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`

14 files changed, 60 insertions, 40 deletions

diff --git a/library/alloc/src/collections/binary_heap.rs b/library/alloc/src/collections/binary_heap.rs
index 4e991018bb4..7fb7686a6e2 100644
--- a/library/alloc/src/collections/binary_heap.rs
+++ b/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
diff --git a/library/alloc/src/collections/btree/map.rs b/library/alloc/src/collections/btree/map.rs
index 224795876fe..2ff7b0fbb75 100644
--- a/library/alloc/src/collections/btree/map.rs
+++ b/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 {
diff --git a/library/alloc/src/collections/btree/set.rs b/library/alloc/src/collections/btree/set.rs
index 59f1ca76b0b..0322cabccde 100644
--- a/library/alloc/src/collections/btree/set.rs
+++ b/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 {
diff --git a/library/alloc/src/collections/btree/set/tests.rs b/library/alloc/src/collections/btree/set/tests.rs
index 01cf62b32ec..2fc17a7c860 100644
--- a/library/alloc/src/collections/btree/set/tests.rs
+++ b/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();
diff --git a/library/alloc/src/collections/linked_list.rs b/library/alloc/src/collections/linked_list.rs
index 4c741133387..e4913b16adb 100644
--- a/library/alloc/src/collections/linked_list.rs
+++ b/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
diff --git a/library/alloc/src/string.rs b/library/alloc/src/string.rs
index 677942a1820..906b0187f7b 100644
--- a/library/alloc/src/string.rs
+++ b/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
diff --git a/library/core/src/ptr/non_null.rs b/library/core/src/ptr/non_null.rs
index 50dd451b5d1..58110b06809 100644
--- a/library/core/src/ptr/non_null.rs
+++ b/library/core/src/ptr/non_null.rs
@@ -449,6 +449,7 @@ 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()
diff --git a/library/core/src/str/mod.rs b/library/core/src/str/mod.rs
index 8f02ecbd009..cd5ed35be79 100644
--- a/library/core/src/str/mod.rs
+++ b/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
     }
diff --git a/library/std/src/ffi/os_str.rs b/library/std/src/ffi/os_str.rs
index 49e268eb99b..d7587e85c66 100644
--- a/library/std/src/ffi/os_str.rs
+++ b/library/std/src/ffi/os_str.rs
@@ -669,6 +669,7 @@ 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()
@@ -700,6 +701,7 @@ 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()
diff --git a/library/std/src/fs.rs b/library/std/src/fs.rs
index 2b76a411a0f..69eca507c71 100644
--- a/library/std/src/fs.rs
+++ b/library/std/src/fs.rs
@@ -1080,6 +1080,7 @@ impl Metadata {
     ///     Ok(())
     /// }
     /// ```
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn len(&self) -> u64 {
         self.0.size()
diff --git a/library/std/src/os/unix/net/ancillary.rs b/library/std/src/os/unix/net/ancillary.rs
index 57bb61903c1..353bf2f1166 100644
--- a/library/std/src/os/unix/net/ancillary.rs
+++ b/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
diff --git a/src/tools/clippy/tests/ui/iter_count.fixed b/src/tools/clippy/tests/ui/iter_count.fixed
index 97c5929783d..90a6eef7526 100644
--- a/src/tools/clippy/tests/ui/iter_count.fixed
+++ b/src/tools/clippy/tests/ui/iter_count.fixed
@@ -33,6 +33,7 @@ impl HasIter {
     }
 }
 
+#[allow(unused_must_use)]
 fn main() {
     let mut vec = vec![0, 1, 2, 3];
     let mut boxed_slice: Box<[u8]> = Box::new([0, 1, 2, 3]);
@@ -50,7 +51,7 @@ fn main() {
     linked_list.push_back(1);
     binary_heap.push(1);
 
-    let _ = &vec[..].len();
+    &vec[..].len();
     vec.len();
     boxed_slice.len();
     vec_deque.len();
@@ -62,13 +63,13 @@ fn main() {
     binary_heap.len();
 
     vec.len();
-    let _ = &vec[..].len();
+    &vec[..].len();
     vec_deque.len();
     hash_map.len();
     b_tree_map.len();
     linked_list.len();
 
