Should `Option<u128>` use a smaller (maybe usize) tag?

Looking at some examples in https://github.com/rust-lang/rust/pull/137278#issue-2863728153 shows that Option<u128> is using a full u128 for its stored tag. That tag, of course, can only be 0 or 1.

As a result things like returning Some(…) need to write two qwords on x64 into the return value, even though of course one of them is only ever all-zeroes.

Could we maybe give it a layout equivalent to (usize, u128) instead?

Or, in general, can we cap the size of tags to a native machine word, given a small discriminant that could fit in just about any type?

(Aside: It's good for Option<u32> to still be (u32, MaybeUninit<u32>), since that has more niches than (u8, MaybeUninit<u32>). But going so big that it takes multiple instructions to write the tag seems like overkill, and for unconstrained repr(Rust) we should probably stop at something reasonable for the target.)

Start by reviewing the examples in the linked pull request and the Rust compiler's type-layout handling for Option. Determine whether discriminant sizing can be bounded without losing niche optimizations; done means reaching a clear implementation or design conclusion, backed by relevant compiler tests if the change is feasible.