src/librustc/traits/object_safety.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 //! "Object safety" refers to the ability for a trait to be converted
  12 //! to an object. In general, traits may only be converted to an
  13 //! object if all of their methods meet certain criteria. In particular,
  14 //! they must:
  15 //!
  16 //!   - have a suitable receiver from which we can extract a vtable;
  17 //!   - not reference the erased type `Self` except for in this receiver;
  18 //!   - not have generic type parameters
  19
  20 use super::elaborate_predicates;
  21
  22 use hir::def_id::DefId;
  23 use traits;
  24 use ty::{self, Ty, TyCtxt, TypeFoldable};
  25 use ty::subst::Substs;
  26 use std::borrow::Cow;
  27 use syntax::ast;
  28
  29 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
  30 pub enum ObjectSafetyViolation {
  31     /// Self : Sized declared on the trait
  32     SizedSelf,
  33
  34     /// Supertrait reference references `Self` an in illegal location
  35     /// (e.g. `trait Foo : Bar<Self>`)
  36     SupertraitSelf,
  37
  38     /// Method has something illegal
  39     Method(ast::Name, MethodViolationCode),
  40 }
  41
  42 impl ObjectSafetyViolation {
  43     pub fn error_msg(&self) -> Cow<'static, str> {
  44         match *self {
  45             ObjectSafetyViolation::SizedSelf =>
  46                 "the trait cannot require that `Self : Sized`".into(),
  47             ObjectSafetyViolation::SupertraitSelf =>
  48                 "the trait cannot use `Self` as a type parameter \
  49                  in the supertraits or where-clauses".into(),
  50             ObjectSafetyViolation::Method(name, MethodViolationCode::StaticMethod) =>
  51                 format!("method `{}` has no receiver", name).into(),
  52             ObjectSafetyViolation::Method(name, MethodViolationCode::ReferencesSelf) =>
  53                 format!("method `{}` references the `Self` type \
  54                          in its arguments or return type", name).into(),
  55             ObjectSafetyViolation::Method(name, MethodViolationCode::Generic) =>
  56                 format!("method `{}` has generic type parameters", name).into(),
  57         }
  58     }
  59 }
  60
  61 /// Reasons a method might not be object-safe.
  62 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
  63 pub enum MethodViolationCode {
  64     /// e.g., `fn foo()`
  65     StaticMethod,
  66
  67     /// e.g., `fn foo(&self, x: Self)` or `fn foo(&self) -> Self`
  68     ReferencesSelf,
  69
  70     /// e.g., `fn foo<A>()`
  71     Generic,
  72 }
  73
  74 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
  75     pub fn is_object_safe(self, trait_def_id: DefId) -> bool {
  76         // Because we query yes/no results frequently, we keep a cache:
  77         let def = self.lookup_trait_def(trait_def_id);
  78
  79         let result = def.object_safety().unwrap_or_else(|| {
  80             let result = self.object_safety_violations(trait_def_id).is_empty();
  81
  82             // Record just a yes/no result in the cache; this is what is
  83             // queried most frequently. Note that this may overwrite a
  84             // previous result, but always with the same thing.
  85             def.set_object_safety(result);
  86
  87             result
  88         });
  89
  90         debug!("is_object_safe({:?}) = {}", trait_def_id, result);
  91
  92         result
  93     }
  94
  95     /// Returns the object safety violations that affect
  96     /// astconv - currently, Self in supertraits. This is needed
  97     /// because `object_safety_violations` can't be used during
  98     /// type collection.
  99     pub fn astconv_object_safety_violations(self, trait_def_id: DefId)
 100                                             -> Vec<ObjectSafetyViolation>
 101     {
 102         let mut violations = vec![];
 103
 104         for def_id in traits::supertrait_def_ids(self, trait_def_id) {
 105             if self.predicates_reference_self(def_id, true) {
 106                 violations.push(ObjectSafetyViolation::SupertraitSelf);
 107             }
 108         }
 109
 110         debug!("astconv_object_safety_violations(trait_def_id={:?}) = {:?}",
 111                trait_def_id,
 112                violations);
 113
 114         violations
 115     }
 116
 117     pub fn object_safety_violations(self, trait_def_id: DefId)
 118                                     -> Vec<ObjectSafetyViolation>
 119     {
 120         traits::supertrait_def_ids(self, trait_def_id)
 121             .flat_map(|def_id| self.object_safety_violations_for_trait(def_id))
 122             .collect()
 123     }
 124
 125     fn object_safety_violations_for_trait(self, trait_def_id: DefId)
 126                                           -> Vec<ObjectSafetyViolation>
 127     {
 128         // Check methods for violations.
 129         let mut violations: Vec<_> = self.associated_items(trait_def_id)
 130             .filter(|item| item.kind == ty::AssociatedKind::Method)
 131             .filter_map(|item| {
 132                 self.object_safety_violation_for_method(trait_def_id, &item)
 133                     .map(|code| ObjectSafetyViolation::Method(item.name, code))
 134             }).collect();
 135
 136         // Check the trait itself.
