Rust with! macro, aka tap, methods cascade, method chaining…

with.rs

// Sources:
// * https://internals.rust-lang.org/t/method-chaining-special-form/342
// * https://github.com/rust-lang/rust/issues/6679
// * https://gist.github.com/huonw/2812cd9433dc5f0cf7f2
// * https://www.reddit.com/r/rust/comments/2wssch/function_chaining/
//
// TODO:
// * How to correctly process generic methods (e.g. collect::<Vec<_>>() ?)

macro_rules! with {
    (
        $subj:expr;
        $(
            $(
                .$meth:ident($( $arg:expr ),*)
            )+
        ),+
        $(,)*  // permit trailing commas
    ) => {{
        let mut subj = $subj;
        $(
            subj$( .$meth($($arg),*) )+;
        )+
        subj
    }};
}


#[cfg(test)]
mod tests {

    #[test]
    fn with() {
        let actual = with!(vec![1, 2]; .push(4), .push(8));

        assert_eq!(actual, vec![1, 2, 4, 8])
    }

    #[test]
    fn with_trailing_comma() {
        let actual = with!(vec![1, 2]; .push(4), .push(8),);

        assert_eq!(actual, vec![1, 2, 4, 8])
    }

    #[test]
    fn with_method_calls() {
        let actual = with! { vec!["hi".to_string()];
            .push("there".into()),
            .get_mut(1).map(|v| v.push('?')),
        };
        assert_eq!(actual, vec!["hi".to_string(), "there?".to_string()])
    }
}