-    let _ = &vec[..].len();
+    &vec[..].len();
     vec.len();
     vec_deque.len();
     hash_set.len();
diff --git a/src/tools/clippy/tests/ui/iter_count.rs b/src/tools/clippy/tests/ui/iter_count.rs
index 70bb734763f..6681a480a28 100644
--- a/src/tools/clippy/tests/ui/iter_count.rs
+++ b/src/tools/clippy/tests/ui/iter_count.rs
@@ -33,6 +33,7 @@ impl HasIter {
     }
 }
 
+#[allow(unused_must_use)]
 fn main() {
     let mut vec = vec![0, 1, 2, 3];
     let mut boxed_slice: Box<[u8]> = Box::new([0, 1, 2, 3]);
@@ -50,7 +51,7 @@ fn main() {
     linked_list.push_back(1);
     binary_heap.push(1);
 
-    let _ = &vec[..].iter().count();
+    &vec[..].iter().count();
     vec.iter().count();
     boxed_slice.iter().count();
     vec_deque.iter().count();
@@ -62,13 +63,13 @@ fn main() {
     binary_heap.iter().count();
 
     vec.iter_mut().count();
-    let _ = &vec[..].iter_mut().count();
+    &vec[..].iter_mut().count();
     vec_deque.iter_mut().count();
     hash_map.iter_mut().count();
     b_tree_map.iter_mut().count();
     linked_list.iter_mut().count();
 
-    let _ = &vec[..].into_iter().count();
+    &vec[..].into_iter().count();
     vec.into_iter().count();
     vec_deque.into_iter().count();
     hash_set.into_iter().count();
diff --git a/src/tools/clippy/tests/ui/iter_count.stderr b/src/tools/clippy/tests/ui/iter_count.stderr
index 1d2c22f9dfa..2e3d7fc35de 100644
--- a/src/tools/clippy/tests/ui/iter_count.stderr
+++ b/src/tools/clippy/tests/ui/iter_count.stderr
@@ -1,151 +1,151 @@
 error: called `.iter().count()` on a `slice`
-  --> $DIR/iter_count.rs:53:14
+  --> $DIR/iter_count.rs:54:6
    |
-LL |     let _ = &vec[..].iter().count();
-   |              ^^^^^^^^^^^^^^^^^^^^^^ help: try: `vec[..].len()`
+LL |     &vec[..].iter().count();
+   |      ^^^^^^^^^^^^^^^^^^^^^^ help: try: `vec[..].len()`
    |
    = note: `-D clippy::iter-count` implied by `-D warnings`
 
 error: called `.iter().count()` on a `Vec`
-  --> $DIR/iter_count.rs:54:5
+  --> $DIR/iter_count.rs:55:5
    |
 LL |     vec.iter().count();
    |     ^^^^^^^^^^^^^^^^^^ help: try: `vec.len()`
 
 error: called `.iter().count()` on a `slice`
-  --> $DIR/iter_count.rs:55:5
+  --> $DIR/iter_count.rs:56:5
    |
 LL |     boxed_slice.iter().count();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `boxed_slice.len()`
 
 error: called `.iter().count()` on a `VecDeque`
-  --> $DIR/iter_count.rs:56:5
+  --> $DIR/iter_count.rs:57:5
    |
 LL |     vec_deque.iter().count();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `vec_deque.len()`
 
 error: called `.iter().count()` on a `HashSet`
-  --> $DIR/iter_count.rs:57:5
+  --> $DIR/iter_count.rs:58:5
    |
 LL |     hash_set.iter().count();
    |     ^^^^^^^^^^^^^^^^^^^^^^^ help: try: `hash_set.len()`
 
 error: called `.iter().count()` on a `HashMap`
-  --> $DIR/iter_count.rs:58:5
+  --> $DIR/iter_count.rs:59:5
    |
 LL |     hash_map.iter().count();
    |     ^^^^^^^^^^^^^^^^^^^^^^^ help: try: `hash_map.len()`
 
 error: called `.iter().count()` on a `BTreeMap`
-  --> $DIR/iter_count.rs:59:5
+  --> $DIR/iter_count.rs:60:5
    |
 LL |     b_tree_map.iter().count();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `b_tree_map.len()`
 
 error: called `.iter().count()` on a `BTreeSet`
-  --> $DIR/iter_count.rs:60:5
+  --> $DIR/iter_count.rs:61:5
    |
 LL |     b_tree_set.iter().count();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `b_tree_set.len()`
 
 error: called `.iter().count()` on a `LinkedList`
-  --> $DIR/iter_count.rs:61:5
+  --> $DIR/iter_count.rs:62:5
    |
 LL |     linked_list.iter().count();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `linked_list.len()`
 
