bufreader::Buffer::backshift: don't move the uninit bytes

lolbinarycat · lolbinarycat · commit ab19755630ea · 2025-08-17T14:13:05.000-05:00
previous code was perfectly sound because of MaybeUninit,
but it did waste cycles on copying memory that is
known to be uninitialized.
diff --git 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;
     }

Original file line number	Diff line number	Diff line change
`@@ -122,7 +122,7 @@ impl Buffer {`
`122`	`122`
`123`	`123`	`/// Remove bytes that have already been read from the buffer.`
`124`	`124`	`pub fn backshift(&mut self) {`
`125`		`- self.buf.copy_within(self.pos.., 0);`
	`125`	`+ self.buf.copy_within(self.pos..self.filled, 0);`
`126`	`126`	`self.filled -= self.pos;`
`127`	`127`	`self.pos = 0;`
`128`	`128`	`}`