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.github.io/rustc-guide/method-lookup.html
  14
  15 mod confirm;
  16 pub mod probe;
  17 mod suggest;
  18
  19 pub use self::MethodError::*;
  20 pub use self::CandidateSource::*;
  21 pub use self::suggest::TraitInfo;
  22
  23 use check::FnCtxt;
  24 use namespace::Namespace;
  25 use rustc_data_structures::sync::Lrc;
  26 use rustc::hir;
  27 use rustc::hir::def::Def;
  28 use rustc::hir::def_id::DefId;
  29 use rustc::traits;
  30 use rustc::ty::subst::Substs;
  31 use rustc::ty::{self, Ty, ToPredicate, ToPolyTraitRef, TraitRef, TypeFoldable};
  32 use rustc::ty::GenericParamDefKind;
  33 use rustc::ty::subst::Subst;
  34 use rustc::infer::{self, InferOk};
  35 use syntax::ast;
  36 use syntax_pos::Span;
  37
  38 use self::probe::{IsSuggestion, ProbeScope};
  39
  40 pub fn provide(providers: &mut ty::query::Providers) {
  41     suggest::provide(providers);
  42 }
  43
  44 #[derive(Clone, Copy, Debug)]
  45 pub struct MethodCallee<'tcx> {
  46     /// Impl method ID, for inherent methods, or trait method ID, otherwise.
  47     pub def_id: DefId,
  48     pub substs: &'tcx Substs<'tcx>,
  49
  50     /// Instantiated method signature, i.e., it has been
  51     /// substituted, normalized, and has had late-bound
  52     /// lifetimes replaced with inference variables.
  53     pub sig: ty::FnSig<'tcx>,
  54 }
  55
  56 pub enum MethodError<'tcx> {
  57     // Did not find an applicable method, but we did find various near-misses that may work.
  58     NoMatch(NoMatchData<'tcx>),
  59
  60     // Multiple methods might apply.
  61     Ambiguity(Vec<CandidateSource>),
  62
  63     // Found an applicable method, but it is not visible. The second argument contains a list of
  64     // not-in-scope traits which may work.
  65     PrivateMatch(Def, Vec<DefId>),
  66
  67     // Found a `Self: Sized` bound where `Self` is a trait object, also the caller may have
  68     // forgotten to import a trait.
  69     IllegalSizedBound(Vec<DefId>),
  70
  71     // Found a match, but the return type is wrong
  72     BadReturnType,
  73 }
  74
  75 // Contains a list of static methods that may apply, a list of unsatisfied trait predicates which
  76 // could lead to matches if satisfied, and a list of not-in-scope traits which may work.
  77 pub struct NoMatchData<'tcx> {
  78     pub static_candidates: Vec<CandidateSource>,
  79     pub unsatisfied_predicates: Vec<TraitRef<'tcx>>,
  80     pub out_of_scope_traits: Vec<DefId>,
  81     pub lev_candidate: Option<ty::AssociatedItem>,
  82     pub mode: probe::Mode,
  83 }
  84
  85 impl<'tcx> NoMatchData<'tcx> {
  86     pub fn new(static_candidates: Vec<CandidateSource>,
  87                unsatisfied_predicates: Vec<TraitRef<'tcx>>,
  88                out_of_scope_traits: Vec<DefId>,
  89                lev_candidate: Option<ty::AssociatedItem>,
  90                mode: probe::Mode)
  91                -> Self {
  92         NoMatchData {
  93             static_candidates,
  94             unsatisfied_predicates,
  95             out_of_scope_traits,
  96             lev_candidate,
  97             mode,
  98         }
  99     }
 100 }
 101
 102 // A pared down enum describing just the places from which a method
 103 // candidate can arise. Used for error reporting only.
 104 #[derive(Copy, Clone, Debug, Eq, Ord, PartialEq, PartialOrd)]
 105 pub enum CandidateSource {
 106     ImplSource(DefId),
 107     TraitSource(DefId /* trait id */),
 108 }
 109
 110 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
 111     /// Determines whether the type `self_ty` supports a method name `method_name` or not.
 112     pub fn method_exists(&self,
 113                          method_name: ast::Ident,
 114                          self_ty: Ty<'tcx>,
 115                          call_expr_id: ast::NodeId,
 116                          allow_private: bool)
 117                          -> bool {
 118         let mode = probe::Mode::MethodCall;
 119         match self.probe_for_name(method_name.span, mode, method_name,
 120                                   IsSuggestion(false), self_ty, call_expr_id,
 121                                   ProbeScope::TraitsInScope) {
 122             Ok(..) => true,
 123             Err(NoMatch(..)) => false,
 124             Err(Ambiguity(..)) => true,
 125             Err(PrivateMatch(..)) => allow_private,
 126             Err(IllegalSizedBound(..)) => true,
 127             Err(BadReturnType) => {
 128                 bug!("no return type expectations but got BadReturnType")
 129             }
 130
 131         }
 132     }
 133
 134     /// Performs method lookup. If lookup is successful, it will return the callee
 135     /// and store an appropriate adjustment for the self-expr. In some cases it may
 136     /// report an error (e.g., invoking the `drop` method).
