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 crate::ObligationCause;
  16 use rustc_data_structures::sync::Lrc;
  17 use rustc_errors::{Applicability, DiagnosticBuilder};
  18 use rustc_hir as hir;
  19 use rustc_hir::def::{CtorOf, DefKind, Namespace};
  20 use rustc_hir::def_id::DefId;
  21 use rustc_infer::infer::{self, InferOk};
  22 use rustc_middle::ty::subst::Subst;
  23 use rustc_middle::ty::subst::{InternalSubsts, SubstsRef};
  24 use rustc_middle::ty::GenericParamDefKind;
  25 use rustc_middle::ty::{self, ToPredicate, Ty, TypeFoldable, WithConstness};
  26 use rustc_span::symbol::Ident;
  27 use rustc_span::Span;
  28 use rustc_trait_selection::traits;
  29 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
  30
  31 use self::probe::{IsSuggestion, ProbeScope};
  32
  33 pub fn provide(providers: &mut ty::query::Providers) {
  34     suggest::provide(providers);
  35     probe::provide(providers);
  36 }
  37
  38 #[derive(Clone, Copy, Debug)]
  39 pub struct MethodCallee<'tcx> {
  40     /// Impl method ID, for inherent methods, or trait method ID, otherwise.
  41     pub def_id: DefId,
  42     pub substs: SubstsRef<'tcx>,
  43
  44     /// Instantiated method signature, i.e., it has been
  45     /// substituted, normalized, and has had late-bound
  46     /// lifetimes replaced with inference variables.
  47     pub sig: ty::FnSig<'tcx>,
  48 }
  49
  50 #[derive(Debug)]
  51 pub enum MethodError<'tcx> {
  52     // Did not find an applicable method, but we did find various near-misses that may work.
  53     NoMatch(NoMatchData<'tcx>),
  54
  55     // Multiple methods might apply.
  56     Ambiguity(Vec<CandidateSource>),
  57
  58     // Found an applicable method, but it is not visible. The third argument contains a list of
  59     // not-in-scope traits which may work.
  60     PrivateMatch(DefKind, DefId, Vec<DefId>),
  61
  62     // Found a `Self: Sized` bound where `Self` is a trait object, also the caller may have
  63     // forgotten to import a trait.
  64     IllegalSizedBound(Vec<DefId>, bool, Span),
  65
  66     // Found a match, but the return type is wrong
  67     BadReturnType,
  68 }
  69
  70 // Contains a list of static methods that may apply, a list of unsatisfied trait predicates which
  71 // could lead to matches if satisfied, and a list of not-in-scope traits which may work.
  72 #[derive(Debug)]
  73 pub struct NoMatchData<'tcx> {
  74     pub static_candidates: Vec<CandidateSource>,
  75     pub unsatisfied_predicates:
  76         Vec<(ty::Predicate<'tcx>, Option<ty::Predicate<'tcx>>, Option<ObligationCause<'tcx>>)>,
  77     pub out_of_scope_traits: Vec<DefId>,
  78     pub lev_candidate: Option<ty::AssocItem>,
  79     pub mode: probe::Mode,
  80 }
  81
  82 impl<'tcx> NoMatchData<'tcx> {
  83     pub fn new(
  84         static_candidates: Vec<CandidateSource>,
  85         unsatisfied_predicates: Vec<(
  86             ty::Predicate<'tcx>,
  87             Option<ty::Predicate<'tcx>>,
  88             Option<ObligationCause<'tcx>>,
  89         )>,
  90         out_of_scope_traits: Vec<DefId>,
  91         lev_candidate: Option<ty::AssocItem>,
  92         mode: probe::Mode,
  93     ) -> Self {
  94         NoMatchData {
  95             static_candidates,
  96             unsatisfied_predicates,
  97             out_of_scope_traits,
  98             lev_candidate,
  99             mode,
 100         }
 101     }
 102 }
 103
 104 // A pared down enum describing just the places from which a method
 105 // candidate can arise. Used for error reporting only.
 106 #[derive(Copy, Clone, Debug, Eq, Ord, PartialEq, PartialOrd)]
 107 pub enum CandidateSource {
 108     ImplSource(DefId),
 109     TraitSource(DefId /* trait id */),
 110 }
 111
 112 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 113     /// Determines whether the type `self_ty` supports a method name `method_name` or not.
