[promise] Context improvements (#35592)

A few improvements to the promise context system (more coming)

Allow subclassed contexts:
If we have multiple different kinds of a base context, allow `GetContext<Derived>()` to mean `down_cast<Derived*>(GetContext<Base>())` everywhere for brevity.

Allow custom context lookup:
For a base context type, allow customization of how that context is looked up.

These two together allow:
1. normalization of activity lookup and context lookup to the same syntax (so we can write `GetContext<Activity>()` everywhere now
2. Party & Activity to share a context, so that anywhere we need to do a party specific operation we can write `GetContext<Party>()->...` and safely know that it's the current activity *and* it's a party.

Closes #35592

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35592 from ctiller:contextual-types 37ef948a36
PiperOrigin-RevId: 599651708

CMakeLists.txt

@@ -2617,6 +2617,7 @@ target_link_libraries(grpc
   ${_gRPC_RE2_LIBRARIES}
   ${_gRPC_ZLIB_LIBRARIES}
   absl::algorithm_container
+  absl::config
   absl::cleanup
   absl::flat_hash_map
   absl::flat_hash_set
@@ -3317,6 +3318,7 @@ target_link_libraries(grpc_unsecure
   utf8_range_lib
   ${_gRPC_ZLIB_LIBRARIES}
   absl::algorithm_container
+  absl::config
   absl::cleanup
   absl::flat_hash_map
   absl::flat_hash_set
@@ -5395,6 +5397,7 @@ target_link_libraries(grpc_authorization_provider
   ${_gRPC_RE2_LIBRARIES}
   utf8_range_lib
   ${_gRPC_ZLIB_LIBRARIES}
+  absl::config
   absl::cleanup
   absl::flat_hash_map
   absl::flat_hash_set
@@ -5906,6 +5909,7 @@ target_include_directories(activity_test
 target_link_libraries(activity_test
   ${_gRPC_ALLTARGETS_LIBRARIES}
   gtest
+  absl::config
   absl::hash
   absl::type_traits
   absl::statusor
@@ -8475,6 +8479,7 @@ target_link_libraries(call_filters_test
   gtest
   upb_message_lib
   utf8_range_lib
+  absl::config
   absl::inlined_vector
   absl::function_ref
   absl::hash
@@ -10059,6 +10064,7 @@ target_link_libraries(chunked_vector_test
   gtest
   upb_message_lib
   utf8_range_lib
+  absl::config
   absl::function_ref
   absl::hash
   absl::type_traits
@@ -11425,6 +11431,8 @@ target_include_directories(context_test
 target_link_libraries(context_test
   ${_gRPC_ALLTARGETS_LIBRARIES}
   gtest
+  absl::config
+  absl::type_traits
   gpr
 )
 
