library/core/tests/iter rearrange & add back missed doc comments

18 files changed, 274 insertions, 270 deletions

diff --git a/library/core/tests/iter/adapters/chain.rs b/library/core/tests/iter/adapters/chain.rs
index b20105259aa..ca5ae12ae26 100644
--- a/library/core/tests/iter/adapters/chain.rs
+++ b/library/core/tests/iter/adapters/chain.rs
@@ -226,3 +226,47 @@ fn test_chain_try_folds() {
     iter.nth(14); // skip the first 15, ending in state Back
     assert_eq!(iter.try_rfold(7, f), (15..20).try_rfold(7, f));
 }
+
+#[test]
+fn test_double_ended_chain() {
+    let xs = [1, 2, 3, 4, 5];
+    let ys = [7, 9, 11];
+    let mut it = xs.iter().chain(&ys).rev();
+    assert_eq!(it.next().unwrap(), &11);
+    assert_eq!(it.next().unwrap(), &9);
+    assert_eq!(it.next_back().unwrap(), &1);
+    assert_eq!(it.next_back().unwrap(), &2);
+    assert_eq!(it.next_back().unwrap(), &3);
+    assert_eq!(it.next_back().unwrap(), &4);
+    assert_eq!(it.next_back().unwrap(), &5);
+    assert_eq!(it.next_back().unwrap(), &7);
+    assert_eq!(it.next_back(), None);
+
+    // test that .chain() is well behaved with an unfused iterator
+    struct CrazyIterator(bool);
+    impl CrazyIterator {
+        fn new() -> CrazyIterator {
+            CrazyIterator(false)
+        }
+    }
+    impl Iterator for CrazyIterator {
+        type Item = i32;
+        fn next(&mut self) -> Option<i32> {
+            if self.0 {
+                Some(99)
+            } else {
+                self.0 = true;
+                None
+            }
+        }
+    }
+
+    impl DoubleEndedIterator for CrazyIterator {
+        fn next_back(&mut self) -> Option<i32> {
+            self.next()
+        }
+    }
+
+    assert_eq!(CrazyIterator::new().chain(0..10).rev().last(), Some(0));
+    assert!((0..10).chain(CrazyIterator::new()).rev().any(|i| i == 0));
+}
diff --git a/library/core/tests/iter/adapters/enumerate.rs b/library/core/tests/iter/adapters/enumerate.rs
index 9a11a28f9b0..0e603387847 100644
--- a/library/core/tests/iter/adapters/enumerate.rs
+++ b/library/core/tests/iter/adapters/enumerate.rs
@@ -92,3 +92,16 @@ fn test_enumerate_try_folds() {
     assert_eq!(iter.try_rfold(0, f), None);
     assert_eq!(iter.next_back(), Some((11, 111)));
 }
+
+#[test]
+fn test_double_ended_enumerate() {
+    let xs = [1, 2, 3, 4, 5, 6];
+    let mut it = xs.iter().cloned().enumerate();
+    assert_eq!(it.next(), Some((0, 1)));
+    assert_eq!(it.next(), Some((1, 2)));
+    assert_eq!(it.next_back(), Some((5, 6)));
+    assert_eq!(it.next_back(), Some((4, 5)));
+    assert_eq!(it.next_back(), Some((3, 4)));
+    assert_eq!(it.next_back(), Some((2, 3)));
+    assert_eq!(it.next(), None);
+}
diff --git a/library/core/tests/iter/adapters/filter.rs b/library/core/tests/iter/adapters/filter.rs
index e39cb7e39c6..a2050d89d85 100644
--- a/library/core/tests/iter/adapters/filter.rs
+++ b/library/core/tests/iter/adapters/filter.rs
@@ -40,3 +40,13 @@ fn test_filter_try_folds() {
     assert_eq!(iter.try_rfold(0, i8::checked_add), None);
     assert_eq!(iter.next_back(), Some(31));
 }
+
+#[test]
+fn test_double_ended_filter() {
+    let xs = [1, 2, 3, 4, 5, 6];
+    let mut it = xs.iter().filter(|&x| *x & 1 == 0);
