause.span, E0275,
 441                                        "overflow evaluating the requirement `{}`",
 442                                        predicate);
 443
 444         if suggest_increasing_limit {
 445             self.suggest_new_overflow_limit(&mut err);
 446         }
 447
 448         self.note_obligation_cause(&mut err, obligation);
 449
 450         err.emit();
 451         self.tcx.sess.abort_if_errors();
 452         bug!();
 453     }
 454
 455     /// Reports that a cycle was detected which led to overflow and halts
 456     /// compilation. This is equivalent to `report_overflow_error` except
 457     /// that we can give a more helpful error message (and, in particular,
 458     /// we do not suggest increasing the overflow limit, which is not
 459     /// going to help).
 460     pub fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 461         let cycle = self.resolve_type_vars_if_possible(&cycle.to_owned());
 462         assert!(cycle.len() > 0);
 463
 464         debug!("report_overflow_error_cycle: cycle={:?}", cycle);
 465
 466         self.report_overflow_error(&cycle[0], false);
 467     }
 468
 469     pub fn report_extra_impl_obligation(&self,
 470                                         error_span: Span,
 471                                         item_name: ast::Name,
 472                                         _impl_item_def_id: DefId,
 473                                         trait_item_def_id: DefId,
 474                                         requirement: &fmt::Display,
 475                                         lint_id: Option<ast::NodeId>) // (*)
 476                                         -> DiagnosticBuilder<'tcx>
 477     {
 478         // (*) This parameter is temporary and used only for phasing
 479         // in the bug fix to #18937. If it is `Some`, it has a kind of
 480         // weird effect -- the diagnostic is reported as a lint, and
 481         // the builder which is returned is marked as canceled.
 482
 483         let msg = "impl has stricter requirements than trait";
 484         let mut err = match lint_id {
 485             Some(node_id) => {
 486                 self.tcx.struct_span_lint_node(EXTRA_REQUIREMENT_IN_IMPL,
 487                                                node_id,
 488                                                error_span,
 489                                                msg)
 490             }
 491             None => {
 492                 struct_span_err!(self.tcx.sess,
 493                                  error_span,
 494                                  E0276,
 495                                  "{}", msg)
 496             }
 497         };
 498
 499         if let Some(trait_item_span) = self.tcx.hir.span_if_local(trait_item_def_id) {
 500             let span = self.tcx.sess.codemap().def_span(trait_item_span);
 501             err.span_label(span, format!("definition of `{}` from trait", item_name));
 502         }
 503
 504         err.span_label(
 505             error_span,
 506             format!("impl has extra requirement {}", requirement));
 507
 508         err
 509     }
 510
 511
 512     /// Get the parent trait chain start
 513     fn get_parent_trait_ref(&self, code: &ObligationCauseCode<'tcx>) -> Option<String> {
 514         match code {
 515             &ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
 516                 let parent_trait_ref = self.resolve_type_vars_if_possible(
 517                     &data.parent_trait_ref);
 518                 match self.get_parent_trait_ref(&data.parent_code) {
 519                     Some(t) => Some(t),
 520                     None => Some(format!("{}", parent_trait_ref.0.self_ty())),
 521                 }
 522             }
 523             _ => None,
 524         }
 525     }
 526
 527     pub fn report_selection_error(&self,
 528                                   obligation: &PredicateObligation<'tcx>,
 529                                   error: &SelectionError<'tcx>)
 530     {
 531         let span = obligation.cause.span;
 532
 533         let mut err = match *error {
 534             SelectionError::Unimplemented => {
 535                 if let ObligationCauseCode::CompareImplMethodObligation {
 536                     item_name, impl_item_def_id, trait_item_def_id, lint_id
 537                 } = obligation.cause.code {
 538                     self.report_extra_impl_obligation(
 539                         span,
 540                         item_name,
 541                         impl_item_def_id,
 542                         trait_item_def_id,
 543                         &format!("`{}`", obligation.predicate),
 544                         lint_id)
 545                         .emit();
 546                     return;
 547                 }
 548                 match obligation.predicate {
 549                     ty::Predicate::Trait(ref trait_predicate) => {
 550                         let trait_predicate =
 551                             self.resolve_type_vars_if_possible(trait_predicate);
 552
 553                         if self.tcx.sess.has_errors() && trait_predicate.references_error() {
 554                             return;
 555                         }
 556                         let trait_ref = trait_predicate.to_poly_trait_ref();
 557                         let (post_message, pre_message) =
 558                             self.get_parent_trait_ref(&obligation.cause.code)
 559                                 .map(|t| (format!(" in `{}`", t), format!("within `{}`, ", t)))
 560                             .unwrap_or((String::new(), String::new()));
 561
 562                         let OnUnimplementedNote { message, label }
 563                             = self.on_unimplemented_note(trait_ref, obligation);
 564                         let have_alt_message = message.is_some() || label.is_some();
 565
 566                         let mut err = struct_span_err!(
 567                             self.tcx.sess,
 568                             span,
 569                             E0277,
 570                             "{}",
 571                             message.unwrap_or_else(|| {
 572                                 format!("the trait bound `{}` is not satisfied{}",
 573                                          trait_ref.to_predicate(), post_message)
 574                             }));
 575
 576                         if let Some(ref s) = label {
 577                             // If it has a custom "#[rustc_on_unimplemented]"
 578                             // error message, let's display it as the label!
