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     probe::provide(providers);
  43 }
  44
  45 #[derive(Clone, Copy, Debug)]
  46 pub struct MethodCallee<'tcx> {
  47     /// Impl method ID, for inherent methods, or trait method ID, otherwise.
  48     pub def_id: DefId,
  49     pub substs: &'tcx Substs<'tcx>,
  50
  51     /// Instantiated method signature, i.e., it has been
  52     /// substituted, normalized, and has had late-bound
  53     /// lifetimes replaced with inference variables.
  54     pub sig: ty::FnSig<'tcx>,
  55 }
  56
  57 pub enum MethodError<'tcx> {
  58     // Did not find an applicable method, but we did find various near-misses that may work.
  59     NoMatch(NoMatchData<'tcx>),
  60
  61     // Multiple methods might apply.
  62     Ambiguity(Vec<CandidateSource>),
  63
  64     // Found an applicable method, but it is not visible. The second argument contains a list of
  65     // not-in-scope traits which may work.
  66     PrivateMatch(Def, Vec<DefId>),
  67
  68     // Found a `Self: Sized` bound where `Self` is a trait object, also the caller may have
  69     // forgotten to import a trait.
  70     IllegalSizedBound(Vec<DefId>),
  71
  72     // Found a match, but the return type is wrong
  73     BadReturnType,
  74 }
  75
  76 // Contains a list of static methods that may apply, a list of unsatisfied trait predicates which
  77 // could lead to matches if satisfied, and a list of not-in-scope traits which may work.
  78 pub struct NoMatchData<'tcx> {
  79     pub static_candidates: Vec<CandidateSource>,
  80     pub unsatisfied_predicates: Vec<TraitRef<'tcx>>,
  81     pub out_of_scope_traits: Vec<DefId>,
  82     pub lev_candidate: Option<ty::AssociatedItem>,
  83     pub mode: probe::Mode,
  84 }
  85
  86 impl<'tcx> NoMatchData<'tcx> {
  87     pub fn new(static_candidates: Vec<CandidateSource>,
  88                unsatisfied_predicates: Vec<TraitRef<'tcx>>,
  89                out_of_scope_traits: Vec<DefId>,
  90                lev_candidate: Option<ty::AssociatedItem>,
  91                mode: probe::Mode)
  92                -> Self {
  93         NoMatchData {
  94             static_candidates,
  95             unsatisfied_predicates,
  96             out_of_scope_traits,
  97             lev_candidate,
  98             mode,
  99         }
 100     }
 101 }
 102
 103 // A pared down enum describing just the places from which a method
 104 // candidate can arise. Used for error reporting only.
 105 #[derive(Copy, Clone, Debug, Eq, Ord, PartialEq, PartialOrd)]
 106 pub enum CandidateSource {
 107     ImplSource(DefId),
 108     TraitSource(DefId /* trait id */),
 109 }
 110
 111 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
 112     /// Determines whether the type `self_ty` supports a method name `method_name` or not.
 113     pub fn method_exists(&self,
 114                          method_name: ast::Ident,
 115                          self_ty: Ty<'tcx>,
 116                          call_expr_id: ast::NodeId,
 117                          allow_private: bool)
 118                          -> bool {
 119         let mode = probe::Mode::MethodCall;
 120         match self.probe_for_name(method_name.span, mode, method_name,
 121                                   IsSuggestion(false), self_ty, call_expr_id,
 122                                   ProbeScope::TraitsInScope) {
 123             Ok(..) => true,
 124             Err(NoMatch(..)) => false,
 125             Err(Ambiguity(..)) => true,
 126             Err(PrivateMatch(..)) => allow_private,
 127             Err(IllegalSizedBound(..)) => true,
 128             Err(BadReturnType) => {
 129                 bug!("no return type expectations but got BadReturnType")
 130             }
 131
 132         }
 133     }
 134
 135     /// Performs method lookup. If lookup is successful, it will return the callee
 136     /// and store an appropriate adjustment for the self-expr. In some cases it may
 137     /// report an error (e.g., invoking the `drop` method).
