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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 | ||
1a4d82fc JJ |
20 | use super::elaborate_predicates; |
21 | ||
54a0048b | 22 | use hir::def_id::DefId; |
54a0048b SL |
23 | use traits; |
24 | use ty::{self, ToPolyTraitRef, Ty, TyCtxt, TypeFoldable}; | |
1a4d82fc JJ |
25 | use std::rc::Rc; |
26 | use syntax::ast; | |
1a4d82fc | 27 | |
e9174d1e | 28 | #[derive(Clone, Debug, PartialEq, Eq, Hash)] |
1a4d82fc JJ |
29 | pub enum ObjectSafetyViolation<'tcx> { |
30 | /// Self : Sized declared on the trait | |
31 | SizedSelf, | |
32 | ||
85aaf69f SL |
33 | /// Supertrait reference references `Self` an in illegal location |
34 | /// (e.g. `trait Foo : Bar<Self>`) | |
35 | SupertraitSelf, | |
36 | ||
1a4d82fc JJ |
37 | /// Method has something illegal |
38 | Method(Rc<ty::Method<'tcx>>, MethodViolationCode), | |
39 | } | |
40 | ||
41 | /// Reasons a method might not be object-safe. | |
e9174d1e | 42 | #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)] |
1a4d82fc | 43 | pub enum MethodViolationCode { |
1a4d82fc JJ |
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 | ||
a7813a04 XL |
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); | |
1a4d82fc | 58 | |
a7813a04 XL |
59 | let result = def.object_safety().unwrap_or_else(|| { |
60 | let result = self.object_safety_violations(trait_def_id).is_empty(); | |
1a4d82fc | 61 | |
a7813a04 XL |
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); | |
1a4d82fc | 66 | |
a7813a04 XL |
67 | result |
68 | }); | |
1a4d82fc | 69 | |
a7813a04 | 70 | debug!("is_object_safe({:?}) = {}", trait_def_id, result); |
1a4d82fc | 71 | |
a7813a04 | 72 | result |
b039eaaf SL |
73 | } |
74 | ||
a7813a04 XL |
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<'tcx>> | |
81 | { | |
82 | let mut violations = vec![]; | |
b039eaaf | 83 | |
a7813a04 XL |
84 | if self.supertraits_reference_self(trait_def_id) { |
85 | violations.push(ObjectSafetyViolation::SupertraitSelf); | |
86 | } | |
1a4d82fc | 87 | |
a7813a04 XL |
88 | debug!("astconv_object_safety_violations(trait_def_id={:?}) = {:?}", |
89 | trait_def_id, | |
90 | violations); | |
1a4d82fc | 91 | |
a7813a04 | 92 | violations |
1a4d82fc | 93 | } |
a7813a04 XL |
94 | |
95 | pub fn object_safety_violations(self, trait_def_id: DefId) | |
96 | -> Vec<ObjectSafetyViolation<'tcx>> | |
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() | |
85aaf69f | 101 | } |
1a4d82fc | 102 | |
a7813a04 XL |
103 | fn object_safety_violations_for_trait(self, trait_def_id: DefId) |
104 | -> Vec<ObjectSafetyViolation<'tcx>> | |
105 | { | |
106 | // Check methods for violations. | |
107 | let mut violations: Vec<_> = | |
108 | self.trait_items(trait_def_id).iter() | |
109 | .filter_map(|item| { | |
110 | match *item { | |
111 | ty::MethodTraitItem(ref m) => { | |
112 | self.object_safety_violation_for_method(trait_def_id, &m) | |
113 | .map(|code| ObjectSafetyViolation::Method(m.clone(), code)) | |
114 | } | |
115 | _ => None, | |
116 | } | |
117 | }) | |
118 | .collect(); | |
1a4d82fc | 119 | |
a7813a04 XL |
120 | // Check the trait itself. |
121 | if self.trait_has_sized_self(trait_def_id) { | |
122 | violations.push(ObjectSafetyViolation::SizedSelf); | |
123 | } | |
124 | if self.supertraits_reference_self(trait_def_id) { | |
