use std::ops::Deref;

enum Foo {
    A(i32),
    B(i32),
    C(i32),
}

struct SBox<T> {
    val: T,
}

impl<T> SBox<T> {
    pub fn new(val: T) -> Self {
        SBox { val }
    }
}

impl<T> Deref for SBox<T> {
    type Target = T;
    fn deref(&self) -> &T {
        &self.val
    }
}

fn foo(foo: SBox<Box<Foo>>) -> SBox<Box<Foo>> {
    match **foo {
        Foo::A(ref f) => {
            let f = SBox::new(Box::new(Foo::B(-f)));
            f
        }
        _ => foo,
    }
}

fn main() {
    let mut a = SBox::new(Box::new(Foo::A(5)));
    a = foo(a);
}