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::Equate(ref predicate) => {
 635                         let predicate = self.resolve_type_vars_if_possible(predicate);
 636                         let err = self.equality_predicate(&obligation.cause,
 637                                                           obligation.param_env,
 638                                                           &predicate).err().unwrap();
 639                         struct_span_err!(self.tcx.sess, span, E0278,
 640                             "the requirement `{}` is not satisfied (`{}`)",
 641                             predicate, err)
 642                     }
 643
 644                     ty::Predicate::RegionOutlives(ref predicate) => {
 645                         let predicate = self.resolve_type_vars_if_possible(predicate);
 646                         let err = self.region_outlives_predicate(&obligation.cause,
 647                                                                     &predicate).err().unwrap();
 648                         struct_span_err!(self.tcx.sess, span, E0279,
 649                             "the requirement `{}` is not satisfied (`{}`)",
 650                             predicate, err)
 651                     }
 652
 653                     ty::Predicate::Projection(..) | ty::Predicate::TypeOutlives(..) => {
 654                         let predicate =
 655                             self.resolve_type_vars_if_possible(&obligation.predicate);
 656                         struct_span_err!(self.tcx.sess, span, E0280,
 657                             "the requirement `{}` is not satisfied",
 658                             predicate)
 659                     }
 660
 661                     ty::Predicate::ObjectSafe(trait_def_id) => {
 662                         let violations = self.tcx.object_safety_violations(trait_def_id);
 663                         self.tcx.report_object_safety_error(span,
 664                                                             trait_def_id,
 665                                                             violations)
 666                     }
 667
 668                     ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => {
 669                         let found_kind = self.closure_kind(closure_def_id, closure_substs).unwrap();
 670                         let closure_span = self.tcx.sess.codemap()
 671                             .def_span(self.tcx.hir.span_if_local(closure_def_id).unwrap());
 672                         let node_id = self.tcx.hir.as_local_node_id(closure_def_id).unwrap();
 673                         let mut err = struct_span_err!(
 674                             self.tcx.sess, closure_span, E0525,
 675                             "expected a closure that implements the `{}` trait, \
 676                                 but this closure only implements `{}`",
 677                             kind,
 678                             found_kind);
 679
 680                         err.span_label(
 681                             closure_span,
 682                             format!("this closure implements `{}`, not `{}`", found_kind, kind));
 683                         err.span_label(
 684                             obligation.cause.span,
 685                             format!("the requirement to implement `{}` derives from here", kind));
 686
 687                         // Additional context information explaining why the closure only implements
 688                         // a particular trait.
 689                         if let Some(tables) = self.in_progress_tables {
 690                             let tables = tables.borrow();
 691                             let closure_hir_id = self.tcx.hir.node_to_hir_id(node_id);
 692                             match (found_kind, tables.closure_kind_origins().get(closure_hir_id)) {
 693                                 (ty::ClosureKind::FnOnce, Some((span, name))) => {
 694                                     err.span_label(*span, format!(
 695                                         "closure is `FnOnce` because it moves the \
 696                                          variable `{}` out of its environment", name));
 697                                 },
 698                                 (ty::ClosureKind::FnMut, Some((span, name))) => {
 699                                     err.span_label(*span, format!(
 700                                         "closure is `FnMut` because it mutates the \
 701                                          variable `{}` here", name));
 702                                 },
 703                                 _ => {}
 704                             }
 705                         }
 706
 707                         err.emit();
 708                         return;
 709                     }
 710
 711                     ty::Predicate::WellFormed(ty) => {
 712                         // WF predicates cannot themselves make
 713                         // errors. They can only block due to
 714                         // ambiguity; otherwise, they always
 715                         // degenerate into other obligations
 716                         // (which may fail).
 717                         span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 718                     }
 719
 720                     ty::Predicate::ConstEvaluatable(..) => {
 721                         // Errors for `ConstEvaluatable` predicates show up as
 722                         // `SelectionError::ConstEvalFailure`,
 723                         // not `Unimplemented`.
