src/librustc/traits/error_reporting.rs

   1 use super::{
   2     ConstEvalFailure,
   3     EvaluationResult,
   4     FulfillmentError,
   5     FulfillmentErrorCode,
   6     MismatchedProjectionTypes,
   7     ObjectSafetyViolation,
   8     Obligation,
   9     ObligationCause,
  10     ObligationCauseCode,
  11     OnUnimplementedDirective,
  12     OnUnimplementedNote,
  13     OutputTypeParameterMismatch,
  14     Overflow,
  15     PredicateObligation,
  16     SelectionContext,
  17     SelectionError,
  18     TraitNotObjectSafe,
  19 };
  20
  21 use crate::hir;
  22 use crate::hir::Node;
  23 use crate::hir::def_id::DefId;
  24 use crate::infer::{self, InferCtxt};
  25 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  26 use crate::session::DiagnosticMessageId;
  27 use crate::ty::{self, AdtKind, DefIdTree, ToPredicate, ToPolyTraitRef, Ty, TyCtxt, TypeFoldable};
  28 use crate::ty::GenericParamDefKind;
  29 use crate::ty::error::ExpectedFound;
  30 use crate::ty::fast_reject;
  31 use crate::ty::fold::TypeFolder;
  32 use crate::ty::subst::Subst;
  33 use crate::ty::SubtypePredicate;
  34 use crate::util::nodemap::{FxHashMap, FxHashSet};
  35
  36 use errors::{Applicability, DiagnosticBuilder, pluralize};
  37 use std::fmt;
  38 use syntax::ast;
  39 use syntax::symbol::{sym, kw};
  40 use syntax_pos::{DUMMY_SP, Span, ExpnKind, MultiSpan};
  41
  42 use rustc_error_codes::*;
  43
  44 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
  45     pub fn report_fulfillment_errors(
  46         &self,
  47         errors: &[FulfillmentError<'tcx>],
  48         body_id: Option<hir::BodyId>,
  49         fallback_has_occurred: bool,
  50     ) {
  51         #[derive(Debug)]
  52         struct ErrorDescriptor<'tcx> {
  53             predicate: ty::Predicate<'tcx>,
  54             index: Option<usize>, // None if this is an old error
  55         }
  56
  57         let mut error_map: FxHashMap<_, Vec<_>> =
  58             self.reported_trait_errors.borrow().iter().map(|(&span, predicates)| {
  59                 (span, predicates.iter().map(|predicate| ErrorDescriptor {
  60                     predicate: predicate.clone(),
  61                     index: None
  62                 }).collect())
  63             }).collect();
  64
  65         for (index, error) in errors.iter().enumerate() {
  66             // We want to ignore desugarings here: spans are equivalent even
  67             // if one is the result of a desugaring and the other is not.
  68             let mut span = error.obligation.cause.span;
  69             let expn_data = span.ctxt().outer_expn_data();
  70             if let ExpnKind::Desugaring(_) = expn_data.kind {
  71                 span = expn_data.call_site;
  72             }
  73
  74             error_map.entry(span).or_default().push(
  75                 ErrorDescriptor {
  76                     predicate: error.obligation.predicate.clone(),
  77                     index: Some(index)
  78                 }
  79             );
  80
  81             self.reported_trait_errors.borrow_mut()
  82                 .entry(span).or_default()
  83                 .push(error.obligation.predicate.clone());
  84         }
  85
  86         // We do this in 2 passes because we want to display errors in order, though
  87         // maybe it *is* better to sort errors by span or something.
  88         let mut is_suppressed = vec![false; errors.len()];
  89         for (_, error_set) in error_map.iter() {
  90             // We want to suppress "duplicate" errors with the same span.
  91             for error in error_set {
  92                 if let Some(index) = error.index {
  93                     // Suppress errors that are either:
  94                     // 1) strictly implied by another error.
  95                     // 2) implied by an error with a smaller index.
  96                     for error2 in error_set {
  97                         if error2.index.map_or(false, |index2| is_suppressed[index2]) {
  98                             // Avoid errors being suppressed by already-suppressed
  99                             // errors, to prevent all errors from being suppressed
 100                             // at once.
 101                             continue
 102                         }
 103
 104                         if self.error_implies(&error2.predicate, &error.predicate) &&
 105                             !(error2.index >= error.index &&
 106                               self.error_implies(&error.predicate, &error2.predicate))
 107                         {
 108                             info!("skipping {:?} (implied by {:?})", error, error2);
 109                             is_suppressed[index] = true;
 110                             break
 111                         }
 112                     }
 113                 }
 114             }
 115         }
 116
 117         for (error, suppressed) in errors.iter().zip(is_suppressed) {
 118             if !suppressed {
 119                 self.report_fulfillment_error(error, body_id, fallback_has_occurred);
 120             }
 121         }
 122     }
 123
 124     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
 125     // `error` occurring implies that `cond` occurs.
 126     fn error_implies(
 127         &self,
 128         cond: &ty::Predicate<'tcx>,
 129         error: &ty::Predicate<'tcx>,
 130     ) -> bool {
 131         if cond == error {
 132             return true
 133         }
 134
 135         let (cond, error) = match (cond, error) {
 136             (&ty::Predicate::Trait(..), &ty::Predicate::Trait(ref error))
 137                 => (cond, error),
 138             _ => {
 139                 // FIXME: make this work in other cases too.
 140                 return false
 141             }
 142         };
 143
 144         for implication in super::elaborate_predicates(self.tcx, vec![cond.clone()]) {
 145             if let ty::Predicate::Trait(implication) = implication {
 146                 let error = error.to_poly_trait_ref();
 147                 let implication = implication.to_poly_trait_ref();
 148                 // FIXME: I'm just not taking associated types at all here.
 149                 // Eventually I'll need to implement param-env-aware
 150                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
 151                 let param_env = ty::ParamEnv::empty();
 152                 if self.can_sub(param_env, error, implication).is_ok() {
 153                     debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
 154                     return true
 155                 }
 156             }
 157         }
 158
 159         false
 160     }
 161
 162     fn report_fulfillment_error(
 163         &self,
 164         error: &FulfillmentError<'tcx>,
 165         body_id: Option<hir::BodyId>,
 166         fallback_has_occurred: bool,
 167     ) {
 168         debug!("report_fulfillment_errors({:?})", error);
 169         match error.code {
 170             FulfillmentErrorCode::CodeSelectionError(ref selection_error) => {
 171                 self.report_selection_error(
 172                     &error.obligation,
 173                     selection_error,
 174                     fallback_has_occurred,
 175                     error.points_at_arg_span,
 176                 );
 177             }
 178             FulfillmentErrorCode::CodeProjectionError(ref e) => {
 179                 self.report_projection_error(&error.obligation, e);
 180             }
 181             FulfillmentErrorCode::CodeAmbiguity => {
 182                 self.maybe_report_ambiguity(&error.obligation, body_id);
 183             }
 184             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
 185                 self.report_mismatched_types(
 186                     &error.obligation.cause,
 187                     expected_found.expected,
 188                     expected_found.found,
 189                     err.clone(),
 190                 ).emit();
 191             }
 192         }
 193     }
 194
 195     fn report_projection_error(
 196         &self,
 197         obligation: &PredicateObligation<'tcx>,
 198         error: &MismatchedProjectionTypes<'tcx>,
 199     ) {
 200         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
 201
 202         if predicate.references_error() {
 203             return
 204         }
 205
 206         self.probe(|_| {
 207             let err_buf;
 208             let mut err = &error.err;
 209             let mut values = None;
 210
 211             // try to find the mismatched types to report the error with.
 212             //
 213             // this can fail if the problem was higher-ranked, in which
 214             // cause I have no idea for a good error message.
 215             if let ty::Predicate::Projection(ref data) = predicate {
 216                 let mut selcx = SelectionContext::new(self);
 217                 let (data, _) = self.replace_bound_vars_with_fresh_vars(
 218                     obligation.cause.span,
 219                     infer::LateBoundRegionConversionTime::HigherRankedType,
 220                     data
 221                 );
 222                 let mut obligations = vec![];
 223                 let normalized_ty = super::normalize_projection_type(
 224                     &mut selcx,
 225                     obligation.param_env,
 226                     data.projection_ty,
 227                     obligation.cause.clone(),
 228                     0,
 229                     &mut obligations
 230                 );
 231
 232                 debug!("report_projection_error obligation.cause={:?} obligation.param_env={:?}",
 233                        obligation.cause, obligation.param_env);
 234
 235                 debug!("report_projection_error normalized_ty={:?} data.ty={:?}",
 236                        normalized_ty, data.ty);
 237
 238                 let is_normalized_ty_expected = match &obligation.cause.code {
 239                     ObligationCauseCode::ItemObligation(_) |
 240                     ObligationCauseCode::BindingObligation(_, _) |
 241                     ObligationCauseCode::ObjectCastObligation(_) => false,
 242                     _ => true,
 243                 };
 244
 245                 if let Err(error) = self.at(&obligation.cause, obligation.param_env)
 246                     .eq_exp(is_normalized_ty_expected, normalized_ty, data.ty)
 247                 {
 248                     values = Some(infer::ValuePairs::Types(
 249                         ExpectedFound::new(is_normalized_ty_expected, normalized_ty, data.ty)));
 250
 251                     err_buf = error;
 252                     err = &err_buf;
 253                 }
 254             }
 255
 256             let msg = format!("type mismatch resolving `{}`", predicate);
 257             let error_id = (
 258                 DiagnosticMessageId::ErrorId(271),
 259                 Some(obligation.cause.span),
 260                 msg,
 261             );
 262             let fresh = self.tcx.sess.one_time_diagnostics.borrow_mut().insert(error_id);
 263             if fresh {
 264                 let mut diag = struct_span_err!(
 265                     self.tcx.sess,
 266                     obligation.cause.span,
 267                     E0271,
 268                     "type mismatch resolving `{}`",
 269                     predicate
 270                 );
 271                 self.note_type_err(&mut diag, &obligation.cause, None, values, err);
 272                 self.note_obligation_cause(&mut diag, obligation);
 273                 diag.emit();
 274             }
 275         });
 276     }
 277
 278     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
 279         /// returns the fuzzy category of a given type, or None
 280         /// if the type can be equated to any type.
