SWR should throw when trying to suspend on the server

[kara]

Bug report

Description / Observed Behavior

Server-side streaming with useSWR appears to work, in that you can see the data fetched on the server and sent through script tags later on. However, it isn't currently set up to share data between the server and client, causing a client-side fetch to happen in addition to the server-side fetch. Since nothing breaks or warns, it looks like this is fully working when it's not.

Expected Behavior

As suggested by @devknoll here, useSWR should throw a friendly error when the the suspense flag is on and it tries to suspend on the server, so folks know this isn't yet implemented fully.

Repro Steps / Code Example

1. Have the following code:
   index.js

<Suspense fallback={<div>Loading profile…</div>}>
    <Profile />
</Suspense>

profile.jsx

function Profile(props) {
   const data = useSWR(fetcher, '/some/profile', {suspense: true})
   return (
        <div> Profile ID: { data.id } </div>
   );
}

2. Turn on the concurrentFeatures flag.
3. Check the HTML response in the network tab and see the script tags being sent from the server with the data.
4. See that there are also client-side fetches for the same data in the network tab.

[chungwu]

One use case that this breaks is when doing rendering in some "simulated" runtime on the server, like react-ssr-prepass, which depends on useSWR() throwing these promises so that it can wait for them to resolve and use the fetched data to pre-populate the data cache. Unfortunately, this now errors out with React 18 on the server. There is no "fallback" data to specify, since the point of using react-ssr-prepass is to populate that fallback data cache.

Would you consider a flag for SWR config that would preserve the previous behavior, throwing the promise even on React 18 + server?

Thanks!