Rollup merge of #134644 - kpreid:duplicates, r=Mark-Simulacrum

Document collection `From` and `FromIterator` impls that drop duplicate keys.

This behavior is worth documenting because there are other plausible alternatives, such as panicking when a duplicate is encountered, and it reminds the programmer to consider whether they should, for example, coalesce duplicate keys first.

Followup to #89869.

2 files changed, 13 insertions, 1 deletions

diff --git a/library/alloc/src/collections/btree/map.rs b/library/alloc/src/collections/btree/map.rs
index d1ce4e215ed..6d305386dbf 100644
--- a/library/alloc/src/collections/btree/map.rs
+++ b/library/alloc/src/collections/btree/map.rs
@@ -2289,6 +2289,10 @@ impl<K, V> FusedIterator for RangeMut<'_, K, V> {}
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<K: Ord, V> FromIterator<(K, V)> for BTreeMap<K, V> {
+    /// Constructs a `BTreeMap<K, V>` from an iterator of key-value pairs.
+    ///
+    /// If the iterator produces any pairs with equal keys,
+    /// all but one of the corresponding values will be dropped.
     fn from_iter<T: IntoIterator<Item = (K, V)>>(iter: T) -> BTreeMap<K, V> {
         let mut inputs: Vec<_> = iter.into_iter().collect();
 
@@ -2403,7 +2407,10 @@ where
 
 #[stable(feature = "std_collections_from_array", since = "1.56.0")]
 impl<K: Ord, V, const N: usize> From<[(K, V); N]> for BTreeMap<K, V> {
-    /// Converts a `[(K, V); N]` into a `BTreeMap<(K, V)>`.
+    /// Converts a `[(K, V); N]` into a `BTreeMap<K, V>`.
+    ///
+    /// If any entries in the array have equal keys,
+    /// all but one of the corresponding values will be dropped.
     ///
     /// ```
     /// use std::collections::BTreeMap;
diff --git a/library/alloc/src/collections/btree/set.rs b/library/alloc/src/collections/btree/set.rs
index 6f8c3b2d152..9660023d694 100644
--- a/library/alloc/src/collections/btree/set.rs
+++ b/library/alloc/src/collections/btree/set.rs
@@ -1491,6 +1491,11 @@ impl<T: Ord, A: Allocator + Clone> BTreeSet<T, A> {
 impl<T: Ord, const N: usize> From<[T; N]> for BTreeSet<T> {
     /// Converts a `[T; N]` into a `BTreeSet<T>`.
     ///
+    /// If the array contains any equal values,
+    /// all but one will be dropped.
+    ///
+    /// # Examples
+    ///
     /// ```
     /// use std::collections::BTreeSet;
     ///