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(param_env.universe,
 296                                                              obligation.cause.span,
 297                                                              def_id);
 298                 let impl_trait_ref = tcx
 299                     .impl_trait_ref(def_id)
 300                     .unwrap()
 301                     .subst(tcx, impl_substs);
 302
 303                 let impl_self_ty = impl_trait_ref.self_ty();
 304
 305                 if let Ok(..) = self.can_eq(param_env, trait_self_ty, impl_self_ty) {
 306                     self_match_impls.push(def_id);
 307
 308                     if trait_ref.substs.types().skip(1)
 309                         .zip(impl_trait_ref.substs.types().skip(1))
 310                         .all(|(u,v)| self.fuzzy_match_tys(u, v))
 311                     {
 312                         fuzzy_match_impls.push(def_id);
 313                     }
 314                 }
 315             });
 316
 317         let impl_def_id = if self_match_impls.len() == 1 {
 318             self_match_impls[0]
 319         } else if fuzzy_match_impls.len() == 1 {
 320             fuzzy_match_impls[0]
 321         } else {
 322             return None
 323         };
 324
 325         if tcx.has_attr(impl_def_id, "rustc_on_unimplemented") {
 326             Some(impl_def_id)
 327         } else {
 328             None
 329         }
 330     }
 331
 332     fn on_unimplemented_note(
 333         &self,
 334         trait_ref: ty::PolyTraitRef<'tcx>,
 335         obligation: &PredicateObligation<'tcx>) ->
 336         OnUnimplementedNote
 337     {
 338         let def_id = self.impl_similar_to(trait_ref, obligation)
 339             .unwrap_or(trait_ref.def_id());
 340         let trait_ref = *trait_ref.skip_binder();
 341
 342         let desugaring;
 343         let method;
 344         let mut flags = vec![];
 345         let direct = match obligation.cause.code {
 346             ObligationCauseCode::BuiltinDerivedObligation(..) |
 347             ObligationCauseCode::ImplDerivedObligation(..) => false,
 348             _ => true
 349         };
 350         if direct {
 351             // this is a "direct", user-specified, rather than derived,
 352             // obligation.
 353             flags.push(("direct".to_string(), None));
 354         }
 355
 356         if let ObligationCauseCode::ItemObligation(item) = obligation.cause.code {
 357             // FIXME: maybe also have some way of handling methods
 358             // from other traits? That would require name resolution,
 359             // which we might want to be some sort of hygienic.
 360             //
 361             // Currently I'm leaving it for what I need for `try`.
 362             if self.tcx.trait_of_item(item) == Some(trait_ref.def_id) {
 363                 method = self.tcx.item_name(item);
 364                 flags.push(("from_method".to_string(), None));
 365                 flags.push(("from_method".to_string(), Some(method.to_string())));
 366             }
 367         }
 368
 369         if let Some(k) = obligation.cause.span.compiler_desugaring_kind() {
 370             desugaring = k.as_symbol().as_str();
 371             flags.push(("from_desugaring".to_string(), None));
 372             flags.push(("from_desugaring".to_string(), Some(desugaring.to_string())));
 373         }
 374         let generics = self.tcx.generics_of(def_id);
 375         let self_ty = trait_ref.self_ty();
 376         let self_ty_str = self_ty.to_string();
 377         flags.push(("_Self".to_string(), Some(self_ty_str.clone())));
 378
 379         for param in generics.types.iter() {
 380             let name = param.name.as_str().to_string();
 381             let ty = trait_ref.substs.type_for_def(param);
 382             let ty_str = ty.to_string();
 383             flags.push((name.clone(),
 384                         Some(ty_str.clone())));
 385         }
 386
 387         if let Some(true) = self_ty.ty_to_def_id().map(|def_id| def_id.is_local()) {
 388             flags.push(("crate_local".to_string(), None));
 389         }
 390
 391         if let Ok(Some(command)) = OnUnimplementedDirective::of_item(
 392             self.tcx, trait_ref.def_id, def_id
 393         ) {
 394             command.evaluate(self.tcx, trait_ref, &flags[..])
 395         } else {
 396             OnUnimplementedNote::empty()
 397         }
 398     }
 399
 400     fn find_similar_impl_candidates(&self,
 401                                     trait_ref: ty::PolyTraitRef<'tcx>)
 402                                     -> Vec<ty::TraitRef<'tcx>>
 403     {
 404         let simp = fast_reject::simplify_type(self.tcx,
 405                                               trait_ref.skip_binder().self_ty(),
 406                                               true);
 407         let mut impl_candidates = Vec::new();
 408
 409         match simp {
 410             Some(simp) => self.tcx.for_each_impl(trait_ref.def_id(), |def_id| {
 411                 let imp = self.tcx.impl_trait_ref(def_id).unwrap();
 412                 let imp_simp = fast_reject::simplify_type(self.tcx,
 413                                                           imp.self_ty(),
 414                                                           true);
 415                 if let Some(imp_simp) = imp_simp {
 416                     if simp != imp_simp {
 417                         return;
 418                     }
 419                 }
 420                 impl_candidates.push(imp);
 421             }),
 422             None => self.tcx.for_each_impl(trait_ref.def_id(), |def_id| {
 423                 impl_candidates.push(
 424                     self.tcx.impl_trait_ref(def_id).unwrap());
 425             })
 426         };
 427         impl_candidates
 428     }
 429
 430     fn report_similar_impl_candidates(&self,
 431                                       impl_candidates: Vec<ty::TraitRef<'tcx>>,
 432                                       err: &mut DiagnosticBuilder)
 433     {
 434         if impl_candidates.is_empty() {
 435             return;
 436         }
 437
 438         let end = if impl_candidates.len() <= 5 {
 439             impl_candidates.len()
 440         } else {
 441             4
 442         };
 443         err.help(&format!("the following implementations were found:{}{}",
 444                           &impl_candidates[0..end].iter().map(|candidate| {
 445                               format!("\n  {:?}", candidate)
 446                           }).collect::<String>(),
 447                           if impl_candidates.len() > 5 {
 448                               format!("\nand {} others", impl_candidates.len() - 4)
 449                           } else {
 450                               "".to_owned()
 451                           }
 452                           ));
 453     }
 454
 455     /// Reports that an overflow has occurred and halts compilation. We
 456     /// halt compilation unconditionally because it is important that
 457     /// overflows never be masked -- they basically represent computations
 458     /// whose result could not be truly determined and thus we can't say
 459     /// if the program type checks or not -- and they are unusual
 460     /// occurrences in any case.
