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 rustc::ty::subst::Substs;
  17 use rustc::traits;
  18 use rustc::ty::{self, Ty, ToPredicate, ToPolyTraitRef, TraitRef, TypeFoldable};
  19 use rustc::ty::subst::Subst;
  20 use rustc::infer::{self, InferOk};
  21
  22 use syntax::ast;
  23 use syntax_pos::Span;
  24
  25 use rustc::hir;
  26
  27 pub use self::MethodError::*;
  28 pub use self::CandidateSource::*;
  29
  30 mod confirm;
  31 pub mod probe;
  32 mod suggest;
  33
  34 use self::probe::{IsSuggestion, ProbeScope};
  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: &'tcx Substs<'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 second argument contains a list of
  56     // not-in-scope traits which may work.
  57     PrivateMatch(Def, 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>),
  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<TraitRef<'tcx>>,
  72     pub out_of_scope_traits: Vec<DefId>,
  73     pub lev_candidate: Option<ty::AssociatedItem>,
  74     pub mode: probe::Mode,
  75 }
  76
  77 impl<'tcx> NoMatchData<'tcx> {
  78     pub fn new(static_candidates: Vec<CandidateSource>,
  79                unsatisfied_predicates: Vec<TraitRef<'tcx>>,
  80                out_of_scope_traits: Vec<DefId>,
  81                lev_candidate: Option<ty::AssociatedItem>,
  82                mode: probe::Mode)
  83                -> Self {
  84         NoMatchData {
  85             static_candidates,
  86             unsatisfied_predicates,
  87             out_of_scope_traits,
  88             lev_candidate,
  89             mode,
  90         }
  91     }
  92 }
  93
  94 // A pared down enum describing just the places from which a method
  95 // candidate can arise. Used for error reporting only.
  96 #[derive(Copy, Clone, Debug, Eq, Ord, PartialEq, PartialOrd)]
  97 pub enum CandidateSource {
  98     ImplSource(DefId),
  99     TraitSource(// trait id
 100                 DefId),
 101 }
 102
 103 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
 104     /// Determines whether the type `self_ty` supports a method name `method_name` or not.
 105     pub fn method_exists(&self,
 106                          span: Span,
 107                          method_name: ast::Name,
 108                          self_ty: Ty<'tcx>,
 109                          call_expr_id: ast::NodeId,
 110                          allow_private: bool)
 111                          -> bool {
 112         let mode = probe::Mode::MethodCall;
 113         match self.probe_for_name(span, mode, method_name, IsSuggestion(false),
 114                                   self_ty, call_expr_id, ProbeScope::TraitsInScope) {
 115             Ok(..) => true,
 116             Err(NoMatch(..)) => false,
 117             Err(Ambiguity(..)) => true,
 118             Err(PrivateMatch(..)) => allow_private,
 119             Err(IllegalSizedBound(..)) => true,
 120             Err(BadReturnType) => {
 121                 bug!("no return type expectations but got BadReturnType")
 122             }
 123
 124         }
 125     }
 126
 127     /// Performs method lookup. If lookup is successful, it will return the callee
 128     /// and store an appropriate adjustment for the self-expr. In some cases it may
 129     /// report an error (e.g., invoking the `drop` method).
 130     ///
 131     /// # Arguments
 132     ///
 133     /// Given a method call like `foo.bar::<T1,...Tn>(...)`:
 134     ///
 135     /// * `fcx`:                   the surrounding `FnCtxt` (!)
 136     /// * `span`:                  the span for the method call
 137     /// * `method_name`:           the name of the method being called (`bar`)
 138     /// * `self_ty`:               the (unadjusted) type of the self expression (`foo`)
 139     /// * `supplied_method_types`: the explicit method type parameters, if any (`T1..Tn`)
 140     /// * `self_expr`:             the self expression (`foo`)
 141     pub fn lookup_method(&self,
 142                          self_ty: Ty<'tcx>,
 143                          segment: &hir::PathSegment,
 144                          span: Span,
 145                          call_expr: &'gcx hir::Expr,
 146                          self_expr: &'gcx hir::Expr)
 147                          -> Result<MethodCallee<'tcx>, MethodError<'tcx>> {
 148         debug!("lookup(method_name={}, self_ty={:?}, call_expr={:?}, self_expr={:?})",
 149                segment.name,
 150                self_ty,
 151                call_expr,
 152                self_expr);
 153
 154         let pick = self.lookup_probe(
 155             span,
 156             segment.name,
 157             self_ty,
 158             call_expr,
 159             ProbeScope::TraitsInScope
 160         )?;
 161
 162         if let Some(import_id) = pick.import_id {
 163             let import_def_id = self.tcx.hir.local_def_id(import_id);
 164             debug!("used_trait_import: {:?}", import_def_id);
 165             self.tables.borrow_mut().used_trait_imports.insert(import_def_id);
 166         }
 167
 168         self.tcx.check_stability(pick.item.def_id, call_expr.id, span);
 169
 170         let result = self.confirm_method(span,
 171                                          self_expr,
 172                                          call_expr,
 173                                          self_ty,
 174                                          pick.clone(),
 175                                          segment);
 176
 177         if result.illegal_sized_bound {
 178             // We probe again, taking all traits into account (not only those in scope).
