compiler/rustc_hir_typeck/src/method/mod.rs

   1 //! Method lookup: the secret sauce of Rust. See the [rustc dev guide] for more information.
   2 //!
   3 //! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/method-lookup.html
   4
   5 mod confirm;
   6 mod prelude2021;
   7 pub mod probe;
   8 mod suggest;
   9
  10 pub use self::suggest::SelfSource;
  11 pub use self::MethodError::*;
  12
  13 use crate::errors::OpMethodGenericParams;
  14 use crate::{Expectation, FnCtxt};
  15 use rustc_data_structures::sync::Lrc;
  16 use rustc_errors::{Applicability, Diagnostic};
  17 use rustc_hir as hir;
  18 use rustc_hir::def::{CtorOf, DefKind, Namespace};
  19 use rustc_hir::def_id::DefId;
  20 use rustc_infer::infer::{self, InferOk};
  21 use rustc_middle::traits::ObligationCause;
  22 use rustc_middle::ty::subst::{InternalSubsts, SubstsRef};
  23 use rustc_middle::ty::{self, GenericParamDefKind, Ty, TypeVisitable};
  24 use rustc_span::symbol::Ident;
  25 use rustc_span::Span;
  26 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
  27 use rustc_trait_selection::traits::{self, NormalizeExt};
  28
  29 use self::probe::{IsSuggestion, ProbeScope};
  30
  31 pub fn provide(providers: &mut ty::query::Providers) {
  32     probe::provide(providers);
  33 }
  34
  35 #[derive(Clone, Copy, Debug)]
  36 pub struct MethodCallee<'tcx> {
  37     /// Impl method ID, for inherent methods, or trait method ID, otherwise.
  38     pub def_id: DefId,
  39     pub substs: SubstsRef<'tcx>,
  40
  41     /// Instantiated method signature, i.e., it has been
  42     /// substituted, normalized, and has had late-bound
  43     /// lifetimes replaced with inference variables.
  44     pub sig: ty::FnSig<'tcx>,
  45 }
  46
  47 #[derive(Debug)]
  48 pub enum MethodError<'tcx> {
  49     // Did not find an applicable method, but we did find various near-misses that may work.
  50     NoMatch(NoMatchData<'tcx>),
  51
  52     // Multiple methods might apply.
  53     Ambiguity(Vec<CandidateSource>),
  54
  55     // Found an applicable method, but it is not visible. The third argument contains a list of
  56     // not-in-scope traits which may work.
  57     PrivateMatch(DefKind, DefId, Vec<DefId>),
  58
  59     // Found a `Self: Sized` bound where `Self` is a trait object.
  60     IllegalSizedBound(Vec<DefId>, bool, Span),
  61
  62     // Found a match, but the return type is wrong
  63     BadReturnType,
  64 }
  65
  66 // Contains a list of static methods that may apply, a list of unsatisfied trait predicates which
  67 // could lead to matches if satisfied, and a list of not-in-scope traits which may work.
  68 #[derive(Debug)]
  69 pub struct NoMatchData<'tcx> {
  70     pub static_candidates: Vec<CandidateSource>,
  71     pub unsatisfied_predicates:
  72         Vec<(ty::Predicate<'tcx>, Option<ty::Predicate<'tcx>>, Option<ObligationCause<'tcx>>)>,
  73     pub out_of_scope_traits: Vec<DefId>,
  74     pub lev_candidate: Option<ty::AssocItem>,
  75     pub mode: probe::Mode,
  76 }
  77
  78 // A pared down enum describing just the places from which a method
  79 // candidate can arise. Used for error reporting only.
  80 #[derive(Copy, Clone, Debug, Eq, Ord, PartialEq, PartialOrd)]
  81 pub enum CandidateSource {
  82     Impl(DefId),
  83     Trait(DefId /* trait id */),
  84 }
  85
  86 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  87     /// Determines whether the type `self_ty` supports a method name `method_name` or not.
