Revamped selection API

selection.rs

struct GlobalState {
    name: String,
    age: i32,
    user: User,
}

struct User {
    name: String,
    age: i32,
    config: Config,
}

struct Config {
    friends: Vec<String>,
}

struct ProjectedGlobalState {
    age: Projected<i32>,
    name: Projected<String>,
    user: ProjectedUser,
}
struct ProjectedUser {
    age: Projected<i32>,
    name: Projected<String>,
    config: ProjectedConfig,
}

struct ProjectedConfig {
    friends: Projected<Vec<String>>,
}

struct Projected<T> {
    _phantom: std::marker::PhantomData<T>,
}

impl<T> Clone for Projected<T> {
    fn clone(&self) -> Self {
        unimplemented!()
    }
}
impl<T> Copy for Projected<T> {}

fn use_selector<T>(f: impl FnOnce(ProjectedGlobalState) -> Projected<T>) -> T {
    todo!()
}

fn main() {
    let age = use_selector(|state| state.user.config.friends);
}