> At this point I'm unclear on how much of this stuff is solid and > enabled-by-default (in particular, what's the difference between "arc" and > "orc"?)
ARC is in version 1.2 with significant stability improvements around the corner in 1.2.2. Many Nimble packages already work with `--gc:arc`. While the stability is still not good enough for Nim compiler bootstrapping, for new projects I wouldn't use anything else because the tooling is so much better. All the sanitizers from C++ simply work, compile your code with `nim c --gc:arc --debuginfo -d:useMalloc y.nim && valgrind ./y` and you can be assured the remaining ARC bugs (sorry!) don't affect you. You can also slowly move from the C++ code to Nim, the interop between Nim and C++ is superb and only getting better with ARC.