125 | violations.push(ObjectSafetyViolation::SupertraitSelf); | |
126 | } | |
1a4d82fc | 127 | |
a7813a04 XL |
128 | debug!("object_safety_violations_for_trait(trait_def_id={:?}) = {:?}", |
129 | trait_def_id, | |
130 | violations); | |
85aaf69f | 131 | |
a7813a04 XL |
132 | violations |
133 | } | |
c34b1796 | 134 | |
a7813a04 XL |
135 | fn supertraits_reference_self(self, trait_def_id: DefId) -> bool { |
136 | let trait_def = self.lookup_trait_def(trait_def_id); | |
137 | let trait_ref = trait_def.trait_ref.clone(); | |
138 | let trait_ref = trait_ref.to_poly_trait_ref(); | |
139 | let predicates = self.lookup_super_predicates(trait_def_id); | |
140 | predicates | |
141 | .predicates | |
142 | .into_iter() | |
143 | .map(|predicate| predicate.subst_supertrait(self, &trait_ref)) | |
144 | .any(|predicate| { | |
145 | match predicate { | |
146 | ty::Predicate::Trait(ref data) => { | |
147 | // In the case of a trait predicate, we can skip the "self" type. | |
9e0c209e | 148 | data.skip_binder().input_types().skip(1).any(|t| t.has_self_ty()) |
a7813a04 XL |
149 | } |
150 | ty::Predicate::Projection(..) | | |
151 | ty::Predicate::WellFormed(..) | | |
152 | ty::Predicate::ObjectSafe(..) | | |
153 | ty::Predicate::TypeOutlives(..) | | |
154 | ty::Predicate::RegionOutlives(..) | | |
155 | ty::Predicate::ClosureKind(..) | | |
a7813a04 XL |
156 | ty::Predicate::Equate(..) => { |
157 | false | |
158 | } | |
1a4d82fc | 159 | } |
a7813a04 | 160 | }) |
c34b1796 AL |
161 | } |
162 | ||
a7813a04 | 163 | fn trait_has_sized_self(self, trait_def_id: DefId) -> bool { |
9e0c209e | 164 | self.generics_require_sized_self(trait_def_id) |
a7813a04 | 165 | } |
1a4d82fc | 166 | |
9e0c209e | 167 | fn generics_require_sized_self(self, def_id: DefId) -> bool { |
a7813a04 XL |
168 | let sized_def_id = match self.lang_items.sized_trait() { |
169 | Some(def_id) => def_id, | |
170 | None => { return false; /* No Sized trait, can't require it! */ } | |
171 | }; | |
172 | ||
173 | // Search for a predicate like `Self : Sized` amongst the trait bounds. | |
9e0c209e | 174 | let free_substs = self.construct_free_substs(def_id, |
a7813a04 | 175 | self.region_maps.node_extent(ast::DUMMY_NODE_ID)); |
9e0c209e SL |
176 | let predicates = self.lookup_predicates(def_id); |
177 | let predicates = predicates.instantiate(self, free_substs).predicates; | |
a7813a04 XL |
178 | elaborate_predicates(self, predicates) |
179 | .any(|predicate| { | |
180 | match predicate { | |
181 | ty::Predicate::Trait(ref trait_pred) if trait_pred.def_id() == sized_def_id => { | |
182 | trait_pred.0.self_ty().is_self() | |
183 | } | |
184 | ty::Predicate::Projection(..) | | |
185 | ty::Predicate::Trait(..) | | |
a7813a04 XL |
186 | ty::Predicate::Equate(..) | |
187 | ty::Predicate::RegionOutlives(..) | | |
188 | ty::Predicate::WellFormed(..) | | |
189 | ty::Predicate::ObjectSafe(..) | | |
190 | ty::Predicate::ClosureKind(..) | | |
191 | ty::Predicate::TypeOutlives(..) => { | |
192 | false | |
193 | } | |
194 | } | |
195 | }) | |
196 | } | |
c34b1796 | 197 | |
a7813a04 XL |
198 | /// Returns `Some(_)` if this method makes the containing trait not object safe. |
199 | fn object_safety_violation_for_method(self, | |
200 | trait_def_id: DefId, | |
201 | method: &ty::Method<'gcx>) | |
202 | -> Option<MethodViolationCode> | |
203 | { | |
204 | // Any method that has a `Self : Sized` requisite is otherwise | |