 281         fn type_category(t: Ty<'_>) -> Option<u32> {
 282             match t.kind {
 283                 ty::Bool => Some(0),
 284                 ty::Char => Some(1),
 285                 ty::Str => Some(2),
 286                 ty::Int(..) | ty::Uint(..) | ty::Infer(ty::IntVar(..)) => Some(3),
 287                 ty::Float(..) | ty::Infer(ty::FloatVar(..)) => Some(4),
 288                 ty::Ref(..) | ty::RawPtr(..) => Some(5),
 289                 ty::Array(..) | ty::Slice(..) => Some(6),
 290                 ty::FnDef(..) | ty::FnPtr(..) => Some(7),
 291                 ty::Dynamic(..) => Some(8),
 292                 ty::Closure(..) => Some(9),
 293                 ty::Tuple(..) => Some(10),
 294                 ty::Projection(..) => Some(11),
 295                 ty::Param(..) => Some(12),
 296                 ty::Opaque(..) => Some(13),
 297                 ty::Never => Some(14),
 298                 ty::Adt(adt, ..) => match adt.adt_kind() {
 299                     AdtKind::Struct => Some(15),
 300                     AdtKind::Union => Some(16),
 301                     AdtKind::Enum => Some(17),
 302                 },
 303                 ty::Generator(..) => Some(18),
 304                 ty::Foreign(..) => Some(19),
 305                 ty::GeneratorWitness(..) => Some(20),
 306                 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error => None,
 307                 ty::UnnormalizedProjection(..) => bug!("only used with chalk-engine"),
 308             }
 309         }
 310
 311         match (type_category(a), type_category(b)) {
 312             (Some(cat_a), Some(cat_b)) => match (&a.kind, &b.kind) {
 313                 (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => def_a == def_b,
 314                 _ => cat_a == cat_b
 315             },
 316             // infer and error can be equated to all types
 317             _ => true
 318         }
 319     }
 320
 321     fn impl_similar_to(&self,
 322                        trait_ref: ty::PolyTraitRef<'tcx>,
 323                        obligation: &PredicateObligation<'tcx>)
 324                        -> Option<DefId>
 325     {
 326         let tcx = self.tcx;
 327         let param_env = obligation.param_env;
 328         let trait_ref = tcx.erase_late_bound_regions(&trait_ref);
 329         let trait_self_ty = trait_ref.self_ty();
 330
 331         let mut self_match_impls = vec![];
 332         let mut fuzzy_match_impls = vec![];
 333
 334         self.tcx.for_each_relevant_impl(
 335             trait_ref.def_id, trait_self_ty, |def_id| {
 336                 let impl_substs = self.fresh_substs_for_item(obligation.cause.span, def_id);
 337                 let impl_trait_ref = tcx
 338                     .impl_trait_ref(def_id)
 339                     .unwrap()
 340                     .subst(tcx, impl_substs);
 341
 342                 let impl_self_ty = impl_trait_ref.self_ty();
 343
 344                 if let Ok(..) = self.can_eq(param_env, trait_self_ty, impl_self_ty) {
 345                     self_match_impls.push(def_id);
 346
 347                     if trait_ref.substs.types().skip(1)
 348                         .zip(impl_trait_ref.substs.types().skip(1))
 349                         .all(|(u,v)| self.fuzzy_match_tys(u, v))
 350                     {
 351                         fuzzy_match_impls.push(def_id);
 352                     }
 353                 }
 354             });
 355
 356         let impl_def_id = if self_match_impls.len() == 1 {
 357             self_match_impls[0]
 358         } else if fuzzy_match_impls.len() == 1 {
 359             fuzzy_match_impls[0]
 360         } else {
 361             return None
 362         };
 363
 364         if tcx.has_attr(impl_def_id, sym::rustc_on_unimplemented) {
 365             Some(impl_def_id)
 366         } else {
 367             None
 368         }
 369     }
 370
 371     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str> {
 372         self.tcx.hir().body(body_id).generator_kind.map(|gen_kind| {
 373             match gen_kind {
 374                 hir::GeneratorKind::Gen => "a generator",
 375                 hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block) => "an async block",
 376                 hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn) => "an async function",
 377                 hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Closure) => "an async closure",
 378             }
 379         })
 380     }
 381
 382     /// Used to set on_unimplemented's `ItemContext`
 383     /// to be the enclosing (async) block/function/closure
 384     fn describe_enclosure(&self, hir_id: hir::HirId) -> Option<&'static str> {
 385         let hir = &self.tcx.hir();
 386         let node = hir.find(hir_id)?;
 387         if let hir::Node::Item(
 388             hir::Item{kind: hir::ItemKind::Fn(sig, _, body_id), .. }) = &node {
 389             self.describe_generator(*body_id).or_else(||
 390                 Some(if let hir::FnHeader{ asyncness: hir::IsAsync::Async, .. } = sig.header {
 391                     "an async function"
 392                 } else {
 393                     "a function"
 394                 })
 395             )
 396         } else if let hir::Node::Expr(hir::Expr {
 397             kind: hir::ExprKind::Closure(_is_move, _, body_id, _, gen_movability), .. }) = &node {
 398             self.describe_generator(*body_id).or_else(||
 399                 Some(if gen_movability.is_some() {
 400                     "an async closure"
 401                 } else {
 402                     "a closure"
 403                 })
 404             )
 405         } else if let hir::Node::Expr(hir::Expr { .. }) = &node {
 406             let parent_hid = hir.get_parent_node(hir_id);
 407             if parent_hid != hir_id {
 408                 return self.describe_enclosure(parent_hid);
 409             } else {
 410                 None
 411             }
 412         } else {
 413             None
 414         }
 415     }
 416
 417     fn on_unimplemented_note(
 418         &self,
 419         trait_ref: ty::PolyTraitRef<'tcx>,
 420         obligation: &PredicateObligation<'tcx>,
 421     ) -> OnUnimplementedNote {
 422         let def_id = self.impl_similar_to(trait_ref, obligation)
 423             .unwrap_or_else(|| trait_ref.def_id());
 424         let trait_ref = *trait_ref.skip_binder();
 425
 426         let mut flags = vec![];
 427         flags.push((sym::item_context,
 428             self.describe_enclosure(obligation.cause.body_id).map(|s|s.to_owned())));
 429
 430         match obligation.cause.code {
 431             ObligationCauseCode::BuiltinDerivedObligation(..) |
 432             ObligationCauseCode::ImplDerivedObligation(..) => {}
 433             _ => {
 434                 // this is a "direct", user-specified, rather than derived,
 435                 // obligation.
 436                 flags.push((sym::direct, None));
 437             }
 438         }
 439
 440         if let ObligationCauseCode::ItemObligation(item) = obligation.cause.code {
 441             // FIXME: maybe also have some way of handling methods
 442             // from other traits? That would require name resolution,
 443             // which we might want to be some sort of hygienic.
 444             //
 445             // Currently I'm leaving it for what I need for `try`.
 446             if self.tcx.trait_of_item(item) == Some(trait_ref.def_id) {
 447                 let method = self.tcx.item_name(item);
 448                 flags.push((sym::from_method, None));
 449                 flags.push((sym::from_method, Some(method.to_string())));
 450             }
 451         }
 452         if let Some(t) = self.get_parent_trait_ref(&obligation.cause.code) {
 453             flags.push((sym::parent_trait, Some(t)));
 454         }
 455
 456         if let Some(k) = obligation.cause.span.desugaring_kind() {
 457             flags.push((sym::from_desugaring, None));
 458             flags.push((sym::from_desugaring, Some(format!("{:?}", k))));
 459         }
 460         let generics = self.tcx.generics_of(def_id);
 461         let self_ty = trait_ref.self_ty();
 462         // This is also included through the generics list as `Self`,
 463         // but the parser won't allow you to use it
 464         flags.push((sym::_Self, Some(self_ty.to_string())));
 465         if let Some(def) = self_ty.ty_adt_def() {
 466             // We also want to be able to select self's original
 467             // signature with no type arguments resolved
 468             flags.push((sym::_Self, Some(self.tcx.type_of(def.did).to_string())));
 469         }
 470
 471         for param in generics.params.iter() {
 472             let value = match param.kind {
 473                 GenericParamDefKind::Type { .. } |
 474                 GenericParamDefKind::Const => {
 475                     trait_ref.substs[param.index as usize].to_string()
 476                 },
 477                 GenericParamDefKind::Lifetime => continue,
 478             };
 479             let name = param.name;
 480             flags.push((name, Some(value)));
 481         }
 482
 483         if let Some(true) = self_ty.ty_adt_def().map(|def| def.did.is_local()) {
 484             flags.push((sym::crate_local, None));
 485         }
 486
 487         // Allow targeting all integers using `{integral}`, even if the exact type was resolved
 488         if self_ty.is_integral() {
 489             flags.push((sym::_Self, Some("{integral}".to_owned())));
 490         }
 491
 492         if let ty::Array(aty, len) = self_ty.kind {
 493             flags.push((sym::_Self, Some("[]".to_owned())));
 494             flags.push((sym::_Self, Some(format!("[{}]", aty))));
 495             if let Some(def) = aty.ty_adt_def() {
 496                 // We also want to be able to select the array's type's original
 497                 // signature with no type arguments resolved
 498                 flags.push((
 499                     sym::_Self,
 500                     Some(format!("[{}]", self.tcx.type_of(def.did).to_string())),
 501                 ));
 502                 let tcx = self.tcx;
 503                 if let Some(len) = len.try_eval_usize(tcx, ty::ParamEnv::empty()) {
 504                     flags.push((
 505                         sym::_Self,
 506                         Some(format!("[{}; {}]", self.tcx.type_of(def.did).to_string(), len)),
 507                     ));
 508                 } else {
 509                     flags.push((
 510                         sym::_Self,
 511                         Some(format!("[{}; _]", self.tcx.type_of(def.did).to_string())),
 512                     ));
 513                 }
 514             }
 515         }
 516
 517         if let Ok(Some(command)) = OnUnimplementedDirective::of_item(
 518             self.tcx, trait_ref.def_id, def_id
 519         ) {
 520             command.evaluate(self.tcx, trait_ref, &flags[..])
 521         } else {
 522             OnUnimplementedNote::empty()
 523         }
 524     }
 525
 526     fn find_similar_impl_candidates(
 527         &self,
 528         trait_ref: ty::PolyTraitRef<'tcx>,
 529     ) -> Vec<ty::TraitRef<'tcx>> {
 530         let simp = fast_reject::simplify_type(self.tcx, trait_ref.skip_binder().self_ty(), true);
 531         let all_impls = self.tcx.all_impls(trait_ref.def_id());
 532
 533         match simp {
 534             Some(simp) => all_impls.iter().filter_map(|&def_id| {
 535                 let imp = self.tcx.impl_trait_ref(def_id).unwrap();
 536                 let imp_simp = fast_reject::simplify_type(self.tcx, imp.self_ty(), true);
 537                 if let Some(imp_simp) = imp_simp {
 538                     if simp != imp_simp {
 539                         return None
 540                     }
 541                 }
 542
 543                 Some(imp)
 544             }).collect(),
 545             None => all_impls.iter().map(|&def_id|
 546                 self.tcx.impl_trait_ref(def_id).unwrap()
 547             ).collect()
 548         }
 549     }
 550
 551     fn report_similar_impl_candidates(
 552         &self,
 553         impl_candidates: Vec<ty::TraitRef<'tcx>>,
 554         err: &mut DiagnosticBuilder<'_>,
 555     ) {
 556         if impl_candidates.is_empty() {
 557             return;
 558         }
 559
 560         let len = impl_candidates.len();
 561         let end = if impl_candidates.len() <= 5 {
 562             impl_candidates.len()
 563         } else {
 564             4
 565         };
 566
 567         let normalize = |candidate| self.tcx.infer_ctxt().enter(|ref infcx| {
 568             let normalized = infcx
 569                 .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
 570                 .normalize(candidate)
 571                 .ok();
 572             match normalized {
 573                 Some(normalized) => format!("\n  {:?}", normalized.value),
 574                 None => format!("\n  {:?}", candidate),
 575             }
 576         });
 577
 578         // Sort impl candidates so that ordering is consistent for UI tests.
 579         let mut normalized_impl_candidates = impl_candidates
 580             .iter()
 581             .map(normalize)
 582             .collect::<Vec<String>>();
 583
 584         // Sort before taking the `..end` range,
 585         // because the ordering of `impl_candidates` may not be deterministic:
 586         // https://github.com/rust-lang/rust/pull/57475#issuecomment-455519507
 587         normalized_impl_candidates.sort();
 588
 589         err.help(&format!("the following implementations were found:{}{}",
 590                           normalized_impl_candidates[..end].join(""),
 591                           if len > 5 {
 592                               format!("\nand {} others", len - 4)
 593                           } else {
 594                               String::new()
 595                           }
 596                           ));
 597     }
 598
 599     /// Reports that an overflow has occurred and halts compilation. We
 600     /// halt compilation unconditionally because it is important that
 601     /// overflows never be masked -- they basically represent computations
 602     /// whose result could not be truly determined and thus we can't say
 603     /// if the program type checks or not -- and they are unusual
 604     /// occurrences in any case.
 605     pub fn report_overflow_error<T>(
 606         &self,
 607         obligation: &Obligation<'tcx, T>,
 608         suggest_increasing_limit: bool,
 609     ) -> !
 610         where T: fmt::Display + TypeFoldable<'tcx>
 611     {
 612         let predicate =
 613             self.resolve_vars_if_possible(&obligation.predicate);
 614         let mut err = struct_span_err!(
 615             self.tcx.sess,
 616             obligation.cause.span,
 617             E0275,
 618             "overflow evaluating the requirement `{}`",
 619             predicate
 620         );
 621
 622         if suggest_increasing_limit {
 623             self.suggest_new_overflow_limit(&mut err);
 624         }
 625
 626         self.note_obligation_cause_code(
 627             &mut err,
 628             &obligation.predicate,
 629             &obligation.cause.code,
 630             &mut vec![],
 631         );
 632
 633         err.emit();
 634         self.tcx.sess.abort_if_errors();
 635         bug!();
 636     }
 637
 638     /// Reports that a cycle was detected which led to overflow and halts
 639     /// compilation. This is equivalent to `report_overflow_error` except
 640     /// that we can give a more helpful error message (and, in particular,
 641     /// we do not suggest increasing the overflow limit, which is not
 642     /// going to help).
