1use crate::iter::{FusedIterator, TrustedLen};
2use crate::{fmt, marker};
3
4/// Creates an iterator that yields nothing.
5///
6/// # Examples
7///
8/// Basic usage:
9///
10/// ```
11/// use std::iter;
12///
13/// // this could have been an iterator over i32, but alas, it's just not.
14/// let mut nope = iter::empty::<i32>();
15///
16/// assert_eq!(None, nope.next());
17/// ```
18#[stable(feature = "iter_empty", since = "1.2.0")]
19#[rustc_const_stable(feature = "const_iter_empty", since = "1.32.0")]
20pub const fn empty<T>() -> Empty<T> {
21 Empty(marker::PhantomData)
22}
23
24/// An iterator that yields nothing.
25///
26/// This `struct` is created by the [`empty()`] function. See its documentation for more.
27#[must_use = "iterators are lazy and do nothing unless consumed"]
28#[stable(feature = "iter_empty", since = "1.2.0")]
29#[rustc_diagnostic_item = "IterEmpty"]
30pub struct Empty<T>(marker::PhantomData<fn() -> T>);
31
32#[stable(feature = "core_impl_debug", since = "1.9.0")]
33impl<T> fmt::Debug for Empty<T> {
34 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
35 f.debug_struct(name:"Empty").finish()
36 }
37}
38
39#[stable(feature = "iter_empty", since = "1.2.0")]
40impl<T> Iterator for Empty<T> {
41 type Item = T;
42
43 fn next(&mut self) -> Option<T> {
44 None
45 }
46
47 fn size_hint(&self) -> (usize, Option<usize>) {
48 (0, Some(0))
49 }
50}
51
52#[stable(feature = "iter_empty", since = "1.2.0")]
53impl<T> DoubleEndedIterator for Empty<T> {
54 fn next_back(&mut self) -> Option<T> {
55 None
56 }
57}
58
59#[stable(feature = "iter_empty", since = "1.2.0")]
60impl<T> ExactSizeIterator for Empty<T> {
61 fn len(&self) -> usize {
62 0
63 }
64}
65
66#[unstable(feature = "trusted_len", issue = "37572")]
67unsafe impl<T> TrustedLen for Empty<T> {}
68
69#[stable(feature = "fused", since = "1.26.0")]
70impl<T> FusedIterator for Empty<T> {}
71
72// not #[derive] because that adds a Clone bound on T,
73// which isn't necessary.
74#[stable(feature = "iter_empty", since = "1.2.0")]
75impl<T> Clone for Empty<T> {
76 fn clone(&self) -> Empty<T> {
77 Empty(marker::PhantomData)
78 }
79}
80
81// not #[derive] because that adds a Default bound on T,
82// which isn't necessary.
83#[stable(feature = "iter_empty", since = "1.2.0")]
84impl<T> Default for Empty<T> {
85 fn default() -> Empty<T> {
86 Empty(marker::PhantomData)
87 }
88}
89