 error: called `.iter().count()` on a `BinaryHeap`
-  --> $DIR/iter_count.rs:62:5
+  --> $DIR/iter_count.rs:63:5
    |
 LL |     binary_heap.iter().count();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `binary_heap.len()`
 
 error: called `.iter_mut().count()` on a `Vec`
-  --> $DIR/iter_count.rs:64:5
+  --> $DIR/iter_count.rs:65:5
    |
 LL |     vec.iter_mut().count();
    |     ^^^^^^^^^^^^^^^^^^^^^^ help: try: `vec.len()`
 
 error: called `.iter_mut().count()` on a `slice`
-  --> $DIR/iter_count.rs:65:14
+  --> $DIR/iter_count.rs:66:6
    |
-LL |     let _ = &vec[..].iter_mut().count();
-   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `vec[..].len()`
+LL |     &vec[..].iter_mut().count();
+   |      ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `vec[..].len()`
 
 error: called `.iter_mut().count()` on a `VecDeque`
-  --> $DIR/iter_count.rs:66:5
+  --> $DIR/iter_count.rs:67:5
    |
 LL |     vec_deque.iter_mut().count();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `vec_deque.len()`
 
 error: called `.iter_mut().count()` on a `HashMap`
-  --> $DIR/iter_count.rs:67:5
+  --> $DIR/iter_count.rs:68:5
    |
 LL |     hash_map.iter_mut().count();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `hash_map.len()`
 
 error: called `.iter_mut().count()` on a `BTreeMap`
-  --> $DIR/iter_count.rs:68:5
+  --> $DIR/iter_count.rs:69:5
    |
 LL |     b_tree_map.iter_mut().count();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `b_tree_map.len()`
 
 error: called `.iter_mut().count()` on a `LinkedList`
-  --> $DIR/iter_count.rs:69:5
+  --> $DIR/iter_count.rs:70:5
    |
 LL |     linked_list.iter_mut().count();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `linked_list.len()`
 
 error: called `.into_iter().count()` on a `slice`
-  --> $DIR/iter_count.rs:71:14
+  --> $DIR/iter_count.rs:72:6
    |
-LL |     let _ = &vec[..].into_iter().count();
-   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `vec[..].len()`
+LL |     &vec[..].into_iter().count();
+   |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `vec[..].len()`
 
 error: called `.into_iter().count()` on a `Vec`
-  --> $DIR/iter_count.rs:72:5
+  --> $DIR/iter_count.rs:73:5
    |
 LL |     vec.into_iter().count();
    |     ^^^^^^^^^^^^^^^^^^^^^^^ help: try: `vec.len()`
 
 error: called `.into_iter().count()` on a `VecDeque`
-  --> $DIR/iter_count.rs:73:5
+  --> $DIR/iter_count.rs:74:5
    |
 LL |     vec_deque.into_iter().count();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `vec_deque.len()`
 
 error: called `.into_iter().count()` on a `HashSet`
-  --> $DIR/iter_count.rs:74:5
+  --> $DIR/iter_count.rs:75:5
    |
 LL |     hash_set.into_iter().count();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `hash_set.len()`
 
 error: called `.into_iter().count()` on a `HashMap`
-  --> $DIR/iter_count.rs:75:5
+  --> $DIR/iter_count.rs:76:5
    |
 LL |     hash_map.into_iter().count();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `hash_map.len()`
 
 error: called `.into_iter().count()` on a `BTreeMap`
-  --> $DIR/iter_count.rs:76:5
+  --> $DIR/iter_count.rs:77:5
    |
 LL |     b_tree_map.into_iter().count();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `b_tree_map.len()`
 
 error: called `.into_iter().count()` on a `BTreeSet`
-  --> $DIR/iter_count.rs:77:5
+  --> $DIR/iter_count.rs:78:5
    |
 LL |     b_tree_set.into_iter().count();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `b_tree_set.len()`
 
 error: called `.into_iter().count()` on a `LinkedList`
-  --> $DIR/iter_count.rs:78:5
+  --> $DIR/iter_count.rs:79:5
    |
 LL |     linked_list.into_iter().count();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `linked_list.len()`
 
 error: called `.into_iter().count()` on a `BinaryHeap`
-  --> $DIR/iter_count.rs:79:5
+  --> $DIR/iter_count.rs:80:5
    |
 LL |     binary_heap.into_iter().count();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `binary_heap.len()`