Check for internal invariant violations in simple parser target (#165)

cedar-drt/fuzz/fuzz_targets/simple-parser.rs

@@ -17,13 +17,32 @@
 #![no_main]
 
 use cedar_drt_inner::fuzz_target;
+use cedar_policy_core::parser::err::{ParseError, ToASTError};
 use cedar_policy_core::parser::parse_policyset;
 
 fuzz_target!(|input: String| {
     // Ensure the parser does not crash
     #[allow(clippy::single_match)]
     match parse_policyset(&input) {
         Ok(_) => (),
-        Err(_) => (),
+        Err(errs) => {
+            // Also check that we don't see a few specific errors.
+            // `AnnotationInvariantViolation` and `MembershipInvariantViolation`
+            // are documented as only being returned for internal invariant violations.  It's not
+            // entirely clear when `MissingNodeData` might be returned, but I don't believe it
+            // should be possible, and, practically, it doesn't make this target fail.
+            assert!(
+                !errs.0.iter().any(|e| matches!(
+                    e,
+                    ParseError::ToAST(
+                        ToASTError::AnnotationInvariantViolation
+                            | ToASTError::MembershipInvariantViolation
+                            | ToASTError::MissingNodeData
+                    )
+                )),
+                "{:?}",
+                errs
+            )
+        }
     };
 });