Optimize nth and nth_back for BoundListIterator

[Owen-CH-Leung]

See #4787

This PR optimizes nth and nth_back for BoundListIterator, and added unittest & benchmarks for these 2 APIs. Here are the benchmark of the optimized nth and nth_back:

With Optimization

list_nth                time:   [293.43 ns 294.83 ns 296.46 ns]
Found 2 outliers among 100 measurements (2.00%)
  2 (2.00%) high severe

list_nth_back           time:   [303.82 ns 307.94 ns 313.12 ns]
Found 9 outliers among 100 measurements (9.00%)

The default nth and nth_back implementation

list_nth                time:   [2.9155 µs 2.9410 µs 2.9742 µs]
                        change: [+903.75% +913.27% +921.36%] (p = 0.00 < 0.05)
                        Performance has regressed.
Found 5 outliers among 100 measurements (5.00%)
  2 (2.00%) high mild
  3 (3.00%) high severe

list_nth_back           time:   [3.6843 µs 3.6950 µs 3.7053 µs]
                        change: [+1106.5% +1121.1% +1133.2%] (p = 0.00 < 0.05)
                        Performance has regressed.

[davidhewitt]

Awesome, thanks! Will seek to review soon. I guess we can do the same thing for tuple iteration?

[Owen-CH-Leung]

Awesome, thanks! Will seek to review soon. I guess we can do the same thing for tuple iteration?

Yup - I'll file a PR to optimise tuple iterator after the holiday =)

[davidhewitt]

I wonder, is there a time-of-check to time-of-use bug here on the length? Not one for this PR, but a follow up I will try not to forget...

[Owen-CH-Leung]

Yup I think that's a potential TOCTOU bug here. This particular implementation assumes the user ensures proper synchronization if they intend to use the iterator in a multi-threaded or mutable environment.

[Owen-CH-Leung]

I think the current implementation of next also has a potential TOCTOU bug:

https://github.com/PyO3/pyo3/blob/main/src/types/list.rs#L495-L498

[ngoldbaum]

Thanks for working on this! Hopefully my PR should be merged in the next few days and then you can rebase this.

[ngoldbaum]

It might also make sense to implement advance_by on nightly. nth is implemented in terms of advance_by, so this improvement would fall out "for free" on nightly if you implement that function.