src/librustc/traits/error_reporting.rs

   1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 use super::{
  12     FulfillmentError,
  13     FulfillmentErrorCode,
  14     MismatchedProjectionTypes,
  15     Obligation,
  16     ObligationCause,
  17     ObligationCauseCode,
  18     OnUnimplementedDirective,
  19     OnUnimplementedNote,
  20     OutputTypeParameterMismatch,
  21     TraitNotObjectSafe,
  22     ConstEvalFailure,
  23     PredicateObligation,
  24     Reveal,
  25     SelectionContext,
  26     SelectionError,
  27     ObjectSafetyViolation,
  28 };
  29
  30 use errors::DiagnosticBuilder;
  31 use hir;
  32 use hir::def_id::DefId;
  33 use infer::{self, InferCtxt};
  34 use infer::type_variable::TypeVariableOrigin;
  35 use middle::const_val;
  36 use std::fmt;
  37 use syntax::ast;
  38 use session::DiagnosticMessageId;
  39 use ty::{self, AdtKind, ToPredicate, ToPolyTraitRef, Ty, TyCtxt, TypeFoldable};
  40 use ty::error::ExpectedFound;
  41 use ty::fast_reject;
  42 use ty::fold::TypeFolder;
  43 use ty::subst::Subst;
  44 use ty::SubtypePredicate;
  45 use util::nodemap::{FxHashMap, FxHashSet};
  46
  47 use syntax_pos::{DUMMY_SP, Span};
  48
  49 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
  50     pub fn report_fulfillment_errors(&self,
  51                                      errors: &Vec<FulfillmentError<'tcx>>,
  52                                      body_id: Option<hir::BodyId>) {
  53         #[derive(Debug)]
  54         struct ErrorDescriptor<'tcx> {
  55             predicate: ty::Predicate<'tcx>,
  56             index: Option<usize>, // None if this is an old error
  57         }
  58
  59         let mut error_map : FxHashMap<_, _> =
  60             self.reported_trait_errors.borrow().iter().map(|(&span, predicates)| {
  61                 (span, predicates.iter().map(|predicate| ErrorDescriptor {
  62                     predicate: predicate.clone(),
  63                     index: None
  64                 }).collect())
  65             }).collect();
  66
  67         for (index, error) in errors.iter().enumerate() {
  68             error_map.entry(error.obligation.cause.span).or_insert(Vec::new()).push(
  69                 ErrorDescriptor {
  70                     predicate: error.obligation.predicate.clone(),
  71                     index: Some(index)
  72                 });
  73
  74             self.reported_trait_errors.borrow_mut()
  75                 .entry(error.obligation.cause.span).or_insert(Vec::new())
  76                 .push(error.obligation.predicate.clone());
  77         }
  78
  79         // We do this in 2 passes because we want to display errors in order, tho
  80         // maybe it *is* better to sort errors by span or something.
  81         let mut is_suppressed: Vec<bool> = errors.iter().map(|_| false).collect();
  82         for (_, error_set) in error_map.iter() {
  83             // We want to suppress "duplicate" errors with the same span.
  84             for error in error_set {
  85                 if let Some(index) = error.index {
  86                     // Suppress errors that are either:
  87                     // 1) strictly implied by another error.
  88                     // 2) implied by an error with a smaller index.
  89                     for error2 in error_set {
  90                         if error2.index.map_or(false, |index2| is_suppressed[index2]) {
  91                             // Avoid errors being suppressed by already-suppressed
  92                             // errors, to prevent all errors from being suppressed
  93                             // at once.
  94                             continue
  95                         }
  96
  97                         if self.error_implies(&error2.predicate, &error.predicate) &&
  98                             !(error2.index >= error.index &&
  99                               self.error_implies(&error.predicate, &error2.predicate))
 100                         {
 101                             info!("skipping {:?} (implied by {:?})", error, error2);
 102                             is_suppressed[index] = true;
 103                             break
 104                         }
 105                     }
 106                 }
 107             }
 108         }
 109
 110         for (error, suppressed) in errors.iter().zip(is_suppressed) {
 111             if !suppressed {
 112                 self.report_fulfillment_error(error, body_id);
 113             }
 114         }
 115     }
 116
 117     // returns if `cond` not occurring implies that `error` does not occur - i.e. that
 118     // `error` occurring implies that `cond` occurs.
 119     fn error_implies(&self,
 120                      cond: &ty::Predicate<'tcx>,
 121                      error: &ty::Predicate<'tcx>)
 122                      -> bool
 123     {
 124         if cond == error {
 125             return true
 126         }
 127
 128         let (cond, error) = match (cond, error) {
 129             (&ty::Predicate::Trait(..), &ty::Predicate::Trait(ref error))
 130                 => (cond, error),
 131             _ => {
 132                 // FIXME: make this work in other cases too.
 133                 return false
 134             }
 135         };
 136
 137         for implication in super::elaborate_predicates(self.tcx, vec![cond.clone()]) {
 138             if let ty::Predicate::Trait(implication) = implication {
 139                 let error = error.to_poly_trait_ref();
 140                 let implication = implication.to_poly_trait_ref();
 141                 // FIXME: I'm just not taking associated types at all here.
 142                 // Eventually I'll need to implement param-env-aware
 143                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
 144                 let param_env = ty::ParamEnv::empty(Reveal::UserFacing);
 145                 if let Ok(_) = self.can_sub(param_env, error, implication) {
 146                     debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
 147                     return true
 148                 }
 149             }
 150         }
 151
 152         false
 153     }
 154
 155     fn report_fulfillment_error(&self, error: &FulfillmentError<'tcx>,
 156                                 body_id: Option<hir::BodyId>) {
 157         debug!("report_fulfillment_errors({:?})", error);
 158         match error.code {
 159             FulfillmentErrorCode::CodeSelectionError(ref e) => {
 160                 self.report_selection_error(&error.obligation, e);
 161             }
 162             FulfillmentErrorCode::CodeProjectionError(ref e) => {
 163                 self.report_projection_error(&error.obligation, e);
 164             }
 165             FulfillmentErrorCode::CodeAmbiguity => {
 166                 self.maybe_report_ambiguity(&error.obligation, body_id);
 167             }
 168             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
 169                 self.report_mismatched_types(&error.obligation.cause,
 170                                              expected_found.expected,
 171                                              expected_found.found,
 172                                              err.clone())
 173                     .emit();
 174             }
 175         }
 176     }
 177
 178     fn report_projection_error(&self,
 179                                obligation: &PredicateObligation<'tcx>,
 180                                error: &MismatchedProjectionTypes<'tcx>)
 181     {
 182         let predicate =
 183             self.resolve_type_vars_if_possible(&obligation.predicate);
 184
 185         if predicate.references_error() {
 186             return
 187         }
 188
 189         self.probe(|_| {
 190             let err_buf;
 191             let mut err = &error.err;
 192             let mut values = None;
 193
 194             // try to find the mismatched types to report the error with.
