also known as magic that go itself won't support.

Why not? Isn’t this just a consolidated effort to create a tool invokable by the toolexec flag?

The difference between compiled code vs the code you wrote makes behaviours non-obvious.

https://github.com/golang/go/issues/69887

The written code is the code that is executed. What’s wrong with that? I’d take macros instead of that any time of the day.

Java has the same mechanism, the -javaagent flag. It has exactly the same drawbacks you mention.