Make all functions part of alloc_layout_extra usable in const contexts

[Raekye]

Proposal

Problem statement

The Rust standard library provides the Layout type for use with the allocator API. Of the functions in rust-lang/rust#55724, some are already marked as const, but the following currently are not:

• align_to
• pad_to_align
• repeat
• extend
• repeat_packed
• extend_packed
• array

This makes them unusable in const contexts.

Motivation, use-cases

Layout is also documented by example as being able to calculate the offsets of fields in a #[repr(C)] struct. This is useful since there is no equivalent of C's offsetof in the standard library, and users must rely on a third party crate.

Layout can also be used to correctly determine the offset of trailing elements in a dynamically sized type. It may be desirable to know this offset at compile time (as a const variable). I believe this can currently be emulated by including a zero-length array of the trailing element in the struct definition and using std::mem::size_of, but the intent is not as clear.

Solution sketches

Add the const keyword to the above function definitions. A few functions currently use the std::cmp::max function which relies on the Ord trait, which can't be used in const contexts. So std::cmp::max would need to be replaced by a plain if-else block expression.

Links and related work

• Tracking issue for alloc_layout_extra rust#55724
• Make functions part of alloc_layout_extra (#55724) const. rust#99016

What happens now?

This issue is part of the libs-api team API change proposal process. Once this issue is filed the libs-api team will review open proposals in its weekly meeting. You should receive feedback within a week or two.

[shepmaster]

See also rust-lang/rfcs#3308

[thomcc]

This is useful since there is no equivalent of C's offsetof in the standard library, and users must rely on a third party crate.

FWIW, I think allowing Layout's functions to be used in const contexts is useful even if a const-supporting offset_of! is added to the standard library.

[Raekye]

The proposed changes have been made and merged in the (later submitted) PR rust-lang/rust#102207 (note, I didn't see an ACP for said newer PR).