 138     ///
 139     /// # Arguments
 140     ///
 141     /// Given a method call like `foo.bar::<T1,...Tn>(...)`:
 142     ///
 143     /// * `fcx`:                   the surrounding `FnCtxt` (!)
 144     /// * `span`:                  the span for the method call
 145     /// * `method_name`:           the name of the method being called (`bar`)
 146     /// * `self_ty`:               the (unadjusted) type of the self expression (`foo`)
 147     /// * `supplied_method_types`: the explicit method type parameters, if any (`T1..Tn`)
 148     /// * `self_expr`:             the self expression (`foo`)
 149     pub fn lookup_method(&self,
 150                          self_ty: Ty<'tcx>,
 151                          segment: &hir::PathSegment,
 152                          span: Span,
 153                          call_expr: &'gcx hir::Expr,
 154                          self_expr: &'gcx hir::Expr)
 155                          -> Result<MethodCallee<'tcx>, MethodError<'tcx>> {
 156         debug!("lookup(method_name={}, self_ty={:?}, call_expr={:?}, self_expr={:?})",
 157                segment.ident,
 158                self_ty,
 159                call_expr,
 160                self_expr);
 161
 162         let pick = self.lookup_probe(
 163             span,
 164             segment.ident,
 165             self_ty,
 166             call_expr,
 167             ProbeScope::TraitsInScope
 168         )?;
 169
 170         if let Some(import_id) = pick.import_id {
 171             let import_def_id = self.tcx.hir().local_def_id(import_id);
 172             debug!("used_trait_import: {:?}", import_def_id);
 173             Lrc::get_mut(&mut self.tables.borrow_mut().used_trait_imports)
 174                 .unwrap().insert(import_def_id);
 175         }
 176
 177         self.tcx.check_stability(pick.item.def_id, Some(call_expr.id), span);
 178
 179         let result = self.confirm_method(
 180             span,
 181             self_expr,
 182             call_expr,
 183             self_ty,
 184             pick.clone(),
 185             segment,
 186         );
 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.ident,
 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::Ident,
 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::Ident,
 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, trait_def_id, |param, _| {
 257             match param.kind {
 258                 GenericParamDefKind::Lifetime => {}
 259                 GenericParamDefKind::Type {..} => {
 260                     if param.index == 0 {
 261                         return self_ty.into();
 262                     } else if let Some(ref input_types) = opt_input_types {
 263                         return input_types[param.index as usize - 1].into();
 264                     }
 265                 }
 266             }
 267             self.var_for_def(span, param)
 268         });
 269
 270         let trait_ref = ty::TraitRef::new(trait_def_id, substs);
 271
 272         // Construct an obligation
 273         let poly_trait_ref = trait_ref.to_poly_trait_ref();
 274         let obligation =
 275             traits::Obligation::misc(span,
 276                                      self.body_id,
 277                                      self.param_env,
 278                                      poly_trait_ref.to_predicate());
 279
 280         // Now we want to know if this can be matched
 281         if !self.predicate_may_hold(&obligation) {
 282             debug!("--> Cannot match obligation");
 283             return None; // Cannot be matched, no such method resolution is possible.
 284         }
 285
 286         // Trait must have a method named `m_name` and it should not have
 287         // type parameters or early-bound regions.
 288         let tcx = self.tcx;
 289         let method_item = match self.associated_item(trait_def_id, m_name, Namespace::Value) {
 290             Some(method_item) => method_item,
 291             None => {
 292                 tcx.sess.delay_span_bug(span,
 293                     "operator trait does not have corresponding operator method");
 294                 return None;
 295             }
 296         };
 297         let def_id = method_item.def_id;
 298         let generics = tcx.generics_of(def_id);
 299         assert_eq!(generics.params.len(), 0);
 300
 301         debug!("lookup_in_trait_adjusted: method_item={:?}", method_item);
 302         let mut obligations = vec![];
 303
 304         // Instantiate late-bound regions and substitute the trait
 305         // parameters into the method type to get the actual method type.
