There is no way to set/unset a Flag without losing its span information

[LikeLakers2]

Hi! I was doing some derive-macro practice over at https://github.com/LikeLakers2/michis_rand_distr_derive.

In the midst of this, I found myself wanting to set a Flag value - but found there was no way to do so without losing its span information.

In my specific case, I have two options that can be applied to enum variants - a weight (how likely that variant is to be selected) and a skip option (if specified, the variant will never be selected). What I wanted to do was, if the weight was a zero-like value, erase the weight value and enable the skip flag. Unfortunately, I found there was no way to do this with Flag alone.

I did find a solution, SpannedValue<Flag>, which allows me to recreate the Flag value without (technically) losing the span information. However, this feels hacky, as now I'm storing the Span twice - and more importantly, I am forced to add #[darling(default)] to that field, because SpannedValue<Flag> does not inherit Flag's property of being inherently optional.

I'm hoping something can be done to handle this.

(P.S. If you're curious about the relevant code: https://github.com/LikeLakers2/michis_rand_distr_derive/blob/98200c0ed81ddace432787e4fe4214db6085049e/src/standard_distribution/derive_variant.rs#L19-L27)