Add a dummy CorrectnessProof trait, to be extended elsewhere

CHANGELOG.md

@@ -34,6 +34,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Added `Message::destination()`. ([#56])
 - `PartyId` trait alias for the combination of bounds needed for a party identifier. ([#59])
 - An impl of `ProtocolError` for `()` for protocols that don't use errors. ([#60])
+- A dummy `CorrectnessProof` trait. ([#60])
 
 
 [#32]: https://github.com/entropyxyz/manul/pull/32

manul/src/protocol.rs

@@ -23,7 +23,9 @@ pub use errors::{
 };
 pub use message::{DirectMessage, EchoBroadcast, NormalBroadcast, ProtocolMessagePart};
 pub use object_safe::BoxedRound;
-pub use round::{Artifact, EntryPoint, FinalizeOutcome, PartyId, Payload, Protocol, ProtocolError, Round, RoundId};
+pub use round::{
+    Artifact, CorrectnessProof, EntryPoint, FinalizeOutcome, PartyId, Payload, Protocol, ProtocolError, Round, RoundId,
+};
 pub use serialization::{Deserializer, Serializer};
 
 pub(crate) use errors::ReceiveErrorType;

manul/src/protocol/round.rs

@@ -89,7 +89,7 @@ pub trait Protocol: 'static {
     /// An object of this type will be returned when an unattributable error happens during [`Round::finalize`].
     ///
     /// It proves that the node did its job correctly, to be adjudicated by a third party.
-    type CorrectnessProof: Send + Serialize + for<'de> Deserialize<'de> + Debug;
+    type CorrectnessProof: CorrectnessProof + Serialize + for<'de> Deserialize<'de>;
 
     /// Returns `Ok(())` if the given direct message cannot be deserialized
     /// assuming it is a direct message from the round `round_id`.
@@ -223,6 +223,18 @@ impl ProtocolError for () {
     }
 }
 
+/// Describes unattributable errors originating during protocol execution.
+///
+/// In the situations where no specific message can be blamed for an error,
+/// each node must generate a correctness proof proving that they performed their duties correctly,
+/// and the collection of proofs is verified by a third party.
+/// One of the proofs will be necessarily missing or invalid.
+pub trait CorrectnessProof: Debug + Clone + Send {}
+
+// A convenience implementation for protocols that don't define any errors.
+// Have to do it for `()`, since `!` is unstable.
+impl CorrectnessProof for () {}
+
 /// Message payload created in [`Round::receive_message`].
 #[derive(Debug)]
 pub struct Payload(pub Box<dyn Any + Send + Sync>);