Only maintain at most 1 UnlockChunk per era (#10670)

* Only maintain at most 1 `UnlockChunk` per era

* Bound `unlocking`

* Run cargo +nightly-2021-10-29 fmt

* Make benchmarks stuff compile

* Update frame/staking/src/lib.rs

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* Remove DerefMut; Implement neccesary methods directly

* Doc comments for new BoundedVec methods

* Fix benchmarks

* wip bonded_vec macro

* Correct rust doc

* Apply suggestions from code review

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* Update staking::Config impls

* Add MaxUnlockingChunks to more places

* Use defensive saturating add

* FMT

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

substrate/bin/node/runtime/src/lib.rs

@@ -558,6 +558,7 @@ impl pallet_staking::Config for Runtime {
 	// Alternatively, use pallet_staking::UseNominatorsMap<Runtime> to just use the nominators map.
 	// Note that the aforementioned does not scale to a very large number of nominators.
 	type SortedListProvider = BagsList;
+	type MaxUnlockingChunks = ConstU32<32>;
 	type WeightInfo = pallet_staking::weights::SubstrateWeight<Runtime>;
 	type BenchmarkingConfig = StakingBenchmarkingConfig;
 }

substrate/frame/babe/src/mock.rs

@@ -198,6 +198,7 @@ impl pallet_staking::Config for Test {
 	type ElectionProvider = onchain::OnChainSequentialPhragmen<Self>;
 	type GenesisElectionProvider = Self::ElectionProvider;
 	type SortedListProvider = pallet_staking::UseNominatorsMap<Self>;
+	type MaxUnlockingChunks = ConstU32<32>;
 	type BenchmarkingConfig = pallet_staking::TestBenchmarkingConfig;
 	type WeightInfo = ();
 }

substrate/frame/grandpa/src/mock.rs

@@ -206,6 +206,7 @@ impl pallet_staking::Config for Test {
 	type ElectionProvider = onchain::OnChainSequentialPhragmen<Self>;
 	type GenesisElectionProvider = Self::ElectionProvider;
 	type SortedListProvider = pallet_staking::UseNominatorsMap<Self>;
+	type MaxUnlockingChunks = ConstU32<32>;
 	type BenchmarkingConfig = pallet_staking::TestBenchmarkingConfig;
 	type WeightInfo = ();
 }

substrate/frame/offences/benchmarking/src/mock.rs

@@ -176,6 +176,7 @@ impl pallet_staking::Config for Test {
 	type ElectionProvider = onchain::OnChainSequentialPhragmen<Self>;
 	type GenesisElectionProvider = Self::ElectionProvider;
 	type SortedListProvider = pallet_staking::UseNominatorsMap<Self>;
+	type MaxUnlockingChunks = ConstU32<32>;
 	type BenchmarkingConfig = pallet_staking::TestBenchmarkingConfig;
 	type WeightInfo = ();
 }

substrate/frame/session/benchmarking/src/mock.rs

@@ -181,6 +181,7 @@ impl pallet_staking::Config for Test {
 	type OffendingValidatorsThreshold = ();
 	type ElectionProvider = onchain::OnChainSequentialPhragmen<Self>;
 	type GenesisElectionProvider = Self::ElectionProvider;
+	type MaxUnlockingChunks = ConstU32<32>;
 	type SortedListProvider = pallet_staking::UseNominatorsMap<Self>;
 	type BenchmarkingConfig = pallet_staking::TestBenchmarkingConfig;
 	type WeightInfo = ();

substrate/frame/staking/src/benchmarking.rs

@@ -618,7 +618,7 @@ benchmarks! {
 	}
 
 	rebond {
-		let l in 1 .. MAX_UNLOCKING_CHUNKS as u32;
+		let l in 1 .. MaxUnlockingChunks::get() as u32;
 
