bounding staking: BoundedElectionProvider trait (#12362)

* add a bounded election provider trait

* extract common trait election provider base

* fmt

* only bound the outer support vector

* fix tests

* docs

* fix rust docs

* fmt

* fix rustdocs

* docs

* improve docs

* small doc change

substrate/frame/fast-unstake/src/lib.rs

@@ -80,7 +80,7 @@ macro_rules! log {
 pub mod pallet {
 	use super::*;
 	use crate::types::*;
-	use frame_election_provider_support::ElectionProvider;
+	use frame_election_provider_support::ElectionProviderBase;
 	use frame_support::{
 		pallet_prelude::*,
 		traits::{Defensive, ReservableCurrency},
@@ -330,7 +330,8 @@ pub mod pallet {
 				}
 			}
 
-			if <T as pallet_staking::Config>::ElectionProvider::ongoing() {
+			if <<T as pallet_staking::Config>::ElectionProvider as ElectionProviderBase>::ongoing()
+			{
 				// NOTE: we assume `ongoing` does not consume any weight.
 				// there is an ongoing election -- we better not do anything. Imagine someone is not
 				// exposed anywhere in the last era, and the snapshot for the election is already

substrate/frame/fast-unstake/src/mock.rs

@@ -104,7 +104,7 @@ parameter_types! {
 }
 
 pub struct MockElection;
-impl frame_election_provider_support::ElectionProvider for MockElection {
+impl frame_election_provider_support::ElectionProviderBase for MockElection {
 	type AccountId = AccountId;
 	type BlockNumber = BlockNumber;
 	type DataProvider = Staking;
@@ -113,7 +113,9 @@ impl frame_election_provider_support::ElectionProvider for MockElection {
 	fn ongoing() -> bool {
 		Ongoing::get()
 	}
+}
 
+impl frame_election_provider_support::ElectionProvider for MockElection {
 	fn elect() -> Result<frame_election_provider_support::Supports<AccountId>, Self::Error> {
 		Err(())
 	}