Feature Request: Union Account Loaders

[mgild]

If I am versioning an account type in an action, it would be great to support something like

#[derive(Accounts)]
pub struct ExampleAction<'info> {
    pub union: UnionAccountLoader<'info, AccountDataV1, AccountDataV2>,
}

This could be extended to support optional accounts as well

#[derive(Accounts)]
pub struct ExampleAction<'info> {
    pub optional: UnionAccountLoader<'info, OptionalAccount, None>,
}

Perhaps None could be a special case since it is not a type, but then load and load_mut could also take the type you are trying to load as well.

[Henry-E]

I'm not sure how feasible it is for us to support a list of possible generic types for accounts. However, there is an optional account feature coming to anchor that has syntax more like Option<UnionAccountLoader<'info, OptionalAccount>>. Check out #2101