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, DefIdTree, GenericParamDefKind, ToPredicate, Ty, TypeVisitable};
  24 use rustc_span::symbol::Ident;
  25 use rustc_span::Span;
  26 use rustc_trait_selection::traits;
  27 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
  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             method_name.span,
  98             probe::Mode::MethodCall,
  99             method_name,
 100             IsSuggestion(false),
 101             self_ty,
 102             call_expr_id,
 103             ProbeScope::TraitsInScope,
 104         ) {
 105             Ok(..) => true,
 106             Err(NoMatch(..)) => false,
 107             Err(Ambiguity(..)) => true,
 108             Err(PrivateMatch(..)) => allow_private,
 109             Err(IllegalSizedBound(..)) => true,
 110             Err(BadReturnType) => bug!("no return type expectations but got BadReturnType"),
 111         }
 112     }
 113
 114     /// Adds a suggestion to call the given method to the provided diagnostic.
 115     #[instrument(level = "debug", skip(self, err, call_expr))]
 116     pub(crate) fn suggest_method_call(
 117         &self,
 118         err: &mut Diagnostic,
 119         msg: &str,
 120         method_name: Ident,
 121         self_ty: Ty<'tcx>,
 122         call_expr: &hir::Expr<'_>,
 123         span: Option<Span>,
 124     ) {
 125         let params = self
 126             .probe_for_name(
 127                 method_name.span,
 128                 probe::Mode::MethodCall,
 129                 method_name,
 130                 IsSuggestion(true),
 131                 self_ty,
 132                 call_expr.hir_id,
 133                 ProbeScope::TraitsInScope,
 134             )
 135             .map(|pick| {
 136                 let sig = self.tcx.fn_sig(pick.item.def_id);
 137                 sig.inputs().skip_binder().len().saturating_sub(1)
 138             })
 139             .unwrap_or(0);
 140
 141         // Account for `foo.bar<T>`;
 142         let sugg_span = span.unwrap_or(call_expr.span).shrink_to_hi();
 143         let (suggestion, applicability) = (
 144             format!("({})", (0..params).map(|_| "_").collect::<Vec<_>>().join(", ")),
 145             if params > 0 { Applicability::HasPlaceholders } else { Applicability::MaybeIncorrect },
 146         );
 147
 148         err.span_suggestion_verbose(sugg_span, msg, suggestion, applicability);
 149     }
 150
 151     /// Performs method lookup. If lookup is successful, it will return the callee
 152     /// and store an appropriate adjustment for the self-expr. In some cases it may
 153     /// report an error (e.g., invoking the `drop` method).
 154     ///
 155     /// # Arguments
 156     ///
 157     /// Given a method call like `foo.bar::<T1,...Tn>(a, b + 1, ...)`:
 158     ///
 159     /// * `self`:                  the surrounding `FnCtxt` (!)
 160     /// * `self_ty`:               the (unadjusted) type of the self expression (`foo`)
 161     /// * `segment`:               the name and generic arguments of the method (`bar::<T1, ...Tn>`)
 162     /// * `span`:                  the span for the method call
 163     /// * `call_expr`:             the complete method call: (`foo.bar::<T1,...Tn>(...)`)
 164     /// * `self_expr`:             the self expression (`foo`)
 165     /// * `args`:                  the expressions of the arguments (`a, b + 1, ...`)
 166     #[instrument(level = "debug", skip(self))]
 167     pub fn lookup_method(
 168         &self,
 169         self_ty: Ty<'tcx>,
 170         segment: &hir::PathSegment<'_>,
 171         span: Span,
 172         call_expr: &'tcx hir::Expr<'tcx>,
 173         self_expr: &'tcx hir::Expr<'tcx>,
 174         args: &'tcx [hir::Expr<'tcx>],
 175     ) -> Result<MethodCallee<'tcx>, MethodError<'tcx>> {
 176         let pick =
 177             self.lookup_probe(span, segment.ident, self_ty, call_expr, ProbeScope::TraitsInScope)?;
 178
 179         self.lint_dot_call_from_2018(self_ty, segment, span, call_expr, self_expr, &pick, args);
 180
 181         for import_id in &pick.import_ids {
 182             debug!("used_trait_import: {:?}", import_id);
 183             Lrc::get_mut(&mut self.typeck_results.borrow_mut().used_trait_imports)
 184                 .unwrap()
 185                 .insert(*import_id);
 186         }
 187
 188         self.tcx.check_stability(pick.item.def_id, Some(call_expr.hir_id), span, None);
 189
 190         let result =
 191             self.confirm_method(span, self_expr, call_expr, self_ty, pick.clone(), segment);
 192         debug!("result = {:?}", result);
 193
 194         if let Some(span) = result.illegal_sized_bound {
 195             let mut needs_mut = false;
 196             if let ty::Ref(region, t_type, mutability) = self_ty.kind() {
 197                 let trait_type = self
 198                     .tcx
 199                     .mk_ref(*region, ty::TypeAndMut { ty: *t_type, mutbl: mutability.invert() });
 200                 // We probe again to see if there might be a borrow mutability discrepancy.
