The issue is that mrustc itself and rustc for it are in a single port, and while mrustc is functional, FWIW, rustc fails to build, on all archs (I tried x86_64 and arm64) and with different errors, which is annoying, to say the least LOL
And I am hesitant to commit a half-fixed port.
I will probably try to fix 1.54 for LLVM-7, because if that works, we know for sure that at least on x86 bootstrapping worked fine. While re 1.39 I am not sure it ever worked for anyone at all (not in MacPorts, but whether upstream tested it on macOS and in what set-up).