1 files changed, 21 insertions, 0 deletions
diff --git a/library/std/src/io/buffered/bufreader/buffer.rs b/library/std/src/io/buffered/bufreader/buffer.rs
index 796137c0123..ccd67fafb45 100644
--- a/library/std/src/io/buffered/bufreader/buffer.rs
+++ b/library/std/src/io/buffered/bufreader/buffer.rs
@@ -97,6 +97,27 @@ impl Buffer {
         self.pos = self.pos.saturating_sub(amt);
     }
 
+    /// Read more bytes into the buffer without discarding any of its contents
+    pub fn read_more(&mut self, mut reader: impl Read) -> io::Result<()> {
+        let mut buf = BorrowedBuf::from(&mut self.buf[self.pos..]);
+        let old_init = self.initialized - self.pos;
+        unsafe {
+            buf.set_init(old_init);
+        }
+        reader.read_buf(buf.unfilled())?;
+        self.filled += buf.len();
+        self.initialized += buf.init_len() - old_init;
+        Ok(())
+    }
+
+    /// Remove bytes that have already been read from the buffer.
+    pub fn backshift(&mut self) {
+        self.buf.copy_within(self.pos.., 0);
+        self.initialized -= self.pos;
+        self.filled -= self.pos;
+        self.pos = 0;
+    }
+
     #[inline]
     pub fn fill_buf(&mut self, mut reader: impl Read) -> io::Result<&[u8]> {
         // If we've reached the end of our internal buffer then we need to fetch