toLosslessStringMaybe is not used by anything other than lib/tests,
so it can be private to that file.
I don't think this function was terribly well thought-through. If
people start using it, we will become permanently dependent on the
ability to test platforms for equality. It also makes the
elaboration process more fragile, because it encourages code outside
of nixpkgs to become sensitive to the minute details of how
elaboration happens.
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