 		// clean up any existing state.
 		clear_validators_and_nominators::<T>();
@@ -652,7 +652,7 @@ benchmarks! {
 		let mut staking_ledger = Ledger::<T>::get(controller.clone()).unwrap();
 
 		for _ in 0 .. l {
-			staking_ledger.unlocking.push(unlock_chunk.clone())
+			staking_ledger.unlocking.try_push(unlock_chunk.clone()).unwrap()
 		}
 		Ledger::<T>::insert(controller.clone(), staking_ledger.clone());
 		let original_bonded: BalanceOf<T> = staking_ledger.active;
@@ -702,7 +702,7 @@ benchmarks! {
 			stash: stash.clone(),
 			active: T::Currency::minimum_balance() - One::one(),
 			total: T::Currency::minimum_balance() - One::one(),
-			unlocking: vec![],
+			unlocking: Default::default(),
 			claimed_rewards: vec![],
 		};
 		Ledger::<T>::insert(&controller, l);
@@ -788,7 +788,7 @@ benchmarks! {
 
 	#[extra]
 	do_slash {
-		let l in 1 .. MAX_UNLOCKING_CHUNKS as u32;
+		let l in 1 .. MaxUnlockingChunks::get() as u32;
 		let (stash, controller) = create_stash_controller::<T>(0, 100, Default::default())?;
 		let mut staking_ledger = Ledger::<T>::get(controller.clone()).unwrap();
 		let unlock_chunk = UnlockChunk::<BalanceOf<T>> {
@@ -796,7 +796,7 @@ benchmarks! {
 			era: EraIndex::zero(),
 		};
 		for _ in 0 .. l {
-			staking_ledger.unlocking.push(unlock_chunk.clone())
+			staking_ledger.unlocking.try_push(unlock_chunk.clone()).unwrap();
 		}
 		Ledger::<T>::insert(controller, staking_ledger);
 		let slash_amount = T::Currency::minimum_balance() * 10u32.into();

substrate/frame/staking/src/lib.rs

@@ -301,6 +301,7 @@ mod pallet;
 
 use codec::{Decode, Encode, HasCompact};
 use frame_support::{
+	parameter_types,
 	traits::{ConstU32, Currency, Get},
 	weights::Weight,
 	BoundedVec, EqNoBound, PartialEqNoBound, RuntimeDebugNoBound,
@@ -347,6 +348,10 @@ type NegativeImbalanceOf<T> = <<T as Config>::Currency as Currency<
 	<T as frame_system::Config>::AccountId,
 >>::NegativeImbalance;
 
+parameter_types! {
+	pub MaxUnlockingChunks: u32 = 32;
+}
+
 /// Information regarding the active era (era in used in session).
 #[derive(Encode, Decode, RuntimeDebug, TypeInfo)]
 pub struct ActiveEraInfo {
@@ -446,9 +451,10 @@ pub struct StakingLedger<AccountId, Balance: HasCompact> {
 	/// rounds.
 	#[codec(compact)]
 	pub active: Balance,
-	/// Any balance that is becoming free, which may eventually be transferred out
-	/// of the stash (assuming it doesn't get slashed first).
-	pub unlocking: Vec<UnlockChunk<Balance>>,
+	/// Any balance that is becoming free, which may eventually be transferred out of the stash
+	/// (assuming it doesn't get slashed first). It is assumed that this will be treated as a first
+	/// in, first out queue where the new (higher value) eras get pushed on the back.
+	pub unlocking: BoundedVec<UnlockChunk<Balance>, MaxUnlockingChunks>,
 	/// List of eras for which the stakers behind a validator have claimed rewards. Only updated
 	/// for validators.
 	pub claimed_rewards: Vec<EraIndex>,
@@ -463,7 +469,7 @@ impl<AccountId, Balance: HasCompact + Copy + Saturating + AtLeast32BitUnsigned +
 			stash,
 			total: Zero::zero(),
 			active: Zero::zero(),
-			unlocking: vec![],
+			unlocking: Default::default(),
 			claimed_rewards: vec![],
 		}
 	}
@@ -472,7 +478,7 @@ impl<AccountId, Balance: HasCompact + Copy + Saturating + AtLeast32BitUnsigned +
 	/// total by the sum of their balances.
 	fn consolidate_unlocked(self, current_era: EraIndex) -> Self {
 		let mut total = self.total;
-		let unlocking = self
+		let unlocking: BoundedVec<_, _> = self
 			.unlocking
 			.into_iter()
 			.filter(|chunk| {
@@ -483,7 +489,11 @@ impl<AccountId, Balance: HasCompact + Copy + Saturating + AtLeast32BitUnsigned +
 					false
 				}
 			})
-			.collect();
+			.collect::<Vec<_>>()
+			.try_into()
+			.expect(
+				"filtering items from a bounded vec always leaves length less than bounds. qed",
+			);
 
