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                                 Some(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| {
 768                             let (found_span, found) = self.get_fn_like_arguments(node);
 769                             (Some(found_span), found)
 770                         }).unwrap_or((found_span, found));
 771
 772                     self.report_arg_count_mismatch(span,
 773                                                    closure_span,
 774                                                    expected,
 775                                                    found,
 776                                                    found_trait_ty.is_closure())
 777                 }
 778             }
 779
 780             TraitNotObjectSafe(did) => {
 781                 let violations = self.tcx.object_safety_violations(did);
 782                 self.tcx.report_object_safety_error(span, did,
 783                                                     violations)
 784             }
 785
 786             ConstEvalFailure(ref err) => {
 787                 if let const_val::ErrKind::TypeckError = err.kind {
 788                     return;
 789                 }
 790                 err.struct_error(self.tcx, span, "constant expression")
 791             }
 792         };
 793         self.note_obligation_cause(&mut err, obligation);
 794         err.emit();
 795     }
 796
 797     /// When encountering an assignment of an unsized trait, like `let x = ""[..];`, provide a
 798     /// suggestion to borrow the initializer in order to use have a slice instead.
 799     fn suggest_borrow_on_unsized_slice(&self,
 800                                        code: &ObligationCauseCode<'tcx>,
 801                                        err: &mut DiagnosticBuilder<'tcx>) {
 802         if let &ObligationCauseCode::VariableType(node_id) = code {
 803             let parent_node = self.tcx.hir.get_parent_node(node_id);
 804             if let Some(hir::map::NodeLocal(ref local)) = self.tcx.hir.find(parent_node) {
 805                 if let Some(ref expr) = local.init {
 806                     if let hir::ExprIndex(_, _) = expr.node {
 807                         if let Ok(snippet) = self.tcx.sess.codemap().span_to_snippet(expr.span) {
 808                             err.span_suggestion(expr.span,
 809                                                 "consider borrowing here",
 810                                                 format!("&{}", snippet));
 811                         }
 812                     }
 813                 }
 814             }
 815         }
 816     }
 817
 818     /// Given some node representing a fn-like thing in the HIR map,
 819     /// returns a span and `ArgKind` information that describes the
 820     /// arguments it expects. This can be supplied to
 821     /// `report_arg_count_mismatch`.
 822     pub fn get_fn_like_arguments(&self, node: hir::map::Node) -> (Span, Vec<ArgKind>) {
 823         match node {
 824             hir::map::NodeExpr(&hir::Expr {
 825                 node: hir::ExprClosure(_, ref _decl, id, span, _),
 826                 ..
 827             }) => {
 828                 (self.tcx.sess.codemap().def_span(span), self.tcx.hir.body(id).arguments.iter()
 829                     .map(|arg| {
 830                         if let hir::Pat {
 831                             node: hir::PatKind::Tuple(args, _),
 832                             span,
 833                             ..
 834                         } = arg.pat.clone().into_inner() {
 835                             ArgKind::Tuple(
 836                                 Some(span),
 837                                 args.iter().map(|pat| {
 838                                     let snippet = self.tcx.sess.codemap()
 839                                         .span_to_snippet(pat.span).unwrap();
 840                                     (snippet, "_".to_owned())
 841                                 }).collect::<Vec<_>>(),
 842                             )
 843                         } else {
 844                             let name = self.tcx.sess.codemap()
 845                                 .span_to_snippet(arg.pat.span).unwrap();
 846                             ArgKind::Arg(name, "_".to_owned())
 847                         }
 848                     })
 849                     .collect::<Vec<ArgKind>>())
 850             }
 851             hir::map::NodeItem(&hir::Item {
 852                 span,
 853                 node: hir::ItemFn(ref decl, ..),
 854                 ..
 855             }) |
 856             hir::map::NodeImplItem(&hir::ImplItem {
 857                 span,
 858                 node: hir::ImplItemKind::Method(hir::MethodSig { ref decl, .. }, _),
 859                 ..
