Adding upstream version 86.0.1.upstream/86.0.1upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

17 files changed, 3669 insertions, 0 deletions

diff --git a/third_party/rust/tracing/tests/event.rs b/third_party/rust/tracing/tests/event.rs
new file mode 100644
index 0000000000..d61ef748b8
--- /dev/null
+++ b/third_party/rust/tracing/tests/event.rs
@@ -0,0 +1,375 @@
+// These tests require the thread-local scoped dispatcher, which only works when
+// we have a standard library. The behaviour being tested should be the same
+// with the standard lib disabled.
+//
+// The alternative would be for each of these tests to be defined in a separate
+// file, which is :(
+#![cfg(feature = "std")]
+
+#[macro_use]
+extern crate tracing;
+mod support;
+
+use self::support::*;
+
+use tracing::{
+    field::{debug, display},
+    subscriber::with_default,
+    Level,
+};
+
+macro_rules! event_without_message {
+    ($name:ident: $e:expr) => {
+        #[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+        #[test]
+        fn $name() {
+            let (subscriber, handle) = subscriber::mock()
+                .event(
+                    event::mock().with_fields(
+                        field::mock("answer")
+                            .with_value(&42)
+                            .and(
+                                field::mock("to_question")
+                                    .with_value(&"life, the universe, and everything"),
+                            )
+                            .only(),
+                    ),
+                )
+                .done()
+                .run_with_handle();
+
+            with_default(subscriber, || {
+                info!(
+                    answer = $e,
+                    to_question = "life, the universe, and everything"
+                );
+            });
+
+            handle.assert_finished();
+        }
+    };
+}
+
+event_without_message! {event_without_message: 42}
+event_without_message! {wrapping_event_without_message: std::num::Wrapping(42)}
+event_without_message! {nonzeroi32_event_without_message: std::num::NonZeroI32::new(42).unwrap()}
+// needs API breakage
+//event_without_message!{nonzerou128_event_without_message: std::num::NonZeroU128::new(42).unwrap()}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn event_with_message() {
+    let (subscriber, handle) = subscriber::mock()
+        .event(event::mock().with_fields(field::mock("message").with_value(
+            &tracing::field::debug(format_args!("hello from my event! yak shaved = {:?}", true)),
+        )))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        debug!("hello from my event! yak shaved = {:?}", true);
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn message_without_delims() {
+    let (subscriber, handle) = subscriber::mock()
+        .event(
+            event::mock().with_fields(
+                field::mock("answer")
+                    .with_value(&42)
+                    .and(field::mock("question").with_value(&"life, the universe, and everything"))
+                    .and(
+                        field::mock("message").with_value(&tracing::field::debug(format_args!(
+                            "hello from my event! tricky? {:?}!",
+                            true
+                        ))),
+                    )
+                    .only(),
+            ),
+        )
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        let question = "life, the universe, and everything";
+        debug!(answer = 42, question, "hello from {where}! tricky? {:?}!", true, where = "my event");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn string_message_without_delims() {
+    let (subscriber, handle) = subscriber::mock()
+        .event(
+            event::mock().with_fields(
+                field::mock("answer")
+                    .with_value(&42)
+                    .and(field::mock("question").with_value(&"life, the universe, and everything"))
+                    .and(
+                        field::mock("message").with_value(&tracing::field::debug(format_args!(
+                            "hello from my event"
+                        ))),
+                    )
+                    .only(),
+            ),
+        )
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        let question = "life, the universe, and everything";
+        debug!(answer = 42, question, "hello from my event");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn one_with_everything() {
+    let (subscriber, handle) = subscriber::mock()
+        .event(
+            event::mock()
+                .with_fields(
+                    field::mock("message")
+                        .with_value(&tracing::field::debug(format_args!(
+                            "{:#x} make me one with{what:.>20}",
+                            4_277_009_102u64,
+                            what = "everything"
+                        )))
+                        .and(field::mock("foo").with_value(&666))
+                        .and(field::mock("bar").with_value(&false))
+                        .only(),
+                )
+                .at_level(Level::ERROR)
+                .with_target("whatever"),
+        )
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        event!(
+            target: "whatever",
+            Level::ERROR,
+            { foo = 666, bar = false },
+             "{:#x} make me one with{what:.>20}", 4_277_009_102u64, what = "everything"
+        );
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn moved_field() {
+    let (subscriber, handle) = subscriber::mock()
+        .event(
+            event::mock().with_fields(
+                field::mock("foo")
+                    .with_value(&display("hello from my event"))
+                    .only(),
+            ),
+        )
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        let from = "my event";
+        event!(Level::INFO, foo = display(format!("hello from {}", from)))
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn dotted_field_name() {
+    let (subscriber, handle) = subscriber::mock()
+        .event(
+            event::mock().with_fields(
+                field::mock("foo.bar")
+                    .with_value(&true)
+                    .and(field::mock("foo.baz").with_value(&false))
+                    .only(),
+            ),
+        )
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        event!(Level::INFO, foo.bar = true, foo.baz = false);
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn borrowed_field() {
+    let (subscriber, handle) = subscriber::mock()
+        .event(
+            event::mock().with_fields(
+                field::mock("foo")
+                    .with_value(&display("hello from my event"))
+                    .only(),
+            ),
+        )
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        let from = "my event";
+        let mut message = format!("hello from {}", from);
+        event!(Level::INFO, foo = display(&message));
+        message.push_str(", which happened!");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+// If emitting log instrumentation, this gets moved anyway, breaking the test.
+#[cfg(not(feature = "log"))]
+fn move_field_out_of_struct() {
+    use tracing::field::debug;
+
+    #[derive(Debug)]
+    struct Position {
+        x: f32,
+        y: f32,
+    }
+
+    let pos = Position {
+        x: 3.234,
+        y: -1.223,
+    };
+    let (subscriber, handle) = subscriber::mock()
+        .event(
+            event::mock().with_fields(
+                field::mock("x")
+                    .with_value(&debug(3.234))
+                    .and(field::mock("y").with_value(&debug(-1.223)))
+                    .only(),
+            ),
+        )
+        .event(event::mock().with_fields(field::mock("position").with_value(&debug(&pos))))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        let pos = Position {
+            x: 3.234,
+            y: -1.223,
+        };
+        debug!(x = debug(pos.x), y = debug(pos.y));
+        debug!(target: "app_events", { position = debug(pos) }, "New position");
+    });
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn display_shorthand() {
+    let (subscriber, handle) = subscriber::mock()
+        .event(
+            event::mock().with_fields(
+                field::mock("my_field")
+                    .with_value(&display("hello world"))
+                    .only(),
+            ),
+        )
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        event!(Level::TRACE, my_field = %"hello world");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn debug_shorthand() {
+    let (subscriber, handle) = subscriber::mock()
+        .event(
+            event::mock().with_fields(
+                field::mock("my_field")
+                    .with_value(&debug("hello world"))
+                    .only(),
+            ),
+        )
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        event!(Level::TRACE, my_field = ?"hello world");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn both_shorthands() {
+    let (subscriber, handle) = subscriber::mock()
+        .event(
+            event::mock().with_fields(
+                field::mock("display_field")
+                    .with_value(&display("hello world"))
+                    .and(field::mock("debug_field").with_value(&debug("hello world")))
+                    .only(),
+            ),
+        )
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        event!(Level::TRACE, display_field = %"hello world", debug_field = ?"hello world");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn explicit_child() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(span::mock().named("foo"))
+        .event(event::mock().with_explicit_parent(Some("foo")))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        let foo = span!(Level::TRACE, "foo");
+        event!(parent: foo.id(), Level::TRACE, "bar");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn explicit_child_at_levels() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(span::mock().named("foo"))
+        .event(event::mock().with_explicit_parent(Some("foo")))
+        .event(event::mock().with_explicit_parent(Some("foo")))
+        .event(event::mock().with_explicit_parent(Some("foo")))
+        .event(event::mock().with_explicit_parent(Some("foo")))
+        .event(event::mock().with_explicit_parent(Some("foo")))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        let foo = span!(Level::TRACE, "foo");
+        trace!(parent: foo.id(), "a");
+        debug!(parent: foo.id(), "b");
+        info!(parent: foo.id(), "c");
+        warn!(parent: foo.id(), "d");
+        error!(parent: foo.id(), "e");
+    });
+
+    handle.assert_finished();
+}
diff --git a/third_party/rust/tracing/tests/filter_caching_is_lexically_scoped.rs b/third_party/rust/tracing/tests/filter_caching_is_lexically_scoped.rs
new file mode 100644
index 0000000000..9b997a0c6b
--- /dev/null
+++ b/third_party/rust/tracing/tests/filter_caching_is_lexically_scoped.rs
@@ -0,0 +1,69 @@
+// Tests that depend on a count of the number of times their filter is evaluated
+// can't exist in the same file with other tests that add subscribers to the
+// registry. The registry was changed so that each time a new dispatcher is
+// added all filters are re-evaluated. The tests being run only in separate
+// threads with shared global state lets them interfere with each other
+
+#[cfg(not(feature = "std"))]
+extern crate std;
+
+#[macro_use]
+extern crate tracing;
+mod support;
+
+use self::support::*;
+use tracing::Level;
+
+use std::sync::{
+    atomic::{AtomicUsize, Ordering},
+    Arc,
+};
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn filter_caching_is_lexically_scoped() {
+    pub fn my_great_function() -> bool {
+        span!(Level::TRACE, "emily").in_scope(|| true)
+    }
+
+    pub fn my_other_function() -> bool {
+        span!(Level::TRACE, "frank").in_scope(|| true)
+    }
+
+    let count = Arc::new(AtomicUsize::new(0));
+    let count2 = count.clone();
+
+    let subscriber = subscriber::mock()
+        .with_filter(move |meta| match meta.name() {
+            "emily" | "frank" => {
+                count2.fetch_add(1, Ordering::Relaxed);
+                true
+            }
+            _ => false,
+        })
+        .run();
+
+    // Since this test is in its own file anyway, we can do this. Thus, this
+    // test will work even with no-std.
+    tracing::subscriber::set_global_default(subscriber).unwrap();
+
+    // Call the function once. The filter should be re-evaluated.
+    assert!(my_great_function());
+    assert_eq!(count.load(Ordering::Relaxed), 1);
+
+    // Call the function again. The cached result should be used.
+    assert!(my_great_function());
+    assert_eq!(count.load(Ordering::Relaxed), 1);
+
+    assert!(my_other_function());
+    assert_eq!(count.load(Ordering::Relaxed), 2);
+
+    assert!(my_great_function());
+    assert_eq!(count.load(Ordering::Relaxed), 2);
+
+    assert!(my_other_function());
+    assert_eq!(count.load(Ordering::Relaxed), 2);
+
+    assert!(my_great_function());
+    assert_eq!(count.load(Ordering::Relaxed), 2);
+}
diff --git a/third_party/rust/tracing/tests/filters_are_not_reevaluated_for_the_same_span.rs b/third_party/rust/tracing/tests/filters_are_not_reevaluated_for_the_same_span.rs
new file mode 100644
index 0000000000..53e11ef448
--- /dev/null
+++ b/third_party/rust/tracing/tests/filters_are_not_reevaluated_for_the_same_span.rs
@@ -0,0 +1,74 @@
+// Tests that depend on a count of the number of times their filter is evaluated
+// cant exist in the same file with other tests that add subscribers to the
+// registry. The registry was changed so that each time a new dispatcher is
+// added all filters are re-evaluated. The tests being run only in separate
+// threads with shared global state lets them interfere with each other
+#[cfg(not(feature = "std"))]
+extern crate std;
+
+#[macro_use]
+extern crate tracing;
+mod support;
+
+use self::support::*;
+use tracing::Level;
+
+use std::sync::{
+    atomic::{AtomicUsize, Ordering},
+    Arc,
+};
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn filters_are_not_reevaluated_for_the_same_span() {
+    // Asserts that the `span!` macro caches the result of calling
+    // `Subscriber::enabled` for each span.
+    let alice_count = Arc::new(AtomicUsize::new(0));
+    let bob_count = Arc::new(AtomicUsize::new(0));
+    let alice_count2 = alice_count.clone();
+    let bob_count2 = bob_count.clone();
+
+    let (subscriber, handle) = subscriber::mock()
+        .with_filter(move |meta| match meta.name() {
+            "alice" => {
+                alice_count2.fetch_add(1, Ordering::Relaxed);
+                false
+            }
+            "bob" => {
+                bob_count2.fetch_add(1, Ordering::Relaxed);
+                true
+            }
+            _ => false,
+        })
+        .run_with_handle();
+
+    // Since this test is in its own file anyway, we can do this. Thus, this
+    // test will work even with no-std.
+    tracing::subscriber::set_global_default(subscriber).unwrap();
+
+    // Enter "alice" and then "bob". The dispatcher expects to see "bob" but
+    // not "alice."
+    let alice = span!(Level::TRACE, "alice");
+    let bob = alice.in_scope(|| {
+        let bob = span!(Level::TRACE, "bob");
+        bob.in_scope(|| ());
+        bob
+    });
+
+    // The filter should have seen each span a single time.
+    assert_eq!(alice_count.load(Ordering::Relaxed), 1);
+    assert_eq!(bob_count.load(Ordering::Relaxed), 1);
+
+    alice.in_scope(|| bob.in_scope(|| {}));
+
+    // The subscriber should see "bob" again, but the filter should not have
+    // been called.
+    assert_eq!(alice_count.load(Ordering::Relaxed), 1);
+    assert_eq!(bob_count.load(Ordering::Relaxed), 1);
+
+    bob.in_scope(|| {});
+    assert_eq!(alice_count.load(Ordering::Relaxed), 1);
+    assert_eq!(bob_count.load(Ordering::Relaxed), 1);
+
+    handle.assert_finished();
+}
diff --git a/third_party/rust/tracing/tests/filters_are_reevaluated_for_different_call_sites.rs b/third_party/rust/tracing/tests/filters_are_reevaluated_for_different_call_sites.rs
new file mode 100644
index 0000000000..5675e4e678
--- /dev/null
+++ b/third_party/rust/tracing/tests/filters_are_reevaluated_for_different_call_sites.rs
@@ -0,0 +1,84 @@
+// Tests that depend on a count of the number of times their filter is evaluated
+// cant exist in the same file with other tests that add subscribers to the
+// registry. The registry was changed so that each time a new dispatcher is
+// added all filters are re-evaluated. The tests being run only in separate
+// threads with shared global state lets them interfere with each other
+#[cfg(not(feature = "std"))]
+extern crate std;
+
+#[macro_use]
+extern crate tracing;
+mod support;
+
+use self::support::*;
+use tracing::Level;
+
+use std::sync::{
+    atomic::{AtomicUsize, Ordering},
+    Arc,
+};
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn filters_are_reevaluated_for_different_call_sites() {
+    // Asserts that the `span!` macro caches the result of calling
+    // `Subscriber::enabled` for each span.
