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, Style};
  37 use std::fmt;
  38 use syntax::ast;
  39 use syntax::symbol::{sym, kw};
  40 use syntax_pos::{DUMMY_SP, Span, ExpnKind, MultiSpan};
  41 use rustc::hir::def_id::LOCAL_CRATE;
  42 use syntax_pos::source_map::SourceMap;
  43
  44 use rustc_error_codes::*;
  45
  46 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
  47     pub fn report_fulfillment_errors(
  48         &self,
  49         errors: &[FulfillmentError<'tcx>],
  50         body_id: Option<hir::BodyId>,
  51         fallback_has_occurred: bool,
  52     ) {
  53         #[derive(Debug)]
  54         struct ErrorDescriptor<'tcx> {
  55             predicate: ty::Predicate<'tcx>,
  56             index: Option<usize>, // None if this is an old error
  57         }
  58
  59         let mut error_map: FxHashMap<_, Vec<_>> =
  60             self.reported_trait_errors.borrow().iter().map(|(&span, predicates)| {
  61                 (span, predicates.iter().map(|predicate| ErrorDescriptor {
  62                     predicate: predicate.clone(),
  63                     index: None
  64                 }).collect())
  65             }).collect();
  66
  67         for (index, error) in errors.iter().enumerate() {
  68             // We want to ignore desugarings here: spans are equivalent even
  69             // if one is the result of a desugaring and the other is not.
  70             let mut span = error.obligation.cause.span;
  71             let expn_data = span.ctxt().outer_expn_data();
  72             if let ExpnKind::Desugaring(_) = expn_data.kind {
  73                 span = expn_data.call_site;
  74             }
  75
  76             error_map.entry(span).or_default().push(
  77                 ErrorDescriptor {
  78                     predicate: error.obligation.predicate.clone(),
  79                     index: Some(index)
  80                 }
  81             );
  82
  83             self.reported_trait_errors.borrow_mut()
  84                 .entry(span).or_default()
  85                 .push(error.obligation.predicate.clone());
  86         }
  87
  88         // We do this in 2 passes because we want to display errors in order, though
  89         // maybe it *is* better to sort errors by span or something.
  90         let mut is_suppressed = vec![false; errors.len()];
  91         for (_, error_set) in error_map.iter() {
  92             // We want to suppress "duplicate" errors with the same span.
  93             for error in error_set {
  94                 if let Some(index) = error.index {
  95                     // Suppress errors that are either:
  96                     // 1) strictly implied by another error.
  97                     // 2) implied by an error with a smaller index.
  98                     for error2 in error_set {
  99                         if error2.index.map_or(false, |index2| is_suppressed[index2]) {
 100                             // Avoid errors being suppressed by already-suppressed
 101                             // errors, to prevent all errors from being suppressed
 102                             // at once.
 103                             continue
 104                         }
 105
 106                         if self.error_implies(&error2.predicate, &error.predicate) &&
 107                             !(error2.index >= error.index &&
 108                               self.error_implies(&error.predicate, &error2.predicate))
 109                         {
 110                             info!("skipping {:?} (implied by {:?})", error, error2);
 111                             is_suppressed[index] = true;
 112                             break
 113                         }
 114                     }
 115                 }
 116             }
 117         }
 118
 119         for (error, suppressed) in errors.iter().zip(is_suppressed) {
 120             if !suppressed {
 121                 self.report_fulfillment_error(error, body_id, fallback_has_occurred);
 122             }
 123         }
 124     }
 125
 126     // returns if `cond` not occurring implies that `error` does not occur - i.e., that
 127     // `error` occurring implies that `cond` occurs.
 128     fn error_implies(
 129         &self,
 130         cond: &ty::Predicate<'tcx>,
 131         error: &ty::Predicate<'tcx>,
 132     ) -> bool {
 133         if cond == error {
 134             return true
 135         }
 136
 137         let (cond, error) = match (cond, error) {
 138             (&ty::Predicate::Trait(..), &ty::Predicate::Trait(ref error))
 139                 => (cond, error),
 140             _ => {
 141                 // FIXME: make this work in other cases too.
 142                 return false
 143             }
 144         };
 145
 146         for implication in super::elaborate_predicates(self.tcx, vec![cond.clone()]) {
 147             if let ty::Predicate::Trait(implication) = implication {
 148                 let error = error.to_poly_trait_ref();
 149                 let implication = implication.to_poly_trait_ref();
 150                 // FIXME: I'm just not taking associated types at all here.
 151                 // Eventually I'll need to implement param-env-aware
 152                 // `Γ₁ ⊦ φ₁ => Γ₂ ⊦ φ₂` logic.
 153                 let param_env = ty::ParamEnv::empty();
 154                 if self.can_sub(param_env, error, implication).is_ok() {
 155                     debug!("error_implies: {:?} -> {:?} -> {:?}", cond, error, implication);
 156                     return true
 157                 }
 158             }
 159         }
 160
 161         false
 162     }
 163
 164     fn report_fulfillment_error(
 165         &self,
 166         error: &FulfillmentError<'tcx>,
 167         body_id: Option<hir::BodyId>,
 168         fallback_has_occurred: bool,
 169     ) {
 170         debug!("report_fulfillment_error({:?})", error);
 171         match error.code {
 172             FulfillmentErrorCode::CodeSelectionError(ref selection_error) => {
 173                 self.report_selection_error(
 174                     &error.obligation,
 175                     selection_error,
 176                     fallback_has_occurred,
 177                     error.points_at_arg_span,
 178                 );
 179             }
 180             FulfillmentErrorCode::CodeProjectionError(ref e) => {
 181                 self.report_projection_error(&error.obligation, e);
 182             }
 183             FulfillmentErrorCode::CodeAmbiguity => {
 184                 self.maybe_report_ambiguity(&error.obligation, body_id);
 185             }
 186             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
 187                 self.report_mismatched_types(
 188                     &error.obligation.cause,
 189                     expected_found.expected,
 190                     expected_found.found,
 191                     err.clone(),
 192                 ).emit();
 193             }
 194         }
 195     }
 196
 197     fn report_projection_error(
 198         &self,
 199         obligation: &PredicateObligation<'tcx>,
 200         error: &MismatchedProjectionTypes<'tcx>,
 201     ) {
 202         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
 203
 204         if predicate.references_error() {
 205             return
 206         }
 207
 208         self.probe(|_| {
 209             let err_buf;
 210             let mut err = &error.err;
 211             let mut values = None;
 212
 213             // try to find the mismatched types to report the error with.
 214             //
 215             // this can fail if the problem was higher-ranked, in which
 216             // cause I have no idea for a good error message.
 217             if let ty::Predicate::Projection(ref data) = predicate {
 218                 let mut selcx = SelectionContext::new(self);
 219                 let (data, _) = self.replace_bound_vars_with_fresh_vars(
 220                     obligation.cause.span,
 221                     infer::LateBoundRegionConversionTime::HigherRankedType,
 222                     data
 223                 );
 224                 let mut obligations = vec![];
 225                 let normalized_ty = super::normalize_projection_type(
 226                     &mut selcx,
 227                     obligation.param_env,
 228                     data.projection_ty,
 229                     obligation.cause.clone(),
 230                     0,
 231                     &mut obligations
 232                 );
 233
 234                 debug!("report_projection_error obligation.cause={:?} obligation.param_env={:?}",
 235                        obligation.cause, obligation.param_env);
 236
 237                 debug!("report_projection_error normalized_ty={:?} data.ty={:?}",
 238                        normalized_ty, data.ty);
 239
 240                 let is_normalized_ty_expected = match &obligation.cause.code {
 241                     ObligationCauseCode::ItemObligation(_) |
 242                     ObligationCauseCode::BindingObligation(_, _) |
 243                     ObligationCauseCode::ObjectCastObligation(_) => false,
 244                     _ => true,
 245                 };
 246
 247                 if let Err(error) = self.at(&obligation.cause, obligation.param_env)
 248                     .eq_exp(is_normalized_ty_expected, normalized_ty, data.ty)
 249                 {
 250                     values = Some(infer::ValuePairs::Types(
 251                         ExpectedFound::new(is_normalized_ty_expected, normalized_ty, data.ty)));
 252
 253                     err_buf = error;
 254                     err = &err_buf;
 255                 }
 256             }
 257
 258             let msg = format!("type mismatch resolving `{}`", predicate);
 259             let error_id = (
 260                 DiagnosticMessageId::ErrorId(271),
 261                 Some(obligation.cause.span),
 262                 msg,
 263             );
 264             let fresh = self.tcx.sess.one_time_diagnostics.borrow_mut().insert(error_id);
 265             if fresh {
 266                 let mut diag = struct_span_err!(
 267                     self.tcx.sess,
 268                     obligation.cause.span,
 269                     E0271,
 270                     "type mismatch resolving `{}`",
 271                     predicate
 272                 );
 273                 self.note_type_err(&mut diag, &obligation.cause, None, values, err);
 274                 self.note_obligation_cause(&mut diag, obligation);
 275                 diag.emit();
 276             }
 277         });
 278     }
 279
 280     fn fuzzy_match_tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
 281         /// returns the fuzzy category of a given type, or None
 282         /// if the type can be equated to any type.
 283         fn type_category(t: Ty<'_>) -> Option<u32> {
 284             match t.kind {
 285                 ty::Bool => Some(0),
 286                 ty::Char => Some(1),
 287                 ty::Str => Some(2),
 288                 ty::Int(..) | ty::Uint(..) | ty::Infer(ty::IntVar(..)) => Some(3),
 289                 ty::Float(..) | ty::Infer(ty::FloatVar(..)) => Some(4),
 290                 ty::Ref(..) | ty::RawPtr(..) => Some(5),
 291                 ty::Array(..) | ty::Slice(..) => Some(6),
 292                 ty::FnDef(..) | ty::FnPtr(..) => Some(7),
 293                 ty::Dynamic(..) => Some(8),
 294                 ty::Closure(..) => Some(9),
 295                 ty::Tuple(..) => Some(10),
 296                 ty::Projection(..) => Some(11),
 297                 ty::Param(..) => Some(12),
 298                 ty::Opaque(..) => Some(13),
 299                 ty::Never => Some(14),
 300                 ty::Adt(adt, ..) => match adt.adt_kind() {
 301                     AdtKind::Struct => Some(15),
 302                     AdtKind::Union => Some(16),
 303                     AdtKind::Enum => Some(17),
 304                 },
 305                 ty::Generator(..) => Some(18),
 306                 ty::Foreign(..) => Some(19),
 307                 ty::GeneratorWitness(..) => Some(20),
 308                 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) | ty::Error => None,
 309                 ty::UnnormalizedProjection(..) => bug!("only used with chalk-engine"),
 310             }
 311         }
 312
 313         match (type_category(a), type_category(b)) {
 314             (Some(cat_a), Some(cat_b)) => match (&a.kind, &b.kind) {
 315                 (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => def_a == def_b,
 316                 _ => cat_a == cat_b
 317             },
 318             // infer and error can be equated to all types
 319             _ => true
 320         }
 321     }
 322
 323     fn impl_similar_to(&self,
 324                        trait_ref: ty::PolyTraitRef<'tcx>,
 325                        obligation: &PredicateObligation<'tcx>)
 326                        -> Option<DefId>
 327     {
 328         let tcx = self.tcx;
 329         let param_env = obligation.param_env;
 330         let trait_ref = tcx.erase_late_bound_regions(&trait_ref);
 331         let trait_self_ty = trait_ref.self_ty();
 332
 333         let mut self_match_impls = vec![];
 334         let mut fuzzy_match_impls = vec![];
 335
 336         self.tcx.for_each_relevant_impl(
 337             trait_ref.def_id, trait_self_ty, |def_id| {
 338                 let impl_substs = self.fresh_substs_for_item(obligation.cause.span, def_id);
 339                 let impl_trait_ref = tcx
 340                     .impl_trait_ref(def_id)
 341                     .unwrap()
 342                     .subst(tcx, impl_substs);
 343
 344                 let impl_self_ty = impl_trait_ref.self_ty();
 345
 346                 if let Ok(..) = self.can_eq(param_env, trait_self_ty, impl_self_ty) {
 347                     self_match_impls.push(def_id);
 348
 349                     if trait_ref.substs.types().skip(1)
 350                         .zip(impl_trait_ref.substs.types().skip(1))
 351                         .all(|(u,v)| self.fuzzy_match_tys(u, v))
 352                     {
 353                         fuzzy_match_impls.push(def_id);
 354                     }
 355                 }
 356             });
 357
 358         let impl_def_id = if self_match_impls.len() == 1 {
 359             self_match_impls[0]
 360         } else if fuzzy_match_impls.len() == 1 {
 361             fuzzy_match_impls[0]
 362         } else {
 363             return None
 364         };
 365
 366         if tcx.has_attr(impl_def_id, sym::rustc_on_unimplemented) {
 367             Some(impl_def_id)
 368         } else {
 369             None
 370         }
 371     }
 372
 373     fn describe_generator(&self, body_id: hir::BodyId) -> Option<&'static str> {
 374         self.tcx.hir().body(body_id).generator_kind.map(|gen_kind| {
 375             match gen_kind {
 376                 hir::GeneratorKind::Gen => "a generator",
 377                 hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block) => "an async block",
 378                 hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn) => "an async function",
 379                 hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Closure) => "an async closure",
 380             }
 381         })
 382     }
 383
 384     /// Used to set on_unimplemented's `ItemContext`
 385     /// to be the enclosing (async) block/function/closure
 386     fn describe_enclosure(&self, hir_id: hir::HirId) -> Option<&'static str> {
 387         let hir = &self.tcx.hir();
 388         let node = hir.find(hir_id)?;
 389         if let hir::Node::Item(
 390             hir::Item{kind: hir::ItemKind::Fn(sig, _, body_id), .. }) = &node {
 391             self.describe_generator(*body_id).or_else(||
 392                 Some(if let hir::FnHeader{ asyncness: hir::IsAsync::Async, .. } = sig.header {
 393                     "an async function"
 394                 } else {
 395                     "a function"
 396                 })
 397             )
 398         } else if let hir::Node::Expr(hir::Expr {
 399             kind: hir::ExprKind::Closure(_is_move, _, body_id, _, gen_movability), .. }) = &node {
 400             self.describe_generator(*body_id).or_else(||
 401                 Some(if gen_movability.is_some() {
 402                     "an async closure"
 403                 } else {
 404                     "a closure"
 405                 })
 406             )
 407         } else if let hir::Node::Expr(hir::Expr { .. }) = &node {
 408             let parent_hid = hir.get_parent_node(hir_id);
 409             if parent_hid != hir_id {
 410                 return self.describe_enclosure(parent_hid);
 411             } else {
 412                 None
 413             }
 414         } else {
 415             None
 416         }
 417     }
 418
 419     fn on_unimplemented_note(
 420         &self,
 421         trait_ref: ty::PolyTraitRef<'tcx>,
 422         obligation: &PredicateObligation<'tcx>,
 423     ) -> OnUnimplementedNote {
 424         let def_id = self.impl_similar_to(trait_ref, obligation)
 425             .unwrap_or_else(|| trait_ref.def_id());
 426         let trait_ref = *trait_ref.skip_binder();
 427
 428         let mut flags = vec![];
 429         flags.push((sym::item_context,
 430             self.describe_enclosure(obligation.cause.body_id).map(|s|s.to_owned())));
 431
 432         match obligation.cause.code {
 433             ObligationCauseCode::BuiltinDerivedObligation(..) |
 434             ObligationCauseCode::ImplDerivedObligation(..) => {}
 435             _ => {
 436                 // this is a "direct", user-specified, rather than derived,
 437                 // obligation.
