Core documentation shows "This is supported on non-crate feature miri-test-libstd or test or doctest only" annotation

[LunaBorowska]

https://doc.rust-lang.org/nightly/core/index.html shows this annotation:

This is supported on non-crate feature miri-test-libstd or test or doctest only.

This doesn't seem very useful for an end-user, and this annotation shows for every page in the documentation. This issue also affects alloc crate.

[jyn514]

@xfix the fix is to add #![doc(cfg_hide(miri-test-libstd))] to the crate root; it would be a good first PR :)