fix: require Server/ClientRpcParams to be last parameters in RPC methods (backport)

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -25,6 +25,7 @@ Additional documentation and release notes are available at [Multiplayer Documen
 - Fixed error when serializing ConnectionApprovalMessage with scene management disabled when one or more objects is hidden via the CheckObjectVisibility delegate (#1720)
 - Fixed CheckObjectVisibility delegate not being properly invoked for connecting clients when Scene Management is enabled. (#1680)
 - Fixed NetworkList to properly call INetworkSerializable's NetworkSerialize() method (#1682)
+- Fixed ServerRpcParams and ClientRpcParams must be the last parameter of an RPC in order to function properly. Added a compile-time check to ensure this is the case and trigger an error if they're placed elsewhere (#1721)
 - Fixed FastBufferReader being created with a length of 1 if provided an input of length 0 (#1724)
 - Fixed The NetworkConfig's checksum hash includes the NetworkTick so that clients with a different tickrate than the server are identified and not allowed to connect (#1728)
 - Fixed OwnedObjects not being properly modified when using ChangeOwnership (#1731)

com.unity.netcode.gameobjects/Editor/CodeGen/NetworkBehaviourILPP.cs

@@ -942,13 +942,29 @@ private void InjectWriteAndCallBlocks(MethodDefinition methodDefinition, CustomA
                     var paramDef = methodDefinition.Parameters[paramIndex];
                     var paramType = paramDef.ParameterType;
                     // ServerRpcParams
-                    if (paramType.FullName == CodeGenHelpers.ServerRpcParams_FullName && isServerRpc && paramIndex == paramCount - 1)
+                    if (paramType.FullName == CodeGenHelpers.ServerRpcParams_FullName)
                     {
+                        if (paramIndex != paramCount - 1)
+                        {
+                            m_Diagnostics.AddError(methodDefinition, $"{nameof(ServerRpcParams)} must be the last parameter in a ServerRpc.");
+                        }
+                        if (!isServerRpc)
+                        {
+                            m_Diagnostics.AddError($"ClientRpcs may not accept {nameof(ServerRpcParams)} as a parameter.");
+                        }
                         continue;
                     }
                     // ClientRpcParams
-                    if (paramType.FullName == CodeGenHelpers.ClientRpcParams_FullName && !isServerRpc && paramIndex == paramCount - 1)
+                    if (paramType.FullName == CodeGenHelpers.ClientRpcParams_FullName)
                     {
+                        if (paramIndex != paramCount - 1)
+                        {
+                            m_Diagnostics.AddError(methodDefinition, $"{nameof(ClientRpcParams)} must be the last parameter in a ClientRpc.");
+                        }
+                        if (isServerRpc)
+                        {
+                            m_Diagnostics.AddError($"ServerRpcs may not accept {nameof(ClientRpcParams)} as a parameter.");
+                        }
                         continue;
                     }