+    assert_eq!(it.next_back().unwrap(), &6);
+    assert_eq!(it.next_back().unwrap(), &4);
+    assert_eq!(it.next().unwrap(), &2);
+    assert_eq!(it.next_back(), None);
+}
diff --git a/library/core/tests/iter/adapters/filter_map.rs b/library/core/tests/iter/adapters/filter_map.rs
index 083ee85c950..46738eda63f 100644
--- a/library/core/tests/iter/adapters/filter_map.rs
+++ b/library/core/tests/iter/adapters/filter_map.rs
@@ -24,3 +24,27 @@ fn test_filter_map_fold() {
     });
     assert_eq!(i, 0);
 }
+
+#[test]
+fn test_filter_map_try_folds() {
+    let mp = &|x| if 0 <= x && x < 10 { Some(x * 2) } else { None };
+    let f = &|acc, x| i32::checked_add(2 * acc, x);
+    assert_eq!((-9..20).filter_map(mp).try_fold(7, f), (0..10).map(|x| 2 * x).try_fold(7, f));
+    assert_eq!((-9..20).filter_map(mp).try_rfold(7, f), (0..10).map(|x| 2 * x).try_rfold(7, f));
+
+    let mut iter = (0..40).filter_map(|x| if x % 2 == 1 { None } else { Some(x * 2 + 10) });
+    assert_eq!(iter.try_fold(0, i8::checked_add), None);
+    assert_eq!(iter.next(), Some(38));
+    assert_eq!(iter.try_rfold(0, i8::checked_add), None);
+    assert_eq!(iter.next_back(), Some(78));
+}
+
+#[test]
+fn test_double_ended_filter_map() {
+    let xs = [1, 2, 3, 4, 5, 6];
+    let mut it = xs.iter().filter_map(|&x| if x & 1 == 0 { Some(x * 2) } else { None });
+    assert_eq!(it.next_back().unwrap(), 12);
+    assert_eq!(it.next_back().unwrap(), 8);
+    assert_eq!(it.next().unwrap(), 4);
+    assert_eq!(it.next_back(), None);
+}
diff --git a/library/core/tests/iter/adapters/flat_map.rs b/library/core/tests/iter/adapters/flat_map.rs
index e21b82a7fe7..ee945e69801 100644
--- a/library/core/tests/iter/adapters/flat_map.rs
+++ b/library/core/tests/iter/adapters/flat_map.rs
@@ -13,6 +13,8 @@ fn test_iterator_flat_map() {
     assert_eq!(i, ys.len());
 }
 
+/// Tests `FlatMap::fold` with items already picked off the front and back,
+/// to make sure all parts of the `FlatMap` are folded correctly.
 #[test]
 fn test_iterator_flat_map_fold() {
     let xs = [0, 3, 6];
@@ -53,3 +55,20 @@ fn test_flat_map_try_folds() {
     assert_eq!(iter.try_rfold(0, i8::checked_add), None);
     assert_eq!(iter.next_back(), Some(35));
 }
+
+#[test]
+fn test_double_ended_flat_map() {
+    let u = [0, 1];
+    let v = [5, 6, 7, 8];
+    let mut it = u.iter().flat_map(|x| &v[*x..v.len()]);
+    assert_eq!(it.next_back().unwrap(), &8);
+    assert_eq!(it.next().unwrap(), &5);
+    assert_eq!(it.next_back().unwrap(), &7);
+    assert_eq!(it.next_back().unwrap(), &6);
+    assert_eq!(it.next_back().unwrap(), &8);
+    assert_eq!(it.next().unwrap(), &6);
+    assert_eq!(it.next_back().unwrap(), &7);
+    assert_eq!(it.next_back(), None);
+    assert_eq!(it.next(), None);
+    assert_eq!(it.next_back(), None);
+}
diff --git a/library/core/tests/iter/adapters/flatten.rs b/library/core/tests/iter/adapters/flatten.rs
index 084229bb092..bd2c6fd9252 100644
--- a/library/core/tests/iter/adapters/flatten.rs
+++ b/library/core/tests/iter/adapters/flatten.rs
@@ -14,6 +14,8 @@ fn test_iterator_flatten() {
     assert_eq!(i, ys.len());
 }
 
+/// Tests `Flatten::fold` with items already picked off the front and back,
+/// to make sure all parts of the `Flatten` are folded correctly.