+    let charlie_count = Arc::new(AtomicUsize::new(0));
+    let dave_count = Arc::new(AtomicUsize::new(0));
+    let charlie_count2 = charlie_count.clone();
+    let dave_count2 = dave_count.clone();
+
+    let subscriber = subscriber::mock()
+        .with_filter(move |meta| {
+            println!("Filter: {:?}", meta.name());
+            match meta.name() {
+                "charlie" => {
+                    charlie_count2.fetch_add(1, Ordering::Relaxed);
+                    false
+                }
+                "dave" => {
+                    dave_count2.fetch_add(1, Ordering::Relaxed);
+                    true
+                }
+                _ => false,
+            }
+        })
+        .run();
+
+    // Since this test is in its own file anyway, we can do this. Thus, this
+    // test will work even with no-std.
+    tracing::subscriber::set_global_default(subscriber).unwrap();
+
+    // Enter "charlie" and then "dave". The dispatcher expects to see "dave" but
+    // not "charlie."
+    let charlie = span!(Level::TRACE, "charlie");
+    let dave = charlie.in_scope(|| {
+        let dave = span!(Level::TRACE, "dave");
+        dave.in_scope(|| {});
+        dave
+    });
+
+    // The filter should have seen each span a single time.
+    assert_eq!(charlie_count.load(Ordering::Relaxed), 1);
+    assert_eq!(dave_count.load(Ordering::Relaxed), 1);
+
+    charlie.in_scope(|| dave.in_scope(|| {}));
+
+    // The subscriber should see "dave" again, but the filter should not have
+    // been called.
+    assert_eq!(charlie_count.load(Ordering::Relaxed), 1);
+    assert_eq!(dave_count.load(Ordering::Relaxed), 1);
+
+    // A different span with the same name has a different call site, so it
+    // should cause the filter to be reapplied.
+    let charlie2 = span!(Level::TRACE, "charlie");
+    charlie.in_scope(|| {});
+    assert_eq!(charlie_count.load(Ordering::Relaxed), 2);
+    assert_eq!(dave_count.load(Ordering::Relaxed), 1);
+
+    // But, the filter should not be re-evaluated for the new "charlie" span
+    // when it is re-entered.
+    charlie2.in_scope(|| span!(Level::TRACE, "dave").in_scope(|| {}));
+    assert_eq!(charlie_count.load(Ordering::Relaxed), 2);
+    assert_eq!(dave_count.load(Ordering::Relaxed), 2);
+}
diff --git a/third_party/rust/tracing/tests/filters_dont_leak.rs b/third_party/rust/tracing/tests/filters_dont_leak.rs
new file mode 100644
index 0000000000..666335a8a4
--- /dev/null
+++ b/third_party/rust/tracing/tests/filters_dont_leak.rs
@@ -0,0 +1,82 @@
+#![cfg(feature = "std")]
+
+mod support;
+use self::support::*;
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn spans_dont_leak() {
+    fn do_span() {
+        let span = tracing::debug_span!("alice");
+        let _e = span.enter();
+    }
+
+    let (subscriber, handle) = subscriber::mock()
+        .named("spans/subscriber1")
+        .with_filter(|_| false)
+        .done()
+        .run_with_handle();
+
+    let _guard = tracing::subscriber::set_default(subscriber);
+
+    do_span();
+
+    let alice = span::mock().named("alice");
+    let (subscriber2, handle2) = subscriber::mock()
+        .named("spans/subscriber2")
+        .with_filter(|_| true)
+        .new_span(alice.clone())
+        .enter(alice.clone())
+        .exit(alice.clone())
+        .drop_span(alice)
+        .done()
+        .run_with_handle();
+
+    tracing::subscriber::with_default(subscriber2, || {
+        println!("--- subscriber 2 is default ---");
+        do_span()
+    });
+
+    println!("--- subscriber 1 is default ---");
+    do_span();
+
+    handle.assert_finished();
+    handle2.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn events_dont_leak() {
+    fn do_event() {
+        tracing::debug!("alice");
+    }
+
+    let (subscriber, handle) = subscriber::mock()
+        .named("events/subscriber1")
+        .with_filter(|_| false)
+        .done()
+        .run_with_handle();
+
+    let _guard = tracing::subscriber::set_default(subscriber);
+
+    do_event();
+
+    let (subscriber2, handle2) = subscriber::mock()
+        .named("events/subscriber2")
+        .with_filter(|_| true)
+        .event(event::mock())
+        .done()
+        .run_with_handle();
+
+    tracing::subscriber::with_default(subscriber2, || {
+        println!("--- subscriber 2 is default ---");
+        do_event()
+    });
+
+    println!("--- subscriber 1 is default ---");
+
+    do_event();
+
+    handle.assert_finished();
+    handle2.assert_finished();
+}
diff --git a/third_party/rust/tracing/tests/macro_imports.rs b/third_party/rust/tracing/tests/macro_imports.rs
new file mode 100644
index 0000000000..2d0a9d6528
--- /dev/null
+++ b/third_party/rust/tracing/tests/macro_imports.rs
@@ -0,0 +1,23 @@
+use tracing::Level;
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn prefixed_span_macros() {
+    tracing::span!(Level::DEBUG, "foo");
+    tracing::trace_span!("foo");
+    tracing::debug_span!("foo");
+    tracing::info_span!("foo");
+    tracing::warn_span!("foo");
+    tracing::error_span!("foo");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn prefixed_event_macros() {
+    tracing::event!(Level::DEBUG, "foo");
+    tracing::trace!("foo");
+    tracing::debug!("foo");
+    tracing::info!("foo");
+    tracing::warn!("foo");
+    tracing::error!("foo");
+}
diff --git a/third_party/rust/tracing/tests/macros.rs b/third_party/rust/tracing/tests/macros.rs
new file mode 100644
index 0000000000..39642667f7
--- /dev/null
+++ b/third_party/rust/tracing/tests/macros.rs
@@ -0,0 +1,890 @@
+#![deny(warnings)]
+use tracing::Level;
+
+#[macro_use]
+extern crate tracing;
+// Tests that macros work across various invocation syntax.
+//
+// These are quite repetitive, and _could_ be generated by a macro. However,
+// they're compile-time tests, so I want to get line numbers etc out of
+// failures, and producing them with a macro would muddy the waters a bit.
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn span() {
+    span!(target: "foo_events", Level::DEBUG, "foo", bar.baz = ?2, quux = %3, quuux = 4);
+    span!(target: "foo_events", Level::DEBUG, "foo", bar.baz = 2, quux = 3);
+    span!(target: "foo_events", Level::DEBUG, "foo", bar.baz = 2, quux = 4,);
+    span!(target: "foo_events", Level::DEBUG, "foo");
+    span!(target: "foo_events", Level::DEBUG, "bar",);
+    span!(Level::DEBUG, "foo", bar.baz = 2, quux = 3);
+    span!(Level::DEBUG, "foo", bar.baz = 2, quux = 4,);
+    span!(Level::DEBUG, "foo", bar.baz = 2, quux = 3);
+    span!(Level::DEBUG, "foo", bar.baz = 2, quux = 4,);
+    span!(Level::DEBUG, "foo", bar.baz = ?2);
+    span!(Level::DEBUG, "foo", bar.baz = %2);
+    span!(Level::DEBUG, "foo");
+    span!(Level::DEBUG, "bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn trace_span() {
+    trace_span!(target: "foo_events", "foo", bar.baz = ?2, quux = %3, quuux = 4);
+    trace_span!(target: "foo_events", "foo", bar.baz = 2, quux = 3);
+    trace_span!(target: "foo_events", "foo", bar.baz = 2, quux = 4,);
+    trace_span!(target: "foo_events", "foo");
+    trace_span!(target: "foo_events", "bar",);
+    trace_span!("foo", bar.baz = 2, quux = 3);
+    trace_span!("foo", bar.baz = 2, quux = 4,);
+    trace_span!("foo", bar.baz = ?2);
+    trace_span!("foo", bar.baz = %2);
+    trace_span!("bar");
+    trace_span!("bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn debug_span() {
+    debug_span!(target: "foo_events", "foo", bar.baz = ?2, quux = %3, quuux = 4);
+    debug_span!(target: "foo_events", "foo", bar.baz = 2, quux = 3);
+    debug_span!(target: "foo_events", "foo", bar.baz = 2, quux = 4,);
+    debug_span!(target: "foo_events", "foo");
+    debug_span!(target: "foo_events", "bar",);
+    debug_span!("foo", bar.baz = 2, quux = 3);
+    debug_span!("foo", bar.baz = 2, quux = 4,);
+    debug_span!("foo", bar.baz = ?2);
+    debug_span!("foo", bar.baz = %2);
+    debug_span!("bar");
+    debug_span!("bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn info_span() {
+    info_span!(target: "foo_events", "foo", bar.baz = ?2, quux = %3, quuux = 4);
+    info_span!(target: "foo_events", "foo", bar.baz = 2, quux = 3);
+    info_span!(target: "foo_events", "foo", bar.baz = 2, quux = 4,);
+    info_span!(target: "foo_events", "foo");
+    info_span!(target: "foo_events", "bar",);
+    info_span!("foo", bar.baz = 2, quux = 3);
+    info_span!("foo", bar.baz = 2, quux = 4,);
+    info_span!("foo", bar.baz = ?2);
+    info_span!("foo", bar.baz = %2);
+    info_span!("bar");
+    info_span!("bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn warn_span() {
+    warn_span!(target: "foo_events", "foo", bar.baz = ?2, quux = %3, quuux = 4);
+    warn_span!(target: "foo_events", "foo", bar.baz = 2, quux = 3);
+    warn_span!(target: "foo_events", "foo", bar.baz = 2, quux = 4,);
+    warn_span!(target: "foo_events", "foo");
+    warn_span!(target: "foo_events", "bar",);
+    warn_span!("foo", bar.baz = 2, quux = 3);
+    warn_span!("foo", bar.baz = 2, quux = 4,);
+    warn_span!("foo", bar.baz = ?2);
+    warn_span!("foo", bar.baz = %2);
+    warn_span!("bar");
+    warn_span!("bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn error_span() {
+    error_span!(target: "foo_events", "foo", bar.baz = ?2, quux = %3, quuux = 4);
+    error_span!(target: "foo_events", "foo", bar.baz = 2, quux = 3);
+    error_span!(target: "foo_events", "foo", bar.baz = 2, quux = 4,);
+    error_span!(target: "foo_events", "foo");
+    error_span!(target: "foo_events", "bar",);
+    error_span!("foo", bar.baz = 2, quux = 3);
+    error_span!("foo", bar.baz = 2, quux = 4,);
+    error_span!("foo", bar.baz = ?2);
+    error_span!("foo", bar.baz = %2);
+    error_span!("bar");
+    error_span!("bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn span_root() {
+    span!(target: "foo_events", parent: None, Level::TRACE, "foo", bar.baz = 2, quux = 3);
+    span!(target: "foo_events", parent: None, Level::TRACE, "foo", bar.baz = 2, quux = 3);
+    span!(target: "foo_events", parent: None, Level::TRACE, "foo", bar.baz = 2, quux = 4,);
+    span!(target: "foo_events", parent: None, Level::TRACE, "foo");
+    span!(target: "foo_events", parent: None, Level::TRACE, "bar",);
+    span!(parent: None, Level::DEBUG, "foo", bar.baz = 2, quux = 3);
+    span!(parent: None, Level::DEBUG, "foo", bar.baz = 2, quux = 4,);
+    span!(parent: None, Level::DEBUG, "foo");
+    span!(parent: None, Level::DEBUG, "bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn trace_span_root() {
+    trace_span!(target: "foo_events", parent: None, "foo", bar.baz = 2, quux = 3);
+    trace_span!(target: "foo_events", parent: None, "foo", bar.baz = 2, quux = 4,);
+    trace_span!(target: "foo_events", parent: None, "foo");
+    trace_span!(target: "foo_events", parent: None, "bar",);
+    trace_span!(parent: None, "foo", bar.baz = 2, quux = 3);
+    trace_span!(parent: None, "foo", bar.baz = 2, quux = 4,);
+    trace_span!(parent: None, "foo");
+    trace_span!(parent: None, "bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn debug_span_root() {
+    debug_span!(target: "foo_events", parent: None, "foo", bar.baz = 2, quux = 3);
+    debug_span!(target: "foo_events", parent: None, "foo", bar.baz = 2, quux = 4,);
+    debug_span!(target: "foo_events", parent: None, "foo");
+    debug_span!(target: "foo_events", parent: None, "bar",);
+    debug_span!(parent: None, "foo", bar.baz = 2, quux = 3);
+    debug_span!(parent: None, "foo", bar.baz = 2, quux = 4,);
+    debug_span!(parent: None, "foo");
+    debug_span!(parent: None, "bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn info_span_root() {
+    info_span!(target: "foo_events", parent: None, "foo", bar.baz = 2, quux = 3);
+    info_span!(target: "foo_events", parent: None, "foo", bar.baz = 2, quux = 4,);
+    info_span!(target: "foo_events", parent: None, "foo");
+    info_span!(target: "foo_events", parent: None, "bar",);
+    info_span!(parent: None, "foo", bar.baz = 2, quux = 3);
+    info_span!(parent: None, "foo", bar.baz = 2, quux = 4,);
+    info_span!(parent: None, "foo");
+    info_span!(parent: None, "bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn warn_span_root() {
+    warn_span!(target: "foo_events", parent: None, "foo", bar.baz = 2, quux = 3);
+    warn_span!(target: "foo_events", parent: None, "foo", bar.baz = 2, quux = 4,);
+    warn_span!(target: "foo_events", parent: None, "foo");
+    warn_span!(target: "foo_events", parent: None, "bar",);
+    warn_span!(parent: None, "foo", bar.baz = 2, quux = 3);
+    warn_span!(parent: None, "foo", bar.baz = 2, quux = 4,);
+    warn_span!(parent: None, "foo");
+    warn_span!(parent: None, "bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn error_span_root() {
+    error_span!(target: "foo_events", parent: None, "foo", bar.baz = 2, quux = 3);
+    error_span!(target: "foo_events", parent: None, "foo", bar.baz = 2, quux = 4,);
+    error_span!(target: "foo_events", parent: None, "foo");
+    error_span!(target: "foo_events", parent: None, "bar",);
+    error_span!(parent: None, "foo", bar.baz = 2, quux = 3);
+    error_span!(parent: None, "foo", bar.baz = 2, quux = 4,);
+    error_span!(parent: None, "foo");
+    error_span!(parent: None, "bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn span_with_parent() {
+    let p = span!(Level::TRACE, "im_a_parent!");
+    span!(target: "foo_events", parent: &p, Level::TRACE, "foo", bar.baz = 2, quux = 3);
+    span!(target: "foo_events", parent: &p, Level::TRACE, "foo", bar.baz = 2, quux = 4,);
+    span!(target: "foo_events", parent: &p, Level::TRACE, "foo");
+    span!(target: "foo_events", parent: &p, Level::TRACE, "bar",);
+    span!(parent: &p, Level::DEBUG, "foo", bar.baz = 2, quux = 3);
+    span!(parent: &p, Level::DEBUG, "foo", bar.baz = 2, quux = 4,);
+    span!(parent: &p, Level::DEBUG, "foo");
+    span!(parent: &p, Level::DEBUG, "bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn trace_span_with_parent() {
+    let p = span!(Level::TRACE, "im_a_parent!");
+    trace_span!(target: "foo_events", parent: &p, "foo", bar.baz = 2, quux = 3);
+    trace_span!(target: "foo_events", parent: &p, "foo", bar.baz = 2, quux = 4,);
+    trace_span!(target: "foo_events", parent: &p, "foo");
+    trace_span!(target: "foo_events", parent: &p, "bar",);
+
+    trace_span!(parent: &p, "foo", bar.baz = 2, quux = 3);
+    trace_span!(parent: &p, "foo", bar.baz = 2, quux = 4,);
+
+    trace_span!(parent: &p, "foo");
+    trace_span!(parent: &p, "bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn debug_span_with_parent() {