 724                         span_bug!(span,
 725                             "const-evaluatable requirement gave wrong error: `{:?}`", obligation)
 726                     }
 727                 }
 728             }
 729
 730             OutputTypeParameterMismatch(ref found_trait_ref, ref expected_trait_ref, _) => {
 731                 let found_trait_ref = self.resolve_type_vars_if_possible(&*found_trait_ref);
 732                 let expected_trait_ref = self.resolve_type_vars_if_possible(&*expected_trait_ref);
 733                 if expected_trait_ref.self_ty().references_error() {
 734                     return;
 735                 }
 736                 let found_trait_ty = found_trait_ref.self_ty();
 737
 738                 let found_did = found_trait_ty.ty_to_def_id();
 739                 let found_span = found_did.and_then(|did| {
 740                     self.tcx.hir.span_if_local(did)
 741                 }).map(|sp| self.tcx.sess.codemap().def_span(sp)); // the sp could be an fn def
 742
 743                 let found = match found_trait_ref.skip_binder().substs.type_at(1).sty {
 744                     ty::TyTuple(ref tys, _) => tys.iter()
 745                         .map(|_| ArgKind::empty()).collect::<Vec<_>>(),
 746                     _ => vec![ArgKind::empty()],
 747                 };
 748                 let expected = match expected_trait_ref.skip_binder().substs.type_at(1).sty {
 749                     ty::TyTuple(ref tys, _) => tys.iter()
 750                         .map(|t| match t.sty {
 751                             ty::TypeVariants::TyTuple(ref tys, _) => ArgKind::Tuple(
 752                                 Some(span),
 753                                 tys.iter()
 754                                     .map(|ty| ("_".to_owned(), format!("{}", ty.sty)))
 755                                     .collect::<Vec<_>>()
 756                             ),
 757                             _ => ArgKind::Arg("_".to_owned(), format!("{}", t.sty)),
 758                         }).collect(),
 759                     ref sty => vec![ArgKind::Arg("_".to_owned(), format!("{}", sty))],
 760                 };
 761                 if found.len() == expected.len() {
 762                     self.report_closure_arg_mismatch(span,
 763                                                      found_span,
 764                                                      found_trait_ref,
 765                                                      expected_trait_ref)
 766                 } else {
 767                     let (closure_span, found) = found_did
 768                         .and_then(|did| self.tcx.hir.get_if_local(did))
 769                         .map(|node| {
 770                             let (found_span, found) = self.get_fn_like_arguments(node);
 771                             (Some(found_span), found)
 772                         }).unwrap_or((found_span, found));
 773
 774                     self.report_arg_count_mismatch(span,
 775                                                    closure_span,
 776                                                    expected,
 777                                                    found,
 778                                                    found_trait_ty.is_closure())
 779                 }
 780             }
 781
 782             TraitNotObjectSafe(did) => {
 783                 let violations = self.tcx.object_safety_violations(did);
 784                 self.tcx.report_object_safety_error(span, did,
 785                                                     violations)
 786             }
 787
 788             ConstEvalFailure(ref err) => {
 789                 if let const_val::ErrKind::TypeckError = err.kind {
 790                     return;
 791                 }
 792                 err.struct_error(self.tcx, span, "constant expression")
 793             }
 794         };
 795         self.note_obligation_cause(&mut err, obligation);
 796         err.emit();
 797     }
 798
 799     /// When encountering an assignment of an unsized trait, like `let x = ""[..];`, provide a
 800     /// suggestion to borrow the initializer in order to use have a slice instead.
 801     fn suggest_borrow_on_unsized_slice(&self,
 802                                        code: &ObligationCauseCode<'tcx>,
 803                                        err: &mut DiagnosticBuilder<'tcx>) {
 804         if let &ObligationCauseCode::VariableType(node_id) = code {
 805             let parent_node = self.tcx.hir.get_parent_node(node_id);
 806             if let Some(hir::map::NodeLocal(ref local)) = self.tcx.hir.find(parent_node) {
 807                 if let Some(ref expr) = local.init {
 808                     if let hir::ExprIndex(_, _) = expr.node {
 809                         if let Ok(snippet) = self.tcx.sess.codemap().span_to_snippet(expr.span) {
 810                             err.span_suggestion(expr.span,
 811                                                 "consider borrowing here",
 812                                                 format!("&{}", snippet));
 813                         }
 814                     }
 815                 }
 816             }
 817         }
 818     }
 819
 820     /// Given some node representing a fn-like thing in the HIR map,
 821     /// returns a span and `ArgKind` information that describes the
 822     /// arguments it expects. This can be supplied to
 823     /// `report_arg_count_mismatch`.
