src/librustc/traits/error_reporting.rs

   1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 use super::{
  12     FulfillmentError,
  13     FulfillmentErrorCode,
  14     MismatchedProjectionTypes,
  15     Obligation,
  16     ObligationCause,
  17     ObligationCauseCode,
  18     OnUnimplementedDirective,
  19     OnUnimplementedNote,
  20     OutputTypeParameterMismatch,
  21     TraitNotObjectSafe,
  22     ConstEvalFailure,
  23     PredicateObligation,
  24     Reveal,
  25     SelectionContext,
  26     SelectionError,
  27     ObjectSafetyViolation,
  28 };
  29
  30 use errors::DiagnosticBuilder;
  31 use hir;
  32 use hir::def_id::DefId;
  33 use infer::{self, InferCtxt};
  34 use infer::type_variable::TypeVariableOrigin;
  35 use middle::const_val;
  36 use std::fmt;
  37 use syntax::ast;
  38 use session::DiagnosticMessageId;
  39 use ty::{self, AdtKind, ToPredicate, ToPolyTraitRef, Ty, TyCtxt, TypeFoldable};
  40 use ty::error::ExpectedFound;
  41 use ty::fast_reject;
  42 use ty::fold::TypeFolder;
  43 use ty::subst::Subst;
  44 use ty::SubtypePredicate;
  45 use util::nodemap::{FxHashMap, FxHashSet};
  46
  47 use syntax_pos::{DUMMY_SP, Span};
  48
  49 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
  50     pub fn report_fulfillment_errors(&self,
  51                                      errors: &Vec<FulfillmentError<'tcx>>,
  52                                      body_id: Option<hir::BodyId>) {
  53         #[derive(Debug)]
  54         struct ErrorDescriptor<'tcx> {
  55             predicate: ty::Predicate<'tcx>,
  56             index: Option<usize>, // None if this is an old error
  57         }
  58
  59         let mut error_map : FxHashMap<_, _> =
  60             self.reported_trait_errors.borrow().iter().map(|(&span, predicates)| {
  61                 (span, predicates.iter().map(|predicate| ErrorDescriptor {
  62                     predicate: predicate.clone(),
  63                     index: None
  64                 }).collect())
  65             }).collect();
  66
  67         for (index, error) in errors.iter().enumerate() {
  68             error_map.entry(error.obligation.cause.span).or_insert(Vec::new()).push(
  69                 ErrorDescriptor {
  70                     predicate: error.obligation.predicate.clone(),
  71                     index: Some(index)
  72                 });
  73
  74             self.reported_trait_errors.borrow_mut()
  75                 .entry(error.obligation.cause.span).or_insert(Vec::new())
  76                 .push(error.obligation.predicate.clone());
  77         }
  78
  79         // We do this in 2 passes because we want to display errors in order, tho
  80         // maybe it *is* better to sort errors by span or something.
  81         let mut is_suppressed: Vec<bool> = errors.iter().map(|_| false).collect();
  82         for (_, error_set) in error_map.iter() {
  83             // We want to suppress "duplicate" errors with the same span.
  84             for error in error_set {
  85                 if let Some(index) = error.index {
  86                     // Suppress errors that are either:
  87                     // 1) strictly implied by another error.
  88                     // 2) implied by an error with a smaller index.
  89                     for error2 in error_set {
  90                         if error2.index.map_or(false, |index2| is_suppressed[index2]) {
  91                             // Avoid errors being suppressed by already-suppressed
  92                             // errors, to prevent all errors from being suppressed
  93                             // at once.
  94                             continue
  95                         }
  96
  97                         if self.error_implies(&error2.predicate, &error.predicate) &&
  98                             !(error2.index >= error.index &&
  99                               self.error_implies(&error.predicate, &error2.predicate))
 100                         {
 101                             info!("skipping {:?} (implied by {:?})", error, error2);
 102                             is_suppressed[index] = true;
 103                             break
 104                         }
 105                     }
 106                 }
 107             }
 108         }
 109
 110         for (error, suppressed) in errors.iter().zip(is_suppressed) {
 111             if !suppressed {
 112                 self.report_fulfillment_error(error, body_id);
 113             }
 114         }
 115     }
 116
 117     // returns if `cond` not occurring implies that `error` does not occur - i.e. that
 118     // `error` occurring implies that `cond` occurs.
 119     fn error_implies(&self,
 120                      cond: &ty::Predicate<'tcx>,
 121                      error: &ty::Predicate<'tcx>)
 122                      -> bool
 123     {
 124         if cond == error {
 125             return true
 126         }
 127
 128         let (cond, error) = match (cond, error) {
 129             (&ty::Predicate::Trait(..), &ty::Predicate::Trait(ref error))
 130                 => (cond, error),
 131             _ => {
 132                 // FIXME: make this work in other cases too.
 133                 return false
 134             }
 135         };
 136
 137         for implication in super::elaborate_predicates(self.tcx, vec![cond.clone()]) {
 138             if let ty::Predicate::Trait(implication) = implication {
 139                 let error = error.to_poly_trait_ref();
 140                 let implication = implication.to_poly_trait_ref();
 141                 // FIXME: I'm just not taking associated types at all here.
 142                 // Eventually I'll need to implement param-env-aware
 143                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
 144                 let param_env = ty::ParamEnv::empty(Reveal::UserFacing);
 145                 if let Ok(_) = self.can_sub(param_env, error, implication) {
 146                     debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
 147                     return true
 148                 }
 149             }
 150         }
 151
 152         false
 153     }
 154
 155     fn report_fulfillment_error(&self, error: &FulfillmentError<'tcx>,
 156                                 body_id: Option<hir::BodyId>) {
 157         debug!("report_fulfillment_errors({:?})", error);
 158         match error.code {
 159             FulfillmentErrorCode::CodeSelectionError(ref e) => {
 160                 self.report_selection_error(&error.obligation, e);
 161             }
 162             FulfillmentErrorCode::CodeProjectionError(ref e) => {
 163                 self.report_projection_error(&error.obligation, e);
 164             }
 165             FulfillmentErrorCode::CodeAmbiguity => {
 166                 self.maybe_report_ambiguity(&error.obligation, body_id);
 167             }
 168             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
 169                 self.report_mismatched_types(&error.obligation.cause,
 170                                              expected_found.expected,
 171                                              expected_found.found,
 172                                              err.clone())
 173                     .emit();
 174             }
 175         }
 176     }
 177
 178     fn report_projection_error(&self,
 179                                obligation: &PredicateObligation<'tcx>,
 180                                error: &MismatchedProjectionTypes<'tcx>)
 181     {
 182         let predicate =
 183             self.resolve_type_vars_if_possible(&obligation.predicate);
 184
 185         if predicate.references_error() {
 186             return
 187         }
 188
 189         self.probe(|_| {
 190             let err_buf;
 191             let mut err = &error.err;
 192             let mut values = None;
 193
 194             // try to find the mismatched types to report the error with.
