Macro downcast_integer

macro_rules! downcast_integer {
    ($($data_type:expr),+ => ($m:path $(, $args:tt)*), $($p:pat => $fallback:expr $(,)*)*) => { ... };
}

Given one or more expressions evaluating to an integer DataType invokes the provided macro m with the corresponding integer ArrowPrimitiveType, followed by any additional arguments

macro_rules! dictionary_key_size_helper {
  ($t:ty, $o:ty) => {
      std::mem::size_of::<<$t as ArrowPrimitiveType>::Native>() as $o
  };
}

fn dictionary_key_size(t: &DataType) -> u8 {
    match t {
        DataType::Dictionary(k, _) => downcast_integer! {
            k.as_ref() => (dictionary_key_size_helper, u8),
            _ => unreachable!(),
        },
        _ => u8::MAX,
    }
}

assert_eq!(dictionary_key_size(&DataType::Dictionary(Box::new(DataType::Int32), Box::new(DataType::Utf8))), 4);
assert_eq!(dictionary_key_size(&DataType::Dictionary(Box::new(DataType::Int64), Box::new(DataType::Utf8))), 8);
assert_eq!(dictionary_key_size(&DataType::Dictionary(Box::new(DataType::UInt16), Box::new(DataType::Utf8))), 2);