 114     #[instrument(level = "debug", skip(self))]
 115     pub fn method_exists(
 116         &self,
 117         method_name: Ident,
 118         self_ty: Ty<'tcx>,
 119         call_expr_id: hir::HirId,
 120         allow_private: bool,
 121     ) -> bool {
 122         let mode = probe::Mode::MethodCall;
 123         match self.probe_for_name(
 124             method_name.span,
 125             mode,
 126             method_name,
 127             IsSuggestion(false),
 128             self_ty,
 129             call_expr_id,
 130             ProbeScope::TraitsInScope,
 131         ) {
 132             Ok(..) => true,
 133             Err(NoMatch(..)) => false,
 134             Err(Ambiguity(..)) => true,
 135             Err(PrivateMatch(..)) => allow_private,
 136             Err(IllegalSizedBound(..)) => true,
 137             Err(BadReturnType) => bug!("no return type expectations but got BadReturnType"),
 138         }
 139     }
 140
 141     /// Adds a suggestion to call the given method to the provided diagnostic.
 142     #[instrument(level = "debug", skip(self, err, call_expr))]
 143     crate fn suggest_method_call(
 144         &self,
 145         err: &mut DiagnosticBuilder<'a>,
 146         msg: &str,
 147         method_name: Ident,
 148         self_ty: Ty<'tcx>,
 149         call_expr: &hir::Expr<'_>,
 150         span: Option<Span>,
 151     ) {
 152         let params = self
 153             .probe_for_name(
 154                 method_name.span,
 155                 probe::Mode::MethodCall,
 156                 method_name,
 157                 IsSuggestion(false),
 158                 self_ty,
 159                 call_expr.hir_id,
 160                 ProbeScope::TraitsInScope,
 161             )
 162             .map(|pick| {
 163                 let sig = self.tcx.fn_sig(pick.item.def_id);
 164                 sig.inputs().skip_binder().len().saturating_sub(1)
 165             })
 166             .unwrap_or(0);
 167
 168         // Account for `foo.bar<T>`;
 169         let sugg_span = span.unwrap_or(call_expr.span).shrink_to_hi();
 170         let (suggestion, applicability) = (
 171             format!("({})", (0..params).map(|_| "_").collect::<Vec<_>>().join(", ")),
 172             if params > 0 { Applicability::HasPlaceholders } else { Applicability::MaybeIncorrect },
 173         );
 174
 175         err.span_suggestion_verbose(sugg_span, msg, suggestion, applicability);
 176     }
 177
 178     /// Performs method lookup. If lookup is successful, it will return the callee
 179     /// and store an appropriate adjustment for the self-expr. In some cases it may
 180     /// report an error (e.g., invoking the `drop` method).
 181     ///
 182     /// # Arguments
 183     ///
 184     /// Given a method call like `foo.bar::<T1,...Tn>(a, b + 1, ...)`:
 185     ///
 186     /// * `self`:                  the surrounding `FnCtxt` (!)
 187     /// * `self_ty`:               the (unadjusted) type of the self expression (`foo`)
 188     /// * `segment`:               the name and generic arguments of the method (`bar::<T1, ...Tn>`)
 189     /// * `span`:                  the span for the method call
 190     /// * `call_expr`:             the complete method call: (`foo.bar::<T1,...Tn>(...)`)
 191     /// * `self_expr`:             the self expression (`foo`)
 192     /// * `args`:                  the expressions of the arguments (`a, b + 1, ...`)
 193     #[instrument(level = "debug", skip(self, call_expr, self_expr))]
 194     pub fn lookup_method(
 195         &self,
 196         self_ty: Ty<'tcx>,
 197         segment: &hir::PathSegment<'_>,
 198         span: Span,
 199         call_expr: &'tcx hir::Expr<'tcx>,
 200         self_expr: &'tcx hir::Expr<'tcx>,
 201         args: &'tcx [hir::Expr<'tcx>],
 202     ) -> Result<MethodCallee<'tcx>, MethodError<'tcx>> {
 203         debug!(
 204             "lookup(method_name={}, self_ty={:?}, call_expr={:?}, self_expr={:?})",
 205             segment.ident, self_ty, call_expr, self_expr
 206         );
 207
 208         let pick =
 209             self.lookup_probe(span, segment.ident, self_ty, call_expr, ProbeScope::TraitsInScope)?;
 210
 211         self.lint_dot_call_from_2018(self_ty, segment, span, call_expr, self_expr, &pick, args);
 212
 213         for import_id in &pick.import_ids {
 214             debug!("used_trait_import: {:?}", import_id);
 215             Lrc::get_mut(&mut self.typeck_results.borrow_mut().used_trait_imports)
 216                 .unwrap()
 217                 .insert(*import_id);
 218         }
 219
 220         self.tcx.check_stability(pick.item.def_id, Some(call_expr.hir_id), span, None);
 221
 222         let result =
 223             self.confirm_method(span, self_expr, call_expr, self_ty, pick.clone(), segment);
 224         debug!("result = {:?}", result);
 225
 226         if let Some(span) = result.illegal_sized_bound {
 227             let mut needs_mut = false;
 228             if let ty::Ref(region, t_type, mutability) = self_ty.kind() {
 229                 let trait_type = self
 230                     .tcx
 231                     .mk_ref(region, ty::TypeAndMut { ty: t_type, mutbl: mutability.invert() });
 232                 // We probe again to see if there might be a borrow mutability discrepancy.
