1 macro_rules! if_downcast_into {
2     ($in_ty:ty, $out_ty:ty, $val:ident, $body:expr) => ({
3         if std::any::TypeId::of::<$in_ty>() == std::any::TypeId::of::<$out_ty>() {
4             // Store the value in an `Option` so we can `take`
5             // it after casting to `&mut dyn Any`.
6             let mut slot = Some($val);
7             // Re-write the `$val` ident with the downcasted value.
8             let $val = (&mut slot as &mut dyn std::any::Any)
9                 .downcast_mut::<Option<$out_ty>>()
10                 .unwrap()
11                 .take()
12                 .unwrap();
13             // Run the $body in scope of the replaced val.
14             $body
15         }
16     })
17 }
18