When `notFound()` is thrown from metadata, it's caught by a
`<MetadataOutlet />` rendered as a sibling to the page component. But we
currently only pass the custom not-found component to the
`<NotFoundBoundary />` for the `children` slot. This means that if a
parallel route throws a `notFound()` in `generateMetadata`, it'd be
caught by the root not found, which would be unexpected.
This mirrors the logic for determining whether or not a `notFound`
boundary should be provided. A side effect of this is that if you throw
a `notFound()` in `generateMetadata` for a segment that _only_ has
parallel routes, and no `children` slot, it won't be caught by the
boundary. But fixing this will require a larger refactor.
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Closes NEXT-3320
### What
When a page throws a `notFound()` error in `generateMetadata`, and the
page contains parallel route(s), an unhandled runtime error would be
thrown rather than displaying the not found page.
### Why
We use the `<MetadataOutlet />` component to throw any errors caught
during metadata resolution once the metadata is rendered by React so
that it can be caught by an error boundary, so that it doesn't throw
during metadata resolution. A promise is tracked & resolved with an
error once the metadata tree is rendered. Once the promise resolves, the
outlet component will throw.
However, every `__PAGE__` segment that would be rendered as part of the
page the user is on will render this `<MetadataOutlet />` component. We
only need a single outlet per segment as only a single error needs to be
thrown & caught.
### How
This will only render a `MetadataOutlet` for the first parallel route
that is encountered at each segment depth, as we only need a single
handler to throw the error.
Fixes#65013
Closes NEXT-3222
### What & Why?
When visiting a route that attempts to render a slot with no page & no default, the fallback behavior is to trigger a 404. However this can lead to a confusing development experience for complex parallel routing cases as you might not realize a default is missing, or which slot is causing the error.
Previous issues where this caused confusion:
- https://github.com/vercel/next.js/issues/51805
- https://github.com/vercel/next.js/issues/49569
### How?
This is a dev-only modification to track parallel slots that are using the default fallback (aka missing). When the `NotFoundBoundary` is triggered in development mode, this will log a warning about why it 404ed, along with a list of slot(s) that were determined to be missing.
### Future
We should eventually lift this into some sort of dev-only UI to help catch it when not monitoring the browser console (similar to the error overlay). However, this will require some design thought and isn't necessary for the first iteration.
Closes NEXT-1798
