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::TyForeign(..) => Some(19),
 259                 ty::TyInfer(..) | ty::TyError => None
 260             }
 261         }
 262
 263         match (type_category(a), type_category(b)) {
 264             (Some(cat_a), Some(cat_b)) => match (&a.sty, &b.sty) {
 265                 (&ty::TyAdt(def_a, _), &ty::TyAdt(def_b, _)) => def_a == def_b,
 266                 _ => cat_a == cat_b
 267             },
 268             // infer and error can be equated to all types
 269             _ => true
 270         }
 271     }
 272
 273     fn impl_similar_to(&self,
 274                        trait_ref: ty::PolyTraitRef<'tcx>,
 275                        obligation: &PredicateObligation<'tcx>)
 276                        -> Option<DefId>
 277     {
 278         let tcx = self.tcx;
 279         let param_env = obligation.param_env;
 280         let trait_ref = tcx.erase_late_bound_regions(&trait_ref);
 281         let trait_self_ty = trait_ref.self_ty();
 282
 283         let mut self_match_impls = vec![];
 284         let mut fuzzy_match_impls = vec![];
 285
 286         self.tcx.for_each_relevant_impl(
 287             trait_ref.def_id, trait_self_ty, |def_id| {
 288                 let impl_substs = self.fresh_substs_for_item(obligation.cause.span, def_id);
 289                 let impl_trait_ref = tcx
 290                     .impl_trait_ref(def_id)
 291                     .unwrap()
 292                     .subst(tcx, impl_substs);
 293
 294                 let impl_self_ty = impl_trait_ref.self_ty();
 295
 296                 if let Ok(..) = self.can_eq(param_env, trait_self_ty, impl_self_ty) {
 297                     self_match_impls.push(def_id);
 298
 299                     if trait_ref.substs.types().skip(1)
 300                         .zip(impl_trait_ref.substs.types().skip(1))
 301                         .all(|(u,v)| self.fuzzy_match_tys(u, v))
 302                     {
 303                         fuzzy_match_impls.push(def_id);
 304                     }
 305                 }
 306             });
 307
 308         let impl_def_id = if self_match_impls.len() == 1 {
 309             self_match_impls[0]
 310         } else if fuzzy_match_impls.len() == 1 {
 311             fuzzy_match_impls[0]
 312         } else {
 313             return None
 314         };
 315
 316         if tcx.has_attr(impl_def_id, "rustc_on_unimplemented") {
 317             Some(impl_def_id)
 318         } else {
 319             None
 320         }
 321     }
 322
 323     fn on_unimplemented_note(
 324         &self,
 325         trait_ref: ty::PolyTraitRef<'tcx>,
 326         obligation: &PredicateObligation<'tcx>) ->
 327         OnUnimplementedNote
 328     {
 329         let def_id = self.impl_similar_to(trait_ref, obligation)
 330             .unwrap_or(trait_ref.def_id());
 331         let trait_ref = *trait_ref.skip_binder();
 332
 333         let desugaring;
 334         let method;
 335         let mut flags = vec![];
 336         let direct = match obligation.cause.code {
 337             ObligationCauseCode::BuiltinDerivedObligation(..) |
 338             ObligationCauseCode::ImplDerivedObligation(..) => false,
 339             _ => true
 340         };
 341         if direct {
 342             // this is a "direct", user-specified, rather than derived,
 343             // obligation.
 344             flags.push(("direct", None));
 345         }
 346
 347         if let ObligationCauseCode::ItemObligation(item) = obligation.cause.code {
 348             // FIXME: maybe also have some way of handling methods
 349             // from other traits? That would require name resolution,
 350             // which we might want to be some sort of hygienic.
 351             //
 352             // Currently I'm leaving it for what I need for `try`.
