Rollup merge of #89897 - jkugelman:must-use-core, r=joshtriplett

Add #[must_use] to remaining core functions

I've run out of compelling reasons to group functions together across crates so I'm just going to go module-by-module. This is everything remaining from the `core` crate.

Ignored by clippy for reasons unknown:

```rust
core::alloc::Layout   unsafe fn for_value_raw<T: ?Sized>(t: *const T) -> Self;
core::any             const fn type_name_of_val<T: ?Sized>(_val: &T) -> &'static str;
```

Ignored by clippy because of `mut`:

```rust
str   fn split_at_mut(&mut self, mid: usize) -> (&mut str, &mut str);
```

<del>
Ignored by clippy presumably because a caller might want `f` called for side effects. That seems like a bad usage of `map` to me.

```rust
core::cell::Ref<'b, T>   fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, T>;
core::cell::Ref<'b, T>   fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>);
```
</del>

Parent issue: #89692

r? ```@joshtriplett```

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/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;

library/core/src/ops/range.rs

@@ -743,6 +743,7 @@ impl<T: Clone> Bound<&T> {
     /// assert_eq!((1..12).start_bound(), Included(&1));
     /// assert_eq!((1..12).start_bound().cloned(), Included(1));
     /// ```
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[stable(feature = "bound_cloned", since = "1.55.0")]
     pub fn cloned(self) -> Bound<T> {
         match self {

library/core/src/option.rs

@@ -1451,6 +1451,7 @@ impl<T: Copy> Option<&T> {
     /// let copied = opt_x.copied();
     /// assert_eq!(copied, Some(12));
     /// ```
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[stable(feature = "copied", since = "1.35.0")]
     #[rustc_const_unstable(feature = "const_option", issue = "67441")]
     pub const fn copied(self) -> Option<T> {

library/core/src/panic/location.rs

@@ -79,6 +79,7 @@ impl<'a> Location<'a> {
     /// assert_ne!(this_location.line(), another_location.line());
     /// assert_ne!(this_location.column(), another_location.column());
     /// ```
+    #[must_use]
     #[stable(feature = "track_caller", since = "1.46.0")]
     #[rustc_const_unstable(feature = "const_caller_location", issue = "76156")]
     #[track_caller]
@@ -119,6 +120,7 @@ impl<'a> Location<'a> {
     ///
     /// panic!("Normal panic");
     /// ```
+    #[must_use]
     #[stable(feature = "panic_hooks", since = "1.10.0")]
     pub fn file(&self) -> &str {
         self.file
@@ -141,6 +143,7 @@ impl<'a> Location<'a> {
     ///
     /// panic!("Normal panic");
     /// ```
+    #[must_use]
     #[stable(feature = "panic_hooks", since = "1.10.0")]
     pub fn line(&self) -> u32 {
         self.line
@@ -163,6 +166,7 @@ impl<'a> Location<'a> {
     ///
     /// panic!("Normal panic");
     /// ```
+    #[must_use]
     #[stable(feature = "panic_col", since = "1.25.0")]
     pub fn column(&self) -> u32 {
         self.col

library/core/src/panic/panic_info.rs

@@ -81,6 +81,7 @@ impl<'a> PanicInfo<'a> {
     ///
     /// panic!("Normal panic");
     /// ```
+    #[must_use]
     #[stable(feature = "panic_hooks", since = "1.10.0")]
     pub fn payload(&self) -> &(dyn Any + Send) {
         self.payload
@@ -89,6 +90,7 @@ impl<'a> PanicInfo<'a> {
     /// If the `panic!` macro from the `core` crate (not from `std`)
     /// was used with a formatting string and some additional arguments,
     /// returns that message ready to be used for example with [`fmt::write`]
+    #[must_use]
     #[unstable(feature = "panic_info_message", issue = "66745")]
     pub fn message(&self) -> Option<&fmt::Arguments<'_>> {
         self.message
@@ -118,6 +120,7 @@ impl<'a> PanicInfo<'a> {
     ///
     /// panic!("Normal panic");
     /// ```
+    #[must_use]
     #[stable(feature = "panic_hooks", since = "1.10.0")]
     pub fn location(&self) -> Option<&Location<'_>> {
         // NOTE: If this is changed to sometimes return None,

library/core/src/pin.rs

@@ -705,6 +705,7 @@ impl<'a, T: ?Sized> Pin<&'a T> {
     ///
     /// ["pinning projections"]: self#projections-and-structural-pinning
     #[inline(always)]
+    #[must_use]
     #[rustc_const_unstable(feature = "const_pin", issue = "76654")]
     #[stable(feature = "pin", since = "1.33.0")]
     pub const fn get_ref(self) -> &'a T {