 		Self {
 			stash: self.stash,

substrate/frame/staking/src/mock.rs

@@ -271,6 +271,7 @@ impl crate::pallet::pallet::Config for Test {
 	type GenesisElectionProvider = Self::ElectionProvider;
 	// NOTE: consider a macro and use `UseNominatorsMap<Self>` as well.
 	type SortedListProvider = BagsList;
+	type MaxUnlockingChunks = ConstU32<32>;
 	type BenchmarkingConfig = TestBenchmarkingConfig;
 	type WeightInfo = ();
 }

substrate/frame/staking/src/pallet/impls.rs

@@ -928,7 +928,7 @@ impl<T: Config> ElectionDataProvider for Pallet<T> {
 				stash: voter.clone(),
 				active: stake,
 				total: stake,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: vec![],
 			},
 		);
@@ -946,7 +946,7 @@ impl<T: Config> ElectionDataProvider for Pallet<T> {
 				stash: target.clone(),
 				active: stake,
 				total: stake,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: vec![],
 			},
 		);
@@ -983,7 +983,7 @@ impl<T: Config> ElectionDataProvider for Pallet<T> {
 					stash: v.clone(),
 					active: stake,
 					total: stake,
-					unlocking: vec![],
+					unlocking: Default::default(),
 					claimed_rewards: vec![],
 				},
 			);
@@ -1004,7 +1004,7 @@ impl<T: Config> ElectionDataProvider for Pallet<T> {
 					stash: v.clone(),
 					active: stake,
 					total: stake,
-					unlocking: vec![],
+					unlocking: Default::default(),
 					claimed_rewards: vec![],
 				},
 			);

substrate/frame/staking/src/pallet/mod.rs

@@ -21,8 +21,8 @@ use frame_election_provider_support::SortedListProvider;
 use frame_support::{
 	pallet_prelude::*,
 	traits::{
-		Currency, CurrencyToVote, EnsureOrigin, EstimateNextNewSession, Get, LockIdentifier,
-		LockableCurrency, OnUnbalanced, UnixTime,
+		Currency, CurrencyToVote, DefensiveSaturating, EnsureOrigin, EstimateNextNewSession, Get,
+		LockIdentifier, LockableCurrency, OnUnbalanced, UnixTime,
 	},
 	weights::Weight,
 };
@@ -40,12 +40,11 @@ pub use impls::*;
 
 use crate::{
 	log, slashing, weights::WeightInfo, ActiveEraInfo, BalanceOf, EraPayout, EraRewardPoints,
-	Exposure, Forcing, NegativeImbalanceOf, Nominations, PositiveImbalanceOf, Releases,
-	RewardDestination, SessionInterface, StakingLedger, UnappliedSlash, UnlockChunk,
+	Exposure, Forcing, MaxUnlockingChunks, NegativeImbalanceOf, Nominations, PositiveImbalanceOf,
+	Releases, RewardDestination, SessionInterface, StakingLedger, UnappliedSlash, UnlockChunk,
 	ValidatorPrefs,
 };
 
-pub const MAX_UNLOCKING_CHUNKS: usize = 32;
 const STAKING_ID: LockIdentifier = *b"staking ";
 
 #[frame_support::pallet]
@@ -157,6 +156,10 @@ pub mod pallet {
 		/// the bags-list is not desired, [`impls::UseNominatorsMap`] is likely the desired option.
 		type SortedListProvider: SortedListProvider<Self::AccountId>;
 
+		/// The maximum number of `unlocking` chunks a [`StakingLedger`] can have. Effectively
+		/// determines how many unique eras a staker may be unbonding in.
+		type MaxUnlockingChunks: Get<u32>;
+
 		/// Some parameters of the benchmarking.
 		type BenchmarkingConfig: BenchmarkingConfig;
 