 643     pub fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 644         let cycle = self.resolve_vars_if_possible(&cycle.to_owned());
 645         assert!(cycle.len() > 0);
 646
 647         debug!("report_overflow_error_cycle: cycle={:?}", cycle);
 648
 649         self.report_overflow_error(&cycle[0], false);
 650     }
 651
 652     pub fn report_extra_impl_obligation(&self,
 653                                         error_span: Span,
 654                                         item_name: ast::Name,
 655                                         _impl_item_def_id: DefId,
 656                                         trait_item_def_id: DefId,
 657                                         requirement: &dyn fmt::Display)
 658                                         -> DiagnosticBuilder<'tcx>
 659     {
 660         let msg = "impl has stricter requirements than trait";
 661         let sp = self.tcx.sess.source_map().def_span(error_span);
 662
 663         let mut err = struct_span_err!(self.tcx.sess, sp, E0276, "{}", msg);
 664
 665         if let Some(trait_item_span) = self.tcx.hir().span_if_local(trait_item_def_id) {
 666             let span = self.tcx.sess.source_map().def_span(trait_item_span);
 667             err.span_label(span, format!("definition of `{}` from trait", item_name));
 668         }
 669
 670         err.span_label(sp, format!("impl has extra requirement {}", requirement));
 671
 672         err
 673     }
 674
 675
 676     /// Gets the parent trait chain start
 677     fn get_parent_trait_ref(&self, code: &ObligationCauseCode<'tcx>) -> Option<String> {
 678         match code {
 679             &ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
 680                 let parent_trait_ref = self.resolve_vars_if_possible(
 681                     &data.parent_trait_ref);
 682                 match self.get_parent_trait_ref(&data.parent_code) {
 683                     Some(t) => Some(t),
 684                     None => Some(parent_trait_ref.skip_binder().self_ty().to_string()),
 685                 }
 686             }
 687             _ => None,
 688         }
 689     }
 690
 691     pub fn report_selection_error(
 692         &self,
 693         obligation: &PredicateObligation<'tcx>,
 694         error: &SelectionError<'tcx>,
 695         fallback_has_occurred: bool,
 696         points_at_arg: bool,
 697     ) {
 698         let span = obligation.cause.span;
 699
 700         let mut err = match *error {
 701             SelectionError::Unimplemented => {
 702                 if let ObligationCauseCode::CompareImplMethodObligation {
 703                     item_name, impl_item_def_id, trait_item_def_id,
 704                 } = obligation.cause.code {
 705                     self.report_extra_impl_obligation(
 706                         span,
 707                         item_name,
 708                         impl_item_def_id,
 709                         trait_item_def_id,
 710                         &format!("`{}`", obligation.predicate))
 711                         .emit();
 712                     return;
 713                 }
 714                 match obligation.predicate {
 715                     ty::Predicate::Trait(ref trait_predicate) => {
 716                         let trait_predicate =
 717                             self.resolve_vars_if_possible(trait_predicate);
 718
 719                         if self.tcx.sess.has_errors() && trait_predicate.references_error() {
 720                             return;
 721                         }
 722                         let trait_ref = trait_predicate.to_poly_trait_ref();
 723                         let (post_message, pre_message) =
 724                             self.get_parent_trait_ref(&obligation.cause.code)
 725                                 .map(|t| (format!(" in `{}`", t), format!("within `{}`, ", t)))
 726                             .unwrap_or_default();
 727
 728                         let OnUnimplementedNote { message, label, note }
 729                             = self.on_unimplemented_note(trait_ref, obligation);
 730                         let have_alt_message = message.is_some() || label.is_some();
 731                         let is_try = self.tcx.sess.source_map().span_to_snippet(span)
 732                             .map(|s| &s == "?")
 733                             .unwrap_or(false);
 734                         let is_from = format!("{}", trait_ref).starts_with("std::convert::From<");
 735                         let (message, note) = if is_try && is_from {
 736                             (Some(format!(
 737                                 "`?` couldn't convert the error to `{}`",
 738                                 trait_ref.self_ty(),
 739                             )), Some(
 740                                 "the question mark operation (`?`) implicitly performs a \
 741                                  conversion on the error value using the `From` trait".to_owned()
 742                             ))
 743                         } else {
 744                             (message, note)
 745                         };
 746
 747                         let mut err = struct_span_err!(
 748                             self.tcx.sess,
 749                             span,
 750                             E0277,
 751                             "{}",
 752                             message.unwrap_or_else(|| format!(
 753                                 "the trait bound `{}` is not satisfied{}",
 754                                 trait_ref.to_predicate(),
 755                                 post_message,
 756                             )));
 757
 758                         let explanation =
 759                             if obligation.cause.code == ObligationCauseCode::MainFunctionType {
 760                                 "consider using `()`, or a `Result`".to_owned()
 761                             } else {
 762                                 format!(
 763                                     "{}the trait `{}` is not implemented for `{}`",
 764                                     pre_message,
 765                                     trait_ref,
 766                                     trait_ref.self_ty(),
 767                                 )
 768                             };
 769
 770                         if let Some(ref s) = label {
 771                             // If it has a custom `#[rustc_on_unimplemented]`
 772                             // error message, let's display it as the label!
 773                             err.span_label(span, s.as_str());
 774                             err.help(&explanation);
 775                         } else {
 776                             err.span_label(span, explanation);
 777                         }
 778                         if let Some(ref s) = note {
 779                             // If it has a custom `#[rustc_on_unimplemented]` note, let's display it
 780                             err.note(s.as_str());
 781                         }
 782
 783                         self.suggest_borrow_on_unsized_slice(&obligation.cause.code, &mut err);
 784                         self.suggest_fn_call(&obligation, &mut err, &trait_ref, points_at_arg);
 785                         self.suggest_remove_reference(&obligation, &mut err, &trait_ref);
 786                         self.suggest_semicolon_removal(&obligation, &mut err, span, &trait_ref);
 787
 788                         // Try to report a help message
 789                         if !trait_ref.has_infer_types() &&
 790                             self.predicate_can_apply(obligation.param_env, trait_ref) {
 791                             // If a where-clause may be useful, remind the
 792                             // user that they can add it.
 793                             //
 794                             // don't display an on-unimplemented note, as
 795                             // these notes will often be of the form
 796                             //     "the type `T` can't be frobnicated"
 797                             // which is somewhat confusing.
 798                             self.suggest_restricting_param_bound(
 799                                 &mut err,
 800                                 &trait_ref,
 801                                 obligation.cause.body_id,
 802                             );
 803                         } else {
 804                             if !have_alt_message {
 805                                 // Can't show anything else useful, try to find similar impls.
 806                                 let impl_candidates = self.find_similar_impl_candidates(trait_ref);
 807                                 self.report_similar_impl_candidates(impl_candidates, &mut err);
 808                             }
 809                             self.suggest_change_mut(
 810                                 &obligation,
 811                                 &mut err,
 812                                 &trait_ref,
 813                                 points_at_arg,
 814                             );
 815                         }
 816
 817                         // If this error is due to `!: Trait` not implemented but `(): Trait` is
 818                         // implemented, and fallback has occurred, then it could be due to a
 819                         // variable that used to fallback to `()` now falling back to `!`. Issue a
 820                         // note informing about the change in behaviour.
 821                         if trait_predicate.skip_binder().self_ty().is_never()
 822                             && fallback_has_occurred
 823                         {
 824                             let predicate = trait_predicate.map_bound(|mut trait_pred| {
 825                                 trait_pred.trait_ref.substs = self.tcx.mk_substs_trait(
 826                                     self.tcx.mk_unit(),
 827                                     &trait_pred.trait_ref.substs[1..],
 828                                 );
 829                                 trait_pred
 830                             });
 831                             let unit_obligation = Obligation {
 832                                 predicate: ty::Predicate::Trait(predicate),
 833                                 .. obligation.clone()
 834                             };
 835                             if self.predicate_may_hold(&unit_obligation) {
 836                                 err.note("the trait is implemented for `()`. \
 837                                          Possibly this error has been caused by changes to \
 838                                          Rust's type-inference algorithm \
 839                                          (see: https://github.com/rust-lang/rust/issues/48950 \
 840                                          for more info). Consider whether you meant to use the \
 841                                          type `()` here instead.");
 842                             }
 843                         }
 844
 845                         err
 846                     }
 847
 848                     ty::Predicate::Subtype(ref predicate) => {
 849                         // Errors for Subtype predicates show up as
 850                         // `FulfillmentErrorCode::CodeSubtypeError`,
 851                         // not selection error.
 852                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
 853                     }
 854
 855                     ty::Predicate::RegionOutlives(ref predicate) => {
 856                         let predicate = self.resolve_vars_if_possible(predicate);
 857                         let err = self.region_outlives_predicate(&obligation.cause,
 858                                                                  &predicate).err().unwrap();
 859                         struct_span_err!(
 860                             self.tcx.sess, span, E0279,
 861                             "the requirement `{}` is not satisfied (`{}`)",
 862                             predicate, err,
 863                         )
 864                     }
 865
 866                     ty::Predicate::Projection(..) | ty::Predicate::TypeOutlives(..) => {
 867                         let predicate =
 868                             self.resolve_vars_if_possible(&obligation.predicate);
 869                         struct_span_err!(self.tcx.sess, span, E0280,
 870                             "the requirement `{}` is not satisfied",
 871                             predicate)
 872                     }
 873
 874                     ty::Predicate::ObjectSafe(trait_def_id) => {
 875                         let violations = self.tcx.object_safety_violations(trait_def_id);
 876                         self.tcx.report_object_safety_error(
 877                             span,
 878                             trait_def_id,
 879                             violations,
 880                         )
 881                     }
 882
 883                     ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => {
 884                         let found_kind = self.closure_kind(closure_def_id, closure_substs).unwrap();
 885                         let closure_span = self.tcx.sess.source_map()
 886                             .def_span(self.tcx.hir().span_if_local(closure_def_id).unwrap());
 887                         let hir_id = self.tcx.hir().as_local_hir_id(closure_def_id).unwrap();
 888                         let mut err = struct_span_err!(
 889                             self.tcx.sess, closure_span, E0525,
 890                             "expected a closure that implements the `{}` trait, \
 891                              but this closure only implements `{}`",
 892                             kind,
 893                             found_kind);
 894
 895                         err.span_label(
 896                             closure_span,
 897                             format!("this closure implements `{}`, not `{}`", found_kind, kind));
 898                         err.span_label(
 899                             obligation.cause.span,
 900                             format!("the requirement to implement `{}` derives from here", kind));
 901
 902                         // Additional context information explaining why the closure only implements
 903                         // a particular trait.
 904                         if let Some(tables) = self.in_progress_tables {
 905                             let tables = tables.borrow();
 906                             match (found_kind, tables.closure_kind_origins().get(hir_id)) {
 907                                 (ty::ClosureKind::FnOnce, Some((span, name))) => {
 908                                     err.span_label(*span, format!(
 909                                         "closure is `FnOnce` because it moves the \
 910                                          variable `{}` out of its environment", name));
 911                                 },
 912                                 (ty::ClosureKind::FnMut, Some((span, name))) => {
 913                                     err.span_label(*span, format!(
 914                                         "closure is `FnMut` because it mutates the \
 915                                          variable `{}` here", name));
 916                                 },
 917                                 _ => {}
 918                             }
 919                         }
 920
 921                         err.emit();
 922                         return;
 923                     }
 924
 925                     ty::Predicate::WellFormed(ty) => {
 926                         if !self.tcx.sess.opts.debugging_opts.chalk {
 927                             // WF predicates cannot themselves make
 928                             // errors. They can only block due to
 929                             // ambiguity; otherwise, they always
 930                             // degenerate into other obligations
 931                             // (which may fail).
