early return on duplicate span lowerings

Author: jdonszelmann

compiler/rustc_ast_lowering/src/lib.rs

@@ -220,6 +220,23 @@ struct SpanLowerer {
 impl SpanLowerer {
     fn lower(&self, span: Span) -> Span {
         if self.is_incremental {
+            // early return
+            if span.parent().is_some_and(|i| i == self.def_id) {
+                return span;
+            }
+
+            // #[cfg(debug_assertions)]
+            // {
+            //     if !span.is_dummy()
+            //         && let Some(p) = span.parent()
+            //     {
+            //         assert!(
+            //             p != self.def_id,
+            //             "duplicate span lowering detected, parent of span {span:?} was already set to {:?}",
+            //             self.def_id
+            //         );
+            //     }
+            // }
             span.with_parent(Some(self.def_id))
         } else {
             // Do not make spans relative when not using incremental compilation.