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 pub mod probe;
   7 mod suggest;
   8
   9 pub use self::suggest::{SelfSource, TraitInfo};
  10 pub use self::CandidateSource::*;
  11 pub use self::MethodError::*;
  12
  13 use crate::check::FnCtxt;
  14 use rustc_data_structures::sync::Lrc;
  15 use rustc_errors::{Applicability, DiagnosticBuilder};
  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::ty::subst::Subst;
  21 use rustc_middle::ty::subst::{InternalSubsts, SubstsRef};
  22 use rustc_middle::ty::GenericParamDefKind;
  23 use rustc_middle::ty::{self, ToPolyTraitRef, ToPredicate, Ty, TypeFoldable, WithConstness};
  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     suggest::provide(providers);
  33     probe::provide(providers);
  34 }
  35
  36 #[derive(Clone, Copy, Debug)]
  37 pub struct MethodCallee<'tcx> {
  38     /// Impl method ID, for inherent methods, or trait method ID, otherwise.
  39     pub def_id: DefId,
  40     pub substs: SubstsRef<'tcx>,
  41
  42     /// Instantiated method signature, i.e., it has been
  43     /// substituted, normalized, and has had late-bound
  44     /// lifetimes replaced with inference variables.
  45     pub sig: ty::FnSig<'tcx>,
  46 }
  47
  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, also the caller may have
  60     // forgotten to import a trait.
  61     IllegalSizedBound(Vec<DefId>, bool, Span),
  62
  63     // Found a match, but the return type is wrong
  64     BadReturnType,
  65 }
  66
  67 // Contains a list of static methods that may apply, a list of unsatisfied trait predicates which
  68 // could lead to matches if satisfied, and a list of not-in-scope traits which may work.
  69 pub struct NoMatchData<'tcx> {
  70     pub static_candidates: Vec<CandidateSource>,
  71     pub unsatisfied_predicates: Vec<(ty::Predicate<'tcx>, Option<ty::Predicate<'tcx>>)>,
  72     pub out_of_scope_traits: Vec<DefId>,
  73     pub lev_candidate: Option<ty::AssocItem>,
  74     pub mode: probe::Mode,
  75 }
  76
  77 impl<'tcx> NoMatchData<'tcx> {
  78     pub fn new(
  79         static_candidates: Vec<CandidateSource>,
  80         unsatisfied_predicates: Vec<(ty::Predicate<'tcx>, Option<ty::Predicate<'tcx>>)>,
  81         out_of_scope_traits: Vec<DefId>,
  82         lev_candidate: Option<ty::AssocItem>,
  83         mode: probe::Mode,
  84     ) -> Self {
  85         NoMatchData {
  86             static_candidates,
  87             unsatisfied_predicates,
  88             out_of_scope_traits,
  89             lev_candidate,
  90             mode,
  91         }
  92     }
  93 }
  94
  95 // A pared down enum describing just the places from which a method
  96 // candidate can arise. Used for error reporting only.
  97 #[derive(Copy, Clone, Debug, Eq, Ord, PartialEq, PartialOrd)]
  98 pub enum CandidateSource {
  99     ImplSource(DefId),
 100     TraitSource(DefId /* trait id */),
 101 }
 102
 103 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 104     /// Determines whether the type `self_ty` supports a method name `method_name` or not.
 105     #[instrument(level = "debug", skip(self))]
 106     pub fn method_exists(
 107         &self,
 108         method_name: Ident,
 109         self_ty: Ty<'tcx>,
 110         call_expr_id: hir::HirId,
 111         allow_private: bool,
 112     ) -> bool {
 113         let mode = probe::Mode::MethodCall;
 114         match self.probe_for_name(
 115             method_name.span,
 116             mode,
 117             method_name,
 118             IsSuggestion(false),
 119             self_ty,
 120             call_expr_id,
 121             ProbeScope::TraitsInScope,
 122         ) {
 123             Ok(..) => true,
 124             Err(NoMatch(..)) => false,
 125             Err(Ambiguity(..)) => true,
 126             Err(PrivateMatch(..)) => allow_private,
 127             Err(IllegalSizedBound(..)) => true,
 128             Err(BadReturnType) => bug!("no return type expectations but got BadReturnType"),
 129         }
 130     }
 131
 132     /// Adds a suggestion to call the given method to the provided diagnostic.