 #[test]
 fn test_iterator_flatten_fold() {
     let xs = [0, 3, 6];
@@ -73,3 +75,20 @@ fn test_flatten_non_fused_inner() {
     assert_eq!(iter.next(), Some(1));
     assert_eq!(iter.next(), None);
 }
+
+#[test]
+fn test_double_ended_flatten() {
+    let u = [0, 1];
+    let v = [5, 6, 7, 8];
+    let mut it = u.iter().map(|x| &v[*x..v.len()]).flatten();
+    assert_eq!(it.next_back().unwrap(), &8);
+    assert_eq!(it.next().unwrap(), &5);
+    assert_eq!(it.next_back().unwrap(), &7);
+    assert_eq!(it.next_back().unwrap(), &6);
+    assert_eq!(it.next_back().unwrap(), &8);
+    assert_eq!(it.next().unwrap(), &6);
+    assert_eq!(it.next_back().unwrap(), &7);
+    assert_eq!(it.next_back(), None);
+    assert_eq!(it.next(), None);
+    assert_eq!(it.next_back(), None);
+}
diff --git a/library/core/tests/iter/adapters/fuse.rs b/library/core/tests/iter/adapters/fuse.rs
index 5b9f8065fe1..f41b379b3ac 100644
--- a/library/core/tests/iter/adapters/fuse.rs
+++ b/library/core/tests/iter/adapters/fuse.rs
@@ -55,3 +55,21 @@ fn test_fuse_fold() {
     });
     assert_eq!(i, xs.len());
 }
+
+#[test]
+fn test_fuse() {
+    let mut it = 0..3;
+    assert_eq!(it.len(), 3);
+    assert_eq!(it.next(), Some(0));
+    assert_eq!(it.len(), 2);
+    assert_eq!(it.next(), Some(1));
+    assert_eq!(it.len(), 1);
+    assert_eq!(it.next(), Some(2));
+    assert_eq!(it.len(), 0);
+    assert_eq!(it.next(), None);
+    assert_eq!(it.len(), 0);
+    assert_eq!(it.next(), None);
+    assert_eq!(it.len(), 0);
+    assert_eq!(it.next(), None);
+    assert_eq!(it.len(), 0);
+}
diff --git a/library/core/tests/iter/adapters/intersperse.rs b/library/core/tests/iter/adapters/intersperse.rs
index 4f65f1bdee3..9dbe232e4ee 100644
--- a/library/core/tests/iter/adapters/intersperse.rs
+++ b/library/core/tests/iter/adapters/intersperse.rs
@@ -129,3 +129,26 @@ fn test_intersperse_collect_string() {
         .collect::<String>();
     assert_eq!(contents_string, "1, 2, 3");
 }
+
+#[test]
+fn test_try_fold_specialization_intersperse_err() {
+    let orig_iter = ["a", "b"].iter().copied().intersperse("-");
+
+    // Abort after the first item.
+    let mut iter = orig_iter.clone();
+    iter.try_for_each(|_| None::<()>);
+    assert_eq!(iter.next(), Some("-"));
+    assert_eq!(iter.next(), Some("b"));
+    assert_eq!(iter.next(), None);
+
+    // Abort after the second item.
+    let mut iter = orig_iter.clone();
+    iter.try_for_each(|item| if item == "-" { None } else { Some(()) });
+    assert_eq!(iter.next(), Some("b"));
+    assert_eq!(iter.next(), None);
+
+    // Abort after the third item.
