test(derive): close fifth mutation shard

Fail closed when generated form field names cannot map to Rust identifiers and delete the redundant empty generated-parameter branch.

req: form/004

req: diagnostics/003

req: test/022

req: test/023

hemx-derive/src/lib.rs

@@ -670,12 +670,8 @@ fn rust_ident(name: &str) -> Option<String> {
 }
 
 fn missing_handle_params(path: &PathBuf, ident: &str, function: &ItemFn) -> Vec<String> {
-    let required = handle_params(path, ident);
-    if required.is_empty() {
-        return Vec::new();
-    }
     let args = handler_arg_names(function);
-    required
+    handle_params(path, ident)
         .into_iter()
         .filter(|param| !args.contains(param))
         .collect()
@@ -1029,7 +1025,7 @@ mod tests {
         handler_syms_path, has_form_param, has_non_unit_return, is_type_named,
         join_contract_errors, missing_component_handlers, missing_form_generated_files_message,
         missing_handle_params, missing_handler_generated_files_message, parser_type,
-        returns_result, syms_contains_handle, HandlerPlacement,
+        returns_result, rust_ident, syms_contains_handle, HandlerPlacement,
     };
     use quote::{quote, ToTokens};
     use syn::{parse_quote, ItemFn, Type};
@@ -1432,6 +1428,16 @@ mod tests {
         assert!(form_model_type(&foreign_form).is_none());
     }
 
+    #[test]
+    fn generated_form_field_names_map_only_to_valid_rust_identifiers() {
+        assert_eq!(rust_ident("first-name").as_deref(), Some("first_name"));
+        assert_eq!(rust_ident("_private2").as_deref(), Some("_private2"));
+        for invalid in ["", "2fast", "with space", "naïve"] {
+            assert_eq!(rust_ident(invalid), None, "{invalid:?} must fail closed");
+        }
+        // test req: form/004 req: diagnostics/003
+    }
+
     #[test]
     fn handler_params_match_generated_param_names() {
         let path = std::env::temp_dir().join("hemx-derive-param-test.syms");
@@ -1453,6 +1459,7 @@ mod tests {
 
         assert!(missing_handle_params(&path, "show", &complete).is_empty());
         assert_eq!(missing_handle_params(&path, "show", &missing), vec!["mode"]);
+        assert!(missing_handle_params(&path, "unknown", &missing).is_empty());
 
         let _ = std::fs::remove_file(path);
     }