Function with_completion 

pub fn with_completion<F>(ioctx: &IoCtx, f: F) -> RadosResult<Completion<'_>>
where
    F: FnOnce(rados_completion_t) -> c_int,

Completions are only created via this wrapper, in order to ensure that the Completion struct is only constructed around ‘armed’ rados_completion_t instances (i.e. those that have been used to start an I/O).