 137     ///
 138     /// # Arguments
 139     ///
 140     /// Given a method call like `foo.bar::<T1,...Tn>(...)`:
 141     ///
 142     /// * `fcx`:                   the surrounding `FnCtxt` (!)
 143     /// * `span`:                  the span for the method call
 144     /// * `method_name`:           the name of the method being called (`bar`)
 145     /// * `self_ty`:               the (unadjusted) type of the self expression (`foo`)
 146     /// * `supplied_method_types`: the explicit method type parameters, if any (`T1..Tn`)
 147     /// * `self_expr`:             the self expression (`foo`)
 148     pub fn lookup_method(&self,
 149                          self_ty: Ty<'tcx>,
 150                          segment: &hir::PathSegment,
 151                          span: Span,
 152                          call_expr: &'gcx hir::Expr,
 153                          self_expr: &'gcx hir::Expr)
 154                          -> Result<MethodCallee<'tcx>, MethodError<'tcx>> {
 155         debug!("lookup(method_name={}, self_ty={:?}, call_expr={:?}, self_expr={:?})",
 156                segment.ident,
 157                self_ty,
 158                call_expr,
 159                self_expr);
 160
 161         let pick = self.lookup_probe(
 162             span,
 163             segment.ident,
 164             self_ty,
 165             call_expr,
 166             ProbeScope::TraitsInScope
 167         )?;
 168
 169         if let Some(import_id) = pick.import_id {
 170             let import_def_id = self.tcx.hir().local_def_id(import_id);
 171             debug!("used_trait_import: {:?}", import_def_id);
 172             Lrc::get_mut(&mut self.tables.borrow_mut().used_trait_imports)
 173                 .unwrap().insert(import_def_id);
 174         }
 175
 176         self.tcx.check_stability(pick.item.def_id, Some(call_expr.id), span);
 177
 178         let result = self.confirm_method(
 179             span,
 180             self_expr,
 181             call_expr,
 182             self_ty,
 183             pick.clone(),
 184             segment,
 185         );
 186
 187         if result.illegal_sized_bound {
 188             // We probe again, taking all traits into account (not only those in scope).
 189             let candidates =
 190                 match self.lookup_probe(span,
 191                                         segment.ident,
 192                                         self_ty,
 193                                         call_expr,
 194                                         ProbeScope::AllTraits) {
 195
 196                     // If we find a different result the caller probably forgot to import a trait.
 197                     Ok(ref new_pick) if *new_pick != pick => vec![new_pick.item.container.id()],
 198                     Err(Ambiguity(ref sources)) => {
 199                         sources.iter()
 200                                .filter_map(|source| {
 201                                    match *source {
 202                                        // Note: this cannot come from an inherent impl,
 203                                        // because the first probing succeeded.
 204                                        ImplSource(def) => self.tcx.trait_id_of_impl(def),
 205                                        TraitSource(_) => None,
 206                                    }
 207                                })
 208                                .collect()
 209                     }
 210                     _ => Vec::new(),
 211                 };
 212
 213             return Err(IllegalSizedBound(candidates));
 214         }
 215
 216         Ok(result.callee)
 217     }
 218
 219     fn lookup_probe(&self,
 220                     span: Span,
 221                     method_name: ast::Ident,
 222                     self_ty: Ty<'tcx>,
 223                     call_expr: &'gcx hir::Expr,
 224                     scope: ProbeScope)
 225                     -> probe::PickResult<'tcx> {
 226         let mode = probe::Mode::MethodCall;
 227         let self_ty = self.resolve_type_vars_if_possible(&self_ty);
 228         self.probe_for_name(span, mode, method_name, IsSuggestion(false),
 229                             self_ty, call_expr.id, scope)
 230     }
 231
 232     /// `lookup_method_in_trait` is used for overloaded operators.
 233     /// It does a very narrow slice of what the normal probe/confirm path does.
 234     /// In particular, it doesn't really do any probing: it simply constructs
 235     /// an obligation for a particular trait with the given self-type and checks
 236     /// whether that trait is implemented.
 237     ///
 238     /// FIXME(#18741): it seems likely that we can consolidate some of this
 239     /// code with the other method-lookup code. In particular, the second half
 240     /// of this method is basically the same as confirmation.
