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 rustc::lint::builtin::EXTRA_REQUIREMENT_IN_IMPL;
  37 use std::fmt;
  38 use syntax::ast;
  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 mut diag = struct_span_err!(
 223                 self.tcx.sess, obligation.cause.span, E0271,
 224                 "type mismatch resolving `{}`", predicate
 225             );
 226             self.note_type_err(&mut diag, &obligation.cause, None, values, err);
 227             self.note_obligation_cause(&mut diag, obligation);
 228             diag.emit();
 229         });
 230     }
 231
 232     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
 233         /// returns the fuzzy category of a given type, or None
 234         /// if the type can be equated to any type.
 235         fn type_category<'tcx>(t: Ty<'tcx>) -> Option<u32> {
 236             match t.sty {
 237                 ty::TyBool => Some(0),
 238                 ty::TyChar => Some(1),
 239                 ty::TyStr => Some(2),
 240                 ty::TyInt(..) | ty::TyUint(..) | ty::TyInfer(ty::IntVar(..)) => Some(3),
 241                 ty::TyFloat(..) | ty::TyInfer(ty::FloatVar(..)) => Some(4),
 242                 ty::TyRef(..) | ty::TyRawPtr(..) => Some(5),
 243                 ty::TyArray(..) | ty::TySlice(..) => Some(6),
 244                 ty::TyFnDef(..) | ty::TyFnPtr(..) => Some(7),
 245                 ty::TyDynamic(..) => Some(8),
 246                 ty::TyClosure(..) => Some(9),
 247                 ty::TyTuple(..) => Some(10),
 248                 ty::TyProjection(..) => Some(11),
 249                 ty::TyParam(..) => Some(12),
 250                 ty::TyAnon(..) => Some(13),
 251                 ty::TyNever => Some(14),
 252                 ty::TyAdt(adt, ..) => match adt.adt_kind() {
 253                     AdtKind::Struct => Some(15),
 254                     AdtKind::Union => Some(16),
 255                     AdtKind::Enum => Some(17),
 256                 },
 257                 ty::TyGenerator(..) => Some(18),
 258                 ty::TyInfer(..) | ty::TyError => None
 259             }
 260         }
 261
 262         match (type_category(a), type_category(b)) {
 263             (Some(cat_a), Some(cat_b)) => match (&a.sty, &b.sty) {
 264                 (&ty::TyAdt(def_a, _), &ty::TyAdt(def_b, _)) => def_a == def_b,
 265                 _ => cat_a == cat_b
 266             },
 267             // infer and error can be equated to all types
 268             _ => true
 269         }
 270     }
 271
 272     fn impl_similar_to(&self,
 273                        trait_ref: ty::PolyTraitRef<'tcx>,
 274                        obligation: &PredicateObligation<'tcx>)
 275                        -> Option<DefId>
 276     {
 277         let tcx = self.tcx;
 278         let param_env = obligation.param_env;
 279         let trait_ref = tcx.erase_late_bound_regions(&trait_ref);
 280         let trait_self_ty = trait_ref.self_ty();
 281
 282         let mut self_match_impls = vec![];
 283         let mut fuzzy_match_impls = vec![];
 284
 285         self.tcx.for_each_relevant_impl(
 286             trait_ref.def_id, trait_self_ty, |def_id| {
 287                 let impl_substs = self.fresh_substs_for_item(obligation.cause.span, def_id);
 288                 let impl_trait_ref = tcx
 289                     .impl_trait_ref(def_id)
 290                     .unwrap()
 291                     .subst(tcx, impl_substs);
 292
 293                 let impl_self_ty = impl_trait_ref.self_ty();
 294
 295                 if let Ok(..) = self.can_eq(param_env, trait_self_ty, impl_self_ty) {
 296                     self_match_impls.push(def_id);
 297
 298                     if trait_ref.substs.types().skip(1)
 299                         .zip(impl_trait_ref.substs.types().skip(1))
 300                         .all(|(u,v)| self.fuzzy_match_tys(u, v))
 301                     {
 302                         fuzzy_match_impls.push(def_id);
 303                     }
 304                 }
 305             });
 306
 307         let impl_def_id = if self_match_impls.len() == 1 {
 308             self_match_impls[0]
 309         } else if fuzzy_match_impls.len() == 1 {
 310             fuzzy_match_impls[0]
 311         } else {
 312             return None
 313         };
 314
 315         if tcx.has_attr(impl_def_id, "rustc_on_unimplemented") {
 316             Some(impl_def_id)
 317         } else {
 318             None
 319         }
 320     }
 321
 322     fn on_unimplemented_note(
 323         &self,
 324         trait_ref: ty::PolyTraitRef<'tcx>,
 325         obligation: &PredicateObligation<'tcx>) ->
 326         OnUnimplementedNote
 327     {
 328         let def_id = self.impl_similar_to(trait_ref, obligation)
 329             .unwrap_or(trait_ref.def_id());
 330         let trait_ref = *trait_ref.skip_binder();
 331
 332         let desugaring;
 333         let method;
 334         let mut flags = vec![];
 335         let direct = match obligation.cause.code {
 336             ObligationCauseCode::BuiltinDerivedObligation(..) |
 337             ObligationCauseCode::ImplDerivedObligation(..) => false,
 338             _ => true
 339         };
 340         if direct {
 341             // this is a "direct", user-specified, rather than derived,
 342             // obligation.
