indexing:  When you use the index operator (a[i]) on an array/slice/Vec/etc., bounds checks are performed, and a panic can be raised.  For most situations, if let + get() tends to outperform other indexing techniques (including the unsafe ones!) because it cannot panic and has excellent compiler optimization:

let t_holder;
let opt_t = if has_t {
    t_holder = get_t();
    Some(&t_holder)
} else {
    None
}