+    let p = span!(Level::TRACE, "im_a_parent!");
+    debug_span!(target: "foo_events", parent: &p, "foo", bar.baz = 2, quux = 3);
+    debug_span!(target: "foo_events", parent: &p, "foo", bar.baz = 2, quux = 4,);
+    debug_span!(target: "foo_events", parent: &p, "foo");
+    debug_span!(target: "foo_events", parent: &p, "bar",);
+
+    debug_span!(parent: &p, "foo", bar.baz = 2, quux = 3);
+    debug_span!(parent: &p, "foo", bar.baz = 2, quux = 4,);
+
+    debug_span!(parent: &p, "foo");
+    debug_span!(parent: &p, "bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn info_span_with_parent() {
+    let p = span!(Level::TRACE, "im_a_parent!");
+    info_span!(target: "foo_events", parent: &p, "foo", bar.baz = 2, quux = 3);
+    info_span!(target: "foo_events", parent: &p, "foo", bar.baz = 2, quux = 4,);
+    info_span!(target: "foo_events", parent: &p, "foo");
+    info_span!(target: "foo_events", parent: &p, "bar",);
+
+    info_span!(parent: &p, "foo", bar.baz = 2, quux = 3);
+    info_span!(parent: &p, "foo", bar.baz = 2, quux = 4,);
+
+    info_span!(parent: &p, "foo");
+    info_span!(parent: &p, "bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn warn_span_with_parent() {
+    let p = span!(Level::TRACE, "im_a_parent!");
+    warn_span!(target: "foo_events", parent: &p, "foo", bar.baz = 2, quux = 3);
+    warn_span!(target: "foo_events", parent: &p, "foo", bar.baz = 2, quux = 4,);
+    warn_span!(target: "foo_events", parent: &p, "foo");
+    warn_span!(target: "foo_events", parent: &p, "bar",);
+
+    warn_span!(parent: &p, "foo", bar.baz = 2, quux = 3);
+    warn_span!(parent: &p, "foo", bar.baz = 2, quux = 4,);
+
+    warn_span!(parent: &p, "foo");
+    warn_span!(parent: &p, "bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn error_span_with_parent() {
+    let p = span!(Level::TRACE, "im_a_parent!");
+    error_span!(target: "foo_events", parent: &p, "foo", bar.baz = 2, quux = 3);
+    error_span!(target: "foo_events", parent: &p, "foo", bar.baz = 2, quux = 4,);
+    error_span!(target: "foo_events", parent: &p, "foo");
+    error_span!(target: "foo_events", parent: &p, "bar",);
+
+    error_span!(parent: &p, "foo", bar.baz = 2, quux = 3);
+    error_span!(parent: &p, "foo", bar.baz = 2, quux = 4,);
+
+    error_span!(parent: &p, "foo");
+    error_span!(parent: &p, "bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn span_with_non_rust_symbol() {
+    span!(Level::TRACE, "non-rust", "guid:x-request-id" = ?"abcdef", "more {}", 42);
+    span!(Level::TRACE, "non-rust", "guid:x-request-id" = %"abcdef", "more {}", 51);
+    span!(
+        Level::TRACE,
+        "non-rust",
+        "guid:x-request-id" = "abcdef",
+        "more {}",
+        60
+    );
+    span!(Level::TRACE, "non-rust", "guid:x-request-id" = ?"abcdef");
+    span!(Level::TRACE, "non-rust", "guid:x-request-id" = %"abcdef");
+    span!(Level::TRACE, "non-rust", "guid:x-request-id" = "abcdef");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn event() {
+    event!(Level::DEBUG, foo = ?3, bar.baz = %2, quux = false);
+    event!(Level::DEBUG, foo = 3, bar.baz = 2, quux = false);
+    event!(Level::DEBUG, foo = 3, bar.baz = 3,);
+    event!(Level::DEBUG, "foo");
+    event!(Level::DEBUG, "foo: {}", 3);
+    event!(Level::INFO, foo = ?3, bar.baz = %2, quux = false, "hello world {:?}", 42);
+    event!(
+        Level::INFO,
+        foo = 3,
+        bar.baz = 2,
+        quux = false,
+        "hello world {:?}",
+        42
+    );
+    event!(Level::INFO, foo = 3, bar.baz = 3, "hello world {:?}", 42,);
+    event!(Level::DEBUG, { foo = 3, bar.baz = 80 }, "quux");
+    event!(Level::DEBUG, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    event!(Level::DEBUG, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    event!(Level::DEBUG, { foo = ?2, bar.baz = %78 }, "quux");
+    event!(target: "foo_events", Level::DEBUG, foo = 3, bar.baz = 2, quux = false);
+    event!(target: "foo_events", Level::DEBUG, foo = 3, bar.baz = 3,);
+    event!(target: "foo_events", Level::DEBUG, "foo");
+    event!(target: "foo_events", Level::DEBUG, "foo: {}", 3);
+    event!(target: "foo_events", Level::DEBUG, { foo = 3, bar.baz = 80 }, "quux");
+    event!(target: "foo_events", Level::DEBUG, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    event!(target: "foo_events", Level::DEBUG, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    event!(target: "foo_events", Level::DEBUG, { foo = 2, bar.baz = 78, }, "quux");
+    let foo = 1;
+    event!(Level::DEBUG, ?foo);
+    event!(Level::DEBUG, %foo);
+    event!(Level::DEBUG, foo);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn locals_with_message() {
+    let data = (42, "forty-two");
+    let private_data = "private";
+    let error = "a bad error";
+    event!(Level::ERROR, %error, "Received error");
+    event!(
+        target: "app_events",
+        Level::WARN,
+        private_data,
+        ?data,
+        "App warning: {}",
+        error
+    );
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn locals_no_message() {
+    let data = (42, "forty-two");
+    let private_data = "private";
+    let error = "a bad error";
+    event!(
+        target: "app_events",
+        Level::WARN,
+        private_data,
+        ?data,
+    );
+    event!(
+        target: "app_events",
+        Level::WARN,
+        private_data,
+        ?data,
+        error,
+    );
+    event!(
+        target: "app_events",
+        Level::WARN,
+        private_data,
+        ?data,
+        error
+    );
+    event!(Level::WARN, private_data, ?data, error,);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn trace() {
+    trace!(foo = ?3, bar.baz = %2, quux = false);
+    trace!(foo = 3, bar.baz = 2, quux = false);
+    trace!(foo = 3, bar.baz = 3,);
+    trace!("foo");
+    trace!("foo: {}", 3);
+    trace!(foo = ?3, bar.baz = %2, quux = false, "hello world {:?}", 42);
+    trace!(foo = 3, bar.baz = 2, quux = false, "hello world {:?}", 42);
+    trace!(foo = 3, bar.baz = 3, "hello world {:?}", 42,);
+    trace!({ foo = 3, bar.baz = 80 }, "quux");
+    trace!({ foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    trace!({ foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    trace!({ foo = 2, bar.baz = 78 }, "quux");
+    trace!({ foo = ?2, bar.baz = %78 }, "quux");
+    trace!(target: "foo_events", foo = 3, bar.baz = 2, quux = false);
+    trace!(target: "foo_events", foo = 3, bar.baz = 3,);
+    trace!(target: "foo_events", "foo");
+    trace!(target: "foo_events", "foo: {}", 3);
+    trace!(target: "foo_events", { foo = 3, bar.baz = 80 }, "quux");
+    trace!(target: "foo_events", { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    trace!(target: "foo_events", { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    trace!(target: "foo_events", { foo = 2, bar.baz = 78, }, "quux");
+    let foo = 1;
+    trace!(?foo);
+    trace!(%foo);
+    trace!(foo);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn debug() {
+    debug!(foo = ?3, bar.baz = %2, quux = false);
+    debug!(foo = 3, bar.baz = 2, quux = false);
+    debug!(foo = 3, bar.baz = 3,);
+    debug!("foo");
+    debug!("foo: {}", 3);
+    debug!(foo = ?3, bar.baz = %2, quux = false, "hello world {:?}", 42);
+    debug!(foo = 3, bar.baz = 2, quux = false, "hello world {:?}", 42);
+    debug!(foo = 3, bar.baz = 3, "hello world {:?}", 42,);
+    debug!({ foo = 3, bar.baz = 80 }, "quux");
+    debug!({ foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    debug!({ foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    debug!({ foo = 2, bar.baz = 78 }, "quux");
+    debug!({ foo = ?2, bar.baz = %78 }, "quux");
+    debug!(target: "foo_events", foo = 3, bar.baz = 2, quux = false);
+    debug!(target: "foo_events", foo = 3, bar.baz = 3,);
+    debug!(target: "foo_events", "foo");
+    debug!(target: "foo_events", "foo: {}", 3);
+    debug!(target: "foo_events", { foo = 3, bar.baz = 80 }, "quux");
+    debug!(target: "foo_events", { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    debug!(target: "foo_events", { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    debug!(target: "foo_events", { foo = 2, bar.baz = 78, }, "quux");
+    let foo = 1;
+    debug!(?foo);
+    debug!(%foo);
+    debug!(foo);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn info() {
+    info!(foo = ?3, bar.baz = %2, quux = false);
+    info!(foo = 3, bar.baz = 2, quux = false);
+    info!(foo = 3, bar.baz = 3,);
+    info!("foo");
+    info!("foo: {}", 3);
+    info!(foo = ?3, bar.baz = %2, quux = false, "hello world {:?}", 42);
+    info!(foo = 3, bar.baz = 2, quux = false, "hello world {:?}", 42);
+    info!(foo = 3, bar.baz = 3, "hello world {:?}", 42,);
+    info!({ foo = 3, bar.baz = 80 }, "quux");
+    info!({ foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    info!({ foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    info!({ foo = 2, bar.baz = 78 }, "quux");
+    info!({ foo = ?2, bar.baz = %78 }, "quux");
+    info!(target: "foo_events", foo = 3, bar.baz = 2, quux = false);
+    info!(target: "foo_events", foo = 3, bar.baz = 3,);
+    info!(target: "foo_events", "foo");
+    info!(target: "foo_events", "foo: {}", 3);
+    info!(target: "foo_events", { foo = 3, bar.baz = 80 }, "quux");
+    info!(target: "foo_events", { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    info!(target: "foo_events", { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    info!(target: "foo_events", { foo = 2, bar.baz = 78, }, "quux");
+    let foo = 1;
+    info!(?foo);
+    info!(%foo);
+    info!(foo);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn warn() {
+    warn!(foo = ?3, bar.baz = %2, quux = false);
+    warn!(foo = 3, bar.baz = 2, quux = false);
+    warn!(foo = 3, bar.baz = 3,);
+    warn!("foo");
+    warn!("foo: {}", 3);
+    warn!(foo = ?3, bar.baz = %2, quux = false, "hello world {:?}", 42);
+    warn!(foo = 3, bar.baz = 2, quux = false, "hello world {:?}", 42);
+    warn!(foo = 3, bar.baz = 3, "hello world {:?}", 42,);
+    warn!({ foo = 3, bar.baz = 80 }, "quux");
+    warn!({ foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    warn!({ foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    warn!({ foo = 2, bar.baz = 78 }, "quux");
+    warn!({ foo = ?2, bar.baz = %78 }, "quux");
+    warn!(target: "foo_events", foo = 3, bar.baz = 2, quux = false);
+    warn!(target: "foo_events", foo = 3, bar.baz = 3,);
+    warn!(target: "foo_events", "foo");
+    warn!(target: "foo_events", "foo: {}", 3);
+    warn!(target: "foo_events", { foo = 3, bar.baz = 80 }, "quux");
+    warn!(target: "foo_events", { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    warn!(target: "foo_events", { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    warn!(target: "foo_events", { foo = 2, bar.baz = 78, }, "quux");
+    let foo = 1;
+    warn!(?foo);
+    warn!(%foo);
+    warn!(foo);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn error() {
+    error!(foo = ?3, bar.baz = %2, quux = false);
+    error!(foo = 3, bar.baz = 2, quux = false);
+    error!(foo = 3, bar.baz = 3,);
+    error!("foo");
+    error!("foo: {}", 3);
+    error!(foo = ?3, bar.baz = %2, quux = false, "hello world {:?}", 42);
+    error!(foo = 3, bar.baz = 2, quux = false, "hello world {:?}", 42);
+    error!(foo = 3, bar.baz = 3, "hello world {:?}", 42,);
+    error!({ foo = 3, bar.baz = 80 }, "quux");
+    error!({ foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    error!({ foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    error!({ foo = 2, bar.baz = 78, }, "quux");
+    error!({ foo = ?2, bar.baz = %78 }, "quux");
+    error!(target: "foo_events", foo = 3, bar.baz = 2, quux = false);
+    error!(target: "foo_events", foo = 3, bar.baz = 3,);
+    error!(target: "foo_events", "foo");
+    error!(target: "foo_events", "foo: {}", 3);
+    error!(target: "foo_events", { foo = 3, bar.baz = 80 }, "quux");
+    error!(target: "foo_events", { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    error!(target: "foo_events", { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    error!(target: "foo_events", { foo = 2, bar.baz = 78, }, "quux");
+    let foo = 1;
+    error!(?foo);
+    error!(%foo);
+    error!(foo);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn event_root() {
+    event!(parent: None, Level::DEBUG, foo = ?3, bar.baz = %2, quux = false);
+    event!(
+        parent: None,
+        Level::DEBUG,
+        foo = 3,
+        bar.baz = 2,
+        quux = false
+    );
+    event!(parent: None, Level::DEBUG, foo = 3, bar.baz = 3,);
+    event!(parent: None, Level::DEBUG, "foo");
+    event!(parent: None, Level::DEBUG, "foo: {}", 3);
+    event!(parent: None, Level::DEBUG, { foo = 3, bar.baz = 80 }, "quux");
+    event!(parent: None, Level::DEBUG, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    event!(parent: None, Level::DEBUG, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    event!(parent: None, Level::DEBUG, { foo = ?2, bar.baz = %78 }, "quux");
+    event!(target: "foo_events", parent: None, Level::DEBUG, foo = 3, bar.baz = 2, quux = false);
+    event!(target: "foo_events", parent: None, Level::DEBUG, foo = 3, bar.baz = 3,);
+    event!(target: "foo_events", parent: None, Level::DEBUG, "foo");
+    event!(target: "foo_events", parent: None, Level::DEBUG, "foo: {}", 3);
+    event!(target: "foo_events", parent: None, Level::DEBUG, { foo = 3, bar.baz = 80 }, "quux");
+    event!(target: "foo_events", parent: None, Level::DEBUG, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    event!(target: "foo_events", parent: None, Level::DEBUG, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    event!(target: "foo_events", parent: None, Level::DEBUG, { foo = 2, bar.baz = 78, }, "quux");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn trace_root() {
+    trace!(parent: None, foo = ?3, bar.baz = %2, quux = false);
+    trace!(parent: None, foo = 3, bar.baz = 2, quux = false);
+    trace!(parent: None, foo = 3, bar.baz = 3,);
+    trace!(parent: None, "foo");
+    trace!(parent: None, "foo: {}", 3);
+    trace!(parent: None, { foo = 3, bar.baz = 80 }, "quux");
+    trace!(parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    trace!(parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    trace!(parent: None, { foo = 2, bar.baz = 78 }, "quux");
+    trace!(parent: None, { foo = ?2, bar.baz = %78 }, "quux");
+    trace!(target: "foo_events", parent: None, foo = 3, bar.baz = 2, quux = false);
+    trace!(target: "foo_events", parent: None, foo = 3, bar.baz = 3,);
+    trace!(target: "foo_events", parent: None, "foo");