 343             flags.push(("direct", None));
 344         }
 345
 346         if let ObligationCauseCode::ItemObligation(item) = obligation.cause.code {
 347             // FIXME: maybe also have some way of handling methods
 348             // from other traits? That would require name resolution,
 349             // which we might want to be some sort of hygienic.
 350             //
 351             // Currently I'm leaving it for what I need for `try`.
 352             if self.tcx.trait_of_item(item) == Some(trait_ref.def_id) {
 353                 method = self.tcx.item_name(item);
 354                 flags.push(("from_method", None));
 355                 flags.push(("from_method", Some(&*method)));
 356             }
 357         }
 358
 359         if let Some(k) = obligation.cause.span.compiler_desugaring_kind() {
 360             desugaring = k.as_symbol().as_str();
 361             flags.push(("from_desugaring", None));
 362             flags.push(("from_desugaring", Some(&*desugaring)));
 363         }
 364
 365         if let Ok(Some(command)) = OnUnimplementedDirective::of_item(
 366             self.tcx, trait_ref.def_id, def_id
 367         ) {
 368             command.evaluate(self.tcx, trait_ref, &flags)
 369         } else {
 370             OnUnimplementedNote::empty()
 371         }
 372     }
 373
 374     fn find_similar_impl_candidates(&self,
 375                                     trait_ref: ty::PolyTraitRef<'tcx>)
 376                                     -> Vec<ty::TraitRef<'tcx>>
 377     {
 378         let simp = fast_reject::simplify_type(self.tcx,
 379                                               trait_ref.skip_binder().self_ty(),
 380                                               true);
 381         let mut impl_candidates = Vec::new();
 382
 383         match simp {
 384             Some(simp) => self.tcx.for_each_impl(trait_ref.def_id(), |def_id| {
 385                 let imp = self.tcx.impl_trait_ref(def_id).unwrap();
 386                 let imp_simp = fast_reject::simplify_type(self.tcx,
 387                                                           imp.self_ty(),
 388                                                           true);
 389                 if let Some(imp_simp) = imp_simp {
 390                     if simp != imp_simp {
 391                         return;
 392                     }
 393                 }
 394                 impl_candidates.push(imp);
 395             }),
 396             None => self.tcx.for_each_impl(trait_ref.def_id(), |def_id| {
 397                 impl_candidates.push(
 398                     self.tcx.impl_trait_ref(def_id).unwrap());
 399             })
 400         };
 401         impl_candidates
 402     }
 403
 404     fn report_similar_impl_candidates(&self,
 405                                       impl_candidates: Vec<ty::TraitRef<'tcx>>,
 406                                       err: &mut DiagnosticBuilder)
 407     {
 408         if impl_candidates.is_empty() {
 409             return;
 410         }
 411
 412         let end = if impl_candidates.len() <= 5 {
 413             impl_candidates.len()
 414         } else {
 415             4
 416         };
 417         err.help(&format!("the following implementations were found:{}{}",
 418                           &impl_candidates[0..end].iter().map(|candidate| {
 419                               format!("\n  {:?}", candidate)
 420                           }).collect::<String>(),
 421                           if impl_candidates.len() > 5 {
 422                               format!("\nand {} others", impl_candidates.len() - 4)
 423                           } else {
 424                               "".to_owned()
 425                           }
 426                           ));
 427     }
 428
 429     /// Reports that an overflow has occurred and halts compilation. We
 430     /// halt compilation unconditionally because it is important that
 431     /// overflows never be masked -- they basically represent computations
 432     /// whose result could not be truly determined and thus we can't say
 433     /// if the program type checks or not -- and they are unusual
 434     /// occurrences in any case.
 435     pub fn report_overflow_error<T>(&self,
 436                                     obligation: &Obligation<'tcx, T>,
 437                                     suggest_increasing_limit: bool) -> !
 438         where T: fmt::Display + TypeFoldable<'tcx>
 439     {
 440         let predicate =
 441             self.resolve_type_vars_if_possible(&obligation.predicate);
 442         let mut err = struct_span_err!(self.tcx.sess, obligation.cause.span, E0275,
 443                                        "overflow evaluating the requirement `{}`",
 444                                        predicate);
 445
 446         if suggest_increasing_limit {
 447             self.suggest_new_overflow_limit(&mut err);
 448         }
 449
 450         self.note_obligation_cause(&mut err, obligation);
 451
 452         err.emit();
 453         self.tcx.sess.abort_if_errors();
 454         bug!();
 455     }
 456
 457     /// Reports that a cycle was detected which led to overflow and halts
 458     /// compilation. This is equivalent to `report_overflow_error` except
 459     /// that we can give a more helpful error message (and, in particular,
 460     /// we do not suggest increasing the overflow limit, which is not
 461     /// going to help).
 462     pub fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 463         let cycle = self.resolve_type_vars_if_possible(&cycle.to_owned());
 464         assert!(cycle.len() > 0);
 465
 466         debug!("report_overflow_error_cycle: cycle={:?}", cycle);
 467
 468         self.report_overflow_error(&cycle[0], false);
 469     }
 470
 471     pub fn report_extra_impl_obligation(&self,
 472                                         error_span: Span,
 473                                         item_name: ast::Name,
 474                                         _impl_item_def_id: DefId,
 475                                         trait_item_def_id: DefId,
 476                                         requirement: &fmt::Display,
 477                                         lint_id: Option<ast::NodeId>) // (*)
 478                                         -> DiagnosticBuilder<'tcx>
 479     {
 480         // (*) This parameter is temporary and used only for phasing
 481         // in the bug fix to #18937. If it is `Some`, it has a kind of
 482         // weird effect -- the diagnostic is reported as a lint, and
 483         // the builder which is returned is marked as canceled.
