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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; |
8faf50e0 | 23 | use lint; |
54a0048b | 24 | use traits; |
476ff2be | 25 | use ty::{self, Ty, TyCtxt, TypeFoldable}; |
abe05a73 | 26 | use ty::util::ExplicitSelf; |
8bb4bdeb | 27 | use std::borrow::Cow; |
1a4d82fc | 28 | use syntax::ast; |
8faf50e0 | 29 | use syntax_pos::Span; |
1a4d82fc | 30 | |
8faf50e0 | 31 | #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)] |
476ff2be | 32 | pub enum ObjectSafetyViolation { |
1a4d82fc JJ |
33 | /// Self : Sized declared on the trait |
34 | SizedSelf, | |
35 | ||
85aaf69f SL |
36 | /// Supertrait reference references `Self` an in illegal location |
37 | /// (e.g. `trait Foo : Bar<Self>`) | |
38 | SupertraitSelf, | |
39 | ||
1a4d82fc | 40 | /// Method has something illegal |
476ff2be | 41 | Method(ast::Name, MethodViolationCode), |
cc61c64b XL |
42 | |
43 | /// Associated const | |
44 | AssociatedConst(ast::Name), | |
1a4d82fc JJ |
45 | } |
46 | ||
8bb4bdeb XL |
47 | impl ObjectSafetyViolation { |
48 | pub fn error_msg(&self) -> Cow<'static, str> { | |
49 | match *self { | |
50 | ObjectSafetyViolation::SizedSelf => | |
51 | "the trait cannot require that `Self : Sized`".into(), | |
52 | ObjectSafetyViolation::SupertraitSelf => | |
53 | "the trait cannot use `Self` as a type parameter \ | |
cc61c64b | 54 | in the supertraits or where-clauses".into(), |
8bb4bdeb XL |
55 | ObjectSafetyViolation::Method(name, MethodViolationCode::StaticMethod) => |
56 | format!("method `{}` has no receiver", name).into(), | |
57 | ObjectSafetyViolation::Method(name, MethodViolationCode::ReferencesSelf) => | |
58 | format!("method `{}` references the `Self` type \ | |
59 | in its arguments or return type", name).into(), | |
8faf50e0 XL |
60 | ObjectSafetyViolation::Method(name, |
61 | MethodViolationCode::WhereClauseReferencesSelf(_)) => | |
62 | format!("method `{}` references the `Self` type in where clauses", name).into(), | |
8bb4bdeb XL |
63 | ObjectSafetyViolation::Method(name, MethodViolationCode::Generic) => |
64 | format!("method `{}` has generic type parameters", name).into(), | |
abe05a73 | 65 | ObjectSafetyViolation::Method(name, MethodViolationCode::NonStandardSelfType) => |
ff7c6d11 | 66 | format!("method `{}` has a non-standard `self` type", name).into(), |
cc61c64b XL |
67 | ObjectSafetyViolation::AssociatedConst(name) => |
68 | format!("the trait cannot contain associated consts like `{}`", name).into(), | |
8bb4bdeb XL |
69 | } |
70 | } | |
71 | } | |
72 | ||
1a4d82fc | 73 | /// Reasons a method might not be object-safe. |
e9174d1e | 74 | #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)] |
1a4d82fc | 75 | pub enum MethodViolationCode { |
1a4d82fc JJ |
76 | /// e.g., `fn foo()` |
77 | StaticMethod, | |
78 | ||
79 | /// e.g., `fn foo(&self, x: Self)` or `fn foo(&self) -> Self` | |
80 | ReferencesSelf, | |
81 | ||
8faf50e0 XL |
82 | /// e.g. `fn foo(&self) where Self: Clone` |
83 | WhereClauseReferencesSelf(Span), | |
84 | ||
1a4d82fc JJ |
85 | /// e.g., `fn foo<A>()` |
86 | Generic, | |
abe05a73 XL |
87 | |
88 | /// arbitrary `self` type, e.g. `self: Rc<Self>` | |
89 | NonStandardSelfType, | |
1a4d82fc JJ |
90 | } |
91 | ||
a7813a04 | 92 | impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> { |
b039eaaf | 93 | |
a7813a04 XL |
94 | /// Returns the object safety violations that affect |
95 | /// astconv - currently, Self in supertraits. This is needed | |
96 | /// because `object_safety_violations` can't be used during | |
97 | /// type collection. | |
