filter_next: check for filter().next_back() (#15748)

Jarcho · web-flow · commit abdc61fb88fa · 2025-09-26T08:02:18.000Z
Closes: #15715 changelog: [`filter_next`]: suggest replacing `filter().next_back()` with `rfind()` for `DoubleEndedIterator`
diff --git a/clippy_lints/src/methods/filter_next.rs b/clippy_lints/src/methods/filter_next.rs
@@ -10,51 +10,68 @@ use rustc_span::sym;
 
 use super::FILTER_NEXT;
 
-/// lint use of `filter().next()` for `Iterators`
+#[derive(Copy, Clone)]
+pub(super) enum Direction {
+    Forward,
+    Backward,
+}
+
+/// lint use of `filter().next()` for `Iterator` and `filter().next_back()` for
+/// `DoubleEndedIterator`
 pub(super) fn check<'tcx>(
     cx: &LateContext<'tcx>,
     expr: &'tcx hir::Expr<'_>,
     recv: &'tcx hir::Expr<'_>,
     filter_arg: &'tcx hir::Expr<'_>,
+    direction: Direction,
 ) {
-    // lint if caller of `.filter().next()` is an Iterator
-    let recv_impls_iterator = cx
+    // lint if caller of `.filter().next()` is an Iterator or `.filter().next_back()` is a
+    // DoubleEndedIterator
+    let (required_trait, next_method, find_method) = match direction {
+        Direction::Forward => (sym::Iterator, "next", "find"),
+        Direction::Backward => (sym::DoubleEndedIterator, "next_back", "rfind"),
+    };
+    if !cx
         .tcx
-        .get_diagnostic_item(sym::Iterator)
-        .is_some_and(|id| implements_trait(cx, cx.typeck_results().expr_ty(recv), id, &[]));
-    if recv_impls_iterator {
-        let msg = "called `filter(..).next()` on an `Iterator`. This is more succinctly expressed by calling \
-                   `.find(..)` instead";
-        let filter_snippet = snippet(cx, filter_arg.span, "..");
-        if filter_snippet.lines().count() <= 1 {
-            let iter_snippet = snippet(cx, recv.span, "..");
-            // add note if not multi-line
-            span_lint_and_then(cx, FILTER_NEXT, expr.span, msg, |diag| {
-                let (applicability, pat) = if let Some(id) = path_to_local_with_projections(recv)
-                    && let hir::Node::Pat(pat) = cx.tcx.hir_node(id)
-                    && let hir::PatKind::Binding(BindingMode(_, Mutability::Not), _, ident, _) = pat.kind
-                {
-                    (Applicability::Unspecified, Some((pat.span, ident)))
-                } else {
-                    (Applicability::MachineApplicable, None)
-                };
+        .get_diagnostic_item(required_trait)
+        .is_some_and(|id| implements_trait(cx, cx.typeck_results().expr_ty(recv), id, &[]))
+    {
+        return;
+    }
+    let msg = format!(
+        "called `filter(..).{next_method}()` on an `{}`. This is more succinctly expressed by calling \
+                   `.{find_method}(..)` instead",
+        required_trait.as_str()
+    );
+    let filter_snippet = snippet(cx, filter_arg.span, "..");
+    if filter_snippet.lines().count() <= 1 {
+        let iter_snippet = snippet(cx, recv.span, "..");
+        // add note if not multi-line
+        span_lint_and_then(cx, FILTER_NEXT, expr.span, msg, |diag| {
+            let (applicability, pat) = if let Some(id) = path_to_local_with_projections(recv)
+                && let hir::Node::Pat(pat) = cx.tcx.hir_node(id)
+                && let hir::PatKind::Binding(BindingMode(_, Mutability::Not), _, ident, _) = pat.kind
+            {
+                (Applicability::Unspecified, Some((pat.span, ident)))
+            } else {
+                (Applicability::MachineApplicable, None)
+            };
 
-                diag.span_suggestion(
-                    expr.span,
-                    "try",
-                    format!("{iter_snippet}.find({filter_snippet})"),
-                    applicability,
-                );
+            diag.span_suggestion(
+                expr.span,
+                "try",
+                format!("{iter_snippet}.{find_method}({filter_snippet})"),
+                applicability,
+            );
 