 579                             err.span_label(span, s.as_str());
 580                             err.help(&format!("{}the trait `{}` is not implemented for `{}`",
 581                                               pre_message,
 582                                               trait_ref,
 583                                               trait_ref.self_ty()));
 584                         } else {
 585                             err.span_label(span,
 586                                            &*format!("{}the trait `{}` is not implemented for `{}`",
 587                                                      pre_message,
 588                                                      trait_ref,
 589                                                      trait_ref.self_ty()));
 590                         }
 591
 592                         // Try to report a help message
 593                         if !trait_ref.has_infer_types() &&
 594                             self.predicate_can_apply(obligation.param_env, trait_ref) {
 595                             // If a where-clause may be useful, remind the
 596                             // user that they can add it.
 597                             //
 598                             // don't display an on-unimplemented note, as
 599                             // these notes will often be of the form
 600                             //     "the type `T` can't be frobnicated"
 601                             // which is somewhat confusing.
 602                             err.help(&format!("consider adding a `where {}` bound",
 603                                                 trait_ref.to_predicate()));
 604                         } else if !have_alt_message {
 605                             // Can't show anything else useful, try to find similar impls.
 606                             let impl_candidates = self.find_similar_impl_candidates(trait_ref);
 607                             self.report_similar_impl_candidates(impl_candidates, &mut err);
 608                         }
 609
 610                         err
 611                     }
 612
 613                     ty::Predicate::Subtype(ref predicate) => {
 614                         // Errors for Subtype predicates show up as
 615                         // `FulfillmentErrorCode::CodeSubtypeError`,
 616                         // not selection error.
 617                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
 618                     }
 619
 620                     ty::Predicate::Equate(ref predicate) => {
 621                         let predicate = self.resolve_type_vars_if_possible(predicate);
 622                         let err = self.equality_predicate(&obligation.cause,
 623                                                           obligation.param_env,
 624                                                           &predicate).err().unwrap();
 625                         struct_span_err!(self.tcx.sess, span, E0278,
 626                             "the requirement `{}` is not satisfied (`{}`)",
 627                             predicate, err)
 628                     }
 629
 630                     ty::Predicate::RegionOutlives(ref predicate) => {
 631                         let predicate = self.resolve_type_vars_if_possible(predicate);
 632                         let err = self.region_outlives_predicate(&obligation.cause,
 633                                                                     &predicate).err().unwrap();
 634                         struct_span_err!(self.tcx.sess, span, E0279,
 635                             "the requirement `{}` is not satisfied (`{}`)",
 636                             predicate, err)
 637                     }
 638
 639                     ty::Predicate::Projection(..) | ty::Predicate::TypeOutlives(..) => {
 640                         let predicate =
 641                             self.resolve_type_vars_if_possible(&obligation.predicate);
 642                         struct_span_err!(self.tcx.sess, span, E0280,
 643                             "the requirement `{}` is not satisfied",
 644                             predicate)
 645                     }
 646
 647                     ty::Predicate::ObjectSafe(trait_def_id) => {
 648                         let violations = self.tcx.object_safety_violations(trait_def_id);
 649                         self.tcx.report_object_safety_error(span,
 650                                                             trait_def_id,
 651                                                             violations)
 652                     }
 653
 654                     ty::Predicate::ClosureKind(closure_def_id, kind) => {
 655                         let found_kind = self.closure_kind(closure_def_id).unwrap();
 656                         let closure_span = self.tcx.hir.span_if_local(closure_def_id).unwrap();
 657                         let node_id = self.tcx.hir.as_local_node_id(closure_def_id).unwrap();
 658                         let mut err = struct_span_err!(
 659                             self.tcx.sess, closure_span, E0525,
 660                             "expected a closure that implements the `{}` trait, \
 661                                 but this closure only implements `{}`",
 662                             kind,
 663                             found_kind);
 664
 665                         err.span_label(
 666                             obligation.cause.span,
 667                             format!("the requirement to implement `{}` derives from here", kind));
 668
 669                         // Additional context information explaining why the closure only implements
 670                         // a particular trait.
