From 698f8c2f01ea549d77d7dc3338a12e04c11057b9 Mon Sep 17 00:00:00 2001
From: Daniel Baumann <daniel.baumann@progress-linux.org>
Date: Wed, 17 Apr 2024 14:02:58 +0200
Subject: Adding upstream version 1.64.0+dfsg1.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
---
 library/core/src/iter/adapters/flatten.rs | 599 ++++++++++++++++++++++++++++++
 1 file changed, 599 insertions(+)
 create mode 100644 library/core/src/iter/adapters/flatten.rs

(limited to 'library/core/src/iter/adapters/flatten.rs')

diff --git a/library/core/src/iter/adapters/flatten.rs b/library/core/src/iter/adapters/flatten.rs
new file mode 100644
index 000000000..15a120e35
--- /dev/null
+++ b/library/core/src/iter/adapters/flatten.rs
@@ -0,0 +1,599 @@
+use crate::fmt;
+use crate::iter::{DoubleEndedIterator, Fuse, FusedIterator, Iterator, Map, TrustedLen};
+use crate::ops::Try;
+
+/// An iterator that maps each element to an iterator, and yields the elements
+/// of the produced iterators.
+///
+/// This `struct` is created by [`Iterator::flat_map`]. See its documentation
+/// for more.
+#[must_use = "iterators are lazy and do nothing unless consumed"]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct FlatMap<I, U: IntoIterator, F> {
+    inner: FlattenCompat<Map<I, F>, <U as IntoIterator>::IntoIter>,
+}
+
+impl<I: Iterator, U: IntoIterator, F: FnMut(I::Item) -> U> FlatMap<I, U, F> {
+    pub(in crate::iter) fn new(iter: I, f: F) -> FlatMap<I, U, F> {
+        FlatMap { inner: FlattenCompat::new(iter.map(f)) }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<I: Clone, U, F: Clone> Clone for FlatMap<I, U, F>
+where
+    U: Clone + IntoIterator<IntoIter: Clone>,
+{
+    fn clone(&self) -> Self {
+        FlatMap { inner: self.inner.clone() }
+    }
+}
+
+#[stable(feature = "core_impl_debug", since = "1.9.0")]
+impl<I: fmt::Debug, U, F> fmt::Debug for FlatMap<I, U, F>
+where
+    U: IntoIterator<IntoIter: fmt::Debug>,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("FlatMap").field("inner", &self.inner).finish()
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<I: Iterator, U: IntoIterator, F> Iterator for FlatMap<I, U, F>
+where
+    F: FnMut(I::Item) -> U,
+{
+    type Item = U::Item;
+
+    #[inline]
+    fn next(&mut self) -> Option<U::Item> {
+        self.inner.next()
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.inner.size_hint()
+    }
+
+    #[inline]
+    fn try_fold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
+    where
+        Self: Sized,
+        Fold: FnMut(Acc, Self::Item) -> R,
+        R: Try<Output = Acc>,
+    {
+        self.inner.try_fold(init, fold)
+    }
+
+    #[inline]
+    fn fold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
+    where
+        Fold: FnMut(Acc, Self::Item) -> Acc,
+    {
+        self.inner.fold(init, fold)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<I: DoubleEndedIterator, U, F> DoubleEndedIterator for FlatMap<I, U, F>
+where
+    F: FnMut(I::Item) -> U,
+    U: IntoIterator<IntoIter: DoubleEndedIterator>,
+{
+    #[inline]
+    fn next_back(&mut self) -> Option<U::Item> {
+        self.inner.next_back()
+    }
+
+    #[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.inner.try_rfold(init, fold)
+    }
+
+    #[inline]
+    fn rfold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
+    where
+        Fold: FnMut(Acc, Self::Item) -> Acc,
+    {
+        self.inner.rfold(init, fold)
+    }
+}
+
+#[stable(feature = "fused", since = "1.26.0")]
+impl<I, U, F> FusedIterator for FlatMap<I, U, F>
+where
+    I: FusedIterator,
+    U: IntoIterator,
+    F: FnMut(I::Item) -> U,
+{
+}
+
+#[unstable(feature = "trusted_len", issue = "37572")]
+unsafe impl<T, I, F, const N: usize> TrustedLen for FlatMap<I, [T; N], F>
+where
+    I: TrustedLen,
+    F: FnMut(I::Item) -> [T; N],
+{
+}
+
+#[unstable(feature = "trusted_len", issue = "37572")]
+unsafe impl<'a, T, I, F, const N: usize> TrustedLen for FlatMap<I, &'a [T; N], F>
+where
+    I: TrustedLen,
+    F: FnMut(I::Item) -> &'a [T; N],
+{
+}
+
+#[unstable(feature = "trusted_len", issue = "37572")]
+unsafe impl<'a, T, I, F, const N: usize> TrustedLen for FlatMap<I, &'a mut [T; N], F>
+where
+    I: TrustedLen,
+    F: FnMut(I::Item) -> &'a mut [T; N],
+{
+}
+
+/// An iterator that flattens one level of nesting in an iterator of things
+/// that can be turned into iterators.
