fixed bug that had to deal with mut and non mut suggestion

tests/ui/extend_with_drain.fixed

@@ -41,7 +41,12 @@ fn main() {
 
     let mut heap = BinaryHeap::from(vec![1, 3]);
     let mut heap2 = BinaryHeap::from(vec![]);
-    heap2.extend(heap.drain())
+    heap2.extend(heap.drain());
+
+    let mut x = vec![0, 1, 2, 3, 5];
+    let ref_x = &mut x;
+    let mut y = Vec::new();
+    y.append(ref_x);
 }
 
 fn return_vector() -> Vec<u8> {

tests/ui/extend_with_drain.rs

@@ -41,7 +41,12 @@ fn main() {
 
     let mut heap = BinaryHeap::from(vec![1, 3]);
     let mut heap2 = BinaryHeap::from(vec![]);
-    heap2.extend(heap.drain())
+    heap2.extend(heap.drain());
+
+    let mut x = vec![0, 1, 2, 3, 5];
+    let ref_x = &mut x;
+    let mut y = Vec::new();
+    y.extend(ref_x.drain(..));
 }
 
 fn return_vector() -> Vec<u8> {

tests/ui/extend_with_drain.stderr

@@ -18,5 +18,11 @@ error: use of `extend` instead of `append` for adding the full range of a second
 LL |     vec11.extend(return_vector().drain(..));
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `vec11.append(&mut return_vector())`
 
-error: aborting due to 3 previous errors
+error: use of `extend` instead of `append` for adding the full range of a second vector
+  --> $DIR/extend_with_drain.rs:49:5
+   |
+LL |     y.extend(ref_x.drain(..));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `y.append(ref_x)`
+
+error: aborting due to 4 previous errors