+    let mut iter = orig_iter.clone();
+    iter.try_for_each(|item| if item == "b" { None } else { Some(()) });
+    assert_eq!(iter.next(), None);
+}
diff --git a/library/core/tests/iter/adapters/map.rs b/library/core/tests/iter/adapters/map.rs
index 704acc12307..77ce3819b32 100644
--- a/library/core/tests/iter/adapters/map.rs
+++ b/library/core/tests/iter/adapters/map.rs
@@ -1,29 +1,6 @@
 use core::iter::*;
 
 #[test]
-fn test_find_map() {
-    let xs: &[isize] = &[];
-    assert_eq!(xs.iter().find_map(half_if_even), None);
-    let xs: &[isize] = &[3, 5];
-    assert_eq!(xs.iter().find_map(half_if_even), None);
-    let xs: &[isize] = &[4, 5];
-    assert_eq!(xs.iter().find_map(half_if_even), Some(2));
-    let xs: &[isize] = &[3, 6];
-    assert_eq!(xs.iter().find_map(half_if_even), Some(3));
-
-    let xs: &[isize] = &[1, 2, 3, 4, 5, 6, 7];
-    let mut iter = xs.iter();
-    assert_eq!(iter.find_map(half_if_even), Some(1));
-    assert_eq!(iter.find_map(half_if_even), Some(2));
-    assert_eq!(iter.find_map(half_if_even), Some(3));
-    assert_eq!(iter.next(), Some(&7));
-
-    fn half_if_even(x: &isize) -> Option<isize> {
-        if x % 2 == 0 { Some(x / 2) } else { None }
-    }
-}
-
-#[test]
 fn test_map_try_folds() {
     let f = &|acc, x| i32::checked_add(2 * acc, x);
     assert_eq!((0..10).map(|x| x + 3).try_fold(7, f), (3..13).try_fold(7, f));
@@ -37,15 +14,14 @@ fn test_map_try_folds() {
 }
 
 #[test]
-fn test_filter_map_try_folds() {
-    let mp = &|x| if 0 <= x && x < 10 { Some(x * 2) } else { None };
-    let f = &|acc, x| i32::checked_add(2 * acc, x);
-    assert_eq!((-9..20).filter_map(mp).try_fold(7, f), (0..10).map(|x| 2 * x).try_fold(7, f));
-    assert_eq!((-9..20).filter_map(mp).try_rfold(7, f), (0..10).map(|x| 2 * x).try_rfold(7, f));
-
-    let mut iter = (0..40).filter_map(|x| if x % 2 == 1 { None } else { Some(x * 2 + 10) });
-    assert_eq!(iter.try_fold(0, i8::checked_add), None);
-    assert_eq!(iter.next(), Some(38));
-    assert_eq!(iter.try_rfold(0, i8::checked_add), None);
-    assert_eq!(iter.next_back(), Some(78));
+fn test_double_ended_map() {
+    let xs = [1, 2, 3, 4, 5, 6];
+    let mut it = xs.iter().map(|&x| x * -1);
+    assert_eq!(it.next(), Some(-1));
+    assert_eq!(it.next(), Some(-2));
+    assert_eq!(it.next_back(), Some(-6));
+    assert_eq!(it.next_back(), Some(-5));
+    assert_eq!(it.next(), Some(-3));
+    assert_eq!(it.next_back(), Some(-4));
+    assert_eq!(it.next(), None);
 }
diff --git a/library/core/tests/iter/adapters/mod.rs b/library/core/tests/iter/adapters/mod.rs
index de4b6c01a6d..96a53be1eaa 100644
--- a/library/core/tests/iter/adapters/mod.rs
+++ b/library/core/tests/iter/adapters/mod.rs
@@ -15,7 +15,6 @@ mod peekable;
 mod scan;
 mod skip;
 mod skip_while;
-mod step;
 mod step_by;
 mod take;
 mod take_while;
diff --git a/library/core/tests/iter/adapters/zip.rs b/library/core/tests/iter/adapters/zip.rs
index a5bf19c43c9..1fce0951e36 100644
--- a/library/core/tests/iter/adapters/zip.rs
+++ b/library/core/tests/iter/adapters/zip.rs
@@ -231,3 +231,17 @@ fn test_zip_trusted_random_access_composition() {
     assert_trusted_random_access(&z2);
     assert_eq!(z2.next().unwrap(), ((1, 1), 1));
 }
+
+#[test]
+fn test_double_ended_zip() {
+    let xs = [1, 2, 3, 4, 5, 6];
+    let ys = [1, 2, 3, 7];
+    let a = xs.iter().cloned();
+    let b = ys.iter().cloned();
+    let mut it = a.zip(b);
+    assert_eq!(it.next(), Some((1, 1)));
+    assert_eq!(it.next(), Some((2, 2)));
+    assert_eq!(it.next_back(), Some((4, 7)));
+    assert_eq!(it.next_back(), Some((3, 3)));
+    assert_eq!(it.next(), None);
+}
diff --git a/library/core/tests/iter/mod.rs b/library/core/tests/iter/mod.rs
index ac51ec797ac..b88454a50c4 100644
--- a/library/core/tests/iter/mod.rs
+++ b/library/core/tests/iter/mod.rs
@@ -83,26 +83,3 @@ pub fn extend_for_unit() {
     }
     assert_eq!(x, 5);
 }
-
-#[test]
-fn test_try_fold_specialization_intersperse_err() {
-    let orig_iter = ["a", "b"].iter().copied().intersperse("-");
-
-    // Abort after the first item.
