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.types.iter() {
 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(),
 386                         Some(ty_str.clone())));
 387         }
 388
 389         if let Some(true) = self_ty.ty_to_def_id().map(|def_id| def_id.is_local()) {
 390             flags.push(("crate_local".to_string(), None));
 391         }
 392
 393         if let Ok(Some(command)) = OnUnimplementedDirective::of_item(
 394             self.tcx, trait_ref.def_id, def_id
 395         ) {
 396             command.evaluate(self.tcx, trait_ref, &flags[..])
 397         } else {
 398             OnUnimplementedNote::empty()
 399         }
 400     }
 401
 402     fn find_similar_impl_candidates(&self,
 403                                     trait_ref: ty::PolyTraitRef<'tcx>)
 404                                     -> Vec<ty::TraitRef<'tcx>>
 405     {
 406         let simp = fast_reject::simplify_type(self.tcx,
 407                                               trait_ref.skip_binder().self_ty(),
 408                                               true);
 409         let mut impl_candidates = Vec::new();
 410
 411         match simp {
 412             Some(simp) => self.tcx.for_each_impl(trait_ref.def_id(), |def_id| {
 413                 let imp = self.tcx.impl_trait_ref(def_id).unwrap();
 414                 let imp_simp = fast_reject::simplify_type(self.tcx,
 415                                                           imp.self_ty(),
 416                                                           true);
 417                 if let Some(imp_simp) = imp_simp {
 418                     if simp != imp_simp {
 419                         return;
 420                     }
 421                 }
 422                 impl_candidates.push(imp);
 423             }),
 424             None => self.tcx.for_each_impl(trait_ref.def_id(), |def_id| {
 425                 impl_candidates.push(
 426                     self.tcx.impl_trait_ref(def_id).unwrap());
 427             })
 428         };
 429         impl_candidates
 430     }
 431
 432     fn report_similar_impl_candidates(&self,
 433                                       impl_candidates: Vec<ty::TraitRef<'tcx>>,
 434                                       err: &mut DiagnosticBuilder)
 435     {
 436         if impl_candidates.is_empty() {
 437             return;
 438         }
 439
 440         let end = if impl_candidates.len() <= 5 {
 441             impl_candidates.len()
 442         } else {
 443             4
 444         };
 445
 446         let normalize = |candidate| self.tcx.global_tcx().infer_ctxt().enter(|ref infcx| {
 447             let normalized = infcx
 448                 .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
 449                 .normalize(candidate)
 450                 .ok();
 451             match normalized {
 452                 Some(normalized) => format!("\n  {:?}", normalized.value),
 453                 None => format!("\n  {:?}", candidate),
 454             }
 455         });
 456
 457         err.help(&format!("the following implementations were found:{}{}",
 458                           &impl_candidates[0..end].iter().map(normalize).collect::<String>(),
 459                           if impl_candidates.len() > 5 {
 460                               format!("\nand {} others", impl_candidates.len() - 4)
 461                           } else {
 462                               "".to_owned()
 463                           }
 464                           ));
 465     }
 466
 467     /// Reports that an overflow has occurred and halts compilation. We
 468     /// halt compilation unconditionally because it is important that
 469     /// overflows never be masked -- they basically represent computations
 470     /// whose result could not be truly determined and thus we can't say
 471     /// if the program type checks or not -- and they are unusual
 472     /// occurrences in any case.
 473     pub fn report_overflow_error<T>(&self,
 474                                     obligation: &Obligation<'tcx, T>,
 475                                     suggest_increasing_limit: bool) -> !
 476         where T: fmt::Display + TypeFoldable<'tcx>
 477     {
 478         let predicate =
 479             self.resolve_type_vars_if_possible(&obligation.predicate);
 480         let mut err = struct_span_err!(self.tcx.sess, obligation.cause.span, E0275,
 481                                        "overflow evaluating the requirement `{}`",
 482                                        predicate);
 483
 484         if suggest_increasing_limit {
 485             self.suggest_new_overflow_limit(&mut err);
 486         }
 487
 488         self.note_obligation_cause(&mut err, obligation);
 489
 490         err.emit();
 491         self.tcx.sess.abort_if_errors();
 492         bug!();
 493     }
 494
 495     /// Reports that a cycle was detected which led to overflow and halts
 496     /// compilation. This is equivalent to `report_overflow_error` except
 497     /// that we can give a more helpful error message (and, in particular,
 498     /// we do not suggest increasing the overflow limit, which is not
 499     /// going to help).