 671                         if let Some(tables) = self.in_progress_tables {
 672                             let tables = tables.borrow();
 673                             let closure_hir_id = self.tcx.hir.node_to_hir_id(node_id);
 674                             match tables.closure_kinds().get(closure_hir_id) {
 675                                 Some(&(ty::ClosureKind::FnOnce, Some((span, name)))) => {
 676                                     err.span_note(span, &format!(
 677                                         "closure is `FnOnce` because it moves the \
 678                                          variable `{}` out of its environment", name));
 679                                 },
 680                                 Some(&(ty::ClosureKind::FnMut, Some((span, name)))) => {
 681                                     err.span_note(span, &format!(
 682                                         "closure is `FnMut` because it mutates the \
 683                                          variable `{}` here", name));
 684                                 },
 685                                 _ => {}
 686                             }
 687                         }
 688
 689                         err.emit();
 690                         return;
 691                     }
 692
 693                     ty::Predicate::WellFormed(ty) => {
 694                         // WF predicates cannot themselves make
 695                         // errors. They can only block due to
 696                         // ambiguity; otherwise, they always
 697                         // degenerate into other obligations
 698                         // (which may fail).
 699                         span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 700                     }
 701                 }
 702             }
 703
 704             OutputTypeParameterMismatch(ref expected_trait_ref, ref actual_trait_ref, ref e) => {
 705                 let expected_trait_ref = self.resolve_type_vars_if_possible(&*expected_trait_ref);
 706                 let actual_trait_ref = self.resolve_type_vars_if_possible(&*actual_trait_ref);
 707                 if actual_trait_ref.self_ty().references_error() {
 708                     return;
 709                 }
 710                 let expected_trait_ty = expected_trait_ref.self_ty();
 711                 let found_span = expected_trait_ty.ty_to_def_id().and_then(|did| {
 712                     self.tcx.hir.span_if_local(did)
 713                 });
 714
 715                 if let &TypeError::TupleSize(ref expected_found) = e {
 716                     // Expected `|x| { }`, found `|x, y| { }`
 717                     self.report_arg_count_mismatch(span,
 718                                                    found_span,
 719                                                    expected_found.expected,
 720                                                    expected_found.found,
 721                                                    expected_trait_ty.is_closure())
 722                 } else if let &TypeError::Sorts(ref expected_found) = e {
 723                     let expected = if let ty::TyTuple(tys, _) = expected_found.expected.sty {
 724                         tys.len()
 725                     } else {
 726                         1
 727                     };
 728                     let found = if let ty::TyTuple(tys, _) = expected_found.found.sty {
 729                         tys.len()
 730                     } else {
 731                         1
 732                     };
 733
 734                     if expected != found {
 735                         // Expected `|| { }`, found `|x, y| { }`
 736                         // Expected `fn(x) -> ()`, found `|| { }`
 737                         self.report_arg_count_mismatch(span,
 738                                                        found_span,
 739                                                        expected,
 740                                                        found,
 741                                                        expected_trait_ty.is_closure())
 742                     } else {
 743                         self.report_type_argument_mismatch(span,
 744                                                             found_span,
 745                                                             expected_trait_ty,
 746                                                             expected_trait_ref,
 747                                                             actual_trait_ref,
 748                                                             e)
 749                     }
 750                 } else {
 751                     self.report_type_argument_mismatch(span,
 752                                                         found_span,
 753                                                         expected_trait_ty,
 754                                                         expected_trait_ref,
 755                                                         actual_trait_ref,
 756                                                         e)
 757                 }
 758             }
 759
 760             TraitNotObjectSafe(did) => {
 761                 let violations = self.tcx.object_safety_violations(did);
