To work with GC, you need some way to track if the GC'd object is accessible in WASM itself. You can't just have gc.release $addr because then you need to introduce a check everywhere you try and do something with $addr in WASM as WASM is supposed to be memory safe.
The reason why you probably need a custom string type is so you can actually embed string literals without relying on interop with the environment. If a WASM module tries to simulate this by having an initialization function that constructs all your string constants in linear memory or something, I could see that getting pretty expensive and/or difficult to optimize.
> To work with GC, you need some way to track if the GC'd object is accessible in WASM itself.
I've never heard of a GC with that kind of API. Usually any native code that holds a GC reference would either mark that reference as a root explicitly (eg. https://github.com/WebAssembly/design/issues/1459) or ensure that it can be traced from a parent object. Either way, this should prevent collection of the object for as long as the reference is reachable from a root. I agree that explicitly checking whether a GC'd object has been freed would not make any sense.
> The reason why you probably need a custom string type is so you can actually embed string literals without relying on interop with the environment.
WASM already has ways of embedding flat string data. This can be materialized into GC/heap objects at module startup. This must happen in some form anyway, as all GC-able objects must be registered with the GC upon creation, for them to be discoverable as candidates for collection.
Overall I still don't understand the issue. There is so much prior art for these patterns in native extensions for Python, PHP, Ruby, etc.
Take the simple example of memory.copy. How is it worse to compile memcpy() to import+memcpy() rather than memory.copy?
Why can't a string type be efficiently GC'd via an imported type?