Forward OsStr::clone_into to the inner Vec

Despite OS differences, they're all just `Vec<u8>` inside, so we can
just forward `clone_into` calls to that optimized implementation.

src/libstd/ffi/os_str.rs

@@ -1120,8 +1120,7 @@ impl ToOwned for OsStr {
         self.to_os_string()
     }
     fn clone_into(&self, target: &mut OsString) {
-        target.clear();
+        self.inner.clone_into(&mut target.inner)
-        target.push(self);
     }
 }
 

src/libstd/sys/windows/os_str.rs

@@ -159,6 +159,10 @@ impl Slice {
         Buf { inner: buf }
     }
 
+    pub fn clone_into(&self, buf: &mut Buf) {
+        self.inner.clone_into(&mut buf.inner)
+    }
+
     #[inline]
     pub fn into_box(&self) -> Box<Slice> {
         unsafe { mem::transmute(self.inner.into_box()) }

src/libstd/sys_common/os_str_bytes.rs

@@ -173,6 +173,10 @@ impl Slice {
         Buf { inner: self.inner.to_vec() }
     }
 
+    pub fn clone_into(&self, buf: &mut Buf) {
+        self.inner.clone_into(&mut buf.inner)
+    }
+
     #[inline]
     pub fn into_box(&self) -> Box<Slice> {
         let boxed: Box<[u8]> = self.inner.into();

src/libstd/sys_common/wtf8.rs

@@ -613,6 +613,10 @@ impl Wtf8 {
         }
     }
 
+    pub fn clone_into(&self, buf: &mut Wtf8Buf) {
+        self.bytes.clone_into(&mut buf.bytes)
+    }
+
     /// Boxes this `Wtf8`.
     #[inline]
     pub fn into_box(&self) -> Box<Wtf8> {