implement BufReader::peek

library/std/src/io/buffered/bufreader.rs

@@ -94,6 +94,32 @@ impl<R: Read> BufReader<R> {
     pub fn with_capacity(capacity: usize, inner: R) -> BufReader<R> {
         BufReader { inner, buf: Buffer::with_capacity(capacity) }
     }
+
+    /// Attempt to look ahead `n` bytes.
+    ///
+    /// `n` must be less than `capacity`.
+    ///
+    /// ## Examples
+    ///
+    /// ```rust
+    /// #![feature(bufreader_peek)]
+    /// use std::io::{Read, BufReader};
+    ///
+    /// let mut bytes = &b"hello"[..];
+    /// let mut rdr = BufReader::with_capacity(6, &mut bytes);
+    /// assert_eq!(rdr.peek(2).unwrap(), b"he");
+    /// let mut s = String::new();
+    /// rdr.read_to_string(&mut s).unwrap();
+    /// assert_eq!(&s, "hello");
+    /// ```
+    #[unstable(feature = "bufreader_peek", issue = "128405")]
+    pub fn peek(&mut self, n: usize) -> io::Result<&[u8]> {
+        assert!(n < self.capacity());
+        while n > self.buf.buffer().len() {
+            self.buf.read_more(&mut self.inner)?;
+        }
+        Ok(&self.buf.buffer()[..n])
+    }
 }
 
 impl<R: ?Sized> BufReader<R> {

library/std/src/io/buffered/bufreader/buffer.rs

@@ -97,6 +97,18 @@ impl Buffer {
         self.pos = self.pos.saturating_sub(amt);
     }
 
+    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(())
+    }
+
     #[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