auto merge of #19303 : nodakai/rust/libsyntax-reject-dirs, r=alexcrichton

On *BSD systems, we can `open(2)` a directory and directly `read(2)` from it due to an old tradition.  We should avoid doing so by explicitly calling `fstat(2)` to check the type of the opened file.

Opening a directory as a module file can't always be avoided.  Even when there's no "path" attribute trick involved, there can always be a *directory* named `my_module.rs`.

Incidentally, remove unnecessary mutability of `&self` from `io::fs::File::stat()`.

src/test/compile-fail/issue-5806.rs

@@ -18,9 +18,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// ignore-freebsd FIXME #12460
-
 #[path = "../compile-fail"]
-mod foo; //~ ERROR: illegal operation on a directory
+mod foo; //~ ERROR: a directory
 
 fn main() {}