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 `README.md`.
  12
  13 use check::FnCtxt;
  14 use hir::def::Def;
  15 use hir::def_id::DefId;
  16 use namespace::Namespace;
  17 use rustc::ty::subst::Substs;
  18 use rustc::traits;
  19 use rustc::ty::{self, Ty, ToPredicate, ToPolyTraitRef, TraitRef, TypeFoldable};
  20 use rustc::ty::subst::Subst;
  21 use rustc::infer::{self, InferOk};
  22
  23 use syntax::ast;
  24 use syntax_pos::Span;
  25
  26 use rustc::hir;
  27
  28 use std::rc::Rc;
  29
  30 pub use self::MethodError::*;
  31 pub use self::CandidateSource::*;
  32
  33 mod confirm;
  34 pub mod probe;
  35 mod suggest;
  36
  37 use self::probe::{IsSuggestion, ProbeScope};
  38
  39 #[derive(Clone, Copy, Debug)]
  40 pub struct MethodCallee<'tcx> {
  41     /// Impl method ID, for inherent methods, or trait method ID, otherwise.
  42     pub def_id: DefId,
  43     pub substs: &'tcx Substs<'tcx>,
  44
  45     /// Instantiated method signature, i.e. it has been
  46     /// substituted, normalized, and has had late-bound
  47     /// lifetimes replaced with inference variables.
  48     pub sig: ty::FnSig<'tcx>,
  49 }
  50
  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 second argument contains a list of
  59     // not-in-scope traits which may work.
  60     PrivateMatch(Def, 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>),
  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 pub struct NoMatchData<'tcx> {
  73     pub static_candidates: Vec<CandidateSource>,
  74     pub unsatisfied_predicates: Vec<TraitRef<'tcx>>,
  75     pub out_of_scope_traits: Vec<DefId>,
  76     pub lev_candidate: Option<ty::AssociatedItem>,
  77     pub mode: probe::Mode,
  78 }
  79
  80 impl<'tcx> NoMatchData<'tcx> {
  81     pub fn new(static_candidates: Vec<CandidateSource>,
  82                unsatisfied_predicates: Vec<TraitRef<'tcx>>,
  83                out_of_scope_traits: Vec<DefId>,
  84                lev_candidate: Option<ty::AssociatedItem>,
  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(// trait id
 103                 DefId),
 104 }
 105
 106 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
 107     /// Determines whether the type `self_ty` supports a method name `method_name` or not.
 108     pub fn method_exists(&self,
 109                          span: Span,
 110                          method_name: ast::Name,
 111                          self_ty: Ty<'tcx>,
 112                          call_expr_id: ast::NodeId,
 113                          allow_private: bool)
 114                          -> bool {
 115         let mode = probe::Mode::MethodCall;
 116         match self.probe_for_name(span, mode, method_name, IsSuggestion(false),
 117                                   self_ty, call_expr_id, ProbeScope::TraitsInScope) {
 118             Ok(..) => true,
 119             Err(NoMatch(..)) => false,
 120             Err(Ambiguity(..)) => true,
 121             Err(PrivateMatch(..)) => allow_private,
 122             Err(IllegalSizedBound(..)) => true,
 123             Err(BadReturnType) => {
 124                 bug!("no return type expectations but got BadReturnType")
 125             }
 126
 127         }
 128     }
 129
 130     /// Performs method lookup. If lookup is successful, it will return the callee
 131     /// and store an appropriate adjustment for the self-expr. In some cases it may
 132     /// report an error (e.g., invoking the `drop` method).
 133     ///
 134     /// # Arguments
 135     ///
 136     /// Given a method call like `foo.bar::<T1,...Tn>(...)`:
 137     ///
 138     /// * `fcx`:                   the surrounding `FnCtxt` (!)
 139     /// * `span`:                  the span for the method call
 140     /// * `method_name`:           the name of the method being called (`bar`)
 141     /// * `self_ty`:               the (unadjusted) type of the self expression (`foo`)
 142     /// * `supplied_method_types`: the explicit method type parameters, if any (`T1..Tn`)
 143     /// * `self_expr`:             the self expression (`foo`)
 144     pub fn lookup_method(&self,
 145                          self_ty: Ty<'tcx>,
 146                          segment: &hir::PathSegment,
 147                          span: Span,
 148                          call_expr: &'gcx hir::Expr,
 149                          self_expr: &'gcx hir::Expr)
 150                          -> Result<MethodCallee<'tcx>, MethodError<'tcx>> {
 151         debug!("lookup(method_name={}, self_ty={:?}, call_expr={:?}, self_expr={:?})",
 152                segment.name,
 153                self_ty,
 154                call_expr,
 155                self_expr);
 156
 157         let pick = self.lookup_probe(
 158             span,
 159             segment.name,
 160             self_ty,
 161             call_expr,
 162             ProbeScope::TraitsInScope
 163         )?;
 164
 165         if let Some(import_id) = pick.import_id {
 166             let import_def_id = self.tcx.hir.local_def_id(import_id);
 167             debug!("used_trait_import: {:?}", import_def_id);
 168             Rc::get_mut(&mut self.tables.borrow_mut().used_trait_imports)
 169                                         .unwrap().insert(import_def_id);
 170         }
 171
 172         self.tcx.check_stability(pick.item.def_id, call_expr.id, span);
 173
 174         let result = self.confirm_method(span,
 175                                          self_expr,
 176                                          call_expr,
 177                                          self_ty,
 178                                          pick.clone(),
 179                                          segment);
 180
 181         if result.illegal_sized_bound {
 182             // We probe again, taking all traits into account (not only those in scope).
