[FRAME] Make core-fellowship ans salary work for swapped members (#3156)

Fixup for https://github.com/paritytech/polkadot-sdk/pull/2587 to make
the `core-fellowship` crate work with swapped members.

Adds a `MemberSwappedHandler` to the `ranked-collective` pallet that are
implemented by `core-fellowship+salary`.
There is are exhaustive tests
[here](https://github.com/paritytech/polkadot-sdk/blob/72aa7ac17a0e5b16faab5d2992aa2db2e01b05d0/substrate/frame/core-fellowship/src/tests/integration.rs#L338)
and
[here](https://github.com/paritytech/polkadot-sdk/blob/ab3cdb05a5ebc1ff841f8dda67edef0ea40bbba5/substrate/frame/salary/src/tests/integration.rs#L224)
to check that adding member `1` is equivalent to adding member `0` and
then swapping.

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

substrate/frame/ranked-collective/src/lib.rs

@@ -54,7 +54,10 @@ use sp_std::{marker::PhantomData, prelude::*};
 use frame_support::{
 	dispatch::{DispatchResultWithPostInfo, PostDispatchInfo},
 	ensure, impl_ensure_origin_with_arg_ignoring_arg,
-	traits::{EnsureOrigin, EnsureOriginWithArg, PollStatus, Polling, RankedMembers, VoteTally},
+	traits::{
+		EnsureOrigin, EnsureOriginWithArg, PollStatus, Polling, RankedMembers,
+		RankedMembersSwapHandler, VoteTally,
+	},
 	CloneNoBound, EqNoBound, PartialEqNoBound, RuntimeDebugNoBound,
 };
 
@@ -365,6 +368,13 @@ impl_ensure_origin_with_arg_ignoring_arg! {
 	{}
 }
 
+/// Helper functions to setup benchmarking.
+#[impl_trait_for_tuples::impl_for_tuples(8)]
+pub trait BenchmarkSetup<AccountId> {
+	/// Ensure that this member is registered correctly.
+	fn ensure_member(acc: &AccountId);
+}
+
 #[frame_support::pallet]
 pub mod pallet {
 	use super::*;
@@ -402,11 +412,21 @@ pub mod pallet {
 		/// "a rank of at least the poll class".
 		type MinRankOfClass: Convert<ClassOf<Self, I>, Rank>;
 
+		/// An external handler that will be notified when two members are swapped.
+		type MemberSwappedHandler: RankedMembersSwapHandler<
+			<Pallet<Self, I> as RankedMembers>::AccountId,
+			<Pallet<Self, I> as RankedMembers>::Rank,
+		>;
+
 		/// Convert a rank_delta into a number of votes the rank gets.
 		///
 		/// Rank_delta is defined as the number of ranks above the minimum required to take part
 		/// in the poll.
 		type VoteWeight: Convert<Rank, Votes>;
+
+		/// Setup a member for benchmarking.
+		#[cfg(feature = "runtime-benchmarks")]
+		type BenchmarkSetup: BenchmarkSetup<Self::AccountId>;
 	}
 
 	/// The number of members in the collective who have at least the rank according to the index
@@ -679,7 +699,12 @@ pub mod pallet {
 			Self::do_remove_member_from_rank(&who, rank)?;
 			Self::do_add_member_to_rank(new_who.clone(), rank, false)?;
 
-			Self::deposit_event(Event::MemberExchanged { who, new_who });
+			Self::deposit_event(Event::MemberExchanged {
+				who: who.clone(),
+				new_who: new_who.clone(),
+			});
+			T::MemberSwappedHandler::swapped(&who, &new_who, rank);
+
 			Ok(())
 		}
 	}