 233                 match self.lookup_probe(
 234                     span,
 235                     segment.ident,
 236                     trait_type,
 237                     call_expr,
 238                     ProbeScope::TraitsInScope,
 239                 ) {
 240                     Ok(ref new_pick) if *new_pick != pick => {
 241                         needs_mut = true;
 242                     }
 243                     _ => {}
 244                 }
 245             }
 246
 247             // We probe again, taking all traits into account (not only those in scope).
 248             let candidates = match self.lookup_probe(
 249                 span,
 250                 segment.ident,
 251                 self_ty,
 252                 call_expr,
 253                 ProbeScope::AllTraits,
 254             ) {
 255                 // If we find a different result the caller probably forgot to import a trait.
 256                 Ok(ref new_pick) if *new_pick != pick => vec![new_pick.item.container.id()],
 257                 Err(Ambiguity(ref sources)) => sources
 258                     .iter()
 259                     .filter_map(|source| {
 260                         match *source {
 261                             // Note: this cannot come from an inherent impl,
 262                             // because the first probing succeeded.
 263                             ImplSource(def) => self.tcx.trait_id_of_impl(def),
 264                             TraitSource(_) => None,
 265                         }
 266                     })
 267                     .collect(),
 268                 _ => Vec::new(),
 269             };
 270
 271             return Err(IllegalSizedBound(candidates, needs_mut, span));
 272         }
 273
 274         Ok(result.callee)
 275     }
 276
 277     #[instrument(level = "debug", skip(self, call_expr))]
 278     pub fn lookup_probe(
 279         &self,
 280         span: Span,
 281         method_name: Ident,
 282         self_ty: Ty<'tcx>,
 283         call_expr: &'tcx hir::Expr<'tcx>,
 284         scope: ProbeScope,
 285     ) -> probe::PickResult<'tcx> {
 286         let mode = probe::Mode::MethodCall;
 287         let self_ty = self.resolve_vars_if_possible(self_ty);
 288         self.probe_for_name(
 289             span,
 290             mode,
 291             method_name,
 292             IsSuggestion(false),
 293             self_ty,
 294             call_expr.hir_id,
 295             scope,
 296         )
 297     }
 298
 299     pub(super) fn obligation_for_method(
 300         &self,
 301         span: Span,
 302         trait_def_id: DefId,
 303         self_ty: Ty<'tcx>,
 304         opt_input_types: Option<&[Ty<'tcx>]>,
 305     ) -> (traits::Obligation<'tcx, ty::Predicate<'tcx>>, &'tcx ty::List<ty::subst::GenericArg<'tcx>>)
 306     {
 307         // Construct a trait-reference `self_ty : Trait<input_tys>`
 308         let substs = InternalSubsts::for_item(self.tcx, trait_def_id, |param, _| {
 309             match param.kind {
 310                 GenericParamDefKind::Lifetime | GenericParamDefKind::Const { .. } => {}
 311                 GenericParamDefKind::Type { .. } => {
 312                     if param.index == 0 {
 313                         return self_ty.into();
 314                     } else if let Some(input_types) = opt_input_types {
 315                         return input_types[param.index as usize - 1].into();
 316                     }
 317                 }
 318             }
 319             self.var_for_def(span, param)
 320         });
 321
 322         let trait_ref = ty::TraitRef::new(trait_def_id, substs);
 323
 324         // Construct an obligation
 325         let poly_trait_ref = ty::Binder::dummy(trait_ref);
 326         (
 327             traits::Obligation::misc(
 328                 span,
 329                 self.body_id,
 330                 self.param_env,
 331                 poly_trait_ref.without_const().to_predicate(self.tcx),
 332             ),
 333             substs,
 334         )
 335     }
 336
 337     /// `lookup_method_in_trait` is used for overloaded operators.
 338     /// It does a very narrow slice of what the normal probe/confirm path does.
 339     /// In particular, it doesn't really do any probing: it simply constructs
 340     /// an obligation for a particular trait with the given self type and checks
 341     /// whether that trait is implemented.
