use std::ops::{Deref, DerefMut};

struct System1 {

}

impl System1 {
    fn mut_it(&mut self) { }
}

struct System2 {

}

impl System2 {
    fn mut_it(&mut self) { }
}

struct Context<'a> {
    s1: Option<&'a mut System1>,
    s2: Option<&'a mut System2>
}

struct ContextRef<'a> {
    u: Context<'a>,
}

impl<'a> Deref for ContextRef<'a> {
    type Target = Context<'a>;

    fn deref(&self) -> &Self::Target { &self.u }
}

impl<'a> DerefMut for ContextRef<'a> {
    fn deref_mut(&mut self) -> &mut Self::Target { &mut self.u }
}

impl<'a> Context<'a> {
    fn s1<'b>(&'b mut self) -> (&'b mut System1, impl DerefMut<Target=Context> + 'b) { (self.s1.as_mut().unwrap(), ContextRef { u: Context::<'b> { s1: None, s2: if self.s2.is_none() { None } else { Some(self.s2.as_mut().unwrap()) } } }) }
    fn s2<'b>(&'b mut self) -> (&'b mut System2, ContextRef<'b>) { (self.s2.as_mut().unwrap(), ContextRef { u: Context::<'b> { s1: if self.s1.is_none() { None } else { Some(self.s1.as_mut().unwrap()) }, s2: None } }) }
}

fn action<'a: 'b, 'b>(c: &'b mut Context<'a>) {
    let (a, mut d) = c.s1();
    let (b, _) = d.s2();
    a.mut_it();
    b.mut_it();
}

fn main() {
    action(&mut Context { s1: Some(&mut System1 { }), s2: Some(&mut System2 { }) })
}