Rust macros: counting with an accumulator

countmacro.rs

// cargo-deps: lazy_static

#[macro_use] extern crate lazy_static;

macro_rules! declare_array {
    // INTERNAL

    // last element (proceed to output)
    (@parse $size:expr, ($val:expr) -> [$($accs:expr),*] $thru:tt) => {
        declare_array!(@output $size + 1usize, [$($accs,)* $val] $thru);
    };
    // more elements (keep parsing)
    (@parse $size:expr, ($val:expr, $($vals:expr),*) -> [$($accs:expr),*] $thru:tt) => {
        declare_array!(@parse $size + 1usize, ($($vals),*) -> [$($accs,)* $val] $thru);
    };
    
    // output a local variable
    (@output $size:expr, $acc:tt (() let $n:ident $t:ty)) => {
        declare_array!(@as_stmt let $n: [$t; $size] = $acc)
    };
    // output a lazy static
    (@output $size:expr, $acc:tt (($($p:tt)*) lazy_static $n:ident $t:ty)) => {
        lazy_static!{ $($p)* static ref $n: [$t; $size] = $acc; }
    };
    // output a static or const item
    (@output $size:expr, $acc:tt (($($p:tt)*) $s:ident $n:ident $t:ty)) => {
        declare_array!(@as_item $($p)* $s $n: [$t; $size] = $acc;);
    };
    
    // screw you, lexer
    (@as_stmt $s:stmt) => ($s);
    (@as_item $i:item) => ($i);
    
    // EXTERNAL
    
    // entry point
    (pub $restr:tt $storage:ident $n:ident: [$t:ty; _] = [$($vals:expr),* $(,)*]) => {
        declare_array!(@parse 0usize, ($($vals),*) -> [] ((pub $restr) $storage $n $t));
    };
    (pub $storage:ident $n:ident: [$t:ty; _] = [$($vals:expr),* $(,)*]) => {
        declare_array!(@parse 0usize, ($($vals),*) -> [] ((pub) $storage $n $t));
    };
    ($storage:ident $n:ident: [$t:ty; _] = [$($vals:expr),* $(,)*]) => {
        declare_array!(@parse 0usize, ($($vals),*) -> [] (() $storage $n $t));
    };
}

use std::time::{SystemTime, UNIX_EPOCH};

declare_array!(const ARR: [i32; _] = [1, 2, 3]);
declare_array!(lazy_static LAZY: [u64; _] = [3, 2, SystemTime::now().duration_since(UNIX_EPOCH).unwrap().as_secs()]);

fn main() {
    declare_array!(let arr: [i32; _] = [4, 5, 6]);
    declare_array!(static LOCAL_ARR: [i32; _] = [7, 8, 9, 10]);
    println!("{:?}", ARR);
    println!("{:?}", arr);
    println!("{:?}", LOCAL_ARR);
    println!("{:?}", *LAZY);
}