 137         if self.trait_has_sized_self(trait_def_id) {
 138             violations.push(ObjectSafetyViolation::SizedSelf);
 139         }
 140         if self.predicates_reference_self(trait_def_id, false) {
 141             violations.push(ObjectSafetyViolation::SupertraitSelf);
 142         }
 143
 144         debug!("object_safety_violations_for_trait(trait_def_id={:?}) = {:?}",
 145                trait_def_id,
 146                violations);
 147
 148         violations
 149     }
 150
 151     fn predicates_reference_self(
 152         self,
 153         trait_def_id: DefId,
 154         supertraits_only: bool) -> bool
 155     {
 156         let trait_ref = ty::Binder(ty::TraitRef {
 157             def_id: trait_def_id,
 158             substs: Substs::identity_for_item(self, trait_def_id)
 159         });
 160         let predicates = if supertraits_only {
 161             self.item_super_predicates(trait_def_id)
 162         } else {
 163             self.item_predicates(trait_def_id)
 164         };
 165         predicates
 166             .predicates
 167             .into_iter()
 168             .map(|predicate| predicate.subst_supertrait(self, &trait_ref))
 169             .any(|predicate| {
 170                 match predicate {
 171                     ty::Predicate::Trait(ref data) => {
 172                         // In the case of a trait predicate, we can skip the "self" type.
 173                         data.skip_binder().input_types().skip(1).any(|t| t.has_self_ty())
 174                     }
 175                     ty::Predicate::Projection(..) |
 176                     ty::Predicate::WellFormed(..) |
 177                     ty::Predicate::ObjectSafe(..) |
 178                     ty::Predicate::TypeOutlives(..) |
 179                     ty::Predicate::RegionOutlives(..) |
 180                     ty::Predicate::ClosureKind(..) |
 181                     ty::Predicate::Subtype(..) |
 182                     ty::Predicate::Equate(..) => {
 183                         false
 184                     }
 185                 }
 186             })
 187     }
 188
 189     fn trait_has_sized_self(self, trait_def_id: DefId) -> bool {
 190         self.generics_require_sized_self(trait_def_id)
 191     }
 192
 193     fn generics_require_sized_self(self, def_id: DefId) -> bool {
 194         let sized_def_id = match self.lang_items.sized_trait() {
 195             Some(def_id) => def_id,
 196             None => { return false; /* No Sized trait, can't require it! */ }
 197         };
 198
 199         // Search for a predicate like `Self : Sized` amongst the trait bounds.
 200         let free_substs = self.construct_free_substs(def_id,
 201             self.region_maps.node_extent(ast::DUMMY_NODE_ID));
 202         let predicates = self.item_predicates(def_id);
 203         let predicates = predicates.instantiate(self, free_substs).predicates;
 204         elaborate_predicates(self, predicates)
 205             .any(|predicate| {
 206                 match predicate {
 207                     ty::Predicate::Trait(ref trait_pred) if trait_pred.def_id() == sized_def_id => {
 208                         trait_pred.0.self_ty().is_self()
 209                     }
 210                     ty::Predicate::Projection(..) |
 211                     ty::Predicate::Trait(..) |
 212                     ty::Predicate::Equate(..) |
 213                     ty::Predicate::Subtype(..) |
 214                     ty::Predicate::RegionOutlives(..) |
 215                     ty::Predicate::WellFormed(..) |
 216                     ty::Predicate::ObjectSafe(..) |
 217                     ty::Predicate::ClosureKind(..) |
 218                     ty::Predicate::TypeOutlives(..) => {
 219                         false
 220                     }
 221                 }
 222             })
 223     }
 224
 225     /// Returns `Some(_)` if this method makes the containing trait not object safe.
 226     fn object_safety_violation_for_method(self,
 227                                           trait_def_id: DefId,
 228                                           method: &ty::AssociatedItem)
 229                                           -> Option<MethodViolationCode>
 230     {
 231         // Any method that has a `Self : Sized` requisite is otherwise
 232         // exempt from the regulations.
 233         if self.generics_require_sized_self(method.def_id) {
 234             return None;
 235         }
 236
 237         self.virtual_call_violation_for_method(trait_def_id, method)
 238     }
 239
 240     /// We say a method is *vtable safe* if it can be invoked on a trait
 241     /// object.  Note that object-safe traits can have some
 242     /// non-vtable-safe methods, so long as they require `Self:Sized` or
 243     /// otherwise ensure that they cannot be used when `Self=Trait`.
 244     pub fn is_vtable_safe_method(self,
 245                                  trait_def_id: DefId,
 246                                  method: &ty::AssociatedItem)
 247                                  -> bool
 248     {
 249         // Any method that has a `Self : Sized` requisite can't be called.
 250         if self.generics_require_sized_self(method.def_id) {
 251             return false;
 252         }
 253
 254         self.virtual_call_violation_for_method(trait_def_id, method).is_none()
 255     }
 256
 257     /// Returns `Some(_)` if this method cannot be called on a trait
 258     /// object; this does not necessarily imply that the enclosing trait
 259     /// is not object safe, because the method might have a where clause
 260     /// `Self:Sized`.
