Add lint for calling `Iterator::last()` on `DoubleEndedIterator` (#13922)

I [recently realized that `.last()` might not call `next_back()` when it
is
available](https://qsantos.fr/2025/01/01/rust-gotcha-last-on-doubleendediterator/).
Although the implementor could make sure to implement `last()` to do so,
this is not what will happen by default. As a result, I think it is
useful to add a lint to promote using `.next_back()` over `.last()` on
`DoubleEndedIterator`.

If this is merged, we might want to close #1822.

changelog: [`double_ended_iterator_last`]: Add lint for calling
`Iterator::last()` on `DoubleEndedIterator`

9 files changed, 157 insertions, 24 deletions

diff --git a/tests/ui/double_ended_iterator_last.fixed b/tests/ui/double_ended_iterator_last.fixed
new file mode 100644
index 00000000000..06c48e33753
--- /dev/null
+++ b/tests/ui/double_ended_iterator_last.fixed
@@ -0,0 +1,53 @@
+#![warn(clippy::double_ended_iterator_last)]
+
+// Typical case
+pub fn last_arg(s: &str) -> Option<&str> {
+    s.split(' ').next_back()
+}
+
+fn main() {
+    // General case
+    struct DeIterator;
+    impl Iterator for DeIterator {
+        type Item = ();
+        fn next(&mut self) -> Option<Self::Item> {
+            Some(())
+        }
+    }
+    impl DoubleEndedIterator for DeIterator {
+        fn next_back(&mut self) -> Option<Self::Item> {
+            Some(())
+        }
+    }
+    let _ = DeIterator.next_back();
+    // Should not apply to other methods of Iterator
+    let _ = DeIterator.count();
+
+    // Should not apply to simple iterators
+    struct SimpleIterator;
+    impl Iterator for SimpleIterator {
+        type Item = ();
+        fn next(&mut self) -> Option<Self::Item> {
+            Some(())
+        }
+    }
+    let _ = SimpleIterator.last();
+
+    // Should not apply to custom implementations of last()
+    struct CustomLast;
+    impl Iterator for CustomLast {
+        type Item = ();
+        fn next(&mut self) -> Option<Self::Item> {
+            Some(())
+        }
+        fn last(self) -> Option<Self::Item> {
+            Some(())
+        }
+    }
+    impl DoubleEndedIterator for CustomLast {
+        fn next_back(&mut self) -> Option<Self::Item> {
+            Some(())
+        }
+    }
+    let _ = CustomLast.last();
+}
diff --git a/tests/ui/double_ended_iterator_last.rs b/tests/ui/double_ended_iterator_last.rs
new file mode 100644
index 00000000000..9c13b496d11
--- /dev/null
+++ b/tests/ui/double_ended_iterator_last.rs
@@ -0,0 +1,53 @@
+#![warn(clippy::double_ended_iterator_last)]
+
+// Typical case
+pub fn last_arg(s: &str) -> Option<&str> {
+    s.split(' ').last()
+}
+
+fn main() {
+    // General case
+    struct DeIterator;
+    impl Iterator for DeIterator {
+        type Item = ();
+        fn next(&mut self) -> Option<Self::Item> {
+            Some(())
+        }
+    }
+    impl DoubleEndedIterator for DeIterator {
+        fn next_back(&mut self) -> Option<Self::Item> {
+            Some(())
+        }
+    }
+    let _ = DeIterator.last();
+    // Should not apply to other methods of Iterator
+    let _ = DeIterator.count();
+
+    // Should not apply to simple iterators
+    struct SimpleIterator;
+    impl Iterator for SimpleIterator {
+        type Item = ();
+        fn next(&mut self) -> Option<Self::Item> {
+            Some(())
+        }
+    }
+    let _ = SimpleIterator.last();
+
+    // Should not apply to custom implementations of last()
+    struct CustomLast;
+    impl Iterator for CustomLast {
+        type Item = ();
+        fn next(&mut self) -> Option<Self::Item> {
+            Some(())
+        }
+        fn last(self) -> Option<Self::Item> {
+            Some(())
+        }
+    }
+    impl DoubleEndedIterator for CustomLast {
+        fn next_back(&mut self) -> Option<Self::Item> {
+            Some(())
+        }
+    }
+    let _ = CustomLast.last();
+}
diff --git a/tests/ui/double_ended_iterator_last.stderr b/tests/ui/double_ended_iterator_last.stderr
new file mode 100644
index 00000000000..b795c18a736
--- /dev/null
+++ b/tests/ui/double_ended_iterator_last.stderr
@@ -0,0 +1,17 @@
+error: called `Iterator::last` on a `DoubleEndedIterator`; this will needlessly iterate the entire iterator
+  --> tests/ui/double_ended_iterator_last.rs:5:18
+   |
+LL |     s.split(' ').last()
+   |                  ^^^^^^ help: try: `next_back()`
+   |
+   = 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.rs:22:24
+   |
+LL |     let _ = DeIterator.last();
+   |                        ^^^^^^ help: try: `next_back()`
+
+error: aborting due to 2 previous errors
+
diff --git a/tests/ui/infinite_iter.rs b/tests/ui/infinite_iter.rs
index da95ba04b82..178e300ff5b 100644
--- a/tests/ui/infinite_iter.rs
+++ b/tests/ui/infinite_iter.rs
@@ -1,4 +1,4 @@
-#![allow(clippy::uninlined_format_args)]
+#![allow(clippy::uninlined_format_args, clippy::double_ended_iterator_last)]
 
