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 =
 196             self.confirm_method(span, self_expr, call_expr, self_ty, pick.clone(), segment);
 197         debug!("result = {:?}", result);
 198
 199         if let Some(span) = result.illegal_sized_bound {
 200             let mut needs_mut = false;
 201             if let ty::Ref(region, t_type, mutability) = self_ty.kind() {
 202                 let trait_type = self
 203                     .tcx
 204                     .mk_ref(*region, ty::TypeAndMut { ty: *t_type, mutbl: mutability.invert() });
 205                 // We probe again to see if there might be a borrow mutability discrepancy.
 206                 match self.lookup_probe(
 207                     segment.ident,
 208                     trait_type,
 209                     call_expr,
 210                     ProbeScope::TraitsInScope,
 211                 ) {
 212                     Ok(ref new_pick) if pick.differs_from(new_pick) => {
 213                         needs_mut = true;
 214                     }
 215                     _ => {}
 216                 }
 217             }
 218
 219             // We probe again, taking all traits into account (not only those in scope).
 220             let candidates =
 221                 match self.lookup_probe(segment.ident, self_ty, call_expr, ProbeScope::AllTraits) {
 222                     // If we find a different result the caller probably forgot to import a trait.
 223                     Ok(ref new_pick) if pick.differs_from(new_pick) => {
 224                         vec![new_pick.item.container_id(self.tcx)]
 225                     }
 226                     Err(Ambiguity(ref sources)) => sources
 227                         .iter()
 228                         .filter_map(|source| {
 229                             match *source {
 230                                 // Note: this cannot come from an inherent impl,
 231                                 // because the first probing succeeded.
 232                                 CandidateSource::Impl(def) => self.tcx.trait_id_of_impl(def),
 233                                 CandidateSource::Trait(_) => None,
 234                             }
 235                         })
 236                         .collect(),
 237                     _ => Vec::new(),
 238                 };
 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         method_name: Ident,
 250         self_ty: Ty<'tcx>,
 251         call_expr: &'tcx hir::Expr<'tcx>,
 252         scope: ProbeScope,
 253     ) -> probe::PickResult<'tcx> {
 254         let pick = self.probe_for_name(
 255             probe::Mode::MethodCall,
 256             method_name,
 257             IsSuggestion(false),
 258             self_ty,
 259             call_expr.hir_id,
 260             scope,
 261         )?;
 262         pick.maybe_emit_unstable_name_collision_hint(self.tcx, method_name.span, call_expr.hir_id);
 263         Ok(pick)
 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                 self.tcx,
 296                 span,
 297                 self.body_id,
 298                 self.param_env,
 299                 poly_trait_ref.without_const(),
 300             ),
 301             substs,
 302         )
 303     }
 304
 305     pub(super) fn obligation_for_op_method(
 306         &self,
 307         span: Span,
 308         trait_def_id: DefId,
 309         self_ty: Ty<'tcx>,
 310         opt_input_type: Option<Ty<'tcx>>,
 311         opt_input_expr: Option<&'tcx hir::Expr<'tcx>>,
 312         expected: Expectation<'tcx>,
 313     ) -> (traits::Obligation<'tcx, ty::Predicate<'tcx>>, &'tcx ty::List<ty::subst::GenericArg<'tcx>>)
 314     {
 315         // Construct a trait-reference `self_ty : Trait<input_tys>`
 316         let substs = InternalSubsts::for_item(self.tcx, trait_def_id, |param, _| {
 317             match param.kind {
 318                 GenericParamDefKind::Lifetime | GenericParamDefKind::Const { .. } => {}
 319                 GenericParamDefKind::Type { .. } => {
 320                     if param.index == 0 {
 321                         return self_ty.into();
 322                     } else if let Some(input_type) = opt_input_type {
 323                         return input_type.into();
 324                     }
 325                 }
 326             }
 327             self.var_for_def(span, param)
 328         });
 329
 330         let trait_ref = ty::TraitRef::new(trait_def_id, substs);
 331
 332         // Construct an obligation
 333         let poly_trait_ref = ty::Binder::dummy(trait_ref);
 334         let output_ty = expected.only_has_type(self).and_then(|ty| (!ty.needs_infer()).then(|| ty));
 335
 336         (
 337             traits::Obligation::new(
 338                 self.tcx,
 339                 traits::ObligationCause::new(
 340                     span,
 341                     self.body_id,
 342                     traits::BinOp {
 343                         rhs_span: opt_input_expr.map(|expr| expr.span),
 344                         is_lit: opt_input_expr
 345                             .map_or(false, |expr| matches!(expr.kind, hir::ExprKind::Lit(_))),
 346                         output_ty,
 347                     },
 348                 ),
 349                 self.param_env,
 350                 poly_trait_ref,
 351             ),
 352             substs,
 353         )
 354     }
 355
 356     /// `lookup_method_in_trait` is used for overloaded operators.
