diff --git a/src/libcore/iter/mod.rs b/src/libcore/iter/mod.rs
index 509068843d193..62e1f9fcb640c 100644
--- a/src/libcore/iter/mod.rs
+++ b/src/libcore/iter/mod.rs
@@ -112,10 +112,10 @@
//!
//! // next() is the only required method
//! fn next(&mut self) -> Option<usize> {
-//! // increment our count. This is why we started at zero.
+//! // Increment our count. This is why we started at zero.
//! self.count += 1;
//!
-//! // check to see if we've finished counting or not.
+//! // Check to see if we've finished counting or not.
//! if self.count < 6 {
//! Some(self.count)
//! } else {
@@ -339,6 +339,8 @@ pub use self::sources::{RepeatWith, repeat_with};
pub use self::sources::{Empty, empty};
#[stable(feature = "iter_once", since = "1.2.0")]
pub use self::sources::{Once, once};
+#[unstable(feature = "iter_unfold", issue = "55977")]
+pub use self::sources::{Unfold, unfold, Successors, successors};
#[stable(feature = "rust1", since = "1.0.0")]
pub use self::traits::{FromIterator, IntoIterator, DoubleEndedIterator, Extend};
diff --git a/src/libcore/iter/sources.rs b/src/libcore/iter/sources.rs
index 7fa3a4bcce7bb..f6a4a7a6fa80a 100644
--- a/src/libcore/iter/sources.rs
+++ b/src/libcore/iter/sources.rs
@@ -386,3 +386,164 @@ impl<T> FusedIterator for Once<T> {}
pub fn once<T>(value: T) -> Once<T> {
Once { inner: Some(value).into_iter() }
}
+
+/// Creates a new iterator where each iteration calls the provided closure
+/// `F: FnMut(&mut St) -> Option<T>`.
+///
+/// This allows creating a custom iterator with any behavior
+/// without using the more verbose syntax of creating a dedicated type
+/// and implementing the `Iterator` trait for it.
+///
+/// In addition to its captures and environment,
+/// the closure is given a mutable reference to some state
+/// that is preserved across iterations.
+/// That state starts as the given `initial_state` value.
+///
+/// Note that the `Unfold` iterator doesn’t make assumptions about the behavior of the closure,
+/// and therefore conservatively does not implement [`FusedIterator`],
+/// or override [`Iterator::size_hint`] from its default `(0, None)`.
+///
+/// [`FusedIterator`]: trait.FusedIterator.html
+/// [`Iterator::size_hint`]: trait.Iterator.html#method.size_hint
+///
+/// # Examples
+///
+/// Let’s re-implement the counter iterator from [module-level documentation]:
+///
+/// [module-level documentation]: index.html
+///
+/// ```
+/// #![feature(iter_unfold)]
+/// let counter = std::iter::unfold(0, |count| {
+/// // Increment our count. This is why we started at zero.
+/// *count += 1;
+///
+/// // Check to see if we've finished counting or not.
+/// if *count < 6 {
+/// Some(*count)
+/// } else {
+/// None
+/// }
+/// });
+/// assert_eq!(counter.collect::<Vec<_>>(), &[1, 2, 3, 4, 5]);
+/// ```
+#[inline]
+#[unstable(feature = "iter_unfold", issue = "55977")]
+pub fn unfold<St, T, F>(initial_state: St, f: F) -> Unfold<St, F>
+ where F: FnMut(&mut St) -> Option<T>
+{
+ Unfold {
+ state: initial_state,
+ f,
+ }
+}
+
+/// An iterator where each iteration calls the provided closure `F: FnMut(&mut St) -> Option<T>`.
+///
+/// This `struct` is created by the [`unfold`] function.
+/// See its documentation for more.
+///
+/// [`unfold`]: fn.unfold.html
+#[derive(Clone)]
+#[unstable(feature = "iter_unfold", issue = "55977")]
+pub struct Unfold<St, F> {
+ state: St,
+ f: F,
+}
+
+#[unstable(feature = "iter_unfold", issue = "55977")]
+impl<St, T, F> Iterator for Unfold<St, F>
+ where F: FnMut(&mut St) -> Option<T>
+{
+ type Item = T;
+
+ #[inline]
+ fn next(&mut self) -> Option<Self::Item> {
+ (self.f)(&mut self.state)
+ }
+}
+
+#[unstable(feature = "iter_unfold", issue = "55977")]
+impl<St: fmt::Debug, F> fmt::Debug for Unfold<St, F> {
+ fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+ f.debug_struct("Unfold")
+ .field("state", &self.state)
+ .finish()
+ }
+}
+
+/// Creates a new iterator where each successive item is computed based on the preceding one.
