Rollup merge of #73627 - ssomers:btree_iter_min_max, r=Mark-Simulacrum

Shortcuts for min/max on double-ended BTreeMap/BTreeSet iterators

Closes #59947: a performance tweak that might benefit some. Optimizes `min` and `max ` on all btree double-ended iterators that do not drop, i.e. the iterators created by:

- `BTreeMap::iter`
- `BTreeMap::iter_mut`
- `BTreeMap::keys` and `BTreeSet::iter`
- `BTreeMap::range` and `BTreeSet::range`
- `BTreeMap::range_mut`

Also in these (currently) single-ended iterators, but obviously for `min` only:
- `BTreeSet::difference`
- `BTreeSet::intersection`
- `BTreeSet::symmetric_difference`
- `BTreeSet::union`

Did not do this in iterators created by `into_iter` to preserve drop order, as outlined in #62316.

Did not do this in iterators created by `drain_filter`, possibly to preserve drop order, possibly to preserve predicate invocation, mostly to not have to think about it too hard (I guess maybe it wouldn't be a change for `min`, which is the only shortcut possible in this single-ended iterator).

src/liballoc/collections/btree/map.rs

@@ -1396,6 +1396,14 @@ impl<'a, K: 'a, V: 'a> Iterator for Iter<'a, K, V> {
     fn last(mut self) -> Option<(&'a K, &'a V)> {
         self.next_back()
     }
+
+    fn min(mut self) -> Option<(&'a K, &'a V)> {
+        self.next()
+    }
+
+    fn max(mut self) -> Option<(&'a K, &'a V)> {
+        self.next_back()
+    }
 }
 
 #[stable(feature = "fused", since = "1.26.0")]
@@ -1458,6 +1466,14 @@ impl<'a, K: 'a, V: 'a> Iterator for IterMut<'a, K, V> {
     fn last(mut self) -> Option<(&'a K, &'a mut V)> {
         self.next_back()
     }
+
+    fn min(mut self) -> Option<(&'a K, &'a mut V)> {
+        self.next()
+    }
+
+    fn max(mut self) -> Option<(&'a K, &'a mut V)> {
+        self.next_back()
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -1595,6 +1611,14 @@ impl<'a, K, V> Iterator for Keys<'a, K, V> {
     fn last(mut self) -> Option<&'a K> {
         self.next_back()
     }
+
+    fn min(mut self) -> Option<&'a K> {
+        self.next()
+    }
+
+    fn max(mut self) -> Option<&'a K> {
+        self.next_back()
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -1768,6 +1792,14 @@ impl<'a, K, V> Iterator for Range<'a, K, V> {
     fn last(mut self) -> Option<(&'a K, &'a V)> {
         self.next_back()
     }
+
+    fn min(mut self) -> Option<(&'a K, &'a V)> {
+        self.next()
+    }
+
+    fn max(mut self) -> Option<(&'a K, &'a V)> {
+        self.next_back()
+    }
 }
 
 #[stable(feature = "map_values_mut", since = "1.10.0")]
@@ -1853,6 +1885,14 @@ impl<'a, K, V> Iterator for RangeMut<'a, K, V> {
     fn last(mut self) -> Option<(&'a K, &'a mut V)> {
         self.next_back()
     }
+
+    fn min(mut self) -> Option<(&'a K, &'a mut V)> {
+        self.next()
+    }
+
+    fn max(mut self) -> Option<(&'a K, &'a mut V)> {
+        self.next_back()
+    }
 }
 
 impl<'a, K, V> RangeMut<'a, K, V> {

src/liballoc/collections/btree/set.rs

@@ -1291,12 +1291,22 @@ impl<'a, T> Iterator for Iter<'a, T> {
     fn next(&mut self) -> Option<&'a T> {
         self.iter.next()
     }
+
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.iter.size_hint()
     }
+
     fn last(mut self) -> Option<&'a T> {
         self.next_back()
     }
+
+    fn min(mut self) -> Option<&'a T> {
+        self.next()
+    }
+
+    fn max(mut self) -> Option<&'a T> {
+        self.next_back()
+    }
 }
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
@@ -1321,6 +1331,7 @@ impl<T> Iterator for IntoIter<T> {
     fn next(&mut self) -> Option<T> {
         self.iter.next().map(|(k, _)| k)
     }
+
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.iter.size_hint()
     }
@@ -1359,6 +1370,14 @@ impl<'a, T> Iterator for Range<'a, T> {
     fn last(mut self) -> Option<&'a T> {
         self.next_back()
     }
+
+    fn min(mut self) -> Option<&'a T> {
+        self.next()
+    }
+
+    fn max(mut self) -> Option<&'a T> {
+        self.next_back()
+    }
 }
 
 #[stable(feature = "btree_range", since = "1.17.0")]
@@ -1429,6 +1448,10 @@ impl<'a, T: Ord> Iterator for Difference<'a, T> {
         };
         (self_len.saturating_sub(other_len), Some(self_len))
     }
+
+    fn min(mut self) -> Option<&'a T> {
+        self.next()
+    }
 }
 
 #[stable(feature = "fused", since = "1.26.0")]
@@ -1460,6 +1483,10 @@ impl<'a, T: Ord> Iterator for SymmetricDifference<'a, T> {
         // the number of elements to less than half the range of usize.
         (0, Some(a_len + b_len))
     }
+
+    fn min(mut self) -> Option<&'a T> {
+        self.next()
+    }
 }
 
