Specialize Zip::nth for TrustedRandomAccess

[scottmcm]

Fixes the performance difference observed in https://users.rust-lang.org/t/performance-difference-between-iterator-zip-and-skip-order/15743

Before:

test iter::bench_skip_then_zip                     ... bench:         154 ns/iter (+/- 4)
test iter::bench_zip_then_skip                     ... bench:       4,725 ns/iter (+/- 225)

After:

test iter::bench_skip_then_zip                     ... bench:         154 ns/iter (+/- 2)
test iter::bench_zip_then_skip                     ... bench:          81 ns/iter (+/- 19)

[rust-highfive]

r? @kennytm

(rust_highfive has picked a reviewer for you, use r? to override)

[kennytm]

Could you add a test to ensure side effects are executed?

fn main() {
    let mut a = Vec::new();
    let mut b = Vec::new();
    let value = (1..7)
        .map(|n| {
            a.push(n);
            n * 10
        })
        .zip((2..9).map(|n| {
            b.push(n * 100);
            n * 1000
        }))
        .skip(1)
        .nth(3);
    assert_eq!(value, Some((50, 6000)));
    assert_eq!(a, vec![1, 2, 3, 4, 5]);
    assert_eq!(b, vec![200, 300, 400, 500, 600]);
}

[scottmcm]

@kennytm Good call.

It looks like Range: !TrustedRandomAccess, though, so I swapped it out for cloning slice iterators.

[kennytm]

Minor nit: why this { is on a new line 😄

[scottmcm]

Now that is an excellent question 😅

[kennytm]

(This also)

[kennytm]

Travis passed, so

@bors r+

[bors]

📌 Commit 5105fc1 has been approved by kennytm