+///
+/// The iterator starts with the given first item (if any)
+/// and calls the given `FnMut(&T) -> Option<T>` closure to compute each item’s successor.
+///
+/// ```
+/// #![feature(iter_unfold)]
+/// use std::iter::successors;
+///
+/// let powers_of_10 = successors(Some(1_u16), |n| n.checked_mul(10));
+/// assert_eq!(powers_of_10.collect::<Vec<_>>(), &[1, 10, 100, 1_000, 10_000]);
+/// ```
+#[unstable(feature = "iter_unfold", issue = "55977")]
+pub fn successors<T, F>(first: Option<T>, succ: F) -> Successors<T, F>
+ where F: FnMut(&T) -> Option<T>
+{
+ // If this function returned `impl Iterator<Item=T>`
+ // it could be based on `unfold` and not need a dedicated type.
+ // However having a named `Successors<T, F>` type allows it to be `Clone` when `T` and `F` are.
+ Successors {
+ next: first,
+ succ,
+ }
+}
+
+/// An new iterator where each successive item is computed based on the preceding one.
+///
+/// This `struct` is created by the [`successors`] function.
+/// See its documentation for more.
+///
+/// [`successors`]: fn.successors.html
+#[derive(Clone)]
+#[unstable(feature = "iter_unfold", issue = "55977")]
+pub struct Successors<T, F> {
+ next: Option<T>,
+ succ: F,
+}
+
+#[unstable(feature = "iter_unfold", issue = "55977")]
+impl<T, F> Iterator for Successors<T, F>
+ where F: FnMut(&T) -> Option<T>
+{
+ type Item = T;
+
+ #[inline]
+ fn next(&mut self) -> Option<Self::Item> {
+ self.next.take().map(|item| {
+ self.next = (self.succ)(&item);
+ item
+ })
+ }
+
+ #[inline]
+ fn size_hint(&self) -> (usize, Option<usize>) {
+ if self.next.is_some() {
+ (1, None)
+ } else {
+ (0, Some(0))
+ }
+ }
+}
+
+#[unstable(feature = "iter_unfold", issue = "55977")]
+impl<T, F> FusedIterator for Successors<T, F>
+ where F: FnMut(&T) -> Option<T>
+{}
+
+#[unstable(feature = "iter_unfold", issue = "55977")]
+impl<T: fmt::Debug, F> fmt::Debug for Successors<T, F> {
+ fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+ f.debug_struct("Successors")
+ .field("next", &self.next)
+ .finish()
+ }
+}
diff --git a/src/libcore/tests/iter.rs b/src/libcore/tests/iter.rs
index ec09071b3d0f0..495483db5551c 100644
--- a/src/libcore/tests/iter.rs
+++ b/src/libcore/tests/iter.rs
@@ -1759,6 +1759,17 @@ fn test_repeat_with_take_collect() {
assert_eq!(v, vec![1, 2, 4, 8, 16]);
}
+#[test]
+fn test_successors() {
+ let mut powers_of_10 = successors(Some(1_u16), |n| n.checked_mul(10));
+ assert_eq!(powers_of_10.by_ref().collect::<Vec<_>>(), &[1, 10, 100, 1_000, 10_000]);
+ assert_eq!(powers_of_10.next(), None);
+
+ let mut empty = successors(None::<u32>, |_| unimplemented!());
+ assert_eq!(empty.next(), None);
+ assert_eq!(empty.next(), None);
+}
+
#[test]
fn test_fuse() {
let mut it = 0..3;
diff --git a/src/libcore/tests/lib.rs b/src/libcore/tests/lib.rs
index 5ac8991226898..7d62b4fa90f20 100644
--- a/src/libcore/tests/lib.rs
+++ b/src/libcore/tests/lib.rs
@@ -19,6 +19,7 @@
#![feature(flt2dec)]
#![feature(fmt_internals)]
#![feature(hashmap_internals)]
+#![feature(iter_unfold)]
#![feature(pattern)]
#![feature(range_is_empty)]
#![feature(raw)]