+    trace!(target: "foo_events", parent: None, "foo: {}", 3);
+    trace!(target: "foo_events", parent: None, { foo = 3, bar.baz = 80 }, "quux");
+    trace!(target: "foo_events", parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    trace!(target: "foo_events", parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    trace!(target: "foo_events", parent: None, { foo = 2, bar.baz = 78, }, "quux");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn debug_root() {
+    debug!(parent: None, foo = ?3, bar.baz = %2, quux = false);
+    debug!(parent: None, foo = 3, bar.baz = 2, quux = false);
+    debug!(parent: None, foo = 3, bar.baz = 3,);
+    debug!(parent: None, "foo");
+    debug!(parent: None, "foo: {}", 3);
+    debug!(parent: None, { foo = 3, bar.baz = 80 }, "quux");
+    debug!(parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    debug!(parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    debug!(parent: None, { foo = 2, bar.baz = 78 }, "quux");
+    debug!(parent: None, { foo = ?2, bar.baz = %78 }, "quux");
+    debug!(target: "foo_events", parent: None, foo = 3, bar.baz = 2, quux = false);
+    debug!(target: "foo_events", parent: None, foo = 3, bar.baz = 3,);
+    debug!(target: "foo_events", parent: None, "foo");
+    debug!(target: "foo_events", parent: None, "foo: {}", 3);
+    debug!(target: "foo_events", parent: None, { foo = 3, bar.baz = 80 }, "quux");
+    debug!(target: "foo_events", parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    debug!(target: "foo_events", parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    debug!(target: "foo_events", parent: None, { foo = 2, bar.baz = 78, }, "quux");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn info_root() {
+    info!(parent: None, foo = ?3, bar.baz = %2, quux = false);
+    info!(parent: None, foo = 3, bar.baz = 2, quux = false);
+    info!(parent: None, foo = 3, bar.baz = 3,);
+    info!(parent: None, "foo");
+    info!(parent: None, "foo: {}", 3);
+    info!(parent: None, { foo = 3, bar.baz = 80 }, "quux");
+    info!(parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    info!(parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    info!(parent: None, { foo = 2, bar.baz = 78 }, "quux");
+    info!(parent: None, { foo = ?2, bar.baz = %78 }, "quux");
+    info!(target: "foo_events", parent: None, foo = 3, bar.baz = 2, quux = false);
+    info!(target: "foo_events", parent: None, foo = 3, bar.baz = 3,);
+    info!(target: "foo_events", parent: None, "foo");
+    info!(target: "foo_events", parent: None, "foo: {}", 3);
+    info!(target: "foo_events", parent: None, { foo = 3, bar.baz = 80 }, "quux");
+    info!(target: "foo_events", parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    info!(target: "foo_events", parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    info!(target: "foo_events", parent: None, { foo = 2, bar.baz = 78, }, "quux");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn warn_root() {
+    warn!(parent: None, foo = ?3, bar.baz = %2, quux = false);
+    warn!(parent: None, foo = 3, bar.baz = 2, quux = false);
+    warn!(parent: None, foo = 3, bar.baz = 3,);
+    warn!(parent: None, "foo");
+    warn!(parent: None, "foo: {}", 3);
+    warn!(parent: None, { foo = 3, bar.baz = 80 }, "quux");
+    warn!(parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    warn!(parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    warn!(parent: None, { foo = 2, bar.baz = 78 }, "quux");
+    warn!(parent: None, { foo = ?2, bar.baz = %78 }, "quux");
+    warn!(target: "foo_events", parent: None, foo = 3, bar.baz = 2, quux = false);
+    warn!(target: "foo_events", parent: None, foo = 3, bar.baz = 3,);
+    warn!(target: "foo_events", parent: None, "foo");
+    warn!(target: "foo_events", parent: None, "foo: {}", 3);
+    warn!(target: "foo_events", parent: None, { foo = 3, bar.baz = 80 }, "quux");
+    warn!(target: "foo_events", parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    warn!(target: "foo_events", parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    warn!(target: "foo_events", parent: None, { foo = 2, bar.baz = 78, }, "quux");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn error_root() {
+    error!(parent: None, foo = ?3, bar.baz = %2, quux = false);
+    error!(parent: None, foo = 3, bar.baz = 2, quux = false);
+    error!(parent: None, foo = 3, bar.baz = 3,);
+    error!(parent: None, "foo");
+    error!(parent: None, "foo: {}", 3);
+    error!(parent: None, { foo = 3, bar.baz = 80 }, "quux");
+    error!(parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    error!(parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    error!(parent: None, { foo = 2, bar.baz = 78 }, "quux");
+    error!(parent: None, { foo = ?2, bar.baz = %78 }, "quux");
+    error!(target: "foo_events", parent: None, foo = 3, bar.baz = 2, quux = false);
+    error!(target: "foo_events", parent: None, foo = 3, bar.baz = 3,);
+    error!(target: "foo_events", parent: None, "foo");
+    error!(target: "foo_events", parent: None, "foo: {}", 3);
+    error!(target: "foo_events", parent: None, { foo = 3, bar.baz = 80 }, "quux");
+    error!(target: "foo_events", parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    error!(target: "foo_events", parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    error!(target: "foo_events", parent: None, { foo = 2, bar.baz = 78, }, "quux");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn event_with_parent() {
+    let p = span!(Level::TRACE, "im_a_parent!");
+    event!(parent: &p, Level::DEBUG, foo = ?3, bar.baz = %2, quux = false);
+    event!(parent: &p, Level::DEBUG, foo = 3, bar.baz = 2, quux = false);
+    event!(parent: &p, Level::DEBUG, foo = 3, bar.baz = 3,);
+    event!(parent: &p, Level::DEBUG, "foo");
+    event!(parent: &p, Level::DEBUG, "foo: {}", 3);
+    event!(parent: &p, Level::DEBUG, { foo = 3, bar.baz = 80 }, "quux");
+    event!(parent: &p, Level::DEBUG, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    event!(parent: &p, Level::DEBUG, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    event!(parent: &p, Level::DEBUG, { foo = ?2, bar.baz = %78 }, "quux");
+    event!(target: "foo_events", parent: &p, Level::DEBUG, foo = 3, bar.baz = 2, quux = false);
+    event!(target: "foo_events", parent: &p, Level::DEBUG, foo = 3, bar.baz = 3,);
+    event!(target: "foo_events", parent: &p, Level::DEBUG, "foo");
+    event!(target: "foo_events", parent: &p, Level::DEBUG, "foo: {}", 3);
+    event!(target: "foo_events", parent: &p, Level::DEBUG, { foo = 3, bar.baz = 80 }, "quux");
+    event!(target: "foo_events", parent: &p, Level::DEBUG, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    event!(target: "foo_events", parent: &p, Level::DEBUG, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    event!(target: "foo_events", parent: &p, Level::DEBUG, { foo = 2, bar.baz = 78, }, "quux");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn trace_with_parent() {
+    let p = span!(Level::TRACE, "im_a_parent!");
+    trace!(parent: &p, foo = ?3, bar.baz = %2, quux = false);
+    trace!(parent: &p, foo = 3, bar.baz = 2, quux = false);
+    trace!(parent: &p, foo = 3, bar.baz = 3,);
+    trace!(parent: &p, "foo");
+    trace!(parent: &p, "foo: {}", 3);
+    trace!(parent: &p, { foo = 3, bar.baz = 80 }, "quux");
+    trace!(parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    trace!(parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    trace!(parent: &p, { foo = 2, bar.baz = 78 }, "quux");
+    trace!(parent: &p, { foo = ?2, bar.baz = %78 }, "quux");
+    trace!(target: "foo_events", parent: &p, foo = 3, bar.baz = 2, quux = false);
+    trace!(target: "foo_events", parent: &p, foo = 3, bar.baz = 3,);
+    trace!(target: "foo_events", parent: &p, "foo");
+    trace!(target: "foo_events", parent: &p, "foo: {}", 3);
+    trace!(target: "foo_events", parent: &p, { foo = 3, bar.baz = 80 }, "quux");
+    trace!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    trace!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    trace!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 78, }, "quux");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn debug_with_parent() {
+    let p = span!(Level::TRACE, "im_a_parent!");
+    debug!(parent: &p, foo = ?3, bar.baz = %2, quux = false);
+    debug!(parent: &p, foo = 3, bar.baz = 2, quux = false);
+    debug!(parent: &p, foo = 3, bar.baz = 3,);
+    debug!(parent: &p, "foo");
+    debug!(parent: &p, "foo: {}", 3);
+    debug!(parent: &p, { foo = 3, bar.baz = 80 }, "quux");
+    debug!(parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    debug!(parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    debug!(parent: &p, { foo = 2, bar.baz = 78 }, "quux");
+    debug!(parent: &p, { foo = ?2, bar.baz = %78 }, "quux");
+    debug!(target: "foo_events", parent: &p, foo = 3, bar.baz = 2, quux = false);
+    debug!(target: "foo_events", parent: &p, foo = 3, bar.baz = 3,);
+    debug!(target: "foo_events", parent: &p, "foo");
+    debug!(target: "foo_events", parent: &p, "foo: {}", 3);
+    debug!(target: "foo_events", parent: &p, { foo = 3, bar.baz = 80 }, "quux");
+    debug!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    debug!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    debug!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 78, }, "quux");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn info_with_parent() {
+    let p = span!(Level::TRACE, "im_a_parent!");
+    info!(parent: &p, foo = ?3, bar.baz = %2, quux = false);
+    info!(parent: &p, foo = 3, bar.baz = 2, quux = false);
+    info!(parent: &p, foo = 3, bar.baz = 3,);
+    info!(parent: &p, "foo");
+    info!(parent: &p, "foo: {}", 3);
+    info!(parent: &p, { foo = 3, bar.baz = 80 }, "quux");
+    info!(parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    info!(parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    info!(parent: &p, { foo = 2, bar.baz = 78 }, "quux");
+    info!(parent: &p, { foo = ?2, bar.baz = %78 }, "quux");
+    info!(target: "foo_events", parent: &p, foo = 3, bar.baz = 2, quux = false);
+    info!(target: "foo_events", parent: &p, foo = 3, bar.baz = 3,);
+    info!(target: "foo_events", parent: &p, "foo");
+    info!(target: "foo_events", parent: &p, "foo: {}", 3);
+    info!(target: "foo_events", parent: &p, { foo = 3, bar.baz = 80 }, "quux");
+    info!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    info!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    info!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 78, }, "quux");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn warn_with_parent() {
+    let p = span!(Level::TRACE, "im_a_parent!");
+    warn!(parent: &p, foo = ?3, bar.baz = %2, quux = false);
+    warn!(parent: &p, foo = 3, bar.baz = 2, quux = false);
+    warn!(parent: &p, foo = 3, bar.baz = 3,);
+    warn!(parent: &p, "foo");
+    warn!(parent: &p, "foo: {}", 3);
+    warn!(parent: &p, { foo = 3, bar.baz = 80 }, "quux");
+    warn!(parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    warn!(parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    warn!(parent: &p, { foo = 2, bar.baz = 78 }, "quux");
+    warn!(parent: &p, { foo = ?2, bar.baz = %78 }, "quux");
+    warn!(target: "foo_events", parent: &p, foo = 3, bar.baz = 2, quux = false);
+    warn!(target: "foo_events", parent: &p, foo = 3, bar.baz = 3,);
+    warn!(target: "foo_events", parent: &p, "foo");
+    warn!(target: "foo_events", parent: &p, "foo: {}", 3);
+    warn!(target: "foo_events", parent: &p, { foo = 3, bar.baz = 80 }, "quux");
+    warn!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    warn!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    warn!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 78, }, "quux");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn error_with_parent() {
+    let p = span!(Level::TRACE, "im_a_parent!");
+    error!(parent: &p, foo = ?3, bar.baz = %2, quux = false);
+    error!(parent: &p, foo = 3, bar.baz = 2, quux = false);
+    error!(parent: &p, foo = 3, bar.baz = 3,);
+    error!(parent: &p, "foo");
+    error!(parent: &p, "foo: {}", 3);
+    error!(parent: &p, { foo = 3, bar.baz = 80 }, "quux");
+    error!(parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    error!(parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    error!(parent: &p, { foo = 2, bar.baz = 78 }, "quux");
+    error!(parent: &p, { foo = ?2, bar.baz = %78 }, "quux");
+    error!(target: "foo_events", parent: &p, foo = 3, bar.baz = 2, quux = false);
+    error!(target: "foo_events", parent: &p, foo = 3, bar.baz = 3,);
+    error!(target: "foo_events", parent: &p, "foo");
+    error!(target: "foo_events", parent: &p, "foo: {}", 3);
+    error!(target: "foo_events", parent: &p, { foo = 3, bar.baz = 80 }, "quux");
+    error!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    error!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    error!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 78, }, "quux");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn field_shorthand_only() {
+    #[derive(Debug)]
+    struct Position {
+        x: f32,
+        y: f32,
+    }
+    let pos = Position {
+        x: 3.234,
+        y: -1.223,
+    };
+
+    trace!(?pos.x, ?pos.y);
+    debug!(?pos.x, ?pos.y);
+    info!(?pos.x, ?pos.y);
+    warn!(?pos.x, ?pos.y);
+    error!(?pos.x, ?pos.y);
+    event!(Level::TRACE, ?pos.x, ?pos.y);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn borrow_val_events() {
+    // Reproduces https://github.com/tokio-rs/tracing/issues/954
+    let mut foo = (String::new(), String::new());
+    let zero = &mut foo.0;
+    trace!(one = ?foo.1);
+    debug!(one = ?foo.1);
+    info!(one = ?foo.1);
+    warn!(one = ?foo.1);
+    error!(one = ?foo.1);
+    zero.push_str("hello world");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn borrow_val_spans() {
+    // Reproduces https://github.com/tokio-rs/tracing/issues/954
+    let mut foo = (String::new(), String::new());
+    let zero = &mut foo.0;
+    let _span = trace_span!("span", one = ?foo.1);
+    let _span = debug_span!("span", one = ?foo.1);
+    let _span = info_span!("span", one = ?foo.1);
+    let _span = warn_span!("span", one = ?foo.1);
+    let _span = error_span!("span", one = ?foo.1);
+    zero.push_str("hello world");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn callsite_macro_api() {
+    // This test should catch any inadvertent breaking changes
+    // caused by changes to the macro.
