Can we get multiplicity with attributes?

I'd like to define a macro version of tor-rpcbase's decl_make_cast_table!.

To do so, I need to take a list of traits as an argument. The logical way to do that would be something like this...

#[derive(Deftly)]
#[derive_deftly(HasCastTable_)]
#[deftly(trait="Trait1", trait="Trait2")]
pub struct MyStruct {

}

but as far as I can tell, there's no great way to do that. As it stands, I'm stuck using a weird macro-based workaround.