Don't trigger unused_result on functions returning empty enums

src/librustc_lint/unused.rs

@@ -148,7 +148,13 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for UnusedResults {
         let ty_warned = match t.sty {
             ty::TyTuple(ref tys, _) if tys.is_empty() => return,
             ty::TyNever => return,
-            ty::TyAdt(def, _) => check_must_use(cx, def.did, s.span, ""),
+            ty::TyAdt(def, _) => {
+                if def.variants.is_empty() {
+                    return;
+                } else {
+                    check_must_use(cx, def.did, s.span, "")
+                }
+            },
             _ => false,
         };
 

src/test/ui/issue-43806.rs

@@ -12,15 +12,22 @@
 
 #![deny(unused_results)]
 
+enum Void {}
+
 fn foo() {}
 
 fn bar() -> ! {
     loop {}
 }
 
+fn baz() -> Void {
+    loop {}
+}
+
 fn qux() {
     foo();
     bar();
+    baz();
 }
 
 fn main() {}