1 files changed, 246 insertions, 0 deletions
diff --git a/library/core/tests/iter/adapters/step_by.rs b/library/core/tests/iter/adapters/step_by.rs
new file mode 100644
index 000000000..94f2fa8c2
--- /dev/null
+++ b/library/core/tests/iter/adapters/step_by.rs
@@ -0,0 +1,246 @@
+use core::iter::*;
+
+#[test]
+fn test_iterator_step_by() {
+    // Identity
+    let mut it = (0..).step_by(1).take(3);
+    assert_eq!(it.next(), Some(0));
+    assert_eq!(it.next(), Some(1));
+    assert_eq!(it.next(), Some(2));
+    assert_eq!(it.next(), None);
+
+    let mut it = (0..).step_by(3).take(4);
+    assert_eq!(it.next(), Some(0));
+    assert_eq!(it.next(), Some(3));
+    assert_eq!(it.next(), Some(6));
+    assert_eq!(it.next(), Some(9));
+    assert_eq!(it.next(), None);
+
+    let mut it = (0..3).step_by(1);
+    assert_eq!(it.next_back(), Some(2));
+    assert_eq!(it.next_back(), Some(1));
+    assert_eq!(it.next_back(), Some(0));
+    assert_eq!(it.next_back(), None);
+
+    let mut it = (0..11).step_by(3);
+    assert_eq!(it.next_back(), Some(9));
+    assert_eq!(it.next_back(), Some(6));
+    assert_eq!(it.next_back(), Some(3));
+    assert_eq!(it.next_back(), Some(0));
+    assert_eq!(it.next_back(), None);
+}
+
+#[test]
+fn test_iterator_step_by_nth() {
+    let mut it = (0..16).step_by(5);
+    assert_eq!(it.nth(0), Some(0));
+    assert_eq!(it.nth(0), Some(5));
+    assert_eq!(it.nth(0), Some(10));
+    assert_eq!(it.nth(0), Some(15));
+    assert_eq!(it.nth(0), None);
+
+    let it = (0..18).step_by(5);
+    assert_eq!(it.clone().nth(0), Some(0));
+    assert_eq!(it.clone().nth(1), Some(5));
+    assert_eq!(it.clone().nth(2), Some(10));
+    assert_eq!(it.clone().nth(3), Some(15));
+    assert_eq!(it.clone().nth(4), None);
+    assert_eq!(it.clone().nth(42), None);
+}
+
+#[test]
+fn test_iterator_step_by_nth_overflow() {
+    #[cfg(target_pointer_width = "16")]
+    type Bigger = u32;
+    #[cfg(target_pointer_width = "32")]
+    type Bigger = u64;
+    #[cfg(target_pointer_width = "64")]
+    type Bigger = u128;
+
+    #[derive(Clone)]
+    struct Test(Bigger);
+    impl Iterator for &mut Test {
+        type Item = i32;
+        fn next(&mut self) -> Option<Self::Item> {
+            Some(21)
+        }
+        fn nth(&mut self, n: usize) -> Option<Self::Item> {
+            self.0 += n as Bigger + 1;
+            Some(42)
+        }
+    }
+
+    let mut it = Test(0);
+    let root = usize::MAX >> (usize::BITS / 2);
+    let n = root + 20;
+    (&mut it).step_by(n).nth(n);
+    assert_eq!(it.0, n as Bigger * n as Bigger);
+
+    // large step
+    let mut it = Test(0);
+    (&mut it).step_by(usize::MAX).nth(5);
+    assert_eq!(it.0, (usize::MAX as Bigger) * 5);
+
+    // n + 1 overflows
+    let mut it = Test(0);
+    (&mut it).step_by(2).nth(usize::MAX);
+    assert_eq!(it.0, (usize::MAX as Bigger) * 2);
+
+    // n + 1 overflows
+    let mut it = Test(0);
+    (&mut it).step_by(1).nth(usize::MAX);
+    assert_eq!(it.0, (usize::MAX as Bigger) * 1);
+}
+
+#[test]
+fn test_iterator_step_by_nth_try_fold() {
+    let mut it = (0..).step_by(10);
+    assert_eq!(it.try_fold(0, i8::checked_add), None);
+    assert_eq!(it.next(), Some(60));
+    assert_eq!(it.try_fold(0, i8::checked_add), None);
+    assert_eq!(it.next(), Some(90));
+
+    let mut it = (100..).step_by(10);
+    assert_eq!(it.try_fold(50, i8::checked_add), None);
+    assert_eq!(it.next(), Some(110));
+
+    let mut it = (100..=100).step_by(10);
+    assert_eq!(it.next(), Some(100));
+    assert_eq!(it.try_fold(0, i8::checked_add), Some(0));
+}
+
+#[test]
+fn test_iterator_step_by_nth_back() {
+    let mut it = (0..16).step_by(5);
+    assert_eq!(it.nth_back(0), Some(15));
+    assert_eq!(it.nth_back(0), Some(10));
+    assert_eq!(it.nth_back(0), Some(5));
+    assert_eq!(it.nth_back(0), Some(0));
+    assert_eq!(it.nth_back(0), None);
+
+    let mut it = (0..16).step_by(5);
+    assert_eq!(it.next(), Some(0)); // to set `first_take` to `false`