-    let mut iter = orig_iter.clone();
-    iter.try_for_each(|_| None::<()>);
-    assert_eq!(iter.next(), Some("-"));
-    assert_eq!(iter.next(), Some("b"));
-    assert_eq!(iter.next(), None);
-
-    // Abort after the second item.
-    let mut iter = orig_iter.clone();
-    iter.try_for_each(|item| if item == "-" { None } else { Some(()) });
-    assert_eq!(iter.next(), Some("b"));
-    assert_eq!(iter.next(), None);
-
-    // Abort after the third item.
-    let mut iter = orig_iter.clone();
-    iter.try_for_each(|item| if item == "b" { None } else { Some(()) });
-    assert_eq!(iter.next(), None);
-}
diff --git a/library/core/tests/iter/range.rs b/library/core/tests/iter/range.rs
index 68cc34d5ddb..44adc3c58d2 100644
--- a/library/core/tests/iter/range.rs
+++ b/library/core/tests/iter/range.rs
@@ -431,3 +431,16 @@ fn test_range_inclusive_size_hint() {
     assert_eq!((imin..=imax).size_hint(), (usize::MAX, None));
     assert_eq!((imin..=imax + 1).size_hint(), (usize::MAX, None));
 }
+
+#[test]
+fn test_double_ended_range() {
+    assert_eq!((11..14).rev().collect::<Vec<_>>(), [13, 12, 11]);
+    for _ in (10..0).rev() {
+        panic!("unreachable");
+    }
+
+    assert_eq!((11..14).rev().collect::<Vec<_>>(), [13, 12, 11]);
+    for _ in (10..0).rev() {
+        panic!("unreachable");
+    }
+}
diff --git a/library/core/tests/iter/traits/double_ended.rs b/library/core/tests/iter/traits/double_ended.rs
index 24523883516..2728be29593 100644
--- a/library/core/tests/iter/traits/double_ended.rs
+++ b/library/core/tests/iter/traits/double_ended.rs
@@ -17,14 +17,6 @@ fn test_iterator_rev_nth() {
 }
 
 #[test]
-fn test_iterator_len() {
-    let v: &[_] = &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
-    assert_eq!(v[..4].iter().count(), 4);
-    assert_eq!(v[..10].iter().count(), 10);
-    assert_eq!(v[..0].iter().count(), 0);
-}
-
-#[test]
 fn test_rev() {
     let xs = [2, 4, 6, 8, 10, 12, 14, 16];
     let mut it = xs.iter();
@@ -34,167 +26,52 @@ fn test_rev() {
 }
 
 #[test]
-fn test_double_ended_map() {
-    let xs = [1, 2, 3, 4, 5, 6];
-    let mut it = xs.iter().map(|&x| x * -1);
-    assert_eq!(it.next(), Some(-1));
-    assert_eq!(it.next(), Some(-2));
-    assert_eq!(it.next_back(), Some(-6));
-    assert_eq!(it.next_back(), Some(-5));
-    assert_eq!(it.next(), Some(-3));
-    assert_eq!(it.next_back(), Some(-4));
-    assert_eq!(it.next(), None);
-}
-
-#[test]
-fn test_double_ended_enumerate() {
-    let xs = [1, 2, 3, 4, 5, 6];
-    let mut it = xs.iter().cloned().enumerate();
-    assert_eq!(it.next(), Some((0, 1)));
-    assert_eq!(it.next(), Some((1, 2)));
-    assert_eq!(it.next_back(), Some((5, 6)));
-    assert_eq!(it.next_back(), Some((4, 5)));
-    assert_eq!(it.next_back(), Some((3, 4)));
-    assert_eq!(it.next_back(), Some((2, 3)));
-    assert_eq!(it.next(), None);
-}
-
-#[test]
-fn test_double_ended_zip() {
-    let xs = [1, 2, 3, 4, 5, 6];
-    let ys = [1, 2, 3, 7];
-    let a = xs.iter().cloned();
-    let b = ys.iter().cloned();
-    let mut it = a.zip(b);