+    let _callsite = callsite! {
+        name: "test callsite",
+        kind: tracing::metadata::Kind::EVENT,
+        target: "test target",
+        level: tracing::Level::TRACE,
+        fields: foo, bar,
+    };
+    let _callsite = callsite! {
+        name: "test callsite",
+        kind: tracing::metadata::Kind::SPAN,
+        level: tracing::Level::TRACE,
+        fields: foo,
+    };
+    let _callsite = callsite! {
+        name: "test callsite",
+        kind: tracing::metadata::Kind::SPAN,
+        fields: foo,
+    };
+}
diff --git a/third_party/rust/tracing/tests/max_level_hint.rs b/third_party/rust/tracing/tests/max_level_hint.rs
new file mode 100644
index 0000000000..49f14d55fe
--- /dev/null
+++ b/third_party/rust/tracing/tests/max_level_hint.rs
@@ -0,0 +1,39 @@
+mod support;
+
+use self::support::*;
+use tracing::Level;
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn max_level_hints() {
+    // This test asserts that when a subscriber provides us with the global
+    // maximum level that it will enable (by implementing the
+    // `Subscriber::max_level_hint` method), we will never call
+    // `Subscriber::enabled` for events above that maximum level.
+    //
+    // In this case, we test that by making the `enabled` method assert that no
+    // `Metadata` for spans or events at the `TRACE` or `DEBUG` levels.
+    let (subscriber, handle) = subscriber::mock()
+        .with_max_level_hint(Level::INFO)
+        .with_filter(|meta| {
+            assert!(
+                dbg!(meta).level() <= &Level::INFO,
+                "a TRACE or DEBUG event was dynamically filtered: "
+            );
+            true
+        })
+        .event(event::mock().at_level(Level::INFO))
+        .event(event::mock().at_level(Level::WARN))
+        .event(event::mock().at_level(Level::ERROR))
+        .done()
+        .run_with_handle();
+
+    tracing::subscriber::set_global_default(subscriber).unwrap();
+
+    tracing::info!("doing a thing that you might care about");
+    tracing::debug!("charging turboencabulator with interocitor");
+    tracing::warn!("extremely serious warning, pay attention");
+    tracing::trace!("interocitor charge level is 10%");
+    tracing::error!("everything is on fire");
+    handle.assert_finished();
+}
diff --git a/third_party/rust/tracing/tests/multiple_max_level_hints.rs b/third_party/rust/tracing/tests/multiple_max_level_hints.rs
new file mode 100644
index 0000000000..b2be6351cf
--- /dev/null
+++ b/third_party/rust/tracing/tests/multiple_max_level_hints.rs
@@ -0,0 +1,70 @@
+#![cfg(feature = "std")]
+mod support;
+
+use self::support::*;
+use tracing::Level;
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn multiple_max_level_hints() {
+    // This test ensures that when multiple subscribers are active, their max
+    // level hints are handled correctly. The global max level should be the
+    // maximum of the level filters returned by the two `Subscriber`'s
+    // `max_level_hint` method.
+    //
+    // In this test, we create a subscriber whose max level is `INFO`, and
+    // another whose max level is `DEBUG`. We then add an assertion to both of
+    // those subscribers' `enabled` method that no metadata for `TRACE` spans or
+    // events are filtered, since they are disabled by the global max filter.
+
+    fn do_events() {
+        tracing::info!("doing a thing that you might care about");
+        tracing::debug!("charging turboencabulator with interocitor");
+        tracing::warn!("extremely serious warning, pay attention");
+        tracing::trace!("interocitor charge level is 10%");
+        tracing::error!("everything is on fire");
+    }
+
+    let (subscriber1, handle1) = subscriber::mock()
+        .named("subscriber1")
+        .with_max_level_hint(Level::INFO)
+        .with_filter(|meta| {
+            let level = dbg!(meta.level());
+            assert!(
+                level <= &Level::DEBUG,
+                "a TRACE event was dynamically filtered by subscriber1"
+            );
+            level <= &Level::INFO
+        })
+        .event(event::mock().at_level(Level::INFO))
+        .event(event::mock().at_level(Level::WARN))
+        .event(event::mock().at_level(Level::ERROR))
+        .done()
+        .run_with_handle();
+    let (subscriber2, handle2) = subscriber::mock()
+        .named("subscriber2")
+        .with_max_level_hint(Level::DEBUG)
+        .with_filter(|meta| {
+            let level = dbg!(meta.level());
+            assert!(
+                level <= &Level::DEBUG,
+                "a TRACE event was dynamically filtered by subscriber2"
+            );
+            level <= &Level::DEBUG
+        })
+        .event(event::mock().at_level(Level::INFO))
+        .event(event::mock().at_level(Level::DEBUG))
+        .event(event::mock().at_level(Level::WARN))
+        .event(event::mock().at_level(Level::ERROR))
+        .done()
+        .run_with_handle();
+
+    let dispatch1 = tracing::Dispatch::new(subscriber1);
+
+    tracing::dispatcher::with_default(&dispatch1, do_events);
+    handle1.assert_finished();
+
+    let dispatch2 = tracing::Dispatch::new(subscriber2);
+    tracing::dispatcher::with_default(&dispatch2, do_events);
+    handle2.assert_finished();
+}
diff --git a/third_party/rust/tracing/tests/span.rs b/third_party/rust/tracing/tests/span.rs
new file mode 100644
index 0000000000..79914950c2
--- /dev/null
+++ b/third_party/rust/tracing/tests/span.rs
@@ -0,0 +1,767 @@
+// These tests require the thread-local scoped dispatcher, which only works when
+// we have a standard library. The behaviour being tested should be the same
+// with the standard lib disabled.
+#![cfg(feature = "std")]
+
+#[macro_use]
+extern crate tracing;
+mod support;
+
+use self::support::*;
+use std::thread;
+use tracing::{
+    field::{debug, display},
+    subscriber::with_default,
+    Level, Span,
+};
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn handles_to_the_same_span_are_equal() {
+    // Create a mock subscriber that will return `true` on calls to
+    // `Subscriber::enabled`, so that the spans will be constructed. We
+    // won't enter any spans in this test, so the subscriber won't actually
+    // expect to see any spans.
+    with_default(subscriber::mock().run(), || {
+        let foo1 = span!(Level::TRACE, "foo");
+        let foo2 = foo1.clone();
+        // Two handles that point to the same span are equal.
+        assert_eq!(foo1, foo2);
+    });
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn handles_to_different_spans_are_not_equal() {
+    with_default(subscriber::mock().run(), || {
+        // Even though these spans have the same name and fields, they will have
+        // differing metadata, since they were created on different lines.
+        let foo1 = span!(Level::TRACE, "foo", bar = 1u64, baz = false);
+        let foo2 = span!(Level::TRACE, "foo", bar = 1u64, baz = false);
+
+        assert_ne!(foo1, foo2);
+    });
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn handles_to_different_spans_with_the_same_metadata_are_not_equal() {
+    // Every time time this function is called, it will return a _new
+    // instance_ of a span with the same metadata, name, and fields.
+    fn make_span() -> Span {
+        span!(Level::TRACE, "foo", bar = 1u64, baz = false)
+    }
+
+    with_default(subscriber::mock().run(), || {
+        let foo1 = make_span();
+        let foo2 = make_span();
+
+        assert_ne!(foo1, foo2);
+        // assert_ne!(foo1.data(), foo2.data());
+    });
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn spans_always_go_to_the_subscriber_that_tagged_them() {
+    let subscriber1 = subscriber::mock()
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .done()
+        .run();
+    let subscriber2 = subscriber::mock().run();
+
+    let foo = with_default(subscriber1, || {
+        let foo = span!(Level::TRACE, "foo");
+        foo.in_scope(|| {});
+        foo
+    });
+    // Even though we enter subscriber 2's context, the subscriber that
+    // tagged the span should see the enter/exit.
+    with_default(subscriber2, move || foo.in_scope(|| {}));
+}
+
+// This gets exempt from testing in wasm because of: `thread::spawn` which is
+// not yet possible to do in WASM. There is work going on see:
+// <https://rustwasm.github.io/2018/10/24/multithreading-rust-and-wasm.html>
+//
+// But for now since it's not possible we don't need to test for it :)
+#[test]
+fn spans_always_go_to_the_subscriber_that_tagged_them_even_across_threads() {
+    let subscriber1 = subscriber::mock()
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .done()
+        .run();
+    let foo = with_default(subscriber1, || {
+        let foo = span!(Level::TRACE, "foo");
+        foo.in_scope(|| {});
+        foo
+    });
+
+    // Even though we enter subscriber 2's context, the subscriber that
+    // tagged the span should see the enter/exit.
+    thread::spawn(move || {
+        with_default(subscriber::mock().run(), || {
+            foo.in_scope(|| {});
+        })
+    })
+    .join()
+    .unwrap();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn dropping_a_span_calls_drop_span() {
+    let (subscriber, handle) = subscriber::mock()
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        let span = span!(Level::TRACE, "foo");
+        span.in_scope(|| {});
+        drop(span);
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn span_closes_after_event() {
+    let (subscriber, handle) = subscriber::mock()
+        .enter(span::mock().named("foo"))
+        .event(event::mock())
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        span!(Level::TRACE, "foo").in_scope(|| {
+            event!(Level::DEBUG, {}, "my event!");
+        });
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn new_span_after_event() {
+    let (subscriber, handle) = subscriber::mock()
+        .enter(span::mock().named("foo"))
+        .event(event::mock())
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .enter(span::mock().named("bar"))
+        .exit(span::mock().named("bar"))
+        .drop_span(span::mock().named("bar"))
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        span!(Level::TRACE, "foo").in_scope(|| {
+            event!(Level::DEBUG, {}, "my event!");
+        });
+        span!(Level::TRACE, "bar").in_scope(|| {});
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn event_outside_of_span() {
+    let (subscriber, handle) = subscriber::mock()
+        .event(event::mock())
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        debug!("my event!");
+        span!(Level::TRACE, "foo").in_scope(|| {});
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn cloning_a_span_calls_clone_span() {
+    let (subscriber, handle) = subscriber::mock()
+        .clone_span(span::mock().named("foo"))
+        .run_with_handle();
+    with_default(subscriber, || {
+        let span = span!(Level::TRACE, "foo");
+        // Allow the "redundant" `.clone` since it is used to call into the `.clone_span` hook.
+        #[allow(clippy::redundant_clone)]
+        let _span2 = span.clone();
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn drop_span_when_exiting_dispatchers_context() {
+    let (subscriber, handle) = subscriber::mock()
+        .clone_span(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .run_with_handle();
+    with_default(subscriber, || {
+        let span = span!(Level::TRACE, "foo");
+        let _span2 = span.clone();
+        drop(span);
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn clone_and_drop_span_always_go_to_the_subscriber_that_tagged_the_span() {
+    let (subscriber1, handle1) = subscriber::mock()
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .clone_span(span::mock().named("foo"))
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .run_with_handle();
+    let subscriber2 = subscriber::mock().done().run();
+
+    let foo = with_default(subscriber1, || {
+        let foo = span!(Level::TRACE, "foo");
+        foo.in_scope(|| {});
+        foo
+    });
+    // Even though we enter subscriber 2's context, the subscriber that
+    // tagged the span should see the enter/exit.
+    with_default(subscriber2, move || {
+        let foo2 = foo.clone();
+        foo.in_scope(|| {});
+        drop(foo);
+        drop(foo2);
+    });
+
+    handle1.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn span_closes_when_exited() {
+    let (subscriber, handle) = subscriber::mock()
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        let foo = span!(Level::TRACE, "foo");
+
+        foo.in_scope(|| {});
+
+        drop(foo);
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn enter() {
+    let (subscriber, handle) = subscriber::mock()
+        .enter(span::mock().named("foo"))
+        .event(event::mock())
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        let foo = span!(Level::TRACE, "foo");
+        let _enter = foo.enter();
+        debug!("dropping guard...");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn moved_field() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(
+            span::mock().named("foo").with_field(
+                field::mock("bar")
+                    .with_value(&display("hello from my span"))
+                    .only(),
+            ),
+        )
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        let from = "my span";
+        let span = span!(
+            Level::TRACE,
+            "foo",
+            bar = display(format!("hello from {}", from))
+        );
+        span.in_scope(|| {});
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn dotted_field_name() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(
+            span::mock()
+                .named("foo")
+                .with_field(field::mock("fields.bar").with_value(&true).only()),
+        )
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        span!(Level::TRACE, "foo", fields.bar = true);
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn borrowed_field() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(
+            span::mock().named("foo").with_field(
+                field::mock("bar")
+                    .with_value(&display("hello from my span"))
+                    .only(),
+            ),
+        )
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        let from = "my span";
+        let mut message = format!("hello from {}", from);
+        let span = span!(Level::TRACE, "foo", bar = display(&message));
+        span.in_scope(|| {
+            message.insert_str(10, " inside");
+        });
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+// If emitting log instrumentation, this gets moved anyway, breaking the test.
