Test for #[must_use] on traits

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

@@ -0,0 +1,22 @@
+#![deny(unused_must_use)]
+
+#[must_use]
+trait Critical {}
+
+trait NotSoCritical {}
+
+trait DecidedlyUnimportant {}
+
+struct Anon;
+
+impl Critical for Anon {}
+impl NotSoCritical for Anon {}
+impl DecidedlyUnimportant for Anon {}
+
+fn get_critical() -> impl NotSoCritical + Critical + DecidedlyUnimportant {
+    Anon {}
+}
+
+fn main() {
+    get_critical(); //~ ERROR unused implementer of `Critical` that must be used
+}

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

@@ -0,0 +1,14 @@
+error: unused implementer of `Critical` that must be used
+  --> $DIR/must_use-trait.rs:21:5
+   |
+LL |     get_critical(); //~ ERROR unused implementer of `Critical` that must be used
+   |     ^^^^^^^^^^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/must_use-trait.rs:1:9
+   |
+LL | #![deny(unused_must_use)]
+   |         ^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+