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     SelectionContext,
  25     SelectionError,
  26     ObjectSafetyViolation,
  27     Overflow,
  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 std::fmt;
  36 use syntax::ast;
  37 use session::DiagnosticMessageId;
  38 use ty::{self, AdtKind, ToPredicate, ToPolyTraitRef, Ty, TyCtxt, TypeFoldable};
  39 use ty::error::ExpectedFound;
  40 use ty::fast_reject;
  41 use ty::fold::TypeFolder;
  42 use ty::subst::Subst;
  43 use ty::SubtypePredicate;
  44 use util::nodemap::{FxHashMap, FxHashSet};
  45
  46 use syntax_pos::{DUMMY_SP, Span};
  47
  48 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
  49     pub fn report_fulfillment_errors(&self,
  50                                      errors: &Vec<FulfillmentError<'tcx>>,
  51                                      body_id: Option<hir::BodyId>,
  52                                      fallback_has_occurred: bool) {
  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, fallback_has_occurred);
 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();
 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                                 fallback_has_occurred: bool) {
 158         debug!("report_fulfillment_errors({:?})", error);
 159         match error.code {
 160             FulfillmentErrorCode::CodeSelectionError(ref e) => {
 161                 self.report_selection_error(&error.obligation, e, fallback_has_occurred);
 162             }
 163             FulfillmentErrorCode::CodeProjectionError(ref e) => {
 164                 self.report_projection_error(&error.obligation, e);
 165             }
 166             FulfillmentErrorCode::CodeAmbiguity => {
 167                 self.maybe_report_ambiguity(&error.obligation, body_id);
 168             }
 169             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
 170                 self.report_mismatched_types(&error.obligation.cause,
 171                                              expected_found.expected,
 172                                              expected_found.found,
 173                                              err.clone())
 174                     .emit();
 175             }
 176         }
 177     }
 178
 179     fn report_projection_error(&self,
 180                                obligation: &PredicateObligation<'tcx>,
 181                                error: &MismatchedProjectionTypes<'tcx>)
 182     {
 183         let predicate =
 184             self.resolve_type_vars_if_possible(&obligation.predicate);
 185
 186         if predicate.references_error() {
 187             return
 188         }
 189
 190         self.probe(|_| {
 191             let err_buf;
 192             let mut err = &error.err;
 193             let mut values = None;
 194
 195             // try to find the mismatched types to report the error with.
 196             //
 197             // this can fail if the problem was higher-ranked, in which
 198             // cause I have no idea for a good error message.
 199             if let ty::Predicate::Projection(ref data) = predicate {
 200                 let mut selcx = SelectionContext::new(self);
 201                 let (data, _) = self.replace_late_bound_regions_with_fresh_var(
 202                     obligation.cause.span,
 203                     infer::LateBoundRegionConversionTime::HigherRankedType,
 204                     data);
 205                 let normalized = super::normalize_projection_type(
 206                     &mut selcx,
 207                     obligation.param_env,
 208                     data.projection_ty,
 209                     obligation.cause.clone(),
 210                     0
 211                 );
 212                 if let Err(error) = self.at(&obligation.cause, obligation.param_env)
 213                                         .eq(normalized.value, data.ty) {
 214                     values = Some(infer::ValuePairs::Types(ExpectedFound {
 215                         expected: normalized.value,
 216                         found: data.ty,
 217                     }));
 218                     err_buf = error;
 219                     err = &err_buf;
 220                 }
 221             }
 222
 223             let msg = format!("type mismatch resolving `{}`", predicate);
 224             let error_id = (DiagnosticMessageId::ErrorId(271),
 225                             Some(obligation.cause.span), msg.clone());
 226             let fresh = self.tcx.sess.one_time_diagnostics.borrow_mut().insert(error_id);
 227             if fresh {
 228                 let mut diag = struct_span_err!(
 229                     self.tcx.sess, obligation.cause.span, E0271,
 230                     "type mismatch resolving `{}`", predicate
 231                 );
 232                 self.note_type_err(&mut diag, &obligation.cause, None, values, err);
 233                 self.note_obligation_cause(&mut diag, obligation);
 234                 diag.emit();
 235             }
 236         });
 237     }
 238
 239     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
 240         /// returns the fuzzy category of a given type, or None
 241         /// if the type can be equated to any type.
 242         fn type_category<'tcx>(t: Ty<'tcx>) -> Option<u32> {
 243             match t.sty {
 244                 ty::TyBool => Some(0),
 245                 ty::TyChar => Some(1),
 246                 ty::TyStr => Some(2),
 247                 ty::TyInt(..) | ty::TyUint(..) | ty::TyInfer(ty::IntVar(..)) => Some(3),
 248                 ty::TyFloat(..) | ty::TyInfer(ty::FloatVar(..)) => Some(4),
 249                 ty::TyRef(..) | ty::TyRawPtr(..) => Some(5),
 250                 ty::TyArray(..) | ty::TySlice(..) => Some(6),
 251                 ty::TyFnDef(..) | ty::TyFnPtr(..) => Some(7),
 252                 ty::TyDynamic(..) => Some(8),
 253                 ty::TyClosure(..) => Some(9),
 254                 ty::TyTuple(..) => Some(10),
 255                 ty::TyProjection(..) => Some(11),
 256                 ty::TyParam(..) => Some(12),
 257                 ty::TyAnon(..) => Some(13),
 258                 ty::TyNever => Some(14),
 259                 ty::TyAdt(adt, ..) => match adt.adt_kind() {
 260                     AdtKind::Struct => Some(15),
 261                     AdtKind::Union => Some(16),
 262                     AdtKind::Enum => Some(17),
 263                 },
 264                 ty::TyGenerator(..) => Some(18),
 265                 ty::TyForeign(..) => Some(19),
 266                 ty::TyGeneratorWitness(..) => Some(20),
 267                 ty::TyInfer(..) | ty::TyError => None
 268             }
 269         }
 270
 271         match (type_category(a), type_category(b)) {
 272             (Some(cat_a), Some(cat_b)) => match (&a.sty, &b.sty) {
 273                 (&ty::TyAdt(def_a, _), &ty::TyAdt(def_b, _)) => def_a == def_b,
 274                 _ => cat_a == cat_b
 275             },
 276             // infer and error can be equated to all types
 277             _ => true
 278         }
 279     }
 280
 281     fn impl_similar_to(&self,
 282                        trait_ref: ty::PolyTraitRef<'tcx>,
 283                        obligation: &PredicateObligation<'tcx>)
 284                        -> Option<DefId>
 285     {
 286         let tcx = self.tcx;
 287         let param_env = obligation.param_env;
 288         let trait_ref = tcx.erase_late_bound_regions(&trait_ref);
 289         let trait_self_ty = trait_ref.self_ty();
 290
 291         let mut self_match_impls = vec![];
 292         let mut fuzzy_match_impls = vec![];
 293
 294         self.tcx.for_each_relevant_impl(
 295             trait_ref.def_id, trait_self_ty, |def_id| {
 296                 let impl_substs = self.fresh_substs_for_item(obligation.cause.span, def_id);
 297                 let impl_trait_ref = tcx
 298                     .impl_trait_ref(def_id)
 299                     .unwrap()
 300                     .subst(tcx, impl_substs);
 301
 302                 let impl_self_ty = impl_trait_ref.self_ty();
 303
 304                 if let Ok(..) = self.can_eq(param_env, trait_self_ty, impl_self_ty) {
 305                     self_match_impls.push(def_id);
 306
 307                     if trait_ref.substs.types().skip(1)
 308                         .zip(impl_trait_ref.substs.types().skip(1))
 309                         .all(|(u,v)| self.fuzzy_match_tys(u, v))
 310                     {
 311                         fuzzy_match_impls.push(def_id);
 312                     }
 313                 }
 314             });
 315
 316         let impl_def_id = if self_match_impls.len() == 1 {
 317             self_match_impls[0]
 318         } else if fuzzy_match_impls.len() == 1 {
 319             fuzzy_match_impls[0]
 320         } else {
 321             return None
 322         };
 323
 324         if tcx.has_attr(impl_def_id, "rustc_on_unimplemented") {
 325             Some(impl_def_id)
 326         } else {
 327             None
 328         }
 329     }
 330
 331     fn on_unimplemented_note(
 332         &self,
 333         trait_ref: ty::PolyTraitRef<'tcx>,
 334         obligation: &PredicateObligation<'tcx>) ->
 335         OnUnimplementedNote
 336     {
 337         let def_id = self.impl_similar_to(trait_ref, obligation)
 338             .unwrap_or(trait_ref.def_id());
 339         let trait_ref = *trait_ref.skip_binder();
 340
 341         let mut flags = vec![];
 342         match obligation.cause.code {
 343             ObligationCauseCode::BuiltinDerivedObligation(..) |
 344             ObligationCauseCode::ImplDerivedObligation(..) => {}
 345             _ => {
 346                 // this is a "direct", user-specified, rather than derived,
 347                 // obligation.