 824     pub fn get_fn_like_arguments(&self, node: hir::map::Node) -> (Span, Vec<ArgKind>) {
 825         match node {
 826             hir::map::NodeExpr(&hir::Expr {
 827                 node: hir::ExprClosure(_, ref _decl, id, span, _),
 828                 ..
 829             }) => {
 830                 (self.tcx.sess.codemap().def_span(span), self.tcx.hir.body(id).arguments.iter()
 831                     .map(|arg| {
 832                         if let hir::Pat {
 833                             node: hir::PatKind::Tuple(args, _),
 834                             span,
 835                             ..
 836                         } = arg.pat.clone().into_inner() {
 837                             ArgKind::Tuple(
 838                                 Some(span),
 839                                 args.iter().map(|pat| {
 840                                     let snippet = self.tcx.sess.codemap()
 841                                         .span_to_snippet(pat.span).unwrap();
 842                                     (snippet, "_".to_owned())
 843                                 }).collect::<Vec<_>>(),
 844                             )
 845                         } else {
 846                             let name = self.tcx.sess.codemap()
 847                                 .span_to_snippet(arg.pat.span).unwrap();
 848                             ArgKind::Arg(name, "_".to_owned())
 849                         }
 850                     })
 851                     .collect::<Vec<ArgKind>>())
 852             }
 853             hir::map::NodeItem(&hir::Item {
 854                 span,
 855                 node: hir::ItemFn(ref decl, ..),
 856                 ..
 857             }) |
 858             hir::map::NodeImplItem(&hir::ImplItem {
 859                 span,
 860                 node: hir::ImplItemKind::Method(hir::MethodSig { ref decl, .. }, _),
 861                 ..
 862             }) |
 863             hir::map::NodeTraitItem(&hir::TraitItem {
 864                 span,
 865                 node: hir::TraitItemKind::Method(hir::MethodSig { ref decl, .. }, _),
 866                 ..
 867             }) => {
 868                 (self.tcx.sess.codemap().def_span(span), decl.inputs.iter()
 869                         .map(|arg| match arg.clone().into_inner().node {
 870                     hir::TyTup(ref tys) => ArgKind::Tuple(
 871                         Some(arg.span),
 872                         tys.iter()
 873                             .map(|_| ("_".to_owned(), "_".to_owned()))
 874                             .collect::<Vec<_>>(),
 875                     ),
 876                     _ => ArgKind::Arg("_".to_owned(), "_".to_owned())
 877                 }).collect::<Vec<ArgKind>>())
 878             }
 879             hir::map::NodeVariant(&hir::Variant {
 880                 span,
 881                 node: hir::Variant_ {
 882                     data: hir::VariantData::Tuple(ref fields, _),
 883                     ..
 884                 },
 885                 ..
 886             }) => {
 887                 (self.tcx.sess.codemap().def_span(span),
 888                  fields.iter().map(|field| {
 889                      ArgKind::Arg(format!("{}", field.name), "_".to_string())
 890                  }).collect::<Vec<_>>())
 891             }
 892             _ => panic!("non-FnLike node found: {:?}", node),
 893         }
 894     }
 895
 896     /// Reports an error when the number of arguments needed by a
 897     /// trait match doesn't match the number that the expression
 898     /// provides.