 133     #[instrument(level = "debug", skip(self, err, call_expr))]
 134     crate fn suggest_method_call(
 135         &self,
 136         err: &mut DiagnosticBuilder<'a>,
 137         msg: &str,
 138         method_name: Ident,
 139         self_ty: Ty<'tcx>,
 140         call_expr: &hir::Expr<'_>,
 141     ) {
 142         let params = self
 143             .probe_for_name(
 144                 method_name.span,
 145                 probe::Mode::MethodCall,
 146                 method_name,
 147                 IsSuggestion(false),
 148                 self_ty,
 149                 call_expr.hir_id,
 150                 ProbeScope::TraitsInScope,
 151             )
 152             .map(|pick| {
 153                 let sig = self.tcx.fn_sig(pick.item.def_id);
 154                 sig.inputs().skip_binder().len().saturating_sub(1)
 155             })
 156             .unwrap_or(0);
 157
 158         // Account for `foo.bar<T>`;
 159         let sugg_span = call_expr.span.shrink_to_hi();
 160         let (suggestion, applicability) = (
 161             format!("({})", (0..params).map(|_| "_").collect::<Vec<_>>().join(", ")),
 162             if params > 0 { Applicability::HasPlaceholders } else { Applicability::MaybeIncorrect },
 163         );
 164
 165         err.span_suggestion_verbose(sugg_span, msg, suggestion, applicability);
 166     }
 167
 168     /// Performs method lookup. If lookup is successful, it will return the callee
 169     /// and store an appropriate adjustment for the self-expr. In some cases it may
 170     /// report an error (e.g., invoking the `drop` method).
 171     ///
 172     /// # Arguments
 173     ///
 174     /// Given a method call like `foo.bar::<T1,...Tn>(...)`:
 175     ///
 176     /// * `self`:                  the surrounding `FnCtxt` (!)
 177     /// * `self_ty`:               the (unadjusted) type of the self expression (`foo`)
 178     /// * `segment`:               the name and generic arguments of the method (`bar::<T1, ...Tn>`)
 179     /// * `span`:                  the span for the method call
 180     /// * `call_expr`:             the complete method call: (`foo.bar::<T1,...Tn>(...)`)
 181     /// * `self_expr`:             the self expression (`foo`)
 182     #[instrument(level = "debug", skip(self, call_expr, self_expr))]
 183     pub fn lookup_method(
 184         &self,
 185         self_ty: Ty<'tcx>,
 186         segment: &hir::PathSegment<'_>,
 187         span: Span,
 188         call_expr: &'tcx hir::Expr<'tcx>,
 189         self_expr: &'tcx hir::Expr<'tcx>,
 190     ) -> Result<MethodCallee<'tcx>, MethodError<'tcx>> {
 191         debug!(
 192             "lookup(method_name={}, self_ty={:?}, call_expr={:?}, self_expr={:?})",
 193             segment.ident, self_ty, call_expr, self_expr
 194         );
 195
 196         let pick =
 197             self.lookup_probe(span, segment.ident, self_ty, call_expr, ProbeScope::TraitsInScope)?;
 198
 199         for import_id in &pick.import_ids {
 200             debug!("used_trait_import: {:?}", import_id);
 201             Lrc::get_mut(&mut self.typeck_results.borrow_mut().used_trait_imports)
 202                 .unwrap()
 203                 .insert(*import_id);
 204         }
 205
 206         self.tcx.check_stability(pick.item.def_id, Some(call_expr.hir_id), span);
 207
 208         let result =
 209             self.confirm_method(span, self_expr, call_expr, self_ty, pick.clone(), segment);
 210         debug!("result = {:?}", result);
 211
 212         if let Some(span) = result.illegal_sized_bound {
 213             let mut needs_mut = false;
 214             if let ty::Ref(region, t_type, mutability) = self_ty.kind() {
 215                 let trait_type = self
 216                     .tcx
 217                     .mk_ref(region, ty::TypeAndMut { ty: t_type, mutbl: mutability.invert() });
 218                 // We probe again to see if there might be a borrow mutability discrepancy.
 219                 match self.lookup_probe(
 220                     span,
 221                     segment.ident,
 222                     trait_type,
 223                     call_expr,
 224                     ProbeScope::TraitsInScope,
 225                 ) {
 226                     Ok(ref new_pick) if *new_pick != pick => {
 227                         needs_mut = true;
 228                     }
 229                     _ => {}
 230                 }
 231             }
 232
 233             // We probe again, taking all traits into account (not only those in scope).
 234             let candidates = match self.lookup_probe(
 235                 span,
 236                 segment.ident,
 237                 self_ty,
 238                 call_expr,
 239                 ProbeScope::AllTraits,
 240             ) {
 241                 // If we find a different result the caller probably forgot to import a trait.
 242                 Ok(ref new_pick) if *new_pick != pick => vec![new_pick.item.container.id()],
 243                 Err(Ambiguity(ref sources)) => sources
 244                     .iter()
 245                     .filter_map(|source| {
 246                         match *source {
 247                             // Note: this cannot come from an inherent impl,
 248                             // because the first probing succeeded.
