Update compiler/rustc_pattern_analysis/src/constructor.rs

add note that `missing_empty` is cleared now

Co-authored-by: Nadrieril <Nadrieril@users.noreply.github.com>

compiler/rustc_pattern_analysis/src/constructor.rs

@@ -979,6 +979,7 @@ impl ConstructorSet {
             && !(pcx.is_top_level && matches!(self, Self::NoConstructors))
         {
             // Treat all missing constructors as nonempty.
+            // This clears `missing_empty`.
             missing.append(&mut missing_empty);
         }