Rollup of 7 pull requests

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

@@ -137,6 +137,8 @@ pub struct Range<'a, T: 'a> {
 /// See its documentation for more.
 ///
 /// [`difference`]: BTreeSet::difference
+#[must_use = "this returns the difference as an iterator, \
+              without modifying either input set"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Difference<'a, T: 'a> {
     inner: DifferenceInner<'a, T>,
@@ -169,6 +171,8 @@ impl<T: fmt::Debug> fmt::Debug for Difference<'_, T> {
 /// [`BTreeSet`]. See its documentation for more.
 ///
 /// [`symmetric_difference`]: BTreeSet::symmetric_difference
+#[must_use = "this returns the difference as an iterator, \
+              without modifying either input set"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct SymmetricDifference<'a, T: 'a>(MergeIterInner<Iter<'a, T>>);
 
@@ -185,6 +189,8 @@ impl<T: fmt::Debug> fmt::Debug for SymmetricDifference<'_, T> {
 /// See its documentation for more.
 ///
 /// [`intersection`]: BTreeSet::intersection
+#[must_use = "this returns the intersection as an iterator, \
+              without modifying either input set"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Intersection<'a, T: 'a> {
     inner: IntersectionInner<'a, T>,
@@ -217,6 +223,8 @@ impl<T: fmt::Debug> fmt::Debug for Intersection<'_, T> {
 /// See its documentation for more.
 ///
 /// [`union`]: BTreeSet::union
+#[must_use = "this returns the union as an iterator, \
+              without modifying either input set"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Union<'a, T: 'a>(MergeIterInner<Iter<'a, T>>);
 

library/alloc/src/lib.rs

@@ -112,7 +112,6 @@
 #![feature(maybe_uninit_slice)]
 #![cfg_attr(test, feature(new_uninit))]
 #![feature(nonnull_slice_from_raw_parts)]
-#![feature(option_result_unwrap_unchecked)]
 #![feature(pattern)]
 #![feature(ptr_internals)]
 #![feature(receiver_trait)]

library/alloc/src/rc.rs

@@ -41,7 +41,7 @@
 //! use std::rc::Rc;
 //!
 //! let my_rc = Rc::new(());
-//! Rc::downgrade(&my_rc);
+//! let my_weak = Rc::downgrade(&my_rc);
 //! ```
 //!
 //! `Rc<T>`'s implementations of traits like `Clone` may also be called using
@@ -889,6 +889,8 @@ impl<T: ?Sized> Rc<T> {
     ///
     /// let weak_five = Rc::downgrade(&five);
     /// ```
+    #[must_use = "this returns a new `Weak` pointer, \
+                  without modifying the original `Rc`"]
     #[stable(feature = "rc_weak", since = "1.4.0")]
     pub fn downgrade(this: &Self) -> Weak<T> {
         this.inner().inc_weak();

library/alloc/src/string.rs

@@ -552,6 +552,7 @@ impl String {
     ///
     /// assert_eq!("Hello �World", output);
     /// ```
+    #[must_use]
     #[cfg(not(no_global_oom_handling))]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn from_utf8_lossy(v: &[u8]) -> Cow<'_, str> {
@@ -646,6 +647,7 @@ impl String {
     ///            String::from_utf16_lossy(v));
     /// ```
     #[cfg(not(no_global_oom_handling))]
+    #[must_use]
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn from_utf16_lossy(v: &[u16]) -> String {

library/alloc/tests/str.rs

@@ -1031,7 +1031,7 @@ fn test_split_at_mut() {
 #[should_panic]
 fn test_split_at_boundscheck() {
     let s = "ศไทย中华Việt Nam";
-    s.split_at(1);
+    let _ = s.split_at(1);
 }
 
 #[test]

library/core/src/alloc/layout.rs

@@ -104,6 +104,7 @@ impl Layout {
     /// The minimum size in bytes 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]
     #[inline]
     pub const fn size(&self) -> usize {
         self.size_
@@ -137,6 +138,7 @@ impl Layout {
     /// allocate backing structure for `T` (which could be a trait
     /// or other unsized type like a slice).
     #[stable(feature = "alloc_layout", since = "1.28.0")]
+    #[must_use]
     #[inline]
     pub fn for_value<T: ?Sized>(t: &T) -> Self {
         let (size, align) = (mem::size_of_val(t), mem::align_of_val(t));
@@ -171,6 +173,7 @@ impl Layout {
     /// [trait object]: ../../book/ch17-02-trait-objects.html
     /// [extern type]: ../../unstable-book/language-features/extern-types.html
     #[unstable(feature = "layout_for_ptr", issue = "69835")]
+    #[must_use]
     pub unsafe fn for_value_raw<T: ?Sized>(t: *const T) -> Self {
         // SAFETY: we pass along the prerequisites of these functions to the caller
         let (size, align) = unsafe { (mem::size_of_val_raw(t), mem::align_of_val_raw(t)) };
@@ -187,6 +190,7 @@ impl Layout {
     /// some other means.
     #[unstable(feature = "alloc_layout_extra", issue = "55724")]
     #[rustc_const_unstable(feature = "alloc_layout_extra", issue = "55724")]
+    #[must_use]
     #[inline]
     pub const fn dangling(&self) -> NonNull<u8> {
         // SAFETY: align is guaranteed to be non-zero