 438                 flags.push((sym::direct, None));
 439             }
 440         }
 441
 442         if let ObligationCauseCode::ItemObligation(item) = obligation.cause.code {
 443             // FIXME: maybe also have some way of handling methods
 444             // from other traits? That would require name resolution,
 445             // which we might want to be some sort of hygienic.
 446             //
 447             // Currently I'm leaving it for what I need for `try`.
 448             if self.tcx.trait_of_item(item) == Some(trait_ref.def_id) {
 449                 let method = self.tcx.item_name(item);
 450                 flags.push((sym::from_method, None));
 451                 flags.push((sym::from_method, Some(method.to_string())));
 452             }
 453         }
 454         if let Some(t) = self.get_parent_trait_ref(&obligation.cause.code) {
 455             flags.push((sym::parent_trait, Some(t)));
 456         }
 457
 458         if let Some(k) = obligation.cause.span.desugaring_kind() {
 459             flags.push((sym::from_desugaring, None));
 460             flags.push((sym::from_desugaring, Some(format!("{:?}", k))));
 461         }
 462         let generics = self.tcx.generics_of(def_id);
 463         let self_ty = trait_ref.self_ty();
 464         // This is also included through the generics list as `Self`,
 465         // but the parser won't allow you to use it
 466         flags.push((sym::_Self, Some(self_ty.to_string())));
 467         if let Some(def) = self_ty.ty_adt_def() {
 468             // We also want to be able to select self's original
 469             // signature with no type arguments resolved
 470             flags.push((sym::_Self, Some(self.tcx.type_of(def.did).to_string())));
 471         }
 472
 473         for param in generics.params.iter() {
 474             let value = match param.kind {
 475                 GenericParamDefKind::Type { .. } |
 476                 GenericParamDefKind::Const => {
 477                     trait_ref.substs[param.index as usize].to_string()
 478                 },
 479                 GenericParamDefKind::Lifetime => continue,
 480             };
 481             let name = param.name;
 482             flags.push((name, Some(value)));
 483         }
 484
 485         if let Some(true) = self_ty.ty_adt_def().map(|def| def.did.is_local()) {
 486             flags.push((sym::crate_local, None));
 487         }
 488
 489         // Allow targeting all integers using `{integral}`, even if the exact type was resolved
 490         if self_ty.is_integral() {
 491             flags.push((sym::_Self, Some("{integral}".to_owned())));
 492         }
 493
 494         if let ty::Array(aty, len) = self_ty.kind {
 495             flags.push((sym::_Self, Some("[]".to_owned())));
 496             flags.push((sym::_Self, Some(format!("[{}]", aty))));
 497             if let Some(def) = aty.ty_adt_def() {
 498                 // We also want to be able to select the array's type's original
 499                 // signature with no type arguments resolved
 500                 flags.push((
 501                     sym::_Self,
 502                     Some(format!("[{}]", self.tcx.type_of(def.did).to_string())),
 503                 ));
 504                 let tcx = self.tcx;
 505                 if let Some(len) = len.try_eval_usize(tcx, ty::ParamEnv::empty()) {
 506                     flags.push((
 507                         sym::_Self,
 508                         Some(format!("[{}; {}]", self.tcx.type_of(def.did).to_string(), len)),
 509                     ));
 510                 } else {
 511                     flags.push((
 512                         sym::_Self,
 513                         Some(format!("[{}; _]", self.tcx.type_of(def.did).to_string())),
 514                     ));
 515                 }
 516             }
 517         }
 518
 519         if let Ok(Some(command)) = OnUnimplementedDirective::of_item(
 520             self.tcx, trait_ref.def_id, def_id
 521         ) {
 522             command.evaluate(self.tcx, trait_ref, &flags[..])
 523         } else {
 524             OnUnimplementedNote::default()
 525         }
 526     }
 527
 528     fn find_similar_impl_candidates(
 529         &self,
 530         trait_ref: ty::PolyTraitRef<'tcx>,
 531     ) -> Vec<ty::TraitRef<'tcx>> {
 532         let simp = fast_reject::simplify_type(self.tcx, trait_ref.skip_binder().self_ty(), true);
 533         let all_impls = self.tcx.all_impls(trait_ref.def_id());
 534
 535         match simp {
 536             Some(simp) => all_impls.iter().filter_map(|&def_id| {
 537                 let imp = self.tcx.impl_trait_ref(def_id).unwrap();
 538                 let imp_simp = fast_reject::simplify_type(self.tcx, imp.self_ty(), true);
 539                 if let Some(imp_simp) = imp_simp {
 540                     if simp != imp_simp {
 541                         return None
 542                     }
 543                 }
 544
 545                 Some(imp)
 546             }).collect(),
 547             None => all_impls.iter().map(|&def_id|
 548                 self.tcx.impl_trait_ref(def_id).unwrap()
 549             ).collect()
 550         }
 551     }
 552
 553     fn report_similar_impl_candidates(
 554         &self,
 555         impl_candidates: Vec<ty::TraitRef<'tcx>>,
 556         err: &mut DiagnosticBuilder<'_>,
 557     ) {
 558         if impl_candidates.is_empty() {
 559             return;
 560         }
 561
 562         let len = impl_candidates.len();
 563         let end = if impl_candidates.len() <= 5 {
 564             impl_candidates.len()
 565         } else {
 566             4
 567         };
 568
 569         let normalize = |candidate| self.tcx.infer_ctxt().enter(|ref infcx| {
 570             let normalized = infcx
 571                 .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
 572                 .normalize(candidate)
 573                 .ok();
 574             match normalized {
 575                 Some(normalized) => format!("\n  {:?}", normalized.value),
 576                 None => format!("\n  {:?}", candidate),
 577             }
 578         });
 579
 580         // Sort impl candidates so that ordering is consistent for UI tests.
 581         let mut normalized_impl_candidates = impl_candidates
 582             .iter()
 583             .map(normalize)
 584             .collect::<Vec<String>>();
 585
 586         // Sort before taking the `..end` range,
 587         // because the ordering of `impl_candidates` may not be deterministic:
 588         // https://github.com/rust-lang/rust/pull/57475#issuecomment-455519507
 589         normalized_impl_candidates.sort();
 590
 591         err.help(&format!("the following implementations were found:{}{}",
 592                           normalized_impl_candidates[..end].join(""),
 593                           if len > 5 {
 594                               format!("\nand {} others", len - 4)
 595                           } else {
 596                               String::new()
 597                           }
 598                           ));
 599     }
 600
 601     /// Reports that an overflow has occurred and halts compilation. We
 602     /// halt compilation unconditionally because it is important that
 603     /// overflows never be masked -- they basically represent computations
 604     /// whose result could not be truly determined and thus we can't say
 605     /// if the program type checks or not -- and they are unusual
 606     /// occurrences in any case.
 607     pub fn report_overflow_error<T>(
 608         &self,
 609         obligation: &Obligation<'tcx, T>,
 610         suggest_increasing_limit: bool,
 611     ) -> !
 612         where T: fmt::Display + TypeFoldable<'tcx>
 613     {
 614         let predicate =
 615             self.resolve_vars_if_possible(&obligation.predicate);
 616         let mut err = struct_span_err!(
 617             self.tcx.sess,
 618             obligation.cause.span,
 619             E0275,
 620             "overflow evaluating the requirement `{}`",
 621             predicate
 622         );
 623
 624         if suggest_increasing_limit {
 625             self.suggest_new_overflow_limit(&mut err);
 626         }
 627
 628         self.note_obligation_cause_code(
 629             &mut err,
 630             &obligation.predicate,
 631             &obligation.cause.code,
 632             &mut vec![],
 633         );
 634
 635         err.emit();
 636         self.tcx.sess.abort_if_errors();
 637         bug!();
 638     }
 639
 640     /// Reports that a cycle was detected which led to overflow and halts
 641     /// compilation. This is equivalent to `report_overflow_error` except
 642     /// that we can give a more helpful error message (and, in particular,
 643     /// we do not suggest increasing the overflow limit, which is not
 644     /// going to help).
 645     pub fn report_overflow_error_cycle(&self, cycle: &[PredicateObligation<'tcx>]) -> ! {
 646         let cycle = self.resolve_vars_if_possible(&cycle.to_owned());
 647         assert!(cycle.len() > 0);
 648
 649         debug!("report_overflow_error_cycle: cycle={:?}", cycle);
 650
 651         self.report_overflow_error(&cycle[0], false);
 652     }
 653
 654     pub fn report_extra_impl_obligation(&self,
 655                                         error_span: Span,
 656                                         item_name: ast::Name,
 657                                         _impl_item_def_id: DefId,
 658                                         trait_item_def_id: DefId,
 659                                         requirement: &dyn fmt::Display)
 660                                         -> DiagnosticBuilder<'tcx>
 661     {
 662         let msg = "impl has stricter requirements than trait";
 663         let sp = self.tcx.sess.source_map().def_span(error_span);
 664
 665         let mut err = struct_span_err!(self.tcx.sess, sp, E0276, "{}", msg);
 666
 667         if let Some(trait_item_span) = self.tcx.hir().span_if_local(trait_item_def_id) {
 668             let span = self.tcx.sess.source_map().def_span(trait_item_span);
 669             err.span_label(span, format!("definition of `{}` from trait", item_name));
 670         }
 671
 672         err.span_label(sp, format!("impl has extra requirement {}", requirement));
 673
 674         err
 675     }
 676
 677
 678     /// Gets the parent trait chain start
 679     fn get_parent_trait_ref(&self, code: &ObligationCauseCode<'tcx>) -> Option<String> {
 680         match code {
 681             &ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
 682                 let parent_trait_ref = self.resolve_vars_if_possible(
 683                     &data.parent_trait_ref);
 684                 match self.get_parent_trait_ref(&data.parent_code) {
 685                     Some(t) => Some(t),
 686                     None => Some(parent_trait_ref.skip_binder().self_ty().to_string()),
 687                 }
 688             }
 689             _ => None,
 690         }
 691     }
 692
 693     pub fn report_selection_error(
 694         &self,
 695         obligation: &PredicateObligation<'tcx>,
 696         error: &SelectionError<'tcx>,
 697         fallback_has_occurred: bool,
 698         points_at_arg: bool,
 699     ) {
 700         let tcx = self.tcx;
 701         let span = obligation.cause.span;
 702
 703         let mut err = match *error {
 704             SelectionError::Unimplemented => {
 705                 if let ObligationCauseCode::CompareImplMethodObligation {
 706                     item_name, impl_item_def_id, trait_item_def_id,
 707                 } = obligation.cause.code {
 708                     self.report_extra_impl_obligation(
 709                         span,
 710                         item_name,
 711                         impl_item_def_id,
 712                         trait_item_def_id,
 713                         &format!("`{}`", obligation.predicate))
 714                         .emit();
 715                     return;
 716                 }
 717                 match obligation.predicate {
 718                     ty::Predicate::Trait(ref trait_predicate) => {
 719                         let trait_predicate = self.resolve_vars_if_possible(trait_predicate);
 720
 721                         if self.tcx.sess.has_errors() && trait_predicate.references_error() {
 722                             return;
 723                         }
 724                         let trait_ref = trait_predicate.to_poly_trait_ref();
 725                         let (
 726                             post_message,
 727                             pre_message,
 728                         ) = self.get_parent_trait_ref(&obligation.cause.code)
 729                             .map(|t| (format!(" in `{}`", t), format!("within `{}`, ", t)))
 730                             .unwrap_or_default();
 731
 732                         let OnUnimplementedNote {
 733                             message,
 734                             label,
 735                             note,
 736                             enclosing_scope,
 737                         } = self.on_unimplemented_note(trait_ref, obligation);
 738                         let have_alt_message = message.is_some() || label.is_some();
 739                         let is_try = self.tcx.sess.source_map().span_to_snippet(span)
 740                             .map(|s| &s == "?")