 179             let candidates =
 180                 match self.lookup_probe(span,
 181                                         segment.name,
 182                                         self_ty,
 183                                         call_expr,
 184                                         ProbeScope::AllTraits) {
 185
 186                     // If we find a different result the caller probably forgot to import a trait.
 187                     Ok(ref new_pick) if *new_pick != pick => vec![new_pick.item.container.id()],
 188                     Err(Ambiguity(ref sources)) => {
 189                         sources.iter()
 190                                .filter_map(|source| {
 191                                    match *source {
 192                                        // Note: this cannot come from an inherent impl,
 193                                        // because the first probing succeeded.
 194                                        ImplSource(def) => self.tcx.trait_id_of_impl(def),
 195                                        TraitSource(_) => None,
 196                                    }
 197                                })
 198                                .collect()
 199                     }
 200                     _ => Vec::new(),
 201                 };
 202
 203             return Err(IllegalSizedBound(candidates));
 204         }
 205
 206         Ok(result.callee)
 207     }
 208
 209     fn lookup_probe(&self,
 210                     span: Span,
 211                     method_name: ast::Name,
 212                     self_ty: Ty<'tcx>,
 213                     call_expr: &'gcx hir::Expr,
 214                     scope: ProbeScope)
 215                     -> probe::PickResult<'tcx> {
 216         let mode = probe::Mode::MethodCall;
 217         let self_ty = self.resolve_type_vars_if_possible(&self_ty);
 218         self.probe_for_name(span, mode, method_name, IsSuggestion(false),
 219                             self_ty, call_expr.id, scope)
 220     }
 221
 222     /// `lookup_method_in_trait` is used for overloaded operators.
 223     /// It does a very narrow slice of what the normal probe/confirm path does.
 224     /// In particular, it doesn't really do any probing: it simply constructs
 225     /// an obligation for a particular trait with the given self-type and checks
 226     /// whether that trait is implemented.
 227     ///
 228     /// FIXME(#18741) -- It seems likely that we can consolidate some of this
 229     /// code with the other method-lookup code. In particular, the second half
 230     /// of this method is basically the same as confirmation.
 231     pub fn lookup_method_in_trait(&self,
 232                                   span: Span,
 233                                   m_name: ast::Name,
 234                                   trait_def_id: DefId,
 235                                   self_ty: Ty<'tcx>,
 236                                   opt_input_types: Option<&[Ty<'tcx>]>)
 237                                   -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
 238         debug!("lookup_in_trait_adjusted(self_ty={:?}, \
 239                 m_name={}, trait_def_id={:?})",
 240                self_ty,
 241                m_name,
 242                trait_def_id);
 243
 244         // Construct a trait-reference `self_ty : Trait<input_tys>`
 245         let substs = Substs::for_item(self.tcx,
 246                                       trait_def_id,
 247                                       |def, _| self.region_var_for_def(span, def),
 248                                       |def, substs| {
 249             if def.index == 0 {
 250                 self_ty
 251             } else if let Some(ref input_types) = opt_input_types {
 252                 input_types[def.index as usize - 1]
 253             } else {
 254                 self.type_var_for_def(span, def, substs)
 255             }
 256         });
 257
 258         let trait_ref = ty::TraitRef::new(trait_def_id, substs);
 259
 260         // Construct an obligation
 261         let poly_trait_ref = trait_ref.to_poly_trait_ref();
 262         let obligation =
 263             traits::Obligation::misc(span,
 264                                      self.body_id,
 265                                      self.param_env,
 266                                      poly_trait_ref.to_predicate());
 267
 268         // Now we want to know if this can be matched
 269         let mut selcx = traits::SelectionContext::new(self);
 270         if !selcx.evaluate_obligation(&obligation) {
 271             debug!("--> Cannot match obligation");
 272             return None; // Cannot be matched, no such method resolution is possible.