 353             if self.tcx.trait_of_item(item) == Some(trait_ref.def_id) {
 354                 method = self.tcx.item_name(item);
 355                 flags.push(("from_method", None));
 356                 flags.push(("from_method", Some(&*method)));
 357             }
 358         }
 359
 360         if let Some(k) = obligation.cause.span.compiler_desugaring_kind() {
 361             desugaring = k.as_symbol().as_str();
 362             flags.push(("from_desugaring", None));
 363             flags.push(("from_desugaring", Some(&*desugaring)));
 364         }
 365
 366         if let Ok(Some(command)) = OnUnimplementedDirective::of_item(
 367             self.tcx, trait_ref.def_id, def_id
 368         ) {
 369             command.evaluate(self.tcx, trait_ref, &flags)
 370         } else {
 371             OnUnimplementedNote::empty()
 372         }
 373     }
 374
 375     fn find_similar_impl_candidates(&self,
 376                                     trait_ref: ty::PolyTraitRef<'tcx>)
 377                                     -> Vec<ty::TraitRef<'tcx>>
 378     {
 379         let simp = fast_reject::simplify_type(self.tcx,
 380                                               trait_ref.skip_binder().self_ty(),
 381                                               true);
 382         let mut impl_candidates = Vec::new();
 383
 384         match simp {
 385             Some(simp) => self.tcx.for_each_impl(trait_ref.def_id(), |def_id| {
 386                 let imp = self.tcx.impl_trait_ref(def_id).unwrap();
 387                 let imp_simp = fast_reject::simplify_type(self.tcx,
 388                                                           imp.self_ty(),
 389                                                           true);
 390                 if let Some(imp_simp) = imp_simp {
 391                     if simp != imp_simp {
 392                         return;
 393                     }
 394                 }
 395                 impl_candidates.push(imp);
 396             }),
 397             None => self.tcx.for_each_impl(trait_ref.def_id(), |def_id| {
 398                 impl_candidates.push(
 399                     self.tcx.impl_trait_ref(def_id).unwrap());
 400             })
 401         };
 402         impl_candidates
 403     }
 404
 405     fn report_similar_impl_candidates(&self,
 406                                       impl_candidates: Vec<ty::TraitRef<'tcx>>,
 407                                       err: &mut DiagnosticBuilder)
 408     {
 409         if impl_candidates.is_empty() {
 410             return;
 411         }
 412
 413         let end = if impl_candidates.len() <= 5 {
 414             impl_candidates.len()
 415         } else {
 416             4
 417         };
 418         err.help(&format!("the following implementations were found:{}{}",
 419                           &impl_candidates[0..end].iter().map(|candidate| {
 420                               format!("\n  {:?}", candidate)
 421                           }).collect::<String>(),
 422                           if impl_candidates.len() > 5 {
 423                               format!("\nand {} others", impl_candidates.len() - 4)
 424                           } else {
 425                               "".to_owned()
 426                           }
 427                           ));
 428     }
 429
 430     /// Reports that an overflow has occurred and halts compilation. We
 431     /// halt compilation unconditionally because it is important that
 432     /// overflows never be masked -- they basically represent computations
 433     /// whose result could not be truly determined and thus we can't say
 434     /// if the program type checks or not -- and they are unusual
 435     /// occurrences in any case.
 436     pub fn report_overflow_error<T>(&self,
 437                                     obligation: &Obligation<'tcx, T>,
 438                                     suggest_increasing_limit: bool) -> !
 439         where T: fmt::Display + TypeFoldable<'tcx>
 440     {
 441         let predicate =
 442             self.resolve_type_vars_if_possible(&obligation.predicate);
 443         let mut err = struct_span_err!(self.tcx.sess, obligation.cause.span, E0275,
 444                                        "overflow evaluating the requirement `{}`",
 445                                        predicate);
 446
 447         if suggest_increasing_limit {
 448             self.suggest_new_overflow_limit(&mut err);
 449         }
 450
 451         self.note_obligation_cause(&mut err, obligation);
 452
 453         err.emit();
 454         self.tcx.sess.abort_if_errors();
 455         bug!();
 456     }
 457
 458     /// Reports that a cycle was detected which led to overflow and halts
 459     /// compilation. This is equivalent to `report_overflow_error` except
 460     /// that we can give a more helpful error message (and, in particular,
 461     /// we do not suggest increasing the overflow limit, which is not
 462     /// going to help).
 463     pub fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 464         let cycle = self.resolve_type_vars_if_possible(&cycle.to_owned());
 465         assert!(cycle.len() > 0);
 466
 467         debug!("report_overflow_error_cycle: cycle={:?}", cycle);
 468
 469         self.report_overflow_error(&cycle[0], false);
 470     }
 471
 472     pub fn report_extra_impl_obligation(&self,
 473                                         error_span: Span,
 474                                         item_name: ast::Name,
 475                                         _impl_item_def_id: DefId,
 476                                         trait_item_def_id: DefId,
 477                                         requirement: &fmt::Display,
 478                                         lint_id: Option<ast::NodeId>) // (*)
 479                                         -> DiagnosticBuilder<'tcx>
 480     {
 481         // (*) This parameter is temporary and used only for phasing
 482         // in the bug fix to #18937. If it is `Some`, it has a kind of
 483         // weird effect -- the diagnostic is reported as a lint, and
 484         // the builder which is returned is marked as canceled.
