Add a test that fails to compile if generated code triggers unused lint warning. (#2145)

* Add a test that fails to compile if generated code triggers unused lint warning.

* Fix formatting.

tests/must_use.rs

@@ -0,0 +1,10 @@
+//! This test validates that the generated bindings don't cause linting warnings
+//! when used with structs annotated with `#[must_use]`.
+
+#![deny(unused)]
+
+use wasm_bindgen::prelude::*;
+
+#[wasm_bindgen]
+#[must_use]
+pub struct MustUse {}