hygiene: Make sure expansion info is set at most once for a given Mark

src/libsyntax_pos/hygiene.rs

@@ -85,7 +85,14 @@ pub fn expn_info(self) -> Option<ExpnInfo> {
 
     #[inline]
     pub fn set_expn_info(self, info: ExpnInfo) {
-        HygieneData::with(|data| data.marks[self.0 as usize].expn_info = Some(info))
+        HygieneData::with(|data| {
+            let old_info = &mut data.marks[self.0 as usize].expn_info;
+            if let Some(old_info) = old_info {
+                panic!("expansion info is reset for the mark {}\nold: {:#?}\nnew: {:#?}",
+                       self.0, old_info, info);
+            }
+            *old_info = Some(info);
+        })
     }
 
     pub fn modern(mut self) -> Mark {