 860             }) |
 861             hir::map::NodeTraitItem(&hir::TraitItem {
 862                 span,
 863                 node: hir::TraitItemKind::Method(hir::MethodSig { ref decl, .. }, _),
 864                 ..
 865             }) => {
 866                 (self.tcx.sess.codemap().def_span(span), decl.inputs.iter()
 867                         .map(|arg| match arg.clone().into_inner().node {
 868                     hir::TyTup(ref tys) => ArgKind::Tuple(
 869                         Some(arg.span),
 870                         tys.iter()
 871                             .map(|_| ("_".to_owned(), "_".to_owned()))
 872                             .collect::<Vec<_>>(),
 873                     ),
 874                     _ => ArgKind::Arg("_".to_owned(), "_".to_owned())
 875                 }).collect::<Vec<ArgKind>>())
 876             }
 877             hir::map::NodeVariant(&hir::Variant {
 878                 span,
 879                 node: hir::Variant_ {
 880                     data: hir::VariantData::Tuple(ref fields, _),
 881                     ..
 882                 },
 883                 ..
 884             }) => {
 885                 (self.tcx.sess.codemap().def_span(span),
 886                  fields.iter().map(|field| {
 887                      ArgKind::Arg(format!("{}", field.name), "_".to_string())
 888                  }).collect::<Vec<_>>())
 889             }
 890             _ => panic!("non-FnLike node found: {:?}", node),
 891         }
 892     }
 893
 894     /// Reports an error when the number of arguments needed by a
 895     /// trait match doesn't match the number that the expression
 896     /// provides.
 897     pub fn report_arg_count_mismatch(
 898         &self,
 899         span: Span,
 900         found_span: Option<Span>,
 901         expected_args: Vec<ArgKind>,
 902         found_args: Vec<ArgKind>,
 903         is_closure: bool,
 904     ) -> DiagnosticBuilder<'tcx> {
 905         let kind = if is_closure { "closure" } else { "function" };
 906
 907         let args_str = |arguments: &Vec<ArgKind>, other: &Vec<ArgKind>| {
 908             let arg_length = arguments.len();
 909             let distinct = match &other[..] {
 910                 &[ArgKind::Tuple(..)] => true,
 911                 _ => false,
 912             };
 913             match (arg_length, arguments.get(0)) {
 914                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
 915                     format!("a single {}-tuple as argument", fields.len())
 916                 }
 917                 _ => format!("{} {}argument{}",
 918                              arg_length,
 919                              if distinct && arg_length > 1 { "distinct " } else { "" },
 920                              if arg_length == 1 { "" } else { "s" }),
 921             }
 922         };
 923
 924         let expected_str = args_str(&expected_args, &found_args);
 925         let found_str = args_str(&found_args, &expected_args);
 926
 927         let mut err = struct_span_err!(
 928             self.tcx.sess,
 929             span,
 930             E0593,
 931             "{} is expected to take {}, but it takes {}",
 932             kind,
 933             expected_str,
 934             found_str,
 935         );
 936
 937         err.span_label(span, format!( "expected {} that takes {}", kind, expected_str));
 938
 939         if let Some(found_span) = found_span {
 940             err.span_label(found_span, format!("takes {}", found_str));
 941
 942             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
 943                 if fields.len() == expected_args.len() {
 944                     let sugg = fields.iter()
 945                         .map(|(name, _)| name.to_owned())
 946                         .collect::<Vec<String>>().join(", ");
 947                     err.span_suggestion(found_span,
 948                                         "change the closure to take multiple arguments instead of \
 949                                          a single tuple",
 950                                         format!("|{}|", sugg));
 951                 }
 952             }
 953             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..] {
 954                 if fields.len() == found_args.len() && is_closure {
 955                     let sugg = format!(
 956                         "|({}){}|",
 957                         found_args.iter()
 958                             .map(|arg| match arg {
 959                                 ArgKind::Arg(name, _) => name.to_owned(),
 960                                 _ => "_".to_owned(),
 961                             })
 962                             .collect::<Vec<String>>()
 963                             .join(", "),
 964                         // add type annotations if available