 500     pub fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 501         let cycle = self.resolve_type_vars_if_possible(&cycle.to_owned());
 502         assert!(cycle.len() > 0);
 503
 504         debug!("report_overflow_error_cycle: cycle={:?}", cycle);
 505
 506         self.report_overflow_error(&cycle[0], false);
 507     }
 508
 509     pub fn report_extra_impl_obligation(&self,
 510                                         error_span: Span,
 511                                         item_name: ast::Name,
 512                                         _impl_item_def_id: DefId,
 513                                         trait_item_def_id: DefId,
 514                                         requirement: &dyn fmt::Display)
 515                                         -> DiagnosticBuilder<'tcx>
 516     {
 517         let msg = "impl has stricter requirements than trait";
 518         let sp = self.tcx.sess.codemap().def_span(error_span);
 519
 520         let mut err = struct_span_err!(self.tcx.sess, sp, E0276, "{}", msg);
 521
 522         if let Some(trait_item_span) = self.tcx.hir.span_if_local(trait_item_def_id) {
 523             let span = self.tcx.sess.codemap().def_span(trait_item_span);
 524             err.span_label(span, format!("definition of `{}` from trait", item_name));
 525         }
 526
 527         err.span_label(sp, format!("impl has extra requirement {}", requirement));
 528
 529         err
 530     }
 531
 532
 533     /// Get the parent trait chain start
 534     fn get_parent_trait_ref(&self, code: &ObligationCauseCode<'tcx>) -> Option<String> {
 535         match code {
 536             &ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
 537                 let parent_trait_ref = self.resolve_type_vars_if_possible(
 538                     &data.parent_trait_ref);
 539                 match self.get_parent_trait_ref(&data.parent_code) {
 540                     Some(t) => Some(t),
 541                     None => Some(format!("{}", parent_trait_ref.skip_binder().self_ty())),
 542                 }
 543             }
 544             _ => None,
 545         }
 546     }
 547
 548     pub fn report_selection_error(&self,
 549                                   obligation: &PredicateObligation<'tcx>,
 550                                   error: &SelectionError<'tcx>,
 551                                   fallback_has_occurred: bool)
 552     {
 553         let span = obligation.cause.span;
 554
 555         let mut err = match *error {
 556             SelectionError::Unimplemented => {
 557                 if let ObligationCauseCode::CompareImplMethodObligation {
 558                     item_name, impl_item_def_id, trait_item_def_id,
 559                 } = obligation.cause.code {
 560                     self.report_extra_impl_obligation(
 561                         span,
 562                         item_name,
 563                         impl_item_def_id,
 564                         trait_item_def_id,
 565                         &format!("`{}`", obligation.predicate))
 566                         .emit();
 567                     return;
 568                 }
 569                 match obligation.predicate {
 570                     ty::Predicate::Trait(ref trait_predicate) => {
 571                         let trait_predicate =
 572                             self.resolve_type_vars_if_possible(trait_predicate);
 573
 574                         if self.tcx.sess.has_errors() && trait_predicate.references_error() {
 575                             return;
 576                         }
 577                         let trait_ref = trait_predicate.to_poly_trait_ref();
 578                         let (post_message, pre_message) =
 579                             self.get_parent_trait_ref(&obligation.cause.code)
 580                                 .map(|t| (format!(" in `{}`", t), format!("within `{}`, ", t)))
 581                             .unwrap_or((String::new(), String::new()));
 582
 583                         let OnUnimplementedNote { message, label, note }
 584                             = self.on_unimplemented_note(trait_ref, obligation);
 585                         let have_alt_message = message.is_some() || label.is_some();
 586
 587                         let mut err = struct_span_err!(
 588                             self.tcx.sess,
 589                             span,
 590                             E0277,
 591                             "{}",
 592                             message.unwrap_or_else(|| {
 593                                 format!("the trait bound `{}` is not satisfied{}",
 594                                          trait_ref.to_predicate(), post_message)
 595                             }));
 596
 597                         let explanation =
 598                             if obligation.cause.code == ObligationCauseCode::MainFunctionType {
 599                                 "consider using `()`, or a `Result`".to_owned()
 600                             } else {
 601                                 format!("{}the trait `{}` is not implemented for `{}`",
 602                                         pre_message,
 603                                         trait_ref,
 604                                         trait_ref.self_ty())
 605                             };
 606
 607                         if let Some(ref s) = label {
 608                             // If it has a custom "#[rustc_on_unimplemented]"
 609                             // error message, let's display it as the label!
 610                             err.span_label(span, s.as_str());
 611                             err.help(&explanation);
 612                         } else {
 613                             err.span_label(span, explanation);
 614                         }
 615                         if let Some(ref s) = note {
 616                             // If it has a custom "#[rustc_on_unimplemented]" note, let's display it
 617                             err.note(s.as_str());
 618                         }
 619
 620                         self.suggest_borrow_on_unsized_slice(&obligation.cause.code, &mut err);
 621                         self.suggest_remove_reference(&obligation, &mut err, &trait_ref);
 622
 623                         // Try to report a help message
 624                         if !trait_ref.has_infer_types() &&
 625                             self.predicate_can_apply(obligation.param_env, trait_ref) {
 626                             // If a where-clause may be useful, remind the
 627                             // user that they can add it.
 628                             //
 629                             // don't display an on-unimplemented note, as
 630                             // these notes will often be of the form
 631                             //     "the type `T` can't be frobnicated"
 632                             // which is somewhat confusing.
 633                             err.help(&format!("consider adding a `where {}` bound",
 634                                                 trait_ref.to_predicate()));
 635                         } else if !have_alt_message {
 636                             // Can't show anything else useful, try to find similar impls.
 637                             let impl_candidates = self.find_similar_impl_candidates(trait_ref);
 638                             self.report_similar_impl_candidates(impl_candidates, &mut err);
 639                         }
 640
 641                         // If this error is due to `!: Trait` not implemented but `(): Trait` is
 642                         // implemented, and fallback has occured, then it could be due to a
 643                         // variable that used to fallback to `()` now falling back to `!`. Issue a
 644                         // note informing about the change in behaviour.
 645                         if trait_predicate.skip_binder().self_ty().is_never()
 646                             && fallback_has_occurred
 647                         {
 648                             let predicate = trait_predicate.map_bound(|mut trait_pred| {
 649                                 {
 650                                     let trait_ref = &mut trait_pred.trait_ref;
 651                                     let never_substs = trait_ref.substs;
 652                                     let mut unit_substs = Vec::with_capacity(never_substs.len());
 653                                     unit_substs.push(self.tcx.mk_nil().into());
 654                                     unit_substs.extend(&never_substs[1..]);
 655                                     trait_ref.substs = self.tcx.intern_substs(&unit_substs);
 656                                 }
 657                                 trait_pred
 658                             });
 659                             let unit_obligation = Obligation {
 660                                 predicate: ty::Predicate::Trait(predicate),
 661                                 .. obligation.clone()
 662                             };
 663                             if self.predicate_may_hold(&unit_obligation) {
 664                                 err.note("the trait is implemented for `()`. \
 665                                          Possibly this error has been caused by changes to \
 666                                          Rust's type-inference algorithm \
 667                                          (see: https://github.com/rust-lang/rust/issues/48950 \
 668                                          for more info). Consider whether you meant to use the \
 669                                          type `()` here instead.");
 670                             }
 671                         }
 672
 673                         err
 674                     }
 675
 676                     ty::Predicate::Subtype(ref predicate) => {
 677                         // Errors for Subtype predicates show up as
 678                         // `FulfillmentErrorCode::CodeSubtypeError`,
 679                         // not selection error.
