DoubleEndedIterator for Args

The number of arguments given to a process is always known, which
makes implementing DoubleEndedIterator possible.

That way, the Iterator::rev() method becomes usable, among others.

Signed-off-by: Sebastian Thiel <byronimo@gmail.com>

Tidy for DoubleEndedIterator

I chose to not create a new feature for it, even though
technically, this makes me lie about the original availability
of the implementation.

Verify with @alexchrichton

Setup feature flag for new std::env::Args iterators

Add test for Args reverse iterator

It's somewhat depending on the input of the test program,
but made in such a way that should be somewhat flexible to changes
to the way it is called.

Deduplicate windows ArgsOS code for DEI

DEI = DoubleEndedIterator

Move env::args().rev() test to run-pass

It must be controlling it's arguments for full isolation.

Remove superfluous feature name

Assert all arguments returned by env::args().rev()

Let's be very sure it works as we expect, why take chances.

Fix rval of os_string_from_ptr

A trait cannot be returned, but only the corresponding object.

Deref pointers to actually operate on the argument

Put unsafe to correct location

2 files changed, 21 insertions, 10 deletions

diff --git a/src/libstd/sys/unix/os.rs b/src/libstd/sys/unix/os.rs
index 21ce6b19ceb..a8cb1ce49d2 100644
--- a/src/libstd/sys/unix/os.rs
+++ b/src/libstd/sys/unix/os.rs
@@ -308,6 +308,10 @@ impl ExactSizeIterator for Args {
     fn len(&self) -> usize { self.iter.len() }
 }
 
+impl DoubleEndedIterator for Args {
+    fn next_back(&mut self) -> Option<OsString> { self.iter.next_back() }
+}
+
 /// Returns the command line arguments
 ///
 /// Returns a list of the command line arguments.
diff --git a/src/libstd/sys/windows/os.rs b/src/libstd/sys/windows/os.rs
index 32ca32e76cb..0cea7f81e36 100644
--- a/src/libstd/sys/windows/os.rs
+++ b/src/libstd/sys/windows/os.rs
@@ -278,23 +278,30 @@ pub struct Args {
     cur: *mut *mut u16,
 }
 
+unsafe fn os_string_from_ptr(ptr: *mut u16) -> OsString {
+    let mut len = 0;
+    while *ptr.offset(len) != 0 { len += 1; }
+
+    // Push it onto the list.
+    let ptr = ptr as *const u16;
+    let buf = slice::from_raw_parts(ptr, len as usize);
+    OsStringExt::from_wide(buf)
+}
+
 impl Iterator for Args {
     type Item = OsString;
     fn next(&mut self) -> Option<OsString> {
-        self.range.next().map(|i| unsafe {
-            let ptr = *self.cur.offset(i);
-            let mut len = 0;
-            while *ptr.offset(len) != 0 { len += 1; }
-
-            // Push it onto the list.
-            let ptr = ptr as *const u16;
-            let buf = slice::from_raw_parts(ptr, len as usize);
-            OsStringExt::from_wide(buf)
-        })
+        self.range.next().map(|i| unsafe { os_string_from_ptr(*self.cur.offset(i)) } )
     }
     fn size_hint(&self) -> (usize, Option<usize>) { self.range.size_hint() }
 }
 
+impl DoubleEndedIterator for Args {
+    fn next_back(&mut self) -> Option<OsString> {
+        self.range.next_back().map(|i| unsafe { os_string_from_ptr(*self.cur.offset(i)) } )
+    }
+}
+
 impl ExactSizeIterator for Args {
     fn len(&self) -> usize { self.range.len() }
 }