 461     pub fn report_overflow_error<T>(&self,
 462                                     obligation: &Obligation<'tcx, T>,
 463                                     suggest_increasing_limit: bool) -> !
 464         where T: fmt::Display + TypeFoldable<'tcx>
 465     {
 466         let predicate =
 467             self.resolve_type_vars_if_possible(&obligation.predicate);
 468         let mut err = struct_span_err!(self.tcx.sess, obligation.cause.span, E0275,
 469                                        "overflow evaluating the requirement `{}`",
 470                                        predicate);
 471
 472         if suggest_increasing_limit {
 473             self.suggest_new_overflow_limit(&mut err);
 474         }
 475
 476         self.note_obligation_cause(&mut err, obligation);
 477
 478         err.emit();
 479         self.tcx.sess.abort_if_errors();
 480         bug!();
 481     }
 482
 483     /// Reports that a cycle was detected which led to overflow and halts
 484     /// compilation. This is equivalent to `report_overflow_error` except
 485     /// that we can give a more helpful error message (and, in particular,
 486     /// we do not suggest increasing the overflow limit, which is not
 487     /// going to help).
 488     pub fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 489         let cycle = self.resolve_type_vars_if_possible(&cycle.to_owned());
 490         assert!(cycle.len() > 0);
 491
 492         debug!("report_overflow_error_cycle: cycle={:?}", cycle);
 493
 494         self.report_overflow_error(&cycle[0], false);
 495     }
 496
 497     pub fn report_extra_impl_obligation(&self,
 498                                         error_span: Span,
 499                                         item_name: ast::Name,
 500                                         _impl_item_def_id: DefId,
 501                                         trait_item_def_id: DefId,
 502                                         requirement: &dyn fmt::Display)
 503                                         -> DiagnosticBuilder<'tcx>
 504     {
 505         let msg = "impl has stricter requirements than trait";
 506         let sp = self.tcx.sess.codemap().def_span(error_span);
 507
 508         let mut err = struct_span_err!(self.tcx.sess, sp, E0276, "{}", msg);
 509
 510         if let Some(trait_item_span) = self.tcx.hir.span_if_local(trait_item_def_id) {
 511             let span = self.tcx.sess.codemap().def_span(trait_item_span);
 512             err.span_label(span, format!("definition of `{}` from trait", item_name));
 513         }
 514
 515         err.span_label(sp, format!("impl has extra requirement {}", requirement));
 516
 517         err
 518     }
 519
 520
 521     /// Get the parent trait chain start
 522     fn get_parent_trait_ref(&self, code: &ObligationCauseCode<'tcx>) -> Option<String> {
 523         match code {
 524             &ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
 525                 let parent_trait_ref = self.resolve_type_vars_if_possible(
 526                     &data.parent_trait_ref);
 527                 match self.get_parent_trait_ref(&data.parent_code) {
 528                     Some(t) => Some(t),
 529                     None => Some(format!("{}", parent_trait_ref.0.self_ty())),
 530                 }
 531             }
 532             _ => None,
 533         }
 534     }
 535
 536     pub fn report_selection_error(&self,
 537                                   obligation: &PredicateObligation<'tcx>,
 538                                   error: &SelectionError<'tcx>)
 539     {
 540         let span = obligation.cause.span;
 541
 542         let mut err = match *error {
 543             SelectionError::Unimplemented => {
 544                 if let ObligationCauseCode::CompareImplMethodObligation {
 545                     item_name, impl_item_def_id, trait_item_def_id,
 546                 } = obligation.cause.code {
 547                     self.report_extra_impl_obligation(
 548                         span,
 549                         item_name,
 550                         impl_item_def_id,
 551                         trait_item_def_id,
 552                         &format!("`{}`", obligation.predicate))
 553                         .emit();
 554                     return;
 555                 }
 556                 match obligation.predicate {
 557                     ty::Predicate::Trait(ref trait_predicate) => {
 558                         let trait_predicate =
 559                             self.resolve_type_vars_if_possible(trait_predicate);
 560
 561                         if self.tcx.sess.has_errors() && trait_predicate.references_error() {
 562                             return;
 563                         }
 564                         let trait_ref = trait_predicate.to_poly_trait_ref();
 565                         let (post_message, pre_message) =
 566                             self.get_parent_trait_ref(&obligation.cause.code)
 567                                 .map(|t| (format!(" in `{}`", t), format!("within `{}`, ", t)))
 568                             .unwrap_or((String::new(), String::new()));
 569
 570                         let OnUnimplementedNote { message, label, note }
 571                             = self.on_unimplemented_note(trait_ref, obligation);
 572                         let have_alt_message = message.is_some() || label.is_some();
 573
 574                         let mut err = struct_span_err!(
 575                             self.tcx.sess,
 576                             span,
 577                             E0277,
 578                             "{}",
 579                             message.unwrap_or_else(|| {
 580                                 format!("the trait bound `{}` is not satisfied{}",
 581                                          trait_ref.to_predicate(), post_message)
 582                             }));
 583
 584                         if let Some(ref s) = label {
 585                             // If it has a custom "#[rustc_on_unimplemented]"
 586                             // error message, let's display it as the label!
