Allow checking whether try_from would succeed without consuming the value

[CasBex]

I have a use case where I want to check whether something is a variant of a subenum like below, but keep ownership of the value if it is not. Below code will not compile because the try_from moves a.

use subenum::subenum;
#[subenum(BB)]
#[derive(Debug)]
enum T {
    A,
    #[subenum(BB)]
    B,
}

fn main() {
    let a = T::A;
    if let Ok(b) = BB::try_from(a) {
        println!("It's a BB {:?}", b);
    } else {
        println!("It's not a BB {:?}", a);
    }
}

Could you add support for something like below?

use subenum::subenum;
#[subenum(BB)]
#[derive(Debug)]
enum T {
    A,
    #[subenum(BB)]
    B,
}

// automatically generate this
impl BB {
    fn isvariant(other: &T) -> bool {
        match other {
            T::B => true,
            _ => false,
        }
    }
}

fn main() {
    let a = T::A;
    if BB::isvariant(&a) {
        println!("It's a BB {:?}", BB::try_from(a).unwrap());
    } else {
        println!("It's not a BB {:?}", a);
    }
}

[max-kamps]

Yes, I could also really use this. An alternative way of implementing this would be having the TryFrom::ChildConvertError contain the parent value that failed to convert. Then you could write code like this:

match Child::try_from(parent) {
    Ok(child) => println!("Was a child! {child:?}"),
    Err(ChildConvertError(parent)) => println!("Wasn't a child, but {parent:?}")
}