 932                             span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 933                         } else {
 934                             // FIXME: we'll need a better message which takes into account
 935                             // which bounds actually failed to hold.
 936                             self.tcx.sess.struct_span_err(
 937                                 span,
 938                                 &format!("the type `{}` is not well-formed (chalk)", ty)
 939                             )
 940                         }
 941                     }
 942
 943                     ty::Predicate::ConstEvaluatable(..) => {
 944                         // Errors for `ConstEvaluatable` predicates show up as
 945                         // `SelectionError::ConstEvalFailure`,
 946                         // not `Unimplemented`.
 947                         span_bug!(span,
 948                             "const-evaluatable requirement gave wrong error: `{:?}`", obligation)
 949                     }
 950                 }
 951             }
 952
 953             OutputTypeParameterMismatch(ref found_trait_ref, ref expected_trait_ref, _) => {
 954                 let found_trait_ref = self.resolve_vars_if_possible(&*found_trait_ref);
 955                 let expected_trait_ref = self.resolve_vars_if_possible(&*expected_trait_ref);
 956
 957                 if expected_trait_ref.self_ty().references_error() {
 958                     return;
 959                 }
 960
 961                 let found_trait_ty = found_trait_ref.self_ty();
 962
 963                 let found_did = match found_trait_ty.kind {
 964                     ty::Closure(did, _) | ty::Foreign(did) | ty::FnDef(did, _) => Some(did),
 965                     ty::Adt(def, _) => Some(def.did),
 966                     _ => None,
 967                 };
 968
 969                 let found_span = found_did.and_then(|did|
 970                     self.tcx.hir().span_if_local(did)
 971                 ).map(|sp| self.tcx.sess.source_map().def_span(sp)); // the sp could be an fn def
 972
 973                 if self.reported_closure_mismatch.borrow().contains(&(span, found_span)) {
 974                     // We check closures twice, with obligations flowing in different directions,
 975                     // but we want to complain about them only once.
 976                     return;
 977                 }
 978
 979                 self.reported_closure_mismatch.borrow_mut().insert((span, found_span));
 980
 981                 let found = match found_trait_ref.skip_binder().substs.type_at(1).kind {
 982                     ty::Tuple(ref tys) => vec![ArgKind::empty(); tys.len()],
 983                     _ => vec![ArgKind::empty()],
 984                 };
 985
 986                 let expected_ty = expected_trait_ref.skip_binder().substs.type_at(1);
 987                 let expected = match expected_ty.kind {
 988                     ty::Tuple(ref tys) => tys.iter()
 989                         .map(|t| ArgKind::from_expected_ty(t.expect_ty(), Some(span))).collect(),
 990                     _ => vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())],
 991                 };
 992
 993                 if found.len() == expected.len() {
 994                     self.report_closure_arg_mismatch(span,
 995                                                      found_span,
 996                                                      found_trait_ref,
 997                                                      expected_trait_ref)
 998                 } else {
 999                     let (closure_span, found) = found_did
1000                         .and_then(|did| self.tcx.hir().get_if_local(did))
1001                         .map(|node| {
1002                             let (found_span, found) = self.get_fn_like_arguments(node);
1003                             (Some(found_span), found)
1004                         }).unwrap_or((found_span, found));
1005
1006                     self.report_arg_count_mismatch(span,
1007                                                    closure_span,
1008                                                    expected,
1009                                                    found,
1010                                                    found_trait_ty.is_closure())
1011                 }
1012             }
1013
1014             TraitNotObjectSafe(did) => {
1015                 let violations = self.tcx.object_safety_violations(did);
1016                 self.tcx.report_object_safety_error(span, did, violations)
1017             }
1018
1019             // already reported in the query
1020             ConstEvalFailure(err) => {
1021                 self.tcx.sess.delay_span_bug(
1022                     span,
1023                     &format!("constant in type had an ignored error: {:?}", err),
1024                 );
1025                 return;
1026             }
1027
1028             Overflow => {
1029                 bug!("overflow should be handled before the `report_selection_error` path");
1030             }
1031         };
1032
1033         self.note_obligation_cause(&mut err, obligation);
1034
1035         err.emit();
1036     }
1037
1038     fn suggest_restricting_param_bound(
1039         &self,
1040         err: &mut DiagnosticBuilder<'_>,
1041         trait_ref: &ty::PolyTraitRef<'_>,
1042         body_id: hir::HirId,
1043     ) {
1044         let self_ty = trait_ref.self_ty();
1045         let (param_ty, projection) = match &self_ty.kind {
1046             ty::Param(_) => (true, None),
1047             ty::Projection(projection) => (false, Some(projection)),
1048             _ => return,
1049         };
1050
1051         let mut suggest_restriction = |generics: &hir::Generics, msg| {
1052             let span = generics.where_clause.span_for_predicates_or_empty_place();
1053             if !span.from_expansion() && span.desugaring_kind().is_none() {
1054                 err.span_suggestion(
1055                     generics.where_clause.span_for_predicates_or_empty_place().shrink_to_hi(),
1056                     &format!("consider further restricting {}", msg),
1057                     format!(
1058                         "{} {} ",
1059                         if !generics.where_clause.predicates.is_empty() {
1060                             ","
1061                         } else {
1062                             " where"
1063                         },
1064                         trait_ref.to_predicate(),
1065                     ),
1066                     Applicability::MachineApplicable,
1067                 );
1068             }
1069         };
1070
1071         // FIXME: Add check for trait bound that is already present, particularly `?Sized` so we
1072         //        don't suggest `T: Sized + ?Sized`.
1073         let mut hir_id = body_id;
1074         while let Some(node) = self.tcx.hir().find(hir_id) {
1075             match node {
1076                 hir::Node::TraitItem(hir::TraitItem {
1077                     generics,
1078                     kind: hir::TraitItemKind::Method(..), ..
1079                 }) if param_ty && self_ty == self.tcx.types.self_param => {
1080                     // Restricting `Self` for a single method.
1081                     suggest_restriction(&generics, "`Self`");
1082                     return;
1083                 }
1084
1085                 hir::Node::Item(hir::Item {
1086                     kind: hir::ItemKind::Fn(_, generics, _), ..
1087                 }) |
1088                 hir::Node::TraitItem(hir::TraitItem {
1089                     generics,
1090                     kind: hir::TraitItemKind::Method(..), ..
1091                 }) |
1092                 hir::Node::ImplItem(hir::ImplItem {
1093                     generics,
1094                     kind: hir::ImplItemKind::Method(..), ..
1095                 }) |
1096                 hir::Node::Item(hir::Item {
1097                     kind: hir::ItemKind::Trait(_, _, generics, _, _), ..
1098                 }) |
1099                 hir::Node::Item(hir::Item {
1100                     kind: hir::ItemKind::Impl(_, _, _, generics, ..), ..
1101                 }) if projection.is_some() => {
1102                     // Missing associated type bound.
1103                     suggest_restriction(&generics, "the associated type");
1104                     return;
1105                 }
1106
1107                 hir::Node::Item(hir::Item { kind: hir::ItemKind::Struct(_, generics), span, .. }) |
1108                 hir::Node::Item(hir::Item { kind: hir::ItemKind::Enum(_, generics), span, .. }) |
1109                 hir::Node::Item(hir::Item { kind: hir::ItemKind::Union(_, generics), span, .. }) |
1110                 hir::Node::Item(hir::Item {
1111                     kind: hir::ItemKind::Trait(_, _, generics, ..), span, ..
1112                 }) |
1113                 hir::Node::Item(hir::Item {
1114                     kind: hir::ItemKind::Impl(_, _, _, generics, ..), span, ..
1115                 }) |
1116                 hir::Node::Item(hir::Item {
1117                     kind: hir::ItemKind::Fn(_, generics, _), span, ..
1118                 }) |
1119                 hir::Node::Item(hir::Item {
1120                     kind: hir::ItemKind::TyAlias(_, generics), span, ..
1121                 }) |
1122                 hir::Node::Item(hir::Item {
1123                     kind: hir::ItemKind::TraitAlias(generics, _), span, ..
1124                 }) |
1125                 hir::Node::Item(hir::Item {
1126                     kind: hir::ItemKind::OpaqueTy(hir::OpaqueTy { generics, .. }), span, ..
1127                 }) |
1128                 hir::Node::TraitItem(hir::TraitItem { generics, span, .. }) |
1129                 hir::Node::ImplItem(hir::ImplItem { generics, span, .. })
1130                 if param_ty => {
1131                     // Missing generic type parameter bound.
1132                     let restrict_msg = "consider further restricting this bound";
1133                     let param_name = self_ty.to_string();
1134                     for param in generics.params.iter().filter(|p| {
1135                         p.name.ident().as_str() == param_name
1136                     }) {
1137                         if param_name.starts_with("impl ") {
1138                             // `impl Trait` in argument:
1139                             // `fn foo(x: impl Trait) {}` → `fn foo(t: impl Trait + Trait2) {}`
1140                             err.span_suggestion(
1141                                 param.span,
1142                                 restrict_msg,
1143                                 // `impl CurrentTrait + MissingTrait`
1144                                 format!("{} + {}", param.name.ident(), trait_ref),
1145                                 Applicability::MachineApplicable,
1146                             );
1147                         } else if generics.where_clause.predicates.is_empty() &&
1148                                 param.bounds.is_empty()
1149                         {
1150                             // If there are no bounds whatsoever, suggest adding a constraint
1151                             // to the type parameter:
1152                             // `fn foo<T>(t: T) {}` → `fn foo<T: Trait>(t: T) {}`
1153                             err.span_suggestion(
1154                                 param.span,
1155                                 "consider restricting this bound",
1156                                 format!("{}", trait_ref.to_predicate()),
1157                                 Applicability::MachineApplicable,
1158                             );
1159                         } else if !generics.where_clause.predicates.is_empty() {
1160                             // There is a `where` clause, so suggest expanding it:
1161                             // `fn foo<T>(t: T) where T: Debug {}` →
1162                             // `fn foo<T>(t: T) where T: Debug, T: Trait {}`
1163                             err.span_suggestion(
1164                                 generics.where_clause.span().unwrap().shrink_to_hi(),
1165                                 &format!(
1166                                     "consider further restricting type parameter `{}`",
1167                                     param_name,
1168                                 ),
1169                                 format!(", {}", trait_ref.to_predicate()),
1170                                 Applicability::MachineApplicable,
1171                             );
1172                         } else {
1173                             // If there is no `where` clause lean towards constraining to the
1174                             // type parameter:
1175                             // `fn foo<X: Bar, T>(t: T, x: X) {}` → `fn foo<T: Trait>(t: T) {}`
1176                             // `fn foo<T: Bar>(t: T) {}` → `fn foo<T: Bar + Trait>(t: T) {}`
1177                             let sp = param.span.with_hi(span.hi());
1178                             let span = self.tcx.sess.source_map()
1179                                 .span_through_char(sp, ':');
1180                             if sp != param.span && sp != span {
1181                                 // Only suggest if we have high certainty that the span
1182                                 // covers the colon in `foo<T: Trait>`.
1183                                 err.span_suggestion(span, restrict_msg, format!(
1184                                     "{} + ",
1185                                     trait_ref.to_predicate(),
1186                                 ), Applicability::MachineApplicable);
1187                             } else {
1188                                 err.span_label(param.span, &format!(
1189                                     "consider adding a `where {}` bound",
1190                                     trait_ref.to_predicate(),
1191                                 ));
1192                             }
1193                         }
1194                         return;
1195                     }
1196                 }
1197
1198                 hir::Node::Crate => return,
1199
1200                 _ => {}
1201             }
1202
1203             hir_id = self.tcx.hir().get_parent_item(hir_id);
1204         }
1205     }
1206
1207     /// When encountering an assignment of an unsized trait, like `let x = ""[..];`, provide a
1208     /// suggestion to borrow the initializer in order to use have a slice instead.