  88     #[instrument(level = "debug", skip(self))]
  89     pub fn method_exists(
  90         &self,
  91         method_name: Ident,
  92         self_ty: Ty<'tcx>,
  93         call_expr_id: hir::HirId,
  94         allow_private: bool,
  95     ) -> bool {
  96         match self.probe_for_name(
  97             probe::Mode::MethodCall,
  98             method_name,
  99             IsSuggestion(false),
 100             self_ty,
 101             call_expr_id,
 102             ProbeScope::TraitsInScope,
 103         ) {
 104             Ok(pick) => {
 105                 pick.maybe_emit_unstable_name_collision_hint(
 106                     self.tcx,
 107                     method_name.span,
 108                     call_expr_id,
 109                 );
 110                 true
 111             }
 112             Err(NoMatch(..)) => false,
 113             Err(Ambiguity(..)) => true,
 114             Err(PrivateMatch(..)) => allow_private,
 115             Err(IllegalSizedBound(..)) => true,
 116             Err(BadReturnType) => bug!("no return type expectations but got BadReturnType"),
 117         }
 118     }
 119
 120     /// Adds a suggestion to call the given method to the provided diagnostic.
 121     #[instrument(level = "debug", skip(self, err, call_expr))]
 122     pub(crate) fn suggest_method_call(
 123         &self,
 124         err: &mut Diagnostic,
 125         msg: &str,
 126         method_name: Ident,
 127         self_ty: Ty<'tcx>,
 128         call_expr: &hir::Expr<'_>,
 129         span: Option<Span>,
 130     ) {
 131         let params = self
 132             .probe_for_name(
 133                 probe::Mode::MethodCall,
 134                 method_name,
 135                 IsSuggestion(true),
 136                 self_ty,
 137                 call_expr.hir_id,
 138                 ProbeScope::TraitsInScope,
 139             )
 140             .map(|pick| {
 141                 let sig = self.tcx.fn_sig(pick.item.def_id);
 142                 sig.inputs().skip_binder().len().saturating_sub(1)
 143             })
 144             .unwrap_or(0);
 145
 146         // Account for `foo.bar<T>`;
 147         let sugg_span = span.unwrap_or(call_expr.span).shrink_to_hi();
 148         let (suggestion, applicability) = (
 149             format!("({})", (0..params).map(|_| "_").collect::<Vec<_>>().join(", ")),
 150             if params > 0 { Applicability::HasPlaceholders } else { Applicability::MaybeIncorrect },
 151         );
 152
 153         err.span_suggestion_verbose(sugg_span, msg, suggestion, applicability);
 154     }
 155
 156     /// Performs method lookup. If lookup is successful, it will return the callee
 157     /// and store an appropriate adjustment for the self-expr. In some cases it may
 158     /// report an error (e.g., invoking the `drop` method).
 159     ///
 160     /// # Arguments
 161     ///
 162     /// Given a method call like `foo.bar::<T1,...Tn>(a, b + 1, ...)`:
 163     ///
 164     /// * `self`:                  the surrounding `FnCtxt` (!)
 165     /// * `self_ty`:               the (unadjusted) type of the self expression (`foo`)
 166     /// * `segment`:               the name and generic arguments of the method (`bar::<T1, ...Tn>`)
 167     /// * `span`:                  the span for the method call
 168     /// * `call_expr`:             the complete method call: (`foo.bar::<T1,...Tn>(...)`)
 169     /// * `self_expr`:             the self expression (`foo`)
 170     /// * `args`:                  the expressions of the arguments (`a, b + 1, ...`)
 171     #[instrument(level = "debug", skip(self))]
 172     pub fn lookup_method(
 173         &self,
 174         self_ty: Ty<'tcx>,
 175         segment: &hir::PathSegment<'_>,
 176         span: Span,
 177         call_expr: &'tcx hir::Expr<'tcx>,
 178         self_expr: &'tcx hir::Expr<'tcx>,
 179         args: &'tcx [hir::Expr<'tcx>],
 180     ) -> Result<MethodCallee<'tcx>, MethodError<'tcx>> {
 181         let pick =
 182             self.lookup_probe(segment.ident, self_ty, call_expr, ProbeScope::TraitsInScope)?;
 183
 184         self.lint_dot_call_from_2018(self_ty, segment, span, call_expr, self_expr, &pick, args);
 185
 186         for import_id in &pick.import_ids {
 187             debug!("used_trait_import: {:?}", import_id);
 188             Lrc::get_mut(&mut self.typeck_results.borrow_mut().used_trait_imports)
 189                 .unwrap()
 190                 .insert(*import_id);
 191         }
 192
 193         self.tcx.check_stability(pick.item.def_id, Some(call_expr.hir_id), span, None);
 194
 195         let result = self.confirm_method(span, self_expr, call_expr, self_ty, &pick, segment);
 196         debug!("result = {:?}", result);
 197
 198         if let Some(span) = result.illegal_sized_bound {
 199             let mut needs_mut = false;
 200             if let ty::Ref(region, t_type, mutability) = self_ty.kind() {
 201                 let trait_type = self
 202                     .tcx
 203                     .mk_ref(*region, ty::TypeAndMut { ty: *t_type, mutbl: mutability.invert() });
 204                 // We probe again to see if there might be a borrow mutability discrepancy.
