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kotlinlang

This is Idea/AndroidStudio feature (maybe also Android Lint)

```
"$ARG_DOC_STUBS ", "Generate documentation stub source files for the API. Documentation stub " +
                "files are similar to regular stub files, but there are some differences. For example, in " +
                "the stub files, we'll use special annotations like @RecentlyNonNull instead of @NonNull to " +
                "indicate that an element is recently marked as non null, whereas in the documentation stubs we'll " +
                "just list this as @NonNull...
```

This is just for their documentation generation

(1) There are some annotations here which are not in the support library, such as @RecentlyNullable and @RecentlyNonNull. These are used only in the stubs to automatically mark code as recently annotated with null/non-null. We do not want these annotations in the source code; the recent-ness is computed at build time and injected into the stubs in place of the normal null annotations.

Yes, I see what you mean, they use it only for their own code

one more reason to create a feature request in Kotlin issue tracker