src/librustc_typeck/check/method/mod.rs

   1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! Method lookup: the secret sauce of Rust. See the [rustc guide] chapter.
  12 //!
  13 //! [rustc guide]: https://rust-lang-nursery.github.io/rustc-guide/method-lookup.html
  14
  15 use check::FnCtxt;
  16 use hir::def::Def;
  17 use hir::def_id::DefId;
  18 use namespace::Namespace;
  19 use rustc::ty::subst::Substs;
  20 use rustc::traits;
  21 use rustc::ty::{self, Ty, ToPredicate, ToPolyTraitRef, TraitRef, TypeFoldable};
  22 use rustc::ty::subst::Subst;
  23 use rustc::infer::{self, InferOk};
  24
  25 use syntax::ast;
  26 use syntax_pos::Span;
  27
  28 use rustc::hir;
  29
  30 use rustc_data_structures::sync::Lrc;
  31
  32 pub use self::MethodError::*;
  33 pub use self::CandidateSource::*;
  34 pub use self::suggest::TraitInfo;
  35
  36 mod confirm;
  37 pub mod probe;
  38 mod suggest;
  39
  40 use self::probe::{IsSuggestion, ProbeScope};
  41
  42 pub fn provide(providers: &mut ty::maps::Providers) {
  43     suggest::provide(providers);
  44 }
  45
  46 #[derive(Clone, Copy, Debug)]
  47 pub struct MethodCallee<'tcx> {
  48     /// Impl method ID, for inherent methods, or trait method ID, otherwise.
  49     pub def_id: DefId,
  50     pub substs: &'tcx Substs<'tcx>,
  51
  52     /// Instantiated method signature, i.e. it has been
  53     /// substituted, normalized, and has had late-bound
  54     /// lifetimes replaced with inference variables.
  55     pub sig: ty::FnSig<'tcx>,
  56 }
  57
  58 pub enum MethodError<'tcx> {
  59     // Did not find an applicable method, but we did find various near-misses that may work.
  60     NoMatch(NoMatchData<'tcx>),
  61
  62     // Multiple methods might apply.
  63     Ambiguity(Vec<CandidateSource>),
  64
  65     // Found an applicable method, but it is not visible. The second argument contains a list of
  66     // not-in-scope traits which may work.
  67     PrivateMatch(Def, Vec<DefId>),
  68
  69     // Found a `Self: Sized` bound where `Self` is a trait object, also the caller may have
  70     // forgotten to import a trait.
  71     IllegalSizedBound(Vec<DefId>),
  72
  73     // Found a match, but the return type is wrong
  74     BadReturnType,
  75 }
  76
  77 // Contains a list of static methods that may apply, a list of unsatisfied trait predicates which
  78 // could lead to matches if satisfied, and a list of not-in-scope traits which may work.
  79 pub struct NoMatchData<'tcx> {
  80     pub static_candidates: Vec<CandidateSource>,
  81     pub unsatisfied_predicates: Vec<TraitRef<'tcx>>,
  82     pub out_of_scope_traits: Vec<DefId>,
  83     pub lev_candidate: Option<ty::AssociatedItem>,
  84     pub mode: probe::Mode,
  85 }
  86
  87 impl<'tcx> NoMatchData<'tcx> {
  88     pub fn new(static_candidates: Vec<CandidateSource>,
  89                unsatisfied_predicates: Vec<TraitRef<'tcx>>,
  90                out_of_scope_traits: Vec<DefId>,
  91                lev_candidate: Option<ty::AssociatedItem>,
  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(// trait id
 110                 DefId),
 111 }
 112
 113 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
 114     /// Determines whether the type `self_ty` supports a method name `method_name` or not.
 115     pub fn method_exists(&self,
 116                          span: Span,
 117                          method_name: ast::Name,
 118                          self_ty: Ty<'tcx>,
 119                          call_expr_id: ast::NodeId,
 120                          allow_private: bool)
 121                          -> bool {
 122         let mode = probe::Mode::MethodCall;
 123         match self.probe_for_name(span, mode, method_name, IsSuggestion(false),
 124                                   self_ty, call_expr_id, ProbeScope::TraitsInScope) {
 125             Ok(..) => true,
 126             Err(NoMatch(..)) => false,
 127             Err(Ambiguity(..)) => true,
 128             Err(PrivateMatch(..)) => allow_private,
 129             Err(IllegalSizedBound(..)) => true,
 130             Err(BadReturnType) => {
 131                 bug!("no return type expectations but got BadReturnType")
 132             }
 133
 134         }
 135     }
 136
 137     /// Performs method lookup. If lookup is successful, it will return the callee
 138     /// and store an appropriate adjustment for the self-expr. In some cases it may
 139     /// report an error (e.g., invoking the `drop` method).
 140     ///
 141     /// # Arguments
 142     ///
 143     /// Given a method call like `foo.bar::<T1,...Tn>(...)`:
 144     ///
 145     /// * `fcx`:                   the surrounding `FnCtxt` (!)
