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.
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.
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,
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
20 use super::supertraits;
21 use super::elaborate_predicates;
23 use middle::def_id::DefId;
24 use middle::subst::{self, SelfSpace, TypeSpace};
26 use middle::ty::{self, HasTypeFlags, ToPolyTraitRef, Ty};
30 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
31 pub enum ObjectSafetyViolation<'tcx> {
32 /// Self : Sized declared on the trait
35 /// Supertrait reference references `Self` an in illegal location
36 /// (e.g. `trait Foo : Bar<Self>`)
39 /// Method has something illegal
40 Method(Rc<ty::Method<'tcx>>, MethodViolationCode),
43 /// Reasons a method might not be object-safe.
44 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
45 pub enum MethodViolationCode {
49 /// e.g., `fn foo(&self, x: Self)` or `fn foo(&self) -> Self`
52 /// e.g., `fn foo<A>()`
56 pub fn is_object_safe<'tcx>(tcx: &ty::ctxt<'tcx>,
60 // Because we query yes/no results frequently, we keep a cache:
61 let def = tcx.lookup_trait_def(trait_def_id);
63 let result = def.object_safety().unwrap_or_else(|| {
64 let result = object_safety_violations(tcx, trait_def_id).is_empty();
66 // Record just a yes/no result in the cache; this is what is
67 // queried most frequently. Note that this may overwrite a
68 // previous result, but always with the same thing.
69 def.set_object_safety(result);
74 debug!("is_object_safe({:?}) = {}", trait_def_id, result);
79 /// Returns the object safety violations that affect
80 /// astconv - currently, Self in supertraits. This is needed
81 /// because `object_safety_violations` can't be used during
83 pub fn astconv_object_safety_violations<'tcx>(tcx: &ty::ctxt<'tcx>,
85 -> Vec<ObjectSafetyViolation<'tcx>>
87 let mut violations = vec![];
89 if supertraits_reference_self(tcx, trait_def_id) {
90 violations.push(ObjectSafetyViolation::SupertraitSelf);
93 debug!("object_safety_violations_for_trait(trait_def_id={:?}) = {:?}",
100 pub fn object_safety_violations<'tcx>(tcx: &ty::ctxt<'tcx>,
102 -> Vec<ObjectSafetyViolation<'tcx>>
104 traits::supertrait_def_ids(tcx, trait_def_id)
105 .flat_map(|def_id| object_safety_violations_for_trait(tcx, def_id))
109 fn object_safety_violations_for_trait<'tcx>(tcx: &ty::ctxt<'tcx>,
111 -> Vec<ObjectSafetyViolation<'tcx>>
113 // Check methods for violations.
114 let mut violations: Vec<_> =
115 tcx.trait_items(trait_def_id).iter()
118 ty::MethodTraitItem(ref m) => {
119 object_safety_violation_for_method(tcx, trait_def_id, &**m)
120 .map(|code| ObjectSafetyViolation::Method(m.clone(), code))
127 // Check the trait itself.
128 if trait_has_sized_self(tcx, trait_def_id) {
129 violations.push(ObjectSafetyViolation::SizedSelf);
131 if supertraits_reference_self(tcx, trait_def_id) {
132 violations.push(ObjectSafetyViolation::SupertraitSelf);
135 debug!("object_safety_violations_for_trait(trait_def_id={:?}) = {:?}",
142 pub fn supertraits_reference_self<'tcx>(tcx: &ty::ctxt<'tcx>,
146 let trait_def = tcx.lookup_trait_def(trait_def_id);
147 let trait_ref = trait_def.trait_ref.clone();
148 let trait_ref = trait_ref.to_poly_trait_ref();
149 let predicates = tcx.lookup_super_predicates(trait_def_id);
153 .map(|predicate| predicate.subst_supertrait(tcx, &trait_ref))
156 ty::Predicate::Trait(ref data) => {
157 // In the case of a trait predicate, we can skip the "self" type.
158 data.0.trait_ref.substs.types.get_slice(TypeSpace)
161 .any(|t| t.has_self_ty())
163 ty::Predicate::Projection(..) |
164 ty::Predicate::WellFormed(..) |
165 ty::Predicate::ObjectSafe(..) |
166 ty::Predicate::TypeOutlives(..) |
167 ty::Predicate::RegionOutlives(..) |
168 ty::Predicate::Equate(..) => {
175 fn trait_has_sized_self<'tcx>(tcx: &ty::ctxt<'tcx>,
179 let trait_def = tcx.lookup_trait_def(trait_def_id);
180 let trait_predicates = tcx.lookup_predicates(trait_def_id);
181 generics_require_sized_self(tcx, &trait_def.generics, &trait_predicates)
184 fn generics_require_sized_self<'tcx>(tcx: &ty::ctxt<'tcx>,
185 generics: &ty::Generics<'tcx>,
186 predicates: &ty::GenericPredicates<'tcx>)
189 let sized_def_id = match tcx.lang_items.sized_trait() {
190 Some(def_id) => def_id,
191 None => { return false; /* No Sized trait, can't require it! */ }
194 // Search for a predicate like `Self : Sized` amongst the trait bounds.
195 let free_substs = tcx.construct_free_substs(generics,
196 tcx.region_maps.node_extent(ast::DUMMY_NODE_ID));
197 let predicates = predicates.instantiate(tcx, &free_substs).predicates.into_vec();
198 elaborate_predicates(tcx, predicates)
201 ty::Predicate::Trait(ref trait_pred) if trait_pred.def_id() == sized_def_id => {
202 trait_pred.0.self_ty().is_self()
204 ty::Predicate::Projection(..) |
205 ty::Predicate::Trait(..) |
206 ty::Predicate::Equate(..) |
207 ty::Predicate::RegionOutlives(..) |
208 ty::Predicate::WellFormed(..) |
209 ty::Predicate::ObjectSafe(..) |
210 ty::Predicate::TypeOutlives(..) => {
217 /// Returns `Some(_)` if this method makes the containing trait not object safe.
