Check signatures as "Compact" in statement distribution (#5071)

* allow converting payloads _up_

* convert to superpayload in statement-distribution

* Update primitives/src/v2/signed.rs

Co-authored-by: Andronik <write@reusable.software>

Co-authored-by: Andronik <write@reusable.software>

polkadot/primitives/src/v2/mod.rs

@@ -660,6 +660,8 @@ impl From<BitVec<u8, bitvec::order::Lsb0>> for AvailabilityBitfield {
 
 /// A signed compact statement, suitable to be sent to the chain.
 pub type SignedStatement = Signed<CompactStatement>;
+/// A signed compact statement, with signature not yet checked.
+pub type UncheckedSignedStatement = UncheckedSigned<CompactStatement>;
 
 /// A bitfield signed by a particular validator about the availability of pending candidates.
 pub type SignedAvailabilityBitfield = Signed<AvailabilityBitfield>;

polkadot/primitives/src/v2/signed.rs

@@ -148,6 +148,30 @@ impl<Payload: EncodeAs<RealPayload>, RealPayload: Encode> Signed<Payload, RealPa
 	{
 		Signed(self.0.unchecked_convert_payload())
 	}
+
+	/// Convert `Payload` into some claimed `SuperPayload` if the encoding matches.
+	///
+	/// Succeeds if and only if the super-payload provided actually encodes as
+	/// the expected payload.
+	pub fn convert_to_superpayload<SuperPayload>(
+		self,
+		claimed: SuperPayload,
+	) -> Result<Signed<SuperPayload, RealPayload>, (Self, SuperPayload)>
+	where
+		SuperPayload: EncodeAs<RealPayload>,
+		Payload: Encode,
+	{
+		if claimed.encode_as() == self.0.payload.encode_as() {
+			Ok(Signed(UncheckedSigned {
+				payload: claimed,
+				validator_index: self.0.validator_index,
+				signature: self.0.signature,
+				real_payload: sp_std::marker::PhantomData,
+			}))
+		} else {
+			Err((self, claimed))
+		}
+	}
 }
 
 // We can't bound this on `Payload: Into<RealPayload>` because that conversion consumes