Auto merge of rust-lang#89755 - jkugelman:must-use-conversions-that-move-self, r=joshtriplett

Add #[must_use] to conversions that move self

Everything here got the same message. Is the wording okay?

```rust
#[must_use = "`self` will be dropped if the result is not used"]
```

I want to draw attention to these methods in particular:

```rust
alloc::sync::Arc<MaybeUninit<T>>     unsafe fn assume_init(self) -> Arc<T>;
alloc::sync::Arc<[MaybeUninit<T>]>   unsafe fn assume_init(self) -> Arc<[T]>;
core::pin::Pin<&'a mut T>            const fn into_ref(self) -> Pin<&'a T>;
core::pin::Pin<&'a mut T>            const fn get_mut(self) -> &'a mut T;
core::pin::Pin<&'a mut T>            const unsafe fn get_unchecked_mut(self) -> &'a mut T;
core::pin::Pin<&'a mut T>            unsafe fn map_unchecked_mut(self, func: F) -> Pin<&'a mut U>;
core::pin::Pin<&'a mut Pin<P>>       fn as_deref_mut(self) -> Pin<&'a mut P::Target>;
```

Parent issue: rust-lang#89692

r? `@joshtriplett`

Author: bors

library/alloc/src/collections/binary_heap.rs

@@ -848,6 +848,7 @@ impl<T> BinaryHeap<T> {
     ///
     /// assert_eq!(heap.into_iter_sorted().take(2).collect::<Vec<_>>(), vec![5, 4]);
     /// ```
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[unstable(feature = "binary_heap_into_iter_sorted", issue = "59278")]
     pub fn into_iter_sorted(self) -> IntoIterSorted<T> {
         IntoIterSorted { inner: self }
@@ -1028,6 +1029,7 @@ impl<T> BinaryHeap<T> {
     ///     println!("{}", x);
     /// }
     /// ```
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[stable(feature = "binary_heap_extras_15", since = "1.5.0")]
     pub fn into_vec(self) -> Vec<T> {
         self.into()

library/alloc/src/collections/btree/map.rs

@@ -1264,6 +1264,7 @@ impl<K, V> BTreeMap<K, V> {
     /// assert_eq!(keys, [1, 2]);
     /// ```
     #[inline]
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[stable(feature = "map_into_keys_values", since = "1.54.0")]
     pub fn into_keys(self) -> IntoKeys<K, V> {
         IntoKeys { inner: self.into_iter() }
@@ -1286,6 +1287,7 @@ impl<K, V> BTreeMap<K, V> {
     /// assert_eq!(values, ["hello", "goodbye"]);
     /// ```
     #[inline]
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[stable(feature = "map_into_keys_values", since = "1.54.0")]
     pub fn into_values(self) -> IntoValues<K, V> {
         IntoValues { inner: self.into_iter() }

library/alloc/src/collections/btree/map/entry.rs

@@ -448,6 +448,7 @@ impl<'a, K: Ord, V> OccupiedEntry<'a, K, V> {
     /// }
     /// assert_eq!(map["poneyland"], 22);
     /// ```
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn into_mut(self) -> &'a mut V {
         self.handle.into_val_mut()

library/alloc/src/collections/btree/map/tests.rs

@@ -1755,20 +1755,20 @@ fn test_send() {
 #[test]
 fn test_ord_absence() {
     fn map<K>(mut map: BTreeMap<K, ()>) {
-        map.is_empty();
-        map.len();
+        let _ = map.is_empty();
+        let _ = map.len();
         map.clear();
-        map.iter();
-        map.iter_mut();
-        map.keys();
-        map.values();
-        map.values_mut();
+        let _ = map.iter();
+        let _ = map.iter_mut();
+        let _ = map.keys();
+        let _ = map.values();
+        let _ = map.values_mut();
         if true {
-            map.into_values();
+            let _ = map.into_values();
         } else if true {
-            map.into_iter();
+            let _ = map.into_iter();
         } else {
-            map.into_keys();
+            let _ = map.into_keys();
         }
     }
 

library/alloc/src/rc.rs

@@ -2130,6 +2130,7 @@ impl<T: ?Sized> Weak<T> {
     ///
     /// [`from_raw`]: Weak::from_raw
     /// [`as_ptr`]: Weak::as_ptr
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[stable(feature = "weak_into_raw", since = "1.45.0")]
     pub fn into_raw(self) -> *const T {
         let result = self.as_ptr();

