Optimize nth and nth_back for BoundListIterator (#4810)

* Optimize nth and nth_back for BoundListIterator. Add unit test and benchmarks

* Fix fmt and newsfragment CI

* Fix clippy and changelog CI

* Revise Impl of nth and nth_back. Impl advance_by

* Fix failing fmt

* Fix failing ruff test

* branch out nth, nth_unchecked, nth_back, nth_back_unchecked.

* Fix fmt

* Revise advance_by impl. add advance_by unittest.

* Fix fmt

* Fix clippy unused function warning

* Set appropriate Py_LIMITED_API flag

* Rewrite nth & nth_back using conditional compilation. Rearrange flags for proper compilation

* fix fmt

* fix failing CI

* Impl advance_back_by. Remove cfg flag for with_critical_section

* refactor advance_by and advance_back_by. Add back cfg for with_critical_section

* Put allow deadcode for with_critical_section

* Remove use of get_item. Revise changelog

newsfragments/4810.added.md

@@ -0,0 +1 @@
+Optimizes `nth`, `nth_back`, `advance_by` and `advance_back_by` for `BoundListIterator`

pyo3-benches/benches/bench_list.rs

@@ -39,7 +39,33 @@ fn list_get_item(b: &mut Bencher<'_>) {
     });
 }
 
-#[cfg(not(any(Py_LIMITED_API, Py_GIL_DISABLED)))]
+fn list_nth(b: &mut Bencher<'_>) {
+    Python::with_gil(|py| {
+        const LEN: usize = 50;
+        let list = PyList::new_bound(py, 0..LEN);
+        let mut sum = 0;
+        b.iter(|| {
+            for i in 0..LEN {
+                sum += list.iter().nth(i).unwrap().extract::<usize>().unwrap();
+            }
+        });
+    });
+}
+
+fn list_nth_back(b: &mut Bencher<'_>) {
+    Python::with_gil(|py| {
+        const LEN: usize = 50;
+        let list = PyList::new_bound(py, 0..LEN);
+        let mut sum = 0;
+        b.iter(|| {
+            for i in 0..LEN {
+                sum += list.iter().nth_back(i).unwrap().extract::<usize>().unwrap();
+            }
+        });
+    });
+}
+
+#[cfg(not(Py_LIMITED_API))]
 fn list_get_item_unchecked(b: &mut Bencher<'_>) {
     Python::with_gil(|py| {
         const LEN: usize = 50_000;
@@ -66,6 +92,8 @@ fn sequence_from_list(b: &mut Bencher<'_>) {
 fn criterion_benchmark(c: &mut Criterion) {
     c.bench_function("iter_list", iter_list);
     c.bench_function("list_new", list_new);
+    c.bench_function("list_nth", list_nth);
+    c.bench_function("list_nth_back", list_nth_back);
     c.bench_function("list_get_item", list_get_item);
     #[cfg(not(any(Py_LIMITED_API, Py_GIL_DISABLED)))]
     c.bench_function("list_get_item_unchecked", list_get_item_unchecked);

src/lib.rs

@@ -1,7 +1,7 @@
 #![warn(missing_docs)]
 #![cfg_attr(
     feature = "nightly",
-    feature(auto_traits, negative_impls, try_trait_v2)
+    feature(auto_traits, negative_impls, try_trait_v2, iter_advance_by)
 )]
 #![cfg_attr(docsrs, feature(doc_cfg, doc_auto_cfg))]
 // Deny some lints in doctests.