From a8340413fe8aa29864ce05875cc63f29af305e31 Mon Sep 17 00:00:00 2001
From: Waffle <waffle.lapkin@gmail.com>
Date: Tue, 4 Feb 2020 11:38:07 +0300
Subject: [PATCH] Remove `finished` flag from `MapWhile`

---
 src/libcore/iter/adapters/mod.rs    | 68 +++++++++--------------------
 src/libcore/iter/traits/iterator.rs | 15 +++++++
 2 files changed, 36 insertions(+), 47 deletions(-)

diff --git a/src/libcore/iter/adapters/mod.rs b/src/libcore/iter/adapters/mod.rs
index 7d10ef3d28219..177e48b05b3db 100644
--- a/src/libcore/iter/adapters/mod.rs
+++ b/src/libcore/iter/adapters/mod.rs
@@ -1752,6 +1752,14 @@ where
     }
 }
 
+#[stable(feature = "fused", since = "1.26.0")]
+impl<I, P> FusedIterator for TakeWhile<I, P>
+where
+    I: FusedIterator,
+    P: FnMut(&I::Item) -> bool,
+{
+}
+
 /// An iterator that only accepts elements while `predicate` returns `Some(_)`.
 ///
 /// This `struct` is created by the [`map_while`] method on [`Iterator`]. See its
@@ -1764,20 +1772,19 @@ where
 #[derive(Clone)]
 pub struct MapWhile<I, P> {
     iter: I,
-    finished: bool,
     predicate: P,
 }
 
 impl<I, P> MapWhile<I, P> {
     pub(super) fn new(iter: I, predicate: P) -> MapWhile<I, P> {
-        MapWhile { iter, finished: false, predicate }
+        MapWhile { iter, predicate }
     }
 }
 
 #[unstable(feature = "iter_map_while", reason = "recently added", issue = "68537")]
 impl<I: fmt::Debug, P> fmt::Debug for MapWhile<I, P> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.debug_struct("MapWhile").field("iter", &self.iter).field("flag", &self.finished).finish()
+        f.debug_struct("MapWhile").field("iter", &self.iter).finish()
     }
 }
 
@@ -1790,65 +1797,32 @@ where
 
     #[inline]
     fn next(&mut self) -> Option<B> {
-        if self.finished {
-            None
-        } else {
-            let x = self.iter.next()?;
-            let ret = (self.predicate)(x);
-            self.finished = ret.is_none();
-            ret
-        }
+        let x = self.iter.next()?;
+        (self.predicate)(x)
     }
 
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) {
-        if self.finished {
-            (0, Some(0))
-        } else {
-            let (_, upper) = self.iter.size_hint();
-            (0, upper) // can't know a lower bound, due to the predicate
-        }
+        let (_, upper) = self.iter.size_hint();
+        (0, upper) // can't know a lower bound, due to the predicate
     }
 
     #[inline]
-    fn try_fold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
+    fn try_fold<Acc, Fold, R>(&mut self, init: Acc, mut fold: Fold) -> R
     where
         Self: Sized,
         Fold: FnMut(Acc, Self::Item) -> R,
         R: Try<Ok = Acc>,
     {
-        fn check<'a, B, T, Acc, R: Try<Ok = Acc>>(
-            flag: &'a mut bool,
-            p: &'a mut impl FnMut(T) -> Option<B>,
-            mut fold: impl FnMut(Acc, B) -> R + 'a,
-        ) -> impl FnMut(Acc, T) -> LoopState<Acc, R> + 'a {
-            move |acc, x| match p(x) {
-                Some(item) => LoopState::from_try(fold(acc, item)),
-                None => {
-                    *flag = true;
-                    LoopState::Break(Try::from_ok(acc))
-                }
-            }
-        }
-
-        if self.finished {
-            Try::from_ok(init)
-        } else {
-            let flag = &mut self.finished;
-            let p = &mut self.predicate;
-            self.iter.try_fold(init, check(flag, p, fold)).into_try()
-        }
+        let Self { iter, predicate } = self;
+        iter.try_fold(init, |acc, x| match predicate(x) {
+            Some(item) => LoopState::from_try(fold(acc, item)),
+            None => LoopState::Break(Try::from_ok(acc)),
+        })
+        .into_try()
     }
 }
 
-#[stable(feature = "fused", since = "1.26.0")]
-impl<I, P> FusedIterator for TakeWhile<I, P>
-where
-    I: FusedIterator,
-    P: FnMut(&I::Item) -> bool,
-{
-}
-
 /// An iterator that skips over `n` elements of `iter`.
 ///
 /// This `struct` is created by the [`skip`] method on [`Iterator`]. See its
diff --git a/src/libcore/iter/traits/iterator.rs b/src/libcore/iter/traits/iterator.rs
index 1d055676c7708..3b1f3b3a4ead6 100644
--- a/src/libcore/iter/traits/iterator.rs
+++ b/src/libcore/iter/traits/iterator.rs
@@ -1114,8 +1114,23 @@ pub trait Iterator {
     /// The `-3` is no longer there, because it was consumed in order to see if
     /// the iteration should stop, but wasn't placed back into the iterator.
     ///
+    /// Note that unlike [`take_while`] this iterator is **not** fused:
+    ///
+    /// ```
+    /// #![feature(iter_map_while)]
+    /// use std::convert::identity;
+    ///
+    /// let mut iter = vec![Some(0), None, Some(1)].into_iter().map_while(identity);
+    /// assert_eq!(iter.next(), Some(0));
+    /// assert_eq!(iter.next(), None);
+    /// assert_eq!(iter.next(), Some(1));
+    /// ```
+    ///
+    /// If you need fused iterator, use [`fuse`].
+    ///
     /// [`Some`]: ../../std/option/enum.Option.html#variant.Some
     /// [`None`]: ../../std/option/enum.Option.html#variant.None
+    /// [`fuse`]: #method.fuse
     #[inline]
     #[unstable(feature = "iter_map_while", reason = "recently added", issue = "68537")]
     fn map_while<B, P>(self, predicate: P) -> MapWhile<Self, P>