library/alloc/src/string.rs

@@ -676,6 +676,7 @@ impl String {
     /// let rebuilt = unsafe { String::from_raw_parts(ptr, len, cap) };
     /// assert_eq!(rebuilt, "hello");
     /// ```
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[unstable(feature = "vec_into_raw_parts", reason = "new API", issue = "65816")]
     pub fn into_raw_parts(self) -> (*mut u8, usize, usize) {
         self.vec.into_raw_parts()
@@ -781,6 +782,7 @@ impl String {
     /// assert_eq!(&[104, 101, 108, 108, 111][..], &bytes[..]);
     /// ```
     #[inline]
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn into_bytes(self) -> Vec<u8> {
         self.vec
@@ -1738,6 +1740,7 @@ impl String {
     /// ```
     #[cfg(not(no_global_oom_handling))]
     #[stable(feature = "box_str", since = "1.4.0")]
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[inline]
     pub fn into_boxed_str(self) -> Box<str> {
         let slice = self.vec.into_boxed_slice();
@@ -1783,6 +1786,7 @@ impl FromUtf8Error {
     ///
     /// assert_eq!(vec![0, 159], value.unwrap_err().into_bytes());
     /// ```
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn into_bytes(self) -> Vec<u8> {
         self.bytes

library/alloc/src/sync.rs

@@ -735,6 +735,7 @@ impl<T> Arc<mem::MaybeUninit<T>> {
     /// assert_eq!(*five, 5)
     /// ```
     #[unstable(feature = "new_uninit", issue = "63291")]
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[inline]
     pub unsafe fn assume_init(self) -> Arc<T> {
         Arc::from_inner(mem::ManuallyDrop::new(self).ptr.cast())
@@ -776,6 +777,7 @@ impl<T> Arc<[mem::MaybeUninit<T>]> {
     /// assert_eq!(*values, [1, 2, 3])
     /// ```
     #[unstable(feature = "new_uninit", issue = "63291")]
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[inline]
     pub unsafe fn assume_init(self) -> Arc<[T]> {
         unsafe { Arc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _) }
@@ -1759,6 +1761,7 @@ impl<T: ?Sized> Weak<T> {
     ///
     /// [`from_raw`]: Weak::from_raw
     /// [`as_ptr`]: Weak::as_ptr
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[stable(feature = "weak_into_raw", since = "1.45.0")]
     pub fn into_raw(self) -> *const T {
         let result = self.as_ptr();

library/core/src/option.rs

@@ -1473,6 +1473,7 @@ impl<T: Copy> Option<&mut 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")]
     pub fn copied(self) -> Option<T> {
         self.map(|&mut t| t)
@@ -1492,6 +1493,7 @@ impl<T: Clone> Option<&T> {
     /// let cloned = opt_x.cloned();
     /// assert_eq!(cloned, Some(12));
     /// ```
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn cloned(self) -> Option<T> {
         self.map(|t| t.clone())

library/core/src/pin.rs

@@ -715,6 +715,7 @@ impl<'a, T: ?Sized> Pin<&'a T> {
 impl<'a, T: ?Sized> Pin<&'a mut T> {
     /// Converts this `Pin<&mut T>` into a `Pin<&T>` with the same lifetime.
     #[inline(always)]
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[rustc_const_unstable(feature = "const_pin", issue = "76654")]
     #[stable(feature = "pin", since = "1.33.0")]
     pub const fn into_ref(self) -> Pin<&'a T> {
@@ -731,6 +732,7 @@ impl<'a, T: ?Sized> Pin<&'a mut T> {
     /// the `Pin` itself. This method allows turning the `Pin` into a reference
     /// with the same lifetime as the original `Pin`.
     #[inline(always)]
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[stable(feature = "pin", since = "1.33.0")]
     #[rustc_const_unstable(feature = "const_pin", issue = "76654")]
     pub const fn get_mut(self) -> &'a mut T
@@ -751,6 +753,7 @@ impl<'a, T: ?Sized> Pin<&'a mut T> {
     /// If the underlying data is `Unpin`, `Pin::get_mut` should be used
     /// instead.
     #[inline(always)]
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[stable(feature = "pin", since = "1.33.0")]
     #[rustc_const_unstable(feature = "const_pin", issue = "76654")]
     pub const unsafe fn get_unchecked_mut(self) -> &'a mut T {
@@ -772,6 +775,7 @@ impl<'a, T: ?Sized> Pin<&'a mut T> {
     /// not move out of the argument you receive to the interior function.
     ///
     /// [`pin` module]: self#projections-and-structural-pinning
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[stable(feature = "pin", since = "1.33.0")]
     pub unsafe fn map_unchecked_mut<U, F>(self, func: F) -> Pin<&'a mut U>
     where
@@ -811,6 +815,7 @@ impl<'a, P: DerefMut> Pin<&'a mut Pin<P>> {
     /// implementations of `P::DerefMut` are likewise ruled out by the contract of
     /// `Pin::new_unchecked`.
     #[unstable(feature = "pin_deref_mut", issue = "86918")]
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[inline(always)]
     pub fn as_deref_mut(self) -> Pin<&'a mut P::Target> {
         // SAFETY: What we're asserting here is that going from