@@ -772,7 +775,7 @@ pub mod pallet {
 				stash,
 				total: value,
 				active: value,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: (last_reward_era..current_era).collect(),
 			};
 			Self::update_ledger(&controller, &item);
@@ -837,7 +840,7 @@ pub mod pallet {
 		/// Once the unlock period is done, you can call `withdraw_unbonded` to actually move
 		/// the funds out of management ready for transfer.
 		///
-		/// No more than a limited number of unlocking chunks (see `MAX_UNLOCKING_CHUNKS`)
+		/// No more than a limited number of unlocking chunks (see `MaxUnlockingChunks`)
 		/// can co-exists at the same time. In that case, [`Call::withdraw_unbonded`] need
 		/// to be called first to remove some of the chunks (if possible).
 		///
@@ -854,7 +857,10 @@ pub mod pallet {
 		) -> DispatchResult {
 			let controller = ensure_signed(origin)?;
 			let mut ledger = Self::ledger(&controller).ok_or(Error::<T>::NotController)?;
-			ensure!(ledger.unlocking.len() < MAX_UNLOCKING_CHUNKS, Error::<T>::NoMoreChunks,);
+			ensure!(
+				ledger.unlocking.len() < MaxUnlockingChunks::get() as usize,
+				Error::<T>::NoMoreChunks,
+			);
 
 			let mut value = value.min(ledger.active);
 
@@ -881,7 +887,19 @@ pub mod pallet {
 
 				// Note: in case there is no current era it is fine to bond one era more.
 				let era = Self::current_era().unwrap_or(0) + T::BondingDuration::get();
-				ledger.unlocking.push(UnlockChunk { value, era });
+				if let Some(mut chunk) =
+					ledger.unlocking.last_mut().filter(|chunk| chunk.era == era)
+				{
+					// To keep the chunk count down, we only keep one chunk per era. Since
+					// `unlocking` is a FiFo queue, if a chunk exists for `era` we know that it will
+					// be the last one.
+					chunk.value = chunk.value.defensive_saturating_add(value)
+				} else {
+					ledger
+						.unlocking
+						.try_push(UnlockChunk { value, era })
+						.map_err(|_| Error::<T>::NoMoreChunks)?;
+				};
 				// NOTE: ledger must be updated prior to calling `Self::weight_of`.
 				Self::update_ledger(&controller, &ledger);
 
@@ -1348,10 +1366,10 @@ pub mod pallet {
 		///
 		/// # <weight>
 		/// - Time complexity: O(L), where L is unlocking chunks
-		/// - Bounded by `MAX_UNLOCKING_CHUNKS`.
+		/// - Bounded by `MaxUnlockingChunks`.
 		/// - Storage changes: Can't increase storage, only decrease it.
 		/// # </weight>
-		#[pallet::weight(T::WeightInfo::rebond(MAX_UNLOCKING_CHUNKS as u32))]
+		#[pallet::weight(T::WeightInfo::rebond(MaxUnlockingChunks::get() as u32))]
 		pub fn rebond(
 			origin: OriginFor<T>,
 			#[pallet::compact] value: BalanceOf<T>,

substrate/frame/staking/src/tests.rs

@@ -17,10 +17,10 @@
 
 //! Tests for the module.
 