@@ -13146,6 +13154,7 @@ target_link_libraries(exec_ctx_wakeup_scheduler_test
   gtest
   upb_message_lib
   utf8_range_lib
+  absl::config
   absl::hash
   absl::type_traits
   absl::statusor
@@ -13973,6 +13982,7 @@ target_link_libraries(flow_control_test
   gtest
   upb_message_lib
   utf8_range_lib
+  absl::config
   absl::function_ref
   absl::hash
   absl::type_traits
@@ -14058,6 +14068,7 @@ target_link_libraries(for_each_test
   gtest
   upb_message_lib
   utf8_range_lib
+  absl::config
   absl::function_ref
   absl::hash
   absl::type_traits
@@ -17331,6 +17342,7 @@ target_include_directories(inter_activity_pipe_test
 target_link_libraries(inter_activity_pipe_test
   ${_gRPC_ALLTARGETS_LIBRARIES}
   gtest
+  absl::config
   absl::hash
   absl::type_traits
   absl::statusor
@@ -17414,6 +17426,7 @@ target_link_libraries(interceptor_list_test
   gtest
   upb_message_lib
   utf8_range_lib
+  absl::config
   absl::function_ref
   absl::hash
   absl::type_traits
@@ -18144,6 +18157,7 @@ target_include_directories(latch_test
 target_link_libraries(latch_test
   ${_gRPC_ALLTARGETS_LIBRARIES}
   gtest
+  absl::config
   absl::hash
   absl::type_traits
   absl::statusor
@@ -18536,6 +18550,7 @@ target_link_libraries(map_pipe_test
   gtest
   upb_message_lib
   utf8_range_lib
+  absl::config
   absl::function_ref
   absl::hash
   absl::type_traits
@@ -19408,6 +19423,7 @@ target_include_directories(mpsc_test
 target_link_libraries(mpsc_test
   ${_gRPC_ALLTARGETS_LIBRARIES}
   gtest
+  absl::config
   absl::hash
   absl::type_traits
   absl::statusor
@@ -21658,6 +21674,7 @@ target_include_directories(promise_mutex_test
 target_link_libraries(promise_mutex_test
   ${_gRPC_ALLTARGETS_LIBRARIES}
   gtest
+  absl::config
   absl::hash
   absl::type_traits
   absl::statusor
@@ -30760,6 +30777,7 @@ target_include_directories(wait_for_callback_test
 target_link_libraries(wait_for_callback_test
   ${_gRPC_ALLTARGETS_LIBRARIES}
   gtest
+  absl::config
   absl::hash
   absl::type_traits
   absl::statusor
@@ -35274,7 +35292,7 @@ generate_pkgconfig(
   "gRPC"
   "high performance general RPC framework"
   "${gRPC_CORE_VERSION}"
-  "absl_algorithm_container absl_any_invocable absl_base absl_bind_front absl_cleanup absl_cord absl_core_headers absl_flags absl_flags_marshalling absl_flat_hash_map absl_flat_hash_set absl_function_ref absl_hash absl_inlined_vector absl_memory absl_optional absl_random_bit_gen_ref absl_random_distributions absl_random_random absl_span absl_status absl_statusor absl_str_format absl_strings absl_synchronization absl_time absl_type_traits absl_utility absl_variant gpr"
+  "absl_algorithm_container absl_any_invocable absl_base absl_bind_front absl_cleanup absl_config absl_cord absl_core_headers absl_flags absl_flags_marshalling absl_flat_hash_map absl_flat_hash_set absl_function_ref absl_hash absl_inlined_vector absl_memory absl_optional absl_random_bit_gen_ref absl_random_distributions absl_random_random absl_span absl_status absl_statusor absl_str_format absl_strings absl_synchronization absl_time absl_type_traits absl_utility absl_variant gpr"
   "libcares openssl re2 zlib"
   "-lgrpc"
   "-laddress_sorting -lupb_textformat_lib -lupb_json_lib -lutf8_range_lib -lupb_message_lib -lupb_mem_lib -lupb_base_lib"
@@ -35285,7 +35303,7 @@ generate_pkgconfig(
   "gRPC unsecure"
   "high performance general RPC framework without SSL"
   "${gRPC_CORE_VERSION}"
-  "absl_algorithm_container absl_any_invocable absl_base absl_bind_front absl_cleanup absl_cord absl_core_headers absl_flags absl_flags_marshalling absl_flat_hash_map absl_flat_hash_set absl_function_ref absl_hash absl_inlined_vector absl_memory absl_optional absl_random_bit_gen_ref absl_random_distributions absl_random_random absl_span absl_status absl_statusor absl_str_format absl_strings absl_synchronization absl_time absl_type_traits absl_utility absl_variant gpr"
+  "absl_algorithm_container absl_any_invocable absl_base absl_bind_front absl_cleanup absl_config absl_cord absl_core_headers absl_flags absl_flags_marshalling absl_flat_hash_map absl_flat_hash_set absl_function_ref absl_hash absl_inlined_vector absl_memory absl_optional absl_random_bit_gen_ref absl_random_distributions absl_random_random absl_span absl_status absl_statusor absl_str_format absl_strings absl_synchronization absl_time absl_type_traits absl_utility absl_variant gpr"
   "libcares zlib"
   "-lgrpc_unsecure"
   "-laddress_sorting -lutf8_range_lib -lupb_message_lib -lupb_mem_lib -lupb_base_lib"
@@ -35296,7 +35314,7 @@ generate_pkgconfig(
   "gRPC++"
   "C++ wrapper for gRPC"
   "${gRPC_CPP_VERSION}"
-  "absl_algorithm_container absl_any_invocable absl_base absl_bind_front absl_cleanup absl_cord absl_core_headers absl_flags absl_flags_marshalling absl_flat_hash_map absl_flat_hash_set absl_function_ref absl_hash absl_inlined_vector absl_memory absl_optional absl_random_bit_gen_ref absl_random_distributions absl_random_random absl_span absl_status absl_statusor absl_str_format absl_strings absl_synchronization absl_time absl_type_traits absl_utility absl_variant gpr grpc"
+  "absl_algorithm_container absl_any_invocable absl_base absl_bind_front absl_cleanup absl_config absl_cord absl_core_headers absl_flags absl_flags_marshalling absl_flat_hash_map absl_flat_hash_set absl_function_ref absl_hash absl_inlined_vector absl_memory absl_optional absl_random_bit_gen_ref absl_random_distributions absl_random_random absl_span absl_status absl_statusor absl_str_format absl_strings absl_synchronization absl_time absl_type_traits absl_utility absl_variant gpr grpc"
   "libcares openssl re2 zlib"
   "-lgrpc++"
   "-laddress_sorting -lupb_textformat_lib -lupb_json_lib -lutf8_range_lib -lupb_message_lib -lupb_mem_lib -lupb_base_lib"
@@ -35307,7 +35325,7 @@ generate_pkgconfig(
   "gRPC++ unsecure"
   "C++ wrapper for gRPC without SSL"
   "${gRPC_CPP_VERSION}"
-  "absl_algorithm_container absl_any_invocable absl_base absl_bind_front absl_cleanup absl_cord absl_core_headers absl_flags absl_flags_marshalling absl_flat_hash_map absl_flat_hash_set absl_function_ref absl_hash absl_inlined_vector absl_memory absl_optional absl_random_bit_gen_ref absl_random_distributions absl_random_random absl_span absl_status absl_statusor absl_str_format absl_strings absl_synchronization absl_time absl_type_traits absl_utility absl_variant gpr grpc_unsecure"
+  "absl_algorithm_container absl_any_invocable absl_base absl_bind_front absl_cleanup absl_config absl_cord absl_core_headers absl_flags absl_flags_marshalling absl_flat_hash_map absl_flat_hash_set absl_function_ref absl_hash absl_inlined_vector absl_memory absl_optional absl_random_bit_gen_ref absl_random_distributions absl_random_random absl_span absl_status absl_statusor absl_str_format absl_strings absl_synchronization absl_time absl_type_traits absl_utility absl_variant gpr grpc_unsecure"
   "libcares zlib"
   "-lgrpc++_unsecure"
   "-laddress_sorting -lutf8_range_lib -lupb_message_lib -lupb_mem_lib -lupb_base_lib"

Package.swift

@@ -1408,6 +1408,7 @@ let package = Package(
         "src/core/lib/gprpp/crash.h",
         "src/core/lib/gprpp/debug_location.h",
         "src/core/lib/gprpp/directory_reader.h",
+        "src/core/lib/gprpp/down_cast.h",
         "src/core/lib/gprpp/dual_ref_counted.h",
         "src/core/lib/gprpp/env.h",
         "src/core/lib/gprpp/examine_stack.cc",

