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