llvm-mos compiles a considerable subset of C++ to 6502 machine code out of the box. IIRC it's just missing "runtime support"; things like RTTI, exceptions, and the runtime layout of VTables. [...]
I came across this answer about a week later, so just in time for ICFP 2021 to start. I also had my Haskell Love talk coming up, and I really wanted to get the RetroClash page in shape by then, so that I can direct people there at the end of my talk.
But still! LLVM, and consequently, Rust, on a C64! Damn if that isn't the perfect nerd-sniping for me. So I looked around to find this 6502.org forum post that shows compiling a Rust implementation of the Fibonacci function and linking it with a small C main. I've reproduced their code here in a Git repo for easier fiddling.
So after ICFP, although other things kept piling up, including a GM-ing gig playing Trail of Cthulhu's excellent The Dance in the Blood and coordinating with the awesome David Thinnes on getting Arrow DECA support into clash-pong in time for Haskell Love, I carved out a weekend to dust off my Rust-on-AVR CHIP-8 implementation to try to get it running on the C64.