 741                             .unwrap_or(false);
 742                         let is_from =
 743                             format!("{}", trait_ref.print_only_trait_path())
 744                             .starts_with("std::convert::From<");
 745                         let (message, note) = if is_try && is_from {
 746                             (Some(format!(
 747                                 "`?` couldn't convert the error to `{}`",
 748                                 trait_ref.self_ty(),
 749                             )), Some(
 750                                 "the question mark operation (`?`) implicitly performs a \
 751                                  conversion on the error value using the `From` trait".to_owned()
 752                             ))
 753                         } else {
 754                             (message, note)
 755                         };
 756
 757                         let mut err = struct_span_err!(
 758                             self.tcx.sess,
 759                             span,
 760                             E0277,
 761                             "{}",
 762                             message.unwrap_or_else(|| format!(
 763                                 "the trait bound `{}` is not satisfied{}",
 764                                 trait_ref.to_predicate(),
 765                                 post_message,
 766                             )));
 767
 768                         let explanation =
 769                             if obligation.cause.code == ObligationCauseCode::MainFunctionType {
 770                                 "consider using `()`, or a `Result`".to_owned()
 771                             } else {
 772                                 format!(
 773                                     "{}the trait `{}` is not implemented for `{}`",
 774                                     pre_message,
 775                                     trait_ref.print_only_trait_path(),
 776                                     trait_ref.self_ty(),
 777                                 )
 778                             };
 779
 780                         if self.suggest_add_reference_to_arg(
 781                             &obligation,
 782                             &mut err,
 783                             &trait_ref,
 784                             points_at_arg,
 785                             have_alt_message,
 786                         ) {
 787                             self.note_obligation_cause(&mut err, obligation);
 788                             err.emit();
 789                             return;
 790                         }
 791                         if let Some(ref s) = label {
 792                             // If it has a custom `#[rustc_on_unimplemented]`
 793                             // error message, let's display it as the label!
 794                             err.span_label(span, s.as_str());
 795                             err.help(&explanation);
 796                         } else {
 797                             err.span_label(span, explanation);
 798                         }
 799                         if let Some(ref s) = note {
 800                             // If it has a custom `#[rustc_on_unimplemented]` note, let's display it
 801                             err.note(s.as_str());
 802                         }
 803                         if let Some(ref s) = enclosing_scope {
 804                             let enclosing_scope_span = tcx.def_span(
 805                                 tcx.hir()
 806                                     .opt_local_def_id(obligation.cause.body_id)
 807                                     .unwrap_or_else(|| {
 808                                         tcx.hir().body_owner_def_id(hir::BodyId {
 809                                             hir_id: obligation.cause.body_id,
 810                                         })
 811                                     }),
 812                             );
 813
 814                             err.span_label(enclosing_scope_span, s.as_str());
 815                         }
 816
 817                         self.suggest_borrow_on_unsized_slice(&obligation.cause.code, &mut err);
 818                         self.suggest_fn_call(&obligation, &mut err, &trait_ref, points_at_arg);
 819                         self.suggest_remove_reference(&obligation, &mut err, &trait_ref);
 820                         self.suggest_semicolon_removal(&obligation, &mut err, span, &trait_ref);
 821                         self.note_version_mismatch(&mut err, &trait_ref);
 822
 823                         // Try to report a help message
 824                         if !trait_ref.has_infer_types() &&
 825                             self.predicate_can_apply(obligation.param_env, trait_ref) {
 826                             // If a where-clause may be useful, remind the
 827                             // user that they can add it.
 828                             //
 829                             // don't display an on-unimplemented note, as
 830                             // these notes will often be of the form
 831                             //     "the type `T` can't be frobnicated"
 832                             // which is somewhat confusing.
 833                             self.suggest_restricting_param_bound(
 834                                 &mut err,
 835                                 &trait_ref,
 836                                 obligation.cause.body_id,
 837                             );
 838                         } else {
 839                             if !have_alt_message {
 840                                 // Can't show anything else useful, try to find similar impls.
 841                                 let impl_candidates = self.find_similar_impl_candidates(trait_ref);
 842                                 self.report_similar_impl_candidates(impl_candidates, &mut err);
 843                             }
 844                             self.suggest_change_mut(
 845                                 &obligation,
 846                                 &mut err,
 847                                 &trait_ref,
 848                                 points_at_arg,
 849                             );
 850                         }
 851
 852                         // If this error is due to `!: Trait` not implemented but `(): Trait` is
 853                         // implemented, and fallback has occurred, then it could be due to a
 854                         // variable that used to fallback to `()` now falling back to `!`. Issue a
 855                         // note informing about the change in behaviour.
 856                         if trait_predicate.skip_binder().self_ty().is_never()
 857                             && fallback_has_occurred
 858                         {
 859                             let predicate = trait_predicate.map_bound(|mut trait_pred| {
 860                                 trait_pred.trait_ref.substs = self.tcx.mk_substs_trait(
 861                                     self.tcx.mk_unit(),
 862                                     &trait_pred.trait_ref.substs[1..],
 863                                 );
 864                                 trait_pred
 865                             });
 866                             let unit_obligation = Obligation {
 867                                 predicate: ty::Predicate::Trait(predicate),
 868                                 .. obligation.clone()
 869                             };
 870                             if self.predicate_may_hold(&unit_obligation) {
 871                                 err.note("the trait is implemented for `()`. \
 872                                          Possibly this error has been caused by changes to \
 873                                          Rust's type-inference algorithm \
 874                                          (see: https://github.com/rust-lang/rust/issues/48950 \
 875                                          for more info). Consider whether you meant to use the \
 876                                          type `()` here instead.");
 877                             }
 878                         }
 879
 880                         err
 881                     }
 882
 883                     ty::Predicate::Subtype(ref predicate) => {
 884                         // Errors for Subtype predicates show up as
 885                         // `FulfillmentErrorCode::CodeSubtypeError`,
 886                         // not selection error.
 887                         span_bug!(span, "subtype requirement gave wrong error: `{:?}`", predicate)
 888                     }
 889
 890                     ty::Predicate::RegionOutlives(ref predicate) => {
 891                         let predicate = self.resolve_vars_if_possible(predicate);
 892                         let err = self.region_outlives_predicate(&obligation.cause,
 893                                                                  &predicate).err().unwrap();
 894                         struct_span_err!(
 895                             self.tcx.sess, span, E0279,
 896                             "the requirement `{}` is not satisfied (`{}`)",
 897                             predicate, err,
 898                         )
 899                     }
 900
 901                     ty::Predicate::Projection(..) | ty::Predicate::TypeOutlives(..) => {
 902                         let predicate =
 903                             self.resolve_vars_if_possible(&obligation.predicate);
 904                         struct_span_err!(self.tcx.sess, span, E0280,
 905                             "the requirement `{}` is not satisfied",
 906                             predicate)
 907                     }
 908
 909                     ty::Predicate::ObjectSafe(trait_def_id) => {
 910                         let violations = self.tcx.object_safety_violations(trait_def_id);
 911                         self.tcx.report_object_safety_error(
 912                             span,
 913                             trait_def_id,
 914                             violations,
 915                         )
 916                     }
 917
 918                     ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => {
 919                         let found_kind = self.closure_kind(closure_def_id, closure_substs).unwrap();
 920                         let closure_span = self.tcx.sess.source_map()
 921                             .def_span(self.tcx.hir().span_if_local(closure_def_id).unwrap());
 922                         let hir_id = self.tcx.hir().as_local_hir_id(closure_def_id).unwrap();
 923                         let mut err = struct_span_err!(
 924                             self.tcx.sess, closure_span, E0525,
 925                             "expected a closure that implements the `{}` trait, \
 926                              but this closure only implements `{}`",
 927                             kind,
 928                             found_kind);
 929
 930                         err.span_label(
 931                             closure_span,
 932                             format!("this closure implements `{}`, not `{}`", found_kind, kind));
 933                         err.span_label(
 934                             obligation.cause.span,
 935                             format!("the requirement to implement `{}` derives from here", kind));
 936
 937                         // Additional context information explaining why the closure only implements
 938                         // a particular trait.
 939                         if let Some(tables) = self.in_progress_tables {
 940                             let tables = tables.borrow();
 941                             match (found_kind, tables.closure_kind_origins().get(hir_id)) {
 942                                 (ty::ClosureKind::FnOnce, Some((span, name))) => {
 943                                     err.span_label(*span, format!(
 944                                         "closure is `FnOnce` because it moves the \
 945                                          variable `{}` out of its environment", name));
 946                                 },
 947                                 (ty::ClosureKind::FnMut, Some((span, name))) => {
 948                                     err.span_label(*span, format!(
 949                                         "closure is `FnMut` because it mutates the \
 950                                          variable `{}` here", name));
 951                                 },
 952                                 _ => {}
 953                             }
 954                         }
 955
 956                         err.emit();
 957                         return;
 958                     }
 959
 960                     ty::Predicate::WellFormed(ty) => {
 961                         if !self.tcx.sess.opts.debugging_opts.chalk {
 962                             // WF predicates cannot themselves make
 963                             // errors. They can only block due to
 964                             // ambiguity; otherwise, they always
 965                             // degenerate into other obligations
 966                             // (which may fail).
 967                             span_bug!(span, "WF predicate not satisfied for {:?}", ty);
 968                         } else {
 969                             // FIXME: we'll need a better message which takes into account
 970                             // which bounds actually failed to hold.
 971                             self.tcx.sess.struct_span_err(
 972                                 span,
 973                                 &format!("the type `{}` is not well-formed (chalk)", ty)
 974                             )
 975                         }
 976                     }
 977
 978                     ty::Predicate::ConstEvaluatable(..) => {
 979                         // Errors for `ConstEvaluatable` predicates show up as
 980                         // `SelectionError::ConstEvalFailure`,
 981                         // not `Unimplemented`.
 982                         span_bug!(span,
 983                             "const-evaluatable requirement gave wrong error: `{:?}`", obligation)
 984                     }
 985                 }
 986             }
 987
 988             OutputTypeParameterMismatch(ref found_trait_ref, ref expected_trait_ref, _) => {
 989                 let found_trait_ref = self.resolve_vars_if_possible(&*found_trait_ref);
 990                 let expected_trait_ref = self.resolve_vars_if_possible(&*expected_trait_ref);
 991
 992                 if expected_trait_ref.self_ty().references_error() {
 993                     return;
 994                 }
 995
 996                 let found_trait_ty = found_trait_ref.self_ty();
 997
 998                 let found_did = match found_trait_ty.kind {
 999                     ty::Closure(did, _) | ty::Foreign(did) | ty::FnDef(did, _) => Some(did),
1000                     ty::Adt(def, _) => Some(def.did),
1001                     _ => None,
1002                 };
1003
1004                 let found_span = found_did.and_then(|did|
1005                     self.tcx.hir().span_if_local(did)
1006                 ).map(|sp| self.tcx.sess.source_map().def_span(sp)); // the sp could be an fn def
1007
1008                 if self.reported_closure_mismatch.borrow().contains(&(span, found_span)) {
1009                     // We check closures twice, with obligations flowing in different directions,
1010                     // but we want to complain about them only once.