 680                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
 681                     }
 682
 683                     ty::Predicate::RegionOutlives(ref predicate) => {
 684                         let predicate = self.resolve_type_vars_if_possible(predicate);
 685                         let err = self.region_outlives_predicate(&obligation.cause,
 686                                                                     &predicate).err().unwrap();
 687                         struct_span_err!(self.tcx.sess, span, E0279,
 688                             "the requirement `{}` is not satisfied (`{}`)",
 689                             predicate, err)
 690                     }
 691
 692                     ty::Predicate::Projection(..) | ty::Predicate::TypeOutlives(..) => {
 693                         let predicate =
 694                             self.resolve_type_vars_if_possible(&obligation.predicate);
 695                         struct_span_err!(self.tcx.sess, span, E0280,
 696                             "the requirement `{}` is not satisfied",
 697                             predicate)
 698                     }
 699
 700                     ty::Predicate::ObjectSafe(trait_def_id) => {
 701                         let violations = self.tcx.object_safety_violations(trait_def_id);
 702                         self.tcx.report_object_safety_error(span,
 703                                                             trait_def_id,
 704                                                             violations)
 705                     }
 706
 707                     ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => {
 708                         let found_kind = self.closure_kind(closure_def_id, closure_substs).unwrap();
 709                         let closure_span = self.tcx.sess.codemap()
 710                             .def_span(self.tcx.hir.span_if_local(closure_def_id).unwrap());
 711                         let node_id = self.tcx.hir.as_local_node_id(closure_def_id).unwrap();
 712                         let mut err = struct_span_err!(
 713                             self.tcx.sess, closure_span, E0525,
 714                             "expected a closure that implements the `{}` trait, \
 715                                 but this closure only implements `{}`",
 716                             kind,
 717                             found_kind);
 718
 719                         err.span_label(
 720                             closure_span,
 721                             format!("this closure implements `{}`, not `{}`", found_kind, kind));
 722                         err.span_label(
 723                             obligation.cause.span,
 724                             format!("the requirement to implement `{}` derives from here", kind));
 725
 726                         // Additional context information explaining why the closure only implements
 727                         // a particular trait.
 728                         if let Some(tables) = self.in_progress_tables {
 729                             let tables = tables.borrow();
 730                             let closure_hir_id = self.tcx.hir.node_to_hir_id(node_id);
 731                             match (found_kind, tables.closure_kind_origins().get(closure_hir_id)) {
 732                                 (ty::ClosureKind::FnOnce, Some((span, name))) => {
 733                                     err.span_label(*span, format!(
 734                                         "closure is `FnOnce` because it moves the \
 735                                          variable `{}` out of its environment", name));
 736                                 },
 737                                 (ty::ClosureKind::FnMut, Some((span, name))) => {
 738                                     err.span_label(*span, format!(
 739                                         "closure is `FnMut` because it mutates the \
 740                                          variable `{}` here", name));
 741                                 },
 742                                 _ => {}
 743                             }
 744                         }
 745
 746                         err.emit();
 747                         return;
 748                     }
 749
 750                     ty::Predicate::WellFormed(ty) => {
 751                         // WF predicates cannot themselves make
 752                         // errors. They can only block due to
 753                         // ambiguity; otherwise, they always
 754                         // degenerate into other obligations
 755                         // (which may fail).
 756                         span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 757                     }
 758
 759                     ty::Predicate::ConstEvaluatable(..) => {
 760                         // Errors for `ConstEvaluatable` predicates show up as
 761                         // `SelectionError::ConstEvalFailure`,
 762                         // not `Unimplemented`.
 763                         span_bug!(span,
 764                             "const-evaluatable requirement gave wrong error: `{:?}`", obligation)
 765                     }
 766                 }
 767             }
 768
 769             OutputTypeParameterMismatch(ref found_trait_ref, ref expected_trait_ref, _) => {
 770                 let found_trait_ref = self.resolve_type_vars_if_possible(&*found_trait_ref);
 771                 let expected_trait_ref = self.resolve_type_vars_if_possible(&*expected_trait_ref);
 772                 if expected_trait_ref.self_ty().references_error() {
 773                     return;
 774                 }
 775                 let found_trait_ty = found_trait_ref.self_ty();
 776
 777                 let found_did = found_trait_ty.ty_to_def_id();
 778                 let found_span = found_did.and_then(|did| {
 779                     self.tcx.hir.span_if_local(did)
 780                 }).map(|sp| self.tcx.sess.codemap().def_span(sp)); // the sp could be an fn def
 781
 782                 let found = match found_trait_ref.skip_binder().substs.type_at(1).sty {
 783                     ty::TyTuple(ref tys) => tys.iter()
 784                         .map(|_| ArgKind::empty()).collect::<Vec<_>>(),
 785                     _ => vec![ArgKind::empty()],
 786                 };
 787                 let expected = match expected_trait_ref.skip_binder().substs.type_at(1).sty {
 788                     ty::TyTuple(ref tys) => tys.iter()
 789                         .map(|t| match t.sty {
 790                             ty::TypeVariants::TyTuple(ref tys) => ArgKind::Tuple(
 791                                 Some(span),
 792                                 tys.iter()
 793                                     .map(|ty| ("_".to_owned(), format!("{}", ty.sty)))
 794                                     .collect::<Vec<_>>()
 795                             ),
 796                             _ => ArgKind::Arg("_".to_owned(), format!("{}", t.sty)),
 797                         }).collect(),
 798                     ref sty => vec![ArgKind::Arg("_".to_owned(), format!("{}", sty))],
 799                 };
 800                 if found.len() == expected.len() {
 801                     self.report_closure_arg_mismatch(span,
 802                                                      found_span,
 803                                                      found_trait_ref,
 804                                                      expected_trait_ref)
 805                 } else {
 806                     let (closure_span, found) = found_did
 807                         .and_then(|did| self.tcx.hir.get_if_local(did))
 808                         .map(|node| {
 809                             let (found_span, found) = self.get_fn_like_arguments(node);
 810                             (Some(found_span), found)
 811                         }).unwrap_or((found_span, found));
 812
 813                     self.report_arg_count_mismatch(span,
 814                                                    closure_span,
 815                                                    expected,
 816                                                    found,
 817                                                    found_trait_ty.is_closure())
 818                 }
 819             }
 820
 821             TraitNotObjectSafe(did) => {
 822                 let violations = self.tcx.object_safety_violations(did);
 823                 self.tcx.report_object_safety_error(span, did,
 824                                                     violations)
 825             }
 826
 827             ConstEvalFailure(ref err) => {
 828                 if let ::middle::const_val::ErrKind::TypeckError = *err.kind {
 829                     return;
 830                 }
 831                 err.struct_error(self.tcx, span, "constant expression")
 832             }
 833
 834             Overflow => {
 835                 bug!("overflow should be handled before the `report_selection_error` path");
 836             }
 837         };
 838         self.note_obligation_cause(&mut err, obligation);
 839         err.emit();
 840     }
 841
 842     /// When encountering an assignment of an unsized trait, like `let x = ""[..];`, provide a
 843     /// suggestion to borrow the initializer in order to use have a slice instead.
