io: remove zeroing for AsyncRead implementors (#2525)

tokio/src/fs/file.rs

@@ -537,6 +537,11 @@ impl File {
 }
 
 impl AsyncRead for File {
+    unsafe fn prepare_uninitialized_buffer(&self, _buf: &mut [std::mem::MaybeUninit<u8>]) -> bool {
+        // https://github.com/rust-lang/rust/blob/09c817eeb29e764cfc12d0a8d94841e3ffe34023/src/libstd/fs.rs#L668
+        false
+    }
+
     fn poll_read(
         mut self: Pin<&mut Self>,
         cx: &mut Context<'_>,

tokio/src/io/async_read.rs

@@ -76,7 +76,7 @@ pub trait AsyncRead {
     /// [`poll_read_buf`]: #method.poll_read_buf
     unsafe fn prepare_uninitialized_buffer(&self, buf: &mut [MaybeUninit<u8>]) -> bool {
         for x in buf {
-            *x.as_mut_ptr() = 0;
+            *x = MaybeUninit::new(0);
         }
 
         true

tokio/src/io/mod.rs

@@ -183,6 +183,7 @@ mod async_buf_read;
 pub use self::async_buf_read::AsyncBufRead;
 
 mod async_read;
+
 pub use self::async_read::AsyncRead;
 
 mod async_seek;

tokio/src/io/stdin.rs

@@ -63,6 +63,11 @@ impl std::os::windows::io::AsRawHandle for Stdin {
 }
 
 impl AsyncRead for Stdin {
+    unsafe fn prepare_uninitialized_buffer(&self, _buf: &mut [std::mem::MaybeUninit<u8>]) -> bool {
+        // https://github.com/rust-lang/rust/blob/09c817eeb29e764cfc12d0a8d94841e3ffe34023/src/libstd/io/stdio.rs#L97
+        false
+    }
+
     fn poll_read(
         mut self: Pin<&mut Self>,
         cx: &mut Context<'_>,

tokio/src/io/util/chain.rs

@@ -84,6 +84,15 @@ where
     T: AsyncRead,
     U: AsyncRead,
 {
+    unsafe fn prepare_uninitialized_buffer(&self, buf: &mut [std::mem::MaybeUninit<u8>]) -> bool {
+        if self.first.prepare_uninitialized_buffer(buf) {
+            return true;
+        }
+        if self.second.prepare_uninitialized_buffer(buf) {
+            return true;
+        }
+        false
+    }
     fn poll_read(
         self: Pin<&mut Self>,
         cx: &mut Context<'_>,

tokio/src/io/util/empty.rs

@@ -47,6 +47,9 @@ cfg_io_util! {
 }
 
 impl AsyncRead for Empty {
+    unsafe fn prepare_uninitialized_buffer(&self, _buf: &mut [std::mem::MaybeUninit<u8>]) -> bool {
+        false
+    }
     #[inline]
     fn poll_read(
         self: Pin<&mut Self>,

tokio/src/io/util/repeat.rs

@@ -47,6 +47,9 @@ cfg_io_util! {
 }
 
 impl AsyncRead for Repeat {
+    unsafe fn prepare_uninitialized_buffer(&self, _buf: &mut [std::mem::MaybeUninit<u8>]) -> bool {
+        false
+    }
     #[inline]
     fn poll_read(
         self: Pin<&mut Self>,

tokio/src/process/mod.rs

@@ -879,6 +879,11 @@ 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<'_>,
@@ -889,6 +894,11 @@ impl AsyncRead for ChildStdout {
 }
 
 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<'_>,