 357     /// It does a very narrow slice of what the normal probe/confirm path does.
 358     /// In particular, it doesn't really do any probing: it simply constructs
 359     /// an obligation for a particular trait with the given self type and checks
 360     /// whether that trait is implemented.
 361     #[instrument(level = "debug", skip(self, span))]
 362     pub(super) fn lookup_method_in_trait(
 363         &self,
 364         span: Span,
 365         m_name: Ident,
 366         trait_def_id: DefId,
 367         self_ty: Ty<'tcx>,
 368         opt_input_types: Option<&[Ty<'tcx>]>,
 369     ) -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
 370         let (obligation, substs) =
 371             self.obligation_for_method(span, trait_def_id, self_ty, opt_input_types);
 372         self.construct_obligation_for_trait(
 373             span,
 374             m_name,
 375             trait_def_id,
 376             obligation,
 377             substs,
 378             None,
 379             false,
 380         )
 381     }
 382
 383     pub(super) fn lookup_op_method_in_trait(
 384         &self,
 385         span: Span,
 386         m_name: Ident,
 387         trait_def_id: DefId,
 388         self_ty: Ty<'tcx>,
 389         opt_input_type: Option<Ty<'tcx>>,
 390         opt_input_expr: Option<&'tcx hir::Expr<'tcx>>,
 391         expected: Expectation<'tcx>,
 392     ) -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
 393         let (obligation, substs) = self.obligation_for_op_method(
 394             span,
 395             trait_def_id,
 396             self_ty,
 397             opt_input_type,
 398             opt_input_expr,
 399             expected,
 400         );
 401         self.construct_obligation_for_trait(
 402             span,
 403             m_name,
 404             trait_def_id,
 405             obligation,
 406             substs,
 407             opt_input_expr,
 408             true,
 409         )
 410     }
 411
 412     // FIXME(#18741): it seems likely that we can consolidate some of this
 413     // code with the other method-lookup code. In particular, the second half
 414     // of this method is basically the same as confirmation.
 415     fn construct_obligation_for_trait(
 416         &self,
 417         span: Span,
 418         m_name: Ident,
 419         trait_def_id: DefId,
 420         obligation: traits::PredicateObligation<'tcx>,
 421         substs: &'tcx ty::List<ty::subst::GenericArg<'tcx>>,
 422         opt_input_expr: Option<&'tcx hir::Expr<'tcx>>,
 423         is_op: bool,
 424     ) -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
 425         debug!(?obligation);
 426
 427         // Now we want to know if this can be matched
 428         if !self.predicate_may_hold(&obligation) {
 429             debug!("--> Cannot match obligation");
 430             // Cannot be matched, no such method resolution is possible.
 431             return None;
 432         }
 433
 434         // Trait must have a method named `m_name` and it should not have
 435         // type parameters or early-bound regions.
