Auto merge of #117756 - a1phyr:hashmap_fold, r=the8472

`HashMap`/`HashSet`: forward `fold` implementations of iterators

Use [rust-lang/hasbrown#480](https://github.com/rust-lang/hashbrown/pull/480) in `std`

Note: this needs a version bump of hashbrown before merging

library/std/src/collections/hash/map.rs

@@ -2232,6 +2232,18 @@ impl<'a, K, V> Iterator for Iter<'a, K, V> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.base.size_hint()
     }
+    #[inline]
+    fn count(self) -> usize {
+        self.base.len()
+    }
+    #[inline]
+    fn fold<B, F>(self, init: B, f: F) -> B
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> B,
+    {
+        self.base.fold(init, f)
+    }
 }
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<K, V> ExactSizeIterator for Iter<'_, K, V> {
@@ -2256,6 +2268,18 @@ impl<'a, K, V> Iterator for IterMut<'a, K, V> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.base.size_hint()
     }
+    #[inline]
+    fn count(self) -> usize {
+        self.base.len()
+    }
+    #[inline]
+    fn fold<B, F>(self, init: B, f: F) -> B
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> B,
+    {
+        self.base.fold(init, f)
+    }
 }
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<K, V> ExactSizeIterator for IterMut<'_, K, V> {
@@ -2290,6 +2314,18 @@ impl<K, V> Iterator for IntoIter<K, V> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.base.size_hint()
     }
+    #[inline]
+    fn count(self) -> usize {
+        self.base.len()
+    }
+    #[inline]
+    fn fold<B, F>(self, init: B, f: F) -> B
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> B,
+    {
+        self.base.fold(init, f)
+    }
 }
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<K, V> ExactSizeIterator for IntoIter<K, V> {
@@ -2320,6 +2356,18 @@ impl<'a, K, V> Iterator for Keys<'a, K, V> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.inner.size_hint()
     }
+    #[inline]
+    fn count(self) -> usize {
+        self.inner.len()
+    }
+    #[inline]
+    fn fold<B, F>(self, init: B, mut f: F) -> B
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> B,
+    {
+        self.inner.fold(init, |acc, (k, _)| f(acc, k))
+    }
 }
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<K, V> ExactSizeIterator for Keys<'_, K, V> {
@@ -2343,6 +2391,18 @@ impl<'a, K, V> Iterator for Values<'a, K, V> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.inner.size_hint()
     }
+    #[inline]
+    fn count(self) -> usize {
+        self.inner.len()
+    }
+    #[inline]
+    fn fold<B, F>(self, init: B, mut f: F) -> B
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> B,
+    {
+        self.inner.fold(init, |acc, (_, v)| f(acc, v))
+    }
 }
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<K, V> ExactSizeIterator for Values<'_, K, V> {
@@ -2366,6 +2426,18 @@ impl<'a, K, V> Iterator for ValuesMut<'a, K, V> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.inner.size_hint()
     }
+    #[inline]
+    fn count(self) -> usize {
+        self.inner.len()
+    }
+    #[inline]
+    fn fold<B, F>(self, init: B, mut f: F) -> B
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> B,
+    {
+        self.inner.fold(init, |acc, (_, v)| f(acc, v))
+    }
 }
 #[stable(feature = "map_values_mut", since = "1.10.0")]
 impl<K, V> ExactSizeIterator for ValuesMut<'_, K, V> {
@@ -2396,6 +2468,18 @@ impl<K, V> Iterator for IntoKeys<K, V> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.inner.size_hint()
     }
+    #[inline]
+    fn count(self) -> usize {
+        self.inner.len()
+    }
+    #[inline]
+    fn fold<B, F>(self, init: B, mut f: F) -> B
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> B,
+    {
+        self.inner.fold(init, |acc, (k, _)| f(acc, k))
+    }
 }
 #[stable(feature = "map_into_keys_values", since = "1.54.0")]
 impl<K, V> ExactSizeIterator for IntoKeys<K, V> {
@@ -2426,6 +2510,18 @@ impl<K, V> Iterator for IntoValues<K, V> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.inner.size_hint()
     }
+    #[inline]
+    fn count(self) -> usize {
+        self.inner.len()
+    }
+    #[inline]
+    fn fold<B, F>(self, init: B, mut f: F) -> B
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> B,
+    {
+        self.inner.fold(init, |acc, (_, v)| f(acc, v))
+    }
 }
 #[stable(feature = "map_into_keys_values", since = "1.54.0")]
 impl<K, V> ExactSizeIterator for IntoValues<K, V> {
@@ -2456,6 +2552,14 @@ impl<'a, K, V> Iterator for Drain<'a, K, V> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.base.size_hint()
     }
+    #[inline]
+    fn fold<B, F>(self, init: B, f: F) -> B
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> B,
+    {
+        self.base.fold(init, f)
+    }
 }
 #[stable(feature = "drain", since = "1.6.0")]
 impl<K, V> ExactSizeIterator for Drain<'_, K, V> {