 205                 match self.lookup_probe(
 206                     segment.ident,
 207                     trait_type,
 208                     call_expr,
 209                     ProbeScope::TraitsInScope,
 210                 ) {
 211                     Ok(ref new_pick) if pick.differs_from(new_pick) => {
 212                         needs_mut = true;
 213                     }
 214                     _ => {}
 215                 }
 216             }
 217
 218             // We probe again, taking all traits into account (not only those in scope).
 219             let candidates =
 220                 match self.lookup_probe(segment.ident, self_ty, call_expr, ProbeScope::AllTraits) {
 221                     // If we find a different result the caller probably forgot to import a trait.
 222                     Ok(ref new_pick) if pick.differs_from(new_pick) => {
 223                         vec![new_pick.item.container_id(self.tcx)]
 224                     }
 225                     Err(Ambiguity(ref sources)) => sources
 226                         .iter()
 227                         .filter_map(|source| {
 228                             match *source {
 229                                 // Note: this cannot come from an inherent impl,
 230                                 // because the first probing succeeded.
 231                                 CandidateSource::Impl(def) => self.tcx.trait_id_of_impl(def),
 232                                 CandidateSource::Trait(_) => None,
 233                             }
 234                         })
 235                         .collect(),
 236                     _ => Vec::new(),
 237                 };
 238
 239             return Err(IllegalSizedBound(candidates, needs_mut, span));
 240         }
 241
 242         Ok(result.callee)
 243     }
 244
 245     #[instrument(level = "debug", skip(self, call_expr))]
 246     pub fn lookup_probe(
 247         &self,
 248         method_name: Ident,
 249         self_ty: Ty<'tcx>,
 250         call_expr: &'tcx hir::Expr<'tcx>,
 251         scope: ProbeScope,
 252     ) -> probe::PickResult<'tcx> {
 253         let pick = self.probe_for_name(
 254             probe::Mode::MethodCall,
 255             method_name,
 256             IsSuggestion(false),
 257             self_ty,
 258             call_expr.hir_id,
 259             scope,
 260         )?;
 261         pick.maybe_emit_unstable_name_collision_hint(self.tcx, method_name.span, call_expr.hir_id);
 262         Ok(pick)
 263     }
 264
 265     pub(super) fn obligation_for_method(
 266         &self,
 267         span: Span,
 268         trait_def_id: DefId,
 269         self_ty: Ty<'tcx>,
 270         opt_input_types: Option<&[Ty<'tcx>]>,
 271     ) -> (traits::Obligation<'tcx, ty::Predicate<'tcx>>, &'tcx ty::List<ty::subst::GenericArg<'tcx>>)
 272     {
 273         // Construct a trait-reference `self_ty : Trait<input_tys>`
 274         let substs = InternalSubsts::for_item(self.tcx, trait_def_id, |param, _| {
 275             match param.kind {
 276                 GenericParamDefKind::Lifetime | GenericParamDefKind::Const { .. } => {}
 277                 GenericParamDefKind::Type { .. } => {
 278                     if param.index == 0 {
 279                         return self_ty.into();
 280                     } else if let Some(input_types) = opt_input_types {
 281                         return input_types[param.index as usize - 1].into();
 282                     }
 283                 }
 284             }
 285             self.var_for_def(span, param)
 286         });
 287
 288         let trait_ref = ty::TraitRef::new(trait_def_id, substs);
 289
 290         // Construct an obligation
 291         let poly_trait_ref = ty::Binder::dummy(trait_ref);
 292         (
 293             traits::Obligation::misc(
 294                 self.tcx,
 295                 span,
 296                 self.body_id,
 297                 self.param_env,
 298                 poly_trait_ref.without_const(),
 299             ),
 300             substs,
 301         )
 302     }
 303
 304     pub(super) fn obligation_for_op_method(
 305         &self,
 306         span: Span,
 307         trait_def_id: DefId,
 308         self_ty: Ty<'tcx>,
