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A naive AST traversal might also interpret some bit of fancy macro DSL as being “regular” code and have a spurious match. Any reliable whole-program source transformation in CL needs to leverage compiler-specific extensions (code walker, environment, etc.).

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I was assuming that at minimum the forms would be fully macroexpanded first (though macrolet is its own problem there). I also have the sense this might be easier in some Scheme dialects, but I don't know any of them well enough to say (thus that's part of my question).

IIRC it's difficult to walk macroexpanded code in Common Lisp because you have to deal with lots of implementation-specific special forms.

(Edit) This article has some good info: https://zenodo.org/record/3254669#.YL3-WzYzb2w

Exactly! If you walk the expanded form you have implementation-specific junk to deal with. Also, it may not be possible to map a form post-expansion to a source location. If you walk the pre-expansion form you’re going to get bad results. There’s no winning here unless you have help from the implementation.

CL being CL, it's important to remember that macroexpansion itself can do anything a real program can. Including writing files, opening network connections, etc.

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