 899     pub fn report_arg_count_mismatch(
 900         &self,
 901         span: Span,
 902         found_span: Option<Span>,
 903         expected_args: Vec<ArgKind>,
 904         found_args: Vec<ArgKind>,
 905         is_closure: bool,
 906     ) -> DiagnosticBuilder<'tcx> {
 907         let kind = if is_closure { "closure" } else { "function" };
 908
 909         let args_str = |arguments: &Vec<ArgKind>, other: &Vec<ArgKind>| {
 910             let arg_length = arguments.len();
 911             let distinct = match &other[..] {
 912                 &[ArgKind::Tuple(..)] => true,
 913                 _ => false,
 914             };
 915             match (arg_length, arguments.get(0)) {
 916                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
 917                     format!("a single {}-tuple as argument", fields.len())
 918                 }
 919                 _ => format!("{} {}argument{}",
 920                              arg_length,
 921                              if distinct && arg_length > 1 { "distinct " } else { "" },
 922                              if arg_length == 1 { "" } else { "s" }),
 923             }
 924         };
 925
 926         let expected_str = args_str(&expected_args, &found_args);
 927         let found_str = args_str(&found_args, &expected_args);
 928
 929         let mut err = struct_span_err!(
 930             self.tcx.sess,
 931             span,
 932             E0593,
 933             "{} is expected to take {}, but it takes {}",
 934             kind,
 935             expected_str,
 936             found_str,
 937         );
 938
 939         err.span_label(span, format!( "expected {} that takes {}", kind, expected_str));
 940
 941         if let Some(found_span) = found_span {
 942             err.span_label(found_span, format!("takes {}", found_str));
 943
 944             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
 945                 if fields.len() == expected_args.len() {
 946                     let sugg = fields.iter()
 947                         .map(|(name, _)| name.to_owned())
 948                         .collect::<Vec<String>>().join(", ");
 949                     err.span_suggestion(found_span,
 950                                         "change the closure to take multiple arguments instead of \
 951                                          a single tuple",
 952                                         format!("|{}|", sugg));
 953                 }
 954             }
 955             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..] {
 956                 if fields.len() == found_args.len() && is_closure {
 957                     let sugg = format!(
 958                         "|({}){}|",
 959                         found_args.iter()
 960                             .map(|arg| match arg {
 961                                 ArgKind::Arg(name, _) => name.to_owned(),
 962                                 _ => "_".to_owned(),
 963                             })
 964                             .collect::<Vec<String>>()
 965                             .join(", "),
 966                         // add type annotations if available
 967                         if found_args.iter().any(|arg| match arg {
 968                             ArgKind::Arg(_, ty) => ty != "_",
 969                             _ => false,
 970                         }) {
 971                             format!(": ({})",
 972                                     fields.iter()
 973                                         .map(|(_, ty)| ty.to_owned())
 974                                         .collect::<Vec<String>>()
 975                                         .join(", "))
 976                         } else {
 977                             "".to_owned()
 978                         },
 979                     );
 980                     err.span_suggestion(found_span,
 981                                         "change the closure to accept a tuple instead of \
 982                                          individual arguments",
 983                                         sugg);
 984                 }
 985             }
 986         }
 987
 988         err
 989     }
 990
 991     fn report_closure_arg_mismatch(&self,
 992                            span: Span,
 993                            found_span: Option<Span>,
 994                            expected_ref: ty::PolyTraitRef<'tcx>,
 995                            found: ty::PolyTraitRef<'tcx>)
 996         -> DiagnosticBuilder<'tcx>
 997     {
 998         fn build_fn_sig_string<'a, 'gcx, 'tcx>(tcx: ty::TyCtxt<'a, 'gcx, 'tcx>,
 999                                                trait_ref: &ty::TraitRef<'tcx>) -> String {
1000             let inputs = trait_ref.substs.type_at(1);
1001             let sig = if let ty::TyTuple(inputs, _) = inputs.sty {
1002                 tcx.mk_fn_sig(
1003                     inputs.iter().map(|&x| x),
1004                     tcx.mk_infer(ty::TyVar(ty::TyVid { index: 0 })),
1005                     false,
1006                     hir::Unsafety::Normal,
1007                     ::syntax::abi::Abi::Rust
1008                 )
1009             } else {
1010                 tcx.mk_fn_sig(
1011                     ::std::iter::once(inputs),
1012                     tcx.mk_infer(ty::TyVar(ty::TyVid { index: 0 })),
1013                     false,
1014                     hir::Unsafety::Normal,
1015                     ::syntax::abi::Abi::Rust
1016                 )
1017             };
1018             format!("{}", ty::Binder(sig))
1019         }
1020
1021         let argument_is_closure = expected_ref.skip_binder().substs.type_at(0).is_closure();
1022         let mut err = struct_span_err!(self.tcx.sess, span, E0631,
1023                                        "type mismatch in {} arguments",
