Enforce MaxEncodedLen impl for NposSolution (#11103)

* Fail if `MaxVoters` too small

* Fixing benchmarking test, better naming of error

* reverting accidental change

* use fully qualified syntax
no need to interate to calculate len

* Fail directly if too many voters

substrate/frame/election-provider-support/solution-type/src/single_page.rs

@@ -124,6 +124,11 @@ pub(crate) fn generate(def: crate::SolutionDef) -> Result<TokenStream2> {
 					for<'r> FV: Fn(&'r A) -> Option<Self::VoterIndex>,
 					for<'r> FT: Fn(&'r A) -> Option<Self::TargetIndex>,
 			{
+				// Make sure that the voter bound is binding.
+				// `assignments.len()` actually represents the number of voters
+				if assignments.len() as u32 > <#max_voters as _feps::Get<u32>>::get() {
+					return Err(_feps::Error::TooManyVoters);
+				}
 				let mut #struct_name: #ident = Default::default();
 				for _feps::Assignment { who, distribution } in assignments {
 					match distribution.len() {
@@ -134,6 +139,7 @@ pub(crate) fn generate(def: crate::SolutionDef) -> Result<TokenStream2> {
 						}
 					}
 				};
+
 				Ok(#struct_name)
 			}
 

substrate/frame/election-provider-support/src/lib.rs

@@ -170,12 +170,13 @@ pub mod onchain;
 pub mod traits;
 #[cfg(feature = "std")]
 use codec::{Decode, Encode};
-use frame_support::{traits::Get, BoundedVec, RuntimeDebug};
+use frame_support::{BoundedVec, RuntimeDebug};
 use sp_runtime::traits::Bounded;
 use sp_std::{fmt::Debug, prelude::*};
 
 /// Re-export the solution generation macro.
 pub use frame_election_provider_solution_type::generate_solution_type;
+pub use frame_support::traits::Get;
 /// Re-export some type as they are used in the interface.
 pub use sp_arithmetic::PerThing;
 pub use sp_npos_elections::{

substrate/frame/election-provider-support/src/mock.rs

@@ -47,7 +47,7 @@ crate::generate_solution_type! {
 		VoterIndex = u32,
 		TargetIndex = u16,
 		Accuracy = TestAccuracy,
-		MaxVoters = frame_support::traits::ConstU32::<20>,
+		MaxVoters = frame_support::traits::ConstU32::<2_500>,
 	>(16)
 }
 

substrate/frame/election-provider-support/src/tests.rs

@@ -91,6 +91,33 @@ mod solution_type {
 		assert!(with_compact < without_compact);
 	}
 
+	#[test]
+	fn from_assignment_fail_too_many_voters() {
+		let rng = rand::rngs::SmallRng::seed_from_u64(0);
+
+		// This will produce 24 voters..
+		let (voters, assignments, candidates) = generate_random_votes(10, 25, rng);
+		let voter_index = make_voter_fn(&voters);
+		let target_index = make_target_fn(&candidates);
+
+		// Limit the voters to 20..
+		generate_solution_type!(
+			pub struct InnerTestSolution::<
+				VoterIndex = u32,
+				TargetIndex = u16,
+				Accuracy = TestAccuracy,
+				MaxVoters = frame_support::traits::ConstU32::<20>,
+			>(16)
+		);
+
+		// 24 > 20, so this should fail.
+		assert_eq!(
+			InnerTestSolution::from_assignment(&assignments, &voter_index, &target_index)
+				.unwrap_err(),
+			NposError::TooManyVoters,
+		);
+	}
+
 	#[test]
 	fn max_encoded_len_too_small() {
 		generate_solution_type!(