1209     fn suggest_borrow_on_unsized_slice(
1210         &self,
1211         code: &ObligationCauseCode<'tcx>,
1212         err: &mut DiagnosticBuilder<'tcx>,
1213     ) {
1214         if let &ObligationCauseCode::VariableType(hir_id) = code {
1215             let parent_node = self.tcx.hir().get_parent_node(hir_id);
1216             if let Some(Node::Local(ref local)) = self.tcx.hir().find(parent_node) {
1217                 if let Some(ref expr) = local.init {
1218                     if let hir::ExprKind::Index(_, _) = expr.kind {
1219                         if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(expr.span) {
1220                             err.span_suggestion(
1221                                 expr.span,
1222                                 "consider borrowing here",
1223                                 format!("&{}", snippet),
1224                                 Applicability::MachineApplicable
1225                             );
1226                         }
1227                     }
1228                 }
1229             }
1230         }
1231     }
1232
1233     fn mk_obligation_for_def_id(
1234         &self,
1235         def_id: DefId,
1236         output_ty: Ty<'tcx>,
1237         cause: ObligationCause<'tcx>,
1238         param_env: ty::ParamEnv<'tcx>,
1239     ) -> PredicateObligation<'tcx> {
1240         let new_trait_ref = ty::TraitRef {
1241             def_id,
1242             substs: self.tcx.mk_substs_trait(output_ty, &[]),
1243         };
1244         Obligation::new(cause, param_env, new_trait_ref.to_predicate())
1245     }
1246
1247
1248     /// We tried to apply the bound to an `fn` or closure. Check whether calling it would
1249     /// evaluate to a type that *would* satisfy the trait binding. If it would, suggest calling
1250     /// it: `bar(foo)` → `bar(foo())`. This case is *very* likely to be hit if `foo` is `async`.
1251     fn suggest_fn_call(
1252         &self,
1253         obligation: &PredicateObligation<'tcx>,
1254         err: &mut DiagnosticBuilder<'_>,
1255         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
1256         points_at_arg: bool,
1257     ) {
1258         let self_ty = trait_ref.self_ty();
1259         let (def_id, output_ty, callable) = match self_ty.kind {
1260             ty::Closure(def_id, substs) => {
1261                 (def_id, self.closure_sig(def_id, substs).output(), "closure")
1262             }
1263             ty::FnDef(def_id, _) => {
1264                 (def_id, self_ty.fn_sig(self.tcx).output(), "function")
1265             }
1266             _ => return,
1267         };
1268         let msg = format!("use parentheses to call the {}", callable);
1269
1270         let obligation = self.mk_obligation_for_def_id(
1271             trait_ref.def_id(),
1272             output_ty.skip_binder(),
1273             obligation.cause.clone(),
1274             obligation.param_env,
1275         );
1276
1277         let get_name = |err: &mut DiagnosticBuilder<'_>, kind: &hir::PatKind| -> Option<String> {
1278             // Get the local name of this closure. This can be inaccurate because
1279             // of the possibility of reassignment, but this should be good enough.
1280             match &kind {
1281                 hir::PatKind::Binding(hir::BindingAnnotation::Unannotated, _, name, None) => {
1282                     Some(format!("{}", name))
1283                 }
1284                 _ => {
1285                     err.note(&msg);
1286                     None
1287                 }
1288             }
1289         };
1290         match self.evaluate_obligation(&obligation) {
1291             Ok(EvaluationResult::EvaluatedToOk) |
1292             Ok(EvaluationResult::EvaluatedToOkModuloRegions) |
1293             Ok(EvaluationResult::EvaluatedToAmbig) => {}
1294             _ => return,
1295         }
1296         let hir = self.tcx.hir();
1297         // Get the name of the callable and the arguments to be used in the suggestion.
1298         let snippet = match hir.get_if_local(def_id) {
1299             Some(hir::Node::Expr(hir::Expr {
1300                 kind: hir::ExprKind::Closure(_, decl, _, span, ..),
1301                 ..
1302             })) => {
1303                 err.span_label(*span, "consider calling this closure");
1304                 let hir_id = match hir.as_local_hir_id(def_id) {
1305                     Some(hir_id) => hir_id,
1306                     None => return,
1307                 };
1308                 let parent_node = hir.get_parent_node(hir_id);
1309                 let name = match hir.find(parent_node) {
1310                     Some(hir::Node::Stmt(hir::Stmt {
1311                         kind: hir::StmtKind::Local(local), ..
1312                     })) => match get_name(err, &local.pat.kind) {
1313                         Some(name) => name,
1314                         None => return,
1315                     },
1316                     // Different to previous arm because one is `&hir::Local` and the other
1317                     // is `P<hir::Local>`.
1318                     Some(hir::Node::Local(local)) => match get_name(err, &local.pat.kind) {
1319                         Some(name) => name,
1320                         None => return,
1321                     },
1322                     _ => return,
1323                 };
1324                 let args = decl.inputs.iter()
1325                     .map(|_| "_")
1326                     .collect::<Vec<_>>().join(", ");
1327                 format!("{}({})", name, args)
1328             }
1329             Some(hir::Node::Item(hir::Item {
1330                 ident,
1331                 kind: hir::ItemKind::Fn(.., body_id),
1332                 ..
1333             })) => {
1334                 err.span_label(ident.span, "consider calling this function");
1335                 let body = hir.body(*body_id);
1336                 let args = body.params.iter()
1337                     .map(|arg| match &arg.pat.kind {
1338                         hir::PatKind::Binding(_, _, ident, None)
1339                         if ident.name != kw::SelfLower => ident.to_string(),
1340                         _ => "_".to_string(),
1341                     }).collect::<Vec<_>>().join(", ");
1342                 format!("{}({})", ident, args)
1343             }
1344             _ => return,
1345         };
1346         if points_at_arg {
1347             // When the obligation error has been ensured to have been caused by
1348             // an argument, the `obligation.cause.span` points at the expression
1349             // of the argument, so we can provide a suggestion. This is signaled
1350             // by `points_at_arg`. Otherwise, we give a more general note.
1351             err.span_suggestion(
1352                 obligation.cause.span,
1353                 &msg,
1354                 snippet,
1355                 Applicability::HasPlaceholders,
1356             );
1357         } else {
1358             err.help(&format!("{}: `{}`", msg, snippet));
1359         }
1360     }
1361
1362     /// Whenever references are used by mistake, like `for (i, e) in &vec.iter().enumerate()`,
1363     /// suggest removing these references until we reach a type that implements the trait.
1364     fn suggest_remove_reference(
1365         &self,
1366         obligation: &PredicateObligation<'tcx>,
1367         err: &mut DiagnosticBuilder<'tcx>,
1368         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
1369     ) {
1370         let trait_ref = trait_ref.skip_binder();
1371         let span = obligation.cause.span;
1372
1373         if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
1374             let refs_number = snippet.chars()
1375                 .filter(|c| !c.is_whitespace())
1376                 .take_while(|c| *c == '&')
1377                 .count();
1378             if let Some('\'') = snippet.chars()
1379                 .filter(|c| !c.is_whitespace())
1380                 .skip(refs_number)
1381                 .next()
1382             { // Do not suggest removal of borrow from type arguments.
1383                 return;
1384             }
1385
1386             let mut trait_type = trait_ref.self_ty();
1387
1388             for refs_remaining in 0..refs_number {
1389                 if let ty::Ref(_, t_type, _) = trait_type.kind {
1390                     trait_type = t_type;
1391
1392                     let new_obligation = self.mk_obligation_for_def_id(
1393                         trait_ref.def_id,
1394                         trait_type,
1395                         ObligationCause::dummy(),
1396                         obligation.param_env,
1397                     );
1398
1399                     if self.predicate_may_hold(&new_obligation) {
1400                         let sp = self.tcx.sess.source_map()
1401                             .span_take_while(span, |c| c.is_whitespace() || *c == '&');
1402
1403                         let remove_refs = refs_remaining + 1;
1404                         let format_str = format!("consider removing {} leading `&`-references",
1405                                                  remove_refs);
1406
1407                         err.span_suggestion_short(
1408                             sp, &format_str, String::new(), Applicability::MachineApplicable
1409                         );
1410                         break;
1411                     }
1412                 } else {
1413                     break;
1414                 }
1415             }
1416         }
1417     }
1418
1419     /// Check if the trait bound is implemented for a different mutability and note it in the
1420     /// final error.
1421     fn suggest_change_mut(
1422         &self,
1423         obligation: &PredicateObligation<'tcx>,
1424         err: &mut DiagnosticBuilder<'tcx>,
1425         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
1426         points_at_arg: bool,
1427     ) {
1428         let span = obligation.cause.span;
1429         if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
1430             let refs_number = snippet.chars()
1431                 .filter(|c| !c.is_whitespace())
1432                 .take_while(|c| *c == '&')
1433                 .count();
1434             if let Some('\'') = snippet.chars()
1435                 .filter(|c| !c.is_whitespace())
1436                 .skip(refs_number)
1437                 .next()
1438             { // Do not suggest removal of borrow from type arguments.
1439                 return;
1440             }
1441             let trait_ref = self.resolve_vars_if_possible(trait_ref);
1442             if trait_ref.has_infer_types() {
1443                 // Do not ICE while trying to find if a reborrow would succeed on a trait with
1444                 // unresolved bindings.
1445                 return;
1446             }
1447
1448             if let ty::Ref(region, t_type, mutability) = trait_ref.skip_binder().self_ty().kind {
1449                 let trait_type = match mutability {
1450                     hir::Mutability::Mutable => self.tcx.mk_imm_ref(region, t_type),
1451                     hir::Mutability::Immutable => self.tcx.mk_mut_ref(region, t_type),
1452                 };
1453
1454                 let new_obligation = self.mk_obligation_for_def_id(
1455                     trait_ref.skip_binder().def_id,
1456                     trait_type,
1457                     ObligationCause::dummy(),
1458                     obligation.param_env,
1459                 );
1460
1461                 if self.evaluate_obligation_no_overflow(
1462                     &new_obligation,
1463                 ).must_apply_modulo_regions() {
1464                     let sp = self.tcx.sess.source_map()
1465                         .span_take_while(span, |c| c.is_whitespace() || *c == '&');
1466                     if points_at_arg &&
1467                         mutability == hir::Mutability::Immutable &&
1468                         refs_number > 0
1469                     {
1470                         err.span_suggestion(
1471                             sp,
1472                             "consider changing this borrow's mutability",
1473                             "&mut ".to_string(),
1474                             Applicability::MachineApplicable,
1475                         );
1476                     } else {
1477                         err.note(&format!(
1478                             "`{}` is implemented for `{:?}`, but not for `{:?}`",
1479                             trait_ref,
1480                             trait_type,
1481                             trait_ref.skip_binder().self_ty(),
1482                         ));
1483                     }
1484                 }
1485             }
1486         }
1487     }
1488
1489     fn suggest_semicolon_removal(
1490         &self,
1491         obligation: &PredicateObligation<'tcx>,
1492         err: &mut DiagnosticBuilder<'tcx>,
1493         span: Span,
1494         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
1495     ) {
1496         let hir = self.tcx.hir();
1497         let parent_node = hir.get_parent_node(obligation.cause.body_id);
1498         let node = hir.find(parent_node);
1499         if let Some(hir::Node::Item(hir::Item {
1500             kind: hir::ItemKind::Fn(sig, _, body_id),
1501             ..
1502         })) = node {
1503             let body = hir.body(*body_id);
1504             if let hir::ExprKind::Block(blk, _) = &body.value.kind {
1505                 if sig.decl.output.span().overlaps(span) && blk.expr.is_none() &&
1506                     "()" == &trait_ref.self_ty().to_string()
1507                 {
1508                     // FIXME(estebank): When encountering a method with a trait
1509                     // bound not satisfied in the return type with a body that has
1510                     // no return, suggest removal of semicolon on last statement.
1511                     // Once that is added, close #54771.
1512                     if let Some(ref stmt) = blk.stmts.last() {
1513                         let sp = self.tcx.sess.source_map().end_point(stmt.span);
1514                         err.span_label(sp, "consider removing this semicolon");
1515                     }
1516                 }
1517             }
1518         }
1519     }
1520
1521     /// Given some node representing a fn-like thing in the HIR map,
1522     /// returns a span and `ArgKind` information that describes the
1523     /// arguments it expects. This can be supplied to
1524     /// `report_arg_count_mismatch`.
