It should be noted that the standard library is quite inappropriate for embedded development as I understand it. It allocates quite liberally, and provides no hooks for handling things like OOM. The stdlibs will just abort (not even panic) on OOM.
Personally this seems like totally reasonable behaviour for userland applications on non-embedded platforms where OOM is considered rare or just impractical to handle. Having ever operation that allocates bubble up a Result that you need to handle would be super annoying IMHO. Maybe you disagree, though.
Anyway, libcore doesn't allocate, so that's totally fine to use.