IMO the typical use case is for portability to platforms that don't have those features, or implementing a lower-layer: bootloader/OS kernel/etc. Regardless of

IMO the typical use case is for portability to platforms that don't have those features, or implementing a lower-layer: bootloader/OS kernel/etc. Regardless of the runtime cost - you don't have a C/C++ library to rely on (or you're implementing one). I'd always assumed rust may have taken this from C compiler arguments like -nostdlib / -nostdinc (and corresponding C++ ones -nostdlib++, -nostdinc++).