 436         let tcx = self.tcx;
 437         let Some(method_item) = self.associated_value(trait_def_id, m_name) else {
 438             tcx.sess.delay_span_bug(
 439                 span,
 440                 "operator trait does not have corresponding operator method",
 441             );
 442             return None;
 443         };
 444         let def_id = method_item.def_id;
 445         let generics = tcx.generics_of(def_id);
 446
 447         if generics.params.len() != 0 {
 448             tcx.sess.emit_fatal(OpMethodGenericParams {
 449                 span: tcx.def_span(method_item.def_id),
 450                 method_name: m_name.to_string(),
 451             });
 452         }
 453
 454         debug!("lookup_in_trait_adjusted: method_item={:?}", method_item);
 455         let mut obligations = vec![];
 456
 457         // Instantiate late-bound regions and substitute the trait
 458         // parameters into the method type to get the actual method type.
 459         //
 460         // N.B., instantiate late-bound regions first so that
 461         // `instantiate_type_scheme` can normalize associated types that
 462         // may reference those regions.
 463         let fn_sig = tcx.bound_fn_sig(def_id);
 464         let fn_sig = fn_sig.subst(self.tcx, substs);
 465         let fn_sig = self.replace_bound_vars_with_fresh_vars(span, infer::FnCall, fn_sig);
 466
 467         let cause = if is_op {
 468             ObligationCause::new(
 469                 span,
 470                 self.body_id,
 471                 traits::BinOp {
 472                     rhs_span: opt_input_expr.map(|expr| expr.span),
 473                     is_lit: opt_input_expr
 474                         .map_or(false, |expr| matches!(expr.kind, hir::ExprKind::Lit(_))),
 475                     output_ty: None,
 476                 },
 477             )
 478         } else {
 479             traits::ObligationCause::misc(span, self.body_id)
 480         };
 481
 482         let InferOk { value, obligations: o } = self.at(&cause, self.param_env).normalize(fn_sig);
 483         let fn_sig = {
 484             obligations.extend(o);
 485             value
 486         };
 487
 488         // Register obligations for the parameters. This will include the
 489         // `Self` parameter, which in turn has a bound of the main trait,
 490         // so this also effectively registers `obligation` as well.  (We
 491         // used to register `obligation` explicitly, but that resulted in
 492         // double error messages being reported.)
 493         //
 494         // Note that as the method comes from a trait, it should not have
 495         // any late-bound regions appearing in its bounds.
 496         let bounds = self.tcx.predicates_of(def_id).instantiate(self.tcx, substs);
 497
 498         let InferOk { value, obligations: o } = self.at(&cause, self.param_env).normalize(bounds);
 499         let bounds = {
 500             obligations.extend(o);
 501             value
 502         };
 503
 504         assert!(!bounds.has_escaping_bound_vars());
 505
 506         let predicates_cause = cause.clone();
 507         obligations.extend(traits::predicates_for_generics(
 508             move |_, _| predicates_cause.clone(),
 509             self.param_env,
 510             bounds,
 511         ));
 512
 513         // Also add an obligation for the method type being well-formed.
 514         let method_ty = tcx.mk_fn_ptr(ty::Binder::dummy(fn_sig));
 515         debug!(
 516             "lookup_in_trait_adjusted: matched method method_ty={:?} obligation={:?}",
 517             method_ty, obligation
 518         );
 519         obligations.push(traits::Obligation::new(
 520             tcx,
 521             cause,
 522             self.param_env,
 523             ty::Binder::dummy(ty::PredicateKind::WellFormed(method_ty.into())),
 524         ));
 525
 526         let callee = MethodCallee { def_id, substs, sig: fn_sig };
 527
 528         debug!("callee = {:?}", callee);
 529
 530         Some(InferOk { obligations, value: callee })
 531     }
 532
 533     /// Performs a [full-qualified function call] (formerly "universal function call") lookup. If
 534     /// lookup is successful, it will return the type of definition and the [`DefId`] of the found
 535     /// function definition.