build_autogenerated.yaml

@@ -930,6 +930,7 @@ libs:
   - src/core/lib/gprpp/chunked_vector.h
   - src/core/lib/gprpp/cpp_impl_of.h
   - src/core/lib/gprpp/directory_reader.h
+  - src/core/lib/gprpp/down_cast.h
   - src/core/lib/gprpp/dual_ref_counted.h
   - src/core/lib/gprpp/if_list.h
   - src/core/lib/gprpp/load_file.h
@@ -2024,6 +2025,7 @@ libs:
   - re2
   - z
   - absl/algorithm:container
+  - absl/base:config
   - absl/cleanup:cleanup
   - absl/container:flat_hash_map
   - absl/container:flat_hash_set
@@ -2406,6 +2408,7 @@ libs:
   - src/core/lib/gprpp/bitset.h
   - src/core/lib/gprpp/chunked_vector.h
   - src/core/lib/gprpp/cpp_impl_of.h
+  - src/core/lib/gprpp/down_cast.h
   - src/core/lib/gprpp/dual_ref_counted.h
   - src/core/lib/gprpp/if_list.h
   - src/core/lib/gprpp/load_file.h
@@ -3069,6 +3072,7 @@ libs:
   - utf8_range_lib
   - z
   - absl/algorithm:container
+  - absl/base:config
   - absl/cleanup:cleanup
   - absl/container:flat_hash_map
   - absl/container:flat_hash_set
@@ -4447,6 +4451,7 @@ libs:
   - src/core/lib/gprpp/bitset.h
   - src/core/lib/gprpp/chunked_vector.h
   - src/core/lib/gprpp/cpp_impl_of.h
+  - src/core/lib/gprpp/down_cast.h
   - src/core/lib/gprpp/dual_ref_counted.h
   - src/core/lib/gprpp/if_list.h
   - src/core/lib/gprpp/load_file.h
@@ -4974,6 +4979,7 @@ libs:
   - re2
   - utf8_range_lib
   - z
+  - absl/base:config
   - absl/cleanup:cleanup
   - absl/container:flat_hash_map
   - absl/container:flat_hash_set
@@ -5158,6 +5164,7 @@ targets:
   - src/core/lib/debug/trace.h
   - src/core/lib/gprpp/atomic_utils.h
   - src/core/lib/gprpp/bitset.h
+  - src/core/lib/gprpp/down_cast.h
   - src/core/lib/gprpp/orphanable.h
   - src/core/lib/gprpp/ref_counted.h
   - src/core/lib/gprpp/ref_counted_ptr.h
@@ -5184,6 +5191,7 @@ targets:
   - test/core/promise/activity_test.cc
   deps:
   - gtest
+  - absl/base:config
   - absl/hash:hash
   - absl/meta:type_traits
   - absl/status:statusor
@@ -6256,6 +6264,7 @@ targets:
   - src/core/lib/gprpp/atomic_utils.h
   - src/core/lib/gprpp/bitset.h
   - src/core/lib/gprpp/chunked_vector.h
+  - src/core/lib/gprpp/down_cast.h
   - src/core/lib/gprpp/dual_ref_counted.h
   - src/core/lib/gprpp/if_list.h
   - src/core/lib/gprpp/manual_constructor.h
@@ -6391,6 +6400,7 @@ targets:
   - gtest
   - upb_message_lib
   - utf8_range_lib
+  - absl/base:config
   - absl/container:inlined_vector
   - absl/functional:function_ref
   - absl/hash:hash
@@ -7334,6 +7344,7 @@ targets:
   - src/core/lib/gprpp/bitset.h
   - src/core/lib/gprpp/chunked_vector.h
   - src/core/lib/gprpp/cpp_impl_of.h
+  - src/core/lib/gprpp/down_cast.h
   - src/core/lib/gprpp/manual_constructor.h
   - src/core/lib/gprpp/orphanable.h
   - src/core/lib/gprpp/ref_counted.h
@@ -7435,6 +7446,7 @@ targets:
   - gtest
   - upb_message_lib
   - utf8_range_lib
+  - absl/base:config
   - absl/functional:function_ref
   - absl/hash:hash
   - absl/meta:type_traits
@@ -7960,11 +7972,14 @@ targets:
   build: test
   language: c++
   headers:
+  - src/core/lib/gprpp/down_cast.h
   - src/core/lib/promise/context.h
   src:
   - test/core/promise/context_test.cc
   deps:
   - gtest
+  - absl/base:config
+  - absl/meta:type_traits
   - gpr
   uses_polling: false
 - name: core_configuration_test
@@ -8683,6 +8698,7 @@ targets:
   - src/core/lib/gpr/spinlock.h
   - src/core/lib/gprpp/atomic_utils.h
   - src/core/lib/gprpp/bitset.h
+  - src/core/lib/gprpp/down_cast.h
   - src/core/lib/gprpp/manual_constructor.h
   - src/core/lib/gprpp/orphanable.h
   - src/core/lib/gprpp/ref_counted.h
@@ -8764,6 +8780,7 @@ targets:
   - gtest
   - upb_message_lib
   - utf8_range_lib
+  - absl/base:config
   - absl/hash:hash
   - absl/meta:type_traits
   - absl/status:statusor
@@ -9190,6 +9207,7 @@ targets:
   - src/core/lib/gprpp/atomic_utils.h
   - src/core/lib/gprpp/bitset.h
   - src/core/lib/gprpp/cpp_impl_of.h
+  - src/core/lib/gprpp/down_cast.h
   - src/core/lib/gprpp/manual_constructor.h
   - src/core/lib/gprpp/orphanable.h
   - src/core/lib/gprpp/ref_counted.h
@@ -9297,6 +9315,7 @@ targets:
   - gtest
   - upb_message_lib
   - utf8_range_lib
+  - absl/base:config
   - absl/functional:function_ref
   - absl/hash:hash
   - absl/meta:type_traits
@@ -9321,6 +9340,7 @@ targets:
   - src/core/lib/gprpp/atomic_utils.h
   - src/core/lib/gprpp/bitset.h
   - src/core/lib/gprpp/cpp_impl_of.h
+  - src/core/lib/gprpp/down_cast.h
   - src/core/lib/gprpp/manual_constructor.h
   - src/core/lib/gprpp/orphanable.h
   - src/core/lib/gprpp/ref_counted.h
@@ -9432,6 +9452,7 @@ targets:
   - gtest
   - upb_message_lib
   - utf8_range_lib
+  - absl/base:config
   - absl/functional:function_ref
   - absl/hash:hash
   - absl/meta:type_traits
@@ -10807,6 +10828,7 @@ targets:
   headers:
   - src/core/lib/debug/trace.h
   - src/core/lib/gprpp/atomic_utils.h
+  - src/core/lib/gprpp/down_cast.h
   - src/core/lib/gprpp/orphanable.h
   - src/core/lib/gprpp/ref_counted.h
   - src/core/lib/gprpp/ref_counted_ptr.h
@@ -10829,6 +10851,7 @@ targets:
   - test/core/promise/inter_activity_pipe_test.cc
   deps:
   - gtest
+  - absl/base:config
   - absl/hash:hash
   - absl/meta:type_traits
   - absl/status:statusor
@@ -10851,6 +10874,7 @@ targets:
   - src/core/lib/gprpp/atomic_utils.h
   - src/core/lib/gprpp/bitset.h
   - src/core/lib/gprpp/cpp_impl_of.h
+  - src/core/lib/gprpp/down_cast.h
   - src/core/lib/gprpp/manual_constructor.h
   - src/core/lib/gprpp/orphanable.h
   - src/core/lib/gprpp/ref_counted.h
@@ -10954,6 +10978,7 @@ targets:
   - gtest
   - upb_message_lib
   - utf8_range_lib
+  - absl/base:config
   - absl/functional:function_ref
   - absl/hash:hash
   - absl/meta:type_traits
@@ -11281,6 +11306,7 @@ targets:
   - src/core/lib/debug/trace.h
   - src/core/lib/gprpp/atomic_utils.h
   - src/core/lib/gprpp/bitset.h
+  - src/core/lib/gprpp/down_cast.h
   - src/core/lib/gprpp/orphanable.h
   - src/core/lib/gprpp/ref_counted.h
   - src/core/lib/gprpp/ref_counted_ptr.h
@@ -11306,6 +11332,7 @@ targets:
   - test/core/promise/latch_test.cc
   deps:
   - gtest
+  - absl/base:config
   - absl/hash:hash
   - absl/meta:type_traits
   - absl/status:statusor
@@ -11435,6 +11462,7 @@ targets:
   - src/core/lib/gprpp/atomic_utils.h
   - src/core/lib/gprpp/bitset.h
   - src/core/lib/gprpp/cpp_impl_of.h
+  - src/core/lib/gprpp/down_cast.h
   - src/core/lib/gprpp/manual_constructor.h
   - src/core/lib/gprpp/orphanable.h
   - src/core/lib/gprpp/ref_counted.h
@@ -11546,6 +11574,7 @@ targets:
   - gtest
   - upb_message_lib
   - utf8_range_lib
+  - absl/base:config
   - absl/functional:function_ref
   - absl/hash:hash
   - absl/meta:type_traits
@@ -11925,6 +11954,7 @@ targets:
   language: c++
   headers:
   - src/core/lib/gprpp/atomic_utils.h
+  - src/core/lib/gprpp/down_cast.h
   - src/core/lib/gprpp/orphanable.h
   - src/core/lib/gprpp/ref_counted.h
   - src/core/lib/gprpp/ref_counted_ptr.h
@@ -11942,6 +11972,7 @@ targets:
   - test/core/promise/mpsc_test.cc
   deps:
   - gtest
+  - absl/base:config
   - absl/hash:hash
   - absl/meta:type_traits
   - absl/status:statusor
@@ -12954,6 +12985,7 @@ targets:
   - src/core/lib/debug/trace.h
   - src/core/lib/gprpp/atomic_utils.h
   - src/core/lib/gprpp/bitset.h
+  - src/core/lib/gprpp/down_cast.h
   - src/core/lib/gprpp/orphanable.h
   - src/core/lib/gprpp/ref_counted.h
   - src/core/lib/gprpp/ref_counted_ptr.h
@@ -12980,6 +13012,7 @@ targets:
   - test/core/promise/promise_mutex_test.cc
   deps:
   - gtest
+  - absl/base:config
   - absl/hash:hash
   - absl/meta:type_traits
   - absl/status:statusor
@@ -17274,6 +17307,7 @@ targets:
   language: c++
   headers:
   - src/core/lib/gprpp/atomic_utils.h
+  - src/core/lib/gprpp/down_cast.h
   - src/core/lib/gprpp/notification.h
   - src/core/lib/gprpp/orphanable.h
   - src/core/lib/gprpp/ref_counted.h
@@ -17292,6 +17326,7 @@ targets:
   - test/core/promise/wait_for_callback_test.cc
   deps:
   - gtest
+  - absl/base:config
   - absl/hash:hash
   - absl/meta:type_traits
   - absl/status:statusor