 485
 486         let msg = "impl has stricter requirements than trait";
 487         let mut err = match lint_id {
 488             Some(node_id) => {
 489                 self.tcx.struct_span_lint_node(EXTRA_REQUIREMENT_IN_IMPL,
 490                                                node_id,
 491                                                error_span,
 492                                                msg)
 493             }
 494             None => {
 495                 struct_span_err!(self.tcx.sess,
 496                                  error_span,
 497                                  E0276,
 498                                  "{}", msg)
 499             }
 500         };
 501
 502         if let Some(trait_item_span) = self.tcx.hir.span_if_local(trait_item_def_id) {
 503             let span = self.tcx.sess.codemap().def_span(trait_item_span);
 504             err.span_label(span, format!("definition of `{}` from trait", item_name));
 505         }
 506
 507         err.span_label(
 508             error_span,
 509             format!("impl has extra requirement {}", requirement));
 510
 511         err
 512     }
 513
 514
 515     /// Get the parent trait chain start
 516     fn get_parent_trait_ref(&self, code: &ObligationCauseCode<'tcx>) -> Option<String> {
 517         match code {
 518             &ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
 519                 let parent_trait_ref = self.resolve_type_vars_if_possible(
 520                     &data.parent_trait_ref);
 521                 match self.get_parent_trait_ref(&data.parent_code) {
 522                     Some(t) => Some(t),
 523                     None => Some(format!("{}", parent_trait_ref.0.self_ty())),
 524                 }
 525             }
 526             _ => None,
 527         }
 528     }
 529
 530     pub fn report_selection_error(&self,
 531                                   obligation: &PredicateObligation<'tcx>,
 532                                   error: &SelectionError<'tcx>)
 533     {
 534         let span = obligation.cause.span;
 535
 536         let mut err = match *error {
 537             SelectionError::Unimplemented => {
 538                 if let ObligationCauseCode::CompareImplMethodObligation {
 539                     item_name, impl_item_def_id, trait_item_def_id, lint_id
 540                 } = obligation.cause.code {
 541                     self.report_extra_impl_obligation(
 542                         span,
 543                         item_name,
 544                         impl_item_def_id,
 545                         trait_item_def_id,
 546                         &format!("`{}`", obligation.predicate),
 547                         lint_id)
 548                         .emit();
 549                     return;
 550                 }
 551                 match obligation.predicate {
 552                     ty::Predicate::Trait(ref trait_predicate) => {
 553                         let trait_predicate =
 554                             self.resolve_type_vars_if_possible(trait_predicate);
 555
 556                         if self.tcx.sess.has_errors() && trait_predicate.references_error() {
 557                             return;
 558                         }
 559                         let trait_ref = trait_predicate.to_poly_trait_ref();
 560                         let (post_message, pre_message) =
 561                             self.get_parent_trait_ref(&obligation.cause.code)
 562                                 .map(|t| (format!(" in `{}`", t), format!("within `{}`, ", t)))
 563                             .unwrap_or((String::new(), String::new()));
 564
 565                         let OnUnimplementedNote { message, label }
 566                             = self.on_unimplemented_note(trait_ref, obligation);
 567                         let have_alt_message = message.is_some() || label.is_some();
 568
 569                         let mut err = struct_span_err!(
 570                             self.tcx.sess,
 571                             span,
 572                             E0277,
 573                             "{}",
 574                             message.unwrap_or_else(|| {
 575                                 format!("the trait bound `{}` is not satisfied{}",
 576                                          trait_ref.to_predicate(), post_message)
 577                             }));
 578
 579                         if let Some(ref s) = label {
 580                             // If it has a custom "#[rustc_on_unimplemented]"
 581                             // error message, let's display it as the label!
 582                             err.span_label(span, s.as_str());
 583                             err.help(&format!("{}the trait `{}` is not implemented for `{}`",
 584                                               pre_message,
 585                                               trait_ref,
 586                                               trait_ref.self_ty()));
 587                         } else {
 588                             err.span_label(span,
 589                                            &*format!("{}the trait `{}` is not implemented for `{}`",
 590                                                      pre_message,
 591                                                      trait_ref,
 592                                                      trait_ref.self_ty()));
 593                         }
 594
 595                         // Try to report a help message
 596                         if !trait_ref.has_infer_types() &&
 597                             self.predicate_can_apply(obligation.param_env, trait_ref) {
 598                             // If a where-clause may be useful, remind the
 599                             // user that they can add it.
 600                             //
 601                             // don't display an on-unimplemented note, as
 602                             // these notes will often be of the form
 603                             //     "the type `T` can't be frobnicated"
 604                             // which is somewhat confusing.
 605                             err.help(&format!("consider adding a `where {}` bound",
 606                                                 trait_ref.to_predicate()));
 607                         } else if !have_alt_message {
 608                             // Can't show anything else useful, try to find similar impls.
 609                             let impl_candidates = self.find_similar_impl_candidates(trait_ref);
 610                             self.report_similar_impl_candidates(impl_candidates, &mut err);
 611                         }
 612
 613                         err
 614                     }
 615
 616                     ty::Predicate::Subtype(ref predicate) => {
 617                         // Errors for Subtype predicates show up as
 618                         // `FulfillmentErrorCode::CodeSubtypeError`,
 619                         // not selection error.
 620                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
 621                     }
 622
 623                     ty::Predicate::Equate(ref predicate) => {
 624                         let predicate = self.resolve_type_vars_if_possible(predicate);
 625                         let err = self.equality_predicate(&obligation.cause,
 626                                                           obligation.param_env,
 627                                                           &predicate).err().unwrap();
 628                         struct_span_err!(self.tcx.sess, span, E0278,
 629                             "the requirement `{}` is not satisfied (`{}`)",
 630                             predicate, err)
 631                     }
 632
 633                     ty::Predicate::RegionOutlives(ref predicate) => {
 634                         let predicate = self.resolve_type_vars_if_possible(predicate);
 635                         let err = self.region_outlives_predicate(&obligation.cause,
 636                                                                     &predicate).err().unwrap();
 637                         struct_span_err!(self.tcx.sess, span, E0279,
 638                             "the requirement `{}` is not satisfied (`{}`)",
 639                             predicate, err)
 640                     }
 641
 642                     ty::Predicate::Projection(..) | ty::Predicate::TypeOutlives(..) => {
 643                         let predicate =
 644                             self.resolve_type_vars_if_possible(&obligation.predicate);
 645                         struct_span_err!(self.tcx.sess, span, E0280,
 646                             "the requirement `{}` is not satisfied",
 647                             predicate)
 648                     }
 649
 650                     ty::Predicate::ObjectSafe(trait_def_id) => {
 651                         let violations = self.tcx.object_safety_violations(trait_def_id);
 652                         self.tcx.report_object_safety_error(span,
 653                                                             trait_def_id,
 654                                                             violations)
 655                     }
 656
 657                     ty::Predicate::ClosureKind(closure_def_id, kind) => {
 658                         let found_kind = self.closure_kind(closure_def_id).unwrap();
 659                         let closure_span = self.tcx.hir.span_if_local(closure_def_id).unwrap();
 660                         let node_id = self.tcx.hir.as_local_node_id(closure_def_id).unwrap();
 661                         let mut err = struct_span_err!(
 662                             self.tcx.sess, closure_span, E0525,
 663                             "expected a closure that implements the `{}` trait, \
 664                                 but this closure only implements `{}`",
 665                             kind,
 666                             found_kind);
 667
 668                         err.span_label(
 669                             obligation.cause.span,
 670                             format!("the requirement to implement `{}` derives from here", kind));
 671
 672                         // Additional context information explaining why the closure only implements
 673                         // a particular trait.
