Function eii 

pub(crate) fn eii(
    ecx: &mut ExtCtxt<'_>,
    span: Span,
    meta_item: &MetaItem,
    item: Annotatable,
) -> Vec<Annotatable>

#[eii]
fn panic_handler();

// or:

#[eii(panic_handler)]
fn panic_handler();

// expansion:

extern "Rust" {
    fn panic_handler();
}

#[rustc_builtin_macro(eii_shared_macro)]
#[eii_extern_target(panic_handler)]
macro panic_handler() {}