Unsupported cyclic reference when implementing CoerceUnsized for pointer to wrapper

[thepowersgang]

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

[eddyb]

This might be fixed by #41913, do you know if the latest nightly contains it?

EDIT: yes so it didn't get fixed. cc @nikomatsakis

[thepowersgang]

Present in latest nightly on playpen, rustc 1.19.0-nightly (978d2cfee 2017-05-10), also in current nightly - rustc 1.19.0-nightly (e40beb3af 2017-05-11)

[kennytm]

Still repro.

Error message, same as OP's

error: main function not found

error[E0391]: unsupported cyclic reference between types/traits detected
 --> 1.rs:7:19
  |
7 | struct LogDataBuf([u8;8]);
  |                   ^^^^^^^ cyclic reference
  |
note: the cycle begins when processing `LogDataBuf::0`...
 --> 1.rs:7:19
  |
7 | struct LogDataBuf([u8;8]);
  |                   ^^^^^^^
note: ...which then requires const-evaluating `LogDataBuf::{{initializer}}`...
 --> 1.rs:7:23
  |
7 | struct LogDataBuf([u8;8]);
  |                       ^
note: ...which then requires processing `LogDataBuf::{{initializer}}`...
 --> 1.rs:7:23
  |
7 | struct LogDataBuf([u8;8]);
  |                       ^
note: ...which then requires coherence checking all impls of trait `std::ops::CoerceUnsized`...
note: ...which then requires processing `<impl at 1.rs:14:1: 14:90>`...
 --> 1.rs:14:1
  |
14| impl<T: ?Sized + marker::Unsize<U>, U: ?Sized> ops::CoerceUnsized<Aref<U>> for Aref<T> {}
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...which then requires processing `ArefInner`...
 --> 1.rs:16:1
  |
16| / struct ArefInner<T: ?Sized>
17| | {
18| |     // Even with this field commented out, the error is raised.
19| |     data: T,
20| | }
  | |_^
note: ...which then requires computing the variances for items in this crate...
  = note: ...which then again requires processing `LogDataBuf::0`, completing the cycle.

error: aborting due to previous error

$ rustc -vV
rustc 1.19.0-nightly (e40beb3af 2017-05-11)
binary: rustc
commit-hash: e40beb3af162e676e40704854dd6547ee8d4cf60
commit-date: 2017-05-11
host: x86_64-apple-darwin
release: 1.19.0-nightly
LLVM version: 4.0

e40beb3 is the commit merging #41913.