From e9f56f949d2d1c11f1331151323beb8e85181c29 Mon Sep 17 00:00:00 2001
From: dswij <dswijj@gmail.com>
Date: Fri, 13 Aug 2021 14:34:33 +0800
Subject: [PATCH 1/2] Add false positive test for iterator method

---
 tests/ui/auxiliary/option_helpers.rs | 9 +++++++++
 tests/ui/methods.rs                  | 5 ++++-
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/tests/ui/auxiliary/option_helpers.rs b/tests/ui/auxiliary/option_helpers.rs
index 7dc3f4ebd4d46..86a637ce3093c 100644
--- a/tests/ui/auxiliary/option_helpers.rs
+++ b/tests/ui/auxiliary/option_helpers.rs
@@ -53,3 +53,12 @@ impl IteratorFalsePositives {
         self.foo as usize
     }
 }
+
+#[derive(Copy, Clone)]
+pub struct IteratorMethodFalsePositives;
+
+impl IteratorMethodFalsePositives {
+    pub fn filter(&self, _s: i32) -> std::vec::IntoIter<i32> {
+        unimplemented!();
+    }
+}
diff --git a/tests/ui/methods.rs b/tests/ui/methods.rs
index 513d930e05687..c441b35b99203 100644
--- a/tests/ui/methods.rs
+++ b/tests/ui/methods.rs
@@ -32,7 +32,7 @@ use std::ops::Mul;
 use std::rc::{self, Rc};
 use std::sync::{self, Arc};
 
-use option_helpers::IteratorFalsePositives;
+use option_helpers::{IteratorFalsePositives, IteratorMethodFalsePositives};
 
 struct Lt<'a> {
     foo: &'a u32,
@@ -131,6 +131,9 @@ fn filter_next() {
     // Check that we don't lint if the caller is not an `Iterator`.
     let foo = IteratorFalsePositives { foo: 0 };
     let _ = foo.filter().next();
+
+    let foo = IteratorMethodFalsePositives {};
+    let _ = foo.filter(42).next();
 }
 
 fn main() {

From 91b598a8e484da328325ece3400e8612b6f854df Mon Sep 17 00:00:00 2001
From: dswij <dswijj@gmail.com>
Date: Fri, 13 Aug 2021 14:36:40 +0800
Subject: [PATCH 2/2] Fix false positive on `filter_next`

---
 clippy_lints/src/methods/filter_next.rs | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/clippy_lints/src/methods/filter_next.rs b/clippy_lints/src/methods/filter_next.rs
index 172714f6b01c6..bcf8d93b602ef 100644
--- a/clippy_lints/src/methods/filter_next.rs
+++ b/clippy_lints/src/methods/filter_next.rs
@@ -1,6 +1,6 @@
 use clippy_utils::diagnostics::{span_lint, span_lint_and_sugg};
-use clippy_utils::is_trait_method;
 use clippy_utils::source::snippet;
+use clippy_utils::ty::implements_trait;
 use rustc_errors::Applicability;
 use rustc_hir as hir;
 use rustc_lint::LateContext;
@@ -16,7 +16,10 @@ pub(super) fn check<'tcx>(
     filter_arg: &'tcx hir::Expr<'_>,
 ) {
     // lint if caller of `.filter().next()` is an Iterator
-    if is_trait_method(cx, expr, sym::Iterator) {
+    let recv_impls_iterator = cx.tcx.get_diagnostic_item(sym::Iterator).map_or(false, |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, "..");