 348                 flags.push(("direct".to_string(), None));
 349             }
 350         }
 351
 352         if let ObligationCauseCode::ItemObligation(item) = obligation.cause.code {
 353             // FIXME: maybe also have some way of handling methods
 354             // from other traits? That would require name resolution,
 355             // which we might want to be some sort of hygienic.
 356             //
 357             // Currently I'm leaving it for what I need for `try`.
 358             if self.tcx.trait_of_item(item) == Some(trait_ref.def_id) {
 359                 let method = self.tcx.item_name(item);
 360                 flags.push(("from_method".to_string(), None));
 361                 flags.push(("from_method".to_string(), Some(method.to_string())));
 362             }
 363         }
 364
 365         if let Some(k) = obligation.cause.span.compiler_desugaring_kind() {
 366             let desugaring = k.as_symbol().as_str();
 367             flags.push(("from_desugaring".to_string(), None));
 368             flags.push(("from_desugaring".to_string(), Some(desugaring.to_string())));
 369         }
 370         let generics = self.tcx.generics_of(def_id);
 371         let self_ty = trait_ref.self_ty();
 372         // This is also included through the generics list as `Self`,
 373         // but the parser won't allow you to use it
 374         flags.push(("_Self".to_string(), Some(self_ty.to_string())));
 375         if let Some(def) = self_ty.ty_adt_def() {
 376             // We also want to be able to select self's original
 377             // signature with no type arguments resolved
 378             flags.push(("_Self".to_string(), Some(self.tcx.type_of(def.did).to_string())));
 379         }
 380
 381         for param in generics.params.iter().filter_map(|param| param.get_type()) {
 382             let name = param.name.to_string();
 383             let ty = trait_ref.substs.type_for_def(&param);
 384             let ty_str = ty.to_string();
 385             flags.push((name.clone(), Some(ty_str.clone())));
 386         }
 387
 388         if let Some(true) = self_ty.ty_to_def_id().map(|def_id| def_id.is_local()) {
 389             flags.push(("crate_local".to_string(), None));
 390         }
 391
 392         if let Ok(Some(command)) = OnUnimplementedDirective::of_item(
 393             self.tcx, trait_ref.def_id, def_id
 394         ) {
 395             command.evaluate(self.tcx, trait_ref, &flags[..])
 396         } else {
 397             OnUnimplementedNote::empty()
 398         }
 399     }
 400
 401     fn find_similar_impl_candidates(&self,
 402                                     trait_ref: ty::PolyTraitRef<'tcx>)
 403                                     -> Vec<ty::TraitRef<'tcx>>
 404     {
 405         let simp = fast_reject::simplify_type(self.tcx,
 406                                               trait_ref.skip_binder().self_ty(),
 407                                               true);
 408         let mut impl_candidates = Vec::new();
 409
 410         match simp {
 411             Some(simp) => self.tcx.for_each_impl(trait_ref.def_id(), |def_id| {
 412                 let imp = self.tcx.impl_trait_ref(def_id).unwrap();
 413                 let imp_simp = fast_reject::simplify_type(self.tcx,
 414                                                           imp.self_ty(),
 415                                                           true);
 416                 if let Some(imp_simp) = imp_simp {
 417                     if simp != imp_simp {
 418                         return;
 419                     }
 420                 }
 421                 impl_candidates.push(imp);
 422             }),
 423             None => self.tcx.for_each_impl(trait_ref.def_id(), |def_id| {
 424                 impl_candidates.push(
 425                     self.tcx.impl_trait_ref(def_id).unwrap());
 426             })
 427         };
 428         impl_candidates
 429     }
 430
 431     fn report_similar_impl_candidates(&self,
 432                                       impl_candidates: Vec<ty::TraitRef<'tcx>>,
 433                                       err: &mut DiagnosticBuilder)
 434     {
 435         if impl_candidates.is_empty() {
 436             return;
 437         }
 438
 439         let end = if impl_candidates.len() <= 5 {
 440             impl_candidates.len()
 441         } else {
 442             4
 443         };
 444
 445         let normalize = |candidate| self.tcx.global_tcx().infer_ctxt().enter(|ref infcx| {
 446             let normalized = infcx
 447                 .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
 448                 .normalize(candidate)
 449                 .ok();
 450             match normalized {
 451                 Some(normalized) => format!("\n  {:?}", normalized.value),
 452                 None => format!("\n  {:?}", candidate),
 453             }
 454         });
 455
 456         err.help(&format!("the following implementations were found:{}{}",
 457                           &impl_candidates[0..end].iter().map(normalize).collect::<String>(),
 458                           if impl_candidates.len() > 5 {
 459                               format!("\nand {} others", impl_candidates.len() - 4)
 460                           } else {
 461                               "".to_owned()
 462                           }
 463                           ));
 464     }
 465
 466     /// Reports that an overflow has occurred and halts compilation. We
 467     /// halt compilation unconditionally because it is important that
 468     /// overflows never be masked -- they basically represent computations
 469     /// whose result could not be truly determined and thus we can't say
 470     /// if the program type checks or not -- and they are unusual
 471     /// occurrences in any case.
 472     pub fn report_overflow_error<T>(&self,
 473                                     obligation: &Obligation<'tcx, T>,
 474                                     suggest_increasing_limit: bool) -> !