 195             //
 196             // this can fail if the problem was higher-ranked, in which
 197             // cause I have no idea for a good error message.
 198             if let ty::Predicate::Projection(ref data) = predicate {
 199                 let mut selcx = SelectionContext::new(self);
 200                 let (data, _) = self.replace_late_bound_regions_with_fresh_var(
 201                     obligation.cause.span,
 202                     infer::LateBoundRegionConversionTime::HigherRankedType,
 203                     data);
 204                 let normalized = super::normalize_projection_type(
 205                     &mut selcx,
 206                     obligation.param_env,
 207                     data.projection_ty,
 208                     obligation.cause.clone(),
 209                     0
 210                 );
 211                 if let Err(error) = self.at(&obligation.cause, obligation.param_env)
 212                                         .eq(normalized.value, data.ty) {
 213                     values = Some(infer::ValuePairs::Types(ExpectedFound {
 214                         expected: normalized.value,
 215                         found: data.ty,
 216                     }));
 217                     err_buf = error;
 218                     err = &err_buf;
 219                 }
 220             }
 221
 222             let msg = format!("type mismatch resolving `{}`", predicate);
 223             let error_id = (DiagnosticMessageId::ErrorId(271),
 224                             Some(obligation.cause.span), msg.clone());
 225             let fresh = self.tcx.sess.one_time_diagnostics.borrow_mut().insert(error_id);
 226             if fresh {
 227                 let mut diag = struct_span_err!(
 228                     self.tcx.sess, obligation.cause.span, E0271,
 229                     "type mismatch resolving `{}`", predicate
 230                 );
 231                 self.note_type_err(&mut diag, &obligation.cause, None, values, err);
 232                 self.note_obligation_cause(&mut diag, obligation);
 233                 diag.emit();
 234             }
 235         });
 236     }
 237
 238     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
 239         /// returns the fuzzy category of a given type, or None
 240         /// if the type can be equated to any type.
 241         fn type_category<'tcx>(t: Ty<'tcx>) -> Option<u32> {
 242             match t.sty {
 243                 ty::TyBool => Some(0),
 244                 ty::TyChar => Some(1),
 245                 ty::TyStr => Some(2),
 246                 ty::TyInt(..) | ty::TyUint(..) | ty::TyInfer(ty::IntVar(..)) => Some(3),
 247                 ty::TyFloat(..) | ty::TyInfer(ty::FloatVar(..)) => Some(4),
 248                 ty::TyRef(..) | ty::TyRawPtr(..) => Some(5),
 249                 ty::TyArray(..) | ty::TySlice(..) => Some(6),
 250                 ty::TyFnDef(..) | ty::TyFnPtr(..) => Some(7),
 251                 ty::TyDynamic(..) => Some(8),
 252                 ty::TyClosure(..) => Some(9),
 253                 ty::TyTuple(..) => Some(10),
 254                 ty::TyProjection(..) => Some(11),
 255                 ty::TyParam(..) => Some(12),
 256                 ty::TyAnon(..) => Some(13),
 257                 ty::TyNever => Some(14),
 258                 ty::TyAdt(adt, ..) => match adt.adt_kind() {
 259                     AdtKind::Struct => Some(15),
 260                     AdtKind::Union => Some(16),
 261                     AdtKind::Enum => Some(17),
 262                 },
 263                 ty::TyGenerator(..) => Some(18),
 264                 ty::TyForeign(..) => Some(19),
 265                 ty::TyInfer(..) | ty::TyError => None
 266             }
 267         }
 268
 269         match (type_category(a), type_category(b)) {
 270             (Some(cat_a), Some(cat_b)) => match (&a.sty, &b.sty) {
 271                 (&ty::TyAdt(def_a, _), &ty::TyAdt(def_b, _)) => def_a == def_b,
 272                 _ => cat_a == cat_b
 273             },
 274             // infer and error can be equated to all types
 275             _ => true
 276         }
 277     }
 278
 279     fn impl_similar_to(&self,
 280                        trait_ref: ty::PolyTraitRef<'tcx>,
 281                        obligation: &PredicateObligation<'tcx>)
 282                        -> Option<DefId>
 283     {
 284         let tcx = self.tcx;
 285         let param_env = obligation.param_env;
 286         let trait_ref = tcx.erase_late_bound_regions(&trait_ref);
 287         let trait_self_ty = trait_ref.self_ty();
 288
 289         let mut self_match_impls = vec![];
 290         let mut fuzzy_match_impls = vec![];
 291
 292         self.tcx.for_each_relevant_impl(
 293             trait_ref.def_id, trait_self_ty, |def_id| {
 294                 let impl_substs = self.fresh_substs_for_item(obligation.cause.span, def_id);
 295                 let impl_trait_ref = tcx
 296                     .impl_trait_ref(def_id)
 297                     .unwrap()
 298                     .subst(tcx, impl_substs);
 299
 300                 let impl_self_ty = impl_trait_ref.self_ty();
 301
 302                 if let Ok(..) = self.can_eq(param_env, trait_self_ty, impl_self_ty) {
 303                     self_match_impls.push(def_id);
 304
 305                     if trait_ref.substs.types().skip(1)
 306                         .zip(impl_trait_ref.substs.types().skip(1))
 307                         .all(|(u,v)| self.fuzzy_match_tys(u, v))
 308                     {
 309                         fuzzy_match_impls.push(def_id);
 310                     }
 311                 }
 312             });
 313
 314         let impl_def_id = if self_match_impls.len() == 1 {
 315             self_match_impls[0]
 316         } else if fuzzy_match_impls.len() == 1 {
 317             fuzzy_match_impls[0]
 318         } else {
 319             return None
 320         };
 321
 322         if tcx.has_attr(impl_def_id, "rustc_on_unimplemented") {
 323             Some(impl_def_id)
 324         } else {
 325             None
 326         }
 327     }
 328
 329     fn on_unimplemented_note(
 330         &self,
 331         trait_ref: ty::PolyTraitRef<'tcx>,
 332         obligation: &PredicateObligation<'tcx>) ->
 333         OnUnimplementedNote
 334     {
 335         let def_id = self.impl_similar_to(trait_ref, obligation)
 336             .unwrap_or(trait_ref.def_id());
 337         let trait_ref = *trait_ref.skip_binder();
 338
 339         let desugaring;
 340         let method;
 341         let mut flags = vec![];
 342         let direct = match obligation.cause.code {
 343             ObligationCauseCode::BuiltinDerivedObligation(..) |
 344             ObligationCauseCode::ImplDerivedObligation(..) => false,
 345             _ => true
 346         };
 347         if direct {
 348             // this is a "direct", user-specified, rather than derived,
 349             // obligation.