-    assert_eq!(it.next(), Some((1, 1)));
-    assert_eq!(it.next(), Some((2, 2)));
-    assert_eq!(it.next_back(), Some((4, 7)));
-    assert_eq!(it.next_back(), Some((3, 3)));
-    assert_eq!(it.next(), None);
-}
-
-#[test]
-fn test_double_ended_filter() {
-    let xs = [1, 2, 3, 4, 5, 6];
-    let mut it = xs.iter().filter(|&x| *x & 1 == 0);
-    assert_eq!(it.next_back().unwrap(), &6);
-    assert_eq!(it.next_back().unwrap(), &4);
-    assert_eq!(it.next().unwrap(), &2);
-    assert_eq!(it.next_back(), None);
-}
+fn test_rev_try_folds() {
+    let f = &|acc, x| i32::checked_add(2 * acc, x);
+    assert_eq!((1..10).rev().try_fold(7, f), (1..10).try_rfold(7, f));
+    assert_eq!((1..10).rev().try_rfold(7, f), (1..10).try_fold(7, f));
 
-#[test]
-fn test_double_ended_filter_map() {
-    let xs = [1, 2, 3, 4, 5, 6];
-    let mut it = xs.iter().filter_map(|&x| if x & 1 == 0 { Some(x * 2) } else { None });
-    assert_eq!(it.next_back().unwrap(), 12);
-    assert_eq!(it.next_back().unwrap(), 8);
-    assert_eq!(it.next().unwrap(), 4);
-    assert_eq!(it.next_back(), None);
+    let a = [10, 20, 30, 40, 100, 60, 70, 80, 90];
+    let mut iter = a.iter().rev();
+    assert_eq!(iter.try_fold(0_i8, |acc, &x| acc.checked_add(x)), None);
+    assert_eq!(iter.next(), Some(&70));
+    let mut iter = a.iter().rev();
+    assert_eq!(iter.try_rfold(0_i8, |acc, &x| acc.checked_add(x)), None);
+    assert_eq!(iter.next_back(), Some(&60));
 }
 
 #[test]
-fn test_double_ended_chain() {
-    let xs = [1, 2, 3, 4, 5];
-    let ys = [7, 9, 11];
-    let mut it = xs.iter().chain(&ys).rev();
-    assert_eq!(it.next().unwrap(), &11);
-    assert_eq!(it.next().unwrap(), &9);
-    assert_eq!(it.next_back().unwrap(), &1);
-    assert_eq!(it.next_back().unwrap(), &2);
-    assert_eq!(it.next_back().unwrap(), &3);
-    assert_eq!(it.next_back().unwrap(), &4);
-    assert_eq!(it.next_back().unwrap(), &5);
-    assert_eq!(it.next_back().unwrap(), &7);
-    assert_eq!(it.next_back(), None);
-
-    // test that .chain() is well behaved with an unfused iterator
-    struct CrazyIterator(bool);
-    impl CrazyIterator {
-        fn new() -> CrazyIterator {
-            CrazyIterator(false)
-        }
+fn test_rposition() {
+    fn f(xy: &(isize, char)) -> bool {
+        let (_x, y) = *xy;
+        y == 'b'
     }
-    impl Iterator for CrazyIterator {
-        type Item = i32;
-        fn next(&mut self) -> Option<i32> {
-            if self.0 {
-                Some(99)
-            } else {
-                self.0 = true;
-                None
-            }
-        }
-    }
-
-    impl DoubleEndedIterator for CrazyIterator {
-        fn next_back(&mut self) -> Option<i32> {
-            self.next()
-        }
+    fn g(xy: &(isize, char)) -> bool {
+        let (_x, y) = *xy;
+        y == 'd'
     }
+    let v = [(0, 'a'), (1, 'b'), (2, 'c'), (3, 'b')];
 
-    assert_eq!(CrazyIterator::new().chain(0..10).rev().last(), Some(0));
-    assert!((0..10).chain(CrazyIterator::new()).rev().any(|i| i == 0));
-}
-
-#[test]