 use std::iter::repeat;
 fn square_is_lower_64(x: &u32) -> bool {
diff --git a/tests/ui/iter_overeager_cloned.fixed b/tests/ui/iter_overeager_cloned.fixed
index 7d8a584b022..d7d3d299349 100644
--- a/tests/ui/iter_overeager_cloned.fixed
+++ b/tests/ui/iter_overeager_cloned.fixed
@@ -1,5 +1,10 @@
 #![warn(clippy::iter_overeager_cloned, clippy::redundant_clone, clippy::filter_next)]
-#![allow(dead_code, clippy::let_unit_value, clippy::useless_vec)]
+#![allow(
+    dead_code,
+    clippy::let_unit_value,
+    clippy::useless_vec,
+    clippy::double_ended_iterator_last
+)]
 
 fn main() {
     let vec = vec!["1".to_string(), "2".to_string(), "3".to_string()];
diff --git a/tests/ui/iter_overeager_cloned.rs b/tests/ui/iter_overeager_cloned.rs
index 58c374ab8cd..45e1349febd 100644
--- a/tests/ui/iter_overeager_cloned.rs
+++ b/tests/ui/iter_overeager_cloned.rs
@@ -1,5 +1,10 @@
 #![warn(clippy::iter_overeager_cloned, clippy::redundant_clone, clippy::filter_next)]
-#![allow(dead_code, clippy::let_unit_value, clippy::useless_vec)]
+#![allow(
+    dead_code,
+    clippy::let_unit_value,
+    clippy::useless_vec,
+    clippy::double_ended_iterator_last
+)]
 
 fn main() {
     let vec = vec!["1".to_string(), "2".to_string(), "3".to_string()];
diff --git a/tests/ui/iter_overeager_cloned.stderr b/tests/ui/iter_overeager_cloned.stderr
index 7a822a79494..e6680266f10 100644
--- a/tests/ui/iter_overeager_cloned.stderr
+++ b/tests/ui/iter_overeager_cloned.stderr
@@ -1,5 +1,5 @@
 error: unnecessarily eager cloning of iterator items
-  --> tests/ui/iter_overeager_cloned.rs:7:29
+  --> tests/ui/iter_overeager_cloned.rs:12:29
    |
 LL |     let _: Option<String> = vec.iter().cloned().last();
    |                             ^^^^^^^^^^----------------
@@ -10,7 +10,7 @@ LL |     let _: Option<String> = vec.iter().cloned().last();
    = help: to override `-D warnings` add `#[allow(clippy::iter_overeager_cloned)]`
 
 error: unnecessarily eager cloning of iterator items
-  --> tests/ui/iter_overeager_cloned.rs:9:29
+  --> tests/ui/iter_overeager_cloned.rs:14:29
    |
 LL |     let _: Option<String> = vec.iter().chain(vec.iter()).cloned().next();
    |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^----------------
@@ -18,7 +18,7 @@ LL |     let _: Option<String> = vec.iter().chain(vec.iter()).cloned().next();
    |                                                         help: try: `.next().cloned()`
 
 error: unneeded cloning of iterator items
-  --> tests/ui/iter_overeager_cloned.rs:11:20
+  --> tests/ui/iter_overeager_cloned.rs:16:20
    |
 LL |     let _: usize = vec.iter().filter(|x| x == &"2").cloned().count();
    |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^-----------------
@@ -29,7 +29,7 @@ LL |     let _: usize = vec.iter().filter(|x| x == &"2").cloned().count();
    = help: to override `-D warnings` add `#[allow(clippy::redundant_clone)]`
 
