Guarantee alignment of fixed-width integer primitives

[joshlf]

Follows up on #2200 (comment)

As an optional extension, we could consider guaranteeing equality between the alignments of:

• u32 and char
• u32 and f32
• u64 and f64

Those equalities strike me as less important, and I'm not sure if they're even valid (ie, whether they hold on all platforms).

[traviscross]

Thanks. I pushed a clarification and separated out the rules.

Clearly we want to guarantee this for fixed-width integers of the same size.

@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.
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[scottmcm]

Good to see this being specifically about for the stated N. I hope we don't need usize ever aligned differently from the uN of the same size, but that's less "well obviously", so sticking with just "the i vs u doesn't impact alignment" as the guarantee sounds good.

@rfcbot reviewed

[rust-rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[joshlf]

Now that the usize/isize language has merged (#2200), the diff in this PR is slightly changed. PTAL and confirm that the updated language and organization still looks good.