How to conditionally include an extrinsic with a rustc feature?

Question

Hey Substrate Community,

is there a way to annotate a extrinsic call function with the Rust cfg condition compile execution?

#[cfg_attr(feature = "with-global-disputes", pallet::weight(T::WeightInfo::start_global_dispute(CacheSize::get())))]
#[cfg_attr(feature = "with-global-disputes", transactional)]
#[cfg(feature = "with-global-disputes")]
pub fn start_global_dispute(origin: OriginFor<T>, #[pallet::compact] market_id: MarketIdOf<T>) -> DispatchResultWithPostInfo {
   ...
}

This produces an error:

  error: Invalid pallet::call, requires weight attribute i.e. `#[pallet::weight($expr)]`
     --> /Users/tesst/dev/random/test-project.rs
      |
  963 |         pub fn start_global_dispute(origin: OriginFor<T>, #[pallet::compact] market_id: MarketIdOf<T>) -> DispatchResultWithPostInfo {

Answer 1

This is not currently possible, and something I might not encourage anyway due to the intended deterministic nature of the runtime.

You can see here we have had a similar need, and there is note that this is not handled yet:

https://github.com/paritytech/substrate/blob/9476d2157a0d266677dee144a8a7bc44a6b821e1/frame/system/src/lib.rs#L371

If this is a feature you think is needed, please open an issue on Substrate, and describe how you would like to use this feature.

Answer 2

You can use a config variable like EnableFeatureX and then require that to be true in the Extrinsic.
This should be a safer way of archiving what you want since you are not reliant on rust features, which must be handled carefully to fulfill their objective.
The extrinsic ordering also stays the same in this approach, allowing you to re-use metadata definitions.