 587                             err.span_label(span, s.as_str());
 588                             err.help(&format!("{}the trait `{}` is not implemented for `{}`",
 589                                               pre_message,
 590                                               trait_ref,
 591                                               trait_ref.self_ty()));
 592                         } else {
 593                             err.span_label(span,
 594                                            &*format!("{}the trait `{}` is not implemented for `{}`",
 595                                                      pre_message,
 596                                                      trait_ref,
 597                                                      trait_ref.self_ty()));
 598                         }
 599                         if let Some(ref s) = note {
 600                             // If it has a custom "#[rustc_on_unimplemented]" note, let's display it
 601                             err.note(s.as_str());
 602                         }
 603
 604                         self.suggest_borrow_on_unsized_slice(&obligation.cause.code, &mut err);
 605
 606                         // Try to report a help message
 607                         if !trait_ref.has_infer_types() &&
 608                             self.predicate_can_apply(obligation.param_env, trait_ref) {
 609                             // If a where-clause may be useful, remind the
 610                             // user that they can add it.
 611                             //
 612                             // don't display an on-unimplemented note, as
 613                             // these notes will often be of the form
 614                             //     "the type `T` can't be frobnicated"
 615                             // which is somewhat confusing.
 616                             err.help(&format!("consider adding a `where {}` bound",
 617                                                 trait_ref.to_predicate()));
 618                         } else if !have_alt_message {
 619                             // Can't show anything else useful, try to find similar impls.
 620                             let impl_candidates = self.find_similar_impl_candidates(trait_ref);
 621                             self.report_similar_impl_candidates(impl_candidates, &mut err);
 622                         }
 623
 624                         err
 625                     }
 626
 627                     ty::Predicate::Subtype(ref predicate) => {
 628                         // Errors for Subtype predicates show up as
 629                         // `FulfillmentErrorCode::CodeSubtypeError`,
 630                         // not selection error.
 631                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
 632                     }
 633
 634                     ty::Predicate::RegionOutlives(ref predicate) => {
 635                         let predicate = self.resolve_type_vars_if_possible(predicate);
 636                         let err = self.region_outlives_predicate(&obligation.cause,
 637                                                                     &predicate).err().unwrap();
 638                         struct_span_err!(self.tcx.sess, span, E0279,
 639                             "the requirement `{}` is not satisfied (`{}`)",
 640                             predicate, err)
 641                     }
 642
 643                     ty::Predicate::Projection(..) | ty::Predicate::TypeOutlives(..) => {
 644                         let predicate =
 645                             self.resolve_type_vars_if_possible(&obligation.predicate);
 646                         struct_span_err!(self.tcx.sess, span, E0280,
 647                             "the requirement `{}` is not satisfied",
 648                             predicate)
 649                     }
 650
 651                     ty::Predicate::ObjectSafe(trait_def_id) => {
 652                         let violations = self.tcx.object_safety_violations(trait_def_id);
 653                         self.tcx.report_object_safety_error(span,
 654                                                             trait_def_id,
 655                                                             violations)
 656                     }
 657
 658                     ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => {
 659                         let found_kind = self.closure_kind(closure_def_id, closure_substs).unwrap();
 660                         let closure_span = self.tcx.sess.codemap()
 661                             .def_span(self.tcx.hir.span_if_local(closure_def_id).unwrap());
 662                         let node_id = self.tcx.hir.as_local_node_id(closure_def_id).unwrap();
 663                         let mut err = struct_span_err!(
 664                             self.tcx.sess, closure_span, E0525,
 665                             "expected a closure that implements the `{}` trait, \
 666                                 but this closure only implements `{}`",
 667                             kind,
 668                             found_kind);
 669
 670                         err.span_label(
 671                             closure_span,
 672                             format!("this closure implements `{}`, not `{}`", found_kind, kind));
 673                         err.span_label(
 674                             obligation.cause.span,
 675                             format!("the requirement to implement `{}` derives from here", kind));
 676
 677                         // Additional context information explaining why the closure only implements
 678                         // a particular trait.
 679                         if let Some(tables) = self.in_progress_tables {
 680                             let tables = tables.borrow();
 681                             let closure_hir_id = self.tcx.hir.node_to_hir_id(node_id);
 682                             match (found_kind, tables.closure_kind_origins().get(closure_hir_id)) {
 683                                 (ty::ClosureKind::FnOnce, Some((span, name))) => {
 684                                     err.span_label(*span, format!(
 685                                         "closure is `FnOnce` because it moves the \
 686                                          variable `{}` out of its environment", name));
 687                                 },
 688                                 (ty::ClosureKind::FnMut, Some((span, name))) => {
 689                                     err.span_label(*span, format!(
 690                                         "closure is `FnMut` because it mutates the \
 691                                          variable `{}` here", name));
 692                                 },
 693                                 _ => {}
 694                             }
 695                         }
 696
 697                         err.emit();
 698                         return;
 699                     }
 700
 701                     ty::Predicate::WellFormed(ty) => {
 702                         // WF predicates cannot themselves make
 703                         // errors. They can only block due to
 704                         // ambiguity; otherwise, they always
 705                         // degenerate into other obligations
 706                         // (which may fail).
 707                         span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 708                     }
 709
 710                     ty::Predicate::ConstEvaluatable(..) => {
 711                         // Errors for `ConstEvaluatable` predicates show up as
 712                         // `SelectionError::ConstEvalFailure`,
 713                         // not `Unimplemented`.