 342     //
 343     // FIXME(#18741): it seems likely that we can consolidate some of this
 344     // code with the other method-lookup code. In particular, the second half
 345     // of this method is basically the same as confirmation.
 346     #[instrument(level = "debug", skip(self, span, opt_input_types))]
 347     pub(super) fn lookup_method_in_trait(
 348         &self,
 349         span: Span,
 350         m_name: Ident,
 351         trait_def_id: DefId,
 352         self_ty: Ty<'tcx>,
 353         opt_input_types: Option<&[Ty<'tcx>]>,
 354     ) -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
 355         debug!(
 356             "lookup_in_trait_adjusted(self_ty={:?}, m_name={}, trait_def_id={:?}, opt_input_types={:?})",
 357             self_ty, m_name, trait_def_id, opt_input_types
 358         );
 359
 360         let (obligation, substs) =
 361             self.obligation_for_method(span, trait_def_id, self_ty, opt_input_types);
 362
 363         // Now we want to know if this can be matched
 364         if !self.predicate_may_hold(&obligation) {
 365             debug!("--> Cannot match obligation");
 366             // Cannot be matched, no such method resolution is possible.
 367             return None;
 368         }
 369
 370         // Trait must have a method named `m_name` and it should not have
 371         // type parameters or early-bound regions.
 372         let tcx = self.tcx;
 373         let method_item = match self.associated_item(trait_def_id, m_name, Namespace::ValueNS) {
 374             Some(method_item) => method_item,
 375             None => {
 376                 tcx.sess.delay_span_bug(
 377                     span,
 378                     "operator trait does not have corresponding operator method",
 379                 );
 380                 return None;
 381             }
 382         };
 383         let def_id = method_item.def_id;
 384         let generics = tcx.generics_of(def_id);
 385         assert_eq!(generics.params.len(), 0);
 386
 387         debug!("lookup_in_trait_adjusted: method_item={:?}", method_item);
 388         let mut obligations = vec![];
 389
 390         // Instantiate late-bound regions and substitute the trait
 391         // parameters into the method type to get the actual method type.
 392         //
 393         // N.B., instantiate late-bound regions first so that
 394         // `instantiate_type_scheme` can normalize associated types that
 395         // may reference those regions.
 396         let fn_sig = tcx.fn_sig(def_id);
 397         let fn_sig = self.replace_bound_vars_with_fresh_vars(span, infer::FnCall, fn_sig).0;
 398         let fn_sig = fn_sig.subst(self.tcx, substs);
 399
 400         let InferOk { value, obligations: o } =
 401             self.normalize_associated_types_in_as_infer_ok(span, fn_sig);
 402         let fn_sig = {
 403             obligations.extend(o);
 404             value
 405         };
 406
 407         // Register obligations for the parameters. This will include the
 408         // `Self` parameter, which in turn has a bound of the main trait,
 409         // so this also effectively registers `obligation` as well.  (We
 410         // used to register `obligation` explicitly, but that resulted in
 411         // double error messages being reported.)
 412         //
 413         // Note that as the method comes from a trait, it should not have
 414         // any late-bound regions appearing in its bounds.
 415         let bounds = self.tcx.predicates_of(def_id).instantiate(self.tcx, substs);
 416
 417         let InferOk { value, obligations: o } =
 418             self.normalize_associated_types_in_as_infer_ok(span, bounds);
 419         let bounds = {
 420             obligations.extend(o);
 421             value
 422         };
 423
 424         assert!(!bounds.has_escaping_bound_vars());
 425
 426         let cause = traits::ObligationCause::misc(span, self.body_id);
 427         obligations.extend(traits::predicates_for_generics(cause.clone(), self.param_env, bounds));
 428
 429         // Also add an obligation for the method type being well-formed.
 430         let method_ty = tcx.mk_fn_ptr(ty::Binder::dummy(fn_sig));
 431         debug!(
 432             "lookup_in_trait_adjusted: matched method method_ty={:?} obligation={:?}",
 433             method_ty, obligation
 434         );
 435         obligations.push(traits::Obligation::new(
 436             cause,
 437             self.param_env,
 438             ty::Binder::dummy(ty::PredicateKind::WellFormed(method_ty.into())).to_predicate(tcx),
 439         ));
 440
 441         let callee = MethodCallee { def_id, substs, sig: fn_sig };
 442
 443         debug!("callee = {:?}", callee);
 444
 445         Some(InferOk { obligations, value: callee })
 446     }
 447
 448     /// Performs a [full-qualified function call] (formerly "universal function call") lookup. If
 449     /// lookup is successful, it will return the type of definition and the [`DefId`] of the found
 450     /// function definition.