205 | // exempt from the regulations. | |
9e0c209e | 206 | if self.generics_require_sized_self(method.def_id) { |
a7813a04 | 207 | return None; |
1a4d82fc JJ |
208 | } |
209 | ||
a7813a04 | 210 | self.virtual_call_violation_for_method(trait_def_id, method) |
1a4d82fc JJ |
211 | } |
212 | ||
a7813a04 XL |
213 | /// We say a method is *vtable safe* if it can be invoked on a trait |
214 | /// object. Note that object-safe traits can have some | |
215 | /// non-vtable-safe methods, so long as they require `Self:Sized` or | |
216 | /// otherwise ensure that they cannot be used when `Self=Trait`. | |
217 | pub fn is_vtable_safe_method(self, | |
218 | trait_def_id: DefId, | |
219 | method: &ty::Method<'gcx>) | |
220 | -> bool | |
221 | { | |
222 | // Any method that has a `Self : Sized` requisite can't be called. | |
9e0c209e | 223 | if self.generics_require_sized_self(method.def_id) { |
a7813a04 | 224 | return false; |
1a4d82fc | 225 | } |
1a4d82fc | 226 | |
a7813a04 | 227 | self.virtual_call_violation_for_method(trait_def_id, method).is_none() |
1a4d82fc JJ |
228 | } |
229 | ||
a7813a04 XL |
230 | /// Returns `Some(_)` if this method cannot be called on a trait |
231 | /// object; this does not necessarily imply that the enclosing trait | |
232 | /// is not object safe, because the method might have a where clause | |
233 | /// `Self:Sized`. | |
234 | fn virtual_call_violation_for_method(self, | |
235 | trait_def_id: DefId, | |
236 | method: &ty::Method<'tcx>) | |
237 | -> Option<MethodViolationCode> | |
238 | { | |
239 | // The method's first parameter must be something that derefs (or | |
240 | // autorefs) to `&self`. For now, we only accept `self`, `&self` | |
241 | // and `Box<Self>`. | |
242 | match method.explicit_self { | |
243 | ty::ExplicitSelfCategory::Static => { | |
244 | return Some(MethodViolationCode::StaticMethod); | |
245 | } | |
1a4d82fc | 246 | |
a7813a04 XL |
247 | ty::ExplicitSelfCategory::ByValue | |
248 | ty::ExplicitSelfCategory::ByReference(..) | | |
249 | ty::ExplicitSelfCategory::ByBox => { | |
250 | } | |
251 | } | |
1a4d82fc | 252 | |
a7813a04 XL |
253 | // The `Self` type is erased, so it should not appear in list of |
254 | // arguments or return type apart from the receiver. | |
255 | let ref sig = method.fty.sig; | |
256 | for &input_ty in &sig.0.inputs[1..] { | |
257 | if self.contains_illegal_self_type_reference(trait_def_id, input_ty) { | |
258 | return Some(MethodViolationCode::ReferencesSelf); | |
259 | } | |
260 | } | |
5bcae85e SL |
261 | if self.contains_illegal_self_type_reference(trait_def_id, sig.0.output) { |
262 | return Some(MethodViolationCode::ReferencesSelf); | |
a7813a04 | 263 | } |
1a4d82fc | 264 | |
a7813a04 | 265 | // We can't monomorphize things like `fn foo<A>(...)`. |
9e0c209e | 266 | if !method.generics.types.is_empty() { |
a7813a04 XL |
267 | return Some(MethodViolationCode::Generic); |
268 | } | |
1a4d82fc | 269 | |
a7813a04 XL |
270 | None |
271 | } | |
272 | ||
273 | fn contains_illegal_self_type_reference(self, | |
274 | trait_def_id: DefId, | |
275 | ty: Ty<'tcx>) | |
276 | -> bool | |
277 | { | |
278 | // This is somewhat subtle. In general, we want to forbid | |
279 | // references to `Self` in the argument and return types, | |
280 | // since the value of `Self` is erased. However, there is one | |
