[Shish2k]

It is specifically both:

- trying to access an arbitrary element in a slice, the compiler will emit bounds checks (`if index > len: panic()`) to avoid an uncontrolled out-of-bounds memory access — https://godbolt.org/z/cbY5ebzvK (note how if you comment out the assert, the code barely changes, because the compiler is adding an invisible assert of its own)

- if the compiler can infer that `index` will always be less than `len`, then it will omit the bounds check — https://godbolt.org/z/TTashYnjd