 475         where T: fmt::Display + TypeFoldable<'tcx>
 476     {
 477         let predicate =
 478             self.resolve_type_vars_if_possible(&obligation.predicate);
 479         let mut err = struct_span_err!(self.tcx.sess, obligation.cause.span, E0275,
 480                                        "overflow evaluating the requirement `{}`",
 481                                        predicate);
 482
 483         if suggest_increasing_limit {
 484             self.suggest_new_overflow_limit(&mut err);
 485         }
 486
 487         self.note_obligation_cause(&mut err, obligation);
 488
 489         err.emit();
 490         self.tcx.sess.abort_if_errors();
 491         bug!();
 492     }
 493
 494     /// Reports that a cycle was detected which led to overflow and halts
 495     /// compilation. This is equivalent to `report_overflow_error` except
 496     /// that we can give a more helpful error message (and, in particular,
 497     /// we do not suggest increasing the overflow limit, which is not
 498     /// going to help).
 499     pub fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 500         let cycle = self.resolve_type_vars_if_possible(&cycle.to_owned());
 501         assert!(cycle.len() > 0);
 502
 503         debug!("report_overflow_error_cycle: cycle={:?}", cycle);
 504
 505         self.report_overflow_error(&cycle[0], false);
 506     }
 507
 508     pub fn report_extra_impl_obligation(&self,
 509                                         error_span: Span,
 510                                         item_name: ast::Name,
 511                                         _impl_item_def_id: DefId,
 512                                         trait_item_def_id: DefId,
 513                                         requirement: &dyn fmt::Display)
 514                                         -> DiagnosticBuilder<'tcx>
 515     {
 516         let msg = "impl has stricter requirements than trait";
 517         let sp = self.tcx.sess.codemap().def_span(error_span);
 518
 519         let mut err = struct_span_err!(self.tcx.sess, sp, E0276, "{}", msg);
 520
 521         if let Some(trait_item_span) = self.tcx.hir.span_if_local(trait_item_def_id) {
 522             let span = self.tcx.sess.codemap().def_span(trait_item_span);
 523             err.span_label(span, format!("definition of `{}` from trait", item_name));
 524         }
 525
 526         err.span_label(sp, format!("impl has extra requirement {}", requirement));
 527
 528         err
 529     }
 530
 531
 532     /// Get the parent trait chain start
 533     fn get_parent_trait_ref(&self, code: &ObligationCauseCode<'tcx>) -> Option<String> {
 534         match code {
 535             &ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
 536                 let parent_trait_ref = self.resolve_type_vars_if_possible(
 537                     &data.parent_trait_ref);
 538                 match self.get_parent_trait_ref(&data.parent_code) {
 539                     Some(t) => Some(t),
 540                     None => Some(format!("{}", parent_trait_ref.skip_binder().self_ty())),
 541                 }
 542             }
 543             _ => None,
 544         }
 545     }
 546
 547     pub fn report_selection_error(&self,
 548                                   obligation: &PredicateObligation<'tcx>,
 549                                   error: &SelectionError<'tcx>,
 550                                   fallback_has_occurred: bool)
 551     {
 552         let span = obligation.cause.span;
 553
 554         let mut err = match *error {
 555             SelectionError::Unimplemented => {
 556                 if let ObligationCauseCode::CompareImplMethodObligation {
 557                     item_name, impl_item_def_id, trait_item_def_id,
 558                 } = obligation.cause.code {
 559                     self.report_extra_impl_obligation(
 560                         span,
 561                         item_name,
 562                         impl_item_def_id,
 563                         trait_item_def_id,
 564                         &format!("`{}`", obligation.predicate))
 565                         .emit();
 566                     return;
 567                 }
 568                 match obligation.predicate {
 569                     ty::Predicate::Trait(ref trait_predicate) => {
 570                         let trait_predicate =
 571                             self.resolve_type_vars_if_possible(trait_predicate);
 572
 573                         if self.tcx.sess.has_errors() && trait_predicate.references_error() {
 574                             return;
 575                         }
 576                         let trait_ref = trait_predicate.to_poly_trait_ref();
 577                         let (post_message, pre_message) =
 578                             self.get_parent_trait_ref(&obligation.cause.code)
 579                                 .map(|t| (format!(" in `{}`", t), format!("within `{}`, ", t)))
 580                             .unwrap_or((String::new(), String::new()));
 581
 582                         let OnUnimplementedNote { message, label, note }
 583                             = self.on_unimplemented_note(trait_ref, obligation);
 584                         let have_alt_message = message.is_some() || label.is_some();
 585
 586                         let mut err = struct_span_err!(
 587                             self.tcx.sess,
 588                             span,
 589                             E0277,
 590                             "{}",
 591                             message.unwrap_or_else(|| {
 592                                 format!("the trait bound `{}` is not satisfied{}",
 593                                          trait_ref.to_predicate(), post_message)
 594                             }));
 595
 596                         let explanation =
 597                             if obligation.cause.code == ObligationCauseCode::MainFunctionType {
 598                                 "consider using `()`, or a `Result`".to_owned()
 599                             } else {
 600                                 format!("{}the trait `{}` is not implemented for `{}`",
 601                                         pre_message,
 602                                         trait_ref,
 603                                         trait_ref.self_ty())
 604                             };
 605
 606                         if let Some(ref s) = label {
 607                             // If it has a custom "#[rustc_on_unimplemented]"
 608                             // error message, let's display it as the label!
 609                             err.span_label(span, s.as_str());
 610                             err.help(&explanation);
 611                         } else {
 612                             err.span_label(span, explanation);
 613                         }
 614                         if let Some(ref s) = note {
 615                             // If it has a custom "#[rustc_on_unimplemented]" note, let's display it
 616                             err.note(s.as_str());
 617                         }
 618
 619                         self.suggest_borrow_on_unsized_slice(&obligation.cause.code, &mut err);
 620                         self.suggest_remove_reference(&obligation, &mut err, &trait_ref);
 621
 622                         // Try to report a help message
 623                         if !trait_ref.has_infer_types() &&
 624                             self.predicate_can_apply(obligation.param_env, trait_ref) {
 625                             // If a where-clause may be useful, remind the
 626                             // user that they can add it.
 627                             //
 628                             // don't display an on-unimplemented note, as
 629                             // these notes will often be of the form
 630                             //     "the type `T` can't be frobnicated"
 631                             // which is somewhat confusing.
 632                             err.help(&format!("consider adding a `where {}` bound",
 633                                                 trait_ref.to_predicate()));
 634                         } else if !have_alt_message {
 635                             // Can't show anything else useful, try to find similar impls.
 636                             let impl_candidates = self.find_similar_impl_candidates(trait_ref);
 637                             self.report_similar_impl_candidates(impl_candidates, &mut err);
 638                         }
 639
 640                         // If this error is due to `!: Trait` not implemented but `(): Trait` is
 641                         // implemented, and fallback has occured, then it could be due to a
 642                         // variable that used to fallback to `()` now falling back to `!`. Issue a
 643                         // note informing about the change in behaviour.
