How does `builtins.fetchTree` work without hashes?

I’m seeing some flakes starting to use builtins.fetchTree to fetch remote git repositories, but I’m noticing that the hash is missing. How does this work? Surely we’re not just completely giving up on reproducibility with this new API right?

The docstring suggests that you should only be able to omit the hash in --impure mode. (I have no idea whether that’s actually true.)

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