Reuse capacity when possible in <BytesMut as Buf>::advance impl

src/bytes_mut.rs

@@ -1064,6 +1064,15 @@ impl Buf for BytesMut {
             cnt,
             self.remaining(),
         );
+
+        // Advancing by the length is the same as resetting the length to 0,
+        // except this way we get to reuse the full capacity.
+        if cnt == self.len {
+            // SAFETY: Zero is not greater than the capacity.
+            unsafe { self.set_len(0) };
+            return;
+        }
+
         unsafe {
             // SAFETY: We've checked that `cnt` <= `self.remaining()` and we know that
             // `self.remaining()` <= `self.cap`.