error[E0277]: `dyn futures_io::if_std::AsyncWrite + std::marker::Unpin` cannot be sent between threads safely
   --> src/intrinsics.rs:350:4
    |
350 |   .to_boxed()
    |    ^^^^^^^^ `dyn futures_io::if_std::AsyncWrite + std::marker::Unpin` cannot be sent between threads safely
    |
    = help: the trait `std::marker::Send` is not implemented for `dyn futures_io::if_std::AsyncWrite + std::marker::Unpin`
    = note: required because of the requirements on the impl of `std::marker::Send` for `&mut dyn futures_io::if_std::AsyncWrite + std::marker::Unpin`