Support Dagger KSP in Anvil

[ZacSweers]

Filing an issue for posterity/tracking. I'm planning to take a stab at this.

Pasting what I wrote in kotlin-lang here: https://kotlinlang.slack.com/archives/C013BA8EQSE/p1669155272374909

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With Dagger’s upcoming support for KSP, we should make Anvil work with this new pattern

It’s a bit tricky, as it has always worked up to this point by running a special IR plugin during kapt stub generation to modify/add annotations to certain classes for Dagger’s use in kapt. Obviously that won’t work in a KSP world since it’s all one task.

What I’m thinking currently is something clever like this:
Anvil exposes its own delegating SymbolProcessor that handles running Dagger’s.
It would then decorate KSP’s Resolver APIs to intercept them and add this metadata (annotations, etc) to the KSP types on their way through.

So for example, this type

@MergeComponent(...)
interface AppComponent

This processor would then intercept that and construct a new KSClassDeclaration that almost entirely mirrors the previous except that it adds the merged @Component annotation that anvil synthesizes instead.

Lastly, when resolving annotated elements, it would intercept searches for Component types and forward the request as a @MergeComponent request instead.

Same patterns would be used for merged modules.

While weird and obviously not something KSP could officially support, I do think this could work more or less entirely within the bounds of KSP’s public API.

[ZacSweers]

I think an initial start to this may require limiting Anvil to only running in a separate compilation. Anvil supports generating more sources and incurring subsequent rounds, but it's not totally clear if that will be possible while running under KSP, because this information would need to be conveyed to KSP and delay the component processing to a later round.