 762                 self.tcx.report_object_safety_error(span, did,
 763                                                     violations)
 764             }
 765         };
 766         self.note_obligation_cause(&mut err, obligation);
 767         err.emit();
 768     }
 769
 770     fn report_type_argument_mismatch(&self,
 771                                       span: Span,
 772                                       found_span: Option<Span>,
 773                                       expected_ty: Ty<'tcx>,
 774                                       expected_ref: ty::PolyTraitRef<'tcx>,
 775                                       found_ref: ty::PolyTraitRef<'tcx>,
 776                                       type_error: &TypeError<'tcx>)
 777         -> DiagnosticBuilder<'tcx>
 778     {
 779         let mut err = struct_span_err!(self.tcx.sess, span, E0281,
 780             "type mismatch: `{}` implements the trait `{}`, but the trait `{}` is required",
 781             expected_ty,
 782             expected_ref,
 783             found_ref);
 784
 785         err.span_label(span, format!("{}", type_error));
 786
 787         if let Some(sp) = found_span {
 788             err.span_label(span, format!("requires `{}`", found_ref));
 789             err.span_label(sp, format!("implements `{}`", expected_ref));
 790         }
 791
 792         err
 793     }
 794
 795     fn report_arg_count_mismatch(&self,
 796                                  span: Span,
 797                                  found_span: Option<Span>,
 798                                  expected: usize,
 799                                  found: usize,
 800                                  is_closure: bool)
 801         -> DiagnosticBuilder<'tcx>
 802     {
 803         let mut err = struct_span_err!(self.tcx.sess, span, E0593,
 804             "{} takes {} argument{} but {} argument{} {} required",
 805             if is_closure { "closure" } else { "function" },
 806             found,
 807             if found == 1 { "" } else { "s" },
 808             expected,
 809             if expected == 1 { "" } else { "s" },
 810             if expected == 1 { "is" } else { "are" });
 811
 812         err.span_label(span, format!("expected {} that takes {} argument{}",
 813                                       if is_closure { "closure" } else { "function" },
 814                                       expected,
 815                                       if expected == 1 { "" } else { "s" }));
 816         if let Some(span) = found_span {
 817             err.span_label(span, format!("takes {} argument{}",
 818                                           found,
 819                                           if found == 1 { "" } else { "s" }));
 820         }
 821         err
 822     }
 823 }
 824
 825 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
 826     pub fn recursive_type_with_infinite_size_error(self,
 827                                                    type_def_id: DefId)
 828                                                    -> DiagnosticBuilder<'tcx>
 829     {
 830         assert!(type_def_id.is_local());
 831         let span = self.hir.span_if_local(type_def_id).unwrap();
 832         let span = self.sess.codemap().def_span(span);
 833         let mut err = struct_span_err!(self.sess, span, E0072,
 834                                        "recursive type `{}` has infinite size",
 835                                        self.item_path_str(type_def_id));
 836         err.span_label(span, "recursive type has infinite size");
 837         err.help(&format!("insert indirection (e.g., a `Box`, `Rc`, or `&`) \
 838                            at some point to make `{}` representable",
 839                           self.item_path_str(type_def_id)));
 840         err
 841     }
 842
 843     pub fn report_object_safety_error(self,
 844                                       span: Span,
 845                                       trait_def_id: DefId,
 846                                       violations: Vec<ObjectSafetyViolation>)
 847                                       -> DiagnosticBuilder<'tcx>
 848     {
 849         let trait_str = self.item_path_str(trait_def_id);
 850         let span = self.sess.codemap().def_span(span);
 851         let mut err = struct_span_err!(
 852             self.sess, span, E0038,
 853             "the trait `{}` cannot be made into an object",
 854             trait_str);
 855         err.span_label(span, format!("the trait `{}` cannot be made into an object", trait_str));
 856
 857         let mut reported_violations = FxHashSet();
 858         for violation in violations {
 859             if !reported_violations.insert(violation.clone()) {
 860                 continue;
 861             }
 862             err.note(&violation.error_msg());
 863         }
 864         err
 865     }
 866 }
 867
 868 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
 869     fn maybe_report_ambiguity(&self, obligation: &PredicateObligation<'tcx>,
 870                               body_id: Option<hir::BodyId>) {
 871         // Unable to successfully determine, probably means
 872         // insufficient type information, but could mean
 873         // ambiguous impls. The latter *ought* to be a
 874         // coherence violation, so we don't report it here.
