rust-lang · matthiaskrgr · Oct 11, 2021 · Oct 11, 2021 · Oct 11, 2021 · Oct 11, 2021
diff --git a/library/alloc/src/collections/binary_heap.rs b/library/alloc/src/collections/binary_heap.rs
@@ -1009,6 +1009,7 @@ impl<T> BinaryHeap<T> {
     ///
     /// io::sink().write(heap.as_slice()).unwrap();
     /// ```
+    #[must_use]
     #[unstable(feature = "binary_heap_as_slice", issue = "83659")]
     pub fn as_slice(&self) -> &[T] {
         self.data.as_slice()
@@ -1049,6 +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()
@@ -1072,6 +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

diff --git a/library/alloc/src/collections/btree/map.rs b/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 {

diff --git a/library/alloc/src/collections/btree/set.rs b/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 {

diff --git a/library/alloc/src/collections/btree/set/tests.rs b/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>) {

diff --git a/library/alloc/src/collections/linked_list.rs b/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
@@ -1385,6 +1387,7 @@ impl<'a, T> CursorMut<'a, T> {
     /// The lifetime of the returned `Cursor` is bound to that of the
     /// `CursorMut`, which means it cannot outlive the `CursorMut` and that the
     /// `CursorMut` is frozen for the lifetime of the `Cursor`.
+    #[must_use]
     #[unstable(feature = "linked_list_cursors", issue = "58533")]
     pub fn as_cursor(&self) -> Cursor<'_, T> {
         Cursor { list: self.list, current: self.current, index: self.index }

diff --git a/library/alloc/src/rc.rs b/library/alloc/src/rc.rs
@@ -2093,6 +2093,7 @@ impl<T: ?Sized> Weak<T> {
     /// ```
     ///
     /// [`null`]: ptr::null
+    #[must_use]
     #[stable(feature = "rc_as_ptr", since = "1.45.0")]
     pub fn as_ptr(&self) -> *const T {
         let ptr: *mut RcBox<T> = NonNull::as_ptr(self.ptr);
@@ -2229,6 +2230,8 @@ impl<T: ?Sized> Weak<T> {
     ///
     /// assert!(weak_five.upgrade().is_none());
     /// ```
+    #[must_use = "this returns a new `Rc`, \
+                  without modifying the original weak pointer"]
     #[stable(feature = "rc_weak", since = "1.4.0")]
     pub fn upgrade(&self) -> Option<Rc<T>> {
         let inner = self.inner()?;

diff --git a/library/alloc/src/string.rs b/library/alloc/src/string.rs
@@ -803,6 +803,7 @@ impl String {
     /// assert_eq!("foo", s.as_str());
     /// ```
     #[inline]
+    #[must_use]
     #[stable(feature = "string_as_str", since = "1.7.0")]
     pub fn as_str(&self) -> &str {
         self
@@ -823,6 +824,7 @@ impl String {
     /// assert_eq!("FOOBAR", s_mut_str);
     /// ```
     #[inline]
+    #[must_use]
     #[stable(feature = "string_as_str", since = "1.7.0")]
     pub fn as_mut_str(&mut self) -> &mut str {
         self
@@ -1163,6 +1165,7 @@ impl String {
     /// assert_eq!(&[104, 101, 108, 108, 111], s.as_bytes());
     /// ```
     #[inline]
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn as_bytes(&self) -> &[u8] {
         &self.vec
@@ -1540,6 +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()
@@ -1559,6 +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
@@ -1766,6 +1771,7 @@ impl FromUtf8Error {
     ///
     /// assert_eq!(&[0, 159], value.unwrap_err().as_bytes());
     /// ```
+    #[must_use]
     #[stable(feature = "from_utf8_error_as_bytes", since = "1.26.0")]
     pub fn as_bytes(&self) -> &[u8] {
         &self.bytes[..]
@@ -2782,6 +2788,7 @@ impl<'a> Drain<'a> {
     /// let _ = drain.next().unwrap();
     /// assert_eq!(drain.as_str(), "bc");
     /// ```
+    #[must_use]
     #[stable(feature = "string_drain_as_str", since = "1.55.0")]
     pub fn as_str(&self) -> &str {
         self.iter.as_str()

