I recently got quite stuck on a bug in my project which boils down to this example:

use std::any::Any;

#[derive(Debug)]
pub struct Foo(pub u32);

fn boxit<T: 'static>(x: T) -> Box<dyn Any> {
    Box::new(x)
}

fn unboxit<T: 'static>(b: Box<dyn Any>) -> T {
    *b.downcast::<T>().unwrap()
}

fn main() {
    let b = boxit(Foo);
    let x: Foo = unboxit(b);
    println!("{x:?}");
}

The footgun here is that once Foo was a ZST type and the program worked, but then someone added a field to it, and while the code kept compiling the semantics changed. Suddenly Foo in boxit(Foo) was a function fn() -> Foo and main panicked.

What is the right strategy to avoid this problem?

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