 674                         if let Some(tables) = self.in_progress_tables {
 675                             let tables = tables.borrow();
 676                             let closure_hir_id = self.tcx.hir.node_to_hir_id(node_id);
 677                             match tables.closure_kinds().get(closure_hir_id) {
 678                                 Some(&(ty::ClosureKind::FnOnce, Some((span, name)))) => {
 679                                     err.span_note(span, &format!(
 680                                         "closure is `FnOnce` because it moves the \
 681                                          variable `{}` out of its environment", name));
 682                                 },
 683                                 Some(&(ty::ClosureKind::FnMut, Some((span, name)))) => {
 684                                     err.span_note(span, &format!(
 685                                         "closure is `FnMut` because it mutates the \
 686                                          variable `{}` here", name));
 687                                 },
 688                                 _ => {}
 689                             }
 690                         }
 691
 692                         err.emit();
 693                         return;
 694                     }
 695
 696                     ty::Predicate::WellFormed(ty) => {
 697                         // WF predicates cannot themselves make
 698                         // errors. They can only block due to
 699                         // ambiguity; otherwise, they always
 700                         // degenerate into other obligations
 701                         // (which may fail).
 702                         span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 703                     }
 704
 705                     ty::Predicate::ConstEvaluatable(..) => {
 706                         // Errors for `ConstEvaluatable` predicates show up as
 707                         // `SelectionError::ConstEvalFailure`,
 708                         // not `Unimplemented`.
 709                         span_bug!(span,
 710                             "const-evaluatable requirement gave wrong error: `{:?}`", obligation)
 711                     }
 712                 }
 713             }
 714
 715             OutputTypeParameterMismatch(ref found_trait_ref, ref expected_trait_ref, _) => {
 716                 let found_trait_ref = self.resolve_type_vars_if_possible(&*found_trait_ref);
 717                 let expected_trait_ref = self.resolve_type_vars_if_possible(&*expected_trait_ref);
 718                 if expected_trait_ref.self_ty().references_error() {
 719                     return;
 720                 }
 721                 let found_trait_ty = found_trait_ref.self_ty();
 722
 723                 let found_did = found_trait_ty.ty_to_def_id();
 724                 let found_span = found_did.and_then(|did| {
 725                     self.tcx.hir.span_if_local(did)
 726                 });
 727
 728                 let found_ty_count =
 729                     match found_trait_ref.skip_binder().substs.type_at(1).sty {
 730                         ty::TyTuple(ref tys, _) => tys.len(),
 731                         _ => 1,
 732                     };
 733                 let (expected_tys, expected_ty_count) =
 734                     match expected_trait_ref.skip_binder().substs.type_at(1).sty {
 735                         ty::TyTuple(ref tys, _) =>
 736                             (tys.iter().map(|t| &t.sty).collect(), tys.len()),
 737                         ref sty => (vec![sty], 1),
 738                     };
 739                 if found_ty_count == expected_ty_count {
 740                     self.report_closure_arg_mismatch(span,
 741                                                      found_span,
 742                                                      found_trait_ref,
 743                                                      expected_trait_ref)
 744                 } else {
 745                     let expected_tuple = if expected_ty_count == 1 {
 746                         expected_tys.first().and_then(|t| {
 747                             if let &&ty::TyTuple(ref tuptys, _) = t {
 748                                 Some(tuptys.len())
 749                             } else {
 750                                 None
 751                             }
 752                         })
 753                     } else {
 754                         None
 755                     };
 756
 757                     // FIXME(#44150): Expand this to "N args expected but a N-tuple found."
 758                     // Type of the 1st expected argument is somehow provided as type of a
 759                     // found one in that case.
 760                     //
 761                     // ```
 762                     // [1i32, 2, 3].sort_by(|(a, b)| ..)
 763                     // //                   ^^^^^^^^
 764                     // // expected_trait_ref:  std::ops::FnMut<(&i32, &i32)>
 765                     // //    found_trait_ref:  std::ops::FnMut<(&i32,)>
 766                     // ```
 767
 768                     let (closure_span, closure_args) = found_did
 769                         .and_then(|did| self.tcx.hir.get_if_local(did))
 770                         .and_then(|node| {
 771                             if let hir::map::NodeExpr(
 772                                 &hir::Expr {
 773                                     node: hir::ExprClosure(_, ref decl, id, span, _),
 774                                     ..
