Allow macro annotation to transform companion

[hamzaremmal]

Allow MacroAnnotations to update the companion of a definition

We extend the MacroAnnotation api to allow to modify the companion of a class or an object.

Specification

1. Order of expansion

• We expand the definitions in program order.
• We expand the annotations of the outer scope first, then we expand the inner definitions.
• Annotations are expanded from the outer annotation to the inner annotation.

In the following example, we expand the annotations in this order: a1, a2, a3.

@a1 @a2
class Foo:
  @a3 def foo = ???

2. Expansion of the companion

We always expand the latest available tree. If an annotation defined on class Foo changes its companion (object Foo) and the class is defined before object, the expansion of the annotations on the object will be performed on the result of the expansion of class.

3. The program order is maintained

We maintain the program order in the definitions that were expanded.

4. Backtrack and reprocess

Example:

@a1 class Foo
@a2 object Foo

If the @a2 annotation changes the definitions in class Foo, we will rerun the algorithm on the result of this new expansion. Please note that we don't allow to generate code with MacroAnnotations, the reason for rerunning the algorithm is to expand and inline possible macros that we generated.

---

Closes #19676

[Gedochao]

It has been decided to be included in the 3.5.0 release.

[hamzaremmal]

I will add support for stoping macro expansion in a second PR (this one is already contained and has many changes in the tests)

[nicolasstucki]

I still have not looked into compiler/src/dotty/tools/dotc/transform/MacroAnnotations.scala

[nicolasstucki]

What kind of error handling?

[hamzaremmal]

What if the interpretation of the annotation failed and returned None ? We might add a better handling here and avoid a possible compiler crash in the future

[nicolasstucki]

Then we should fix this now. Maybe this would be enough. Just check that the interpretation error has been reported.

    val annotInstance = interpreter.interpret[MacroAnnotation](annot.tree) match
      case Some(annotInstance) => annotInstance
      case None => return (List(tree), companion)

[hamzaremmal]

We agreed to investigate this later

[nicolasstucki]

Then we should fix this now. Maybe this would be enough. Just check that the interpretation error has been reported.

    val annotInstance = interpreter.interpret[MacroAnnotation](annot.tree) match
      case Some(annotInstance) => annotInstance
      case None => return (List(tree), companion)

[jchyb]

Looks good to me, I cannot find anything wrong here. Just one semi-related question. I can't find where do we check if the generated tree contains a macro annotation. Is that in TreeChecker?

[hamzaremmal]

I can't find where do we check if the generated tree contains a macro annotation. Is that in TreeChecker?

Generated trees cannot have annotations. We do a second run to inline code in the new defintions if necessary (last paragraph in the description).