docs: private Description checkSubtype(ClassTree tree, VisitorState state) {
      ClassSymbol sym = ASTHelpers.getSymbol(tree);
      if (sym == null) {
        return NO_MATCH;
      }
      Type superType = immutableSupertype(sym, state);
      if (superType == null) {
        return NO_MATCH;
      }
      String message =
          String.format(
              "Class extends @Immutable type %s, but is not annotated as immutable", superType);
      Fix fix =
          SuggestedFix.builder()
              .prefixWith(tree, "@Immutable ")
              .addImport(Immutable.class.getName())
              .build();
      return buildDescription(tree).setMessage(message).addFix(fix).build();
    }