-use super::{Event, *};
+use super::{Event, MaxUnlockingChunks, *};
 use frame_election_provider_support::{ElectionProvider, SortedListProvider, Support};
 use frame_support::{
-	assert_noop, assert_ok,
+	assert_noop, assert_ok, bounded_vec,
 	dispatch::WithPostDispatchInfo,
 	pallet_prelude::*,
 	traits::{Currency, Get, ReservableCurrency},
@@ -104,7 +104,7 @@ fn basic_setup_works() {
 				stash: 11,
 				total: 1000,
 				active: 1000,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: vec![]
 			})
 		);
@@ -115,7 +115,7 @@ fn basic_setup_works() {
 				stash: 21,
 				total: 1000,
 				active: 1000,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: vec![]
 			})
 		);
@@ -138,7 +138,7 @@ fn basic_setup_works() {
 				stash: 101,
 				total: 500,
 				active: 500,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: vec![]
 			})
 		);
@@ -382,7 +382,7 @@ fn staking_should_work() {
 				stash: 3,
 				total: 1500,
 				active: 1500,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: vec![0],
 			})
 		);
@@ -936,7 +936,7 @@ fn reward_destination_works() {
 				stash: 11,
 				total: 1000,
 				active: 1000,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: vec![],
 			})
 		);
@@ -959,7 +959,7 @@ fn reward_destination_works() {
 				stash: 11,
 				total: 1000 + total_payout_0,
 				active: 1000 + total_payout_0,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: vec![0],
 			})
 		);
@@ -987,7 +987,7 @@ fn reward_destination_works() {
 				stash: 11,
 				total: 1000 + total_payout_0,
 				active: 1000 + total_payout_0,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: vec![0, 1],
 			})
 		);
@@ -1016,7 +1016,7 @@ fn reward_destination_works() {
 				stash: 11,
 				total: 1000 + total_payout_0,
 				active: 1000 + total_payout_0,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: vec![0, 1, 2],
 			})
 		);
@@ -1081,7 +1081,7 @@ fn bond_extra_works() {
 				stash: 11,
 				total: 1000,
 				active: 1000,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: vec![],
 			})
 		);
@@ -1098,7 +1098,7 @@ fn bond_extra_works() {
 				stash: 11,
 				total: 1000 + 100,
 				active: 1000 + 100,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: vec![],
 			})
 		);
@@ -1112,7 +1112,7 @@ fn bond_extra_works() {
 				stash: 11,
 				total: 1000000,
 				active: 1000000,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: vec![],
 			})
 		);
@@ -1150,7 +1150,7 @@ fn bond_extra_and_withdraw_unbonded_works() {
 				stash: 11,
 				total: 1000,
 				active: 1000,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: vec![],
 			})
 		);
@@ -1168,7 +1168,7 @@ fn bond_extra_and_withdraw_unbonded_works() {
 				stash: 11,
 				total: 1000 + 100,
 				active: 1000 + 100,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: vec![],
 			})
 		);
@@ -1189,7 +1189,7 @@ fn bond_extra_and_withdraw_unbonded_works() {
 				stash: 11,
 				total: 1000 + 100,
 				active: 1000 + 100,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: vec![],
 			})
 		);
@@ -1207,7 +1207,7 @@ fn bond_extra_and_withdraw_unbonded_works() {
 				stash: 11,
 				total: 1000 + 100,
 				active: 100,
-				unlocking: vec![UnlockChunk { value: 1000, era: 2 + 3 }],
+				unlocking: bounded_vec![UnlockChunk { value: 1000, era: 2 + 3 }],
 				claimed_rewards: vec![]
 			}),
 		);
@@ -1220,7 +1220,7 @@ fn bond_extra_and_withdraw_unbonded_works() {
 				stash: 11,
 				total: 1000 + 100,
 				active: 100,
-				unlocking: vec![UnlockChunk { value: 1000, era: 2 + 3 }],
+				unlocking: bounded_vec![UnlockChunk { value: 1000, era: 2 + 3 }],
 				claimed_rewards: vec![]
 			}),
 		);
@@ -1236,7 +1236,7 @@ fn bond_extra_and_withdraw_unbonded_works() {
 				stash: 11,
 				total: 1000 + 100,
 				active: 100,
-				unlocking: vec![UnlockChunk { value: 1000, era: 2 + 3 }],
+				unlocking: bounded_vec![UnlockChunk { value: 1000, era: 2 + 3 }],
 				claimed_rewards: vec![]
 			}),
 		);
@@ -1252,7 +1252,7 @@ fn bond_extra_and_withdraw_unbonded_works() {
 				stash: 11,
 				total: 100,
 				active: 100,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: vec![]
 			}),
 		);
@@ -1262,23 +1262,35 @@ fn bond_extra_and_withdraw_unbonded_works() {
 #[test]
 fn too_many_unbond_calls_should_not_work() {
 	ExtBuilder::default().build_and_execute(|| {
-		// locked at era 0 until 3
-		for _ in 0..MAX_UNLOCKING_CHUNKS - 1 {
+		let mut current_era = 0;
+		// locked at era MaxUnlockingChunks - 1 until 3
+		for i in 0..MaxUnlockingChunks::get() - 1 {
+			// There is only 1 chunk per era, so we need to be in a new era to create a chunk.
+			current_era = i as u32;
+			mock::start_active_era(current_era);
 			assert_ok!(Staking::unbond(Origin::signed(10), 1));
 		}
 