 844     fn suggest_borrow_on_unsized_slice(&self,
 845                                        code: &ObligationCauseCode<'tcx>,
 846                                        err: &mut DiagnosticBuilder<'tcx>) {
 847         if let &ObligationCauseCode::VariableType(node_id) = code {
 848             let parent_node = self.tcx.hir.get_parent_node(node_id);
 849             if let Some(hir::map::NodeLocal(ref local)) = self.tcx.hir.find(parent_node) {
 850                 if let Some(ref expr) = local.init {
 851                     if let hir::ExprIndex(_, _) = expr.node {
 852                         if let Ok(snippet) = self.tcx.sess.codemap().span_to_snippet(expr.span) {
 853                             err.span_suggestion(expr.span,
 854                                                 "consider borrowing here",
 855                                                 format!("&{}", snippet));
 856                         }
 857                     }
 858                 }
 859             }
 860         }
 861     }
 862
 863     /// Whenever references are used by mistake, like `for (i, e) in &vec.iter().enumerate()`,
 864     /// suggest removing these references until we reach a type that implements the trait.
 865     fn suggest_remove_reference(&self,
 866                                 obligation: &PredicateObligation<'tcx>,
 867                                 err: &mut DiagnosticBuilder<'tcx>,
 868                                 trait_ref: &ty::Binder<ty::TraitRef<'tcx>>) {
 869         let trait_ref = trait_ref.skip_binder();
 870         let span = obligation.cause.span;
 871
 872         if let Ok(snippet) = self.tcx.sess.codemap().span_to_snippet(span) {
 873             let refs_number = snippet.chars()
 874                 .filter(|c| !c.is_whitespace())
 875                 .take_while(|c| *c == '&')
 876                 .count();
 877
 878             let mut trait_type = trait_ref.self_ty();
 879
 880             for refs_remaining in 0..refs_number {
 881                 if let ty::TypeVariants::TyRef(_, t_type, _) = trait_type.sty {
 882                     trait_type = t_type;
 883
 884                     let substs = self.tcx.mk_substs_trait(trait_type, &[]);
 885                     let new_trait_ref = ty::TraitRef::new(trait_ref.def_id, substs);
 886                     let new_obligation = Obligation::new(ObligationCause::dummy(),
 887                                                          obligation.param_env,
 888                                                          new_trait_ref.to_predicate());
 889
 890                     if self.predicate_may_hold(&new_obligation) {
 891                         let sp = self.tcx.sess.codemap()
 892                             .span_take_while(span, |c| c.is_whitespace() || *c == '&');
 893
 894                         let remove_refs = refs_remaining + 1;
 895                         let format_str = format!("consider removing {} leading `&`-references",
 896                                                  remove_refs);
 897
 898                         err.span_suggestion_short(sp, &format_str, String::from(""));
 899                         break;
 900                     }
 901                 } else {
 902                     break;
 903                 }
 904             }
 905         }
 906     }
 907
 908     /// Given some node representing a fn-like thing in the HIR map,
 909     /// returns a span and `ArgKind` information that describes the
 910     /// arguments it expects. This can be supplied to
 911     /// `report_arg_count_mismatch`.
 912     pub fn get_fn_like_arguments(&self, node: hir::map::Node) -> (Span, Vec<ArgKind>) {
 913         match node {
 914             hir::map::NodeExpr(&hir::Expr {
 915                 node: hir::ExprClosure(_, ref _decl, id, span, _),
 916                 ..
 917             }) => {
 918                 (self.tcx.sess.codemap().def_span(span), self.tcx.hir.body(id).arguments.iter()
 919                     .map(|arg| {
 920                         if let hir::Pat {
 921                             node: hir::PatKind::Tuple(args, _),
 922                             span,
 923                             ..
 924                         } = arg.pat.clone().into_inner() {
 925                             ArgKind::Tuple(
 926                                 Some(span),
 927                                 args.iter().map(|pat| {
 928                                     let snippet = self.tcx.sess.codemap()
 929                                         .span_to_snippet(pat.span).unwrap();
 930                                     (snippet, "_".to_owned())
 931                                 }).collect::<Vec<_>>(),
 932                             )
 933                         } else {
 934                             let name = self.tcx.sess.codemap()
 935                                 .span_to_snippet(arg.pat.span).unwrap();
 936                             ArgKind::Arg(name, "_".to_owned())
 937                         }
 938                     })
 939                     .collect::<Vec<ArgKind>>())
 940             }
 941             hir::map::NodeItem(&hir::Item {
 942                 span,
 943                 node: hir::ItemFn(ref decl, ..),
 944                 ..