 775                                 }) = node
 776                             {
 777                                 let ty_snips = decl.inputs.iter()
 778                                     .map(|ty| {
 779                                         self.tcx.sess.codemap().span_to_snippet(ty.span).ok()
 780                                             .and_then(|snip| {
 781                                                 // filter out dummy spans
 782                                                 if snip == "," || snip == "|" {
 783                                                     None
 784                                                 } else {
 785                                                     Some(snip)
 786                                                 }
 787                                             })
 788                                     })
 789                                     .collect::<Vec<Option<String>>>();
 790
 791                                 let body = self.tcx.hir.body(id);
 792                                 let pat_snips = body.arguments.iter()
 793                                     .map(|arg|
 794                                         self.tcx.sess.codemap().span_to_snippet(arg.pat.span).ok())
 795                                     .collect::<Option<Vec<String>>>();
 796
 797                                 Some((span, pat_snips, ty_snips))
 798                             } else {
 799                                 None
 800                             }
 801                         })
 802                         .map(|(span, pat, ty)| (Some(span), Some((pat, ty))))
 803                         .unwrap_or((None, None));
 804                     let closure_args = closure_args.and_then(|(pat, ty)| Some((pat?, ty)));
 805
 806                     self.report_arg_count_mismatch(
 807                         span,
 808                         closure_span.or(found_span),
 809                         expected_ty_count,
 810                         expected_tuple,
 811                         found_ty_count,
 812                         closure_args,
 813                         found_trait_ty.is_closure()
 814                     )
 815                 }
 816             }
 817
 818             TraitNotObjectSafe(did) => {
 819                 let violations = self.tcx.object_safety_violations(did);
 820                 self.tcx.report_object_safety_error(span, did,
 821                                                     violations)
 822             }
 823
 824             ConstEvalFailure(ref err) => {
 825                 if let const_val::ErrKind::TypeckError = err.kind {
 826                     return;
 827                 }
 828                 err.struct_error(self.tcx, span, "constant expression")
 829             }
 830         };
 831         self.note_obligation_cause(&mut err, obligation);
 832         err.emit();
 833     }
 834
 835     fn report_arg_count_mismatch(
 836         &self,
 837         span: Span,
 838         found_span: Option<Span>,
 839         expected: usize,
 840         expected_tuple: Option<usize>,
 841         found: usize,
 842         closure_args: Option<(Vec<String>, Vec<Option<String>>)>,
 843         is_closure: bool
 844     ) -> DiagnosticBuilder<'tcx> {
 845         use std::borrow::Cow;
 846
 847         let kind = if is_closure { "closure" } else { "function" };
 848
 849         let args_str = |n, distinct| format!(
 850                 "{} {}argument{}",
 851                 n,
 852                 if distinct && n >= 2 { "distinct " } else { "" },
 853                 if n == 1 { "" } else { "s" },
 854             );
 855
 856         let expected_str = if let Some(n) = expected_tuple {
 857             assert!(expected == 1);
 858             if closure_args.as_ref().map(|&(ref pats, _)| pats.len()) == Some(n) {
 859                 Cow::from("a single tuple as argument")
 860             } else {
 861                 // be verbose when numbers differ
 862                 Cow::from(format!("a single {}-tuple as argument", n))
 863             }
 864         } else {
 865             Cow::from(args_str(expected, false))
 866         };
 867
 868         let found_str = if expected_tuple.is_some() {
 869             args_str(found, true)
 870         } else {
 871             args_str(found, false)
 872         };
 873
 874
 875         let mut err = struct_span_err!(self.tcx.sess, span, E0593,
 876             "{} is expected to take {}, but it takes {}",
 877             kind,
 878             expected_str,
 879             found_str,
 880         );
 881
 882         err.span_label(
 883             span,
 884             format!(
 885                 "expected {} that takes {}",
 886                 kind,
 887                 expected_str,
 888             )
 889         );
 890
 891         if let Some(span) = found_span {
 892             if let (Some(expected_tuple), Some((pats, tys))) = (expected_tuple, closure_args) {
 893                 if expected_tuple != found || pats.len() != found {
 894                     err.span_label(span, format!("takes {}", found_str));
 895                 } else {
 896                     let sugg = format!(
 897                         "|({}){}|",
 898                         pats.join(", "),
 899
 900                         // add type annotations if available
 901                         if tys.iter().any(|ty| ty.is_some()) {
 902                             Cow::from(format!(
 903                                 ": ({})",
 904                                 tys.into_iter().map(|ty| if let Some(ty) = ty {
 905                                     ty
 906                                 } else {
 907                                     "_".to_string()
 908                                 }).collect::<Vec<String>>().join(", ")
 909                             ))
 910                         } else {
 911                             Cow::from("")
 912                         },
 913                     );
 914
 915                     err.span_suggestion(
 916                         span,
 917                         "consider changing the closure to accept a tuple",
 918                         sugg
 919                     );
 920                 }
 921             } else {
 922                 err.span_label(span, format!("takes {}", found_str));
