I think we're suffering from some fuzziness about what bounds checks we're referring to. Even in your example, you only need to check the size of the Vec<T> when you instantiate the iterator, not each time the iterator accesses an element, because at the time the iterator over the Vec<T>'s contents is instantiated, the Vec<T>'s size is known, and it can't change over the life of the iterator (because mutation is disallowed when there's an outstanding borrow). With a regular for-loop:

    for i in 0..v.len() {
        println!("{:?}", v[i]);
    }

you check the length at the top (the `v.len()`) and also for each `v[i]`. The first is unavoidable, but the second can be skipped when using an iterator instead, because it can be statically guaranteed that, even if you don't know at compile time what concretely the length is, whatever it ends up being, the index will never exceed it. Rust specifically differs from C++ in this respect, because nothing in that language prevents the underlying vector's length from changing while the iterator exists, so without per-access bounds checks it's still possible for an iterator to walk past the end.