-		mock::start_active_era(1);
+		current_era += 1;
+		mock::start_active_era(current_era);
 
-		// locked at era 1 until 4
+		// This chunk is locked at `current_era` through `current_era + 2` (because BondingDuration
+		// == 3).
 		assert_ok!(Staking::unbond(Origin::signed(10), 1));
+		assert_eq!(
+			Staking::ledger(&10).unwrap().unlocking.len(),
+			MaxUnlockingChunks::get() as usize
+		);
 		// can't do more.
 		assert_noop!(Staking::unbond(Origin::signed(10), 1), Error::<Test>::NoMoreChunks);
 
-		mock::start_active_era(3);
+		current_era += 2;
+		mock::start_active_era(current_era);
 
 		assert_noop!(Staking::unbond(Origin::signed(10), 1), Error::<Test>::NoMoreChunks);
-		// free up.
+		// free up everything except the most recently added chunk.
 		assert_ok!(Staking::withdraw_unbonded(Origin::signed(10), 0));
+		assert_eq!(Staking::ledger(&10).unwrap().unlocking.len(), 1);
 
 		// Can add again.
 		assert_ok!(Staking::unbond(Origin::signed(10), 1));
@@ -1310,7 +1322,7 @@ fn rebond_works() {
 				stash: 11,
 				total: 1000,
 				active: 1000,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: vec![],
 			})
 		);
@@ -1329,7 +1341,7 @@ fn rebond_works() {
 				stash: 11,
 				total: 1000,
 				active: 100,
-				unlocking: vec![UnlockChunk { value: 900, era: 2 + 3 }],
+				unlocking: bounded_vec![UnlockChunk { value: 900, era: 2 + 3 }],
 				claimed_rewards: vec![],
 			})
 		);
@@ -1342,7 +1354,7 @@ fn rebond_works() {
 				stash: 11,
 				total: 1000,
 				active: 1000,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: vec![],
 			})
 		);
@@ -1355,7 +1367,7 @@ fn rebond_works() {
 				stash: 11,
 				total: 1000,
 				active: 100,
-				unlocking: vec![UnlockChunk { value: 900, era: 5 }],
+				unlocking: bounded_vec![UnlockChunk { value: 900, era: 5 }],
 				claimed_rewards: vec![],
 			})
 		);
@@ -1368,7 +1380,7 @@ fn rebond_works() {
 				stash: 11,
 				total: 1000,
 				active: 600,
-				unlocking: vec![UnlockChunk { value: 400, era: 5 }],
+				unlocking: bounded_vec![UnlockChunk { value: 400, era: 5 }],
 				claimed_rewards: vec![],
 			})
 		);
@@ -1381,7 +1393,7 @@ fn rebond_works() {
 				stash: 11,
 				total: 1000,
 				active: 1000,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: vec![],
 			})
 		);
@@ -1396,11 +1408,7 @@ fn rebond_works() {
 				stash: 11,
 				total: 1000,
 				active: 100,
-				unlocking: vec![
-					UnlockChunk { value: 300, era: 5 },
-					UnlockChunk { value: 300, era: 5 },
-					UnlockChunk { value: 300, era: 5 },
-				],
+				unlocking: bounded_vec![UnlockChunk { value: 900, era: 5 }],
 				claimed_rewards: vec![],
 			})
 		);
@@ -1413,10 +1421,7 @@ fn rebond_works() {
 				stash: 11,
 				total: 1000,
 				active: 600,
-				unlocking: vec![
-					UnlockChunk { value: 300, era: 5 },
-					UnlockChunk { value: 100, era: 5 },
-				],
+				unlocking: bounded_vec![UnlockChunk { value: 400, era: 5 }],
 				claimed_rewards: vec![],
 			})
 		);
@@ -1443,7 +1448,7 @@ fn rebond_is_fifo() {
 				stash: 11,
 				total: 1000,
 				active: 1000,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: vec![],
 			})
 		);
@@ -1458,7 +1463,7 @@ fn rebond_is_fifo() {
 				stash: 11,
 				total: 1000,