 923             }
 924         }
 925
 926         err
 927     }
 928
 929     fn report_closure_arg_mismatch(&self,
 930                            span: Span,
 931                            found_span: Option<Span>,
 932                            expected_ref: ty::PolyTraitRef<'tcx>,
 933                            found: ty::PolyTraitRef<'tcx>)
 934         -> DiagnosticBuilder<'tcx>
 935     {
 936         fn build_fn_sig_string<'a, 'gcx, 'tcx>(tcx: ty::TyCtxt<'a, 'gcx, 'tcx>,
 937                                                trait_ref: &ty::TraitRef<'tcx>) -> String {
 938             let inputs = trait_ref.substs.type_at(1);
 939             let sig = if let ty::TyTuple(inputs, _) = inputs.sty {
 940                 tcx.mk_fn_sig(
 941                     inputs.iter().map(|&x| x),
 942                     tcx.mk_infer(ty::TyVar(ty::TyVid { index: 0 })),
 943                     false,
 944                     hir::Unsafety::Normal,
 945                     ::syntax::abi::Abi::Rust
 946                 )
 947             } else {
 948                 tcx.mk_fn_sig(
 949                     ::std::iter::once(inputs),
 950                     tcx.mk_infer(ty::TyVar(ty::TyVid { index: 0 })),
 951                     false,
 952                     hir::Unsafety::Normal,
 953                     ::syntax::abi::Abi::Rust
 954                 )
 955             };
 956             format!("{}", ty::Binder(sig))
 957         }
 958
 959         let argument_is_closure = expected_ref.skip_binder().substs.type_at(0).is_closure();
 960         let mut err = struct_span_err!(self.tcx.sess, span, E0631,
 961                                        "type mismatch in {} arguments",
 962                                        if argument_is_closure { "closure" } else { "function" });
 963
 964         let found_str = format!(
 965             "expected signature of `{}`",
 966             build_fn_sig_string(self.tcx, found.skip_binder())
 967         );
 968         err.span_label(span, found_str);
 969
 970         let found_span = found_span.unwrap_or(span);
 971         let expected_str = format!(
 972             "found signature of `{}`",
 973             build_fn_sig_string(self.tcx, expected_ref.skip_binder())
 974         );
 975         err.span_label(found_span, expected_str);
 976
 977         err
 978     }
 979 }
 980
 981 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
 982     pub fn recursive_type_with_infinite_size_error(self,
 983                                                    type_def_id: DefId)
 984                                                    -> DiagnosticBuilder<'tcx>
 985     {
 986         assert!(type_def_id.is_local());
 987         let span = self.hir.span_if_local(type_def_id).unwrap();
 988         let span = self.sess.codemap().def_span(span);
 989         let mut err = struct_span_err!(self.sess, span, E0072,
 990                                        "recursive type `{}` has infinite size",
 991                                        self.item_path_str(type_def_id));
 992         err.span_label(span, "recursive type has infinite size");
 993         err.help(&format!("insert indirection (e.g., a `Box`, `Rc`, or `&`) \
 994                            at some point to make `{}` representable",
 995                           self.item_path_str(type_def_id)));
 996         err
 997     }
 998
 999     pub fn report_object_safety_error(self,
1000                                       span: Span,
1001                                       trait_def_id: DefId,
1002                                       violations: Vec<ObjectSafetyViolation>)
1003                                       -> DiagnosticBuilder<'tcx>
1004     {
1005         let trait_str = self.item_path_str(trait_def_id);
1006         let span = self.sess.codemap().def_span(span);
1007         let mut err = struct_span_err!(
1008             self.sess, span, E0038,
1009             "the trait `{}` cannot be made into an object",
1010             trait_str);
1011         err.span_label(span, format!("the trait `{}` cannot be made into an object", trait_str));
1012
1013         let mut reported_violations = FxHashSet();
1014         for violation in violations {
1015             if !reported_violations.insert(violation.clone()) {
1016                 continue;
1017             }
1018             err.note(&violation.error_msg());
1019         }
1020         err
1021     }
1022 }
1023
1024 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
1025     fn maybe_report_ambiguity(&self, obligation: &PredicateObligation<'tcx>,
1026                               body_id: Option<hir::BodyId>) {
1027         // Unable to successfully determine, probably means
1028         // insufficient type information, but could mean
1029         // ambiguous impls. The latter *ought* to be a
1030         // coherence violation, so we don't report it here.
1031
1032         let predicate = self.resolve_type_vars_if_possible(&obligation.predicate);
1033         let span = obligation.cause.span;
1034
1035         debug!("maybe_report_ambiguity(predicate={:?}, obligation={:?})",
1036                predicate,
1037                obligation);
1038
1039         // Ambiguity errors are often caused as fallout from earlier
1040         // errors. So just ignore them if this infcx is tainted.
1041         if self.is_tainted_by_errors() {
1042             return;
1043         }
1044
1045         match predicate {
1046             ty::Predicate::Trait(ref data) => {
1047                 let trait_ref = data.to_poly_trait_ref();
1048                 let self_ty = trait_ref.self_ty();
1049                 if predicate.references_error() {
1050                     return;
1051                 }
1052                 // Typically, this ambiguity should only happen if
1053                 // there are unresolved type inference variables
1054                 // (otherwise it would suggest a coherence
1055                 // failure). But given #21974 that is not necessarily
1056                 // the case -- we can have multiple where clauses that
1057                 // are only distinguished by a region, which results
1058                 // in an ambiguity even when all types are fully
1059                 // known, since we don't dispatch based on region
1060                 // relationships.