 350             flags.push(("direct", None));
 351         }
 352
 353         if let ObligationCauseCode::ItemObligation(item) = obligation.cause.code {
 354             // FIXME: maybe also have some way of handling methods
 355             // from other traits? That would require name resolution,
 356             // which we might want to be some sort of hygienic.
 357             //
 358             // Currently I'm leaving it for what I need for `try`.
 359             if self.tcx.trait_of_item(item) == Some(trait_ref.def_id) {
 360                 method = self.tcx.item_name(item);
 361                 flags.push(("from_method", None));
 362                 flags.push(("from_method", Some(&*method)));
 363             }
 364         }
 365
 366         if let Some(k) = obligation.cause.span.compiler_desugaring_kind() {
 367             desugaring = k.as_symbol().as_str();
 368             flags.push(("from_desugaring", None));
 369             flags.push(("from_desugaring", Some(&*desugaring)));
 370         }
 371
 372         if let Ok(Some(command)) = OnUnimplementedDirective::of_item(
 373             self.tcx, trait_ref.def_id, def_id
 374         ) {
 375             command.evaluate(self.tcx, trait_ref, &flags)
 376         } else {
 377             OnUnimplementedNote::empty()
 378         }
 379     }
 380
 381     fn find_similar_impl_candidates(&self,
 382                                     trait_ref: ty::PolyTraitRef<'tcx>)
 383                                     -> Vec<ty::TraitRef<'tcx>>
 384     {
 385         let simp = fast_reject::simplify_type(self.tcx,
 386                                               trait_ref.skip_binder().self_ty(),
 387                                               true);
 388         let mut impl_candidates = Vec::new();
 389
 390         match simp {
 391             Some(simp) => self.tcx.for_each_impl(trait_ref.def_id(), |def_id| {
 392                 let imp = self.tcx.impl_trait_ref(def_id).unwrap();
 393                 let imp_simp = fast_reject::simplify_type(self.tcx,
 394                                                           imp.self_ty(),
 395                                                           true);
 396                 if let Some(imp_simp) = imp_simp {
 397                     if simp != imp_simp {
 398                         return;
 399                     }
 400                 }
 401                 impl_candidates.push(imp);
 402             }),
 403             None => self.tcx.for_each_impl(trait_ref.def_id(), |def_id| {
 404                 impl_candidates.push(
 405                     self.tcx.impl_trait_ref(def_id).unwrap());
 406             })
 407         };
 408         impl_candidates
 409     }
 410
 411     fn report_similar_impl_candidates(&self,
 412                                       impl_candidates: Vec<ty::TraitRef<'tcx>>,
 413                                       err: &mut DiagnosticBuilder)
 414     {
 415         if impl_candidates.is_empty() {
 416             return;
 417         }
 418
 419         let end = if impl_candidates.len() <= 5 {
 420             impl_candidates.len()
 421         } else {
 422             4
 423         };
 424         err.help(&format!("the following implementations were found:{}{}",
 425                           &impl_candidates[0..end].iter().map(|candidate| {
 426                               format!("\n  {:?}", candidate)
 427                           }).collect::<String>(),
 428                           if impl_candidates.len() > 5 {
 429                               format!("\nand {} others", impl_candidates.len() - 4)
 430                           } else {
 431                               "".to_owned()
 432                           }
 433                           ));
 434     }
 435
 436     /// Reports that an overflow has occurred and halts compilation. We
 437     /// halt compilation unconditionally because it is important that
 438     /// overflows never be masked -- they basically represent computations
 439     /// whose result could not be truly determined and thus we can't say
 440     /// if the program type checks or not -- and they are unusual
 441     /// occurrences in any case.
 442     pub fn report_overflow_error<T>(&self,
 443                                     obligation: &Obligation<'tcx, T>,
 444                                     suggest_increasing_limit: bool) -> !
 445         where T: fmt::Display + TypeFoldable<'tcx>
 446     {
 447         let predicate =
 448             self.resolve_type_vars_if_possible(&obligation.predicate);
 449         let mut err = struct_span_err!(self.tcx.sess, obligation.cause.span, E0275,
 450                                        "overflow evaluating the requirement `{}`",
 451                                        predicate);
 452
 453         if suggest_increasing_limit {
 454             self.suggest_new_overflow_limit(&mut err);
 455         }
 456
 457         self.note_obligation_cause(&mut err, obligation);
 458
 459         err.emit();
 460         self.tcx.sess.abort_if_errors();
 461         bug!();
 462     }
 463
 464     /// Reports that a cycle was detected which led to overflow and halts
 465     /// compilation. This is equivalent to `report_overflow_error` except
 466     /// that we can give a more helpful error message (and, in particular,
 467     /// we do not suggest increasing the overflow limit, which is not
 468     /// going to help).
