In the first case, it often is optimized out. But the optimizer isn't perfect, and can't detect every case where it is unreachable.

If you have the second case, I would much rather have a panic than undefined behavior. As mentioned in another comment, in c indexing an array is semantically equivalent to:

    if (i < len(arr)) arr[i] else UB()

In fact a c compiler could put in a check and abort if it is out of bounds, like rust does and still be in spec. But the undefined behavior could also cause memory corruption, or cause some other subtle bug.