| 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")] |
| #[rustc_diagnostic_item = "IterOnce"] |
| 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> {} |