1macro_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