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         let mode = probe::Mode::MethodCall;
  97         match self.probe_for_name(
  98             method_name.span,
  99             mode,
 100             method_name,
 101             IsSuggestion(false),
 102             self_ty,
 103             call_expr_id,
 104             ProbeScope::TraitsInScope,
 105         ) {
 106             Ok(..) => true,
 107             Err(NoMatch(..)) => false,
 108             Err(Ambiguity(..)) => true,
 109             Err(PrivateMatch(..)) => allow_private,
 110             Err(IllegalSizedBound(..)) => true,
 111             Err(BadReturnType) => bug!("no return type expectations but got BadReturnType"),
 112         }
 113     }
 114
 115     /// Adds a suggestion to call the given method to the provided diagnostic.
 116     #[instrument(level = "debug", skip(self, err, call_expr))]
 117     pub(crate) fn suggest_method_call(
 118         &self,
 119         err: &mut Diagnostic,
 120         msg: &str,
 121         method_name: Ident,
 122         self_ty: Ty<'tcx>,
 123         call_expr: &hir::Expr<'_>,
 124         span: Option<Span>,
 125     ) {
 126         let params = self
 127             .probe_for_name(
 128                 method_name.span,
 129                 probe::Mode::MethodCall,
 130                 method_name,
 131                 IsSuggestion(false),
 132                 self_ty,
 133                 call_expr.hir_id,
 134                 ProbeScope::TraitsInScope,
 135             )
 136             .map(|pick| {
 137                 let sig = self.tcx.fn_sig(pick.item.def_id);
 138                 sig.inputs().skip_binder().len().saturating_sub(1)
 139             })
 140             .unwrap_or(0);
 141
 142         // Account for `foo.bar<T>`;
 143         let sugg_span = span.unwrap_or(call_expr.span).shrink_to_hi();
 144         let (suggestion, applicability) = (
 145             format!("({})", (0..params).map(|_| "_").collect::<Vec<_>>().join(", ")),
 146             if params > 0 { Applicability::HasPlaceholders } else { Applicability::MaybeIncorrect },
 147         );
 148
 149         err.span_suggestion_verbose(sugg_span, msg, suggestion, applicability);
 150     }
 151
 152     /// Performs method lookup. If lookup is successful, it will return the callee
 153     /// and store an appropriate adjustment for the self-expr. In some cases it may
 154     /// report an error (e.g., invoking the `drop` method).
 155     ///
 156     /// # Arguments
 157     ///
 158     /// Given a method call like `foo.bar::<T1,...Tn>(a, b + 1, ...)`:
 159     ///
 160     /// * `self`:                  the surrounding `FnCtxt` (!)
 161     /// * `self_ty`:               the (unadjusted) type of the self expression (`foo`)
 162     /// * `segment`:               the name and generic arguments of the method (`bar::<T1, ...Tn>`)
 163     /// * `span`:                  the span for the method call
 164     /// * `call_expr`:             the complete method call: (`foo.bar::<T1,...Tn>(...)`)
 165     /// * `self_expr`:             the self expression (`foo`)
 166     /// * `args`:                  the expressions of the arguments (`a, b + 1, ...`)
 167     #[instrument(level = "debug", skip(self))]
 168     pub fn lookup_method(
 169         &self,
 170         self_ty: Ty<'tcx>,
 171         segment: &hir::PathSegment<'_>,
 172         span: Span,
 173         call_expr: &'tcx hir::Expr<'tcx>,
 174         self_expr: &'tcx hir::Expr<'tcx>,
 175         args: &'tcx [hir::Expr<'tcx>],
 176     ) -> Result<MethodCallee<'tcx>, MethodError<'tcx>> {
 177         let pick =
 178             self.lookup_probe(span, segment.ident, self_ty, call_expr, ProbeScope::TraitsInScope)?;
 179
 180         self.lint_dot_call_from_2018(self_ty, segment, span, call_expr, self_expr, &pick, args);
 181
 182         for import_id in &pick.import_ids {
 183             debug!("used_trait_import: {:?}", import_id);
 184             Lrc::get_mut(&mut self.typeck_results.borrow_mut().used_trait_imports)
 185                 .unwrap()
 186                 .insert(*import_id);
 187         }
 188
 189         self.tcx.check_stability(pick.item.def_id, Some(call_expr.hir_id), span, None);
 190
 191         let result =
 192             self.confirm_method(span, self_expr, call_expr, self_ty, pick.clone(), segment);
 193         debug!("result = {:?}", result);
 194
 195         if let Some(span) = result.illegal_sized_bound {
 196             let mut needs_mut = false;
 197             if let ty::Ref(region, t_type, mutability) = self_ty.kind() {
 198                 let trait_type = self
 199                     .tcx
 200                     .mk_ref(*region, ty::TypeAndMut { ty: *t_type, mutbl: mutability.invert() });
 201                 // We probe again to see if there might be a borrow mutability discrepancy.
