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