 195             //
 196             // this can fail if the problem was higher-ranked, in which
 197             // cause I have no idea for a good error message.
 198             if let ty::Predicate::Projection(ref data) = predicate {
 199                 let mut selcx = SelectionContext::new(self);
 200                 let (data, _) = self.replace_late_bound_regions_with_fresh_var(
 201                     obligation.cause.span,
 202                     infer::LateBoundRegionConversionTime::HigherRankedType,
 203                     data);
 204                 let normalized = super::normalize_projection_type(
 205                     &mut selcx,
 206                     obligation.param_env,
 207                     data.projection_ty,
 208                     obligation.cause.clone(),
 209                     0
 210                 );
 211                 if let Err(error) = self.at(&obligation.cause, obligation.param_env)
 212                                         .eq(normalized.value, data.ty) {
 213                     values = Some(infer::ValuePairs::Types(ExpectedFound {
 214                         expected: normalized.value,
 215                         found: data.ty,
 216                     }));
 217                     err_buf = error;
 218                     err = &err_buf;
 219                 }
 220             }
 221
 222             let msg = format!("type mismatch resolving `{}`", predicate);
 223             let error_id = (DiagnosticMessageId::ErrorId(271),
 224                             Some(obligation.cause.span), msg.clone());
 225             let fresh = self.tcx.sess.one_time_diagnostics.borrow_mut().insert(error_id);
 226             if fresh {
 227                 let mut diag = struct_span_err!(
 228                     self.tcx.sess, obligation.cause.span, E0271,
 229                     "type mismatch resolving `{}`", predicate
 230                 );
 231                 self.note_type_err(&mut diag, &obligation.cause, None, values, err);
 232                 self.note_obligation_cause(&mut diag, obligation);
 233                 diag.emit();
 234             }
 235         });
 236     }
 237
 238     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
 239         /// returns the fuzzy category of a given type, or None
 240         /// if the type can be equated to any type.
 241         fn type_category<'tcx>(t: Ty<'tcx>) -> Option<u32> {
 242             match t.sty {
 243                 ty::TyBool => Some(0),
 244                 ty::TyChar => Some(1),
 245                 ty::TyStr => Some(2),
 246                 ty::TyInt(..) | ty::TyUint(..) | ty::TyInfer(ty::IntVar(..)) => Some(3),
 247                 ty::TyFloat(..) | ty::TyInfer(ty::FloatVar(..)) => Some(4),
 248                 ty::TyRef(..) | ty::TyRawPtr(..) => Some(5),
 249                 ty::TyArray(..) | ty::TySlice(..) => Some(6),
 250                 ty::TyFnDef(..) | ty::TyFnPtr(..) => Some(7),
 251                 ty::TyDynamic(..) => Some(8),
 252                 ty::TyClosure(..) => Some(9),
 253                 ty::TyTuple(..) => Some(10),
 254                 ty::TyProjection(..) => Some(11),
 255                 ty::TyParam(..) => Some(12),
 256                 ty::TyAnon(..) => Some(13),
 257                 ty::TyNever => Some(14),
 258                 ty::TyAdt(adt, ..) => match adt.adt_kind() {
 259                     AdtKind::Struct => Some(15),
 260                     AdtKind::Union => Some(16),
 261                     AdtKind::Enum => Some(17),
 262                 },
 263                 ty::TyGenerator(..) => Some(18),
 264                 ty::TyForeign(..) => Some(19),
 265                 ty::TyGeneratorWitness(..) => Some(20),
 266                 ty::TyInfer(..) | ty::TyError => None
 267             }
 268         }
 269
 270         match (type_category(a), type_category(b)) {
 271             (Some(cat_a), Some(cat_b)) => match (&a.sty, &b.sty) {
 272                 (&ty::TyAdt(def_a, _), &ty::TyAdt(def_b, _)) => def_a == def_b,
 273                 _ => cat_a == cat_b
 274             },
 275             // infer and error can be equated to all types
 276             _ => true
 277         }
 278     }
 279
 280     fn impl_similar_to(&self,
 281                        trait_ref: ty::PolyTraitRef<'tcx>,
 282                        obligation: &PredicateObligation<'tcx>)
 283                        -> Option<DefId>
 284     {
 285         let tcx = self.tcx;
 286         let param_env = obligation.param_env;
 287         let trait_ref = tcx.erase_late_bound_regions(&trait_ref);
 288         let trait_self_ty = trait_ref.self_ty();
 289
 290         let mut self_match_impls = vec![];
 291         let mut fuzzy_match_impls = vec![];
 292
 293         self.tcx.for_each_relevant_impl(
 294             trait_ref.def_id, trait_self_ty, |def_id| {
 295                 let impl_substs = self.fresh_substs_for_item(obligation.cause.span, def_id);
 296                 let impl_trait_ref = tcx
 297                     .impl_trait_ref(def_id)
 298                     .unwrap()
 299                     .subst(tcx, impl_substs);
 300
 301                 let impl_self_ty = impl_trait_ref.self_ty();
 302
 303                 if let Ok(..) = self.can_eq(param_env, trait_self_ty, impl_self_ty) {
 304                     self_match_impls.push(def_id);
 305
 306                     if trait_ref.substs.types().skip(1)
 307                         .zip(impl_trait_ref.substs.types().skip(1))
 308                         .all(|(u,v)| self.fuzzy_match_tys(u, v))
 309                     {
 310                         fuzzy_match_impls.push(def_id);
 311                     }
 312                 }
 313             });
 314
 315         let impl_def_id = if self_match_impls.len() == 1 {
 316             self_match_impls[0]
 317         } else if fuzzy_match_impls.len() == 1 {
 318             fuzzy_match_impls[0]
 319         } else {
 320             return None
 321         };
 322
 323         if tcx.has_attr(impl_def_id, "rustc_on_unimplemented") {
 324             Some(impl_def_id)
 325         } else {
 326             None
 327         }
 328     }
 329
 330     fn on_unimplemented_note(
 331         &self,
 332         trait_ref: ty::PolyTraitRef<'tcx>,
 333         obligation: &PredicateObligation<'tcx>) ->
 334         OnUnimplementedNote
 335     {
 336         let def_id = self.impl_similar_to(trait_ref, obligation)
 337             .unwrap_or(trait_ref.def_id());
 338         let trait_ref = *trait_ref.skip_binder();
 339
 340         let desugaring;
 341         let method;
 342         let mut flags = vec![];
 343         let direct = match obligation.cause.code {
 344             ObligationCauseCode::BuiltinDerivedObligation(..) |
 345             ObligationCauseCode::ImplDerivedObligation(..) => false,
 346             _ => true
 347         };
 348         if direct {
 349             // this is a "direct", user-specified, rather than derived,
 350             // obligation.
 351             flags.push(("direct".to_string(), None));
 352         }
 353
 354         if let ObligationCauseCode::ItemObligation(item) = obligation.cause.code {
 355             // FIXME: maybe also have some way of handling methods
 356             // from other traits? That would require name resolution,
 357             // which we might want to be some sort of hygienic.
