rust downcast

downcast-rs = “1.2”

// Can call macro via namespace since rust 1.30.

use downcast_rs::Downcast;
use std::fmt::Debug;
// To create a trait with downcasting methods, extend `Downcast` or `DowncastSync`
// and run `impl_downcast!()` on the trait.
trait Base<T: Clone>: Downcast { type H: Copy; }

downcast_rs::impl_downcast!(Base<T> assoc H where T: Clone, H: Copy);
// or: impl_downcast!(concrete Base<u32> assoc H=f32)

// Concrete types implementing Base.
#[derive(Debug)]
struct Foo(u32);

impl Base<u32> for Foo { type H = f32; }
#[derive(Debug)]
struct Bar(f64);
impl Base<u32> for Bar { type H = f32; }

fn main() {
    // Create a trait object.
    let mut base: Box<dyn Base<u32, H=f32>> = Box::new(Bar(42.0));

    // Try sequential downcasts.
    if let Some(x) = base.downcast_ref::<Foo>() {
        println!("111111: {:?}", x);
        // assert_eq!(foo.0, 42);
    } else if let Some(y) = base.downcast_ref::<Bar>() {
        println!("22222: {:?}", y);
        // assert_eq!(bar.0, 42.0);
    }
    assert!(base.is::<Bar>());
}