Is it possible to create a macro which counts the number of expanded items?

Question

Is it possible to create a macro which counts the number of expanded items?

macro_rules! count {
    ($($name:ident),*) => {
        pub enum Count {
           $(
               $name = 1 << $i // $i is the current expansion index
            ),*
        }
    }
}

count!(A, B, C);

Answer 1

Here is a macro that counts the number of matched items:

macro_rules! count_items {
    ($name:ident) => { 1 };
    ($first:ident, $($rest:ident),*) => {
        1 + count_items!($($rest),*)
    }
}

fn main() {
    const X: usize = count_items!(a);
    const Y: usize = count_items!(a, b);
    const Z: usize = count_items!(a, b, c);
    assert_eq!(1, X);
    assert_eq!(2, Y);
    assert_eq!(3, Z);
}

Note that the counting is computed at compile time.

---

For your example, you can do it using accumulation:

macro_rules! count {
    ($first:ident, $($rest:ident),*) => (
        count!($($rest),+ ; 0; $first = 0)
    );
    ($cur:ident, $($rest:ident),* ; $last_index: expr ; $($var:ident = $index:expr)+) => (
        count!($($rest),* ; $last_index + 1; $($var = $index)* $cur = $last_index + 1)
    );
    ($cur:ident; $last_index:expr ; $($var:ident = $index:expr)+) => (
        #[repr(C)]
        enum Count {
            $($var = 1 << $index),*,
            $cur = 1 << ($last_index + 1),
        }
    );
}

pub fn main() {
    count!(A, B, C, D);
    assert_eq!(1, Count::A as usize);
    assert_eq!(2, Count::B as usize);
    assert_eq!(4, Count::C as usize);
    assert_eq!(8, Count::D as usize);
}