Shared via Rust Playground

playground.rs

trait GetSet<G,S> {
    fn get(&self) -> G;
    fn set(&self) -> S;
}

macro_rules! field {
    ($s:ident . $f:ident) => ($s.get().$f(&$s));
    ($s:ident . $f:ident = $v:expr) => ($s.set().$f(&mut $s, $v));
    
    (
        struct $s:ident: $m:ident {
            $(
                $n:ident: $t:ty =>
                    |$getprev:ident| $getbody:expr,
                    |$setprev:ident, $setnew:ident| $setbody:expr
            )*
        }
    ) => {
        #[allow(non_snake_case)] // concat_idents! can't come soon enough
        mod $m {
            struct Get;
            impl Get {
                $(
                    pub fn $n(self, s: &$s) -> $t {
                        let $getprev = s.$n;
                        $getbody
                    }
                ),*
            }
            struct Set;
            impl Set {
                $(
                    pub fn $n(self, s: &mut $s, $setnew: $t) {
                        let $setprev = s.$n;
                        s.$n = $setbody;
                    }
                ),*
            }
            pub struct $s {
                $(
                    pub $n: $t
                ),*
            }
            impl super::GetSet<Get, Set> for $s {
                fn get(&self) -> Get { Get }
                fn set(&self) -> Set { Set }
            }
        }
        use $m::$s;
    }
}

field! {
    struct A: MA {
        i: i32 =>
            |prev| { // just print the value as it is read
                println!("get i which is {}", prev);
                prev
            },
            |prev, new| { // clamp the value to a maximum of 42
                println!("set i which was {} to {}", prev, new);
                if new <= 42 { new } else { 42 }
            }
    }
}

impl A {
    fn new(i: i32) -> A {
        A { i: i }
    }
}

fn main() {
    let mut a = A::new(42);
    println!("{:?}", field!(a.i));
    field!(a.i = 43);
    println!("{:?}", field!(a.i));
    field!(a.i = 41);
    println!("{:?}", field!(a.i));
}