1525     pub fn get_fn_like_arguments(&self, node: Node<'_>) -> (Span, Vec<ArgKind>) {
1526         match node {
1527             Node::Expr(&hir::Expr {
1528                 kind: hir::ExprKind::Closure(_, ref _decl, id, span, _),
1529                 ..
1530             }) => {
1531                 (self.tcx.sess.source_map().def_span(span),
1532                  self.tcx.hir().body(id).params.iter()
1533                     .map(|arg| {
1534                         if let hir::Pat {
1535                             kind: hir::PatKind::Tuple(ref args, _),
1536                             span,
1537                             ..
1538                         } = *arg.pat {
1539                             ArgKind::Tuple(
1540                                 Some(span),
1541                                 args.iter().map(|pat| {
1542                                     let snippet = self.tcx.sess.source_map()
1543                                         .span_to_snippet(pat.span).unwrap();
1544                                     (snippet, "_".to_owned())
1545                                 }).collect::<Vec<_>>(),
1546                             )
1547                         } else {
1548                             let name = self.tcx.sess.source_map()
1549                                 .span_to_snippet(arg.pat.span).unwrap();
1550                             ArgKind::Arg(name, "_".to_owned())
1551                         }
1552                     })
1553                     .collect::<Vec<ArgKind>>())
1554             }
1555             Node::Item(&hir::Item {
1556                 span,
1557                 kind: hir::ItemKind::Fn(ref sig, ..),
1558                 ..
1559             }) |
1560             Node::ImplItem(&hir::ImplItem {
1561                 span,
1562                 kind: hir::ImplItemKind::Method(ref sig, _),
1563                 ..
1564             }) |
1565             Node::TraitItem(&hir::TraitItem {
1566                 span,
1567                 kind: hir::TraitItemKind::Method(ref sig, _),
1568                 ..
1569             }) => {
1570                 (self.tcx.sess.source_map().def_span(span), sig.decl.inputs.iter()
1571                         .map(|arg| match arg.clone().kind {
1572                     hir::TyKind::Tup(ref tys) => ArgKind::Tuple(
1573                         Some(arg.span),
1574                         vec![("_".to_owned(), "_".to_owned()); tys.len()]
1575                     ),
1576                     _ => ArgKind::empty()
1577                 }).collect::<Vec<ArgKind>>())
1578             }
1579             Node::Ctor(ref variant_data) => {
1580                 let span = variant_data.ctor_hir_id()
1581                     .map(|hir_id| self.tcx.hir().span(hir_id))
1582                     .unwrap_or(DUMMY_SP);
1583                 let span = self.tcx.sess.source_map().def_span(span);
1584
1585                 (span, vec![ArgKind::empty(); variant_data.fields().len()])
1586             }
1587             _ => panic!("non-FnLike node found: {:?}", node),
1588         }
1589     }
1590
1591     /// Reports an error when the number of arguments needed by a
1592     /// trait match doesn't match the number that the expression
1593     /// provides.
1594     pub fn report_arg_count_mismatch(
1595         &self,
1596         span: Span,
1597         found_span: Option<Span>,
1598         expected_args: Vec<ArgKind>,
1599         found_args: Vec<ArgKind>,
1600         is_closure: bool,
1601     ) -> DiagnosticBuilder<'tcx> {
1602         let kind = if is_closure { "closure" } else { "function" };
1603
1604         let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
1605             let arg_length = arguments.len();
1606             let distinct = match &other[..] {
1607                 &[ArgKind::Tuple(..)] => true,
1608                 _ => false,
1609             };
1610             match (arg_length, arguments.get(0)) {
1611                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
1612                     format!("a single {}-tuple as argument", fields.len())
1613                 }
1614                 _ => format!("{} {}argument{}",
1615                              arg_length,
1616                              if distinct && arg_length > 1 { "distinct " } else { "" },
1617                              pluralize!(arg_length))
1618             }
1619         };
1620
1621         let expected_str = args_str(&expected_args, &found_args);
1622         let found_str = args_str(&found_args, &expected_args);
1623
1624         let mut err = struct_span_err!(
1625             self.tcx.sess,
1626             span,
1627             E0593,
1628             "{} is expected to take {}, but it takes {}",
1629             kind,
1630             expected_str,
1631             found_str,
1632         );
1633
1634         err.span_label(span, format!("expected {} that takes {}", kind, expected_str));
1635
1636         if let Some(found_span) = found_span {
1637             err.span_label(found_span, format!("takes {}", found_str));
1638
1639             // move |_| { ... }
1640             // ^^^^^^^^-- def_span
1641             //
1642             // move |_| { ... }
1643             // ^^^^^-- prefix
1644             let prefix_span = self.tcx.sess.source_map().span_until_non_whitespace(found_span);
1645             // move |_| { ... }
1646             //      ^^^-- pipe_span
1647             let pipe_span = if let Some(span) = found_span.trim_start(prefix_span) {
1648                 span
1649             } else {
1650                 found_span
1651             };
1652
1653             // Suggest to take and ignore the arguments with expected_args_length `_`s if
1654             // found arguments is empty (assume the user just wants to ignore args in this case).
1655             // For example, if `expected_args_length` is 2, suggest `|_, _|`.
1656             if found_args.is_empty() && is_closure {
1657                 let underscores = vec!["_"; expected_args.len()].join(", ");
1658                 err.span_suggestion(
1659                     pipe_span,
1660                     &format!(
1661                         "consider changing the closure to take and ignore the expected argument{}",
1662                         if expected_args.len() < 2 {
1663                             ""
1664                         } else {
1665                             "s"
1666                         }
1667                     ),
1668                     format!("|{}|", underscores),
1669                     Applicability::MachineApplicable,
1670                 );
1671             }
1672
1673             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
1674                 if fields.len() == expected_args.len() {
1675                     let sugg = fields.iter()
1676                         .map(|(name, _)| name.to_owned())
1677                         .collect::<Vec<String>>()
1678                         .join(", ");
1679                     err.span_suggestion(
1680                         found_span,
1681                         "change the closure to take multiple arguments instead of a single tuple",
1682                         format!("|{}|", sugg),
1683                         Applicability::MachineApplicable,
1684                     );
1685                 }
1686             }
1687             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..] {
1688                 if fields.len() == found_args.len() && is_closure {
1689                     let sugg = format!(
1690                         "|({}){}|",
1691                         found_args.iter()
1692                             .map(|arg| match arg {
1693                                 ArgKind::Arg(name, _) => name.to_owned(),
1694                                 _ => "_".to_owned(),
1695                             })
1696                             .collect::<Vec<String>>()
1697                             .join(", "),
1698                         // add type annotations if available
1699                         if found_args.iter().any(|arg| match arg {
1700                             ArgKind::Arg(_, ty) => ty != "_",
1701                             _ => false,
1702                         }) {
1703                             format!(": ({})",
1704                                     fields.iter()
1705                                         .map(|(_, ty)| ty.to_owned())
1706                                         .collect::<Vec<String>>()
1707                                         .join(", "))
1708                         } else {
1709                             String::new()
1710                         },
1711                     );
1712                     err.span_suggestion(
1713                         found_span,
1714                         "change the closure to accept a tuple instead of individual arguments",
1715                         sugg,
1716                         Applicability::MachineApplicable,
1717                     );
1718                 }
1719             }
1720         }
1721
1722         err
1723     }
1724
1725     fn report_closure_arg_mismatch(
1726         &self,
1727         span: Span,
1728         found_span: Option<Span>,
1729         expected_ref: ty::PolyTraitRef<'tcx>,
1730         found: ty::PolyTraitRef<'tcx>,
1731     ) -> DiagnosticBuilder<'tcx> {
1732         fn build_fn_sig_string<'tcx>(tcx: TyCtxt<'tcx>, trait_ref: &ty::TraitRef<'tcx>) -> String {
1733             let inputs = trait_ref.substs.type_at(1);
1734             let sig = if let ty::Tuple(inputs) = inputs.kind {
1735                 tcx.mk_fn_sig(
1736                     inputs.iter().map(|k| k.expect_ty()),
1737                     tcx.mk_ty_infer(ty::TyVar(ty::TyVid { index: 0 })),
1738                     false,
1739                     hir::Unsafety::Normal,
1740                     ::rustc_target::spec::abi::Abi::Rust
1741                 )
1742             } else {
1743                 tcx.mk_fn_sig(
1744                     ::std::iter::once(inputs),
1745                     tcx.mk_ty_infer(ty::TyVar(ty::TyVid { index: 0 })),
1746                     false,
1747                     hir::Unsafety::Normal,
1748                     ::rustc_target::spec::abi::Abi::Rust
1749                 )
1750             };
1751             ty::Binder::bind(sig).to_string()
1752         }
1753
1754         let argument_is_closure = expected_ref.skip_binder().substs.type_at(0).is_closure();
1755         let mut err = struct_span_err!(self.tcx.sess, span, E0631,
1756                                        "type mismatch in {} arguments",
1757                                        if argument_is_closure { "closure" } else { "function" });
1758
1759         let found_str = format!(
1760             "expected signature of `{}`",
1761             build_fn_sig_string(self.tcx, found.skip_binder())
1762         );
1763         err.span_label(span, found_str);
1764
1765         let found_span = found_span.unwrap_or(span);
1766         let expected_str = format!(
1767             "found signature of `{}`",
1768             build_fn_sig_string(self.tcx, expected_ref.skip_binder())
1769         );
1770         err.span_label(found_span, expected_str);
1771
1772         err
1773     }
1774 }
1775
1776 impl<'tcx> TyCtxt<'tcx> {
1777     pub fn recursive_type_with_infinite_size_error(self,
1778                                                    type_def_id: DefId)
1779                                                    -> DiagnosticBuilder<'tcx>
1780     {
1781         assert!(type_def_id.is_local());
1782         let span = self.hir().span_if_local(type_def_id).unwrap();
1783         let span = self.sess.source_map().def_span(span);
1784         let mut err = struct_span_err!(self.sess, span, E0072,
1785                                        "recursive type `{}` has infinite size",
1786                                        self.def_path_str(type_def_id));
1787         err.span_label(span, "recursive type has infinite size");
1788         err.help(&format!("insert indirection (e.g., a `Box`, `Rc`, or `&`) \
1789                            at some point to make `{}` representable",
1790                           self.def_path_str(type_def_id)));
1791         err
1792     }
1793
1794     pub fn report_object_safety_error(
1795         self,
1796         span: Span,
1797         trait_def_id: DefId,
1798         violations: Vec<ObjectSafetyViolation>,
1799     ) -> DiagnosticBuilder<'tcx> {
1800         let trait_str = self.def_path_str(trait_def_id);
1801         let span = self.sess.source_map().def_span(span);
1802         let mut err = struct_span_err!(
1803             self.sess, span, E0038,
1804             "the trait `{}` cannot be made into an object",
1805             trait_str);
1806         err.span_label(span, format!("the trait `{}` cannot be made into an object", trait_str));
1807
1808         let mut reported_violations = FxHashSet::default();
1809         for violation in violations {
1810             if reported_violations.insert(violation.clone()) {
1811                 match violation.span() {
1812                     Some(span) => err.span_label(span, violation.error_msg()),
1813                     None => err.note(&violation.error_msg()),
1814                 };
1815             }
1816         }
1817
1818         if self.sess.trait_methods_not_found.borrow().contains(&span) {
1819             // Avoid emitting error caused by non-existing method (#58734)
1820             err.cancel();
1821         }
1822
1823         err
1824     }
1825 }
1826
1827 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
1828     fn maybe_report_ambiguity(
1829         &self,
1830         obligation: &PredicateObligation<'tcx>,
1831         body_id: Option<hir::BodyId>,
1832     ) {
1833         // Unable to successfully determine, probably means
1834         // insufficient type information, but could mean
1835         // ambiguous impls. The latter *ought* to be a
1836         // coherence violation, so we don't report it here.
1837
1838         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
1839         let span = obligation.cause.span;
1840
1841         debug!(
1842             "maybe_report_ambiguity(predicate={:?}, obligation={:?} body_id={:?}, code={:?})",
1843             predicate,
1844             obligation,
1845             body_id,
1846             obligation.cause.code,
1847         );
1848
1849         // Ambiguity errors are often caused as fallout from earlier
1850         // errors. So just ignore them if this infcx is tainted.
