Implement BufMut for &mut [MaybeUninit<u8>]

This will allow using BufMut without having to initialise memory
first.  In particular, this will allow simple integration with tokio:

    #![feature(maybe_uninit_slice)]

    use tokio::io::AsyncReadExt;

    let mut tokio_file = /* ... */;
    let mut buf = [MaybeUninit::uninit(); 4096];
    let len = tokio_file.read_buf(&mut &mut buf[..]);
    // SAFETY: read initialised first len bytes of the buffer.
    let buf = unsafe {
        MaybeUninit::slice_assume_init_mut(&buf[..len])
    };

src/buf/buf_mut.rs

@@ -1419,6 +1419,41 @@ unsafe impl BufMut for &mut [u8] {
     }
 }
 
+unsafe impl BufMut for &mut [core::mem::MaybeUninit<u8>] {
+    #[inline]
+    fn remaining_mut(&self) -> usize {
+        self.len()
+    }
+
+    #[inline]
+    fn chunk_mut(&mut self) -> &mut UninitSlice {
+        UninitSlice::from_slice(self)
+    }
+
+    #[inline]
+    unsafe fn advance_mut(&mut self, cnt: usize) {
+        // Lifetime dance taken from `impl Write for &mut [u8]`.
+        let (_, b) = core::mem::replace(self, &mut []).split_at_mut(cnt);
+        *self = b;
+    }
+
+    #[inline]
+    fn put_slice(&mut self, src: &[u8]) {
+        self.chunk_mut()[..src.len()].copy_from_slice(src);
+        unsafe {
+            self.advance_mut(src.len());
+        }
+    }
+
+    fn put_bytes(&mut self, val: u8, cnt: usize) {
+        assert!(self.remaining_mut() >= cnt);
+        unsafe {
+            ptr::write_bytes(self.as_mut_ptr() as *mut u8, val, cnt);
+            self.advance_mut(cnt);
+        }
+    }
+}
+
 unsafe impl BufMut for Vec<u8> {
     #[inline]
     fn remaining_mut(&self) -> usize {

tests/test_buf_mut.rs

@@ -3,6 +3,7 @@
 use bytes::buf::UninitSlice;
 use bytes::{BufMut, BytesMut};
 use core::fmt::Write;
+use core::mem::{transmute, MaybeUninit};
 use core::usize;
 
 #[test]
@@ -121,6 +122,41 @@ fn test_slice_put_bytes() {
     assert_eq!(&[17, 19, 19, 0], &v[..]);
 }
 
+#[test]
+fn test_uninit_buf_mut() {
+    let mut buf = [b'X'; 100];
+    let fill = [b'Z'; 100];
+
+    fn make(slice: &mut [u8]) -> &mut [MaybeUninit<u8>] {
+        // SAFETY: [T] and [MaybeUninit<T>] have the same layout.
+        unsafe { transmute(slice) }
+    }
+
+    {
+        let mut slice = make(&mut buf[..]);
+        slice.put_bytes(b'A', 2);
+        slice.put_slice(b"BBCC");
+        assert_eq!(b"AABBCCXX", &buf[..8]);
+    }
+
+    for buf_len in 0..100 {
+        for fill_len in 0..=buf_len {
+            let mut slice = make(&mut buf[..buf_len]);
+            slice.put_slice(&fill[..fill_len]);
+            assert_eq!(buf_len - fill_len, slice.remaining_mut());
+        }
+    }
+}
+
+#[test]
+#[should_panic]
+fn test_uninit_buf_mut_overflow() {
+    let mut buf = [b'X'; 4];
+    // SAFETY: [T] and [MaybeUninit<T>] have the same layout.
+    let mut slice: &mut [MaybeUninit<u8>] = unsafe { transmute(&mut buf[..]) };
+    slice.put_slice(b"AABBCC");
+}
+
 #[test]
 fn test_deref_bufmut_forwards() {
     struct Special;