+    assert_eq!(it.nth_back(0), Some(15));
+    assert_eq!(it.nth_back(0), Some(10));
+    assert_eq!(it.nth_back(0), Some(5));
+    assert_eq!(it.nth_back(0), None);
+
+    let it = || (0..18).step_by(5);
+    assert_eq!(it().nth_back(0), Some(15));
+    assert_eq!(it().nth_back(1), Some(10));
+    assert_eq!(it().nth_back(2), Some(5));
+    assert_eq!(it().nth_back(3), Some(0));
+    assert_eq!(it().nth_back(4), None);
+    assert_eq!(it().nth_back(42), None);
+}
+
+#[test]
+fn test_iterator_step_by_nth_try_rfold() {
+    let mut it = (0..100).step_by(10);
+    assert_eq!(it.try_rfold(0, i8::checked_add), None);
+    assert_eq!(it.next_back(), Some(70));
+    assert_eq!(it.next(), Some(0));
+    assert_eq!(it.try_rfold(0, i8::checked_add), None);
+    assert_eq!(it.next_back(), Some(30));
+
+    let mut it = (0..100).step_by(10);
+    assert_eq!(it.try_rfold(50, i8::checked_add), None);
+    assert_eq!(it.next_back(), Some(80));
+
+    let mut it = (100..=100).step_by(10);
+    assert_eq!(it.next_back(), Some(100));
+    assert_eq!(it.try_fold(0, i8::checked_add), Some(0));
+}
+
+#[test]
+#[should_panic]
+fn test_iterator_step_by_zero() {
+    let mut it = (0..).step_by(0);
+    it.next();
+}
+
+#[test]
+fn test_iterator_step_by_size_hint() {
+    struct StubSizeHint(usize, Option<usize>);
+    impl Iterator for StubSizeHint {
+        type Item = ();
+        fn next(&mut self) -> Option<()> {
+            self.0 -= 1;
+            if let Some(ref mut upper) = self.1 {
+                *upper -= 1;
+            }
+            Some(())
+        }
+        fn size_hint(&self) -> (usize, Option<usize>) {
+            (self.0, self.1)
+        }
+    }
+
+    // The two checks in each case are needed because the logic
+    // is different before the first call to `next()`.
+
+    let mut it = StubSizeHint(10, Some(10)).step_by(1);
+    assert_eq!(it.size_hint(), (10, Some(10)));
+    it.next();
+    assert_eq!(it.size_hint(), (9, Some(9)));
+
+    // exact multiple
+    let mut it = StubSizeHint(10, Some(10)).step_by(3);
+    assert_eq!(it.size_hint(), (4, Some(4)));
+    it.next();
+    assert_eq!(it.size_hint(), (3, Some(3)));
+
+    // larger base range, but not enough to get another element
+    let mut it = StubSizeHint(12, Some(12)).step_by(3);
+    assert_eq!(it.size_hint(), (4, Some(4)));
+    it.next();
+    assert_eq!(it.size_hint(), (3, Some(3)));
+
+    // smaller base range, so fewer resulting elements
+    let mut it = StubSizeHint(9, Some(9)).step_by(3);
+    assert_eq!(it.size_hint(), (3, Some(3)));
+    it.next();
+    assert_eq!(it.size_hint(), (2, Some(2)));
+
+    // infinite upper bound
+    let mut it = StubSizeHint(usize::MAX, None).step_by(1);
+    assert_eq!(it.size_hint(), (usize::MAX, None));
+    it.next();
+    assert_eq!(it.size_hint(), (usize::MAX - 1, None));
+
+    // still infinite with larger step
+    let mut it = StubSizeHint(7, None).step_by(3);
+    assert_eq!(it.size_hint(), (3, None));
+    it.next();
+    assert_eq!(it.size_hint(), (2, None));
+
+    // propagates ExactSizeIterator
+    let a = [1, 2, 3, 4, 5];
+    let it = a.iter().step_by(2);
+    assert_eq!(it.len(), 3);
+
+    // Cannot be TrustedLen as a step greater than one makes an iterator
+    // with (usize::MAX, None) no longer meet the safety requirements
+    trait TrustedLenCheck {
+        fn test(self) -> bool;
+    }
+    impl<T: Iterator> TrustedLenCheck for T {
+        default fn test(self) -> bool {
+            false
+        }
+    }
+    impl<T: TrustedLen> TrustedLenCheck for T {
+        fn test(self) -> bool {
+            true
+        }
+    }
+    assert!(TrustedLenCheck::test(a.iter()));
+    assert!(!TrustedLenCheck::test(a.iter().step_by(1)));
+}
+
+#[test]
+fn test_step_by_skip() {
+    assert_eq!((0..640).step_by(128).skip(1).collect::<Vec<_>>(), [128, 256, 384, 512]);
+    assert_eq!((0..=50).step_by(10).nth(3), Some(30));
+    assert_eq!((200..=255u8).step_by(10).nth(3), Some(230));
+}