library/core/src/any.rs

@@ -458,6 +458,7 @@ impl TypeId {
     /// assert_eq!(is_string(&0), false);
     /// assert_eq!(is_string(&"cookie monster".to_string()), true);
     /// ```
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_unstable(feature = "const_type_id", issue = "77125")]
     pub const fn of<T: ?Sized + 'static>() -> TypeId {
@@ -492,6 +493,7 @@ impl TypeId {
 ///     "core::option::Option<alloc::string::String>",
 /// );
 /// ```
+#[must_use]
 #[stable(feature = "type_name", since = "1.38.0")]
 #[rustc_const_unstable(feature = "const_type_name", issue = "63084")]
 pub const fn type_name<T: ?Sized>() -> &'static str {
@@ -534,6 +536,7 @@ pub const fn type_name<T: ?Sized>() -> &'static str {
 /// let y = 1.0;
 /// println!("{}", type_name_of_val(&y));
 /// ```
+#[must_use]
 #[unstable(feature = "type_name_of_val", issue = "66359")]
 #[rustc_const_unstable(feature = "const_type_name", issue = "63084")]
 pub const fn type_name_of_val<T: ?Sized>(_val: &T) -> &'static str {

library/core/src/ascii.rs

@@ -18,6 +18,7 @@ use crate::str::from_utf8_unchecked;
 ///
 /// This `struct` is created by the [`escape_default`] function. See its
 /// documentation for more.
+#[must_use = "iterators are lazy and do nothing unless consumed"]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[derive(Clone)]
 pub struct EscapeDefault {

library/core/src/cell.rs

@@ -1335,6 +1335,7 @@ impl<'b, T: ?Sized> Ref<'b, T> {
     /// with the widespread use of `r.borrow().clone()` to clone the contents of
     /// a `RefCell`.
     #[stable(feature = "cell_extras", since = "1.15.0")]
+    #[must_use]
     #[inline]
     pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
         Ref { value: orig.value, borrow: orig.borrow.clone() }

library/core/src/char/decode.rs

@@ -128,6 +128,7 @@ impl<I: Iterator<Item = u16>> Iterator for DecodeUtf16<I> {
 
 impl DecodeUtf16Error {
     /// Returns the unpaired surrogate which caused this error.
+    #[must_use]
     #[stable(feature = "decode_utf16", since = "1.9.0")]
     pub fn unpaired_surrogate(&self) -> u16 {
         self.code

library/core/src/default.rs

@@ -155,6 +155,7 @@ pub trait Default: Sized {
 /// }
 /// ```
 #[unstable(feature = "default_free_fn", issue = "73014")]
+#[must_use]
 #[inline]
 pub fn default<T: Default>() -> T {
     Default::default()

