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