GC doesn't matter much for a simple compiler, as you either don't need to allocate much (single pass, Wirth-style compilers that generate code inline) or most of what you allocate becomes garbage all at once at the end (AST).

In my half-finished Ruby compiler prototype, even before I added type tagging, and so allocated every integer on the heap, I just didn't add a GC for a long time because it was fine to just leak, because the compiler isn't generally long running.

Yeah, I never got around to writing a GC for Ur-Scheme either—you could argue that it's also "half-finished" but it does successfully compile itself correctly. It's not so much the GC that matters as it is the ability to allocate on the heap without much ceremony. I should have thought of that before I posted, and I appreciate the correction.