+#[cfg(not(feature = "log"))]
+fn move_field_out_of_struct() {
+    use tracing::field::debug;
+
+    #[derive(Debug)]
+    struct Position {
+        x: f32,
+        y: f32,
+    }
+
+    let pos = Position {
+        x: 3.234,
+        y: -1.223,
+    };
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(
+            span::mock().named("foo").with_field(
+                field::mock("x")
+                    .with_value(&debug(3.234))
+                    .and(field::mock("y").with_value(&debug(-1.223)))
+                    .only(),
+            ),
+        )
+        .new_span(
+            span::mock()
+                .named("bar")
+                .with_field(field::mock("position").with_value(&debug(&pos)).only()),
+        )
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        let pos = Position {
+            x: 3.234,
+            y: -1.223,
+        };
+        let foo = span!(Level::TRACE, "foo", x = debug(pos.x), y = debug(pos.y));
+        let bar = span!(Level::TRACE, "bar", position = debug(pos));
+        foo.in_scope(|| {});
+        bar.in_scope(|| {});
+    });
+
+    handle.assert_finished();
+}
+
+// TODO(#1138): determine a new syntax for uninitialized span fields, and
+// re-enable these.
+/*
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn add_field_after_new_span() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(
+            span::mock()
+                .named("foo")
+                .with_field(field::mock("bar").with_value(&5)
+                .and(field::mock("baz").with_value).only()),
+        )
+        .record(
+            span::mock().named("foo"),
+            field::mock("baz").with_value(&true).only(),
+        )
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        let span = span!(Level::TRACE, "foo", bar = 5, baz = false);
+        span.record("baz", &true);
+        span.in_scope(|| {})
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn add_fields_only_after_new_span() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(span::mock().named("foo"))
+        .record(
+            span::mock().named("foo"),
+            field::mock("bar").with_value(&5).only(),
+        )
+        .record(
+            span::mock().named("foo"),
+            field::mock("baz").with_value(&true).only(),
+        )
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        let span = span!(Level::TRACE, "foo", bar = _, baz = _);
+        span.record("bar", &5);
+        span.record("baz", &true);
+        span.in_scope(|| {})
+    });
+
+    handle.assert_finished();
+}
+*/
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn record_new_value_for_field() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(
+            span::mock().named("foo").with_field(
+                field::mock("bar")
+                    .with_value(&5)
+                    .and(field::mock("baz").with_value(&false))
+                    .only(),
+            ),
+        )
+        .record(
+            span::mock().named("foo"),
+            field::mock("baz").with_value(&true).only(),
+        )
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        let span = span!(Level::TRACE, "foo", bar = 5, baz = false);
+        span.record("baz", &true);
+        span.in_scope(|| {})
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn record_new_values_for_fields() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(
+            span::mock().named("foo").with_field(
+                field::mock("bar")
+                    .with_value(&4)
+                    .and(field::mock("baz").with_value(&false))
+                    .only(),
+            ),
+        )
+        .record(
+            span::mock().named("foo"),
+            field::mock("bar").with_value(&5).only(),
+        )
+        .record(
+            span::mock().named("foo"),
+            field::mock("baz").with_value(&true).only(),
+        )
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        let span = span!(Level::TRACE, "foo", bar = 4, baz = false);
+        span.record("bar", &5);
+        span.record("baz", &true);
+        span.in_scope(|| {})
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn new_span_with_target_and_log_level() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(
+            span::mock()
+                .named("foo")
+                .with_target("app_span")
+                .at_level(Level::DEBUG),
+        )
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        span!(target: "app_span", Level::DEBUG, "foo");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn explicit_root_span_is_root() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(span::mock().named("foo").with_explicit_parent(None))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        span!(parent: None, Level::TRACE, "foo");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn explicit_root_span_is_root_regardless_of_ctx() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(span::mock().named("foo"))
+        .enter(span::mock().named("foo"))
+        .new_span(span::mock().named("bar").with_explicit_parent(None))
+        .exit(span::mock().named("foo"))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        span!(Level::TRACE, "foo").in_scope(|| {
+            span!(parent: None, Level::TRACE, "bar");
+        })
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn explicit_child() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(span::mock().named("foo"))
+        .new_span(span::mock().named("bar").with_explicit_parent(Some("foo")))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        let foo = span!(Level::TRACE, "foo");
+        span!(parent: foo.id(), Level::TRACE, "bar");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn explicit_child_at_levels() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(span::mock().named("foo"))
+        .new_span(span::mock().named("a").with_explicit_parent(Some("foo")))
+        .new_span(span::mock().named("b").with_explicit_parent(Some("foo")))
+        .new_span(span::mock().named("c").with_explicit_parent(Some("foo")))
+        .new_span(span::mock().named("d").with_explicit_parent(Some("foo")))
+        .new_span(span::mock().named("e").with_explicit_parent(Some("foo")))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        let foo = span!(Level::TRACE, "foo");
+        trace_span!(parent: foo.id(), "a");
+        debug_span!(parent: foo.id(), "b");
+        info_span!(parent: foo.id(), "c");
+        warn_span!(parent: foo.id(), "d");
+        error_span!(parent: foo.id(), "e");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn explicit_child_regardless_of_ctx() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(span::mock().named("foo"))
+        .new_span(span::mock().named("bar"))
+        .enter(span::mock().named("bar"))
+        .new_span(span::mock().named("baz").with_explicit_parent(Some("foo")))
+        .exit(span::mock().named("bar"))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        let foo = span!(Level::TRACE, "foo");
+        span!(Level::TRACE, "bar").in_scope(|| span!(parent: foo.id(), Level::TRACE, "baz"))
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn contextual_root() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(span::mock().named("foo").with_contextual_parent(None))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        span!(Level::TRACE, "foo");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn contextual_child() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(span::mock().named("foo"))
+        .enter(span::mock().named("foo"))
+        .new_span(
+            span::mock()
+                .named("bar")
+                .with_contextual_parent(Some("foo")),
+        )
+        .exit(span::mock().named("foo"))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        span!(Level::TRACE, "foo").in_scope(|| {
+            span!(Level::TRACE, "bar");
+        })
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn display_shorthand() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(
+            span::mock().named("my_span").with_field(
+                field::mock("my_field")
+                    .with_value(&display("hello world"))
+                    .only(),
+            ),
+        )
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        span!(Level::TRACE, "my_span", my_field = %"hello world");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn debug_shorthand() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(
+            span::mock().named("my_span").with_field(
+                field::mock("my_field")
+                    .with_value(&debug("hello world"))
+                    .only(),
+            ),
+        )
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        span!(Level::TRACE, "my_span", my_field = ?"hello world");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn both_shorthands() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(
+            span::mock().named("my_span").with_field(
+                field::mock("display_field")
+                    .with_value(&display("hello world"))
+                    .and(field::mock("debug_field").with_value(&debug("hello world")))
+                    .only(),
+            ),
+        )
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        span!(Level::TRACE, "my_span", display_field = %"hello world", debug_field = ?"hello world");
+    });
+
+    handle.assert_finished();
+}
diff --git a/third_party/rust/tracing/tests/subscriber.rs b/third_party/rust/tracing/tests/subscriber.rs
new file mode 100644
index 0000000000..2a810c9111
--- /dev/null
+++ b/third_party/rust/tracing/tests/subscriber.rs
@@ -0,0 +1,57 @@
+// These tests require the thread-local scoped dispatcher, which only works when
+// we have a standard library. The behaviour being tested should be the same
+// with the standard lib disabled.
+//
+// The alternative would be for each of these tests to be defined in a separate
+// file, which is :(
+#![cfg(feature = "std")]
+
+#[macro_use]
+extern crate tracing;
+use tracing::{
+    span,
+    subscriber::{with_default, Interest, Subscriber},
+    Event, Level, Metadata,
+};
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn event_macros_dont_infinite_loop() {
+    // This test ensures that an event macro within a subscriber
+    // won't cause an infinite loop of events.
+    struct TestSubscriber;
+    impl Subscriber for TestSubscriber {
+        fn register_callsite(&self, _: &Metadata<'_>) -> Interest {
+            // Always return sometimes so that `enabled` will be called
+            // (which can loop).
+            Interest::sometimes()
+        }
+
+        fn enabled(&self, meta: &Metadata<'_>) -> bool {
+            assert!(meta.fields().iter().any(|f| f.name() == "foo"));
+            event!(Level::TRACE, bar = false);
+            true
+        }
+
+        fn new_span(&self, _: &span::Attributes<'_>) -> span::Id {
+            span::Id::from_u64(0xAAAA)
+        }
+
+        fn record(&self, _: &span::Id, _: &span::Record<'_>) {}
+
+        fn record_follows_from(&self, _: &span::Id, _: &span::Id) {}
+
+        fn event(&self, event: &Event<'_>) {
+            assert!(event.metadata().fields().iter().any(|f| f.name() == "foo"));
+            event!(Level::TRACE, baz = false);
+        }
+
+        fn enter(&self, _: &span::Id) {}
+
+        fn exit(&self, _: &span::Id) {}
+    }
+
+    with_default(TestSubscriber, || {
+        event!(Level::TRACE, foo = false);
+    })
+}
diff --git a/third_party/rust/tracing/tests/support/event.rs b/third_party/rust/tracing/tests/support/event.rs
new file mode 100644
index 0000000000..7033d8a134
--- /dev/null
+++ b/third_party/rust/tracing/tests/support/event.rs
@@ -0,0 +1,99 @@
+#![allow(missing_docs)]
+use super::{field, metadata, Parent};
+
+use std::fmt;
+
+/// A mock event.
+///
+/// This is intended for use with the mock subscriber API in the
+/// `subscriber` module.
+#[derive(Debug, Default, Eq, PartialEq)]
+pub struct MockEvent {
+    pub fields: Option<field::Expect>,
+    pub(in crate::support) parent: Option<Parent>,
+    metadata: metadata::Expect,
+}
+
+pub fn mock() -> MockEvent {
+    MockEvent {
+        ..Default::default()
+    }
+}
+
+impl MockEvent {
+    pub fn named<I>(self, name: I) -> Self
+    where
+        I: Into<String>,
+    {
+        Self {
+            metadata: metadata::Expect {
+                name: Some(name.into()),
+                ..self.metadata
+            },
+            ..self
+        }
+    }
+
+    pub fn with_fields<I>(self, fields: I) -> Self
+    where
+        I: Into<field::Expect>,
+    {
+        Self {
+            fields: Some(fields.into()),
+            ..self
+        }
+    }
+
+    pub fn at_level(self, level: tracing::Level) -> Self {
+        Self {
+            metadata: metadata::Expect {
+                level: Some(level),
+                ..self.metadata
+            },
+            ..self
+        }
+    }
+
+    pub fn with_target<I>(self, target: I) -> Self
+    where
+        I: Into<String>,
+    {
+        Self {
+            metadata: metadata::Expect {
+                target: Some(target.into()),
+                ..self.metadata
+            },
+            ..self
+        }
+    }
+
+    pub fn with_explicit_parent(self, parent: Option<&str>) -> MockEvent {
+        let parent = match parent {
+            Some(name) => Parent::Explicit(name.into()),
+            None => Parent::ExplicitRoot,
+        };
+        Self {
+            parent: Some(parent),
+            ..self
+        }
+    }
+
+    pub(in crate::support) fn check(&mut self, event: &tracing::Event<'_>) {
+        let meta = event.metadata();
+        let name = meta.name();
+        self.metadata
+            .check(meta, format_args!("event \"{}\"", name));
+        assert!(meta.is_event(), "expected {}, but got {:?}", self, event);
+        if let Some(ref mut expected_fields) = self.fields {
+            let mut checker = expected_fields.checker(name.to_string());
+            event.record(&mut checker);
+            checker.finish();
+        }
+    }
+}
+
+impl fmt::Display for MockEvent {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "an event{}", self.metadata)
+    }
+}
diff --git a/third_party/rust/tracing/tests/support/field.rs b/third_party/rust/tracing/tests/support/field.rs
new file mode 100644
index 0000000000..3667cf08b4
--- /dev/null
+++ b/third_party/rust/tracing/tests/support/field.rs
@@ -0,0 +1,226 @@
+use tracing::{
+    callsite,
+    callsite::Callsite,
+    field::{self, Field, Value, Visit},
+    metadata::Kind,
+};
+
+use std::{collections::HashMap, fmt};
+
+#[derive(Default, Debug, Eq, PartialEq)]
+pub struct Expect {
+    fields: HashMap<String, MockValue>,
+    only: bool,
+}
+
+#[derive(Debug)]
+pub struct MockField {
+    name: String,
+    value: MockValue,
+}
+
+#[derive(Debug, Eq, PartialEq)]
+pub enum MockValue {
+    I64(i64),
+    U64(u64),
+    Bool(bool),
+    Str(String),
+    Debug(String),
+    Any,
+}
+
+pub fn mock<K>(name: K) -> MockField
+where
+    String: From<K>,
+{
+    MockField {
+        name: name.into(),
+        value: MockValue::Any,
+    }
+}
+
+impl MockField {
+    /// Expect a field with the given name and value.
+    pub fn with_value(self, value: &dyn Value) -> Self {
+        Self {
+            value: MockValue::from(value),
+            ..self
+        }
+    }
+
+    pub fn and(self, other: MockField) -> Expect {
+        Expect {
+            fields: HashMap::new(),
+            only: false,
+        }
+        .and(self)
+        .and(other)
+    }
+
+    pub fn only(self) -> Expect {
+        Expect {
+            fields: HashMap::new(),
+            only: true,
+        }
+        .and(self)
+    }
+}
+
+impl Into<Expect> for MockField {
+    fn into(self) -> Expect {
+        Expect {
+            fields: HashMap::new(),
+            only: false,
+        }
+        .and(self)
+    }
+}
+
+impl Expect {
+    pub fn and(mut self, field: MockField) -> Self {
+        self.fields.insert(field.name, field.value);
+        self
+    }
+
+    /// Indicates that no fields other than those specified should be expected.