98 | pub fn astconv_object_safety_violations(self, trait_def_id: DefId) | |
476ff2be | 99 | -> Vec<ObjectSafetyViolation> |
a7813a04 | 100 | { |
8faf50e0 XL |
101 | let violations = traits::supertrait_def_ids(self, trait_def_id) |
102 | .filter(|&def_id| self.predicates_reference_self(def_id, true)) | |
103 | .map(|_| ObjectSafetyViolation::SupertraitSelf) | |
104 | .collect(); | |
1a4d82fc | 105 | |
a7813a04 XL |
106 | debug!("astconv_object_safety_violations(trait_def_id={:?}) = {:?}", |
107 | trait_def_id, | |
108 | violations); | |
1a4d82fc | 109 | |
a7813a04 | 110 | violations |
1a4d82fc | 111 | } |
a7813a04 XL |
112 | |
113 | pub fn object_safety_violations(self, trait_def_id: DefId) | |
476ff2be | 114 | -> Vec<ObjectSafetyViolation> |
a7813a04 XL |
115 | { |
116 | traits::supertrait_def_ids(self, trait_def_id) | |
117 | .flat_map(|def_id| self.object_safety_violations_for_trait(def_id)) | |
118 | .collect() | |
85aaf69f | 119 | } |
1a4d82fc | 120 | |
a7813a04 | 121 | fn object_safety_violations_for_trait(self, trait_def_id: DefId) |
476ff2be | 122 | -> Vec<ObjectSafetyViolation> |
a7813a04 XL |
123 | { |
124 | // Check methods for violations. | |
476ff2be SL |
125 | let mut violations: Vec<_> = self.associated_items(trait_def_id) |
126 | .filter(|item| item.kind == ty::AssociatedKind::Method) | |
0bf4aa26 | 127 | .filter_map(|item| |
476ff2be | 128 | self.object_safety_violation_for_method(trait_def_id, &item) |
8faf50e0 | 129 | .map(|code| ObjectSafetyViolation::Method(item.ident.name, code)) |
0bf4aa26 | 130 | ).filter(|violation| { |
8faf50e0 | 131 | if let ObjectSafetyViolation::Method(_, |
0bf4aa26 XL |
132 | MethodViolationCode::WhereClauseReferencesSelf(span)) = violation |
133 | { | |
134 | // Using `CRATE_NODE_ID` is wrong, but it's hard to get a more precise id. | |
8faf50e0 XL |
135 | // It's also hard to get a use site span, so we use the method definition span. |
136 | self.lint_node_note( | |
137 | lint::builtin::WHERE_CLAUSES_OBJECT_SAFETY, | |
138 | ast::CRATE_NODE_ID, | |
139 | *span, | |
140 | &format!("the trait `{}` cannot be made into an object", | |
0bf4aa26 | 141 | self.item_path_str(trait_def_id)), |
8faf50e0 XL |
142 | &violation.error_msg()); |
143 | false | |
144 | } else { | |
145 | true | |
146 | } | |
476ff2be | 147 | }).collect(); |
1a4d82fc | 148 | |
a7813a04 XL |
149 | // Check the trait itself. |
150 | if self.trait_has_sized_self(trait_def_id) { | |
151 | violations.push(ObjectSafetyViolation::SizedSelf); | |
152 | } | |
32a655c1 | 153 | if self.predicates_reference_self(trait_def_id, false) { |
a7813a04 XL |
154 | violations.push(ObjectSafetyViolation::SupertraitSelf); |
155 | } | |
1a4d82fc | 156 | |
cc61c64b XL |
157 | violations.extend(self.associated_items(trait_def_id) |
158 | .filter(|item| item.kind == ty::AssociatedKind::Const) | |
8faf50e0 | 159 | .map(|item| ObjectSafetyViolation::AssociatedConst(item.ident.name))); |
cc61c64b | 160 | |
a7813a04 XL |
161 | debug!("object_safety_violations_for_trait(trait_def_id={:?}) = {:?}", |
162 | trait_def_id, | |
163 | violations); | |
85aaf69f | 164 | |
a7813a04 XL |
165 | violations |
166 | } | |
c34b1796 | 167 | |
32a655c1 SL |
168 | fn predicates_reference_self( |
169 | self, | |
170 | trait_def_id: DefId, | |
171 | supertraits_only: bool) -> bool | |
172 | { | |
8faf50e0 | 173 | let trait_ref = ty::Binder::dummy(ty::TraitRef::identity(self, trait_def_id)); |
32a655c1 | 174 | let predicates = if supertraits_only { |
7cac9316 | 175 | self.super_predicates_of(trait_def_id) |
32a655c1 | 176 | } else { |
7cac9316 | 177 | self.predicates_of(trait_def_id) |
32a655c1 | 178 | }; |
