Implement Iterator::last for vec::IntoIter

Author: thaliaarchi

Diff for: library/alloc/src/vec/into_iter.rs

@@ -258,6 +258,11 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
         self.len()
     }
 
+    #[inline]
+    fn last(mut self) -> Option<T> {
+        self.next_back()
+    }
+
     #[inline]
     fn next_chunk<const N: usize>(&mut self) -> Result<[T; N], core::array::IntoIter<T, N>> {
         let mut raw_ary = [const { MaybeUninit::uninit() }; N];

Diff for: src/tools/clippy/tests/ui/double_ended_iterator_last.fixed

@@ -85,8 +85,7 @@ fn drop_order() {
     }
 
     let v = vec![S("one"), S("two"), S("three")];
-    let mut v = v.into_iter();
-    println!("Last element is {}", v.next_back().unwrap().0);
-    //~^ ERROR: called `Iterator::last` on a `DoubleEndedIterator`
+    let v = v.into_iter();
+    println!("Last element is {}", v.last().unwrap().0);
     println!("Done");
 }

Diff for: src/tools/clippy/tests/ui/double_ended_iterator_last.rs

@@ -87,6 +87,5 @@ fn drop_order() {
     let v = vec![S("one"), S("two"), S("three")];
     let v = v.into_iter();
     println!("Last element is {}", v.last().unwrap().0);
-    //~^ ERROR: called `Iterator::last` on a `DoubleEndedIterator`
     println!("Done");
 }

Diff for: src/tools/clippy/tests/ui/double_ended_iterator_last.stderr

@@ -65,18 +65,5 @@ LL ~     let (mut subindex, _) = (index.by_ref().take(3), 42);
 LL ~     let _ = subindex.next_back();
    |
 
-error: called `Iterator::last` on a `DoubleEndedIterator`; this will needlessly iterate the entire iterator
-  --> tests/ui/double_ended_iterator_last.rs:89:36
-   |
-LL |     println!("Last element is {}", v.last().unwrap().0);
-   |                                    ^^^^^^^^
-   |
-   = note: this change will alter drop order which may be undesirable
-help: try
-   |
-LL ~     let mut v = v.into_iter();
-LL ~     println!("Last element is {}", v.next_back().unwrap().0);
-   |
-
-error: aborting due to 8 previous errors
+error: aborting due to 7 previous errors
 

Diff for: src/tools/clippy/tests/ui/double_ended_iterator_last_unfixable.rs

@@ -18,6 +18,5 @@ fn drop_order() {
     let v = vec![S("one"), S("two"), S("three")];
     let v = (v.into_iter(), 42);
     println!("Last element is {}", v.0.last().unwrap().0);
-    //~^ ERROR: called `Iterator::last` on a `DoubleEndedIterator`
     println!("Done");
 }

Diff for: src/tools/clippy/tests/ui/double_ended_iterator_last_unfixable.stderr

@@ -14,20 +14,5 @@ LL |     let _ = subindex.0.last();
    = note: `-D clippy::double-ended-iterator-last` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::double_ended_iterator_last)]`
 
-error: called `Iterator::last` on a `DoubleEndedIterator`; this will needlessly iterate the entire iterator
-  --> tests/ui/double_ended_iterator_last_unfixable.rs:20:36
-   |
-LL |     println!("Last element is {}", v.0.last().unwrap().0);
-   |                                    ^^^^------
-   |                                        |
-   |                                        help: try: `next_back()`
-   |
-   = note: this change will alter drop order which may be undesirable
-note: this must be made mutable to use `.next_back()`
-  --> tests/ui/double_ended_iterator_last_unfixable.rs:20:36
-   |
-LL |     println!("Last element is {}", v.0.last().unwrap().0);
-   |                                    ^^^
-
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error