repl: Don't prefix free variables with _ (#16494)

This PR is a small refactor to remove the leading `_` for some free
variables, as this unintentionally marks them as unused to the compiler.

While the fields on the struct _are_ unused, the free variables should
participate in usage tracking, as we want to make sure they get stored
on the struct.

Release Notes:

- N/A

crates/repl/src/kernels.rs

@@ -261,7 +261,7 @@ impl RunningKernel {
             messages_rx.push(control_reply_rx);
             messages_rx.push(shell_reply_rx);
 
-            let _iopub_task = cx.background_executor().spawn({
+            let iopub_task = cx.background_executor().spawn({
                 async move {
                     while let Ok(message) = iopub_socket.read().await {
                         iopub.send(message).await?;
@@ -274,7 +274,7 @@ impl RunningKernel {
                 futures::channel::mpsc::channel(100);
             let (mut shell_request_tx, mut shell_request_rx) = futures::channel::mpsc::channel(100);
 
-            let _routing_task = cx.background_executor().spawn({
+            let routing_task = cx.background_executor().spawn({
                 async move {
                     while let Some(message) = request_rx.next().await {
                         match message.content {
@@ -292,7 +292,7 @@ impl RunningKernel {
                 }
             });
 
-            let _shell_task = cx.background_executor().spawn({
+            let shell_task = cx.background_executor().spawn({
                 async move {
                     while let Some(message) = shell_request_rx.next().await {
                         shell_socket.send(message).await.ok();
@@ -303,7 +303,7 @@ impl RunningKernel {
                 }
             });
 
-            let _control_task = cx.background_executor().spawn({
+            let control_task = cx.background_executor().spawn({
                 async move {
                     while let Some(message) = control_request_rx.next().await {
                         control_socket.send(message).await.ok();
@@ -319,10 +319,10 @@ impl RunningKernel {
                     process,
                     request_tx,
                     working_directory,
-                    _shell_task,
-                    _iopub_task,
-                    _control_task,
-                    _routing_task,
+                    _shell_task: shell_task,
+                    _iopub_task: iopub_task,
+                    _control_task: control_task,
+                    _routing_task: routing_task,
                     connection_path,
                     execution_state: ExecutionState::Busy,
                     kernel_info: None,