1024                                        if argument_is_closure { "closure" } else { "function" });
1025
1026         let found_str = format!(
1027             "expected signature of `{}`",
1028             build_fn_sig_string(self.tcx, found.skip_binder())
1029         );
1030         err.span_label(span, found_str);
1031
1032         let found_span = found_span.unwrap_or(span);
1033         let expected_str = format!(
1034             "found signature of `{}`",
1035             build_fn_sig_string(self.tcx, expected_ref.skip_binder())
1036         );
1037         err.span_label(found_span, expected_str);
1038
1039         err
1040     }
1041 }
1042
1043 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
1044     pub fn recursive_type_with_infinite_size_error(self,
1045                                                    type_def_id: DefId)
1046                                                    -> DiagnosticBuilder<'tcx>
1047     {
1048         assert!(type_def_id.is_local());
1049         let span = self.hir.span_if_local(type_def_id).unwrap();
1050         let span = self.sess.codemap().def_span(span);
1051         let mut err = struct_span_err!(self.sess, span, E0072,
1052                                        "recursive type `{}` has infinite size",
1053                                        self.item_path_str(type_def_id));
1054         err.span_label(span, "recursive type has infinite size");
1055         err.help(&format!("insert indirection (e.g., a `Box`, `Rc`, or `&`) \
1056                            at some point to make `{}` representable",
1057                           self.item_path_str(type_def_id)));
1058         err
1059     }
1060
1061     pub fn report_object_safety_error(self,
1062                                       span: Span,
1063                                       trait_def_id: DefId,
1064                                       violations: Vec<ObjectSafetyViolation>)
1065                                       -> DiagnosticBuilder<'tcx>
1066     {
1067         let trait_str = self.item_path_str(trait_def_id);
1068         let span = self.sess.codemap().def_span(span);
1069         let mut err = struct_span_err!(
1070             self.sess, span, E0038,
1071             "the trait `{}` cannot be made into an object",
1072             trait_str);
1073         err.span_label(span, format!("the trait `{}` cannot be made into an object", trait_str));
1074
1075         let mut reported_violations = FxHashSet();
1076         for violation in violations {
1077             if !reported_violations.insert(violation.clone()) {
1078                 continue;
1079             }
1080             err.note(&violation.error_msg());
1081         }
1082         err
1083     }
1084 }
1085
1086 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
1087     fn maybe_report_ambiguity(&self, obligation: &PredicateObligation<'tcx>,
1088                               body_id: Option<hir::BodyId>) {
1089         // Unable to successfully determine, probably means
1090         // insufficient type information, but could mean
1091         // ambiguous impls. The latter *ought* to be a
1092         // coherence violation, so we don't report it here.
1093
1094         let predicate = self.resolve_type_vars_if_possible(&obligation.predicate);
1095         let span = obligation.cause.span;
1096
1097         debug!("maybe_report_ambiguity(predicate={:?}, obligation={:?})",
1098                predicate,
1099                obligation);
1100
1101         // Ambiguity errors are often caused as fallout from earlier
1102         // errors. So just ignore them if this infcx is tainted.
1103         if self.is_tainted_by_errors() {
1104             return;
1105         }
1106
1107         match predicate {
1108             ty::Predicate::Trait(ref data) => {
1109                 let trait_ref = data.to_poly_trait_ref();
1110                 let self_ty = trait_ref.self_ty();
1111                 if predicate.references_error() {
1112                     return;
1113                 }
1114                 // Typically, this ambiguity should only happen if
1115                 // there are unresolved type inference variables
1116                 // (otherwise it would suggest a coherence
1117                 // failure). But given #21974 that is not necessarily
1118                 // the case -- we can have multiple where clauses that
1119                 // are only distinguished by a region, which results
1120                 // in an ambiguity even when all types are fully
1121                 // known, since we don't dispatch based on region
1122                 // relationships.
1123
1124                 // This is kind of a hack: it frequently happens that some earlier
1125                 // error prevents types from being fully inferred, and then we get
1126                 // a bunch of uninteresting errors saying something like "<generic
1127                 // #0> doesn't implement Sized".  It may even be true that we
1128                 // could just skip over all checks where the self-ty is an
1129                 // inference variable, but I was afraid that there might be an
1130                 // inference variable created, registered as an obligation, and
1131                 // then never forced by writeback, and hence by skipping here we'd
1132                 // be ignoring the fact that we don't KNOW the type works
1133                 // out. Though even that would probably be harmless, given that
1134                 // we're only talking about builtin traits, which are known to be
1135                 // inhabited. But in any case I just threw in this check for
1136                 // has_errors() to be sure that compilation isn't happening
1137                 // anyway. In that case, why inundate the user.