 202                 match self.lookup_probe(
 203                     span,
 204                     segment.ident,
 205                     trait_type,
 206                     call_expr,
 207                     ProbeScope::TraitsInScope,
 208                 ) {
 209                     Ok(ref new_pick) if *new_pick != pick => {
 210                         needs_mut = true;
 211                     }
 212                     _ => {}
 213                 }
 214             }
 215
 216             // We probe again, taking all traits into account (not only those in scope).
 217             let mut candidates = match self.lookup_probe(
 218                 span,
 219                 segment.ident,
 220                 self_ty,
 221                 call_expr,
 222                 ProbeScope::AllTraits,
 223             ) {
 224                 // If we find a different result the caller probably forgot to import a trait.
 225                 Ok(ref new_pick) if *new_pick != pick => vec![new_pick.item.container_id(self.tcx)],
 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             candidates.retain(|candidate| *candidate != self.tcx.parent(result.callee.def_id));
 240
 241             return Err(IllegalSizedBound(candidates, needs_mut, span));
 242         }
 243
 244         Ok(result.callee)
 245     }
 246
 247     #[instrument(level = "debug", skip(self, call_expr))]
 248     pub fn lookup_probe(
 249         &self,
 250         span: Span,
 251         method_name: Ident,
 252         self_ty: Ty<'tcx>,
 253         call_expr: &'tcx hir::Expr<'tcx>,
 254         scope: ProbeScope,
 255     ) -> probe::PickResult<'tcx> {
 256         let mode = probe::Mode::MethodCall;
 257         let self_ty = self.resolve_vars_if_possible(self_ty);
 258         self.probe_for_name(
 259             span,
 260             mode,
 261             method_name,
 262             IsSuggestion(false),
 263             self_ty,
 264             call_expr.hir_id,
 265             scope,
 266         )
 267     }
 268
 269     pub(super) fn obligation_for_method(
 270         &self,
 271         span: Span,
 272         trait_def_id: DefId,
 273         self_ty: Ty<'tcx>,
 274         opt_input_types: Option<&[Ty<'tcx>]>,
 275     ) -> (traits::Obligation<'tcx, ty::Predicate<'tcx>>, &'tcx ty::List<ty::subst::GenericArg<'tcx>>)
 276     {
 277         // Construct a trait-reference `self_ty : Trait<input_tys>`
 278         let substs = InternalSubsts::for_item(self.tcx, trait_def_id, |param, _| {
 279             match param.kind {
 280                 GenericParamDefKind::Lifetime | GenericParamDefKind::Const { .. } => {}
 281                 GenericParamDefKind::Type { .. } => {
 282                     if param.index == 0 {
 283                         return self_ty.into();
 284                     } else if let Some(input_types) = opt_input_types {
 285                         return input_types[param.index as usize - 1].into();
 286                     }
 287                 }
 288             }
 289             self.var_for_def(span, param)
 290         });
 291
 292         let trait_ref = ty::TraitRef::new(trait_def_id, substs);
 293
 294         // Construct an obligation
 295         let poly_trait_ref = ty::Binder::dummy(trait_ref);
 296         (
 297             traits::Obligation::misc(
 298                 span,
 299                 self.body_id,
 300                 self.param_env,
 301                 poly_trait_ref.without_const().to_predicate(self.tcx),
 302             ),
 303             substs,
 304         )
 305     }
 306
 307     pub(super) fn obligation_for_op_method(
 308         &self,
 309         span: Span,
 310         trait_def_id: DefId,
 311         self_ty: Ty<'tcx>,
 312         opt_input_type: Option<Ty<'tcx>>,
 313         opt_input_expr: Option<&'tcx hir::Expr<'tcx>>,
 314         expected: Expectation<'tcx>,
 315     ) -> (traits::Obligation<'tcx, ty::Predicate<'tcx>>, &'tcx ty::List<ty::subst::GenericArg<'tcx>>)
 316     {