 358             //
 359             // Currently I'm leaving it for what I need for `try`.
 360             if self.tcx.trait_of_item(item) == Some(trait_ref.def_id) {
 361                 method = self.tcx.item_name(item);
 362                 flags.push(("from_method".to_string(), None));
 363                 flags.push(("from_method".to_string(), Some(method.to_string())));
 364             }
 365         }
 366
 367         if let Some(k) = obligation.cause.span.compiler_desugaring_kind() {
 368             desugaring = k.as_symbol().as_str();
 369             flags.push(("from_desugaring".to_string(), None));
 370             flags.push(("from_desugaring".to_string(), Some(desugaring.to_string())));
 371         }
 372         let generics = self.tcx.generics_of(def_id);
 373         let self_ty = trait_ref.self_ty();
 374         let self_ty_str = self_ty.to_string();
 375         flags.push(("_Self".to_string(), Some(self_ty_str.clone())));
 376
 377         for param in generics.types.iter() {
 378             let name = param.name.as_str().to_string();
 379             let ty = trait_ref.substs.type_for_def(param);
 380             let ty_str = ty.to_string();
 381             flags.push((name.clone(),
 382                         Some(ty_str.clone())));
 383         }
 384
 385         if let Some(true) = self_ty.ty_to_def_id().map(|def_id| def_id.is_local()) {
 386             flags.push(("crate_local".to_string(), None));
 387         }
 388
 389         if let Ok(Some(command)) = OnUnimplementedDirective::of_item(
 390             self.tcx, trait_ref.def_id, def_id
 391         ) {
 392             command.evaluate(self.tcx, trait_ref, &flags[..])
 393         } else {
 394             OnUnimplementedNote::empty()
 395         }
 396     }
 397
 398     fn find_similar_impl_candidates(&self,
 399                                     trait_ref: ty::PolyTraitRef<'tcx>)
 400                                     -> Vec<ty::TraitRef<'tcx>>
 401     {
 402         let simp = fast_reject::simplify_type(self.tcx,
 403                                               trait_ref.skip_binder().self_ty(),
 404                                               true);
 405         let mut impl_candidates = Vec::new();
 406
 407         match simp {
 408             Some(simp) => self.tcx.for_each_impl(trait_ref.def_id(), |def_id| {
 409                 let imp = self.tcx.impl_trait_ref(def_id).unwrap();
 410                 let imp_simp = fast_reject::simplify_type(self.tcx,
 411                                                           imp.self_ty(),
 412                                                           true);
 413                 if let Some(imp_simp) = imp_simp {
 414                     if simp != imp_simp {
 415                         return;
 416                     }
 417                 }
 418                 impl_candidates.push(imp);
 419             }),
 420             None => self.tcx.for_each_impl(trait_ref.def_id(), |def_id| {
 421                 impl_candidates.push(
 422                     self.tcx.impl_trait_ref(def_id).unwrap());
 423             })
 424         };
 425         impl_candidates
 426     }
 427
 428     fn report_similar_impl_candidates(&self,
 429                                       impl_candidates: Vec<ty::TraitRef<'tcx>>,
 430                                       err: &mut DiagnosticBuilder)
 431     {
 432         if impl_candidates.is_empty() {
 433             return;
 434         }
 435
 436         let end = if impl_candidates.len() <= 5 {
 437             impl_candidates.len()
 438         } else {
 439             4
 440         };
 441         err.help(&format!("the following implementations were found:{}{}",
 442                           &impl_candidates[0..end].iter().map(|candidate| {
 443                               format!("\n  {:?}", candidate)
 444                           }).collect::<String>(),
 445                           if impl_candidates.len() > 5 {
 446                               format!("\nand {} others", impl_candidates.len() - 4)
 447                           } else {
 448                               "".to_owned()
 449                           }
 450                           ));
 451     }
 452
 453     /// Reports that an overflow has occurred and halts compilation. We
 454     /// halt compilation unconditionally because it is important that
 455     /// overflows never be masked -- they basically represent computations
 456     /// whose result could not be truly determined and thus we can't say
 457     /// if the program type checks or not -- and they are unusual
 458     /// occurrences in any case.
 459     pub fn report_overflow_error<T>(&self,
 460                                     obligation: &Obligation<'tcx, T>,
 461                                     suggest_increasing_limit: bool) -> !
 462         where T: fmt::Display + TypeFoldable<'tcx>
 463     {
 464         let predicate =
 465             self.resolve_type_vars_if_possible(&obligation.predicate);
 466         let mut err = struct_span_err!(self.tcx.sess, obligation.cause.span, E0275,
 467                                        "overflow evaluating the requirement `{}`",
 468                                        predicate);
 469
 470         if suggest_increasing_limit {
 471             self.suggest_new_overflow_limit(&mut err);
 472         }
 473
 474         self.note_obligation_cause(&mut err, obligation);
 475
 476         err.emit();
 477         self.tcx.sess.abort_if_errors();
 478         bug!();
 479     }
 480
 481     /// Reports that a cycle was detected which led to overflow and halts
 482     /// compilation. This is equivalent to `report_overflow_error` except
 483     /// that we can give a more helpful error message (and, in particular,
 484     /// we do not suggest increasing the overflow limit, which is not
 485     /// going to help).
