Rollup merge of #90431 - jkugelman:must-use-std-o-through-z, r=joshtriplett

Add #[must_use] to remaining std functions (O-Z)

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 half of the remaining items from the `std` crate, from O-Z.

`panicking::take_hook` has a side effect: it unregisters the current panic hook, returning it. I almost ignored it, but the documentation example shows `let _ = panic::take_hook();`, so following suit I went ahead and added a `#[must_use]`.

```rust
std::panicking   fn take_hook() -> Box<dyn Fn(&PanicInfo<'_>) + 'static + Sync + Send>;
```

I added these functions that clippy did not flag:

```rust
std::path::Path   fn starts_with<P: AsRef<Path>>(&self, base: P) -> bool;
std::path::Path   fn ends_with<P: AsRef<Path>>(&self, child: P) -> bool;
std::path::Path   fn with_file_name<S: AsRef<OsStr>>(&self, file_name: S) -> PathBuf;
std::path::Path   fn with_extension<S: AsRef<OsStr>>(&self, extension: S) -> PathBuf;
```

Parent issue: #89692

r? `@joshtriplett`

library/std/src/os/unix/net/ancillary.rs

@@ -201,6 +201,7 @@ impl SocketCred {
     }
 
     /// Get the current PID.
+    #[must_use]
     #[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
     pub fn get_pid(&self) -> libc::pid_t {
         self.0.pid
@@ -213,6 +214,7 @@ impl SocketCred {
     }
 
     /// Get the current UID.
+    #[must_use]
     #[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
     pub fn get_uid(&self) -> libc::uid_t {
         self.0.uid
@@ -225,6 +227,7 @@ impl SocketCred {
     }
 
     /// Get the current GID.
+    #[must_use]
     #[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
     pub fn get_gid(&self) -> libc::gid_t {
         self.0.gid
@@ -330,6 +333,7 @@ impl<'a> AncillaryData<'a> {
 }
 
 /// This struct is used to iterate through the control messages.
+#[must_use = "iterators are lazy and do nothing unless consumed"]
 #[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
 pub struct Messages<'a> {
     buffer: &'a [u8],
@@ -425,6 +429,7 @@ impl<'a> SocketAncillary<'a> {
     }
 
     /// Returns the capacity of the buffer.
+    #[must_use]
     #[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
     pub fn capacity(&self) -> usize {
         self.buffer.len()
@@ -473,6 +478,7 @@ impl<'a> SocketAncillary<'a> {
     ///     Ok(())
     /// }
     /// ```
+    #[must_use]
     #[unstable(feature = "unix_socket_ancillary_data", issue = "76915")]
     pub fn truncated(&self) -> bool {
         self.truncated

library/std/src/os/unix/net/listener.rs

@@ -365,6 +365,7 @@ impl<'a> IntoIterator for &'a UnixListener {
 /// }
 /// ```
 #[derive(Debug)]
+#[must_use = "iterators are lazy and do nothing unless consumed"]
 #[stable(feature = "unix_socket", since = "1.10.0")]
 pub struct Incoming<'a> {
     listener: &'a UnixListener,

library/std/src/os/unix/process.rs

@@ -436,6 +436,7 @@ impl From<crate::process::ChildStderr> for OwnedFd {
 }
 
 /// Returns the OS-assigned process identifier associated with this process's parent.
+#[must_use]
 #[stable(feature = "unix_ppid", since = "1.27.0")]
 pub fn parent_id() -> u32 {
     crate::sys::os::getppid()

library/std/src/panicking.rs

@@ -160,6 +160,7 @@ pub fn set_hook(hook: Box<dyn Fn(&PanicInfo<'_>) + 'static + Sync + Send>) {
 ///
 /// panic!("Normal panic");
 /// ```
+#[must_use]
 #[stable(feature = "panic_hooks", since = "1.10.0")]
 pub fn take_hook() -> Box<dyn Fn(&PanicInfo<'_>) + 'static + Sync + Send> {
     if thread::panicking() {
@@ -284,11 +285,13 @@ pub mod panic_count {
     }
 
