runtime: mark EnterGuard with must_use
@@ -39,6 +39,7 @@ pub struct Handle {
///
/// [`Runtime::enter`]: fn@crate::runtime::Runtime::enter
#[derive(Debug)]
+#[must_use = "Creating and dropping a guard does nothing"]
pub struct EnterGuard<'a> {
handle: &'a Handle,
guard: context::EnterGuard,
