src/librustc/traits/error_reporting.rs

   1 use super::{
   2     FulfillmentError,
   3     FulfillmentErrorCode,
   4     MismatchedProjectionTypes,
   5     Obligation,
   6     ObligationCause,
   7     ObligationCauseCode,
   8     OnUnimplementedDirective,
   9     OnUnimplementedNote,
  10     OutputTypeParameterMismatch,
  11     TraitNotObjectSafe,
  12     ConstEvalFailure,
  13     PredicateObligation,
  14     SelectionContext,
  15     SelectionError,
  16     ObjectSafetyViolation,
  17     Overflow,
  18 };
  19
  20 use crate::hir;
  21 use crate::hir::Node;
  22 use crate::hir::def_id::DefId;
  23 use crate::infer::{self, InferCtxt};
  24 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  25 use crate::session::DiagnosticMessageId;
  26 use crate::ty::{self, AdtKind, ToPredicate, ToPolyTraitRef, Ty, TyCtxt, TypeFoldable};
  27 use crate::ty::GenericParamDefKind;
  28 use crate::ty::error::ExpectedFound;
  29 use crate::ty::fast_reject;
  30 use crate::ty::fold::TypeFolder;
  31 use crate::ty::subst::Subst;
  32 use crate::ty::SubtypePredicate;
  33 use crate::util::nodemap::{FxHashMap, FxHashSet};
  34
  35 use errors::{Applicability, DiagnosticBuilder};
  36 use std::fmt;
  37 use syntax::ast;
  38 use syntax::symbol::sym;
  39 use syntax_pos::{DUMMY_SP, Span, ExpnInfo, ExpnFormat};
  40
  41 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
  42     pub fn report_fulfillment_errors(&self,
  43                                      errors: &[FulfillmentError<'tcx>],
  44                                      body_id: Option<hir::BodyId>,
  45                                      fallback_has_occurred: bool) {
  46         #[derive(Debug)]
  47         struct ErrorDescriptor<'tcx> {
  48             predicate: ty::Predicate<'tcx>,
  49             index: Option<usize>, // None if this is an old error
  50         }
  51
  52         let mut error_map: FxHashMap<_, Vec<_>> =
  53             self.reported_trait_errors.borrow().iter().map(|(&span, predicates)| {
  54                 (span, predicates.iter().map(|predicate| ErrorDescriptor {
  55                     predicate: predicate.clone(),
  56                     index: None
  57                 }).collect())
  58             }).collect();
  59
  60         for (index, error) in errors.iter().enumerate() {
  61             // We want to ignore desugarings here: spans are equivalent even
  62             // if one is the result of a desugaring and the other is not.
  63             let mut span = error.obligation.cause.span;
  64             if let Some(ExpnInfo {
  65                 format: ExpnFormat::CompilerDesugaring(_),
  66                 def_site: Some(def_span),
  67                 ..
  68             }) = span.ctxt().outer_expn_info() {
  69                 span = def_span;
  70             }
  71
  72             error_map.entry(span).or_default().push(
  73                 ErrorDescriptor {
  74                     predicate: error.obligation.predicate.clone(),
  75                     index: Some(index)
  76                 }
  77             );
  78
  79             self.reported_trait_errors.borrow_mut()
  80                 .entry(span).or_default()
  81                 .push(error.obligation.predicate.clone());
  82         }
  83
  84         // We do this in 2 passes because we want to display errors in order, though
  85         // maybe it *is* better to sort errors by span or something.
  86         let mut is_suppressed = vec![false; errors.len()];
  87         for (_, error_set) in error_map.iter() {
  88             // We want to suppress "duplicate" errors with the same span.
  89             for error in error_set {
  90                 if let Some(index) = error.index {
  91                     // Suppress errors that are either:
  92                     // 1) strictly implied by another error.
  93                     // 2) implied by an error with a smaller index.
  94                     for error2 in error_set {
  95                         if error2.index.map_or(false, |index2| is_suppressed[index2]) {
  96                             // Avoid errors being suppressed by already-suppressed
  97                             // errors, to prevent all errors from being suppressed
  98                             // at once.
  99                             continue
 100                         }
 101
 102                         if self.error_implies(&error2.predicate, &error.predicate) &&
 103                             !(error2.index >= error.index &&
 104                               self.error_implies(&error.predicate, &error2.predicate))
 105                         {
 106                             info!("skipping {:?} (implied by {:?})", error, error2);
 107                             is_suppressed[index] = true;
 108                             break
 109                         }
 110                     }
 111                 }
 112             }
 113         }
 114
 115         for (error, suppressed) in errors.iter().zip(is_suppressed) {
 116             if !suppressed {
 117                 self.report_fulfillment_error(error, body_id, fallback_has_occurred);
 118             }
 119         }
 120     }
 121
 122     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
 123     // `error` occurring implies that `cond` occurs.
 124     fn error_implies(&self,
 125                      cond: &ty::Predicate<'tcx>,
 126                      error: &ty::Predicate<'tcx>)
 127                      -> bool
 128     {
 129         if cond == error {
 130             return true
 131         }
 132
 133         let (cond, error) = match (cond, error) {
 134             (&ty::Predicate::Trait(..), &ty::Predicate::Trait(ref error))
 135                 => (cond, error),
 136             _ => {
 137                 // FIXME: make this work in other cases too.
 138                 return false
 139             }
 140         };
 141
 142         for implication in super::elaborate_predicates(self.tcx, vec![cond.clone()]) {
 143             if let ty::Predicate::Trait(implication) = implication {
 144                 let error = error.to_poly_trait_ref();
 145                 let implication = implication.to_poly_trait_ref();
 146                 // FIXME: I'm just not taking associated types at all here.
 147                 // Eventually I'll need to implement param-env-aware
 148                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
 149                 let param_env = ty::ParamEnv::empty();
 150                 if self.can_sub(param_env, error, implication).is_ok() {
 151                     debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
 152                     return true
 153                 }
 154             }
 155         }
 156
 157         false
 158     }
 159
 160     fn report_fulfillment_error(&self, error: &FulfillmentError<'tcx>,
 161                                 body_id: Option<hir::BodyId>,
 162                                 fallback_has_occurred: bool) {
 163         debug!("report_fulfillment_errors({:?})", error);
 164         match error.code {
 165             FulfillmentErrorCode::CodeSelectionError(ref e) => {
 166                 self.report_selection_error(&error.obligation, e, fallback_has_occurred);
 167             }
 168             FulfillmentErrorCode::CodeProjectionError(ref e) => {
 169                 self.report_projection_error(&error.obligation, e);
 170             }
 171             FulfillmentErrorCode::CodeAmbiguity => {
 172                 self.maybe_report_ambiguity(&error.obligation, body_id);
 173             }
 174             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
 175                 self.report_mismatched_types(&error.obligation.cause,
 176                                              expected_found.expected,
 177                                              expected_found.found,
 178                                              err.clone())
 179                     .emit();
 180             }
 181         }
 182     }
 183
 184     fn report_projection_error(&self,
 185                                obligation: &PredicateObligation<'tcx>,
 186                                error: &MismatchedProjectionTypes<'tcx>)
 187     {
 188         let predicate =
 189             self.resolve_vars_if_possible(&obligation.predicate);
 190
 191         if predicate.references_error() {
 192             return
 193         }
 194
 195         self.probe(|_| {
 196             let err_buf;
 197             let mut err = &error.err;
 198             let mut values = None;
 199
 200             // try to find the mismatched types to report the error with.
 201             //
 202             // this can fail if the problem was higher-ranked, in which
 203             // cause I have no idea for a good error message.
 204             if let ty::Predicate::Projection(ref data) = predicate {
 205                 let mut selcx = SelectionContext::new(self);
 206                 let (data, _) = self.replace_bound_vars_with_fresh_vars(
 207                     obligation.cause.span,
 208                     infer::LateBoundRegionConversionTime::HigherRankedType,
 209                     data
 210                 );
 211                 let mut obligations = vec![];
 212                 let normalized_ty = super::normalize_projection_type(
 213                     &mut selcx,
 214                     obligation.param_env,
 215                     data.projection_ty,
 216                     obligation.cause.clone(),
 217                     0,
 218                     &mut obligations
 219                 );
 220                 if let Err(error) = self.at(&obligation.cause, obligation.param_env)
 221                                         .eq(normalized_ty, data.ty) {
 222                     values = Some(infer::ValuePairs::Types(ExpectedFound {
 223                         expected: normalized_ty,
 224                         found: data.ty,
 225                     }));
 226                     err_buf = error;
 227                     err = &err_buf;
 228                 }
 229             }
 230
 231             let msg = format!("type mismatch resolving `{}`", predicate);
 232             let error_id = (DiagnosticMessageId::ErrorId(271),
 233                             Some(obligation.cause.span), msg);
 234             let fresh = self.tcx.sess.one_time_diagnostics.borrow_mut().insert(error_id);
 235             if fresh {
 236                 let mut diag = struct_span_err!(
 237                     self.tcx.sess, obligation.cause.span, E0271,
 238                     "type mismatch resolving `{}`", predicate
 239                 );
 240                 self.note_type_err(&mut diag, &obligation.cause, None, values, err);
 241                 self.note_obligation_cause(&mut diag, obligation);
 242                 diag.emit();
 243             }
 244         });
 245     }
 246
 247     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
 248         /// returns the fuzzy category of a given type, or None
 249         /// if the type can be equated to any type.
