Implement DoubleEnded and ExactSize for Take<Repeat> and Take<RepeatWith>

Repeat iterator always returns the same element and behaves the same way
backwards and forwards.  Take iterator can trivially implement backwards
iteration over Repeat inner iterator by simply doing forwards iteration.

DoubleEndedIterator is not currently implemented for Take<Repeat<T>>
because Repeat doesn’t implement ExactSizeIterator which is a required
bound on DEI implementation for Take.

Similarly, since Repeat is an infinite iterator which never stops, Take
can trivially know how many elements it’s going to return.  This allows
implementing ExactSizeIterator on Take<Repeat<T>>.

While at it, observe that ExactSizeIterator can also be implemented for
Take<RepeatWhile<F>> so add that implementation too.  Since in contrast
to Repeat, RepeatWhile doesn’t guarante to always return the same value,
DoubleEndedIterator isn’t implemented.

Those changes render core::iter::repeat_n somewhat redundant.

Issue: rust-lang#104434
Issue: rust-lang#104729

Author: mina86

library/core/src/iter/adapters/take.rs

@@ -312,3 +312,60 @@ impl<I: Iterator + TrustedRandomAccess> SpecTake for Take<I> {
         }
     }
 }
+
+#[stable(feature = "exact_size_take_repeat", since = "CURRENT_RUSTC_VERSION")]
+impl<T: Clone> DoubleEndedIterator for Take<crate::iter::Repeat<T>> {
+    #[inline]
+    fn next_back(&mut self) -> Option<Self::Item> {
+        self.next()
+    }
+
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
+        self.nth(n)
+    }
+
+    #[inline]
+    fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
+    where
+        Self: Sized,
+        Fold: FnMut(Acc, Self::Item) -> R,
+        R: Try<Output = Acc>,
+    {
+        self.try_fold(init, fold)
+    }
+
+    #[inline]
+    fn rfold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
+    where
+        Self: Sized,
+        Fold: FnMut(Acc, Self::Item) -> Acc,
+    {
+        self.fold(init, fold)
+    }
+
+    #[inline]
+    #[rustc_inherit_overflow_checks]
+    fn advance_back_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
+        self.advance_by(n)
+    }
+}
+
+// Note: It may be tempting to impl DoubleEndedIterator for Take<RepeatWith>.
+// One must fight that temptation since such implementation wouldn’t be correct
+// because we have no way to return value of nth invocation of repeater followed
+// by n-1st without remembering all results.
+
+#[stable(feature = "exact_size_take_repeat", since = "CURRENT_RUSTC_VERSION")]
+impl<T: Clone> ExactSizeIterator for Take<crate::iter::Repeat<T>> {
+    fn len(&self) -> usize {
+        self.n
+    }
+}
+
+#[stable(feature = "exact_size_take_repeat", since = "CURRENT_RUSTC_VERSION")]
+impl<F: FnMut() -> A, A> ExactSizeIterator for Take<crate::iter::RepeatWith<F>> {
+    fn len(&self) -> usize {
+        self.n
+    }
+}

library/core/tests/iter/adapters/take.rs

@@ -167,3 +167,92 @@ fn test_byref_take_consumed_items() {
     assert_eq!(count, 70);
     assert_eq!(inner, 90..90);
 }
+
+#[test]
+fn test_exact_size_take_repeat() {
+    let mut iter = core::iter::repeat(42).take(40);
+    assert_eq!((40, Some(40)), iter.size_hint());
+    assert_eq!(40, iter.len());
+
+    assert_eq!(Some(42), iter.next());
+    assert_eq!((39, Some(39)), iter.size_hint());
+    assert_eq!(39, iter.len());
+
+    assert_eq!(Some(42), iter.next_back());
+    assert_eq!((38, Some(38)), iter.size_hint());
+    assert_eq!(38, iter.len());
+
+    assert_eq!(Some(42), iter.nth(3));
+    assert_eq!((34, Some(34)), iter.size_hint());
+    assert_eq!(34, iter.len());
+
+    assert_eq!(Some(42), iter.nth_back(3));
+    assert_eq!((30, Some(30)), iter.size_hint());
+    assert_eq!(30, iter.len());
+
+    assert_eq!(Ok(()), iter.advance_by(10));
+    assert_eq!((20, Some(20)), iter.size_hint());
+    assert_eq!(20, iter.len());
+
+    assert_eq!(Ok(()), iter.advance_back_by(10));
+    assert_eq!((10, Some(10)), iter.size_hint());
+    assert_eq!(10, iter.len());
+}
+
+#[test]
+fn test_exact_size_take_repeat_with() {
+    let mut counter = 0;
+    let mut iter = core::iter::repeat_with(move || {
+        counter += 1;
+        counter
+    }).take(40);
+    assert_eq!((40, Some(40)), iter.size_hint());
+    assert_eq!(40, iter.len());
+
+    assert_eq!(Some(1), iter.next());
+    assert_eq!((39, Some(39)), iter.size_hint());
+    assert_eq!(39, iter.len());
+
+    assert_eq!(Some(5), iter.nth(3));
+    assert_eq!((35, Some(35)), iter.size_hint());
+    assert_eq!(35, iter.len());
+
+    assert_eq!(Ok(()), iter.advance_by(10));
+    assert_eq!((25, Some(25)), iter.size_hint());
+    assert_eq!(25, iter.len());
+
+    assert_eq!(Some(16), iter.next());
+    assert_eq!((24, Some(24)), iter.size_hint());
+    assert_eq!(24, iter.len());
+}
+
+// This is https://github.com/rust-lang/rust/issues/104729 with all uses of
+// repeat(0) were replaced by repeat(0).take(20).
+#[test]
+fn test_reverse_on_zip() {
+    let vec_1 = [1; 10];
+
+    let zipped_iter = vec_1.iter().copied().zip(core::iter::repeat(0).take(20));
+
+    // Forward
+    for (one, zero) in zipped_iter {
+        assert_eq!((1, 0), (one, zero));
+    }
+
+    let rev_vec_iter = vec_1.iter().rev();
+    let rev_repeat_iter = std::iter::repeat(0).take(20).rev();
+
+    // Manual reversed zip
+    let rev_zipped_iter = rev_vec_iter.zip(rev_repeat_iter);
+
+    for (&one, zero) in rev_zipped_iter {
+        assert_eq!((1, 0), (one, zero));
+    }
+
+    let zipped_iter = vec_1.iter().zip(core::iter::repeat(0).take(20));
+
+    // Cannot call rev here for automatic reversed zip constuction
+    for (&one, zero) in zipped_iter.rev() {
+        assert_eq!((1, 0), (one, zero));
+    }
+}