[type layout] usize and isize have the same size and alignment

[joshlf]

Everybody assumes this to be true, and I assume it was intended to be guaranteed to be true.

[joshlf]

r? @RalfJung

[rustbot]

Failed to set assignee to RalfJung: invalid assignee

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[joshlf]

r? @ehuss

[traviscross]

Makes sense. It is a new language guarantee, though, so...

@rfcbot fcp merge lang

[rust-rfcbot]

Team member @traviscross has proposed to merge this. The next step is review by the rest of the tagged team members:

• @joshtriplett
• @nikomatsakis
• @scottmcm
• @tmandry
• @traviscross

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

[scottmcm]

Is there an equivalent statement about alignment that iN and uN have the same alignment in the reference?

[joshlf]

Is there an equivalent statement about alignment that iN and uN have the same alignment in the reference?

No. I'd be happy to relax this to just saying that sizes are equal, which is really what I (personally) care about.

[scottmcm]

TBH I think that both are obvious things to provide. Was just seeing that we have the size equality guarantees for uN & iN (via the table specifying their exact sizes) so this is blatantly obvious for size. Was just thinking that if the alignment equality was worth guaranteeing for usize & isize then it's probably worth doing for the others too.

(I have no strong feelings about the best way to do that. If you wanted to just just make this be size that's fine; if you wanted to just open another PR to also give the alignment guarantee for the specific-sized integer types that'd also be fine.)