Add private Span field to LexError

Author: dtolnay

src/fallback.rs

@@ -35,13 +35,17 @@ pub(crate) struct TokenStream {
 }
 
 #[derive(Debug)]
-pub(crate) struct LexError {}
+pub(crate) struct LexError {
+    span: Span,
+}
 
 impl LexError {
     // Codepaths that will need a meaningful span attached as part of
     // https://github.com/alexcrichton/proc-macro2/issues/178
     pub(crate) fn todo() -> Self {
-        LexError {}
+        LexError {
+            span: Span::call_site(),
+        }
     }
 }