library/core/src/ptr/unique.rs

@@ -101,6 +101,7 @@ impl<T: ?Sized> Unique<T> {
     }
 
     /// Acquires the underlying `*mut` pointer.
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[inline]
     pub const fn as_ptr(self) -> *mut T {
         self.pointer as *mut T
@@ -131,6 +132,7 @@ impl<T: ?Sized> Unique<T> {
     }
 
     /// Casts to a pointer of another type.
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[inline]
     pub const fn cast<U>(self) -> Unique<U> {
         // SAFETY: Unique::new_unchecked() creates a new unique and needs

library/core/src/slice/iter.rs

@@ -267,6 +267,7 @@ impl<'a, T> IterMut<'a, T> {
     /// // Now slice is "[2, 2, 3]":
     /// println!("{:?}", slice);
     /// ```
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[stable(feature = "iter_to_slice", since = "1.4.0")]
     pub fn into_slice(self) -> &'a mut [T] {
         // SAFETY: the iterator was created from a mutable slice with pointer
@@ -1869,6 +1870,7 @@ impl<'a, T> ChunksExactMut<'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 = "`self` will be dropped if the result is not used"]
     #[stable(feature = "chunks_exact", since = "1.31.0")]
     pub fn into_remainder(self) -> &'a mut [T] {
         self.rem
@@ -2264,6 +2266,7 @@ impl<'a, T, const N: usize> ArrayChunksMut<'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 = "`self` will be dropped if the result is not used"]
     #[unstable(feature = "array_chunks", issue = "74985")]
     pub fn into_remainder(self) -> &'a mut [T] {
         self.rem
@@ -2875,6 +2878,7 @@ impl<'a, T> RChunksExactMut<'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 = "`self` will be dropped if the result is not used"]
     #[stable(feature = "rchunks", since = "1.31.0")]
     pub fn into_remainder(self) -> &'a mut [T] {
         self.rem

library/std/src/collections/hash/map.rs

@@ -1720,6 +1720,7 @@ impl<'a, K, V, S> RawOccupiedEntryMut<'a, K, V, S> {
     /// Converts the entry into a mutable reference to the key in the entry
     /// with a lifetime bound to the map itself.
     #[inline]
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[unstable(feature = "hash_raw_entry", issue = "56167")]
     pub fn into_key(self) -> &'a mut K {
         self.base.into_key()
@@ -1735,6 +1736,7 @@ impl<'a, K, V, S> RawOccupiedEntryMut<'a, K, V, S> {
     /// Converts the `OccupiedEntry` into a mutable reference to the value in the entry
     /// with a lifetime bound to the map itself.
     #[inline]
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[unstable(feature = "hash_raw_entry", issue = "56167")]
     pub fn into_mut(self) -> &'a mut V {
         self.base.into_mut()
@@ -1764,6 +1766,7 @@ impl<'a, K, V, S> RawOccupiedEntryMut<'a, K, V, S> {
     /// Converts the `OccupiedEntry` into a mutable reference to the key and value in the entry
     /// with a lifetime bound to the map itself.
     #[inline]
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[unstable(feature = "hash_raw_entry", issue = "56167")]
     pub fn into_key_value(self) -> (&'a mut K, &'a mut V) {
         self.base.into_key_value()

