io: change AsyncRead to use a ReadBuf (#2758)

Works towards #2716. Changes the argument to `AsyncRead::poll_read` to
take a `ReadBuf` struct that safely manages writes to uninitialized memory.

tokio/src/process/mod.rs

@@ -120,7 +120,7 @@ mod imp;
 
 mod kill;
 
-use crate::io::{AsyncRead, AsyncWrite};
+use crate::io::{AsyncRead, AsyncWrite, ReadBuf};
 use crate::process::kill::Kill;
 
 use std::ffi::OsStr;
@@ -909,31 +909,21 @@ impl AsyncWrite for ChildStdin {
 }
 
 impl AsyncRead for ChildStdout {
-    unsafe fn prepare_uninitialized_buffer(&self, _buf: &mut [std::mem::MaybeUninit<u8>]) -> bool {
-        // https://github.com/rust-lang/rust/blob/09c817eeb29e764cfc12d0a8d94841e3ffe34023/src/libstd/process.rs#L314
-        false
-    }
-
     fn poll_read(
         mut self: Pin<&mut Self>,
         cx: &mut Context<'_>,
-        buf: &mut [u8],
-    ) -> Poll<io::Result<usize>> {
+        buf: &mut ReadBuf<'_>,
+    ) -> Poll<io::Result<()>> {
         Pin::new(&mut self.inner).poll_read(cx, buf)
     }
 }
 
 impl AsyncRead for ChildStderr {
-    unsafe fn prepare_uninitialized_buffer(&self, _buf: &mut [std::mem::MaybeUninit<u8>]) -> bool {
-        // https://github.com/rust-lang/rust/blob/09c817eeb29e764cfc12d0a8d94841e3ffe34023/src/libstd/process.rs#L375
-        false
-    }
-
     fn poll_read(
         mut self: Pin<&mut Self>,
         cx: &mut Context<'_>,
-        buf: &mut [u8],
-    ) -> Poll<io::Result<usize>> {
+        buf: &mut ReadBuf<'_>,
+    ) -> Poll<io::Result<()>> {
         Pin::new(&mut self.inner).poll_read(cx, buf)
     }
 }