I agree with your point 1 and it’s a weakness of this proposal, point 6 seems like a pretty rare occurence that could be addressed by using different source files for your different cross-builds, but I don’t think that points 2 to 5 apply to this proposal: I’m not suggesting that the compiler starts rewriting your code under your feet, that would be pretty chaotic indeed. Instead, the compiler would simply emit an error if the annotated code did not contain the expected generated code, this is something that can already be accomplished today as demonstrated by [Proof of Concept] Code generation via rewriting errors in macro annotations by smarter · Pull Request #16545 · lampepfl/dotty · GitHub, it’s just awkward to do because there’s no nice API for it.
The other piece of the puzzle is IDE integration via code actions generated from the compiler messages, this is something that is actively being worked on in both Scala 2 and Scala 3 for built-in rewrites in Roadmap for actionable diagnostics and it’s natural to expose that ability in the macro API too.
So really the only thing specific to this proposal is the idea of having some extra macro APIs around pretty-printing and diffing trees to make the proposed pattern easier to implement in a robust way. It doesn’t even need to be part of the standard library: if we can’t reach consensus that this is worth having, someone else might decide to make their own macro library that implements these APIs to promote this pattern.