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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 | ||
20 | use super::supertraits; | |
21 | use super::elaborate_predicates; | |
22 | ||
85aaf69f | 23 | use middle::subst::{self, SelfSpace, TypeSpace}; |
1a4d82fc | 24 | use middle::traits; |
c34b1796 | 25 | use middle::ty::{self, ToPolyTraitRef, Ty}; |
1a4d82fc JJ |
26 | use std::rc::Rc; |
27 | use syntax::ast; | |
1a4d82fc | 28 | |
62682a34 | 29 | #[derive(Debug)] |
1a4d82fc JJ |
30 | pub enum ObjectSafetyViolation<'tcx> { |
31 | /// Self : Sized declared on the trait | |
32 | SizedSelf, | |
33 | ||
85aaf69f SL |
34 | /// Supertrait reference references `Self` an in illegal location |
35 | /// (e.g. `trait Foo : Bar<Self>`) | |
36 | SupertraitSelf, | |
37 | ||
1a4d82fc JJ |
38 | /// Method has something illegal |
39 | Method(Rc<ty::Method<'tcx>>, MethodViolationCode), | |
40 | } | |
41 | ||
42 | /// Reasons a method might not be object-safe. | |
85aaf69f | 43 | #[derive(Copy,Clone,Debug)] |
1a4d82fc | 44 | pub enum MethodViolationCode { |
1a4d82fc JJ |
45 | /// e.g., `fn foo()` |
46 | StaticMethod, | |
47 | ||
48 | /// e.g., `fn foo(&self, x: Self)` or `fn foo(&self) -> Self` | |
49 | ReferencesSelf, | |
50 | ||
51 | /// e.g., `fn foo<A>()` | |
52 | Generic, | |
53 | } | |
54 | ||
55 | pub fn is_object_safe<'tcx>(tcx: &ty::ctxt<'tcx>, | |
c34b1796 | 56 | trait_def_id: ast::DefId) |
1a4d82fc JJ |
57 | -> bool |
58 | { | |
59 | // Because we query yes/no results frequently, we keep a cache: | |
d9579d0f | 60 | let def = ty::lookup_trait_def(tcx, trait_def_id); |
1a4d82fc | 61 | |
d9579d0f AL |
62 | let result = def.object_safety().unwrap_or_else(|| { |
63 | let result = object_safety_violations(tcx, trait_def_id).is_empty(); | |
1a4d82fc | 64 | |
d9579d0f AL |
65 | // Record just a yes/no result in the cache; this is what is |
66 | // queried most frequently. Note that this may overwrite a | |
67 | // previous result, but always with the same thing. | |
68 | def.set_object_safety(result); | |
1a4d82fc | 69 | |
d9579d0f AL |
70 | result |
71 | }); | |
1a4d82fc | 72 | |
62682a34 | 73 | debug!("is_object_safe({:?}) = {}", trait_def_id, result); |
1a4d82fc JJ |
74 | |
75 | result | |
76 | } | |
77 | ||
78 | pub fn object_safety_violations<'tcx>(tcx: &ty::ctxt<'tcx>, | |
c34b1796 | 79 | trait_def_id: ast::DefId) |
1a4d82fc JJ |
80 | -> Vec<ObjectSafetyViolation<'tcx>> |
81 | { | |
c34b1796 | 82 | traits::supertrait_def_ids(tcx, trait_def_id) |
62682a34 | 83 | .flat_map(|def_id| object_safety_violations_for_trait(tcx, def_id)) |
1a4d82fc JJ |
84 | .collect() |
85 | } | |
86 | ||
87 | fn object_safety_violations_for_trait<'tcx>(tcx: &ty::ctxt<'tcx>, | |
88 | trait_def_id: ast::DefId) | |
89 | -> Vec<ObjectSafetyViolation<'tcx>> | |
90 | { | |
91 | // Check methods for violations. | |
92 | let mut violations: Vec<_> = | |
93 | ty::trait_items(tcx, trait_def_id).iter() | |
94 | .flat_map(|item| { | |
95 | match *item { | |
96 | ty::MethodTraitItem(ref m) => { | |
c34b1796 | 97 | object_safety_violation_for_method(tcx, trait_def_id, &**m) |
