Support var.len

[katat]

It will be handy to directly obtain the length of an array variable.

For example

fn init_arr(const LEN: Field) -> [Field; LEN + 2] {
 return [0; LEN + 2];
}

fn main() {
  let arr = init_arr(10);
  assert_eq(arr.len, 12);

  // should also support being used to instantiate functions in mast phase
  let arr2 = init_arr(arr.len);
  assert_eq(arr2.len, 14);
}

[cyberbono3]

let me take it

[mimoo]

I think .len() would be better, or even a len() builtin function that works on any type :D

[cyberbono3]

I think .len() would be better, or even a len() builtin function that works on any type :D

I am adding new builtin len function into BUILTIN_FN_NAMES

I am debugging now this test to find out where assert_fn_eq gets invoked, but it looks like it is not called anywhere.

#[test]
fn test_array_len() {
    let code = r#"
        fn main(pub xx: Field) {
            let aa = xx + 1;
            assert_eq(aa, xx);
        }
    "#;

    if let Err(err) = mast_pass(code) {
        println!("err: {:?}", err);
    }

    assert_eq!(1, 2);
}