+///
+/// This `struct` is created by the [`flatten`] method on [`Iterator`]. See its
+/// documentation for more.
+///
+/// [`flatten`]: Iterator::flatten()
+#[must_use = "iterators are lazy and do nothing unless consumed"]
+#[stable(feature = "iterator_flatten", since = "1.29.0")]
+pub struct Flatten<I: Iterator<Item: IntoIterator>> {
+    inner: FlattenCompat<I, <I::Item as IntoIterator>::IntoIter>,
+}
+
+impl<I: Iterator<Item: IntoIterator>> Flatten<I> {
+    pub(in super::super) fn new(iter: I) -> Flatten<I> {
+        Flatten { inner: FlattenCompat::new(iter) }
+    }
+}
+
+#[stable(feature = "iterator_flatten", since = "1.29.0")]
+impl<I, U> fmt::Debug for Flatten<I>
+where
+    I: fmt::Debug + Iterator<Item: IntoIterator<IntoIter = U, Item = U::Item>>,
+    U: fmt::Debug + Iterator,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Flatten").field("inner", &self.inner).finish()
+    }
+}
+
+#[stable(feature = "iterator_flatten", since = "1.29.0")]
+impl<I, U> Clone for Flatten<I>
+where
+    I: Clone + Iterator<Item: IntoIterator<IntoIter = U, Item = U::Item>>,
+    U: Clone + Iterator,
+{
+    fn clone(&self) -> Self {
+        Flatten { inner: self.inner.clone() }
+    }
+}
+
+#[stable(feature = "iterator_flatten", since = "1.29.0")]
+impl<I, U> Iterator for Flatten<I>
+where
+    I: Iterator<Item: IntoIterator<IntoIter = U, Item = U::Item>>,
+    U: Iterator,
+{
+    type Item = U::Item;
+
+    #[inline]
+    fn next(&mut self) -> Option<U::Item> {
+        self.inner.next()
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.inner.size_hint()
+    }
+
+    #[inline]
+    fn try_fold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
+    where
+        Self: Sized,
+        Fold: FnMut(Acc, Self::Item) -> R,
+        R: Try<Output = Acc>,
+    {
+        self.inner.try_fold(init, fold)
+    }
+
+    #[inline]
+    fn fold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
+    where
+        Fold: FnMut(Acc, Self::Item) -> Acc,
+    {
+        self.inner.fold(init, fold)
+    }
+}
+
+#[stable(feature = "iterator_flatten", since = "1.29.0")]
+impl<I, U> DoubleEndedIterator for Flatten<I>
+where
+    I: DoubleEndedIterator<Item: IntoIterator<IntoIter = U, Item = U::Item>>,
+    U: DoubleEndedIterator,
+{
+    #[inline]
+    fn next_back(&mut self) -> Option<U::Item> {
+        self.inner.next_back()
+    }
+
+    #[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.inner.try_rfold(init, fold)
+    }
+
+    #[inline]
+    fn rfold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
+    where
+        Fold: FnMut(Acc, Self::Item) -> Acc,
+    {
+        self.inner.rfold(init, fold)
+    }
+}
+
+#[stable(feature = "iterator_flatten", since = "1.29.0")]
+impl<I, U> FusedIterator for Flatten<I>
+where
+    I: FusedIterator<Item: IntoIterator<IntoIter = U, Item = U::Item>>,
+    U: Iterator,
+{
+}
+
+#[unstable(feature = "trusted_len", issue = "37572")]
+unsafe impl<I> TrustedLen for Flatten<I>
+where
+    I: TrustedLen,
+    <I as Iterator>::Item: TrustedConstSize,
+{
+}
+
+/// Real logic of both `Flatten` and `FlatMap` which simply delegate to
+/// this type.
