Try using both types of join

prusti-contracts/prusti-specs/src/specifications/preparser.rs

@@ -905,8 +905,11 @@ impl RustOp {
 }
 
 fn join_spans(s1: Span, s2: Span) -> Span {
-    // This works even when compiled with stable, compared to `s1.join(s2)`
-    s1.unwrap().join(s2.unwrap()).expect("Failed to join spans!").into()
+    // This will not work on stable, but will work outside of proc macros (e.g. tests)
+    s1.join(s2).unwrap_or_else(||
+        // This works even when compiled with stable, but panics when outside of the proc macro context
+        s1.unwrap().join(s2.unwrap()).expect("Failed to join spans!").into()
+    )
 }
 
 #[cfg(test)]