 469     pub fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 470         let cycle = self.resolve_type_vars_if_possible(&cycle.to_owned());
 471         assert!(cycle.len() > 0);
 472
 473         debug!("report_overflow_error_cycle: cycle={:?}", cycle);
 474
 475         self.report_overflow_error(&cycle[0], false);
 476     }
 477
 478     pub fn report_extra_impl_obligation(&self,
 479                                         error_span: Span,
 480                                         item_name: ast::Name,
 481                                         _impl_item_def_id: DefId,
 482                                         trait_item_def_id: DefId,
 483                                         requirement: &fmt::Display)
 484                                         -> DiagnosticBuilder<'tcx>
 485     {
 486         let msg = "impl has stricter requirements than trait";
 487         let mut err = struct_span_err!(self.tcx.sess,
 488                                        error_span,
 489                                        E0276,
 490                                        "{}", msg);
 491
 492         if let Some(trait_item_span) = self.tcx.hir.span_if_local(trait_item_def_id) {
 493             let span = self.tcx.sess.codemap().def_span(trait_item_span);
 494             err.span_label(span, format!("definition of `{}` from trait", item_name));
 495         }
 496
 497         err.span_label(
 498             error_span,
 499             format!("impl has extra requirement {}", requirement));
 500
 501         err
 502     }
 503
 504
 505     /// Get the parent trait chain start
 506     fn get_parent_trait_ref(&self, code: &ObligationCauseCode<'tcx>) -> Option<String> {
 507         match code {
 508             &ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
 509                 let parent_trait_ref = self.resolve_type_vars_if_possible(
 510                     &data.parent_trait_ref);
 511                 match self.get_parent_trait_ref(&data.parent_code) {
 512                     Some(t) => Some(t),
 513                     None => Some(format!("{}", parent_trait_ref.0.self_ty())),
 514                 }
 515             }
 516             _ => None,
 517         }
 518     }
 519
 520     pub fn report_selection_error(&self,
 521                                   obligation: &PredicateObligation<'tcx>,
 522                                   error: &SelectionError<'tcx>)
 523     {
 524         let span = obligation.cause.span;
 525
 526         let mut err = match *error {
 527             SelectionError::Unimplemented => {
 528                 if let ObligationCauseCode::CompareImplMethodObligation {
 529                     item_name, impl_item_def_id, trait_item_def_id,
 530                 } = obligation.cause.code {
 531                     self.report_extra_impl_obligation(
 532                         span,
 533                         item_name,
 534                         impl_item_def_id,
 535                         trait_item_def_id,
 536                         &format!("`{}`", obligation.predicate))
 537                         .emit();
 538                     return;
 539                 }
 540                 match obligation.predicate {
 541                     ty::Predicate::Trait(ref trait_predicate) => {
 542                         let trait_predicate =
 543                             self.resolve_type_vars_if_possible(trait_predicate);
 544
 545                         if self.tcx.sess.has_errors() && trait_predicate.references_error() {
 546                             return;
 547                         }
 548                         let trait_ref = trait_predicate.to_poly_trait_ref();
 549                         let (post_message, pre_message) =
 550                             self.get_parent_trait_ref(&obligation.cause.code)
 551                                 .map(|t| (format!(" in `{}`", t), format!("within `{}`, ", t)))
 552                             .unwrap_or((String::new(), String::new()));
 553
 554                         let OnUnimplementedNote { message, label }
 555                             = self.on_unimplemented_note(trait_ref, obligation);
 556                         let have_alt_message = message.is_some() || label.is_some();
 557
 558                         let mut err = struct_span_err!(
 559                             self.tcx.sess,
 560                             span,
 561                             E0277,
 562                             "{}",
 563                             message.unwrap_or_else(|| {
 564                                 format!("the trait bound `{}` is not satisfied{}",
 565                                          trait_ref.to_predicate(), post_message)
 566                             }));
 567
 568                         if let Some(ref s) = label {
 569                             // If it has a custom "#[rustc_on_unimplemented]"
 570                             // error message, let's display it as the label!
 571                             err.span_label(span, s.as_str());
 572                             err.help(&format!("{}the trait `{}` is not implemented for `{}`",
 573                                               pre_message,
 574                                               trait_ref,
 575                                               trait_ref.self_ty()));
 576                         } else {
 577                             err.span_label(span,
 578                                            &*format!("{}the trait `{}` is not implemented for `{}`",
 579                                                      pre_message,
 580                                                      trait_ref,
 581                                                      trait_ref.self_ty()));
 582                         }
 583
 584                         // Try to report a help message
 585                         if !trait_ref.has_infer_types() &&
 586                             self.predicate_can_apply(obligation.param_env, trait_ref) {
 587                             // If a where-clause may be useful, remind the
 588                             // user that they can add it.
 589                             //
 590                             // don't display an on-unimplemented note, as
 591                             // these notes will often be of the form
 592                             //     "the type `T` can't be frobnicated"
 593                             // which is somewhat confusing.
 594                             err.help(&format!("consider adding a `where {}` bound",
 595                                                 trait_ref.to_predicate()));
 596                         } else if !have_alt_message {
 597                             // Can't show anything else useful, try to find similar impls.
 598                             let impl_candidates = self.find_similar_impl_candidates(trait_ref);
 599                             self.report_similar_impl_candidates(impl_candidates, &mut err);
 600                         }
 601
 602                         err
 603                     }
 604
 605                     ty::Predicate::Subtype(ref predicate) => {
 606                         // Errors for Subtype predicates show up as
 607                         // `FulfillmentErrorCode::CodeSubtypeError`,
 608                         // not selection error.
 609                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
 610                     }
 611
 612                     ty::Predicate::Equate(ref predicate) => {
 613                         let predicate = self.resolve_type_vars_if_possible(predicate);
 614                         let err = self.equality_predicate(&obligation.cause,
 615                                                           obligation.param_env,
 616                                                           &predicate).err().unwrap();
 617                         struct_span_err!(self.tcx.sess, span, E0278,
 618                             "the requirement `{}` is not satisfied (`{}`)",
 619                             predicate, err)
 620                     }
 621
 622                     ty::Predicate::RegionOutlives(ref predicate) => {
 623                         let predicate = self.resolve_type_vars_if_possible(predicate);
 624                         let err = self.region_outlives_predicate(&obligation.cause,
 625                                                                     &predicate).err().unwrap();
 626                         struct_span_err!(self.tcx.sess, span, E0279,
 627                             "the requirement `{}` is not satisfied (`{}`)",
 628                             predicate, err)
 629                     }
 630
 631                     ty::Predicate::Projection(..) | ty::Predicate::TypeOutlives(..) => {
 632                         let predicate =
 633                             self.resolve_type_vars_if_possible(&obligation.predicate);
 634                         struct_span_err!(self.tcx.sess, span, E0280,
 635                             "the requirement `{}` is not satisfied",
 636                             predicate)
 637                     }
 638
 639                     ty::Predicate::ObjectSafe(trait_def_id) => {
 640                         let violations = self.tcx.object_safety_violations(trait_def_id);
 641                         self.tcx.report_object_safety_error(span,
 642                                                             trait_def_id,
 643                                                             violations)
 644                     }
 645
 646                     ty::Predicate::ClosureKind(closure_def_id, kind) => {
 647                         let found_kind = self.closure_kind(closure_def_id).unwrap();
 648                         let closure_span = self.tcx.hir.span_if_local(closure_def_id).unwrap();
 649                         let node_id = self.tcx.hir.as_local_node_id(closure_def_id).unwrap();
 650                         let mut err = struct_span_err!(
 651                             self.tcx.sess, closure_span, E0525,
 652                             "expected a closure that implements the `{}` trait, \
 653                                 but this closure only implements `{}`",
 654                             kind,
 655                             found_kind);
 656
 657                         err.span_label(
 658                             obligation.cause.span,
 659                             format!("the requirement to implement `{}` derives from here", kind));
 660
 661                         // Additional context information explaining why the closure only implements
 662                         // a particular trait.
