Syntax extension plugin without attribute annotation?

Say I have the syntax extension compiler plugin and a source file in which you want to use the extension.

struct ExtensionType;

impl MultiItemModifier for SomeType {
    fn expand(
        ecx: &mut ExtCtxt,
        span: Span,
        meta_item: &MetaItem,
        annotatable: base::Annotatable,
    ) -> Vec<base::Annotatable> {
        // do something

pub fn registrar(reg: &mut Registry) {

fn main() {
    // do some stuff

Now my question is, is it somehow possible to use the syntax extension without any kind of attribute annotation within the source file?

I have come across the -Z --extra-plugins=val rustc compiler option, which allows to load a plugin without the #![plugin(my_plugin_name)] crate attribute.
But as far as I know, the expand function is only invoked (and thus the actual modification is performed) when there is some kind of attribute either #![my_plugin_name] (at the top of the module/file) or #[my_plugin_name] (before a top level item) present in the source file.

Is there a way I can avoid having to add either of these attributes?
Thank you very much