Fix false positives

clippy_lints/src/lifetimes.rs

@@ -565,6 +565,7 @@ fn report_extra_impl_lifetimes<'tcx>(cx: &LateContext<'tcx>, impl_: &'tcx Impl<'
         .collect();
     let mut checker = LifetimeChecker::<mir_nested_filter::All>::new(cx, hs);
 
+    walk_generics(&mut checker, &impl_.generics);
     if let Some(ref trait_ref) = impl_.of_trait {
         walk_trait_ref(&mut checker, trait_ref);
     }

tests/ui/extra_unused_lifetimes.rs

@@ -88,4 +88,30 @@ mod issue6437 {
     }
 }
 
+// https://github.com/rust-lang/rust-clippy/pull/8737#pullrequestreview-951268213
+mod first_case {
+    use serde::de::Visitor;
+    pub trait Expected {
+        fn fmt(&self, formatter: &mut std::fmt::Formatter);
+    }
+
+    impl<'de, T> Expected for T
+    where
+        T: Visitor<'de>,
+    {
+        fn fmt(&self, formatter: &mut std::fmt::Formatter) {}
+    }
+}
+
+// https://github.com/rust-lang/rust-clippy/pull/8737#pullrequestreview-951268213
+mod second_case {
+    pub trait Source {
+        fn hey();
+    }
+
+    impl<'a, T: Source + ?Sized + 'a> Source for Box<T> {
+        fn hey() {}
+    }
+}
+
 fn main() {}