 309         opt_input_type: Option<Ty<'tcx>>,
 310         opt_input_expr: Option<&'tcx hir::Expr<'tcx>>,
 311         expected: Expectation<'tcx>,
 312     ) -> (traits::Obligation<'tcx, ty::Predicate<'tcx>>, &'tcx ty::List<ty::subst::GenericArg<'tcx>>)
 313     {
 314         // Construct a trait-reference `self_ty : Trait<input_tys>`
 315         let substs = InternalSubsts::for_item(self.tcx, trait_def_id, |param, _| {
 316             match param.kind {
 317                 GenericParamDefKind::Lifetime | GenericParamDefKind::Const { .. } => {}
 318                 GenericParamDefKind::Type { .. } => {
 319                     if param.index == 0 {
 320                         return self_ty.into();
 321                     } else if let Some(input_type) = opt_input_type {
 322                         return input_type.into();
 323                     }
 324                 }
 325             }
 326             self.var_for_def(span, param)
 327         });
 328
 329         let trait_ref = ty::TraitRef::new(trait_def_id, substs);
 330
 331         // Construct an obligation
 332         let poly_trait_ref = ty::Binder::dummy(trait_ref);
 333         let output_ty = expected.only_has_type(self).and_then(|ty| (!ty.needs_infer()).then(|| ty));
 334
 335         (
 336             traits::Obligation::new(
 337                 self.tcx,
 338                 traits::ObligationCause::new(
 339                     span,
 340                     self.body_id,
 341                     traits::BinOp {
 342                         rhs_span: opt_input_expr.map(|expr| expr.span),
 343                         is_lit: opt_input_expr
 344                             .map_or(false, |expr| matches!(expr.kind, hir::ExprKind::Lit(_))),
 345                         output_ty,
 346                     },
 347                 ),
 348                 self.param_env,
 349                 poly_trait_ref,
 350             ),
 351             substs,
 352         )
 353     }
 354
 355     /// `lookup_method_in_trait` is used for overloaded operators.
 356     /// It does a very narrow slice of what the normal probe/confirm path does.
 357     /// In particular, it doesn't really do any probing: it simply constructs
 358     /// an obligation for a particular trait with the given self type and checks
 359     /// whether that trait is implemented.
 360     #[instrument(level = "debug", skip(self, span))]
 361     pub(super) fn lookup_method_in_trait(
 362         &self,
 363         span: Span,
 364         m_name: Ident,
 365         trait_def_id: DefId,
 366         self_ty: Ty<'tcx>,
 367         opt_input_types: Option<&[Ty<'tcx>]>,
 368     ) -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
 369         let (obligation, substs) =
 370             self.obligation_for_method(span, trait_def_id, self_ty, opt_input_types);
 371         self.construct_obligation_for_trait(
 372             span,
 373             m_name,
 374             trait_def_id,
 375             obligation,
 376             substs,
 377             None,
 378             false,
 379         )
 380     }
 381
 382     pub(super) fn lookup_op_method_in_trait(
 383         &self,
 384         span: Span,
 385         m_name: Ident,
 386         trait_def_id: DefId,
 387         self_ty: Ty<'tcx>,
 388         opt_input_type: Option<Ty<'tcx>>,
 389         opt_input_expr: Option<&'tcx hir::Expr<'tcx>>,
 390         expected: Expectation<'tcx>,
 391     ) -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
 392         let (obligation, substs) = self.obligation_for_op_method(
 393             span,
 394             trait_def_id,
 395             self_ty,
 396             opt_input_type,
 397             opt_input_expr,
 398             expected,
 399         );
 400         self.construct_obligation_for_trait(
 401             span,
 402             m_name,
 403             trait_def_id,
 404             obligation,
 405             substs,
 406             opt_input_expr,
 407             true,
 408         )
 409     }
 410
 411     // FIXME(#18741): it seems likely that we can consolidate some of this
 412     // code with the other method-lookup code. In particular, the second half
 413     // of this method is basically the same as confirmation.