library/core/src/fmt/mod.rs

@@ -1604,6 +1604,7 @@ impl<'a> Formatter<'a> {
     }
 
     /// Flags for formatting
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_deprecated(
         since = "1.24.0",
@@ -1641,6 +1642,7 @@ impl<'a> Formatter<'a> {
     /// assert_eq!(&format!("{:G>3}", Foo), "GGG");
     /// assert_eq!(&format!("{:t>6}", Foo), "tttttt");
     /// ```
+    #[must_use]
     #[stable(feature = "fmt_flags", since = "1.5.0")]
     pub fn fill(&self) -> char {
         self.fill
@@ -1677,6 +1679,7 @@ impl<'a> Formatter<'a> {
     /// assert_eq!(&format!("{:^}", Foo), "center");
     /// assert_eq!(&format!("{}", Foo), "into the void");
     /// ```
+    #[must_use]
     #[stable(feature = "fmt_flags_align", since = "1.28.0")]
     pub fn align(&self) -> Option<Alignment> {
         match self.align {
@@ -1711,6 +1714,7 @@ impl<'a> Formatter<'a> {
     /// assert_eq!(&format!("{:10}", Foo(23)), "Foo(23)   ");
     /// assert_eq!(&format!("{}", Foo(23)), "Foo(23)");
     /// ```
+    #[must_use]
     #[stable(feature = "fmt_flags", since = "1.5.0")]
     pub fn width(&self) -> Option<usize> {
         self.width
@@ -1741,6 +1745,7 @@ impl<'a> Formatter<'a> {
     /// assert_eq!(&format!("{:.4}", Foo(23.2)), "Foo(23.2000)");
     /// assert_eq!(&format!("{}", Foo(23.2)), "Foo(23.20)");
     /// ```
+    #[must_use]
     #[stable(feature = "fmt_flags", since = "1.5.0")]
     pub fn precision(&self) -> Option<usize> {
         self.precision
@@ -1771,6 +1776,7 @@ impl<'a> Formatter<'a> {
     /// assert_eq!(&format!("{:+}", Foo(23)), "Foo(+23)");
     /// assert_eq!(&format!("{}", Foo(23)), "Foo(23)");
     /// ```
+    #[must_use]
     #[stable(feature = "fmt_flags", since = "1.5.0")]
     pub fn sign_plus(&self) -> bool {
         self.flags & (1 << FlagV1::SignPlus as u32) != 0
@@ -1799,6 +1805,7 @@ impl<'a> Formatter<'a> {
     /// assert_eq!(&format!("{:-}", Foo(23)), "-Foo(23)");
     /// assert_eq!(&format!("{}", Foo(23)), "Foo(23)");
     /// ```
+    #[must_use]
     #[stable(feature = "fmt_flags", since = "1.5.0")]
     pub fn sign_minus(&self) -> bool {
         self.flags & (1 << FlagV1::SignMinus as u32) != 0
@@ -1826,6 +1833,7 @@ impl<'a> Formatter<'a> {
     /// assert_eq!(&format!("{:#}", Foo(23)), "Foo(23)");
     /// assert_eq!(&format!("{}", Foo(23)), "23");
     /// ```
+    #[must_use]
     #[stable(feature = "fmt_flags", since = "1.5.0")]
     pub fn alternate(&self) -> bool {
         self.flags & (1 << FlagV1::Alternate as u32) != 0
@@ -1851,6 +1859,7 @@ impl<'a> Formatter<'a> {
     ///
     /// assert_eq!(&format!("{:04}", Foo(23)), "23");
     /// ```
+    #[must_use]
     #[stable(feature = "fmt_flags", since = "1.5.0")]
     pub fn sign_aware_zero_pad(&self) -> bool {
         self.flags & (1 << FlagV1::SignAwareZeroPad as u32) != 0

library/core/src/future/mod.rs

@@ -90,6 +90,7 @@ where
 #[lang = "get_context"]
 #[doc(hidden)]
 #[unstable(feature = "gen_future", issue = "50547")]
+#[must_use]
 #[inline]
 pub unsafe fn get_context<'a, 'b>(cx: ResumeTy) -> &'a mut Context<'b> {
     // SAFETY: the caller must guarantee that `cx.0` is a valid pointer

library/core/src/iter/sources/empty.rs

@@ -25,6 +25,7 @@ pub const fn empty<T>() -> Empty<T> {
 /// An iterator that yields nothing.
 ///
 /// This `struct` is created by the [`empty()`] function. See its documentation for more.
+#[must_use = "iterators are lazy and do nothing unless consumed"]
 #[stable(feature = "iter_empty", since = "1.2.0")]
 pub struct Empty<T>(marker::PhantomData<T>);
 

