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Orphanck: Consider opaque types to never cover type parameters
1 parent 0a59f11 commit cad4e89

10 files changed

+143
-32
lines changed

‎compiler/rustc_trait_selection/src/traits/coherence.rs

Lines changed: 37 additions & 31 deletions
Original file line numberDiff line numberDiff line change
@@ -924,9 +924,9 @@ where
924924
}
925925
}
926926

927-
ty::Alias(kind @ (ty::Projection | ty::Inherent | ty::Weak), ..) => {
927+
ty::Alias(kind @ (ty::Projection | ty::Inherent | ty::Weak), _) => {
928928
if ty.has_type_flags(ty::TypeFlags::HAS_TY_PARAM) {
929-
bug!("unexpected ty param in alias ty");
929+
bug!("orphanck: unexpected ty param in alias ty");
930930
}
931931

932932
if ty.has_type_flags(
@@ -952,6 +952,41 @@ where
952952
}
953953
}
954954

955+
ty::Alias(ty::Opaque, _) => {
956+
// This merits some explanation.
957+
//
958+
// We choose to treat all opaque types as non-local, even
959+
// those that appear within the same crate. This seems
960+
// somewhat surprising at first, but makes sense when
961+
// you consider that opaque types are supposed to hide
962+
// the underlying type *within the same crate*. When an
963+
// opaque type is used from outside the module
964+
// where it is declared, it should be impossible to observe
965+
// anything about it other than the traits that it implements.
966+
//
967+
// The alternative would be to look at the underlying type
968+
// to determine whether or not the opaque type itself should
969+
// be considered local. However, this could make it a breaking change
970+
// to switch the underlying ('defining') type from a local type
971+
// to a remote type. This would violate the rule that opaque
972+
// types should be completely opaque apart from the traits
973+
// that they implement, so we don't use this behavior.
974+
975+
if ty.has_type_flags(ty::TypeFlags::HAS_TY_PARAM) {
976+
bug!("orphanck: unexpected ty param in opaque ty");
977+
}
978+
979+
if ty.has_type_flags(
980+
ty::TypeFlags::HAS_TY_PLACEHOLDER
981+
| ty::TypeFlags::HAS_TY_BOUND
982+
| ty::TypeFlags::HAS_TY_INFER,
983+
) {
984+
self.found_uncovered_ty_param(ty)
985+
} else {
986+
self.found_non_local_ty(ty)
987+
}
988+
}
989+
955990
// For fundamental types, we just look inside of them.
956991
ty::Ref(_, ty, _) => ty.visit_with(self),
957992
ty::Adt(def, args) => {
@@ -990,35 +1025,6 @@ where
9901025
// auto trait impl applies. There will never be multiple impls, so we can just
9911026
// act as if it were a local type here.
9921027
ty::CoroutineWitness(..) => ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty)),
993-
ty::Alias(ty::Opaque, ..) => {
994-
// This merits some explanation.
995-
// Normally, opaque types are not involved when performing
996-
// coherence checking, since it is illegal to directly
997-
// implement a trait on an opaque type. However, we might
998-
// end up looking at an opaque type during coherence checking
999-
// if an opaque type gets used within another type (e.g. as
1000-
// the type of a field) when checking for auto trait or `Sized`
1001-
// impls. This requires us to decide whether or not an opaque
1002-
// type should be considered 'local' or not.
1003-
//
1004-
// We choose to treat all opaque types as non-local, even
1005-
// those that appear within the same crate. This seems
1006-
// somewhat surprising at first, but makes sense when
1007-
// you consider that opaque types are supposed to hide
1008-
// the underlying type *within the same crate*. When an
1009-
// opaque type is used from outside the module
1010-
// where it is declared, it should be impossible to observe
1011-
// anything about it other than the traits that it implements.
1012-
//
1013-
// The alternative would be to look at the underlying type
1014-
// to determine whether or not the opaque type itself should
1015-
// be considered local. However, this could make it a breaking change
1016-
// to switch the underlying ('defining') type from a local type
1017-
// to a remote type. This would violate the rule that opaque
1018-
// types should be completely opaque apart from the traits
1019-
// that they implement, so we don't use this behavior.
1020-
self.found_non_local_ty(ty)
1021-
}
10221028
};
10231029
// A bit of a hack, the `OrphanChecker` is only used to visit a `TraitRef`, so
10241030
// the first type we visit is always the self type.

‎tests/ui/coherence/orphan-check-opaque-types-not-covering-0.classic.stderr

Lines changed: 12 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,12 @@
1+
error[E0210]: type parameter `T` must be covered by another type when it appears before the first local type (`Local`)
2+
--> $DIR/orphan-check-opaque-types-not-covering-0.rs:17:6
3+
|
4+
LL | impl<T> foreign::Trait1<Local, T> for Identity<T> {}
5+
| ^ type parameter `T` must be covered by another type when it appears before the first local type (`Local`)
6+
|
7+
= note: implementing a foreign trait is only possible if at least one of the types for which it is implemented is local, and no uncovered type parameters appear before that first local type
8+
= note: in this case, 'before' refers to the following order: `impl<..> ForeignTrait<T1, ..., Tn> for T0`, where `T0` is the first and `Tn` is the last
9+
10+
error: aborting due to 1 previous error
11+
12+
For more information about this error, try `rustc --explain E0210`.

