2 files changed, 22 insertions, 8 deletions
diff --git a/tvix/eval/src/observer.rs b/tvix/eval/src/observer.rs
index 8572c2c1eb..74ed316a6a 100644
--- a/tvix/eval/src/observer.rs
+++ b/tvix/eval/src/observer.rs
@@ -42,7 +42,7 @@ pub trait RuntimeObserver {
     fn observe_enter_frame(&mut self, _arg_count: usize, _: &Rc<Lambda>, _call_depth: usize) {}
 
     /// Called when the runtime exits a call frame.
-    fn observe_exit_frame(&mut self, _frame_at: usize) {}
+    fn observe_exit_frame(&mut self, _frame_at: usize, _stack: &[Value]) {}
 
     /// Called when the runtime replaces the current call frame for a
     /// tail call.
@@ -52,7 +52,7 @@ pub trait RuntimeObserver {
     fn observe_enter_builtin(&mut self, _name: &'static str) {}
 
     /// Called when the runtime exits a builtin.
-    fn observe_exit_builtin(&mut self, _name: &'static str) {}
+    fn observe_exit_builtin(&mut self, _name: &'static str, _stack: &[Value]) {}
 
     /// Called when the runtime *begins* executing an instruction. The
     /// provided stack is the state at the beginning of the operation.
@@ -146,16 +146,28 @@ impl<W: Write> RuntimeObserver for TracingObserver<W> {
         );
     }
 
-    fn observe_exit_frame(&mut self, frame_at: usize) {
-        let _ = writeln!(&mut self.writer, "=== exiting frame {} ===", frame_at);
+    fn observe_exit_frame(&mut self, frame_at: usize, stack: &[Value]) {
+        let _ = write!(&mut self.writer, "=== exiting frame {} ===\t[ ", frame_at);
+
+        for val in stack {
+            let _ = write!(&mut self.writer, "{} ", val);
+        }
+
+        let _ = writeln!(&mut self.writer, "]");
     }
 
     fn observe_enter_builtin(&mut self, name: &'static str) {
         let _ = writeln!(&mut self.writer, "=== entering builtin {} ===", name);
     }
 
-    fn observe_exit_builtin(&mut self, name: &'static str) {
-        let _ = writeln!(&mut self.writer, "=== exiting builtin {} ===", name);
+    fn observe_exit_builtin(&mut self, name: &'static str, stack: &[Value]) {
+        let _ = write!(&mut self.writer, "=== exiting builtin {} ===\t[ ", name);
+
+        for val in stack {
+            let _ = write!(&mut self.writer, "{} ", val);
+        }
+
+        let _ = writeln!(&mut self.writer, "]");
     }
 
     fn observe_tail_call(&mut self, frame_at: usize, lambda: &Rc<Lambda>) {
diff --git a/tvix/eval/src/vm.rs b/tvix/eval/src/vm.rs
index 9a8d07ea63..daee8c886c 100644
--- a/tvix/eval/src/vm.rs
+++ b/tvix/eval/src/vm.rs
@@ -336,7 +336,8 @@ impl<'o> VM<'o> {
         self.frames.push(frame);
         let result = self.run();
 
-        self.observer.observe_exit_frame(self.frames.len() + 1);
+        self.observer
+            .observe_exit_frame(self.frames.len() + 1, &self.stack);
 
         result
     }
@@ -855,7 +856,8 @@ impl<'o> VM<'o> {
         let arg = self.pop();
         let result = fallible!(self, builtin.apply(self, arg));
 
-        self.observer.observe_exit_builtin(builtin_name);
+        self.observer
+            .observe_exit_builtin(builtin_name, &self.stack);
 
         self.push(result);