-fn test_double_ended_flat_map() {
-    let u = [0, 1];
-    let v = [5, 6, 7, 8];
-    let mut it = u.iter().flat_map(|x| &v[*x..v.len()]);
-    assert_eq!(it.next_back().unwrap(), &8);
-    assert_eq!(it.next().unwrap(), &5);
-    assert_eq!(it.next_back().unwrap(), &7);
-    assert_eq!(it.next_back().unwrap(), &6);
-    assert_eq!(it.next_back().unwrap(), &8);
-    assert_eq!(it.next().unwrap(), &6);
-    assert_eq!(it.next_back().unwrap(), &7);
-    assert_eq!(it.next_back(), None);
-    assert_eq!(it.next(), None);
-    assert_eq!(it.next_back(), None);
+    assert_eq!(v.iter().rposition(f), Some(3));
+    assert!(v.iter().rposition(g).is_none());
 }
 
 #[test]
-fn test_double_ended_flatten() {
-    let u = [0, 1];
-    let v = [5, 6, 7, 8];
-    let mut it = u.iter().map(|x| &v[*x..v.len()]).flatten();
-    assert_eq!(it.next_back().unwrap(), &8);
-    assert_eq!(it.next().unwrap(), &5);
-    assert_eq!(it.next_back().unwrap(), &7);
-    assert_eq!(it.next_back().unwrap(), &6);
-    assert_eq!(it.next_back().unwrap(), &8);
-    assert_eq!(it.next().unwrap(), &6);
-    assert_eq!(it.next_back().unwrap(), &7);
-    assert_eq!(it.next_back(), None);
-    assert_eq!(it.next(), None);
-    assert_eq!(it.next_back(), None);
-}
-
-#[test]
-fn test_double_ended_range() {
-    assert_eq!((11..14).rev().collect::<Vec<_>>(), [13, 12, 11]);
-    for _ in (10..0).rev() {
-        panic!("unreachable");
-    }
-
-    assert_eq!((11..14).rev().collect::<Vec<_>>(), [13, 12, 11]);
-    for _ in (10..0).rev() {
-        panic!("unreachable");
-    }
+fn test_rev_rposition() {
+    let v = [0, 0, 1, 1];
+    assert_eq!(v.iter().rev().rposition(|&x| x == 1), Some(1));
 }
 
 #[test]
-fn test_rev_try_folds() {
-    let f = &|acc, x| i32::checked_add(2 * acc, x);
-    assert_eq!((1..10).rev().try_fold(7, f), (1..10).try_rfold(7, f));
-    assert_eq!((1..10).rev().try_rfold(7, f), (1..10).try_fold(7, f));
-
-    let a = [10, 20, 30, 40, 100, 60, 70, 80, 90];
-    let mut iter = a.iter().rev();
-    assert_eq!(iter.try_fold(0_i8, |acc, &x| acc.checked_add(x)), None);
-    assert_eq!(iter.next(), Some(&70));
-    let mut iter = a.iter().rev();
-    assert_eq!(iter.try_rfold(0_i8, |acc, &x| acc.checked_add(x)), None);
-    assert_eq!(iter.next_back(), Some(&60));
+#[should_panic]
+fn test_rposition_panic() {
+    let v: [(Box<_>, Box<_>); 4] = [(box 0, box 0), (box 0, box 0), (box 0, box 0), (box 0, box 0)];
+    let mut i = 0;
+    v.iter().rposition(|_elt| {
+        if i == 2 {
+            panic!()
+        }
+        i += 1;
+        false
+    });
 }
diff --git a/library/core/tests/iter/traits/iterator.rs b/library/core/tests/iter/traits/iterator.rs
index d3799b55e40..f31686378eb 100644
--- a/library/core/tests/iter/traits/iterator.rs
+++ b/library/core/tests/iter/traits/iterator.rs
@@ -430,3 +430,34 @@ fn test_iterator_rev_advance_by() {
     assert_eq!(v.iter().rev().advance_by(v.len()), Ok(()));
     assert_eq!(v.iter().rev().advance_by(100), Err(v.len()));
 }
+
+#[test]
+fn test_find_map() {
+    let xs: &[isize] = &[];