1851         if self.is_tainted_by_errors() {
1852             return;
1853         }
1854
1855         match predicate {
1856             ty::Predicate::Trait(ref data) => {
1857                 let trait_ref = data.to_poly_trait_ref();
1858                 let self_ty = trait_ref.self_ty();
1859                 debug!("self_ty {:?} {:?} trait_ref {:?}", self_ty, self_ty.kind, trait_ref);
1860
1861                 if predicate.references_error() {
1862                     return;
1863                 }
1864                 // Typically, this ambiguity should only happen if
1865                 // there are unresolved type inference variables
1866                 // (otherwise it would suggest a coherence
1867                 // failure). But given #21974 that is not necessarily
1868                 // the case -- we can have multiple where clauses that
1869                 // are only distinguished by a region, which results
1870                 // in an ambiguity even when all types are fully
1871                 // known, since we don't dispatch based on region
1872                 // relationships.
1873
1874                 // This is kind of a hack: it frequently happens that some earlier
1875                 // error prevents types from being fully inferred, and then we get
1876                 // a bunch of uninteresting errors saying something like "<generic
1877                 // #0> doesn't implement Sized".  It may even be true that we
1878                 // could just skip over all checks where the self-ty is an
1879                 // inference variable, but I was afraid that there might be an
1880                 // inference variable created, registered as an obligation, and
1881                 // then never forced by writeback, and hence by skipping here we'd
1882                 // be ignoring the fact that we don't KNOW the type works
1883                 // out. Though even that would probably be harmless, given that
1884                 // we're only talking about builtin traits, which are known to be
1885                 // inhabited. We used to check for `self.tcx.sess.has_errors()` to
1886                 // avoid inundating the user with unnecessary errors, but we now
1887                 // check upstream for type errors and dont add the obligations to
1888                 // begin with in those cases.
1889                 if
1890                     self.tcx.lang_items().sized_trait()
1891                     .map_or(false, |sized_id| sized_id == trait_ref.def_id())
1892                 {
1893                     self.need_type_info_err(body_id, span, self_ty).emit();
1894                 } else {
1895                     let mut err = struct_span_err!(
1896                         self.tcx.sess,
1897                         span,
1898                         E0283,
1899                         "type annotations needed: cannot resolve `{}`",
1900                         predicate,
1901                     );
1902                     self.note_obligation_cause(&mut err, obligation);
1903                     err.emit();
1904                 }
1905             }
1906
1907             ty::Predicate::WellFormed(ty) => {
1908                 // Same hacky approach as above to avoid deluging user
1909                 // with error messages.
1910                 if !ty.references_error() && !self.tcx.sess.has_errors() {
1911                     self.need_type_info_err(body_id, span, ty).emit();
1912                 }
1913             }
1914
1915             ty::Predicate::Subtype(ref data) => {
1916                 if data.references_error() || self.tcx.sess.has_errors() {
1917                     // no need to overload user in such cases
1918                 } else {
1919                     let &SubtypePredicate { a_is_expected: _, a, b } = data.skip_binder();
1920                     // both must be type variables, or the other would've been instantiated
1921                     assert!(a.is_ty_var() && b.is_ty_var());
1922                     self.need_type_info_err(body_id,
1923                                             obligation.cause.span,
1924                                             a).emit();
1925                 }
1926             }
1927
1928             _ => {
1929                 if !self.tcx.sess.has_errors() {
1930                     let mut err = struct_span_err!(
1931                         self.tcx.sess,
1932                         obligation.cause.span,
1933                         E0284,
1934                         "type annotations needed: cannot resolve `{}`",
1935                         predicate,
1936                     );
1937                     self.note_obligation_cause(&mut err, obligation);
1938                     err.emit();
1939                 }
1940             }
1941         }
1942     }
1943
1944     /// Returns `true` if the trait predicate may apply for *some* assignment
1945     /// to the type parameters.
1946     fn predicate_can_apply(
1947         &self,
1948         param_env: ty::ParamEnv<'tcx>,
1949         pred: ty::PolyTraitRef<'tcx>,
1950     ) -> bool {
1951         struct ParamToVarFolder<'a, 'tcx> {
1952             infcx: &'a InferCtxt<'a, 'tcx>,
1953             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
1954         }
1955
1956         impl<'a, 'tcx> TypeFolder<'tcx> for ParamToVarFolder<'a, 'tcx> {
1957             fn tcx<'b>(&'b self) -> TyCtxt<'tcx> { self.infcx.tcx }
1958
1959             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
1960                 if let ty::Param(ty::ParamTy {name, .. }) = ty.kind {
1961                     let infcx = self.infcx;
1962                     self.var_map.entry(ty).or_insert_with(||
1963                         infcx.next_ty_var(
1964                             TypeVariableOrigin {
1965                                 kind: TypeVariableOriginKind::TypeParameterDefinition(name),
1966                                 span: DUMMY_SP,
1967                             }
1968                         )
1969                     )
1970                 } else {
1971                     ty.super_fold_with(self)
1972                 }
1973             }
1974         }
1975
1976         self.probe(|_| {
1977             let mut selcx = SelectionContext::new(self);
1978
1979             let cleaned_pred = pred.fold_with(&mut ParamToVarFolder {
1980                 infcx: self,
1981                 var_map: Default::default()
1982             });
1983
1984             let cleaned_pred = super::project::normalize(
1985                 &mut selcx,
1986                 param_env,
1987                 ObligationCause::dummy(),
1988                 &cleaned_pred
1989             ).value;
1990
1991             let obligation = Obligation::new(
1992                 ObligationCause::dummy(),
1993                 param_env,
1994                 cleaned_pred.to_predicate()
1995             );
1996
1997             self.predicate_may_hold(&obligation)
1998         })
1999     }
2000
2001     fn note_obligation_cause(
2002         &self,
2003         err: &mut DiagnosticBuilder<'_>,
2004         obligation: &PredicateObligation<'tcx>,
2005     ) {
2006         // First, attempt to add note to this error with an async-await-specific
2007         // message, and fall back to regular note otherwise.
2008         if !self.note_obligation_cause_for_async_await(err, obligation) {
2009             self.note_obligation_cause_code(err, &obligation.predicate, &obligation.cause.code,
2010                                             &mut vec![]);
2011         }
2012     }
2013
2014     /// Adds an async-await specific note to the diagnostic:
2015     ///
2016     /// ```ignore (diagnostic)
2017     /// note: future does not implement `std::marker::Send` because this value is used across an
2018     ///       await
2019     ///   --> $DIR/issue-64130-non-send-future-diags.rs:15:5
2020     ///    |
2021     /// LL |     let g = x.lock().unwrap();
2022     ///    |         - has type `std::sync::MutexGuard<'_, u32>`
2023     /// LL |     baz().await;
2024     ///    |     ^^^^^^^^^^^ await occurs here, with `g` maybe used later
2025     /// LL | }
2026     ///    | - `g` is later dropped here
2027     /// ```
2028     ///
2029     /// Returns `true` if an async-await specific note was added to the diagnostic.
2030     fn note_obligation_cause_for_async_await(
2031         &self,
2032         err: &mut DiagnosticBuilder<'_>,
2033         obligation: &PredicateObligation<'tcx>,
2034     ) -> bool {
2035         debug!("note_obligation_cause_for_async_await: obligation.predicate={:?} \
2036                 obligation.cause.span={:?}", obligation.predicate, obligation.cause.span);
2037         let source_map = self.tcx.sess.source_map();
2038
2039         // Look into the obligation predicate to determine the type in the generator which meant
2040         // that the predicate was not satisifed.
2041         let (trait_ref, target_ty) = match obligation.predicate {
2042             ty::Predicate::Trait(trait_predicate) =>
2043                 (trait_predicate.skip_binder().trait_ref, trait_predicate.skip_binder().self_ty()),
2044             _ => return false,
2045         };
2046         debug!("note_obligation_cause_for_async_await: target_ty={:?}", target_ty);
2047
2048         // Attempt to detect an async-await error by looking at the obligation causes, looking
2049         // for only generators, generator witnesses, opaque types or `std::future::GenFuture` to
2050         // be present.
2051         //
2052         // When a future does not implement a trait because of a captured type in one of the
2053         // generators somewhere in the call stack, then the result is a chain of obligations.
2054         // Given a `async fn` A that calls a `async fn` B which captures a non-send type and that
2055         // future is passed as an argument to a function C which requires a `Send` type, then the
2056         // chain looks something like this:
2057         //
2058         // - `BuiltinDerivedObligation` with a generator witness (B)
2059         // - `BuiltinDerivedObligation` with a generator (B)
2060         // - `BuiltinDerivedObligation` with `std::future::GenFuture` (B)
2061         // - `BuiltinDerivedObligation` with `impl std::future::Future` (B)
2062         // - `BuiltinDerivedObligation` with `impl std::future::Future` (B)
2063         // - `BuiltinDerivedObligation` with a generator witness (A)
2064         // - `BuiltinDerivedObligation` with a generator (A)
2065         // - `BuiltinDerivedObligation` with `std::future::GenFuture` (A)
2066         // - `BuiltinDerivedObligation` with `impl std::future::Future` (A)
2067         // - `BuiltinDerivedObligation` with `impl std::future::Future` (A)
2068         // - `BindingObligation` with `impl_send (Send requirement)
2069         //
2070         // The first obligations in the chain can be used to get the details of the type that is
2071         // captured but the entire chain must be inspected to detect this case.
2072         let mut generator = None;
2073         let mut next_code = Some(&obligation.cause.code);
2074         while let Some(code) = next_code {
2075             debug!("note_obligation_cause_for_async_await: code={:?}", code);
2076             match code {
2077                 ObligationCauseCode::BuiltinDerivedObligation(derived_obligation) |
2078                 ObligationCauseCode::ImplDerivedObligation(derived_obligation) => {
2079                     debug!("note_obligation_cause_for_async_await: self_ty.kind={:?}",
2080                            derived_obligation.parent_trait_ref.self_ty().kind);
2081                     match derived_obligation.parent_trait_ref.self_ty().kind {
2082                         ty::Adt(ty::AdtDef { did, .. }, ..) if
2083                             self.tcx.is_diagnostic_item(sym::gen_future, *did) => {},
2084                         ty::Generator(did, ..) => generator = generator.or(Some(did)),
2085                         ty::GeneratorWitness(_) | ty::Opaque(..) => {},
2086                         _ => return false,
2087                     }
2088
2089                     next_code = Some(derived_obligation.parent_code.as_ref());
2090                 },
2091                 ObligationCauseCode::ItemObligation(_) | ObligationCauseCode::BindingObligation(..)
2092                     if generator.is_some() => break,
2093                 _ => return false,
2094             }
2095         }
2096
2097         let generator_did = generator.expect("can only reach this if there was a generator");
2098
2099         // Only continue to add a note if the generator is from an `async` function.
2100         let parent_node = self.tcx.parent(generator_did)
2101             .and_then(|parent_did| self.tcx.hir().get_if_local(parent_did));
2102         debug!("note_obligation_cause_for_async_await: parent_node={:?}", parent_node);
2103         if let Some(hir::Node::Item(hir::Item {
2104             kind: hir::ItemKind::Fn(sig, _, _),
2105             ..
2106         })) = parent_node {
2107             debug!("note_obligation_cause_for_async_await: header={:?}", sig.header);
2108             if sig.header.asyncness != hir::IsAsync::Async {
2109                 return false;
2110             }
2111         }
2112
2113         let span = self.tcx.def_span(generator_did);
2114         let tables = self.tcx.typeck_tables_of(generator_did);
2115         debug!("note_obligation_cause_for_async_await: generator_did={:?} span={:?} ",
2116                generator_did, span);
2117
2118         // Look for a type inside the generator interior that matches the target type to get
2119         // a span.
2120         let target_span = tables.generator_interior_types.iter()
2121             .find(|ty::GeneratorInteriorTypeCause { ty, .. }| ty::TyS::same_type(*ty, target_ty))
2122             .map(|ty::GeneratorInteriorTypeCause { span, scope_span, .. }|
2123                  (span, source_map.span_to_snippet(*span), scope_span));
2124         if let Some((target_span, Ok(snippet), scope_span)) = target_span {
2125             // Look at the last interior type to get a span for the `.await`.
