Once { inner: Some(value).into_iter() }
}
+/// An iterator that repeats elements of type `A` endlessly by
+/// applying the provided closure `F: FnMut() -> A`.
+///
+/// This `struct` is created by the [`once_with`] function.
+/// See its documentation for more.
+///
+/// [`once_with`]: fn.once_with.html
+#[derive(Copy, Clone, Debug)]
+#[unstable(feature = "iter_once_with", issue = "0")]
+pub struct OnceWith<F> {
+ gen: Option<F>,
+}
+
+#[unstable(feature = "iter_once_with", issue = "0")]
+impl<A, F: FnOnce() -> A> Iterator for OnceWith<F> {
+ type Item = A;
+
+ #[inline]
+ fn next(&mut self) -> Option<A> {
+ self.gen.take().map(|f| f())
+ }
+
+ #[inline]
+ fn size_hint(&self) -> (usize, Option<usize>) {
+ self.gen.iter().size_hint()
+ }
+}
+
+#[unstable(feature = "iter_once_with", issue = "0")]
+impl<A, F: FnOnce() -> A> DoubleEndedIterator for OnceWith<F> {
+ fn next_back(&mut self) -> Option<A> {
+ self.next()
+ }
+}
+
+#[unstable(feature = "iter_once_with", issue = "0")]
+impl<A, F: FnOnce() -> A> ExactSizeIterator for OnceWith<F> {
+ fn len(&self) -> usize {
+ self.gen.iter().len()
+ }
+}
+
+#[unstable(feature = "iter_once_with", issue = "0")]
+impl<A, F: FnOnce() -> A> FusedIterator for OnceWith<F> {}
+
+#[unstable(feature = "iter_once_with", issue = "0")]
+unsafe impl<A, F: FnOnce() -> A> TrustedLen for OnceWith<F> {}
+
+/// Creates an iterator that lazily generates a value exactly once by invoking
+/// the provided closure.
+///
+/// This is commonly used to adapt a single value generator into a [`chain`] of
+/// other kinds of iteration. Maybe you have an iterator that covers almost
+/// everything, but you need an extra special case. Maybe you have a function
+/// which works on iterators, but you only need to process one value.
+///
+/// Unlike [`once`], this function will lazily generate the value on request.
+///
+/// [`once`]: fn.once.html
+///
+/// # Examples
+///
+/// Basic usage:
+///
+/// ```
+/// use std::iter;
+///
+/// // one is the loneliest number
+/// let mut one = iter::once_with(|| 1);
+///
+/// assert_eq!(Some(1), one.next());
+///
+/// // just one, that's all we get
+/// assert_eq!(None, one.next());
+/// ```
+///
+/// Chaining together with another iterator. Let's say that we want to iterate
+/// over each file of the `.foo` directory, but also a configuration file,
+/// `.foorc`:
+///
+/// ```no_run
+/// use std::iter;
+/// use std::fs;
+/// use std::path::PathBuf;
+///
+/// let dirs = fs::read_dir(".foo").unwrap();
+///
+/// // we need to convert from an iterator of DirEntry-s to an iterator of
+/// // PathBufs, so we use map
+/// let dirs = dirs.map(|file| file.unwrap().path());
+///
+/// // now, our iterator just for our config file
+/// let config = iter::once_with(|| PathBuf::from(".foorc"));
+///
+/// // chain the two iterators together into one big iterator
+/// let files = dirs.chain(config);
+///
+/// // this will give us all of the files in .foo as well as .foorc
+/// for f in files {
+/// println!("{:?}", f);
+/// }
+/// ```
+#[inline]
+#[unstable(feature = "iter_once_with", issue = "0")]
+pub fn once_with<A, F: FnOnce() -> A>(gen: F) -> OnceWith<F> {
+ OnceWith { gen: Some(gen) }
+}
+
/// Creates a new iterator where each iteration calls the provided closure
/// `F: FnMut(&mut St) -> Option<T>`.
///