Deprecate ClassfileAnnotation, introduce ConstantAnnotation

[lrytz]

Annotations that need to be written to the classfile in the
Java-compatible format need to be defined in Java.

Scala annotations extending ConstantAnnotation can only accept
compile-time constants as argument. The implementation represents
constant annotations using the assocs field in the AnnotationInfo,
just like for Java-defined annotations. The implementation for getting
the constants and building the assocs is the same as for Java
annotations, and the same restrictions apply: arguments need to be
passed as named arguments (except if there's a single argument defining
the value parameter). This is a bit of an ad-hoc restriction for
Scala-defined constant annotations, but changing it to accept positional
args went over my time budget (there's no easy way to reuse the code
that deals with named arguments in general).

Internally, we still add the StaticAnnotation parent to Java-defined
annotations (in the Classfile parser, Java source parser, runtime
reflection) to make sure that Java annotations are not only emitted to
the classfile in the Java format, but also pickled.

[adriaanm]

Looks good. I got 99% through the review and then interrupted again.

[adriaanm]

How about the restriction on classfile annotations that they could not be inner classes?

[adriaanm]

Ah, I see this is caught by https://github.com/scala/scala/pull/6143/files#diff-4eab1aad4533a31c10565971e90f73eaR3738 (else if (!annType.typeSymbol.isJavaDefined))

[adriaanm]

I tried to resolve the conflicts with the web editor (cool!), but the UI hasn't caught up?

[xeno-by]

@adriaanm Wow the web editor is pretty cool indeed 😍

[adriaanm]

Some by-name fails due to my incomplete merge?

!! 680 - neg/t6895.scala                           [expected compilation failure]
!! 687 - neg/t6895b.scala                          [expected compilation failure]
!!  62 - run/byname.scala                          [compilation failed]

byname.scala:55: error: type mismatch;
 found   : Int
 required: () => ?
val testC00R = testC00 _
               ^
byname.scala:58: error: type mismatch;
 found   : Int
 required: () => ?
val testC00RR = testC00() _
                       ^
two errors found

[lrytz]

These test failures are in current 2.13.x, since the merge of #6092 / https://github.com/scala/scala/commits/8272151ac87628e2f1165547ff4b794f3226bbbc

[lrytz]

ping @adriaanm

[retronym]

FTR, this change seems to be causing some temporary problems when working on the 2.13.x branch. The reference compiler, 2.13.0-M2, incorrectly typechecks @SerialVersionUID(0L) to put the arguments in args, rather than assocs, because SerialVersionID no longer extends ClassfileAnnotation. The backend then omits the static field, because the pattern match doesn't look at args.

This causes a failure in SerializationStabilityTest.

[lrytz]

@retronym the failure only shows up when using the reference compiler with a current 2.13.x version of the library on the class- / sourcepath, not on CI where we bootstrap, correct? Would you like me to do something about it? M3 is coming out soon...

[retronym]

Yes, the problem appears specific to local development without bootstrapping.

I think we can just wait for the next release. I just wanted to note the problem for posterity on the off chance that it helps someone else avoid debugging it :)

With the benefit of hindsight, we might have been better to defer changing our annotations to the non-deprecated parent types until the next STARR update.