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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 | ||
e9174d1e | 23 | use middle::def_id::DefId; |
85aaf69f | 24 | use middle::subst::{self, SelfSpace, TypeSpace}; |
1a4d82fc | 25 | use middle::traits; |
9cc50fc6 | 26 | use middle::ty::{self, ToPolyTraitRef, Ty, TypeFoldable}; |
1a4d82fc JJ |
27 | use std::rc::Rc; |
28 | use syntax::ast; | |
1a4d82fc | 29 | |
e9174d1e | 30 | #[derive(Clone, Debug, PartialEq, Eq, Hash)] |
1a4d82fc JJ |
31 | pub enum ObjectSafetyViolation<'tcx> { |
32 | /// Self : Sized declared on the trait | |
33 | SizedSelf, | |
34 | ||
85aaf69f SL |
35 | /// Supertrait reference references `Self` an in illegal location |
36 | /// (e.g. `trait Foo : Bar<Self>`) | |
37 | SupertraitSelf, | |
38 | ||
1a4d82fc JJ |
39 | /// Method has something illegal |
40 | Method(Rc<ty::Method<'tcx>>, MethodViolationCode), | |
41 | } | |
42 | ||
43 | /// Reasons a method might not be object-safe. | |
e9174d1e | 44 | #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)] |
1a4d82fc | 45 | pub enum MethodViolationCode { |
1a4d82fc JJ |
46 | /// e.g., `fn foo()` |
47 | StaticMethod, | |
48 | ||
49 | /// e.g., `fn foo(&self, x: Self)` or `fn foo(&self) -> Self` | |
50 | ReferencesSelf, | |
51 | ||
52 | /// e.g., `fn foo<A>()` | |
53 | Generic, | |
54 | } | |
55 | ||
56 | pub fn is_object_safe<'tcx>(tcx: &ty::ctxt<'tcx>, | |
e9174d1e | 57 | trait_def_id: DefId) |
1a4d82fc JJ |
58 | -> bool |
59 | { | |
60 | // Because we query yes/no results frequently, we keep a cache: | |
c1a9b12d | 61 | let def = tcx.lookup_trait_def(trait_def_id); |
1a4d82fc | 62 | |
d9579d0f AL |
63 | let result = def.object_safety().unwrap_or_else(|| { |
64 | let result = object_safety_violations(tcx, trait_def_id).is_empty(); | |
1a4d82fc | 65 | |
d9579d0f AL |
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); | |
1a4d82fc | 70 | |
d9579d0f AL |
71 | result |
72 | }); | |
1a4d82fc | 73 | |
62682a34 | 74 | debug!("is_object_safe({:?}) = {}", trait_def_id, result); |
1a4d82fc JJ |
75 | |
76 | result | |
77 | } | |
78 | ||
b039eaaf SL |
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 | |
82 | /// type collection. | |
83 | pub fn astconv_object_safety_violations<'tcx>(tcx: &ty::ctxt<'tcx>, | |
84 | trait_def_id: DefId) | |
85 | -> Vec<ObjectSafetyViolation<'tcx>> | |
86 | { | |
87 | let mut violations = vec![]; | |
88 | ||
89 | if supertraits_reference_self(tcx, trait_def_id) { | |
90 | violations.push(ObjectSafetyViolation::SupertraitSelf); | |
91 | } | |
92 | ||
93 | debug!("object_safety_violations_for_trait(trait_def_id={:?}) = {:?}", | |
94 | trait_def_id, | |
95 | violations); | |
96 | ||
97 | violations | |
98 | } | |
99 | ||
1a4d82fc | 100 | pub fn object_safety_violations<'tcx>(tcx: &ty::ctxt<'tcx>, |
e9174d1e | 101 | trait_def_id: DefId) |
1a4d82fc JJ |
102 | -> Vec<ObjectSafetyViolation<'tcx>> |
103 | { | |
c34b1796 | 104 | traits::supertrait_def_ids(tcx, trait_def_id) |
62682a34 | 105 | .flat_map(|def_id| object_safety_violations_for_trait(tcx, def_id)) |
1a4d82fc JJ |
106 | .collect() |
107 | } | |
108 | ||
109 | fn object_safety_violations_for_trait<'tcx>(tcx: &ty::ctxt<'tcx>, | |
e9174d1e | 110 | trait_def_id: DefId) |
1a4d82fc JJ |
111 | -> Vec<ObjectSafetyViolation<'tcx>> |
112 | { | |
113 | // Check methods for violations. | |
114 | let mut violations: Vec<_> = | |
c1a9b12d | 115 | tcx.trait_items(trait_def_id).iter() |
e9174d1e | 116 | .filter_map(|item| { |
1a4d82fc JJ |
117 | match *item { |
118 | ty::MethodTraitItem(ref m) => { | |
7453a54e | 119 | object_safety_violation_for_method(tcx, trait_def_id, &m) |
1a4d82fc | 120 | .map(|code| ObjectSafetyViolation::Method(m.clone(), code)) |
1a4d82fc | 121 | } |
e9174d1e | 122 | _ => None, |
1a4d82fc JJ |
123 | } |
124 | }) | |
125 | .collect(); | |
126 | ||
127 | // Check the trait itself. | |
128 | if trait_has_sized_self(tcx, trait_def_id) { | |
