Rollup merge of #90430 - jkugelman:must-use-std-a-through-n, r=joshtriplett

Add #[must_use] to remaining std functions (A-N)

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 A-N.

I added these functions myself. Clippy predictably ignored the `mut` ones, but I don't know why the rest weren't flagged. Check them closely, please? Maybe I overlooked good reasons.

```rust
std::backtrace::Backtrace                                   const fn disabled() -> Backtrace;
std::backtrace::Backtrace<'a>                               fn frames(&'a self) -> &'a [BacktraceFrame];
std::collections::hash_map::RawOccupiedEntryMut<'a, K, V>   fn key_mut(&mut self) -> &mut K;
std::collections::hash_map::RawOccupiedEntryMut<'a, K, V>   fn get_mut(&mut self) -> &mut V;
std::collections::hash_map::RawOccupiedEntryMut<'a, K, V>   fn get_key_value(&mut self) -> (&K, &V);
std::collections::hash_map::RawOccupiedEntryMut<'a, K, V>   fn get_key_value_mut(&mut self) -> (&mut K, &mut V);
std::env                                                    fn var_os<K: AsRef<OsStr>>(key: K) -> Option<OsString>;
std::env                                                    fn split_paths<T: AsRef<OsStr> + ?Sized>(unparsed: &T) -> SplitPaths<'_>;
std::io::Error                                              fn get_mut(&mut self) -> Option<&mut (dyn error::Error + Send + Sync + 'static)>;
```

Parent issue: #89692

r? `@joshtriplett`

2 files changed, 5 insertions, 0 deletions

diff --git a/library/std/src/ffi/c_str.rs b/library/std/src/ffi/c_str.rs
index b7822b40a7c..465bbae8631 100644
--- a/library/std/src/ffi/c_str.rs
+++ b/library/std/src/ffi/c_str.rs
@@ -1009,6 +1009,7 @@ impl NulError {
     /// let nul_error = CString::new("foo bar\0").unwrap_err();
     /// assert_eq!(nul_error.nul_position(), 7);
     /// ```
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn nul_position(&self) -> usize {
         self.0
@@ -1107,6 +1108,7 @@ impl IntoStringError {
     }
 
     /// Access the underlying UTF-8 error that was the cause of this error.
+    #[must_use]
     #[stable(feature = "cstring_into", since = "1.7.0")]
     pub fn utf8_error(&self) -> Utf8Error {
         self.error
@@ -1456,6 +1458,7 @@ impl CStr {
     /// let boxed = c_string.into_boxed_c_str();
     /// assert_eq!(boxed.into_c_string(), CString::new("foo").expect("CString::new failed"));
     /// ```
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[stable(feature = "into_boxed_c_str", since = "1.20.0")]
     pub fn into_c_string(self: Box<CStr>) -> CString {
         let raw = Box::into_raw(self) as *mut [u8];
diff --git a/library/std/src/ffi/os_str.rs b/library/std/src/ffi/os_str.rs
index d7587e85c66..0f9912fa64d 100644
--- a/library/std/src/ffi/os_str.rs
+++ b/library/std/src/ffi/os_str.rs
@@ -239,6 +239,7 @@ impl OsString {
     /// assert!(os_string.capacity() >= 10);
     /// ```
     #[stable(feature = "osstring_simple_functions", since = "1.9.0")]
+    #[must_use]
     #[inline]
     pub fn capacity(&self) -> usize {
         self.inner.capacity()
@@ -709,6 +710,7 @@ impl OsStr {
 
     /// Converts a <code>[Box]<[OsStr]></code> into an [`OsString`] without copying or allocating.
     #[stable(feature = "into_boxed_os_str", since = "1.20.0")]
+    #[must_use = "`self` will be dropped if the result is not used"]
     pub fn into_os_string(self: Box<OsStr>) -> OsString {
         let boxed = unsafe { Box::from_raw(Box::into_raw(self) as *mut Slice) };
         OsString { inner: Buf::from_box(boxed) }