Add macro bench_specializations

Cargo.toml

@@ -71,3 +71,7 @@ harness = false
 [[bench]]
 name = "powerset"
 harness = false
+
+[[bench]]
+name = "specializations"
+harness = false

benches/specializations.rs

@@ -0,0 +1,130 @@
+use criterion::black_box;
+use itertools::Itertools;
+
+/// Create multiple functions each defining a benchmark group about iterator methods.
+///
+/// Each created group has functions with the following ids:
+///
+/// - `next`, `size_hint`, `count`, `last`, `nth`, `collect`, `fold`
+/// - and when marked as `DoubleEndedIterator`: `next_back`, `rfold`
+/// - and when marked as `ExactSizeIterator`: `len`
+///
+/// Note that this macro can be called only once.
+macro_rules! bench_specializations {
+    (
+        $(
+            $name:ident {
+                $($extra:ident)*
+                {$(
+                    $init:stmt;
+                )*}
+                $iterator:expr
+            }
+        )*
+    ) => {
+        $(
+            fn $name(c: &mut ::criterion::Criterion) {
+                let mut bench_group = c.benchmark_group(stringify!($name));
+                $(
+                    $init
+                )*
+                let bench_first_its = {
+                    let mut bench_idx = 0;
+                    [0; 1000].map(|_| {
+                        let mut it = $iterator;
+                        if bench_idx != 0 {
+                            it.nth(bench_idx - 1);
+                        }
+                        bench_idx += 1;
+                        it
+                    })
+                };
+                bench_specializations!(@Iterator bench_group bench_first_its: $iterator);
+                $(
+                    bench_specializations!(@$extra bench_group bench_first_its: $iterator);
+                )*
+                bench_group.finish();
+            }
+        )*
+
+        ::criterion::criterion_group!(benches, $($name, )*);
+        ::criterion::criterion_main!(benches);
+    };
+
+    (@Iterator $group:ident $first_its:ident: $iterator:expr) => {
+        $group.bench_function("next", |bencher| bencher.iter(|| {
+            let mut it = $iterator;
+            while let Some(x) = it.next() {
+                black_box(x);
+            }
+        }));
+        $group.bench_function("size_hint", |bencher| bencher.iter(|| {
+            $first_its.iter().for_each(|it| {
+                black_box(it.size_hint());
+            })
+        }));
+        $group.bench_function("count", |bencher| bencher.iter(|| {
+            $iterator.count()
+        }));
+        $group.bench_function("last", |bencher| bencher.iter(|| {
+            $iterator.last()
+        }));
+        $group.bench_function("nth", |bencher| bencher.iter(|| {
+            for start in 0_usize..10 {
+                for n in 0..10 {
+                    let mut it = $iterator;
+                    if let Some(s) = start.checked_sub(1) {
+                        black_box(it.nth(s));
+                    }
+                    while let Some(x) = it.nth(n) {
+                        black_box(x);
+                    }
+                }
+            }
+        }));
+        $group.bench_function("collect", |bencher| bencher.iter(|| {
+            $iterator.collect::<Vec<_>>()
+        }));
+        $group.bench_function("fold", |bencher| bencher.iter(|| {
+            $iterator.fold((), |(), x| {
+                black_box(x);
+            })
+        }));
+    };
+
+    (@DoubleEndedIterator $group:ident $_first_its:ident: $iterator:expr) => {
+        $group.bench_function("next_back", |bencher| bencher.iter(|| {
+            let mut it = $iterator;
+            while let Some(x) = it.next_back() {
+                black_box(x);
+            }
+        }));
+        $group.bench_function("rfold", |bencher| bencher.iter(|| {
+            $iterator.rfold((), |(), x| {
+                black_box(x);
+            })
+        }));
+    };
+
+    (@ExactSizeIterator $group:ident $first_its:ident: $_iterator:expr) => {
+        $group.bench_function("len", |bencher| bencher.iter(|| {
+            $first_its.iter().for_each(|it| {
+                black_box(it.len());
+            })
+        }));
+    };
+}
+
+// Example: To bench only `ZipLongest::fold`, you can do
+//     cargo bench --bench specializations zip_longest/fold
+bench_specializations! {
+    zip_longest {
+        DoubleEndedIterator
+        ExactSizeIterator
+        {
+            let xs = black_box(vec![0; 1024]);
+            let ys = black_box(vec![0; 768]);
+        }
+        xs.iter().zip_longest(ys.iter())
+    }
+}