 250         fn type_category<'tcx>(t: Ty<'tcx>) -> Option<u32> {
 251             match t.sty {
 252                 ty::Bool => Some(0),
 253                 ty::Char => Some(1),
 254                 ty::Str => Some(2),
 255                 ty::Int(..) | ty::Uint(..) | ty::Infer(ty::IntVar(..)) => Some(3),
 256                 ty::Float(..) | ty::Infer(ty::FloatVar(..)) => Some(4),
 257                 ty::Ref(..) | ty::RawPtr(..) => Some(5),
 258                 ty::Array(..) | ty::Slice(..) => Some(6),
 259                 ty::FnDef(..) | ty::FnPtr(..) => Some(7),
 260                 ty::Dynamic(..) => Some(8),
 261                 ty::Closure(..) => Some(9),
 262                 ty::Tuple(..) => Some(10),
 263                 ty::Projection(..) => Some(11),
 264                 ty::Param(..) => Some(12),
 265                 ty::Opaque(..) => Some(13),
 266                 ty::Never => Some(14),
 267                 ty::Adt(adt, ..) => match adt.adt_kind() {
 268                     AdtKind::Struct => Some(15),
 269                     AdtKind::Union => Some(16),
 270                     AdtKind::Enum => Some(17),
 271                 },
 272                 ty::Generator(..) => Some(18),
 273                 ty::Foreign(..) => Some(19),
 274                 ty::GeneratorWitness(..) => Some(20),
 275                 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error => None,
 276                 ty::UnnormalizedProjection(..) => bug!("only used with chalk-engine"),
 277             }
 278         }
 279
 280         match (type_category(a), type_category(b)) {
 281             (Some(cat_a), Some(cat_b)) => match (&a.sty, &b.sty) {
 282                 (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => def_a == def_b,
 283                 _ => cat_a == cat_b
 284             },
 285             // infer and error can be equated to all types
 286             _ => true
 287         }
 288     }
 289
 290     fn impl_similar_to(&self,
 291                        trait_ref: ty::PolyTraitRef<'tcx>,
 292                        obligation: &PredicateObligation<'tcx>)
 293                        -> Option<DefId>
 294     {
 295         let tcx = self.tcx;
 296         let param_env = obligation.param_env;
 297         let trait_ref = tcx.erase_late_bound_regions(&trait_ref);
 298         let trait_self_ty = trait_ref.self_ty();
 299
 300         let mut self_match_impls = vec![];
 301         let mut fuzzy_match_impls = vec![];
 302
 303         self.tcx.for_each_relevant_impl(
 304             trait_ref.def_id, trait_self_ty, |def_id| {
 305                 let impl_substs = self.fresh_substs_for_item(obligation.cause.span, def_id);
 306                 let impl_trait_ref = tcx
 307                     .impl_trait_ref(def_id)
 308                     .unwrap()
 309                     .subst(tcx, impl_substs);
 310
 311                 let impl_self_ty = impl_trait_ref.self_ty();
 312
 313                 if let Ok(..) = self.can_eq(param_env, trait_self_ty, impl_self_ty) {
 314                     self_match_impls.push(def_id);
 315
 316                     if trait_ref.substs.types().skip(1)
 317                         .zip(impl_trait_ref.substs.types().skip(1))
 318                         .all(|(u,v)| self.fuzzy_match_tys(u, v))
 319                     {
 320                         fuzzy_match_impls.push(def_id);
 321                     }
 322                 }
 323             });
 324
 325         let impl_def_id = if self_match_impls.len() == 1 {
 326             self_match_impls[0]
 327         } else if fuzzy_match_impls.len() == 1 {
 328             fuzzy_match_impls[0]
 329         } else {
 330             return None
 331         };
 332
 333         if tcx.has_attr(impl_def_id, sym::rustc_on_unimplemented) {
 334             Some(impl_def_id)
 335         } else {
 336             None
 337         }
 338     }
 339
 340     fn on_unimplemented_note(
 341         &self,
 342         trait_ref: ty::PolyTraitRef<'tcx>,
 343         obligation: &PredicateObligation<'tcx>,
 344     ) -> OnUnimplementedNote {
 345         let def_id = self.impl_similar_to(trait_ref, obligation)
 346             .unwrap_or_else(|| trait_ref.def_id());
 347         let trait_ref = *trait_ref.skip_binder();
 348
 349         let mut flags = vec![];
 350         match obligation.cause.code {
 351             ObligationCauseCode::BuiltinDerivedObligation(..) |
 352             ObligationCauseCode::ImplDerivedObligation(..) => {}
 353             _ => {
 354                 // this is a "direct", user-specified, rather than derived,
 355                 // obligation.
 356                 flags.push((sym::direct, None));
 357             }
 358         }
 359
 360         if let ObligationCauseCode::ItemObligation(item) = obligation.cause.code {
 361             // FIXME: maybe also have some way of handling methods
 362             // from other traits? That would require name resolution,
 363             // which we might want to be some sort of hygienic.
 364             //
 365             // Currently I'm leaving it for what I need for `try`.
 366             if self.tcx.trait_of_item(item) == Some(trait_ref.def_id) {
 367                 let method = self.tcx.item_name(item);
 368                 flags.push((sym::from_method, None));
 369                 flags.push((sym::from_method, Some(method.to_string())));
 370             }
 371         }
 372         if let Some(t) = self.get_parent_trait_ref(&obligation.cause.code) {
 373             flags.push((sym::parent_trait, Some(t)));
 374         }
 375
 376         if let Some(k) = obligation.cause.span.compiler_desugaring_kind() {
 377             flags.push((sym::from_desugaring, None));
 378             flags.push((sym::from_desugaring, Some(k.name().to_string())));
 379         }
 380         let generics = self.tcx.generics_of(def_id);
 381         let self_ty = trait_ref.self_ty();
 382         // This is also included through the generics list as `Self`,
 383         // but the parser won't allow you to use it
 384         flags.push((sym::_Self, Some(self_ty.to_string())));
 385         if let Some(def) = self_ty.ty_adt_def() {
 386             // We also want to be able to select self's original
 387             // signature with no type arguments resolved
 388             flags.push((sym::_Self, Some(self.tcx.type_of(def.did).to_string())));
 389         }
 390
 391         for param in generics.params.iter() {
 392             let value = match param.kind {
 393                 GenericParamDefKind::Type { .. } |
 394                 GenericParamDefKind::Const => {
 395                     trait_ref.substs[param.index as usize].to_string()
 396                 },
 397                 GenericParamDefKind::Lifetime => continue,
 398             };
 399             let name = param.name.as_symbol();
 400             flags.push((name, Some(value)));
 401         }
 402
 403         if let Some(true) = self_ty.ty_adt_def().map(|def| def.did.is_local()) {
 404             flags.push((sym::crate_local, None));
 405         }
 406
 407         // Allow targeting all integers using `{integral}`, even if the exact type was resolved
 408         if self_ty.is_integral() {
 409             flags.push((sym::_Self, Some("{integral}".to_owned())));
 410         }
 411
 412         if let ty::Array(aty, len) = self_ty.sty {
 413             flags.push((sym::_Self, Some("[]".to_owned())));
 414             flags.push((sym::_Self, Some(format!("[{}]", aty))));
 415             if let Some(def) = aty.ty_adt_def() {
 416                 // We also want to be able to select the array's type's original
 417                 // signature with no type arguments resolved
 418                 flags.push((
 419                     sym::_Self,
 420                     Some(format!("[{}]", self.tcx.type_of(def.did).to_string())),
 421                 ));
 422                 let tcx = self.tcx;
 423                 if let Some(len) = len.assert_usize(tcx) {
 424                     flags.push((
 425                         sym::_Self,
 426                         Some(format!("[{}; {}]", self.tcx.type_of(def.did).to_string(), len)),
 427                     ));
 428                 } else {
 429                     flags.push((
 430                         sym::_Self,
 431                         Some(format!("[{}; _]", self.tcx.type_of(def.did).to_string())),
 432                     ));
 433                 }
 434             }
 435         }
 436
 437         if let Ok(Some(command)) = OnUnimplementedDirective::of_item(
 438             self.tcx, trait_ref.def_id, def_id
 439         ) {
 440             command.evaluate(self.tcx, trait_ref, &flags[..])
