Function generate_attribute_macro_to_implement 

fn generate_attribute_macro_to_implement(
    ecx: &mut ExtCtxt<'_>,
    span: Span,
    macro_name: Ident,
    foreign_item_name: Ident,
    impl_unsafe: bool,
    decl_span: Span,
) -> Item

Generate a stub macro (a bit like in core) that will roughly look like:

// Since this a stub macro, the actual code that expands it lives in the compiler.
// This attribute tells the compiler that
#[builtin_macro(eii_shared_macro)]
// the metadata to link this macro to the generated foreign item.
#[eii_extern_target(<related_reign_item>)]
macro macro_name { () => {} }