2126             let await_span = tables.generator_interior_types.iter().map(|i| i.span).last().unwrap();
2127             let mut span = MultiSpan::from_span(await_span);
2128             span.push_span_label(
2129                 await_span, format!("await occurs here, with `{}` maybe used later", snippet));
2130
2131             span.push_span_label(*target_span, format!("has type `{}`", target_ty));
2132
2133             // If available, use the scope span to annotate the drop location.
2134             if let Some(scope_span) = scope_span {
2135                 span.push_span_label(
2136                     source_map.end_point(*scope_span),
2137                     format!("`{}` is later dropped here", snippet),
2138                 );
2139             }
2140
2141             err.span_note(span, &format!(
2142                 "future does not implement `{}` as this value is used across an await",
2143                 trait_ref,
2144             ));
2145
2146             // Add a note for the item obligation that remains - normally a note pointing to the
2147             // bound that introduced the obligation (e.g. `T: Send`).
2148             debug!("note_obligation_cause_for_async_await: next_code={:?}", next_code);
2149             self.note_obligation_cause_code(
2150                 err,
2151                 &obligation.predicate,
2152                 next_code.unwrap(),
2153                 &mut Vec::new(),
2154             );
2155
2156             true
2157         } else {
2158             false
2159         }
2160     }
2161
2162     fn note_obligation_cause_code<T>(&self,
2163                                      err: &mut DiagnosticBuilder<'_>,
2164                                      predicate: &T,
2165                                      cause_code: &ObligationCauseCode<'tcx>,
2166                                      obligated_types: &mut Vec<&ty::TyS<'tcx>>)
2167         where T: fmt::Display
2168     {
2169         let tcx = self.tcx;
2170         match *cause_code {
2171             ObligationCauseCode::ExprAssignable |
2172             ObligationCauseCode::MatchExpressionArm { .. } |
2173             ObligationCauseCode::MatchExpressionArmPattern { .. } |
2174             ObligationCauseCode::IfExpression { .. } |
2175             ObligationCauseCode::IfExpressionWithNoElse |
2176             ObligationCauseCode::MainFunctionType |
2177             ObligationCauseCode::StartFunctionType |
2178             ObligationCauseCode::IntrinsicType |
2179             ObligationCauseCode::MethodReceiver |
2180             ObligationCauseCode::ReturnNoExpression |
2181             ObligationCauseCode::MiscObligation => {}
2182             ObligationCauseCode::SliceOrArrayElem => {
2183                 err.note("slice and array elements must have `Sized` type");
2184             }
2185             ObligationCauseCode::TupleElem => {
2186                 err.note("only the last element of a tuple may have a dynamically sized type");
2187             }
2188             ObligationCauseCode::ProjectionWf(data) => {
2189                 err.note(&format!(
2190                     "required so that the projection `{}` is well-formed",
2191                     data,
2192                 ));
2193             }
2194             ObligationCauseCode::ReferenceOutlivesReferent(ref_ty) => {
2195                 err.note(&format!(
2196                     "required so that reference `{}` does not outlive its referent",
2197                     ref_ty,
2198                 ));
2199             }
2200             ObligationCauseCode::ObjectTypeBound(object_ty, region) => {
2201                 err.note(&format!(
2202                     "required so that the lifetime bound of `{}` for `{}` is satisfied",
2203                     region,
2204                     object_ty,
2205                 ));
2206             }
2207             ObligationCauseCode::ItemObligation(item_def_id) => {
2208                 let item_name = tcx.def_path_str(item_def_id);
2209                 let msg = format!("required by `{}`", item_name);
2210
2211                 if let Some(sp) = tcx.hir().span_if_local(item_def_id) {
2212                     let sp = tcx.sess.source_map().def_span(sp);
2213                     err.span_label(sp, &msg);
2214                 } else {
2215                     err.note(&msg);
2216                 }
2217             }
2218             ObligationCauseCode::BindingObligation(item_def_id, span) => {
2219                 let item_name = tcx.def_path_str(item_def_id);
2220                 let msg = format!("required by this bound in `{}`", item_name);
2221                 if let Some(ident) = tcx.opt_item_name(item_def_id) {
2222                     err.span_label(ident.span, "");
2223                 }
2224                 if span != DUMMY_SP {
2225                     err.span_label(span, &msg);
2226                 } else {
2227                     err.note(&msg);
2228                 }
2229             }
2230             ObligationCauseCode::ObjectCastObligation(object_ty) => {
2231                 err.note(&format!("required for the cast to the object type `{}`",
2232                                   self.ty_to_string(object_ty)));
2233             }
2234             ObligationCauseCode::Coercion { source: _, target } => {
2235                 err.note(&format!("required by cast to type `{}`",
2236                                   self.ty_to_string(target)));
2237             }
2238             ObligationCauseCode::RepeatVec(suggest_const_in_array_repeat_expressions) => {
2239                 err.note("the `Copy` trait is required because the \
2240                           repeated element will be copied");
2241                 if suggest_const_in_array_repeat_expressions {
2242                     err.note("this array initializer can be evaluated at compile-time, for more \
2243                               information, see issue \
2244                               https://github.com/rust-lang/rust/issues/49147");
2245                     if tcx.sess.opts.unstable_features.is_nightly_build() {
2246                         err.help("add `#![feature(const_in_array_repeat_expressions)]` to the \
2247                                   crate attributes to enable");
2248                     }
2249                 }
2250             }
2251             ObligationCauseCode::VariableType(_) => {
2252                 err.note("all local variables must have a statically known size");
2253                 if !self.tcx.features().unsized_locals {
2254                     err.help("unsized locals are gated as an unstable feature");
2255                 }
2256             }
2257             ObligationCauseCode::SizedArgumentType => {
2258                 err.note("all function arguments must have a statically known size");
2259                 if !self.tcx.features().unsized_locals {
2260                     err.help("unsized locals are gated as an unstable feature");
2261                 }
2262             }
2263             ObligationCauseCode::SizedReturnType => {
2264                 err.note("the return type of a function must have a \
2265                           statically known size");
2266             }
2267             ObligationCauseCode::SizedYieldType => {
2268                 err.note("the yield type of a generator must have a \
2269                           statically known size");
2270             }
2271             ObligationCauseCode::AssignmentLhsSized => {
2272                 err.note("the left-hand-side of an assignment must have a statically known size");
2273             }
2274             ObligationCauseCode::TupleInitializerSized => {
2275                 err.note("tuples must have a statically known size to be initialized");
2276             }
2277             ObligationCauseCode::StructInitializerSized => {
2278                 err.note("structs must have a statically known size to be initialized");
2279             }
2280             ObligationCauseCode::FieldSized { adt_kind: ref item, last } => {
2281                 match *item {
2282                     AdtKind::Struct => {
2283                         if last {
2284                             err.note("the last field of a packed struct may only have a \
2285                                       dynamically sized type if it does not need drop to be run");
2286                         } else {
2287                             err.note("only the last field of a struct may have a dynamically \
2288                                       sized type");
2289                         }
2290                     }
2291                     AdtKind::Union => {
2292                         err.note("no field of a union may have a dynamically sized type");
2293                     }
2294                     AdtKind::Enum => {
2295                         err.note("no field of an enum variant may have a dynamically sized type");
2296                     }
2297                 }
2298             }
2299             ObligationCauseCode::ConstSized => {
2300                 err.note("constant expressions must have a statically known size");
2301             }
2302             ObligationCauseCode::ConstPatternStructural => {
2303                 err.note("constants used for pattern-matching must derive `PartialEq` and `Eq`");
2304             }
2305             ObligationCauseCode::SharedStatic => {
2306                 err.note("shared static variables must have a type that implements `Sync`");
2307             }
2308             ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
2309                 let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
2310                 let ty = parent_trait_ref.skip_binder().self_ty();
2311                 err.note(&format!("required because it appears within the type `{}`", ty));
2312                 obligated_types.push(ty);
2313
2314                 let parent_predicate = parent_trait_ref.to_predicate();
2315                 if !self.is_recursive_obligation(obligated_types, &data.parent_code) {
2316                     self.note_obligation_cause_code(err,
2317                                                     &parent_predicate,
2318                                                     &data.parent_code,
2319                                                     obligated_types);
2320                 }
2321             }
2322             ObligationCauseCode::ImplDerivedObligation(ref data) => {
2323                 let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
2324                 err.note(
2325                     &format!("required because of the requirements on the impl of `{}` for `{}`",
2326                              parent_trait_ref,
2327                              parent_trait_ref.skip_binder().self_ty()));
2328                 let parent_predicate = parent_trait_ref.to_predicate();
2329                 self.note_obligation_cause_code(err,
2330                                                 &parent_predicate,
2331                                                 &data.parent_code,
2332                                                 obligated_types);
2333             }
2334             ObligationCauseCode::CompareImplMethodObligation { .. } => {
2335                 err.note(
2336                     &format!("the requirement `{}` appears on the impl method \
2337                               but not on the corresponding trait method",
2338                              predicate));
2339             }
2340             ObligationCauseCode::ReturnType |
2341             ObligationCauseCode::ReturnValue(_) |
2342             ObligationCauseCode::BlockTailExpression(_) => (),
2343             ObligationCauseCode::TrivialBound => {
2344                 err.help("see issue #48214");
2345                 if tcx.sess.opts.unstable_features.is_nightly_build() {
2346                     err.help("add `#![feature(trivial_bounds)]` to the \
2347                               crate attributes to enable",
2348                     );
2349                 }
2350             }
2351             ObligationCauseCode::AssocTypeBound(ref data) => {
2352                 err.span_label(data.original, "associated type defined here");
2353                 if let Some(sp) = data.impl_span {
2354                     err.span_label(sp, "in this `impl` item");
2355                 }
2356                 for sp in &data.bounds {
2357                     err.span_label(*sp, "restricted in this bound");
2358                 }
2359             }
2360         }
2361     }
2362
2363     fn suggest_new_overflow_limit(&self, err: &mut DiagnosticBuilder<'_>) {
2364         let current_limit = self.tcx.sess.recursion_limit.get();
2365         let suggested_limit = current_limit * 2;
2366         err.help(&format!("consider adding a `#![recursion_limit=\"{}\"]` attribute to your crate",
2367                           suggested_limit));
2368     }
2369
2370     fn is_recursive_obligation(&self,
2371                                obligated_types: &mut Vec<&ty::TyS<'tcx>>,
2372                                cause_code: &ObligationCauseCode<'tcx>) -> bool {
2373         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
2374             let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
2375
2376             if obligated_types.iter().any(|ot| ot == &parent_trait_ref.skip_binder().self_ty()) {
2377                 return true;
2378             }
2379         }
2380         false
2381     }
2382 }
2383
2384 /// Summarizes information
2385 #[derive(Clone)]
2386 pub enum ArgKind {
2387     /// An argument of non-tuple type. Parameters are (name, ty)
2388     Arg(String, String),
2389
2390     /// An argument of tuple type. For a "found" argument, the span is
2391     /// the locationo in the source of the pattern. For a "expected"
2392     /// argument, it will be None. The vector is a list of (name, ty)
2393     /// strings for the components of the tuple.
2394     Tuple(Option<Span>, Vec<(String, String)>),
2395 }
2396
2397 impl ArgKind {
2398     fn empty() -> ArgKind {
2399         ArgKind::Arg("_".to_owned(), "_".to_owned())
2400     }
2401
2402     /// Creates an `ArgKind` from the expected type of an
2403     /// argument. It has no name (`_`) and an optional source span.
2404     pub fn from_expected_ty(t: Ty<'_>, span: Option<Span>) -> ArgKind {
2405         match t.kind {
2406             ty::Tuple(ref tys) => ArgKind::Tuple(
2407                 span,
2408                 tys.iter()
2409                    .map(|ty| ("_".to_owned(), ty.to_string()))
2410                    .collect::<Vec<_>>()
2411             ),
2412             _ => ArgKind::Arg("_".to_owned(), t.to_string()),
2413         }
2414     }
2415 }