The standard library seems to assume calling make_ascii_uppercase() on the underlying [u8] is safe on Windows, seen here:

Would it be reasonable to add a make_ascii_uppercase() and/or make_ascii_lowercase() to Windows OsStrExt and/or OsStringExt to make this available to users of std?

Even if the internal representation of OsString changes down the road, I would assume the code above would still need to be replicated with the new representation, so it seems like it should be okay to have make_ascii_uppercase a stable/public API.

If this is a reasonable request, I am willing to make a PR with the appropriate methods implemented in the appropriate places.