Fix VecDeque::shrink_to UB when handle_alloc_error unwinds.

[Sp00ph]

Fixes #123369

For VecDeque it's relatively simple to restore the buffer into a consistent state so this PR does just that.

Note that with its current implementation, shrink_to may change the internal arrangement of elements in the buffer, so e.g. [D, <uninit>, A, B, C] will become [<uninit>, A, B, C, D] and [<uninit>, <uninit>, A, B, C] may become [B, C, <uninit>, <uninit>, A] if shrink_to unwinds. This shouldn't be an issue though as we don't make any guarantees about the stability of the internal buffer arrangement (and this case is impossible to hit on stable anyways).

This PR also includes a test with code adapted from #123369 which fails without the new shrink_to code. Does this suffice or do we maybe need more exhaustive tests like in #108475?

cc @Amanieu

@rustbot label +T-libs

r? @Mark-Simulacrum

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@bors r+

📌 Commit fb00ac6 has been approved by Mark-Simulacrum

It is now in the queue for this repository.

@bors r- #123910 (comment)

Woops, didn't mean to comment here from that account lol.

This changes the alloc error hook for all concurrently running tests... is that really a good idea?

Hm, is there another mechanism to allow what I want to test there? Something like compiling this specific test with -Zoom=unwind maybe?

You can make it a separate test crate by adding a file like alloc/tests/vec_deque_alloc_error.rs and add it to alloc/Cargo.toml separately.

Hmm, so is this just blocked on rust-lang/miri#3439 then? Or should I just slap a #[cfg(not(miri))] on the test for now?

Yeah disabling the test in Miri is fine for now, but please ping @rust-lang/miri when you do that so we can keep tabs of these tests and re-enable them when the underlying issue is fixed.

@rustbot ready

@bors r+

📌 Commit 5cb53bc has been approved by Mark-Simulacrum

It is now in the queue for this repository.