Optimize behavior of vec.split_off(0) (take all)

library/alloc/src/vec.rs

@@ -1410,6 +1410,11 @@ impl<T> Vec<T> {
             assert_failed(at, self.len());
         }
 
+        if at == 0 {
+            // the new vector can take over the original buffer and avoid the copy
+            return mem::replace(self, Vec::with_capacity(self.capacity()));
+        }
+
         let other_len = self.len - at;
         let mut other = Vec::with_capacity(other_len);
 

library/alloc/tests/string.rs

@@ -278,17 +278,21 @@ fn test_split_off_mid_char() {
 #[test]
 fn test_split_off_ascii() {
     let mut ab = String::from("ABCD");
+    let orig_capacity = ab.capacity();
     let cd = ab.split_off(2);
     assert_eq!(ab, "AB");
     assert_eq!(cd, "CD");
+    assert_eq!(ab.capacity(), orig_capacity);
 }
 
 #[test]
 fn test_split_off_unicode() {
     let mut nihon = String::from("日本語");
+    let orig_capacity = nihon.capacity();
     let go = nihon.split_off("日本".len());
     assert_eq!(nihon, "日本");
     assert_eq!(go, "語");
+    assert_eq!(nihon.capacity(), orig_capacity);
 }
 
 #[test]

library/alloc/tests/vec.rs

@@ -772,9 +772,23 @@ fn test_append() {
 #[test]
 fn test_split_off() {
     let mut vec = vec![1, 2, 3, 4, 5, 6];
+    let orig_capacity = vec.capacity();
     let vec2 = vec.split_off(4);
     assert_eq!(vec, [1, 2, 3, 4]);
     assert_eq!(vec2, [5, 6]);
+    assert_eq!(vec.capacity(), orig_capacity);
+}
+
+#[test]
+fn test_split_off_take_all() {
+    let mut vec = vec![1, 2, 3, 4, 5, 6];
+    let orig_ptr = vec.as_ptr();
+    let orig_capacity = vec.capacity();
+    let vec2 = vec.split_off(0);
+    assert_eq!(vec, []);
+    assert_eq!(vec2, [1, 2, 3, 4, 5, 6]);
+    assert_eq!(vec.capacity(), orig_capacity);
+    assert_eq!(vec2.as_ptr(), orig_ptr);
 }
 
 #[test]