early return on duplicate span lowerings

Author: jdonszelmann

compiler/rustc_ast_lowering/src/lib.rs

@@ -220,6 +220,13 @@ struct SpanLowerer {
 impl SpanLowerer {
     fn lower(&self, span: Span) -> Span {
         if self.is_incremental {
+            // early return: span lowering takes some time since it accesses the query dependency graph
+            // to make sure we rerun the right code paths when spans change. When we've already lowered a span,
+            // or don't have to, bail out ASAP.
+            if span.is_dummy() || span.parent().is_some_and(|i| i == self.def_id) {
+                return span;
+            }
+
             span.with_parent(Some(self.def_id))
         } else {
             // Do not make spans relative when not using incremental compilation.