add test reproducing #2587, #2436, and #2411

tracing-core/tests/local_dispatch_before_init.rs

@@ -0,0 +1,44 @@
+mod common;
+
+use common::*;
+use tracing_core::{subscriber::NoSubscriber, dispatcher::{Dispatch, self}};
+
+/// This test reproduces the following issues:
+/// - https://github.com/tokio-rs/tracing/issues/2587
+/// - https://github.com/tokio-rs/tracing/issues/2411
+/// - https://github.com/tokio-rs/tracing/issues/2436
+#[test]
+fn local_dispatch_before_init() {
+    dispatcher::get_default(|current| assert!(current.is::<NoSubscriber>()));
+
+    // Temporarily override the default dispatcher with a scoped dispatcher.
+    //
+    // This will suppress the "SCOPED_COUNT" optimization and cause threads to
+    // attempt to cache the global default after the scoped dispatcher has been
+    // unset.
+    #[cfg(feature = "std")] {
+        dispatcher::with_default(&Dispatch::new(TestSubscriberB), || {
+            dispatcher::get_default(|current| {
+                assert!(
+                    current.is::<TestSubscriberB>(),
+                    "overriden subscriber not set; current: {:?}",
+                    current,
+                );
+            })
+        })
+    }
+
+    dispatcher::get_default(|current| assert!(current.is::<NoSubscriber>()));
+
+    dispatcher::set_global_default(Dispatch::new(TestSubscriberA))
+        .expect("set global dispatch failed");
+
+    dispatcher::get_default(|current| {
+        assert!(
+            current.is::<TestSubscriberA>(),
+            "default subscriber not set; current: {:?}",
+            current,
+        );
+    });
+
+}