Add test for #[must_use] in tuples

src/test/ui/lint/must_use-tuple.rs

@@ -0,0 +1,5 @@
+#![deny(unused_must_use)]
+
+fn main() {
+    (Ok::<(), ()>(()),); //~ ERROR unused `std::result::Result` that must be used
+}

src/test/ui/lint/must_use-tuple.stderr

@@ -0,0 +1,15 @@
+error: unused `std::result::Result` that must be used
+  --> $DIR/must_use-tuple.rs:4:5
+   |
+LL |     (Ok::<(), ()>(()),);
+   |     ^^^^^^^^^^^^^^^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/must_use-tuple.rs:1:9
+   |
+LL | #![deny(unused_must_use)]
+   |         ^^^^^^^^^^^^^^^
+   = note: this `Result` may be an `Err` variant, which should be handled
+
+error: aborting due to previous error
+