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