1a4d82fc JJ |
98 | .map(|code| ObjectSafetyViolation::Method(m.clone(), code)) |
99 | .into_iter() | |
100 | } | |
d9579d0f | 101 | _ => None.into_iter(), |
1a4d82fc JJ |
102 | } |
103 | }) | |
104 | .collect(); | |
105 | ||
106 | // Check the trait itself. | |
107 | if trait_has_sized_self(tcx, trait_def_id) { | |
108 | violations.push(ObjectSafetyViolation::SizedSelf); | |
109 | } | |
85aaf69f SL |
110 | if supertraits_reference_self(tcx, trait_def_id) { |
111 | violations.push(ObjectSafetyViolation::SupertraitSelf); | |
112 | } | |
1a4d82fc | 113 | |
62682a34 SL |
114 | debug!("object_safety_violations_for_trait(trait_def_id={:?}) = {:?}", |
115 | trait_def_id, | |
116 | violations); | |
1a4d82fc JJ |
117 | |
118 | violations | |
119 | } | |
120 | ||
85aaf69f SL |
121 | fn supertraits_reference_self<'tcx>(tcx: &ty::ctxt<'tcx>, |
122 | trait_def_id: ast::DefId) | |
123 | -> bool | |
124 | { | |
125 | let trait_def = ty::lookup_trait_def(tcx, trait_def_id); | |
126 | let trait_ref = trait_def.trait_ref.clone(); | |
c34b1796 AL |
127 | let trait_ref = trait_ref.to_poly_trait_ref(); |
128 | let predicates = ty::lookup_super_predicates(tcx, trait_def_id); | |
85aaf69f | 129 | predicates |
c34b1796 | 130 | .predicates |
85aaf69f | 131 | .into_iter() |
c34b1796 | 132 | .map(|predicate| predicate.subst_supertrait(tcx, &trait_ref)) |
85aaf69f SL |
133 | .any(|predicate| { |
134 | match predicate { | |
135 | ty::Predicate::Trait(ref data) => { | |
136 | // In the case of a trait predicate, we can skip the "self" type. | |
9346a6ac AL |
137 | data.0.trait_ref.substs.types.get_slice(TypeSpace) |
138 | .iter() | |
139 | .cloned() | |
140 | .any(is_self) | |
85aaf69f SL |
141 | } |
142 | ty::Predicate::Projection(..) | | |
143 | ty::Predicate::TypeOutlives(..) | | |
144 | ty::Predicate::RegionOutlives(..) | | |
145 | ty::Predicate::Equate(..) => { | |
146 | false | |
147 | } | |
148 | } | |
149 | }) | |
150 | } | |
151 | ||
1a4d82fc JJ |
152 | fn trait_has_sized_self<'tcx>(tcx: &ty::ctxt<'tcx>, |
153 | trait_def_id: ast::DefId) | |
154 | -> bool | |
c34b1796 AL |
155 | { |
156 | let trait_def = ty::lookup_trait_def(tcx, trait_def_id); | |
157 | let trait_predicates = ty::lookup_predicates(tcx, trait_def_id); | |
158 | generics_require_sized_self(tcx, &trait_def.generics, &trait_predicates) | |
159 | } | |
160 | ||
161 | fn generics_require_sized_self<'tcx>(tcx: &ty::ctxt<'tcx>, | |
162 | generics: &ty::Generics<'tcx>, | |
163 | predicates: &ty::GenericPredicates<'tcx>) | |
164 | -> bool | |
1a4d82fc | 165 | { |
1a4d82fc JJ |
166 | let sized_def_id = match tcx.lang_items.sized_trait() { |
167 | Some(def_id) => def_id, | |
168 | None => { return false; /* No Sized trait, can't require it! */ } | |
169 | }; | |
170 | ||
171 | // Search for a predicate like `Self : Sized` amongst the trait bounds. | |
c34b1796 AL |
172 | let free_substs = ty::construct_free_substs(tcx, generics, ast::DUMMY_NODE_ID); |
173 | let predicates = predicates.instantiate(tcx, &free_substs).predicates.into_vec(); | |
1a4d82fc JJ |
