From 698f8c2f01ea549d77d7dc3338a12e04c11057b9 Mon Sep 17 00:00:00 2001
From: Daniel Baumann <daniel.baumann@progress-linux.org>
Date: Wed, 17 Apr 2024 14:02:58 +0200
Subject: Adding upstream version 1.64.0+dfsg1.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
---
 library/core/src/iter/sources/once.rs | 99 +++++++++++++++++++++++++++++++++++
 1 file changed, 99 insertions(+)
 create mode 100644 library/core/src/iter/sources/once.rs

(limited to 'library/core/src/iter/sources/once.rs')

diff --git a/library/core/src/iter/sources/once.rs b/library/core/src/iter/sources/once.rs
new file mode 100644
index 000000000..6e9ed0d3c
--- /dev/null
+++ b/library/core/src/iter/sources/once.rs
@@ -0,0 +1,99 @@
+use crate::iter::{FusedIterator, TrustedLen};
+
+/// Creates an iterator that yields an element exactly once.
+///
+/// This is commonly used to adapt a single value 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.
+///
+/// [`chain()`]: Iterator::chain
+///
+/// # Examples
+///
+/// Basic usage:
+///
+/// ```
+/// use std::iter;
+///
+/// // one is the loneliest number
+/// let mut one = iter::once(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(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:?}");
+/// }
+/// ```
+#[stable(feature = "iter_once", since = "1.2.0")]
+pub fn once<T>(value: T) -> Once<T> {
+    Once { inner: Some(value).into_iter() }
+}
+
+/// An iterator that yields an element exactly once.
+///
+/// This `struct` is created by the [`once()`] function. See its documentation for more.
+#[derive(Clone, Debug)]
+#[stable(feature = "iter_once", since = "1.2.0")]
+pub struct Once<T> {
+    inner: crate::option::IntoIter<T>,
+}
+
+#[stable(feature = "iter_once", since = "1.2.0")]
+impl<T> Iterator for Once<T> {
+    type Item = T;
+
+    fn next(&mut self) -> Option<T> {
+        self.inner.next()
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.inner.size_hint()
+    }
+}
+
+#[stable(feature = "iter_once", since = "1.2.0")]
+impl<T> DoubleEndedIterator for Once<T> {
+    fn next_back(&mut self) -> Option<T> {
+        self.inner.next_back()
+    }
+}
+
+#[stable(feature = "iter_once", since = "1.2.0")]
+impl<T> ExactSizeIterator for Once<T> {
+    fn len(&self) -> usize {
+        self.inner.len()
+    }
+}
+
+#[unstable(feature = "trusted_len", issue = "37572")]
+unsafe impl<T> TrustedLen for Once<T> {}
+
+#[stable(feature = "fused", since = "1.26.0")]
+impl<T> FusedIterator for Once<T> {}
-- 
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