2 use crate::iter::{FusedIterator, TrustedLen};
5 /// Creates an iterator that yields nothing.
14 /// // this could have been an iterator over i32, but alas, it's just not.
15 /// let mut nope = iter::empty::<i32>();
17 /// assert_eq!(None, nope.next());
19 #[stable(feature = "iter_empty", since = "1.2.0")]
20 #[rustc_const_stable(feature = "const_iter_empty", since = "1.32.0")]
21 pub const fn empty<T>() -> Empty<T> {
22 Empty(marker::PhantomData)
25 // Newtype for use in `PhantomData` to avoid
26 // > error: const-stable function cannot use `#[feature(const_fn_fn_ptr_basics)]`
27 // in `const fn empty<T>()` above.
28 struct FnReturning<T>(fn() -> T);
30 /// An iterator that yields nothing.
32 /// This `struct` is created by the [`empty()`] function. See its documentation for more.
33 #[must_use = "iterators are lazy and do nothing unless consumed"]
34 #[stable(feature = "iter_empty", since = "1.2.0")]
35 pub struct Empty<T>(marker::PhantomData<FnReturning<T>>);
37 #[stable(feature = "core_impl_debug", since = "1.9.0")]
38 impl<T> fmt::Debug for Empty<T> {
39 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
40 f.debug_struct("Empty").finish()
44 #[stable(feature = "iter_empty", since = "1.2.0")]
45 impl<T> Iterator for Empty<T> {
48 fn next(&mut self) -> Option<T> {
52 fn size_hint(&self) -> (usize, Option<usize>) {
57 #[stable(feature = "iter_empty", since = "1.2.0")]
58 impl<T> DoubleEndedIterator for Empty<T> {
59 fn next_back(&mut self) -> Option<T> {
64 #[stable(feature = "iter_empty", since = "1.2.0")]
65 impl<T> ExactSizeIterator for Empty<T> {
66 fn len(&self) -> usize {
71 #[unstable(feature = "trusted_len", issue = "37572")]
72 unsafe impl<T> TrustedLen for Empty<T> {}
74 #[stable(feature = "fused", since = "1.26.0")]
75 impl<T> FusedIterator for Empty<T> {}
77 // not #[derive] because that adds a Clone bound on T,
78 // which isn't necessary.
79 #[stable(feature = "iter_empty", since = "1.2.0")]
80 impl<T> Clone for Empty<T> {
81 fn clone(&self) -> Empty<T> {
82 Empty(marker::PhantomData)
86 // not #[derive] because that adds a Default bound on T,
87 // which isn't necessary.
88 #[stable(feature = "iter_empty", since = "1.2.0")]
89 #[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
90 impl<T> const Default for Empty<T> {
91 fn default() -> Empty<T> {
92 Empty(marker::PhantomData)