 965                         if found_args.iter().any(|arg| match arg {
 966                             ArgKind::Arg(_, ty) => ty != "_",
 967                             _ => false,
 968                         }) {
 969                             format!(": ({})",
 970                                     fields.iter()
 971                                         .map(|(_, ty)| ty.to_owned())
 972                                         .collect::<Vec<String>>()
 973                                         .join(", "))
 974                         } else {
 975                             "".to_owned()
 976                         },
 977                     );
 978                     err.span_suggestion(found_span,
 979                                         "change the closure to accept a tuple instead of \
 980                                          individual arguments",
 981                                         sugg);
 982                 }
 983             }
 984         }
 985
 986         err
 987     }
 988
 989     fn report_closure_arg_mismatch(&self,
 990                            span: Span,
 991                            found_span: Option<Span>,
 992                            expected_ref: ty::PolyTraitRef<'tcx>,
 993                            found: ty::PolyTraitRef<'tcx>)
 994         -> DiagnosticBuilder<'tcx>
 995     {
 996         fn build_fn_sig_string<'a, 'gcx, 'tcx>(tcx: ty::TyCtxt<'a, 'gcx, 'tcx>,
 997                                                trait_ref: &ty::TraitRef<'tcx>) -> String {
 998             let inputs = trait_ref.substs.type_at(1);
 999             let sig = if let ty::TyTuple(inputs, _) = inputs.sty {
1000                 tcx.mk_fn_sig(
1001                     inputs.iter().map(|&x| x),
1002                     tcx.mk_infer(ty::TyVar(ty::TyVid { index: 0 })),
1003                     false,
1004                     hir::Unsafety::Normal,
1005                     ::syntax::abi::Abi::Rust
1006                 )
1007             } else {
1008                 tcx.mk_fn_sig(
1009                     ::std::iter::once(inputs),
1010                     tcx.mk_infer(ty::TyVar(ty::TyVid { index: 0 })),
1011                     false,
1012                     hir::Unsafety::Normal,
1013                     ::syntax::abi::Abi::Rust
1014                 )
1015             };
1016             format!("{}", ty::Binder(sig))
1017         }
1018
1019         let argument_is_closure = expected_ref.skip_binder().substs.type_at(0).is_closure();
1020         let mut err = struct_span_err!(self.tcx.sess, span, E0631,
1021                                        "type mismatch in {} arguments",
1022                                        if argument_is_closure { "closure" } else { "function" });
1023
1024         let found_str = format!(
1025             "expected signature of `{}`",
1026             build_fn_sig_string(self.tcx, found.skip_binder())
1027         );
1028         err.span_label(span, found_str);
1029
1030         let found_span = found_span.unwrap_or(span);
1031         let expected_str = format!(
1032             "found signature of `{}`",
1033             build_fn_sig_string(self.tcx, expected_ref.skip_binder())
1034         );
1035         err.span_label(found_span, expected_str);
1036
1037         err
1038     }
1039 }
1040
1041 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
1042     pub fn recursive_type_with_infinite_size_error(self,
1043                                                    type_def_id: DefId)
1044                                                    -> DiagnosticBuilder<'tcx>
1045     {
1046         assert!(type_def_id.is_local());
1047         let span = self.hir.span_if_local(type_def_id).unwrap();
1048         let span = self.sess.codemap().def_span(span);
1049         let mut err = struct_span_err!(self.sess, span, E0072,
1050                                        "recursive type `{}` has infinite size",
1051                                        self.item_path_str(type_def_id));
1052         err.span_label(span, "recursive type has infinite size");
1053         err.help(&format!("insert indirection (e.g., a `Box`, `Rc`, or `&`) \
1054                            at some point to make `{}` representable",
1055                           self.item_path_str(type_def_id)));
1056         err
1057     }
1058
1059     pub fn report_object_safety_error(self,
1060                                       span: Span,
1061                                       trait_def_id: DefId,
1062                                       violations: Vec<ObjectSafetyViolation>)
1063                                       -> DiagnosticBuilder<'tcx>
1064     {
1065         let trait_str = self.item_path_str(trait_def_id);
1066         let span = self.sess.codemap().def_span(span);
