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````

5 files changed, 13 insertions, 0 deletions

diff --git a/library/std/src/ffi/c_str.rs b/library/std/src/ffi/c_str.rs
index 8b51c4450c2..78eb91e4193 100644
--- a/library/std/src/ffi/c_str.rs
+++ b/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 --git a/library/std/src/ffi/os_str.rs b/library/std/src/ffi/os_str.rs
index de745f21649..3b0e8833144 100644
--- a/library/std/src/ffi/os_str.rs
+++ b/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 --git a/library/std/src/os/unix/net/addr.rs b/library/std/src/os/unix/net/addr.rs
index 62bfde8bfd4..d741096893a 100644
--- a/library/std/src/os/unix/net/addr.rs
+++ b/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 }
     }
diff --git a/library/std/src/path.rs b/library/std/src/path.rs
index a8a79fb9c8f..d9f18db2fa7 100644
--- a/library/std/src/path.rs
+++ b/library/std/src/path.rs
@@ -427,6 +427,7 @@ impl<'a> PrefixComponent<'a> {
 
     /// Returns the raw [`OsStr`] slice for this prefix.
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
     #[inline]
     pub fn as_os_str(&self) -> &'a OsStr {
         self.raw
@@ -676,6 +677,7 @@ impl<'a> Components<'a> {
     ///
     /// assert_eq!(Path::new("foo/bar.txt"), components.as_path());
     /// ```
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn as_path(&self) -> &'a Path {
         let mut comps = self.clone();
@@ -821,6 +823,7 @@ impl<'a> Iter<'a> {
     /// assert_eq!(Path::new("foo/bar.txt"), iter.as_path());
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
     #[inline]
     pub fn as_path(&self) -> &'a Path {
         self.inner.as_path()
@@ -1188,6 +1191,7 @@ impl PathBuf {
     /// assert_eq!(Path::new("/test"), p.as_path());
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
     #[inline]
     pub fn as_path(&self) -> &Path {
         self
@@ -1923,6 +1927,7 @@ impl Path {
     /// assert_eq!(os_str, std::ffi::OsStr::new("foo.txt"));
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
     #[inline]
     pub fn as_os_str(&self) -> &OsStr {
         &self.inner
diff --git a/library/std/src/thread/mod.rs b/library/std/src/thread/mod.rs
index 7d404aff30e..82847d83ad2 100644
--- a/library/std/src/thread/mod.rs
+++ b/library/std/src/thread/mod.rs
@@ -1031,6 +1031,7 @@ impl ThreadId {
     /// value is entirely opaque -- only equality testing is stable. Note that
     /// it is not guaranteed which values new threads will return, and this may
     /// change across Rust versions.
+    #[must_use]
     #[unstable(feature = "thread_id_value", issue = "67939")]
     pub fn as_u64(&self) -> NonZeroU64 {
         self.0