core::path::Path

RelativePath<Sep: AsciiChar> could be made platform independent fairly easily. A relative path is just (conceptually at least) an array of OsStr separated by a character. Most of the complexity in Path comes with having different types of root.

There is also the problem that Win32 allows both / and \ as separators but that could be worked around, either by RelativePath taking multiple separators or by it being an error on use in a Windows context (like null in Path is today).