Recognise #[must_use] on traits, affecting impl Trait

src/librustc_lint/unused.rs

@@ -66,9 +66,22 @@ fn check_stmt(&mut self, cx: &LateContext, s: &hir::Stmt) {
         } else {
             match t.sty {
                 ty::Adt(def, _) => check_must_use(cx, def.did, s.span, ""),
+                ty::Opaque(def, _) => {
+                    let mut must_use = false;
+                    for (predicate, _) in cx.tcx.predicates_of(def).predicates {
+                        if let ty::Predicate::Trait(ref poly_trait_predicate) = predicate {
+                            let trait_ref = poly_trait_predicate.skip_binder().trait_ref;
+                            if check_must_use(cx, trait_ref.def_id, s.span, "implementer of ") {
+                                must_use = true;
+                                break;
+                            }
+                        }
+                    }
+                    must_use
+                }
                 _ => false,
             }
-        }
+        };
 
         let mut fn_warned = false;
         let mut op_warned = false;