 945             }) |
 946             hir::map::NodeImplItem(&hir::ImplItem {
 947                 span,
 948                 node: hir::ImplItemKind::Method(hir::MethodSig { ref decl, .. }, _),
 949                 ..
 950             }) |
 951             hir::map::NodeTraitItem(&hir::TraitItem {
 952                 span,
 953                 node: hir::TraitItemKind::Method(hir::MethodSig { ref decl, .. }, _),
 954                 ..
 955             }) => {
 956                 (self.tcx.sess.codemap().def_span(span), decl.inputs.iter()
 957                         .map(|arg| match arg.clone().into_inner().node {
 958                     hir::TyTup(ref tys) => ArgKind::Tuple(
 959                         Some(arg.span),
 960                         tys.iter()
 961                             .map(|_| ("_".to_owned(), "_".to_owned()))
 962                             .collect::<Vec<_>>(),
 963                     ),
 964                     _ => ArgKind::Arg("_".to_owned(), "_".to_owned())
 965                 }).collect::<Vec<ArgKind>>())
 966             }
 967             hir::map::NodeVariant(&hir::Variant {
 968                 span,
 969                 node: hir::Variant_ {
 970                     data: hir::VariantData::Tuple(ref fields, _),
 971                     ..
 972                 },
 973                 ..
 974             }) => {
 975                 (self.tcx.sess.codemap().def_span(span),
 976                  fields.iter().map(|field| {
 977                      ArgKind::Arg(format!("{}", field.name), "_".to_string())
 978                  }).collect::<Vec<_>>())
 979             }
 980             hir::map::NodeStructCtor(ref variant_data) => {
 981                 (self.tcx.sess.codemap().def_span(self.tcx.hir.span(variant_data.id())),
 982                  variant_data.fields()
 983                     .iter().map(|_| ArgKind::Arg("_".to_owned(), "_".to_owned()))
 984                     .collect())
 985             }
 986             _ => panic!("non-FnLike node found: {:?}", node),
 987         }
 988     }
 989
 990     /// Reports an error when the number of arguments needed by a
 991     /// trait match doesn't match the number that the expression
 992     /// provides.
 993     pub fn report_arg_count_mismatch(
 994         &self,
 995         span: Span,
 996         found_span: Option<Span>,
 997         expected_args: Vec<ArgKind>,
 998         found_args: Vec<ArgKind>,
 999         is_closure: bool,
1000     ) -> DiagnosticBuilder<'tcx> {
1001         let kind = if is_closure { "closure" } else { "function" };
1002
1003         let args_str = |arguments: &Vec<ArgKind>, other: &Vec<ArgKind>| {
1004             let arg_length = arguments.len();
1005             let distinct = match &other[..] {
1006                 &[ArgKind::Tuple(..)] => true,
1007                 _ => false,
1008             };
1009             match (arg_length, arguments.get(0)) {
1010                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
1011                     format!("a single {}-tuple as argument", fields.len())
1012                 }
1013                 _ => format!("{} {}argument{}",
1014                              arg_length,
1015                              if distinct && arg_length > 1 { "distinct " } else { "" },
1016                              if arg_length == 1 { "" } else { "s" }),
1017             }
1018         };
1019
1020         let expected_str = args_str(&expected_args, &found_args);
1021         let found_str = args_str(&found_args, &expected_args);
1022
1023         let mut err = struct_span_err!(
1024             self.tcx.sess,
1025             span,
1026             E0593,
1027             "{} is expected to take {}, but it takes {}",
1028             kind,
1029             expected_str,
1030             found_str,
1031         );
1032
1033         err.span_label(span, format!( "expected {} that takes {}", kind, expected_str));
1034
1035         if let Some(found_span) = found_span {
1036             err.span_label(found_span, format!("takes {}", found_str));
1037
1038             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
1039                 if fields.len() == expected_args.len() {
1040                     let sugg = fields.iter()
1041                         .map(|(name, _)| name.to_owned())
1042                         .collect::<Vec<String>>().join(", ");
1043                     err.span_suggestion(found_span,
1044                                         "change the closure to take multiple arguments instead of \
1045                                          a single tuple",
1046                                         format!("|{}|", sugg));
1047                 }
1048             }
1049             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..] {
1050                 if fields.len() == found_args.len() && is_closure {
1051                     let sugg = format!(
1052                         "|({}){}|",
1053                         found_args.iter()
1054                             .map(|arg| match arg {
1055                                 ArgKind::Arg(name, _) => name.to_owned(),
1056                                 _ => "_".to_owned(),
1057                             })
1058                             .collect::<Vec<String>>()
1059                             .join(", "),
1060                         // add type annotations if available
1061                         if found_args.iter().any(|arg| match arg {
1062                             ArgKind::Arg(_, ty) => ty != "_",
1063                             _ => false,
1064                         }) {
1065                             format!(": ({})",
1066                                     fields.iter()
1067                                         .map(|(_, ty)| ty.to_owned())
1068                                         .collect::<Vec<String>>()
1069                                         .join(", "))
1070                         } else {
1071                             "".to_owned()
1072                         },
1073                     );
1074                     err.span_suggestion(found_span,
1075                                         "change the closure to accept a tuple instead of \
1076                                          individual arguments",
1077                                         sugg);
1078                 }
1079             }
1080         }
1081
1082         err
1083     }
1084
1085     fn report_closure_arg_mismatch(&self,
1086                            span: Span,
1087                            found_span: Option<Span>,
1088                            expected_ref: ty::PolyTraitRef<'tcx>,
1089                            found: ty::PolyTraitRef<'tcx>)
1090         -> DiagnosticBuilder<'tcx>
1091     {
1092         fn build_fn_sig_string<'a, 'gcx, 'tcx>(tcx: ty::TyCtxt<'a, 'gcx, 'tcx>,
1093                                                trait_ref: &ty::TraitRef<'tcx>) -> String {