library/core/src/slice/iter.rs

@@ -1724,6 +1724,7 @@ impl<'a, T> ChunksExact<'a, T> {
     /// Returns the remainder of the original slice that is not going to be
     /// returned by the iterator. The returned slice has at most `chunk_size-1`
     /// elements.
+    #[must_use]
     #[stable(feature = "chunks_exact", since = "1.31.0")]
     pub fn remainder(&self) -> &'a [T] {
         self.rem
@@ -2153,6 +2154,7 @@ impl<'a, T, const N: usize> ArrayChunks<'a, T, N> {
     /// Returns the remainder of the original slice that is not going to be
     /// returned by the iterator. The returned slice has at most `N-1`
     /// elements.
+    #[must_use]
     #[unstable(feature = "array_chunks", issue = "74985")]
     pub fn remainder(&self) -> &'a [T] {
         self.rem
@@ -2728,6 +2730,7 @@ impl<'a, T> RChunksExact<'a, T> {
     /// Returns the remainder of the original slice that is not going to be
     /// returned by the iterator. The returned slice has at most `chunk_size-1`
     /// elements.
+    #[must_use]
     #[stable(feature = "rchunks", since = "1.31.0")]
     pub fn remainder(&self) -> &'a [T] {
         self.rem

library/core/src/str/error.rs

@@ -72,6 +72,7 @@ impl Utf8Error {
     /// assert_eq!(1, error.valid_up_to());
     /// ```
     #[stable(feature = "utf8_error", since = "1.5.0")]
+    #[must_use]
     #[inline]
     pub fn valid_up_to(&self) -> usize {
         self.valid_up_to
@@ -93,6 +94,7 @@ impl Utf8Error {
     ///
     /// [U+FFFD]: ../../std/char/constant.REPLACEMENT_CHARACTER.html
     #[stable(feature = "utf8_error_error_len", since = "1.20.0")]
+    #[must_use]
     #[inline]
     pub fn error_len(&self) -> Option<usize> {
         self.error_len.map(|len| len as usize)

library/core/src/str/iter.rs

@@ -213,6 +213,7 @@ impl<'a> CharIndices<'a> {
     /// assert_eq!(chars.next(), None);
     /// ```
     #[inline]
+    #[must_use]
     #[unstable(feature = "char_indices_offset", issue = "83871")]
     pub fn offset(&self) -> usize {
         self.front_offset
@@ -225,6 +226,7 @@ impl<'a> CharIndices<'a> {
 /// See its documentation for more.
 ///
 /// [`bytes`]: str::bytes
+#[must_use = "iterators are lazy and do nothing unless consumed"]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[derive(Clone, Debug)]
 pub struct Bytes<'a>(pub(super) Copied<slice::Iter<'a, u8>>);

library/core/src/str/mod.rs

@@ -498,6 +498,7 @@ impl str {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_deprecated(since = "1.29.0", reason = "use `get_unchecked(begin..end)` instead")]
+    #[must_use]
     #[inline]
     pub unsafe fn slice_unchecked(&self, begin: usize, end: usize) -> &str {
         // SAFETY: the caller must uphold the safety contract for `get_unchecked`;
@@ -570,6 +571,7 @@ impl str {
     /// assert_eq!(" Martin-Löf", last);
     /// ```
     #[inline]
+    #[must_use]
     #[stable(feature = "str_split_at", since = "1.4.0")]
     pub fn split_at(&self, mid: usize) -> (&str, &str) {
         // is_char_boundary checks that the index is in [0, .len()]
@@ -613,6 +615,7 @@ impl str {
     /// assert_eq!("PER Martin-Löf", s);
     /// ```
     #[inline]
+    #[must_use]
     #[stable(feature = "str_split_at", since = "1.4.0")]
     pub fn split_at_mut(&mut self, mid: usize) -> (&mut str, &mut str) {
         // is_char_boundary checks that the index is in [0, .len()]

library/core/src/str/validations.rs

@@ -251,6 +251,7 @@ static UTF8_CHAR_WIDTH: [u8; 256] = [
 
 /// Given a first byte, determines how many bytes are in this UTF-8 character.
 #[unstable(feature = "str_internals", issue = "none")]
+#[must_use]
 #[inline]
 pub fn utf8_char_width(b: u8) -> usize {
     UTF8_CHAR_WIDTH[b as usize] as usize

library/core/src/task/wake.rs

@@ -167,6 +167,7 @@ impl<'a> Context<'a> {
 
     /// Returns a reference to the `Waker` for the current task.
     #[stable(feature = "futures_api", since = "1.36.0")]
+    #[must_use]
     #[inline]
     pub fn waker(&self) -> &'a Waker {
         &self.waker
@@ -242,6 +243,7 @@ impl Waker {
     ///
     /// This function is primarily used for optimization purposes.
     #[inline]
+    #[must_use]
     #[stable(feature = "futures_api", since = "1.36.0")]
     pub fn will_wake(&self, other: &Waker) -> bool {
         self.waker == other.waker