 714                         span_bug!(span,
 715                             "const-evaluatable requirement gave wrong error: `{:?}`", obligation)
 716                     }
 717                 }
 718             }
 719
 720             OutputTypeParameterMismatch(ref found_trait_ref, ref expected_trait_ref, _) => {
 721                 let found_trait_ref = self.resolve_type_vars_if_possible(&*found_trait_ref);
 722                 let expected_trait_ref = self.resolve_type_vars_if_possible(&*expected_trait_ref);
 723                 if expected_trait_ref.self_ty().references_error() {
 724                     return;
 725                 }
 726                 let found_trait_ty = found_trait_ref.self_ty();
 727
 728                 let found_did = found_trait_ty.ty_to_def_id();
 729                 let found_span = found_did.and_then(|did| {
 730                     self.tcx.hir.span_if_local(did)
 731                 }).map(|sp| self.tcx.sess.codemap().def_span(sp)); // the sp could be an fn def
 732
 733                 let found = match found_trait_ref.skip_binder().substs.type_at(1).sty {
 734                     ty::TyTuple(ref tys, _) => tys.iter()
 735                         .map(|_| ArgKind::empty()).collect::<Vec<_>>(),
 736                     _ => vec![ArgKind::empty()],
 737                 };
 738                 let expected = match expected_trait_ref.skip_binder().substs.type_at(1).sty {
 739                     ty::TyTuple(ref tys, _) => tys.iter()
 740                         .map(|t| match t.sty {
 741                             ty::TypeVariants::TyTuple(ref tys, _) => ArgKind::Tuple(
 742                                 Some(span),
 743                                 tys.iter()
 744                                     .map(|ty| ("_".to_owned(), format!("{}", ty.sty)))
 745                                     .collect::<Vec<_>>()
 746                             ),
 747                             _ => ArgKind::Arg("_".to_owned(), format!("{}", t.sty)),
 748                         }).collect(),
 749                     ref sty => vec![ArgKind::Arg("_".to_owned(), format!("{}", sty))],
 750                 };
 751                 if found.len() == expected.len() {
 752                     self.report_closure_arg_mismatch(span,
 753                                                      found_span,
 754                                                      found_trait_ref,
 755                                                      expected_trait_ref)
 756                 } else {
 757                     let (closure_span, found) = found_did
 758                         .and_then(|did| self.tcx.hir.get_if_local(did))
 759                         .map(|node| {
 760                             let (found_span, found) = self.get_fn_like_arguments(node);
 761                             (Some(found_span), found)
 762                         }).unwrap_or((found_span, found));
 763
 764                     self.report_arg_count_mismatch(span,
 765                                                    closure_span,
 766                                                    expected,
 767                                                    found,
 768                                                    found_trait_ty.is_closure())
 769                 }
 770             }
 771
 772             TraitNotObjectSafe(did) => {
 773                 let violations = self.tcx.object_safety_violations(did);
 774                 self.tcx.report_object_safety_error(span, did,
 775                                                     violations)
 776             }
 777
 778             ConstEvalFailure(ref err) => {
 779                 if let const_val::ErrKind::TypeckError = err.kind {
 780                     return;
 781                 }
 782                 err.struct_error(self.tcx, span, "constant expression")
 783             }
 784         };
 785         self.note_obligation_cause(&mut err, obligation);
 786         err.emit();
 787     }
 788
 789     /// When encountering an assignment of an unsized trait, like `let x = ""[..];`, provide a
 790     /// suggestion to borrow the initializer in order to use have a slice instead.
 791     fn suggest_borrow_on_unsized_slice(&self,
 792                                        code: &ObligationCauseCode<'tcx>,
 793                                        err: &mut DiagnosticBuilder<'tcx>) {
 794         if let &ObligationCauseCode::VariableType(node_id) = code {
 795             let parent_node = self.tcx.hir.get_parent_node(node_id);
 796             if let Some(hir::map::NodeLocal(ref local)) = self.tcx.hir.find(parent_node) {
 797                 if let Some(ref expr) = local.init {
 798                     if let hir::ExprIndex(_, _) = expr.node {
 799                         if let Ok(snippet) = self.tcx.sess.codemap().span_to_snippet(expr.span) {
 800                             err.span_suggestion(expr.span,
 801                                                 "consider borrowing here",
 802                                                 format!("&{}", snippet));
 803                         }
 804                     }
 805                 }
 806             }
 807         }
 808     }
 809
 810     /// Given some node representing a fn-like thing in the HIR map,
 811     /// returns a span and `ArgKind` information that describes the
 812     /// arguments it expects. This can be supplied to
 813     /// `report_arg_count_mismatch`.
 814     pub fn get_fn_like_arguments(&self, node: hir::map::Node) -> (Span, Vec<ArgKind>) {
 815         match node {
 816             hir::map::NodeExpr(&hir::Expr {
 817                 node: hir::ExprClosure(_, ref _decl, id, span, _),
 818                 ..
 819             }) => {
 820                 (self.tcx.sess.codemap().def_span(span), self.tcx.hir.body(id).arguments.iter()
 821                     .map(|arg| {
 822                         if let hir::Pat {
 823                             node: hir::PatKind::Tuple(args, _),
 824                             span,
 825                             ..
 826                         } = arg.pat.clone().into_inner() {
 827                             ArgKind::Tuple(
 828                                 Some(span),
 829                                 args.iter().map(|pat| {
 830                                     let snippet = self.tcx.sess.codemap()
 831                                         .span_to_snippet(pat.span).unwrap();
 832                                     (snippet, "_".to_owned())
 833                                 }).collect::<Vec<_>>(),
 834                             )
 835                         } else {
 836                             let name = self.tcx.sess.codemap()
 837                                 .span_to_snippet(arg.pat.span).unwrap();
 838                             ArgKind::Arg(name, "_".to_owned())
 839                         }
 840                     })
 841                     .collect::<Vec<ArgKind>>())
 842             }
 843             hir::map::NodeItem(&hir::Item {
 844                 span,
 845                 node: hir::ItemFn(ref decl, ..),
 846                 ..
 847             }) |
 848             hir::map::NodeImplItem(&hir::ImplItem {
 849                 span,
 850                 node: hir::ImplItemKind::Method(hir::MethodSig { ref decl, .. }, _),
 851                 ..