 317         // Construct a trait-reference `self_ty : Trait<input_tys>`
 318         let substs = InternalSubsts::for_item(self.tcx, trait_def_id, |param, _| {
 319             match param.kind {
 320                 GenericParamDefKind::Lifetime | GenericParamDefKind::Const { .. } => {}
 321                 GenericParamDefKind::Type { .. } => {
 322                     if param.index == 0 {
 323                         return self_ty.into();
 324                     } else if let Some(input_type) = opt_input_type {
 325                         return input_type.into();
 326                     }
 327                 }
 328             }
 329             self.var_for_def(span, param)
 330         });
 331
 332         let trait_ref = ty::TraitRef::new(trait_def_id, substs);
 333
 334         // Construct an obligation
 335         let poly_trait_ref = ty::Binder::dummy(trait_ref);
 336         let output_ty = expected.only_has_type(self).and_then(|ty| (!ty.needs_infer()).then(|| ty));
 337
 338         (
 339             traits::Obligation::new(
 340                 traits::ObligationCause::new(
 341                     span,
 342                     self.body_id,
 343                     traits::BinOp {
 344                         rhs_span: opt_input_expr.map(|expr| expr.span),
 345                         is_lit: opt_input_expr
 346                             .map_or(false, |expr| matches!(expr.kind, hir::ExprKind::Lit(_))),
 347                         output_ty,
 348                     },
 349                 ),
 350                 self.param_env,
 351                 poly_trait_ref.without_const().to_predicate(self.tcx),
 352             ),
 353             substs,
 354         )
 355     }
 356
 357     /// `lookup_method_in_trait` is used for overloaded operators.
 358     /// It does a very narrow slice of what the normal probe/confirm path does.
 359     /// In particular, it doesn't really do any probing: it simply constructs
 360     /// an obligation for a particular trait with the given self type and checks
 361     /// whether that trait is implemented.
 362     #[instrument(level = "debug", skip(self, span))]
 363     pub(super) fn lookup_method_in_trait(
 364         &self,
 365         span: Span,
 366         m_name: Ident,
 367         trait_def_id: DefId,
 368         self_ty: Ty<'tcx>,
 369         opt_input_types: Option<&[Ty<'tcx>]>,
 370     ) -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
 371         let (obligation, substs) =
 372             self.obligation_for_method(span, trait_def_id, self_ty, opt_input_types);
 373         self.construct_obligation_for_trait(
 374             span,
 375             m_name,
 376             trait_def_id,
 377             obligation,
 378             substs,
 379             None,
 380             false,
 381         )
 382     }
 383
 384     pub(super) fn lookup_op_method_in_trait(
 385         &self,
 386         span: Span,
 387         m_name: Ident,
 388         trait_def_id: DefId,
 389         self_ty: Ty<'tcx>,
 390         opt_input_type: Option<Ty<'tcx>>,
 391         opt_input_expr: Option<&'tcx hir::Expr<'tcx>>,
 392         expected: Expectation<'tcx>,
 393     ) -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
 394         let (obligation, substs) = self.obligation_for_op_method(
 395             span,
 396             trait_def_id,
 397             self_ty,
 398             opt_input_type,
 399             opt_input_expr,
 400             expected,
 401         );
 402         self.construct_obligation_for_trait(
 403             span,
 404             m_name,
 405             trait_def_id,
 406             obligation,
 407             substs,
 408             opt_input_expr,
 409             true,
 410         )
 411     }
 412
 413     // FIXME(#18741): it seems likely that we can consolidate some of this
 414     // code with the other method-lookup code. In particular, the second half
 415     // of this method is basically the same as confirmation.