 875
 876         let predicate = self.resolve_type_vars_if_possible(&obligation.predicate);
 877         let span = obligation.cause.span;
 878
 879         debug!("maybe_report_ambiguity(predicate={:?}, obligation={:?})",
 880                predicate,
 881                obligation);
 882
 883         // Ambiguity errors are often caused as fallout from earlier
 884         // errors. So just ignore them if this infcx is tainted.
 885         if self.is_tainted_by_errors() {
 886             return;
 887         }
 888
 889         match predicate {
 890             ty::Predicate::Trait(ref data) => {
 891                 let trait_ref = data.to_poly_trait_ref();
 892                 let self_ty = trait_ref.self_ty();
 893                 if predicate.references_error() {
 894                     return;
 895                 }
 896                 // Typically, this ambiguity should only happen if
 897                 // there are unresolved type inference variables
 898                 // (otherwise it would suggest a coherence
 899                 // failure). But given #21974 that is not necessarily
 900                 // the case -- we can have multiple where clauses that
 901                 // are only distinguished by a region, which results
 902                 // in an ambiguity even when all types are fully
 903                 // known, since we don't dispatch based on region
 904                 // relationships.
 905
 906                 // This is kind of a hack: it frequently happens that some earlier
 907                 // error prevents types from being fully inferred, and then we get
 908                 // a bunch of uninteresting errors saying something like "<generic
 909                 // #0> doesn't implement Sized".  It may even be true that we
 910                 // could just skip over all checks where the self-ty is an
 911                 // inference variable, but I was afraid that there might be an
 912                 // inference variable created, registered as an obligation, and
 913                 // then never forced by writeback, and hence by skipping here we'd
 914                 // be ignoring the fact that we don't KNOW the type works
 915                 // out. Though even that would probably be harmless, given that
 916                 // we're only talking about builtin traits, which are known to be
 917                 // inhabited. But in any case I just threw in this check for
 918                 // has_errors() to be sure that compilation isn't happening
 919                 // anyway. In that case, why inundate the user.
 920                 if !self.tcx.sess.has_errors() {
 921                     if
 922                         self.tcx.lang_items().sized_trait()
 923                         .map_or(false, |sized_id| sized_id == trait_ref.def_id())
 924                     {
 925                         self.need_type_info(body_id, span, self_ty);
 926                     } else {
 927                         let mut err = struct_span_err!(self.tcx.sess,
 928                                                         span, E0283,
 929                                                         "type annotations required: \
 930                                                         cannot resolve `{}`",
 931                                                         predicate);
 932                         self.note_obligation_cause(&mut err, obligation);
 933                         err.emit();
 934                     }
 935                 }
 936             }
 937
 938             ty::Predicate::WellFormed(ty) => {
 939                 // Same hacky approach as above to avoid deluging user
 940                 // with error messages.
