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