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::{Expectation, FnCtxt};
  14 use rustc_data_structures::sync::Lrc;
  15 use rustc_errors::{Applicability, Diagnostic};
  16 use rustc_hir as hir;
  17 use rustc_hir::def::{CtorOf, DefKind, Namespace};
  18 use rustc_hir::def_id::DefId;
  19 use rustc_infer::infer::{self, InferOk};
  20 use rustc_middle::traits::ObligationCause;
  21 use rustc_middle::ty::subst::{InternalSubsts, SubstsRef};
  22 use rustc_middle::ty::{self, DefIdTree, GenericParamDefKind, ToPredicate, Ty, TypeVisitable};
  23 use rustc_span::symbol::Ident;
  24 use rustc_span::Span;
  25 use rustc_trait_selection::traits;
  26 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
  27
  28 use self::probe::{IsSuggestion, ProbeScope};
  29
  30 pub fn provide(providers: &mut ty::query::Providers) {
  31     probe::provide(providers);
  32 }
  33
  34 #[derive(Clone, Copy, Debug)]
  35 pub struct MethodCallee<'tcx> {
  36     /// Impl method ID, for inherent methods, or trait method ID, otherwise.
  37     pub def_id: DefId,
  38     pub substs: SubstsRef<'tcx>,
  39
  40     /// Instantiated method signature, i.e., it has been
  41     /// substituted, normalized, and has had late-bound
  42     /// lifetimes replaced with inference variables.
  43     pub sig: ty::FnSig<'tcx>,
  44 }
  45
  46 #[derive(Debug)]
  47 pub enum MethodError<'tcx> {
  48     // Did not find an applicable method, but we did find various near-misses that may work.
  49     NoMatch(NoMatchData<'tcx>),
  50
  51     // Multiple methods might apply.
  52     Ambiguity(Vec<CandidateSource>),
  53
  54     // Found an applicable method, but it is not visible. The third argument contains a list of
  55     // not-in-scope traits which may work.
  56     PrivateMatch(DefKind, DefId, Vec<DefId>),
  57
  58     // Found a `Self: Sized` bound where `Self` is a trait object.
  59     IllegalSizedBound(Vec<DefId>, bool, Span),
  60
  61     // Found a match, but the return type is wrong
  62     BadReturnType,
  63 }
  64
  65 // Contains a list of static methods that may apply, a list of unsatisfied trait predicates which
  66 // could lead to matches if satisfied, and a list of not-in-scope traits which may work.
  67 #[derive(Debug)]
  68 pub struct NoMatchData<'tcx> {
  69     pub static_candidates: Vec<CandidateSource>,
  70     pub unsatisfied_predicates:
  71         Vec<(ty::Predicate<'tcx>, Option<ty::Predicate<'tcx>>, Option<ObligationCause<'tcx>>)>,
  72     pub out_of_scope_traits: Vec<DefId>,
  73     pub lev_candidate: Option<ty::AssocItem>,
  74     pub mode: probe::Mode,
  75 }
  76
  77 // A pared down enum describing just the places from which a method
  78 // candidate can arise. Used for error reporting only.
  79 #[derive(Copy, Clone, Debug, Eq, Ord, PartialEq, PartialOrd)]
  80 pub enum CandidateSource {
  81     Impl(DefId),
  82     Trait(DefId /* trait id */),
  83 }
  84
  85 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  86     /// Determines whether the type `self_ty` supports a method name `method_name` or not.