 #[stable(feature = "fused", since = "1.26.0")]
@@ -1516,6 +1543,10 @@ impl<'a, T: Ord> Iterator for Intersection<'a, T> {
             IntersectionInner::Answer(Some(_)) => (1, Some(1)),
         }
     }
+
+    fn min(mut self) -> Option<&'a T> {
+        self.next()
+    }
 }
 
 #[stable(feature = "fused", since = "1.26.0")]
@@ -1541,6 +1572,10 @@ impl<'a, T: Ord> Iterator for Union<'a, T> {
         // No checked_add - see SymmetricDifference::size_hint.
         (max(a_len, b_len), Some(a_len + b_len))
     }
+
+    fn min(mut self) -> Option<&'a T> {
+        self.next()
+    }
 }
 
 #[stable(feature = "fused", since = "1.26.0")]

src/liballoc/tests/btree/map.rs

@@ -309,6 +309,41 @@ fn test_iter_mixed() {
     test(size, map.into_iter());
 }
 
+#[test]
+fn test_iter_min_max() {
+    let mut a = BTreeMap::new();
+    assert_eq!(a.iter().min(), None);
+    assert_eq!(a.iter().max(), None);
+    assert_eq!(a.iter_mut().min(), None);
+    assert_eq!(a.iter_mut().max(), None);
+    assert_eq!(a.range(..).min(), None);
+    assert_eq!(a.range(..).max(), None);
+    assert_eq!(a.range_mut(..).min(), None);
+    assert_eq!(a.range_mut(..).max(), None);
+    assert_eq!(a.keys().min(), None);
+    assert_eq!(a.keys().max(), None);
+    assert_eq!(a.values().min(), None);
+    assert_eq!(a.values().max(), None);
+    assert_eq!(a.values_mut().min(), None);
+    assert_eq!(a.values_mut().max(), None);
+    a.insert(1, 42);
+    a.insert(2, 24);
+    assert_eq!(a.iter().min(), Some((&1, &42)));
+    assert_eq!(a.iter().max(), Some((&2, &24)));
+    assert_eq!(a.iter_mut().min(), Some((&1, &mut 42)));
+    assert_eq!(a.iter_mut().max(), Some((&2, &mut 24)));
+    assert_eq!(a.range(..).min(), Some((&1, &42)));
+    assert_eq!(a.range(..).max(), Some((&2, &24)));
+    assert_eq!(a.range_mut(..).min(), Some((&1, &mut 42)));
+    assert_eq!(a.range_mut(..).max(), Some((&2, &mut 24)));
+    assert_eq!(a.keys().min(), Some(&1));
+    assert_eq!(a.keys().max(), Some(&2));
+    assert_eq!(a.values().min(), Some(&24));
+    assert_eq!(a.values().max(), Some(&42));
+    assert_eq!(a.values_mut().min(), Some(&mut 24));
+    assert_eq!(a.values_mut().max(), Some(&mut 42));
+}
+
 fn range_keys(map: &BTreeMap<i32, i32>, range: impl RangeBounds<i32>) -> Vec<i32> {
     map.range(range)
         .map(|(&k, &v)| {

src/liballoc/tests/btree/set.rs

@@ -33,6 +33,37 @@ fn test_hash() {
     assert_eq!(hash(&x), hash(&y));
 }
 
+#[test]
+fn test_iter_min_max() {
+    let mut a = BTreeSet::new();
+    assert_eq!(a.iter().min(), None);
+    assert_eq!(a.iter().max(), None);
+    assert_eq!(a.range(..).min(), None);
+    assert_eq!(a.range(..).max(), None);
+    assert_eq!(a.difference(&BTreeSet::new()).min(), None);
+    assert_eq!(a.difference(&BTreeSet::new()).max(), None);
+    assert_eq!(a.intersection(&a).min(), None);
+    assert_eq!(a.intersection(&a).max(), None);
+    assert_eq!(a.symmetric_difference(&BTreeSet::new()).min(), None);
+    assert_eq!(a.symmetric_difference(&BTreeSet::new()).max(), None);
+    assert_eq!(a.union(&a).min(), None);
+    assert_eq!(a.union(&a).max(), None);
+    a.insert(1);
+    a.insert(2);
+    assert_eq!(a.iter().min(), Some(&1));
+    assert_eq!(a.iter().max(), Some(&2));
+    assert_eq!(a.range(..).min(), Some(&1));
+    assert_eq!(a.range(..).max(), Some(&2));
+    assert_eq!(a.difference(&BTreeSet::new()).min(), Some(&1));
+    assert_eq!(a.difference(&BTreeSet::new()).max(), Some(&2));
+    assert_eq!(a.intersection(&a).min(), Some(&1));
+    assert_eq!(a.intersection(&a).max(), Some(&2));
+    assert_eq!(a.symmetric_difference(&BTreeSet::new()).min(), Some(&1));
+    assert_eq!(a.symmetric_difference(&BTreeSet::new()).max(), Some(&2));
+    assert_eq!(a.union(&a).min(), Some(&1));
+    assert_eq!(a.union(&a).max(), Some(&2));
+}
+
 fn check<F>(a: &[i32], b: &[i32], expected: &[i32], f: F)
 where
     F: FnOnce(&BTreeSet<i32>, &BTreeSet<i32>, &mut dyn FnMut(&i32) -> bool) -> bool,