 484
 485         let msg = "impl has stricter requirements than trait";
 486         let mut err = match lint_id {
 487             Some(node_id) => {
 488                 self.tcx.struct_span_lint_node(EXTRA_REQUIREMENT_IN_IMPL,
 489                                                node_id,
 490                                                error_span,
 491                                                msg)
 492             }
 493             None => {
 494                 struct_span_err!(self.tcx.sess,
 495                                  error_span,
 496                                  E0276,
 497                                  "{}", msg)
 498             }
 499         };
 500
 501         if let Some(trait_item_span) = self.tcx.hir.span_if_local(trait_item_def_id) {
 502             let span = self.tcx.sess.codemap().def_span(trait_item_span);
 503             err.span_label(span, format!("definition of `{}` from trait", item_name));
 504         }
 505
 506         err.span_label(
 507             error_span,
 508             format!("impl has extra requirement {}", requirement));
 509
 510         err
 511     }
 512
 513
 514     /// Get the parent trait chain start
 515     fn get_parent_trait_ref(&self, code: &ObligationCauseCode<'tcx>) -> Option<String> {
 516         match code {
 517             &ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
 518                 let parent_trait_ref = self.resolve_type_vars_if_possible(
 519                     &data.parent_trait_ref);
 520                 match self.get_parent_trait_ref(&data.parent_code) {
 521                     Some(t) => Some(t),
 522                     None => Some(format!("{}", parent_trait_ref.0.self_ty())),
 523                 }
 524             }
 525             _ => None,
 526         }
 527     }
 528
 529     pub fn report_selection_error(&self,
 530                                   obligation: &PredicateObligation<'tcx>,
 531                                   error: &SelectionError<'tcx>)
 532     {
 533         let span = obligation.cause.span;
 534
 535         let mut err = match *error {
 536             SelectionError::Unimplemented => {
 537                 if let ObligationCauseCode::CompareImplMethodObligation {
 538                     item_name, impl_item_def_id, trait_item_def_id, lint_id
 539                 } = obligation.cause.code {
 540                     self.report_extra_impl_obligation(
 541                         span,
 542                         item_name,
 543                         impl_item_def_id,
 544                         trait_item_def_id,
 545                         &format!("`{}`", obligation.predicate),
 546                         lint_id)
 547                         .emit();
 548                     return;
 549                 }
 550                 match obligation.predicate {
 551                     ty::Predicate::Trait(ref trait_predicate) => {
 552                         let trait_predicate =
 553                             self.resolve_type_vars_if_possible(trait_predicate);
 554
 555                         if self.tcx.sess.has_errors() && trait_predicate.references_error() {
 556                             return;
 557                         }
 558                         let trait_ref = trait_predicate.to_poly_trait_ref();
 559                         let (post_message, pre_message) =
 560                             self.get_parent_trait_ref(&obligation.cause.code)
 561                                 .map(|t| (format!(" in `{}`", t), format!("within `{}`, ", t)))
 562                             .unwrap_or((String::new(), String::new()));
 563
 564                         let OnUnimplementedNote { message, label }
 565                             = self.on_unimplemented_note(trait_ref, obligation);
 566                         let have_alt_message = message.is_some() || label.is_some();
 567
 568                         let mut err = struct_span_err!(
 569                             self.tcx.sess,
 570                             span,
 571                             E0277,
 572                             "{}",
 573                             message.unwrap_or_else(|| {
 574                                 format!("the trait bound `{}` is not satisfied{}",
 575                                          trait_ref.to_predicate(), post_message)
 576                             }));
 577
 578                         if let Some(ref s) = label {
 579                             // If it has a custom "#[rustc_on_unimplemented]"
 580                             // error message, let's display it as the label!
 581                             err.span_label(span, s.as_str());
 582                             err.help(&format!("{}the trait `{}` is not implemented for `{}`",
 583                                               pre_message,
 584                                               trait_ref,
 585                                               trait_ref.self_ty()));
 586                         } else {
 587                             err.span_label(span,
 588                                            &*format!("{}the trait `{}` is not implemented for `{}`",
 589                                                      pre_message,
 590                                                      trait_ref,
 591                                                      trait_ref.self_ty()));
 592                         }
 593
 594                         // Try to report a help message
 595                         if !trait_ref.has_infer_types() &&
 596                             self.predicate_can_apply(obligation.param_env, trait_ref) {
 597                             // If a where-clause may be useful, remind the
 598                             // user that they can add it.
 599                             //
 600                             // don't display an on-unimplemented note, as
 601                             // these notes will often be of the form
 602                             //     "the type `T` can't be frobnicated"
 603                             // which is somewhat confusing.
 604                             err.help(&format!("consider adding a `where {}` bound",
 605                                                 trait_ref.to_predicate()));
 606                         } else if !have_alt_message {
 607                             // Can't show anything else useful, try to find similar impls.
 608                             let impl_candidates = self.find_similar_impl_candidates(trait_ref);
 609                             self.report_similar_impl_candidates(impl_candidates, &mut err);
 610                         }
 611
 612                         err
 613                     }
 614
 615                     ty::Predicate::Subtype(ref predicate) => {
 616                         // Errors for Subtype predicates show up as
 617                         // `FulfillmentErrorCode::CodeSubtypeError`,
 618                         // not selection error.