1011                     return;
1012                 }
1013
1014                 self.reported_closure_mismatch.borrow_mut().insert((span, found_span));
1015
1016                 let found = match found_trait_ref.skip_binder().substs.type_at(1).kind {
1017                     ty::Tuple(ref tys) => vec![ArgKind::empty(); tys.len()],
1018                     _ => vec![ArgKind::empty()],
1019                 };
1020
1021                 let expected_ty = expected_trait_ref.skip_binder().substs.type_at(1);
1022                 let expected = match expected_ty.kind {
1023                     ty::Tuple(ref tys) => tys.iter()
1024                         .map(|t| ArgKind::from_expected_ty(t.expect_ty(), Some(span))).collect(),
1025                     _ => vec![ArgKind::Arg("_".to_owned(), expected_ty.to_string())],
1026                 };
1027
1028                 if found.len() == expected.len() {
1029                     self.report_closure_arg_mismatch(span,
1030                                                      found_span,
1031                                                      found_trait_ref,
1032                                                      expected_trait_ref)
1033                 } else {
1034                     let (closure_span, found) = found_did
1035                         .and_then(|did| self.tcx.hir().get_if_local(did))
1036                         .map(|node| {
1037                             let (found_span, found) = self.get_fn_like_arguments(node);
1038                             (Some(found_span), found)
1039                         }).unwrap_or((found_span, found));
1040
1041                     self.report_arg_count_mismatch(span,
1042                                                    closure_span,
1043                                                    expected,
1044                                                    found,
1045                                                    found_trait_ty.is_closure())
1046                 }
1047             }
1048
1049             TraitNotObjectSafe(did) => {
1050                 let violations = self.tcx.object_safety_violations(did);
1051                 self.tcx.report_object_safety_error(span, did, violations)
1052             }
1053
1054             // already reported in the query
1055             ConstEvalFailure(err) => {
1056                 self.tcx.sess.delay_span_bug(
1057                     span,
1058                     &format!("constant in type had an ignored error: {:?}", err),
1059                 );
1060                 return;
1061             }
1062
1063             Overflow => {
1064                 bug!("overflow should be handled before the `report_selection_error` path");
1065             }
1066         };
1067
1068         self.note_obligation_cause(&mut err, obligation);
1069
1070         err.emit();
1071     }
1072
1073     /// If the `Self` type of the unsatisfied trait `trait_ref` implements a trait
1074     /// with the same path as `trait_ref`, a help message about
1075     /// a probable version mismatch is added to `err`
1076     fn note_version_mismatch(
1077         &self,
1078         err: &mut DiagnosticBuilder<'_>,
1079         trait_ref: &ty::PolyTraitRef<'tcx>,
1080     ) {
1081         let get_trait_impl = |trait_def_id| {
1082             let mut trait_impl = None;
1083             self.tcx.for_each_relevant_impl(trait_def_id, trait_ref.self_ty(), |impl_def_id| {
1084                 if trait_impl.is_none() {
1085                     trait_impl = Some(impl_def_id);
1086                 }
1087             });
1088             trait_impl
1089         };
1090         let required_trait_path = self.tcx.def_path_str(trait_ref.def_id());
1091         let all_traits = self.tcx.all_traits(LOCAL_CRATE);
1092         let traits_with_same_path: std::collections::BTreeSet<_> = all_traits
1093             .iter()
1094             .filter(|trait_def_id| **trait_def_id != trait_ref.def_id())
1095             .filter(|trait_def_id| self.tcx.def_path_str(**trait_def_id) == required_trait_path)
1096             .collect();
1097         for trait_with_same_path in traits_with_same_path {
1098             if let Some(impl_def_id) = get_trait_impl(*trait_with_same_path) {
1099                 let impl_span = self.tcx.def_span(impl_def_id);
1100                 err.span_help(impl_span, "trait impl with same name found");
1101                 let trait_crate = self.tcx.crate_name(trait_with_same_path.krate);
1102                 let crate_msg = format!(
1103                     "Perhaps two different versions of crate `{}` are being used?",
1104                     trait_crate
1105                 );
1106                 err.note(&crate_msg);
1107             }
1108         }
1109     }
1110     fn suggest_restricting_param_bound(
1111         &self,
1112         mut err: &mut DiagnosticBuilder<'_>,
1113         trait_ref: &ty::PolyTraitRef<'_>,
1114         body_id: hir::HirId,
1115     ) {
1116         let self_ty = trait_ref.self_ty();
1117         let (param_ty, projection) = match &self_ty.kind {
1118             ty::Param(_) => (true, None),
1119             ty::Projection(projection) => (false, Some(projection)),
1120             _ => return,
1121         };
1122
1123         let suggest_restriction = |
1124             generics: &hir::Generics,
1125             msg,
1126             err: &mut DiagnosticBuilder<'_>,
1127         | {
1128             let span = generics.where_clause.span_for_predicates_or_empty_place();
1129             if !span.from_expansion() && span.desugaring_kind().is_none() {
1130                 err.span_suggestion(
1131                     generics.where_clause.span_for_predicates_or_empty_place().shrink_to_hi(),
1132                     &format!("consider further restricting {}", msg),
1133                     format!(
1134                         "{} {} ",
1135                         if !generics.where_clause.predicates.is_empty() {
1136                             ","
1137                         } else {
1138                             " where"
1139                         },
1140                         trait_ref.to_predicate(),
1141                     ),
1142                     Applicability::MachineApplicable,
1143                 );
1144             }
1145         };
1146
1147         // FIXME: Add check for trait bound that is already present, particularly `?Sized` so we
1148         //        don't suggest `T: Sized + ?Sized`.
1149         let mut hir_id = body_id;
1150         while let Some(node) = self.tcx.hir().find(hir_id) {
1151             match node {
1152                 hir::Node::TraitItem(hir::TraitItem {
1153                     generics,
1154                     kind: hir::TraitItemKind::Method(..), ..
1155                 }) if param_ty && self_ty == self.tcx.types.self_param => {
1156                     // Restricting `Self` for a single method.
1157                     suggest_restriction(&generics, "`Self`", err);
1158                     return;
1159                 }
1160
1161                 hir::Node::Item(hir::Item {
1162                     kind: hir::ItemKind::Fn(_, generics, _), ..
1163                 }) |
1164                 hir::Node::TraitItem(hir::TraitItem {
1165                     generics,
1166                     kind: hir::TraitItemKind::Method(..), ..
1167                 }) |
1168                 hir::Node::ImplItem(hir::ImplItem {
1169                     generics,
1170                     kind: hir::ImplItemKind::Method(..), ..
1171                 }) |
1172                 hir::Node::Item(hir::Item {
1173                     kind: hir::ItemKind::Trait(_, _, generics, _, _), ..
1174                 }) |
1175                 hir::Node::Item(hir::Item {
1176                     kind: hir::ItemKind::Impl(_, _, _, generics, ..), ..
1177                 }) if projection.is_some() => {
1178                     // Missing associated type bound.
1179                     suggest_restriction(&generics, "the associated type", err);
1180                     return;
1181                 }
1182
1183                 hir::Node::Item(hir::Item { kind: hir::ItemKind::Struct(_, generics), span, .. }) |
1184                 hir::Node::Item(hir::Item { kind: hir::ItemKind::Enum(_, generics), span, .. }) |
1185                 hir::Node::Item(hir::Item { kind: hir::ItemKind::Union(_, generics), span, .. }) |
1186                 hir::Node::Item(hir::Item {
1187                     kind: hir::ItemKind::Trait(_, _, generics, ..), span, ..
1188                 }) |
1189                 hir::Node::Item(hir::Item {
1190                     kind: hir::ItemKind::Impl(_, _, _, generics, ..), span, ..
1191                 }) |
1192                 hir::Node::Item(hir::Item {
1193                     kind: hir::ItemKind::Fn(_, generics, _), span, ..
1194                 }) |
1195                 hir::Node::Item(hir::Item {
1196                     kind: hir::ItemKind::TyAlias(_, generics), span, ..
1197                 }) |
1198                 hir::Node::Item(hir::Item {
1199                     kind: hir::ItemKind::TraitAlias(generics, _), span, ..
1200                 }) |
1201                 hir::Node::Item(hir::Item {
1202                     kind: hir::ItemKind::OpaqueTy(hir::OpaqueTy { generics, .. }), span, ..
1203                 }) |
1204                 hir::Node::TraitItem(hir::TraitItem { generics, span, .. }) |
1205                 hir::Node::ImplItem(hir::ImplItem { generics, span, .. })
1206                 if param_ty => {
1207                     // Missing generic type parameter bound.
1208                     let param_name = self_ty.to_string();
1209                     let constraint = trait_ref.print_only_trait_path().to_string();
1210                     if suggest_constraining_type_param(
1211                         generics,
1212                         &mut err,
1213                         &param_name,
1214                         &constraint,
1215                         self.tcx.sess.source_map(),
1216                         *span,
1217                     ) {
1218                         return;
1219                     }
1220                 }
1221
1222                 hir::Node::Crate => return,
1223
1224                 _ => {}
1225             }
1226
1227             hir_id = self.tcx.hir().get_parent_item(hir_id);
1228         }
1229     }
1230
1231     /// When encountering an assignment of an unsized trait, like `let x = ""[..];`, provide a
1232     /// suggestion to borrow the initializer in order to use have a slice instead.
1233     fn suggest_borrow_on_unsized_slice(
1234         &self,
1235         code: &ObligationCauseCode<'tcx>,
1236         err: &mut DiagnosticBuilder<'tcx>,
1237     ) {
1238         if let &ObligationCauseCode::VariableType(hir_id) = code {
1239             let parent_node = self.tcx.hir().get_parent_node(hir_id);
1240             if let Some(Node::Local(ref local)) = self.tcx.hir().find(parent_node) {
1241                 if let Some(ref expr) = local.init {
1242                     if let hir::ExprKind::Index(_, _) = expr.kind {
1243                         if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(expr.span) {
1244                             err.span_suggestion(
1245                                 expr.span,
1246                                 "consider borrowing here",
1247                                 format!("&{}", snippet),
1248                                 Applicability::MachineApplicable
1249                             );
1250                         }
1251                     }
1252                 }
1253             }
1254         }
1255     }
1256
1257     fn mk_obligation_for_def_id(
1258         &self,
1259         def_id: DefId,
1260         output_ty: Ty<'tcx>,
1261         cause: ObligationCause<'tcx>,
1262         param_env: ty::ParamEnv<'tcx>,
1263     ) -> PredicateObligation<'tcx> {
1264         let new_trait_ref = ty::TraitRef {
1265             def_id,
1266             substs: self.tcx.mk_substs_trait(output_ty, &[]),
1267         };
1268         Obligation::new(cause, param_env, new_trait_ref.to_predicate())
1269     }
1270
1271     /// Given a closure's `DefId`, return the given name of the closure.
1272     ///
1273     /// This doesn't account for reassignments, but it's only used for suggestions.
1274     fn get_closure_name(
1275         &self,
1276         def_id: DefId,
1277         err: &mut DiagnosticBuilder<'_>,
1278         msg: &str,
1279     ) -> Option<String> {
1280         let get_name = |err: &mut DiagnosticBuilder<'_>, kind: &hir::PatKind| -> Option<String> {
1281             // Get the local name of this closure. This can be inaccurate because
1282             // of the possibility of reassignment, but this should be good enough.
1283             match &kind {
1284                 hir::PatKind::Binding(hir::BindingAnnotation::Unannotated, _, name, None) => {
1285                     Some(format!("{}", name))
1286                 }
1287                 _ => {
1288                     err.note(&msg);
1289                     None
1290                 }
1291             }
1292         };
1293
1294         let hir = self.tcx.hir();
1295         let hir_id = hir.as_local_hir_id(def_id)?;
1296         let parent_node = hir.get_parent_node(hir_id);
1297         match hir.find(parent_node) {
1298             Some(hir::Node::Stmt(hir::Stmt {
1299                 kind: hir::StmtKind::Local(local), ..
1300             })) => get_name(err, &local.pat.kind),
1301             // Different to previous arm because one is `&hir::Local` and the other
1302             // is `P<hir::Local>`.
1303             Some(hir::Node::Local(local)) => get_name(err, &local.pat.kind),
1304             _ => return None,
1305         }
1306     }
1307
1308     /// We tried to apply the bound to an `fn` or closure. Check whether calling it would
1309     /// evaluate to a type that *would* satisfy the trait binding. If it would, suggest calling
1310     /// it: `bar(foo)` → `bar(foo())`. This case is *very* likely to be hit if `foo` is `async`.