 663                         if let Some(tables) = self.in_progress_tables {
 664                             let tables = tables.borrow();
 665                             let closure_hir_id = self.tcx.hir.node_to_hir_id(node_id);
 666                             match tables.closure_kinds().get(closure_hir_id) {
 667                                 Some(&(ty::ClosureKind::FnOnce, Some((span, name)))) => {
 668                                     err.span_note(span, &format!(
 669                                         "closure is `FnOnce` because it moves the \
 670                                          variable `{}` out of its environment", name));
 671                                 },
 672                                 Some(&(ty::ClosureKind::FnMut, Some((span, name)))) => {
 673                                     err.span_note(span, &format!(
 674                                         "closure is `FnMut` because it mutates the \
 675                                          variable `{}` here", name));
 676                                 },
 677                                 _ => {}
 678                             }
 679                         }
 680
 681                         err.emit();
 682                         return;
 683                     }
 684
 685                     ty::Predicate::WellFormed(ty) => {
 686                         // WF predicates cannot themselves make
 687                         // errors. They can only block due to
 688                         // ambiguity; otherwise, they always
 689                         // degenerate into other obligations
 690                         // (which may fail).
 691                         span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 692                     }
 693
 694                     ty::Predicate::ConstEvaluatable(..) => {
 695                         // Errors for `ConstEvaluatable` predicates show up as
 696                         // `SelectionError::ConstEvalFailure`,
 697                         // not `Unimplemented`.
 698                         span_bug!(span,
 699                             "const-evaluatable requirement gave wrong error: `{:?}`", obligation)
 700                     }
 701                 }
 702             }
 703
 704             OutputTypeParameterMismatch(ref found_trait_ref, ref expected_trait_ref, _) => {
 705                 let found_trait_ref = self.resolve_type_vars_if_possible(&*found_trait_ref);
 706                 let expected_trait_ref = self.resolve_type_vars_if_possible(&*expected_trait_ref);
 707                 if expected_trait_ref.self_ty().references_error() {
 708                     return;
 709                 }
 710                 let found_trait_ty = found_trait_ref.self_ty();
 711
 712                 let found_did = found_trait_ty.ty_to_def_id();
 713                 let found_span = found_did.and_then(|did| {
 714                     self.tcx.hir.span_if_local(did)
 715                 });
 716
 717                 let found_ty_count =
 718                     match found_trait_ref.skip_binder().substs.type_at(1).sty {
 719                         ty::TyTuple(ref tys, _) => tys.len(),
 720                         _ => 1,
 721                     };
 722                 let (expected_tys, expected_ty_count) =
 723                     match expected_trait_ref.skip_binder().substs.type_at(1).sty {
 724                         ty::TyTuple(ref tys, _) =>
 725                             (tys.iter().map(|t| &t.sty).collect(), tys.len()),
 726                         ref sty => (vec![sty], 1),
 727                     };
 728                 if found_ty_count == expected_ty_count {
 729                     self.report_closure_arg_mismatch(span,
 730                                                      found_span,
 731                                                      found_trait_ref,
 732                                                      expected_trait_ref)
 733                 } else {
 734                     let expected_tuple = if expected_ty_count == 1 {
 735                         expected_tys.first().and_then(|t| {
 736                             if let &&ty::TyTuple(ref tuptys, _) = t {
 737                                 Some(tuptys.len())
 738                             } else {
 739                                 None
 740                             }
 741                         })
 742                     } else {
 743                         None
 744                     };
 745
 746                     // FIXME(#44150): Expand this to "N args expected but a N-tuple found."
 747                     // Type of the 1st expected argument is somehow provided as type of a
 748                     // found one in that case.
 749                     //
 750                     // ```
 751                     // [1i32, 2, 3].sort_by(|(a, b)| ..)
 752                     // //                   ^^^^^^^^
 753                     // // expected_trait_ref:  std::ops::FnMut<(&i32, &i32)>
 754                     // //    found_trait_ref:  std::ops::FnMut<(&i32,)>
 755                     // ```
 756
 757                     let (closure_span, closure_args) = found_did
 758                         .and_then(|did| self.tcx.hir.get_if_local(did))
 759                         .and_then(|node| {
 760                             if let hir::map::NodeExpr(
 761                                 &hir::Expr {
 762                                     node: hir::ExprClosure(_, ref decl, id, span, _),
 763                                     ..
