Rollup merge of rust-lang#58145 - taiki-e:poll, r=cramertj

Add #[must_use] to core::task::Poll

cc rust-lang/rfcs#2592

r? @withoutboats

src/libcore/task/poll.rs

@@ -7,6 +7,7 @@ use result::Result;
 
 /// Indicates whether a value is available or if the current task has been
 /// scheduled to receive a wakeup instead.
+#[must_use = "this `Poll` may be a `Pending` variant, which should be handled"]
 #[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd, Hash)]
 pub enum Poll<T> {
     /// Represents that a value is immediately ready.