 441         } else {
 442             OnUnimplementedNote::empty()
 443         }
 444     }
 445
 446     fn find_similar_impl_candidates(&self,
 447                                     trait_ref: ty::PolyTraitRef<'tcx>)
 448                                     -> Vec<ty::TraitRef<'tcx>>
 449     {
 450         let simp = fast_reject::simplify_type(self.tcx,
 451                                               trait_ref.skip_binder().self_ty(),
 452                                               true);
 453         let all_impls = self.tcx.all_impls(trait_ref.def_id());
 454
 455         match simp {
 456             Some(simp) => all_impls.iter().filter_map(|&def_id| {
 457                 let imp = self.tcx.impl_trait_ref(def_id).unwrap();
 458                 let imp_simp = fast_reject::simplify_type(self.tcx,
 459                                                           imp.self_ty(),
 460                                                           true);
 461                 if let Some(imp_simp) = imp_simp {
 462                     if simp != imp_simp {
 463                         return None
 464                     }
 465                 }
 466
 467                 Some(imp)
 468             }).collect(),
 469             None => all_impls.iter().map(|&def_id|
 470                 self.tcx.impl_trait_ref(def_id).unwrap()
 471             ).collect()
 472         }
 473     }
 474
 475     fn report_similar_impl_candidates(&self,
 476                                       impl_candidates: Vec<ty::TraitRef<'tcx>>,
 477                                       err: &mut DiagnosticBuilder<'_>)
 478     {
 479         if impl_candidates.is_empty() {
 480             return;
 481         }
 482
 483         let len = impl_candidates.len();
 484         let end = if impl_candidates.len() <= 5 {
 485             impl_candidates.len()
 486         } else {
 487             4
 488         };
 489
 490         let normalize = |candidate| self.tcx.global_tcx().infer_ctxt().enter(|ref infcx| {
 491             let normalized = infcx
 492                 .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
 493                 .normalize(candidate)
 494                 .ok();
 495             match normalized {
 496                 Some(normalized) => format!("\n  {:?}", normalized.value),
 497                 None => format!("\n  {:?}", candidate),
 498             }
 499         });
 500
 501         // Sort impl candidates so that ordering is consistent for UI tests.
 502         let mut normalized_impl_candidates = impl_candidates
 503             .iter()
 504             .map(normalize)
 505             .collect::<Vec<String>>();
 506
 507         // Sort before taking the `..end` range,
 508         // because the ordering of `impl_candidates` may not be deterministic:
 509         // https://github.com/rust-lang/rust/pull/57475#issuecomment-455519507
 510         normalized_impl_candidates.sort();
 511
 512         err.help(&format!("the following implementations were found:{}{}",
 513                           normalized_impl_candidates[..end].join(""),
 514                           if len > 5 {
 515                               format!("\nand {} others", len - 4)
 516                           } else {
 517                               String::new()
 518                           }
 519                           ));
 520     }
 521
 522     /// Reports that an overflow has occurred and halts compilation. We
 523     /// halt compilation unconditionally because it is important that
 524     /// overflows never be masked -- they basically represent computations
 525     /// whose result could not be truly determined and thus we can't say
 526     /// if the program type checks or not -- and they are unusual
 527     /// occurrences in any case.
 528     pub fn report_overflow_error<T>(&self,
 529                                     obligation: &Obligation<'tcx, T>,
 530                                     suggest_increasing_limit: bool) -> !
 531         where T: fmt::Display + TypeFoldable<'tcx>
 532     {
 533         let predicate =
 534             self.resolve_vars_if_possible(&obligation.predicate);
 535         let mut err = struct_span_err!(self.tcx.sess, obligation.cause.span, E0275,
 536                                        "overflow evaluating the requirement `{}`",
 537                                        predicate);
 538
 539         if suggest_increasing_limit {
 540             self.suggest_new_overflow_limit(&mut err);
 541         }
 542
 543         self.note_obligation_cause(&mut err, obligation);
 544
 545         err.emit();
 546         self.tcx.sess.abort_if_errors();
 547         bug!();
 548     }
 549
 550     /// Reports that a cycle was detected which led to overflow and halts
 551     /// compilation. This is equivalent to `report_overflow_error` except
 552     /// that we can give a more helpful error message (and, in particular,
 553     /// we do not suggest increasing the overflow limit, which is not
 554     /// going to help).
 555     pub fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 556         let cycle = self.resolve_vars_if_possible(&cycle.to_owned());
 557         assert!(cycle.len() > 0);
 558
 559         debug!("report_overflow_error_cycle: cycle={:?}", cycle);
 560
 561         self.report_overflow_error(&cycle[0], false);
 562     }
 563
 564     pub fn report_extra_impl_obligation(&self,
 565                                         error_span: Span,
 566                                         item_name: ast::Name,
 567                                         _impl_item_def_id: DefId,
 568                                         trait_item_def_id: DefId,
 569                                         requirement: &dyn fmt::Display)
 570                                         -> DiagnosticBuilder<'tcx>
 571     {
 572         let msg = "impl has stricter requirements than trait";
 573         let sp = self.tcx.sess.source_map().def_span(error_span);
 574
 575         let mut err = struct_span_err!(self.tcx.sess, sp, E0276, "{}", msg);
 576
 577         if let Some(trait_item_span) = self.tcx.hir().span_if_local(trait_item_def_id) {
 578             let span = self.tcx.sess.source_map().def_span(trait_item_span);
 579             err.span_label(span, format!("definition of `{}` from trait", item_name));
 580         }
 581
 582         err.span_label(sp, format!("impl has extra requirement {}", requirement));
 583
 584         err
 585     }
 586
 587
 588     /// Gets the parent trait chain start
 589     fn get_parent_trait_ref(&self, code: &ObligationCauseCode<'tcx>) -> Option<String> {
 590         match code {
 591             &ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
 592                 let parent_trait_ref = self.resolve_vars_if_possible(
 593                     &data.parent_trait_ref);
 594                 match self.get_parent_trait_ref(&data.parent_code) {
 595                     Some(t) => Some(t),
 596                     None => Some(parent_trait_ref.skip_binder().self_ty().to_string()),
 597                 }
 598             }
 599             _ => None,
 600         }
 601     }
 602
 603     pub fn report_selection_error(
 604         &self,
 605         obligation: &PredicateObligation<'tcx>,
 606         error: &SelectionError<'tcx>,
 607         fallback_has_occurred: bool,
 608     ) {
 609         let span = obligation.cause.span;
 610
 611         let mut err = match *error {
 612             SelectionError::Unimplemented => {
 613                 if let ObligationCauseCode::CompareImplMethodObligation {
 614                     item_name, impl_item_def_id, trait_item_def_id,
 615                 } = obligation.cause.code {
 616                     self.report_extra_impl_obligation(
 617                         span,
 618                         item_name,
 619                         impl_item_def_id,
 620                         trait_item_def_id,
 621                         &format!("`{}`", obligation.predicate))
 622                         .emit();
 623                     return;
 624                 }
 625                 match obligation.predicate {
 626                     ty::Predicate::Trait(ref trait_predicate) => {
 627                         let trait_predicate =
 628                             self.resolve_vars_if_possible(trait_predicate);
 629
 630                         if self.tcx.sess.has_errors() && trait_predicate.references_error() {
 631                             return;
 632                         }
 633                         let trait_ref = trait_predicate.to_poly_trait_ref();
 634                         let (post_message, pre_message) =
 635                             self.get_parent_trait_ref(&obligation.cause.code)
 636                                 .map(|t| (format!(" in `{}`", t), format!("within `{}`, ", t)))
 637                             .unwrap_or_default();
 638
 639                         let OnUnimplementedNote { message, label, note }
 640                             = self.on_unimplemented_note(trait_ref, obligation);
 641                         let have_alt_message = message.is_some() || label.is_some();
 642                         let is_try = self.tcx.sess.source_map().span_to_snippet(span)
 643                             .map(|s| &s == "?")