 error: unnecessarily eager cloning of iterator items
-  --> tests/ui/iter_overeager_cloned.rs:13:21
+  --> tests/ui/iter_overeager_cloned.rs:18:21
    |
 LL |     let _: Vec<_> = vec.iter().cloned().take(2).collect();
    |                     ^^^^^^^^^^-----------------
@@ -37,7 +37,7 @@ LL |     let _: Vec<_> = vec.iter().cloned().take(2).collect();
    |                               help: try: `.take(2).cloned()`
 
 error: unnecessarily eager cloning of iterator items
-  --> tests/ui/iter_overeager_cloned.rs:15:21
+  --> tests/ui/iter_overeager_cloned.rs:20:21
    |
 LL |     let _: Vec<_> = vec.iter().cloned().skip(2).collect();
    |                     ^^^^^^^^^^-----------------
@@ -45,7 +45,7 @@ LL |     let _: Vec<_> = vec.iter().cloned().skip(2).collect();
    |                               help: try: `.skip(2).cloned()`
 
 error: unnecessarily eager cloning of iterator items
-  --> tests/ui/iter_overeager_cloned.rs:17:13
+  --> tests/ui/iter_overeager_cloned.rs:22:13
    |
 LL |     let _ = vec.iter().filter(|x| x == &"2").cloned().nth(2);
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^----------------
@@ -53,7 +53,7 @@ LL |     let _ = vec.iter().filter(|x| x == &"2").cloned().nth(2);
    |                                             help: try: `.nth(2).cloned()`
 
 error: unnecessarily eager cloning of iterator items
-  --> tests/ui/iter_overeager_cloned.rs:19:13
+  --> tests/ui/iter_overeager_cloned.rs:24:13
    |
 LL |       let _ = [Some(Some("str".to_string())), Some(Some("str".to_string()))]
    |  _____________^
@@ -69,7 +69,7 @@ LL ~         .flatten().cloned();
    |
 
 error: unnecessarily eager cloning of iterator items
-  --> tests/ui/iter_overeager_cloned.rs:24:13
+  --> tests/ui/iter_overeager_cloned.rs:29:13
    |
 LL |     let _ = vec.iter().cloned().filter(|x| x.starts_with('2'));
    |             ^^^^^^^^^^----------------------------------------
@@ -77,7 +77,7 @@ LL |     let _ = vec.iter().cloned().filter(|x| x.starts_with('2'));
    |                       help: try: `.filter(|&x| x.starts_with('2')).cloned()`
 
 error: unnecessarily eager cloning of iterator items
-  --> tests/ui/iter_overeager_cloned.rs:26:13
+  --> tests/ui/iter_overeager_cloned.rs:31:13
    |
 LL |     let _ = vec.iter().cloned().find(|x| x == "2");
    |             ^^^^^^^^^^----------------------------
@@ -85,7 +85,7 @@ LL |     let _ = vec.iter().cloned().find(|x| x == "2");
    |                       help: try: `.find(|&x| x == "2").cloned()`
 
 error: unnecessarily eager cloning of iterator items
-  --> tests/ui/iter_overeager_cloned.rs:30:17
+  --> tests/ui/iter_overeager_cloned.rs:35:17
    |
 LL |         let _ = vec.iter().cloned().filter(f);
    |                 ^^^^^^^^^^-------------------
@@ -93,7 +93,7 @@ LL |         let _ = vec.iter().cloned().filter(f);
    |                           help: try: `.filter(|&x| f(x)).cloned()`
 
 error: unnecessarily eager cloning of iterator items
-  --> tests/ui/iter_overeager_cloned.rs:31:17
+  --> tests/ui/iter_overeager_cloned.rs:36:17
    |
 LL |         let _ = vec.iter().cloned().find(f);
    |                 ^^^^^^^^^^-----------------
@@ -101,7 +101,7 @@ LL |         let _ = vec.iter().cloned().find(f);
    |                           help: try: `.find(|&x| f(x)).cloned()`
 
 error: unnecessarily eager cloning of iterator items
-  --> tests/ui/iter_overeager_cloned.rs:37:17
+  --> tests/ui/iter_overeager_cloned.rs:42:17
    |
 LL |         let _ = vec.iter().cloned().filter(f);
    |                 ^^^^^^^^^^-------------------
@@ -109,7 +109,7 @@ LL |         let _ = vec.iter().cloned().filter(f);
    |                           help: try: `.filter(|&x| f(x)).cloned()`
 
