so.rs

use std::ops::Add;

#[derive(Debug)]
pub enum Expr<'a> {
    Constant(f32),
    //       ^^^--this is a simplified version. In the real version,
    //            Constant holds large matrices and I would like to 
    //            avoid copying it. 
    Add(&'a Node<'a>, &'a Node<'a>),
}

#[derive(Debug)]
pub struct Node<'a> {
    pub body: Expr<'a>
}

impl<'a> Add for &'a Node<'a> {
    type Output = Node<'a>;
    fn add(self, other: &'a Node<'a>) -> Node<'a> {
        Node { body: Expr::Add(self, other) }
    }
}

fn main() {
    let a = Node { body: Expr::Constant(1.) };
    let f: Node = &(&a + &a) + &a;
    println!("f: {:#?}", f);
}