 619                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
 620                     }
 621
 622                     ty::Predicate::Equate(ref predicate) => {
 623                         let predicate = self.resolve_type_vars_if_possible(predicate);
 624                         let err = self.equality_predicate(&obligation.cause,
 625                                                           obligation.param_env,
 626                                                           &predicate).err().unwrap();
 627                         struct_span_err!(self.tcx.sess, span, E0278,
 628                             "the requirement `{}` is not satisfied (`{}`)",
 629                             predicate, err)
 630                     }
 631
 632                     ty::Predicate::RegionOutlives(ref predicate) => {
 633                         let predicate = self.resolve_type_vars_if_possible(predicate);
 634                         let err = self.region_outlives_predicate(&obligation.cause,
 635                                                                     &predicate).err().unwrap();
 636                         struct_span_err!(self.tcx.sess, span, E0279,
 637                             "the requirement `{}` is not satisfied (`{}`)",
 638                             predicate, err)
 639                     }
 640
 641                     ty::Predicate::Projection(..) | ty::Predicate::TypeOutlives(..) => {
 642                         let predicate =
 643                             self.resolve_type_vars_if_possible(&obligation.predicate);
 644                         struct_span_err!(self.tcx.sess, span, E0280,
 645                             "the requirement `{}` is not satisfied",
 646                             predicate)
 647                     }
 648
 649                     ty::Predicate::ObjectSafe(trait_def_id) => {
 650                         let violations = self.tcx.object_safety_violations(trait_def_id);
 651                         self.tcx.report_object_safety_error(span,
 652                                                             trait_def_id,
 653                                                             violations)
 654                     }
 655
 656                     ty::Predicate::ClosureKind(closure_def_id, kind) => {
 657                         let found_kind = self.closure_kind(closure_def_id).unwrap();
 658                         let closure_span = self.tcx.hir.span_if_local(closure_def_id).unwrap();
 659                         let node_id = self.tcx.hir.as_local_node_id(closure_def_id).unwrap();
 660                         let mut err = struct_span_err!(
 661                             self.tcx.sess, closure_span, E0525,
 662                             "expected a closure that implements the `{}` trait, \
 663                                 but this closure only implements `{}`",
 664                             kind,
 665                             found_kind);
 666
 667                         err.span_label(
 668                             obligation.cause.span,
 669                             format!("the requirement to implement `{}` derives from here", kind));
 670
 671                         // Additional context information explaining why the closure only implements
 672                         // a particular trait.
 673                         if let Some(tables) = self.in_progress_tables {
 674                             let tables = tables.borrow();
 675                             let closure_hir_id = self.tcx.hir.node_to_hir_id(node_id);
 676                             match tables.closure_kinds().get(closure_hir_id) {
 677                                 Some(&(ty::ClosureKind::FnOnce, Some((span, name)))) => {
 678                                     err.span_note(span, &format!(
 679                                         "closure is `FnOnce` because it moves the \
 680                                          variable `{}` out of its environment", name));
 681                                 },
 682                                 Some(&(ty::ClosureKind::FnMut, Some((span, name)))) => {
 683                                     err.span_note(span, &format!(
 684                                         "closure is `FnMut` because it mutates the \
 685                                          variable `{}` here", name));
 686                                 },
 687                                 _ => {}
 688                             }
 689                         }
 690
 691                         err.emit();
 692                         return;
 693                     }
 694
 695                     ty::Predicate::WellFormed(ty) => {
 696                         // WF predicates cannot themselves make
 697                         // errors. They can only block due to
 698                         // ambiguity; otherwise, they always
 699                         // degenerate into other obligations
 700                         // (which may fail).
 701                         span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 702                     }
 703
 704                     ty::Predicate::ConstEvaluatable(..) => {
 705                         // Errors for `ConstEvaluatable` predicates show up as
 706                         // `SelectionError::ConstEvalFailure`,
 707                         // not `Unimplemented`.
 708                         span_bug!(span,
 709                             "const-evaluatable requirement gave wrong error: `{:?}`", obligation)
 710                     }
 711                 }
 712             }
 713
 714             OutputTypeParameterMismatch(ref found_trait_ref, ref expected_trait_ref, _) => {
 715                 let found_trait_ref = self.resolve_type_vars_if_possible(&*found_trait_ref);
 716                 let expected_trait_ref = self.resolve_type_vars_if_possible(&*expected_trait_ref);
 717                 if expected_trait_ref.self_ty().references_error() {
 718                     return;
 719                 }
 720                 let found_trait_ty = found_trait_ref.self_ty();
 721
 722                 let found_did = found_trait_ty.ty_to_def_id();
 723                 let found_span = found_did.and_then(|did| {
 724                     self.tcx.hir.span_if_local(did)
 725                 });
 726
 727                 let found_ty_count =
 728                     match found_trait_ref.skip_binder().substs.type_at(1).sty {
 729                         ty::TyTuple(ref tys, _) => tys.len(),
 730                         _ => 1,
 731                     };
 732                 let (expected_tys, expected_ty_count) =
 733                     match expected_trait_ref.skip_binder().substs.type_at(1).sty {
 734                         ty::TyTuple(ref tys, _) =>
 735                             (tys.iter().map(|t| &t.sty).collect(), tys.len()),
 736                         ref sty => (vec![sty], 1),
 737                     };
 738                 if found_ty_count == expected_ty_count {
 739                     self.report_closure_arg_mismatch(span,
 740                                                      found_span,
 741                                                      found_trait_ref,
 742                                                      expected_trait_ref)
 743                 } else {
 744                     let expected_tuple = if expected_ty_count == 1 {
 745                         expected_tys.first().and_then(|t| {
 746                             if let &&ty::TyTuple(ref tuptys, _) = t {
 747                                 Some(tuptys.len())
 748                             } else {
 749                                 None
 750                             }
 751                         })
 752                     } else {
 753                         None
 754                     };
 755
 756                     // FIXME(#44150): Expand this to "N args expected but a N-tuple found."