 644                             .unwrap_or(false);
 645                         let is_from = format!("{}", trait_ref).starts_with("std::convert::From<");
 646                         let (message, note) = if is_try && is_from {
 647                             (Some(format!(
 648                                 "`?` couldn't convert the error to `{}`",
 649                                 trait_ref.self_ty(),
 650                             )), Some(
 651                                 "the question mark operation (`?`) implicitly performs a \
 652                                  conversion on the error value using the `From` trait".to_owned()
 653                             ))
 654                         } else {
 655                             (message, note)
 656                         };
 657
 658                         let mut err = struct_span_err!(
 659                             self.tcx.sess,
 660                             span,
 661                             E0277,
 662                             "{}",
 663                             message.unwrap_or_else(||
 664                                 format!("the trait bound `{}` is not satisfied{}",
 665                                         trait_ref.to_predicate(), post_message)
 666                             ));
 667
 668                         let explanation =
 669                             if obligation.cause.code == ObligationCauseCode::MainFunctionType {
 670                                 "consider using `()`, or a `Result`".to_owned()
 671                             } else {
 672                                 format!("{}the trait `{}` is not implemented for `{}`",
 673                                         pre_message,
 674                                         trait_ref,
 675                                         trait_ref.self_ty())
 676                             };
 677
 678                         if let Some(ref s) = label {
 679                             // If it has a custom `#[rustc_on_unimplemented]`
 680                             // error message, let's display it as the label!
 681                             err.span_label(span, s.as_str());
 682                             err.help(&explanation);
 683                         } else {
 684                             err.span_label(span, explanation);
 685                         }
 686                         if let Some(ref s) = note {
 687                             // If it has a custom `#[rustc_on_unimplemented]` note, let's display it
 688                             err.note(s.as_str());
 689                         }
 690
 691                         self.suggest_borrow_on_unsized_slice(&obligation.cause.code, &mut err);
 692                         self.suggest_remove_reference(&obligation, &mut err, &trait_ref);
 693                         self.suggest_semicolon_removal(&obligation, &mut err, span, &trait_ref);
 694
 695                         // Try to report a help message
 696                         if !trait_ref.has_infer_types() &&
 697                             self.predicate_can_apply(obligation.param_env, trait_ref) {
 698                             // If a where-clause may be useful, remind the
 699                             // user that they can add it.
 700                             //
 701                             // don't display an on-unimplemented note, as
 702                             // these notes will often be of the form
 703                             //     "the type `T` can't be frobnicated"
 704                             // which is somewhat confusing.
 705                             err.help(&format!("consider adding a `where {}` bound",
 706                                               trait_ref.to_predicate()));
 707                         } else if !have_alt_message {
 708                             // Can't show anything else useful, try to find similar impls.
 709                             let impl_candidates = self.find_similar_impl_candidates(trait_ref);
 710                             self.report_similar_impl_candidates(impl_candidates, &mut err);
 711                         }
 712
 713                         // If this error is due to `!: Trait` not implemented but `(): Trait` is
 714                         // implemented, and fallback has occurred, then it could be due to a
 715                         // variable that used to fallback to `()` now falling back to `!`. Issue a
 716                         // note informing about the change in behaviour.
 717                         if trait_predicate.skip_binder().self_ty().is_never()
 718                             && fallback_has_occurred
 719                         {
 720                             let predicate = trait_predicate.map_bound(|mut trait_pred| {
 721                                 trait_pred.trait_ref.substs = self.tcx.mk_substs_trait(
 722                                     self.tcx.mk_unit(),
 723                                     &trait_pred.trait_ref.substs[1..],
 724                                 );
 725                                 trait_pred
 726                             });
 727                             let unit_obligation = Obligation {
 728                                 predicate: ty::Predicate::Trait(predicate),
 729                                 .. obligation.clone()
 730                             };
 731                             if self.predicate_may_hold(&unit_obligation) {
 732                                 err.note("the trait is implemented for `()`. \
 733                                          Possibly this error has been caused by changes to \
 734                                          Rust's type-inference algorithm \
 735                                          (see: https://github.com/rust-lang/rust/issues/48950 \
 736                                          for more info). Consider whether you meant to use the \
 737                                          type `()` here instead.");
 738                             }
 739                         }
 740
 741                         err
 742                     }
 743
 744                     ty::Predicate::Subtype(ref predicate) => {
 745                         // Errors for Subtype predicates show up as
 746                         // `FulfillmentErrorCode::CodeSubtypeError`,
 747                         // not selection error.
 748                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
 749                     }
 750
 751                     ty::Predicate::RegionOutlives(ref predicate) => {
 752                         let predicate = self.resolve_vars_if_possible(predicate);
 753                         let err = self.region_outlives_predicate(&obligation.cause,
 754                                                                  &predicate).err().unwrap();
 755                         struct_span_err!(
 756                             self.tcx.sess, span, E0279,
 757                             "the requirement `{}` is not satisfied (`{}`)",
 758                             predicate, err,
 759                         )
 760                     }
 761
 762                     ty::Predicate::Projection(..) | ty::Predicate::TypeOutlives(..) => {
 763                         let predicate =
 764                             self.resolve_vars_if_possible(&obligation.predicate);
 765                         struct_span_err!(self.tcx.sess, span, E0280,
 766                             "the requirement `{}` is not satisfied",
 767                             predicate)
 768                     }
 769
 770                     ty::Predicate::ObjectSafe(trait_def_id) => {
 771                         let violations = self.tcx.global_tcx()
 772                             .object_safety_violations(trait_def_id);
 773                         if let Some(err) = self.tcx.report_object_safety_error(
 774                             span,
 775                             trait_def_id,
 776                             violations,
 777                         ) {
 778                             err
 779                         } else {
 780                             return;
 781                         }
 782                     }
 783
 784                     ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => {
 785                         let found_kind = self.closure_kind(closure_def_id, closure_substs).unwrap();
 786                         let closure_span = self.tcx.sess.source_map()
 787                             .def_span(self.tcx.hir().span_if_local(closure_def_id).unwrap());
 788                         let hir_id = self.tcx.hir().as_local_hir_id(closure_def_id).unwrap();
 789                         let mut err = struct_span_err!(
 790                             self.tcx.sess, closure_span, E0525,
 791                             "expected a closure that implements the `{}` trait, \
 792                              but this closure only implements `{}`",
 793                             kind,
 794                             found_kind);
 795
 796                         err.span_label(
 797                             closure_span,
 798                             format!("this closure implements `{}`, not `{}`", found_kind, kind));
 799                         err.span_label(
 800                             obligation.cause.span,
 801                             format!("the requirement to implement `{}` derives from here", kind));
 802
 803                         // Additional context information explaining why the closure only implements
 804                         // a particular trait.
 805                         if let Some(tables) = self.in_progress_tables {
 806                             let tables = tables.borrow();
 807                             match (found_kind, tables.closure_kind_origins().get(hir_id)) {
 808                                 (ty::ClosureKind::FnOnce, Some((span, name))) => {
 809                                     err.span_label(*span, format!(
 810                                         "closure is `FnOnce` because it moves the \
 811                                          variable `{}` out of its environment", name));
 812                                 },
 813                                 (ty::ClosureKind::FnMut, Some((span, name))) => {
 814                                     err.span_label(*span, format!(
 815                                         "closure is `FnMut` because it mutates the \
 816                                          variable `{}` here", name));
 817                                 },
 818                                 _ => {}
 819                             }
 820                         }
 821
 822                         err.emit();
 823                         return;
 824                     }
 825
 826                     ty::Predicate::WellFormed(ty) => {
 827                         if !self.tcx.sess.opts.debugging_opts.chalk {
 828                             // WF predicates cannot themselves make
 829                             // errors. They can only block due to
 830                             // ambiguity; otherwise, they always
 831                             // degenerate into other obligations
 832                             // (which may fail).
 833                             span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 834                         } else {
 835                             // FIXME: we'll need a better message which takes into account
 836                             // which bounds actually failed to hold.
 837                             self.tcx.sess.struct_span_err(
 838                                 span,
 839                                 &format!("the type `{}` is not well-formed (chalk)", ty)
 840                             )
 841                         }
 842                     }
 843
 844                     ty::Predicate::ConstEvaluatable(..) => {
 845                         // Errors for `ConstEvaluatable` predicates show up as
 846                         // `SelectionError::ConstEvalFailure`,
 847                         // not `Unimplemented`.