 414     fn construct_obligation_for_trait(
 415         &self,
 416         span: Span,
 417         m_name: Ident,
 418         trait_def_id: DefId,
 419         obligation: traits::PredicateObligation<'tcx>,
 420         substs: &'tcx ty::List<ty::subst::GenericArg<'tcx>>,
 421         opt_input_expr: Option<&'tcx hir::Expr<'tcx>>,
 422         is_op: bool,
 423     ) -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
 424         debug!(?obligation);
 425
 426         // Now we want to know if this can be matched
 427         if !self.predicate_may_hold(&obligation) {
 428             debug!("--> Cannot match obligation");
 429             // Cannot be matched, no such method resolution is possible.
 430             return None;
 431         }
 432
 433         // Trait must have a method named `m_name` and it should not have
 434         // type parameters or early-bound regions.
 435         let tcx = self.tcx;
 436         let Some(method_item) = self.associated_value(trait_def_id, m_name) else {
 437             tcx.sess.delay_span_bug(
 438                 span,
 439                 "operator trait does not have corresponding operator method",
 440             );
 441             return None;
 442         };
 443         let def_id = method_item.def_id;
 444         let generics = tcx.generics_of(def_id);
 445
 446         if generics.params.len() != 0 {
 447             tcx.sess.emit_fatal(OpMethodGenericParams {
 448                 span: tcx.def_span(method_item.def_id),
 449                 method_name: m_name.to_string(),
 450             });
 451         }
 452
 453         debug!("lookup_in_trait_adjusted: method_item={:?}", method_item);
 454         let mut obligations = vec![];
 455
 456         // Instantiate late-bound regions and substitute the trait
 457         // parameters into the method type to get the actual method type.
 458         //
 459         // N.B., instantiate late-bound regions first so that
 460         // `instantiate_type_scheme` can normalize associated types that
 461         // may reference those regions.
 462         let fn_sig = tcx.bound_fn_sig(def_id);
 463         let fn_sig = fn_sig.subst(self.tcx, substs);
 464         let fn_sig = self.replace_bound_vars_with_fresh_vars(span, infer::FnCall, fn_sig);
 465
 466         let cause = if is_op {
 467             ObligationCause::new(
 468                 span,
 469                 self.body_id,
 470                 traits::BinOp {
 471                     rhs_span: opt_input_expr.map(|expr| expr.span),
 472                     is_lit: opt_input_expr
 473                         .map_or(false, |expr| matches!(expr.kind, hir::ExprKind::Lit(_))),
 474                     output_ty: None,
 475                 },
 476             )
 477         } else {
 478             traits::ObligationCause::misc(span, self.body_id)
 479         };
 480
 481         let InferOk { value, obligations: o } = self.at(&cause, self.param_env).normalize(fn_sig);
 482         let fn_sig = {
 483             obligations.extend(o);
 484             value
 485         };
 486
 487         // Register obligations for the parameters. This will include the
 488         // `Self` parameter, which in turn has a bound of the main trait,
 489         // so this also effectively registers `obligation` as well.  (We
 490         // used to register `obligation` explicitly, but that resulted in
 491         // double error messages being reported.)
 492         //
 493         // Note that as the method comes from a trait, it should not have
 494         // any late-bound regions appearing in its bounds.
 495         let bounds = self.tcx.predicates_of(def_id).instantiate(self.tcx, substs);
 496
 497         let InferOk { value, obligations: o } = self.at(&cause, self.param_env).normalize(bounds);
 498         let bounds = {
 499             obligations.extend(o);
 500             value
 501         };
 502
 503         assert!(!bounds.has_escaping_bound_vars());
 504
 505         let predicates_cause = cause.clone();
 506         obligations.extend(traits::predicates_for_generics(
 507             move |_, _| predicates_cause.clone(),
 508             self.param_env,
 509             bounds,
 510         ));
 511
 512         // Also add an obligation for the method type being well-formed.
 513         let method_ty = tcx.mk_fn_ptr(ty::Binder::dummy(fn_sig));
 514         debug!(
 515             "lookup_in_trait_adjusted: matched method method_ty={:?} obligation={:?}",
 516             method_ty, obligation
 517         );
 518         obligations.push(traits::Obligation::new(
 519             tcx,
 520             cause,
 521             self.param_env,
 522             ty::Binder::dummy(ty::PredicateKind::WellFormed(method_ty.into())),
 523         ));
 524
 525         let callee = MethodCallee { def_id, substs, sig: fn_sig };
 526
 527         debug!("callee = {:?}", callee);
 528
 529         Some(InferOk { obligations, value: callee })
 530     }
 531
 532     /// Performs a [full-qualified function call] (formerly "universal function call") lookup. If
 533     /// lookup is successful, it will return the type of definition and the [`DefId`] of the found
 534     /// function definition.
