add lint `manual_next_back`

checks for manual reverse iteration (`.rev().next()`) of a
`DoubleEndedIterator`

3 files changed, 88 insertions, 0 deletions

diff --git a/tests/ui/manual_next_back.fixed b/tests/ui/manual_next_back.fixed
new file mode 100644
index 00000000000..e8a47063ad6
--- /dev/null
+++ b/tests/ui/manual_next_back.fixed
@@ -0,0 +1,36 @@
+//@run-rustfix
+
+#![allow(unused)]
+#![warn(clippy::manual_next_back)]
+
+struct FakeIter(std::ops::Range<i32>);
+
+impl FakeIter {
+    fn rev(self) -> Self {
+        self
+    }
+
+    fn next(&self) {}
+}
+
+impl DoubleEndedIterator for FakeIter {
+    fn next_back(&mut self) -> Option<Self::Item> {
+        self.0.next_back()
+    }
+}
+
+impl Iterator for FakeIter {
+    type Item = i32;
+    fn next(&mut self) -> Option<Self::Item> {
+        self.0.next()
+    }
+}
+
+fn main() {
+    // should not lint
+    FakeIter(0..10).rev().next();
+
+    // should lint
+    let _ = (0..10).next_back().unwrap();
+    let _ = "something".bytes().next_back();
+}
diff --git a/tests/ui/manual_next_back.rs b/tests/ui/manual_next_back.rs
new file mode 100644
index 00000000000..9ec89242241
--- /dev/null
+++ b/tests/ui/manual_next_back.rs
@@ -0,0 +1,36 @@
+//@run-rustfix
+
+#![allow(unused)]
+#![warn(clippy::manual_next_back)]
+
+struct FakeIter(std::ops::Range<i32>);
+
+impl FakeIter {
+    fn rev(self) -> Self {
+        self
+    }
+
+    fn next(&self) {}
+}
+
+impl DoubleEndedIterator for FakeIter {
+    fn next_back(&mut self) -> Option<Self::Item> {
+        self.0.next_back()
+    }
+}
+
+impl Iterator for FakeIter {
+    type Item = i32;
+    fn next(&mut self) -> Option<Self::Item> {
+        self.0.next()
+    }
+}
+
+fn main() {
+    // should not lint
+    FakeIter(0..10).rev().next();
+
+    // should lint
+    let _ = (0..10).rev().next().unwrap();
+    let _ = "something".bytes().rev().next();
+}
diff --git a/tests/ui/manual_next_back.stderr b/tests/ui/manual_next_back.stderr
new file mode 100644
index 00000000000..94ccaa9e4cc
--- /dev/null
+++ b/tests/ui/manual_next_back.stderr
@@ -0,0 +1,16 @@
+error: manual backwards iteration
+  --> $DIR/manual_next_back.rs:34:20
+   |
+LL |     let _ = (0..10).rev().next().unwrap();
+   |                    ^^^^^^^^^^^^^ help: use: `.next_back()`
+   |
+   = note: `-D clippy::manual-next-back` implied by `-D warnings`
+
+error: manual backwards iteration
+  --> $DIR/manual_next_back.rs:35:32
+   |
+LL |     let _ = "something".bytes().rev().next();
+   |                                ^^^^^^^^^^^^^ help: use: `.next_back()`
+
+error: aborting due to 2 previous errors
+