1094             let inputs = trait_ref.substs.type_at(1);
1095             let sig = if let ty::TyTuple(inputs) = inputs.sty {
1096                 tcx.mk_fn_sig(
1097                     inputs.iter().map(|&x| x),
1098                     tcx.mk_infer(ty::TyVar(ty::TyVid { index: 0 })),
1099                     false,
1100                     hir::Unsafety::Normal,
1101                     ::rustc_target::spec::abi::Abi::Rust
1102                 )
1103             } else {
1104                 tcx.mk_fn_sig(
1105                     ::std::iter::once(inputs),
1106                     tcx.mk_infer(ty::TyVar(ty::TyVid { index: 0 })),
1107                     false,
1108                     hir::Unsafety::Normal,
1109                     ::rustc_target::spec::abi::Abi::Rust
1110                 )
1111             };
1112             format!("{}", ty::Binder::bind(sig))
1113         }
1114
1115         let argument_is_closure = expected_ref.skip_binder().substs.type_at(0).is_closure();
1116         let mut err = struct_span_err!(self.tcx.sess, span, E0631,
1117                                        "type mismatch in {} arguments",
1118                                        if argument_is_closure { "closure" } else { "function" });
1119
1120         let found_str = format!(
1121             "expected signature of `{}`",
1122             build_fn_sig_string(self.tcx, found.skip_binder())
1123         );
1124         err.span_label(span, found_str);
1125
1126         let found_span = found_span.unwrap_or(span);
1127         let expected_str = format!(
1128             "found signature of `{}`",
1129             build_fn_sig_string(self.tcx, expected_ref.skip_binder())
1130         );
1131         err.span_label(found_span, expected_str);
1132
1133         err
1134     }
1135 }
1136
1137 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
1138     pub fn recursive_type_with_infinite_size_error(self,
1139                                                    type_def_id: DefId)
1140                                                    -> DiagnosticBuilder<'tcx>
1141     {
1142         assert!(type_def_id.is_local());
1143         let span = self.hir.span_if_local(type_def_id).unwrap();
1144         let span = self.sess.codemap().def_span(span);
1145         let mut err = struct_span_err!(self.sess, span, E0072,
1146                                        "recursive type `{}` has infinite size",
1147                                        self.item_path_str(type_def_id));
1148         err.span_label(span, "recursive type has infinite size");
1149         err.help(&format!("insert indirection (e.g., a `Box`, `Rc`, or `&`) \
1150                            at some point to make `{}` representable",
1151                           self.item_path_str(type_def_id)));
1152         err
1153     }
1154
1155     pub fn report_object_safety_error(self,
1156                                       span: Span,
1157                                       trait_def_id: DefId,
1158                                       violations: Vec<ObjectSafetyViolation>)
1159                                       -> DiagnosticBuilder<'tcx>
1160     {
1161         let trait_str = self.item_path_str(trait_def_id);
1162         let span = self.sess.codemap().def_span(span);
1163         let mut err = struct_span_err!(
1164             self.sess, span, E0038,
1165             "the trait `{}` cannot be made into an object",
1166             trait_str);
1167         err.span_label(span, format!("the trait `{}` cannot be made into an object", trait_str));
1168
1169         let mut reported_violations = FxHashSet();
1170         for violation in violations {
1171             if !reported_violations.insert(violation.clone()) {
1172                 continue;
1173             }
1174             err.note(&violation.error_msg());
1175         }
1176         err
1177     }
1178 }
1179
1180 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
1181     fn maybe_report_ambiguity(&self, obligation: &PredicateObligation<'tcx>,
1182                               body_id: Option<hir::BodyId>) {
1183         // Unable to successfully determine, probably means
1184         // insufficient type information, but could mean
1185         // ambiguous impls. The latter *ought* to be a
1186         // coherence violation, so we don't report it here.
1187
1188         let predicate = self.resolve_type_vars_if_possible(&obligation.predicate);
1189         let span = obligation.cause.span;
1190
1191         debug!("maybe_report_ambiguity(predicate={:?}, obligation={:?})",
1192                predicate,
1193                obligation);
1194
1195         // Ambiguity errors are often caused as fallout from earlier
1196         // errors. So just ignore them if this infcx is tainted.
1197         if self.is_tainted_by_errors() {
1198             return;
1199         }
1200
1201         match predicate {
1202             ty::Predicate::Trait(ref data) => {
1203                 let trait_ref = data.to_poly_trait_ref();
1204                 let self_ty = trait_ref.self_ty();
1205                 if predicate.references_error() {
1206                     return;
1207                 }
1208                 // Typically, this ambiguity should only happen if
1209                 // there are unresolved type inference variables
1210                 // (otherwise it would suggest a coherence
1211                 // failure). But given #21974 that is not necessarily
1212                 // the case -- we can have multiple where clauses that
1213                 // are only distinguished by a region, which results
1214                 // in an ambiguity even when all types are fully
1215                 // known, since we don't dispatch based on region
1216                 // relationships.
1217
1218                 // This is kind of a hack: it frequently happens that some earlier
1219                 // error prevents types from being fully inferred, and then we get
1220                 // a bunch of uninteresting errors saying something like "<generic
1221                 // #0> doesn't implement Sized".  It may even be true that we
1222                 // could just skip over all checks where the self-ty is an
1223                 // inference variable, but I was afraid that there might be an
1224                 // inference variable created, registered as an obligation, and
1225                 // then never forced by writeback, and hence by skipping here we'd
1226                 // be ignoring the fact that we don't KNOW the type works
1227                 // out. Though even that would probably be harmless, given that
1228                 // we're only talking about builtin traits, which are known to be
1229                 // inhabited. But in any case I just threw in this check for
1230                 // has_errors() to be sure that compilation isn't happening
1231                 // anyway. In that case, why inundate the user.
