test: Add asserts to catch BorrowMutError's #13650
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What does this PR try to resolve?
This intentionally borrows from
RefCell
s before conditional code to try to prevent regressions like #13646 without exhaustively covering every case that could hit theseBorrowMutError
s with complicated tests.How should we test and review this PR?
I tested this by ensuring the registry code path panicked before rebasing on top of #13647.
Once I rebased, the panic went away.
Additional information
Split off from #13649 to try to avoid appveyor