Describe the bug

The behavior of builtins.dirOf got silently changed in 2.23.

Steps To Reproduce

nix run github:nixos/nix/2.22-maintenance -- eval --expr 'builtins.dirOf "a///b"'
"a//"

nix run github:nixos/nix/2.23-maintenance -- eval --expr 'builtins.dirOf "a///b"'
"a"

It's pretty clear that the >=2.23 behaviour is more "correct" but we don't have the luxury of breaking bug-for-bug compatibility in stable interfaces. Also I think this got changed silently rather than intentionally, because the reason this works like this now is because dirOf started delegating to std::filesystem for this, but since the evaluator must always follow the CanonPath semantics of UNIX paths this isn't correct.

Expected behavior

Bug-for-bug compat is retained. This behavior has existed since forever and should be restored.

Metadata

Not applicable.

Additional context

Checklist

• checked latest Nix manual (source)
• checked open bug issues and pull requests for possible duplicates

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