Auto merge of #262 - Yamakaky:clone_from, r=mbrubeck

Add specialized `clone_from` implementation

Author: bors-servo

src/lib.rs

@@ -1725,6 +1725,21 @@ where
     fn clone(&self) -> SmallVec<A> {
         SmallVec::from(self.as_slice())
     }
+
+    fn clone_from(&mut self, source: &Self) {
+        // Inspired from `impl Clone for Vec`.
+
+        // drop anything that will not be overwritten
+        self.truncate(source.len());
+
+        // self.len <= other.len due to the truncate above, so the
+        // slices here are always in-bounds.
+        let (init, tail) = source.split_at(self.len());
+
+        // reuse the contained values' allocations/resources.
+        self.clone_from_slice(init);
+        self.extend(tail.iter().cloned());
+    }
 }
 
 impl<A: Array, B: Array> PartialEq<SmallVec<B>> for SmallVec<A>

src/tests.rs

@@ -925,3 +925,25 @@ fn test_insert_many_overflow() {
     v.insert_many(0, iter);
     assert_eq!(&*v, &[0, 2, 4, 123]);
 }
+
+#[test]
+fn test_clone_from() {
+    let mut a: SmallVec<[u8; 2]> = SmallVec::new();
+    a.push(1);
+    a.push(2);
+    a.push(3);
+
+    let mut b: SmallVec<[u8; 2]> = SmallVec::new();
+    b.push(10);
+
+    let mut c: SmallVec<[u8; 2]> = SmallVec::new();
+    c.push(20);
+    c.push(21);
+    c.push(22);
+
+    a.clone_from(&b);
+    assert_eq!(&*a, &[10]);
+
+    b.clone_from(&c);
+    assert_eq!(&*b, &[20, 21, 22]);
+}