a7813a04 XL |
179 | predicates |
180 | .predicates | |
181 | .into_iter() | |
0bf4aa26 | 182 | .map(|(predicate, _)| predicate.subst_supertrait(self, &trait_ref)) |
a7813a04 XL |
183 | .any(|predicate| { |
184 | match predicate { | |
185 | ty::Predicate::Trait(ref data) => { | |
186 | // In the case of a trait predicate, we can skip the "self" type. | |
9e0c209e | 187 | data.skip_binder().input_types().skip(1).any(|t| t.has_self_ty()) |
a7813a04 XL |
188 | } |
189 | ty::Predicate::Projection(..) | | |
190 | ty::Predicate::WellFormed(..) | | |
191 | ty::Predicate::ObjectSafe(..) | | |
192 | ty::Predicate::TypeOutlives(..) | | |
193 | ty::Predicate::RegionOutlives(..) | | |
194 | ty::Predicate::ClosureKind(..) | | |
cc61c64b | 195 | ty::Predicate::Subtype(..) | |
ea8adc8c | 196 | ty::Predicate::ConstEvaluatable(..) => { |
a7813a04 XL |
197 | false |
198 | } | |
1a4d82fc | 199 | } |
a7813a04 | 200 | }) |
c34b1796 AL |
201 | } |
202 | ||
a7813a04 | 203 | fn trait_has_sized_self(self, trait_def_id: DefId) -> bool { |
9e0c209e | 204 | self.generics_require_sized_self(trait_def_id) |
a7813a04 | 205 | } |
1a4d82fc | 206 | |
9e0c209e | 207 | fn generics_require_sized_self(self, def_id: DefId) -> bool { |
ea8adc8c | 208 | let sized_def_id = match self.lang_items().sized_trait() { |
a7813a04 XL |
209 | Some(def_id) => def_id, |
210 | None => { return false; /* No Sized trait, can't require it! */ } | |
211 | }; | |
212 | ||
213 | // Search for a predicate like `Self : Sized` amongst the trait bounds. | |
7cac9316 XL |
214 | let predicates = self.predicates_of(def_id); |
215 | let predicates = predicates.instantiate_identity(self).predicates; | |
a7813a04 | 216 | elaborate_predicates(self, predicates) |
0bf4aa26 XL |
217 | .any(|predicate| match predicate { |
218 | ty::Predicate::Trait(ref trait_pred) if trait_pred.def_id() == sized_def_id => { | |
219 | trait_pred.skip_binder().self_ty().is_self() | |
a7813a04 | 220 | } |
0bf4aa26 XL |
221 | ty::Predicate::Projection(..) | |
222 | ty::Predicate::Trait(..) | | |
223 | ty::Predicate::Subtype(..) | | |
224 | ty::Predicate::RegionOutlives(..) | | |
225 | ty::Predicate::WellFormed(..) | | |
226 | ty::Predicate::ObjectSafe(..) | | |
227 | ty::Predicate::ClosureKind(..) | | |
228 | ty::Predicate::TypeOutlives(..) | | |
229 | ty::Predicate::ConstEvaluatable(..) => { | |
230 | false | |
231 | } | |
232 | } | |
233 | ) | |
a7813a04 | 234 | } |
c34b1796 | 235 | |
a7813a04 XL |
236 | /// Returns `Some(_)` if this method makes the containing trait not object safe. |
237 | fn object_safety_violation_for_method(self, | |
238 | trait_def_id: DefId, | |
476ff2be | 239 | method: &ty::AssociatedItem) |
a7813a04 XL |
240 | -> Option<MethodViolationCode> |
241 | { | |
242 | // Any method that has a `Self : Sized` requisite is otherwise | |
243 | // exempt from the regulations. | |
9e0c209e | 244 | if self.generics_require_sized_self(method.def_id) { |
a7813a04 | 245 | return None; |
1a4d82fc JJ |
246 | } |
247 | ||
a7813a04 | 248 | self.virtual_call_violation_for_method(trait_def_id, method) |
1a4d82fc JJ |
249 | } |
250 | ||
a7813a04 XL |
251 | /// We say a method is *vtable safe* if it can be invoked on a trait |
252 | /// object. Note that object-safe traits can have some | |
253 | /// non-vtable-safe methods, so long as they require `Self:Sized` or | |
254 | /// otherwise ensure that they cannot be used when `Self=Trait`. | |
255 | pub fn is_vtable_safe_method(self, | |
256 | trait_def_id: DefId, | |
476ff2be | 257 | method: &ty::AssociatedItem) |
a7813a04 XL |
258 | -> bool |
259 | { | |
