Rollup merge of #89899 - jkugelman:must-use-alloc, r=joshtriplett

Add #[must_use] to remaining alloc functions

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 everything remaining from the `alloc` crate.

I ignored these because they might be used to purposefully leak memory... or other allocator shenanigans? I dunno. I'll add them if y'all tell me to.

```rust
alloc::alloc          unsafe fn alloc(layout: Layout) -> *mut u8;
alloc::alloc          unsafe fn alloc_zeroed(layout: Layout) -> *mut u8;
alloc::sync::Arc<T>   fn into_raw(this: Self) -> *const T;
```

I don't know why clippy ignored these. I added them myself:

```rust
alloc::collections::btree_map::BTreeMap<K, V>   fn range<T: ?Sized, R>(&self, range: R) -> Range<'_, K, V>;
alloc::collections::btree_set::BTreeSet<T>      fn range<K: ?Sized, R>(&self, range: R) -> Range<'_, T>;
```

I added these non-mutating `mut` functions:

```rust
alloc::collections::btree_map::BTreeMap<K, V>     fn range_mut<T: ?Sized, R>(&mut self, range: R) -> RangeMut<'_, K, V>;
alloc::collections::btree_map::BTreeMap<K, V>     fn iter_mut(&mut self) -> IterMut<'_, K, V>;
alloc::collections::btree_map::BTreeMap<K, V>     fn values_mut(&mut self) -> ValuesMut<'_, K, V>;
alloc::collections::linked_list::LinkedList<T>    fn iter_mut(&mut self) -> IterMut<'_, T>;
alloc::collections::linked_list::LinkedList<T>    fn cursor_front_mut(&mut self) -> CursorMut<'_, T>;
alloc::collections::linked_list::LinkedList<T>    fn cursor_back_mut(&mut self) -> CursorMut<'_, T>;
alloc::collections::linked_list::LinkedList<T>    fn front_mut(&mut self) -> Option<&mut T>;
alloc::collections::linked_list::LinkedList<T>    fn back_mut(&mut self) -> Option<&mut T>;
alloc::collections::linked_list::CursorMut<'a, T> fn current(&mut self) -> Option<&mut T>;
alloc::collections::linked_list::CursorMut<'a, T> fn peek_next(&mut self) -> Option<&mut T>;
alloc::collections::linked_list::CursorMut<'a, T> fn peek_prev(&mut self) -> Option<&mut T>;
alloc::collections::linked_list::CursorMut<'a, T> fn front_mut(&mut self) -> Option<&mut T>;
alloc::collections::linked_list::CursorMut<'a, T> fn back_mut(&mut self) -> Option<&mut T>;
```

I moved a few existing `#[must_use]`s from functions onto the iterator types they return: `IntoIterSorted`, `IntoKeys`, `IntoValues`.

Parent issue: #89692

r? `@joshtriplett`

1 files changed, 2 insertions, 0 deletions

diff --git a/library/alloc/src/string.rs b/library/alloc/src/string.rs
index 85ee7ea9438..d2471f164fc 100644
--- a/library/alloc/src/string.rs
+++ b/library/alloc/src/string.rs
@@ -898,6 +898,7 @@ impl String {
     /// assert!(s.capacity() >= 10);
     /// ```
     #[inline]
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn capacity(&self) -> usize {
         self.vec.capacity()
@@ -1823,6 +1824,7 @@ impl FromUtf8Error {
     /// // the first byte is invalid here
     /// assert_eq!(1, error.valid_up_to());
     /// ```
+    #[must_use]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn utf8_error(&self) -> Utf8Error {
         self.error