You'd use CStr::from_ptr if you've received a C string (which does not allocate, that's correct, though not via transmute, just a cast), but since Rust strings aren't null terminated, you'll need to allocate some memory to add the null byte on the end. Which yeah, then I'd want to try to use CString as much as possible, in that case.

(If you wrote to an array on the stack, you could turn it into a Cstr too, instead of heap allocating, but then you'd have a max string size, which may or may not be something you'd want.)