 306         //
 307         // N.B., instantiate late-bound regions first so that
 308         // `instantiate_type_scheme` can normalize associated types that
 309         // may reference those regions.
 310         let fn_sig = tcx.fn_sig(def_id);
 311         let fn_sig = self.replace_bound_vars_with_fresh_vars(
 312             span,
 313             infer::FnCall,
 314             &fn_sig
 315         ).0;
 316         let fn_sig = fn_sig.subst(self.tcx, substs);
 317         let fn_sig = match self.normalize_associated_types_in_as_infer_ok(span, &fn_sig) {
 318             InferOk { value, obligations: o } => {
 319                 obligations.extend(o);
 320                 value
 321             }
 322         };
 323
 324         // Register obligations for the parameters. This will include the
 325         // `Self` parameter, which in turn has a bound of the main trait,
 326         // so this also effectively registers `obligation` as well.  (We
 327         // used to register `obligation` explicitly, but that resulted in
 328         // double error messages being reported.)
 329         //
 330         // Note that as the method comes from a trait, it should not have
 331         // any late-bound regions appearing in its bounds.
 332         let bounds = self.tcx.predicates_of(def_id).instantiate(self.tcx, substs);
 333         let bounds = match self.normalize_associated_types_in_as_infer_ok(span, &bounds) {
 334             InferOk { value, obligations: o } => {
 335                 obligations.extend(o);
 336                 value
 337             }
 338         };
 339         assert!(!bounds.has_escaping_bound_vars());
 340
 341         let cause = traits::ObligationCause::misc(span, self.body_id);
 342         obligations.extend(traits::predicates_for_generics(cause.clone(),
 343                                                            self.param_env,
 344                                                            &bounds));
 345
 346         // Also add an obligation for the method type being well-formed.
 347         let method_ty = tcx.mk_fn_ptr(ty::Binder::bind(fn_sig));
 348         debug!("lookup_in_trait_adjusted: matched method method_ty={:?} obligation={:?}",
 349                method_ty,
 350                obligation);
 351         obligations.push(traits::Obligation::new(cause,
 352                                                  self.param_env,
 353                                                  ty::Predicate::WellFormed(method_ty)));
 354
 355         let callee = MethodCallee {
 356             def_id,
 357             substs: trait_ref.substs,
 358             sig: fn_sig,
 359         };
 360
 361         debug!("callee = {:?}", callee);
 362
 363         Some(InferOk {
 364             obligations,
 365             value: callee
 366         })
 367     }
 368
 369     pub fn resolve_ufcs(&self,
 370                         span: Span,
 371                         method_name: ast::Ident,
 372                         self_ty: Ty<'tcx>,
 373                         expr_id: ast::NodeId)
 374                         -> Result<Def, MethodError<'tcx>> {
 375         let mode = probe::Mode::Path;
 376         let pick = self.probe_for_name(span, mode, method_name, IsSuggestion(false),
 377                                        self_ty, expr_id, ProbeScope::TraitsInScope)?;
 378
 379         if let Some(import_id) = pick.import_id {
 380             let import_def_id = self.tcx.hir().local_def_id(import_id);
 381             debug!("used_trait_import: {:?}", import_def_id);
 382             Lrc::get_mut(&mut self.tables.borrow_mut().used_trait_imports)
 383                                         .unwrap().insert(import_def_id);
 384         }
 385
 386         let def = pick.item.def();
 387         self.tcx.check_stability(def.def_id(), Some(expr_id), span);
 388
 389         Ok(def)
 390     }
 391
 392     /// Find item with name `item_name` defined in impl/trait `def_id`
 393     /// and return it, or `None`, if no such item was defined there.
 394     pub fn associated_item(&self, def_id: DefId, item_name: ast::Ident, ns: Namespace)
 395                            -> Option<ty::AssociatedItem> {
 396         self.tcx.associated_items(def_id).find(|item| {
 397             Namespace::from(item.kind) == ns &&
 398             self.tcx.hygienic_eq(item_name, item.ident, def_id)
 399         })
 400     }
 401 }