CCP_WIN_A_TO_POSIX and encodings

[Jeremy Drake]

In https://github.com/rust-lang/rust/pull/141864 I was going to say that
passing an already-posix path through cygwin_conv_path(CPP_WIN_A_TO_POSIX,
...) was unsafe because Windows ANSI file paths are either CP_ACP or
CP_OEMCP, while Cygwin POSIX path encodings are based on the C locale
encoding (usually UTF-8).  However, when I looked at the code I didn't see
any encoding conversion in the CCP_WIN_A_TO_POSIX path.  Is my
understanding wrong?  Is there a conversion going on somewhere I'm not
seeing?