-                if let Some((pat_span, ident)) = pat {
-                    diag.span_help(
-                        pat_span,
-                        format!("you will also need to make `{ident}` mutable, because `find` takes `&mut self`"),
-                    );
-                }
-            });
-        } else {
-            span_lint(cx, FILTER_NEXT, expr.span, msg);
-        }
+            if let Some((pat_span, ident)) = pat {
+                diag.span_help(
+                    pat_span,
+                    format!("you will also need to make `{ident}` mutable, because `{find_method}` takes `&mut self`"),
+                );
+            }
+        });
+    } else {
+        span_lint(cx, FILTER_NEXT, expr.span, msg);
     }
 }
diff --git a/clippy_lints/src/methods/mod.rs b/clippy_lints/src/methods/mod.rs
@@ -5367,7 +5367,9 @@ impl Methods {
                                 iter_overeager_cloned::Op::LaterCloned,
                                 false,
                             ),
-                            (sym::filter, [arg]) => filter_next::check(cx, expr, recv2, arg),
+                            (sym::filter, [arg]) => {
+                                filter_next::check(cx, expr, recv2, arg, filter_next::Direction::Forward);
+                            },
                             (sym::filter_map, [arg]) => filter_map_next::check(cx, expr, recv2, arg, self.msrv),
                             (sym::iter, []) => iter_next_slice::check(cx, expr, recv2),
                             (sym::skip, [arg]) => iter_skip_next::check(cx, expr, recv2, arg),
@@ -5377,6 +5379,14 @@ impl Methods {
                         }
                     }
                 },
+                (sym::next_back, []) => {
+                    if let Some((name2, recv2, args2, _, _)) = method_call(recv)
+                        && let (sym::filter, [arg]) = (name2, args2)
+                        && self.msrv.meets(cx, msrvs::DOUBLE_ENDED_ITERATOR_RFIND)
+                    {
+                        filter_next::check(cx, expr, recv2, arg, filter_next::Direction::Backward);
+                    }
+                },
                 (sym::nth, [n_arg]) => match method_call(recv) {
                     Some((sym::bytes, recv2, [], _, _)) => bytes_nth::check(cx, expr, recv2, n_arg),
                     Some((sym::cloned, recv2, [], _, _)) => iter_overeager_cloned::check(
diff --git a/clippy_utils/src/msrvs.rs b/clippy_utils/src/msrvs.rs
@@ -72,7 +72,7 @@ msrv_aliases! {
     1,30,0 { ITERATOR_FIND_MAP, TOOL_ATTRIBUTES }
     1,29,0 { ITER_FLATTEN }
     1,28,0 { FROM_BOOL, REPEAT_WITH, SLICE_FROM_REF }
-    1,27,0 { ITERATOR_TRY_FOLD }
+    1,27,0 { ITERATOR_TRY_FOLD, DOUBLE_ENDED_ITERATOR_RFIND }
     1,26,0 { RANGE_INCLUSIVE, STRING_RETAIN, POINTER_ADD_SUB_METHODS }
     1,24,0 { IS_ASCII_DIGIT, PTR_NULL }
     1,18,0 { HASH_MAP_RETAIN, HASH_SET_RETAIN }
diff --git a/tests/ui/auxiliary/option_helpers.rs b/tests/ui/auxiliary/option_helpers.rs
@@ -25,6 +25,10 @@ impl IteratorFalsePositives {
         self
     }
 
+    pub fn next_back(self) -> IteratorFalsePositives {
+        self
+    }
+
     pub fn find(self) -> Option<u32> {
         Some(self.foo)
     }
diff --git a/tests/ui/methods.rs b/tests/ui/methods.rs
@@ -135,6 +135,26 @@ fn filter_next() {
     let _ = foo.filter(42).next();
 }
 
