core: clarify MaybeUninit::assume_init UB for uninitialized integers

[laddery-a]

Fixes #150689.

This clarifies in the MaybeUninit::assume_init safety docs that calling assume_init on uninitialized memory is immediate UB even when T is an integer / POD type, and adds a small incorrect-usage example using u8.

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[scottmcm]

When you're done changing things and have squashed the commits and updated the OP comment, use @rustbot ready to put this back in the queue.

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[rustbot]

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[asder8215]

Isn't this being worked on in #153030?

[RalfJung]

Yeah this is a duplicate.

@laddery-a thanks for helping improve our docs! However, before you work on fixing an opened issue, please check that the issue isn't already claimed by someone. #150689 has an assignee so it is clearly claimed. I will therefore close this PR in favor of #153030. Your feedback is welcome on that PR!

[laddery-a]

Thanks for the quick review. Sorry I missed that this was already being handled in #153030 (and that #150689 was already claimed).
I’ll follow up there with any feedback and won’t duplicate work. Feel free to delete this branch.