+#[derive(Clone, Debug)]
+struct FlattenCompat<I, U> {
+    iter: Fuse<I>,
+    frontiter: Option<U>,
+    backiter: Option<U>,
+}
+impl<I, U> FlattenCompat<I, U>
+where
+    I: Iterator,
+{
+    /// Adapts an iterator by flattening it, for use in `flatten()` and `flat_map()`.
+    fn new(iter: I) -> FlattenCompat<I, U> {
+        FlattenCompat { iter: iter.fuse(), frontiter: None, backiter: None }
+    }
+}
+
+impl<I, U> Iterator for FlattenCompat<I, U>
+where
+    I: Iterator<Item: IntoIterator<IntoIter = U, Item = U::Item>>,
+    U: Iterator,
+{
+    type Item = U::Item;
+
+    #[inline]
+    fn next(&mut self) -> Option<U::Item> {
+        loop {
+            if let elt @ Some(_) = and_then_or_clear(&mut self.frontiter, Iterator::next) {
+                return elt;
+            }
+            match self.iter.next() {
+                None => return and_then_or_clear(&mut self.backiter, Iterator::next),
+                Some(inner) => self.frontiter = Some(inner.into_iter()),
+            }
+        }
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let (flo, fhi) = self.frontiter.as_ref().map_or((0, Some(0)), U::size_hint);
+        let (blo, bhi) = self.backiter.as_ref().map_or((0, Some(0)), U::size_hint);
+        let lo = flo.saturating_add(blo);
+
+        if let Some(fixed_size) = <<I as Iterator>::Item as ConstSizeIntoIterator>::size() {
+            let (lower, upper) = self.iter.size_hint();
+
+            let lower = lower.saturating_mul(fixed_size).saturating_add(lo);
+            let upper =
+                try { fhi?.checked_add(bhi?)?.checked_add(fixed_size.checked_mul(upper?)?)? };
+
+            return (lower, upper);
+        }
+
+        match (self.iter.size_hint(), fhi, bhi) {
+            ((0, Some(0)), Some(a), Some(b)) => (lo, a.checked_add(b)),
+            _ => (lo, None),
+        }
+    }
+
+    #[inline]
+    fn try_fold<Acc, Fold, R>(&mut self, mut init: Acc, mut fold: Fold) -> R
+    where
+        Self: Sized,
+        Fold: FnMut(Acc, Self::Item) -> R,
+        R: Try<Output = Acc>,
+    {
+        #[inline]
+        fn flatten<'a, T: IntoIterator, Acc, R: Try<Output = Acc>>(
+            frontiter: &'a mut Option<T::IntoIter>,
+            fold: &'a mut impl FnMut(Acc, T::Item) -> R,
+        ) -> impl FnMut(Acc, T) -> R + 'a {
+            move |acc, x| {
+                let mut mid = x.into_iter();
+                let r = mid.try_fold(acc, &mut *fold);
+                *frontiter = Some(mid);
+                r
+            }
+        }
+
+        if let Some(ref mut front) = self.frontiter {
+            init = front.try_fold(init, &mut fold)?;
+        }
+        self.frontiter = None;
+
+        init = self.iter.try_fold(init, flatten(&mut self.frontiter, &mut fold))?;
+        self.frontiter = None;
+
+        if let Some(ref mut back) = self.backiter {
+            init = back.try_fold(init, &mut fold)?;
+        }
+        self.backiter = None;
+
+        try { init }
+    }
+
+    #[inline]
+    fn fold<Acc, Fold>(self, mut init: Acc, mut fold: Fold) -> Acc
+    where
+        Fold: FnMut(Acc, Self::Item) -> Acc,
+    {
+        #[inline]
+        fn flatten<T: IntoIterator, Acc>(
+            fold: &mut impl FnMut(Acc, T::Item) -> Acc,
+        ) -> impl FnMut(Acc, T) -> Acc + '_ {