 416     fn construct_obligation_for_trait(
 417         &self,
 418         span: Span,
 419         m_name: Ident,
 420         trait_def_id: DefId,
 421         obligation: traits::PredicateObligation<'tcx>,
 422         substs: &'tcx ty::List<ty::subst::GenericArg<'tcx>>,
 423         opt_input_expr: Option<&'tcx hir::Expr<'tcx>>,
 424         is_op: bool,
 425     ) -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
 426         debug!(?obligation);
 427
 428         // Now we want to know if this can be matched
 429         if !self.predicate_may_hold(&obligation) {
 430             debug!("--> Cannot match obligation");
 431             // Cannot be matched, no such method resolution is possible.
 432             return None;
 433         }
 434
 435         // Trait must have a method named `m_name` and it should not have
 436         // type parameters or early-bound regions.
 437         let tcx = self.tcx;
 438         let Some(method_item) = self.associated_value(trait_def_id, m_name) else {
 439             tcx.sess.delay_span_bug(
 440                 span,
 441                 "operator trait does not have corresponding operator method",
 442             );
 443             return None;
 444         };
 445         let def_id = method_item.def_id;
 446         let generics = tcx.generics_of(def_id);
 447
 448         if generics.params.len() != 0 {
 449             tcx.sess.emit_fatal(OpMethodGenericParams {
 450                 span: tcx.def_span(method_item.def_id),
 451                 method_name: m_name.to_string(),
 452             });
 453         }
 454
 455         debug!("lookup_in_trait_adjusted: method_item={:?}", method_item);
 456         let mut obligations = vec![];
 457
 458         // Instantiate late-bound regions and substitute the trait
 459         // parameters into the method type to get the actual method type.
 460         //
 461         // N.B., instantiate late-bound regions first so that
 462         // `instantiate_type_scheme` can normalize associated types that
 463         // may reference those regions.
 464         let fn_sig = tcx.bound_fn_sig(def_id);
 465         let fn_sig = fn_sig.subst(self.tcx, substs);
 466         let fn_sig = self.replace_bound_vars_with_fresh_vars(span, infer::FnCall, fn_sig);
 467
 468         let InferOk { value, obligations: o } = if is_op {
 469             self.normalize_op_associated_types_in_as_infer_ok(span, fn_sig, opt_input_expr)
 470         } else {
 471             self.normalize_associated_types_in_as_infer_ok(span, fn_sig)
 472         };
 473         let fn_sig = {
 474             obligations.extend(o);
 475             value
 476         };
 477
 478         // Register obligations for the parameters. This will include the
 479         // `Self` parameter, which in turn has a bound of the main trait,
 480         // so this also effectively registers `obligation` as well.  (We
 481         // used to register `obligation` explicitly, but that resulted in
 482         // double error messages being reported.)
 483         //
 484         // Note that as the method comes from a trait, it should not have
 485         // any late-bound regions appearing in its bounds.
 486         let bounds = self.tcx.predicates_of(def_id).instantiate(self.tcx, substs);
 487
 488         let InferOk { value, obligations: o } = if is_op {
 489             self.normalize_op_associated_types_in_as_infer_ok(span, bounds, opt_input_expr)
 490         } else {
 491             self.normalize_associated_types_in_as_infer_ok(span, bounds)
 492         };
 493         let bounds = {
 494             obligations.extend(o);
 495             value
 496         };
 497
 498         assert!(!bounds.has_escaping_bound_vars());
 499
 500         let cause = if is_op {
 501             ObligationCause::new(
 502                 span,
 503                 self.body_id,
 504                 traits::BinOp {
 505                     rhs_span: opt_input_expr.map(|expr| expr.span),
 506                     is_lit: opt_input_expr
 507                         .map_or(false, |expr| matches!(expr.kind, hir::ExprKind::Lit(_))),
 508                     output_ty: None,
 509                 },
 510             )
 511         } else {
 512             traits::ObligationCause::misc(span, self.body_id)
 513         };
 514         let predicates_cause = cause.clone();
 515         obligations.extend(traits::predicates_for_generics(
 516             move |_, _| predicates_cause.clone(),
 517             self.param_env,
 518             bounds,
 519         ));
 520
 521         // Also add an obligation for the method type being well-formed.
