Implement ExactSizeIterator for Args and ArgsOs

src/libstd/env.rs

@@ -488,12 +488,20 @@ impl Iterator for Args {
     fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
 }
 
+impl ExactSizeIterator for Args {
+    fn len(&self) -> usize { self.inner.len() }
+}
+
 impl Iterator for ArgsOs {
     type Item = OsString;
     fn next(&mut self) -> Option<OsString> { self.inner.next() }
     fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
 }
 
+impl ExactSizeIterator for ArgsOs {
+    fn len(&self) -> usize { self.inner.len() }
+}
+
 /// Returns the page size of the current architecture in bytes.
 pub fn page_size() -> usize {
     os_imp::page_size()

src/libstd/sys/unix/os.rs

@@ -247,6 +247,10 @@ impl Iterator for Args {
     fn size_hint(&self) -> (usize, Option<usize>) { self.iter.size_hint() }
 }
 
+impl ExactSizeIterator for Args {
+    fn len(&self) -> usize { self.iter.len() }
+}
+
 /// Returns the command line arguments
 ///
 /// Returns a list of the command line arguments.

src/libstd/sys/windows/os.rs

@@ -303,6 +303,10 @@ impl Iterator for Args {
     fn size_hint(&self) -> (usize, Option<usize>) { self.range.size_hint() }
 }
 
+impl ExactSizeIterator for Args {
+    fn len(&self) -> usize { self.range.len() }
+}
+
 impl Drop for Args {
     fn drop(&mut self) {
         unsafe { c::LocalFree(self.cur as *mut c_void); }