 486     pub fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 487         let cycle = self.resolve_type_vars_if_possible(&cycle.to_owned());
 488         assert!(cycle.len() > 0);
 489
 490         debug!("report_overflow_error_cycle: cycle={:?}", cycle);
 491
 492         self.report_overflow_error(&cycle[0], false);
 493     }
 494
 495     pub fn report_extra_impl_obligation(&self,
 496                                         error_span: Span,
 497                                         item_name: ast::Name,
 498                                         _impl_item_def_id: DefId,
 499                                         trait_item_def_id: DefId,
 500                                         requirement: &fmt::Display)
 501                                         -> DiagnosticBuilder<'tcx>
 502     {
 503         let msg = "impl has stricter requirements than trait";
 504         let sp = self.tcx.sess.codemap().def_span(error_span);
 505
 506         let mut err = struct_span_err!(self.tcx.sess, sp, E0276, "{}", msg);
 507
 508         if let Some(trait_item_span) = self.tcx.hir.span_if_local(trait_item_def_id) {
 509             let span = self.tcx.sess.codemap().def_span(trait_item_span);
 510             err.span_label(span, format!("definition of `{}` from trait", item_name));
 511         }
 512
 513         err.span_label(sp, format!("impl has extra requirement {}", requirement));
 514
 515         err
 516     }
 517
 518
 519     /// Get the parent trait chain start
 520     fn get_parent_trait_ref(&self, code: &ObligationCauseCode<'tcx>) -> Option<String> {
 521         match code {
 522             &ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
 523                 let parent_trait_ref = self.resolve_type_vars_if_possible(
 524                     &data.parent_trait_ref);
 525                 match self.get_parent_trait_ref(&data.parent_code) {
 526                     Some(t) => Some(t),
 527                     None => Some(format!("{}", parent_trait_ref.0.self_ty())),
 528                 }
 529             }
 530             _ => None,
 531         }
 532     }
 533
 534     pub fn report_selection_error(&self,
 535                                   obligation: &PredicateObligation<'tcx>,
 536                                   error: &SelectionError<'tcx>)
 537     {
 538         let span = obligation.cause.span;
 539
 540         let mut err = match *error {
 541             SelectionError::Unimplemented => {
 542                 if let ObligationCauseCode::CompareImplMethodObligation {
 543                     item_name, impl_item_def_id, trait_item_def_id,
 544                 } = obligation.cause.code {
 545                     self.report_extra_impl_obligation(
 546                         span,
 547                         item_name,
 548                         impl_item_def_id,
 549                         trait_item_def_id,
 550                         &format!("`{}`", obligation.predicate))
 551                         .emit();
 552                     return;
 553                 }
 554                 match obligation.predicate {
 555                     ty::Predicate::Trait(ref trait_predicate) => {
 556                         let trait_predicate =
 557                             self.resolve_type_vars_if_possible(trait_predicate);
 558
 559                         if self.tcx.sess.has_errors() && trait_predicate.references_error() {
 560                             return;
 561                         }
 562                         let trait_ref = trait_predicate.to_poly_trait_ref();
 563                         let (post_message, pre_message) =
 564                             self.get_parent_trait_ref(&obligation.cause.code)
 565                                 .map(|t| (format!(" in `{}`", t), format!("within `{}`, ", t)))
 566                             .unwrap_or((String::new(), String::new()));
 567
 568                         let OnUnimplementedNote { message, label, note }
 569                             = self.on_unimplemented_note(trait_ref, obligation);
 570                         let have_alt_message = message.is_some() || label.is_some();
 571
 572                         let mut err = struct_span_err!(
 573                             self.tcx.sess,
 574                             span,
 575                             E0277,
 576                             "{}",
 577                             message.unwrap_or_else(|| {
 578                                 format!("the trait bound `{}` is not satisfied{}",
 579                                          trait_ref.to_predicate(), post_message)
 580                             }));
 581
 582                         if let Some(ref s) = label {
 583                             // If it has a custom "#[rustc_on_unimplemented]"
 584                             // error message, let's display it as the label!
 585                             err.span_label(span, s.as_str());
 586                             err.help(&format!("{}the trait `{}` is not implemented for `{}`",
 587                                               pre_message,
 588                                               trait_ref,
 589                                               trait_ref.self_ty()));
 590                         } else {
 591                             err.span_label(span,
 592                                            &*format!("{}the trait `{}` is not implemented for `{}`",
 593                                                      pre_message,
 594                                                      trait_ref,
 595                                                      trait_ref.self_ty()));
 596                         }
 597                         if let Some(ref s) = note {
 598                             // If it has a custom "#[rustc_on_unimplemented]" note, let's display it
 599                             err.note(s.as_str());
 600                         }
 601
 602                         self.suggest_borrow_on_unsized_slice(&obligation.cause.code, &mut err);
 603
 604                         // Try to report a help message
 605                         if !trait_ref.has_infer_types() &&
 606                             self.predicate_can_apply(obligation.param_env, trait_ref) {
 607                             // If a where-clause may be useful, remind the
 608                             // user that they can add it.
 609                             //
 610                             // don't display an on-unimplemented note, as
 611                             // these notes will often be of the form
 612                             //     "the type `T` can't be frobnicated"
 613                             // which is somewhat confusing.
 614                             err.help(&format!("consider adding a `where {}` bound",
 615                                                 trait_ref.to_predicate()));
 616                         } else if !have_alt_message {
 617                             // Can't show anything else useful, try to find similar impls.
 618                             let impl_candidates = self.find_similar_impl_candidates(trait_ref);
 619                             self.report_similar_impl_candidates(impl_candidates, &mut err);
 620                         }
 621
 622                         err
 623                     }
 624
 625                     ty::Predicate::Subtype(ref predicate) => {
 626                         // Errors for Subtype predicates show up as
 627                         // `FulfillmentErrorCode::CodeSubtypeError`,
 628                         // not selection error.
 629                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
 630                     }
 631
 632                     ty::Predicate::Equate(ref predicate) => {
 633                         let predicate = self.resolve_type_vars_if_possible(predicate);
 634                         let err = self.equality_predicate(&obligation.cause,
 635                                                           obligation.param_env,
 636                                                           &predicate).err().unwrap();
 637                         struct_span_err!(self.tcx.sess, span, E0278,
 638                             "the requirement `{}` is not satisfied (`{}`)",
 639                             predicate, err)
 640                     }
 641
 642                     ty::Predicate::RegionOutlives(ref predicate) => {
 643                         let predicate = self.resolve_type_vars_if_possible(predicate);
 644                         let err = self.region_outlives_predicate(&obligation.cause,
 645                                                                     &predicate).err().unwrap();
 646                         struct_span_err!(self.tcx.sess, span, E0279,
 647                             "the requirement `{}` is not satisfied (`{}`)",
 648                             predicate, err)
 649                     }
 650
 651                     ty::Predicate::Projection(..) | ty::Predicate::TypeOutlives(..) => {
 652                         let predicate =
 653                             self.resolve_type_vars_if_possible(&obligation.predicate);
 654                         struct_span_err!(self.tcx.sess, span, E0280,
 655                             "the requirement `{}` is not satisfied",
 656                             predicate)
 657                     }
 658
 659                     ty::Predicate::ObjectSafe(trait_def_id) => {
 660                         let violations = self.tcx.object_safety_violations(trait_def_id);
 661                         self.tcx.report_object_safety_error(span,
 662                                                             trait_def_id,
 663                                                             violations)
 664                     }
 665
 666                     ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => {
 667                         let found_kind = self.closure_kind(closure_def_id, closure_substs).unwrap();
 668                         let closure_span = self.tcx.sess.codemap()
 669                             .def_span(self.tcx.hir.span_if_local(closure_def_id).unwrap());
 670                         let node_id = self.tcx.hir.as_local_node_id(closure_def_id).unwrap();
 671                         let mut err = struct_span_err!(
 672                             self.tcx.sess, closure_span, E0525,
 673                             "expected a closure that implements the `{}` trait, \
 674                                 but this closure only implements `{}`",
 675                             kind,
 676                             found_kind);
 677
 678                         err.span_label(
 679                             closure_span,
 680                             format!("this closure implements `{}`, not `{}`", found_kind, kind));
 681                         err.span_label(
 682                             obligation.cause.span,
 683                             format!("the requirement to implement `{}` derives from here", kind));
 684
 685                         // Additional context information explaining why the closure only implements
 686                         // a particular trait.