 522         let method_ty = tcx.mk_fn_ptr(ty::Binder::dummy(fn_sig));
 523         debug!(
 524             "lookup_in_trait_adjusted: matched method method_ty={:?} obligation={:?}",
 525             method_ty, obligation
 526         );
 527         obligations.push(traits::Obligation::new(
 528             cause,
 529             self.param_env,
 530             ty::Binder::dummy(ty::PredicateKind::WellFormed(method_ty.into())).to_predicate(tcx),
 531         ));
 532
 533         let callee = MethodCallee { def_id, substs, sig: fn_sig };
 534
 535         debug!("callee = {:?}", callee);
 536
 537         Some(InferOk { obligations, value: callee })
 538     }
 539
 540     /// Performs a [full-qualified function call] (formerly "universal function call") lookup. If
 541     /// lookup is successful, it will return the type of definition and the [`DefId`] of the found
 542     /// function definition.
 543     ///
 544     /// [full-qualified function call]: https://doc.rust-lang.org/reference/expressions/call-expr.html#disambiguating-function-calls
 545     ///
 546     /// # Arguments
 547     ///
 548     /// Given a function call like `Foo::bar::<T1,...Tn>(...)`:
 549     ///
 550     /// * `self`:                  the surrounding `FnCtxt` (!)
 551     /// * `span`:                  the span of the call, excluding arguments (`Foo::bar::<T1, ...Tn>`)
 552     /// * `method_name`:           the identifier of the function within the container type (`bar`)
 553     /// * `self_ty`:               the type to search within (`Foo`)
 554     /// * `self_ty_span`           the span for the type being searched within (span of `Foo`)
 555     /// * `expr_id`:               the [`hir::HirId`] of the expression composing the entire call
 556     #[instrument(level = "debug", skip(self), ret)]
 557     pub fn resolve_fully_qualified_call(
 558         &self,
 559         span: Span,
 560         method_name: Ident,
 561         self_ty: Ty<'tcx>,
 562         self_ty_span: Span,
 563         expr_id: hir::HirId,
 564     ) -> Result<(DefKind, DefId), MethodError<'tcx>> {
 565         let tcx = self.tcx;
 566
 567         // Check if we have an enum variant.
 568         if let ty::Adt(adt_def, _) = self_ty.kind() {
 569             if adt_def.is_enum() {
 570                 let variant_def = adt_def
 571                     .variants()
 572                     .iter()
 573                     .find(|vd| tcx.hygienic_eq(method_name, vd.ident(tcx), adt_def.did()));
 574                 if let Some(variant_def) = variant_def {
 575                     // Braced variants generate unusable names in value namespace (reserved for
 576                     // possible future use), so variants resolved as associated items may refer to
 577                     // them as well. It's ok to use the variant's id as a ctor id since an
 578                     // error will be reported on any use of such resolution anyway.
 579                     let ctor_def_id = variant_def.ctor_def_id.unwrap_or(variant_def.def_id);
 580                     tcx.check_stability(ctor_def_id, Some(expr_id), span, Some(method_name.span));
 581                     return Ok((
 582                         DefKind::Ctor(CtorOf::Variant, variant_def.ctor_kind),
 583                         ctor_def_id,
 584                     ));
 585                 }
 586             }
 587         }
 588
 589         let pick = self.probe_for_name(
 590             span,
 591             probe::Mode::Path,
 592             method_name,
 593             IsSuggestion(false),
 594             self_ty,
 595             expr_id,
 596             ProbeScope::TraitsInScope,
 597         )?;
 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 }