Add default rule match panic branch.

ast-generator/src/leaf_enum_build_fn.rs

@@ -29,7 +29,8 @@ pub fn make_leaf_enum_build_fn(leaf_enum_build_spec: &LeafEnumBuildSpec) -> Toke
         fn #build_fn_ident(#pair_ident: Pair<Rule>) -> #return_type_ident {
             let inner_pair = #pair_ident.into_inner().next().unwrap();
             match inner_pair.as_rule() {
-                #(#rule_branches),*
+                #(#rule_branches,)*
+                _ => panic!("Unexpected rule: {:?}", inner_pair.as_rule())
             }
         }
     }

ast-generator/src/struct_build_fn.rs

@@ -248,6 +248,7 @@ pub fn make_struct_build_fn(build_spec: &StructBuildSpec) -> TokenStream {
         for inner_pair in #pair_ident.into_inner() {
             match inner_pair.as_rule() {
                 #(#rule_matchers)*
+                _ => panic!("Unexpected rule: {:?}", inner_pair.as_rule())
             }
         }
     };