 644                         if trait_predicate.skip_binder().self_ty().is_never()
 645                             && fallback_has_occurred
 646                         {
 647                             let predicate = trait_predicate.map_bound(|mut trait_pred| {
 648                                 {
 649                                     let trait_ref = &mut trait_pred.trait_ref;
 650                                     let never_substs = trait_ref.substs;
 651                                     let mut unit_substs = Vec::with_capacity(never_substs.len());
 652                                     unit_substs.push(self.tcx.mk_nil().into());
 653                                     unit_substs.extend(&never_substs[1..]);
 654                                     trait_ref.substs = self.tcx.intern_substs(&unit_substs);
 655                                 }
 656                                 trait_pred
 657                             });
 658                             let unit_obligation = Obligation {
 659                                 predicate: ty::Predicate::Trait(predicate),
 660                                 .. obligation.clone()
 661                             };
 662                             if self.predicate_may_hold(&unit_obligation) {
 663                                 err.note("the trait is implemented for `()`. \
 664                                          Possibly this error has been caused by changes to \
 665                                          Rust's type-inference algorithm \
 666                                          (see: https://github.com/rust-lang/rust/issues/48950 \
 667                                          for more info). Consider whether you meant to use the \
 668                                          type `()` here instead.");
 669                             }
 670                         }
 671
 672                         err
 673                     }
 674
 675                     ty::Predicate::Subtype(ref predicate) => {
 676                         // Errors for Subtype predicates show up as
 677                         // `FulfillmentErrorCode::CodeSubtypeError`,
 678                         // not selection error.
 679                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
 680                     }
 681
 682                     ty::Predicate::RegionOutlives(ref predicate) => {
 683                         let predicate = self.resolve_type_vars_if_possible(predicate);
 684                         let err = self.region_outlives_predicate(&obligation.cause,
 685                                                                     &predicate).err().unwrap();
 686                         struct_span_err!(self.tcx.sess, span, E0279,
 687                             "the requirement `{}` is not satisfied (`{}`)",
 688                             predicate, err)
 689                     }
 690
 691                     ty::Predicate::Projection(..) | ty::Predicate::TypeOutlives(..) => {
 692                         let predicate =
 693                             self.resolve_type_vars_if_possible(&obligation.predicate);
 694                         struct_span_err!(self.tcx.sess, span, E0280,
 695                             "the requirement `{}` is not satisfied",
 696                             predicate)
 697                     }
 698
 699                     ty::Predicate::ObjectSafe(trait_def_id) => {
 700                         let violations = self.tcx.object_safety_violations(trait_def_id);
 701                         self.tcx.report_object_safety_error(span,
 702                                                             trait_def_id,
 703                                                             violations)
 704                     }
 705
 706                     ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => {
 707                         let found_kind = self.closure_kind(closure_def_id, closure_substs).unwrap();
 708                         let closure_span = self.tcx.sess.codemap()
 709                             .def_span(self.tcx.hir.span_if_local(closure_def_id).unwrap());
 710                         let node_id = self.tcx.hir.as_local_node_id(closure_def_id).unwrap();
 711                         let mut err = struct_span_err!(
 712                             self.tcx.sess, closure_span, E0525,
 713                             "expected a closure that implements the `{}` trait, \
 714                                 but this closure only implements `{}`",
 715                             kind,
 716                             found_kind);
 717
 718                         err.span_label(
 719                             closure_span,
 720                             format!("this closure implements `{}`, not `{}`", found_kind, kind));
 721                         err.span_label(
 722                             obligation.cause.span,
 723                             format!("the requirement to implement `{}` derives from here", kind));
 724
 725                         // Additional context information explaining why the closure only implements
 726                         // a particular trait.
 727                         if let Some(tables) = self.in_progress_tables {
 728                             let tables = tables.borrow();
 729                             let closure_hir_id = self.tcx.hir.node_to_hir_id(node_id);
 730                             match (found_kind, tables.closure_kind_origins().get(closure_hir_id)) {
 731                                 (ty::ClosureKind::FnOnce, Some((span, name))) => {
 732                                     err.span_label(*span, format!(
 733                                         "closure is `FnOnce` because it moves the \
 734                                          variable `{}` out of its environment", name));
 735                                 },
 736                                 (ty::ClosureKind::FnMut, Some((span, name))) => {
 737                                     err.span_label(*span, format!(
 738                                         "closure is `FnMut` because it mutates the \
 739                                          variable `{}` here", name));
 740                                 },
 741                                 _ => {}
 742                             }
 743                         }
 744
 745                         err.emit();
 746                         return;
 747                     }
 748
 749                     ty::Predicate::WellFormed(ty) => {
 750                         // WF predicates cannot themselves make
 751                         // errors. They can only block due to
 752                         // ambiguity; otherwise, they always
 753                         // degenerate into other obligations
 754                         // (which may fail).
 755                         span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 756                     }
 757
 758                     ty::Predicate::ConstEvaluatable(..) => {
 759                         // Errors for `ConstEvaluatable` predicates show up as
 760                         // `SelectionError::ConstEvalFailure`,
 761                         // not `Unimplemented`.
 762                         span_bug!(span,
 763                             "const-evaluatable requirement gave wrong error: `{:?}`", obligation)
 764                     }
 765                 }
 766             }
 767
 768             OutputTypeParameterMismatch(ref found_trait_ref, ref expected_trait_ref, _) => {
 769                 let found_trait_ref = self.resolve_type_vars_if_possible(&*found_trait_ref);
 770                 let expected_trait_ref = self.resolve_type_vars_if_possible(&*expected_trait_ref);
 771                 if expected_trait_ref.self_ty().references_error() {
 772                     return;
 773                 }
 774                 let found_trait_ty = found_trait_ref.self_ty();
 775
 776                 let found_did = found_trait_ty.ty_to_def_id();
 777                 let found_span = found_did.and_then(|did| {
 778                     self.tcx.hir.span_if_local(did)
 779                 }).map(|sp| self.tcx.sess.codemap().def_span(sp)); // the sp could be an fn def
 780
 781                 let found = match found_trait_ref.skip_binder().substs.type_at(1).sty {
 782                     ty::TyTuple(ref tys) => tys.iter()
 783                         .map(|_| ArgKind::empty()).collect::<Vec<_>>(),
 784                     _ => vec![ArgKind::empty()],
 785                 };
 786                 let expected = match expected_trait_ref.skip_binder().substs.type_at(1).sty {
 787                     ty::TyTuple(ref tys) => tys.iter()
 788                         .map(|t| match t.sty {
 789                             ty::TypeVariants::TyTuple(ref tys) => ArgKind::Tuple(
 790                                 Some(span),
 791                                 tys.iter()
 792                                     .map(|ty| ("_".to_owned(), format!("{}", ty.sty)))
 793                                     .collect::<Vec<_>>()
 794                             ),
 795                             _ => ArgKind::Arg("_".to_owned(), format!("{}", t.sty)),
 796                         }).collect(),
 797                     ref sty => vec![ArgKind::Arg("_".to_owned(), format!("{}", sty))],
 798                 };
 799                 if found.len() == expected.len() {
 800                     self.report_closure_arg_mismatch(span,
 801                                                      found_span,
 802                                                      found_trait_ref,
 803                                                      expected_trait_ref)
 804                 } else {
 805                     let (closure_span, found) = found_did
 806                         .and_then(|did| self.tcx.hir.get_if_local(did))
 807                         .map(|node| {
 808                             let (found_span, found) = self.get_fn_like_arguments(node);
 809                             (Some(found_span), found)
 810                         }).unwrap_or((found_span, found));
 811
 812                     self.report_arg_count_mismatch(span,
 813                                                    closure_span,
 814                                                    expected,
 815                                                    found,
 816                                                    found_trait_ty.is_closure())
 817                 }
 818             }
 819
 820             TraitNotObjectSafe(did) => {
 821                 let violations = self.tcx.object_safety_violations(did);
 822                 self.tcx.report_object_safety_error(span, did,
 823                                                     violations)
 824             }
 825
 826             ConstEvalFailure(ref err) => {
 827                 if let ::middle::const_val::ErrKind::TypeckError = *err.kind {
 828                     return;
 829                 }
 830                 err.struct_error(self.tcx, span, "constant expression")
 831             }
 832
 833             Overflow => {
 834                 bug!("overflow should be handled before the `report_selection_error` path");
 835             }
 836         };
 837         self.note_obligation_cause(&mut err, obligation);
 838         err.emit();
 839     }
 840
 841     /// When encountering an assignment of an unsized trait, like `let x = ""[..];`, provide a
 842     /// suggestion to borrow the initializer in order to use have a slice instead.
