---
 src/lib.rs | 1 +
 1 file changed, 1 insertion(+)

diff --git b/src/lib.rs a/src/lib.rs
index 2f31fc65..d1fe77e8 100644
--- b/src/lib.rs
+++ a/src/lib.rs
@@ -3,6 +3,7 @@
 #![crate_type = "proc-macro"]
 #![forbid(unsafe_code)]
 #![warn(rust_2018_idioms, trivial_casts, unused_qualifications)]
+extern crate proc_macro;
 
 use proc_macro2::{Ident, TokenStream};
 use quote::{format_ident, quote};