174 | elaborate_predicates(tcx, predicates) |
175 | .any(|predicate| { | |
176 | match predicate { | |
177 | ty::Predicate::Trait(ref trait_pred) if trait_pred.def_id() == sized_def_id => { | |
85aaf69f | 178 | is_self(trait_pred.0.self_ty()) |
1a4d82fc JJ |
179 | } |
180 | ty::Predicate::Projection(..) | | |
181 | ty::Predicate::Trait(..) | | |
182 | ty::Predicate::Equate(..) | | |
183 | ty::Predicate::RegionOutlives(..) | | |
184 | ty::Predicate::TypeOutlives(..) => { | |
185 | false | |
186 | } | |
187 | } | |
188 | }) | |
189 | } | |
190 | ||
c34b1796 AL |
191 | /// Returns `Some(_)` if this method makes the containing trait not object safe. |
192 | fn object_safety_violation_for_method<'tcx>(tcx: &ty::ctxt<'tcx>, | |
193 | trait_def_id: ast::DefId, | |
194 | method: &ty::Method<'tcx>) | |
195 | -> Option<MethodViolationCode> | |
1a4d82fc | 196 | { |
c34b1796 AL |
197 | // Any method that has a `Self : Sized` requisite is otherwise |
198 | // exempt from the regulations. | |
199 | if generics_require_sized_self(tcx, &method.generics, &method.predicates) { | |
200 | return None; | |
201 | } | |
202 | ||
203 | virtual_call_violation_for_method(tcx, trait_def_id, method) | |
204 | } | |
1a4d82fc | 205 | |
c34b1796 AL |
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<'tcx>(tcx: &ty::ctxt<'tcx>, | |
211 | trait_def_id: ast::DefId, | |
212 | method: &ty::Method<'tcx>) | |
213 | -> bool | |
214 | { | |
215 | virtual_call_violation_for_method(tcx, trait_def_id, method).is_none() | |
216 | } | |
217 | ||
218 | /// Returns `Some(_)` if this method cannot be called on a trait | |
219 | /// object; this does not necessarily imply that the enclosing trait | |
220 | /// is not object safe, because the method might have a where clause | |
221 | /// `Self:Sized`. | |
222 | fn virtual_call_violation_for_method<'tcx>(tcx: &ty::ctxt<'tcx>, | |
223 | trait_def_id: ast::DefId, | |
224 | method: &ty::Method<'tcx>) | |
225 | -> Option<MethodViolationCode> | |
226 | { | |
227 | // The method's first parameter must be something that derefs (or | |
228 | // autorefs) to `&self`. For now, we only accept `self`, `&self` | |
229 | // and `Box<Self>`. | |
230 | match method.explicit_self { | |
1a4d82fc JJ |
231 | ty::StaticExplicitSelfCategory => { |
232 | return Some(MethodViolationCode::StaticMethod); | |
233 | } | |
234 | ||
c34b1796 | 235 | ty::ByValueExplicitSelfCategory | |
1a4d82fc JJ |
236 | ty::ByReferenceExplicitSelfCategory(..) | |
237 | ty::ByBoxExplicitSelfCategory => { | |
238 | } | |
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 = method.fty.sig; | |
85aaf69f | 244 | for &input_ty in &sig.0.inputs[1..] { |
1a4d82fc JJ |
245 | if contains_illegal_self_type_reference(tcx, trait_def_id, input_ty) { |
246 | return Some(MethodViolationCode::ReferencesSelf); | |
247 | } | |
248 | } | |
249 | if let ty::FnConverging(result_type) = sig.0.output { | |
250 | if contains_illegal_self_type_reference(tcx, trait_def_id, result_type) { | |
251 | return Some(MethodViolationCode::ReferencesSelf); | |
252 | } | |
253 | } | |
254 | ||
