Fix conversions of Box<T, A> to Rc<T, A> or Arc<T, A> to use allocator correctly for dropping the Box

[steffahn]

This fixes #119749.

I wanted to add a test, too, but realized there are essentially no tests for almost any of the code handling custom allocators, anyways¹, and I didn’t want to introduce a whole generally usable testing Allocator implementation just for this soundness fix.

Footnotes

1. as far as I could tell. Feel free to point me to anything I might have missed. ↩

[rustbot]

r? @cuviper

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[klensy]

#119801

[steffahn]

Looks like the issue was independently discovered there too, perhaps, otherwise #119749 would have been linked?

I didn't know that miri does differentiate Global from System, that may make adding tests easier - though.. are (all or some) library tests run through miri on CI?

[the8472]

CI doesn't run lib tests under miri, that happens somewhere else as a daily job because it's slow. You can do that manually via https://github.com/rust-lang/miri-test-libstd
In CI miri only runs some basic tests from src/tools/miri/tests

[steffahn]

I meant CI more generally anyways - so I guess a normal library test would be sufficient to make sure a bug being reintroduced wouldn't stay unnoticed. Thanks for the info, good to know.

[steffahn]

Anyways… I guess I’ll close this to remove the duplicate :-)

[zachs18]

Ah sorry, I didn't see there was already an issue for it. I discovered the problem while working on #119761 but wanted to fix it in a separate PR since it was a soundness fix which shouldn't need as much approval as that PR.

[steffahn]

No worries. We had literally seeing the same issue independently around roughly the same time. Great coincidence. I’ve spotted your PR #119761 already after #119801 was mentioned, and figured that’s where you’re coming from. While you were creating #119761, the issue #119749 probably didn't even exist yet. I was coming from explaining ppl how Rc can support unsized contents, and pointed out the Box<T> -> Rc<T> conversion as an example how to move around unsized data; in the process of that noticing the bug in the code.