1067         let mut err = struct_span_err!(
1068             self.sess, span, E0038,
1069             "the trait `{}` cannot be made into an object",
1070             trait_str);
1071         err.span_label(span, format!("the trait `{}` cannot be made into an object", trait_str));
1072
1073         let mut reported_violations = FxHashSet();
1074         for violation in violations {
1075             if !reported_violations.insert(violation.clone()) {
1076                 continue;
1077             }
1078             err.note(&violation.error_msg());
1079         }
1080         err
1081     }
1082 }
1083
1084 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
1085     fn maybe_report_ambiguity(&self, obligation: &PredicateObligation<'tcx>,
1086                               body_id: Option<hir::BodyId>) {
1087         // Unable to successfully determine, probably means
1088         // insufficient type information, but could mean
1089         // ambiguous impls. The latter *ought* to be a
1090         // coherence violation, so we don't report it here.
1091
1092         let predicate = self.resolve_type_vars_if_possible(&obligation.predicate);
1093         let span = obligation.cause.span;
1094
1095         debug!("maybe_report_ambiguity(predicate={:?}, obligation={:?})",
1096                predicate,
1097                obligation);
1098
1099         // Ambiguity errors are often caused as fallout from earlier
1100         // errors. So just ignore them if this infcx is tainted.
1101         if self.is_tainted_by_errors() {
1102             return;
1103         }
1104
1105         match predicate {
1106             ty::Predicate::Trait(ref data) => {
1107                 let trait_ref = data.to_poly_trait_ref();
1108                 let self_ty = trait_ref.self_ty();
1109                 if predicate.references_error() {
1110                     return;
1111                 }
1112                 // Typically, this ambiguity should only happen if
1113                 // there are unresolved type inference variables
1114                 // (otherwise it would suggest a coherence
1115                 // failure). But given #21974 that is not necessarily
1116                 // the case -- we can have multiple where clauses that
1117                 // are only distinguished by a region, which results
1118                 // in an ambiguity even when all types are fully
1119                 // known, since we don't dispatch based on region
1120                 // relationships.
1121
1122                 // This is kind of a hack: it frequently happens that some earlier
1123                 // error prevents types from being fully inferred, and then we get
1124                 // a bunch of uninteresting errors saying something like "<generic
1125                 // #0> doesn't implement Sized".  It may even be true that we
1126                 // could just skip over all checks where the self-ty is an
1127                 // inference variable, but I was afraid that there might be an
1128                 // inference variable created, registered as an obligation, and
1129                 // then never forced by writeback, and hence by skipping here we'd
1130                 // be ignoring the fact that we don't KNOW the type works
1131                 // out. Though even that would probably be harmless, given that
1132                 // we're only talking about builtin traits, which are known to be
1133                 // inhabited. But in any case I just threw in this check for
1134                 // has_errors() to be sure that compilation isn't happening
1135                 // anyway. In that case, why inundate the user.
1136                 if !self.tcx.sess.has_errors() {
1137                     if
1138                         self.tcx.lang_items().sized_trait()
1139                         .map_or(false, |sized_id| sized_id == trait_ref.def_id())
1140                     {
1141                         self.need_type_info(body_id, span, self_ty);
1142                     } else {
1143                         let mut err = struct_span_err!(self.tcx.sess,
1144                                                         span, E0283,
1145                                                         "type annotations required: \
1146                                                         cannot resolve `{}`",
1147                                                         predicate);
1148                         self.note_obligation_cause(&mut err, obligation);
1149                         err.emit();
1150                     }
1151                 }
1152             }
1153
1154             ty::Predicate::WellFormed(ty) => {
1155                 // Same hacky approach as above to avoid deluging user
1156                 // with error messages.