260 | // Any method that has a `Self : Sized` requisite can't be called. | |
9e0c209e | 261 | if self.generics_require_sized_self(method.def_id) { |
a7813a04 | 262 | return false; |
1a4d82fc | 263 | } |
1a4d82fc | 264 | |
8faf50e0 XL |
265 | match self.virtual_call_violation_for_method(trait_def_id, method) { |
266 | None | Some(MethodViolationCode::WhereClauseReferencesSelf(_)) => true, | |
267 | Some(_) => false, | |
268 | } | |
1a4d82fc JJ |
269 | } |
270 | ||
a7813a04 XL |
271 | /// Returns `Some(_)` if this method cannot be called on a trait |
272 | /// object; this does not necessarily imply that the enclosing trait | |
273 | /// is not object safe, because the method might have a where clause | |
274 | /// `Self:Sized`. | |
275 | fn virtual_call_violation_for_method(self, | |
276 | trait_def_id: DefId, | |
476ff2be | 277 | method: &ty::AssociatedItem) |
a7813a04 XL |
278 | -> Option<MethodViolationCode> |
279 | { | |
280 | // The method's first parameter must be something that derefs (or | |
281 | // autorefs) to `&self`. For now, we only accept `self`, `&self` | |
282 | // and `Box<Self>`. | |
476ff2be SL |
283 | if !method.method_has_self_argument { |
284 | return Some(MethodViolationCode::StaticMethod); | |
a7813a04 | 285 | } |
1a4d82fc | 286 | |
abe05a73 XL |
287 | let sig = self.fn_sig(method.def_id); |
288 | ||
289 | let self_ty = self.mk_self_type(); | |
290 | let self_arg_ty = sig.skip_binder().inputs()[0]; | |
291 | if let ExplicitSelf::Other = ExplicitSelf::determine(self_arg_ty, |ty| ty == self_ty) { | |
292 | return Some(MethodViolationCode::NonStandardSelfType); | |
293 | } | |
294 | ||
a7813a04 XL |
295 | // The `Self` type is erased, so it should not appear in list of |
296 | // arguments or return type apart from the receiver. | |
476ff2be | 297 | for input_ty in &sig.skip_binder().inputs()[1..] { |
a7813a04 XL |
298 | if self.contains_illegal_self_type_reference(trait_def_id, input_ty) { |
299 | return Some(MethodViolationCode::ReferencesSelf); | |
300 | } | |
301 | } | |
476ff2be | 302 | if self.contains_illegal_self_type_reference(trait_def_id, sig.output().skip_binder()) { |
5bcae85e | 303 | return Some(MethodViolationCode::ReferencesSelf); |
a7813a04 | 304 | } |
1a4d82fc | 305 | |
a7813a04 | 306 | // We can't monomorphize things like `fn foo<A>(...)`. |
94b46f34 | 307 | if self.generics_of(method.def_id).own_counts().types != 0 { |
a7813a04 XL |
308 | return Some(MethodViolationCode::Generic); |
309 | } | |
1a4d82fc | 310 | |
8faf50e0 XL |
311 | if self.predicates_of(method.def_id).predicates.into_iter() |
312 | // A trait object can't claim to live more than the concrete type, | |
313 | // so outlives predicates will always hold. | |
0bf4aa26 | 314 | .filter(|(p, _)| p.to_opt_type_outlives().is_none()) |
8faf50e0 XL |
315 | .collect::<Vec<_>>() |
316 | // Do a shallow visit so that `contains_illegal_self_type_reference` | |
317 | // may apply it's custom visiting. | |
318 | .visit_tys_shallow(|t| self.contains_illegal_self_type_reference(trait_def_id, t)) { | |
319 | let span = self.def_span(method.def_id); | |
320 | return Some(MethodViolationCode::WhereClauseReferencesSelf(span)); | |
321 | } | |
322 | ||
a7813a04 XL |
323 | None |
324 | } | |
325 | ||
326 | fn contains_illegal_self_type_reference(self, | |
327 | trait_def_id: DefId, | |
328 | ty: Ty<'tcx>) | |
329 | -> bool | |
330 | { | |
331 | // This is somewhat subtle. In general, we want to forbid | |
332 | // references to `Self` in the argument and return types, | |
333 | // since the value of `Self` is erased. However, there is one | |
