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     /// Associated const
  42     AssociatedConst(ast::Name),
  43 }
  44
  45 impl ObjectSafetyViolation {
  46     pub fn error_msg(&self) -> Cow<'static, str> {
  47         match *self {
  48             ObjectSafetyViolation::SizedSelf =>
  49                 "the trait cannot require that `Self : Sized`".into(),
  50             ObjectSafetyViolation::SupertraitSelf =>
  51                 "the trait cannot use `Self` as a type parameter \
  52                  in the supertraits or where-clauses".into(),
  53             ObjectSafetyViolation::Method(name, MethodViolationCode::StaticMethod) =>
  54                 format!("method `{}` has no receiver", name).into(),
  55             ObjectSafetyViolation::Method(name, MethodViolationCode::ReferencesSelf) =>
  56                 format!("method `{}` references the `Self` type \
  57                          in its arguments or return type", name).into(),
  58             ObjectSafetyViolation::Method(name, MethodViolationCode::Generic) =>
  59                 format!("method `{}` has generic type parameters", name).into(),
  60             ObjectSafetyViolation::AssociatedConst(name) =>
  61                 format!("the trait cannot contain associated consts like `{}`", name).into(),
  62         }
  63     }
  64 }
  65
  66 /// Reasons a method might not be object-safe.
  67 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
  68 pub enum MethodViolationCode {
  69     /// e.g., `fn foo()`
  70     StaticMethod,
  71
  72     /// e.g., `fn foo(&self, x: Self)` or `fn foo(&self) -> Self`
  73     ReferencesSelf,
  74
  75     /// e.g., `fn foo<A>()`
  76     Generic,
  77 }
  78
  79 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
  80     pub fn is_object_safe(self, trait_def_id: DefId) -> bool {
  81         // Because we query yes/no results frequently, we keep a cache:
  82         let def = self.trait_def(trait_def_id);
  83
  84         let result = def.object_safety().unwrap_or_else(|| {
  85             let result = self.object_safety_violations(trait_def_id).is_empty();
  86
  87             // Record just a yes/no result in the cache; this is what is
  88             // queried most frequently. Note that this may overwrite a
  89             // previous result, but always with the same thing.
  90             def.set_object_safety(result);
  91
  92             result
  93         });
  94
  95         debug!("is_object_safe({:?}) = {}", trait_def_id, result);
  96
  97         result
  98     }
  99
 100     /// Returns the object safety violations that affect
 101     /// astconv - currently, Self in supertraits. This is needed
 102     /// because `object_safety_violations` can't be used during
 103     /// type collection.
 104     pub fn astconv_object_safety_violations(self, trait_def_id: DefId)
 105                                             -> Vec<ObjectSafetyViolation>
 106     {
 107         let mut violations = vec![];
 108
 109         for def_id in traits::supertrait_def_ids(self, trait_def_id) {
 110             if self.predicates_reference_self(def_id, true) {
 111                 violations.push(ObjectSafetyViolation::SupertraitSelf);
 112             }
 113         }
 114
 115         debug!("astconv_object_safety_violations(trait_def_id={:?}) = {:?}",
 116                trait_def_id,
 117                violations);
 118
 119         violations
 120     }
 121
 122     pub fn object_safety_violations(self, trait_def_id: DefId)
 123                                     -> Vec<ObjectSafetyViolation>
 124     {
 125         traits::supertrait_def_ids(self, trait_def_id)
 126             .flat_map(|def_id| self.object_safety_violations_for_trait(def_id))
 127             .collect()
 128     }
 129
 130     fn object_safety_violations_for_trait(self, trait_def_id: DefId)
 131                                           -> Vec<ObjectSafetyViolation>
 132     {
 133         // Check methods for violations.
 134         let mut violations: Vec<_> = self.associated_items(trait_def_id)
 135             .filter(|item| item.kind == ty::AssociatedKind::Method)
 136             .filter_map(|item| {
 137                 self.object_safety_violation_for_method(trait_def_id, &item)
 138                     .map(|code| ObjectSafetyViolation::Method(item.name, code))
 139             }).collect();
 140
 141         // Check the trait itself.
