support: add lateness trait (#5519)

* support: add lateness trait

* babe: implement the lateness trait

* babe: add docs about lateness entry lifetime

* babe: don't use option for lateness storage entry

substrate/frame/support/src/traits.rs

@@ -24,7 +24,7 @@ use sp_core::u32_trait::Value as U32;
 use sp_runtime::{
 	RuntimeDebug,
 	ConsensusEngineId, DispatchResult, DispatchError,
-	traits::{MaybeSerializeDeserialize, AtLeast32Bit, Saturating, TrailingZeroInput, Bounded},
+	traits::{MaybeSerializeDeserialize, AtLeast32Bit, Saturating, TrailingZeroInput, Bounded, Zero},
 };
 use crate::dispatch::Parameter;
 use crate::storage::StorageMap;
@@ -1032,6 +1032,21 @@ impl<Output: Decode + Default> Randomness<Output> for () {
 	}
 }
 
+/// Trait to be used by block producing consensus engine modules to determine
+/// how late the current block is (e.g. in a slot-based proposal mechanism how
+/// many slots were skipped since the previous block).
+pub trait Lateness<N> {
+	/// Returns a generic measure of how late the current block is compared to
+	/// its parent.
+	fn lateness(&self) -> N;
+}
+
+impl<N: Zero> Lateness<N> for () {
+	fn lateness(&self) -> N {
+		Zero::zero()
+	}
+}
+
 /// Implementors of this trait provide information about whether or not some validator has
 /// been registered with them. The [Session module](../../pallet_session/index.html) is an implementor.
 pub trait ValidatorRegistration<ValidatorId> {