 261     fn virtual_call_violation_for_method(self,
 262                                          trait_def_id: DefId,
 263                                          method: &ty::AssociatedItem)
 264                                          -> Option<MethodViolationCode>
 265     {
 266         // The method's first parameter must be something that derefs (or
 267         // autorefs) to `&self`. For now, we only accept `self`, `&self`
 268         // and `Box<Self>`.
 269         if !method.method_has_self_argument {
 270             return Some(MethodViolationCode::StaticMethod);
 271         }
 272
 273         // The `Self` type is erased, so it should not appear in list of
 274         // arguments or return type apart from the receiver.
 275         let ref sig = self.item_type(method.def_id).fn_sig();
 276         for input_ty in &sig.skip_binder().inputs()[1..] {
 277             if self.contains_illegal_self_type_reference(trait_def_id, input_ty) {
 278                 return Some(MethodViolationCode::ReferencesSelf);
 279             }
 280         }
 281         if self.contains_illegal_self_type_reference(trait_def_id, sig.output().skip_binder()) {
 282             return Some(MethodViolationCode::ReferencesSelf);
 283         }
 284
 285         // We can't monomorphize things like `fn foo<A>(...)`.
 286         if !self.item_generics(method.def_id).types.is_empty() {
 287             return Some(MethodViolationCode::Generic);
 288         }
 289
 290         None
 291     }
 292
 293     fn contains_illegal_self_type_reference(self,
 294                                             trait_def_id: DefId,
 295                                             ty: Ty<'tcx>)
 296                                             -> bool
 297     {
 298         // This is somewhat subtle. In general, we want to forbid
 299         // references to `Self` in the argument and return types,
 300         // since the value of `Self` is erased. However, there is one
 301         // exception: it is ok to reference `Self` in order to access
 302         // an associated type of the current trait, since we retain
 303         // the value of those associated types in the object type
 304         // itself.
 305         //
 306         // ```rust
 307         // trait SuperTrait {
 308         //     type X;
 309         // }
 310         //
 311         // trait Trait : SuperTrait {
 312         //     type Y;
 313         //     fn foo(&self, x: Self) // bad
 314         //     fn foo(&self) -> Self // bad
 315         //     fn foo(&self) -> Option<Self> // bad
 316         //     fn foo(&self) -> Self::Y // OK, desugars to next example
 317         //     fn foo(&self) -> <Self as Trait>::Y // OK
 318         //     fn foo(&self) -> Self::X // OK, desugars to next example
 319         //     fn foo(&self) -> <Self as SuperTrait>::X // OK
 320         // }
 321         // ```
 322         //
 323         // However, it is not as simple as allowing `Self` in a projected
 324         // type, because there are illegal ways to use `Self` as well:
 325         //
 326         // ```rust
 327         // trait Trait : SuperTrait {
 328         //     ...
 329         //     fn foo(&self) -> <Self as SomeOtherTrait>::X;
 330         // }
 331         // ```
 332         //
 333         // Here we will not have the type of `X` recorded in the
 334         // object type, and we cannot resolve `Self as SomeOtherTrait`
 335         // without knowing what `Self` is.
 336
 337         let mut supertraits: Option<Vec<ty::PolyTraitRef<'tcx>>> = None;
 338         let mut error = false;
 339         ty.maybe_walk(|ty| {
 340             match ty.sty {
 341                 ty::TyParam(ref param_ty) => {
 342                     if param_ty.is_self() {
 343                         error = true;
 344                     }
 345
 346                     false // no contained types to walk
 347                 }
 348
 349                 ty::TyProjection(ref data) => {
 350                     // This is a projected type `<Foo as SomeTrait>::X`.
 351
 352                     // Compute supertraits of current trait lazily.
 353                     if supertraits.is_none() {
 354                         let trait_ref = ty::Binder(ty::TraitRef {
 355                             def_id: trait_def_id,
 356                             substs: Substs::identity_for_item(self, trait_def_id)
 357                         });
 358                         supertraits = Some(traits::supertraits(self, trait_ref).collect());
 359                     }
 360
 361                     // Determine whether the trait reference `Foo as
 362                     // SomeTrait` is in fact a supertrait of the
 363                     // current trait. In that case, this type is
 364                     // legal, because the type `X` will be specified
 365                     // in the object type.  Note that we can just use
 366                     // direct equality here because all of these types
 367                     // are part of the formal parameter listing, and
 368                     // hence there should be no inference variables.
 369                     let projection_trait_ref = ty::Binder(data.trait_ref.clone());
 370                     let is_supertrait_of_current_trait =
 371                         supertraits.as_ref().unwrap().contains(&projection_trait_ref);
 372
 373                     if is_supertrait_of_current_trait {
 374                         false // do not walk contained types, do not report error, do collect $200
 375                     } else {
 376                         true // DO walk contained types, POSSIBLY reporting an error
 377                     }
 378                 }
 379
 380                 _ => true, // walk contained types, if any
 381             }
 382         });
 383
 384         error
 385     }
 386 }