 848                         span_bug!(span,
 849                             "const-evaluatable requirement gave wrong error: `{:?}`", obligation)
 850                     }
 851                 }
 852             }
 853
 854             OutputTypeParameterMismatch(ref found_trait_ref, ref expected_trait_ref, _) => {
 855                 let found_trait_ref = self.resolve_vars_if_possible(&*found_trait_ref);
 856                 let expected_trait_ref = self.resolve_vars_if_possible(&*expected_trait_ref);
 857
 858                 if expected_trait_ref.self_ty().references_error() {
 859                     return;
 860                 }
 861
 862                 let found_trait_ty = found_trait_ref.self_ty();
 863
 864                 let found_did = match found_trait_ty.sty {
 865                     ty::Closure(did, _) | ty::Foreign(did) | ty::FnDef(did, _) => Some(did),
 866                     ty::Adt(def, _) => Some(def.did),
 867                     _ => None,
 868                 };
 869
 870                 let found_span = found_did.and_then(|did|
 871                     self.tcx.hir().span_if_local(did)
 872                 ).map(|sp| self.tcx.sess.source_map().def_span(sp)); // the sp could be an fn def
 873
 874                 let found = match found_trait_ref.skip_binder().substs.type_at(1).sty {
 875                     ty::Tuple(ref tys) => vec![ArgKind::empty(); tys.len()],
 876                     _ => vec![ArgKind::empty()],
 877                 };
 878
 879                 let expected_ty = expected_trait_ref.skip_binder().substs.type_at(1);
 880                 let expected = match expected_ty.sty {
 881                     ty::Tuple(ref tys) => tys.iter()
 882                         .map(|t| ArgKind::from_expected_ty(t.expect_ty(), Some(span))).collect(),
 883                     _ => vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())],
 884                 };
 885
 886                 if found.len() == expected.len() {
 887                     self.report_closure_arg_mismatch(span,
 888                                                      found_span,
 889                                                      found_trait_ref,
 890                                                      expected_trait_ref)
 891                 } else {
 892                     let (closure_span, found) = found_did
 893                         .and_then(|did| self.tcx.hir().get_if_local(did))
 894                         .map(|node| {
 895                             let (found_span, found) = self.get_fn_like_arguments(node);
 896                             (Some(found_span), found)
 897                         }).unwrap_or((found_span, found));
 898
 899                     self.report_arg_count_mismatch(span,
 900                                                    closure_span,
 901                                                    expected,
 902                                                    found,
 903                                                    found_trait_ty.is_closure())
 904                 }
 905             }
 906
 907             TraitNotObjectSafe(did) => {
 908                 let violations = self.tcx.global_tcx().object_safety_violations(did);
 909                 if let Some(err) = self.tcx.report_object_safety_error(span, did, violations) {
 910                     err
 911                 } else {
 912                     return;
 913                 }
 914             }
 915
 916             // already reported in the query
 917             ConstEvalFailure(err) => {
 918                 self.tcx.sess.delay_span_bug(
 919                     span,
 920                     &format!("constant in type had an ignored error: {:?}", err),
 921                 );
 922                 return;
 923             }
 924
 925             Overflow => {
 926                 bug!("overflow should be handled before the `report_selection_error` path");
 927             }
 928         };
 929         self.note_obligation_cause(&mut err, obligation);
 930         err.emit();
 931     }
 932
 933     /// When encountering an assignment of an unsized trait, like `let x = ""[..];`, provide a
 934     /// suggestion to borrow the initializer in order to use have a slice instead.
 935     fn suggest_borrow_on_unsized_slice(
 936         &self,
 937         code: &ObligationCauseCode<'tcx>,
 938         err: &mut DiagnosticBuilder<'tcx>,
 939     ) {
 940         if let &ObligationCauseCode::VariableType(hir_id) = code {
 941             let parent_node = self.tcx.hir().get_parent_node(hir_id);
 942             if let Some(Node::Local(ref local)) = self.tcx.hir().find(parent_node) {
 943                 if let Some(ref expr) = local.init {
 944                     if let hir::ExprKind::Index(_, _) = expr.node {
 945                         if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(expr.span) {
 946                             err.span_suggestion(
 947                                 expr.span,
 948                                 "consider borrowing here",
 949                                 format!("&{}", snippet),
 950                                 Applicability::MachineApplicable
 951                             );
 952                         }
 953                     }
 954                 }
 955             }
 956         }
 957     }
 958
 959     /// Whenever references are used by mistake, like `for (i, e) in &vec.iter().enumerate()`,
 960     /// suggest removing these references until we reach a type that implements the trait.
 961     fn suggest_remove_reference(
 962         &self,
 963         obligation: &PredicateObligation<'tcx>,
 964         err: &mut DiagnosticBuilder<'tcx>,
 965         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
 966     ) {
 967         let trait_ref = trait_ref.skip_binder();
 968         let span = obligation.cause.span;
 969
 970         if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
 971             let refs_number = snippet.chars()
 972                 .filter(|c| !c.is_whitespace())
 973                 .take_while(|c| *c == '&')
 974                 .count();
 975
 976             let mut trait_type = trait_ref.self_ty();
 977
 978             for refs_remaining in 0..refs_number {
 979                 if let ty::Ref(_, t_type, _) = trait_type.sty {
 980                     trait_type = t_type;
 981
 982                     let substs = self.tcx.mk_substs_trait(trait_type, &[]);
 983                     let new_trait_ref = ty::TraitRef::new(trait_ref.def_id, substs);
 984                     let new_obligation = Obligation::new(ObligationCause::dummy(),
 985                                                          obligation.param_env,
 986                                                          new_trait_ref.to_predicate());
 987
 988                     if self.predicate_may_hold(&new_obligation) {
 989                         let sp = self.tcx.sess.source_map()
 990                             .span_take_while(span, |c| c.is_whitespace() || *c == '&');
 991
 992                         let remove_refs = refs_remaining + 1;
 993                         let format_str = format!("consider removing {} leading `&`-references",
 994                                                  remove_refs);
 995
 996                         err.span_suggestion_short(
 997                             sp, &format_str, String::new(), Applicability::MachineApplicable
 998                         );
 999                         break;
1000                     }
1001                 } else {
1002                     break;
1003                 }
1004             }
1005         }
1006     }
1007
1008     fn suggest_semicolon_removal(
1009         &self,
1010         obligation: &PredicateObligation<'tcx>,
1011         err: &mut DiagnosticBuilder<'tcx>,
1012         span: Span,
1013         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
1014     ) {
1015         let hir = self.tcx.hir();
1016         let parent_node = hir.get_parent_node(obligation.cause.body_id);
1017         let node = hir.find(parent_node);
1018         if let Some(hir::Node::Item(hir::Item {
1019             node: hir::ItemKind::Fn(decl, _, _, body_id),
1020             ..
1021         })) = node {
1022             let body = hir.body(*body_id);
1023             if let hir::ExprKind::Block(blk, _) = &body.value.node {
1024                 if decl.output.span().overlaps(span) && blk.expr.is_none() &&
1025                     "()" == &trait_ref.self_ty().to_string()
1026                 {
1027                     // FIXME(estebank): When encountering a method with a trait
1028                     // bound not satisfied in the return type with a body that has
1029                     // no return, suggest removal of semicolon on last statement.
1030                     // Once that is added, close #54771.
1031                     if let Some(ref stmt) = blk.stmts.last() {
1032                         let sp = self.tcx.sess.source_map().end_point(stmt.span);
1033                         err.span_label(sp, "consider removing this semicolon");
1034                     }
1035                 }
1036             }
1037         }
1038     }
1039
1040     /// Given some node representing a fn-like thing in the HIR map,
1041     /// returns a span and `ArgKind` information that describes the
1042     /// arguments it expects. This can be supplied to
1043     /// `report_arg_count_mismatch`.
1044     pub fn get_fn_like_arguments(&self, node: Node<'_>) -> (Span, Vec<ArgKind>) {
1045         match node {
1046             Node::Expr(&hir::Expr {
1047                 node: hir::ExprKind::Closure(_, ref _decl, id, span, _),
1048                 ..
1049             }) => {
1050                 (self.tcx.sess.source_map().def_span(span), self.tcx.hir().body(id).arguments.iter()
1051                     .map(|arg| {
1052                         if let hir::Pat {
1053                             node: hir::PatKind::Tuple(ref args, _),
1054                             span,
1055                             ..
1056                         } = *arg.pat {
1057                             ArgKind::Tuple(
1058                                 Some(span),
1059                                 args.iter().map(|pat| {
1060                                     let snippet = self.tcx.sess.source_map()
1061                                         .span_to_snippet(pat.span).unwrap();
1062                                     (snippet, "_".to_owned())
1063                                 }).collect::<Vec<_>>(),
1064                             )
1065                         } else {
1066                             let name = self.tcx.sess.source_map()
1067                                 .span_to_snippet(arg.pat.span).unwrap();
1068                             ArgKind::Arg(name, "_".to_owned())
1069                         }
1070                     })
1071                     .collect::<Vec<ArgKind>>())
1072             }
1073             Node::Item(&hir::Item {
1074                 span,
1075                 node: hir::ItemKind::Fn(ref decl, ..),
1076                 ..
