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

Add #[must_use] to len and is_empty

Parent issue: rust-lang#89692

r? ``@joshtriplett``

Author: the8472

library/alloc/src/collections/binary_heap.rs

@@ -1050,6 +1050,7 @@ 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()
@@ -1073,6 +1074,7 @@ 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

@@ -2205,6 +2205,7 @@ 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 {
@@ -2225,6 +2226,7 @@ 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

@@ -1034,6 +1034,7 @@ 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 {
@@ -1052,6 +1053,7 @@ 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

@@ -610,11 +610,11 @@ 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();
-        set.iter();
-        set.into_iter();
+        let _ = set.iter();
+        let _ = set.into_iter();
     }
 
     fn set_debug<K: Debug>(set: BTreeSet<K>) {

library/alloc/src/collections/linked_list.rs

@@ -577,6 +577,7 @@ 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()
@@ -603,6 +604,7 @@ 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

@@ -1543,6 +1543,7 @@ 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()
@@ -1562,6 +1563,7 @@ 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

library/core/src/ptr/non_null.rs

@@ -443,6 +443,7 @@ impl<T> NonNull<[T]> {
     /// ```
     #[unstable(feature = "slice_ptr_len", issue = "71146")]
     #[rustc_const_unstable(feature = "const_slice_ptr_len", issue = "71146")]
+    #[must_use]
     #[inline]
     pub const fn len(self) -> usize {
         self.as_ptr().len()

library/core/src/str/mod.rs

@@ -140,6 +140,7 @@ impl str {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_stable(feature = "const_str_len", since = "1.39.0")]
+    #[must_use]
     #[inline]
     pub const fn len(&self) -> usize {
         self.as_bytes().len()
@@ -158,9 +159,10 @@ impl str {
     /// let s = "not empty";
     /// assert!(!s.is_empty());
     /// ```
-    #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_stable(feature = "const_str_is_empty", since = "1.39.0")]
+    #[must_use]
+    #[inline]
     pub const fn is_empty(&self) -> bool {
         self.len() == 0
     }

library/std/src/ffi/os_str.rs

@@ -663,6 +663,7 @@ impl OsStr {
     /// assert!(!os_str.is_empty());
     /// ```
     #[stable(feature = "osstring_simple_functions", since = "1.9.0")]
+    #[must_use]
     #[inline]
     pub fn is_empty(&self) -> bool {
         self.inner.inner.is_empty()
@@ -694,6 +695,7 @@ impl OsStr {
     /// assert_eq!(os_str.len(), 3);
     /// ```
     #[stable(feature = "osstring_simple_functions", since = "1.9.0")]
+    #[must_use]
     #[inline]
     pub fn len(&self) -> usize {
         self.inner.inner.len()

library/std/src/fs.rs

@@ -1078,6 +1078,7 @@ impl Metadata {
     ///     Ok(())
     /// }
     /// ```
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn len(&self) -> u64 {
         self.0.size()

library/std/src/os/unix/net/ancillary.rs

@@ -431,12 +431,14 @@ impl<'a> SocketAncillary<'a> {
     }
 
     /// Returns `true` if the ancillary data is empty.
+    #[must_use]
     #[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
     pub fn is_empty(&self) -> bool {
         self.length == 0
     }
 
     /// Returns the number of used bytes.
+    #[must_use]
     #[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
     pub fn len(&self) -> usize {
         self.length