compiles

src/core/client_channel/client_channel.cc

@@ -1842,8 +1842,7 @@ ClientChannel::PickSubchannel(LoadBalancingPolicy::SubchannelPicker& picker,
         // it when the call finishes.
         if (complete_pick->subchannel_call_tracker != nullptr) {
           complete_pick->subchannel_call_tracker->Start();
-          unstarted_handler.SetContext(
-              complete_pick->subchannel_call_tracker.release());
+          SetContext(complete_pick->subchannel_call_tracker.release());
         }
         // Return the connected subchannel.
         return connected_subchannel;

src/core/lib/channel/context.h

@@ -59,6 +59,9 @@ typedef enum {
   /// the server.
   GRPC_CONTEXT_BACKEND_METRIC_PROVIDER,
 
+  /// A LoadBalancingPolicy::SubchannelCallTrackerInterface
+  GRPC_SUBCHANNEL_CALL_TRACKER_INTERFACE,
+
   /// Special Google context
   GRPC_CONTEXT_GOOGLE,
 
@@ -104,6 +107,14 @@ class Context<T, absl::void_t<decltype(OldStyleContext<T>::kIndex)>> {
         GetContext<grpc_call_context_element>()[OldStyleContext<T>::kIndex]
             .value);
   }
+  static void set(T* value) {
+    auto& elem = GetContext<grpc_call_context_element>()[OldStyleContext<T>::kIndex];
+    if (elem.destroy != nullptr) {
+      elem.destroy(elem.value);
+      elem.destroy = nullptr;
+    }
+    elem.value = value;
+  }
 };
 
 }  // namespace promise_detail

src/core/lib/promise/context.h

@@ -119,6 +119,11 @@ T* GetContext() {
   return p;
 }
 
+template <typename T>
+void SetContext(T* p) {
+  promise_detail::Context<T>::set(p);
+}
+
 // Given a promise and a context, return a promise that has that context set.
 template <typename T, typename F>
 promise_detail::WithContext<T, F> WithContext(F f, T* context) {

src/core/load_balancing/lb_policy.h

@@ -484,6 +484,15 @@ class LoadBalancingPolicy : public InternallyRefCounted<LoadBalancingPolicy> {
   ChannelArgs channel_args_;
 };
 
+namespace promise_detail {
+
+template <>
+struct OldStyleContext<LoadBalancingPolicy::SubchannelCallTrackerInterface> {
+  static constexpr grpc_context_index kIndex = GRPC_SUBCHANNEL_CALL_TRACKER_INTERFACE;
+};
+
+}
+
 }  // namespace grpc_core
 
 #endif  // GRPC_SRC_CORE_LOAD_BALANCING_LB_POLICY_H