 764                                 }) = node
 765                             {
 766                                 let ty_snips = decl.inputs.iter()
 767                                     .map(|ty| {
 768                                         self.tcx.sess.codemap().span_to_snippet(ty.span).ok()
 769                                             .and_then(|snip| {
 770                                                 // filter out dummy spans
 771                                                 if snip == "," || snip == "|" {
 772                                                     None
 773                                                 } else {
 774                                                     Some(snip)
 775                                                 }
 776                                             })
 777                                     })
 778                                     .collect::<Vec<Option<String>>>();
 779
 780                                 let body = self.tcx.hir.body(id);
 781                                 let pat_snips = body.arguments.iter()
 782                                     .map(|arg|
 783                                         self.tcx.sess.codemap().span_to_snippet(arg.pat.span).ok())
 784                                     .collect::<Option<Vec<String>>>();
 785
 786                                 Some((span, pat_snips, ty_snips))
 787                             } else {
 788                                 None
 789                             }
 790                         })
 791                         .map(|(span, pat, ty)| (Some(span), Some((pat, ty))))
 792                         .unwrap_or((None, None));
 793                     let closure_args = closure_args.and_then(|(pat, ty)| Some((pat?, ty)));
 794
 795                     self.report_arg_count_mismatch(
 796                         span,
 797                         closure_span.or(found_span),
 798                         expected_ty_count,
 799                         expected_tuple,
 800                         found_ty_count,
 801                         closure_args,
 802                         found_trait_ty.is_closure()
 803                     )
 804                 }
 805             }
 806
 807             TraitNotObjectSafe(did) => {
 808                 let violations = self.tcx.object_safety_violations(did);
 809                 self.tcx.report_object_safety_error(span, did,
 810                                                     violations)
 811             }
 812
 813             ConstEvalFailure(ref err) => {
 814                 if let const_val::ErrKind::TypeckError = err.kind {
 815                     return;
 816                 }
 817                 err.struct_error(self.tcx, span, "constant expression")
 818             }
 819         };
 820         self.note_obligation_cause(&mut err, obligation);
 821         err.emit();
 822     }
 823
 824     fn report_arg_count_mismatch(
 825         &self,
 826         span: Span,
 827         found_span: Option<Span>,
 828         expected: usize,
 829         expected_tuple: Option<usize>,
 830         found: usize,
 831         closure_args: Option<(Vec<String>, Vec<Option<String>>)>,
 832         is_closure: bool
 833     ) -> DiagnosticBuilder<'tcx> {
 834         use std::borrow::Cow;
 835
 836         let kind = if is_closure { "closure" } else { "function" };
 837
 838         let args_str = |n, distinct| format!(
 839                 "{} {}argument{}",
 840                 n,
 841                 if distinct && n >= 2 { "distinct " } else { "" },
 842                 if n == 1 { "" } else { "s" },
 843             );
 844
 845         let expected_str = if let Some(n) = expected_tuple {
 846             assert!(expected == 1);
 847             if closure_args.as_ref().map(|&(ref pats, _)| pats.len()) == Some(n) {
 848                 Cow::from("a single tuple as argument")
 849             } else {
 850                 // be verbose when numbers differ
 851                 Cow::from(format!("a single {}-tuple as argument", n))
 852             }
 853         } else {
 854             Cow::from(args_str(expected, false))
 855         };
 856
 857         let found_str = if expected_tuple.is_some() {
 858             args_str(found, true)
 859         } else {
 860             args_str(found, false)
 861         };
 862
 863
 864         let mut err = struct_span_err!(self.tcx.sess, span, E0593,
 865             "{} is expected to take {}, but it takes {}",
 866             kind,
 867             expected_str,
 868             found_str,
 869         );
 870
 871         err.span_label(
 872             span,
 873             format!(
 874                 "expected {} that takes {}",
 875                 kind,
 876                 expected_str,
 877             )
 878         );
 879
 880         if let Some(span) = found_span {
 881             if let (Some(expected_tuple), Some((pats, tys))) = (expected_tuple, closure_args) {
 882                 if expected_tuple != found || pats.len() != found {
 883                     err.span_label(span, format!("takes {}", found_str));
 884                 } else {
 885                     let sugg = format!(
 886                         "|({}){}|",
 887                         pats.join(", "),
 888
 889                         // add type annotations if available
 890                         if tys.iter().any(|ty| ty.is_some()) {
 891                             Cow::from(format!(
 892                                 ": ({})",
 893                                 tys.into_iter().map(|ty| if let Some(ty) = ty {
 894                                     ty
 895                                 } else {
 896                                     "_".to_string()
 897                                 }).collect::<Vec<String>>().join(", ")
 898                             ))
 899                         } else {
 900                             Cow::from("")
 901                         },
 902                     );
 903
 904                     err.span_suggestion(
 905                         span,
 906                         "consider changing the closure to accept a tuple",
 907                         sugg
 908                     );
 909                 }
 910             } else {
 911                 err.span_label(span, format!("takes {}", found_str));
 912             }
 913         }
 914
 915         err
 916     }
 917
 918     fn report_closure_arg_mismatch(&self,
 919                            span: Span,
 920                            found_span: Option<Span>,
 921                            expected_ref: ty::PolyTraitRef<'tcx>,
 922                            found: ty::PolyTraitRef<'tcx>)
 923         -> DiagnosticBuilder<'tcx>
 924     {
 925         fn build_fn_sig_string<'a, 'gcx, 'tcx>(tcx: ty::TyCtxt<'a, 'gcx, 'tcx>,
 926                                                trait_ref: &ty::TraitRef<'tcx>) -> String {
 927             let inputs = trait_ref.substs.type_at(1);
 928             let sig = if let ty::TyTuple(inputs, _) = inputs.sty {
 929                 tcx.mk_fn_sig(
 930                     inputs.iter().map(|&x| x),
 931                     tcx.mk_infer(ty::TyVar(ty::TyVid { index: 0 })),
 932                     false,
 933                     hir::Unsafety::Normal,
 934                     ::syntax::abi::Abi::Rust
 935                 )
 936             } else {
 937                 tcx.mk_fn_sig(
 938                     ::std::iter::once(inputs),
 939                     tcx.mk_infer(ty::TyVar(ty::TyVid { index: 0 })),
 940                     false,
 941                     hir::Unsafety::Normal,
 942                     ::syntax::abi::Abi::Rust
 943                 )
 944             };
 945             format!("{}", ty::Binder(sig))
 946         }
 947
 948         let argument_is_closure = expected_ref.skip_binder().substs.type_at(0).is_closure();
 949         let mut err = struct_span_err!(self.tcx.sess, span, E0631,
 950                                        "type mismatch in {} arguments",
 951                                        if argument_is_closure { "closure" } else { "function" });
 952
 953         let found_str = format!(
 954             "expected signature of `{}`",
 955             build_fn_sig_string(self.tcx, found.skip_binder())
 956         );
 957         err.span_label(span, found_str);
 958
 959         let found_span = found_span.unwrap_or(span);
 960         let expected_str = format!(
 961             "found signature of `{}`",
 962             build_fn_sig_string(self.tcx, expected_ref.skip_binder())
 963         );
 964         err.span_label(found_span, expected_str);
 965
 966         err
 967     }
 968 }
 969
 970 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
 971     pub fn recursive_type_with_infinite_size_error(self,
 972                                                    type_def_id: DefId)
 973                                                    -> DiagnosticBuilder<'tcx>
 974     {
 975         assert!(type_def_id.is_local());
 976         let span = self.hir.span_if_local(type_def_id).unwrap();
 977         let span = self.sess.codemap().def_span(span);
 978         let mut err = struct_span_err!(self.sess, span, E0072,
 979                                        "recursive type `{}` has infinite size",
 980                                        self.item_path_str(type_def_id));
 981         err.span_label(span, "recursive type has infinite size");
 982         err.help(&format!("insert indirection (e.g., a `Box`, `Rc`, or `&`) \
 983                            at some point to make `{}` representable",
 984                           self.item_path_str(type_def_id)));
 985         err
 986     }
 987
 988     pub fn report_object_safety_error(self,
 989                                       span: Span,
 990                                       trait_def_id: DefId,
 991                                       violations: Vec<ObjectSafetyViolation>)
 992                                       -> DiagnosticBuilder<'tcx>
 993     {
 994         let trait_str = self.item_path_str(trait_def_id);
 995         let span = self.sess.codemap().def_span(span);
 996         let mut err = struct_span_err!(
 997             self.sess, span, E0038,
 998             "the trait `{}` cannot be made into an object",
 999             trait_str);
1000         err.span_label(span, format!("the trait `{}` cannot be made into an object", trait_str));
1001
1002         let mut reported_violations = FxHashSet();
1003         for violation in violations {
1004             if !reported_violations.insert(violation.clone()) {
1005                 continue;
1006             }
1007             err.note(&violation.error_msg());
1008         }
1009         err
1010     }
1011 }
1012
1013 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
1014     fn maybe_report_ambiguity(&self, obligation: &PredicateObligation<'tcx>,
1015                               body_id: Option<hir::BodyId>) {
1016         // Unable to successfully determine, probably means
1017         // insufficient type information, but could mean
1018         // ambiguous impls. The latter *ought* to be a
1019         // coherence violation, so we don't report it here.
