Add #[must_use] to core and std constructors

[jkugelman]

Parent issue: #89692

r? @joshtriplett

[jkugelman]

Heads up, I threw MaybeUninit into the mix. I'm not 100% sure if it belongs.

core::mem::MaybeUninit<T>   const fn uninit() -> MaybeUninit<T>;
core::mem::MaybeUninit<T>   const fn uninit_array<const LEN: usize>() -> [Self; LEN];
core::mem::MaybeUninit<T>   fn zeroed() -> MaybeUninit<T>;

I left MaybeUninit::new(val: T) out since it could be used to nullify T's drop code. Let me know if you think I should add it. I could put a note saying to use ManuallyDrop::new or mem::forget instead.

[joshtriplett]

I do think it's reasonable to add the annotation to MaybeUninit::new as well, but that can happen in another PR. Suggesting mem::forget sounds good.

[joshtriplett]

@bors r+

[bors]

📌 Commit 5b5c12b has been approved by joshtriplett