Make dropping an empty BTreeMap free

[jakedrew]

#161375 (code example) shows that drop of an empty BTree map walks the dying_next path and so is not free. I tracked this down to 7b28036 in 1.56.0 using godbolt’s version comparison.

When the Btree map has no root we use the ManuallyDrop to safely drop the allocator early resulting in a free drop and a ret instead of the dying_next path.

Includes codegen test which correctly catches the prior dying_next case, which I based on uninit-aggregate-field.rs.

Fixes #161375

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[mati865]

Nice, but that requires libs review. r? rust-lang/libs

[ruriww]

Just a side note, but can't the unsafe code be replaced with take(self)?

Hmm, maybe not https://godbolt.org/z/Wzhqh9jre.

[clarfonthey]

I personally would rather this check for calls to dealloc rather than dying_next, since I this test could easily become worthless if this method is renamed.

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[clarfonthey]

@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[rust-bors]

☔ The latest upstream changes (presumably #161990) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.