Add `Span::def_site`-the-API
diff --git a/src/stable.rs b/src/stable.rs
index 911d648..8277d4c 100644
--- a/src/stable.rs
+++ b/src/stable.rs
@@ -137,13 +137,17 @@
     }
 }
 
-#[derive(Clone, Copy, Default, Debug)]
+#[derive(Clone, Copy, Debug)]
 pub struct Span;
 
 impl Span {
     pub fn call_site() -> Span {
         Span
     }
+
+    pub fn def_site() -> Span {
+        Span
+    }
 }
 
 #[derive(Copy, Clone)]