Rollup merge of #48156 - Centril:feature/iterator_repeat_with, r=alexcrichton

Add std/core::iter::repeat_with

Adds an iterator primitive `repeat_with` which is the "lazy" version of `repeat` but also more flexible since you can build up state with the `FnMut`. The design is mostly taken from `repeat`.

r? @rust-lang/libs
cc @withoutboats, @scottmcm

2 files changed, 46 insertions, 0 deletions

diff --git a/src/libcore/tests/iter.rs b/src/libcore/tests/iter.rs
index d8c9dcd8664..f91c919d744 100644
--- a/src/libcore/tests/iter.rs
+++ b/src/libcore/tests/iter.rs
@@ -1593,6 +1593,51 @@ fn test_repeat_take_collect() {
 }
 
 #[test]
+fn test_repeat_with() {
+    #[derive(PartialEq, Debug)]
+    struct NotClone(usize);
+    let mut it = repeat_with(|| NotClone(42));
+    assert_eq!(it.next(), Some(NotClone(42)));
+    assert_eq!(it.next(), Some(NotClone(42)));
+    assert_eq!(it.next(), Some(NotClone(42)));
+    assert_eq!(repeat_with(|| NotClone(42)).size_hint(), (usize::MAX, None));
+}
+
+#[test]
+fn test_repeat_with_rev() {
+    let mut curr = 1;
+    let mut pow2 = repeat_with(|| { let tmp = curr; curr *= 2; tmp })
+                    .rev().take(4);
+    assert_eq!(pow2.next(), Some(1));
+    assert_eq!(pow2.next(), Some(2));
+    assert_eq!(pow2.next(), Some(4));
+    assert_eq!(pow2.next(), Some(8));
+    assert_eq!(pow2.next(), None);
+}
+
+#[test]
+fn test_repeat_with_take() {
+    let mut it = repeat_with(|| 42).take(3);
+    assert_eq!(it.next(), Some(42));
+    assert_eq!(it.next(), Some(42));
+    assert_eq!(it.next(), Some(42));
+    assert_eq!(it.next(), None);
+    is_trusted_len(repeat_with(|| 42).take(3));
+    assert_eq!(repeat_with(|| 42).take(3).size_hint(), (3, Some(3)));
+    assert_eq!(repeat_with(|| 42).take(0).size_hint(), (0, Some(0)));
+    assert_eq!(repeat_with(|| 42).take(usize::MAX).size_hint(),
+               (usize::MAX, Some(usize::MAX)));
+}
+
+#[test]
+fn test_repeat_with_take_collect() {
+    let mut curr = 1;
+    let v: Vec<_> = repeat_with(|| { let tmp = curr; curr *= 2; tmp })
+                      .take(5).collect();
+    assert_eq!(v, vec![1, 2, 4, 8, 16]);
+}
+
+#[test]
 fn test_fuse() {
     let mut it = 0..3;
     assert_eq!(it.len(), 3);
diff --git a/src/libcore/tests/lib.rs b/src/libcore/tests/lib.rs
index 0d55e6eeeb1..3e901a9d442 100644
--- a/src/libcore/tests/lib.rs
+++ b/src/libcore/tests/lib.rs
@@ -27,6 +27,7 @@
 #![feature(iterator_try_fold)]
 #![feature(iter_rfind)]
 #![feature(iter_rfold)]
+#![feature(iterator_repeat_with)]
 #![feature(nonzero)]
 #![feature(pattern)]
 #![feature(range_is_empty)]