 451     ///
 452     /// [full-qualified function call]: https://doc.rust-lang.org/reference/expressions/call-expr.html#disambiguating-function-calls
 453     ///
 454     /// # Arguments
 455     ///
 456     /// Given a function call like `Foo::bar::<T1,...Tn>(...)`:
 457     ///
 458     /// * `self`:                  the surrounding `FnCtxt` (!)
 459     /// * `span`:                  the span of the call, excluding arguments (`Foo::bar::<T1, ...Tn>`)
 460     /// * `method_name`:           the identifier of the function within the container type (`bar`)
 461     /// * `self_ty`:               the type to search within (`Foo`)
 462     /// * `self_ty_span`           the span for the type being searched within (span of `Foo`)
 463     /// * `expr_id`:               the [`hir::HirId`] of the expression composing the entire call
 464     #[instrument(level = "debug", skip(self))]
 465     pub fn resolve_fully_qualified_call(
 466         &self,
 467         span: Span,
 468         method_name: Ident,
 469         self_ty: Ty<'tcx>,
 470         self_ty_span: Span,
 471         expr_id: hir::HirId,
 472     ) -> Result<(DefKind, DefId), MethodError<'tcx>> {
 473         debug!(
 474             "resolve_fully_qualified_call: method_name={:?} self_ty={:?} expr_id={:?}",
 475             method_name, self_ty, expr_id,
 476         );
 477
 478         let tcx = self.tcx;
 479
 480         // Check if we have an enum variant.
 481         if let ty::Adt(adt_def, _) = self_ty.kind() {
 482             if adt_def.is_enum() {
 483                 let variant_def = adt_def
 484                     .variants
 485                     .iter()
 486                     .find(|vd| tcx.hygienic_eq(method_name, vd.ident, adt_def.did));
 487                 if let Some(variant_def) = variant_def {
 488                     // Braced variants generate unusable names in value namespace (reserved for
 489                     // possible future use), so variants resolved as associated items may refer to
 490                     // them as well. It's ok to use the variant's id as a ctor id since an
 491                     // error will be reported on any use of such resolution anyway.
 492                     let ctor_def_id = variant_def.ctor_def_id.unwrap_or(variant_def.def_id);
 493                     tcx.check_stability(ctor_def_id, Some(expr_id), span, Some(method_name.span));
 494                     return Ok((
 495                         DefKind::Ctor(CtorOf::Variant, variant_def.ctor_kind),
 496                         ctor_def_id,
 497                     ));
 498                 }
 499             }
 500         }
 501
 502         let pick = self.probe_for_name(
 503             span,
 504             probe::Mode::Path,
 505             method_name,
 506             IsSuggestion(false),
 507             self_ty,
 508             expr_id,
 509             ProbeScope::TraitsInScope,
 510         )?;
 511
 512         self.lint_fully_qualified_call_from_2018(
 513             span,
 514             method_name,
 515             self_ty,
 516             self_ty_span,
 517             expr_id,
 518             &pick,
 519         );
 520
 521         debug!("resolve_fully_qualified_call: pick={:?}", pick);
 522         {
 523             let mut typeck_results = self.typeck_results.borrow_mut();
 524             let used_trait_imports = Lrc::get_mut(&mut typeck_results.used_trait_imports).unwrap();
 525             for import_id in pick.import_ids {
 526                 debug!("resolve_fully_qualified_call: used_trait_import: {:?}", import_id);
 527                 used_trait_imports.insert(import_id);
 528             }
 529         }
 530
 531         let def_kind = pick.item.kind.as_def_kind();
 532         debug!(
 533             "resolve_fully_qualified_call: def_kind={:?}, def_id={:?}",
 534             def_kind, pick.item.def_id
 535         );
 536         tcx.check_stability(pick.item.def_id, Some(expr_id), span, Some(method_name.span));
 537         Ok((def_kind, pick.item.def_id))
 538     }
 539
 540     /// Finds item with name `item_name` defined in impl/trait `def_id`
 541     /// and return it, or `None`, if no such item was defined there.
 542     pub fn associated_item(
 543         &self,
 544         def_id: DefId,
 545         item_name: Ident,
 546         ns: Namespace,
 547     ) -> Option<ty::AssocItem> {
 548         self.tcx
 549             .associated_items(def_id)
 550             .find_by_name_and_namespace(self.tcx, item_name, ns, def_id)
 551             .copied()
 552     }
 553 }