‎tests/ui/coherence/orphan-check-opaque-types-not-covering-0.next.stderr

Lines changed: 12 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,12 @@
1+
error[E0210]: type parameter `T` must be covered by another type when it appears before the first local type (`Local`)
2+
--> $DIR/orphan-check-opaque-types-not-covering-0.rs:17:6
3+
|
4+
LL | impl<T> foreign::Trait1<Local, T> for Identity<T> {}
5+
| ^ type parameter `T` must be covered by another type when it appears before the first local type (`Local`)
6+
|
7+
= note: implementing a foreign trait is only possible if at least one of the types for which it is implemented is local, and no uncovered type parameters appear before that first local type
8+
= note: in this case, 'before' refers to the following order: `impl<..> ForeignTrait<T1, ..., Tn> for T0`, where `T0` is the first and `Tn` is the last
9+
10+
error: aborting due to 1 previous error
11+
12+
For more information about this error, try `rustc --explain E0210`.

‎tests/ui/coherence/orphan-check-opaque-types-not-covering-0.rs

Lines changed: 22 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,22 @@
1+
// Opaque types never cover type parameters.
2+
3+
//@ revisions: classic next
4+
//@[next] compile-flags: -Znext-solver
5+
6+
//@ aux-crate:foreign=parametrized-trait.rs
7+
//@ edition:2021
8+
9+
#![feature(type_alias_impl_trait)]
10+
11+
type Identity<T> = impl Sized;
12+
13+
fn define<T>(x: T) -> Identity<T> {
14+
x
15+
}
16+
17+
impl<T> foreign::Trait1<Local, T> for Identity<T> {}
18+
//~^ ERROR type parameter `T` must be covered by another type
19+
20+
struct Local;
21+
22+
fn main() {}

‎tests/ui/coherence/orphan-check-opaque-types-not-covering-1.classic.stderr

Lines changed: 12 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,12 @@
1+
error[E0210]: type parameter `T` must be covered by another type when it appears before the first local type (`Local`)
2+
--> $DIR/orphan-check-opaque-types-not-covering-1.rs:17:6
3+
|
4+
LL | impl<T> foreign::Trait1<Local, T> for Opaque<T> {}
5+
| ^ type parameter `T` must be covered by another type when it appears before the first local type (`Local`)
6+
|
7+
= note: implementing a foreign trait is only possible if at least one of the types for which it is implemented is local, and no uncovered type parameters appear before that first local type
8+
= note: in this case, 'before' refers to the following order: `impl<..> ForeignTrait<T1, ..., Tn> for T0`, where `T0` is the first and `Tn` is the last
9+
10+
error: aborting due to 1 previous error
11+
12+
For more information about this error, try `rustc --explain E0210`.

‎tests/ui/coherence/orphan-check-opaque-types-not-covering-1.next.stderr

Lines changed: 12 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,12 @@
1+
error[E0210]: type parameter `T` must be covered by another type when it appears before the first local type (`Local`)
2+
--> $DIR/orphan-check-opaque-types-not-covering-1.rs:17:6
3+
|
4+
LL | impl<T> foreign::Trait1<Local, T> for Opaque<T> {}
5+
| ^ type parameter `T` must be covered by another type when it appears before the first local type (`Local`)
6+
|
7+
= note: implementing a foreign trait is only possible if at least one of the types for which it is implemented is local, and no uncovered type parameters appear before that first local type
8+
= note: in this case, 'before' refers to the following order: `impl<..> ForeignTrait<T1, ..., Tn> for T0`, where `T0` is the first and `Tn` is the last
9+
10+
error: aborting due to 1 previous error
11+
12+
For more information about this error, try `rustc --explain E0210`.

‎tests/ui/coherence/orphan-check-opaque-types-not-covering-1.rs

Lines changed: 22 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,22 @@
1+
// Opaque types never cover type parameters.
2+
3+
//@ revisions: classic next
4+
//@[next] compile-flags: -Znext-solver
5+
6+
//@ aux-crate:foreign=parametrized-trait.rs
7+
//@ edition:2021
8+
9+
#![feature(type_alias_impl_trait)]
10+
11+
type Opaque<T> = impl Sized;
12+
13+
fn define<T>() -> Opaque<T> {
14+
Local
15+
}
16+
17+
impl<T> foreign::Trait1<Local, T> for Opaque<T> {}
18+
//~^ ERROR type parameter `T` must be covered by another type
19+
20+
struct Local;
21+
22+
fn main() {}

‎tests/ui/type-alias-impl-trait/coherence.stderr renamed to ‎tests/ui/type-alias-impl-trait/coherence.classic.stderr

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1,5 +1,5 @@
11
error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
2-
--> $DIR/coherence.rs:14:1
2+
--> $DIR/coherence.rs:16:1
33
|
44
LL | impl foreign_crate::ForeignTrait for AliasOfForeignType<()> {}
55
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^----------------------

‎tests/ui/type-alias-impl-trait/coherence.next.stderr

Lines changed: 11 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,11 @@
1+
error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
2+
--> $DIR/coherence.rs:16:1
3+
|
4+
LL | impl foreign_crate::ForeignTrait for AliasOfForeignType<()> {}
5+
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ impl doesn't use only types from inside the current crate
6+
|
7+
= note: define and implement a trait or new type instead
8+
9+
error: aborting due to 1 previous error
10+
11+
For more information about this error, try `rustc --explain E0117`.

‎tests/ui/type-alias-impl-trait/coherence.rs

Lines changed: 2 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -1,4 +1,6 @@
11
//@ aux-build:foreign-crate.rs
2+
//@ revisions: classic next
3+
//@[next] compile-flags: -Znext-solver
24
#![feature(type_alias_impl_trait)]
35

46
extern crate foreign_crate;

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