Implement Iterator::last for vec::IntoIter

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];

library/std/src/sys/args/common.rs

@@ -49,8 +49,8 @@ impl Iterator for Args {
     }
 
     #[inline]
-    fn last(mut self) -> Option<OsString> {
-        self.iter.next_back()
+    fn last(self) -> Option<OsString> {
+        self.iter.last()
     }
 
     #[inline]

src/tools/clippy/tests/ui/double_ended_iterator_last.fixed

@@ -84,6 +84,19 @@ fn issue_14139() {
 }
 
 fn drop_order() {
+    struct DropDeIterator(std::vec::IntoIter<S>);
+    impl Iterator for DropDeIterator {
+        type Item = S;
+        fn next(&mut self) -> Option<Self::Item> {
+            self.0.next()
+        }
+    }
+    impl DoubleEndedIterator for DropDeIterator {
+        fn next_back(&mut self) -> Option<Self::Item> {
+            self.0.next_back()
+        }
+    }
+
     struct S(&'static str);
     impl std::ops::Drop for S {
         fn drop(&mut self) {
@@ -92,7 +105,7 @@ fn drop_order() {
     }
 
     let v = vec![S("one"), S("two"), S("three")];
-    let mut v = v.into_iter();
+    let mut v = DropDeIterator(v.into_iter());
     println!("Last element is {}", v.next_back().unwrap().0);
     //~^ ERROR: called `Iterator::last` on a `DoubleEndedIterator`
     println!("Done");

src/tools/clippy/tests/ui/double_ended_iterator_last.rs

@@ -84,6 +84,19 @@ fn issue_14139() {
 }
 
 fn drop_order() {
+    struct DropDeIterator(std::vec::IntoIter<S>);
+    impl Iterator for DropDeIterator {
+        type Item = S;
+        fn next(&mut self) -> Option<Self::Item> {
+            self.0.next()
+        }
+    }
+    impl DoubleEndedIterator for DropDeIterator {
+        fn next_back(&mut self) -> Option<Self::Item> {
+            self.0.next_back()
+        }
+    }
+
     struct S(&'static str);
     impl std::ops::Drop for S {
         fn drop(&mut self) {
@@ -92,7 +105,7 @@ fn drop_order() {
     }
 
     let v = vec![S("one"), S("two"), S("three")];
-    let v = v.into_iter();
+    let v = DropDeIterator(v.into_iter());
     println!("Last element is {}", v.last().unwrap().0);
     //~^ ERROR: called `Iterator::last` on a `DoubleEndedIterator`
     println!("Done");

src/tools/clippy/tests/ui/double_ended_iterator_last.stderr

@@ -18,15 +18,15 @@ LL |     let _ = DeIterator.last();
    |                        help: try: `next_back()`
 
 error: called `Iterator::last` on a `DoubleEndedIterator`; this will needlessly iterate the entire iterator
-  --> tests/ui/double_ended_iterator_last.rs:96:36
+  --> tests/ui/double_ended_iterator_last.rs:109:36
    |
 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 ~     let mut v = DropDeIterator(v.into_iter());
 LL ~     println!("Last element is {}", v.next_back().unwrap().0);
    |
 

src/tools/clippy/tests/ui/double_ended_iterator_last_unfixable.rs

@@ -11,6 +11,19 @@ fn main() {
 }
 
 fn drop_order() {
+    struct DropDeIterator(std::vec::IntoIter<S>);
+    impl Iterator for DropDeIterator {
+        type Item = S;
+        fn next(&mut self) -> Option<Self::Item> {
+            self.0.next()
+        }
+    }
+    impl DoubleEndedIterator for DropDeIterator {
+        fn next_back(&mut self) -> Option<Self::Item> {
+            self.0.next_back()
+        }
+    }
+
     struct S(&'static str);
     impl std::ops::Drop for S {
         fn drop(&mut self) {
@@ -19,7 +32,7 @@ fn drop_order() {
     }
 
     let v = vec![S("one"), S("two"), S("three")];
-    let v = (v.into_iter(), 42);
+    let v = (DropDeIterator(v.into_iter()), 42);
     println!("Last element is {}", v.0.last().unwrap().0);
     //~^ ERROR: called `Iterator::last` on a `DoubleEndedIterator`
     println!("Done");

src/tools/clippy/tests/ui/double_ended_iterator_last_unfixable.stderr

@@ -1,5 +1,5 @@
 error: called `Iterator::last` on a `DoubleEndedIterator`; this will needlessly iterate the entire iterator
-  --> tests/ui/double_ended_iterator_last_unfixable.rs:23:36
+  --> tests/ui/double_ended_iterator_last_unfixable.rs:36:36
    |
 LL |     println!("Last element is {}", v.0.last().unwrap().0);
    |                                    ^^^^------
@@ -8,7 +8,7 @@ LL |     println!("Last element is {}", v.0.last().unwrap().0);
    |
    = 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:23:36
+  --> tests/ui/double_ended_iterator_last_unfixable.rs:36:36
    |
 LL |     println!("Last element is {}", v.0.last().unwrap().0);
    |                                    ^^^