 error: unnecessarily eager cloning of iterator items
-  --> tests/ui/iter_overeager_cloned.rs:38:17
+  --> tests/ui/iter_overeager_cloned.rs:43:17
    |
 LL |         let _ = vec.iter().cloned().find(f);
    |                 ^^^^^^^^^^-----------------
@@ -117,7 +117,7 @@ LL |         let _ = vec.iter().cloned().find(f);
    |                           help: try: `.find(|&x| f(x)).cloned()`
 
 error: unnecessarily eager cloning of iterator items
-  --> tests/ui/iter_overeager_cloned.rs:45:9
+  --> tests/ui/iter_overeager_cloned.rs:50:9
    |
 LL |         iter.cloned().filter(move |(&a, b)| a == 1 && b == &target)
    |         ^^^^-------------------------------------------------------
@@ -125,7 +125,7 @@ LL |         iter.cloned().filter(move |(&a, b)| a == 1 && b == &target)
    |             help: try: `.filter(move |&(&a, b)| a == 1 && b == &target).cloned()`
 
 error: unnecessarily eager cloning of iterator items
-  --> tests/ui/iter_overeager_cloned.rs:56:13
+  --> tests/ui/iter_overeager_cloned.rs:61:13
    |
 LL |             iter.cloned().filter(move |S { a, b }| **a == 1 && b == &target)
    |             ^^^^------------------------------------------------------------
@@ -133,7 +133,7 @@ LL |             iter.cloned().filter(move |S { a, b }| **a == 1 && b == &target
    |                 help: try: `.filter(move |&S { a, b }| **a == 1 && b == &target).cloned()`
 
 error: unneeded cloning of iterator items
-  --> tests/ui/iter_overeager_cloned.rs:60:13
+  --> tests/ui/iter_overeager_cloned.rs:65:13
    |
 LL |     let _ = vec.iter().cloned().map(|x| x.len());
    |             ^^^^^^^^^^--------------------------
@@ -141,7 +141,7 @@ LL |     let _ = vec.iter().cloned().map(|x| x.len());
    |                       help: try: `.map(|x| x.len())`
 
 error: unneeded cloning of iterator items
-  --> tests/ui/iter_overeager_cloned.rs:65:13
+  --> tests/ui/iter_overeager_cloned.rs:70:13
    |
 LL |     let _ = vec.iter().cloned().for_each(|x| assert!(!x.is_empty()));
    |             ^^^^^^^^^^----------------------------------------------
@@ -149,7 +149,7 @@ LL |     let _ = vec.iter().cloned().for_each(|x| assert!(!x.is_empty()));
    |                       help: try: `.for_each(|x| assert!(!x.is_empty()))`
 
 error: unneeded cloning of iterator items
-  --> tests/ui/iter_overeager_cloned.rs:67:13
+  --> tests/ui/iter_overeager_cloned.rs:72:13
    |
 LL |     let _ = vec.iter().cloned().all(|x| x.len() == 1);
    |             ^^^^^^^^^^-------------------------------
@@ -157,7 +157,7 @@ LL |     let _ = vec.iter().cloned().all(|x| x.len() == 1);
    |                       help: try: `.all(|x| x.len() == 1)`
 
 error: unneeded cloning of iterator items
-  --> tests/ui/iter_overeager_cloned.rs:69:13
+  --> tests/ui/iter_overeager_cloned.rs:74:13
    |
 LL |     let _ = vec.iter().cloned().any(|x| x.len() == 1);
    |             ^^^^^^^^^^-------------------------------
diff --git a/tests/ui/starts_ends_with.fixed b/tests/ui/starts_ends_with.fixed
index 4a66ca7ec91..252b6e5a98c 100644
--- a/tests/ui/starts_ends_with.fixed
+++ b/tests/ui/starts_ends_with.fixed
@@ -1,4 +1,4 @@
-#![allow(clippy::needless_if, dead_code, unused_must_use)]
+#![allow(clippy::needless_if, dead_code, unused_must_use, clippy::double_ended_iterator_last)]
 
 fn main() {}
 
diff --git a/tests/ui/starts_ends_with.rs b/tests/ui/starts_ends_with.rs
index 16a68e02d66..6c5655f3178 100644
--- a/tests/ui/starts_ends_with.rs
+++ b/tests/ui/starts_ends_with.rs
@@ -1,4 +1,4 @@
-#![allow(clippy::needless_if, dead_code, unused_must_use)]
+#![allow(clippy::needless_if, dead_code, unused_must_use, clippy::double_ended_iterator_last)]
 
 fn main() {}