[Pre-RFC] assert macros simplified

libs
9

You can have ($left:tt == $right:expr), which matches 5 == 2 + 3.

Of course it won’t match the other way, 2 + 3 == 5, since expressions can contain ==, so the parser has no way of knowing when to stop parsing expr and when to parse == in the macro. But instead of an error, there can be a generic fallback in such case:

macro_rules! assert2 {
    ($left:tt == $right:expr) => {
        match (&$left, &$right) {
            (left_val, right_val) => if !(*left_val == *right_val) {
                panic!("{0} == {1} was false\n\t{0} = {2:?}, {1} = {3:?}",
                       stringify!($left), stringify!($right),
                       left_val, right_val);
            }
        }
    };
    ($right:expr) => {assert!($right)};
}

fn main() {
    assert2!(5 == 2 + 3);
    assert2!(2 + 3 == 5);
}