[DO NOT MERGE] Test the hashbrown `add-is_empty-checks` branch.#92239

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nnethercote wants to merge 3 commits intorust-lang:masterfrom

nnethercote:hashbrown-add-is_empty-checks

Commits on Dec 24, 2021

Commits on Jan 5, 2022

More hacks, trying to make this work on CI.
nnethercote
committed