Rollup merge of rust-lang#129034 - henryksloan:coroutine-must-use, r=joboet

Add `#[must_use]` attribute to `Coroutine` trait

[Coroutines tracking issue](rust-lang#43122)

Like closures (`FnOnce`, `AsyncFn`, etc.), coroutines are lazy and do nothing unless called (resumed). Closure traits like `FnOnce` have `#[must_use = "closures are lazy and do nothing unless called"]` to catch likely bugs for users of APIs that produce them. This PR adds such a `#[must_use]` attribute to `trait Coroutine`.

Author: matthiaskrgr

core/src/ops/coroutine.rs

@@ -69,6 +69,7 @@ pub enum CoroutineState<Y, R> {
 #[lang = "coroutine"]
 #[unstable(feature = "coroutine_trait", issue = "43122")]
 #[fundamental]
+#[must_use = "coroutines are lazy and do nothing unless resumed"]
 pub trait Coroutine<R = ()> {
     /// The type of value this coroutine yields.
     ///