diff --git a/library/alloc/src/sync.rs b/library/alloc/src/sync.rs
@@ -146,7 +146,7 @@ macro_rules! acquire {
 /// use std::sync::Arc;
 ///
 /// let my_arc = Arc::new(());
-/// Arc::downgrade(&my_arc);
+/// let my_weak = Arc::downgrade(&my_arc);
 /// ```
 ///
 /// `Arc<T>`'s implementations of traits like `Clone` may also be called using
@@ -827,6 +827,7 @@ impl<T: ?Sized> Arc<T> {
     /// assert_eq!(x_ptr, Arc::as_ptr(&y));
     /// assert_eq!(unsafe { &*x_ptr }, "hello");
     /// ```
+    #[must_use]
     #[stable(feature = "rc_as_ptr", since = "1.45.0")]
     pub fn as_ptr(this: &Self) -> *const T {
         let ptr: *mut ArcInner<T> = NonNull::as_ptr(this.ptr);
@@ -897,6 +898,8 @@ impl<T: ?Sized> Arc<T> {
     ///
     /// let weak_five = Arc::downgrade(&five);
     /// ```
+    #[must_use = "this returns a new `Weak` pointer, \
+                  without modifying the original `Arc`"]
     #[stable(feature = "arc_weak", since = "1.4.0")]
     pub fn downgrade(this: &Self) -> Weak<T> {
         // This Relaxed is OK because we're checking the value in the CAS
@@ -1724,6 +1727,7 @@ impl<T: ?Sized> Weak<T> {
     /// ```
     ///
     /// [`null`]: core::ptr::null "ptr::null"
+    #[must_use]
     #[stable(feature = "weak_into_raw", since = "1.45.0")]
     pub fn as_ptr(&self) -> *const T {
         let ptr: *mut ArcInner<T> = NonNull::as_ptr(self.ptr);
@@ -1861,6 +1865,8 @@ impl<T: ?Sized> Weak<T> {
     ///
     /// assert!(weak_five.upgrade().is_none());
     /// ```
+    #[must_use = "this returns a new `Arc`, \
+                  without modifying the original weak pointer"]
     #[stable(feature = "arc_weak", since = "1.4.0")]
     pub fn upgrade(&self) -> Option<Arc<T>> {
         // We use a CAS loop to increment the strong count instead of a

diff --git a/library/alloc/src/vec/drain.rs b/library/alloc/src/vec/drain.rs
@@ -52,6 +52,7 @@ impl<'a, T, A: Allocator> Drain<'a, T, A> {
     /// let _ = drain.next().unwrap();
     /// assert_eq!(drain.as_slice(), &['b', 'c']);
     /// ```
+    #[must_use]
     #[stable(feature = "vec_drain_as_slice", since = "1.46.0")]
     pub fn as_slice(&self) -> &[T] {
         self.iter.as_slice()

diff --git a/library/core/src/alloc/layout.rs b/library/core/src/alloc/layout.rs
@@ -112,6 +112,8 @@ impl Layout {
     /// The minimum byte alignment for a memory block of this layout.
     #[stable(feature = "alloc_layout", since = "1.28.0")]
     #[rustc_const_stable(feature = "const_alloc_layout", since = "1.50.0")]
+    #[must_use = "this returns the minimum alignment, \
+                  without modifying the layout"]
     #[inline]
     pub const fn align(&self) -> usize {
         self.align_.get()
@@ -229,6 +231,8 @@ impl Layout {
     /// satisfy this constraint is to ensure `align <= self.align()`.
     #[unstable(feature = "alloc_layout_extra", issue = "55724")]
     #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
+    #[must_use = "this returns the padding needed, \
+                  without modifying the `Layout`"]
     #[inline]
     pub const fn padding_needed_for(&self, align: usize) -> usize {
         let len = self.size();
@@ -262,6 +266,8 @@ impl Layout {
     /// This is equivalent to adding the result of `padding_needed_for`
     /// to the layout's current size.
     #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
+    #[must_use = "this returns a new `Layout`, \
+                  without modifying the original"]
     #[inline]
     pub fn pad_to_align(&self) -> Layout {
         let pad = self.padding_needed_for(self.align());