1077             }) |
1078             Node::ImplItem(&hir::ImplItem {
1079                 span,
1080                 node: hir::ImplItemKind::Method(hir::MethodSig { ref decl, .. }, _),
1081                 ..
1082             }) |
1083             Node::TraitItem(&hir::TraitItem {
1084                 span,
1085                 node: hir::TraitItemKind::Method(hir::MethodSig { ref decl, .. }, _),
1086                 ..
1087             }) => {
1088                 (self.tcx.sess.source_map().def_span(span), decl.inputs.iter()
1089                         .map(|arg| match arg.clone().node {
1090                     hir::TyKind::Tup(ref tys) => ArgKind::Tuple(
1091                         Some(arg.span),
1092                         vec![("_".to_owned(), "_".to_owned()); tys.len()]
1093                     ),
1094                     _ => ArgKind::empty()
1095                 }).collect::<Vec<ArgKind>>())
1096             }
1097             Node::Ctor(ref variant_data) => {
1098                 let span = variant_data.ctor_hir_id()
1099                     .map(|hir_id| self.tcx.hir().span(hir_id))
1100                     .unwrap_or(DUMMY_SP);
1101                 let span = self.tcx.sess.source_map().def_span(span);
1102
1103                 (span, vec![ArgKind::empty(); variant_data.fields().len()])
1104             }
1105             _ => panic!("non-FnLike node found: {:?}", node),
1106         }
1107     }
1108
1109     /// Reports an error when the number of arguments needed by a
1110     /// trait match doesn't match the number that the expression
1111     /// provides.
1112     pub fn report_arg_count_mismatch(
1113         &self,
1114         span: Span,
1115         found_span: Option<Span>,
1116         expected_args: Vec<ArgKind>,
1117         found_args: Vec<ArgKind>,
1118         is_closure: bool,
1119     ) -> DiagnosticBuilder<'tcx> {
1120         let kind = if is_closure { "closure" } else { "function" };
1121
1122         let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
1123             let arg_length = arguments.len();
1124             let distinct = match &other[..] {
1125                 &[ArgKind::Tuple(..)] => true,
1126                 _ => false,
1127             };
1128             match (arg_length, arguments.get(0)) {
1129                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
1130                     format!("a single {}-tuple as argument", fields.len())
1131                 }
1132                 _ => format!("{} {}argument{}",
1133                              arg_length,
1134                              if distinct && arg_length > 1 { "distinct " } else { "" },
1135                              if arg_length == 1 { "" } else { "s" }),
1136             }
1137         };
1138
1139         let expected_str = args_str(&expected_args, &found_args);
1140         let found_str = args_str(&found_args, &expected_args);
1141
1142         let mut err = struct_span_err!(
1143             self.tcx.sess,
1144             span,
1145             E0593,
1146             "{} is expected to take {}, but it takes {}",
1147             kind,
1148             expected_str,
1149             found_str,
1150         );
1151
1152         err.span_label(span, format!("expected {} that takes {}", kind, expected_str));
1153
1154         if let Some(found_span) = found_span {
1155             err.span_label(found_span, format!("takes {}", found_str));
1156
1157             // move |_| { ... }
1158             // ^^^^^^^^-- def_span
1159             //
1160             // move |_| { ... }
1161             // ^^^^^-- prefix
1162             let prefix_span = self.tcx.sess.source_map().span_until_non_whitespace(found_span);
1163             // move |_| { ... }
1164             //      ^^^-- pipe_span
1165             let pipe_span = if let Some(span) = found_span.trim_start(prefix_span) {
1166                 span
1167             } else {
1168                 found_span
1169             };
1170
1171             // Suggest to take and ignore the arguments with expected_args_length `_`s if
1172             // found arguments is empty (assume the user just wants to ignore args in this case).
1173             // For example, if `expected_args_length` is 2, suggest `|_, _|`.
1174             if found_args.is_empty() && is_closure {
1175                 let underscores = vec!["_"; expected_args.len()].join(", ");
1176                 err.span_suggestion(
1177                     pipe_span,
1178                     &format!(
1179                         "consider changing the closure to take and ignore the expected argument{}",
1180                         if expected_args.len() < 2 {
1181                             ""
1182                         } else {
1183                             "s"
1184                         }
1185                     ),
1186                     format!("|{}|", underscores),
1187                     Applicability::MachineApplicable,
1188                 );
1189             }
1190
1191             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
1192                 if fields.len() == expected_args.len() {
1193                     let sugg = fields.iter()
1194                         .map(|(name, _)| name.to_owned())
1195                         .collect::<Vec<String>>()
1196                         .join(", ");
1197                     err.span_suggestion(
1198                         found_span,
1199                         "change the closure to take multiple arguments instead of a single tuple",
1200                         format!("|{}|", sugg),
1201                         Applicability::MachineApplicable,
1202                     );
1203                 }
1204             }
1205             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..] {
1206                 if fields.len() == found_args.len() && is_closure {
1207                     let sugg = format!(
1208                         "|({}){}|",
1209                         found_args.iter()
1210                             .map(|arg| match arg {
1211                                 ArgKind::Arg(name, _) => name.to_owned(),
1212                                 _ => "_".to_owned(),
1213                             })
1214                             .collect::<Vec<String>>()
1215                             .join(", "),
1216                         // add type annotations if available
1217                         if found_args.iter().any(|arg| match arg {
1218                             ArgKind::Arg(_, ty) => ty != "_",
1219                             _ => false,
1220                         }) {
1221                             format!(": ({})",
1222                                     fields.iter()
1223                                         .map(|(_, ty)| ty.to_owned())
1224                                         .collect::<Vec<String>>()
1225                                         .join(", "))
1226                         } else {
1227                             String::new()
1228                         },
1229                     );
1230                     err.span_suggestion(
1231                         found_span,
1232                         "change the closure to accept a tuple instead of individual arguments",
1233                         sugg,
1234                         Applicability::MachineApplicable,
1235                     );
1236                 }
1237             }
1238         }
1239
1240         err
1241     }
1242
1243     fn report_closure_arg_mismatch(
1244         &self,
1245         span: Span,
1246         found_span: Option<Span>,
1247         expected_ref: ty::PolyTraitRef<'tcx>,
1248         found: ty::PolyTraitRef<'tcx>,
1249     ) -> DiagnosticBuilder<'tcx> {
1250         fn build_fn_sig_string<'tcx>(tcx: TyCtxt<'tcx>, trait_ref: &ty::TraitRef<'tcx>) -> String {
1251             let inputs = trait_ref.substs.type_at(1);
1252             let sig = if let ty::Tuple(inputs) = inputs.sty {
1253                 tcx.mk_fn_sig(
1254                     inputs.iter().map(|k| k.expect_ty()),
1255                     tcx.mk_ty_infer(ty::TyVar(ty::TyVid { index: 0 })),
1256                     false,
1257                     hir::Unsafety::Normal,
1258                     ::rustc_target::spec::abi::Abi::Rust
1259                 )
1260             } else {
1261                 tcx.mk_fn_sig(
1262                     ::std::iter::once(inputs),
1263                     tcx.mk_ty_infer(ty::TyVar(ty::TyVid { index: 0 })),
1264                     false,
1265                     hir::Unsafety::Normal,
1266                     ::rustc_target::spec::abi::Abi::Rust
1267                 )
1268             };
1269             ty::Binder::bind(sig).to_string()
1270         }
1271
1272         let argument_is_closure = expected_ref.skip_binder().substs.type_at(0).is_closure();
1273         let mut err = struct_span_err!(self.tcx.sess, span, E0631,
1274                                        "type mismatch in {} arguments",
1275                                        if argument_is_closure { "closure" } else { "function" });
1276
1277         let found_str = format!(
1278             "expected signature of `{}`",
1279             build_fn_sig_string(self.tcx, found.skip_binder())
1280         );
1281         err.span_label(span, found_str);
1282
1283         let found_span = found_span.unwrap_or(span);
1284         let expected_str = format!(
1285             "found signature of `{}`",
1286             build_fn_sig_string(self.tcx, expected_ref.skip_binder())
1287         );
1288         err.span_label(found_span, expected_str);
1289
1290         err
1291     }
1292 }
1293
1294 impl<'tcx> TyCtxt<'tcx> {
1295     pub fn recursive_type_with_infinite_size_error(self,
1296                                                    type_def_id: DefId)
1297                                                    -> DiagnosticBuilder<'tcx>
1298     {
1299         assert!(type_def_id.is_local());
1300         let span = self.hir().span_if_local(type_def_id).unwrap();
1301         let span = self.sess.source_map().def_span(span);
1302         let mut err = struct_span_err!(self.sess, span, E0072,
1303                                        "recursive type `{}` has infinite size",
1304                                        self.def_path_str(type_def_id));
1305         err.span_label(span, "recursive type has infinite size");
1306         err.help(&format!("insert indirection (e.g., a `Box`, `Rc`, or `&`) \
1307                            at some point to make `{}` representable",
1308                           self.def_path_str(type_def_id)));
1309         err
1310     }
1311
1312     pub fn report_object_safety_error(
1313         self,
1314         span: Span,
1315         trait_def_id: DefId,
1316         violations: Vec<ObjectSafetyViolation>,
1317     ) -> Option<DiagnosticBuilder<'tcx>> {
1318         if self.sess.trait_methods_not_found.borrow().contains(&span) {
1319             // Avoid emitting error caused by non-existing method (#58734)
1320             return None;
1321         }
1322         let trait_str = self.def_path_str(trait_def_id);
1323         let span = self.sess.source_map().def_span(span);
1324         let mut err = struct_span_err!(
1325             self.sess, span, E0038,
1326             "the trait `{}` cannot be made into an object",
1327             trait_str);
1328         err.span_label(span, format!("the trait `{}` cannot be made into an object", trait_str));
1329
1330         let mut reported_violations = FxHashSet::default();
1331         for violation in violations {
1332             if reported_violations.insert(violation.clone()) {
1333                 err.note(&violation.error_msg());
1334             }
1335         }
1336         Some(err)
1337     }
1338 }
1339
1340 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
1341     fn maybe_report_ambiguity(&self, obligation: &PredicateObligation<'tcx>,
1342                               body_id: Option<hir::BodyId>) {
1343         // Unable to successfully determine, probably means
1344         // insufficient type information, but could mean
1345         // ambiguous impls. The latter *ought* to be a
1346         // coherence violation, so we don't report it here.
