Rollup merge of #89786 - jkugelman:must-use-len-and-is_empty, r=joshtriplett

Add #[must_use] to len and is_empty

Parent issue: #89692

r? `@joshtriplett`

library/alloc/src/collections/binary_heap.rs

@@ -1052,6 +1052,7 @@ impl<T> BinaryHeap<T> {
     ///
     /// assert_eq!(heap.len(), 2);
     /// ```
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn len(&self) -> usize {
         self.data.len()
@@ -1075,6 +1076,7 @@ impl<T> BinaryHeap<T> {
     ///
     /// assert!(!heap.is_empty());
     /// ```
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn is_empty(&self) -> bool {
         self.len() == 0

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

@@ -2212,6 +2212,7 @@ impl<K, V> BTreeMap<K, V> {
     /// a.insert(1, "a");
     /// assert_eq!(a.len(), 1);
     /// ```
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_unstable(feature = "const_btree_new", issue = "71835")]
     pub const fn len(&self) -> usize {
@@ -2232,6 +2233,7 @@ impl<K, V> BTreeMap<K, V> {
     /// a.insert(1, "a");
     /// assert!(!a.is_empty());
     /// ```
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_unstable(feature = "const_btree_new", issue = "71835")]
     pub const fn is_empty(&self) -> bool {

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

@@ -1046,6 +1046,7 @@ impl<T> BTreeSet<T> {
     /// v.insert(1);
     /// assert_eq!(v.len(), 1);
     /// ```
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_unstable(feature = "const_btree_new", issue = "71835")]
     pub const fn len(&self) -> usize {
@@ -1064,6 +1065,7 @@ impl<T> BTreeSet<T> {
     /// v.insert(1);
     /// assert!(!v.is_empty());
     /// ```
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_unstable(feature = "const_btree_new", issue = "71835")]
     pub const fn is_empty(&self) -> bool {

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

@@ -610,8 +610,8 @@ fn test_send() {
 #[test]
 fn test_ord_absence() {
     fn set<K>(mut set: BTreeSet<K>) {
-        set.is_empty();
-        set.len();
+        let _ = set.is_empty();
+        let _ = set.len();
         set.clear();
         let _ = set.iter();
         let _ = set.into_iter();

library/alloc/src/collections/linked_list.rs

@@ -583,6 +583,7 @@ impl<T> LinkedList<T> {
     /// assert!(!dl.is_empty());
     /// ```
     #[inline]
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn is_empty(&self) -> bool {
         self.head.is_none()
@@ -609,6 +610,7 @@ impl<T> LinkedList<T> {
     /// assert_eq!(dl.len(), 3);
     /// ```
     #[inline]
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn len(&self) -> usize {
         self.len

library/alloc/src/string.rs

@@ -1547,6 +1547,7 @@ impl String {
     /// assert_eq!(fancy_f.chars().count(), 3);
     /// ```
     #[inline]
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn len(&self) -> usize {
         self.vec.len()
@@ -1566,6 +1567,7 @@ impl String {
     /// assert!(!v.is_empty());
     /// ```
     #[inline]
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn is_empty(&self) -> bool {
         self.len() == 0