 852             }) |
 853             hir::map::NodeTraitItem(&hir::TraitItem {
 854                 span,
 855                 node: hir::TraitItemKind::Method(hir::MethodSig { ref decl, .. }, _),
 856                 ..
 857             }) => {
 858                 (self.tcx.sess.codemap().def_span(span), decl.inputs.iter()
 859                         .map(|arg| match arg.clone().into_inner().node {
 860                     hir::TyTup(ref tys) => ArgKind::Tuple(
 861                         Some(arg.span),
 862                         tys.iter()
 863                             .map(|_| ("_".to_owned(), "_".to_owned()))
 864                             .collect::<Vec<_>>(),
 865                     ),
 866                     _ => ArgKind::Arg("_".to_owned(), "_".to_owned())
 867                 }).collect::<Vec<ArgKind>>())
 868             }
 869             hir::map::NodeVariant(&hir::Variant {
 870                 span,
 871                 node: hir::Variant_ {
 872                     data: hir::VariantData::Tuple(ref fields, _),
 873                     ..
 874                 },
 875                 ..
 876             }) => {
 877                 (self.tcx.sess.codemap().def_span(span),
 878                  fields.iter().map(|field| {
 879                      ArgKind::Arg(format!("{}", field.name), "_".to_string())
 880                  }).collect::<Vec<_>>())
 881             }
 882             _ => panic!("non-FnLike node found: {:?}", node),
 883         }
 884     }
 885
 886     /// Reports an error when the number of arguments needed by a
 887     /// trait match doesn't match the number that the expression
 888     /// provides.
 889     pub fn report_arg_count_mismatch(
 890         &self,
 891         span: Span,
 892         found_span: Option<Span>,
 893         expected_args: Vec<ArgKind>,
 894         found_args: Vec<ArgKind>,
 895         is_closure: bool,
 896     ) -> DiagnosticBuilder<'tcx> {
 897         let kind = if is_closure { "closure" } else { "function" };
 898
 899         let args_str = |arguments: &Vec<ArgKind>, other: &Vec<ArgKind>| {
 900             let arg_length = arguments.len();
 901             let distinct = match &other[..] {
 902                 &[ArgKind::Tuple(..)] => true,
 903                 _ => false,
 904             };
 905             match (arg_length, arguments.get(0)) {
 906                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
 907                     format!("a single {}-tuple as argument", fields.len())
 908                 }
 909                 _ => format!("{} {}argument{}",
 910                              arg_length,
 911                              if distinct && arg_length > 1 { "distinct " } else { "" },
 912                              if arg_length == 1 { "" } else { "s" }),
 913             }
 914         };
 915
 916         let expected_str = args_str(&expected_args, &found_args);
 917         let found_str = args_str(&found_args, &expected_args);
 918
 919         let mut err = struct_span_err!(
 920             self.tcx.sess,
 921             span,
 922             E0593,
 923             "{} is expected to take {}, but it takes {}",
 924             kind,
 925             expected_str,
 926             found_str,
 927         );
 928
 929         err.span_label(span, format!( "expected {} that takes {}", kind, expected_str));
 930
 931         if let Some(found_span) = found_span {
 932             err.span_label(found_span, format!("takes {}", found_str));
 933
 934             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
 935                 if fields.len() == expected_args.len() {
 936                     let sugg = fields.iter()
 937                         .map(|(name, _)| name.to_owned())
 938                         .collect::<Vec<String>>().join(", ");
 939                     err.span_suggestion(found_span,
 940                                         "change the closure to take multiple arguments instead of \
 941                                          a single tuple",
 942                                         format!("|{}|", sugg));
 943                 }
 944             }
 945             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..] {
 946                 if fields.len() == found_args.len() && is_closure {
 947                     let sugg = format!(
 948                         "|({}){}|",
 949                         found_args.iter()
 950                             .map(|arg| match arg {
 951                                 ArgKind::Arg(name, _) => name.to_owned(),
 952                                 _ => "_".to_owned(),
 953                             })
 954                             .collect::<Vec<String>>()
 955                             .join(", "),
 956                         // add type annotations if available
 957                         if found_args.iter().any(|arg| match arg {
 958                             ArgKind::Arg(_, ty) => ty != "_",
 959                             _ => false,
 960                         }) {
 961                             format!(": ({})",
 962                                     fields.iter()
 963                                         .map(|(_, ty)| ty.to_owned())
 964                                         .collect::<Vec<String>>()
 965                                         .join(", "))
 966                         } else {
 967                             "".to_owned()
 968                         },
 969                     );
 970                     err.span_suggestion(found_span,
 971                                         "change the closure to accept a tuple instead of \
 972                                          individual arguments",
 973                                         sugg);
 974                 }
 975             }
 976         }
 977
 978         err
 979     }
 980
 981     fn report_closure_arg_mismatch(&self,
 982                            span: Span,
 983                            found_span: Option<Span>,
 984                            expected_ref: ty::PolyTraitRef<'tcx>,
 985                            found: ty::PolyTraitRef<'tcx>)
 986         -> DiagnosticBuilder<'tcx>
 987     {
 988         fn build_fn_sig_string<'a, 'gcx, 'tcx>(tcx: ty::TyCtxt<'a, 'gcx, 'tcx>,
 989                                                trait_ref: &ty::TraitRef<'tcx>) -> String {
 990             let inputs = trait_ref.substs.type_at(1);
 991             let sig = if let ty::TyTuple(inputs, _) = inputs.sty {
 992                 tcx.mk_fn_sig(
 993                     inputs.iter().map(|&x| x),
 994                     tcx.mk_infer(ty::TyVar(ty::TyVid { index: 0 })),
 995                     false,
 996                     hir::Unsafety::Normal,
 997                     ::syntax::abi::Abi::Rust
 998                 )
 999             } else {
1000                 tcx.mk_fn_sig(
1001                     ::std::iter::once(inputs),
1002                     tcx.mk_infer(ty::TyVar(ty::TyVid { index: 0 })),
1003                     false,
1004                     hir::Unsafety::Normal,
1005                     ::syntax::abi::Abi::Rust
1006                 )
1007             };
1008             format!("{}", ty::Binder(sig))
