Add test to check library traits have #[must_use] attribute

Author: aelred

src/test/compile-fail/must_use-in-stdlib-traits.rs

@@ -0,0 +1,47 @@
+#![deny(unused_must_use)]
+#![feature(futures_api, pin, arbitrary_self_types)]
+
+use std::iter::Iterator;
+use std::future::Future;
+
+use std::task::{Poll, LocalWaker};
+use std::pin::Pin;
+use std::unimplemented;
+
+struct MyFuture;
+
+impl Future for MyFuture {
+   type Output = u32;
+
+   fn poll(self: Pin<&mut Self>, lw: &LocalWaker) -> Poll<u32> {
+      Poll::Pending
+   }
+}
+
+fn iterator() -> impl Iterator {
+   std::iter::empty::<u32>()
+}
+
+fn future() -> impl Future {
+   MyFuture
+}
+
+fn square_fn_once() -> impl FnOnce(u32) -> u32 {
+   |x| x * x
+}
+
+fn square_fn_mut() -> impl FnMut(u32) -> u32 {
+   |x| x * x
+}
+
+fn square_fn() -> impl Fn(u32) -> u32 {
+   |x| x * x
+}
+
+fn main() {
+   iterator(); //~ ERROR unused implementer of `std::iter::Iterator` that must be used
+   future(); //~ ERROR unused implementer of `std::future::Future` that must be used
+   square_fn_once(); //~ ERROR unused implementer of `std::ops::FnOnce` that must be used
+   square_fn_mut(); //~ ERROR unused implementer of `std::ops::FnMut` that must be used
+   square_fn(); //~ ERROR unused implementer of `std::ops::Fn` that must be used
+}