Add `Span::def_site`-the-API
diff --git a/src/unstable.rs b/src/unstable.rs
index a3ce1fd..6705fb5 100644
--- a/src/unstable.rs
+++ b/src/unstable.rs
@@ -166,10 +166,8 @@
     pub fn call_site() -> Span {
         Span(proc_macro::Span::call_site())
     }
-}
 
-impl Default for Span {
-    fn default() -> Span {
+    pub fn def_site() -> Span {
         Span(proc_macro::Span::def_site())
     }
 }