library/std/src/collections/hash/set.rs

@@ -1500,6 +1500,18 @@ impl<'a, K> Iterator for Iter<'a, K> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.base.size_hint()
     }
+    #[inline]
+    fn count(self) -> usize {
+        self.base.len()
+    }
+    #[inline]
+    fn fold<B, F>(self, init: B, f: F) -> B
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> B,
+    {
+        self.base.fold(init, f)
+    }
 }
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<K> ExactSizeIterator for Iter<'_, K> {
@@ -1530,6 +1542,18 @@ impl<K> Iterator for IntoIter<K> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.base.size_hint()
     }
+    #[inline]
+    fn count(self) -> usize {
+        self.base.len()
+    }
+    #[inline]
+    fn fold<B, F>(self, init: B, f: F) -> B
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> B,
+    {
+        self.base.fold(init, f)
+    }
 }
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<K> ExactSizeIterator for IntoIter<K> {
@@ -1560,6 +1584,14 @@ impl<'a, K> Iterator for Drain<'a, K> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.base.size_hint()
     }
+    #[inline]
+    fn fold<B, F>(self, init: B, f: F) -> B
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> B,
+    {
+        self.base.fold(init, f)
+    }
 }
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<K> ExactSizeIterator for Drain<'_, K> {
@@ -1639,6 +1671,15 @@ where
         let (_, upper) = self.iter.size_hint();
         (0, upper)
     }
+
+    #[inline]
+    fn fold<B, F>(self, init: B, mut f: F) -> B
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> B,
+    {
+        self.iter.fold(init, |acc, elt| if self.other.contains(elt) { f(acc, elt) } else { acc })
+    }
 }
 
 #[stable(feature = "std_debug", since = "1.16.0")]
@@ -1691,6 +1732,15 @@ where
         let (_, upper) = self.iter.size_hint();
         (0, upper)
     }
+
+    #[inline]
+    fn fold<B, F>(self, init: B, mut f: F) -> B
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> B,
+    {
+        self.iter.fold(init, |acc, elt| if self.other.contains(elt) { acc } else { f(acc, elt) })
+    }
 }
 
 #[stable(feature = "fused", since = "1.26.0")]
@@ -1736,6 +1786,14 @@ where
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.iter.size_hint()
     }
+    #[inline]
+    fn fold<B, F>(self, init: B, f: F) -> B
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> B,
+    {
+        self.iter.fold(init, f)
+    }
 }
 
 #[stable(feature = "fused", since = "1.26.0")]
@@ -1800,6 +1858,18 @@ where
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.iter.size_hint()
     }
+    #[inline]
+    fn count(self) -> usize {
+        self.iter.count()
+    }
+    #[inline]
+    fn fold<B, F>(self, init: B, f: F) -> B
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> B,
+    {
+        self.iter.fold(init, f)
+    }
 }
 
 #[allow(dead_code)]