+    pub fn only(self) -> Self {
+        Self { only: true, ..self }
+    }
+
+    fn compare_or_panic(&mut self, name: &str, value: &dyn Value, ctx: &str) {
+        let value = value.into();
+        match self.fields.remove(name) {
+            Some(MockValue::Any) => {}
+            Some(expected) => assert!(
+                expected == value,
+                "\nexpected `{}` to contain:\n\t`{}{}`\nbut got:\n\t`{}{}`",
+                ctx,
+                name,
+                expected,
+                name,
+                value
+            ),
+            None if self.only => panic!(
+                "\nexpected `{}` to contain only:\n\t`{}`\nbut got:\n\t`{}{}`",
+                ctx, self, name, value
+            ),
+            _ => {}
+        }
+    }
+
+    pub fn checker(&mut self, ctx: String) -> CheckVisitor<'_> {
+        CheckVisitor { expect: self, ctx }
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.fields.is_empty()
+    }
+}
+
+impl fmt::Display for MockValue {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            MockValue::I64(v) => write!(f, "i64 = {:?}", v),
+            MockValue::U64(v) => write!(f, "u64 = {:?}", v),
+            MockValue::Bool(v) => write!(f, "bool = {:?}", v),
+            MockValue::Str(v) => write!(f, "&str = {:?}", v),
+            MockValue::Debug(v) => write!(f, "&fmt::Debug = {:?}", v),
+            MockValue::Any => write!(f, "_ = _"),
+        }
+    }
+}
+
+pub struct CheckVisitor<'a> {
+    expect: &'a mut Expect,
+    ctx: String,
+}
+
+impl<'a> Visit for CheckVisitor<'a> {
+    fn record_i64(&mut self, field: &Field, value: i64) {
+        self.expect
+            .compare_or_panic(field.name(), &value, &self.ctx[..])
+    }
+
+    fn record_u64(&mut self, field: &Field, value: u64) {
+        self.expect
+            .compare_or_panic(field.name(), &value, &self.ctx[..])
+    }
+
+    fn record_bool(&mut self, field: &Field, value: bool) {
+        self.expect
+            .compare_or_panic(field.name(), &value, &self.ctx[..])
+    }
+
+    fn record_str(&mut self, field: &Field, value: &str) {
+        self.expect
+            .compare_or_panic(field.name(), &value, &self.ctx[..])
+    }
+
+    fn record_debug(&mut self, field: &Field, value: &dyn fmt::Debug) {
+        self.expect
+            .compare_or_panic(field.name(), &field::debug(value), &self.ctx)
+    }
+}
+
+impl<'a> CheckVisitor<'a> {
+    pub fn finish(self) {
+        assert!(
+            self.expect.fields.is_empty(),
+            "{}missing {}",
+            self.expect,
+            self.ctx
+        );
+    }
+}
+
+impl<'a> From<&'a dyn Value> for MockValue {
+    fn from(value: &'a dyn Value) -> Self {
+        struct MockValueBuilder {
+            value: Option<MockValue>,
+        }
+
+        impl Visit for MockValueBuilder {
+            fn record_i64(&mut self, _: &Field, value: i64) {
+                self.value = Some(MockValue::I64(value));
+            }
+
+            fn record_u64(&mut self, _: &Field, value: u64) {
+                self.value = Some(MockValue::U64(value));
+            }
+
+            fn record_bool(&mut self, _: &Field, value: bool) {
+                self.value = Some(MockValue::Bool(value));
+            }
+
+            fn record_str(&mut self, _: &Field, value: &str) {
+                self.value = Some(MockValue::Str(value.to_owned()));
+            }
+
+            fn record_debug(&mut self, _: &Field, value: &dyn fmt::Debug) {
+                self.value = Some(MockValue::Debug(format!("{:?}", value)));
+            }
+        }
+
+        let fake_field = callsite!(name: "fake", kind: Kind::EVENT, fields: fake_field)
+            .metadata()
+            .fields()
+            .field("fake_field")
+            .unwrap();
+        let mut builder = MockValueBuilder { value: None };
+        value.record(&fake_field, &mut builder);
+        builder
+            .value
+            .expect("finish called before a value was recorded")
+    }
+}
+
+impl fmt::Display for Expect {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "fields ")?;
+        let entries = self
+            .fields
+            .iter()
+            .map(|(k, v)| (field::display(k), field::display(v)));
+        f.debug_map().entries(entries).finish()
+    }
+}
diff --git a/third_party/rust/tracing/tests/support/metadata.rs b/third_party/rust/tracing/tests/support/metadata.rs
new file mode 100644
index 0000000000..2c3606b05e
--- /dev/null
+++ b/third_party/rust/tracing/tests/support/metadata.rs
@@ -0,0 +1,64 @@
+use std::fmt;
+use tracing::Metadata;
+
+#[derive(Clone, Debug, Eq, PartialEq, Default)]
+pub struct Expect {
+    pub name: Option<String>,
+    pub level: Option<tracing::Level>,
+    pub target: Option<String>,
+}
+
+impl Expect {
+    pub(in crate::support) fn check(&self, actual: &Metadata<'_>, ctx: fmt::Arguments<'_>) {
+        if let Some(ref expected_name) = self.name {
+            let name = actual.name();
+            assert!(
+                expected_name == name,
+                "expected {} to be named `{}`, but got one named `{}`",
+                ctx,
+                expected_name,
+                name
+            )
+        }
+
+        if let Some(ref expected_level) = self.level {
+            let level = actual.level();
+            assert!(
+                expected_level == level,
+                "expected {} to be at level `{:?}`, but it was at level `{:?}` instead",
+                ctx,
+                expected_level,
+                level,
+            )
+        }
+
+        if let Some(ref expected_target) = self.target {
+            let target = actual.target();
+            assert!(
+                expected_target == target,
+                "expected {} to have target `{}`, but it had target `{}` instead",
+                ctx,
+                expected_target,
+                target,
+            )
+        }
+    }
+}
+
+impl fmt::Display for Expect {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if let Some(ref name) = self.name {
+            write!(f, " named `{}`", name)?;
+        }
+
+        if let Some(ref level) = self.level {
+            write!(f, " at the `{:?}` level", level)?;
+        }
+
+        if let Some(ref target) = self.target {
+            write!(f, " with target `{}`", target)?;
+        }
+
+        Ok(())
+    }
+}
diff --git a/third_party/rust/tracing/tests/support/mod.rs b/third_party/rust/tracing/tests/support/mod.rs
new file mode 100644
index 0000000000..7900f38c76
--- /dev/null
+++ b/third_party/rust/tracing/tests/support/mod.rs
@@ -0,0 +1,14 @@
+#![allow(dead_code)]
+pub mod event;
+pub mod field;
+mod metadata;
+pub mod span;
+pub mod subscriber;
+
+#[derive(Debug, Eq, PartialEq)]
+pub(in crate::support) enum Parent {
+    ContextualRoot,
+    Contextual(String),
+    ExplicitRoot,
+    Explicit(String),
+}
diff --git a/third_party/rust/tracing/tests/support/span.rs b/third_party/rust/tracing/tests/support/span.rs
new file mode 100644
index 0000000000..023e5b7079
--- /dev/null
+++ b/third_party/rust/tracing/tests/support/span.rs
@@ -0,0 +1,167 @@
+#![allow(missing_docs)]
+use super::{field, metadata, Parent};
+use std::fmt;
+
+/// A mock span.
+///
+/// This is intended for use with the mock subscriber API in the
+/// `subscriber` module.
+#[derive(Clone, Debug, Default, Eq, PartialEq)]
+pub struct MockSpan {
+    pub(in crate::support) metadata: metadata::Expect,
+}
+
+#[derive(Debug, Default, Eq, PartialEq)]
+pub struct NewSpan {
+    pub(in crate::support) span: MockSpan,
+    pub(in crate::support) fields: field::Expect,
+    pub(in crate::support) parent: Option<Parent>,
+}
+
+pub fn mock() -> MockSpan {
+    MockSpan {
+        ..Default::default()
+    }
+}
+
+impl MockSpan {
+    pub fn named<I>(self, name: I) -> Self
+    where
+        I: Into<String>,
+    {
+        Self {
+            metadata: metadata::Expect {
+                name: Some(name.into()),
+                ..self.metadata
+            },
+        }
+    }
+
+    pub fn at_level(self, level: tracing::Level) -> Self {
+        Self {
+            metadata: metadata::Expect {
+                level: Some(level),
+                ..self.metadata
+            },
+        }
+    }
+
+    pub fn with_target<I>(self, target: I) -> Self
+    where
+        I: Into<String>,
+    {
+        Self {
+            metadata: metadata::Expect {
+                target: Some(target.into()),
+                ..self.metadata
+            },
+        }
+    }
+
+    pub fn with_explicit_parent(self, parent: Option<&str>) -> NewSpan {
+        let parent = match parent {
+            Some(name) => Parent::Explicit(name.into()),
+            None => Parent::ExplicitRoot,
+        };
+        NewSpan {
+            parent: Some(parent),
+            span: self,
+            ..Default::default()
+        }
+    }
+
+    pub fn with_contextual_parent(self, parent: Option<&str>) -> NewSpan {
+        let parent = match parent {
+            Some(name) => Parent::Contextual(name.into()),
+            None => Parent::ContextualRoot,
+        };
+        NewSpan {
+            parent: Some(parent),
+            span: self,
+            ..Default::default()
+        }
+    }
+
+    pub fn name(&self) -> Option<&str> {
+        self.metadata.name.as_ref().map(String::as_ref)
+    }
+
+    pub fn with_field<I>(self, fields: I) -> NewSpan
+    where
+        I: Into<field::Expect>,
+    {
+        NewSpan {
+            span: self,
+            fields: fields.into(),
+            ..Default::default()
+        }
+    }
+
+    pub(in crate::support) fn check_metadata(&self, actual: &tracing::Metadata<'_>) {
+        self.metadata.check(actual, format_args!("span {}", self));
+        assert!(actual.is_span(), "expected a span but got {:?}", actual);
+    }
+}
+
+impl fmt::Display for MockSpan {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if self.metadata.name.is_some() {
+            write!(f, "a span{}", self.metadata)
+        } else {
+            write!(f, "any span{}", self.metadata)
+        }
+    }
+}
+
+impl Into<NewSpan> for MockSpan {
+    fn into(self) -> NewSpan {
+        NewSpan {
+            span: self,
+            ..Default::default()
+        }
+    }
+}
+
+impl NewSpan {
+    pub fn with_explicit_parent(self, parent: Option<&str>) -> NewSpan {
+        let parent = match parent {
+            Some(name) => Parent::Explicit(name.into()),
+            None => Parent::ExplicitRoot,
+        };
+        NewSpan {
+            parent: Some(parent),
+            ..self
+        }
+    }
+
+    pub fn with_contextual_parent(self, parent: Option<&str>) -> NewSpan {
+        let parent = match parent {
+            Some(name) => Parent::Contextual(name.into()),
+            None => Parent::ContextualRoot,
+        };
+        NewSpan {
+            parent: Some(parent),
+            ..self
+        }
+    }
+
+    pub fn with_field<I>(self, fields: I) -> NewSpan
+    where
+        I: Into<field::Expect>,
+    {
+        NewSpan {
+            fields: fields.into(),
+            ..self
+        }
+    }
+}
+
+impl fmt::Display for NewSpan {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "a new span{}", self.span.metadata)?;
+        if !self.fields.is_empty() {
+            write!(f, " with {}", self.fields)?;
+        }
+        Ok(())
+    }
+}
diff --git a/third_party/rust/tracing/tests/support/subscriber.rs b/third_party/rust/tracing/tests/support/subscriber.rs
new file mode 100644
index 0000000000..83f1139849
--- /dev/null
+++ b/third_party/rust/tracing/tests/support/subscriber.rs
@@ -0,0 +1,569 @@
+#![allow(missing_docs)]
+use super::{
+    event::MockEvent,
+    field as mock_field,
+    span::{MockSpan, NewSpan},
+    Parent,
+};
+use std::{
+    collections::{HashMap, VecDeque},
+    fmt,
+    sync::{
+        atomic::{AtomicUsize, Ordering},
+        Arc, Mutex,
+    },
+    thread,
+};
+use tracing::{
+    level_filters::LevelFilter,
+    span::{self, Attributes, Id},
+    subscriber::Interest,
+    Event, Metadata, Subscriber,
+};
+
+#[derive(Debug, Eq, PartialEq)]
+enum Expect {
+    Event(MockEvent),
+    Enter(MockSpan),
+    Exit(MockSpan),
+    CloneSpan(MockSpan),
+    DropSpan(MockSpan),
+    Visit(MockSpan, mock_field::Expect),
+    NewSpan(NewSpan),
+    Nothing,
+}
+
+struct SpanState {
+    name: &'static str,
+    refs: usize,
+}
+
+struct Running<F: Fn(&Metadata<'_>) -> bool> {
+    spans: Mutex<HashMap<Id, SpanState>>,
+    expected: Arc<Mutex<VecDeque<Expect>>>,
+    current: Mutex<Vec<Id>>,
+    ids: AtomicUsize,
+    max_level: Option<LevelFilter>,
+    filter: F,
+    name: String,
+}
+
+pub struct MockSubscriber<F: Fn(&Metadata<'_>) -> bool> {
+    expected: VecDeque<Expect>,
+    max_level: Option<LevelFilter>,
+    filter: F,
+    name: String,
+}
+
+pub struct MockHandle(Arc<Mutex<VecDeque<Expect>>>, String);
+
+pub fn mock() -> MockSubscriber<fn(&Metadata<'_>) -> bool> {
+    MockSubscriber {
+        expected: VecDeque::new(),
+        filter: (|_: &Metadata<'_>| true) as for<'r, 's> fn(&'r Metadata<'s>) -> _,
+        max_level: None,
+        name: thread::current()
+            .name()
+            .unwrap_or("mock_subscriber")
+            .to_string(),
+    }
+}
+
+impl<F> MockSubscriber<F>
+where
+    F: Fn(&Metadata<'_>) -> bool + 'static,
+{
+    /// Overrides the name printed by the mock subscriber's debugging output.
+    ///
+    /// The debugging output is displayed if the test panics, or if the test is
+    /// run with `--nocapture`.
+    ///
+    /// By default, the mock subscriber's name is the  name of the test
+    /// (*technically*, the name of the thread where it was created, which is
+    /// the name of the test unless tests are run with `--test-threads=1`).
+    /// When a test has only one mock subscriber, this is sufficient. However,
+    /// some tests may include multiple subscribers, in order to test
+    /// interactions between multiple subscribers. In that case, it can be
+    /// helpful to give each subscriber a separate name to distinguish where the
+    /// debugging output comes from.