 201                 match self.lookup_probe(
 202                     span,
 203                     segment.ident,
 204                     trait_type,
 205                     call_expr,
 206                     ProbeScope::TraitsInScope,
 207                 ) {
 208                     Ok(ref new_pick) if *new_pick != pick => {
 209                         needs_mut = true;
 210                     }
 211                     _ => {}
 212                 }
 213             }
 214
 215             // We probe again, taking all traits into account (not only those in scope).
 216             let mut candidates = match self.lookup_probe(
 217                 span,
 218                 segment.ident,
 219                 self_ty,
 220                 call_expr,
 221                 ProbeScope::AllTraits,
 222             ) {
 223                 // If we find a different result the caller probably forgot to import a trait.
 224                 Ok(ref new_pick) if *new_pick != pick => vec![new_pick.item.container_id(self.tcx)],
 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             candidates.retain(|candidate| *candidate != self.tcx.parent(result.callee.def_id));
 239
 240             return Err(IllegalSizedBound(candidates, needs_mut, span));
 241         }
 242
 243         Ok(result.callee)
 244     }
 245
 246     #[instrument(level = "debug", skip(self, call_expr))]
 247     pub fn lookup_probe(
 248         &self,
 249         span: Span,
 250         method_name: Ident,
 251         self_ty: Ty<'tcx>,
 252         call_expr: &'tcx hir::Expr<'tcx>,
 253         scope: ProbeScope,
 254     ) -> probe::PickResult<'tcx> {
 255         self.probe_for_name(
 256             span,
 257             probe::Mode::MethodCall,
 258             method_name,
 259             IsSuggestion(false),
 260             self_ty,
 261             call_expr.hir_id,
 262             scope,
 263         )
 264     }
 265
 266     pub(super) fn obligation_for_method(
 267         &self,
 268         span: Span,
 269         trait_def_id: DefId,
 270         self_ty: Ty<'tcx>,
 271         opt_input_types: Option<&[Ty<'tcx>]>,
 272     ) -> (traits::Obligation<'tcx, ty::Predicate<'tcx>>, &'tcx ty::List<ty::subst::GenericArg<'tcx>>)
 273     {
 274         // Construct a trait-reference `self_ty : Trait<input_tys>`
 275         let substs = InternalSubsts::for_item(self.tcx, trait_def_id, |param, _| {
 276             match param.kind {
 277                 GenericParamDefKind::Lifetime | GenericParamDefKind::Const { .. } => {}
 278                 GenericParamDefKind::Type { .. } => {
 279                     if param.index == 0 {
 280                         return self_ty.into();
 281                     } else if let Some(input_types) = opt_input_types {
 282                         return input_types[param.index as usize - 1].into();
 283                     }
 284                 }
 285             }
 286             self.var_for_def(span, param)
 287         });
 288
 289         let trait_ref = ty::TraitRef::new(trait_def_id, substs);
 290
 291         // Construct an obligation
 292         let poly_trait_ref = ty::Binder::dummy(trait_ref);
 293         (
 294             traits::Obligation::misc(
 295                 span,
 296                 self.body_id,
 297                 self.param_env,
 298                 poly_trait_ref.without_const().to_predicate(self.tcx),
 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                 traits::ObligationCause::new(
 338                     span,
 339                     self.body_id,
 340                     traits::BinOp {
 341                         rhs_span: opt_input_expr.map(|expr| expr.span),
 342                         is_lit: opt_input_expr
 343                             .map_or(false, |expr| matches!(expr.kind, hir::ExprKind::Lit(_))),
 344                         output_ty,
 345                     },
 346                 ),
 347                 self.param_env,
 348                 poly_trait_ref.without_const().to_predicate(self.tcx),
 349             ),
 350             substs,
 351         )
 352     }
 353
 354     /// `lookup_method_in_trait` is used for overloaded operators.