1157                 if !ty.references_error() && !self.tcx.sess.has_errors() {
1158                     self.need_type_info(body_id, span, ty);
1159                 }
1160             }
1161
1162             ty::Predicate::Subtype(ref data) => {
1163                 if data.references_error() || self.tcx.sess.has_errors() {
1164                     // no need to overload user in such cases
1165                 } else {
1166                     let &SubtypePredicate { a_is_expected: _, a, b } = data.skip_binder();
1167                     // both must be type variables, or the other would've been instantiated
1168                     assert!(a.is_ty_var() && b.is_ty_var());
1169                     self.need_type_info(body_id,
1170                                         obligation.cause.span,
1171                                         a);
1172                 }
1173             }
1174
1175             _ => {
1176                 if !self.tcx.sess.has_errors() {
1177                     let mut err = struct_span_err!(self.tcx.sess,
1178                                                    obligation.cause.span, E0284,
1179                                                    "type annotations required: \
1180                                                     cannot resolve `{}`",
1181                                                    predicate);
1182                     self.note_obligation_cause(&mut err, obligation);
1183                     err.emit();
1184                 }
1185             }
1186         }
1187     }
1188
1189     /// Returns whether the trait predicate may apply for *some* assignment
1190     /// to the type parameters.
1191     fn predicate_can_apply(&self,
1192                            param_env: ty::ParamEnv<'tcx>,
1193                            pred: ty::PolyTraitRef<'tcx>)
1194                            -> bool {
1195         struct ParamToVarFolder<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
1196             infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
1197             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>
1198         }
1199
1200         impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for ParamToVarFolder<'a, 'gcx, 'tcx> {
1201             fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> { self.infcx.tcx }
1202
1203             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
1204                 if let ty::TyParam(ty::ParamTy {name, ..}) = ty.sty {
1205                     let infcx = self.infcx;
1206                     self.var_map.entry(ty).or_insert_with(||
1207                         infcx.next_ty_var(
1208                             TypeVariableOrigin::TypeParameterDefinition(DUMMY_SP, name)))
1209                 } else {
1210                     ty.super_fold_with(self)
1211                 }
1212             }
1213         }
1214
1215         self.probe(|_| {
1216             let mut selcx = SelectionContext::new(self);
1217
1218             let cleaned_pred = pred.fold_with(&mut ParamToVarFolder {
1219                 infcx: self,
1220                 var_map: FxHashMap()
1221             });
1222
1223             let cleaned_pred = super::project::normalize(
1224                 &mut selcx,
1225                 param_env,
1226                 ObligationCause::dummy(),
1227                 &cleaned_pred
1228             ).value;
1229
1230             let obligation = Obligation::new(
1231                 ObligationCause::dummy(),
1232                 param_env,
1233                 cleaned_pred.to_predicate()
1234             );
1235
1236             selcx.evaluate_obligation(&obligation)
1237         })
1238     }
1239
1240     fn note_obligation_cause<T>(&self,
1241                                 err: &mut DiagnosticBuilder,
1242                                 obligation: &Obligation<'tcx, T>)
1243         where T: fmt::Display
1244     {
1245         self.note_obligation_cause_code(err,
1246                                         &obligation.predicate,
1247                                         &obligation.cause.code,
1248                                         &mut vec![]);
1249     }
1250
1251     fn note_obligation_cause_code<T>(&self,
1252                                      err: &mut DiagnosticBuilder,
1253                                      predicate: &T,
1254                                      cause_code: &ObligationCauseCode<'tcx>,
1255                                      obligated_types: &mut Vec<&ty::TyS<'tcx>>)
1256         where T: fmt::Display
1257     {
1258         let tcx = self.tcx;
1259         match *cause_code {