 535     ///
 536     /// [full-qualified function call]: https://doc.rust-lang.org/reference/expressions/call-expr.html#disambiguating-function-calls
 537     ///
 538     /// # Arguments
 539     ///
 540     /// Given a function call like `Foo::bar::<T1,...Tn>(...)`:
 541     ///
 542     /// * `self`:                  the surrounding `FnCtxt` (!)
 543     /// * `span`:                  the span of the call, excluding arguments (`Foo::bar::<T1, ...Tn>`)
 544     /// * `method_name`:           the identifier of the function within the container type (`bar`)
 545     /// * `self_ty`:               the type to search within (`Foo`)
 546     /// * `self_ty_span`           the span for the type being searched within (span of `Foo`)
 547     /// * `expr_id`:               the [`hir::HirId`] of the expression composing the entire call
 548     #[instrument(level = "debug", skip(self), ret)]
 549     pub fn resolve_fully_qualified_call(
 550         &self,
 551         span: Span,
 552         method_name: Ident,
 553         self_ty: Ty<'tcx>,
 554         self_ty_span: Span,
 555         expr_id: hir::HirId,
 556     ) -> Result<(DefKind, DefId), MethodError<'tcx>> {
 557         let tcx = self.tcx;
 558
 559         // Check if we have an enum variant.
 560         let mut struct_variant = None;
 561         if let ty::Adt(adt_def, _) = self_ty.kind() {
 562             if adt_def.is_enum() {
 563                 let variant_def = adt_def
 564                     .variants()
 565                     .iter()
 566                     .find(|vd| tcx.hygienic_eq(method_name, vd.ident(tcx), adt_def.did()));
 567                 if let Some(variant_def) = variant_def {
 568                     if let Some((ctor_kind, ctor_def_id)) = variant_def.ctor {
 569                         tcx.check_stability(
 570                             ctor_def_id,
 571                             Some(expr_id),
 572                             span,
 573                             Some(method_name.span),
 574                         );
 575                         return Ok((DefKind::Ctor(CtorOf::Variant, ctor_kind), ctor_def_id));
 576                     } else {
 577                         struct_variant = Some((DefKind::Variant, variant_def.def_id));
 578                     }
 579                 }
 580             }
 581         }
 582
 583         let pick = self.probe_for_name(
 584             probe::Mode::Path,
 585             method_name,
 586             IsSuggestion(false),
 587             self_ty,
 588             expr_id,
 589             ProbeScope::TraitsInScope,
 590         );
 591         let pick = match (pick, struct_variant) {
 592             // Fall back to a resolution that will produce an error later.
 593             (Err(_), Some(res)) => return Ok(res),
 594             (pick, _) => pick?,
 595         };
 596
 597         pick.maybe_emit_unstable_name_collision_hint(self.tcx, span, expr_id);
 598
 599         self.lint_fully_qualified_call_from_2018(
 600             span,
 601             method_name,
 602             self_ty,
 603             self_ty_span,
 604             expr_id,
 605             &pick,
 606         );
 607
 608         debug!(?pick);
 609         {
 610             let mut typeck_results = self.typeck_results.borrow_mut();
 611             let used_trait_imports = Lrc::get_mut(&mut typeck_results.used_trait_imports).unwrap();
 612             for import_id in pick.import_ids {
 613                 debug!(used_trait_import=?import_id);
 614                 used_trait_imports.insert(import_id);
 615             }
 616         }
 617
 618         let def_kind = pick.item.kind.as_def_kind();
 619         tcx.check_stability(pick.item.def_id, Some(expr_id), span, Some(method_name.span));
 620         Ok((def_kind, pick.item.def_id))
 621     }
 622
 623     /// Finds item with name `item_name` defined in impl/trait `def_id`
 624     /// and return it, or `None`, if no such item was defined there.
 625     pub fn associated_value(&self, def_id: DefId, item_name: Ident) -> Option<ty::AssocItem> {
 626         self.tcx
 627             .associated_items(def_id)
 628             .find_by_name_and_namespace(self.tcx, item_name, Namespace::ValueNS, def_id)
 629             .copied()
 630     }
 631 }