 249                             ImplSource(def) => self.tcx.trait_id_of_impl(def),
 250                             TraitSource(_) => None,
 251                         }
 252                     })
 253                     .collect(),
 254                 _ => Vec::new(),
 255             };
 256
 257             return Err(IllegalSizedBound(candidates, needs_mut, span));
 258         }
 259
 260         Ok(result.callee)
 261     }
 262
 263     #[instrument(level = "debug", skip(self, call_expr))]
 264     pub fn lookup_probe(
 265         &self,
 266         span: Span,
 267         method_name: Ident,
 268         self_ty: Ty<'tcx>,
 269         call_expr: &'tcx hir::Expr<'tcx>,
 270         scope: ProbeScope,
 271     ) -> probe::PickResult<'tcx> {
 272         let mode = probe::Mode::MethodCall;
 273         let self_ty = self.resolve_vars_if_possible(self_ty);
 274         self.probe_for_name(
 275             span,
 276             mode,
 277             method_name,
 278             IsSuggestion(false),
 279             self_ty,
 280             call_expr.hir_id,
 281             scope,
 282         )
 283     }
 284
 285     /// `lookup_method_in_trait` is used for overloaded operators.
 286     /// It does a very narrow slice of what the normal probe/confirm path does.
 287     /// In particular, it doesn't really do any probing: it simply constructs
 288     /// an obligation for a particular trait with the given self type and checks
 289     /// whether that trait is implemented.
 290     //
 291     // FIXME(#18741): it seems likely that we can consolidate some of this
 292     // code with the other method-lookup code. In particular, the second half
 293     // of this method is basically the same as confirmation.
 294     #[instrument(level = "debug", skip(self, span, opt_input_types))]
 295     pub fn lookup_method_in_trait(
 296         &self,
 297         span: Span,
 298         m_name: Ident,
 299         trait_def_id: DefId,
 300         self_ty: Ty<'tcx>,
 301         opt_input_types: Option<&[Ty<'tcx>]>,
 302     ) -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
 303         debug!(
 304             "lookup_in_trait_adjusted(self_ty={:?}, m_name={}, trait_def_id={:?})",
 305             self_ty, m_name, trait_def_id
 306         );
 307
 308         // Construct a trait-reference `self_ty : Trait<input_tys>`
 309         let substs = InternalSubsts::for_item(self.tcx, trait_def_id, |param, _| {
 310             match param.kind {
 311                 GenericParamDefKind::Lifetime | GenericParamDefKind::Const => {}
 312                 GenericParamDefKind::Type { .. } => {
 313                     if param.index == 0 {
 314                         return self_ty.into();
 315                     } else if let Some(ref input_types) = opt_input_types {
 316                         return input_types[param.index as usize - 1].into();
 317                     }
 318                 }
 319             }
 320             self.var_for_def(span, param)
 321         });
 322
 323         let trait_ref = ty::TraitRef::new(trait_def_id, substs);
 324
 325         // Construct an obligation
 326         let poly_trait_ref = trait_ref.to_poly_trait_ref();
 327         let obligation = 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
 334         // Now we want to know if this can be matched
 335         if !self.predicate_may_hold(&obligation) {
 336             debug!("--> Cannot match obligation");
 337             return None; // Cannot be matched, no such method resolution is possible.
 338         }
 339
 340         // Trait must have a method named `m_name` and it should not have
 341         // type parameters or early-bound regions.
 342         let tcx = self.tcx;
 343         let method_item = match self.associated_item(trait_def_id, m_name, Namespace::ValueNS) {
 344             Some(method_item) => method_item,
 345             None => {
 346                 tcx.sess.delay_span_bug(
 347                     span,
 348                     "operator trait does not have corresponding operator method",
 349                 );
 350                 return None;
 351             }
 352         };
 353         let def_id = method_item.def_id;
 354         let generics = tcx.generics_of(def_id);
 355         assert_eq!(generics.params.len(), 0);
 356
 357         debug!("lookup_in_trait_adjusted: method_item={:?}", method_item);
 358         let mut obligations = vec![];
 359
 360         // Instantiate late-bound regions and substitute the trait
 361         // parameters into the method type to get the actual method type.
 362         //
 363         // N.B., instantiate late-bound regions first so that
 364         // `instantiate_type_scheme` can normalize associated types that
 365         // may reference those regions.
 366         let fn_sig = tcx.fn_sig(def_id);
 367         let fn_sig = self.replace_bound_vars_with_fresh_vars(span, infer::FnCall, fn_sig).0;
 368         let fn_sig = fn_sig.subst(self.tcx, substs);
 369
 370         let InferOk { value, obligations: o } =
 371             self.normalize_associated_types_in_as_infer_ok(span, fn_sig);
 372         let fn_sig = {
 373             obligations.extend(o);
 374             value
 375         };
 376
 377         // Register obligations for the parameters. This will include the
 378         // `Self` parameter, which in turn has a bound of the main trait,
 379         // so this also effectively registers `obligation` as well.  (We
 380         // used to register `obligation` explicitly, but that resulted in
 381         // double error messages being reported.)
