wishlist: Allow optimization of uninhabited values

[chrysn]

The uX types could, conceptually, allow the compiler to do optimizations based on their constraints -- for example, an Option could fit in a u64, and (later when mechanisms to make use of Copy are in place to ensure there won't be pointer to the interior; possibly coming out of rust-lang/rust#27060) a Result<u63, u63> could be too.

This can not be done in stable Rust now, and even the means to do it that are available in unstable (rustc_layout_scalar_valid_range_end) are internal attributes never meant to become stable.

This is not an issue that would be solved quickly (unless by a possible added unstable range_optimizations feature that could break on any nightly), but more a long-term point for visibility, and to document towards the language team the usefulness of a public way to achieve this.

[chrysn]

The most tracking-issue-like place for that Rust feature right now is probably rust-lang/rust#94970 (which is asking for stabilization of a bunch of things, including _scalkar_valid_range_).

[Kijewski]

@chrysn, please have a look at #49.

[chrysn]

I think you meant #49. Thanks, looking at it.

[Kijewski]

Oops, yes. Pasting is hard. :)