1061
1062                 // This is kind of a hack: it frequently happens that some earlier
1063                 // error prevents types from being fully inferred, and then we get
1064                 // a bunch of uninteresting errors saying something like "<generic
1065                 // #0> doesn't implement Sized".  It may even be true that we
1066                 // could just skip over all checks where the self-ty is an
1067                 // inference variable, but I was afraid that there might be an
1068                 // inference variable created, registered as an obligation, and
1069                 // then never forced by writeback, and hence by skipping here we'd
1070                 // be ignoring the fact that we don't KNOW the type works
1071                 // out. Though even that would probably be harmless, given that
1072                 // we're only talking about builtin traits, which are known to be
1073                 // inhabited. But in any case I just threw in this check for
1074                 // has_errors() to be sure that compilation isn't happening
1075                 // anyway. In that case, why inundate the user.
1076                 if !self.tcx.sess.has_errors() {
1077                     if
1078                         self.tcx.lang_items().sized_trait()
1079                         .map_or(false, |sized_id| sized_id == trait_ref.def_id())
1080                     {
1081                         self.need_type_info(body_id, span, self_ty);
1082                     } else {
1083                         let mut err = struct_span_err!(self.tcx.sess,
1084                                                         span, E0283,
1085                                                         "type annotations required: \
1086                                                         cannot resolve `{}`",
1087                                                         predicate);
1088                         self.note_obligation_cause(&mut err, obligation);
1089                         err.emit();
1090                     }
1091                 }
1092             }
1093
1094             ty::Predicate::WellFormed(ty) => {
1095                 // Same hacky approach as above to avoid deluging user
1096                 // with error messages.
1097                 if !ty.references_error() && !self.tcx.sess.has_errors() {
1098                     self.need_type_info(body_id, span, ty);
1099                 }
1100             }
1101
1102             ty::Predicate::Subtype(ref data) => {
1103                 if data.references_error() || self.tcx.sess.has_errors() {
1104                     // no need to overload user in such cases
1105                 } else {
1106                     let &SubtypePredicate { a_is_expected: _, a, b } = data.skip_binder();
1107                     // both must be type variables, or the other would've been instantiated
1108                     assert!(a.is_ty_var() && b.is_ty_var());
1109                     self.need_type_info(body_id,
1110                                         obligation.cause.span,
1111                                         a);
1112                 }
1113             }
1114
1115             _ => {
1116                 if !self.tcx.sess.has_errors() {
1117                     let mut err = struct_span_err!(self.tcx.sess,
1118                                                    obligation.cause.span, E0284,
1119                                                    "type annotations required: \
1120                                                     cannot resolve `{}`",
1121                                                    predicate);
1122                     self.note_obligation_cause(&mut err, obligation);
1123                     err.emit();
1124                 }
1125             }
1126         }
1127     }
1128
1129     /// Returns whether the trait predicate may apply for *some* assignment
1130     /// to the type parameters.
1131     fn predicate_can_apply(&self,
1132                            param_env: ty::ParamEnv<'tcx>,
1133                            pred: ty::PolyTraitRef<'tcx>)
1134                            -> bool {
1135         struct ParamToVarFolder<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
1136             infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
1137             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>
1138         }
1139
1140         impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for ParamToVarFolder<'a, 'gcx, 'tcx> {
1141             fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> { self.infcx.tcx }
1142
1143             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
1144                 if let ty::TyParam(ty::ParamTy {name, ..}) = ty.sty {
1145                     let infcx = self.infcx;
1146                     self.var_map.entry(ty).or_insert_with(||
1147                         infcx.next_ty_var(
1148                             TypeVariableOrigin::TypeParameterDefinition(DUMMY_SP, name)))
1149                 } else {
1150                     ty.super_fold_with(self)
1151                 }
1152             }
1153         }
1154
1155         self.probe(|_| {
1156             let mut selcx = SelectionContext::new(self);
1157
1158             let cleaned_pred = pred.fold_with(&mut ParamToVarFolder {
1159                 infcx: self,
1160                 var_map: FxHashMap()
1161             });
1162
1163             let cleaned_pred = super::project::normalize(
1164                 &mut selcx,
1165                 param_env,
1166                 ObligationCause::dummy(),
1167                 &cleaned_pred
1168             ).value;
1169
1170             let obligation = Obligation::new(
1171                 ObligationCause::dummy(),
1172                 param_env,
1173                 cleaned_pred.to_predicate()
1174             );
1175
1176             selcx.evaluate_obligation(&obligation)
1177         })
1178     }
1179
1180     fn note_obligation_cause<T>(&self,
1181                                 err: &mut DiagnosticBuilder,
1182                                 obligation: &Obligation<'tcx, T>)
1183         where T: fmt::Display
1184     {
1185         self.note_obligation_cause_code(err,
1186                                         &obligation.predicate,
1187                                         &obligation.cause.code);
1188     }
1189
1190     fn note_obligation_cause_code<T>(&self,
1191                                      err: &mut DiagnosticBuilder,
