Those functions could be easily done via syscalls to the underlying OS or simple Assembly routines, as they are quite simple to implement. None of those four f

Those functions could be easily done via syscalls to the underlying OS or simple Assembly routines, as they are quite simple to implement. None of those four functions use syscalls, nor are any of them simple to implement efficiently in assembly. Consider that memcpy, under GCC, * generates different code based on any known alignment of the source or destination * generates simpler code if the size is known at compile time * generates only register accesses if one or both of the arguments live in registers * is elided entirely if GCC determines that it may do so safely The other three have similar complex behavior. The Rust developers didn't use them just because they were too dumb to know how to write a for loop in assembly.