CharzBuffer for a generally better alternative. You should only use mallocedStringz where CharzBuffer cannot be used (e.g. when druntime is not usable or you have no stack space for the temporary buffer).
mallocSlice is the function this function calls, so the notes in its documentation applies here too.
Uses C's malloc to allocate a copy of original with an attached zero terminator. It may return a slice with a null pointer (but non-zero length!) if malloc fails and you are responsible for freeing the returned pointer with core.stdc.stdlib.free(ret.ptr).