Function nf_struct_field_mut 

#[unsafe(no_mangle)]
pub(crate) unsafe extern "C" fn nf_struct_field_mut(
    place: *mut Slot,
    index: usize,
) -> *mut Slot

The slot of the field at index in the struct in place, which becomes its only owner’s so the field can be changed. The slot is valid until the struct next changes.

Safety

place holds a struct with a field at index.