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