1311     fn suggest_fn_call(
1312         &self,
1313         obligation: &PredicateObligation<'tcx>,
1314         err: &mut DiagnosticBuilder<'_>,
1315         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
1316         points_at_arg: bool,
1317     ) {
1318         let self_ty = trait_ref.self_ty();
1319         let (def_id, output_ty, callable) = match self_ty.kind {
1320             ty::Closure(def_id, substs) => {
1321                 (def_id, self.closure_sig(def_id, substs).output(), "closure")
1322             }
1323             ty::FnDef(def_id, _) => {
1324                 (def_id, self_ty.fn_sig(self.tcx).output(), "function")
1325             }
1326             _ => return,
1327         };
1328         let msg = format!("use parentheses to call the {}", callable);
1329
1330         let obligation = self.mk_obligation_for_def_id(
1331             trait_ref.def_id(),
1332             output_ty.skip_binder(),
1333             obligation.cause.clone(),
1334             obligation.param_env,
1335         );
1336
1337         match self.evaluate_obligation(&obligation) {
1338             Ok(EvaluationResult::EvaluatedToOk) |
1339             Ok(EvaluationResult::EvaluatedToOkModuloRegions) |
1340             Ok(EvaluationResult::EvaluatedToAmbig) => {}
1341             _ => return,
1342         }
1343         let hir = self.tcx.hir();
1344         // Get the name of the callable and the arguments to be used in the suggestion.
1345         let snippet = match hir.get_if_local(def_id) {
1346             Some(hir::Node::Expr(hir::Expr {
1347                 kind: hir::ExprKind::Closure(_, decl, _, span, ..),
1348                 ..
1349             })) => {
1350                 err.span_label(*span, "consider calling this closure");
1351                 let name = match self.get_closure_name(def_id, err, &msg) {
1352                     Some(name) => name,
1353                     None => return,
1354                 };
1355                 let args = decl.inputs.iter()
1356                     .map(|_| "_")
1357                     .collect::<Vec<_>>()
1358                     .join(", ");
1359                 format!("{}({})", name, args)
1360             }
1361             Some(hir::Node::Item(hir::Item {
1362                 ident,
1363                 kind: hir::ItemKind::Fn(.., body_id),
1364                 ..
1365             })) => {
1366                 err.span_label(ident.span, "consider calling this function");
1367                 let body = hir.body(*body_id);
1368                 let args = body.params.iter()
1369                     .map(|arg| match &arg.pat.kind {
1370                         hir::PatKind::Binding(_, _, ident, None)
1371                         // FIXME: provide a better suggestion when encountering `SelfLower`, it
1372                         // should suggest a method call.
1373                         if ident.name != kw::SelfLower => ident.to_string(),
1374                         _ => "_".to_string(),
1375                     })
1376                     .collect::<Vec<_>>()
1377                     .join(", ");
1378                 format!("{}({})", ident, args)
1379             }
1380             _ => return,
1381         };
1382         if points_at_arg {
1383             // When the obligation error has been ensured to have been caused by
1384             // an argument, the `obligation.cause.span` points at the expression
1385             // of the argument, so we can provide a suggestion. This is signaled
1386             // by `points_at_arg`. Otherwise, we give a more general note.
1387             err.span_suggestion(
1388                 obligation.cause.span,
1389                 &msg,
1390                 snippet,
1391                 Applicability::HasPlaceholders,
1392             );
1393         } else {
1394             err.help(&format!("{}: `{}`", msg, snippet));
1395         }
1396     }
1397
1398     fn suggest_add_reference_to_arg(
1399         &self,
1400         obligation: &PredicateObligation<'tcx>,
1401         err: &mut DiagnosticBuilder<'tcx>,
1402         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
1403         points_at_arg: bool,
1404         has_custom_message: bool,
1405     ) -> bool {
1406         if !points_at_arg {
1407             return false;
1408         }
1409
1410         let span = obligation.cause.span;
1411         let param_env = obligation.param_env;
1412         let trait_ref = trait_ref.skip_binder();
1413
1414         if let ObligationCauseCode::ImplDerivedObligation(obligation) = &obligation.cause.code {
1415             // Try to apply the original trait binding obligation by borrowing.
1416             let self_ty = trait_ref.self_ty();
1417             let found = self_ty.to_string();
1418             let new_self_ty = self.tcx.mk_imm_ref(self.tcx.lifetimes.re_static, self_ty);
1419             let substs = self.tcx.mk_substs_trait(new_self_ty, &[]);
1420             let new_trait_ref = ty::TraitRef::new(obligation.parent_trait_ref.def_id(), substs);
1421             let new_obligation = Obligation::new(
1422                 ObligationCause::dummy(),
1423                 param_env,
1424                 new_trait_ref.to_predicate(),
1425             );
1426             if self.predicate_must_hold_modulo_regions(&new_obligation) {
1427                 if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
1428                     // We have a very specific type of error, where just borrowing this argument
1429                     // might solve the problem. In cases like this, the important part is the
1430                     // original type obligation, not the last one that failed, which is arbitrary.
1431                     // Because of this, we modify the error to refer to the original obligation and
1432                     // return early in the caller.
1433                     let msg = format!(
1434                         "the trait bound `{}: {}` is not satisfied",
1435                         found,
1436                         obligation.parent_trait_ref.skip_binder().print_only_trait_path(),
1437                     );
1438                     if has_custom_message {
1439                         err.note(&msg);
1440                     } else {
1441                         err.message = vec![(msg, Style::NoStyle)];
1442                     }
1443                     if snippet.starts_with('&') {
1444                         // This is already a literal borrow and the obligation is failing
1445                         // somewhere else in the obligation chain. Do not suggest non-sense.
1446                         return false;
1447                     }
1448                     err.span_label(span, &format!(
1449                         "expected an implementor of trait `{}`",
1450                         obligation.parent_trait_ref.skip_binder().print_only_trait_path(),
1451                     ));
1452                     err.span_suggestion(
1453                         span,
1454                         "consider borrowing here",
1455                         format!("&{}", snippet),
1456                         Applicability::MaybeIncorrect,
1457                     );
1458                     return true;
1459                 }
1460             }
1461         }
1462         false
1463     }
1464
1465     /// Whenever references are used by mistake, like `for (i, e) in &vec.iter().enumerate()`,
1466     /// suggest removing these references until we reach a type that implements the trait.
1467     fn suggest_remove_reference(
1468         &self,
1469         obligation: &PredicateObligation<'tcx>,
1470         err: &mut DiagnosticBuilder<'tcx>,
1471         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
1472     ) {
1473         let trait_ref = trait_ref.skip_binder();
1474         let span = obligation.cause.span;
1475
1476         if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
1477             let refs_number = snippet.chars()
1478                 .filter(|c| !c.is_whitespace())
1479                 .take_while(|c| *c == '&')
1480                 .count();
1481             if let Some('\'') = snippet.chars()
1482                 .filter(|c| !c.is_whitespace())
1483                 .skip(refs_number)
1484                 .next()
1485             { // Do not suggest removal of borrow from type arguments.
1486                 return;
1487             }
1488
1489             let mut trait_type = trait_ref.self_ty();
1490
1491             for refs_remaining in 0..refs_number {
1492                 if let ty::Ref(_, t_type, _) = trait_type.kind {
1493                     trait_type = t_type;
1494
1495                     let new_obligation = self.mk_obligation_for_def_id(
1496                         trait_ref.def_id,
1497                         trait_type,
1498                         ObligationCause::dummy(),
1499                         obligation.param_env,
1500                     );
1501
1502                     if self.predicate_may_hold(&new_obligation) {
1503                         let sp = self.tcx.sess.source_map()
1504                             .span_take_while(span, |c| c.is_whitespace() || *c == '&');
1505
1506                         let remove_refs = refs_remaining + 1;
1507                         let format_str = format!("consider removing {} leading `&`-references",
1508                                                  remove_refs);
1509
1510                         err.span_suggestion_short(
1511                             sp, &format_str, String::new(), Applicability::MachineApplicable
1512                         );
1513                         break;
1514                     }
1515                 } else {
1516                     break;
1517                 }
1518             }
1519         }
1520     }
1521
1522     /// Check if the trait bound is implemented for a different mutability and note it in the
1523     /// final error.
1524     fn suggest_change_mut(
1525         &self,
1526         obligation: &PredicateObligation<'tcx>,
1527         err: &mut DiagnosticBuilder<'tcx>,
1528         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
1529         points_at_arg: bool,
1530     ) {
1531         let span = obligation.cause.span;
1532         if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
1533             let refs_number = snippet.chars()
1534                 .filter(|c| !c.is_whitespace())
1535                 .take_while(|c| *c == '&')
1536                 .count();
1537             if let Some('\'') = snippet.chars()
1538                 .filter(|c| !c.is_whitespace())
1539                 .skip(refs_number)
1540                 .next()
1541             { // Do not suggest removal of borrow from type arguments.
1542                 return;
1543             }
1544             let trait_ref = self.resolve_vars_if_possible(trait_ref);
1545             if trait_ref.has_infer_types() {
1546                 // Do not ICE while trying to find if a reborrow would succeed on a trait with
1547                 // unresolved bindings.
1548                 return;
1549             }
1550
1551             if let ty::Ref(region, t_type, mutability) = trait_ref.skip_binder().self_ty().kind {
1552                 let trait_type = match mutability {
1553                     hir::Mutability::Mutable => self.tcx.mk_imm_ref(region, t_type),
1554                     hir::Mutability::Immutable => self.tcx.mk_mut_ref(region, t_type),
1555                 };
1556
1557                 let new_obligation = self.mk_obligation_for_def_id(
1558                     trait_ref.skip_binder().def_id,
1559                     trait_type,
1560                     ObligationCause::dummy(),
1561                     obligation.param_env,
1562                 );
1563
1564                 if self.evaluate_obligation_no_overflow(
1565                     &new_obligation,
1566                 ).must_apply_modulo_regions() {
1567                     let sp = self.tcx.sess.source_map()
1568                         .span_take_while(span, |c| c.is_whitespace() || *c == '&');
1569                     if points_at_arg &&
1570                         mutability == hir::Mutability::Immutable &&
1571                         refs_number > 0
1572                     {
1573                         err.span_suggestion(
1574                             sp,
1575                             "consider changing this borrow's mutability",
1576                             "&mut ".to_string(),
1577                             Applicability::MachineApplicable,
1578                         );
1579                     } else {
1580                         err.note(&format!(
1581                             "`{}` is implemented for `{:?}`, but not for `{:?}`",
1582                             trait_ref.print_only_trait_path(),
1583                             trait_type,
1584                             trait_ref.skip_binder().self_ty(),
1585                         ));
1586                     }
1587                 }
1588             }
1589         }
1590     }
1591
1592     fn suggest_semicolon_removal(
1593         &self,
1594         obligation: &PredicateObligation<'tcx>,
1595         err: &mut DiagnosticBuilder<'tcx>,
1596         span: Span,
1597         trait_ref: &ty::Binder<ty::TraitRef<'tcx>>,
1598     ) {
1599         let hir = self.tcx.hir();
1600         let parent_node = hir.get_parent_node(obligation.cause.body_id);
1601         let node = hir.find(parent_node);
1602         if let Some(hir::Node::Item(hir::Item {
1603             kind: hir::ItemKind::Fn(sig, _, body_id),
1604             ..
1605         })) = node {
1606             let body = hir.body(*body_id);
1607             if let hir::ExprKind::Block(blk, _) = &body.value.kind {
1608                 if sig.decl.output.span().overlaps(span) && blk.expr.is_none() &&
1609                     "()" == &trait_ref.self_ty().to_string()
1610                 {
1611                     // FIXME(estebank): When encountering a method with a trait
1612                     // bound not satisfied in the return type with a body that has
1613                     // no return, suggest removal of semicolon on last statement.
1614                     // Once that is added, close #54771.
1615                     if let Some(ref stmt) = blk.stmts.last() {
1616                         let sp = self.tcx.sess.source_map().end_point(stmt.span);
1617                         err.span_label(sp, "consider removing this semicolon");
1618                     }
1619                 }
1620             }
1621         }
1622     }
1623
1624     /// Given some node representing a fn-like thing in the HIR map,
1625     /// returns a span and `ArgKind` information that describes the
1626     /// arguments it expects. This can be supplied to
1627     /// `report_arg_count_mismatch`.
1628     pub fn get_fn_like_arguments(&self, node: Node<'_>) -> (Span, Vec<ArgKind>) {
1629         match node {
1630             Node::Expr(&hir::Expr {
1631                 kind: hir::ExprKind::Closure(_, ref _decl, id, span, _),
1632                 ..
1633             }) => {
1634                 (self.tcx.sess.source_map().def_span(span),
1635                  self.tcx.hir().body(id).params.iter()
1636                     .map(|arg| {
1637                         if let hir::Pat {
1638                             kind: hir::PatKind::Tuple(ref args, _),
1639                             span,
1640                             ..
1641                         } = *arg.pat {
1642                             ArgKind::Tuple(
1643                                 Some(span),
1644                                 args.iter().map(|pat| {
1645                                     let snippet = self.tcx.sess.source_map()
1646                                         .span_to_snippet(pat.span).unwrap();
1647                                     (snippet, "_".to_owned())
1648                                 }).collect::<Vec<_>>(),
1649                             )
1650                         } else {
1651                             let name = self.tcx.sess.source_map()
1652                                 .span_to_snippet(arg.pat.span).unwrap();
1653                             ArgKind::Arg(name, "_".to_owned())
1654                         }
1655                     })
1656                     .collect::<Vec<ArgKind>>())
1657             }
1658             Node::Item(&hir::Item {
1659                 span,
1660                 kind: hir::ItemKind::Fn(ref sig, ..),
1661                 ..
