Rollup merge of #89778 - jkugelman:must-use-as_type-conversions, r=joshtriplett

Add #[must_use] to as_type conversions

Clippy missed these:

```rust
alloc::string::String   fn as_mut_str(&mut self) -> &mut str;
core::mem::NonNull<T>   unsafe fn as_uninit_mut<'a>(&mut self) -> &'a MaybeUninit<T>;
str                     unsafe fn as_bytes_mut(&mut self) -> &mut [u8];
str                     fn as_mut_ptr(&mut self) -> *mut u8;
```

Parent issue: #89692

r? ````@joshtriplett````

Author: the8472

Diff for: 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()

Diff for: library/alloc/src/collections/linked_list.rs

@@ -1385,6 +1385,7 @@ 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 for: 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);

Diff for: 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
@@ -1766,6 +1769,7 @@ 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 +2786,7 @@ 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 for: library/alloc/src/sync.rs

@@ -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);
@@ -1724,6 +1725,7 @@ 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);

Diff for: 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 for: 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 for: 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 for: 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
@@ -455,6 +460,7 @@ 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 +480,7 @@ 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 +525,7 @@ 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 +587,7 @@ 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 for: 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

Diff for: library/core/src/slice/iter.rs

@@ -124,6 +124,7 @@ impl<'a, T> Iter<'a, T> {
     /// // Now `as_slice` returns "[2, 3]":
     /// println!("{:?}", iter.as_slice());
     /// ```
+    #[must_use]
     #[stable(feature = "iter_to_slice", since = "1.4.0")]
     pub fn as_slice(&self) -> &'a [T] {
         self.make_slice()
@@ -298,6 +299,7 @@ impl<'a, T> IterMut<'a, T> {
     /// // Now `as_slice` returns "[2, 3]":
     /// assert_eq!(iter.as_slice(), &[2, 3]);
     /// ```
+    #[must_use]
     #[stable(feature = "slice_iter_mut_as_slice", since = "1.53.0")]
     pub fn as_slice(&self) -> &[T] {
         self.make_slice()

Diff for: library/core/src/str/iter.rs

@@ -109,6 +109,7 @@ impl<'a> Chars<'a> {
     /// assert_eq!(chars.as_str(), "");
     /// ```
     #[stable(feature = "iter_to_slice", since = "1.4.0")]
+    #[must_use]
     #[inline]
     pub fn as_str(&self) -> &'a str {
         // SAFETY: `Chars` is only made from a str, which guarantees the iter is valid UTF-8.
@@ -185,6 +186,7 @@ impl<'a> CharIndices<'a> {
     /// This has the same lifetime as the original slice, and so the
     /// iterator can continue to be used while this exists.
     #[stable(feature = "iter_to_slice", since = "1.4.0")]
+    #[must_use]
     #[inline]
     pub fn as_str(&self) -> &'a str {
         self.iter.as_str()
@@ -1247,6 +1249,7 @@ impl<'a> SplitWhitespace<'a> {
     /// assert_eq!(split.as_str(), "");
     /// ```
     #[inline]
+    #[must_use]
     #[unstable(feature = "str_split_whitespace_as_str", issue = "77998")]
     pub fn as_str(&self) -> &'a str {
         self.inner.iter.as_str()
@@ -1302,6 +1305,7 @@ impl<'a> SplitAsciiWhitespace<'a> {
     /// assert_eq!(split.as_str(), "");
     /// ```
     #[inline]
+    #[must_use]
     #[unstable(feature = "str_split_whitespace_as_str", issue = "77998")]
     pub fn as_str(&self) -> &'a str {
         if self.inner.iter.iter.finished {

Diff for: library/core/src/str/mod.rs

@@ -230,6 +230,7 @@ impl str {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_stable(feature = "str_as_bytes", since = "1.39.0")]
+    #[must_use]
     #[inline(always)]
     #[allow(unused_attributes)]
     pub const fn as_bytes(&self) -> &[u8] {
@@ -274,6 +275,7 @@ impl str {
     /// assert_eq!("🍔∈🌏", s);
     /// ```
     #[stable(feature = "str_mut_extras", since = "1.20.0")]
+    #[must_use]
     #[inline(always)]
     pub unsafe fn as_bytes_mut(&mut self) -> &mut [u8] {
         // SAFETY: the cast from `&str` to `&[u8]` is safe since `str`
@@ -304,6 +306,7 @@ impl str {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_stable(feature = "rustc_str_as_ptr", since = "1.32.0")]
+    #[must_use]
     #[inline]
     pub const fn as_ptr(&self) -> *const u8 {
         self as *const str as *const u8
@@ -318,6 +321,7 @@ impl str {
     /// It is your responsibility to make sure that the string slice only gets
     /// modified in a way that it remains valid UTF-8.
     #[stable(feature = "str_as_mut_ptr", since = "1.36.0")]
+    #[must_use]
     #[inline]
     pub fn as_mut_ptr(&mut self) -> *mut u8 {
         self as *mut str as *mut u8