1020
1021         let predicate = self.resolve_type_vars_if_possible(&obligation.predicate);
1022         let span = obligation.cause.span;
1023
1024         debug!("maybe_report_ambiguity(predicate={:?}, obligation={:?})",
1025                predicate,
1026                obligation);
1027
1028         // Ambiguity errors are often caused as fallout from earlier
1029         // errors. So just ignore them if this infcx is tainted.
1030         if self.is_tainted_by_errors() {
1031             return;
1032         }
1033
1034         match predicate {
1035             ty::Predicate::Trait(ref data) => {
1036                 let trait_ref = data.to_poly_trait_ref();
1037                 let self_ty = trait_ref.self_ty();
1038                 if predicate.references_error() {
1039                     return;
1040                 }
1041                 // Typically, this ambiguity should only happen if
1042                 // there are unresolved type inference variables
1043                 // (otherwise it would suggest a coherence
1044                 // failure). But given #21974 that is not necessarily
1045                 // the case -- we can have multiple where clauses that
1046                 // are only distinguished by a region, which results
1047                 // in an ambiguity even when all types are fully
1048                 // known, since we don't dispatch based on region
1049                 // relationships.
1050
1051                 // This is kind of a hack: it frequently happens that some earlier
1052                 // error prevents types from being fully inferred, and then we get
1053                 // a bunch of uninteresting errors saying something like "<generic
1054                 // #0> doesn't implement Sized".  It may even be true that we
1055                 // could just skip over all checks where the self-ty is an
1056                 // inference variable, but I was afraid that there might be an
1057                 // inference variable created, registered as an obligation, and
1058                 // then never forced by writeback, and hence by skipping here we'd
1059                 // be ignoring the fact that we don't KNOW the type works
1060                 // out. Though even that would probably be harmless, given that
1061                 // we're only talking about builtin traits, which are known to be
1062                 // inhabited. But in any case I just threw in this check for
1063                 // has_errors() to be sure that compilation isn't happening
1064                 // anyway. In that case, why inundate the user.
1065                 if !self.tcx.sess.has_errors() {
1066                     if
1067                         self.tcx.lang_items().sized_trait()
1068                         .map_or(false, |sized_id| sized_id == trait_ref.def_id())
1069                     {
1070                         self.need_type_info(body_id, span, self_ty);
1071                     } else {
1072                         let mut err = struct_span_err!(self.tcx.sess,
1073                                                         span, E0283,
1074                                                         "type annotations required: \
1075                                                         cannot resolve `{}`",
1076                                                         predicate);
1077                         self.note_obligation_cause(&mut err, obligation);
1078                         err.emit();
1079                     }
1080                 }
1081             }
1082
1083             ty::Predicate::WellFormed(ty) => {
1084                 // Same hacky approach as above to avoid deluging user
1085                 // with error messages.
1086                 if !ty.references_error() && !self.tcx.sess.has_errors() {
1087                     self.need_type_info(body_id, span, ty);
1088                 }
1089             }
1090
1091             ty::Predicate::Subtype(ref data) => {
1092                 if data.references_error() || self.tcx.sess.has_errors() {
1093                     // no need to overload user in such cases
1094                 } else {
1095                     let &SubtypePredicate { a_is_expected: _, a, b } = data.skip_binder();
1096                     // both must be type variables, or the other would've been instantiated
1097                     assert!(a.is_ty_var() && b.is_ty_var());
1098                     self.need_type_info(body_id,
1099                                         obligation.cause.span,
1100                                         a);
1101                 }
1102             }
1103
1104             _ => {
1105                 if !self.tcx.sess.has_errors() {
1106                     let mut err = struct_span_err!(self.tcx.sess,
1107                                                    obligation.cause.span, E0284,
1108                                                    "type annotations required: \
1109                                                     cannot resolve `{}`",
1110                                                    predicate);
1111                     self.note_obligation_cause(&mut err, obligation);
1112                     err.emit();
1113                 }
1114             }
1115         }
1116     }
1117
1118     /// Returns whether the trait predicate may apply for *some* assignment
1119     /// to the type parameters.