1009         }
1010
1011         let argument_is_closure = expected_ref.skip_binder().substs.type_at(0).is_closure();
1012         let mut err = struct_span_err!(self.tcx.sess, span, E0631,
1013                                        "type mismatch in {} arguments",
1014                                        if argument_is_closure { "closure" } else { "function" });
1015
1016         let found_str = format!(
1017             "expected signature of `{}`",
1018             build_fn_sig_string(self.tcx, found.skip_binder())
1019         );
1020         err.span_label(span, found_str);
1021
1022         let found_span = found_span.unwrap_or(span);
1023         let expected_str = format!(
1024             "found signature of `{}`",
1025             build_fn_sig_string(self.tcx, expected_ref.skip_binder())
1026         );
1027         err.span_label(found_span, expected_str);
1028
1029         err
1030     }
1031 }
1032
1033 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
1034     pub fn recursive_type_with_infinite_size_error(self,
1035                                                    type_def_id: DefId)
1036                                                    -> DiagnosticBuilder<'tcx>
1037     {
1038         assert!(type_def_id.is_local());
1039         let span = self.hir.span_if_local(type_def_id).unwrap();
1040         let span = self.sess.codemap().def_span(span);
1041         let mut err = struct_span_err!(self.sess, span, E0072,
1042                                        "recursive type `{}` has infinite size",
1043                                        self.item_path_str(type_def_id));
1044         err.span_label(span, "recursive type has infinite size");
1045         err.help(&format!("insert indirection (e.g., a `Box`, `Rc`, or `&`) \
1046                            at some point to make `{}` representable",
1047                           self.item_path_str(type_def_id)));
1048         err
1049     }
1050
1051     pub fn report_object_safety_error(self,
1052                                       span: Span,
1053                                       trait_def_id: DefId,
1054                                       violations: Vec<ObjectSafetyViolation>)
1055                                       -> DiagnosticBuilder<'tcx>
1056     {
1057         let trait_str = self.item_path_str(trait_def_id);
1058         let span = self.sess.codemap().def_span(span);
1059         let mut err = struct_span_err!(
1060             self.sess, span, E0038,
1061             "the trait `{}` cannot be made into an object",
1062             trait_str);
1063         err.span_label(span, format!("the trait `{}` cannot be made into an object", trait_str));
1064
1065         let mut reported_violations = FxHashSet();
1066         for violation in violations {
1067             if !reported_violations.insert(violation.clone()) {
1068                 continue;
1069             }
1070             err.note(&violation.error_msg());
1071         }
1072         err
1073     }
1074 }
1075
1076 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
1077     fn maybe_report_ambiguity(&self, obligation: &PredicateObligation<'tcx>,
1078                               body_id: Option<hir::BodyId>) {
1079         // Unable to successfully determine, probably means
1080         // insufficient type information, but could mean
1081         // ambiguous impls. The latter *ought* to be a
1082         // coherence violation, so we don't report it here.
1083
1084         let predicate = self.resolve_type_vars_if_possible(&obligation.predicate);
1085         let span = obligation.cause.span;
1086
1087         debug!("maybe_report_ambiguity(predicate={:?}, obligation={:?})",
1088                predicate,
1089                obligation);
1090
1091         // Ambiguity errors are often caused as fallout from earlier
1092         // errors. So just ignore them if this infcx is tainted.
1093         if self.is_tainted_by_errors() {
1094             return;
1095         }
1096
1097         match predicate {
1098             ty::Predicate::Trait(ref data) => {
1099                 let trait_ref = data.to_poly_trait_ref();
1100                 let self_ty = trait_ref.self_ty();
1101                 if predicate.references_error() {
1102                     return;
1103                 }
1104                 // Typically, this ambiguity should only happen if
1105                 // there are unresolved type inference variables
1106                 // (otherwise it would suggest a coherence
1107                 // failure). But given #21974 that is not necessarily
1108                 // the case -- we can have multiple where clauses that
1109                 // are only distinguished by a region, which results
1110                 // in an ambiguity even when all types are fully
1111                 // known, since we don't dispatch based on region
1112                 // relationships.
1113
1114                 // This is kind of a hack: it frequently happens that some earlier
1115                 // error prevents types from being fully inferred, and then we get
1116                 // a bunch of uninteresting errors saying something like "<generic
1117                 // #0> doesn't implement Sized".  It may even be true that we
1118                 // could just skip over all checks where the self-ty is an
1119                 // inference variable, but I was afraid that there might be an
1120                 // inference variable created, registered as an obligation, and
1121                 // then never forced by writeback, and hence by skipping here we'd
1122                 // be ignoring the fact that we don't KNOW the type works
1123                 // out. Though even that would probably be harmless, given that
1124                 // we're only talking about builtin traits, which are known to be
1125                 // inhabited. But in any case I just threw in this check for
1126                 // has_errors() to be sure that compilation isn't happening
1127                 // anyway. In that case, why inundate the user.
1128                 if !self.tcx.sess.has_errors() {
1129                     if
1130                         self.tcx.lang_items().sized_trait()
1131                         .map_or(false, |sized_id| sized_id == trait_ref.def_id())
1132                     {
1133                         self.need_type_info(body_id, span, self_ty);
1134                     } else {
1135                         let mut err = struct_span_err!(self.tcx.sess,
1136                                                         span, E0283,
1137                                                         "type annotations required: \
1138                                                         cannot resolve `{}`",
1139                                                         predicate);
1140                         self.note_obligation_cause(&mut err, obligation);
1141                         err.emit();
1142                     }
1143                 }
1144             }
1145
1146             ty::Predicate::WellFormed(ty) => {
1147                 // Same hacky approach as above to avoid deluging user
1148                 // with error messages.