 183             let candidates =
 184                 match self.lookup_probe(span,
 185                                         segment.name,
 186                                         self_ty,
 187                                         call_expr,
 188                                         ProbeScope::AllTraits) {
 189
 190                     // If we find a different result the caller probably forgot to import a trait.
 191                     Ok(ref new_pick) if *new_pick != pick => vec![new_pick.item.container.id()],
 192                     Err(Ambiguity(ref sources)) => {
 193                         sources.iter()
 194                                .filter_map(|source| {
 195                                    match *source {
 196                                        // Note: this cannot come from an inherent impl,
 197                                        // because the first probing succeeded.
 198                                        ImplSource(def) => self.tcx.trait_id_of_impl(def),
 199                                        TraitSource(_) => None,
 200                                    }
 201                                })
 202                                .collect()
 203                     }
 204                     _ => Vec::new(),
 205                 };
 206
 207             return Err(IllegalSizedBound(candidates));
 208         }
 209
 210         Ok(result.callee)
 211     }
 212
 213     fn lookup_probe(&self,
 214                     span: Span,
 215                     method_name: ast::Name,
 216                     self_ty: Ty<'tcx>,
 217                     call_expr: &'gcx hir::Expr,
 218                     scope: ProbeScope)
 219                     -> probe::PickResult<'tcx> {
 220         let mode = probe::Mode::MethodCall;
 221         let self_ty = self.resolve_type_vars_if_possible(&self_ty);
 222         self.probe_for_name(span, mode, method_name, IsSuggestion(false),
 223                             self_ty, call_expr.id, scope)
 224     }
 225
 226     /// `lookup_method_in_trait` is used for overloaded operators.
 227     /// It does a very narrow slice of what the normal probe/confirm path does.
 228     /// In particular, it doesn't really do any probing: it simply constructs
 229     /// an obligation for a particular trait with the given self-type and checks
 230     /// whether that trait is implemented.
 231     ///
 232     /// FIXME(#18741) -- It seems likely that we can consolidate some of this
 233     /// code with the other method-lookup code. In particular, the second half
 234     /// of this method is basically the same as confirmation.
 235     pub fn lookup_method_in_trait(&self,
 236                                   span: Span,
 237                                   m_name: ast::Name,
 238                                   trait_def_id: DefId,
 239                                   self_ty: Ty<'tcx>,
 240                                   opt_input_types: Option<&[Ty<'tcx>]>)
 241                                   -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
 242         debug!("lookup_in_trait_adjusted(self_ty={:?}, \
 243                 m_name={}, trait_def_id={:?})",
 244                self_ty,
 245                m_name,
 246                trait_def_id);
 247
 248         // Construct a trait-reference `self_ty : Trait<input_tys>`
 249         let substs = Substs::for_item(self.tcx,
 250                                       trait_def_id,
 251                                       |def, _| self.region_var_for_def(span, def),
 252                                       |def, substs| {
 253             if def.index == 0 {
 254                 self_ty
 255             } else if let Some(ref input_types) = opt_input_types {
 256                 input_types[def.index as usize - 1]
 257             } else {
 258                 self.type_var_for_def(span, def, substs)
 259             }
 260         });
 261
 262         let trait_ref = ty::TraitRef::new(trait_def_id, substs);
 263
 264         // Construct an obligation
 265         let poly_trait_ref = trait_ref.to_poly_trait_ref();
 266         let obligation =
 267             traits::Obligation::misc(span,
 268                                      self.body_id,
 269                                      self.param_env,
 270                                      poly_trait_ref.to_predicate());
 271
 272         // Now we want to know if this can be matched
 273         let mut selcx = traits::SelectionContext::new(self);
 274         if !selcx.evaluate_obligation(&obligation) {
 275             debug!("--> Cannot match obligation");
 276             return None; // Cannot be matched, no such method resolution is possible.
