const select_unpredictable

[clarfonthey]

Tracking issue: #145938

Since constants don't actually care about branch prediction, this just uses const_eval_select to use a normal if statement in const-time, instead of having to make the intrinsic work in const context.

Blocked by #146187.

[rustbot]

r? @tgross35

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[clarfonthey]

@rustbot blocked

[bors]

☔ The latest upstream changes (presumably #146052) made this pull request unmergeable. Please resolve the merge conflicts.

[rustbot]

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

[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.

[clarfonthey]

@rustbot ready

Should be good, but will fix if CI shows up with any weird issues. Now that ptr::drop_in_place works in const context, we can just mark this method and its corresponding intrinsic as const. I also updated the existing test to test both at runtime and const-time, so that we verify that the drop behaviour is consistent across both.

[clarfonthey]

Didn't want to block this on #147708 but I'll update this depending on which one gets merged first.