1138                 if !self.tcx.sess.has_errors() {
1139                     if
1140                         self.tcx.lang_items().sized_trait()
1141                         .map_or(false, |sized_id| sized_id == trait_ref.def_id())
1142                     {
1143                         self.need_type_info(body_id, span, self_ty);
1144                     } else {
1145                         let mut err = struct_span_err!(self.tcx.sess,
1146                                                         span, E0283,
1147                                                         "type annotations required: \
1148                                                         cannot resolve `{}`",
1149                                                         predicate);
1150                         self.note_obligation_cause(&mut err, obligation);
1151                         err.emit();
1152                     }
1153                 }
1154             }
1155
1156             ty::Predicate::WellFormed(ty) => {
1157                 // Same hacky approach as above to avoid deluging user
1158                 // with error messages.
1159                 if !ty.references_error() && !self.tcx.sess.has_errors() {
1160                     self.need_type_info(body_id, span, ty);
1161                 }
1162             }
1163
1164             ty::Predicate::Subtype(ref data) => {
1165                 if data.references_error() || self.tcx.sess.has_errors() {
1166                     // no need to overload user in such cases
1167                 } else {
1168                     let &SubtypePredicate { a_is_expected: _, a, b } = data.skip_binder();
1169                     // both must be type variables, or the other would've been instantiated
1170                     assert!(a.is_ty_var() && b.is_ty_var());
1171                     self.need_type_info(body_id,
1172                                         obligation.cause.span,
1173                                         a);
1174                 }
1175             }
1176
1177             _ => {
1178                 if !self.tcx.sess.has_errors() {
1179                     let mut err = struct_span_err!(self.tcx.sess,
1180                                                    obligation.cause.span, E0284,
1181                                                    "type annotations required: \
1182                                                     cannot resolve `{}`",
1183                                                    predicate);
1184                     self.note_obligation_cause(&mut err, obligation);
1185                     err.emit();
1186                 }
1187             }
1188         }
1189     }
1190
1191     /// Returns whether the trait predicate may apply for *some* assignment
1192     /// to the type parameters.
1193     fn predicate_can_apply(&self,
1194                            param_env: ty::ParamEnv<'tcx>,
1195                            pred: ty::PolyTraitRef<'tcx>)
1196                            -> bool {
1197         struct ParamToVarFolder<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
1198             infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
1199             param_env: ty::ParamEnv<'tcx>,
1200             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>
1201         }
1202
1203         impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for ParamToVarFolder<'a, 'gcx, 'tcx> {
1204             fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> { self.infcx.tcx }
1205
1206             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
1207                 if let ty::TyParam(ty::ParamTy {name, ..}) = ty.sty {
1208                     let infcx = self.infcx;
1209                     let param_env = self.param_env;
1210                     self.var_map
1211                         .entry(ty)
1212                         .or_insert_with(|| {
1213                             let origin = TypeVariableOrigin::TypeParameterDefinition(DUMMY_SP,
1214                                                                                      name);
1215                             infcx.next_ty_var(param_env.universe, origin)
1216                         })
1217                 } else {
1218                     ty.super_fold_with(self)
1219                 }
1220             }
1221         }
1222
1223         self.probe(|_| {
1224             let mut selcx = SelectionContext::new(self);
1225
1226             let cleaned_pred = pred.fold_with(&mut ParamToVarFolder {
1227                 infcx: self,
1228                 param_env,
1229                 var_map: FxHashMap()
1230             });
1231
1232             let cleaned_pred = super::project::normalize(
1233                 &mut selcx,
1234                 param_env,
1235                 ObligationCause::dummy(),
1236                 &cleaned_pred
1237             ).value;
1238
1239             let obligation = Obligation::new(
1240                 ObligationCause::dummy(),
1241                 param_env,
1242                 cleaned_pred.to_predicate()
1243             );
1244
