Server `render()` result shape is impractical to work with

[florian-lefebvre]

Describe the bug

When awaiting render() when compilerOptions.experimental.async isn't enabled, the experimental_async_ssr warning is shown. Problem is, there's no way to know if the result is awaitable from its shape because then is always present: https://github.com/sveltejs/svelte/blob/main/packages/svelte/src/internal/server/renderer.js#L352-L384. That's an issue for us in Astro.

My recommendation would be to only set the then property if async_mode_flag is true so we can do checks like 'then' in result:

let result = render(...)
if (isPromise(result)) {
  result = await result
}

That currently forces us to make a pretty big workaround, see withastro/astro#14526.

I'd be happy to open a PR

Reproduction

See withastro/astro#14508 and https://stackblitz.com/edit/github-tpnsqnas?file=src%2Fpages%2Findex.astro

Logs

System Info

N/A

Severity

annoyance

[ascorbic]

That would mean the warning was never logged

[florian-lefebvre]

Which may be fine? Not sure that's needed