The GC proposal now specifies how the GC objects we implement (structs/arrays) should interact with JS. [1]

Essentially, these objects are allowed to flow to and from JS as an opaque value, but every JS operation on one of these objects will throw an exception.

We basically implement this, except for allowing the reading of fields of structs and elements of arrays. We use this pretty extensively for testing. We should add a special testing only builtin to support this wasmReadField(object, fieldIndex) when we implement this.

[1] https://github.com/WebAssembly/gc/blob/main/proposals/gc/MVP-JS.md

https://hg.mozilla.org/mozilla-central/rev/4017661e8be2