From cb5520bc4865473971281ddb3e33a29a256109c8 Mon Sep 17 00:00:00 2001
From: varkor <github@varkor.com>
Date: Sat, 3 Nov 2018 20:08:30 +0000
Subject: [PATCH] Recognise #[must_use] on traits, affecting impl Trait

---
 src/librustc_lint/unused.rs | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/src/librustc_lint/unused.rs b/src/librustc_lint/unused.rs
index 6383ca69fe5c..f8030fa6ec7a 100644
--- a/src/librustc_lint/unused.rs
+++ b/src/librustc_lint/unused.rs
@@ -66,9 +66,22 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for UnusedResults {
         } 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;