library/core/src/mem/mod.rs

@@ -296,6 +296,7 @@ pub fn forget_unsized<T: ?Sized>(t: T) {
 ///
 /// [alignment]: align_of
 #[inline(always)]
+#[must_use]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_promotable]
 #[rustc_const_stable(feature = "const_size_of", since = "1.24.0")]
@@ -324,6 +325,7 @@ pub const fn size_of<T>() -> usize {
 /// assert_eq!(13, mem::size_of_val(y));
 /// ```
 #[inline]
+#[must_use]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_unstable(feature = "const_size_of_val", issue = "46571")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "mem_size_of_val")]
@@ -373,6 +375,7 @@ pub const fn size_of_val<T: ?Sized>(val: &T) -> usize {
 /// assert_eq!(13, unsafe { mem::size_of_val_raw(y) });
 /// ```
 #[inline]
+#[must_use]
 #[unstable(feature = "layout_for_ptr", issue = "69835")]
 #[rustc_const_unstable(feature = "const_size_of_val_raw", issue = "46571")]
 pub const unsafe fn size_of_val_raw<T: ?Sized>(val: *const T) -> usize {
@@ -397,6 +400,7 @@ pub const unsafe fn size_of_val_raw<T: ?Sized>(val: *const T) -> usize {
 /// assert_eq!(4, mem::min_align_of::<i32>());
 /// ```
 #[inline]
+#[must_use]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_deprecated(reason = "use `align_of` instead", since = "1.2.0")]
 pub fn min_align_of<T>() -> usize {
@@ -418,6 +422,7 @@ pub fn min_align_of<T>() -> usize {
 /// assert_eq!(4, mem::min_align_of_val(&5i32));
 /// ```
 #[inline]
+#[must_use]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_deprecated(reason = "use `align_of_val` instead", since = "1.2.0")]
 pub fn min_align_of_val<T: ?Sized>(val: &T) -> usize {
@@ -441,6 +446,7 @@ pub fn min_align_of_val<T: ?Sized>(val: &T) -> usize {
 /// assert_eq!(4, mem::align_of::<i32>());
 /// ```
 #[inline(always)]
+#[must_use]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_promotable]
 #[rustc_const_stable(feature = "const_align_of", since = "1.24.0")]
@@ -462,6 +468,7 @@ pub const fn align_of<T>() -> usize {
 /// assert_eq!(4, mem::align_of_val(&5i32));
 /// ```
 #[inline]
+#[must_use]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_unstable(feature = "const_align_of_val", issue = "46571")]
 #[allow(deprecated)]
@@ -507,6 +514,7 @@ pub const fn align_of_val<T: ?Sized>(val: &T) -> usize {
 /// assert_eq!(4, unsafe { mem::align_of_val_raw(&5i32) });
 /// ```
 #[inline]
+#[must_use]
 #[unstable(feature = "layout_for_ptr", issue = "69835")]
 #[rustc_const_unstable(feature = "const_align_of_val_raw", issue = "46571")]
 pub const unsafe fn align_of_val_raw<T: ?Sized>(val: *const T) -> usize {
@@ -571,6 +579,7 @@ pub const unsafe fn align_of_val_raw<T: ?Sized>(val: *const T) -> usize {
 /// }
 /// ```
 #[inline]
+#[must_use]
 #[stable(feature = "needs_drop", since = "1.21.0")]
 #[rustc_const_stable(feature = "const_needs_drop", since = "1.36.0")]
 #[rustc_diagnostic_item = "needs_drop"]
@@ -618,6 +627,7 @@ pub const fn needs_drop<T>() -> bool {
 /// let _y: fn() = unsafe { mem::zeroed() }; // And again!
 /// ```
 #[inline(always)]
+#[must_use]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[allow(deprecated_in_future)]
 #[allow(deprecated)]
@@ -653,6 +663,7 @@ pub unsafe fn zeroed<T>() -> T {
 /// [assume_init]: MaybeUninit::assume_init
 /// [inv]: MaybeUninit#initialization-invariant
 #[inline(always)]
+#[must_use]
 #[rustc_deprecated(since = "1.39.0", reason = "use `mem::MaybeUninit` instead")]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[allow(deprecated_in_future)]
@@ -938,6 +949,7 @@ pub fn drop<T>(_x: T) {}
 /// assert_eq!(foo_array, [10]);
 /// ```
 #[inline]
+#[must_use]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_unstable(feature = "const_transmute_copy", issue = "83165")]
 pub const unsafe fn transmute_copy<T, U>(src: &T) -> U {
@@ -1051,6 +1063,7 @@ pub const fn discriminant<T>(v: &T) -> Discriminant<T> {
 /// assert_eq!(mem::variant_count::<Result<!, !>>(), 2);
 /// ```
 #[inline(always)]
+#[must_use]
 #[unstable(feature = "variant_count", issue = "73662")]
 #[rustc_const_unstable(feature = "variant_count", issue = "73662")]
 #[rustc_diagnostic_item = "mem_variant_count"]

library/core/src/num/error.rs

@@ -115,6 +115,7 @@ pub enum IntErrorKind {
 
 impl ParseIntError {
     /// Outputs the detailed cause of parsing an integer failing.
+    #[must_use]
     #[stable(feature = "int_error_matching", since = "1.55.0")]
     pub fn kind(&self) -> &IntErrorKind {
         &self.kind

library/core/src/num/f32.rs

@@ -980,6 +980,7 @@ impl f32 {
     /// #     .all(|(a, b)| a.to_bits() == b.to_bits()))
     /// ```
     #[unstable(feature = "total_cmp", issue = "72599")]
+    #[must_use]
     #[inline]
     pub fn total_cmp(&self, other: &Self) -> crate::cmp::Ordering {
         let mut left = self.to_bits() as i32;

library/core/src/num/f64.rs

@@ -996,6 +996,7 @@ impl f64 {
     /// #     .all(|(a, b)| a.to_bits() == b.to_bits()))
     /// ```
     #[unstable(feature = "total_cmp", issue = "72599")]
+    #[must_use]
     #[inline]
     pub fn total_cmp(&self, other: &Self) -> crate::cmp::Ordering {
         let mut left = self.to_bits() as i64;