allow BufReader::peek to be called on unsized types

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

@@ -94,7 +94,9 @@ impl<R: Read> BufReader<R> {
     pub fn with_capacity(capacity: usize, inner: R) -> BufReader<R> {
         BufReader { inner, buf: Buffer::with_capacity(capacity) }
     }
+}
 
+impl<R: Read + ?Sized> BufReader<R> {
     /// Attempt to look ahead `n` bytes.
     ///
     /// `n` must be less than `capacity`.
@@ -117,7 +119,10 @@ impl<R: Read> BufReader<R> {
     /// assert_eq!(&s, "hello");
     /// ```
     #[unstable(feature = "bufreader_peek", issue = "128405")]
-    pub fn peek(&mut self, n: usize) -> io::Result<&[u8]> {
+    pub fn peek(&mut self, n: usize) -> io::Result<&[u8]>
+    where
+        R: ?Sized,
+    {
         assert!(n <= self.capacity());
         while n > self.buf.buffer().len() {
             if self.buf.pos() > 0 {