[Custom Descriptors] Test struct.new execution (#7687)

And fix a missing check for null descriptors. Passing a null descriptor
to struct.new should trap.

Author: tlively

src/wasm-interpreter.h

@@ -1811,6 +1811,9 @@ class ExpressionRunner : public OverriddenVisitor<SubType, Flow> {
     if (desc.breaking()) {
       return desc;
     }
+    if (desc.getSingleValue().isNull()) {
+      trap("null descriptor");
+    }
     return makeGCData(std::move(data), curr->type, desc.getSingleValue());
   }
   Flow visitStructGet(StructGet* curr) {

test/spec/struct.new-desc.wast

@@ -3,22 +3,47 @@
     (type $pair (descriptor $pair.desc (struct (field i32 i64))))
     (type $pair.desc (describes $pair (struct)))
   )
-  (func $struct.new (result (ref (exact $pair)))
-    (local $desc (ref null (exact $pair.desc)))
+  (func $struct.new (param $desc (ref null (exact $pair.desc))) (result (ref (exact $pair)))
     (struct.new $pair
-      (i32.const 0)
-      (i64.const 1)
+      (i32.const 1)
+      (i64.const 2)
       (local.get $desc)
     )
   )
-  (func $struct.new_default (result (ref (exact $pair)))
-    (local $desc (ref null (exact $pair.desc)))
+  (func (export "check-new") (result i32)
+    (local $pair (ref null $pair))
+    (local.set $pair (call $struct.new (struct.new $pair.desc)))
+    (i32.and
+      (i32.eq (struct.get $pair 0 (local.get $pair)) (i32.const 1))
+      (i64.eq (struct.get $pair 1 (local.get $pair)) (i64.const 2))
+    )
+  )
+  (func (export "new-null-desc")
+    (drop (call $struct.new (ref.null none)))
+  )
+  (func $struct.new_default (param $desc (ref null (exact $pair.desc))) (result (ref (exact $pair)))
     (struct.new_default $pair
       (local.get $desc)
     )
   )
+  (func (export "check-new-default") (result i32)
+    (local $pair (ref null $pair))
+    (local.set $pair (call $struct.new_default (struct.new $pair.desc)))
+    (i32.and
+      (i32.eq (struct.get $pair 0 (local.get $pair)) (i32.const 0))
+      (i64.eq (struct.get $pair 1 (local.get $pair)) (i64.const 0))
+    )
+  )
+  (func (export "new-default-null-desc")
+    (drop (call $struct.new_default (ref.null none)))
+  )
 )
 
+(assert_return (invoke "check-new") (i32.const 1))
+(assert_return (invoke "check-new-default") (i32.const 1))
+(assert_trap (invoke "new-null-desc") "null descriptor")
+(assert_trap (invoke "new-default-null-desc") "null descriptor")
+
 (assert_invalid
   (module
     (rec