RFR: JDK-8246704: --release => "unknown enum constant PreviewFeature$Feature.TEXT_BLOCKS"

[Jim Laskey]

Seems like a reasonable approach. Code looks good.

> On Jun 11, 2020, at 10:26 AM, Jan Lahoda <jan.lahoda at oracle.com> wrote:
> 
> Hi,
> 
> When compiling with --release 14, warnings like this may be printed:
> warning: unknown enum constant PreviewFeature$Feature.TEXT_BLOCKS
>  reason: class file for jdk.internal.PreviewFeature$Feature not found
> 
> 
> The cause is fairly simple - the APIs associated with preview features (like new methods in j.l.String) are annotated with a JDK-internal annotation, which allows javac to detect them a provide an error/warning as appropriate. The issues is that the internal annotation is not inside the ct.sym, and loading an element annotated with it leads to resolution of the annotation, and to the warning above.
> 
> Unfortunately, this is not very trivial to fix, as javac needs the annotation to detect the API elements associated with preview features (and so the annotation needs to be in ct.sym). And, when compiling without --release, not resolving an annotation that is part of the classfile would provide inconsistent results.
> 
> So, the proposal is to use a different, synthetic, "annotation" (or rather name) inside ct.sym, and use that name only for detection of APIs associated with preview features and not resolve that annotation.
> 
> Proposed webrev:
> http://cr.openjdk.java.net/~jlahoda/8246704/webrev.00/
> 
> JBS:
> https://bugs.openjdk.java.net/browse/JDK-8246704
> 
> Any comments on this?
> 
> Thanks,
>    Jan