RFR: 8287186: JDK modules participating in preview [v2]

[liach]

On Wed, 8 Jun 2022 18:24:35 GMT, Paul Sandoz <psandoz at openjdk.org> wrote:

>> Allow JDK modules that use preview features (preview language features or preview API features from dependent modules) to participate without the need to compile with `--enable-preview`.
>> 
>> It's difficult to enable participation using an annotation due to the nature in which symbols are encountered when processing source as there is no guaranteed order to the processing of certain symbols.
>> 
>> Instead a JDK module participates if the `java.base` package `jdk.internal.javac` is exported to that module (@lahodaj clever idea!). An internal annotation `jdk.internal.javac.ParticipatesInPreview` can be declared on the module. Such a declaration cannot be enforced but does by its use require the `jdk.internal.javac`'s export list to be updated.
>> 
>> The modules `jdk.incubator.vector` and `jdk.incubator.concurrent` have been updated accordingly, both of which participate in preview APIs (APIs in `java.lang.foreign` and `Thread.ofVirtual`, respectively).
>
> Paul Sandoz has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Let java.management participate in preview features.

Just curious, is it still up to incubator modules' discretion to avoid accidental user access to preview content via the modules without enabling preview, like the `PreviewFeatures.ensureEnabled()` in `StructuredTaskScope`?

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PR: https://git.openjdk.java.net/jdk/pull/9087