 757                     // Type of the 1st expected argument is somehow provided as type of a
 758                     // found one in that case.
 759                     //
 760                     // ```
 761                     // [1i32, 2, 3].sort_by(|(a, b)| ..)
 762                     // //                   ^^^^^^^^
 763                     // // expected_trait_ref:  std::ops::FnMut<(&i32, &i32)>
 764                     // //    found_trait_ref:  std::ops::FnMut<(&i32,)>
 765                     // ```
 766
 767                     let (closure_span, closure_args) = found_did
 768                         .and_then(|did| self.tcx.hir.get_if_local(did))
 769                         .and_then(|node| {
 770                             if let hir::map::NodeExpr(
 771                                 &hir::Expr {
 772                                     node: hir::ExprClosure(_, ref decl, id, span, _),
 773                                     ..
 774                                 }) = node
 775                             {
 776                                 let ty_snips = decl.inputs.iter()
 777                                     .map(|ty| {
 778                                         self.tcx.sess.codemap().span_to_snippet(ty.span).ok()
 779                                             .and_then(|snip| {
 780                                                 // filter out dummy spans
 781                                                 if snip == "," || snip == "|" {
 782                                                     None
 783                                                 } else {
 784                                                     Some(snip)
 785                                                 }
 786                                             })
 787                                     })
 788                                     .collect::<Vec<Option<String>>>();
 789
 790                                 let body = self.tcx.hir.body(id);
 791                                 let pat_snips = body.arguments.iter()
 792                                     .map(|arg|
 793                                         self.tcx.sess.codemap().span_to_snippet(arg.pat.span).ok())
 794                                     .collect::<Option<Vec<String>>>();
 795
 796                                 Some((span, pat_snips, ty_snips))
 797                             } else {
 798                                 None
 799                             }
 800                         })
 801                         .map(|(span, pat, ty)| (Some(span), Some((pat, ty))))
 802                         .unwrap_or((None, None));
 803                     let closure_args = closure_args.and_then(|(pat, ty)| Some((pat?, ty)));
 804
 805                     self.report_arg_count_mismatch(
 806                         span,
 807                         closure_span.or(found_span),
 808                         expected_ty_count,
 809                         expected_tuple,
 810                         found_ty_count,
 811                         closure_args,
 812                         found_trait_ty.is_closure()
 813                     )
 814                 }
 815             }
 816
 817             TraitNotObjectSafe(did) => {
 818                 let violations = self.tcx.object_safety_violations(did);
 819                 self.tcx.report_object_safety_error(span, did,
 820                                                     violations)
 821             }
 822
 823             ConstEvalFailure(ref err) => {
 824                 if let const_val::ErrKind::TypeckError = err.kind {
 825                     return;
 826                 }
 827                 err.struct_error(self.tcx, span, "constant expression")
 828             }
 829         };
 830         self.note_obligation_cause(&mut err, obligation);
 831         err.emit();
 832     }
 833
 834     fn report_arg_count_mismatch(
 835         &self,
 836         span: Span,
 837         found_span: Option<Span>,
 838         expected: usize,
 839         expected_tuple: Option<usize>,
 840         found: usize,
 841         closure_args: Option<(Vec<String>, Vec<Option<String>>)>,
 842         is_closure: bool
 843     ) -> DiagnosticBuilder<'tcx> {
 844         use std::borrow::Cow;
 845
 846         let kind = if is_closure { "closure" } else { "function" };
 847
 848         let args_str = |n, distinct| format!(
 849                 "{} {}argument{}",
 850                 n,
 851                 if distinct && n >= 2 { "distinct " } else { "" },
 852                 if n == 1 { "" } else { "s" },
 853             );
 854
 855         let expected_str = if let Some(n) = expected_tuple {
 856             assert!(expected == 1);
 857             if closure_args.as_ref().map(|&(ref pats, _)| pats.len()) == Some(n) {
 858                 Cow::from("a single tuple as argument")
 859             } else {
 860                 // be verbose when numbers differ
 861                 Cow::from(format!("a single {}-tuple as argument", n))
 862             }
 863         } else {
 864             Cow::from(args_str(expected, false))
 865         };
 866
 867         let found_str = if expected_tuple.is_some() {
 868             args_str(found, true)
 869         } else {
 870             args_str(found, false)
 871         };
 872
 873
 874         let mut err = struct_span_err!(self.tcx.sess, span, E0593,
 875             "{} is expected to take {}, but it takes {}",
 876             kind,
 877             expected_str,
 878             found_str,
 879         );
 880
 881         err.span_label(
 882             span,
 883             format!(
 884                 "expected {} that takes {}",
 885                 kind,
 886                 expected_str,
 887             )
 888         );
 889
 890         if let Some(span) = found_span {
 891             if let (Some(expected_tuple), Some((pats, tys))) = (expected_tuple, closure_args) {
 892                 if expected_tuple != found || pats.len() != found {
 893                     err.span_label(span, format!("takes {}", found_str));
 894                 } else {
 895                     let sugg = format!(
 896                         "|({}){}|",
 897                         pats.join(", "),
 898
 899                         // add type annotations if available