1120     fn predicate_can_apply(&self,
1121                            param_env: ty::ParamEnv<'tcx>,
1122                            pred: ty::PolyTraitRef<'tcx>)
1123                            -> bool {
1124         struct ParamToVarFolder<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
1125             infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
1126             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>
1127         }
1128
1129         impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for ParamToVarFolder<'a, 'gcx, 'tcx> {
1130             fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> { self.infcx.tcx }
1131
1132             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
1133                 if let ty::TyParam(ty::ParamTy {name, ..}) = ty.sty {
1134                     let infcx = self.infcx;
1135                     self.var_map.entry(ty).or_insert_with(||
1136                         infcx.next_ty_var(
1137                             TypeVariableOrigin::TypeParameterDefinition(DUMMY_SP, name)))
1138                 } else {
1139                     ty.super_fold_with(self)
1140                 }
1141             }
1142         }
1143
1144         self.probe(|_| {
1145             let mut selcx = SelectionContext::new(self);
1146
1147             let cleaned_pred = pred.fold_with(&mut ParamToVarFolder {
1148                 infcx: self,
1149                 var_map: FxHashMap()
1150             });
1151
1152             let cleaned_pred = super::project::normalize(
1153                 &mut selcx,
1154                 param_env,
1155                 ObligationCause::dummy(),
1156                 &cleaned_pred
1157             ).value;
1158
1159             let obligation = Obligation::new(
1160                 ObligationCause::dummy(),
1161                 param_env,
1162                 cleaned_pred.to_predicate()
1163             );
1164
1165             selcx.evaluate_obligation(&obligation)
1166         })
1167     }
1168
1169     fn note_obligation_cause<T>(&self,
1170                                 err: &mut DiagnosticBuilder,
1171                                 obligation: &Obligation<'tcx, T>)
1172         where T: fmt::Display
1173     {
1174         self.note_obligation_cause_code(err,
1175                                         &obligation.predicate,
1176                                         &obligation.cause.code);
1177     }
1178
1179     fn note_obligation_cause_code<T>(&self,
1180                                      err: &mut DiagnosticBuilder,
1181                                      predicate: &T,
1182                                      cause_code: &ObligationCauseCode<'tcx>)
1183         where T: fmt::Display
1184     {
1185         let tcx = self.tcx;
1186         match *cause_code {
1187             ObligationCauseCode::ExprAssignable |
1188             ObligationCauseCode::MatchExpressionArm { .. } |
1189             ObligationCauseCode::IfExpression |
1190             ObligationCauseCode::IfExpressionWithNoElse |
1191             ObligationCauseCode::EquatePredicate |
1192             ObligationCauseCode::MainFunctionType |
1193             ObligationCauseCode::StartFunctionType |
1194             ObligationCauseCode::IntrinsicType |
1195             ObligationCauseCode::MethodReceiver |
1196             ObligationCauseCode::ReturnNoExpression |
1197             ObligationCauseCode::MiscObligation => {
1198             }
1199             ObligationCauseCode::SliceOrArrayElem => {
1200                 err.note("slice and array elements must have `Sized` type");
1201             }
1202             ObligationCauseCode::TupleElem => {
1203                 err.note("only the last element of a tuple may have a dynamically sized type");
1204             }
1205             ObligationCauseCode::ProjectionWf(data) => {
1206                 err.note(&format!("required so that the projection `{}` is well-formed",
1207                                   data));
1208             }
1209             ObligationCauseCode::ReferenceOutlivesReferent(ref_ty) => {
1210                 err.note(&format!("required so that reference `{}` does not outlive its referent",
1211                                   ref_ty));
1212             }
1213             ObligationCauseCode::ObjectTypeBound(object_ty, region) => {
1214                 err.note(&format!("required so that the lifetime bound of `{}` for `{}` \
1215                                    is satisfied",
1216                                   region, object_ty));
1217             }
1218             ObligationCauseCode::ItemObligation(item_def_id) => {
1219                 let item_name = tcx.item_path_str(item_def_id);
1220                 err.note(&format!("required by `{}`", item_name));
1221             }
1222             ObligationCauseCode::ObjectCastObligation(object_ty) => {
1223                 err.note(&format!("required for the cast to the object type `{}`",
1224                                   self.ty_to_string(object_ty)));
1225             }
1226             ObligationCauseCode::RepeatVec => {
1227                 err.note("the `Copy` trait is required because the \
1228                           repeated element will be copied");
1229             }
1230             ObligationCauseCode::VariableType(_) => {
1231                 err.note("all local variables must have a statically known size");
1232             }
1233             ObligationCauseCode::SizedReturnType => {
1234                 err.note("the return type of a function must have a \
1235                           statically known size");
1236             }
1237             ObligationCauseCode::AssignmentLhsSized => {
1238                 err.note("the left-hand-side of an assignment must have a statically known size");
1239             }
1240             ObligationCauseCode::TupleInitializerSized => {
1241                 err.note("tuples must have a statically known size to be initialized");
1242             }
1243             ObligationCauseCode::StructInitializerSized => {
1244                 err.note("structs must have a statically known size to be initialized");
1245             }
1246             ObligationCauseCode::FieldSized(ref item) => {
1247                 match *item {
1248                     AdtKind::Struct => {
1249                         err.note("only the last field of a struct may have a dynamically \
1250                                   sized type");
1251                     }
1252                     AdtKind::Union => {
1253                         err.note("no field of a union may have a dynamically sized type");
1254                     }
1255                     AdtKind::Enum => {
1256                         err.note("no field of an enum variant may have a dynamically sized type");
1257                     }
1258                 }
1259             }
1260             ObligationCauseCode::ConstSized => {
1261                 err.note("constant expressions must have a statically known size");
1262             }
1263             ObligationCauseCode::SharedStatic => {
1264                 err.note("shared static variables must have a type that implements `Sync`");
1265             }
1266             ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
1267                 let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1268                 err.note(&format!("required because it appears within the type `{}`",
1269                                   parent_trait_ref.0.self_ty()));
1270                 let parent_predicate = parent_trait_ref.to_predicate();
1271                 self.note_obligation_cause_code(err,
1272                                                 &parent_predicate,
1273                                                 &data.parent_code);
1274             }
1275             ObligationCauseCode::ImplDerivedObligation(ref data) => {
1276                 let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1277                 err.note(
1278                     &format!("required because of the requirements on the impl of `{}` for `{}`",
1279                              parent_trait_ref,
1280                              parent_trait_ref.0.self_ty()));
1281                 let parent_predicate = parent_trait_ref.to_predicate();
1282                 self.note_obligation_cause_code(err,
1283                                                 &parent_predicate,
1284                                                 &data.parent_code);
1285             }
1286             ObligationCauseCode::CompareImplMethodObligation { .. } => {
1287                 err.note(
1288                     &format!("the requirement `{}` appears on the impl method \
1289                               but not on the corresponding trait method",
1290                              predicate));
1291             }
1292             ObligationCauseCode::ReturnType(_) |
1293             ObligationCauseCode::BlockTailExpression(_) => (),
1294         }
1295     }
1296
1297     fn suggest_new_overflow_limit(&self, err: &mut DiagnosticBuilder) {
1298         let current_limit = self.tcx.sess.recursion_limit.get();
1299         let suggested_limit = current_limit * 2;
1300         err.help(&format!("consider adding a `#![recursion_limit=\"{}\"]` attribute to your crate",
1301                           suggested_limit));
1302     }
1303 }