diff --git a/library/core/src/convert/mod.rs b/library/core/src/convert/mod.rs
@@ -273,6 +273,7 @@ pub trait AsMut<T: ?Sized> {
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait Into<T>: Sized {
     /// Performs the conversion.
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn into(self) -> T;
 }
@@ -367,6 +368,7 @@ pub trait Into<T>: Sized {
 pub trait From<T>: Sized {
     /// Performs the conversion.
     #[lang = "from"]
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     fn from(_: T) -> Self;
 }

diff --git a/library/core/src/fmt/mod.rs b/library/core/src/fmt/mod.rs
@@ -491,6 +491,7 @@ impl<'a> Arguments<'a> {
     /// ```
     #[stable(feature = "fmt_as_str", since = "1.52.0")]
     #[rustc_const_unstable(feature = "const_arguments_as_str", issue = "none")]
+    #[must_use]
     #[inline]
     pub const fn as_str(&self) -> Option<&'static str> {
         match (self.pieces, self.args) {

diff --git a/library/core/src/mem/maybe_uninit.rs b/library/core/src/mem/maybe_uninit.rs
@@ -291,6 +291,7 @@ impl<T> MaybeUninit<T> {
     /// [`assume_init`]: MaybeUninit::assume_init
     #[stable(feature = "maybe_uninit", since = "1.36.0")]
     #[rustc_const_stable(feature = "const_maybe_uninit", since = "1.36.0")]
+    #[must_use = "use `forget` to avoid running Drop code"]
     #[inline(always)]
     pub const fn new(val: T) -> MaybeUninit<T> {
         MaybeUninit { value: ManuallyDrop::new(val) }

diff --git a/library/core/src/option.rs b/library/core/src/option.rs
@@ -657,6 +657,7 @@ impl<T> Option<T> {
     ///
     /// [&]: reference "shared reference"
     #[inline]
+    #[must_use]
     #[stable(feature = "pin", since = "1.33.0")]
     pub fn as_pin_ref(self: Pin<&Self>) -> Option<Pin<&T>> {
         // SAFETY: `x` is guaranteed to be pinned because it comes from `self`
@@ -668,6 +669,7 @@ impl<T> Option<T> {
     ///
     /// [&mut]: reference "mutable reference"
     #[inline]
+    #[must_use]
     #[stable(feature = "pin", since = "1.33.0")]
     pub fn as_pin_mut(self: Pin<&mut Self>) -> Option<Pin<&mut T>> {
         // SAFETY: `get_unchecked_mut` is never used to move the `Option` inside `self`.