129 | violations.push(ObjectSafetyViolation::SizedSelf); | |
130 | } | |
85aaf69f SL |
131 | if supertraits_reference_self(tcx, trait_def_id) { |
132 | violations.push(ObjectSafetyViolation::SupertraitSelf); | |
133 | } | |
1a4d82fc | 134 | |
62682a34 SL |
135 | debug!("object_safety_violations_for_trait(trait_def_id={:?}) = {:?}", |
136 | trait_def_id, | |
137 | violations); | |
1a4d82fc JJ |
138 | |
139 | violations | |
140 | } | |
141 | ||
b039eaaf SL |
142 | pub fn supertraits_reference_self<'tcx>(tcx: &ty::ctxt<'tcx>, |
143 | trait_def_id: DefId) | |
144 | -> bool | |
85aaf69f | 145 | { |
c1a9b12d | 146 | let trait_def = tcx.lookup_trait_def(trait_def_id); |
85aaf69f | 147 | let trait_ref = trait_def.trait_ref.clone(); |
c34b1796 | 148 | let trait_ref = trait_ref.to_poly_trait_ref(); |
c1a9b12d | 149 | let predicates = tcx.lookup_super_predicates(trait_def_id); |
85aaf69f | 150 | predicates |
c34b1796 | 151 | .predicates |
85aaf69f | 152 | .into_iter() |
c34b1796 | 153 | .map(|predicate| predicate.subst_supertrait(tcx, &trait_ref)) |
85aaf69f SL |
154 | .any(|predicate| { |
155 | match predicate { | |
156 | ty::Predicate::Trait(ref data) => { | |
157 | // In the case of a trait predicate, we can skip the "self" type. | |
9346a6ac AL |
158 | data.0.trait_ref.substs.types.get_slice(TypeSpace) |
159 | .iter() | |
160 | .cloned() | |
b039eaaf | 161 | .any(|t| t.has_self_ty()) |
85aaf69f SL |
162 | } |
163 | ty::Predicate::Projection(..) | | |
e9174d1e SL |
164 | ty::Predicate::WellFormed(..) | |
165 | ty::Predicate::ObjectSafe(..) | | |
85aaf69f SL |
166 | ty::Predicate::TypeOutlives(..) | |
167 | ty::Predicate::RegionOutlives(..) | | |
168 | ty::Predicate::Equate(..) => { | |
169 | false | |
170 | } | |
171 | } | |
172 | }) | |
173 | } | |
174 | ||
1a4d82fc | 175 | fn trait_has_sized_self<'tcx>(tcx: &ty::ctxt<'tcx>, |
e9174d1e | 176 | trait_def_id: DefId) |
1a4d82fc | 177 | -> bool |
c34b1796 | 178 | { |
c1a9b12d SL |
179 | let trait_def = tcx.lookup_trait_def(trait_def_id); |
180 | let trait_predicates = tcx.lookup_predicates(trait_def_id); | |
c34b1796 AL |
181 | generics_require_sized_self(tcx, &trait_def.generics, &trait_predicates) |
182 | } | |
183 | ||
184 | fn generics_require_sized_self<'tcx>(tcx: &ty::ctxt<'tcx>, | |
185 | generics: &ty::Generics<'tcx>, | |
186 | predicates: &ty::GenericPredicates<'tcx>) | |
187 | -> bool | |
1a4d82fc | 188 | { |
1a4d82fc JJ |
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! */ } | |
192 | }; | |
193 | ||
194 | // Search for a predicate like `Self : Sized` amongst the trait bounds. | |
9cc50fc6 SL |
195 | let free_substs = tcx.construct_free_substs(generics, |
196 | tcx.region_maps.node_extent(ast::DUMMY_NODE_ID)); | |
c34b1796 | 197 | let predicates = predicates.instantiate(tcx, &free_substs).predicates.into_vec(); |
1a4d82fc JJ |
198 | elaborate_predicates(tcx, predicates) |
199 | .any(|predicate| { | |
200 | match predicate { | |
201 | ty::Predicate::Trait(ref trait_pred) if trait_pred.def_id() == sized_def_id => { | |
b039eaaf | 202 | trait_pred.0.self_ty().is_self() |
1a4d82fc JJ |
203 | } |
204 | ty::Predicate::Projection(..) | | |
205 | ty::Predicate::Trait(..) | | |
206 | ty::Predicate::Equate(..) | | |
207 | ty::Predicate::RegionOutlives(..) | | |
e9174d1e SL |
208 | ty::Predicate::WellFormed(..) | |
209 | ty::Predicate::ObjectSafe(..) | | |
1a4d82fc JJ |
210 | ty::Predicate::TypeOutlives(..) => { |
211 | false | |
212 | } | |
213 | } | |
214 | }) | |
215 | } | |
216 | ||
c34b1796 AL |
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>, | |
e9174d1e | 219 | trait_def_id: DefId, |
c34b1796 AL |
220 | method: &ty::Method<'tcx>) |
221 | -> Option<MethodViolationCode> | |
1a4d82fc | 222 | { |
c34b1796 AL |
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) { | |
226 | return None; | |
227 | } | |
228 | ||
229 | virtual_call_violation_for_method(tcx, trait_def_id, method) | |
