with_macro.rs

macro_rules! update {
    ($source:ident with { $($field:ident: $newValue:expr),* }) => {
        {
            let mut result = $source.clone();
            
            $(
                result.$field = $newValue;
            )*
            
            result
        }
    }
}

#[derive(Debug, Clone)]
struct Point {
    x: f32,
    y: f32
}

fn main() {
    let a = Point { x: 1.0, y: 1.0 };
    let b = update!(a with { y: 2.0 });
    let c = update!(b with { x: 3.0, y: -4.0 });
    
    println!("{:?}", a);
    println!("{:?}", b);
    println!("{:?}", c);
}