Code shared from the Rust Playground

playground.rs

#[derive(Debug)]
struct UiState {
    parent: u64,
    id: u64,
}

trait Ui {
    fn state(&mut self) -> &mut UiState;
    
    #[must_use]
    fn horizontal(&mut self) -> UiContext {
        let state = self.state();
        let child = UiState{parent: state.id, id: state.id + 1};
        let mut ctx = UiContext{ui: child, parent: state, result: Result{rect: 0}};
        ctx.enter();
        ctx
    }
}

#[derive(Debug)]
struct Result {
    rect: u64,
}

#[derive(Debug)]
struct UiContext<'a> {
    ui: UiState,
    parent: &'a UiState,
    result: Result,
}

impl Ui for UiState {
    fn state(&mut self) -> &mut UiState { self }
}

impl Ui for UiContext<'_> {
    fn state(&mut self) -> &mut UiState { &mut self.ui }
}

impl UiContext<'_> {
    fn enter(&mut self) {
        println!("enter context {:?}", self);
    }
    
    fn exit(&mut self) {
        println!("exit context {:?}", self);
    }
}

impl Drop for UiContext<'_> {
    fn drop(&mut self) {
        println!("dropping context {:?}", self);
        self.exit();
    }
}

fn main() {
    let mut root = UiState{parent: 0, id: 0};
    println!("root {:?}", root);
    {
        let mut ui = root.horizontal();
        println!("ui 1 {:?}", ui);
        {
            let mut ui = ui.horizontal();
            println!("ui 2 {:?}", ui);
        }
    }
}