+#[rustfmt::skip]
+fn filter_next_back() {
+    let v = vec![3, 2, 1, 0, -1, -2, -3];
+
+    // Multi-line case.
+    let _ = v.iter().filter(|&x| {
+    //~^ filter_next
+                                *x < 0
+                            }
+                   ).next_back();
+    
+    // Check that we don't lint if the caller is not an `Iterator`.
+    let foo = IteratorFalsePositives { foo: 0 };
+    let _ = foo.filter().next_back();
+
+    let foo = IteratorMethodFalsePositives {};
+    let _ = foo.filter(42).next_back();
+}
+
 fn main() {
     filter_next();
+    filter_next_back();
 }
diff --git a/tests/ui/methods.stderr b/tests/ui/methods.stderr
@@ -24,5 +24,16 @@ LL | |                    ).next();
    = note: `-D clippy::filter-next` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::filter_next)]`
 
-error: aborting due to 2 previous errors
+error: called `filter(..).next_back()` on an `DoubleEndedIterator`. This is more succinctly expressed by calling `.rfind(..)` instead
+  --> tests/ui/methods.rs:143:13
+   |
+LL |       let _ = v.iter().filter(|&x| {
+   |  _____________^
+LL | |
+LL | |                                 *x < 0
+LL | |                             }
+LL | |                    ).next_back();
+   | |________________________________^
+
+error: aborting due to 3 previous errors
 
diff --git a/tests/ui/methods_fixable.fixed b/tests/ui/methods_fixable.fixed
@@ -8,4 +8,18 @@ fn main() {
     // Single-line case.
     let _ = v.iter().find(|&x| *x < 0);
     //~^ filter_next
+
+    let _ = v.iter().rfind(|&x| *x < 0);
+    //~^ filter_next
+}
+
+#[clippy::msrv = "1.27"]
+fn msrv_1_27() {
+    let _ = vec![1].into_iter().rfind(|&x| x < 0);
+    //~^ filter_next
+}
+
+#[clippy::msrv = "1.26"]
+fn msrv_1_26() {
+    let _ = vec![1].into_iter().filter(|&x| x < 0).next_back();
 }
diff --git a/tests/ui/methods_fixable.rs b/tests/ui/methods_fixable.rs
@@ -8,4 +8,18 @@ fn main() {
     // Single-line case.
     let _ = v.iter().filter(|&x| *x < 0).next();
     //~^ filter_next
+
+    let _ = v.iter().filter(|&x| *x < 0).next_back();
+    //~^ filter_next
+}
+
+#[clippy::msrv = "1.27"]
+fn msrv_1_27() {
+    let _ = vec![1].into_iter().filter(|&x| x < 0).next_back();
+    //~^ filter_next
+}
+
+#[clippy::msrv = "1.26"]
+fn msrv_1_26() {
+    let _ = vec![1].into_iter().filter(|&x| x < 0).next_back();
 }
diff --git a/tests/ui/methods_fixable.stderr b/tests/ui/methods_fixable.stderr
@@ -7,5 +7,17 @@ LL |     let _ = v.iter().filter(|&x| *x < 0).next();
    = note: `-D clippy::filter-next` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::filter_next)]`
 
-error: aborting due to 1 previous error
+error: called `filter(..).next_back()` on an `DoubleEndedIterator`. This is more succinctly expressed by calling `.rfind(..)` instead
+  --> tests/ui/methods_fixable.rs:12:13
+   |
+LL |     let _ = v.iter().filter(|&x| *x < 0).next_back();
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `v.iter().rfind(|&x| *x < 0)`
+
+error: called `filter(..).next_back()` on an `DoubleEndedIterator`. This is more succinctly expressed by calling `.rfind(..)` instead
+  --> tests/ui/methods_fixable.rs:18:13
+   |
+LL |     let _ = vec![1].into_iter().filter(|&x| x < 0).next_back();
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `vec![1].into_iter().rfind(|&x| x < 0)`
+
+error: aborting due to 3 previous errors
 

Original file line number	Diff line number	Diff line change
`@@ -25,6 +25,10 @@ impl IteratorFalsePositives {`
`25`	`25`	`self`
`26`	`26`	`}`
`27`	`27`
	`28`	`+ pub fn next_back(self) -> IteratorFalsePositives {`
	`29`	`+ self`
	`30`	`+ }`
	`31`	`+`
`28`	`32`	`pub fn find(self) -> Option<u32> {`
`29`	`33`	`Some(self.foo)`
`30`	`34`	`}`