1232                 if !self.tcx.sess.has_errors() {
1233                     if
1234                         self.tcx.lang_items().sized_trait()
1235                         .map_or(false, |sized_id| sized_id == trait_ref.def_id())
1236                     {
1237                         self.need_type_info(body_id, span, self_ty);
1238                     } else {
1239                         let mut err = struct_span_err!(self.tcx.sess,
1240                                                         span, E0283,
1241                                                         "type annotations required: \
1242                                                         cannot resolve `{}`",
1243                                                         predicate);
1244                         self.note_obligation_cause(&mut err, obligation);
1245                         err.emit();
1246                     }
1247                 }
1248             }
1249
1250             ty::Predicate::WellFormed(ty) => {
1251                 // Same hacky approach as above to avoid deluging user
1252                 // with error messages.
1253                 if !ty.references_error() && !self.tcx.sess.has_errors() {
1254                     self.need_type_info(body_id, span, ty);
1255                 }
1256             }
1257
1258             ty::Predicate::Subtype(ref data) => {
1259                 if data.references_error() || self.tcx.sess.has_errors() {
1260                     // no need to overload user in such cases
1261                 } else {
1262                     let &SubtypePredicate { a_is_expected: _, a, b } = data.skip_binder();
1263                     // both must be type variables, or the other would've been instantiated
1264                     assert!(a.is_ty_var() && b.is_ty_var());
1265                     self.need_type_info(body_id,
1266                                         obligation.cause.span,
1267                                         a);
1268                 }
1269             }
1270
1271             _ => {
1272                 if !self.tcx.sess.has_errors() {
1273                     let mut err = struct_span_err!(self.tcx.sess,
1274                                                    obligation.cause.span, E0284,
1275                                                    "type annotations required: \
1276                                                     cannot resolve `{}`",
1277                                                    predicate);
1278                     self.note_obligation_cause(&mut err, obligation);
1279                     err.emit();
1280                 }
1281             }
1282         }
1283     }
1284
1285     /// Returns whether the trait predicate may apply for *some* assignment
1286     /// to the type parameters.
1287     fn predicate_can_apply(&self,
1288                            param_env: ty::ParamEnv<'tcx>,
1289                            pred: ty::PolyTraitRef<'tcx>)
1290                            -> bool {
1291         struct ParamToVarFolder<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
1292             infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
1293             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>
1294         }
1295
1296         impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for ParamToVarFolder<'a, 'gcx, 'tcx> {
1297             fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> { self.infcx.tcx }
1298
1299             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
1300                 if let ty::TyParam(ty::ParamTy {name, ..}) = ty.sty {
1301                     let infcx = self.infcx;
1302                     self.var_map.entry(ty).or_insert_with(||
1303                         infcx.next_ty_var(
1304                             TypeVariableOrigin::TypeParameterDefinition(DUMMY_SP, name)))
1305                 } else {
1306                     ty.super_fold_with(self)
1307                 }
1308             }
1309         }
1310
1311         self.probe(|_| {
1312             let mut selcx = SelectionContext::new(self);
1313
1314             let cleaned_pred = pred.fold_with(&mut ParamToVarFolder {
1315                 infcx: self,
1316                 var_map: FxHashMap()
1317             });
1318
1319             let cleaned_pred = super::project::normalize(
1320                 &mut selcx,
1321                 param_env,
1322                 ObligationCause::dummy(),
1323                 &cleaned_pred
1324             ).value;
1325
1326             let obligation = Obligation::new(
1327                 ObligationCause::dummy(),
1328                 param_env,
1329                 cleaned_pred.to_predicate()
1330             );
1331
1332             self.predicate_may_hold(&obligation)
1333         })
1334     }
1335
1336     fn note_obligation_cause<T>(&self,
1337                                 err: &mut DiagnosticBuilder,
1338                                 obligation: &Obligation<'tcx, T>)
1339         where T: fmt::Display
1340     {
1341         self.note_obligation_cause_code(err,
1342                                         &obligation.predicate,
1343                                         &obligation.cause.code,
1344                                         &mut vec![]);
1345     }
1346
1347     fn note_obligation_cause_code<T>(&self,
1348                                      err: &mut DiagnosticBuilder,
1349                                      predicate: &T,
1350                                      cause_code: &ObligationCauseCode<'tcx>,
1351                                      obligated_types: &mut Vec<&ty::TyS<'tcx>>)
1352         where T: fmt::Display
1353     {
1354         let tcx = self.tcx;
1355         match *cause_code {
1356             ObligationCauseCode::ExprAssignable |
1357             ObligationCauseCode::MatchExpressionArm { .. } |
1358             ObligationCauseCode::IfExpression |
1359             ObligationCauseCode::IfExpressionWithNoElse |
1360             ObligationCauseCode::MainFunctionType |
1361             ObligationCauseCode::StartFunctionType |
1362             ObligationCauseCode::IntrinsicType |
1363             ObligationCauseCode::MethodReceiver |
1364             ObligationCauseCode::ReturnNoExpression |
1365             ObligationCauseCode::MiscObligation => {
1366             }
1367             ObligationCauseCode::SliceOrArrayElem => {
1368                 err.note("slice and array elements must have `Sized` type");
1369             }
1370             ObligationCauseCode::TupleElem => {
1371                 err.note("only the last element of a tuple may have a dynamically sized type");
1372             }
1373             ObligationCauseCode::ProjectionWf(data) => {
1374                 err.note(&format!("required so that the projection `{}` is well-formed",
1375                                   data));
1376             }
1377             ObligationCauseCode::ReferenceOutlivesReferent(ref_ty) => {
1378                 err.note(&format!("required so that reference `{}` does not outlive its referent",
1379                                   ref_ty));
1380             }
1381             ObligationCauseCode::ObjectTypeBound(object_ty, region) => {
1382                 err.note(&format!("required so that the lifetime bound of `{}` for `{}` \
1383                                    is satisfied",
1384                                   region, object_ty));
1385             }