218 fn object_safety_violation_for_method<'tcx>(tcx: &ty::ctxt<'tcx>,
220 method: &ty::Method<'tcx>)
221 -> Option<MethodViolationCode>
223 // Any method that has a `Self : Sized` requisite is otherwise
224 // exempt from the regulations.
225 if generics_require_sized_self(tcx, &method.generics, &method.predicates) {
229 virtual_call_violation_for_method(tcx, trait_def_id, method)
232 /// We say a method is *vtable safe* if it can be invoked on a trait
233 /// object. Note that object-safe traits can have some
234 /// non-vtable-safe methods, so long as they require `Self:Sized` or
235 /// otherwise ensure that they cannot be used when `Self=Trait`.
236 pub fn is_vtable_safe_method<'tcx>(tcx: &ty::ctxt<'tcx>,
238 method: &ty::Method<'tcx>)
241 virtual_call_violation_for_method(tcx, trait_def_id, method).is_none()
244 /// Returns `Some(_)` if this method cannot be called on a trait
245 /// object; this does not necessarily imply that the enclosing trait
246 /// is not object safe, because the method might have a where clause
248 fn virtual_call_violation_for_method<'tcx>(tcx: &ty::ctxt<'tcx>,
250 method: &ty::Method<'tcx>)
251 -> Option<MethodViolationCode>
253 // The method's first parameter must be something that derefs (or
254 // autorefs) to `&self`. For now, we only accept `self`, `&self`
256 match method.explicit_self {
257 ty::ExplicitSelfCategory::Static => {
258 return Some(MethodViolationCode::StaticMethod);
261 ty::ExplicitSelfCategory::ByValue |
262 ty::ExplicitSelfCategory::ByReference(..) |
263 ty::ExplicitSelfCategory::ByBox => {
267 // The `Self` type is erased, so it should not appear in list of
268 // arguments or return type apart from the receiver.
269 let ref sig = method.fty.sig;
270 for &input_ty in &sig.0.inputs[1..] {
271 if contains_illegal_self_type_reference(tcx, trait_def_id, input_ty) {
272 return Some(MethodViolationCode::ReferencesSelf);
275 if let ty::FnConverging(result_type) = sig.0.output {
276 if contains_illegal_self_type_reference(tcx, trait_def_id, result_type) {
277 return Some(MethodViolationCode::ReferencesSelf);
281 // We can't monomorphize things like `fn foo<A>(...)`.
282 if !method.generics.types.is_empty_in(subst::FnSpace) {
283 return Some(MethodViolationCode::Generic);
289 fn contains_illegal_self_type_reference<'tcx>(tcx: &ty::ctxt<'tcx>,
294 // This is somewhat subtle. In general, we want to forbid
295 // references to `Self` in the argument and return types,
296 // since the value of `Self` is erased. However, there is one
297 // exception: it is ok to reference `Self` in order to access
298 // an associated type of the current trait, since we retain
299 // the value of those associated types in the object type
303 // trait SuperTrait {
307 // trait Trait : SuperTrait {
309 // fn foo(&self, x: Self) // bad
310 // fn foo(&self) -> Self // bad
311 // fn foo(&self) -> Option<Self> // bad
312 // fn foo(&self) -> Self::Y // OK, desugars to next example
313 // fn foo(&self) -> <Self as Trait>::Y // OK
314 // fn foo(&self) -> Self::X // OK, desugars to next example
315 // fn foo(&self) -> <Self as SuperTrait>::X // OK
319 // However, it is not as simple as allowing `Self` in a projected
320 // type, because there are illegal ways to use `Self` as well:
323 // trait Trait : SuperTrait {
325 // fn foo(&self) -> <Self as SomeOtherTrait>::X;
329 // Here we will not have the type of `X` recorded in the
330 // object type, and we cannot resolve `Self as SomeOtherTrait`
331 // without knowing what `Self` is.
333 let mut supertraits: Option<Vec<ty::PolyTraitRef<'tcx>>> = None;
334 let mut error = false;
337 ty::TyParam(ref param_ty) => {
338 if param_ty.space == SelfSpace {
342 false // no contained types to walk
345 ty::TyProjection(ref data) => {
346 // This is a projected type `<Foo as SomeTrait>::X`.
348 // Compute supertraits of current trait lazily.
349 if supertraits.is_none() {
350 let trait_def = tcx.lookup_trait_def(trait_def_id);
351 let trait_ref = ty::Binder(trait_def.trait_ref.clone());
352 supertraits = Some(traits::supertraits(tcx, trait_ref).collect());
355 // Determine whether the trait reference `Foo as
356 // SomeTrait` is in fact a supertrait of the
357 // current trait. In that case, this type is
358 // legal, because the type `X` will be specified
359 // in the object type. Note that we can just use
360 // direct equality here because all of these types
361 // are part of the formal parameter listing, and
362 // hence there should be no inference variables.
363 let projection_trait_ref = ty::Binder(data.trait_ref.clone());
364 let is_supertrait_of_current_trait =
365 supertraits.as_ref().unwrap().contains(&projection_trait_ref);
367 if is_supertrait_of_current_trait {
368 false // do not walk contained types, do not report error, do collect $200
370 true // DO walk contained types, POSSIBLY reporting an error
374 _ => true, // walk contained types, if any