 142         if self.trait_has_sized_self(trait_def_id) {
 143             violations.push(ObjectSafetyViolation::SizedSelf);
 144         }
 145         if self.predicates_reference_self(trait_def_id, false) {
 146             violations.push(ObjectSafetyViolation::SupertraitSelf);
 147         }
 148
 149         violations.extend(self.associated_items(trait_def_id)
 150             .filter(|item| item.kind == ty::AssociatedKind::Const)
 151             .map(|item| ObjectSafetyViolation::AssociatedConst(item.name)));
 152
 153         debug!("object_safety_violations_for_trait(trait_def_id={:?}) = {:?}",
 154                trait_def_id,
 155                violations);
 156
 157         violations
 158     }
 159
 160     fn predicates_reference_self(
 161         self,
 162         trait_def_id: DefId,
 163         supertraits_only: bool) -> bool
 164     {
 165         let trait_ref = ty::Binder(ty::TraitRef {
 166             def_id: trait_def_id,
 167             substs: Substs::identity_for_item(self, trait_def_id)
 168         });
 169         let predicates = if supertraits_only {
 170             self.super_predicates_of(trait_def_id)
 171         } else {
 172             self.predicates_of(trait_def_id)
 173         };
 174         predicates
 175             .predicates
 176             .into_iter()
 177             .map(|predicate| predicate.subst_supertrait(self, &trait_ref))
 178             .any(|predicate| {
 179                 match predicate {
 180                     ty::Predicate::Trait(ref data) => {
 181                         // In the case of a trait predicate, we can skip the "self" type.
 182                         data.skip_binder().input_types().skip(1).any(|t| t.has_self_ty())
 183                     }
 184                     ty::Predicate::Projection(..) |
 185                     ty::Predicate::WellFormed(..) |
 186                     ty::Predicate::ObjectSafe(..) |
 187                     ty::Predicate::TypeOutlives(..) |
 188                     ty::Predicate::RegionOutlives(..) |
 189                     ty::Predicate::ClosureKind(..) |
 190                     ty::Predicate::Subtype(..) |
 191                     ty::Predicate::Equate(..) => {
 192                         false
 193                     }
 194                 }
 195             })
 196     }
 197
 198     fn trait_has_sized_self(self, trait_def_id: DefId) -> bool {
 199         self.generics_require_sized_self(trait_def_id)
 200     }
 201
 202     fn generics_require_sized_self(self, def_id: DefId) -> bool {
 203         let sized_def_id = match self.lang_items.sized_trait() {
 204             Some(def_id) => def_id,
 205             None => { return false; /* No Sized trait, can't require it! */ }
 206         };
 207
 208         // Search for a predicate like `Self : Sized` amongst the trait bounds.
 209         let free_substs = self.construct_free_substs(def_id,
 210             self.region_maps.node_extent(ast::DUMMY_NODE_ID));
 211         let predicates = self.predicates_of(def_id);
 212         let predicates = predicates.instantiate(self, free_substs).predicates;
 213         elaborate_predicates(self, predicates)
 214             .any(|predicate| {
 215                 match predicate {
 216                     ty::Predicate::Trait(ref trait_pred) if trait_pred.def_id() == sized_def_id => {
 217                         trait_pred.0.self_ty().is_self()
 218                     }
 219                     ty::Predicate::Projection(..) |
 220                     ty::Predicate::Trait(..) |
 221                     ty::Predicate::Equate(..) |
 222                     ty::Predicate::Subtype(..) |
 223                     ty::Predicate::RegionOutlives(..) |
 224                     ty::Predicate::WellFormed(..) |
 225                     ty::Predicate::ObjectSafe(..) |
 226                     ty::Predicate::ClosureKind(..) |
 227                     ty::Predicate::TypeOutlives(..) => {
 228                         false
 229                     }
 230                 }
 231             })
 232     }
 233
 234     /// Returns `Some(_)` if this method makes the containing trait not object safe.
 235     fn object_safety_violation_for_method(self,
 236                                           trait_def_id: DefId,
 237                                           method: &ty::AssociatedItem)
 238                                           -> Option<MethodViolationCode>
 239     {
 240         // Any method that has a `Self : Sized` requisite is otherwise
 241         // exempt from the regulations.
 242         if self.generics_require_sized_self(method.def_id) {
 243             return None;
 244         }
 245
 246         self.virtual_call_violation_for_method(trait_def_id, method)
 247     }
 248
 249     /// We say a method is *vtable safe* if it can be invoked on a trait
 250     /// object.  Note that object-safe traits can have some
 251     /// non-vtable-safe methods, so long as they require `Self:Sized` or
 252     /// otherwise ensure that they cannot be used when `Self=Trait`.
 253     pub fn is_vtable_safe_method(self,
 254                                  trait_def_id: DefId,
 255                                  method: &ty::AssociatedItem)
 256                                  -> bool
 257     {
 258         // Any method that has a `Self : Sized` requisite can't be called.
 259         if self.generics_require_sized_self(method.def_id) {
 260             return false;
 261         }
 262
 263         self.virtual_call_violation_for_method(trait_def_id, method).is_none()
 264     }
 265
 266     /// Returns `Some(_)` if this method cannot be called on a trait
 267     /// object; this does not necessarily imply that the enclosing trait
 268     /// is not object safe, because the method might have a where clause
 269     /// `Self:Sized`.
