Cond Rust macro with likely/unlikely branch optimizations

likely_unlikely_cond.rs

macro_rules! cold {
    ($expr:expr) => {{
        #[cold]
        #[inline(always)]
        fn __cold() {}
        
        __cold();
        $expr
    }}
}

macro_rules! cond {
    (unlikely default => $body:expr $(,)?) => {
        cold!($body)
    };
    (default => $body:expr $(,)?) => {
        $body
    };
    (unlikely $cond:expr => $body:expr , $($else:tt)*) => {
        if $cond { cold!($body) } else { cond!($($else)*) }
    };
    (likely $cond:expr => $body:expr , $($else:tt)*) => {
        if $cond { $body } else { cold!(cond!($($else)*)) }
    };
    ($cond:expr => $body:expr , $($else:tt)*) => {
        if $cond { $body } else { cond!($($else)*) }
    };
}

fn main() {
    let x = cond! {
        likely 3 == 3 => {
            3 + 3
        },
        unlikely 2 == 1 => 2,
        unlikely default => 1,
    };
    println!("{x}");
}