 				active: 600,
-				unlocking: vec![UnlockChunk { value: 400, era: 2 + 3 },],
+				unlocking: bounded_vec![UnlockChunk { value: 400, era: 2 + 3 }],
 				claimed_rewards: vec![],
 			})
 		);
@@ -1473,9 +1478,9 @@ fn rebond_is_fifo() {
 				stash: 11,
 				total: 1000,
 				active: 300,
-				unlocking: vec![
+				unlocking: bounded_vec![
 					UnlockChunk { value: 400, era: 2 + 3 },
-					UnlockChunk { value: 300, era: 3 + 3 },
+					UnlockChunk { value: 300, era: 3 + 3 }
 				],
 				claimed_rewards: vec![],
 			})
@@ -1491,10 +1496,10 @@ fn rebond_is_fifo() {
 				stash: 11,
 				total: 1000,
 				active: 100,
-				unlocking: vec![
+				unlocking: bounded_vec![
 					UnlockChunk { value: 400, era: 2 + 3 },
 					UnlockChunk { value: 300, era: 3 + 3 },
-					UnlockChunk { value: 200, era: 4 + 3 },
+					UnlockChunk { value: 200, era: 4 + 3 }
 				],
 				claimed_rewards: vec![],
 			})
@@ -1508,9 +1513,9 @@ fn rebond_is_fifo() {
 				stash: 11,
 				total: 1000,
 				active: 500,
-				unlocking: vec![
+				unlocking: bounded_vec![
 					UnlockChunk { value: 400, era: 2 + 3 },
-					UnlockChunk { value: 100, era: 3 + 3 },
+					UnlockChunk { value: 100, era: 3 + 3 }
 				],
 				claimed_rewards: vec![],
 			})
@@ -1540,7 +1545,7 @@ fn rebond_emits_right_value_in_event() {
 				stash: 11,
 				total: 1000,
 				active: 100,
-				unlocking: vec![UnlockChunk { value: 900, era: 1 + 3 }],
+				unlocking: bounded_vec![UnlockChunk { value: 900, era: 1 + 3 }],
 				claimed_rewards: vec![],
 			})
 		);
@@ -1553,7 +1558,7 @@ fn rebond_emits_right_value_in_event() {
 				stash: 11,
 				total: 1000,
 				active: 200,
-				unlocking: vec![UnlockChunk { value: 800, era: 1 + 3 }],
+				unlocking: bounded_vec![UnlockChunk { value: 800, era: 1 + 3 }],
 				claimed_rewards: vec![],
 			})
 		);
@@ -1568,7 +1573,7 @@ fn rebond_emits_right_value_in_event() {
 				stash: 11,
 				total: 1000,
 				active: 1000,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: vec![],
 			})
 		);
@@ -1604,7 +1609,7 @@ fn reward_to_stake_works() {
 					stash: 21,
 					total: 69,
 					active: 69,
-					unlocking: vec![],
+					unlocking: Default::default(),
 					claimed_rewards: vec![],
 				},
 			);
@@ -1665,7 +1670,7 @@ fn reap_stash_works() {
 					stash: 11,
 					total: 5,
 					active: 5,
-					unlocking: vec![],
+					unlocking: Default::default(),
 					claimed_rewards: vec![],
 				},
 			);
@@ -1784,7 +1789,7 @@ fn bond_with_no_staked_value() {
 					stash: 1,
 					active: 0,
 					total: 5,
-					unlocking: vec![UnlockChunk { value: 5, era: 3 }],
+					unlocking: bounded_vec![UnlockChunk { value: 5, era: 3 }],
 					claimed_rewards: vec![],
 				})
 			);
@@ -3354,7 +3359,7 @@ fn test_payout_stakers() {
 				stash: 11,
 				total: 1000,
 				active: 1000,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: vec![1]
 			})
 		);
@@ -3376,7 +3381,7 @@ fn test_payout_stakers() {
 				stash: 11,
 				total: 1000,
 				active: 1000,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: (1..=14).collect()
 			})
 		);
@@ -3397,7 +3402,7 @@ fn test_payout_stakers() {
 				stash: 11,
 				total: 1000,
 				active: 1000,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: vec![15, 98]