 277         }
 278
 279         // Trait must have a method named `m_name` and it should not have
 280         // type parameters or early-bound regions.
 281         let tcx = self.tcx;
 282         let method_item = self.associated_item(trait_def_id, m_name, Namespace::Value).unwrap();
 283         let def_id = method_item.def_id;
 284         let generics = tcx.generics_of(def_id);
 285         assert_eq!(generics.types.len(), 0);
 286         assert_eq!(generics.regions.len(), 0);
 287
 288         debug!("lookup_in_trait_adjusted: method_item={:?}", method_item);
 289         let mut obligations = vec![];
 290
 291         // Instantiate late-bound regions and substitute the trait
 292         // parameters into the method type to get the actual method type.
 293         //
 294         // NB: Instantiate late-bound regions first so that
 295         // `instantiate_type_scheme` can normalize associated types that
 296         // may reference those regions.
 297         let fn_sig = tcx.fn_sig(def_id);
 298         let fn_sig = self.replace_late_bound_regions_with_fresh_var(span,
 299                                                                     infer::FnCall,
 300                                                                     &fn_sig).0;
 301         let fn_sig = fn_sig.subst(self.tcx, substs);
 302         let fn_sig = match self.normalize_associated_types_in_as_infer_ok(span, &fn_sig) {
 303             InferOk { value, obligations: o } => {
 304                 obligations.extend(o);
 305                 value
 306             }
 307         };
 308
 309         // Register obligations for the parameters.  This will include the
 310         // `Self` parameter, which in turn has a bound of the main trait,
 311         // so this also effectively registers `obligation` as well.  (We
 312         // used to register `obligation` explicitly, but that resulted in
 313         // double error messages being reported.)
 314         //
 315         // Note that as the method comes from a trait, it should not have
 316         // any late-bound regions appearing in its bounds.
 317         let bounds = self.tcx.predicates_of(def_id).instantiate(self.tcx, substs);
 318         let bounds = match self.normalize_associated_types_in_as_infer_ok(span, &bounds) {
 319             InferOk { value, obligations: o } => {
 320                 obligations.extend(o);
 321                 value
 322             }
 323         };
 324         assert!(!bounds.has_escaping_regions());
 325
 326         let cause = traits::ObligationCause::misc(span, self.body_id);
 327         obligations.extend(traits::predicates_for_generics(cause.clone(),
 328                                                            self.param_env,
 329                                                            &bounds));
 330
 331         // Also add an obligation for the method type being well-formed.
 332         let method_ty = tcx.mk_fn_ptr(ty::Binder(fn_sig));
 333         debug!("lookup_in_trait_adjusted: matched method method_ty={:?} obligation={:?}",
 334                method_ty,
 335                obligation);
 336         obligations.push(traits::Obligation::new(cause,
 337                                                  self.param_env,
 338                                                  ty::Predicate::WellFormed(method_ty)));
 339
 340         let callee = MethodCallee {
 341             def_id,
 342             substs: trait_ref.substs,
 343             sig: fn_sig,
 344         };
 345
 346         debug!("callee = {:?}", callee);
 347
 348         Some(InferOk {
 349             obligations,
 350             value: callee
 351         })
 352     }
 353
 354     pub fn resolve_ufcs(&self,
 355                         span: Span,
 356                         method_name: ast::Name,
 357                         self_ty: Ty<'tcx>,
 358                         expr_id: ast::NodeId)
 359                         -> Result<Def, MethodError<'tcx>> {
 360         let mode = probe::Mode::Path;
 361         let pick = self.probe_for_name(span, mode, method_name, IsSuggestion(false),
 362                                        self_ty, expr_id, ProbeScope::TraitsInScope)?;
 363
 364         if let Some(import_id) = pick.import_id {
 365             let import_def_id = self.tcx.hir.local_def_id(import_id);
 366             debug!("used_trait_import: {:?}", import_def_id);
 367             Rc::get_mut(&mut self.tables.borrow_mut().used_trait_imports)
 368                                         .unwrap().insert(import_def_id);
 369         }
 370
 371         let def = pick.item.def();
 372         self.tcx.check_stability(def.def_id(), expr_id, span);
 373
 374         Ok(def)
 375     }
 376
 377     /// Find item with name `item_name` defined in impl/trait `def_id`
 378     /// and return it, or `None`, if no such item was defined there.
 379     pub fn associated_item(&self, def_id: DefId, item_name: ast::Name, ns: Namespace)
 380                            -> Option<ty::AssociatedItem> {
 381         self.tcx.associated_items(def_id)
 382                 .find(|item| Namespace::from(item.kind) == ns &&
 383                              self.tcx.hygienic_eq(item_name, item.name, def_id))
 384     }
 385 }