Add a fuzzer utility for ensuring types are inhabitable (#5541)

Some valid GC types, such as non-nullable references to bottom heap types and
types that contain non-nullable references to themselves, are uninhabitable,
meaning it is not possible to construct values of those types. This can cause
problems for the fuzzer, which generally needs to be able to construct values of
arbitrary types.

To simplify things for the fuzzer, introduce a utility for transforming type
graphs such that all their types are inhabitable. The utility performs a DFS to
find cycles of non-nullable references and breaks those cycles by introducing
nullability.

The new utility is itself fuzzed in the type fuzzer.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/wasm-type.h b/src/wasm-type.h
index 67223705e..d72f5477d 100644
--- a/src/wasm-type.h
+++ b/src/wasm-type.h
@@ -405,6 +405,8 @@ public:
   // Returns true if left is a subtype of right. Subtype includes itself.
   static bool isSubType(HeapType left, HeapType right);
 
+  std::vector<Type> getTypeChildren() const;
+
   // Return the ordered HeapType children, looking through child Types.
   std::vector<HeapType> getHeapTypeChildren() const;