gRPC-C++.podspec

@@ -219,6 +219,7 @@ Pod::Spec.new do |s|
     abseil_version = '1.20230802.0'
     ss.dependency 'abseil/algorithm/container', abseil_version
     ss.dependency 'abseil/base/base', abseil_version
+    ss.dependency 'abseil/base/config', abseil_version
     ss.dependency 'abseil/base/core_headers', abseil_version
     ss.dependency 'abseil/cleanup/cleanup', abseil_version
     ss.dependency 'abseil/container/flat_hash_map', abseil_version
@@ -1012,6 +1013,7 @@ Pod::Spec.new do |s|
                       'src/core/lib/gprpp/crash.h',
                       'src/core/lib/gprpp/debug_location.h',
                       'src/core/lib/gprpp/directory_reader.h',
+                      'src/core/lib/gprpp/down_cast.h',
                       'src/core/lib/gprpp/dual_ref_counted.h',
                       'src/core/lib/gprpp/env.h',
                       'src/core/lib/gprpp/examine_stack.h',
@@ -2259,6 +2261,7 @@ Pod::Spec.new do |s|
                               'src/core/lib/gprpp/crash.h',
                               'src/core/lib/gprpp/debug_location.h',
                               'src/core/lib/gprpp/directory_reader.h',
+                              'src/core/lib/gprpp/down_cast.h',
                               'src/core/lib/gprpp/dual_ref_counted.h',
                               'src/core/lib/gprpp/env.h',
                               'src/core/lib/gprpp/examine_stack.h',

gRPC-Core.podspec

@@ -188,6 +188,7 @@ Pod::Spec.new do |s|
     ss.dependency 'BoringSSL-GRPC', '0.0.31'
     ss.dependency 'abseil/algorithm/container', abseil_version
     ss.dependency 'abseil/base/base', abseil_version
+    ss.dependency 'abseil/base/config', abseil_version
     ss.dependency 'abseil/base/core_headers', abseil_version
     ss.dependency 'abseil/cleanup/cleanup', abseil_version
     ss.dependency 'abseil/container/flat_hash_map', abseil_version
@@ -1511,6 +1512,7 @@ Pod::Spec.new do |s|
                       'src/core/lib/gprpp/crash.h',
                       'src/core/lib/gprpp/debug_location.h',
                       'src/core/lib/gprpp/directory_reader.h',
+                      'src/core/lib/gprpp/down_cast.h',
                       'src/core/lib/gprpp/dual_ref_counted.h',
                       'src/core/lib/gprpp/env.h',
                       'src/core/lib/gprpp/examine_stack.cc',
@@ -3033,6 +3035,7 @@ Pod::Spec.new do |s|
                               'src/core/lib/gprpp/crash.h',
                               'src/core/lib/gprpp/debug_location.h',
                               'src/core/lib/gprpp/directory_reader.h',
+                              'src/core/lib/gprpp/down_cast.h',
                               'src/core/lib/gprpp/dual_ref_counted.h',
                               'src/core/lib/gprpp/env.h',
                               'src/core/lib/gprpp/examine_stack.h',

