BTree: add benchmark of legacy ways to obtain first & last

library/alloc/benches/btree/map.rs

@@ -177,7 +177,7 @@ pub fn iteration_mut_100000(b: &mut Bencher) {
     bench_iteration_mut(b, 100000);
 }
 
-fn bench_first_and_last(b: &mut Bencher, size: i32) {
+fn bench_first_and_last_nightly(b: &mut Bencher, size: i32) {
     let map: BTreeMap<_, _> = (0..size).map(|i| (i, i)).collect();
     b.iter(|| {
         for _ in 0..10 {
@@ -187,19 +187,44 @@ fn bench_first_and_last(b: &mut Bencher, size: i32) {
     });
 }
 
+fn bench_first_and_last_stable(b: &mut Bencher, size: i32) {
+    let map: BTreeMap<_, _> = (0..size).map(|i| (i, i)).collect();
+    b.iter(|| {
+        for _ in 0..10 {
+            black_box(map.iter().next());
+            black_box(map.iter().next_back());
+        }
+    });
+}
+
+#[bench]
+pub fn first_and_last_0_nightly(b: &mut Bencher) {
+    bench_first_and_last_nightly(b, 0);
+}
+
+#[bench]
+pub fn first_and_last_0_stable(b: &mut Bencher) {
+    bench_first_and_last_stable(b, 0);
+}
+
+#[bench]
+pub fn first_and_last_100_nightly(b: &mut Bencher) {
+    bench_first_and_last_nightly(b, 100);
+}
+
 #[bench]
-pub fn first_and_last_0(b: &mut Bencher) {
-    bench_first_and_last(b, 0);
+pub fn first_and_last_100_stable(b: &mut Bencher) {
+    bench_first_and_last_stable(b, 100);
 }
 
 #[bench]
-pub fn first_and_last_100(b: &mut Bencher) {
-    bench_first_and_last(b, 100);
+pub fn first_and_last_10k_nightly(b: &mut Bencher) {
+    bench_first_and_last_nightly(b, 10_000);
 }
 
 #[bench]
-pub fn first_and_last_10k(b: &mut Bencher) {
-    bench_first_and_last(b, 10_000);
+pub fn first_and_last_10k_stable(b: &mut Bencher) {
+    bench_first_and_last_stable(b, 10_000);
 }
 
 const BENCH_RANGE_SIZE: i32 = 145;