From a655558b380bf689f806e45fc896b7e6ba739cb7 Mon Sep 17 00:00:00 2001
From: Zalathar <Zalathar@users.noreply.github.com>
Date: Sat, 13 Jan 2024 17:12:15 +1100
Subject: [PATCH] Remove special-case handling of `vec.split_off(0)`

---
 library/alloc/src/vec/mod.rs |  8 --------
 library/alloc/tests/vec.rs   | 24 ++++++++++++++++++------
 2 files changed, 18 insertions(+), 14 deletions(-)

diff --git a/library/alloc/src/vec/mod.rs b/library/alloc/src/vec/mod.rs
index 8aa0c6e7ed6..9603f03d27f 100644
--- a/library/alloc/src/vec/mod.rs
+++ b/library/alloc/src/vec/mod.rs
@@ -2195,14 +2195,6 @@ fn assert_failed(at: usize, len: usize) -> ! {
             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_in(self.capacity(), self.allocator().clone()),
-            );
-        }
-
         let other_len = self.len - at;
         let mut other = Vec::with_capacity_in(other_len, self.allocator().clone());
 
diff --git a/library/alloc/tests/vec.rs b/library/alloc/tests/vec.rs
index 0f5e0d99eca..364acb32f65 100644
--- a/library/alloc/tests/vec.rs
+++ b/library/alloc/tests/vec.rs
@@ -958,23 +958,35 @@ fn test_append() {
 #[test]
 fn test_split_off() {
     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(4);
+
+    let split_off = vec.split_off(4);
     assert_eq!(vec, [1, 2, 3, 4]);
-    assert_eq!(vec2, [5, 6]);
+    assert_eq!(split_off, [5, 6]);
     assert_eq!(vec.capacity(), orig_capacity);
+    assert_eq!(vec.as_ptr(), orig_ptr);
 }
 
 #[test]
 fn test_split_off_take_all() {
-    let mut vec = vec![1, 2, 3, 4, 5, 6];
+    // Allocate enough capacity that we can tell whether the split-off vector's
+    // capacity is based on its size, or (incorrectly) on the original capacity.
+    let mut vec = Vec::with_capacity(1000);
+    vec.extend([1, 2, 3, 4, 5, 6]);
     let orig_ptr = vec.as_ptr();
     let orig_capacity = vec.capacity();
-    let vec2 = vec.split_off(0);
+
+    let split_off = vec.split_off(0);
     assert_eq!(vec, []);
-    assert_eq!(vec2, [1, 2, 3, 4, 5, 6]);
+    assert_eq!(split_off, [1, 2, 3, 4, 5, 6]);
     assert_eq!(vec.capacity(), orig_capacity);
-    assert_eq!(vec2.as_ptr(), orig_ptr);
+    assert_eq!(vec.as_ptr(), orig_ptr);
+
+    // The split-off vector should be newly-allocated, and should not have
+    // stolen the original vector's allocation.
+    assert!(split_off.capacity() < orig_capacity);
+    assert_ne!(split_off.as_ptr(), orig_ptr);
 }
 
 #[test]