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Rollup merge of #60369 - TimDiekmann:dispatch-zst, r=eddyb

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Support ZSTs in DispatchFromDyn

Allows to use ZSTs with 1 byte alignment in `DispatchFromDyn` implementation. This is required for `Box<T, A: Alloc>`

cc #58457
This commit is contained in:
Mazdak Farrokhzad 2019-05-01 08:29:14 +02:00 • committed by GitHub
commit 604176cc10
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4 changed files with 80 additions and 8 deletions
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9
src/test/ui/invalid_dispatch_from_dyn_impls.rs
View file

@ -39,4 +39,13 @@ where
T: Unsize<U>,
{} //~^^^ ERROR [E0378]
#[repr(align(64))]
struct OverAlignedZst;
struct OverAligned<T: ?Sized>(Box<T>, OverAlignedZst);
impl<T: ?Sized, U: ?Sized> DispatchFromDyn<OverAligned<U>> for OverAligned<T>
where
T: Unsize<U>,
{} //~^^^ ERROR [E0378]
fn main() {}

15
src/test/ui/invalid_dispatch_from_dyn_impls.stderr
View file

@ -1,4 +1,4 @@
error[E0378]: the trait `DispatchFromDyn` may only be implemented for structs containing the field being coerced, `PhantomData` fields, and nothing else
error[E0378]: the trait `DispatchFromDyn` may only be implemented for structs containing the field being coerced, ZST fields with 1 byte alignment, and nothing else
--> $DIR/invalid_dispatch_from_dyn_impls.rs:10:1
|
LL | / impl<T, U> DispatchFromDyn<WrapperWithExtraField<U>> for WrapperWithExtraField<T>
@ -36,6 +36,17 @@ LL | | T: Unsize<U>,
LL | | {}
| |__^
error: aborting due to 4 previous errors
error[E0378]: the trait `DispatchFromDyn` may only be implemented for structs containing the field being coerced, ZST fields with 1 byte alignment, and nothing else
--> $DIR/invalid_dispatch_from_dyn_impls.rs:46:1
|
LL | / impl<T: ?Sized, U: ?Sized> DispatchFromDyn<OverAligned<U>> for OverAligned<T>
LL | | where
LL | | T: Unsize<U>,
LL | | {}
| |__^
|
= note: extra field `1` of type `OverAlignedZst` is not allowed
error: aborting due to 5 previous errors
For more information about this error, try `rustc --explain E0378`.