1347
1348         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
1349         let span = obligation.cause.span;
1350
1351         debug!("maybe_report_ambiguity(predicate={:?}, obligation={:?})",
1352                predicate,
1353                obligation);
1354
1355         // Ambiguity errors are often caused as fallout from earlier
1356         // errors. So just ignore them if this infcx is tainted.
1357         if self.is_tainted_by_errors() {
1358             return;
1359         }
1360
1361         match predicate {
1362             ty::Predicate::Trait(ref data) => {
1363                 let trait_ref = data.to_poly_trait_ref();
1364                 let self_ty = trait_ref.self_ty();
1365                 if predicate.references_error() {
1366                     return;
1367                 }
1368                 // Typically, this ambiguity should only happen if
1369                 // there are unresolved type inference variables
1370                 // (otherwise it would suggest a coherence
1371                 // failure). But given #21974 that is not necessarily
1372                 // the case -- we can have multiple where clauses that
1373                 // are only distinguished by a region, which results
1374                 // in an ambiguity even when all types are fully
1375                 // known, since we don't dispatch based on region
1376                 // relationships.
1377
1378                 // This is kind of a hack: it frequently happens that some earlier
1379                 // error prevents types from being fully inferred, and then we get
1380                 // a bunch of uninteresting errors saying something like "<generic
1381                 // #0> doesn't implement Sized".  It may even be true that we
1382                 // could just skip over all checks where the self-ty is an
1383                 // inference variable, but I was afraid that there might be an
1384                 // inference variable created, registered as an obligation, and
1385                 // then never forced by writeback, and hence by skipping here we'd
1386                 // be ignoring the fact that we don't KNOW the type works
1387                 // out. Though even that would probably be harmless, given that
1388                 // we're only talking about builtin traits, which are known to be
1389                 // inhabited. But in any case I just threw in this check for
1390                 // has_errors() to be sure that compilation isn't happening
1391                 // anyway. In that case, why inundate the user.
1392                 if !self.tcx.sess.has_errors() {
1393                     if
1394                         self.tcx.lang_items().sized_trait()
1395                         .map_or(false, |sized_id| sized_id == trait_ref.def_id())
1396                     {
1397                         self.need_type_info_err(body_id, span, self_ty).emit();
1398                     } else {
1399                         let mut err = struct_span_err!(self.tcx.sess,
1400                                                        span, E0283,
1401                                                        "type annotations required: \
1402                                                         cannot resolve `{}`",
1403                                                        predicate);
1404                         self.note_obligation_cause(&mut err, obligation);
1405                         err.emit();
1406                     }
1407                 }
1408             }
1409
1410             ty::Predicate::WellFormed(ty) => {
1411                 // Same hacky approach as above to avoid deluging user
1412                 // with error messages.
1413                 if !ty.references_error() && !self.tcx.sess.has_errors() {
1414                     self.need_type_info_err(body_id, span, ty).emit();
1415                 }
1416             }
1417
1418             ty::Predicate::Subtype(ref data) => {
1419                 if data.references_error() || self.tcx.sess.has_errors() {
1420                     // no need to overload user in such cases
1421                 } else {
1422                     let &SubtypePredicate { a_is_expected: _, a, b } = data.skip_binder();
1423                     // both must be type variables, or the other would've been instantiated
1424                     assert!(a.is_ty_var() && b.is_ty_var());
1425                     self.need_type_info_err(body_id,
1426                                             obligation.cause.span,
1427                                             a).emit();
1428                 }
1429             }
1430
1431             _ => {
1432                 if !self.tcx.sess.has_errors() {
1433                     let mut err = struct_span_err!(self.tcx.sess,
1434                                                    obligation.cause.span, E0284,
1435                                                    "type annotations required: \
1436                                                     cannot resolve `{}`",
1437                                                    predicate);
1438                     self.note_obligation_cause(&mut err, obligation);
1439                     err.emit();
1440                 }
1441             }
1442         }
1443     }
1444
1445     /// Returns `true` if the trait predicate may apply for *some* assignment
1446     /// to the type parameters.
