gen-host-js: Top-level await compatibility via $init promise

[guybedford]

This adds a new compatibility feature, --tla-compat which is enabled under the --compat flag as well. Top-level await support is wide, but still gives some gap on browser compatibility (around 5% of browsers support modules but not TLA), so is still useful for maximum JS support.

When used, all top-level Wasm loads and instantiations that rely on await get wrapped in a single $init promise, which is exported and must be awaited before any component functions can be called. This only applies to ESM instance mode and not --instantiation.

If calling any exported component functions before initialization has been completed, an error is thrown informing that the $init promise must first be awaited. The $init export name is used as it should be an invalid world export name I believe.

[alexcrichton]

This seems like a reasonable feature, but the internals feel pretty heavy-handed with lots of if/else strewn throughout code generation. Instead of this structure could something be generated along the lines of:

async function init() {
    // current init, always using `await`
}

// if --tla-compat
init()

// if not-compat
await init()

That way I think just the exports would get the let-vs-otherwise definition treatment?

[guybedford]

@alexcrichton the problem is I don't know of a single JS optimizer short of closure that will unwrap async function init () {}; await init(). Byte size in JS is important and a feature of this output.

[guybedford]

I've managed to remove one branch with an update commit. The remaining 6 branches are for:

• One branch for the $init function itself
• One branch for exported functions to throw if called when unitialized
• Three branches to hoist the memory, realloc and postReturn variable definitions from const function definitions to out-scope let definitions. I'd prefer not to collapse this branch since that would involve removing const declarations which could be useful optimization information for an optimizer on non-tla-compat source
• One branch to vary the WebAssembly instantiation routine from setting the local const versus global let variable in TLA compat.

With the current output size constraints I'm not sure I can simplify more than this, but I'm open to refactoring suggestions.

[guybedford]

Ok, I've unified on just the single branch as discussed, leaving any unwrapping to rather be the job of an optimizer.