1260             ObligationCauseCode::ExprAssignable |
1261             ObligationCauseCode::MatchExpressionArm { .. } |
1262             ObligationCauseCode::IfExpression |
1263             ObligationCauseCode::IfExpressionWithNoElse |
1264             ObligationCauseCode::EquatePredicate |
1265             ObligationCauseCode::MainFunctionType |
1266             ObligationCauseCode::StartFunctionType |
1267             ObligationCauseCode::IntrinsicType |
1268             ObligationCauseCode::MethodReceiver |
1269             ObligationCauseCode::ReturnNoExpression |
1270             ObligationCauseCode::MiscObligation => {
1271             }
1272             ObligationCauseCode::SliceOrArrayElem => {
1273                 err.note("slice and array elements must have `Sized` type");
1274             }
1275             ObligationCauseCode::TupleElem => {
1276                 err.note("only the last element of a tuple may have a dynamically sized type");
1277             }
1278             ObligationCauseCode::ProjectionWf(data) => {
1279                 err.note(&format!("required so that the projection `{}` is well-formed",
1280                                   data));
1281             }
1282             ObligationCauseCode::ReferenceOutlivesReferent(ref_ty) => {
1283                 err.note(&format!("required so that reference `{}` does not outlive its referent",
1284                                   ref_ty));
1285             }
1286             ObligationCauseCode::ObjectTypeBound(object_ty, region) => {
1287                 err.note(&format!("required so that the lifetime bound of `{}` for `{}` \
1288                                    is satisfied",
1289                                   region, object_ty));
1290             }
1291             ObligationCauseCode::ItemObligation(item_def_id) => {
1292                 let item_name = tcx.item_path_str(item_def_id);
1293                 let msg = format!("required by `{}`", item_name);
1294                 if let Some(sp) = tcx.hir.span_if_local(item_def_id) {
1295                     let sp = tcx.sess.codemap().def_span(sp);
1296                     err.span_note(sp, &msg);
1297                 } else {
1298                     err.note(&msg);
1299                 }
1300             }
1301             ObligationCauseCode::ObjectCastObligation(object_ty) => {
1302                 err.note(&format!("required for the cast to the object type `{}`",
1303                                   self.ty_to_string(object_ty)));
1304             }
1305             ObligationCauseCode::RepeatVec => {
1306                 err.note("the `Copy` trait is required because the \
1307                           repeated element will be copied");
1308             }
1309             ObligationCauseCode::VariableType(_) => {
1310                 err.note("all local variables must have a statically known size");
1311             }
1312             ObligationCauseCode::SizedReturnType => {
1313                 err.note("the return type of a function must have a \
1314                           statically known size");
1315             }
1316             ObligationCauseCode::SizedYieldType => {
1317                 err.note("the yield type of a generator must have a \
1318                           statically known size");
1319             }
1320             ObligationCauseCode::AssignmentLhsSized => {
1321                 err.note("the left-hand-side of an assignment must have a statically known size");
1322             }
1323             ObligationCauseCode::TupleInitializerSized => {
1324                 err.note("tuples must have a statically known size to be initialized");
1325             }
1326             ObligationCauseCode::StructInitializerSized => {
1327                 err.note("structs must have a statically known size to be initialized");
1328             }
1329             ObligationCauseCode::FieldSized(ref item) => {
1330                 match *item {
1331                     AdtKind::Struct => {
1332                         err.note("only the last field of a struct may have a dynamically \
1333                                   sized type");
1334                     }
1335                     AdtKind::Union => {
1336                         err.note("no field of a union may have a dynamically sized type");
1337                     }
1338                     AdtKind::Enum => {
1339                         err.note("no field of an enum variant may have a dynamically sized type");
1340                     }
1341                 }
1342             }
1343             ObligationCauseCode::ConstSized => {