 687                         if let Some(tables) = self.in_progress_tables {
 688                             let tables = tables.borrow();
 689                             let closure_hir_id = self.tcx.hir.node_to_hir_id(node_id);
 690                             match (found_kind, tables.closure_kind_origins().get(closure_hir_id)) {
 691                                 (ty::ClosureKind::FnOnce, Some((span, name))) => {
 692                                     err.span_label(*span, format!(
 693                                         "closure is `FnOnce` because it moves the \
 694                                          variable `{}` out of its environment", name));
 695                                 },
 696                                 (ty::ClosureKind::FnMut, Some((span, name))) => {
 697                                     err.span_label(*span, format!(
 698                                         "closure is `FnMut` because it mutates the \
 699                                          variable `{}` here", name));
 700                                 },
 701                                 _ => {}
 702                             }
 703                         }
 704
 705                         err.emit();
 706                         return;
 707                     }
 708
 709                     ty::Predicate::WellFormed(ty) => {
 710                         // WF predicates cannot themselves make
 711                         // errors. They can only block due to
 712                         // ambiguity; otherwise, they always
 713                         // degenerate into other obligations
 714                         // (which may fail).
 715                         span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 716                     }
 717
 718                     ty::Predicate::ConstEvaluatable(..) => {
 719                         // Errors for `ConstEvaluatable` predicates show up as
 720                         // `SelectionError::ConstEvalFailure`,
 721                         // not `Unimplemented`.
 722                         span_bug!(span,
 723                             "const-evaluatable requirement gave wrong error: `{:?}`", obligation)
 724                     }
 725                 }
 726             }
 727
 728             OutputTypeParameterMismatch(ref found_trait_ref, ref expected_trait_ref, _) => {
 729                 let found_trait_ref = self.resolve_type_vars_if_possible(&*found_trait_ref);
 730                 let expected_trait_ref = self.resolve_type_vars_if_possible(&*expected_trait_ref);
 731                 if expected_trait_ref.self_ty().references_error() {
 732                     return;
 733                 }
 734                 let found_trait_ty = found_trait_ref.self_ty();
 735
 736                 let found_did = found_trait_ty.ty_to_def_id();
 737                 let found_span = found_did.and_then(|did| {
 738                     self.tcx.hir.span_if_local(did)
 739                 }).map(|sp| self.tcx.sess.codemap().def_span(sp)); // the sp could be an fn def
 740
 741                 let found = match found_trait_ref.skip_binder().substs.type_at(1).sty {
 742                     ty::TyTuple(ref tys, _) => tys.iter()
 743                         .map(|_| ArgKind::empty()).collect::<Vec<_>>(),
 744                     _ => vec![ArgKind::empty()],
 745                 };
 746                 let expected = match expected_trait_ref.skip_binder().substs.type_at(1).sty {
 747                     ty::TyTuple(ref tys, _) => tys.iter()
 748                         .map(|t| match t.sty {
 749                             ty::TypeVariants::TyTuple(ref tys, _) => ArgKind::Tuple(
 750                                 span,
 751                                 tys.iter()
 752                                     .map(|ty| ("_".to_owned(), format!("{}", ty.sty)))
 753                                     .collect::<Vec<_>>()
 754                             ),
 755                             _ => ArgKind::Arg("_".to_owned(), format!("{}", t.sty)),
 756                         }).collect(),
 757                     ref sty => vec![ArgKind::Arg("_".to_owned(), format!("{}", sty))],
 758                 };
 759                 if found.len()== expected.len() {
 760                     self.report_closure_arg_mismatch(span,
 761                                                      found_span,
 762                                                      found_trait_ref,
 763                                                      expected_trait_ref)
 764                 } else {
 765                     let (closure_span, found) = found_did
 766                         .and_then(|did| self.tcx.hir.get_if_local(did))
 767                         .map(|node| self.get_fn_like_arguments(node))
 768                         .unwrap_or((found_span.unwrap(), found));
 769
 770                     self.report_arg_count_mismatch(span,
 771                                                    closure_span,
 772                                                    expected,
 773                                                    found,
 774                                                    found_trait_ty.is_closure())
 775                 }
 776             }
 777
 778             TraitNotObjectSafe(did) => {
 779                 let violations = self.tcx.object_safety_violations(did);
 780                 self.tcx.report_object_safety_error(span, did,
 781                                                     violations)
 782             }
 783
 784             ConstEvalFailure(ref err) => {
 785                 if let const_val::ErrKind::TypeckError = err.kind {
 786                     return;
 787                 }
 788                 err.struct_error(self.tcx, span, "constant expression")
 789             }
 790         };
 791         self.note_obligation_cause(&mut err, obligation);
 792         err.emit();
 793     }
 794
 795     /// When encountering an assignment of an unsized trait, like `let x = ""[..];`, provide a
 796     /// suggestion to borrow the initializer in order to use have a slice instead.
 797     fn suggest_borrow_on_unsized_slice(&self,
 798                                        code: &ObligationCauseCode<'tcx>,
 799                                        err: &mut DiagnosticBuilder<'tcx>) {
 800         if let &ObligationCauseCode::VariableType(node_id) = code {
 801             let parent_node = self.tcx.hir.get_parent_node(node_id);
 802             if let Some(hir::map::NodeLocal(ref local)) = self.tcx.hir.find(parent_node) {
 803                 if let Some(ref expr) = local.init {
 804                     if let hir::ExprIndex(_, _) = expr.node {
 805                         if let Ok(snippet) = self.tcx.sess.codemap().span_to_snippet(expr.span) {
 806                             err.span_suggestion(expr.span,
 807                                                 "consider borrowing here",
 808                                                 format!("&{}", snippet));
 809                         }
 810                     }
 811                 }
 812             }
 813         }
 814     }
 815
 816     fn get_fn_like_arguments(&self, node: hir::map::Node) -> (Span, Vec<ArgKind>) {
 817         match node {
 818             hir::map::NodeExpr(&hir::Expr {
 819                 node: hir::ExprClosure(_, ref _decl, id, span, _),
 820                 ..
 821             }) => {
 822                 (self.tcx.sess.codemap().def_span(span), self.tcx.hir.body(id).arguments.iter()
 823                     .map(|arg| {
 824                         if let hir::Pat {
 825                             node: hir::PatKind::Tuple(args, _),
 826                             span,
 827                             ..
 828                         } = arg.pat.clone().into_inner() {
 829                             ArgKind::Tuple(
 830                                 span,
 831                                 args.iter().map(|pat| {
 832                                     let snippet = self.tcx.sess.codemap()
 833                                         .span_to_snippet(pat.span).unwrap();
 834                                     (snippet, "_".to_owned())
 835                                 }).collect::<Vec<_>>(),
 836                             )
 837                         } else {
 838                             let name = self.tcx.sess.codemap()
 839                                 .span_to_snippet(arg.pat.span).unwrap();
 840                             ArgKind::Arg(name, "_".to_owned())
 841                         }
 842                     })
 843                     .collect::<Vec<ArgKind>>())
 844             }
 845             hir::map::NodeItem(&hir::Item {
 846                 span,
 847                 node: hir::ItemFn(ref decl, ..),
 848                 ..