 900                         if tys.iter().any(|ty| ty.is_some()) {
 901                             Cow::from(format!(
 902                                 ": ({})",
 903                                 tys.into_iter().map(|ty| if let Some(ty) = ty {
 904                                     ty
 905                                 } else {
 906                                     "_".to_string()
 907                                 }).collect::<Vec<String>>().join(", ")
 908                             ))
 909                         } else {
 910                             Cow::from("")
 911                         },
 912                     );
 913
 914                     err.span_suggestion(
 915                         span,
 916                         "consider changing the closure to accept a tuple",
 917                         sugg
 918                     );
 919                 }
 920             } else {
 921                 err.span_label(span, format!("takes {}", found_str));
 922             }
 923         }
 924
 925         err
 926     }
 927
 928     fn report_closure_arg_mismatch(&self,
 929                            span: Span,
 930                            found_span: Option<Span>,
 931                            expected_ref: ty::PolyTraitRef<'tcx>,
 932                            found: ty::PolyTraitRef<'tcx>)
 933         -> DiagnosticBuilder<'tcx>
 934     {
 935         fn build_fn_sig_string<'a, 'gcx, 'tcx>(tcx: ty::TyCtxt<'a, 'gcx, 'tcx>,
 936                                                trait_ref: &ty::TraitRef<'tcx>) -> String {
 937             let inputs = trait_ref.substs.type_at(1);
 938             let sig = if let ty::TyTuple(inputs, _) = inputs.sty {
 939                 tcx.mk_fn_sig(
 940                     inputs.iter().map(|&x| x),
 941                     tcx.mk_infer(ty::TyVar(ty::TyVid { index: 0 })),
 942                     false,
 943                     hir::Unsafety::Normal,
 944                     ::syntax::abi::Abi::Rust
 945                 )
 946             } else {
 947                 tcx.mk_fn_sig(
 948                     ::std::iter::once(inputs),
 949                     tcx.mk_infer(ty::TyVar(ty::TyVid { index: 0 })),
 950                     false,
 951                     hir::Unsafety::Normal,
 952                     ::syntax::abi::Abi::Rust
 953                 )
 954             };
 955             format!("{}", ty::Binder(sig))
 956         }
 957
 958         let argument_is_closure = expected_ref.skip_binder().substs.type_at(0).is_closure();
 959         let mut err = struct_span_err!(self.tcx.sess, span, E0631,
 960                                        "type mismatch in {} arguments",
 961                                        if argument_is_closure { "closure" } else { "function" });
 962
 963         let found_str = format!(
 964             "expected signature of `{}`",
 965             build_fn_sig_string(self.tcx, found.skip_binder())
 966         );
 967         err.span_label(span, found_str);
 968
 969         let found_span = found_span.unwrap_or(span);
 970         let expected_str = format!(
 971             "found signature of `{}`",
 972             build_fn_sig_string(self.tcx, expected_ref.skip_binder())
 973         );
 974         err.span_label(found_span, expected_str);
 975
 976         err
 977     }
 978 }
 979
 980 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
 981     pub fn recursive_type_with_infinite_size_error(self,
 982                                                    type_def_id: DefId)
 983                                                    -> DiagnosticBuilder<'tcx>
 984     {
 985         assert!(type_def_id.is_local());
 986         let span = self.hir.span_if_local(type_def_id).unwrap();
 987         let span = self.sess.codemap().def_span(span);
 988         let mut err = struct_span_err!(self.sess, span, E0072,
 989                                        "recursive type `{}` has infinite size",
 990                                        self.item_path_str(type_def_id));
 991         err.span_label(span, "recursive type has infinite size");
 992         err.help(&format!("insert indirection (e.g., a `Box`, `Rc`, or `&`) \
 993                            at some point to make `{}` representable",
 994                           self.item_path_str(type_def_id)));
 995         err
 996     }
 997
 998     pub fn report_object_safety_error(self,
 999                                       span: Span,
1000                                       trait_def_id: DefId,
1001                                       violations: Vec<ObjectSafetyViolation>)
1002                                       -> DiagnosticBuilder<'tcx>
1003     {
1004         let trait_str = self.item_path_str(trait_def_id);
1005         let span = self.sess.codemap().def_span(span);
1006         let mut err = struct_span_err!(
1007             self.sess, span, E0038,
1008             "the trait `{}` cannot be made into an object",
1009             trait_str);
1010         err.span_label(span, format!("the trait `{}` cannot be made into an object", trait_str));
1011
1012         let mut reported_violations = FxHashSet();
1013         for violation in violations {
1014             if !reported_violations.insert(violation.clone()) {
1015                 continue;
1016             }
1017             err.note(&violation.error_msg());
1018         }
1019         err
1020     }
1021 }
1022
1023 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
1024     fn maybe_report_ambiguity(&self, obligation: &PredicateObligation<'tcx>,
1025                               body_id: Option<hir::BodyId>) {
1026         // Unable to successfully determine, probably means
1027         // insufficient type information, but could mean
1028         // ambiguous impls. The latter *ought* to be a
1029         // coherence violation, so we don't report it here.
1030
1031         let predicate = self.resolve_type_vars_if_possible(&obligation.predicate);
1032         let span = obligation.cause.span;
1033
1034         debug!("maybe_report_ambiguity(predicate={:?}, obligation={:?})",
1035                predicate,
1036                obligation);
1037
1038         // Ambiguity errors are often caused as fallout from earlier
1039         // errors. So just ignore them if this infcx is tainted.