 241     pub fn lookup_method_in_trait(&self,
 242                                   span: Span,
 243                                   m_name: ast::Ident,
 244                                   trait_def_id: DefId,
 245                                   self_ty: Ty<'tcx>,
 246                                   opt_input_types: Option<&[Ty<'tcx>]>)
 247                                   -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
 248         debug!("lookup_in_trait_adjusted(self_ty={:?}, \
 249                 m_name={}, trait_def_id={:?})",
 250                self_ty,
 251                m_name,
 252                trait_def_id);
 253
 254         // Construct a trait-reference `self_ty : Trait<input_tys>`
 255         let substs = Substs::for_item(self.tcx, trait_def_id, |param, _| {
 256             match param.kind {
 257                 GenericParamDefKind::Lifetime => {}
 258                 GenericParamDefKind::Type {..} => {
 259                     if param.index == 0 {
 260                         return self_ty.into();
 261                     } else if let Some(ref input_types) = opt_input_types {
 262                         return input_types[param.index as usize - 1].into();
 263                     }
 264                 }
 265             }
 266             self.var_for_def(span, param)
 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 = match self.associated_item(trait_def_id, m_name, Namespace::Value) {
 289             Some(method_item) => method_item,
 290             None => {
 291                 tcx.sess.delay_span_bug(span,
 292                     "operator trait does not have corresponding operator method");
 293                 return None;
 294             }
 295         };
 296         let def_id = method_item.def_id;
 297         let generics = tcx.generics_of(def_id);
 298         assert_eq!(generics.params.len(), 0);
 299
 300         debug!("lookup_in_trait_adjusted: method_item={:?}", method_item);
 301         let mut obligations = vec![];
 302
 303         // Instantiate late-bound regions and substitute the trait
 304         // parameters into the method type to get the actual method type.
 305         //
 306         // N.B., instantiate late-bound regions first so that
 307         // `instantiate_type_scheme` can normalize associated types that
 308         // may reference those regions.
 309         let fn_sig = tcx.fn_sig(def_id);
 310         let fn_sig = self.replace_bound_vars_with_fresh_vars(
 311             span,
 312             infer::FnCall,
 313             &fn_sig
 314         ).0;
 315         let fn_sig = fn_sig.subst(self.tcx, substs);
 316         let fn_sig = match self.normalize_associated_types_in_as_infer_ok(span, &fn_sig) {
 317             InferOk { value, obligations: o } => {
 318                 obligations.extend(o);
 319                 value
 320             }
 321         };
 322
 323         // Register obligations for the parameters. This will include the
 324         // `Self` parameter, which in turn has a bound of the main trait,
 325         // so this also effectively registers `obligation` as well.  (We
 326         // used to register `obligation` explicitly, but that resulted in
 327         // double error messages being reported.)
 328         //
 329         // Note that as the method comes from a trait, it should not have
 330         // any late-bound regions appearing in its bounds.
 331         let bounds = self.tcx.predicates_of(def_id).instantiate(self.tcx, substs);
 332         let bounds = match self.normalize_associated_types_in_as_infer_ok(span, &bounds) {
 333             InferOk { value, obligations: o } => {
 334                 obligations.extend(o);
 335                 value
 336             }
 337         };
 338         assert!(!bounds.has_escaping_bound_vars());
 339
 340         let cause = traits::ObligationCause::misc(span, self.body_id);
 341         obligations.extend(traits::predicates_for_generics(cause.clone(),
 342                                                            self.param_env,
 343                                                            &bounds));
 344
 345         // Also add an obligation for the method type being well-formed.
 346         let method_ty = tcx.mk_fn_ptr(ty::Binder::bind(fn_sig));
 347         debug!("lookup_in_trait_adjusted: matched method method_ty={:?} obligation={:?}",
 348                method_ty,
 349                obligation);
 350         obligations.push(traits::Obligation::new(cause,
 351                                                  self.param_env,
 352                                                  ty::Predicate::WellFormed(method_ty)));
 353
 354         let callee = MethodCallee {
 355             def_id,
 356             substs: trait_ref.substs,
 357             sig: fn_sig,
 358         };
 359
 360         debug!("callee = {:?}", callee);
 361
 362         Some(InferOk {
 363             obligations,
 364             value: callee
 365         })
 366     }
 367
 368     pub fn resolve_ufcs(&self,
 369                         span: Span,
 370                         method_name: ast::Ident,
 371                         self_ty: Ty<'tcx>,
 372                         expr_id: ast::NodeId)
 373                         -> Result<Def, MethodError<'tcx>> {
 374         let mode = probe::Mode::Path;
 375         let pick = self.probe_for_name(span, mode, method_name, IsSuggestion(false),
 376                                        self_ty, expr_id, ProbeScope::TraitsInScope)?;
 377
 378         if let Some(import_id) = pick.import_id {
 379             let import_def_id = self.tcx.hir().local_def_id(import_id);
 380             debug!("used_trait_import: {:?}", import_def_id);
 381             Lrc::get_mut(&mut self.tables.borrow_mut().used_trait_imports)
 382                                         .unwrap().insert(import_def_id);
 383         }
 384
 385         let def = pick.item.def();
 386         self.tcx.check_stability(def.def_id(), Some(expr_id), span);
 387
 388         Ok(def)
 389     }
 390
 391     /// Find item with name `item_name` defined in impl/trait `def_id`
 392     /// and return it, or `None`, if no such item was defined there.
 393     pub fn associated_item(&self, def_id: DefId, item_name: ast::Ident, ns: Namespace)
 394                            -> Option<ty::AssociatedItem> {
 395         self.tcx.associated_items(def_id).find(|item| {
 396             Namespace::from(item.kind) == ns &&
 397             self.tcx.hygienic_eq(item_name, item.ident, def_id)
 398         })
 399     }
 400 }