core/iter/adapters/
map_while.rs

1use crate::fmt;
2use crate::iter::InPlaceIterable;
3use crate::iter::adapters::SourceIter;
4use crate::num::NonZero;
5use crate::ops::{ControlFlow, Try};
6
7/// An iterator that only accepts elements while `predicate` returns `Some(_)`.
8///
9/// This `struct` is created by the [`map_while`] method on [`Iterator`]. See its
10/// documentation for more.
11///
12/// [`map_while`]: Iterator::map_while
13/// [`Iterator`]: trait.Iterator.html
14#[must_use = "iterators are lazy and do nothing unless consumed"]
15#[stable(feature = "iter_map_while", since = "1.57.0")]
16#[derive(Clone)]
17pub struct MapWhile<I, P> {
18    iter: I,
19    predicate: P,
20}
21
22impl<I, P> MapWhile<I, P> {
23    pub(in crate::iter) const fn new(iter: I, predicate: P) -> MapWhile<I, P> {
24        MapWhile { iter, predicate }
25    }
26}
27
28#[stable(feature = "iter_map_while", since = "1.57.0")]
29impl<I: fmt::Debug, P> fmt::Debug for MapWhile<I, P> {
30    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
31        f.debug_struct("MapWhile").field("iter", &self.iter).finish()
32    }
33}
34
35#[stable(feature = "iter_map_while", since = "1.57.0")]
36impl<B, I: Iterator, P> Iterator for MapWhile<I, P>
37where
38    P: FnMut(I::Item) -> Option<B>,
39{
40    type Item = B;
41
42    #[inline]
43    fn next(&mut self) -> Option<B> {
44        let x = self.iter.next()?;
45        (self.predicate)(x)
46    }
47
48    #[inline]
49    fn size_hint(&self) -> (usize, Option<usize>) {
50        let (_, upper) = self.iter.size_hint();
51        (0, upper) // can't know a lower bound, due to the predicate
52    }
53
54    #[inline]
55    #[ferrocene::prevalidated]
56    fn try_fold<Acc, Fold, R>(&mut self, init: Acc, mut fold: Fold) -> R
57    where
58        Self: Sized,
59        Fold: FnMut(Acc, Self::Item) -> R,
60        R: Try<Output = Acc>,
61    {
62        let Self { iter, predicate } = self;
63        iter.try_fold(init, |acc, x| match predicate(x) {
64            Some(item) => ControlFlow::from_try(fold(acc, item)),
65            None => ControlFlow::Break(try { acc }),
66        })
67        .into_try()
68    }
69
70    impl_fold_via_try_fold! { fold -> try_fold }
71}
72
73#[unstable(issue = "none", feature = "inplace_iteration")]
74unsafe impl<I, P> SourceIter for MapWhile<I, P>
75where
76    I: SourceIter,
77{
78    type Source = I::Source;
79
80    #[inline]
81    unsafe fn as_inner(&mut self) -> &mut I::Source {
82        // SAFETY: unsafe function forwarding to unsafe function with the same requirements
83        unsafe { SourceIter::as_inner(&mut self.iter) }
84    }
85}
86
87#[unstable(issue = "none", feature = "inplace_iteration")]
88unsafe impl<I: InPlaceIterable, P> InPlaceIterable for MapWhile<I, P> {
89    const EXPAND_BY: Option<NonZero<usize>> = I::EXPAND_BY;
90    const MERGE_BY: Option<NonZero<usize>> = I::MERGE_BY;
91}
core/iter/adapters/map_while.rs

core/iter/adapters/
map_while.rs