 849             }) |
 850             hir::map::NodeImplItem(&hir::ImplItem {
 851                 span,
 852                 node: hir::ImplItemKind::Method(hir::MethodSig { ref decl, .. }, _),
 853                 ..
 854             }) |
 855             hir::map::NodeTraitItem(&hir::TraitItem {
 856                 span,
 857                 node: hir::TraitItemKind::Method(hir::MethodSig { ref decl, .. }, _),
 858                 ..
 859             }) => {
 860                 (self.tcx.sess.codemap().def_span(span), decl.inputs.iter()
 861                         .map(|arg| match arg.clone().into_inner().node {
 862                     hir::TyTup(ref tys) => ArgKind::Tuple(
 863                         arg.span,
 864                         tys.iter()
 865                             .map(|_| ("_".to_owned(), "_".to_owned()))
 866                             .collect::<Vec<_>>(),
 867                     ),
 868                     _ => ArgKind::Arg("_".to_owned(), "_".to_owned())
 869                 }).collect::<Vec<ArgKind>>())
 870             }
 871             _ => panic!("non-FnLike node found: {:?}", node),
 872         }
 873     }
 874
 875     fn report_arg_count_mismatch(
 876         &self,
 877         span: Span,
 878         found_span: Span,
 879         expected_args: Vec<ArgKind>,
 880         found_args: Vec<ArgKind>,
 881         is_closure: bool,
 882     ) -> DiagnosticBuilder<'tcx> {
 883         let kind = if is_closure { "closure" } else { "function" };
 884
 885         let args_str = |arguments: &Vec<ArgKind>, other: &Vec<ArgKind>| {
 886             let arg_length = arguments.len();
 887             let distinct = match &other[..] {
 888                 &[ArgKind::Tuple(..)] => true,
 889                 _ => false,
 890             };
 891             match (arg_length, arguments.get(0)) {
 892                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
 893                     format!("a single {}-tuple as argument", fields.len())
 894                 }
 895                 _ => format!("{} {}argument{}",
 896                              arg_length,
 897                              if distinct && arg_length > 1 { "distinct " } else { "" },
 898                              if arg_length == 1 { "" } else { "s" }),
 899             }
 900         };
 901
 902         let expected_str = args_str(&expected_args, &found_args);
 903         let found_str = args_str(&found_args, &expected_args);
 904
 905         let mut err = struct_span_err!(
 906             self.tcx.sess,
 907             span,
 908             E0593,
 909             "{} is expected to take {}, but it takes {}",
 910             kind,
 911             expected_str,
 912             found_str,
 913         );
 914
 915         err.span_label(span, format!( "expected {} that takes {}", kind, expected_str));
 916         err.span_label(found_span, format!("takes {}", found_str));
 917
 918         if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
 919             if fields.len() == expected_args.len() {
 920                 let sugg = fields.iter()
 921                     .map(|(name, _)| name.to_owned())
 922                     .collect::<Vec<String>>().join(", ");
 923                 err.span_suggestion(found_span,
 924                                     "change the closure to take multiple arguments instead of \
 925                                      a single tuple",
 926                                     format!("|{}|", sugg));
 927             }
 928         }
 929         if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..] {
 930             if fields.len() == found_args.len() && is_closure {
 931                 let sugg = format!(
 932                     "|({}){}|",
 933                     found_args.iter()
 934                         .map(|arg| match arg {
 935                             ArgKind::Arg(name, _) => name.to_owned(),
 936                             _ => "_".to_owned(),
 937                         })
 938                         .collect::<Vec<String>>()
 939                         .join(", "),
 940                     // add type annotations if available
 941                     if found_args.iter().any(|arg| match arg {
 942                         ArgKind::Arg(_, ty) => ty != "_",
 943                         _ => false,
 944                     }) {
 945                         format!(": ({})",
 946                                 fields.iter()
 947                                     .map(|(_, ty)| ty.to_owned())
 948                                     .collect::<Vec<String>>()
 949                                     .join(", "))
 950                     } else {
 951                         "".to_owned()
 952                     },
 953                 );
 954                 err.span_suggestion(found_span,
 955                                     "change the closure to accept a tuple instead of individual \
 956                                      arguments",
 957                                     sugg);
 958             }
 959         }
 960
 961         err
 962     }
 963
 964     fn report_closure_arg_mismatch(&self,
 965                            span: Span,
 966                            found_span: Option<Span>,
 967                            expected_ref: ty::PolyTraitRef<'tcx>,
 968                            found: ty::PolyTraitRef<'tcx>)
 969         -> DiagnosticBuilder<'tcx>
 970     {
 971         fn build_fn_sig_string<'a, 'gcx, 'tcx>(tcx: ty::TyCtxt<'a, 'gcx, 'tcx>,
 972                                                trait_ref: &ty::TraitRef<'tcx>) -> String {
 973             let inputs = trait_ref.substs.type_at(1);
 974             let sig = if let ty::TyTuple(inputs, _) = inputs.sty {
 975                 tcx.mk_fn_sig(
 976                     inputs.iter().map(|&x| x),
 977                     tcx.mk_infer(ty::TyVar(ty::TyVid { index: 0 })),
 978                     false,
 979                     hir::Unsafety::Normal,
 980                     ::syntax::abi::Abi::Rust
 981                 )
 982             } else {
 983                 tcx.mk_fn_sig(
 984                     ::std::iter::once(inputs),
 985                     tcx.mk_infer(ty::TyVar(ty::TyVid { index: 0 })),
 986                     false,
 987                     hir::Unsafety::Normal,
 988                     ::syntax::abi::Abi::Rust
 989                 )
 990             };
 991             format!("{}", ty::Binder(sig))
 992         }
 993
 994         let argument_is_closure = expected_ref.skip_binder().substs.type_at(0).is_closure();
 995         let mut err = struct_span_err!(self.tcx.sess, span, E0631,
 996                                        "type mismatch in {} arguments",
 997                                        if argument_is_closure { "closure" } else { "function" });
 998
 999         let found_str = format!(
1000             "expected signature of `{}`",
1001             build_fn_sig_string(self.tcx, found.skip_binder())
1002         );
1003         err.span_label(span, found_str);
1004
1005         let found_span = found_span.unwrap_or(span);
1006         let expected_str = format!(
1007             "found signature of `{}`",
1008             build_fn_sig_string(self.tcx, expected_ref.skip_binder())
1009         );
1010         err.span_label(found_span, expected_str);
1011
1012         err
1013     }
1014 }
1015
1016 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
1017     pub fn recursive_type_with_infinite_size_error(self,
1018                                                    type_def_id: DefId)
1019                                                    -> DiagnosticBuilder<'tcx>
1020     {
1021         assert!(type_def_id.is_local());
1022         let span = self.hir.span_if_local(type_def_id).unwrap();
1023         let span = self.sess.codemap().def_span(span);
1024         let mut err = struct_span_err!(self.sess, span, E0072,
1025                                        "recursive type `{}` has infinite size",
1026                                        self.item_path_str(type_def_id));
1027         err.span_label(span, "recursive type has infinite size");
1028         err.help(&format!("insert indirection (e.g., a `Box`, `Rc`, or `&`) \
1029                            at some point to make `{}` representable",
1030                           self.item_path_str(type_def_id)));
1031         err
1032     }
1033
1034     pub fn report_object_safety_error(self,
1035                                       span: Span,
1036                                       trait_def_id: DefId,
1037                                       violations: Vec<ObjectSafetyViolation>)
1038                                       -> DiagnosticBuilder<'tcx>
1039     {
1040         let trait_str = self.item_path_str(trait_def_id);
1041         let span = self.sess.codemap().def_span(span);
1042         let mut err = struct_span_err!(
1043             self.sess, span, E0038,
1044             "the trait `{}` cannot be made into an object",
1045             trait_str);
1046         err.span_label(span, format!("the trait `{}` cannot be made into an object", trait_str));
1047
1048         let mut reported_violations = FxHashSet();
1049         for violation in violations {
1050             if !reported_violations.insert(violation.clone()) {
1051                 continue;
1052             }
1053             err.note(&violation.error_msg());
1054         }
1055         err
1056     }
1057 }
1058
1059 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
1060     fn maybe_report_ambiguity(&self, obligation: &PredicateObligation<'tcx>,
1061                               body_id: Option<hir::BodyId>) {
1062         // Unable to successfully determine, probably means
1063         // insufficient type information, but could mean
1064         // ambiguous impls. The latter *ought* to be a
1065         // coherence violation, so we don't report it here.
