Macro async_io 

macro_rules! async_io {
    () => { ... };
    (if { $($p:tt)* } else { $($n:tt)* }) => { ... };
    ($($p:tt)*) => { ... };
}

Indicates async-io will be used for the implementation of block_on.

This macro suppresses the provided code because #[cfg(feature = "async-io")] is not currently active.