Add support for JSpecify annotations

[coehlrich]

What it does

Adds support for JSpecify annotations

How to test

Author checklist

• I have thoroughly tested my changes
• The change is following the coding conventions
• I have signed the Eclipse Contributor Agreement (ECA)

Currently I have only started implementing support for the null mark annotations (and the kotlin.Metadata check is a part of the spec in the Null-marked Scope section)

There are 4 Nullness Operators:

• UNION_NULL: from the Nullable annotation and includes null
• MINUS_NULL: from the NonNull annotation and excludes null
• UNSPECIFIED: for unannotated code outside of null marked locations and is unspecified
• NO_CHANGE: used internally including for everything recognized inside null marked locations that aren't explicitly annotated where for regular types is the same as MINUS_NULL and for type variables uses the provided type argument

detectStandardAnnotation currently only returns a long and so can't currently return anything defined in ExtendedTagBits. How should I change detectStandardAnnotation to allow using ExtendedTagBits?

Currently there are multiple places where annotations are added to existing types internally. How should they be handled since NullMarked applies NO_CHANGE instead of MINUS_NULL?

[stephan-herrmann]

Originally I was optimistic regarding jspecify filling a relevant gap, but it's been quite a while since. At some point I lost interest as I sensed more focus on making things different rather than unifying prior art. I haven't checked their status for a while (last time I locked it lacked specification at the level of detail that would be necessary for implementing compliant checking).

With the prospect of null-restricted types in standard Java I'm not convinced that now is a good point in time to invest much into supporting jspecify.

As you focus on null marked, can you describe the differences between that one and ecj's @NonNullByDefault?

[coehlrich]

The main difference is:
ECJ's NonNullByDefault applies the NonNull annotation to everything in context that doesn't have a null annotation.

While JSpecify's NonNull annotation applies the nullness operator MINUS_NULL (which should be atleast roughly equivalent to ECJ's NonNull), the NullMarked annotation applies the operator NO_CHANGE which for regular types is equivalent to MINUS_NULL but for type variables it basically copies the nullness operator from the type argument.

[stephan-herrmann]

ECJ's NonNullByDefault applies the NonNull annotation to everything in context that doesn't have a null annotation.

There is more to it:

• the meaning of "everything" is fine tuned by the annotation detail of type DefaultLocation
  • note that omitting details selects the subset {PARAMETER, RETURN_TYPE, FIELD, TYPE_BOUND, TYPE_ARGUMENT, RECORD_COMPONENT} (excluding ARRAY_CONTENT and TYPE_PARAMETER).
• DefaultLocation.TYPE_ARGUMENT specifically excludes wildcards and type variables

Perhaps this allows users to apply NonNullByDefault in the exact same way as NullMarked? Or what exact difference remains?