grpc.gemspec

@@ -1414,6 +1414,7 @@ Gem::Specification.new do |s|
   s.files += %w( src/core/lib/gprpp/crash.h )
   s.files += %w( src/core/lib/gprpp/debug_location.h )
   s.files += %w( src/core/lib/gprpp/directory_reader.h )
+  s.files += %w( src/core/lib/gprpp/down_cast.h )
   s.files += %w( src/core/lib/gprpp/dual_ref_counted.h )
   s.files += %w( src/core/lib/gprpp/env.h )
   s.files += %w( src/core/lib/gprpp/examine_stack.cc )

grpc.gyp

@@ -254,6 +254,7 @@
         're2',
         'z',
         'absl/algorithm:container',
+        'absl/base:config',
         'absl/cleanup:cleanup',
         'absl/container:flat_hash_map',
         'absl/container:flat_hash_set',
@@ -1120,6 +1121,7 @@
         'utf8_range_lib',
         'z',
         'absl/algorithm:container',
+        'absl/base:config',
         'absl/cleanup:cleanup',
         'absl/container:flat_hash_map',
         'absl/container:flat_hash_set',
@@ -2008,6 +2010,7 @@
         're2',
         'utf8_range_lib',
         'z',
+        'absl/base:config',
         'absl/cleanup:cleanup',
         'absl/container:flat_hash_map',
         'absl/container:flat_hash_set',

package.xml

@@ -1396,6 +1396,7 @@
     <file baseinstalldir="/" name="src/core/lib/gprpp/crash.h" role="src" />
     <file baseinstalldir="/" name="src/core/lib/gprpp/debug_location.h" role="src" />
     <file baseinstalldir="/" name="src/core/lib/gprpp/directory_reader.h" role="src" />
+    <file baseinstalldir="/" name="src/core/lib/gprpp/down_cast.h" role="src" />
     <file baseinstalldir="/" name="src/core/lib/gprpp/dual_ref_counted.h" role="src" />
     <file baseinstalldir="/" name="src/core/lib/gprpp/env.h" role="src" />
     <file baseinstalldir="/" name="src/core/lib/gprpp/examine_stack.cc" role="src" />

src/core/BUILD

@@ -495,11 +495,15 @@ grpc_cc_library(
 
 grpc_cc_library(
     name = "context",
+    external_deps = ["absl/meta:type_traits"],
     language = "c++",
     public_hdrs = [
         "lib/promise/context.h",
     ],
-    deps = ["//:gpr"],
+    deps = [
+        "down_cast",
+        "//:gpr",
+    ],
 )
 
 grpc_cc_library(

src/core/ext/filters/client_channel/client_channel.cc

@@ -332,7 +332,7 @@ class ClientChannel::PromiseBasedCallData : public ClientChannel::CallData {
                      GRPC_CHANNEL_IDLE)) {
       if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
         gpr_log(GPR_INFO, "chand=%p calld=%p: %striggering exit idle", chand_,
-                this, Activity::current()->DebugTag().c_str());
+                this, GetContext<Activity>()->DebugTag().c_str());
       }
       // Bounce into the control plane work serializer to start resolving.
       GRPC_CHANNEL_STACK_REF(chand_->owning_stack_, "ExitIdle");
@@ -349,7 +349,7 @@ class ClientChannel::PromiseBasedCallData : public ClientChannel::CallData {
       auto result = CheckResolution(was_queued_);
       if (GRPC_TRACE_FLAG_ENABLED(grpc_client_channel_call_trace)) {
         gpr_log(GPR_INFO, "chand=%p calld=%p: %sCheckResolution returns %s",
-                chand_, this, Activity::current()->DebugTag().c_str(),
+                chand_, this, GetContext<Activity>()->DebugTag().c_str(),
                 result.has_value() ? result->ToString().c_str() : "Pending");
       }
       if (!result.has_value()) return Pending{};
@@ -372,7 +372,7 @@ class ClientChannel::PromiseBasedCallData : public ClientChannel::CallData {
 
   void OnAddToQueueLocked() override
       ABSL_EXCLUSIVE_LOCKS_REQUIRED(&ClientChannel::resolution_mu_) {
-    waker_ = Activity::current()->MakeNonOwningWaker();
+    waker_ = GetContext<Activity>()->MakeNonOwningWaker();
     was_queued_ = true;
   }
 
@@ -3541,7 +3541,7 @@ ClientChannel::PromiseBasedLoadBalancedCall::MakeCallPromise(
                   gpr_log(GPR_INFO,
                           "chand=%p lb_call=%p: %sPickSubchannel() returns %s",
                           chand(), this,
-                          Activity::current()->DebugTag().c_str(),
+                          GetContext<Activity>()->DebugTag().c_str(),
                           result.has_value() ? result->ToString().c_str()
                                              : "Pending");
                 }
@@ -3624,7 +3624,7 @@ ClientChannel::PromiseBasedLoadBalancedCall::send_initial_metadata() const {
 }
 
 void ClientChannel::PromiseBasedLoadBalancedCall::OnAddToQueueLocked() {
-  waker_ = Activity::current()->MakeNonOwningWaker();
+  waker_ = GetContext<Activity>()->MakeNonOwningWaker();
   was_queued_ = true;
 }
 

src/core/ext/filters/http/message_compress/legacy_compression_filter.cc

@@ -291,7 +291,7 @@ LegacyServerCompressionFilter::MakeCallPromise(
         auto r = DecompressMessage(std::move(message), decompress_args);
         if (grpc_call_trace.enabled()) {
           gpr_log(GPR_DEBUG, "%s[compression] DecompressMessage returned %s",
-                  Activity::current()->DebugTag().c_str(),
+                  GetContext<Activity>()->DebugTag().c_str(),
                   r.status().ToString().c_str());
         }
         if (!r.ok()) {
@@ -306,7 +306,7 @@ LegacyServerCompressionFilter::MakeCallPromise(
       [this, compression_algorithm](ServerMetadataHandle md) {
         if (grpc_call_trace.enabled()) {
           gpr_log(GPR_INFO, "%s[compression] Write metadata",
-                  Activity::current()->DebugTag().c_str());
+                  GetContext<Activity>()->DebugTag().c_str());
         }
         // Find the compression algorithm.
         *compression_algorithm = HandleOutgoingMetadata(*md);

