test-box-vs-custom.rs

use std::ops::DerefMut;
use std::ops::Deref;

struct Foo {
    a: i32,
    b: bool,
}
fn foo(_: &mut i32, _: bool) {}

struct Bar {
    f: Foo,
}

impl Deref for Bar {
    type Target = Foo;
    fn deref(&self) -> &Self::Target {
        &self.f
    }
}

impl DerefMut for Bar {
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.f
    }
}

fn main() {
    let f = Foo { a: 42, b: true };

    // let mut zed = Bar { f };
    let mut zed = std::boxed::Box::new(f);

    let xyz = &mut zed.a;
    foo(xyz, zed.b);
}