 355     /// It does a very narrow slice of what the normal probe/confirm path does.
 356     /// In particular, it doesn't really do any probing: it simply constructs
 357     /// an obligation for a particular trait with the given self type and checks
 358     /// whether that trait is implemented.
 359     #[instrument(level = "debug", skip(self, span))]
 360     pub(super) fn lookup_method_in_trait(
 361         &self,
 362         span: Span,
 363         m_name: Ident,
 364         trait_def_id: DefId,
 365         self_ty: Ty<'tcx>,
 366         opt_input_types: Option<&[Ty<'tcx>]>,
 367     ) -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
 368         let (obligation, substs) =
 369             self.obligation_for_method(span, trait_def_id, self_ty, opt_input_types);
 370         self.construct_obligation_for_trait(
 371             span,
 372             m_name,
 373             trait_def_id,
 374             obligation,
 375             substs,
 376             None,
 377             false,
 378         )
 379     }
 380
 381     pub(super) fn lookup_op_method_in_trait(
 382         &self,
 383         span: Span,
 384         m_name: Ident,
 385         trait_def_id: DefId,
 386         self_ty: Ty<'tcx>,
 387         opt_input_type: Option<Ty<'tcx>>,
 388         opt_input_expr: Option<&'tcx hir::Expr<'tcx>>,
 389         expected: Expectation<'tcx>,
 390     ) -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
 391         let (obligation, substs) = self.obligation_for_op_method(
 392             span,
 393             trait_def_id,
 394             self_ty,
 395             opt_input_type,
 396             opt_input_expr,
 397             expected,
 398         );
 399         self.construct_obligation_for_trait(
 400             span,
 401             m_name,
 402             trait_def_id,
 403             obligation,
 404             substs,
 405             opt_input_expr,
 406             true,
 407         )
 408     }
 409
 410     // FIXME(#18741): it seems likely that we can consolidate some of this
 411     // code with the other method-lookup code. In particular, the second half
 412     // of this method is basically the same as confirmation.
 413     fn construct_obligation_for_trait(
 414         &self,
 415         span: Span,
 416         m_name: Ident,
 417         trait_def_id: DefId,
 418         obligation: traits::PredicateObligation<'tcx>,
 419         substs: &'tcx ty::List<ty::subst::GenericArg<'tcx>>,
 420         opt_input_expr: Option<&'tcx hir::Expr<'tcx>>,
 421         is_op: bool,
 422     ) -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
 423         debug!(?obligation);
 424
 425         // Now we want to know if this can be matched
 426         if !self.predicate_may_hold(&obligation) {
 427             debug!("--> Cannot match obligation");
 428             // Cannot be matched, no such method resolution is possible.
 429             return None;
 430         }
 431
 432         // Trait must have a method named `m_name` and it should not have
 433         // type parameters or early-bound regions.