src/core/ext/filters/http/server/http_server_filter.cc

@@ -141,7 +141,7 @@ ServerMetadataHandle HttpServerFilter::Call::OnClientInitialMetadata(
 void HttpServerFilter::Call::OnServerInitialMetadata(ServerMetadata& md) {
   if (grpc_call_trace.enabled()) {
     gpr_log(GPR_INFO, "%s[http-server] Write metadata",
-            Activity::current()->DebugTag().c_str());
+            GetContext<Activity>()->DebugTag().c_str());
   }
   FilterOutgoingMetadata(&md);
   md.Set(HttpStatusMetadata(), 200);

src/core/ext/filters/message_size/message_size_filter.cc

@@ -164,8 +164,8 @@ ServerMetadataHandle CheckPayload(const Message& msg,
   if (!max_length.has_value()) return nullptr;
   if (GRPC_TRACE_FLAG_ENABLED(grpc_call_trace)) {
     gpr_log(GPR_INFO, "%s[message_size] %s len:%" PRIdPTR " max:%d",
-            Activity::current()->DebugTag().c_str(), is_send ? "send" : "recv",
+            GetContext<Activity>()->DebugTag().c_str(),
-            msg.payload()->Length(), *max_length);
+            is_send ? "send" : "recv", msg.payload()->Length(), *max_length);
   }
   if (msg.payload()->Length() <= *max_length) return nullptr;
   auto r = GetContext<Arena>()->MakePooled<ServerMetadata>(GetContext<Arena>());

src/core/lib/channel/connected_channel.cc

@@ -364,7 +364,7 @@ class ConnectedChannelStream : public Orphanable {
   grpc_stream_refcount stream_refcount_;
   StreamPtr stream_;
   Arena* arena_ = GetContext<Arena>();
-  Party* const party_ = static_cast<Party*>(Activity::current());
+  Party* const party_ = GetContext<Party>();
   ExternallyObservableLatch<void> finished_;
 };
 
@@ -383,17 +383,18 @@ auto ConnectedChannelStream::RecvMessages(
               gpr_log(GPR_INFO,
                       "%s[connected] RecvMessage: received payload of %" PRIdPTR
                       " bytes",
-                      Activity::current()->DebugTag().c_str(),
+                      GetContext<Activity>()->DebugTag().c_str(),
                       pending_message->payload()->Length());
             }
             return Map(incoming_messages.Push(std::move(pending_message)),
                        [](bool ok) -> LoopCtl<absl::Status> {
                          if (!ok) {
                            if (grpc_call_trace.enabled()) {
-                             gpr_log(GPR_INFO,
+                             gpr_log(
-                                     "%s[connected] RecvMessage: failed to "
+                                 GPR_INFO,
-                                     "push message towards the application",
+                                 "%s[connected] RecvMessage: failed to "
-                                     Activity::current()->DebugTag().c_str());
+                                 "push message towards the application",
+                                 GetContext<Activity>()->DebugTag().c_str());
                            }
                            return absl::OkStatus();
                          }
@@ -406,7 +407,7 @@ auto ConnectedChannelStream::RecvMessages(
               gpr_log(GPR_INFO,
                       "%s[connected] RecvMessage: reached end of stream with "
                       "status:%s",
-                      Activity::current()->DebugTag().c_str(),
+                      GetContext<Activity>()->DebugTag().c_str(),
                       status.status().ToString().c_str());
             }
             if (cancel_on_error && !status.ok()) {
@@ -444,7 +445,7 @@ ArenaPromise<ServerMetadataHandle> MakeClientCallPromise(Transport* transport,
   transport->filter_stack_transport()->InitStream(stream->stream(),
                                                   stream->stream_refcount(),
                                                   nullptr, GetContext<Arena>());
-  auto* party = static_cast<Party*>(Activity::current());
+  auto* party = GetContext<Party>();
   party->Spawn("set_polling_entity", call_args.polling_entity->Wait(),
                [transport, stream = stream->InternalRef()](
                    grpc_polling_entity polling_entity) {
@@ -474,7 +475,7 @@ ArenaPromise<ServerMetadataHandle> MakeClientCallPromise(Transport* transport,
                if (grpc_call_trace.enabled()) {
                  gpr_log(GPR_DEBUG,
                          "%s[connected] Publish client initial metadata: %s",
-                         Activity::current()->DebugTag().c_str(),
+                         GetContext<Activity>()->DebugTag().c_str(),
                          server_initial_metadata->DebugString().c_str());
                }
                return Map(pipe->Push(std::move(server_initial_metadata)),
@@ -581,7 +582,7 @@ ArenaPromise<ServerMetadataHandle> MakeServerCallPromise(
       stream->stream(), stream->stream_refcount(),
       GetContext<CallContext>()->server_call_context()->server_stream_data(),
       GetContext<Arena>());
-  auto* party = static_cast<Party*>(Activity::current());
+  auto* party = GetContext<Party>();
 
   // Arifacts we need for the lifetime of the call.
   struct CallData {
@@ -747,7 +748,7 @@ ArenaPromise<ServerMetadataHandle> MakeServerCallPromise(
                       : nullptr;
         if (md != nullptr) {
           call_data->sent_initial_metadata = true;
-          auto* party = static_cast<Party*>(Activity::current());
+          auto* party = GetContext<Party>();
           party->Spawn("connected/send_initial_metadata",
                        GetContext<BatchBuilder>()->SendServerInitialMetadata(
                            stream->batch_target(), std::move(md)),
@@ -777,7 +778,7 @@ ArenaPromise<ServerMetadataHandle> MakeServerCallPromise(
               gpr_log(
                   GPR_DEBUG,
                   "%s[connected] Got trailing metadata; status=%s metadata=%s",
-                  Activity::current()->DebugTag().c_str(),
+                  GetContext<Activity>()->DebugTag().c_str(),
                   status.status().ToString().c_str(),
                   status.ok() ? (*status)->DebugString().c_str() : "<none>");
             }

