doc cenum_impl_drop_cast

[ickk]

Add note about as casts being disallowed for enums that implement Drop.

The cenum_impl_drop_cast lint rust-lang/rust#73333 is already deny on stable rust, and I think eventually this will become a hard restriction for enum-to-numeric casts. playground

[ehuss]

Thanks for the PR! We generally are hesitant to document these future-incompat things until they are actually made into a hard error. In which case, it would be written as a normal rule and not a warning block. Is there a planned timeline for this?

[ickk]

Based on the reference, it was a bit surprising to discover that the compiler rejects this (since rust-lang/rust#97652) without an explicit annotation, but I understand it's not technically a requirement. I wasn't sure what the reference guidelines were for such things, sorry.

Is there a planned timeline for this?

On the lint's tracking issue page it appears to have been added to the "Lang Edition 2024" project, but doesn't seem to appear on the project board itself - I'm not sure if this means it was rejected from inclusion in the Edition, or somehow lost and forgotten? Either way, at this point I assume it won't be a hard error until at least the following edition.

[ehuss]

I pushed an update as this is changing to a hard error via rust-lang/rust#135964.