     // Disregards ALWAYS_ABORT_FLAG
+    #[must_use]
     pub fn get_count() -> usize {
         LOCAL_PANIC_COUNT.with(|c| c.get())
     }
 
     // Disregards ALWAYS_ABORT_FLAG
+    #[must_use]
     #[inline]
     pub fn count_is_zero() -> bool {
         if GLOBAL_PANIC_COUNT.load(Ordering::Relaxed) & !ALWAYS_ABORT_FLAG == 0 {

library/std/src/path.rs

@@ -422,6 +422,7 @@ impl<'a> PrefixComponent<'a> {
     /// See [`Prefix`]'s documentation for more information on the different
     /// kinds of prefixes.
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
     #[inline]
     pub fn kind(&self) -> Prefix<'a> {
         self.parsed
@@ -583,6 +584,7 @@ impl AsRef<Path> for Component<'_> {
 ///
 /// [`components`]: Path::components
 #[derive(Clone)]
+#[must_use = "iterators are lazy and do nothing unless consumed"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Components<'a> {
     // The path left to parse components from
@@ -609,6 +611,7 @@ pub struct Components<'a> {
 ///
 /// [`iter`]: Path::iter
 #[derive(Clone)]
+#[must_use = "iterators are lazy and do nothing unless consumed"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Iter<'a> {
     inner: Components<'a>,
@@ -1051,6 +1054,7 @@ fn compare_components(mut left: Components<'_>, mut right: Components<'_>) -> cm
 ///
 /// [`ancestors`]: Path::ancestors
 #[derive(Copy, Clone, Debug)]
+#[must_use = "iterators are lazy and do nothing unless consumed"]
 #[stable(feature = "path_ancestors", since = "1.28.0")]
 pub struct Ancestors<'a> {
     next: Option<&'a Path>,
@@ -1459,6 +1463,7 @@ impl PathBuf {
     ///
     /// [`capacity`]: OsString::capacity
     #[stable(feature = "path_buf_capacity", since = "1.44.0")]
+    #[must_use]
     #[inline]
     pub fn capacity(&self) -> usize {
         self.inner.capacity()
@@ -2103,6 +2108,7 @@ impl Path {
     /// assert_eq!(grand_parent.parent(), None);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
     pub fn parent(&self) -> Option<&Path> {
         let mut comps = self.components();
         let comp = comps.next_back();
@@ -2169,6 +2175,7 @@ impl Path {
     /// assert_eq!(None, Path::new("/").file_name());
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
     pub fn file_name(&self) -> Option<&OsStr> {
         self.components().next_back().and_then(|p| match p {
             Component::Normal(p) => Some(p),
@@ -2241,6 +2248,7 @@ impl Path {
     /// assert!(!Path::new("/etc/foo.rs").starts_with("/etc/foo"));
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
     pub fn starts_with<P: AsRef<Path>>(&self, base: P) -> bool {
         self._starts_with(base.as_ref())
     }
@@ -2268,6 +2276,7 @@ impl Path {
     /// assert!(!path.ends_with("conf")); // use .extension() instead
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
     pub fn ends_with<P: AsRef<Path>>(&self, child: P) -> bool {
         self._ends_with(child.as_ref())
     }
@@ -2303,6 +2312,7 @@ impl Path {
     /// [`Path::file_prefix`]: Path::file_prefix
     ///
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
     pub fn file_stem(&self) -> Option<&OsStr> {
         self.file_name().map(rsplit_file_at_dot).and_then(|(before, after)| before.or(after))
     }
@@ -2336,6 +2346,7 @@ impl Path {
     /// [`Path::file_stem`]: Path::file_stem
     ///
     #[unstable(feature = "path_file_prefix", issue = "86319")]
+    #[must_use]
     pub fn file_prefix(&self) -> Option<&OsStr> {
         self.file_name().map(split_file_at_dot).and_then(|(before, _after)| Some(before))
     }
@@ -2360,6 +2371,7 @@ impl Path {
     /// assert_eq!("gz", Path::new("foo.tar.gz").extension().unwrap());
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
     pub fn extension(&self) -> Option<&OsStr> {
         self.file_name().map(rsplit_file_at_dot).and_then(|(before, after)| before.and(after))
     }
@@ -2403,6 +2415,7 @@ impl Path {
     /// assert_eq!(path.with_file_name("var"), PathBuf::from("/var"));
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
     pub fn with_file_name<S: AsRef<OsStr>>(&self, file_name: S) -> PathBuf {
         self._with_file_name(file_name.as_ref())
     }
@@ -2660,6 +2673,7 @@ impl Path {
     /// This is a convenience function that coerces errors to false. If you want to
     /// check errors, call [`fs::metadata`].
     #[stable(feature = "path_ext", since = "1.5.0")]
+    #[must_use]
     #[inline]
     pub fn exists(&self) -> bool {
         fs::metadata(self).is_ok()
@@ -2786,6 +2800,7 @@ impl Path {
     /// Converts a [`Box<Path>`](Box) into a [`PathBuf`] without copying or
     /// allocating.
     #[stable(feature = "into_boxed_path", since = "1.20.0")]
+    #[must_use = "`self` will be dropped if the result is not used"]
     pub fn into_path_buf(self: Box<Path>) -> PathBuf {
         let rw = Box::into_raw(self) as *mut OsStr;
         let inner = unsafe { Box::from_raw(rw) };