src/core/lib/channel/promise_based_filter.cc

@@ -374,11 +374,11 @@ void BaseCallData::SendMessage::GotPipe(T* pipe_end) {
   switch (state_) {
     case State::kInitial:
       state_ = State::kIdle;
-      Activity::current()->ForceImmediateRepoll();
+      GetContext<Activity>()->ForceImmediateRepoll();
       break;
     case State::kGotBatchNoPipe:
       state_ = State::kGotBatch;
-      Activity::current()->ForceImmediateRepoll();
+      GetContext<Activity>()->ForceImmediateRepoll();
       break;
     case State::kIdle:
     case State::kGotBatch:
@@ -567,7 +567,7 @@ void BaseCallData::SendMessage::WakeInsideCombiner(Flusher* flusher,
       }
       if (completed_status_.ok()) {
         state_ = State::kIdle;
-        Activity::current()->ForceImmediateRepoll();
+        GetContext<Activity>()->ForceImmediateRepoll();
       } else {
         state_ = State::kCancelled;
       }
@@ -675,7 +675,7 @@ void BaseCallData::ReceiveMessage::GotPipe(T* pipe_end) {
       break;
     case State::kBatchCompletedNoPipe:
       state_ = State::kBatchCompleted;
-      Activity::current()->ForceImmediateRepoll();
+      GetContext<Activity>()->ForceImmediateRepoll();
       break;
     case State::kIdle:
     case State::kForwardedBatch:

src/core/lib/promise/activity.h

@@ -282,6 +282,12 @@ class ContextHolder<std::unique_ptr<Context, Deleter>> {
   std::unique_ptr<Context, Deleter> value_;
 };
 
+template <>
+class Context<Activity> {
+ public:
+  static Activity* get() { return Activity::current(); }
+};
+
 template <typename HeldContext>
 using ContextTypeFromHeld = typename ContextHolder<HeldContext>::ContextType;
 
@@ -632,13 +638,13 @@ ActivityPtr MakeActivity(Factory promise_factory,
 }
 
 inline Pending IntraActivityWaiter::pending() {
-  wakeups_ |= Activity::current()->CurrentParticipant();
+  wakeups_ |= GetContext<Activity>()->CurrentParticipant();
   return Pending();
 }
 
 inline void IntraActivityWaiter::Wake() {
   if (wakeups_ == 0) return;
-  Activity::current()->ForceImmediateRepoll(std::exchange(wakeups_, 0));
+  GetContext<Activity>()->ForceImmediateRepoll(std::exchange(wakeups_, 0));
 }
 
 }  // namespace grpc_core

src/core/lib/promise/context.h

@@ -19,8 +19,12 @@
 
 #include <utility>
 
+#include "absl/meta/type_traits.h"
+
 #include <grpc/support/log.h>
 
+#include "src/core/lib/gprpp/down_cast.h"
+
 namespace grpc_core {
 
 // To avoid accidentally creating context types, we require an explicit
@@ -28,17 +32,35 @@ namespace grpc_core {
 // not contain any members, only exist.
 // The reason for avoiding this is that context types each use a thread local.
 template <typename T>
-struct ContextType;  // IWYU pragma: keep
+struct ContextType;
+
+// Some contexts can be subclassed. If the subclass is set as that context
+// then GetContext<Base>() will return the base, and GetContext<Derived>() will
+// down_cast to the derived type.
+// Specializations of this type should be created for each derived type, and
+// should have a single using statement Base pointing to the derived base class.
+// Example:
+//  class SomeContext {};
+//  class SomeDerivedContext : public SomeContext {};
+//  template <> struct ContextType<SomeContext> {};
+//  template <> struct ContextSubclass<SomeDerivedContext> {
+//    using Base = SomeContext;
+//  };
+template <typename Derived>
+struct ContextSubclass;
 
 namespace promise_detail {
 
+template <typename T, typename = void>
+class Context;
+
 template <typename T>
-class Context : public ContextType<T> {
+class ThreadLocalContext : public ContextType<T> {
  public:
-  explicit Context(T* p) : old_(current_) { current_ = p; }
+  explicit ThreadLocalContext(T* p) : old_(current_) { current_ = p; }
-  ~Context() { current_ = old_; }
+  ~ThreadLocalContext() { current_ = old_; }
-  Context(const Context&) = delete;
+  ThreadLocalContext(const ThreadLocalContext&) = delete;
-  Context& operator=(const Context&) = delete;
+  ThreadLocalContext& operator=(const ThreadLocalContext&) = delete;
 
   static T* get() { return current_; }
 
@@ -48,7 +70,23 @@ class Context : public ContextType<T> {
 };
 
 template <typename T>
-thread_local T* Context<T>::current_;
+thread_local T* ThreadLocalContext<T>::current_;
+
+template <typename T>
+class Context<T, absl::void_t<decltype(ContextType<T>())>>
+    : public ThreadLocalContext<T> {
+  using ThreadLocalContext<T>::ThreadLocalContext;
+};
+
+template <typename T>
+class Context<T, absl::void_t<typename ContextSubclass<T>::Base>>
+    : public Context<typename ContextSubclass<T>::Base> {
+ public:
+  using Context<typename ContextSubclass<T>::Base>::Context;
+  static T* get() {
+    return down_cast<T*>(Context<typename ContextSubclass<T>::Base>::get());
+  }
+};
 
 template <typename T, typename F>
 class WithContext {

src/core/lib/promise/for_each.h

@@ -118,7 +118,7 @@ class ForEach {
   };
 
   std::string DebugTag() {
-    return absl::StrCat(Activity::current()->DebugTag(), " FOR_EACH[0x",
+    return absl::StrCat(GetContext<Activity>()->DebugTag(), " FOR_EACH[0x",
                         reinterpret_cast<uintptr_t>(this), "]: ");
   }
 

src/core/lib/promise/inter_activity_latch.h

@@ -56,7 +56,8 @@ class InterActivityLatch<void> {
       if (is_set_) {
         return Empty{};
       } else {
-        return waiters_.AddPending(Activity::current()->MakeNonOwningWaker());
+        return waiters_.AddPending(
+            GetContext<Activity>()->MakeNonOwningWaker());
       }
     };
   }
@@ -78,7 +79,7 @@ class InterActivityLatch<void> {
 
  private:
   std::string DebugTag() {
-    return absl::StrCat(Activity::current()->DebugTag(),
+    return absl::StrCat(GetContext<Activity>()->DebugTag(),
                         " INTER_ACTIVITY_LATCH[0x",
                         reinterpret_cast<uintptr_t>(this), "]: ");
   }

src/core/lib/promise/inter_activity_pipe.h

@@ -42,7 +42,7 @@ class InterActivityPipe {
       ReleasableMutexLock lock(&mu_);
       if (closed_) return false;
       if (count_ == kQueueSize) {
-        on_available_ = Activity::current()->MakeNonOwningWaker();
+        on_available_ = GetContext<Activity>()->MakeNonOwningWaker();
         return Pending{};
       }
       queue_[(first_ + count_) % kQueueSize] = std::move(value);
@@ -59,7 +59,7 @@ class InterActivityPipe {
       ReleasableMutexLock lock(&mu_);
       if (count_ == 0) {
         if (closed_) return absl::nullopt;
-        on_occupied_ = Activity::current()->MakeNonOwningWaker();
+        on_occupied_ = GetContext<Activity>()->MakeNonOwningWaker();
         return Pending{};
       }
       auto value = std::move(queue_[first_]);

src/core/lib/promise/latch.h

@@ -113,7 +113,7 @@ class Latch {
 
  private:
   std::string DebugTag() {
-    return absl::StrCat(Activity::current()->DebugTag(), " LATCH[0x",
+    return absl::StrCat(GetContext<Activity>()->DebugTag(), " LATCH[0x",
                         reinterpret_cast<uintptr_t>(this), "]: ");
   }
 
