/// Runs `$code` if `$old` is of type `$t`.
#[macro_export]
macro_rules! reify {
    ($old:ident, |$new:ident : $t:ty| $code:expr, || $fallback:expr) => {{
        #[allow(unused_imports)]
        use ::std::{any::{Any, TypeId}, mem::{ManuallyDrop, transmute_copy}};
        if TypeId::of::<$t>() == $old.type_id() {
            // SAFETY: This is safe because we check to make sure the two types are
            // actually the same type.
            let $new: $t = unsafe { transmute_copy(&ManuallyDrop::new($old)) };
            $code
        } else {
            $fallback
        }
    }};
    ($old:expr, |$new:ident : $t:ty| $code:expr, || $fallback:expr) => {{
        let old = $old;
        reify!(old, |$new : $t| $code, || $fallback)
    }};
    ($old:ident, |$new:ident : $t:ty| $code:expr) => {
        reify!($old, |$new : $t| $code, || ())
    };
    ($old:expr, |$new:ident : $t:ty| $code:expr) => {
        reify!($old, |$new : $t| $code, || ())
    };
}