Add test for #[must_use] in tuples

src/librustc_lint/unused.rs

@@ -1,7 +1,7 @@
 use rustc::hir::def::{Res, DefKind};
 use rustc::hir::def_id::DefId;
 use rustc::lint;
-use rustc::ty;
+use rustc::ty::{self, Ty};
 use rustc::ty::adjustment;
 use rustc_data_structures::fx::FxHashMap;
 use lint::{LateContext, EarlyContext, LintContext, LintArray};
@@ -137,7 +137,7 @@ fn check_stmt(&mut self, cx: &LateContext<'_, '_>, s: &hir::Stmt) {
         // Returns whether an error has been emitted (and thus another does not need to be later).
         fn check_must_use_ty(
             cx: &LateContext<'_, '_>,
-            ty: ty::Ty<'_>,
+            ty: Ty<'_>,
             span: Span,
         ) -> bool {
             match ty.sty {

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
+