diff --git a/src/libsyntax/parse/token.rs b/src/libsyntax/parse/token.rs
index bb1f8f1d1d9..2b9df61120e 100644
--- a/src/libsyntax/parse/token.rs
+++ b/src/libsyntax/parse/token.rs
@@ -115,7 +115,7 @@ pub enum nonterminal {
     nt_matchers(~[ast::matcher])
 }
 
-pub fn binop_to_str(o: binop) -> ~str {
+pub pure fn binop_to_str(o: binop) -> ~str {
     match o {
       PLUS => ~"+",
       MINUS => ~"-",