io: add ReadBuf::inner_mut

This returns the entire buffer as a `&mut [MaybeUninit<u8>]`, which may
be useful for building abstractions on top.

tokio/src/io/read_buf.rs

@@ -113,6 +113,18 @@ impl<'a> ReadBuf<'a> {
         unsafe { mem::transmute::<&mut [MaybeUninit<u8>], &mut [u8]>(slice) }
     }
 
+    /// Returns a mutable reference to the entire buffer, without ensuring that it has been fully
+    /// initialized.
+    ///
+    /// # Safety
+    ///
+    /// The caller must not de-initialize portions of the buffer that have already been initialized.
+    /// This includes any bytes in the region marked as uninitialized by `ReadBuf`.
+    #[inline]
+    pub unsafe fn inner_mut(&mut self) -> &mut [MaybeUninit<u8>] {
+        &mut self.buf
+    }
+
     /// Returns a mutable reference to the unfilled part of the buffer without ensuring that it has been fully
     /// initialized.
     ///