Diff for: library/core/src/time.rs

@@ -334,6 +334,7 @@ impl Duration {
     /// [`subsec_nanos`]: Duration::subsec_nanos
     #[stable(feature = "duration", since = "1.3.0")]
     #[rustc_const_stable(feature = "duration", since = "1.32.0")]
+    #[must_use]
     #[inline]
     pub const fn as_secs(&self) -> u64 {
         self.secs
@@ -417,6 +418,7 @@ impl Duration {
     /// ```
     #[stable(feature = "duration_as_u128", since = "1.33.0")]
     #[rustc_const_stable(feature = "duration_as_u128", since = "1.33.0")]
+    #[must_use]
     #[inline]
     pub const fn as_millis(&self) -> u128 {
         self.secs as u128 * MILLIS_PER_SEC as u128 + (self.nanos / NANOS_PER_MILLI) as u128
@@ -434,6 +436,7 @@ impl Duration {
     /// ```
     #[stable(feature = "duration_as_u128", since = "1.33.0")]
     #[rustc_const_stable(feature = "duration_as_u128", since = "1.33.0")]
+    #[must_use]
     #[inline]
     pub const fn as_micros(&self) -> u128 {
         self.secs as u128 * MICROS_PER_SEC as u128 + (self.nanos / NANOS_PER_MICRO) as u128
@@ -451,6 +454,7 @@ impl Duration {
     /// ```
     #[stable(feature = "duration_as_u128", since = "1.33.0")]
     #[rustc_const_stable(feature = "duration_as_u128", since = "1.33.0")]
+    #[must_use]
     #[inline]
     pub const fn as_nanos(&self) -> u128 {
         self.secs as u128 * NANOS_PER_SEC as u128 + self.nanos as u128
@@ -674,6 +678,7 @@ impl Duration {
     /// assert_eq!(dur.as_secs_f64(), 2.7);
     /// ```
     #[stable(feature = "duration_float", since = "1.38.0")]
+    #[must_use]
     #[inline]
     #[rustc_const_unstable(feature = "duration_consts_2", issue = "72440")]
     pub const fn as_secs_f64(&self) -> f64 {
@@ -692,6 +697,7 @@ impl Duration {
     /// assert_eq!(dur.as_secs_f32(), 2.7);
     /// ```
     #[stable(feature = "duration_float", since = "1.38.0")]
+    #[must_use]
     #[inline]
     #[rustc_const_unstable(feature = "duration_consts_2", issue = "72440")]
     pub const fn as_secs_f32(&self) -> f32 {

Diff for: library/std/src/ffi/c_str.rs

@@ -297,6 +297,7 @@ impl FromVecWithNulError {
     ///
     /// assert_eq!(&bytes[..], value.unwrap_err().as_bytes());
     /// ```
+    #[must_use]
     pub fn as_bytes(&self) -> &[u8] {
         &self.bytes[..]
     }
@@ -618,6 +619,7 @@ impl CString {
     /// assert_eq!(bytes, &[b'f', b'o', b'o']);
     /// ```
     #[inline]
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn as_bytes(&self) -> &[u8] {
         // SAFETY: CString has a length at least 1
@@ -637,6 +639,7 @@ impl CString {
     /// assert_eq!(bytes, &[b'f', b'o', b'o', b'\0']);
     /// ```
     #[inline]
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn as_bytes_with_nul(&self) -> &[u8] {
         &self.inner
@@ -655,6 +658,7 @@ impl CString {
     ///            CStr::from_bytes_with_nul(b"foo\0").expect("CStr::from_bytes_with_nul failed"));
     /// ```
     #[inline]
+    #[must_use]
     #[stable(feature = "as_c_str", since = "1.20.0")]
     pub fn as_c_str(&self) -> &CStr {
         &*self
@@ -1313,6 +1317,7 @@ impl CStr {
     /// This way, the lifetime of the [`CString`] in `hello` encompasses
     /// the lifetime of `ptr` and the `unsafe` block.
     #[inline]
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_stable(feature = "const_str_as_ptr", since = "1.32.0")]
     pub const fn as_ptr(&self) -> *const c_char {

Diff for: library/std/src/ffi/os_str.rs

@@ -137,6 +137,7 @@ impl OsString {
     /// assert_eq!(os_string.as_os_str(), os_str);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
     #[inline]
     pub fn as_os_str(&self) -> &OsStr {
         self

Diff for: library/std/src/os/unix/net/addr.rs

@@ -192,6 +192,7 @@ impl SocketAddr {
     /// }
     /// ```
     #[stable(feature = "unix_socket", since = "1.10.0")]
+    #[must_use]
     pub fn as_pathname(&self) -> Option<&Path> {
         if let AddressKind::Pathname(path) = self.address() { Some(path) } else { None }
     }