1662             }) |
1663             Node::ImplItem(&hir::ImplItem {
1664                 span,
1665                 kind: hir::ImplItemKind::Method(ref sig, _),
1666                 ..
1667             }) |
1668             Node::TraitItem(&hir::TraitItem {
1669                 span,
1670                 kind: hir::TraitItemKind::Method(ref sig, _),
1671                 ..
1672             }) => {
1673                 (self.tcx.sess.source_map().def_span(span), sig.decl.inputs.iter()
1674                         .map(|arg| match arg.clone().kind {
1675                     hir::TyKind::Tup(ref tys) => ArgKind::Tuple(
1676                         Some(arg.span),
1677                         vec![("_".to_owned(), "_".to_owned()); tys.len()]
1678                     ),
1679                     _ => ArgKind::empty()
1680                 }).collect::<Vec<ArgKind>>())
1681             }
1682             Node::Ctor(ref variant_data) => {
1683                 let span = variant_data.ctor_hir_id()
1684                     .map(|hir_id| self.tcx.hir().span(hir_id))
1685                     .unwrap_or(DUMMY_SP);
1686                 let span = self.tcx.sess.source_map().def_span(span);
1687
1688                 (span, vec![ArgKind::empty(); variant_data.fields().len()])
1689             }
1690             _ => panic!("non-FnLike node found: {:?}", node),
1691         }
1692     }
1693
1694     /// Reports an error when the number of arguments needed by a
1695     /// trait match doesn't match the number that the expression
1696     /// provides.
1697     pub fn report_arg_count_mismatch(
1698         &self,
1699         span: Span,
1700         found_span: Option<Span>,
1701         expected_args: Vec<ArgKind>,
1702         found_args: Vec<ArgKind>,
1703         is_closure: bool,
1704     ) -> DiagnosticBuilder<'tcx> {
1705         let kind = if is_closure { "closure" } else { "function" };
1706
1707         let args_str = |arguments: &[ArgKind], other: &[ArgKind]| {
1708             let arg_length = arguments.len();
1709             let distinct = match &other[..] {
1710                 &[ArgKind::Tuple(..)] => true,
1711                 _ => false,
1712             };
1713             match (arg_length, arguments.get(0)) {
1714                 (1, Some(&ArgKind::Tuple(_, ref fields))) => {
1715                     format!("a single {}-tuple as argument", fields.len())
1716                 }
1717                 _ => format!("{} {}argument{}",
1718                              arg_length,
1719                              if distinct && arg_length > 1 { "distinct " } else { "" },
1720                              pluralize!(arg_length))
1721             }
1722         };
1723
1724         let expected_str = args_str(&expected_args, &found_args);
1725         let found_str = args_str(&found_args, &expected_args);
1726
1727         let mut err = struct_span_err!(
1728             self.tcx.sess,
1729             span,
1730             E0593,
1731             "{} is expected to take {}, but it takes {}",
1732             kind,
1733             expected_str,
1734             found_str,
1735         );
1736
1737         err.span_label(span, format!("expected {} that takes {}", kind, expected_str));
1738
1739         if let Some(found_span) = found_span {
1740             err.span_label(found_span, format!("takes {}", found_str));
1741
1742             // move |_| { ... }
1743             // ^^^^^^^^-- def_span
1744             //
1745             // move |_| { ... }
1746             // ^^^^^-- prefix
1747             let prefix_span = self.tcx.sess.source_map().span_until_non_whitespace(found_span);
1748             // move |_| { ... }
1749             //      ^^^-- pipe_span
1750             let pipe_span = if let Some(span) = found_span.trim_start(prefix_span) {
1751                 span
1752             } else {
1753                 found_span
1754             };
1755
1756             // Suggest to take and ignore the arguments with expected_args_length `_`s if
1757             // found arguments is empty (assume the user just wants to ignore args in this case).
1758             // For example, if `expected_args_length` is 2, suggest `|_, _|`.
1759             if found_args.is_empty() && is_closure {
1760                 let underscores = vec!["_"; expected_args.len()].join(", ");
1761                 err.span_suggestion(
1762                     pipe_span,
1763                     &format!(
1764                         "consider changing the closure to take and ignore the expected argument{}",
1765                         if expected_args.len() < 2 {
1766                             ""
1767                         } else {
1768                             "s"
1769                         }
1770                     ),
1771                     format!("|{}|", underscores),
1772                     Applicability::MachineApplicable,
1773                 );
1774             }
1775
1776             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
1777                 if fields.len() == expected_args.len() {
1778                     let sugg = fields.iter()
1779                         .map(|(name, _)| name.to_owned())
1780                         .collect::<Vec<String>>()
1781                         .join(", ");
1782                     err.span_suggestion(
1783                         found_span,
1784                         "change the closure to take multiple arguments instead of a single tuple",
1785                         format!("|{}|", sugg),
1786                         Applicability::MachineApplicable,
1787                     );
1788                 }
1789             }
1790             if let &[ArgKind::Tuple(_, ref fields)] = &expected_args[..] {
1791                 if fields.len() == found_args.len() && is_closure {
1792                     let sugg = format!(
1793                         "|({}){}|",
1794                         found_args.iter()
1795                             .map(|arg| match arg {
1796                                 ArgKind::Arg(name, _) => name.to_owned(),
1797                                 _ => "_".to_owned(),
1798                             })
1799                             .collect::<Vec<String>>()
1800                             .join(", "),
1801                         // add type annotations if available
1802                         if found_args.iter().any(|arg| match arg {
1803                             ArgKind::Arg(_, ty) => ty != "_",
1804                             _ => false,
1805                         }) {
1806                             format!(": ({})",
1807                                     fields.iter()
1808                                         .map(|(_, ty)| ty.to_owned())
1809                                         .collect::<Vec<String>>()
1810                                         .join(", "))
1811                         } else {
1812                             String::new()
1813                         },
1814                     );
1815                     err.span_suggestion(
1816                         found_span,
1817                         "change the closure to accept a tuple instead of individual arguments",
1818                         sugg,
1819                         Applicability::MachineApplicable,
1820                     );
1821                 }
1822             }
1823         }
1824
1825         err
1826     }
1827
1828     fn report_closure_arg_mismatch(
1829         &self,
1830         span: Span,
1831         found_span: Option<Span>,
1832         expected_ref: ty::PolyTraitRef<'tcx>,
1833         found: ty::PolyTraitRef<'tcx>,
1834     ) -> DiagnosticBuilder<'tcx> {
1835         fn build_fn_sig_string<'tcx>(tcx: TyCtxt<'tcx>, trait_ref: &ty::TraitRef<'tcx>) -> String {
1836             let inputs = trait_ref.substs.type_at(1);
1837             let sig = if let ty::Tuple(inputs) = inputs.kind {
1838                 tcx.mk_fn_sig(
1839                     inputs.iter().map(|k| k.expect_ty()),
1840                     tcx.mk_ty_infer(ty::TyVar(ty::TyVid { index: 0 })),
1841                     false,
1842                     hir::Unsafety::Normal,
1843                     ::rustc_target::spec::abi::Abi::Rust
1844                 )
1845             } else {
1846                 tcx.mk_fn_sig(
1847                     ::std::iter::once(inputs),
1848                     tcx.mk_ty_infer(ty::TyVar(ty::TyVid { index: 0 })),
1849                     false,
1850                     hir::Unsafety::Normal,
1851                     ::rustc_target::spec::abi::Abi::Rust
1852                 )
1853             };
1854             ty::Binder::bind(sig).to_string()
1855         }
1856
1857         let argument_is_closure = expected_ref.skip_binder().substs.type_at(0).is_closure();
1858         let mut err = struct_span_err!(self.tcx.sess, span, E0631,
1859                                        "type mismatch in {} arguments",
1860                                        if argument_is_closure { "closure" } else { "function" });
1861
1862         let found_str = format!(
1863             "expected signature of `{}`",
1864             build_fn_sig_string(self.tcx, found.skip_binder())
1865         );
1866         err.span_label(span, found_str);
1867
1868         let found_span = found_span.unwrap_or(span);
1869         let expected_str = format!(
1870             "found signature of `{}`",
1871             build_fn_sig_string(self.tcx, expected_ref.skip_binder())
1872         );
1873         err.span_label(found_span, expected_str);
1874
1875         err
1876     }
1877 }
1878
1879 impl<'tcx> TyCtxt<'tcx> {
1880     pub fn recursive_type_with_infinite_size_error(self,
1881                                                    type_def_id: DefId)
1882                                                    -> DiagnosticBuilder<'tcx>
1883     {
1884         assert!(type_def_id.is_local());
1885         let span = self.hir().span_if_local(type_def_id).unwrap();
1886         let span = self.sess.source_map().def_span(span);
1887         let mut err = struct_span_err!(self.sess, span, E0072,
1888                                        "recursive type `{}` has infinite size",
1889                                        self.def_path_str(type_def_id));
1890         err.span_label(span, "recursive type has infinite size");
1891         err.help(&format!("insert indirection (e.g., a `Box`, `Rc`, or `&`) \
1892                            at some point to make `{}` representable",
1893                           self.def_path_str(type_def_id)));
1894         err
1895     }
1896
1897     pub fn report_object_safety_error(
1898         self,
1899         span: Span,
1900         trait_def_id: DefId,
1901         violations: Vec<ObjectSafetyViolation>,
1902     ) -> DiagnosticBuilder<'tcx> {
1903         let trait_str = self.def_path_str(trait_def_id);
1904         let span = self.sess.source_map().def_span(span);
1905         let mut err = struct_span_err!(
1906             self.sess, span, E0038,
1907             "the trait `{}` cannot be made into an object",
1908             trait_str);
1909         err.span_label(span, format!("the trait `{}` cannot be made into an object", trait_str));
1910
1911         let mut reported_violations = FxHashSet::default();
1912         for violation in violations {
1913             if reported_violations.insert(violation.clone()) {
1914                 match violation.span() {
1915                     Some(span) => err.span_label(span, violation.error_msg()),
1916                     None => err.note(&violation.error_msg()),
1917                 };
1918             }
1919         }
1920
1921         if self.sess.trait_methods_not_found.borrow().contains(&span) {
1922             // Avoid emitting error caused by non-existing method (#58734)
1923             err.cancel();
1924         }
1925
1926         err
1927     }
1928 }
1929
1930 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
1931     fn maybe_report_ambiguity(
1932         &self,
1933         obligation: &PredicateObligation<'tcx>,
1934         body_id: Option<hir::BodyId>,
1935     ) {
1936         // Unable to successfully determine, probably means
1937         // insufficient type information, but could mean
1938         // ambiguous impls. The latter *ought* to be a
1939         // coherence violation, so we don't report it here.
1940
1941         let predicate = self.resolve_vars_if_possible(&obligation.predicate);
1942         let span = obligation.cause.span;
1943
1944         debug!(
1945             "maybe_report_ambiguity(predicate={:?}, obligation={:?} body_id={:?}, code={:?})",
1946             predicate,
1947             obligation,
1948             body_id,
1949             obligation.cause.code,
1950         );
1951
1952         // Ambiguity errors are often caused as fallout from earlier
1953         // errors. So just ignore them if this infcx is tainted.
1954         if self.is_tainted_by_errors() {
1955             return;
1956         }
1957
1958         match predicate {
1959             ty::Predicate::Trait(ref data) => {
1960                 let trait_ref = data.to_poly_trait_ref();
1961                 let self_ty = trait_ref.self_ty();
1962                 debug!("self_ty {:?} {:?} trait_ref {:?}", self_ty, self_ty.kind, trait_ref);
1963
1964                 if predicate.references_error() {
1965                     return;
1966                 }
1967                 // Typically, this ambiguity should only happen if
1968                 // there are unresolved type inference variables
1969                 // (otherwise it would suggest a coherence
1970                 // failure). But given #21974 that is not necessarily
1971                 // the case -- we can have multiple where clauses that
1972                 // are only distinguished by a region, which results
1973                 // in an ambiguity even when all types are fully
1974                 // known, since we don't dispatch based on region
1975                 // relationships.
1976
1977                 // This is kind of a hack: it frequently happens that some earlier
1978                 // error prevents types from being fully inferred, and then we get
1979                 // a bunch of uninteresting errors saying something like "<generic
1980                 // #0> doesn't implement Sized".  It may even be true that we
1981                 // could just skip over all checks where the self-ty is an
1982                 // inference variable, but I was afraid that there might be an
1983                 // inference variable created, registered as an obligation, and
1984                 // then never forced by writeback, and hence by skipping here we'd
1985                 // be ignoring the fact that we don't KNOW the type works
1986                 // out. Though even that would probably be harmless, given that
1987                 // we're only talking about builtin traits, which are known to be
1988                 // inhabited. We used to check for `self.tcx.sess.has_errors()` to
1989                 // avoid inundating the user with unnecessary errors, but we now
1990                 // check upstream for type errors and dont add the obligations to
1991                 // begin with in those cases.