  87     #[instrument(level = "debug", skip(self))]
  88     pub fn method_exists(
  89         &self,
  90         method_name: Ident,
  91         self_ty: Ty<'tcx>,
  92         call_expr_id: hir::HirId,
  93         allow_private: bool,
  94     ) -> bool {
  95         let mode = probe::Mode::MethodCall;
  96         match self.probe_for_name(
  97             method_name.span,
  98             mode,
  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(false),
 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         let mode = probe::Mode::MethodCall;
 256         let self_ty = self.resolve_vars_if_possible(self_ty);
 257         self.probe_for_name(
 258             span,
 259             mode,
 260             method_name,
 261             IsSuggestion(false),
 262             self_ty,
 263             call_expr.hir_id,
 264             scope,
 265         )
 266     }
 267
 268     pub(super) fn obligation_for_method(
 269         &self,
 270         span: Span,
 271         trait_def_id: DefId,
 272         self_ty: Ty<'tcx>,
 273         opt_input_types: Option<&[Ty<'tcx>]>,
 274     ) -> (traits::Obligation<'tcx, ty::Predicate<'tcx>>, &'tcx ty::List<ty::subst::GenericArg<'tcx>>)
 275     {
 276         // Construct a trait-reference `self_ty : Trait<input_tys>`
 277         let substs = InternalSubsts::for_item(self.tcx, trait_def_id, |param, _| {
 278             match param.kind {
 279                 GenericParamDefKind::Lifetime | GenericParamDefKind::Const { .. } => {}
 280                 GenericParamDefKind::Type { .. } => {
 281                     if param.index == 0 {
 282                         return self_ty.into();
 283                     } else if let Some(input_types) = opt_input_types {
 284                         return input_types[param.index as usize - 1].into();
 285                     }
 286                 }
 287             }
 288             self.var_for_def(span, param)
 289         });
 290
 291         let trait_ref = ty::TraitRef::new(trait_def_id, substs);
 292
 293         // Construct an obligation
 294         let poly_trait_ref = ty::Binder::dummy(trait_ref);
 295         (
 296             traits::Obligation::misc(
 297                 span,
 298                 self.body_id,
 299                 self.param_env,
 300                 poly_trait_ref.without_const().to_predicate(self.tcx),
 301             ),
 302             substs,
 303         )
 304     }
 305
 306     pub(super) fn obligation_for_op_method(
 307         &self,
 308         span: Span,
 309         trait_def_id: DefId,
 310         self_ty: Ty<'tcx>,
 311         opt_input_type: Option<Ty<'tcx>>,
 312         opt_input_expr: Option<&'tcx hir::Expr<'tcx>>,
 313         expected: Expectation<'tcx>,
 314     ) -> (traits::Obligation<'tcx, ty::Predicate<'tcx>>, &'tcx ty::List<ty::subst::GenericArg<'tcx>>)
 315     {
 316         // Construct a trait-reference `self_ty : Trait<input_tys>`
 317         let substs = InternalSubsts::for_item(self.tcx, trait_def_id, |param, _| {
 318             match param.kind {
 319                 GenericParamDefKind::Lifetime | GenericParamDefKind::Const { .. } => {}
 320                 GenericParamDefKind::Type { .. } => {
 321                     if param.index == 0 {
 322                         return self_ty.into();
 323                     } else if let Some(input_type) = opt_input_type {
 324                         return input_type.into();
 325                     }
 326                 }
 327             }
 328             self.var_for_def(span, param)
 329         });
 330
 331         let trait_ref = ty::TraitRef::new(trait_def_id, substs);
 332
 333         // Construct an obligation
 334         let poly_trait_ref = ty::Binder::dummy(trait_ref);
 335         let output_ty = expected.only_has_type(self).and_then(|ty| (!ty.needs_infer()).then(|| ty));
 336
 337         (
 338             traits::Obligation::new(
 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.without_const().to_predicate(self.tcx),
 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         assert_eq!(generics.params.len(), 0);
 447
 448         debug!("lookup_in_trait_adjusted: method_item={:?}", method_item);
 449         let mut obligations = vec![];
 450
 451         // Instantiate late-bound regions and substitute the trait
 452         // parameters into the method type to get the actual method type.
 453         //
 454         // N.B., instantiate late-bound regions first so that
 455         // `instantiate_type_scheme` can normalize associated types that
 456         // may reference those regions.
 457         let fn_sig = tcx.bound_fn_sig(def_id);
 458         let fn_sig = fn_sig.subst(self.tcx, substs);
 459         let fn_sig = self.replace_bound_vars_with_fresh_vars(span, infer::FnCall, fn_sig);
 460
 461         let InferOk { value, obligations: o } = if is_op {
 462             self.normalize_op_associated_types_in_as_infer_ok(span, fn_sig, opt_input_expr)
 463         } else {
 464             self.normalize_associated_types_in_as_infer_ok(span, fn_sig)
 465         };
 466         let fn_sig = {
 467             obligations.extend(o);
 468             value
 469         };
 470
 471         // Register obligations for the parameters. This will include the
 472         // `Self` parameter, which in turn has a bound of the main trait,
 473         // so this also effectively registers `obligation` as well.  (We
 474         // used to register `obligation` explicitly, but that resulted in
 475         // double error messages being reported.)
