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/primitives/npos-elections/src/lib.rs

@@ -74,17 +74,16 @@
 
 #![cfg_attr(not(feature = "std"), no_std)]
 
+use codec::{Decode, Encode, MaxEncodedLen};
 use scale_info::TypeInfo;
+#[cfg(feature = "std")]
+use serde::{Deserialize, Serialize};
 use sp_arithmetic::{traits::Zero, Normalizable, PerThing, Rational128, ThresholdOrd};
-use sp_core::RuntimeDebug;
+use sp_core::{bounded::BoundedVec, RuntimeDebug};
 use sp_std::{
 	cell::RefCell, cmp::Ordering, collections::btree_map::BTreeMap, prelude::*, rc::Rc, vec,
 };
 
-use codec::{Decode, Encode, MaxEncodedLen};
-#[cfg(feature = "std")]
-use serde::{Deserialize, Serialize};
-
 #[cfg(test)]
 mod mock;
 #[cfg(test)]
@@ -451,6 +450,11 @@ impl<AccountId> Default for Support<AccountId> {
 /// The main advantage of this is that it is encodable.
 pub type Supports<A> = Vec<(A, Support<A>)>;
 
+/// Same as `Supports` bounded by `MaxWinners`.
+///
+/// To note, the inner `Support` is still unbounded.
+pub type BoundedSupports<A, MaxWinners> = BoundedVec<(A, Support<A>), MaxWinners>;
+
 /// Linkage from a winner to their [`Support`].
 ///
 /// This is more helpful than a normal [`Supports`] as it allows faster error checking.