Auto merge of #3348 - RalfJung:miri-thread-logging, r=RalfJung

log when we change the active thread

Author: bors

src/bin/miri.rs

@@ -202,7 +202,7 @@ fn rustc_logger_config() -> rustc_log::LoggerConfig {
             // rustc traced, but you can also do `MIRI_LOG=miri=trace,rustc_const_eval::interpret=debug`.
             if tracing::Level::from_str(&var).is_ok() {
                 cfg.filter = Ok(format!(
-                    "rustc_middle::mir::interpret={var},rustc_const_eval::interpret={var}"
+                    "rustc_middle::mir::interpret={var},rustc_const_eval::interpret={var},miri={var}"
                 ));
             } else {
                 cfg.filter = Ok(var);

src/concurrency/thread.rs

@@ -445,10 +445,13 @@ impl<'mir, 'tcx: 'mir> ThreadManager<'mir, 'tcx> {
 
     /// Set an active thread and return the id of the thread that was active before.
     fn set_active_thread_id(&mut self, id: ThreadId) -> ThreadId {
-        let active_thread_id = self.active_thread;
-        self.active_thread = id;
-        assert!(self.active_thread.index() < self.threads.len());
-        active_thread_id
+        assert!(id.index() < self.threads.len());
+        info!(
+            "Now executing on thread `{}` (previous: `{}`)",
+            self.get_thread_display_name(id),
+            self.get_thread_display_name(self.active_thread)
+        );
+        std::mem::replace(&mut self.active_thread, id)
     }
 
     /// Get the id of the currently active thread.
@@ -735,6 +738,11 @@ impl<'mir, 'tcx: 'mir> ThreadManager<'mir, 'tcx> {
         for (id, thread) in threads {
             debug_assert_ne!(self.active_thread, id);
             if thread.state == ThreadState::Enabled {
+                info!(
+                    "Now executing on thread `{}` (previous: `{}`)",
+                    self.get_thread_display_name(id),
+                    self.get_thread_display_name(self.active_thread)
+                );
                 self.active_thread = id;
                 break;
             }