#102750 regresses layout size of Cow<>

[emilio]

    assert_eq!(std::mem::size_of::<std::borrow::Cow<'static, str>>(), 24);

Passes on stable and beta but not on Nightly.

I bisected it, results below which point to #102750:

searched nightlies: from nightly-2022-10-20 to nightly-2022-12-06
regressed nightly: nightly-2022-11-24
searched commit range: ff8c8df...70f8737
regressed commit: 4e0d0d7

bisected with cargo-bisect-rustc v0.6.5

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --start 2022-10-20 

@rustbot modify labels: +regression-from-stable-to-nightly -regression-untriaged

cc: @nnethercote @the8472 @wesleywiser

[the8472]

I'm working on improving the niche placement logic, #104807

[the8472]

Also note that Cow is repr(Rust) so its layout and sizes aren't guaranteed.

[emilio]

Sure, sure, I know that, I wasn't trying to imply otherwise. I just figured that it was probably unintended given the PR that introduced it. Thanks for your work on struct layout optimizations, they're great! :)

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-medium

[khuey]

This shipped :(

[mati865]

@rustbot modify labels -regression-from-stable-to-beta +regression-from-stable-to-stable