+    pub fn named(self, name: impl ToString) -> Self {
+        Self {
+            name: name.to_string(),
+            ..self
+        }
+    }
+
+    pub fn enter(mut self, span: MockSpan) -> Self {
+        self.expected.push_back(Expect::Enter(span));
+        self
+    }
+
+    pub fn event(mut self, event: MockEvent) -> Self {
+        self.expected.push_back(Expect::Event(event));
+        self
+    }
+
+    pub fn exit(mut self, span: MockSpan) -> Self {
+        self.expected.push_back(Expect::Exit(span));
+        self
+    }
+
+    pub fn clone_span(mut self, span: MockSpan) -> Self {
+        self.expected.push_back(Expect::CloneSpan(span));
+        self
+    }
+
+    #[allow(deprecated)]
+    pub fn drop_span(mut self, span: MockSpan) -> Self {
+        self.expected.push_back(Expect::DropSpan(span));
+        self
+    }
+
+    pub fn done(mut self) -> Self {
+        self.expected.push_back(Expect::Nothing);
+        self
+    }
+
+    pub fn record<I>(mut self, span: MockSpan, fields: I) -> Self
+    where
+        I: Into<mock_field::Expect>,
+    {
+        self.expected.push_back(Expect::Visit(span, fields.into()));
+        self
+    }
+
+    pub fn new_span<I>(mut self, new_span: I) -> Self
+    where
+        I: Into<NewSpan>,
+    {
+        self.expected.push_back(Expect::NewSpan(new_span.into()));
+        self
+    }
+
+    pub fn with_filter<G>(self, filter: G) -> MockSubscriber<G>
+    where
+        G: Fn(&Metadata<'_>) -> bool + 'static,
+    {
+        MockSubscriber {
+            expected: self.expected,
+            filter,
+            max_level: self.max_level,
+            name: self.name,
+        }
+    }
+
+    pub fn with_max_level_hint(self, hint: impl Into<LevelFilter>) -> Self {
+        Self {
+            max_level: Some(hint.into()),
+            ..self
+        }
+    }
+
+    pub fn run(self) -> impl Subscriber {
+        let (subscriber, _) = self.run_with_handle();
+        subscriber
+    }
+
+    pub fn run_with_handle(self) -> (impl Subscriber, MockHandle) {
+        let expected = Arc::new(Mutex::new(self.expected));
+        let handle = MockHandle(expected.clone(), self.name.clone());
+        let subscriber = Running {
+            spans: Mutex::new(HashMap::new()),
+            expected,
+            current: Mutex::new(Vec::new()),
+            ids: AtomicUsize::new(1),
+            filter: self.filter,
+            max_level: self.max_level,
+            name: self.name,
+        };
+        (subscriber, handle)
+    }
+}
+
+impl<F> Subscriber for Running<F>
+where
+    F: Fn(&Metadata<'_>) -> bool + 'static,
+{
+    fn enabled(&self, meta: &Metadata<'_>) -> bool {
+        println!("[{}] enabled: {:#?}", self.name, meta);
+        let enabled = (self.filter)(meta);
+        println!("[{}] enabled -> {}", self.name, enabled);
+        enabled
+    }
+
+    fn register_callsite(&self, meta: &'static Metadata<'static>) -> Interest {
+        println!("[{}] register_callsite: {:#?}", self.name, meta);
+        if self.enabled(meta) {
+            Interest::always()
+        } else {
+            Interest::never()
+        }
+    }
+    fn max_level_hint(&self) -> Option<LevelFilter> {
+        self.max_level
+    }
+
+    fn record(&self, id: &Id, values: &span::Record<'_>) {
+        let spans = self.spans.lock().unwrap();
+        let mut expected = self.expected.lock().unwrap();
+        let span = spans
+            .get(id)
+            .unwrap_or_else(|| panic!("[{}] no span for ID {:?}", self.name, id));
+        println!(
+            "[{}] record: {}; id={:?}; values={:?};",
+            self.name, span.name, id, values
+        );
+        let was_expected = if let Some(Expect::Visit(_, _)) = expected.front() {
+            true
+        } else {
+            false
+        };
+        if was_expected {
+            if let Expect::Visit(expected_span, mut expected_values) = expected.pop_front().unwrap()
+            {
+                if let Some(name) = expected_span.name() {
+                    assert_eq!(name, span.name);
+                }
+                let mut checker = expected_values.checker(format!("span {}: ", span.name));
+                values.record(&mut checker);
+                checker.finish();
+            }
+        }
+    }
+
+    fn event(&self, event: &Event<'_>) {
+        let name = event.metadata().name();
+        println!("[{}] event: {};", self.name, name);
+        match self.expected.lock().unwrap().pop_front() {
+            None => {}
+            Some(Expect::Event(mut expected)) => {
+                let spans = self.spans.lock().unwrap();
+                expected.check(event);
+                match expected.parent {
+                    Some(Parent::ExplicitRoot) => {
+                        assert!(
+                            event.is_root(),
+                            "[{}] expected {:?} to be an explicit root event",
+                            self.name,
+                            name
+                        );
+                    }
+                    Some(Parent::Explicit(expected_parent)) => {
+                        let actual_parent =
+                            event.parent().and_then(|id| spans.get(id)).map(|s| s.name);
+                        assert_eq!(
+                            Some(expected_parent.as_ref()),
+                            actual_parent,
+                            "[{}] expected {:?} to have explicit parent {:?}",
+                            self.name,
+                            name,
+                            expected_parent,
+                        );
+                    }
+                    Some(Parent::ContextualRoot) => {
+                        assert!(
+                            event.is_contextual(),
+                            "[{}] expected {:?} to have a contextual parent",
+                            self.name,
+                            name
+                        );
+                        assert!(
+                            self.current.lock().unwrap().last().is_none(),
+                            "[{}] expected {:?} to be a root, but we were inside a span",
+                            self.name,
+                            name
+                        );
+                    }
+                    Some(Parent::Contextual(expected_parent)) => {
+                        assert!(
+                            event.is_contextual(),
+                            "[{}] expected {:?} to have a contextual parent",
+                            self.name,
+                            name
+                        );
+                        let stack = self.current.lock().unwrap();
+                        let actual_parent =
+                            stack.last().and_then(|id| spans.get(id)).map(|s| s.name);
+                        assert_eq!(
+                            Some(expected_parent.as_ref()),
+                            actual_parent,
+                            "[{}] expected {:?} to have contextual parent {:?}",
+                            self.name,
+                            name,
+                            expected_parent,
+                        );
+                    }
+                    None => {}
+                }
+            }
+            Some(ex) => ex.bad(
+                &self.name,
+                format_args!("[{}] observed event {:?}", self.name, event),
+            ),
+        }
+    }
+
+    fn record_follows_from(&self, _span: &Id, _follows: &Id) {
+        // TODO: it should be possible to expect spans to follow from other spans
+    }
+
+    fn new_span(&self, span: &Attributes<'_>) -> Id {
+        let meta = span.metadata();
+        let id = self.ids.fetch_add(1, Ordering::SeqCst);
+        let id = Id::from_u64(id as u64);
+        println!(
+            "[{}] new_span: name={:?}; target={:?}; id={:?};",
+            self.name,
+            meta.name(),
+            meta.target(),
+            id
+        );
+        let mut expected = self.expected.lock().unwrap();
+        let was_expected = match expected.front() {
+            Some(Expect::NewSpan(_)) => true,
+            _ => false,
+        };
+        let mut spans = self.spans.lock().unwrap();
+        if was_expected {
+            if let Expect::NewSpan(mut expected) = expected.pop_front().unwrap() {
+                let name = meta.name();
+                expected
+                    .span
+                    .metadata
+                    .check(meta, format_args!("span `{}`", name));
+                let mut checker = expected.fields.checker(name.to_string());
+                span.record(&mut checker);
+                checker.finish();
+                match expected.parent {
+                    Some(Parent::ExplicitRoot) => {
+                        assert!(
+                            span.is_root(),
+                            "[{}] expected {:?} to be an explicit root span",
+                            self.name,
+                            name
+                        );
+                    }
+                    Some(Parent::Explicit(expected_parent)) => {
+                        let actual_parent =
+                            span.parent().and_then(|id| spans.get(id)).map(|s| s.name);
+                        assert_eq!(
+                            Some(expected_parent.as_ref()),
+                            actual_parent,
+                            "[{}] expected {:?} to have explicit parent {:?}",
+                            self.name,
+                            name,
+                            expected_parent,
+                        );
+                    }
+                    Some(Parent::ContextualRoot) => {
+                        assert!(
+                            span.is_contextual(),
+                            "[{}] expected {:?} to have a contextual parent",
+                            self.name,
+                            name
+                        );
+                        assert!(
+                            self.current.lock().unwrap().last().is_none(),
+                            "[{}] expected {:?} to be a root, but we were inside a span",
+                            self.name,
+                            name
+                        );
+                    }
+                    Some(Parent::Contextual(expected_parent)) => {
+                        assert!(
+                            span.is_contextual(),
+                            "[{}] expected {:?} to have a contextual parent",
+                            self.name,
+                            name
+                        );
+                        let stack = self.current.lock().unwrap();
+                        let actual_parent =
+                            stack.last().and_then(|id| spans.get(id)).map(|s| s.name);
+                        assert_eq!(
+                            Some(expected_parent.as_ref()),
+                            actual_parent,
+                            "[{}] expected {:?} to have contextual parent {:?}",
+                            self.name,
+                            name,
+                            expected_parent,
+                        );
+                    }
+                    None => {}
+                }
+            }
+        }
+        spans.insert(
+            id.clone(),
+            SpanState {
+                name: meta.name(),
+                refs: 1,
+            },
+        );
+        id
+    }
+
+    fn enter(&self, id: &Id) {
+        let spans = self.spans.lock().unwrap();
+        if let Some(span) = spans.get(id) {
+            println!("[{}] enter: {}; id={:?};", self.name, span.name, id);
+            match self.expected.lock().unwrap().pop_front() {
+                None => {}
+                Some(Expect::Enter(ref expected_span)) => {
+                    if let Some(name) = expected_span.name() {
+                        assert_eq!(name, span.name);
+                    }
+                }
+                Some(ex) => ex.bad(&self.name, format_args!("entered span {:?}", span.name)),
+            }
+        };
+        self.current.lock().unwrap().push(id.clone());
+    }
+
+    fn exit(&self, id: &Id) {
+        if std::thread::panicking() {
+            // `exit()` can be called in `drop` impls, so we must guard against
+            // double panics.
+            println!("[{}] exit {:?} while panicking", self.name, id);
+            return;
+        }
+        let spans = self.spans.lock().unwrap();
+        let span = spans
+            .get(id)
+            .unwrap_or_else(|| panic!("[{}] no span for ID {:?}", self.name, id));
+        println!("[{}] exit: {}; id={:?};", self.name, span.name, id);
+        match self.expected.lock().unwrap().pop_front() {
+            None => {}
+            Some(Expect::Exit(ref expected_span)) => {
+                if let Some(name) = expected_span.name() {
+                    assert_eq!(name, span.name);
+                }
+                let curr = self.current.lock().unwrap().pop();
+                assert_eq!(
+                    Some(id),
+                    curr.as_ref(),
+                    "[{}] exited span {:?}, but the current span was {:?}",
+                    self.name,
+                    span.name,
+                    curr.as_ref().and_then(|id| spans.get(id)).map(|s| s.name)
+                );
+            }
+            Some(ex) => ex.bad(&self.name, format_args!("exited span {:?}", span.name)),
+        };
+    }
+
+    fn clone_span(&self, id: &Id) -> Id {
+        let name = self.spans.lock().unwrap().get_mut(id).map(|span| {
+            let name = span.name;
+            println!(
+                "[{}] clone_span: {}; id={:?}; refs={:?};",
+                self.name, name, id, span.refs
+            );
+            span.refs += 1;
+            name
+        });
+        if name.is_none() {
+            println!("[{}] clone_span: id={:?};", self.name, id);
+        }
+        let mut expected = self.expected.lock().unwrap();
+        let was_expected = if let Some(Expect::CloneSpan(ref span)) = expected.front() {
+            assert_eq!(
+                name,
+                span.name(),
+                "[{}] expected to clone a span named {:?}",
+                self.name,
+                span.name()
+            );
+            true
+        } else {
+            false
+        };
+        if was_expected {
+            expected.pop_front();
+        }
+        id.clone()
+    }
+
+    fn drop_span(&self, id: Id) {
+        let mut is_event = false;
+        let name = if let Ok(mut spans) = self.spans.try_lock() {
+            spans.get_mut(&id).map(|span| {
+                let name = span.name;
+                if name.contains("event") {
+                    is_event = true;
+                }
+                println!(
+                    "[{}] drop_span: {}; id={:?}; refs={:?};",
+                    self.name, name, id, span.refs
+                );
+                span.refs -= 1;
+                name
+            })
+        } else {
+            None
+        };
+        if name.is_none() {
+            println!("[{}] drop_span: id={:?}", self.name, id);
+        }
+        if let Ok(mut expected) = self.expected.try_lock() {
+            let was_expected = match expected.front() {
+                Some(Expect::DropSpan(ref span)) => {
+                    // Don't assert if this function was called while panicking,
+                    // as failing the assertion can cause a double panic.
+                    if !::std::thread::panicking() {
+                        assert_eq!(name, span.name());
+                    }
+                    true
+                }
+                Some(Expect::Event(_)) => {
+                    if !::std::thread::panicking() {
+                        assert!(is_event, "[{}] expected an event", self.name);
+                    }
+                    true
+                }
+                _ => false,
+            };
+            if was_expected {
+                expected.pop_front();
+            }
+        }
+    }
+}
+
+impl MockHandle {
+    pub fn assert_finished(&self) {
+        if let Ok(ref expected) = self.0.lock() {
+            assert!(
+                !expected.iter().any(|thing| thing != &Expect::Nothing),
+                "[{}] more notifications expected: {:?}",
+                self.1,
+                **expected
+            );
+        }
+    }
+}
+
+impl Expect {
+    fn bad<'a>(&self, name: impl AsRef<str>, what: fmt::Arguments<'a>) {
+        let name = name.as_ref();
+        match self {
+            Expect::Event(e) => panic!("[{}] expected event {}, but {} instead", name, e, what,),
+            Expect::Enter(e) => panic!("[{}] expected to enter {} but {} instead", name, e, what,),
+            Expect::Exit(e) => panic!("[{}] expected to exit {} but {} instead", name, e, what,),
+            Expect::CloneSpan(e) => {
+                panic!("[{}] expected to clone {} but {} instead", name, e, what,)
+            }
+            Expect::DropSpan(e) => {
+                panic!("[{}] expected to drop {} but {} instead", name, e, what,)
+            }
+            Expect::Visit(e, fields) => panic!(
+                "[{}] expected {} to record {} but {} instead",
+                name, e, fields, what,
+            ),
+            Expect::NewSpan(e) => panic!("[{}] expected {} but {} instead", name, e, what),
+            Expect::Nothing => panic!(
+                "[{}] expected nothing else to happen, but {} instead",
+                name, what,
+            ),
+        }
+    }
+}