+            move |acc, x| x.into_iter().fold(acc, &mut *fold)
+        }
+
+        if let Some(front) = self.frontiter {
+            init = front.fold(init, &mut fold);
+        }
+
+        init = self.iter.fold(init, flatten(&mut fold));
+
+        if let Some(back) = self.backiter {
+            init = back.fold(init, &mut fold);
+        }
+
+        init
+    }
+
+    #[inline]
+    #[rustc_inherit_overflow_checks]
+    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
+        let mut rem = n;
+        loop {
+            if let Some(ref mut front) = self.frontiter {
+                match front.advance_by(rem) {
+                    ret @ Ok(_) => return ret,
+                    Err(advanced) => rem -= advanced,
+                }
+            }
+            self.frontiter = match self.iter.next() {
+                Some(iterable) => Some(iterable.into_iter()),
+                _ => break,
+            }
+        }
+
+        self.frontiter = None;
+
+        if let Some(ref mut back) = self.backiter {
+            match back.advance_by(rem) {
+                ret @ Ok(_) => return ret,
+                Err(advanced) => rem -= advanced,
+            }
+        }
+
+        if rem > 0 {
+            return Err(n - rem);
+        }
+
+        self.backiter = None;
+
+        Ok(())
+    }
+}
+
+impl<I, U> DoubleEndedIterator for FlattenCompat<I, U>
+where
+    I: DoubleEndedIterator<Item: IntoIterator<IntoIter = U, Item = U::Item>>,
+    U: DoubleEndedIterator,
+{
+    #[inline]
+    fn next_back(&mut self) -> Option<U::Item> {
+        loop {
+            if let elt @ Some(_) = and_then_or_clear(&mut self.backiter, |b| b.next_back()) {
+                return elt;
+            }
+            match self.iter.next_back() {
+                None => return and_then_or_clear(&mut self.frontiter, |f| f.next_back()),
+                Some(inner) => self.backiter = Some(inner.into_iter()),
+            }
+        }
+    }
+
+    #[inline]
+    fn try_rfold<Acc, Fold, R>(&mut self, mut init: Acc, mut fold: Fold) -> R
+    where
+        Self: Sized,
+        Fold: FnMut(Acc, Self::Item) -> R,
+        R: Try<Output = Acc>,
+    {
+        #[inline]
+        fn flatten<'a, T: IntoIterator, Acc, R: Try<Output = Acc>>(
+            backiter: &'a mut Option<T::IntoIter>,
+            fold: &'a mut impl FnMut(Acc, T::Item) -> R,
+        ) -> impl FnMut(Acc, T) -> R + 'a
+        where
+            T::IntoIter: DoubleEndedIterator,
+        {
+            move |acc, x| {
+                let mut mid = x.into_iter();
+                let r = mid.try_rfold(acc, &mut *fold);
+                *backiter = Some(mid);
+                r
+            }
+        }
+
+        if let Some(ref mut back) = self.backiter {
+            init = back.try_rfold(init, &mut fold)?;
+        }
+        self.backiter = None;
+
+        init = self.iter.try_rfold(init, flatten(&mut self.backiter, &mut fold))?;
+        self.backiter = None;
+
+        if let Some(ref mut front) = self.frontiter {
+            init = front.try_rfold(init, &mut fold)?;
+        }
+        self.frontiter = None;
+
+        try { init }
+    }
+
+    #[inline]
+    fn rfold<Acc, Fold>(self, mut init: Acc, mut fold: Fold) -> Acc
+    where
+        Fold: FnMut(Acc, Self::Item) -> Acc,