1040         if self.is_tainted_by_errors() {
1041             return;
1042         }
1043
1044         match predicate {
1045             ty::Predicate::Trait(ref data) => {
1046                 let trait_ref = data.to_poly_trait_ref();
1047                 let self_ty = trait_ref.self_ty();
1048                 if predicate.references_error() {
1049                     return;
1050                 }
1051                 // Typically, this ambiguity should only happen if
1052                 // there are unresolved type inference variables
1053                 // (otherwise it would suggest a coherence
1054                 // failure). But given #21974 that is not necessarily
1055                 // the case -- we can have multiple where clauses that
1056                 // are only distinguished by a region, which results
1057                 // in an ambiguity even when all types are fully
1058                 // known, since we don't dispatch based on region
1059                 // relationships.
1060
1061                 // This is kind of a hack: it frequently happens that some earlier
1062                 // error prevents types from being fully inferred, and then we get
1063                 // a bunch of uninteresting errors saying something like "<generic
1064                 // #0> doesn't implement Sized".  It may even be true that we
1065                 // could just skip over all checks where the self-ty is an
1066                 // inference variable, but I was afraid that there might be an
1067                 // inference variable created, registered as an obligation, and
1068                 // then never forced by writeback, and hence by skipping here we'd
1069                 // be ignoring the fact that we don't KNOW the type works
1070                 // out. Though even that would probably be harmless, given that
1071                 // we're only talking about builtin traits, which are known to be
1072                 // inhabited. But in any case I just threw in this check for
1073                 // has_errors() to be sure that compilation isn't happening
1074                 // anyway. In that case, why inundate the user.
1075                 if !self.tcx.sess.has_errors() {
1076                     if
1077                         self.tcx.lang_items().sized_trait()
1078                         .map_or(false, |sized_id| sized_id == trait_ref.def_id())
1079                     {
1080                         self.need_type_info(body_id, span, self_ty);
1081                     } else {
1082                         let mut err = struct_span_err!(self.tcx.sess,
1083                                                         span, E0283,
1084                                                         "type annotations required: \
1085                                                         cannot resolve `{}`",
1086                                                         predicate);
1087                         self.note_obligation_cause(&mut err, obligation);
1088                         err.emit();
1089                     }
1090                 }
1091             }
1092
1093             ty::Predicate::WellFormed(ty) => {
1094                 // Same hacky approach as above to avoid deluging user
1095                 // with error messages.
1096                 if !ty.references_error() && !self.tcx.sess.has_errors() {
1097                     self.need_type_info(body_id, span, ty);
1098                 }
1099             }
1100
1101             ty::Predicate::Subtype(ref data) => {
1102                 if data.references_error() || self.tcx.sess.has_errors() {
1103                     // no need to overload user in such cases
1104                 } else {
1105                     let &SubtypePredicate { a_is_expected: _, a, b } = data.skip_binder();
1106                     // both must be type variables, or the other would've been instantiated
1107                     assert!(a.is_ty_var() && b.is_ty_var());
1108                     self.need_type_info(body_id,
1109                                         obligation.cause.span,
1110                                         a);
1111                 }
1112             }
1113
1114             _ => {
1115                 if !self.tcx.sess.has_errors() {
1116                     let mut err = struct_span_err!(self.tcx.sess,
1117                                                    obligation.cause.span, E0284,
1118                                                    "type annotations required: \
1119                                                     cannot resolve `{}`",
1120                                                    predicate);
1121                     self.note_obligation_cause(&mut err, obligation);
1122                     err.emit();
1123                 }
1124             }
1125         }
1126     }
1127
1128     /// Returns whether the trait predicate may apply for *some* assignment
1129     /// to the type parameters.
1130     fn predicate_can_apply(&self,
1131                            param_env: ty::ParamEnv<'tcx>,
1132                            pred: ty::PolyTraitRef<'tcx>)
1133                            -> bool {
1134         struct ParamToVarFolder<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
1135             infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
1136             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>
1137         }
1138
1139         impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for ParamToVarFolder<'a, 'gcx, 'tcx> {
1140             fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> { self.infcx.tcx }
1141
1142             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
1143                 if let ty::TyParam(ty::ParamTy {name, ..}) = ty.sty {
1144                     let infcx = self.infcx;
1145                     self.var_map.entry(ty).or_insert_with(||
1146                         infcx.next_ty_var(
1147                             TypeVariableOrigin::TypeParameterDefinition(DUMMY_SP, name)))
1148                 } else {
1149                     ty.super_fold_with(self)
1150                 }
1151             }
1152         }
1153
1154         self.probe(|_| {
1155             let mut selcx = SelectionContext::new(self);
1156
1157             let cleaned_pred = pred.fold_with(&mut ParamToVarFolder {
1158                 infcx: self,
1159                 var_map: FxHashMap()
1160             });
1161
1162             let cleaned_pred = super::project::normalize(
1163                 &mut selcx,
1164                 param_env,
1165                 ObligationCause::dummy(),
1166                 &cleaned_pred
1167             ).value;
1168
1169             let obligation = Obligation::new(
1170                 ObligationCause::dummy(),
1171                 param_env,
1172                 cleaned_pred.to_predicate()
1173             );
1174
1175             selcx.evaluate_obligation(&obligation)
1176         })
1177     }
1178
1179     fn note_obligation_cause<T>(&self,
1180                                 err: &mut DiagnosticBuilder,
1181                                 obligation: &Obligation<'tcx, T>)
