From 26b7106b1daf44b5cc850830d4a0f99d0e9a740c Mon Sep 17 00:00:00 2001
From: Paolo Barbolini <paolo.barbolini@m4ss.net>
Date: Tue, 23 Apr 2024 21:55:34 +0200
Subject: [PATCH] Reuse capacity when possible in <BytesMut as Buf>::advance
 impl

---
 src/bytes_mut.rs | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/src/bytes_mut.rs b/src/bytes_mut.rs
index 0ea0272c5..2d77092a8 100644
--- a/src/bytes_mut.rs
+++ b/src/bytes_mut.rs
@@ -1066,12 +1066,21 @@ impl Buf for BytesMut {
 
     #[inline]
     fn advance(&mut self, cnt: usize) {
+        // 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.remaining() {
+            // SAFETY: Zero is not greater than the capacity.
+            unsafe { self.set_len(0) };
+            return;
+        }
+
         assert!(
             cnt <= self.remaining(),
             "cannot advance past `remaining`: {:?} <= {:?}",
             cnt,
             self.remaining(),
         );
+
         unsafe {
             // SAFETY: We've checked that `cnt` <= `self.remaining()` and we know that
             // `self.remaining()` <= `self.cap`.