 273         }
 274
 275         // Trait must have a method named `m_name` and it should not have
 276         // type parameters or early-bound regions.
 277         let tcx = self.tcx;
 278         let method_item = self.associated_item(trait_def_id, m_name).unwrap();
 279         let def_id = method_item.def_id;
 280         let generics = tcx.generics_of(def_id);
 281         assert_eq!(generics.types.len(), 0);
 282         assert_eq!(generics.regions.len(), 0);
 283
 284         debug!("lookup_in_trait_adjusted: method_item={:?}", method_item);
 285         let mut obligations = vec![];
 286
 287         // Instantiate late-bound regions and substitute the trait
 288         // parameters into the method type to get the actual method type.
 289         //
 290         // NB: Instantiate late-bound regions first so that
 291         // `instantiate_type_scheme` can normalize associated types that
 292         // may reference those regions.
 293         let fn_sig = tcx.fn_sig(def_id);
 294         let fn_sig = self.replace_late_bound_regions_with_fresh_var(span,
 295                                                                     infer::FnCall,
 296                                                                     &fn_sig).0;
 297         let fn_sig = fn_sig.subst(self.tcx, substs);
 298         let fn_sig = match self.normalize_associated_types_in_as_infer_ok(span, &fn_sig) {
 299             InferOk { value, obligations: o } => {
 300                 obligations.extend(o);
 301                 value
 302             }
 303         };
 304
 305         // Register obligations for the parameters.  This will include the
 306         // `Self` parameter, which in turn has a bound of the main trait,
 307         // so this also effectively registers `obligation` as well.  (We
 308         // used to register `obligation` explicitly, but that resulted in
 309         // double error messages being reported.)
 310         //
 311         // Note that as the method comes from a trait, it should not have
 312         // any late-bound regions appearing in its bounds.
 313         let bounds = self.tcx.predicates_of(def_id).instantiate(self.tcx, substs);
 314         let bounds = match self.normalize_associated_types_in_as_infer_ok(span, &bounds) {
 315             InferOk { value, obligations: o } => {
 316                 obligations.extend(o);
 317                 value
 318             }
 319         };
 320         assert!(!bounds.has_escaping_regions());
 321
 322         let cause = traits::ObligationCause::misc(span, self.body_id);
 323         obligations.extend(traits::predicates_for_generics(cause.clone(),
 324                                                            self.param_env,
 325                                                            &bounds));
 326
 327         // Also add an obligation for the method type being well-formed.
 328         let method_ty = tcx.mk_fn_ptr(ty::Binder(fn_sig));
 329         debug!("lookup_in_trait_adjusted: matched method method_ty={:?} obligation={:?}",
 330                method_ty,
 331                obligation);
 332         obligations.push(traits::Obligation::new(cause,
 333                                                  self.param_env,
 334                                                  ty::Predicate::WellFormed(method_ty)));
 335
 336         let callee = MethodCallee {
 337             def_id,
 338             substs: trait_ref.substs,
 339             sig: fn_sig,
 340         };
 341
 342         debug!("callee = {:?}", callee);
 343
 344         Some(InferOk {
 345             obligations,
 346             value: callee
 347         })
 348     }
 349
 350     pub fn resolve_ufcs(&self,
 351                         span: Span,
 352                         method_name: ast::Name,
 353                         self_ty: Ty<'tcx>,
 354                         expr_id: ast::NodeId)
 355                         -> Result<Def, MethodError<'tcx>> {
 356         let mode = probe::Mode::Path;
 357         let pick = self.probe_for_name(span, mode, method_name, IsSuggestion(false),
 358                                        self_ty, expr_id, ProbeScope::TraitsInScope)?;
 359
 360         if let Some(import_id) = pick.import_id {
 361             let import_def_id = self.tcx.hir.local_def_id(import_id);
 362             debug!("used_trait_import: {:?}", import_def_id);
 363             self.tables.borrow_mut().used_trait_imports.insert(import_def_id);
 364         }
 365
 366         let def = pick.item.def();
 367         self.tcx.check_stability(def.def_id(), expr_id, span);
 368
 369         Ok(def)
 370     }
 371
 372     /// Find item with name `item_name` defined in impl/trait `def_id`
 373     /// and return it, or `None`, if no such item was defined there.
 374     pub fn associated_item(&self, def_id: DefId, item_name: ast::Name)
 375                            -> Option<ty::AssociatedItem> {
 376         self.tcx.associated_items(def_id)
 377                 .find(|item| self.tcx.hygienic_eq(item_name, item.name, def_id))
 378
 379     }
 380 }