Rollup merge of #153204 - xtqqczze:must-use-map, r=Amanieu,joboet

Add `#[must_use]` attribute to `HashMap` and `HashSet` constructors

- `new_in`
- `with_capacity_and_hasher`
- `with_capacity_and_hasher_in`
- `with_hasher`
- `with_hasher_in`

See also: https://github.com/rust-lang/rust/issues/89692

library/alloc/src/collections/btree/map.rs

@@ -693,6 +693,7 @@ pub fn clear(&mut self) {
     /// map.insert(1, "a");
     /// ```
     #[unstable(feature = "btreemap_alloc", issue = "32838")]
+    #[must_use]
     pub const fn new_in(alloc: A) -> BTreeMap<K, V, A> {
         BTreeMap { root: None, length: 0, alloc: ManuallyDrop::new(alloc), _marker: PhantomData }
     }

library/alloc/src/collections/btree/set.rs

@@ -361,6 +361,7 @@ impl<T, A: Allocator + Clone> BTreeSet<T, A> {
     /// let mut set: BTreeSet<i32> = BTreeSet::new_in(Global);
     /// ```
     #[unstable(feature = "btreemap_alloc", issue = "32838")]
+    #[must_use]
     pub const fn new_in(alloc: A) -> BTreeSet<T, A> {
         BTreeSet { map: BTreeMap::new_in(alloc) }
     }

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

@@ -357,6 +357,7 @@ impl<K, V, S> HashMap<K, V, S> {
     /// map.insert(1, 2);
     /// ```
     #[inline]
+    #[must_use]
     #[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
     #[rustc_const_stable(feature = "const_collections_with_hasher", since = "1.85.0")]
     pub const fn with_hasher(hash_builder: S) -> HashMap<K, V, S> {
@@ -389,6 +390,7 @@ pub const fn with_hasher(hash_builder: S) -> HashMap<K, V, S> {
     /// map.insert(1, 2);
     /// ```
     #[inline]
+    #[must_use]
     #[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
     pub fn with_capacity_and_hasher(capacity: usize, hasher: S) -> HashMap<K, V, S> {
         HashMap { base: base::HashMap::with_capacity_and_hasher(capacity, hasher) }
@@ -409,6 +411,7 @@ impl<K, V, S, A: Allocator> HashMap<K, V, S, A> {
     /// The `hash_builder` passed should implement the [`BuildHasher`] trait for
     /// the `HashMap` to be useful, see its documentation for details.
     #[inline]
+    #[must_use]
     #[unstable(feature = "allocator_api", issue = "32838")]
     pub fn with_hasher_in(hash_builder: S, alloc: A) -> Self {
         HashMap { base: base::HashMap::with_hasher_in(hash_builder, alloc) }
@@ -430,6 +433,7 @@ pub fn with_hasher_in(hash_builder: S, alloc: A) -> Self {
     /// the `HashMap` to be useful, see its documentation for details.
     ///
     #[inline]
+    #[must_use]
     #[unstable(feature = "allocator_api", issue = "32838")]
     pub fn with_capacity_and_hasher_in(capacity: usize, hash_builder: S, alloc: A) -> Self {
         HashMap { base: base::HashMap::with_capacity_and_hasher_in(capacity, hash_builder, alloc) }

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

@@ -229,6 +229,7 @@ impl<T, S> HashSet<T, S> {
     /// set.insert(2);
     /// ```
     #[inline]
+    #[must_use]
     #[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
     #[rustc_const_stable(feature = "const_collections_with_hasher", since = "1.85.0")]
     pub const fn with_hasher(hasher: S) -> HashSet<T, S> {
@@ -261,6 +262,7 @@ pub const fn with_hasher(hasher: S) -> HashSet<T, S> {
     /// set.insert(1);
     /// ```
     #[inline]
+    #[must_use]
     #[stable(feature = "hashmap_build_hasher", since = "1.7.0")]
     pub fn with_capacity_and_hasher(capacity: usize, hasher: S) -> HashSet<T, S> {
         HashSet { base: base::HashSet::with_capacity_and_hasher(capacity, hasher) }
@@ -281,6 +283,7 @@ impl<T, S, A: Allocator> HashSet<T, S, A> {
     /// The `hash_builder` passed should implement the [`BuildHasher`] trait for
     /// the `HashSet` to be useful, see its documentation for details.
     #[inline]
+    #[must_use]
     #[unstable(feature = "allocator_api", issue = "32838")]
     pub fn with_hasher_in(hasher: S, alloc: A) -> HashSet<T, S, A> {
         HashSet { base: base::HashSet::with_hasher_in(hasher, alloc) }
@@ -301,6 +304,7 @@ pub fn with_hasher_in(hasher: S, alloc: A) -> HashSet<T, S, A> {
     /// The `hash_builder` passed should implement the [`BuildHasher`] trait for
     /// the `HashSet` to be useful, see its documentation for details.
     #[inline]
+    #[must_use]
     #[unstable(feature = "allocator_api", issue = "32838")]
     pub fn with_capacity_and_hasher_in(capacity: usize, hasher: S, alloc: A) -> HashSet<T, S, A> {
         HashSet { base: base::HashSet::with_capacity_and_hasher_in(capacity, hasher, alloc) }