334 | // exception: it is ok to reference `Self` in order to access | |
335 | // an associated type of the current trait, since we retain | |
336 | // the value of those associated types in the object type | |
337 | // itself. | |
338 | // | |
339 | // ```rust | |
340 | // trait SuperTrait { | |
341 | // type X; | |
342 | // } | |
343 | // | |
344 | // trait Trait : SuperTrait { | |
345 | // type Y; | |
346 | // fn foo(&self, x: Self) // bad | |
347 | // fn foo(&self) -> Self // bad | |
348 | // fn foo(&self) -> Option<Self> // bad | |
349 | // fn foo(&self) -> Self::Y // OK, desugars to next example | |
350 | // fn foo(&self) -> <Self as Trait>::Y // OK | |
351 | // fn foo(&self) -> Self::X // OK, desugars to next example | |
352 | // fn foo(&self) -> <Self as SuperTrait>::X // OK | |
353 | // } | |
354 | // ``` | |
355 | // | |
356 | // However, it is not as simple as allowing `Self` in a projected | |
357 | // type, because there are illegal ways to use `Self` as well: | |
358 | // | |
359 | // ```rust | |
360 | // trait Trait : SuperTrait { | |
361 | // ... | |
362 | // fn foo(&self) -> <Self as SomeOtherTrait>::X; | |
363 | // } | |
364 | // ``` | |
365 | // | |
366 | // Here we will not have the type of `X` recorded in the | |
367 | // object type, and we cannot resolve `Self as SomeOtherTrait` | |
368 | // without knowing what `Self` is. | |
369 | ||
370 | let mut supertraits: Option<Vec<ty::PolyTraitRef<'tcx>>> = None; | |
371 | let mut error = false; | |
372 | ty.maybe_walk(|ty| { | |
373 | match ty.sty { | |
b7449926 | 374 | ty::Param(ref param_ty) => { |
9e0c209e | 375 | if param_ty.is_self() { |
a7813a04 XL |
376 | error = true; |
377 | } | |
378 | ||
379 | false // no contained types to walk | |
1a4d82fc JJ |
380 | } |
381 | ||
b7449926 | 382 | ty::Projection(ref data) => { |
a7813a04 XL |
383 | // This is a projected type `<Foo as SomeTrait>::X`. |
384 | ||
385 | // Compute supertraits of current trait lazily. | |
386 | if supertraits.is_none() { | |
8faf50e0 XL |
387 | let trait_ref = ty::Binder::bind( |
388 | ty::TraitRef::identity(self, trait_def_id), | |
389 | ); | |
a7813a04 XL |
390 | supertraits = Some(traits::supertraits(self, trait_ref).collect()); |
391 | } | |
392 | ||
393 | // Determine whether the trait reference `Foo as | |
394 | // SomeTrait` is in fact a supertrait of the | |
395 | // current trait. In that case, this type is | |
396 | // legal, because the type `X` will be specified | |
397 | // in the object type. Note that we can just use | |
398 | // direct equality here because all of these types | |
399 | // are part of the formal parameter listing, and | |
400 | // hence there should be no inference variables. | |
83c7162d | 401 | let projection_trait_ref = ty::Binder::bind(data.trait_ref(self)); |
a7813a04 XL |
402 | let is_supertrait_of_current_trait = |
403 | supertraits.as_ref().unwrap().contains(&projection_trait_ref); | |
404 | ||
405 | if is_supertrait_of_current_trait { | |
406 | false // do not walk contained types, do not report error, do collect $200 | |
407 | } else { | |
408 | true // DO walk contained types, POSSIBLY reporting an error | |
409 | } | |
1a4d82fc | 410 | } |
1a4d82fc | 411 | |
a7813a04 XL |
412 | _ => true, // walk contained types, if any |
413 | } | |
414 | }); | |
1a4d82fc | 415 | |
a7813a04 XL |
416 | error |
417 | } | |
1a4d82fc | 418 | } |
7cac9316 XL |
419 | |
420 | pub(super) fn is_object_safe_provider<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, | |
94b46f34 | 421 | trait_def_id: DefId) -> bool { |
7cac9316 XL |
422 | tcx.object_safety_violations(trait_def_id).is_empty() |
423 | } |