255 | // We can't monomorphize things like `fn foo<A>(...)`. | |
256 | if !method.generics.types.is_empty_in(subst::FnSpace) { | |
257 | return Some(MethodViolationCode::Generic); | |
258 | } | |
259 | ||
260 | None | |
261 | } | |
262 | ||
263 | fn contains_illegal_self_type_reference<'tcx>(tcx: &ty::ctxt<'tcx>, | |
264 | trait_def_id: ast::DefId, | |
265 | ty: Ty<'tcx>) | |
266 | -> bool | |
267 | { | |
268 | // This is somewhat subtle. In general, we want to forbid | |
269 | // references to `Self` in the argument and return types, | |
270 | // since the value of `Self` is erased. However, there is one | |
271 | // exception: it is ok to reference `Self` in order to access | |
272 | // an associated type of the current trait, since we retain | |
273 | // the value of those associated types in the object type | |
274 | // itself. | |
275 | // | |
276 | // ```rust | |
277 | // trait SuperTrait { | |
278 | // type X; | |
279 | // } | |
280 | // | |
281 | // trait Trait : SuperTrait { | |
282 | // type Y; | |
283 | // fn foo(&self, x: Self) // bad | |
284 | // fn foo(&self) -> Self // bad | |
285 | // fn foo(&self) -> Option<Self> // bad | |
286 | // fn foo(&self) -> Self::Y // OK, desugars to next example | |
287 | // fn foo(&self) -> <Self as Trait>::Y // OK | |
288 | // fn foo(&self) -> Self::X // OK, desugars to next example | |
289 | // fn foo(&self) -> <Self as SuperTrait>::X // OK | |
290 | // } | |
291 | // ``` | |
292 | // | |
293 | // However, it is not as simple as allowing `Self` in a projected | |
294 | // type, because there are illegal ways to use `Self` as well: | |
295 | // | |
296 | // ```rust | |
297 | // trait Trait : SuperTrait { | |
298 | // ... | |
299 | // fn foo(&self) -> <Self as SomeOtherTrait>::X; | |
300 | // } | |
301 | // ``` | |
302 | // | |
303 | // Here we will not have the type of `X` recorded in the | |
304 | // object type, and we cannot resolve `Self as SomeOtherTrait` | |
305 | // without knowing what `Self` is. | |
306 | ||
307 | let mut supertraits: Option<Vec<ty::PolyTraitRef<'tcx>>> = None; | |
308 | let mut error = false; | |
309 | ty::maybe_walk_ty(ty, |ty| { | |
310 | match ty.sty { | |
62682a34 | 311 | ty::TyParam(ref param_ty) => { |
1a4d82fc JJ |
312 | if param_ty.space == SelfSpace { |
313 | error = true; | |
314 | } | |
315 | ||
316 | false // no contained types to walk | |
317 | } | |
318 | ||
62682a34 | 319 | ty::TyProjection(ref data) => { |
1a4d82fc JJ |
320 | // This is a projected type `<Foo as SomeTrait>::X`. |
321 | ||
322 | // Compute supertraits of current trait lazily. | |
323 | if supertraits.is_none() { | |
324 | let trait_def = ty::lookup_trait_def(tcx, trait_def_id); | |
325 | let trait_ref = ty::Binder(trait_def.trait_ref.clone()); | |
326 | supertraits = Some(traits::supertraits(tcx, 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 | ||
85aaf69f SL |
355 | fn is_self<'tcx>(ty: Ty<'tcx>) -> bool { |
356 | match ty.sty { | |
62682a34 | 357 | ty::TyParam(ref data) => data.space == subst::SelfSpace, |
85aaf69f SL |
358 | _ => false, |
359 | } | |
360 | } |