1066
1067         let predicate = self.resolve_type_vars_if_possible(&obligation.predicate);
1068         let span = obligation.cause.span;
1069
1070         debug!("maybe_report_ambiguity(predicate={:?}, obligation={:?})",
1071                predicate,
1072                obligation);
1073
1074         // Ambiguity errors are often caused as fallout from earlier
1075         // errors. So just ignore them if this infcx is tainted.
1076         if self.is_tainted_by_errors() {
1077             return;
1078         }
1079
1080         match predicate {
1081             ty::Predicate::Trait(ref data) => {
1082                 let trait_ref = data.to_poly_trait_ref();
1083                 let self_ty = trait_ref.self_ty();
1084                 if predicate.references_error() {
1085                     return;
1086                 }
1087                 // Typically, this ambiguity should only happen if
1088                 // there are unresolved type inference variables
1089                 // (otherwise it would suggest a coherence
1090                 // failure). But given #21974 that is not necessarily
1091                 // the case -- we can have multiple where clauses that
1092                 // are only distinguished by a region, which results
1093                 // in an ambiguity even when all types are fully
1094                 // known, since we don't dispatch based on region
1095                 // relationships.
1096
1097                 // This is kind of a hack: it frequently happens that some earlier
1098                 // error prevents types from being fully inferred, and then we get
1099                 // a bunch of uninteresting errors saying something like "<generic
1100                 // #0> doesn't implement Sized".  It may even be true that we
1101                 // could just skip over all checks where the self-ty is an
1102                 // inference variable, but I was afraid that there might be an
1103                 // inference variable created, registered as an obligation, and
1104                 // then never forced by writeback, and hence by skipping here we'd
1105                 // be ignoring the fact that we don't KNOW the type works
1106                 // out. Though even that would probably be harmless, given that
1107                 // we're only talking about builtin traits, which are known to be
1108                 // inhabited. But in any case I just threw in this check for
1109                 // has_errors() to be sure that compilation isn't happening
1110                 // anyway. In that case, why inundate the user.
1111                 if !self.tcx.sess.has_errors() {
1112                     if
1113                         self.tcx.lang_items().sized_trait()
1114                         .map_or(false, |sized_id| sized_id == trait_ref.def_id())
1115                     {
1116                         self.need_type_info(body_id, span, self_ty);
1117                     } else {
1118                         let mut err = struct_span_err!(self.tcx.sess,
1119                                                         span, E0283,
1120                                                         "type annotations required: \
1121                                                         cannot resolve `{}`",
1122                                                         predicate);
1123                         self.note_obligation_cause(&mut err, obligation);
1124                         err.emit();
1125                     }
1126                 }
1127             }
1128
1129             ty::Predicate::WellFormed(ty) => {
1130                 // Same hacky approach as above to avoid deluging user
1131                 // with error messages.
1132                 if !ty.references_error() && !self.tcx.sess.has_errors() {
1133                     self.need_type_info(body_id, span, ty);
1134                 }
1135             }
1136
1137             ty::Predicate::Subtype(ref data) => {
1138                 if data.references_error() || self.tcx.sess.has_errors() {
1139                     // no need to overload user in such cases
1140                 } else {
1141                     let &SubtypePredicate { a_is_expected: _, a, b } = data.skip_binder();
1142                     // both must be type variables, or the other would've been instantiated
1143                     assert!(a.is_ty_var() && b.is_ty_var());
1144                     self.need_type_info(body_id,
1145                                         obligation.cause.span,
1146                                         a);
1147                 }
1148             }
1149
1150             _ => {
1151                 if !self.tcx.sess.has_errors() {
1152                     let mut err = struct_span_err!(self.tcx.sess,
1153                                                    obligation.cause.span, E0284,
1154                                                    "type annotations required: \
1155                                                     cannot resolve `{}`",
1156                                                    predicate);
1157                     self.note_obligation_cause(&mut err, obligation);
1158                     err.emit();
1159                 }
1160             }
1161         }
1162     }
1163
1164     /// Returns whether the trait predicate may apply for *some* assignment
1165     /// to the type parameters.
