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* Internal `act`: Unwrapping resolved promises This update our internal implementation of `act` to support React's new behavior for unwrapping promises. Like we did with Scheduler, when something suspends, it will yield to the main thread so the microtasks can run, then continue in a new task. I need to implement the same behavior in the public version of `act`, but there are some additional considerations so I'll do that in a separate commit. * Move throwException to after work loop resumes throwException is the function that finds the nearest boundary and schedules it for a second render pass. We should only call it right before we unwind the stack — not if we receive an immediate ping and render the fiber again. This was an oversight in 8ef3a7c that I didn't notice because it happens to mostly work, anyway. What made me notice the mistake is that throwException also marks the entire render phase as suspended (RootDidSuspend or RootDidSuspendWithDelay), which is only supposed to be happen if we show a fallback. One consequence was that, in the RootDidSuspendWithDelay case, the entire commit phase was blocked, because that's the exit status we use to block a bad fallback from appearing. * Use expando to check whether promise has resolved Add a `status` expando to a thrown thenable to track when its value has resolved. In a later step, we'll also use `value` and `reason` expandos to track the resolved value. This is not part of the official JavaScript spec — think of it as an extension of the Promise API, or a custom interface that is a superset of Thenable. However, it's inspired by the terminology used by `Promise.allSettled`. The intent is that this will be a public API — Suspense implementations can set these expandos to allow React to unwrap the value synchronously without waiting a microtask. * Scaffolding for `experimental_use` hook Sets up a new experimental hook behind a feature flag, but does not implement it yet. * use(promise) Adds experimental support to Fiber for unwrapping the value of a promise inside a component. It is not yet implemented for Server Components, but that is planned. If promise has already resolved, the value can be unwrapped "immediately" without showing a fallback. The trick we use to implement this is to yield to the main thread (literally suspending the work loop), wait for the microtask queue to drain, then check if the promise resolved in the meantime. If so, we can resume the last attempted fiber without unwinding the stack. This functionality was implemented in previous commits. Another feature is that the promises do not need to be cached between attempts. Because we assume idempotent execution of components, React will track the promises that were used during the previous attempt and reuse the result. You shouldn't rely on this property, but during initial render it mostly just works. Updates are trickier, though, because if you used an uncached promise, we have no way of knowing whether the underlying data has changed, so we have to unwrap the promise every time. It will still work, but it's inefficient and can lead to unnecessary fallbacks if it happens during a discrete update. When we implement this for Server Components, this will be less of an issue because there are no updates in that environment. However, it's still better for performance to cache data requests, so the same principles largely apply. The intention is that this will eventually be the only supported way to suspend on arbitrary promises. Throwing a promise directly will be deprecated.
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