281 | // exception: it is ok to reference `Self` in order to access | |
282 | // an associated type of the current trait, since we retain | |
283 | // the value of those associated types in the object type | |
284 | // itself. | |
285 | // | |
286 | // ```rust | |
287 | // trait SuperTrait { | |
288 | // type X; | |
289 | // } | |
290 | // | |
291 | // trait Trait : SuperTrait { | |
292 | // type Y; | |
293 | // fn foo(&self, x: Self) // bad | |
294 | // fn foo(&self) -> Self // bad | |
295 | // fn foo(&self) -> Option<Self> // bad | |
296 | // fn foo(&self) -> Self::Y // OK, desugars to next example | |
297 | // fn foo(&self) -> <Self as Trait>::Y // OK | |
298 | // fn foo(&self) -> Self::X // OK, desugars to next example | |
299 | // fn foo(&self) -> <Self as SuperTrait>::X // OK | |
300 | // } | |
301 | // ``` | |
302 | // | |
303 | // However, it is not as simple as allowing `Self` in a projected | |
304 | // type, because there are illegal ways to use `Self` as well: | |
305 | // | |
306 | // ```rust | |
307 | // trait Trait : SuperTrait { | |
308 | // ... | |
309 | // fn foo(&self) -> <Self as SomeOtherTrait>::X; | |
310 | // } | |
311 | // ``` | |
312 | // | |
313 | // Here we will not have the type of `X` recorded in the | |
314 | // object type, and we cannot resolve `Self as SomeOtherTrait` | |
315 | // without knowing what `Self` is. | |
316 | ||
317 | let mut supertraits: Option<Vec<ty::PolyTraitRef<'tcx>>> = None; | |
318 | let mut error = false; | |
319 | ty.maybe_walk(|ty| { | |
320 | match ty.sty { | |
321 | ty::TyParam(ref param_ty) => { | |
9e0c209e | 322 | if param_ty.is_self() { |
a7813a04 XL |
323 | error = true; |
324 | } | |
325 | ||
326 | false // no contained types to walk | |
1a4d82fc JJ |
327 | } |
328 | ||
a7813a04 XL |
329 | ty::TyProjection(ref data) => { |
330 | // This is a projected type `<Foo as SomeTrait>::X`. | |
331 | ||
332 | // Compute supertraits of current trait lazily. | |
333 | if supertraits.is_none() { | |
334 | let trait_def = self.lookup_trait_def(trait_def_id); | |
335 | let trait_ref = ty::Binder(trait_def.trait_ref.clone()); | |
336 | supertraits = Some(traits::supertraits(self, trait_ref).collect()); | |
337 | } | |
338 | ||
339 | // Determine whether the trait reference `Foo as | |
340 | // SomeTrait` is in fact a supertrait of the | |
341 | // current trait. In that case, this type is | |
342 | // legal, because the type `X` will be specified | |
343 | // in the object type. Note that we can just use | |
344 | // direct equality here because all of these types | |
345 | // are part of the formal parameter listing, and | |
346 | // hence there should be no inference variables. | |
347 | let projection_trait_ref = ty::Binder(data.trait_ref.clone()); | |
348 | let is_supertrait_of_current_trait = | |
349 | supertraits.as_ref().unwrap().contains(&projection_trait_ref); | |
350 | ||
351 | if is_supertrait_of_current_trait { | |
352 | false // do not walk contained types, do not report error, do collect $200 | |
353 | } else { | |
354 | true // DO walk contained types, POSSIBLY reporting an error | |
355 | } | |
1a4d82fc | 356 | } |
1a4d82fc | 357 | |
a7813a04 XL |
358 | _ => true, // walk contained types, if any |
359 | } | |
360 | }); | |
1a4d82fc | 361 | |
a7813a04 XL |
362 | error |
363 | } | |
1a4d82fc | 364 | } |