macro_rules! cfg_prefix {
    (
        #[cfg($($cond:tt)*)]($($pos:tt)*)
        #[cfg(else)]($($neg:tt)*)
        $($rest:tt)*
    ) => {
        #[cfg($($cond)*)]
        $($pos)* $($rest)*

        #[cfg(not($($cond)*))]
        $($neg)* $($rest)*
    }
}

pub struct MyStruct<T, #[cfg(test)] U>(Option<T>, #[cfg(test)] Option<U>);

cfg_prefix! {
    #[cfg(test)](impl<T, U> MyStruct<T, U>)
    #[cfg(else)](impl<T> MyStruct<T>)
    {
        pub fn new() -> Self {
            #[cfg(test)]
            return MyStruct(None, None);

            #[cfg(not(test))]
            return MyStruct(None);
        }
    }
}

#[test]
fn test() {
    <MyStruct<i32, i32>>::new();
    panic!("In a test");
}

#[cfg(not(test))]
pub fn main() {
    <MyStruct<i32>>::new();
    panic!("Not in a test");
}