for .. { .. } else { .. } macro for Rust

playground.rs

macro_rules! for_expr {
    ( $lhs:pat in $iter:ident $body:block else $else:block ) => {
        for_expr!($lhs in ($iter) $body else $else)
    };
    ( $lhs:pat in ($iter:expr) $body:block else $else:block ) => {{
        let mut iter = ::core::iter::IntoIterator::into_iter($iter);
        if let Some(mut item) = iter.next() {
            loop {
                let $lhs = item;
                let result = $body;
                item = if let Some(i) = iter.next() {
                    i
                } else {
                    break result;
                };
            }
        } else $else
    }}
}

fn main() {
    // let values: &[i32] = &[2, 3, 5];
    let values: &[i32] = &[];
    let result = for_expr!(num in values {
        num * 3
    } else {
        -1
    });
    println!("{}", result);
}