 941                 if !ty.references_error() && !self.tcx.sess.has_errors() {
 942                     self.need_type_info(body_id, span, ty);
 943                 }
 944             }
 945
 946             ty::Predicate::Subtype(ref data) => {
 947                 if data.references_error() || self.tcx.sess.has_errors() {
 948                     // no need to overload user in such cases
 949                 } else {
 950                     let &SubtypePredicate { a_is_expected: _, a, b } = data.skip_binder();
 951                     // both must be type variables, or the other would've been instantiated
 952                     assert!(a.is_ty_var() && b.is_ty_var());
 953                     self.need_type_info(body_id,
 954                                         obligation.cause.span,
 955                                         a);
 956                 }
 957             }
 958
 959             _ => {
 960                 if !self.tcx.sess.has_errors() {
 961                     let mut err = struct_span_err!(self.tcx.sess,
 962                                                    obligation.cause.span, E0284,
 963                                                    "type annotations required: \
 964                                                     cannot resolve `{}`",
 965                                                    predicate);
 966                     self.note_obligation_cause(&mut err, obligation);
 967                     err.emit();
 968                 }
 969             }
 970         }
 971     }
 972
 973     /// Returns whether the trait predicate may apply for *some* assignment
 974     /// to the type parameters.
 975     fn predicate_can_apply(&self,
 976                            param_env: ty::ParamEnv<'tcx>,
 977                            pred: ty::PolyTraitRef<'tcx>)
 978                            -> bool {
 979         struct ParamToVarFolder<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
 980             infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
 981             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>
 982         }
 983
 984         impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for ParamToVarFolder<'a, 'gcx, 'tcx> {
 985             fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> { self.infcx.tcx }
 986
 987             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
 988                 if let ty::TyParam(ty::ParamTy {name, ..}) = ty.sty {
 989                     let infcx = self.infcx;
 990                     self.var_map.entry(ty).or_insert_with(||
 991                         infcx.next_ty_var(
 992                             TypeVariableOrigin::TypeParameterDefinition(DUMMY_SP, name)))
 993                 } else {
 994                     ty.super_fold_with(self)
 995                 }
 996             }
 997         }
 998
 999         self.probe(|_| {
1000             let mut selcx = SelectionContext::new(self);
1001
1002             let cleaned_pred = pred.fold_with(&mut ParamToVarFolder {
1003                 infcx: self,
1004                 var_map: FxHashMap()
1005             });
1006
1007             let cleaned_pred = super::project::normalize(
1008                 &mut selcx,
1009                 param_env,
1010                 ObligationCause::dummy(),
1011                 &cleaned_pred
1012             ).value;
1013
1014             let obligation = Obligation::new(
1015                 ObligationCause::dummy(),
1016                 param_env,
1017                 cleaned_pred.to_predicate()
1018             );
1019
1020             selcx.evaluate_obligation(&obligation)
1021         })
1022     }
1023
1024     fn note_obligation_cause<T>(&self,
1025                                 err: &mut DiagnosticBuilder,
1026                                 obligation: &Obligation<'tcx, T>)
1027         where T: fmt::Display
1028     {
1029         self.note_obligation_cause_code(err,
1030                                         &obligation.predicate,
1031                                         &obligation.cause.code);
1032     }
1033
1034     fn note_obligation_cause_code<T>(&self,
1035                                      err: &mut DiagnosticBuilder,
1036                                      predicate: &T,
1037                                      cause_code: &ObligationCauseCode<'tcx>)
1038         where T: fmt::Display
1039     {
1040         let tcx = self.tcx;
1041         match *cause_code {
1042             ObligationCauseCode::ExprAssignable |
1043             ObligationCauseCode::MatchExpressionArm { .. } |
1044             ObligationCauseCode::IfExpression |
1045             ObligationCauseCode::IfExpressionWithNoElse |
1046             ObligationCauseCode::EquatePredicate |
1047             ObligationCauseCode::MainFunctionType |
1048             ObligationCauseCode::StartFunctionType |
1049             ObligationCauseCode::IntrinsicType |
1050             ObligationCauseCode::MethodReceiver |
1051             ObligationCauseCode::ReturnNoExpression |
1052             ObligationCauseCode::MiscObligation => {
1053             }
1054             ObligationCauseCode::SliceOrArrayElem => {
1055                 err.note("slice and array elements must have `Sized` type");
1056             }
1057             ObligationCauseCode::TupleElem => {
1058                 err.note("only the last element of a tuple may have a dynamically sized type");
1059             }
1060             ObligationCauseCode::ProjectionWf(data) => {
