chore: add safety comments on unsafe blocks (#827)

tests/test_bytes.rs

@@ -1543,6 +1543,65 @@ fn split_to_empty_addr_mut() {
     let _ = &buf[..];
 }
 
+#[test]
+fn bytes_mut_split_boundary_capacities() {
+    // VEC mode
+    for at in [0, 5, 11] {
+        let mut buf = BytesMut::with_capacity(64);
+        buf.extend_from_slice(b"hello world");
+
+        let other = buf.split_off(at);
+        assert_eq!(
+            buf.capacity() + other.capacity(),
+            64,
+            "split_off at {} should preserve total capacity",
+            at
+        );
+    }
+
+    for at in [0, 5, 11] {
+        let mut buf = BytesMut::with_capacity(64);
+        buf.extend_from_slice(b"hello world");
+
+        let other = buf.split_to(at);
+        assert_eq!(
+            buf.capacity() + other.capacity(),
+            64,
+            "split_to at {} should preserve total capacity",
+            at
+        );
+    }
+
+    // ARC mode (promote via a no-op split)
+    for at in [0, 5, 11] {
+        let mut buf = BytesMut::with_capacity(64);
+        buf.extend_from_slice(b"hello world");
+        let _ = buf.split_to(0); // promotes to ARC
+
+        let other = buf.split_off(at);
+        assert_eq!(
+            buf.capacity() + other.capacity(),
+            64,
+            "ARC split_off at {} should preserve total capacity",
+            at
+        );
+    }
+
+    for at in [0, 5, 11] {
+        let mut buf = BytesMut::with_capacity(64);
+        buf.extend_from_slice(b"hello world");
+        let _ = buf.split_to(0); // promotes to ARC
+
+        let other = buf.split_to(at);
+        assert_eq!(
+            buf.capacity() + other.capacity(),
+            64,
+            "ARC split_to at {} should preserve total capacity",
+            at
+        );
+    }
+}
+
 #[derive(Clone)]
 struct SharedAtomicCounter(Arc<AtomicUsize>);