1245             selcx.evaluate_obligation(&obligation)
1246         })
1247     }
1248
1249     fn note_obligation_cause<T>(&self,
1250                                 err: &mut DiagnosticBuilder,
1251                                 obligation: &Obligation<'tcx, T>)
1252         where T: fmt::Display
1253     {
1254         self.note_obligation_cause_code(err,
1255                                         &obligation.predicate,
1256                                         &obligation.cause.code,
1257                                         &mut vec![]);
1258     }
1259
1260     fn note_obligation_cause_code<T>(&self,
1261                                      err: &mut DiagnosticBuilder,
1262                                      predicate: &T,
1263                                      cause_code: &ObligationCauseCode<'tcx>,
1264                                      obligated_types: &mut Vec<&ty::TyS<'tcx>>)
1265         where T: fmt::Display
1266     {
1267         let tcx = self.tcx;
1268         match *cause_code {
1269             ObligationCauseCode::ExprAssignable |
1270             ObligationCauseCode::MatchExpressionArm { .. } |
1271             ObligationCauseCode::IfExpression |
1272             ObligationCauseCode::IfExpressionWithNoElse |
1273             ObligationCauseCode::EquatePredicate |
1274             ObligationCauseCode::MainFunctionType |
1275             ObligationCauseCode::StartFunctionType |
1276             ObligationCauseCode::IntrinsicType |
1277             ObligationCauseCode::MethodReceiver |
1278             ObligationCauseCode::ReturnNoExpression |
1279             ObligationCauseCode::MiscObligation => {
1280             }
1281             ObligationCauseCode::SliceOrArrayElem => {
1282                 err.note("slice and array elements must have `Sized` type");
1283             }
1284             ObligationCauseCode::TupleElem => {
1285                 err.note("only the last element of a tuple may have a dynamically sized type");
1286             }
1287             ObligationCauseCode::ProjectionWf(data) => {
1288                 err.note(&format!("required so that the projection `{}` is well-formed",
1289                                   data));
1290             }
1291             ObligationCauseCode::ReferenceOutlivesReferent(ref_ty) => {
1292                 err.note(&format!("required so that reference `{}` does not outlive its referent",
1293                                   ref_ty));
1294             }
1295             ObligationCauseCode::ObjectTypeBound(object_ty, region) => {
1296                 err.note(&format!("required so that the lifetime bound of `{}` for `{}` \
1297                                    is satisfied",
1298                                   region, object_ty));
1299             }
1300             ObligationCauseCode::ItemObligation(item_def_id) => {
1301                 let item_name = tcx.item_path_str(item_def_id);
1302                 let msg = format!("required by `{}`", item_name);
1303                 if let Some(sp) = tcx.hir.span_if_local(item_def_id) {
1304                     let sp = tcx.sess.codemap().def_span(sp);
1305                     err.span_note(sp, &msg);
1306                 } else {
1307                     err.note(&msg);
1308                 }
1309             }
1310             ObligationCauseCode::ObjectCastObligation(object_ty) => {
1311                 err.note(&format!("required for the cast to the object type `{}`",
1312                                   self.ty_to_string(object_ty)));
1313             }
1314             ObligationCauseCode::RepeatVec => {
1315                 err.note("the `Copy` trait is required because the \
1316                           repeated element will be copied");
1317             }
1318             ObligationCauseCode::VariableType(_) => {
1319                 err.note("all local variables must have a statically known size");
1320             }
1321             ObligationCauseCode::SizedReturnType => {
1322                 err.note("the return type of a function must have a \
1323                           statically known size");
1324             }
1325             ObligationCauseCode::SizedYieldType => {
1326                 err.note("the yield type of a generator must have a \
1327                           statically known size");
1328             }
1329             ObligationCauseCode::AssignmentLhsSized => {
1330                 err.note("the left-hand-side of an assignment must have a statically known size");
1331             }
1332             ObligationCauseCode::TupleInitializerSized => {
1333                 err.note("tuples must have a statically known size to be initialized");
1334             }
1335             ObligationCauseCode::StructInitializerSized => {
1336                 err.note("structs must have a statically known size to be initialized");
1337             }
1338             ObligationCauseCode::FieldSized(ref item) => {
1339                 match *item {
1340                     AdtKind::Struct => {
1341                         err.note("only the last field of a struct may have a dynamically \
