How to use WebAssembly Reference Types with Clang

I'm writing a WebAssembly application in C using Clang and LLD.

How do I write a C function that takes an externref as a parameter and how do I call this function from JavaScript? How do I use the externref value in C to call an imported function that is defined in JavaScript?

Support for WebAssembly reference types was added to Clang on Feb 18, 2023.

Here is a simple program that uses reference types. A function foo is implemented in C that takes an externref as an argument and uses that argument to call a function bar which is implemented in JavaScript.

typedef __externref_t externref_t;

#define WASM_IMPORT(module, name) __attribute__((import_module(module), import_name(name)))
#define WASM_EXPORT(name) __attribute__((export_name(name)))

WASM_IMPORT("env", "bar") void bar(externref_t arg);

WASM_EXPORT("foo") void foo(externref_t arg) {
    bar(arg);
}

The program can be compiled with clang foo.c -o foo.wasm -target wasm32 -mreference-types -nostdlib -Wl,--no-entry and it can be used from JavaScript as follows:

const imports = {
    env: {
        bar(arg) {
            console.log(arg);
        }
    }
};
const obj = await WebAssembly.instantiateStreaming(fetch("foo.wasm"), imports);
obj.instance.exports.foo("hello");