1166     fn predicate_can_apply(&self,
1167                            param_env: ty::ParamEnv<'tcx>,
1168                            pred: ty::PolyTraitRef<'tcx>)
1169                            -> bool {
1170         struct ParamToVarFolder<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
1171             infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
1172             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>
1173         }
1174
1175         impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for ParamToVarFolder<'a, 'gcx, 'tcx> {
1176             fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> { self.infcx.tcx }
1177
1178             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
1179                 if let ty::TyParam(ty::ParamTy {name, ..}) = ty.sty {
1180                     let infcx = self.infcx;
1181                     self.var_map.entry(ty).or_insert_with(||
1182                         infcx.next_ty_var(
1183                             TypeVariableOrigin::TypeParameterDefinition(DUMMY_SP, name)))
1184                 } else {
1185                     ty.super_fold_with(self)
1186                 }
1187             }
1188         }
1189
1190         self.probe(|_| {
1191             let mut selcx = SelectionContext::new(self);
1192
1193             let cleaned_pred = pred.fold_with(&mut ParamToVarFolder {
1194                 infcx: self,
1195                 var_map: FxHashMap()
1196             });
1197
1198             let cleaned_pred = super::project::normalize(
1199                 &mut selcx,
1200                 param_env,
1201                 ObligationCause::dummy(),
1202                 &cleaned_pred
1203             ).value;
1204
1205             let obligation = Obligation::new(
1206                 ObligationCause::dummy(),
1207                 param_env,
1208                 cleaned_pred.to_predicate()
1209             );
1210
1211             selcx.evaluate_obligation(&obligation)
1212         })
1213     }
1214
1215     fn note_obligation_cause<T>(&self,
1216                                 err: &mut DiagnosticBuilder,
1217                                 obligation: &Obligation<'tcx, T>)
1218         where T: fmt::Display
1219     {
1220         self.note_obligation_cause_code(err,
1221                                         &obligation.predicate,
1222                                         &obligation.cause.code);
1223     }
1224
1225     fn note_obligation_cause_code<T>(&self,
1226                                      err: &mut DiagnosticBuilder,
1227                                      predicate: &T,
1228                                      cause_code: &ObligationCauseCode<'tcx>)
1229         where T: fmt::Display
1230     {
1231         let tcx = self.tcx;
1232         match *cause_code {
1233             ObligationCauseCode::ExprAssignable |
1234             ObligationCauseCode::MatchExpressionArm { .. } |
1235             ObligationCauseCode::IfExpression |
1236             ObligationCauseCode::IfExpressionWithNoElse |
1237             ObligationCauseCode::EquatePredicate |
1238             ObligationCauseCode::MainFunctionType |
1239             ObligationCauseCode::StartFunctionType |
1240             ObligationCauseCode::IntrinsicType |
1241             ObligationCauseCode::MethodReceiver |
1242             ObligationCauseCode::ReturnNoExpression |
1243             ObligationCauseCode::MiscObligation => {
1244             }
1245             ObligationCauseCode::SliceOrArrayElem => {
1246                 err.note("slice and array elements must have `Sized` type");
1247             }
1248             ObligationCauseCode::TupleElem => {
1249                 err.note("only the last element of a tuple may have a dynamically sized type");
1250             }
1251             ObligationCauseCode::ProjectionWf(data) => {
1252                 err.note(&format!("required so that the projection `{}` is well-formed",
1253                                   data));
1254             }
1255             ObligationCauseCode::ReferenceOutlivesReferent(ref_ty) => {
1256                 err.note(&format!("required so that reference `{}` does not outlive its referent",
1257                                   ref_ty));
1258             }
1259             ObligationCauseCode::ObjectTypeBound(object_ty, region) => {
1260                 err.note(&format!("required so that the lifetime bound of `{}` for `{}` \
1261                                    is satisfied",
1262                                   region, object_ty));
1263             }
1264             ObligationCauseCode::ItemObligation(item_def_id) => {
1265                 let item_name = tcx.item_path_str(item_def_id);
1266                 let msg = format!("required by `{}`", item_name);
1267                 if let Some(sp) = tcx.hir.span_if_local(item_def_id) {
1268                     let sp = tcx.sess.codemap().def_span(sp);
1269                     err.span_note(sp, &msg);
1270                 } else {
1271                     err.note(&msg);
1272                 }
1273             }
1274             ObligationCauseCode::ObjectCastObligation(object_ty) => {
1275                 err.note(&format!("required for the cast to the object type `{}`",
1276                                   self.ty_to_string(object_ty)));
1277             }
1278             ObligationCauseCode::RepeatVec => {
1279                 err.note("the `Copy` trait is required because the \
1280                           repeated element will be copied");
1281             }
1282             ObligationCauseCode::VariableType(_) => {
1283                 err.note("all local variables must have a statically known size");
1284             }
1285             ObligationCauseCode::SizedReturnType => {
1286                 err.note("the return type of a function must have a \
1287                           statically known size");
1288             }
1289             ObligationCauseCode::SizedYieldType => {
1290                 err.note("the yield type of a generator must have a \
1291                           statically known size");
1292             }
1293             ObligationCauseCode::AssignmentLhsSized => {
1294                 err.note("the left-hand-side of an assignment must have a statically known size");
1295             }
1296             ObligationCauseCode::TupleInitializerSized => {
1297                 err.note("tuples must have a statically known size to be initialized");
1298             }
1299             ObligationCauseCode::StructInitializerSized => {
1300                 err.note("structs must have a statically known size to be initialized");
1301             }
1302             ObligationCauseCode::FieldSized(ref item) => {
1303                 match *item {
1304                     AdtKind::Struct => {
1305                         err.note("only the last field of a struct may have a dynamically \
1306                                   sized type");
1307                     }
1308                     AdtKind::Union => {
1309                         err.note("no field of a union may have a dynamically sized type");
1310                     }
1311                     AdtKind::Enum => {
1312                         err.note("no field of an enum variant may have a dynamically sized type");
1313                     }
1314                 }
1315             }
1316             ObligationCauseCode::ConstSized => {
1317                 err.note("constant expressions must have a statically known size");
1318             }
1319             ObligationCauseCode::SharedStatic => {
1320                 err.note("shared static variables must have a type that implements `Sync`");
1321             }
1322             ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
1323                 let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1324                 err.note(&format!("required because it appears within the type `{}`",
1325                                   parent_trait_ref.0.self_ty()));
1326                 let parent_predicate = parent_trait_ref.to_predicate();
1327                 self.note_obligation_cause_code(err,
1328                                                 &parent_predicate,
1329                                                 &data.parent_code);
1330             }
1331             ObligationCauseCode::ImplDerivedObligation(ref data) => {
1332                 let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1333                 err.note(
1334                     &format!("required because of the requirements on the impl of `{}` for `{}`",
1335                              parent_trait_ref,
1336                              parent_trait_ref.0.self_ty()));
1337                 let parent_predicate = parent_trait_ref.to_predicate();
1338                 self.note_obligation_cause_code(err,
1339                                                 &parent_predicate,
1340                                                 &data.parent_code);
1341             }
1342             ObligationCauseCode::CompareImplMethodObligation { .. } => {
1343                 err.note(
1344                     &format!("the requirement `{}` appears on the impl method \
1345                               but not on the corresponding trait method",
1346                              predicate));
1347             }
1348             ObligationCauseCode::ReturnType(_) |
1349             ObligationCauseCode::BlockTailExpression(_) => (),
1350         }
1351     }
1352
1353     fn suggest_new_overflow_limit(&self, err: &mut DiagnosticBuilder) {
1354         let current_limit = self.tcx.sess.recursion_limit.get();
1355         let suggested_limit = current_limit * 2;
1356         err.help(&format!("consider adding a `#![recursion_limit=\"{}\"]` attribute to your crate",
1357                           suggested_limit));
1358     }
1359 }
1360
1361 enum ArgKind {
1362     Arg(String, String),
1363     Tuple(Span, Vec<(String, String)>),
1364 }
1365
1366 impl ArgKind {
1367     fn empty() -> ArgKind {
1368         ArgKind::Arg("_".to_owned(), "_".to_owned())
1369     }
1370 }