io::Chain: specialize some BufRead methods

library/std/src/io/mod.rs

@@ -2429,9 +2429,7 @@ impl<T: BufRead, U: BufRead> BufRead for Chain<T, U> {
     fn fill_buf(&mut self) -> Result<&[u8]> {
         if !self.done_first {
             match self.first.fill_buf()? {
-                buf if buf.is_empty() => {
+                buf if buf.is_empty() => self.done_first = true,
-                    self.done_first = true;
-                }
                 buf => return Ok(buf),
             }
         }
@@ -2441,6 +2439,21 @@ fn fill_buf(&mut self) -> Result<&[u8]> {
     fn consume(&mut self, amt: usize) {
         if !self.done_first { self.first.consume(amt) } else { self.second.consume(amt) }
     }
+
+    fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> Result<usize> {
+        let mut read = 0;
+        if !self.done_first {
+            let n = self.first.read_until(byte, buf)?;
+            read += n;
+
+            match buf.last() {
+                Some(b) if *b == byte && n != 0 => return Ok(read),
+                _ => self.done_first = true,
+            }
+        }
+        read += self.second.read_until(byte, buf)?;
+        Ok(read)
+    }
 }
 
 impl<T, U> SizeHint for Chain<T, U> {