 843     fn suggest_borrow_on_unsized_slice(&self,
 844                                        code: &ObligationCauseCode<'tcx>,
 845                                        err: &mut DiagnosticBuilder<'tcx>) {
 846         if let &ObligationCauseCode::VariableType(node_id) = code {
 847             let parent_node = self.tcx.hir.get_parent_node(node_id);
 848             if let Some(hir::map::NodeLocal(ref local)) = self.tcx.hir.find(parent_node) {
 849                 if let Some(ref expr) = local.init {
 850                     if let hir::ExprIndex(_, _) = expr.node {
 851                         if let Ok(snippet) = self.tcx.sess.codemap().span_to_snippet(expr.span) {
 852                             err.span_suggestion(expr.span,
 853                                                 "consider borrowing here",
 854                                                 format!("&{}", snippet));
 855                         }
 856                     }
 857                 }
 858             }
 859         }
 860     }
 861
 862     /// Whenever references are used by mistake, like `for (i, e) in &vec.iter().enumerate()`,
 863     /// suggest removing these references until we reach a type that implements the trait.
 864     fn suggest_remove_reference(&self,
 865                                 obligation: &PredicateObligation<'tcx>,
 866                                 err: &mut DiagnosticBuilder<'tcx>,
 867                                 trait_ref: &ty::Binder<ty::TraitRef<'tcx>>) {
 868         let trait_ref = trait_ref.skip_binder();
 869         let span = obligation.cause.span;
 870
 871         if let Ok(snippet) = self.tcx.sess.codemap().span_to_snippet(span) {
 872             let refs_number = snippet.chars()
 873                 .filter(|c| !c.is_whitespace())
 874                 .take_while(|c| *c == '&')
 875                 .count();
 876
 877             let mut trait_type = trait_ref.self_ty();
 878
 879             for refs_remaining in 0..refs_number {
 880                 if let ty::TypeVariants::TyRef(_, t_type, _) = trait_type.sty {
 881                     trait_type = t_type;
 882
 883                     let substs = self.tcx.mk_substs_trait(trait_type, &[]);
 884                     let new_trait_ref = ty::TraitRef::new(trait_ref.def_id, substs);
 885                     let new_obligation = Obligation::new(ObligationCause::dummy(),
 886                                                          obligation.param_env,
 887                                                          new_trait_ref.to_predicate());
 888
 889                     if self.predicate_may_hold(&new_obligation) {
 890                         let sp = self.tcx.sess.codemap()
 891                             .span_take_while(span, |c| c.is_whitespace() || *c == '&');
 892
 893                         let remove_refs = refs_remaining + 1;
 894                         let format_str = format!("consider removing {} leading `&`-references",
 895                                                  remove_refs);
 896
 897                         err.span_suggestion_short(sp, &format_str, String::from(""));
 898                         break;
 899                     }
 900                 } else {
 901                     break;
 902                 }
 903             }
 904         }
 905     }
 906
 907     /// Given some node representing a fn-like thing in the HIR map,
 908     /// returns a span and `ArgKind` information that describes the
 909     /// arguments it expects. This can be supplied to
 910     /// `report_arg_count_mismatch`.
 911     pub fn get_fn_like_arguments(&self, node: hir::map::Node) -> (Span, Vec<ArgKind>) {
 912         match node {
 913             hir::map::NodeExpr(&hir::Expr {
 914                 node: hir::ExprClosure(_, ref _decl, id, span, _),
 915                 ..
 916             }) => {
 917                 (self.tcx.sess.codemap().def_span(span), self.tcx.hir.body(id).arguments.iter()
 918                     .map(|arg| {
 919                         if let hir::Pat {
 920                             node: hir::PatKind::Tuple(args, _),
 921                             span,
 922                             ..
 923                         } = arg.pat.clone().into_inner() {
 924                             ArgKind::Tuple(
 925                                 Some(span),
 926                                 args.iter().map(|pat| {
 927                                     let snippet = self.tcx.sess.codemap()
 928                                         .span_to_snippet(pat.span).unwrap();
 929                                     (snippet, "_".to_owned())
 930                                 }).collect::<Vec<_>>(),
 931                             )
 932                         } else {
 933                             let name = self.tcx.sess.codemap()
 934                                 .span_to_snippet(arg.pat.span).unwrap();
 935                             ArgKind::Arg(name, "_".to_owned())
 936                         }
 937                     })
 938                     .collect::<Vec<ArgKind>>())
 939             }
 940             hir::map::NodeItem(&hir::Item {
 941                 span,
 942                 node: hir::ItemFn(ref decl, ..),
 943                 ..
 944             }) |
 945             hir::map::NodeImplItem(&hir::ImplItem {
 946                 span,
 947                 node: hir::ImplItemKind::Method(hir::MethodSig { ref decl, .. }, _),
 948                 ..
 949             }) |
 950             hir::map::NodeTraitItem(&hir::TraitItem {
 951                 span,
 952                 node: hir::TraitItemKind::Method(hir::MethodSig { ref decl, .. }, _),
 953                 ..
 954             }) => {
 955                 (self.tcx.sess.codemap().def_span(span), decl.inputs.iter()
 956                         .map(|arg| match arg.clone().into_inner().node {
 957                     hir::TyTup(ref tys) => ArgKind::Tuple(
 958                         Some(arg.span),
 959                         tys.iter()
 960                             .map(|_| ("_".to_owned(), "_".to_owned()))
 961                             .collect::<Vec<_>>(),
 962                     ),
 963                     _ => ArgKind::Arg("_".to_owned(), "_".to_owned())
 964                 }).collect::<Vec<ArgKind>>())
 965             }
 966             hir::map::NodeVariant(&hir::Variant {
 967                 span,
 968                 node: hir::Variant_ {
 969                     data: hir::VariantData::Tuple(ref fields, _),
 970                     ..
 971                 },
 972                 ..
 973             }) => {
 974                 (self.tcx.sess.codemap().def_span(span),
 975                  fields.iter().map(|field| {
 976                      ArgKind::Arg(format!("{}", field.name), "_".to_string())
 977                  }).collect::<Vec<_>>())
 978             }
 979             hir::map::NodeStructCtor(ref variant_data) => {
 980                 (self.tcx.sess.codemap().def_span(self.tcx.hir.span(variant_data.id())),
 981                  variant_data.fields()
 982                     .iter().map(|_| ArgKind::Arg("_".to_owned(), "_".to_owned()))
 983                     .collect())
 984             }
 985             _ => panic!("non-FnLike node found: {:?}", node),
 986         }
 987     }
 988
 989     /// Reports an error when the number of arguments needed by a
 990     /// trait match doesn't match the number that the expression
 991     /// provides.