library/std/src/process.rs

@@ -948,6 +948,7 @@ impl Command {
     /// let cmd = Command::new("echo");
     /// assert_eq!(cmd.get_program(), "echo");
     /// ```
+    #[must_use]
     #[stable(feature = "command_access", since = "1.57.0")]
     pub fn get_program(&self) -> &OsStr {
         self.inner.get_program()
@@ -1021,6 +1022,7 @@ impl Command {
     /// cmd.current_dir("/bin");
     /// assert_eq!(cmd.get_current_dir(), Some(Path::new("/bin")));
     /// ```
+    #[must_use]
     #[stable(feature = "command_access", since = "1.57.0")]
     pub fn get_current_dir(&self) -> Option<&Path> {
         self.inner.get_current_dir()
@@ -1053,6 +1055,7 @@ impl AsInnerMut<imp::Command> for Command {
 ///
 /// This struct is created by [`Command::get_args`]. See its documentation for
 /// more.
+#[must_use = "iterators are lazy and do nothing unless consumed"]
 #[stable(feature = "command_access", since = "1.57.0")]
 #[derive(Debug)]
 pub struct CommandArgs<'a> {
@@ -1183,6 +1186,7 @@ impl Stdio {
     /// its entire stdin before writing more than a pipe buffer's worth of output.
     /// The size of a pipe buffer varies on different targets.
     ///
+    #[must_use]
     #[stable(feature = "process", since = "1.0.0")]
     pub fn piped() -> Stdio {
         Stdio(imp::Stdio::MakePipe)
@@ -1222,6 +1226,7 @@ impl Stdio {
     /// print!("You piped in the reverse of: ");
     /// io::stdout().write_all(&output.stdout).unwrap();
     /// ```
+    #[must_use]
     #[stable(feature = "process", since = "1.0.0")]
     pub fn inherit() -> Stdio {
         Stdio(imp::Stdio::Inherit)
@@ -1261,6 +1266,7 @@ impl Stdio {
     /// assert_eq!(String::from_utf8_lossy(&output.stdout), "");
     /// // Ignores any piped-in input
     /// ```
+    #[must_use]
     #[stable(feature = "process", since = "1.0.0")]
     pub fn null() -> Stdio {
         Stdio(imp::Stdio::Null)
@@ -1462,6 +1468,7 @@ impl ExitStatus {
     ///     println!("failed to create 'projects/' directory: {}", status);
     /// }
     /// ```
+    #[must_use]
     #[stable(feature = "process", since = "1.0.0")]
     pub fn success(&self) -> bool {
         self.0.exit_ok().is_ok()
@@ -1493,6 +1500,7 @@ impl ExitStatus {
     ///     None       => println!("Process terminated by signal")
     /// }
     /// ```
+    #[must_use]
     #[stable(feature = "process", since = "1.0.0")]
     pub fn code(&self) -> Option<i32> {
         self.0.code()
@@ -1580,6 +1588,7 @@ impl ExitStatusError {
     /// assert_eq!(bad.code(), Some(1));
     /// # } // #[cfg(unix)]
     /// ```
+    #[must_use]
     pub fn code(&self) -> Option<i32> {
         self.code_nonzero().map(Into::into)
     }
@@ -1605,11 +1614,13 @@ impl ExitStatusError {
     /// assert_eq!(bad.code_nonzero().unwrap(), NonZeroI32::try_from(1).unwrap());
     /// # } // cfg!(unix)
     /// ```
+    #[must_use]
     pub fn code_nonzero(&self) -> Option<NonZeroI32> {
         self.0.code()
     }
 