1182         where T: fmt::Display
1183     {
1184         self.note_obligation_cause_code(err,
1185                                         &obligation.predicate,
1186                                         &obligation.cause.code);
1187     }
1188
1189     fn note_obligation_cause_code<T>(&self,
1190                                      err: &mut DiagnosticBuilder,
1191                                      predicate: &T,
1192                                      cause_code: &ObligationCauseCode<'tcx>)
1193         where T: fmt::Display
1194     {
1195         let tcx = self.tcx;
1196         match *cause_code {
1197             ObligationCauseCode::ExprAssignable |
1198             ObligationCauseCode::MatchExpressionArm { .. } |
1199             ObligationCauseCode::IfExpression |
1200             ObligationCauseCode::IfExpressionWithNoElse |
1201             ObligationCauseCode::EquatePredicate |
1202             ObligationCauseCode::MainFunctionType |
1203             ObligationCauseCode::StartFunctionType |
1204             ObligationCauseCode::IntrinsicType |
1205             ObligationCauseCode::MethodReceiver |
1206             ObligationCauseCode::ReturnNoExpression |
1207             ObligationCauseCode::MiscObligation => {
1208             }
1209             ObligationCauseCode::SliceOrArrayElem => {
1210                 err.note("slice and array elements must have `Sized` type");
1211             }
1212             ObligationCauseCode::TupleElem => {
1213                 err.note("only the last element of a tuple may have a dynamically sized type");
1214             }
1215             ObligationCauseCode::ProjectionWf(data) => {
1216                 err.note(&format!("required so that the projection `{}` is well-formed",
1217                                   data));
1218             }
1219             ObligationCauseCode::ReferenceOutlivesReferent(ref_ty) => {
1220                 err.note(&format!("required so that reference `{}` does not outlive its referent",
1221                                   ref_ty));
1222             }
1223             ObligationCauseCode::ObjectTypeBound(object_ty, region) => {
1224                 err.note(&format!("required so that the lifetime bound of `{}` for `{}` \
1225                                    is satisfied",
1226                                   region, object_ty));
1227             }
1228             ObligationCauseCode::ItemObligation(item_def_id) => {
1229                 let item_name = tcx.item_path_str(item_def_id);
1230                 err.note(&format!("required by `{}`", item_name));
1231             }
1232             ObligationCauseCode::ObjectCastObligation(object_ty) => {
1233                 err.note(&format!("required for the cast to the object type `{}`",
1234                                   self.ty_to_string(object_ty)));
1235             }
1236             ObligationCauseCode::RepeatVec => {
1237                 err.note("the `Copy` trait is required because the \
1238                           repeated element will be copied");
1239             }
1240             ObligationCauseCode::VariableType(_) => {
1241                 err.note("all local variables must have a statically known size");
1242             }
1243             ObligationCauseCode::SizedReturnType => {
1244                 err.note("the return type of a function must have a \
1245                           statically known size");
1246             }
1247             ObligationCauseCode::AssignmentLhsSized => {
1248                 err.note("the left-hand-side of an assignment must have a statically known size");
1249             }
1250             ObligationCauseCode::TupleInitializerSized => {
1251                 err.note("tuples must have a statically known size to be initialized");
1252             }
1253             ObligationCauseCode::StructInitializerSized => {
1254                 err.note("structs must have a statically known size to be initialized");
1255             }
1256             ObligationCauseCode::FieldSized(ref item) => {
1257                 match *item {
1258                     AdtKind::Struct => {
1259                         err.note("only the last field of a struct may have a dynamically \
1260                                   sized type");
1261                     }
1262                     AdtKind::Union => {
1263                         err.note("no field of a union may have a dynamically sized type");
1264                     }
1265                     AdtKind::Enum => {
1266                         err.note("no field of an enum variant may have a dynamically sized type");
1267                     }
1268                 }
1269             }
1270             ObligationCauseCode::ConstSized => {
1271                 err.note("constant expressions must have a statically known size");
1272             }
1273             ObligationCauseCode::SharedStatic => {
1274                 err.note("shared static variables must have a type that implements `Sync`");
1275             }
1276             ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
1277                 let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1278                 err.note(&format!("required because it appears within the type `{}`",
1279                                   parent_trait_ref.0.self_ty()));
1280                 let parent_predicate = parent_trait_ref.to_predicate();
1281                 self.note_obligation_cause_code(err,
1282                                                 &parent_predicate,
1283                                                 &data.parent_code);
1284             }
1285             ObligationCauseCode::ImplDerivedObligation(ref data) => {
1286                 let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1287                 err.note(
1288                     &format!("required because of the requirements on the impl of `{}` for `{}`",
1289                              parent_trait_ref,
1290                              parent_trait_ref.0.self_ty()));
1291                 let parent_predicate = parent_trait_ref.to_predicate();
1292                 self.note_obligation_cause_code(err,
1293                                                 &parent_predicate,
1294                                                 &data.parent_code);
1295             }
1296             ObligationCauseCode::CompareImplMethodObligation { .. } => {
1297                 err.note(
1298                     &format!("the requirement `{}` appears on the impl method \
1299                               but not on the corresponding trait method",
1300                              predicate));
1301             }
1302             ObligationCauseCode::ReturnType(_) |
1303             ObligationCauseCode::BlockTailExpression(_) => (),
1304         }
1305     }
1306
1307     fn suggest_new_overflow_limit(&self, err: &mut DiagnosticBuilder) {
1308         let current_limit = self.tcx.sess.recursion_limit.get();
1309         let suggested_limit = current_limit * 2;
1310         err.help(&format!("consider adding a `#![recursion_limit=\"{}\"]` attribute to your crate",
1311                           suggested_limit));
1312     }
1313 }