A macro for cached computed fields in rust. Generates a function that checks the cached value of a field, and computes it if it's is not present.

cached_field.rs

use std::fmt::Debug;

#[derive(Debug)]
struct S {
    x: Option<u32>,
    y: u32,
}

/// Generates a funtion named $field the returns the existing value for
/// $field in self if it's not None, and otherwise computes it in
/// self.$field (of type Option<$typ>)
macro_rules! cached_prop {
    ( $field:ident, $typ:ty, $compute_field:expr ) => {
        fn $field(&mut self) -> $typ {
            if let Some(value) = self.$field {value}
            else {
                let result = ($compute_field)(self);
                self.$field = Some(result);
                result
            }
        }
    };
}

impl S {
    fn compute_x(&self) -> u32 {
        self.y * 2
    }
    cached_prop!{x, u32, S::compute_x}
}

fn main() {
    let mut s = S{x:None, y:9};
    dbg!(&s);
    dbg!(&s.x());
    dbg!(&s);
    // This prints:
    // s = S {x: None, y: 9,}
    // s.x() = 18
    // s = S {x: Some(18), y: 9,}
}