Proper support for type annotations

[Alexander--]

In ideal world the following snippet would both compile and be inspectable from annotation processors:

void methodWithListParameter(List<@NonNull ? extends CustomParcelable>);

In reality ECJ does not appear to recognize type annotations as anything, but opaque source code tokens, and so does Javac.

Current situation

There is a way to make introspection on type annotations "work" in Javac:

1. Postpone your compilation until type analysis completes using Oracle-proprietary JavacTask API
2. Obtain the annotated type using Oracle proprietary Trees.getTypeMirror API.

Unfortunately, as of Javac 1.8.0_112 doing this requires significant effort (manually walking trees using tree API) and does not really work for any types, only for simple, non-composite types. The following TreePath will have no annotations:

List<@NonNull ? extends CustomParcelable>

You will have to cast it to parametrized tree and walk down to

@NonNull ? extends CustomParcelable

For which Trees.getTypeMirror will finally produce correctly annotated WildcardType.

The alternative is to use non-documented Javac classes, which will likely become unavailable in Java 9. And that still does not guarantee correct outcome.

Oncoming improvements

Javac have had a major injection of code to actually support type annotations. That code isn't included in Java 8 and will be available in Java 9 only.