core: Fail with a better error message when list_dir gets an empty path

(Yes, this did happen in real life...)

Author: catamorphism

src/libstd/os.rs

@@ -675,6 +675,11 @@ pub fn mkdir_recursive(p: &Path, mode: c_int) -> bool {
 /// Lists the contents of a directory
 #[allow(non_implicitly_copyable_typarams)]
 pub fn list_dir(p: &Path) -> ~[~str] {
+    if p.components.is_empty() {
+        // Not sure what the right behavior is here, but this
+        // prevents a bounds check failure later
+        return ~[];
+    }
     unsafe {
         #[cfg(target_os = "linux")]
         #[cfg(target_os = "android")]
@@ -1596,6 +1601,12 @@ mod tests {
         }
     }
 
+    #[test]
+    fn list_dir_empty_path() {
+        let dirs = os::list_dir(&Path(""));
+        assert!(dirs.is_empty());
+    }
+
     #[test]
     fn path_is_dir() {
         assert!((os::path_is_dir(&Path("."))));