Cast a Rust enum to a sub-enum

Question

I'm creating subsets of std::sync::atomic::Ordering:

use std::sync::atomic::Ordering;

pub enum StoreOrdering {
    Relaxed,
    Release,
    SeqCst
}
impl Into<Ordering> for StoreOrdering {
    fn into(self) -> Ordering {
        match self {
            Self::Relaxed => Ordering::Relaxed,
            Self::Release => Ordering::Release,
            Self::SeqCst  => Ordering::SeqCst
        }
    }
}
impl std::convert::TryFrom<Ordering> for StoreOrdering {
    type Error = (); // HACK
    fn try_from(ord: Ordering) -> Result<Self, Self::Error> {
        match ord {
            Ordering::Relaxed => Ok(Self::Relaxed),
            Ordering::Release => Ok(Self::Release),
            Ordering::SeqCst  => Ok(Self::SeqCst),
            _ => Err(())
        }
    }
}

enum LoadOrdering {
    Acquire,
    Relaxed,
    SeqCst
}
// ???

As you can see, now I need to write those two impls with matches again for StoreOrdering <-> LoadOrdering and maybe even for StoreOrdering <-> LoadOrdering - as well as for any enum subset. How to avoid such boilerplate?

Answer 1

Rust doesn't support duck typing like C++ does with templates. The only functionality that generics can access is determined by the trait bounds.

So any duck-type-like behaviour must be done with macros.

For this, you could use the given macro below. It only works for simple C-style macros. It creates the enum and auto-generates the conversions to the given super-enum.

use std::sync::atomic::Ordering;
use std::convert::TryInto;

// Create the store ordering
sub_enum!(StoreOrdering of Ordering {
    Relaxed,
    Release,
    SeqCst
});

// Create the load ordering
sub_enum!(LoadOrdering of Ordering {
    Acquire,
    Relaxed,
    SeqCst
});

#[macro_export]
macro_rules! sub_enum {
    ($sub_enum_name:ident of $super_enum_name:ty {
        $($variant:ident),* $(,)?
    }) => {
        pub enum $sub_enum_name {
            $($variant,)*
        }
        
        impl From<$sub_enum_name> for $super_enum_name {
            fn from(val: $sub_enum_name) -> $super_enum_name {
                match val {
                    $(<$sub_enum_name>::$variant => <$super_enum_name>::$variant,)*
                }
            }
        }
        
        impl std::convert::TryFrom<$super_enum_name> for $sub_enum_name {
            type Error = ();
            fn try_from(val: $super_enum_name) -> Result<Self, Self::Error> {
                match val {
                    $(<$super_enum_name>::$variant => Ok(Self::$variant),)*
                    _ => Err(())
                }
            }
        }
    }
}

fn main() {
    let store = StoreOrdering::SeqCst;
    let general: Ordering = store.into();
    let load: LoadOrdering = general.try_into().unwrap();
}

Playground link

A lot could be improved still, of course. However, this should do for your problem right now.