I'm unsure what you're referring to here -- Zig doesn't have any runtime, it doesn't even depend on libc.

The only thing I can think of that you might be referring to is compiler-rt: if so, this is a thing in C too! It's just a small collection of implementations for operations the code generator wants to call into (e.g. memset, arithmetic for integers larger than CPU word size). Clang uses compiler-rt when compiling C code, and GCC's equivalent is libgcc. Nonetheless, Zig lets you disable it using `-fno-compiler-rt`, in which case you'll need to provide the relevant symbols yourself somehow.