diff --git a/library/core/src/ptr/non_null.rs b/library/core/src/ptr/non_null.rs
@@ -119,6 +119,7 @@ impl<T: Sized> NonNull<T> {
     ///
     /// [the module documentation]: crate::ptr#safety
     #[inline]
+    #[must_use]
     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
     pub unsafe fn as_uninit_ref<'a>(&self) -> &'a MaybeUninit<T> {
         // SAFETY: the caller must guarantee that `self` meets all the
@@ -151,6 +152,7 @@ impl<T: Sized> NonNull<T> {
     ///
     /// [the module documentation]: crate::ptr#safety
     #[inline]
+    #[must_use]
     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
     pub unsafe fn as_uninit_mut<'a>(&mut self) -> &'a mut MaybeUninit<T> {
         // SAFETY: the caller must guarantee that `self` meets all the
@@ -264,6 +266,7 @@ impl<T: ?Sized> NonNull<T> {
     /// ```
     #[stable(feature = "nonnull", since = "1.25.0")]
     #[rustc_const_stable(feature = "const_nonnull_as_ptr", since = "1.32.0")]
+    #[must_use]
     #[inline]
     pub const fn as_ptr(self) -> *mut T {
         self.pointer as *mut T
@@ -310,6 +313,7 @@ impl<T: ?Sized> NonNull<T> {
     ///
     /// [the module documentation]: crate::ptr#safety
     #[stable(feature = "nonnull", since = "1.25.0")]
+    #[must_use]
     #[inline]
     pub unsafe fn as_ref<'a>(&self) -> &'a T {
         // SAFETY: the caller must guarantee that `self` meets all the
@@ -359,6 +363,7 @@ impl<T: ?Sized> NonNull<T> {
     ///
     /// [the module documentation]: crate::ptr#safety
     #[stable(feature = "nonnull", since = "1.25.0")]
+    #[must_use]
     #[inline]
     pub unsafe fn as_mut<'a>(&mut self) -> &'a mut T {
         // SAFETY: the caller must guarantee that `self` meets all the
@@ -438,6 +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()
@@ -455,6 +461,7 @@ impl<T> NonNull<[T]> {
     /// assert_eq!(slice.as_non_null_ptr(), NonNull::new(1 as *mut i8).unwrap());
     /// ```
     #[inline]
+    #[must_use]
     #[unstable(feature = "slice_ptr_get", issue = "74265")]
     #[rustc_const_unstable(feature = "slice_ptr_get", issue = "74265")]
     pub const fn as_non_null_ptr(self) -> NonNull<T> {
@@ -474,6 +481,7 @@ impl<T> NonNull<[T]> {
     /// assert_eq!(slice.as_mut_ptr(), 1 as *mut i8);
     /// ```
     #[inline]
+    #[must_use]
     #[unstable(feature = "slice_ptr_get", issue = "74265")]
     #[rustc_const_unstable(feature = "slice_ptr_get", issue = "74265")]
     pub const fn as_mut_ptr(self) -> *mut T {
@@ -518,6 +526,7 @@ impl<T> NonNull<[T]> {
     ///
     /// [valid]: crate::ptr#safety
     #[inline]
+    #[must_use]
     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
     pub unsafe fn as_uninit_slice<'a>(&self) -> &'a [MaybeUninit<T>] {
         // SAFETY: the caller must uphold the safety contract for `as_uninit_slice`.
@@ -579,6 +588,7 @@ impl<T> NonNull<[T]> {
     /// # Ok::<_, std::alloc::AllocError>(())
     /// ```
     #[inline]
+    #[must_use]
     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
     pub unsafe fn as_uninit_slice_mut<'a>(&self) -> &'a mut [MaybeUninit<T>] {
         // SAFETY: the caller must uphold the safety contract for `as_uninit_slice_mut`.

diff --git a/library/core/src/ptr/unique.rs b/library/core/src/ptr/unique.rs
@@ -112,6 +112,7 @@ impl<T: ?Sized> Unique<T> {
     /// The resulting lifetime is bound to self so this behaves "as if"
     /// it were actually an instance of T that is getting borrowed. If a longer
     /// (unbound) lifetime is needed, use `&*my_ptr.as_ptr()`.
+    #[must_use]
     #[inline]
     pub unsafe fn as_ref(&self) -> &T {
         // SAFETY: the caller must guarantee that `self` meets all the
@@ -124,6 +125,7 @@ impl<T: ?Sized> Unique<T> {
     /// The resulting lifetime is bound to self so this behaves "as if"
     /// it were actually an instance of T that is getting borrowed. If a longer
     /// (unbound) lifetime is needed, use `&mut *my_ptr.as_ptr()`.
+    #[must_use]
     #[inline]
     pub unsafe fn as_mut(&mut self) -> &mut T {
         // SAFETY: the caller must guarantee that `self` meets all the