trait WithContext {
    fn with_context(&mut self, f: &mut FnMut(&mut Context<'_>));

    fn with_context_once<'b>(&mut self, f: Box<dyn 'b + FnOnce(&mut Context<'_>)>);
}

impl<'a> WithContext for Context<'a> {
    fn with_context(&mut self, f: &mut FnMut(&mut Context<'_>)) {
        f(self)
    }

    fn with_context_once<'b>(&mut self, f: Box<dyn 'b + FnOnce(&mut Context<'_>)>) {
        f(self)
    }
}

pub struct Context<'a> {
    parent: Option<&'a mut dyn WithContext>
}

impl<'a> Context<'a> {
    fn child(&mut self) -> Context<'_> {
        Context {
            parent: Some(self),
        }
    }

    pub fn with_parent<F: FnMut(&mut Context<'_>)>(&mut self, mut f: F) {
        if let Some(parent) = self.parent.as_mut() {
            parent.with_context(&mut f)
        }
    }

    pub fn with_parent_once<F: FnOnce(&mut Context<'_>)>(&mut self, f: F) {
        if let Some(parent) = self.parent.as_mut() {
            parent.with_context_once(Box::new(f))
        }
    }
}

fn this_doesnt_work(context: &mut Context) {
    let mut child = context.child();

    let mut x : u32 = 0;

    child.with_parent(|parent| {
        x = 0;
    });

    let x = x;
}