Add helper macro to implement bincode serialization of program-specific errors

sdk/src/native_program.rs

@@ -51,6 +51,20 @@ impl std::fmt::Display for ProgramError {
 }
 impl std::error::Error for ProgramError {}
 
+// Convenience macro to serialize (and potentially truncate) a program-specific error to pass in
+// ProgramError::CustomError
+#[macro_export]
+macro_rules! custom_error(
+    ($program_error:expr) => ({
+        use bincode::serialize;
+        let mut error = serialize(&$program_error).expect("failed to serialize program error");
+        if error.len() > 32 {
+            error.truncate(32);
+        }
+        error
+    });
+);
+
 // All native programs export a symbol named process()
 pub const ENTRYPOINT: &str = "process";