 434         let tcx = self.tcx;
 435         let Some(method_item) = self.associated_value(trait_def_id, m_name) else {
 436             tcx.sess.delay_span_bug(
 437                 span,
 438                 "operator trait does not have corresponding operator method",
 439             );
 440             return None;
 441         };
 442         let def_id = method_item.def_id;
 443         let generics = tcx.generics_of(def_id);
 444
 445         if generics.params.len() != 0 {
 446             tcx.sess.emit_fatal(OpMethodGenericParams {
 447                 span: tcx.def_span(method_item.def_id),
 448                 method_name: m_name.to_string(),
 449             });
 450         }
 451
 452         debug!("lookup_in_trait_adjusted: method_item={:?}", method_item);
 453         let mut obligations = vec![];
 454
 455         // Instantiate late-bound regions and substitute the trait
 456         // parameters into the method type to get the actual method type.
 457         //
 458         // N.B., instantiate late-bound regions first so that
 459         // `instantiate_type_scheme` can normalize associated types that
 460         // may reference those regions.
 461         let fn_sig = tcx.bound_fn_sig(def_id);
 462         let fn_sig = fn_sig.subst(self.tcx, substs);
 463         let fn_sig = self.replace_bound_vars_with_fresh_vars(span, infer::FnCall, fn_sig);
 464
 465         let InferOk { value, obligations: o } = if is_op {
 466             self.normalize_op_associated_types_in_as_infer_ok(span, fn_sig, opt_input_expr)
 467         } else {
 468             self.normalize_associated_types_in_as_infer_ok(span, fn_sig)
 469         };
 470         let fn_sig = {
 471             obligations.extend(o);
 472             value
 473         };
 474
 475         // Register obligations for the parameters. This will include the
 476         // `Self` parameter, which in turn has a bound of the main trait,
 477         // so this also effectively registers `obligation` as well.  (We
 478         // used to register `obligation` explicitly, but that resulted in
 479         // double error messages being reported.)
 480         //
 481         // Note that as the method comes from a trait, it should not have
 482         // any late-bound regions appearing in its bounds.
 483         let bounds = self.tcx.predicates_of(def_id).instantiate(self.tcx, substs);
 484
 485         let InferOk { value, obligations: o } = if is_op {
 486             self.normalize_op_associated_types_in_as_infer_ok(span, bounds, opt_input_expr)
 487         } else {
 488             self.normalize_associated_types_in_as_infer_ok(span, bounds)
 489         };
 490         let bounds = {
 491             obligations.extend(o);
 492             value
 493         };
 494
 495         assert!(!bounds.has_escaping_bound_vars());
 496
 497         let cause = if is_op {
 498             ObligationCause::new(
 499                 span,
 500                 self.body_id,
 501                 traits::BinOp {
 502                     rhs_span: opt_input_expr.map(|expr| expr.span),
 503                     is_lit: opt_input_expr
 504                         .map_or(false, |expr| matches!(expr.kind, hir::ExprKind::Lit(_))),
 505                     output_ty: None,
 506                 },
 507             )
 508         } else {
 509             traits::ObligationCause::misc(span, self.body_id)
 510         };
 511         let predicates_cause = cause.clone();
 512         obligations.extend(traits::predicates_for_generics(
 513             move |_, _| predicates_cause.clone(),
 514             self.param_env,
 515             bounds,
 516         ));
 517
 518         // Also add an obligation for the method type being well-formed.
 519         let method_ty = tcx.mk_fn_ptr(ty::Binder::dummy(fn_sig));
 520         debug!(
 521             "lookup_in_trait_adjusted: matched method method_ty={:?} obligation={:?}",
 522             method_ty, obligation
 523         );
 524         obligations.push(traits::Obligation::new(
 525             cause,
 526             self.param_env,
 527             ty::Binder::dummy(ty::PredicateKind::WellFormed(method_ty.into())).to_predicate(tcx),
 528         ));
 529
 530         let callee = MethodCallee { def_id, substs, sig: fn_sig };
 531
 532         debug!("callee = {:?}", callee);
 533
 534         Some(InferOk { obligations, value: callee })
 535     }
 536
 537     /// Performs a [full-qualified function call] (formerly "universal function call") lookup. If
 538     /// lookup is successful, it will return the type of definition and the [`DefId`] of the found
 539     /// function definition.