1149                 if !ty.references_error() && !self.tcx.sess.has_errors() {
1150                     self.need_type_info(body_id, span, ty);
1151                 }
1152             }
1153
1154             ty::Predicate::Subtype(ref data) => {
1155                 if data.references_error() || self.tcx.sess.has_errors() {
1156                     // no need to overload user in such cases
1157                 } else {
1158                     let &SubtypePredicate { a_is_expected: _, a, b } = data.skip_binder();
1159                     // both must be type variables, or the other would've been instantiated
1160                     assert!(a.is_ty_var() && b.is_ty_var());
1161                     self.need_type_info(body_id,
1162                                         obligation.cause.span,
1163                                         a);
1164                 }
1165             }
1166
1167             _ => {
1168                 if !self.tcx.sess.has_errors() {
1169                     let mut err = struct_span_err!(self.tcx.sess,
1170                                                    obligation.cause.span, E0284,
1171                                                    "type annotations required: \
1172                                                     cannot resolve `{}`",
1173                                                    predicate);
1174                     self.note_obligation_cause(&mut err, obligation);
1175                     err.emit();
1176                 }
1177             }
1178         }
1179     }
1180
1181     /// Returns whether the trait predicate may apply for *some* assignment
1182     /// to the type parameters.
1183     fn predicate_can_apply(&self,
1184                            param_env: ty::ParamEnv<'tcx>,
1185                            pred: ty::PolyTraitRef<'tcx>)
1186                            -> bool {
1187         struct ParamToVarFolder<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
1188             infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
1189             param_env: ty::ParamEnv<'tcx>,
1190             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>
1191         }
1192
1193         impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for ParamToVarFolder<'a, 'gcx, 'tcx> {
1194             fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> { self.infcx.tcx }
1195
1196             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
1197                 if let ty::TyParam(ty::ParamTy {name, ..}) = ty.sty {
1198                     let infcx = self.infcx;
1199                     let param_env = self.param_env;
1200                     self.var_map
1201                         .entry(ty)
1202                         .or_insert_with(|| {
1203                             let origin = TypeVariableOrigin::TypeParameterDefinition(DUMMY_SP,
1204                                                                                      name);
1205                             infcx.next_ty_var(param_env.universe, origin)
1206                         })
1207                 } else {
1208                     ty.super_fold_with(self)
1209                 }
1210             }
1211         }
1212
1213         self.probe(|_| {
1214             let mut selcx = SelectionContext::new(self);
1215
1216             let cleaned_pred = pred.fold_with(&mut ParamToVarFolder {
1217                 infcx: self,
1218                 param_env,
1219                 var_map: FxHashMap()
1220             });
1221
1222             let cleaned_pred = super::project::normalize(
1223                 &mut selcx,
1224                 param_env,
1225                 ObligationCause::dummy(),
1226                 &cleaned_pred
1227             ).value;
1228
1229             let obligation = Obligation::new(
1230                 ObligationCause::dummy(),
1231                 param_env,
1232                 cleaned_pred.to_predicate()
1233             );
1234
1235             selcx.evaluate_obligation(&obligation)
1236         })
1237     }
1238
1239     fn note_obligation_cause<T>(&self,
1240                                 err: &mut DiagnosticBuilder,
1241                                 obligation: &Obligation<'tcx, T>)
1242         where T: fmt::Display
1243     {
1244         self.note_obligation_cause_code(err,
1245                                         &obligation.predicate,
1246                                         &obligation.cause.code,
1247                                         &mut vec![]);
1248     }
1249
1250     fn note_obligation_cause_code<T>(&self,
1251                                      err: &mut DiagnosticBuilder,
1252                                      predicate: &T,
1253                                      cause_code: &ObligationCauseCode<'tcx>,
1254                                      obligated_types: &mut Vec<&ty::TyS<'tcx>>)
1255         where T: fmt::Display
1256     {
1257         let tcx = self.tcx;
1258         match *cause_code {
1259             ObligationCauseCode::ExprAssignable |
1260             ObligationCauseCode::MatchExpressionArm { .. } |
1261             ObligationCauseCode::IfExpression |
1262             ObligationCauseCode::IfExpressionWithNoElse |
1263             ObligationCauseCode::MainFunctionType |
1264             ObligationCauseCode::StartFunctionType |
1265             ObligationCauseCode::IntrinsicType |
1266             ObligationCauseCode::MethodReceiver |
1267             ObligationCauseCode::ReturnNoExpression |
1268             ObligationCauseCode::MiscObligation => {
1269             }
1270             ObligationCauseCode::SliceOrArrayElem => {
1271                 err.note("slice and array elements must have `Sized` type");
1272             }
1273             ObligationCauseCode::TupleElem => {
1274                 err.note("only the last element of a tuple may have a dynamically sized type");
1275             }
1276             ObligationCauseCode::ProjectionWf(data) => {
1277                 err.note(&format!("required so that the projection `{}` is well-formed",
1278                                   data));
1279             }
1280             ObligationCauseCode::ReferenceOutlivesReferent(ref_ty) => {
1281                 err.note(&format!("required so that reference `{}` does not outlive its referent",
1282                                   ref_ty));
1283             }
1284             ObligationCauseCode::ObjectTypeBound(object_ty, region) => {
1285                 err.note(&format!("required so that the lifetime bound of `{}` for `{}` \
1286                                    is satisfied",
1287                                   region, object_ty));
1288             }
1289             ObligationCauseCode::ItemObligation(item_def_id) => {
1290                 let item_name = tcx.item_path_str(item_def_id);
1291                 let msg = format!("required by `{}`", item_name);
1292                 if let Some(sp) = tcx.hir.span_if_local(item_def_id) {
1293                     let sp = tcx.sess.codemap().def_span(sp);
1294                     err.span_note(sp, &msg);
