Modify nested UserData in Arc<Mutex<_>>/Rc<RefCell<_>>

[tkr-sh]

Following #546 (reply in thread)
Let's say that I have 2 depth of UserData like:

struct A { b: B }
struct B { c: String }

If you have a Arc<Mutex<A>> and that you want to modify the field c you would do something like:

a.b.c = "new string"

BUT, this actually won't work, because what is happening behind the scene is __indexing a with b, BUT you're __newindexing b with - therefore - the cloned b.
Therefore, it won't work. And even more than that, it will clone some unnecessary data that is not needed in this case.

Do any of you have a clean solution for this ?

---

Minimal example:

#[derive(Debug, Clone)]
struct A {
    b: B,
}
#[derive(Debug, Clone)]
struct B {
    c: String,
}

impl UserData for A {
    fn add_fields<F: mlua::UserDataFields<Self>>(fields: &mut F) {
        fields.add_field_method_get("b", |_, this| Ok(this.b.clone()));
    }
}
impl UserData for B {
    fn add_fields<F: mlua::UserDataFields<Self>>(fields: &mut F) {
        fields.add_field_method_get("c", |_, this| Ok(this.c.clone()));
        fields.add_field_method_set("c", |_, this, v: String| {
            this.c = v;
            Ok(())
        });
    }
}

let lua = mlua::Lua::new();
let a = Arc::new(Mutex::new(A {
    b: B { c: String::new() },
})); // `feature = ["userdata-wrappers"]`

lua.globals().set("a", a.clone())?;
lua.load("a.b.c = 'hello'").exec()?;
lua.load("assert(a.b.c == 'hello')").exec()?;