Fix bug with fork choice, tidy

eth2/state_processing/src/per_block_processing/errors.rs

@@ -136,6 +136,9 @@ pub enum AttestationInvalid {
         delay: u64,
         attestation: Slot,
     },
+    /// The attestation is attesting to a state that is later than itself. (Viz., attesting to the
+    /// future).
+    AttestsToFutureState { state: Slot, attestation: Slot },
     /// Attestation slot is too far in the past to be included in a block.
     IncludedTooLate { state: Slot, attestation: Slot },
     /// Attestation target epoch does not match the current or previous epoch.

eth2/state_processing/src/per_block_processing/verify_attestation.rs

@@ -62,6 +62,17 @@ pub fn verify_attestation_for_state<T: EthSpec>(
         Invalid::BadShard
     );
 
+    let attestation_slot = state.get_attestation_data_slot(&data)?;
+
+    // An attestation cannot attest to a state that is later than itself.
+    verify!(
+        attestation_slot <= state.slot,
+        Invalid::AttestsToFutureState {
+            state: state.slot,
+            attestation: attestation_slot
+        }
+    );
+
     // Verify the Casper FFG vote and crosslink data.
     let parent_crosslink = verify_casper_ffg_vote(attestation, state)?;