+    assert_eq!(xs.iter().find_map(half_if_even), None);
+    let xs: &[isize] = &[3, 5];
+    assert_eq!(xs.iter().find_map(half_if_even), None);
+    let xs: &[isize] = &[4, 5];
+    assert_eq!(xs.iter().find_map(half_if_even), Some(2));
+    let xs: &[isize] = &[3, 6];
+    assert_eq!(xs.iter().find_map(half_if_even), Some(3));
+
+    let xs: &[isize] = &[1, 2, 3, 4, 5, 6, 7];
+    let mut iter = xs.iter();
+    assert_eq!(iter.find_map(half_if_even), Some(1));
+    assert_eq!(iter.find_map(half_if_even), Some(2));
+    assert_eq!(iter.find_map(half_if_even), Some(3));
+    assert_eq!(iter.next(), Some(&7));
+
+    fn half_if_even(x: &isize) -> Option<isize> {
+        if x % 2 == 0 { Some(x / 2) } else { None }
+    }
+}
+
+#[test]
+fn test_iterator_len() {
+    let v: &[_] = &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
+    assert_eq!(v[..4].iter().count(), 4);
+    assert_eq!(v[..10].iter().count(), 10);
+    assert_eq!(v[..0].iter().count(), 0);
+}
diff --git a/library/core/tests/iter/traits/marker.rs b/library/core/tests/iter/traits/marker.rs
deleted file mode 100644
index d3510703a2d..00000000000
--- a/library/core/tests/iter/traits/marker.rs
+++ /dev/null
@@ -1,17 +0,0 @@
-#[test]
-fn test_fuse() {
-    let mut it = 0..3;
-    assert_eq!(it.len(), 3);
-    assert_eq!(it.next(), Some(0));
-    assert_eq!(it.len(), 2);
-    assert_eq!(it.next(), Some(1));
-    assert_eq!(it.len(), 1);
-    assert_eq!(it.next(), Some(2));
-    assert_eq!(it.len(), 0);
-    assert_eq!(it.next(), None);
-    assert_eq!(it.len(), 0);
-    assert_eq!(it.next(), None);
-    assert_eq!(it.len(), 0);
-    assert_eq!(it.next(), None);
-    assert_eq!(it.len(), 0);
-}
diff --git a/library/core/tests/iter/traits/mod.rs b/library/core/tests/iter/traits/mod.rs
index a31b622addf..b00e49b0696 100644
--- a/library/core/tests/iter/traits/mod.rs
+++ b/library/core/tests/iter/traits/mod.rs
@@ -2,40 +2,4 @@ mod accum;
 mod collect;
 mod double_ended;
 mod iterator;
-mod marker;
-
-#[test]
-fn test_rposition() {
-    fn f(xy: &(isize, char)) -> bool {
-        let (_x, y) = *xy;
-        y == 'b'
-    }
-    fn g(xy: &(isize, char)) -> bool {
-        let (_x, y) = *xy;
-        y == 'd'
-    }
-    let v = [(0, 'a'), (1, 'b'), (2, 'c'), (3, 'b')];
-
-    assert_eq!(v.iter().rposition(f), Some(3));
-    assert!(v.iter().rposition(g).is_none());
-}
-
-#[test]
-fn test_rev_rposition() {
-    let v = [0, 0, 1, 1];
-    assert_eq!(v.iter().rev().rposition(|&x| x == 1), Some(1));
-}
-
-#[test]
-#[should_panic]
-fn test_rposition_panic() {
-    let v: [(Box<_>, Box<_>); 4] = [(box 0, box 0), (box 0, box 0), (box 0, box 0), (box 0, box 0)];
-    let mut i = 0;
-    v.iter().rposition(|_elt| {
-        if i == 2 {
-            panic!()
-        }
-        i += 1;
-        false
-    });
-}
+mod step;
diff --git a/library/core/tests/iter/adapters/step.rs b/library/core/tests/iter/traits/step.rs
index 3d82a40cd29..3d82a40cd29 100644
--- a/library/core/tests/iter/adapters/step.rs
+++ b/library/core/tests/iter/traits/step.rs