[NominalFuzzing] GTO: trap on null ref in removed struct.set (#4607)

When a field has no reads, we remove all its writes, but we did this:

(struct.set $foo A B)
=>
(drop A) (drop B)

We also need to trap if A, the reference, is null, which this PR
fixes,

(struct.set $foo A B)
=>
(drop (ref.as_non_null A)) (drop B)

1 files changed, 5 insertions, 3 deletions

diff --git a/src/passes/GlobalTypeOptimization.cpp b/src/passes/GlobalTypeOptimization.cpp
index 7e1ebf04e..53c76ca6f 100644
--- a/src/passes/GlobalTypeOptimization.cpp
+++ b/src/passes/GlobalTypeOptimization.cpp
@@ -391,10 +391,12 @@ struct GlobalTypeOptimization : public Pass {
           // Map to the new index.
           curr->index = newIndex;
         } else {
-          // This field was removed, so just emit drops of our children.
+          // This field was removed, so just emit drops of our children (plus a
+          // trap if the input is null).
           Builder builder(*getModule());
-          replaceCurrent(builder.makeSequence(builder.makeDrop(curr->ref),
-                                              builder.makeDrop(curr->value)));
+          replaceCurrent(builder.makeSequence(
+            builder.makeDrop(builder.makeRefAs(RefAsNonNull, curr->ref)),
+            builder.makeDrop(curr->value)));
         }
       }