 146     /// * `span`:                  the span for the method call
 147     /// * `method_name`:           the name of the method being called (`bar`)
 148     /// * `self_ty`:               the (unadjusted) type of the self expression (`foo`)
 149     /// * `supplied_method_types`: the explicit method type parameters, if any (`T1..Tn`)
 150     /// * `self_expr`:             the self expression (`foo`)
 151     pub fn lookup_method(&self,
 152                          self_ty: Ty<'tcx>,
 153                          segment: &hir::PathSegment,
 154                          span: Span,
 155                          call_expr: &'gcx hir::Expr,
 156                          self_expr: &'gcx hir::Expr)
 157                          -> Result<MethodCallee<'tcx>, MethodError<'tcx>> {
 158         debug!("lookup(method_name={}, self_ty={:?}, call_expr={:?}, self_expr={:?})",
 159                segment.name,
 160                self_ty,
 161                call_expr,
 162                self_expr);
 163
 164         let pick = self.lookup_probe(
 165             span,
 166             segment.name,
 167             self_ty,
 168             call_expr,
 169             ProbeScope::TraitsInScope
 170         )?;
 171
 172         if let Some(import_id) = pick.import_id {
 173             let import_def_id = self.tcx.hir.local_def_id(import_id);
 174             debug!("used_trait_import: {:?}", import_def_id);
 175             Lrc::get_mut(&mut self.tables.borrow_mut().used_trait_imports)
 176                                         .unwrap().insert(import_def_id);
 177         }
 178
 179         self.tcx.check_stability(pick.item.def_id, Some(call_expr.id), span);
 180
 181         let result = self.confirm_method(span,
 182                                          self_expr,
 183                                          call_expr,
 184                                          self_ty,
 185                                          pick.clone(),
 186                                          segment);
 187
 188         if result.illegal_sized_bound {
 189             // We probe again, taking all traits into account (not only those in scope).
 190             let candidates =
 191                 match self.lookup_probe(span,
 192                                         segment.name,
 193                                         self_ty,
 194                                         call_expr,
 195                                         ProbeScope::AllTraits) {
 196
 197                     // If we find a different result the caller probably forgot to import a trait.
 198                     Ok(ref new_pick) if *new_pick != pick => vec![new_pick.item.container.id()],
 199                     Err(Ambiguity(ref sources)) => {
 200                         sources.iter()
 201                                .filter_map(|source| {
 202                                    match *source {
 203                                        // Note: this cannot come from an inherent impl,
 204                                        // because the first probing succeeded.
 205                                        ImplSource(def) => self.tcx.trait_id_of_impl(def),
 206                                        TraitSource(_) => None,
 207                                    }
 208                                })
 209                                .collect()
 210                     }
 211                     _ => Vec::new(),
 212                 };
 213
 214             return Err(IllegalSizedBound(candidates));
 215         }
 216
 217         Ok(result.callee)
 218     }
 219
 220     fn lookup_probe(&self,
 221                     span: Span,
 222                     method_name: ast::Name,
 223                     self_ty: Ty<'tcx>,
 224                     call_expr: &'gcx hir::Expr,
 225                     scope: ProbeScope)
 226                     -> probe::PickResult<'tcx> {
 227         let mode = probe::Mode::MethodCall;
 228         let self_ty = self.resolve_type_vars_if_possible(&self_ty);
 229         self.probe_for_name(span, mode, method_name, IsSuggestion(false),
 230                             self_ty, call_expr.id, scope)
 231     }
 232
 233     /// `lookup_method_in_trait` is used for overloaded operators.
 234     /// It does a very narrow slice of what the normal probe/confirm path does.
 235     /// In particular, it doesn't really do any probing: it simply constructs
 236     /// an obligation for a particular trait with the given self-type and checks
 237     /// whether that trait is implemented.
 238     ///
 239     /// FIXME(#18741) -- It seems likely that we can consolidate some of this
 240     /// code with the other method-lookup code. In particular, the second half
 241     /// of this method is basically the same as confirmation.
 242     pub fn lookup_method_in_trait(&self,
 243                                   span: Span,
 244                                   m_name: ast::Name,
 245                                   trait_def_id: DefId,
 246                                   self_ty: Ty<'tcx>,
 247                                   opt_input_types: Option<&[Ty<'tcx>]>)
 248                                   -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
 249         debug!("lookup_in_trait_adjusted(self_ty={:?}, \
 250                 m_name={}, trait_def_id={:?})",
 251                self_ty,
 252                m_name,
 253                trait_def_id);
 254
 255         // Construct a trait-reference `self_ty : Trait<input_tys>`
 256         let substs = Substs::for_item(self.tcx,
 257                                       trait_def_id,
 258                                       |def, _| self.region_var_for_def(span, def),
 259                                       |def, _substs| {
 260             if def.index == 0 {
 261                 self_ty
 262             } else if let Some(ref input_types) = opt_input_types {
 263                 input_types[def.index as usize - 1]
 264             } else {
 265                 self.type_var_for_def(span, def)
 266             }
 267         });
 268
 269         let trait_ref = ty::TraitRef::new(trait_def_id, substs);
 270
 271         // Construct an obligation
 272         let poly_trait_ref = trait_ref.to_poly_trait_ref();
 273         let obligation =
 274             traits::Obligation::misc(span,
 275                                      self.body_id,
 276                                      self.param_env,
 277                                      poly_trait_ref.to_predicate());
 278
 279         // Now we want to know if this can be matched
 280         if !self.predicate_may_hold(&obligation) {
 281             debug!("--> Cannot match obligation");
 282             return None; // Cannot be matched, no such method resolution is possible.
