std::fs::walk_dir never closes directories

[clee]

I wrote a small utility to recursively parse time data from files in a given root directory, and when I try to run it on my $HOME it errors out with:

Error { repr: Os(24) }

According to the devs in #rust, this is the error code for 'too many open files'. After looking at the code for walk_dir, it looks like it never calls closedir, which would explain why this is happening for me.

[alexcrichton]

We do actually call closedir when the ReadDir value goes out of scope, so I don't believe that's a problem. This is likely a dupe of #23715, however, so I'm going to close in favor of that, but thanks for the report!