1342                                   sized type");
1343                     }
1344                     AdtKind::Union => {
1345                         err.note("no field of a union may have a dynamically sized type");
1346                     }
1347                     AdtKind::Enum => {
1348                         err.note("no field of an enum variant may have a dynamically sized type");
1349                     }
1350                 }
1351             }
1352             ObligationCauseCode::ConstSized => {
1353                 err.note("constant expressions must have a statically known size");
1354             }
1355             ObligationCauseCode::SharedStatic => {
1356                 err.note("shared static variables must have a type that implements `Sync`");
1357             }
1358             ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
1359                 let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1360                 let ty = parent_trait_ref.0.self_ty();
1361                 err.note(&format!("required because it appears within the type `{}`", ty));
1362                 obligated_types.push(ty);
1363
1364                 let parent_predicate = parent_trait_ref.to_predicate();
1365                 if !self.is_recursive_obligation(obligated_types, &data.parent_code) {
1366                     self.note_obligation_cause_code(err,
1367                                                     &parent_predicate,
1368                                                     &data.parent_code,
1369                                                     obligated_types);
1370                 }
1371             }
1372             ObligationCauseCode::ImplDerivedObligation(ref data) => {
1373                 let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1374                 err.note(
1375                     &format!("required because of the requirements on the impl of `{}` for `{}`",
1376                              parent_trait_ref,
1377                              parent_trait_ref.0.self_ty()));
1378                 let parent_predicate = parent_trait_ref.to_predicate();
1379                 self.note_obligation_cause_code(err,
1380                                             &parent_predicate,
1381                                             &data.parent_code,
1382                                             obligated_types);
1383             }
1384             ObligationCauseCode::CompareImplMethodObligation { .. } => {
1385                 err.note(
1386                     &format!("the requirement `{}` appears on the impl method \
1387                               but not on the corresponding trait method",
1388                              predicate));
1389             }
1390             ObligationCauseCode::ReturnType(_) |
1391             ObligationCauseCode::BlockTailExpression(_) => (),
1392         }
1393     }
1394
1395     fn suggest_new_overflow_limit(&self, err: &mut DiagnosticBuilder) {
1396         let current_limit = self.tcx.sess.recursion_limit.get();
1397         let suggested_limit = current_limit * 2;
1398         err.help(&format!("consider adding a `#![recursion_limit=\"{}\"]` attribute to your crate",
1399                           suggested_limit));
1400     }
1401
1402     fn is_recursive_obligation(&self,
1403                                    obligated_types: &mut Vec<&ty::TyS<'tcx>>,
1404                                    cause_code: &ObligationCauseCode<'tcx>) -> bool {
1405         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
1406             let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1407             for obligated_type in obligated_types {
1408                 if obligated_type == &parent_trait_ref.0.self_ty() {
1409                     return true;
1410                 }
1411             }
1412         }
1413         return false;
1414     }
1415 }
1416
1417 /// Summarizes information
1418 pub enum ArgKind {
1419     /// An argument of non-tuple type. Parameters are (name, ty)
1420     Arg(String, String),
1421
1422     /// An argument of tuple type. For a "found" argument, the span is
1423     /// the locationo in the source of the pattern. For a "expected"
1424     /// argument, it will be None. The vector is a list of (name, ty)
1425     /// strings for the components of the tuple.
1426     Tuple(Option<Span>, Vec<(String, String)>),
1427 }
1428
1429 impl ArgKind {
1430     fn empty() -> ArgKind {
1431         ArgKind::Arg("_".to_owned(), "_".to_owned())
1432     }
1433
1434     /// Creates an `ArgKind` from the expected type of an
1435     /// argument. This has no name (`_`) and no source spans..
1436     pub fn from_expected_ty(t: Ty<'_>) -> ArgKind {
1437         match t.sty {
1438             ty::TyTuple(ref tys, _) => ArgKind::Tuple(
1439                 None,
1440                 tys.iter()
1441                    .map(|ty| ("_".to_owned(), format!("{}", ty.sty)))
1442                    .collect::<Vec<_>>()
1443             ),
1444             _ => ArgKind::Arg("_".to_owned(), format!("{}", t.sty)),
1445         }
1446     }
1447 }