 270     fn virtual_call_violation_for_method(self,
 271                                          trait_def_id: DefId,
 272                                          method: &ty::AssociatedItem)
 273                                          -> Option<MethodViolationCode>
 274     {
 275         // The method's first parameter must be something that derefs (or
 276         // autorefs) to `&self`. For now, we only accept `self`, `&self`
 277         // and `Box<Self>`.
 278         if !method.method_has_self_argument {
 279             return Some(MethodViolationCode::StaticMethod);
 280         }
 281
 282         // The `Self` type is erased, so it should not appear in list of
 283         // arguments or return type apart from the receiver.
 284         let ref sig = self.type_of(method.def_id).fn_sig();
 285         for input_ty in &sig.skip_binder().inputs()[1..] {
 286             if self.contains_illegal_self_type_reference(trait_def_id, input_ty) {
 287                 return Some(MethodViolationCode::ReferencesSelf);
 288             }
 289         }
 290         if self.contains_illegal_self_type_reference(trait_def_id, sig.output().skip_binder()) {
 291             return Some(MethodViolationCode::ReferencesSelf);
 292         }
 293
 294         // We can't monomorphize things like `fn foo<A>(...)`.
 295         if !self.generics_of(method.def_id).types.is_empty() {
 296             return Some(MethodViolationCode::Generic);
 297         }
 298
 299         None
 300     }
 301
 302     fn contains_illegal_self_type_reference(self,
 303                                             trait_def_id: DefId,
 304                                             ty: Ty<'tcx>)
 305                                             -> bool
 306     {
 307         // This is somewhat subtle. In general, we want to forbid
 308         // references to `Self` in the argument and return types,
 309         // since the value of `Self` is erased. However, there is one
 310         // exception: it is ok to reference `Self` in order to access
 311         // an associated type of the current trait, since we retain
 312         // the value of those associated types in the object type
 313         // itself.
 314         //
 315         // ```rust
 316         // trait SuperTrait {
 317         //     type X;
 318         // }
 319         //
 320         // trait Trait : SuperTrait {
 321         //     type Y;
 322         //     fn foo(&self, x: Self) // bad
 323         //     fn foo(&self) -> Self // bad
 324         //     fn foo(&self) -> Option<Self> // bad
 325         //     fn foo(&self) -> Self::Y // OK, desugars to next example
 326         //     fn foo(&self) -> <Self as Trait>::Y // OK
 327         //     fn foo(&self) -> Self::X // OK, desugars to next example
 328         //     fn foo(&self) -> <Self as SuperTrait>::X // OK
 329         // }
 330         // ```
 331         //
 332         // However, it is not as simple as allowing `Self` in a projected
 333         // type, because there are illegal ways to use `Self` as well:
 334         //
 335         // ```rust
 336         // trait Trait : SuperTrait {
 337         //     ...
 338         //     fn foo(&self) -> <Self as SomeOtherTrait>::X;
 339         // }
 340         // ```
 341         //
 342         // Here we will not have the type of `X` recorded in the
 343         // object type, and we cannot resolve `Self as SomeOtherTrait`
 344         // without knowing what `Self` is.
 345
 346         let mut supertraits: Option<Vec<ty::PolyTraitRef<'tcx>>> = None;
 347         let mut error = false;
 348         ty.maybe_walk(|ty| {
 349             match ty.sty {
 350                 ty::TyParam(ref param_ty) => {
 351                     if param_ty.is_self() {
 352                         error = true;
 353                     }
 354
 355                     false // no contained types to walk
 356                 }
 357
 358                 ty::TyProjection(ref data) => {
 359                     // This is a projected type `<Foo as SomeTrait>::X`.
 360
 361                     // Compute supertraits of current trait lazily.
 362                     if supertraits.is_none() {
 363                         let trait_ref = ty::Binder(ty::TraitRef {
 364                             def_id: trait_def_id,
 365                             substs: Substs::identity_for_item(self, trait_def_id)
 366                         });
 367                         supertraits = Some(traits::supertraits(self, trait_ref).collect());
 368                     }
 369
 370                     // Determine whether the trait reference `Foo as
 371                     // SomeTrait` is in fact a supertrait of the
 372                     // current trait. In that case, this type is
 373                     // legal, because the type `X` will be specified
 374                     // in the object type.  Note that we can just use
 375                     // direct equality here because all of these types
 376                     // are part of the formal parameter listing, and
 377                     // hence there should be no inference variables.
 378                     let projection_trait_ref = ty::Binder(data.trait_ref.clone());
 379                     let is_supertrait_of_current_trait =
 380                         supertraits.as_ref().unwrap().contains(&projection_trait_ref);
 381
 382                     if is_supertrait_of_current_trait {
 383                         false // do not walk contained types, do not report error, do collect $200
 384                     } else {
 385                         true // DO walk contained types, POSSIBLY reporting an error
 386                     }
 387                 }
 388
 389                 _ => true, // walk contained types, if any
 390             }
 391         });
 392
 393         error
 394     }
 395 }