1295                 } else {
1296                     err.note(&msg);
1297                 }
1298             }
1299             ObligationCauseCode::ObjectCastObligation(object_ty) => {
1300                 err.note(&format!("required for the cast to the object type `{}`",
1301                                   self.ty_to_string(object_ty)));
1302             }
1303             ObligationCauseCode::RepeatVec => {
1304                 err.note("the `Copy` trait is required because the \
1305                           repeated element will be copied");
1306             }
1307             ObligationCauseCode::VariableType(_) => {
1308                 err.note("all local variables must have a statically known size");
1309             }
1310             ObligationCauseCode::SizedReturnType => {
1311                 err.note("the return type of a function must have a \
1312                           statically known size");
1313             }
1314             ObligationCauseCode::SizedYieldType => {
1315                 err.note("the yield type of a generator must have a \
1316                           statically known size");
1317             }
1318             ObligationCauseCode::AssignmentLhsSized => {
1319                 err.note("the left-hand-side of an assignment must have a statically known size");
1320             }
1321             ObligationCauseCode::TupleInitializerSized => {
1322                 err.note("tuples must have a statically known size to be initialized");
1323             }
1324             ObligationCauseCode::StructInitializerSized => {
1325                 err.note("structs must have a statically known size to be initialized");
1326             }
1327             ObligationCauseCode::FieldSized(ref item) => {
1328                 match *item {
1329                     AdtKind::Struct => {
1330                         err.note("only the last field of a struct may have a dynamically \
1331                                   sized type");
1332                     }
1333                     AdtKind::Union => {
1334                         err.note("no field of a union may have a dynamically sized type");
1335                     }
1336                     AdtKind::Enum => {
1337                         err.note("no field of an enum variant may have a dynamically sized type");
1338                     }
1339                 }
1340             }
1341             ObligationCauseCode::ConstSized => {
1342                 err.note("constant expressions must have a statically known size");
1343             }
1344             ObligationCauseCode::SharedStatic => {
1345                 err.note("shared static variables must have a type that implements `Sync`");
1346             }
1347             ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
1348                 let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1349                 let ty = parent_trait_ref.0.self_ty();
1350                 err.note(&format!("required because it appears within the type `{}`", ty));
1351                 obligated_types.push(ty);
1352
1353                 let parent_predicate = parent_trait_ref.to_predicate();
1354                 if !self.is_recursive_obligation(obligated_types, &data.parent_code) {
1355                     self.note_obligation_cause_code(err,
1356                                                     &parent_predicate,
1357                                                     &data.parent_code,
1358                                                     obligated_types);
1359                 }
1360             }
1361             ObligationCauseCode::ImplDerivedObligation(ref data) => {
1362                 let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1363                 err.note(
1364                     &format!("required because of the requirements on the impl of `{}` for `{}`",
1365                              parent_trait_ref,
1366                              parent_trait_ref.0.self_ty()));
1367                 let parent_predicate = parent_trait_ref.to_predicate();
1368                 self.note_obligation_cause_code(err,
1369                                             &parent_predicate,
1370                                             &data.parent_code,
1371                                             obligated_types);
1372             }
1373             ObligationCauseCode::CompareImplMethodObligation { .. } => {
1374                 err.note(
1375                     &format!("the requirement `{}` appears on the impl method \
1376                               but not on the corresponding trait method",
1377                              predicate));
1378             }
1379             ObligationCauseCode::ReturnType(_) |
1380             ObligationCauseCode::BlockTailExpression(_) => (),
1381         }
1382     }
1383
1384     fn suggest_new_overflow_limit(&self, err: &mut DiagnosticBuilder) {
1385         let current_limit = self.tcx.sess.recursion_limit.get();
1386         let suggested_limit = current_limit * 2;
1387         err.help(&format!("consider adding a `#![recursion_limit=\"{}\"]` attribute to your crate",
1388                           suggested_limit));
1389     }
1390
1391     fn is_recursive_obligation(&self,
1392                                    obligated_types: &mut Vec<&ty::TyS<'tcx>>,
1393                                    cause_code: &ObligationCauseCode<'tcx>) -> bool {
1394         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
1395             let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1396             for obligated_type in obligated_types {
1397                 if obligated_type == &parent_trait_ref.0.self_ty() {
1398                     return true;
1399                 }
1400             }
1401         }
1402         return false;
1403     }
1404 }
1405
1406 /// Summarizes information
1407 pub enum ArgKind {
1408     /// An argument of non-tuple type. Parameters are (name, ty)
1409     Arg(String, String),
1410
1411     /// An argument of tuple type. For a "found" argument, the span is
1412     /// the locationo in the source of the pattern. For a "expected"
1413     /// argument, it will be None. The vector is a list of (name, ty)
1414     /// strings for the components of the tuple.
1415     Tuple(Option<Span>, Vec<(String, String)>),
1416 }
1417
1418 impl ArgKind {
1419     fn empty() -> ArgKind {
1420         ArgKind::Arg("_".to_owned(), "_".to_owned())
1421     }
1422
1423     /// Creates an `ArgKind` from the expected type of an
1424     /// argument. This has no name (`_`) and no source spans..
1425     pub fn from_expected_ty(t: Ty<'_>) -> ArgKind {
1426         match t.sty {
1427             ty::TyTuple(ref tys, _) => ArgKind::Tuple(
1428                 None,
1429                 tys.iter()
1430                    .map(|ty| ("_".to_owned(), format!("{}", ty.sty)))
1431                    .collect::<Vec<_>>()
1432             ),
1433             _ => ArgKind::Arg("_".to_owned(), format!("{}", t.sty)),
1434         }
1435     }
1436 }