1992                 if
1993                     self.tcx.lang_items().sized_trait()
1994                     .map_or(false, |sized_id| sized_id == trait_ref.def_id())
1995                 {
1996                     self.need_type_info_err(body_id, span, self_ty).emit();
1997                 } else {
1998                     let mut err = struct_span_err!(
1999                         self.tcx.sess,
2000                         span,
2001                         E0283,
2002                         "type annotations needed: cannot resolve `{}`",
2003                         predicate,
2004                     );
2005                     self.note_obligation_cause(&mut err, obligation);
2006                     err.emit();
2007                 }
2008             }
2009
2010             ty::Predicate::WellFormed(ty) => {
2011                 // Same hacky approach as above to avoid deluging user
2012                 // with error messages.
2013                 if !ty.references_error() && !self.tcx.sess.has_errors() {
2014                     self.need_type_info_err(body_id, span, ty).emit();
2015                 }
2016             }
2017
2018             ty::Predicate::Subtype(ref data) => {
2019                 if data.references_error() || self.tcx.sess.has_errors() {
2020                     // no need to overload user in such cases
2021                 } else {
2022                     let &SubtypePredicate { a_is_expected: _, a, b } = data.skip_binder();
2023                     // both must be type variables, or the other would've been instantiated
2024                     assert!(a.is_ty_var() && b.is_ty_var());
2025                     self.need_type_info_err(body_id,
2026                                             obligation.cause.span,
2027                                             a).emit();
2028                 }
2029             }
2030
2031             _ => {
2032                 if !self.tcx.sess.has_errors() {
2033                     let mut err = struct_span_err!(
2034                         self.tcx.sess,
2035                         obligation.cause.span,
2036                         E0284,
2037                         "type annotations needed: cannot resolve `{}`",
2038                         predicate,
2039                     );
2040                     self.note_obligation_cause(&mut err, obligation);
2041                     err.emit();
2042                 }
2043             }
2044         }
2045     }
2046
2047     /// Returns `true` if the trait predicate may apply for *some* assignment
2048     /// to the type parameters.
2049     fn predicate_can_apply(
2050         &self,
2051         param_env: ty::ParamEnv<'tcx>,
2052         pred: ty::PolyTraitRef<'tcx>,
2053     ) -> bool {
2054         struct ParamToVarFolder<'a, 'tcx> {
2055             infcx: &'a InferCtxt<'a, 'tcx>,
2056             var_map: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
2057         }
2058
2059         impl<'a, 'tcx> TypeFolder<'tcx> for ParamToVarFolder<'a, 'tcx> {
2060             fn tcx<'b>(&'b self) -> TyCtxt<'tcx> { self.infcx.tcx }
2061
2062             fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
2063                 if let ty::Param(ty::ParamTy {name, .. }) = ty.kind {
2064                     let infcx = self.infcx;
2065                     self.var_map.entry(ty).or_insert_with(||
2066                         infcx.next_ty_var(
2067                             TypeVariableOrigin {
2068                                 kind: TypeVariableOriginKind::TypeParameterDefinition(name),
2069                                 span: DUMMY_SP,
2070                             }
2071                         )
2072                     )
2073                 } else {
2074                     ty.super_fold_with(self)
2075                 }
2076             }
2077         }
2078
2079         self.probe(|_| {
2080             let mut selcx = SelectionContext::new(self);
2081
2082             let cleaned_pred = pred.fold_with(&mut ParamToVarFolder {
2083                 infcx: self,
2084                 var_map: Default::default()
2085             });
2086
2087             let cleaned_pred = super::project::normalize(
2088                 &mut selcx,
2089                 param_env,
2090                 ObligationCause::dummy(),
2091                 &cleaned_pred
2092             ).value;
2093
2094             let obligation = Obligation::new(
2095                 ObligationCause::dummy(),
2096                 param_env,
2097                 cleaned_pred.to_predicate()
2098             );
2099
2100             self.predicate_may_hold(&obligation)
2101         })
2102     }
2103
2104     fn note_obligation_cause(
2105         &self,
2106         err: &mut DiagnosticBuilder<'_>,
2107         obligation: &PredicateObligation<'tcx>,
2108     ) {
2109         // First, attempt to add note to this error with an async-await-specific
2110         // message, and fall back to regular note otherwise.
2111         if !self.note_obligation_cause_for_async_await(err, obligation) {
2112             self.note_obligation_cause_code(err, &obligation.predicate, &obligation.cause.code,
2113                                             &mut vec![]);
2114         }
2115     }
2116
2117     /// Adds an async-await specific note to the diagnostic:
2118     ///
2119     /// ```ignore (diagnostic)
2120     /// note: future does not implement `std::marker::Send` because this value is used across an
2121     ///       await
2122     ///   --> $DIR/issue-64130-non-send-future-diags.rs:15:5
2123     ///    |
2124     /// LL |     let g = x.lock().unwrap();
2125     ///    |         - has type `std::sync::MutexGuard<'_, u32>`
2126     /// LL |     baz().await;
2127     ///    |     ^^^^^^^^^^^ await occurs here, with `g` maybe used later
2128     /// LL | }
2129     ///    | - `g` is later dropped here
2130     /// ```
2131     ///
2132     /// Returns `true` if an async-await specific note was added to the diagnostic.
2133     fn note_obligation_cause_for_async_await(
2134         &self,
2135         err: &mut DiagnosticBuilder<'_>,
2136         obligation: &PredicateObligation<'tcx>,
2137     ) -> bool {
2138         debug!("note_obligation_cause_for_async_await: obligation.predicate={:?} \
2139                 obligation.cause.span={:?}", obligation.predicate, obligation.cause.span);
2140         let source_map = self.tcx.sess.source_map();
2141
2142         // Look into the obligation predicate to determine the type in the generator which meant
2143         // that the predicate was not satisifed.
2144         let (trait_ref, target_ty) = match obligation.predicate {
2145             ty::Predicate::Trait(trait_predicate) =>
2146                 (trait_predicate.skip_binder().trait_ref, trait_predicate.skip_binder().self_ty()),
2147             _ => return false,
2148         };
2149         debug!("note_obligation_cause_for_async_await: target_ty={:?}", target_ty);
2150
2151         // Attempt to detect an async-await error by looking at the obligation causes, looking
2152         // for only generators, generator witnesses, opaque types or `std::future::GenFuture` to
2153         // be present.
2154         //
2155         // When a future does not implement a trait because of a captured type in one of the
2156         // generators somewhere in the call stack, then the result is a chain of obligations.
2157         // Given a `async fn` A that calls a `async fn` B which captures a non-send type and that
2158         // future is passed as an argument to a function C which requires a `Send` type, then the
2159         // chain looks something like this:
2160         //
2161         // - `BuiltinDerivedObligation` with a generator witness (B)
2162         // - `BuiltinDerivedObligation` with a generator (B)
2163         // - `BuiltinDerivedObligation` with `std::future::GenFuture` (B)
2164         // - `BuiltinDerivedObligation` with `impl std::future::Future` (B)
2165         // - `BuiltinDerivedObligation` with `impl std::future::Future` (B)
2166         // - `BuiltinDerivedObligation` with a generator witness (A)
2167         // - `BuiltinDerivedObligation` with a generator (A)
2168         // - `BuiltinDerivedObligation` with `std::future::GenFuture` (A)
2169         // - `BuiltinDerivedObligation` with `impl std::future::Future` (A)
2170         // - `BuiltinDerivedObligation` with `impl std::future::Future` (A)
2171         // - `BindingObligation` with `impl_send (Send requirement)
2172         //
2173         // The first obligations in the chain can be used to get the details of the type that is
2174         // captured but the entire chain must be inspected to detect this case.
2175         let mut generator = None;
2176         let mut next_code = Some(&obligation.cause.code);
2177         while let Some(code) = next_code {
2178             debug!("note_obligation_cause_for_async_await: code={:?}", code);
2179             match code {
2180                 ObligationCauseCode::BuiltinDerivedObligation(derived_obligation) |
2181                 ObligationCauseCode::ImplDerivedObligation(derived_obligation) => {
2182                     debug!("note_obligation_cause_for_async_await: self_ty.kind={:?}",
2183                            derived_obligation.parent_trait_ref.self_ty().kind);
2184                     match derived_obligation.parent_trait_ref.self_ty().kind {
2185                         ty::Adt(ty::AdtDef { did, .. }, ..) if
2186                             self.tcx.is_diagnostic_item(sym::gen_future, *did) => {},
2187                         ty::Generator(did, ..) => generator = generator.or(Some(did)),
2188                         ty::GeneratorWitness(_) | ty::Opaque(..) => {},
2189                         _ => return false,
2190                     }
2191
2192                     next_code = Some(derived_obligation.parent_code.as_ref());
2193                 },
2194                 ObligationCauseCode::ItemObligation(_) | ObligationCauseCode::BindingObligation(..)
2195                     if generator.is_some() => break,
2196                 _ => return false,
2197             }
2198         }
2199
2200         let generator_did = generator.expect("can only reach this if there was a generator");
2201
2202         // Only continue to add a note if the generator is from an `async` function.
2203         let parent_node = self.tcx.parent(generator_did)
2204             .and_then(|parent_did| self.tcx.hir().get_if_local(parent_did));
2205         debug!("note_obligation_cause_for_async_await: parent_node={:?}", parent_node);
2206         if let Some(hir::Node::Item(hir::Item {
2207             kind: hir::ItemKind::Fn(sig, _, _),
2208             ..
2209         })) = parent_node {
2210             debug!("note_obligation_cause_for_async_await: header={:?}", sig.header);
2211             if sig.header.asyncness != hir::IsAsync::Async {
2212                 return false;
2213             }
2214         }
2215
2216         let span = self.tcx.def_span(generator_did);
2217         let tables = self.tcx.typeck_tables_of(generator_did);
2218         debug!("note_obligation_cause_for_async_await: generator_did={:?} span={:?} ",
2219                generator_did, span);
2220
2221         // Look for a type inside the generator interior that matches the target type to get
2222         // a span.
2223         let target_span = tables.generator_interior_types.iter()
2224             .find(|ty::GeneratorInteriorTypeCause { ty, .. }| ty::TyS::same_type(*ty, target_ty))
2225             .map(|ty::GeneratorInteriorTypeCause { span, scope_span, .. }|
2226                  (span, source_map.span_to_snippet(*span), scope_span));
2227         if let Some((target_span, Ok(snippet), scope_span)) = target_span {
2228             // Look at the last interior type to get a span for the `.await`.
2229             let await_span = tables.generator_interior_types.iter().map(|i| i.span).last().unwrap();
2230             let mut span = MultiSpan::from_span(await_span);
2231             span.push_span_label(
2232                 await_span, format!("await occurs here, with `{}` maybe used later", snippet));
2233
2234             span.push_span_label(*target_span, format!("has type `{}`", target_ty));
2235
2236             // If available, use the scope span to annotate the drop location.
2237             if let Some(scope_span) = scope_span {
2238                 span.push_span_label(
2239                     source_map.end_point(*scope_span),
2240                     format!("`{}` is later dropped here", snippet),
2241                 );
2242             }
2243
2244             err.span_note(span, &format!(
2245                 "future does not implement `{}` as this value is used across an await",
2246                 trait_ref.print_only_trait_path(),
2247             ));
2248
2249             // Add a note for the item obligation that remains - normally a note pointing to the
2250             // bound that introduced the obligation (e.g. `T: Send`).
