Extend comment in UsedLocals::visit_lhs

compiler/rustc_mir/src/transform/simplify.rs

@@ -422,7 +422,9 @@ impl UsedLocals<'tcx> {
             // A use, not a definition.
             self.visit_place(place, PlaceContext::MutatingUse(MutatingUseContext::Store), location);
         } else {
-            // A definition. Although, it still might use other locals for indexing.
+            // A definition. The base local itself is not visited, so this occurrence is not counted
+            // toward its use count. There might be other locals still, used in an indexing
+            // projection.
             self.super_projection(
                 place.as_ref(),
                 PlaceContext::MutatingUse(MutatingUseContext::Projection),