Variadic generics design sketch

language design
69

It could, but it doesn't scale well to multiple #[cfg] cases.

#[cfg(all(not(feature = "first"), not(feature = "second")))]
type MySoA = VariadicSoa<
    TypeA,
    TypeB,
    TypeC,
    TypeD,
>;

#[cfg(all(feature = "first", not(feature = "second")))]
type MySoA = VariadicSoa<
    TypeA,
    TypeB,
    TypeC,
    TypeD,
    TypeE,
>;

#[cfg(all(not(feature = "first"), feature = "second"))]
type MySoA = VariadicSoa<
    TypeA,
    TypeB,
    TypeC,
    TypeD,
    TypeF,
>;

#[cfg(all(feature = "first", feature = "second"))]
type MySoA = VariadicSoa<
    TypeA,
    TypeB,
    TypeC,
    TypeD,
    TypeE,
    TypeF,
>;

It would certainly preferable to be able to do

type MySoA = VariadicSoa<
    TypeA,
    TypeB,
    TypeC,
    TypeD,
    #[cfg(feature = "first")] TypeE,
    #[cfg(feature = "second")] TypeF,
>;

This can already be done with non-variadic types, I just want to make sure it's accounted for in the grammar for variadics as well. There are a handful of lexical positions where #[cfg] isn't allowed largely out of oversight, and probably should be permitted -- RFC 3399 is one such example.