From ab19755630ea6eb3c2f2a364300380ab01eff923 Mon Sep 17 00:00:00 2001
From: binarycat <binarycat@envs.net>
Date: Sun, 17 Aug 2025 14:13:05 -0500
Subject: [PATCH] bufreader::Buffer::backshift: don't move the uninit bytes

previous code was perfectly sound because of MaybeUninit,
but it did waste cycles on copying memory that is
known to be uninitialized.
---
 library/std/src/io/buffered/bufreader/buffer.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/library/std/src/io/buffered/bufreader/buffer.rs b/library/std/src/io/buffered/bufreader/buffer.rs
index 574288e579e0b..9b600cd55758b 100644
--- a/library/std/src/io/buffered/bufreader/buffer.rs
+++ b/library/std/src/io/buffered/bufreader/buffer.rs
@@ -122,7 +122,7 @@ impl Buffer {
 
     /// Remove bytes that have already been read from the buffer.
     pub fn backshift(&mut self) {
-        self.buf.copy_within(self.pos.., 0);
+        self.buf.copy_within(self.pos..self.filled, 0);
         self.filled -= self.pos;
         self.pos = 0;
     }