 992     pub fn report_arg_count_mismatch(
 993         &self,
 994         span: Span,
 995         found_span: Option<Span>,
 996         expected_args: Vec<ArgKind>,
 997         found_args: Vec<ArgKind>,
 998         is_closure: bool,
 999     ) -> DiagnosticBuilder<'tcx> {
1000         let kind = if is_closure { "closure" } else { "function" };
1001
1002         let args_str = |arguments: &Vec<ArgKind>, other: &Vec<ArgKind>| {
1003             let arg_length = arguments.len();
1004             let distinct = match &other[..] {
1005                 &[ArgKind::Tuple(..)] => true,
1006                 _ => false,
1007             };
1008             match (arg_length, arguments.get(0)) {
1009                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
1010                     format!("a single {}-tuple as argument", fields.len())
1011                 }
1012                 _ => format!("{} {}argument{}",
1013                              arg_length,
1014                              if distinct && arg_length > 1 { "distinct " } else { "" },
1015                              if arg_length == 1 { "" } else { "s" }),
1016             }
1017         };
1018
1019         let expected_str = args_str(&expected_args, &found_args);
1020         let found_str = args_str(&found_args, &expected_args);
1021
1022         let mut err = struct_span_err!(
1023             self.tcx.sess,
1024             span,
1025             E0593,
1026             "{} is expected to take {}, but it takes {}",
1027             kind,
1028             expected_str,
1029             found_str,
1030         );
1031
1032         err.span_label(span, format!( "expected {} that takes {}", kind, expected_str));
1033
1034         if let Some(found_span) = found_span {
1035             err.span_label(found_span, format!("takes {}", found_str));
1036
1037             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
1038                 if fields.len() == expected_args.len() {
1039                     let sugg = fields.iter()
1040                         .map(|(name, _)| name.to_owned())
1041                         .collect::<Vec<String>>().join(", ");
1042                     err.span_suggestion(found_span,
1043                                         "change the closure to take multiple arguments instead of \
1044                                          a single tuple",
1045                                         format!("|{}|", sugg));
1046                 }
1047             }
1048             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..] {
1049                 if fields.len() == found_args.len() && is_closure {
1050                     let sugg = format!(
1051                         "|({}){}|",
1052                         found_args.iter()
1053                             .map(|arg| match arg {
1054                                 ArgKind::Arg(name, _) => name.to_owned(),
1055                                 _ => "_".to_owned(),
1056                             })
1057                             .collect::<Vec<String>>()
1058                             .join(", "),
1059                         // add type annotations if available
1060                         if found_args.iter().any(|arg| match arg {
1061                             ArgKind::Arg(_, ty) => ty != "_",
1062                             _ => false,
1063                         }) {
1064                             format!(": ({})",
1065                                     fields.iter()
1066                                         .map(|(_, ty)| ty.to_owned())
1067                                         .collect::<Vec<String>>()
1068                                         .join(", "))
1069                         } else {
1070                             "".to_owned()
1071                         },
1072                     );
1073                     err.span_suggestion(found_span,
1074                                         "change the closure to accept a tuple instead of \
1075                                          individual arguments",
1076                                         sugg);
1077                 }
1078             }
1079         }
1080
1081         err
1082     }
1083
1084     fn report_closure_arg_mismatch(&self,
1085                            span: Span,
1086                            found_span: Option<Span>,
1087                            expected_ref: ty::PolyTraitRef<'tcx>,
1088                            found: ty::PolyTraitRef<'tcx>)
1089         -> DiagnosticBuilder<'tcx>
1090     {
1091         fn build_fn_sig_string<'a, 'gcx, 'tcx>(tcx: ty::TyCtxt<'a, 'gcx, 'tcx>,
1092                                                trait_ref: &ty::TraitRef<'tcx>) -> String {
1093             let inputs = trait_ref.substs.type_at(1);
1094             let sig = if let ty::TyTuple(inputs) = inputs.sty {
1095                 tcx.mk_fn_sig(
1096                     inputs.iter().map(|&x| x),
1097                     tcx.mk_infer(ty::TyVar(ty::TyVid { index: 0 })),
1098                     false,
1099                     hir::Unsafety::Normal,
1100                     ::rustc_target::spec::abi::Abi::Rust
1101                 )
1102             } else {
1103                 tcx.mk_fn_sig(
1104                     ::std::iter::once(inputs),
1105                     tcx.mk_infer(ty::TyVar(ty::TyVid { index: 0 })),
1106                     false,
1107                     hir::Unsafety::Normal,
1108                     ::rustc_target::spec::abi::Abi::Rust
1109                 )
1110             };
1111             format!("{}", ty::Binder::bind(sig))
1112         }
1113
1114         let argument_is_closure = expected_ref.skip_binder().substs.type_at(0).is_closure();
1115         let mut err = struct_span_err!(self.tcx.sess, span, E0631,
1116                                        "type mismatch in {} arguments",
1117                                        if argument_is_closure { "closure" } else { "function" });
1118
1119         let found_str = format!(
1120             "expected signature of `{}`",
1121             build_fn_sig_string(self.tcx, found.skip_binder())
1122         );
1123         err.span_label(span, found_str);
1124
1125         let found_span = found_span.unwrap_or(span);
1126         let expected_str = format!(
1127             "found signature of `{}`",
1128             build_fn_sig_string(self.tcx, expected_ref.skip_binder())
1129         );
1130         err.span_label(found_span, expected_str);
1131
1132         err
1133     }
1134 }
1135
1136 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
1137     pub fn recursive_type_with_infinite_size_error(self,
1138                                                    type_def_id: DefId)
1139                                                    -> DiagnosticBuilder<'tcx>
1140     {
1141         assert!(type_def_id.is_local());
1142         let span = self.hir.span_if_local(type_def_id).unwrap();
1143         let span = self.sess.codemap().def_span(span);
1144         let mut err = struct_span_err!(self.sess, span, E0072,
1145                                        "recursive type `{}` has infinite size",
1146                                        self.item_path_str(type_def_id));
1147         err.span_label(span, "recursive type has infinite size");
1148         err.help(&format!("insert indirection (e.g., a `Box`, `Rc`, or `&`) \
1149                            at some point to make `{}` representable",
1150                           self.item_path_str(type_def_id)));
1151         err
1152     }
1153
1154     pub fn report_object_safety_error(self,
1155                                       span: Span,
1156                                       trait_def_id: DefId,
1157                                       violations: Vec<ObjectSafetyViolation>)
1158                                       -> DiagnosticBuilder<'tcx>
1159     {
1160         let trait_str = self.item_path_str(trait_def_id);
1161         let span = self.sess.codemap().def_span(span);
1162         let mut err = struct_span_err!(
1163             self.sess, span, E0038,
1164             "the trait `{}` cannot be made into an object",
1165             trait_str);
1166         err.span_label(span, format!("the trait `{}` cannot be made into an object", trait_str));
1167
1168         let mut reported_violations = FxHashSet();
1169         for violation in violations {
1170             if !reported_violations.insert(violation.clone()) {
1171                 continue;
1172             }
1173             err.note(&violation.error_msg());
1174         }
1175         err
1176     }
1177 }
1178
1179 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
1180     fn maybe_report_ambiguity(&self, obligation: &PredicateObligation<'tcx>,
1181                               body_id: Option<hir::BodyId>) {
1182         // Unable to successfully determine, probably means
1183         // insufficient type information, but could mean
1184         // ambiguous impls. The latter *ought* to be a
1185         // coherence violation, so we don't report it here.
