The function filepath.WalkDir uses the callback fs.WalkDirFunc.

Within path/filepath, paths generally use the slash separator of the host platform, so the root parameter to filepath.WalkDir is clearly a platform-specific path.

However, it wasn't obvious to me upon reading the documentation whether the path passed to the WalkDirFunc was a platform-specific path or used / as the path separator. In fact, all the paths use the platform separator, but I had to read the code to be sure. Here are some reasons I thought the WalkDirFunc path might be /-separated:

• Other types/functions inside io/fs, such as fs.FS, use /-separated paths.

• When the same callback function fs.WalkDirFunc is called via fs.WalkDir (rather than filepath.WalkDir) it sees /-separated paths.

• The fs.WalkDirFunc documentation mentions /-separated paths:

      The path argument contains the argument to WalkDir as a prefix. That is, if
      WalkDir is called with root argument "dir" and finds a file named "a" in
      that directory, the walk function will be called with argument "dir/a".
  

  (I believe that on Windows, if this is coming from filepath.WalkDir, then it would be dir\a rather than dir/a.)

I think the documentation of filepath.WalkDir and/or fs.WalkDirFunc could be clearer on this point.

/cc @rsc who wrote most of this stuff as part of io/fs for Go 1.16.