Fix extra_unused_lifetimes false positive

Fixes #4291

Author: Michael Wright

clippy_lints/src/lifetimes.rs

@@ -9,7 +9,7 @@ use syntax::source_map::Span;
 use syntax::symbol::kw;
 
 use crate::reexport::*;
-use crate::utils::{last_path_segment, span_lint};
+use crate::utils::{last_path_segment, span_lint, trait_ref_of_method};
 
 declare_clippy_lint! {
     /// **What it does:** Checks for lifetime annotations which can be removed by
@@ -66,7 +66,9 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Lifetimes {
 
     fn check_impl_item(&mut self, cx: &LateContext<'a, 'tcx>, item: &'tcx ImplItem) {
         if let ImplItemKind::Method(ref sig, id) = item.node {
-            check_fn_inner(cx, &sig.decl, Some(id), &item.generics, item.span);
+            if trait_ref_of_method(cx, item.hir_id).is_none() {
+                check_fn_inner(cx, &sig.decl, Some(id), &item.generics, item.span);
+            }
         }
     }
 

tests/ui/extra_unused_lifetimes.rs

@@ -61,4 +61,23 @@ impl X {
     fn explicit_self_with_lifetime<'a>(self: &'a Self) {}
 }
 
+// Methods implementing traits must have matching lifetimes
+mod issue4291 {
+    #[derive(Debug)]
+    pub struct Foo<'a>(&'a std::marker::PhantomData<u8>);
+
+    #[derive(Debug)]
+    pub struct Bar<'a: 'b, 'b>(Foo<'a>, &'b std::marker::PhantomData<u8>);
+
+    trait LT {
+        fn test<'a: 'b, 'b>(foo: &Foo<'a>, bar: &Bar<'a, 'b>);
+    }
+
+    pub struct Baz;
+
+    impl LT for Baz {
+        fn test<'a: 'b, 'b>(_foo: &Foo, _bar: &Bar) {}
+    }
+}
+
 fn main() {}