1186
1187         let predicate = self.resolve_type_vars_if_possible(&obligation.predicate);
1188         let span = obligation.cause.span;
1189
1190         debug!("maybe_report_ambiguity(predicate={:?}, obligation={:?})",
1191                predicate,
1192                obligation);
1193
1194         // Ambiguity errors are often caused as fallout from earlier
1195         // errors. So just ignore them if this infcx is tainted.
1196         if self.is_tainted_by_errors() {
1197             return;
1198         }
1199
1200         match predicate {
1201             ty::Predicate::Trait(ref data) => {
1202                 let trait_ref = data.to_poly_trait_ref();
1203                 let self_ty = trait_ref.self_ty();
1204                 if predicate.references_error() {
1205                     return;
1206                 }
1207                 // Typically, this ambiguity should only happen if
1208                 // there are unresolved type inference variables
1209                 // (otherwise it would suggest a coherence
1210                 // failure). But given #21974 that is not necessarily
1211                 // the case -- we can have multiple where clauses that
1212                 // are only distinguished by a region, which results
1213                 // in an ambiguity even when all types are fully
1214                 // known, since we don't dispatch based on region
1215                 // relationships.
1216
1217                 // This is kind of a hack: it frequently happens that some earlier
1218                 // error prevents types from being fully inferred, and then we get
1219                 // a bunch of uninteresting errors saying something like "<generic
1220                 // #0> doesn't implement Sized".  It may even be true that we
1221                 // could just skip over all checks where the self-ty is an
1222                 // inference variable, but I was afraid that there might be an
1223                 // inference variable created, registered as an obligation, and
1224                 // then never forced by writeback, and hence by skipping here we'd
1225                 // be ignoring the fact that we don't KNOW the type works
1226                 // out. Though even that would probably be harmless, given that
1227                 // we're only talking about builtin traits, which are known to be
1228                 // inhabited. But in any case I just threw in this check for
1229                 // has_errors() to be sure that compilation isn't happening
1230                 // anyway. In that case, why inundate the user.
1231                 if !self.tcx.sess.has_errors() {
1232                     if
1233                         self.tcx.lang_items().sized_trait()
1234                         .map_or(false, |sized_id| sized_id == trait_ref.def_id())
1235                     {
1236                         self.need_type_info(body_id, span, self_ty);
1237                     } else {
1238                         let mut err = struct_span_err!(self.tcx.sess,
1239                                                         span, E0283,
1240                                                         "type annotations required: \
1241                                                         cannot resolve `{}`",
1242                                                         predicate);
1243                         self.note_obligation_cause(&mut err, obligation);
1244                         err.emit();
1245                     }
1246                 }
1247             }
1248
1249             ty::Predicate::WellFormed(ty) => {
1250                 // Same hacky approach as above to avoid deluging user
1251                 // with error messages.
1252                 if !ty.references_error() && !self.tcx.sess.has_errors() {
1253                     self.need_type_info(body_id, span, ty);
1254                 }
1255             }
1256
1257             ty::Predicate::Subtype(ref data) => {
1258                 if data.references_error() || self.tcx.sess.has_errors() {
1259                     // no need to overload user in such cases
1260                 } else {
1261                     let &SubtypePredicate { a_is_expected: _, a, b } = data.skip_binder();
1262                     // both must be type variables, or the other would've been instantiated
1263                     assert!(a.is_ty_var() && b.is_ty_var());
1264                     self.need_type_info(body_id,
1265                                         obligation.cause.span,
1266                                         a);
1267                 }
1268             }
1269
1270             _ => {
1271                 if !self.tcx.sess.has_errors() {
1272                     let mut err = struct_span_err!(self.tcx.sess,
1273                                                    obligation.cause.span, E0284,
1274                                                    "type annotations required: \
1275                                                     cannot resolve `{}`",
1276                                                    predicate);
1277                     self.note_obligation_cause(&mut err, obligation);
1278                     err.emit();
1279                 }
1280             }
1281         }
1282     }
1283
1284     /// Returns whether the trait predicate may apply for *some* assignment
1285     /// to the type parameters.
1286     fn predicate_can_apply(&self,
1287                            param_env: ty::ParamEnv<'tcx>,
1288                            pred: ty::PolyTraitRef<'tcx>)
1289                            -> bool {
1290         struct ParamToVarFolder<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
1291             infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
1292             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>
1293         }
1294
1295         impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for ParamToVarFolder<'a, 'gcx, 'tcx> {
1296             fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> { self.infcx.tcx }
1297
1298             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
1299                 if let ty::TyParam(ty::ParamTy {name, ..}) = ty.sty {
1300                     let infcx = self.infcx;
1301                     self.var_map.entry(ty).or_insert_with(||
1302                         infcx.next_ty_var(
1303                             TypeVariableOrigin::TypeParameterDefinition(DUMMY_SP, name)))
1304                 } else {
1305                     ty.super_fold_with(self)
1306                 }
1307             }
1308         }
1309
1310         self.probe(|_| {
1311             let mut selcx = SelectionContext::new(self);
1312
1313             let cleaned_pred = pred.fold_with(&mut ParamToVarFolder {
1314                 infcx: self,
1315                 var_map: FxHashMap()
1316             });
1317
1318             let cleaned_pred = super::project::normalize(
1319                 &mut selcx,
1320                 param_env,
1321                 ObligationCause::dummy(),
1322                 &cleaned_pred
1323             ).value;
1324
1325             let obligation = Obligation::new(
1326                 ObligationCause::dummy(),
1327                 param_env,
1328                 cleaned_pred.to_predicate()
1329             );
1330
1331             self.predicate_may_hold(&obligation)
1332         })
1333     }
1334
1335     fn note_obligation_cause<T>(&self,
1336                                 err: &mut DiagnosticBuilder,
1337                                 obligation: &Obligation<'tcx, T>)
1338         where T: fmt::Display
1339     {
1340         self.note_obligation_cause_code(err,
1341                                         &obligation.predicate,
1342                                         &obligation.cause.code,
1343                                         &mut vec![]);
1344     }
1345
1346     fn note_obligation_cause_code<T>(&self,
1347                                      err: &mut DiagnosticBuilder,
1348                                      predicate: &T,
1349                                      cause_code: &ObligationCauseCode<'tcx>,
1350                                      obligated_types: &mut Vec<&ty::TyS<'tcx>>)
1351         where T: fmt::Display
1352     {
1353         let tcx = self.tcx;
1354         match *cause_code {
1355             ObligationCauseCode::ExprAssignable |
1356             ObligationCauseCode::MatchExpressionArm { .. } |
1357             ObligationCauseCode::IfExpression |
1358             ObligationCauseCode::IfExpressionWithNoElse |
1359             ObligationCauseCode::MainFunctionType |
1360             ObligationCauseCode::StartFunctionType |
1361             ObligationCauseCode::IntrinsicType |
