#![feature(overlapping_marker_traits)] breaks the guarantee for Unpin provided by pin-project

[taiki-e]

UPDATE2(2020/01/27):

This will be fixed in rust-lang/rust#68544

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UPDATE(2019/09/26):

overlapping_marker_traits is replaced with marker_trait_attr.
marker_trait_attr doesn't seem to break the guarantee (added test in #110). So, once overlapping_marker_traits is removed, close this issue.

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pin-project provides an appropriate Unpin implementation by default. Since overlapping implementations are prohibited, this ensures that users cannot add inappropriate Unpin implementations.

However, currently, this guarantee can be broken by enabling #[feature(overlapping_marker_traits)]: playground

@cramertj has already pointed out this in rust-lang/rust#29864 (comment).
And it seems that it is planned to make #[feature(overlapping_marker_traits)] available only with explicitly allowed traits, but it doesn't seem to be fully implemented yet.

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Thanks to @RalfJung for pointing out the interaction with this feature.

[scottmcm]

The gate for the new behavior is https://doc.rust-lang.org/nightly/unstable-book/language-features/marker-trait-attr.html -- it was implemented in parallel without removing the old feature.

[taiki-e]

@scottmcm Thanks! I think I missed it.

However, even if it is deprecated, it breaks soundness, so I believe that it makes sense to leave this issue open until the feature is removed.

[taiki-e]

This will be fixed in rust-lang/rust#68544 (Thanks @Aaron1011!)

[Aaron1011]

Now that rust-lang/rust#68544 has been merged, I believe that this issue can be closed.