 382         //
 383         // Note that as the method comes from a trait, it should not have
 384         // any late-bound regions appearing in its bounds.
 385         let bounds = self.tcx.predicates_of(def_id).instantiate(self.tcx, substs);
 386
 387         let InferOk { value, obligations: o } =
 388             self.normalize_associated_types_in_as_infer_ok(span, bounds);
 389         let bounds = {
 390             obligations.extend(o);
 391             value
 392         };
 393
 394         assert!(!bounds.has_escaping_bound_vars());
 395
 396         let cause = traits::ObligationCause::misc(span, self.body_id);
 397         obligations.extend(traits::predicates_for_generics(cause.clone(), self.param_env, bounds));
 398
 399         // Also add an obligation for the method type being well-formed.
 400         let method_ty = tcx.mk_fn_ptr(ty::Binder::bind(fn_sig));
 401         debug!(
 402             "lookup_in_trait_adjusted: matched method method_ty={:?} obligation={:?}",
 403             method_ty, obligation
 404         );
 405         obligations.push(traits::Obligation::new(
 406             cause,
 407             self.param_env,
 408             ty::PredicateKind::WellFormed(method_ty.into()).to_predicate(tcx),
 409         ));
 410
 411         let callee = MethodCallee { def_id, substs: trait_ref.substs, sig: fn_sig };
 412
 413         debug!("callee = {:?}", callee);
 414
 415         Some(InferOk { obligations, value: callee })
 416     }
 417
 418     #[instrument(level = "debug", skip(self))]
 419     pub fn resolve_ufcs(
 420         &self,
 421         span: Span,
 422         method_name: Ident,
 423         self_ty: Ty<'tcx>,
 424         expr_id: hir::HirId,
 425     ) -> Result<(DefKind, DefId), MethodError<'tcx>> {
 426         debug!(
 427             "resolve_ufcs: method_name={:?} self_ty={:?} expr_id={:?}",
 428             method_name, self_ty, expr_id,
 429         );
 430
 431         let tcx = self.tcx;
 432
 433         // Check if we have an enum variant.
 434         if let ty::Adt(adt_def, _) = self_ty.kind() {
 435             if adt_def.is_enum() {
 436                 let variant_def = adt_def
 437                     .variants
 438                     .iter()
 439                     .find(|vd| tcx.hygienic_eq(method_name, vd.ident, adt_def.did));
 440                 if let Some(variant_def) = variant_def {
 441                     // Braced variants generate unusable names in value namespace (reserved for
 442                     // possible future use), so variants resolved as associated items may refer to
 443                     // them as well. It's ok to use the variant's id as a ctor id since an
 444                     // error will be reported on any use of such resolution anyway.
 445                     let ctor_def_id = variant_def.ctor_def_id.unwrap_or(variant_def.def_id);
 446                     tcx.check_stability(ctor_def_id, Some(expr_id), span);
 447                     return Ok((
 448                         DefKind::Ctor(CtorOf::Variant, variant_def.ctor_kind),
 449                         ctor_def_id,
 450                     ));
 451                 }
 452             }
 453         }
 454
 455         let pick = self.probe_for_name(
 456             span,
 457             probe::Mode::Path,
 458             method_name,
 459             IsSuggestion(false),
 460             self_ty,
 461             expr_id,
 462             ProbeScope::TraitsInScope,
 463         )?;
 464         debug!("resolve_ufcs: pick={:?}", pick);
 465         {
 466             let mut typeck_results = self.typeck_results.borrow_mut();
 467             let used_trait_imports = Lrc::get_mut(&mut typeck_results.used_trait_imports).unwrap();
 468             for import_id in pick.import_ids {
 469                 debug!("resolve_ufcs: used_trait_import: {:?}", import_id);
 470                 used_trait_imports.insert(import_id);
 471             }
 472         }
 473
 474         let def_kind = pick.item.kind.as_def_kind();
 475         debug!("resolve_ufcs: def_kind={:?}, def_id={:?}", def_kind, pick.item.def_id);
 476         tcx.check_stability(pick.item.def_id, Some(expr_id), span);
 477         Ok((def_kind, pick.item.def_id))
 478     }
 479
 480     /// Finds item with name `item_name` defined in impl/trait `def_id`
 481     /// and return it, or `None`, if no such item was defined there.
 482     pub fn associated_item(
 483         &self,
 484         def_id: DefId,
 485         item_name: Ident,
 486         ns: Namespace,
 487     ) -> Option<ty::AssocItem> {
 488         self.tcx
 489             .associated_items(def_id)
 490             .find_by_name_and_namespace(self.tcx, item_name, ns, def_id)
 491             .copied()
 492     }
 493 }