 476         //
 477         // Note that as the method comes from a trait, it should not have
 478         // any late-bound regions appearing in its bounds.
 479         let bounds = self.tcx.predicates_of(def_id).instantiate(self.tcx, substs);
 480
 481         let InferOk { value, obligations: o } = if is_op {
 482             self.normalize_op_associated_types_in_as_infer_ok(span, bounds, opt_input_expr)
 483         } else {
 484             self.normalize_associated_types_in_as_infer_ok(span, bounds)
 485         };
 486         let bounds = {
 487             obligations.extend(o);
 488             value
 489         };
 490
 491         assert!(!bounds.has_escaping_bound_vars());
 492
 493         let cause = if is_op {
 494             ObligationCause::new(
 495                 span,
 496                 self.body_id,
 497                 traits::BinOp {
 498                     rhs_span: opt_input_expr.map(|expr| expr.span),
 499                     is_lit: opt_input_expr
 500                         .map_or(false, |expr| matches!(expr.kind, hir::ExprKind::Lit(_))),
 501                     output_ty: None,
 502                 },
 503             )
 504         } else {
 505             traits::ObligationCause::misc(span, self.body_id)
 506         };
 507         let predicates_cause = cause.clone();
 508         obligations.extend(traits::predicates_for_generics(
 509             move |_, _| predicates_cause.clone(),
 510             self.param_env,
 511             bounds,
 512         ));
 513
 514         // Also add an obligation for the method type being well-formed.
 515         let method_ty = tcx.mk_fn_ptr(ty::Binder::dummy(fn_sig));
 516         debug!(
 517             "lookup_in_trait_adjusted: matched method method_ty={:?} obligation={:?}",
 518             method_ty, obligation
 519         );
 520         obligations.push(traits::Obligation::new(
 521             cause,
 522             self.param_env,
 523             ty::Binder::dummy(ty::PredicateKind::WellFormed(method_ty.into())).to_predicate(tcx),
 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         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                     // Braced variants generate unusable names in value namespace (reserved for
 569                     // possible future use), so variants resolved as associated items may refer to
 570                     // them as well. It's ok to use the variant's id as a ctor id since an
 571                     // error will be reported on any use of such resolution anyway.
 572                     let ctor_def_id = variant_def.ctor_def_id.unwrap_or(variant_def.def_id);
 573                     tcx.check_stability(ctor_def_id, Some(expr_id), span, Some(method_name.span));
 574                     return Ok((
 575                         DefKind::Ctor(CtorOf::Variant, variant_def.ctor_kind),
 576                         ctor_def_id,
 577                     ));
 578                 }
 579             }
 580         }
 581
 582         let pick = self.probe_for_name(
 583             span,
 584             probe::Mode::Path,
 585             method_name,
 586             IsSuggestion(false),
 587             self_ty,
 588             expr_id,
 589             ProbeScope::TraitsInScope,
 590         )?;
 591
 592         self.lint_fully_qualified_call_from_2018(
 593             span,
 594             method_name,
 595             self_ty,
 596             self_ty_span,
 597             expr_id,
 598             &pick,
 599         );
 600
 601         debug!(?pick);
 602         {
 603             let mut typeck_results = self.typeck_results.borrow_mut();
 604             let used_trait_imports = Lrc::get_mut(&mut typeck_results.used_trait_imports).unwrap();
 605             for import_id in pick.import_ids {
 606                 debug!(used_trait_import=?import_id);
 607                 used_trait_imports.insert(import_id);
 608             }
 609         }
 610
 611         let def_kind = pick.item.kind.as_def_kind();
 612         tcx.check_stability(pick.item.def_id, Some(expr_id), span, Some(method_name.span));
 613         Ok((def_kind, pick.item.def_id))
 614     }
 615
 616     /// Finds item with name `item_name` defined in impl/trait `def_id`
 617     /// and return it, or `None`, if no such item was defined there.
 618     pub fn associated_value(&self, def_id: DefId, item_name: Ident) -> Option<ty::AssocItem> {
 619         self.tcx
 620             .associated_items(def_id)
 621             .find_by_name_and_namespace(self.tcx, item_name, Namespace::ValueNS, def_id)
 622             .copied()
 623     }
 624 }