test: fix miri failure due to tracing deps

cordyceps/src/list/tests.rs

@@ -117,14 +117,28 @@ fn const_new() {
     const _: List<Entry> = List::new();
 }
 
-fn trace_init() -> tracing::dispatcher::DefaultGuard {
-    use tracing_subscriber::prelude::*;
-    tracing_subscriber::fmt()
-        .with_test_writer()
-        .with_max_level(tracing::Level::TRACE)
-        .with_target(false)
-        .with_timer(())
-        .set_default()
+fn trace_init() -> impl Drop {
+    // XXX(eliza): unfortunately, attempting to set the default `tracing`
+    // subscriber inside of a Miri test makes Miri angry, because `tracing` uses
+    // `once_cell` internally, and `once_cell` apparently does a Naughty and
+    // Evil int-to-ptr cast. Sigh.
+    //
+    // re-enable this with Miri when `once_cell` is fixed.
+    #[cfg(not(miri))]
+    {
+        use tracing_subscriber::prelude::*;
+        tracing_subscriber::fmt()
+            .with_test_writer()
+            .with_max_level(tracing::Level::TRACE)
+            .with_target(false)
+            .with_timer(())
+            .set_default()
+    }
+
+    // Return an empty `Vec` when Miri is enabled: it has a `Drop` impl but
+    // doesn't actually *do* anything.
+    #[cfg(miri)]
+    Vec::<()>::new()
 }
 
 #[test]