1344                 err.note("constant expressions must have a statically known size");
1345             }
1346             ObligationCauseCode::SharedStatic => {
1347                 err.note("shared static variables must have a type that implements `Sync`");
1348             }
1349             ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
1350                 let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1351                 let ty = parent_trait_ref.0.self_ty();
1352                 err.note(&format!("required because it appears within the type `{}`", ty));
1353                 obligated_types.push(ty);
1354
1355                 let parent_predicate = parent_trait_ref.to_predicate();
1356                 if !self.is_recursive_obligation(obligated_types, &data.parent_code) {
1357                     self.note_obligation_cause_code(err,
1358                                                     &parent_predicate,
1359                                                     &data.parent_code,
1360                                                     obligated_types);
1361                 }
1362             }
1363             ObligationCauseCode::ImplDerivedObligation(ref data) => {
1364                 let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1365                 err.note(
1366                     &format!("required because of the requirements on the impl of `{}` for `{}`",
1367                              parent_trait_ref,
1368                              parent_trait_ref.0.self_ty()));
1369                 let parent_predicate = parent_trait_ref.to_predicate();
1370                 self.note_obligation_cause_code(err,
1371                                             &parent_predicate,
1372                                             &data.parent_code,
1373                                             obligated_types);
1374             }
1375             ObligationCauseCode::CompareImplMethodObligation { .. } => {
1376                 err.note(
1377                     &format!("the requirement `{}` appears on the impl method \
1378                               but not on the corresponding trait method",
1379                              predicate));
1380             }
1381             ObligationCauseCode::ReturnType(_) |
1382             ObligationCauseCode::BlockTailExpression(_) => (),
1383         }
1384     }
1385
1386     fn suggest_new_overflow_limit(&self, err: &mut DiagnosticBuilder) {
1387         let current_limit = self.tcx.sess.recursion_limit.get();
1388         let suggested_limit = current_limit * 2;
1389         err.help(&format!("consider adding a `#![recursion_limit=\"{}\"]` attribute to your crate",
1390                           suggested_limit));
1391     }
1392
1393     fn is_recursive_obligation(&self,
1394                                    obligated_types: &mut Vec<&ty::TyS<'tcx>>,
1395                                    cause_code: &ObligationCauseCode<'tcx>) -> bool {
1396         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
1397             let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1398             for obligated_type in obligated_types {
1399                 if obligated_type == &parent_trait_ref.0.self_ty() {
1400                     return true;
1401                 }
1402             }
1403         }
1404         return false;
1405     }
1406 }
1407
1408 /// Summarizes information
1409 pub enum ArgKind {
1410     /// An argument of non-tuple type. Parameters are (name, ty)
1411     Arg(String, String),
1412
1413     /// An argument of tuple type. For a "found" argument, the span is
1414     /// the locationo in the source of the pattern. For a "expected"
1415     /// argument, it will be None. The vector is a list of (name, ty)
1416     /// strings for the components of the tuple.
1417     Tuple(Option<Span>, Vec<(String, String)>),
1418 }
1419
1420 impl ArgKind {
1421     fn empty() -> ArgKind {
1422         ArgKind::Arg("_".to_owned(), "_".to_owned())
1423     }
1424
1425     /// Creates an `ArgKind` from the expected type of an
1426     /// argument. This has no name (`_`) and no source spans..
1427     pub fn from_expected_ty(t: Ty<'_>) -> ArgKind {
1428         match t.sty {
1429             ty::TyTuple(ref tys, _) => ArgKind::Tuple(
1430                 None,
1431                 tys.iter()
1432                    .map(|ty| ("_".to_owned(), format!("{}", ty.sty)))
1433                    .collect::<Vec<_>>()
1434             ),
1435             _ => ArgKind::Arg("_".to_owned(), format!("{}", t.sty)),
1436         }
1437     }
1438 }