1447     fn predicate_can_apply(
1448         &self,
1449         param_env: ty::ParamEnv<'tcx>,
1450         pred: ty::PolyTraitRef<'tcx>,
1451     ) -> bool {
1452         struct ParamToVarFolder<'a, 'tcx> {
1453             infcx: &'a InferCtxt<'a, 'tcx>,
1454             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
1455         }
1456
1457         impl<'a, 'tcx> TypeFolder<'tcx> for ParamToVarFolder<'a, 'tcx> {
1458             fn tcx<'b>(&'b self) -> TyCtxt<'tcx> { self.infcx.tcx }
1459
1460             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
1461                 if let ty::Param(ty::ParamTy {name, .. }) = ty.sty {
1462                     let infcx = self.infcx;
1463                     self.var_map.entry(ty).or_insert_with(||
1464                         infcx.next_ty_var(
1465                             TypeVariableOrigin {
1466                                 kind: TypeVariableOriginKind::TypeParameterDefinition(name),
1467                                 span: DUMMY_SP,
1468                             }
1469                         )
1470                     )
1471                 } else {
1472                     ty.super_fold_with(self)
1473                 }
1474             }
1475         }
1476
1477         self.probe(|_| {
1478             let mut selcx = SelectionContext::new(self);
1479
1480             let cleaned_pred = pred.fold_with(&mut ParamToVarFolder {
1481                 infcx: self,
1482                 var_map: Default::default()
1483             });
1484
1485             let cleaned_pred = super::project::normalize(
1486                 &mut selcx,
1487                 param_env,
1488                 ObligationCause::dummy(),
1489                 &cleaned_pred
1490             ).value;
1491
1492             let obligation = Obligation::new(
1493                 ObligationCause::dummy(),
1494                 param_env,
1495                 cleaned_pred.to_predicate()
1496             );
1497
1498             self.predicate_may_hold(&obligation)
1499         })
1500     }
1501
1502     fn note_obligation_cause<T>(&self,
1503                                 err: &mut DiagnosticBuilder<'_>,
1504                                 obligation: &Obligation<'tcx, T>)
1505         where T: fmt::Display
1506     {
1507         self.note_obligation_cause_code(err,
1508                                         &obligation.predicate,
1509                                         &obligation.cause.code,
1510                                         &mut vec![]);
1511     }
1512
1513     fn note_obligation_cause_code<T>(&self,
1514                                      err: &mut DiagnosticBuilder<'_>,
1515                                      predicate: &T,
1516                                      cause_code: &ObligationCauseCode<'tcx>,
1517                                      obligated_types: &mut Vec<&ty::TyS<'tcx>>)
1518         where T: fmt::Display
1519     {
1520         let tcx = self.tcx;
1521         match *cause_code {
1522             ObligationCauseCode::ExprAssignable |
1523             ObligationCauseCode::MatchExpressionArm { .. } |
1524             ObligationCauseCode::MatchExpressionArmPattern { .. } |
1525             ObligationCauseCode::IfExpression { .. } |
1526             ObligationCauseCode::IfExpressionWithNoElse |
1527             ObligationCauseCode::MainFunctionType |
1528             ObligationCauseCode::StartFunctionType |
1529             ObligationCauseCode::IntrinsicType |
1530             ObligationCauseCode::MethodReceiver |
1531             ObligationCauseCode::ReturnNoExpression |
1532             ObligationCauseCode::MiscObligation => {}
1533             ObligationCauseCode::SliceOrArrayElem => {
1534                 err.note("slice and array elements must have `Sized` type");
1535             }
1536             ObligationCauseCode::TupleElem => {
1537                 err.note("only the last element of a tuple may have a dynamically sized type");
1538             }
1539             ObligationCauseCode::ProjectionWf(data) => {
1540                 err.note(&format!("required so that the projection `{}` is well-formed",
1541                                   data));
1542             }
1543             ObligationCauseCode::ReferenceOutlivesReferent(ref_ty) => {
1544                 err.note(&format!("required so that reference `{}` does not outlive its referent",
1545                                   ref_ty));
1546             }
1547             ObligationCauseCode::ObjectTypeBound(object_ty, region) => {
1548                 err.note(&format!("required so that the lifetime bound of `{}` for `{}` \
1549                                    is satisfied",
1550                                   region, object_ty));
1551             }
1552             ObligationCauseCode::ItemObligation(item_def_id) => {
1553                 let item_name = tcx.def_path_str(item_def_id);
1554                 let msg = format!("required by `{}`", item_name);
1555
1556                 if let Some(sp) = tcx.hir().span_if_local(item_def_id) {
1557                     let sp = tcx.sess.source_map().def_span(sp);
1558                     err.span_note(sp, &msg);
1559                 } else {
1560                     err.note(&msg);
1561                 }
1562             }
1563             ObligationCauseCode::ObjectCastObligation(object_ty) => {
1564                 err.note(&format!("required for the cast to the object type `{}`",
1565                                   self.ty_to_string(object_ty)));
1566             }
1567             ObligationCauseCode::RepeatVec => {
1568                 err.note("the `Copy` trait is required because the \
1569                           repeated element will be copied");
1570             }
1571             ObligationCauseCode::VariableType(_) => {
1572                 err.note("all local variables must have a statically known size");
1573                 if !self.tcx.features().unsized_locals {
1574                     err.help("unsized locals are gated as an unstable feature");
1575                 }
1576             }
1577             ObligationCauseCode::SizedArgumentType => {
1578                 err.note("all function arguments must have a statically known size");
1579                 if !self.tcx.features().unsized_locals {
1580                     err.help("unsized locals are gated as an unstable feature");
1581                 }
1582             }
1583             ObligationCauseCode::SizedReturnType => {
1584                 err.note("the return type of a function must have a \
1585                           statically known size");
1586             }
1587             ObligationCauseCode::SizedYieldType => {
1588                 err.note("the yield type of a generator must have a \
1589                           statically known size");
1590             }
1591             ObligationCauseCode::AssignmentLhsSized => {
1592                 err.note("the left-hand-side of an assignment must have a statically known size");
1593             }
1594             ObligationCauseCode::TupleInitializerSized => {
1595                 err.note("tuples must have a statically known size to be initialized");
1596             }
1597             ObligationCauseCode::StructInitializerSized => {
1598                 err.note("structs must have a statically known size to be initialized");
1599             }
1600             ObligationCauseCode::FieldSized { adt_kind: ref item, last } => {
1601                 match *item {
1602                     AdtKind::Struct => {
1603                         if last {
1604                             err.note("the last field of a packed struct may only have a \
1605                                       dynamically sized type if it does not need drop to be run");
1606                         } else {
1607                             err.note("only the last field of a struct may have a dynamically \
1608                                       sized type");
1609                         }
1610                     }
1611                     AdtKind::Union => {
1612                         err.note("no field of a union may have a dynamically sized type");
1613                     }
1614                     AdtKind::Enum => {
1615                         err.note("no field of an enum variant may have a dynamically sized type");
1616                     }
1617                 }
1618             }
1619             ObligationCauseCode::ConstSized => {
1620                 err.note("constant expressions must have a statically known size");
1621             }
1622             ObligationCauseCode::SharedStatic => {
1623                 err.note("shared static variables must have a type that implements `Sync`");
1624             }
1625             ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
1626                 let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
1627                 let ty = parent_trait_ref.skip_binder().self_ty();
1628                 err.note(&format!("required because it appears within the type `{}`", ty));
1629                 obligated_types.push(ty);
1630
1631                 let parent_predicate = parent_trait_ref.to_predicate();
1632                 if !self.is_recursive_obligation(obligated_types, &data.parent_code) {
1633                     self.note_obligation_cause_code(err,
1634                                                     &parent_predicate,
1635                                                     &data.parent_code,
1636                                                     obligated_types);
1637                 }
1638             }
1639             ObligationCauseCode::ImplDerivedObligation(ref data) => {
1640                 let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
1641                 err.note(
1642                     &format!("required because of the requirements on the impl of `{}` for `{}`",
1643                              parent_trait_ref,
1644                              parent_trait_ref.skip_binder().self_ty()));
1645                 let parent_predicate = parent_trait_ref.to_predicate();
1646                 self.note_obligation_cause_code(err,
1647                                                 &parent_predicate,
1648                                                 &data.parent_code,
1649                                                 obligated_types);
1650             }
1651             ObligationCauseCode::CompareImplMethodObligation { .. } => {
1652                 err.note(
1653                     &format!("the requirement `{}` appears on the impl method \
1654                               but not on the corresponding trait method",
1655                              predicate));
1656             }
1657             ObligationCauseCode::ReturnType(_) |
1658             ObligationCauseCode::BlockTailExpression(_) => (),
1659             ObligationCauseCode::TrivialBound => {
1660                 err.help("see issue #48214");
1661                 if tcx.sess.opts.unstable_features.is_nightly_build() {
1662                     err.help("add #![feature(trivial_bounds)] to the \
1663                               crate attributes to enable",
1664                     );
1665                 }
1666             }
1667         }
1668     }
1669
1670     fn suggest_new_overflow_limit(&self, err: &mut DiagnosticBuilder<'_>) {
1671         let current_limit = self.tcx.sess.recursion_limit.get();
1672         let suggested_limit = current_limit * 2;
1673         err.help(&format!("consider adding a `#![recursion_limit=\"{}\"]` attribute to your crate",
1674                           suggested_limit));
1675     }
1676
1677     fn is_recursive_obligation(&self,
1678                                obligated_types: &mut Vec<&ty::TyS<'tcx>>,
1679                                cause_code: &ObligationCauseCode<'tcx>) -> bool {
1680         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
1681             let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
1682
1683             if obligated_types.iter().any(|ot| ot == &parent_trait_ref.skip_binder().self_ty()) {
1684                 return true;
1685             }
1686         }
1687         false
1688     }
1689 }
1690
1691 /// Summarizes information
1692 #[derive(Clone)]
1693 pub enum ArgKind {
1694     /// An argument of non-tuple type. Parameters are (name, ty)
1695     Arg(String, String),
1696
1697     /// An argument of tuple type. For a "found" argument, the span is
1698     /// the locationo in the source of the pattern. For a "expected"
1699     /// argument, it will be None. The vector is a list of (name, ty)
1700     /// strings for the components of the tuple.
1701     Tuple(Option<Span>, Vec<(String, String)>),
1702 }
1703
1704 impl ArgKind {
1705     fn empty() -> ArgKind {
1706         ArgKind::Arg("_".to_owned(), "_".to_owned())
1707     }
1708
1709     /// Creates an `ArgKind` from the expected type of an
1710     /// argument. It has no name (`_`) and an optional source span.
1711     pub fn from_expected_ty(t: Ty<'_>, span: Option<Span>) -> ArgKind {
1712         match t.sty {
1713             ty::Tuple(ref tys) => ArgKind::Tuple(
1714                 span,
1715                 tys.iter()
1716                    .map(|ty| ("_".to_owned(), ty.to_string()))
1717                    .collect::<Vec<_>>()
1718             ),
1719             _ => ArgKind::Arg("_".to_owned(), t.to_string()),
1720         }
1721     }
1722 }