1061                 err.note(&format!("required so that the projection `{}` is well-formed",
1062                                   data));
1063             }
1064             ObligationCauseCode::ReferenceOutlivesReferent(ref_ty) => {
1065                 err.note(&format!("required so that reference `{}` does not outlive its referent",
1066                                   ref_ty));
1067             }
1068             ObligationCauseCode::ObjectTypeBound(object_ty, region) => {
1069                 err.note(&format!("required so that the lifetime bound of `{}` for `{}` \
1070                                    is satisfied",
1071                                   region, object_ty));
1072             }
1073             ObligationCauseCode::ItemObligation(item_def_id) => {
1074                 let item_name = tcx.item_path_str(item_def_id);
1075                 err.note(&format!("required by `{}`", item_name));
1076             }
1077             ObligationCauseCode::ObjectCastObligation(object_ty) => {
1078                 err.note(&format!("required for the cast to the object type `{}`",
1079                                   self.ty_to_string(object_ty)));
1080             }
1081             ObligationCauseCode::RepeatVec => {
1082                 err.note("the `Copy` trait is required because the \
1083                           repeated element will be copied");
1084             }
1085             ObligationCauseCode::VariableType(_) => {
1086                 err.note("all local variables must have a statically known size");
1087             }
1088             ObligationCauseCode::SizedReturnType => {
1089                 err.note("the return type of a function must have a \
1090                           statically known size");
1091             }
1092             ObligationCauseCode::AssignmentLhsSized => {
1093                 err.note("the left-hand-side of an assignment must have a statically known size");
1094             }
1095             ObligationCauseCode::TupleInitializerSized => {
1096                 err.note("tuples must have a statically known size to be initialized");
1097             }
1098             ObligationCauseCode::StructInitializerSized => {
1099                 err.note("structs must have a statically known size to be initialized");
1100             }
1101             ObligationCauseCode::FieldSized(ref item) => {
1102                 match *item {
1103                     AdtKind::Struct => {
1104                         err.note("only the last field of a struct may have a dynamically \
1105                                   sized type");
1106                     }
1107                     AdtKind::Union => {
1108                         err.note("no field of a union may have a dynamically sized type");
1109                     }
1110                     AdtKind::Enum => {
1111                         err.note("no field of an enum variant may have a dynamically sized type");
1112                     }
1113                 }
1114             }
1115             ObligationCauseCode::ConstSized => {
1116                 err.note("constant expressions must have a statically known size");
1117             }
1118             ObligationCauseCode::SharedStatic => {
1119                 err.note("shared static variables must have a type that implements `Sync`");
1120             }
1121             ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
1122                 let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1123                 err.note(&format!("required because it appears within the type `{}`",
1124                                   parent_trait_ref.0.self_ty()));
1125                 let parent_predicate = parent_trait_ref.to_predicate();
1126                 self.note_obligation_cause_code(err,
1127                                                 &parent_predicate,
1128                                                 &data.parent_code);
1129             }
1130             ObligationCauseCode::ImplDerivedObligation(ref data) => {
1131                 let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1132                 err.note(
1133                     &format!("required because of the requirements on the impl of `{}` for `{}`",
1134                              parent_trait_ref,
1135                              parent_trait_ref.0.self_ty()));
1136                 let parent_predicate = parent_trait_ref.to_predicate();
1137                 self.note_obligation_cause_code(err,
1138                                                 &parent_predicate,
1139                                                 &data.parent_code);
1140             }
1141             ObligationCauseCode::CompareImplMethodObligation { .. } => {
1142                 err.note(
1143                     &format!("the requirement `{}` appears on the impl method \
1144                               but not on the corresponding trait method",
1145                              predicate));
1146             }
1147             ObligationCauseCode::ReturnType(_) |
1148             ObligationCauseCode::BlockTailExpression(_) => (),
1149         }
1150     }
1151
1152     fn suggest_new_overflow_limit(&self, err: &mut DiagnosticBuilder) {
1153         let current_limit = self.tcx.sess.recursion_limit.get();
1154         let suggested_limit = current_limit * 2;
1155         err.help(&format!("consider adding a `#![recursion_limit=\"{}\"]` attribute to your crate",
1156                           suggested_limit));
1157     }
1158 }