1362             ObligationCauseCode::MethodReceiver |
1363             ObligationCauseCode::ReturnNoExpression |
1364             ObligationCauseCode::MiscObligation => {
1365             }
1366             ObligationCauseCode::SliceOrArrayElem => {
1367                 err.note("slice and array elements must have `Sized` type");
1368             }
1369             ObligationCauseCode::TupleElem => {
1370                 err.note("only the last element of a tuple may have a dynamically sized type");
1371             }
1372             ObligationCauseCode::ProjectionWf(data) => {
1373                 err.note(&format!("required so that the projection `{}` is well-formed",
1374                                   data));
1375             }
1376             ObligationCauseCode::ReferenceOutlivesReferent(ref_ty) => {
1377                 err.note(&format!("required so that reference `{}` does not outlive its referent",
1378                                   ref_ty));
1379             }
1380             ObligationCauseCode::ObjectTypeBound(object_ty, region) => {
1381                 err.note(&format!("required so that the lifetime bound of `{}` for `{}` \
1382                                    is satisfied",
1383                                   region, object_ty));
1384             }
1385             ObligationCauseCode::ItemObligation(item_def_id) => {
1386                 let item_name = tcx.item_path_str(item_def_id);
1387                 let msg = format!("required by `{}`", item_name);
1388                 if let Some(sp) = tcx.hir.span_if_local(item_def_id) {
1389                     let sp = tcx.sess.codemap().def_span(sp);
1390                     err.span_note(sp, &msg);
1391                 } else {
1392                     err.note(&msg);
1393                 }
1394             }
1395             ObligationCauseCode::ObjectCastObligation(object_ty) => {
1396                 err.note(&format!("required for the cast to the object type `{}`",
1397                                   self.ty_to_string(object_ty)));
1398             }
1399             ObligationCauseCode::RepeatVec => {
1400                 err.note("the `Copy` trait is required because the \
1401                           repeated element will be copied");
1402             }
1403             ObligationCauseCode::VariableType(_) => {
1404                 err.note("all local variables must have a statically known size");
1405             }
1406             ObligationCauseCode::SizedReturnType => {
1407                 err.note("the return type of a function must have a \
1408                           statically known size");
1409             }
1410             ObligationCauseCode::SizedYieldType => {
1411                 err.note("the yield type of a generator must have a \
1412                           statically known size");
1413             }
1414             ObligationCauseCode::AssignmentLhsSized => {
1415                 err.note("the left-hand-side of an assignment must have a statically known size");
1416             }
1417             ObligationCauseCode::TupleInitializerSized => {
1418                 err.note("tuples must have a statically known size to be initialized");
1419             }
1420             ObligationCauseCode::StructInitializerSized => {
1421                 err.note("structs must have a statically known size to be initialized");
1422             }
1423             ObligationCauseCode::FieldSized(ref item) => {
1424                 match *item {
1425                     AdtKind::Struct => {
1426                         err.note("only the last field of a struct may have a dynamically \
1427                                   sized type");
1428                     }
1429                     AdtKind::Union => {
1430                         err.note("no field of a union may have a dynamically sized type");
1431                     }
1432                     AdtKind::Enum => {
1433                         err.note("no field of an enum variant may have a dynamically sized type");
1434                     }
1435                 }
1436             }
1437             ObligationCauseCode::ConstSized => {
1438                 err.note("constant expressions must have a statically known size");
1439             }
1440             ObligationCauseCode::SharedStatic => {
1441                 err.note("shared static variables must have a type that implements `Sync`");
1442             }
1443             ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
1444                 let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1445                 let ty = parent_trait_ref.skip_binder().self_ty();
1446                 err.note(&format!("required because it appears within the type `{}`", ty));
1447                 obligated_types.push(ty);
1448
1449                 let parent_predicate = parent_trait_ref.to_predicate();
1450                 if !self.is_recursive_obligation(obligated_types, &data.parent_code) {
1451                     self.note_obligation_cause_code(err,
1452                                                     &parent_predicate,
1453                                                     &data.parent_code,
1454                                                     obligated_types);
1455                 }
1456             }
1457             ObligationCauseCode::ImplDerivedObligation(ref data) => {
1458                 let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1459                 err.note(
1460                     &format!("required because of the requirements on the impl of `{}` for `{}`",
1461                              parent_trait_ref,
1462                              parent_trait_ref.skip_binder().self_ty()));
1463                 let parent_predicate = parent_trait_ref.to_predicate();
1464                 self.note_obligation_cause_code(err,
1465                                             &parent_predicate,
1466                                             &data.parent_code,
1467                                             obligated_types);
1468             }
1469             ObligationCauseCode::CompareImplMethodObligation { .. } => {
1470                 err.note(
1471                     &format!("the requirement `{}` appears on the impl method \
1472                               but not on the corresponding trait method",
1473                              predicate));
1474             }
1475             ObligationCauseCode::ReturnType(_) |
1476             ObligationCauseCode::BlockTailExpression(_) => (),
1477         }
1478     }
1479
1480     fn suggest_new_overflow_limit(&self, err: &mut DiagnosticBuilder) {
1481         let current_limit = self.tcx.sess.recursion_limit.get();
1482         let suggested_limit = current_limit * 2;
1483         err.help(&format!("consider adding a `#![recursion_limit=\"{}\"]` attribute to your crate",
1484                           suggested_limit));
1485     }
1486
1487     fn is_recursive_obligation(&self,
1488                                    obligated_types: &mut Vec<&ty::TyS<'tcx>>,
1489                                    cause_code: &ObligationCauseCode<'tcx>) -> bool {
1490         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
1491             let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1492             for obligated_type in obligated_types {
1493                 if obligated_type == &parent_trait_ref.skip_binder().self_ty() {
1494                     return true;
1495                 }
1496             }
1497         }
1498         return false;
1499     }
1500 }
1501
1502 /// Summarizes information
1503 pub enum ArgKind {
1504     /// An argument of non-tuple type. Parameters are (name, ty)
1505     Arg(String, String),
1506
1507     /// An argument of tuple type. For a "found" argument, the span is
1508     /// the locationo in the source of the pattern. For a "expected"
1509     /// argument, it will be None. The vector is a list of (name, ty)
1510     /// strings for the components of the tuple.
1511     Tuple(Option<Span>, Vec<(String, String)>),
1512 }
1513
1514 impl ArgKind {
1515     fn empty() -> ArgKind {
1516         ArgKind::Arg("_".to_owned(), "_".to_owned())
1517     }
1518
1519     /// Creates an `ArgKind` from the expected type of an
1520     /// argument. This has no name (`_`) and no source spans..
1521     pub fn from_expected_ty(t: Ty<'_>) -> ArgKind {
1522         match t.sty {
1523             ty::TyTuple(ref tys) => ArgKind::Tuple(
1524                 None,
1525                 tys.iter()
1526                    .map(|ty| ("_".to_owned(), format!("{}", ty.sty)))
1527                    .collect::<Vec<_>>()
1528             ),
1529             _ => ArgKind::Arg("_".to_owned(), format!("{}", t.sty)),
1530         }
1531     }
1532 }