230 | } | |
1a4d82fc | 231 | |
c34b1796 AL |
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>, | |
e9174d1e | 237 | trait_def_id: DefId, |
c34b1796 AL |
238 | method: &ty::Method<'tcx>) |
239 | -> bool | |
240 | { | |
241 | virtual_call_violation_for_method(tcx, trait_def_id, method).is_none() | |
242 | } | |
243 | ||
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 | |
247 | /// `Self:Sized`. | |
248 | fn virtual_call_violation_for_method<'tcx>(tcx: &ty::ctxt<'tcx>, | |
e9174d1e | 249 | trait_def_id: DefId, |
c34b1796 AL |
250 | method: &ty::Method<'tcx>) |
251 | -> Option<MethodViolationCode> | |
252 | { | |
253 | // The method's first parameter must be something that derefs (or | |
254 | // autorefs) to `&self`. For now, we only accept `self`, `&self` | |
255 | // and `Box<Self>`. | |
256 | match method.explicit_self { | |
9cc50fc6 | 257 | ty::ExplicitSelfCategory::Static => { |
1a4d82fc JJ |
258 | return Some(MethodViolationCode::StaticMethod); |
259 | } | |
260 | ||
9cc50fc6 SL |
261 | ty::ExplicitSelfCategory::ByValue | |
262 | ty::ExplicitSelfCategory::ByReference(..) | | |
263 | ty::ExplicitSelfCategory::ByBox => { | |
1a4d82fc JJ |
264 | } |
265 | } | |
266 | ||
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; | |
85aaf69f | 270 | for &input_ty in &sig.0.inputs[1..] { |
1a4d82fc JJ |
271 | if contains_illegal_self_type_reference(tcx, trait_def_id, input_ty) { |
272 | return Some(MethodViolationCode::ReferencesSelf); | |
273 | } | |
274 | } | |
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); | |
278 | } | |
279 | } | |
280 | ||
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); | |
284 | } | |
285 | ||
286 | None | |
287 | } | |
288 | ||
289 | fn contains_illegal_self_type_reference<'tcx>(tcx: &ty::ctxt<'tcx>, | |
e9174d1e | 290 | trait_def_id: DefId, |
1a4d82fc JJ |
291 | ty: Ty<'tcx>) |
292 | -> bool | |
293 | { | |
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 | |
300 | // itself. | |
301 | // | |
302 | // ```rust | |
303 | // trait SuperTrait { | |
304 | // type X; | |
305 | // } | |
306 | // | |
307 | // trait Trait : SuperTrait { | |
308 | // type Y; | |
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 | |
316 | // } | |
317 | // ``` | |
318 | // | |
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: | |
321 | // | |
322 | // ```rust | |
323 | // trait Trait : SuperTrait { | |
324 | // ... | |
325 | // fn foo(&self) -> <Self as SomeOtherTrait>::X; | |
326 | // } | |
327 | // ``` | |
328 | // | |
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. | |
332 | ||
333 | let mut supertraits: Option<Vec<ty::PolyTraitRef<'tcx>>> = None; | |
334 | let mut error = false; | |
c1a9b12d | 335 | ty.maybe_walk(|ty| { |
1a4d82fc | 336 | match ty.sty { |
62682a34 | 337 | ty::TyParam(ref param_ty) => { |
1a4d82fc JJ |
338 | if param_ty.space == SelfSpace { |
339 | error = true; | |
340 | } | |
341 | ||
342 | false // no contained types to walk | |
343 | } | |
344 | ||
62682a34 | 345 | ty::TyProjection(ref data) => { |
1a4d82fc JJ |
346 | // This is a projected type `<Foo as SomeTrait>::X`. |
347 | ||
348 | // Compute supertraits of current trait lazily. | |
349 | if supertraits.is_none() { | |
c1a9b12d | 350 | let trait_def = tcx.lookup_trait_def(trait_def_id); |
1a4d82fc JJ |
351 | let trait_ref = ty::Binder(trait_def.trait_ref.clone()); |
352 | supertraits = Some(traits::supertraits(tcx, trait_ref).collect()); | |
353 | } | |
354 | ||
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); | |
366 | ||
367 | if is_supertrait_of_current_trait { | |
368 | false // do not walk contained types, do not report error, do collect $200 | |
369 | } else { | |
370 | true // DO walk contained types, POSSIBLY reporting an error | |
371 | } | |
372 | } | |
373 | ||
374 | _ => true, // walk contained types, if any | |
375 | } | |
376 | }); | |
377 | ||
378 | error | |
379 | } |