+    {
+        #[inline]
+        fn flatten<T: IntoIterator, Acc>(
+            fold: &mut impl FnMut(Acc, T::Item) -> Acc,
+        ) -> impl FnMut(Acc, T) -> Acc + '_
+        where
+            T::IntoIter: DoubleEndedIterator,
+        {
+            move |acc, x| x.into_iter().rfold(acc, &mut *fold)
+        }
+
+        if let Some(back) = self.backiter {
+            init = back.rfold(init, &mut fold);
+        }
+
+        init = self.iter.rfold(init, flatten(&mut fold));
+
+        if let Some(front) = self.frontiter {
+            init = front.rfold(init, &mut fold);
+        }
+
+        init
+    }
+
+    #[inline]
+    #[rustc_inherit_overflow_checks]
+    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+        let mut rem = n;
+        loop {
+            if let Some(ref mut back) = self.backiter {
+                match back.advance_back_by(rem) {
+                    ret @ Ok(_) => return ret,
+                    Err(advanced) => rem -= advanced,
+                }
+            }
+            match self.iter.next_back() {
+                Some(iterable) => self.backiter = Some(iterable.into_iter()),
+                _ => break,
+            }
+        }
+
+        self.backiter = None;
+
+        if let Some(ref mut front) = self.frontiter {
+            match front.advance_back_by(rem) {
+                ret @ Ok(_) => return ret,
+                Err(advanced) => rem -= advanced,
+            }
+        }
+
+        if rem > 0 {
+            return Err(n - rem);
+        }
+
+        self.frontiter = None;
+
+        Ok(())
+    }
+}
+
+trait ConstSizeIntoIterator: IntoIterator {
+    // FIXME(#31844): convert to an associated const once specialization supports that
+    fn size() -> Option<usize>;
+}
+
+impl<T> ConstSizeIntoIterator for T
+where
+    T: IntoIterator,
+{
+    #[inline]
+    default fn size() -> Option<usize> {
+        None
+    }
+}
+
+impl<T, const N: usize> ConstSizeIntoIterator for [T; N] {
+    #[inline]
+    fn size() -> Option<usize> {
+        Some(N)
+    }
+}
+
+impl<T, const N: usize> ConstSizeIntoIterator for &[T; N] {
+    #[inline]
+    fn size() -> Option<usize> {
+        Some(N)
+    }
+}
+
+impl<T, const N: usize> ConstSizeIntoIterator for &mut [T; N] {
+    #[inline]
+    fn size() -> Option<usize> {
+        Some(N)
+    }
+}
+
+#[doc(hidden)]
+#[unstable(feature = "std_internals", issue = "none")]
+// FIXME(#20400): Instead of this helper trait there should be multiple impl TrustedLen for Flatten<>
+//   blocks with different bounds on Iterator::Item but the compiler erroneously considers them overlapping
+pub unsafe trait TrustedConstSize: IntoIterator {}
+
+#[unstable(feature = "std_internals", issue = "none")]
+unsafe impl<T, const N: usize> TrustedConstSize for [T; N] {}
+#[unstable(feature = "std_internals", issue = "none")]
+unsafe impl<T, const N: usize> TrustedConstSize for &'_ [T; N] {}
+#[unstable(feature = "std_internals", issue = "none")]
+unsafe impl<T, const N: usize> TrustedConstSize for &'_ mut [T; N] {}
+
+#[inline]
+fn and_then_or_clear<T, U>(opt: &mut Option<T>, f: impl FnOnce(&mut T) -> Option<U>) -> Option<U> {
+    let x = f(opt.as_mut()?);
+    if x.is_none() {
+        *opt = None;
+    }
+    x
+}
-- 
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