1192                                      predicate: &T,
1193                                      cause_code: &ObligationCauseCode<'tcx>)
1194         where T: fmt::Display
1195     {
1196         let tcx = self.tcx;
1197         match *cause_code {
1198             ObligationCauseCode::ExprAssignable |
1199             ObligationCauseCode::MatchExpressionArm { .. } |
1200             ObligationCauseCode::IfExpression |
1201             ObligationCauseCode::IfExpressionWithNoElse |
1202             ObligationCauseCode::EquatePredicate |
1203             ObligationCauseCode::MainFunctionType |
1204             ObligationCauseCode::StartFunctionType |
1205             ObligationCauseCode::IntrinsicType |
1206             ObligationCauseCode::MethodReceiver |
1207             ObligationCauseCode::ReturnNoExpression |
1208             ObligationCauseCode::MiscObligation => {
1209             }
1210             ObligationCauseCode::SliceOrArrayElem => {
1211                 err.note("slice and array elements must have `Sized` type");
1212             }
1213             ObligationCauseCode::TupleElem => {
1214                 err.note("only the last element of a tuple may have a dynamically sized type");
1215             }
1216             ObligationCauseCode::ProjectionWf(data) => {
1217                 err.note(&format!("required so that the projection `{}` is well-formed",
1218                                   data));
1219             }
1220             ObligationCauseCode::ReferenceOutlivesReferent(ref_ty) => {
1221                 err.note(&format!("required so that reference `{}` does not outlive its referent",
1222                                   ref_ty));
1223             }
1224             ObligationCauseCode::ObjectTypeBound(object_ty, region) => {
1225                 err.note(&format!("required so that the lifetime bound of `{}` for `{}` \
1226                                    is satisfied",
1227                                   region, object_ty));
1228             }
1229             ObligationCauseCode::ItemObligation(item_def_id) => {
1230                 let item_name = tcx.item_path_str(item_def_id);
1231                 err.note(&format!("required by `{}`", item_name));
1232             }
1233             ObligationCauseCode::ObjectCastObligation(object_ty) => {
1234                 err.note(&format!("required for the cast to the object type `{}`",
1235                                   self.ty_to_string(object_ty)));
1236             }
1237             ObligationCauseCode::RepeatVec => {
1238                 err.note("the `Copy` trait is required because the \
1239                           repeated element will be copied");
1240             }
1241             ObligationCauseCode::VariableType(_) => {
1242                 err.note("all local variables must have a statically known size");
1243             }
1244             ObligationCauseCode::SizedReturnType => {
1245                 err.note("the return type of a function must have a \
1246                           statically known size");
1247             }
1248             ObligationCauseCode::AssignmentLhsSized => {
1249                 err.note("the left-hand-side of an assignment must have a statically known size");
1250             }
1251             ObligationCauseCode::TupleInitializerSized => {
1252                 err.note("tuples must have a statically known size to be initialized");
1253             }
1254             ObligationCauseCode::StructInitializerSized => {
1255                 err.note("structs must have a statically known size to be initialized");
1256             }
1257             ObligationCauseCode::FieldSized(ref item) => {
1258                 match *item {
1259                     AdtKind::Struct => {
1260                         err.note("only the last field of a struct may have a dynamically \
1261                                   sized type");
1262                     }
1263                     AdtKind::Union => {
1264                         err.note("no field of a union may have a dynamically sized type");
1265                     }
1266                     AdtKind::Enum => {
1267                         err.note("no field of an enum variant may have a dynamically sized type");
1268                     }
1269                 }
1270             }
1271             ObligationCauseCode::ConstSized => {
1272                 err.note("constant expressions must have a statically known size");
1273             }
1274             ObligationCauseCode::SharedStatic => {
1275                 err.note("shared static variables must have a type that implements `Sync`");
1276             }
1277             ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
1278                 let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1279                 err.note(&format!("required because it appears within the type `{}`",
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::ImplDerivedObligation(ref data) => {
1287                 let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1288                 err.note(
1289                     &format!("required because of the requirements on the impl of `{}` for `{}`",
1290                              parent_trait_ref,
1291                              parent_trait_ref.0.self_ty()));
1292                 let parent_predicate = parent_trait_ref.to_predicate();
1293                 self.note_obligation_cause_code(err,
1294                                                 &parent_predicate,
1295                                                 &data.parent_code);
1296             }
1297             ObligationCauseCode::CompareImplMethodObligation { .. } => {
1298                 err.note(
1299                     &format!("the requirement `{}` appears on the impl method \
1300                               but not on the corresponding trait method",
1301                              predicate));
1302             }
1303             ObligationCauseCode::ReturnType(_) |
1304             ObligationCauseCode::BlockTailExpression(_) => (),
1305         }
1306     }
1307
1308     fn suggest_new_overflow_limit(&self, err: &mut DiagnosticBuilder) {
1309         let current_limit = self.tcx.sess.recursion_limit.get();
1310         let suggested_limit = current_limit * 2;
1311         err.help(&format!("consider adding a `#![recursion_limit=\"{}\"]` attribute to your crate",
1312                           suggested_limit));
1313     }
1314 }