io::readdir (apparently) has access to useful information about its child entries

[huonw]

According to http://legacy.python.org/dev/peps/pep-0471/

[...] -- it executes the stat() system call or GetFileAttributes() on each file to determine whether the entry is a directory or not.

But the underlying system calls -- FindFirstFile / FindNextFile on Windows and readdir on POSIX systems -- already tell you whether the files returned are directories or not, so no further system calls are needed. Further, the Windows system calls return all the information for a stat_result object on the directory entry, such as file size and last modification time.

Our current readdir just returns the raw Paths, discarding this information.

Reusing that information would likely allow us to see speed-ups for walk_dir similar to those seen in that PEP:

In practice, removing all those extra system calls makes os.walk() about 8-9 times as fast on Windows, and about 2-3 times as fast on POSIX systems

[huonw]

cc #16236

cc @alexcrichton

[aturon]

cc me

[lucy]

Linux readdir doesn't return the file type on all file systems, according to readdir(3).

[alexcrichton]

This has now been exposed in terms of the yielded value from the ReadDir iterator, we should be free to add various accessors in the future if necessary.