Function proc_macro_error::entry_point

pub fn entry_point<F>(f: F) -> TokenStream
where
    F: FnOnce() -> TokenStream + UnwindSafe,

This is the entry point for your proc-macro. It is must to be used on the top level of the proc-macro (a function annotated with `#[proc_macro*] attribute).

Typically, you use #[proc_macro_error] instead, see module level docs.