2251             debug!("note_obligation_cause_for_async_await: next_code={:?}", next_code);
2252             self.note_obligation_cause_code(
2253                 err,
2254                 &obligation.predicate,
2255                 next_code.unwrap(),
2256                 &mut Vec::new(),
2257             );
2258
2259             true
2260         } else {
2261             false
2262         }
2263     }
2264
2265     fn note_obligation_cause_code<T>(&self,
2266                                      err: &mut DiagnosticBuilder<'_>,
2267                                      predicate: &T,
2268                                      cause_code: &ObligationCauseCode<'tcx>,
2269                                      obligated_types: &mut Vec<&ty::TyS<'tcx>>)
2270         where T: fmt::Display
2271     {
2272         let tcx = self.tcx;
2273         match *cause_code {
2274             ObligationCauseCode::ExprAssignable |
2275             ObligationCauseCode::MatchExpressionArm { .. } |
2276             ObligationCauseCode::MatchExpressionArmPattern { .. } |
2277             ObligationCauseCode::IfExpression { .. } |
2278             ObligationCauseCode::IfExpressionWithNoElse |
2279             ObligationCauseCode::MainFunctionType |
2280             ObligationCauseCode::StartFunctionType |
2281             ObligationCauseCode::IntrinsicType |
2282             ObligationCauseCode::MethodReceiver |
2283             ObligationCauseCode::ReturnNoExpression |
2284             ObligationCauseCode::MiscObligation => {}
2285             ObligationCauseCode::SliceOrArrayElem => {
2286                 err.note("slice and array elements must have `Sized` type");
2287             }
2288             ObligationCauseCode::TupleElem => {
2289                 err.note("only the last element of a tuple may have a dynamically sized type");
2290             }
2291             ObligationCauseCode::ProjectionWf(data) => {
2292                 err.note(&format!(
2293                     "required so that the projection `{}` is well-formed",
2294                     data,
2295                 ));
2296             }
2297             ObligationCauseCode::ReferenceOutlivesReferent(ref_ty) => {
2298                 err.note(&format!(
2299                     "required so that reference `{}` does not outlive its referent",
2300                     ref_ty,
2301                 ));
2302             }
2303             ObligationCauseCode::ObjectTypeBound(object_ty, region) => {
2304                 err.note(&format!(
2305                     "required so that the lifetime bound of `{}` for `{}` is satisfied",
2306                     region,
2307                     object_ty,
2308                 ));
2309             }
2310             ObligationCauseCode::ItemObligation(item_def_id) => {
2311                 let item_name = tcx.def_path_str(item_def_id);
2312                 let msg = format!("required by `{}`", item_name);
2313
2314                 if let Some(sp) = tcx.hir().span_if_local(item_def_id) {
2315                     let sp = tcx.sess.source_map().def_span(sp);
2316                     err.span_label(sp, &msg);
2317                 } else {
2318                     err.note(&msg);
2319                 }
2320             }
2321             ObligationCauseCode::BindingObligation(item_def_id, span) => {
2322                 let item_name = tcx.def_path_str(item_def_id);
2323                 let msg = format!("required by this bound in `{}`", item_name);
2324                 if let Some(ident) = tcx.opt_item_name(item_def_id) {
2325                     err.span_label(ident.span, "");
2326                 }
2327                 if span != DUMMY_SP {
2328                     err.span_label(span, &msg);
2329                 } else {
2330                     err.note(&msg);
2331                 }
2332             }
2333             ObligationCauseCode::ObjectCastObligation(object_ty) => {
2334                 err.note(&format!("required for the cast to the object type `{}`",
2335                                   self.ty_to_string(object_ty)));
2336             }
2337             ObligationCauseCode::Coercion { source: _, target } => {
2338                 err.note(&format!("required by cast to type `{}`",
2339                                   self.ty_to_string(target)));
2340             }
2341             ObligationCauseCode::RepeatVec(suggest_const_in_array_repeat_expressions) => {
2342                 err.note("the `Copy` trait is required because the \
2343                           repeated element will be copied");
2344                 if suggest_const_in_array_repeat_expressions {
2345                     err.note("this array initializer can be evaluated at compile-time, for more \
2346                               information, see issue \
2347                               https://github.com/rust-lang/rust/issues/49147");
2348                     if tcx.sess.opts.unstable_features.is_nightly_build() {
2349                         err.help("add `#![feature(const_in_array_repeat_expressions)]` to the \
2350                                   crate attributes to enable");
2351                     }
2352                 }
2353             }
2354             ObligationCauseCode::VariableType(_) => {
2355                 err.note("all local variables must have a statically known size");
2356                 if !self.tcx.features().unsized_locals {
2357                     err.help("unsized locals are gated as an unstable feature");
2358                 }
2359             }
2360             ObligationCauseCode::SizedArgumentType => {
2361                 err.note("all function arguments must have a statically known size");
2362                 if !self.tcx.features().unsized_locals {
2363                     err.help("unsized locals are gated as an unstable feature");
2364                 }
2365             }
2366             ObligationCauseCode::SizedReturnType => {
2367                 err.note("the return type of a function must have a \
2368                           statically known size");
2369             }
2370             ObligationCauseCode::SizedYieldType => {
2371                 err.note("the yield type of a generator must have a \
2372                           statically known size");
2373             }
2374             ObligationCauseCode::AssignmentLhsSized => {
2375                 err.note("the left-hand-side of an assignment must have a statically known size");
2376             }
2377             ObligationCauseCode::TupleInitializerSized => {
2378                 err.note("tuples must have a statically known size to be initialized");
2379             }
2380             ObligationCauseCode::StructInitializerSized => {
2381                 err.note("structs must have a statically known size to be initialized");
2382             }
2383             ObligationCauseCode::FieldSized { adt_kind: ref item, last } => {
2384                 match *item {
2385                     AdtKind::Struct => {
2386                         if last {
2387                             err.note("the last field of a packed struct may only have a \
2388                                       dynamically sized type if it does not need drop to be run");
2389                         } else {
2390                             err.note("only the last field of a struct may have a dynamically \
2391                                       sized type");
2392                         }
2393                     }
2394                     AdtKind::Union => {
2395                         err.note("no field of a union may have a dynamically sized type");
2396                     }
2397                     AdtKind::Enum => {
2398                         err.note("no field of an enum variant may have a dynamically sized type");
2399                     }
2400                 }
2401             }
2402             ObligationCauseCode::ConstSized => {
2403                 err.note("constant expressions must have a statically known size");
2404             }
2405             ObligationCauseCode::ConstPatternStructural => {
2406                 err.note("constants used for pattern-matching must derive `PartialEq` and `Eq`");
2407             }
2408             ObligationCauseCode::SharedStatic => {
2409                 err.note("shared static variables must have a type that implements `Sync`");
2410             }
2411             ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
2412                 let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
2413                 let ty = parent_trait_ref.skip_binder().self_ty();
2414                 err.note(&format!("required because it appears within the type `{}`", ty));
2415                 obligated_types.push(ty);
2416
2417                 let parent_predicate = parent_trait_ref.to_predicate();
2418                 if !self.is_recursive_obligation(obligated_types, &data.parent_code) {
2419                     self.note_obligation_cause_code(err,
2420                                                     &parent_predicate,
2421                                                     &data.parent_code,
2422                                                     obligated_types);
2423                 }
2424             }
2425             ObligationCauseCode::ImplDerivedObligation(ref data) => {
2426                 let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
2427                 err.note(
2428                     &format!("required because of the requirements on the impl of `{}` for `{}`",
2429                              parent_trait_ref.print_only_trait_path(),
2430                              parent_trait_ref.skip_binder().self_ty()));
2431                 let parent_predicate = parent_trait_ref.to_predicate();
2432                 self.note_obligation_cause_code(err,
2433                                                 &parent_predicate,
2434                                                 &data.parent_code,
2435                                                 obligated_types);
2436             }
2437             ObligationCauseCode::CompareImplMethodObligation { .. } => {
2438                 err.note(
2439                     &format!("the requirement `{}` appears on the impl method \
2440                               but not on the corresponding trait method",
2441                              predicate));
2442             }
2443             ObligationCauseCode::ReturnType |
2444             ObligationCauseCode::ReturnValue(_) |
2445             ObligationCauseCode::BlockTailExpression(_) => (),
2446             ObligationCauseCode::TrivialBound => {
2447                 err.help("see issue #48214");
2448                 if tcx.sess.opts.unstable_features.is_nightly_build() {
2449                     err.help("add `#![feature(trivial_bounds)]` to the \
2450                               crate attributes to enable",
2451                     );
2452                 }
2453             }
2454             ObligationCauseCode::AssocTypeBound(ref data) => {
2455                 err.span_label(data.original, "associated type defined here");
2456                 if let Some(sp) = data.impl_span {
2457                     err.span_label(sp, "in this `impl` item");
2458                 }
2459                 for sp in &data.bounds {
2460                     err.span_label(*sp, "restricted in this bound");
2461                 }
2462             }
2463         }
2464     }
2465
2466     fn suggest_new_overflow_limit(&self, err: &mut DiagnosticBuilder<'_>) {
2467         let current_limit = self.tcx.sess.recursion_limit.get();
2468         let suggested_limit = current_limit * 2;
2469         err.help(&format!("consider adding a `#![recursion_limit=\"{}\"]` attribute to your crate",
2470                           suggested_limit));
2471     }
2472
2473     fn is_recursive_obligation(&self,
2474                                obligated_types: &mut Vec<&ty::TyS<'tcx>>,
2475                                cause_code: &ObligationCauseCode<'tcx>) -> bool {
2476         if let ObligationCauseCode::BuiltinDerivedObligation(ref data) = cause_code {
2477             let parent_trait_ref = self.resolve_vars_if_possible(&data.parent_trait_ref);
2478
2479             if obligated_types.iter().any(|ot| ot == &parent_trait_ref.skip_binder().self_ty()) {
2480                 return true;
2481             }
2482         }
2483         false
2484     }
2485 }
2486
2487 /// Summarizes information
2488 #[derive(Clone)]
2489 pub enum ArgKind {
2490     /// An argument of non-tuple type. Parameters are (name, ty)
2491     Arg(String, String),
2492
2493     /// An argument of tuple type. For a "found" argument, the span is
2494     /// the locationo in the source of the pattern. For a "expected"
2495     /// argument, it will be None. The vector is a list of (name, ty)
2496     /// strings for the components of the tuple.
2497     Tuple(Option<Span>, Vec<(String, String)>),
2498 }
2499
2500 impl ArgKind {
2501     fn empty() -> ArgKind {
2502         ArgKind::Arg("_".to_owned(), "_".to_owned())
2503     }
2504
2505     /// Creates an `ArgKind` from the expected type of an
2506     /// argument. It has no name (`_`) and an optional source span.
2507     pub fn from_expected_ty(t: Ty<'_>, span: Option<Span>) -> ArgKind {
2508         match t.kind {
2509             ty::Tuple(ref tys) => ArgKind::Tuple(
2510                 span,
2511                 tys.iter()
2512                    .map(|ty| ("_".to_owned(), ty.to_string()))
2513                    .collect::<Vec<_>>()
2514             ),
2515             _ => ArgKind::Arg("_".to_owned(), t.to_string()),
2516         }
2517     }
2518 }
2519
2520 /// Suggest restricting a type param with a new bound.
2521 pub fn suggest_constraining_type_param(
2522     generics: &hir::Generics,
2523     err: &mut DiagnosticBuilder<'_>,
2524     param_name: &str,
2525     constraint: &str,
2526     source_map: &SourceMap,
2527     span: Span,
2528 ) -> bool {
2529     let restrict_msg = "consider further restricting this bound";
2530     if let Some(param) = generics.params.iter().filter(|p| {
2531         p.name.ident().as_str() == param_name
2532     }).next() {
2533         if param_name.starts_with("impl ") {
2534             // `impl Trait` in argument:
2535             // `fn foo(x: impl Trait) {}` → `fn foo(t: impl Trait + Trait2) {}`
2536             err.span_suggestion(
2537                 param.span,
2538                 restrict_msg,
2539                 // `impl CurrentTrait + MissingTrait`
2540                 format!("{} + {}", param_name, constraint),
2541                 Applicability::MachineApplicable,
2542             );
2543         } else if generics.where_clause.predicates.is_empty() &&
2544                 param.bounds.is_empty()
2545         {
2546             // If there are no bounds whatsoever, suggest adding a constraint
2547             // to the type parameter:
2548             // `fn foo<T>(t: T) {}` → `fn foo<T: Trait>(t: T) {}`
2549             err.span_suggestion(
2550                 param.span,
2551                 "consider restricting this bound",
2552                 format!("{}: {}", param_name, constraint),
2553                 Applicability::MachineApplicable,
2554             );
2555         } else if !generics.where_clause.predicates.is_empty() {
2556             // There is a `where` clause, so suggest expanding it:
2557             // `fn foo<T>(t: T) where T: Debug {}` →
2558             // `fn foo<T>(t: T) where T: Debug, T: Trait {}`
2559             err.span_suggestion(
2560                 generics.where_clause.span().unwrap().shrink_to_hi(),
2561                 &format!("consider further restricting type parameter `{}`", param_name),
2562                 format!(", {}: {}", param_name, constraint),
2563                 Applicability::MachineApplicable,
2564             );
2565         } else {
2566             // If there is no `where` clause lean towards constraining to the
2567             // type parameter:
2568             // `fn foo<X: Bar, T>(t: T, x: X) {}` → `fn foo<T: Trait>(t: T) {}`
2569             // `fn foo<T: Bar>(t: T) {}` → `fn foo<T: Bar + Trait>(t: T) {}`
2570             let sp = param.span.with_hi(span.hi());
2571             let span = source_map.span_through_char(sp, ':');
2572             if sp != param.span && sp != span {
2573                 // Only suggest if we have high certainty that the span
2574                 // covers the colon in `foo<T: Trait>`.
2575                 err.span_suggestion(
2576                     span,
2577                     restrict_msg,
2578                     format!("{}: {} + ", param_name, constraint),
2579                     Applicability::MachineApplicable,
2580                 );
2581             } else {
2582                 err.span_label(
2583                     param.span,
2584                     &format!("consider adding a `where {}: {}` bound", param_name, constraint),
2585                 );
2586             }
2587         }
2588         return true;
2589     }
2590     false
2591 }