allow BufReader::peek to be called on unsized types

Author: lolbinarycat

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 {