1386             ObligationCauseCode::ItemObligation(item_def_id) => {
1387                 let item_name = tcx.item_path_str(item_def_id);
1388                 let msg = format!("required by `{}`", item_name);
1389                 if let Some(sp) = tcx.hir.span_if_local(item_def_id) {
1390                     let sp = tcx.sess.codemap().def_span(sp);
1391                     err.span_note(sp, &msg);
1392                 } else {
1393                     err.note(&msg);
1394                 }
1395             }
1396             ObligationCauseCode::ObjectCastObligation(object_ty) => {
1397                 err.note(&format!("required for the cast to the object type `{}`",
1398                                   self.ty_to_string(object_ty)));
1399             }
1400             ObligationCauseCode::RepeatVec => {
1401                 err.note("the `Copy` trait is required because the \
1402                           repeated element will be copied");
1403             }
1404             ObligationCauseCode::VariableType(_) => {
1405                 err.note("all local variables must have a statically known size");
1406             }
1407             ObligationCauseCode::SizedReturnType => {
1408                 err.note("the return type of a function must have a \
1409                           statically known size");
1410             }
1411             ObligationCauseCode::SizedYieldType => {
1412                 err.note("the yield type of a generator must have a \
1413                           statically known size");
1414             }
1415             ObligationCauseCode::AssignmentLhsSized => {
1416                 err.note("the left-hand-side of an assignment must have a statically known size");
1417             }
1418             ObligationCauseCode::TupleInitializerSized => {
1419                 err.note("tuples must have a statically known size to be initialized");
1420             }
1421             ObligationCauseCode::StructInitializerSized => {
1422                 err.note("structs must have a statically known size to be initialized");
1423             }
1424             ObligationCauseCode::FieldSized(ref item) => {
1425                 match *item {
1426                     AdtKind::Struct => {
1427                         err.note("only the last field of a struct may have a dynamically \
1428                                   sized type");
1429                     }
1430                     AdtKind::Union => {
1431                         err.note("no field of a union may have a dynamically sized type");
1432                     }
1433                     AdtKind::Enum => {
1434                         err.note("no field of an enum variant may have a dynamically sized type");
1435                     }
1436                 }
1437             }
1438             ObligationCauseCode::ConstSized => {
1439                 err.note("constant expressions must have a statically known size");
1440             }
1441             ObligationCauseCode::SharedStatic => {
1442                 err.note("shared static variables must have a type that implements `Sync`");
1443             }
1444             ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
1445                 let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1446                 let ty = parent_trait_ref.skip_binder().self_ty();
1447                 err.note(&format!("required because it appears within the type `{}`", ty));
1448                 obligated_types.push(ty);
1449
1450                 let parent_predicate = parent_trait_ref.to_predicate();
1451                 if !self.is_recursive_obligation(obligated_types, &data.parent_code) {
1452                     self.note_obligation_cause_code(err,
1453                                                     &parent_predicate,
1454                                                     &data.parent_code,
1455                                                     obligated_types);
1456                 }
1457             }
1458             ObligationCauseCode::ImplDerivedObligation(ref data) => {
1459                 let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1460                 err.note(
1461                     &format!("required because of the requirements on the impl of `{}` for `{}`",
1462                              parent_trait_ref,
1463                              parent_trait_ref.skip_binder().self_ty()));
1464                 let parent_predicate = parent_trait_ref.to_predicate();
1465                 self.note_obligation_cause_code(err,
1466                                             &parent_predicate,
1467                                             &data.parent_code,
1468                                             obligated_types);
1469             }
1470             ObligationCauseCode::CompareImplMethodObligation { .. } => {
1471                 err.note(
1472                     &format!("the requirement `{}` appears on the impl method \
1473                               but not on the corresponding trait method",
1474                              predicate));
1475             }
1476             ObligationCauseCode::ReturnType(_) |
1477             ObligationCauseCode::BlockTailExpression(_) => (),
1478         }
1479     }
1480
1481     fn suggest_new_overflow_limit(&self, err: &mut DiagnosticBuilder) {
1482         let current_limit = self.tcx.sess.recursion_limit.get();
1483         let suggested_limit = current_limit * 2;
1484         err.help(&format!("consider adding a `#![recursion_limit=\"{}\"]` attribute to your crate",
1485                           suggested_limit));
1486     }
1487
1488     fn is_recursive_obligation(&self,
1489                                    obligated_types: &mut Vec<&ty::TyS<'tcx>>,
1490                                    cause_code: &ObligationCauseCode<'tcx>) -> bool {
1491         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
1492             let parent_trait_ref = self.resolve_type_vars_if_possible(&data.parent_trait_ref);
1493             for obligated_type in obligated_types {
1494                 if obligated_type == &parent_trait_ref.skip_binder().self_ty() {
1495                     return true;
1496                 }
1497             }
1498         }
1499         return false;
1500     }
1501 }
1502
1503 /// Summarizes information
1504 pub enum ArgKind {
1505     /// An argument of non-tuple type. Parameters are (name, ty)
1506     Arg(String, String),
1507
1508     /// An argument of tuple type. For a "found" argument, the span is
1509     /// the locationo in the source of the pattern. For a "expected"
1510     /// argument, it will be None. The vector is a list of (name, ty)
1511     /// strings for the components of the tuple.
1512     Tuple(Option<Span>, Vec<(String, String)>),
1513 }
1514
1515 impl ArgKind {
1516     fn empty() -> ArgKind {
1517         ArgKind::Arg("_".to_owned(), "_".to_owned())
1518     }
1519
1520     /// Creates an `ArgKind` from the expected type of an
1521     /// argument. This has no name (`_`) and no source spans..
1522     pub fn from_expected_ty(t: Ty<'_>) -> ArgKind {
1523         match t.sty {
1524             ty::TyTuple(ref tys) => ArgKind::Tuple(
1525                 None,
1526                 tys.iter()
1527                    .map(|ty| ("_".to_owned(), format!("{}", ty.sty)))
1528                    .collect::<Vec<_>>()
1529             ),
1530             _ => ArgKind::Arg("_".to_owned(), format!("{}", t.sty)),
1531         }
1532     }
1533 }