 283         }
 284
 285         // Trait must have a method named `m_name` and it should not have
 286         // type parameters or early-bound regions.
 287         let tcx = self.tcx;
 288         let method_item = self.associated_item(trait_def_id, m_name, Namespace::Value).unwrap();
 289         let def_id = method_item.def_id;
 290         let generics = tcx.generics_of(def_id);
 291         assert_eq!(generics.types.len(), 0);
 292         assert_eq!(generics.regions.len(), 0);
 293
 294         debug!("lookup_in_trait_adjusted: method_item={:?}", method_item);
 295         let mut obligations = vec![];
 296
 297         // Instantiate late-bound regions and substitute the trait
 298         // parameters into the method type to get the actual method type.
 299         //
 300         // NB: Instantiate late-bound regions first so that
 301         // `instantiate_type_scheme` can normalize associated types that
 302         // may reference those regions.
 303         let fn_sig = tcx.fn_sig(def_id);
 304         let fn_sig = self.replace_late_bound_regions_with_fresh_var(span,
 305                                                                     infer::FnCall,
 306                                                                     &fn_sig).0;
 307         let fn_sig = fn_sig.subst(self.tcx, substs);
 308         let fn_sig = match self.normalize_associated_types_in_as_infer_ok(span, &fn_sig) {
 309             InferOk { value, obligations: o } => {
 310                 obligations.extend(o);
 311                 value
 312             }
 313         };
 314
 315         // Register obligations for the parameters.  This will include the
 316         // `Self` parameter, which in turn has a bound of the main trait,
 317         // so this also effectively registers `obligation` as well.  (We
 318         // used to register `obligation` explicitly, but that resulted in
 319         // double error messages being reported.)
 320         //
 321         // Note that as the method comes from a trait, it should not have
 322         // any late-bound regions appearing in its bounds.
 323         let bounds = self.tcx.predicates_of(def_id).instantiate(self.tcx, substs);
 324         let bounds = match self.normalize_associated_types_in_as_infer_ok(span, &bounds) {
 325             InferOk { value, obligations: o } => {
 326                 obligations.extend(o);
 327                 value
 328             }
 329         };
 330         assert!(!bounds.has_escaping_regions());
 331
 332         let cause = traits::ObligationCause::misc(span, self.body_id);
 333         obligations.extend(traits::predicates_for_generics(cause.clone(),
 334                                                            self.param_env,
 335                                                            &bounds));
 336
 337         // Also add an obligation for the method type being well-formed.
 338         let method_ty = tcx.mk_fn_ptr(ty::Binder::bind(fn_sig));
 339         debug!("lookup_in_trait_adjusted: matched method method_ty={:?} obligation={:?}",
 340                method_ty,
 341                obligation);
 342         obligations.push(traits::Obligation::new(cause,
 343                                                  self.param_env,
 344                                                  ty::Predicate::WellFormed(method_ty)));
 345
 346         let callee = MethodCallee {
 347             def_id,
 348             substs: trait_ref.substs,
 349             sig: fn_sig,
 350         };
 351
 352         debug!("callee = {:?}", callee);
 353
 354         Some(InferOk {
 355             obligations,
 356             value: callee
 357         })
 358     }
 359
 360     pub fn resolve_ufcs(&self,
 361                         span: Span,
 362                         method_name: ast::Name,
 363                         self_ty: Ty<'tcx>,
 364                         expr_id: ast::NodeId)
 365                         -> Result<Def, MethodError<'tcx>> {
 366         let mode = probe::Mode::Path;
 367         let pick = self.probe_for_name(span, mode, method_name, IsSuggestion(false),
 368                                        self_ty, expr_id, ProbeScope::TraitsInScope)?;
 369
 370         if let Some(import_id) = pick.import_id {
 371             let import_def_id = self.tcx.hir.local_def_id(import_id);
 372             debug!("used_trait_import: {:?}", import_def_id);
 373             Lrc::get_mut(&mut self.tables.borrow_mut().used_trait_imports)
 374                                         .unwrap().insert(import_def_id);
 375         }
 376
 377         let def = pick.item.def();
 378         self.tcx.check_stability(def.def_id(), Some(expr_id), span);
 379
 380         Ok(def)
 381     }
 382
 383     /// Find item with name `item_name` defined in impl/trait `def_id`
 384     /// and return it, or `None`, if no such item was defined there.
 385     pub fn associated_item(&self, def_id: DefId, item_name: ast::Name, ns: Namespace)
 386                            -> Option<ty::AssociatedItem> {
 387         self.tcx.associated_items(def_id)
 388                 .find(|item| Namespace::from(item.kind) == ns &&
 389                              self.tcx.hygienic_eq(item_name, item.name, def_id))
 390     }
 391 }