 540     ///
 541     /// [full-qualified function call]: https://doc.rust-lang.org/reference/expressions/call-expr.html#disambiguating-function-calls
 542     ///
 543     /// # Arguments
 544     ///
 545     /// Given a function call like `Foo::bar::<T1,...Tn>(...)`:
 546     ///
 547     /// * `self`:                  the surrounding `FnCtxt` (!)
 548     /// * `span`:                  the span of the call, excluding arguments (`Foo::bar::<T1, ...Tn>`)
 549     /// * `method_name`:           the identifier of the function within the container type (`bar`)
 550     /// * `self_ty`:               the type to search within (`Foo`)
 551     /// * `self_ty_span`           the span for the type being searched within (span of `Foo`)
 552     /// * `expr_id`:               the [`hir::HirId`] of the expression composing the entire call
 553     #[instrument(level = "debug", skip(self), ret)]
 554     pub fn resolve_fully_qualified_call(
 555         &self,
 556         span: Span,
 557         method_name: Ident,
 558         self_ty: Ty<'tcx>,
 559         self_ty_span: Span,
 560         expr_id: hir::HirId,
 561     ) -> Result<(DefKind, DefId), MethodError<'tcx>> {
 562         let tcx = self.tcx;
 563
 564         // Check if we have an enum variant.
 565         if let ty::Adt(adt_def, _) = self_ty.kind() {
 566             if adt_def.is_enum() {
 567                 let variant_def = adt_def
 568                     .variants()
 569                     .iter()
 570                     .find(|vd| tcx.hygienic_eq(method_name, vd.ident(tcx), adt_def.did()));
 571                 if let Some(variant_def) = variant_def {
 572                     // Braced variants generate unusable names in value namespace (reserved for
 573                     // possible future use), so variants resolved as associated items may refer to
 574                     // them as well. It's ok to use the variant's id as a ctor id since an
 575                     // error will be reported on any use of such resolution anyway.
 576                     let ctor_def_id = variant_def.ctor_def_id.unwrap_or(variant_def.def_id);
 577                     tcx.check_stability(ctor_def_id, Some(expr_id), span, Some(method_name.span));
 578                     return Ok((
 579                         DefKind::Ctor(CtorOf::Variant, variant_def.ctor_kind),
 580                         ctor_def_id,
 581                     ));
 582                 }
 583             }
 584         }
 585
 586         let pick = self.probe_for_name(
 587             span,
 588             probe::Mode::Path,
 589             method_name,
 590             IsSuggestion(false),
 591             self_ty,
 592             expr_id,
 593             ProbeScope::TraitsInScope,
 594         )?;
 595
 596         self.lint_fully_qualified_call_from_2018(
 597             span,
 598             method_name,
 599             self_ty,
 600             self_ty_span,
 601             expr_id,
 602             &pick,
 603         );
 604
 605         debug!(?pick);
 606         {
 607             let mut typeck_results = self.typeck_results.borrow_mut();
 608             let used_trait_imports = Lrc::get_mut(&mut typeck_results.used_trait_imports).unwrap();
 609             for import_id in pick.import_ids {
 610                 debug!(used_trait_import=?import_id);
 611                 used_trait_imports.insert(import_id);
 612             }
 613         }
 614
 615         let def_kind = pick.item.kind.as_def_kind();
 616         tcx.check_stability(pick.item.def_id, Some(expr_id), span, Some(method_name.span));
 617         Ok((def_kind, pick.item.def_id))
 618     }
 619
 620     /// Finds item with name `item_name` defined in impl/trait `def_id`
 621     /// and return it, or `None`, if no such item was defined there.
 622     pub fn associated_value(&self, def_id: DefId, item_name: Ident) -> Option<ty::AssocItem> {
 623         self.tcx
 624             .associated_items(def_id)
 625             .find_by_name_and_namespace(self.tcx, item_name, Namespace::ValueNS, def_id)
 626             .copied()
 627     }
 628 }