 			})
 		);
@@ -3412,7 +3417,7 @@ fn test_payout_stakers() {
 				stash: 11,
 				total: 1000,
 				active: 1000,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: vec![15, 23, 42, 69, 98]
 			})
 		);
@@ -3607,7 +3612,7 @@ fn bond_during_era_correctly_populates_claimed_rewards() {
 				stash: 9,
 				total: 1000,
 				active: 1000,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: vec![],
 			})
 		);
@@ -3619,7 +3624,7 @@ fn bond_during_era_correctly_populates_claimed_rewards() {
 				stash: 11,
 				total: 1000,
 				active: 1000,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: (0..5).collect(),
 			})
 		);
@@ -3631,7 +3636,7 @@ fn bond_during_era_correctly_populates_claimed_rewards() {
 				stash: 13,
 				total: 1000,
 				active: 1000,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: (15..99).collect(),
 			})
 		);
@@ -3850,7 +3855,7 @@ fn cannot_rebond_to_lower_than_ed() {
 					stash: 21,
 					total: 10 * 1000,
 					active: 10 * 1000,
-					unlocking: vec![],
+					unlocking: Default::default(),
 					claimed_rewards: vec![]
 				}
 			);
@@ -3864,7 +3869,7 @@ fn cannot_rebond_to_lower_than_ed() {
 					stash: 21,
 					total: 10 * 1000,
 					active: 0,
-					unlocking: vec![UnlockChunk { value: 10 * 1000, era: 3 }],
+					unlocking: bounded_vec![UnlockChunk { value: 10 * 1000, era: 3 }],
 					claimed_rewards: vec![]
 				}
 			);
@@ -3887,7 +3892,7 @@ fn cannot_bond_extra_to_lower_than_ed() {
 					stash: 21,
 					total: 10 * 1000,
 					active: 10 * 1000,
-					unlocking: vec![],
+					unlocking: Default::default(),
 					claimed_rewards: vec![]
 				}
 			);
@@ -3901,7 +3906,7 @@ fn cannot_bond_extra_to_lower_than_ed() {
 					stash: 21,
 					total: 10 * 1000,
 					active: 0,
-					unlocking: vec![UnlockChunk { value: 10 * 1000, era: 3 }],
+					unlocking: bounded_vec![UnlockChunk { value: 10 * 1000, era: 3 }],
 					claimed_rewards: vec![]
 				}
 			);
@@ -3928,7 +3933,7 @@ fn do_not_die_when_active_is_ed() {
 					stash: 21,
 					total: 1000 * ed,
 					active: 1000 * ed,
-					unlocking: vec![],
+					unlocking: Default::default(),
 					claimed_rewards: vec![]
 				}
 			);
@@ -3945,7 +3950,7 @@ fn do_not_die_when_active_is_ed() {
 					stash: 21,
 					total: ed,
 					active: ed,
-					unlocking: vec![],
+					unlocking: Default::default(),
 					claimed_rewards: vec![]
 				}
 			);

substrate/frame/support/src/storage/bounded_vec.rs

@@ -25,7 +25,7 @@ use crate::{
 };
 use codec::{Decode, Encode, EncodeLike, MaxEncodedLen};
 use core::{
-	ops::{Deref, Index, IndexMut},
+	ops::{Deref, Index, IndexMut, RangeBounds},
 	slice::SliceIndex,
 };
 use sp_std::{marker::PhantomData, prelude::*};
@@ -178,6 +178,19 @@ impl<T, S> BoundedVec<T, S> {
 	pub fn iter_mut(&mut self) -> core::slice::IterMut<'_, T> {
 		self.0.iter_mut()
 	}
+
+	/// Exactly the same semantics as [`slice::last_mut`].
+	pub fn last_mut(&mut self) -> Option<&mut T> {
+		self.0.last_mut()
+	}
+
+	/// Exact same semantics as [`Vec::drain`].
+	pub fn drain<R>(&mut self, range: R) -> sp_std::vec::Drain<'_, T>
+	where
+		R: RangeBounds<usize>,
+	{
+		self.0.drain(range)
+	}
 }
 
 impl<T, S: Get<u32>> From<BoundedVec<T, S>> for Vec<T> {