#[derive(Debug)]
enum TestEnum {
    Empty,
    Single(isize),
    Pair(isize, char),
    Struct{first: char, second: char},
}

macro_rules! assert_variant {
    ($value:expr, $pattern:pat) => ({
        let value = &$value;

        if let $pattern = value {} else {
            panic!(r#"assertion failed (value doesn't match pattern):
   value: `{:?}`,
 pattern: `{}`"#, value, stringify!($pattern))
        }
    })

    // TODO: Additional patterns for trailing args, like assert and assert_eq
}

fn main() {
    let value = TestEnum::Pair(10, 'a');

    assert_variant!(value, TestEnum::Pair(..));
    assert_variant!(value, TestEnum::Pair(10, _));

    // This one fails
    assert_variant!(value, TestEnum::Struct{..});
}