Closed
Description
Encountered when forward-porting my kernel to latest nightly.
Implementing CoerceUnsized on a type that wraps a pointer to a wrapping type errors when there is also a struct that wraps an array in the same crate.
error[E0391]: unsupported cyclic reference between types/traits detected
--> <anon>:8:19
|
8 | struct LogDataBuf([u8;8]);
| ^^^^^^^ cyclic reference
|
note: the cycle begins when processing `LogDataBuf::0`...
--> <anon>:8:19
|
8 | struct LogDataBuf([u8;8]);
| ^^^^^^^
note: ...which then requires const-evaluating `LogDataBuf::{{initializer}}`...
--> <anon>:8:23
|
8 | struct LogDataBuf([u8;8]);
| ^
note: ...which then requires processing `LogDataBuf::{{initializer}}`...
--> <anon>:8:23
|
8 | struct LogDataBuf([u8;8]);
| ^
note: ...which then requires coherence checking all impls of trait `std::ops::CoerceUnsized`...
note: ...which then requires processing `<impl at <anon>:15:1: 15:90>`...
--> <anon>:15:1
|
15| impl<T: ?Sized + marker::Unsize<U>, U: ?Sized> ops::CoerceUnsized<Aref<U>> for Aref<T> {}
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...which then requires processing `ArefInner`...
--> <anon>:17:1
|
17| / struct ArefInner<T: ?Sized>
18| | {
19| | // Even with this field commented out, the error is raised.
20| | data: T,
21| | }
| |_^
note: ...which then requires computing the variances for items in this crate...
= note: ...which then again requires processing `LogDataBuf::0`, completing the cycle.
use std::{marker,ops};
// Change the array to a non-array, and error disappears
// Adding a new field to the end keeps the error
struct LogDataBuf([u8;8]);
struct Aref<T: ?Sized>
{
// Inner structure triggers the error, removing the inner removes the message.
ptr: Box<ArefInner<T>>,
}
impl<T: ?Sized + marker::Unsize<U>, U: ?Sized> ops::CoerceUnsized<Aref<U>> for Aref<T> {}
struct ArefInner<T: ?Sized>
{
// Even with this field commented out, the error is raised.
data: T,
}Playground version
https://play.rust-lang.org/?gist=854452b1dea1fc197d3bd9d6b1848eed&version=nightly&backtrace=0