Rollup merge of #86790 - janikrabe:retain-iter-order-doc, r=m-ou-se

Document iteration order of `retain` functions

For `HashSet` and `HashMap`, this simply copies the comment from
`BinaryHeap::retain`.

For `BTreeSet` and `BTreeMap`, this adds an additional guarantee that
wasn't previously documented. I think that because these data structures
are inherently ordered and other functions guarantee ordered iteration,
it makes sense to provide this guarantee for `retain` as well.

4 files changed, 4 insertions, 0 deletions

diff --git a/library/alloc/src/collections/btree/map.rs b/library/alloc/src/collections/btree/map.rs
index dfd693d13b3..79042ab739c 100644
--- a/library/alloc/src/collections/btree/map.rs
+++ b/library/alloc/src/collections/btree/map.rs
@@ -935,6 +935,7 @@ impl<K, V> BTreeMap<K, V> {
     /// Retains only the elements specified by the predicate.
     ///
     /// In other words, remove all pairs `(k, v)` such that `f(&k, &mut v)` returns `false`.
+    /// The elements are visited in ascending key order.
     ///
     /// # Examples
     ///
diff --git a/library/alloc/src/collections/btree/set.rs b/library/alloc/src/collections/btree/set.rs
index be740b05fb8..9711214aea6 100644
--- a/library/alloc/src/collections/btree/set.rs
+++ b/library/alloc/src/collections/btree/set.rs
@@ -846,6 +846,7 @@ impl<T> BTreeSet<T> {
     /// Retains only the elements specified by the predicate.
     ///
     /// In other words, remove all elements `e` such that `f(&e)` returns `false`.
+    /// The elements are visited in ascending order.
     ///
     /// # Examples
     ///
diff --git a/library/std/src/collections/hash/map.rs b/library/std/src/collections/hash/map.rs
index fac285c96f0..4a5efab9055 100644
--- a/library/std/src/collections/hash/map.rs
+++ b/library/std/src/collections/hash/map.rs
@@ -934,6 +934,7 @@ where
     /// Retains only the elements specified by the predicate.
     ///
     /// In other words, remove all pairs `(k, v)` such that `f(&k, &mut v)` returns `false`.
+    /// The elements are visited in unsorted (and unspecified) order.
     ///
     /// # Examples
     ///
diff --git a/library/std/src/collections/hash/set.rs b/library/std/src/collections/hash/set.rs
index 272e1c2be2b..1a2a8afac8b 100644
--- a/library/std/src/collections/hash/set.rs
+++ b/library/std/src/collections/hash/set.rs
@@ -912,6 +912,7 @@ where
     /// Retains only the elements specified by the predicate.
     ///
     /// In other words, remove all elements `e` such that `f(&e)` returns `false`.
+    /// The elements are visited in unsorted (and unspecified) order.
     ///
     /// # Examples
     ///