Reason safety for unsafe blocks for uwp stdin

Author: bdbai

Diff for: library/std/src/sys/windows/stdio_uwp.rs

@@ -26,6 +26,7 @@ pub fn get_handle(handle_id: c::DWORD) -> io::Result<c::HANDLE> {
 
 fn write(handle_id: c::DWORD, data: &[u8]) -> io::Result<usize> {
     let handle = get_handle(handle_id)?;
+    // SAFETY: The handle returned from `get_handle` must be valid and non-null.
     let handle = unsafe { Handle::from_raw_handle(handle) };
     ManuallyDrop::new(handle).write(data)
 }
@@ -39,6 +40,7 @@ impl Stdin {
 impl io::Read for Stdin {
     fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
         let handle = get_handle(c::STD_INPUT_HANDLE)?;
+        // SAFETY: The handle returned from `get_handle` must be valid and non-null.
         let handle = unsafe { Handle::from_raw_handle(handle) };
         ManuallyDrop::new(handle).read(buf)
     }