     /// Converts an `ExitStatusError` (back) to an `ExitStatus`.
+    #[must_use]
     pub fn into_status(&self) -> ExitStatus {
         ExitStatus(self.0.into())
     }
@@ -1718,6 +1729,7 @@ impl Child {
     ///     println!("ls command didn't start");
     /// }
     /// ```
+    #[must_use]
     #[stable(feature = "process_id", since = "1.3.0")]
     pub fn id(&self) -> u32 {
         self.handle.id()
@@ -1988,6 +2000,7 @@ pub fn abort() -> ! {
 /// ```
 ///
 ///
+#[must_use]
 #[stable(feature = "getpid", since = "1.26.0")]
 pub fn id() -> u32 {
     crate::sys::os::getpid()

library/std/src/sync/condvar.rs

@@ -61,6 +61,7 @@ impl WaitTimeoutResult {
     ///     }
     /// }
     /// ```
+    #[must_use]
     #[stable(feature = "wait_timeout", since = "1.5.0")]
     pub fn timed_out(&self) -> bool {
         self.0

library/std/src/sync/mpsc/mod.rs

@@ -707,6 +707,7 @@ impl<T> UnsafeFlavor<T> for Receiver<T> {
 /// // Let's see what that answer was
 /// println!("{:?}", receiver.recv().unwrap());
 /// ```
+#[must_use]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn channel<T>() -> (Sender<T>, Receiver<T>) {
     let a = Arc::new(oneshot::Packet::new());
@@ -755,6 +756,7 @@ pub fn channel<T>() -> (Sender<T>, Receiver<T>) {
 /// assert_eq!(receiver.recv().unwrap(), 1);
 /// assert_eq!(receiver.recv().unwrap(), 2);
 /// ```
+#[must_use]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn sync_channel<T>(bound: usize) -> (SyncSender<T>, Receiver<T>) {
     let a = Arc::new(sync::Packet::new(bound));

library/std/src/sys_common/process.rs

@@ -106,6 +106,7 @@ impl CommandEnv {
 /// This struct is created by
 /// [`Command::get_envs`][crate::process::Command::get_envs]. See its
 /// documentation for more.
+#[must_use = "iterators are lazy and do nothing unless consumed"]
 #[stable(feature = "command_access", since = "1.57.0")]
 #[derive(Debug)]
 pub struct CommandEnvs<'a> {

