rule/rule.go
func (r *Rule) Attr(key string) bzl.Expr {
	attr, ok := r.attrs[key]
	if !ok {
		return nil
	}
	return attr.RHS
}
func (r *Rule) Attr(key string) bzl.Expr {
	attr, ok := r.attrs[key]
	if !ok {
		return nil
	}
	return attr.RHS
}
// Attr returns the value of the named attribute. nil is returned when the
// attribute is not set.