 536     ///
 537     /// [full-qualified function call]: https://doc.rust-lang.org/reference/expressions/call-expr.html#disambiguating-function-calls
 538     ///
 539     /// # Arguments
 540     ///
 541     /// Given a function call like `Foo::bar::<T1,...Tn>(...)`:
 542     ///
 543     /// * `self`:                  the surrounding `FnCtxt` (!)
 544     /// * `span`:                  the span of the call, excluding arguments (`Foo::bar::<T1, ...Tn>`)
 545     /// * `method_name`:           the identifier of the function within the container type (`bar`)
 546     /// * `self_ty`:               the type to search within (`Foo`)
 547     /// * `self_ty_span`           the span for the type being searched within (span of `Foo`)
 548     /// * `expr_id`:               the [`hir::HirId`] of the expression composing the entire call
 549     #[instrument(level = "debug", skip(self), ret)]
 550     pub fn resolve_fully_qualified_call(
 551         &self,
 552         span: Span,
 553         method_name: Ident,
 554         self_ty: Ty<'tcx>,
 555         self_ty_span: Span,
 556         expr_id: hir::HirId,
 557     ) -> Result<(DefKind, DefId), MethodError<'tcx>> {
 558         let tcx = self.tcx;
 559
 560         // Check if we have an enum variant.
 561         let mut struct_variant = None;
 562         if let ty::Adt(adt_def, _) = self_ty.kind() {
 563             if adt_def.is_enum() {
 564                 let variant_def = adt_def
 565                     .variants()
 566                     .iter()
 567                     .find(|vd| tcx.hygienic_eq(method_name, vd.ident(tcx), adt_def.did()));
 568                 if let Some(variant_def) = variant_def {
 569                     if let Some((ctor_kind, ctor_def_id)) = variant_def.ctor {
 570                         tcx.check_stability(
 571                             ctor_def_id,
 572                             Some(expr_id),
 573                             span,
 574                             Some(method_name.span),
 575                         );
 576                         return Ok((DefKind::Ctor(CtorOf::Variant, ctor_kind), ctor_def_id));
 577                     } else {
 578                         struct_variant = Some((DefKind::Variant, variant_def.def_id));
 579                     }
 580                 }
 581             }
 582         }
 583
 584         let pick = self.probe_for_name(
 585             probe::Mode::Path,
 586             method_name,
 587             IsSuggestion(false),
 588             self_ty,
 589             expr_id,
 590             ProbeScope::TraitsInScope,
 591         );
 592         let pick = match (pick, struct_variant) {
 593             // Fall back to a resolution that will produce an error later.
 594             (Err(_), Some(res)) => return Ok(res),
 595             (pick, _) => pick?,
 596         };
 597
 598         pick.maybe_emit_unstable_name_collision_hint(self.tcx, span, expr_id);
 599
 600         self.lint_fully_qualified_call_from_2018(
 601             span,
 602             method_name,
 603             self_ty,
 604             self_ty_span,
 605             expr_id,
 606             &pick,
 607         );
 608
 609         debug!(?pick);
 610         {
 611             let mut typeck_results = self.typeck_results.borrow_mut();
 612             let used_trait_imports = Lrc::get_mut(&mut typeck_results.used_trait_imports).unwrap();
 613             for import_id in pick.import_ids {
 614                 debug!(used_trait_import=?import_id);
 615                 used_trait_imports.insert(import_id);
 616             }
 617         }
 618
 619         let def_kind = pick.item.kind.as_def_kind();
 620         tcx.check_stability(pick.item.def_id, Some(expr_id), span, Some(method_name.span));
 621         Ok((def_kind, pick.item.def_id))
 622     }
 623
 624     /// Finds item with name `item_name` defined in impl/trait `def_id`
 625     /// and return it, or `None`, if no such item was defined there.
 626     pub fn associated_value(&self, def_id: DefId, item_name: Ident) -> Option<ty::AssocItem> {
 627         self.tcx
 628             .associated_items(def_id)
 629             .find_by_name_and_namespace(self.tcx, item_name, Namespace::ValueNS, def_id)
 630             .copied()
 631     }
 632 }