library/std/src/ffi/c_str.rs

@@ -322,6 +322,7 @@ impl FromVecWithNulError {
     ///
     /// assert_eq!(bytes, value.unwrap_err().into_bytes());
     /// ```
+    #[must_use = "`self` will be dropped if the result is not used"]
     pub fn into_bytes(self) -> Vec<u8> {
         self.bytes
     }
@@ -524,6 +525,7 @@ impl CString {
     /// }
     /// ```
     #[inline]
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[stable(feature = "cstr_memory", since = "1.4.0")]
     pub fn into_raw(self) -> *mut c_char {
         Box::into_raw(self.into_inner()) as *mut c_char
@@ -547,7 +549,6 @@ impl CString {
     /// let err = cstring.into_string().err().expect("into_string().err() failed");
     /// assert_eq!(err.utf8_error().valid_up_to(), 1);
     /// ```
-
     #[stable(feature = "cstring_into", since = "1.7.0")]
     pub fn into_string(self) -> Result<String, IntoStringError> {
         String::from_utf8(self.into_bytes()).map_err(|e| IntoStringError {
@@ -571,6 +572,7 @@ impl CString {
     /// let bytes = c_string.into_bytes();
     /// assert_eq!(bytes, vec![b'f', b'o', b'o']);
     /// ```
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[stable(feature = "cstring_into", since = "1.7.0")]
     pub fn into_bytes(self) -> Vec<u8> {
         let mut vec = self.into_inner().into_vec();
@@ -591,6 +593,7 @@ impl CString {
     /// let bytes = c_string.into_bytes_with_nul();
     /// assert_eq!(bytes, vec![b'f', b'o', b'o', b'\0']);
     /// ```
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[stable(feature = "cstring_into", since = "1.7.0")]
     pub fn into_bytes_with_nul(self) -> Vec<u8> {
         self.into_inner().into_vec()
@@ -667,6 +670,7 @@ impl CString {
     /// assert_eq!(&*boxed,
     ///            CStr::from_bytes_with_nul(b"foo\0").expect("CStr::from_bytes_with_nul 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_boxed_c_str(self) -> Box<CStr> {
         unsafe { Box::from_raw(Box::into_raw(self.into_inner()) as *mut CStr) }
@@ -1018,6 +1022,7 @@ impl NulError {
     /// let nul_error = CString::new("foo\0bar").unwrap_err();
     /// assert_eq!(nul_error.into_vec(), b"foo\0bar");
     /// ```
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn into_vec(self) -> Vec<u8> {
         self.1
@@ -1092,6 +1097,7 @@ impl fmt::Display for FromVecWithNulError {
 impl IntoStringError {
     /// Consumes this error, returning original [`CString`] which generated the
     /// error.
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[stable(feature = "cstring_into", since = "1.7.0")]
     pub fn into_cstring(self) -> CString {
         self.inner

library/std/src/ffi/os_str.rs

@@ -346,6 +346,7 @@ impl OsString {
     ///
     /// let b: Box<OsStr> = s.into_boxed_os_str();
     /// ```
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[stable(feature = "into_boxed_os_str", since = "1.20.0")]
     pub fn into_boxed_os_str(self) -> Box<OsStr> {
         let rw = Box::into_raw(self.inner.into_box()) as *mut OsStr;

library/std/src/io/buffered/bufwriter.rs

@@ -476,6 +476,7 @@ pub struct WriterPanicked {
 impl WriterPanicked {
     /// Returns the perhaps-unwritten data.  Some of this data may have been written by the
     /// panicking call(s) to the underlying writer, so simply writing it again is not a good idea.
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[stable(feature = "bufwriter_into_parts", since = "1.56.0")]
     pub fn into_inner(self) -> Vec<u8> {
         self.buf

library/std/src/io/error.rs

@@ -657,6 +657,7 @@ impl Error {
     /// }
     /// ```
     #[stable(feature = "io_error_inner", since = "1.3.0")]
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[inline]
     pub fn into_inner(self) -> Option<Box<dyn error::Error + Send + Sync>> {
         match self.repr {

library/std/src/io/stdio.rs

@@ -464,6 +464,7 @@ impl Stdin {
     ///     println!("got a line: {}", line.unwrap());
     /// }
     /// ```
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[unstable(feature = "stdin_forwarders", issue = "87096")]
     pub fn lines(self) -> Lines<StdinLock<'static>> {
         self.into_locked().lines()

library/std/src/net/tcp.rs

@@ -883,6 +883,7 @@ impl TcpListener {
     ///     Ok(())
     /// }
     /// ```
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[unstable(feature = "tcplistener_into_incoming", issue = "88339")]
     pub fn into_incoming(self) -> IntoIncoming {
         IntoIncoming { listener: self }

library/std/src/path.rs

@@ -532,6 +532,7 @@ impl<'a> Component<'a> {
     /// let components: Vec<_> = path.components().map(|comp| comp.as_os_str()).collect();
     /// assert_eq!(&components, &[".", "tmp", "foo", "bar.txt"]);
     /// ```
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn as_os_str(self) -> &'a OsStr {
         match self {
@@ -1428,13 +1429,15 @@ impl PathBuf {
     /// let os_str = p.into_os_string();
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[inline]
     pub fn into_os_string(self) -> OsString {
         self.inner
     }
 
     /// Converts this `PathBuf` into a [boxed](Box) [`Path`].
     #[stable(feature = "into_boxed_path", since = "1.20.0")]
+    #[must_use = "`self` will be dropped if the result is not used"]
     #[inline]
     pub fn into_boxed_path(self) -> Box<Path> {
         let rw = Box::into_raw(self.inner.into_boxed_os_str()) as *mut Path;