library/std/src/thread/mod.rs

@@ -650,6 +650,7 @@ where
 ///
 /// handler.join().unwrap();
 /// ```
+#[must_use]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn current() -> Thread {
     thread_info::current_thread().expect(
@@ -738,6 +739,7 @@ pub fn yield_now() {
 ///
 /// [Mutex]: crate::sync::Mutex
 #[inline]
+#[must_use]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn panicking() -> bool {
     panicking::panicking()
@@ -1132,6 +1134,7 @@ impl Thread {
     /// assert!(thread::current().id() != other_thread_id);
     /// ```
     #[stable(feature = "thread_id", since = "1.19.0")]
+    #[must_use]
     pub fn id(&self) -> ThreadId {
         self.inner.id
     }
@@ -1174,6 +1177,7 @@ impl Thread {
     ///
     /// [naming-threads]: ./index.html#naming-threads
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
     pub fn name(&self) -> Option<&str> {
         self.cname().map(|s| unsafe { str::from_utf8_unchecked(s.to_bytes()) })
     }
@@ -1361,6 +1365,7 @@ impl<T> JoinHandle<T> {
     /// println!("thread id: {:?}", thread.id());
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
     pub fn thread(&self) -> &Thread {
         &self.0.thread
     }

library/std/src/time.rs

@@ -239,6 +239,7 @@ impl Instant {
     ///
     /// let now = Instant::now();
     /// ```
+    #[must_use]
     #[stable(feature = "time2", since = "1.8.0")]
     pub fn now() -> Instant {
         let os_now = time::Instant::now();
@@ -306,6 +307,7 @@ impl Instant {
     /// let new_now = Instant::now();
     /// println!("{:?}", new_now.duration_since(now));
     /// ```
+    #[must_use]
     #[stable(feature = "time2", since = "1.8.0")]
     pub fn duration_since(&self, earlier: Instant) -> Duration {
         self.0.checked_sub_instant(&earlier.0).expect("supplied instant is later than self")
@@ -326,6 +328,7 @@ impl Instant {
     /// println!("{:?}", new_now.checked_duration_since(now));
     /// println!("{:?}", now.checked_duration_since(new_now)); // None
     /// ```
+    #[must_use]
     #[stable(feature = "checked_duration_since", since = "1.39.0")]
     pub fn checked_duration_since(&self, earlier: Instant) -> Option<Duration> {
         self.0.checked_sub_instant(&earlier.0)
@@ -346,6 +349,7 @@ impl Instant {
     /// println!("{:?}", new_now.saturating_duration_since(now));
     /// println!("{:?}", now.saturating_duration_since(new_now)); // 0ns
     /// ```
+    #[must_use]
     #[stable(feature = "checked_duration_since", since = "1.39.0")]
     pub fn saturating_duration_since(&self, earlier: Instant) -> Duration {
         self.checked_duration_since(earlier).unwrap_or_default()
@@ -370,6 +374,7 @@ impl Instant {
     /// sleep(three_secs);
     /// assert!(instant.elapsed() >= three_secs);
     /// ```
+    #[must_use]
     #[stable(feature = "time2", since = "1.8.0")]
     pub fn elapsed(&self) -> Duration {
         Instant::now() - *self
@@ -476,6 +481,7 @@ impl SystemTime {
     ///
     /// let sys_time = SystemTime::now();
     /// ```
+    #[must_use]
     #[stable(feature = "time2", since = "1.8.0")]
     pub fn now() -> SystemTime {
         SystemTime(time::SystemTime::now())
@@ -644,6 +650,7 @@ impl SystemTimeError {
     ///     Err(e) => println!("SystemTimeError difference: {:?}", e.duration()),
     /// }
     /// ```
+    #[must_use]
     #[stable(feature = "time2", since = "1.8.0")]
     pub fn duration(&self) -> Duration {
         self.0

library/std/src/time/tests.rs

@@ -48,7 +48,7 @@ fn instant_monotonic_concurrent() -> crate::thread::Result<()> {
 #[test]
 fn instant_elapsed() {
     let a = Instant::now();
-    a.elapsed();
+    let _ = a.elapsed();
 }
 
 #[test]
@@ -93,7 +93,7 @@ fn instant_math_is_associative() {
 #[should_panic]
 fn instant_duration_since_panic() {
     let a = Instant::now();
-    (a - Duration::SECOND).duration_since(a);
+    let _ = (a - Duration::SECOND).duration_since(a);
 }
 
 #[test]