@@ -189,7 +189,7 @@ class Latch<void> {
 
  private:
   std::string DebugTag() {
-    return absl::StrCat(Activity::current()->DebugTag(), " LATCH(void)[0x",
+    return absl::StrCat(GetContext<Activity>()->DebugTag(), " LATCH(void)[0x",
                         reinterpret_cast<uintptr_t>(this), "]: ");
   }
 
@@ -259,7 +259,7 @@ class ExternallyObservableLatch<void> {
 
  private:
   std::string DebugTag() {
-    return absl::StrCat(Activity::current()->DebugTag(), " LATCH(void)[0x",
+    return absl::StrCat(GetContext<Activity>()->DebugTag(), " LATCH(void)[0x",
                         reinterpret_cast<uintptr_t>(this), "]: ");
   }
 

src/core/lib/promise/mpsc.h

@@ -62,7 +62,7 @@ class Center : public RefCounted<Center<T>> {
   bool PollReceiveBatch(std::vector<T>& dest) {
     ReleasableMutexLock lock(&mu_);
     if (queue_.empty()) {
-      receive_waker_ = Activity::current()->MakeNonOwningWaker();
+      receive_waker_ = GetContext<Activity>()->MakeNonOwningWaker();
       return false;
     }
     dest.swap(queue_);
@@ -87,7 +87,7 @@ class Center : public RefCounted<Center<T>> {
       receive_waker.Wakeup();
       return Poll<bool>(true);
     }
-    send_wakers_.AddPending(Activity::current()->MakeNonOwningWaker());
+    send_wakers_.AddPending(GetContext<Activity>()->MakeNonOwningWaker());
     return Pending{};
   }
 

src/core/lib/promise/party.h

@@ -385,10 +385,10 @@ class Party : public Activity, private Wakeable {
   // This is useful for implementing batching and the like: we can hold some
   // action until the rest of the party resolves itself.
   auto AfterCurrentPoll() {
-    GPR_DEBUG_ASSERT(Activity::current() == this);
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == this);
     sync_.WakeAfterPoll(CurrentParticipant());
     return [this, iteration = sync_.iteration()]() -> Poll<Empty> {
-      GPR_DEBUG_ASSERT(Activity::current() == this);
+      GPR_DEBUG_ASSERT(GetContext<Activity>() == this);
       if (iteration == sync_.iteration()) return Pending{};
       return Empty{};
     };
@@ -561,7 +561,7 @@ class Party : public Activity, private Wakeable {
       GPR_NO_UNIQUE_ADDRESS Promise promise_;
       GPR_NO_UNIQUE_ADDRESS Result result_;
     };
-    Waker waiter_{Activity::current()->MakeOwningWaker()};
+    Waker waiter_{GetContext<Activity>()->MakeOwningWaker()};
     std::atomic<State> state_{State::kFactory};
   };
 
@@ -608,6 +608,11 @@ class Party : public Activity, private Wakeable {
   std::atomic<Participant*> participants_[party_detail::kMaxParticipants] = {};
 };
 
+template <>
+struct ContextSubclass<Party> {
+  using Base = Activity;
+};
+
 template <typename Factory, typename OnComplete>
 void Party::BulkSpawner::Spawn(absl::string_view name, Factory promise_factory,
                                OnComplete on_complete) {

src/core/lib/promise/pipe.h

@@ -374,7 +374,7 @@ class Center : public InterceptorList<T> {
   const T& value() const { return value_; }
 
   std::string DebugTag() {
-    if (auto* activity = Activity::current()) {
+    if (auto* activity = GetContext<Activity>()) {
       return absl::StrCat(activity->DebugTag(), " PIPE[0x", absl::Hex(this),
                           "]: ");
     } else {
@@ -661,7 +661,7 @@ class Push {
     if (center_ == nullptr) {
       if (grpc_trace_promise_primitives.enabled()) {
         gpr_log(GPR_DEBUG, "%s Pipe push has a null center",
-                Activity::current()->DebugTag().c_str());
+                GetContext<Activity>()->DebugTag().c_str());
       }
       return false;
     }

src/core/lib/promise/sleep.cc

@@ -54,7 +54,7 @@ Poll<absl::Status> Sleep::operator()() {
 }
 
 Sleep::ActiveClosure::ActiveClosure(Timestamp deadline)
-    : waker_(Activity::current()->MakeOwningWaker()),
+    : waker_(GetContext<Activity>()->MakeOwningWaker()),
       timer_handle_(GetContext<EventEngine>()->RunAfter(
           deadline - Timestamp::Now(), this)) {}
 

src/core/lib/promise/wait_for_callback.h

@@ -39,7 +39,7 @@ class WaitForCallback {
     return [state = state_]() -> Poll<Empty> {
       MutexLock lock(&state->mutex);
       if (state->done) return Empty{};
-      state->waker = Activity::current()->MakeNonOwningWaker();
+      state->waker = GetContext<Activity>()->MakeNonOwningWaker();
       return Pending{};
     };
   }

src/core/lib/resource_quota/memory_quota.cc

@@ -232,10 +232,10 @@ Poll<RefCountedPtr<ReclaimerQueue::Handle>> ReclaimerQueue::PollNext() {
   if (!empty) {
     // If we don't, but the queue is probably not empty, schedule an immediate
     // repoll.
-    Activity::current()->ForceImmediateRepoll();
+    GetContext<Activity>()->ForceImmediateRepoll();
   } else {
     // Otherwise, schedule a wakeup for whenever something is pushed.
-    state_->waker = Activity::current()->MakeNonOwningWaker();
+    state_->waker = GetContext<Activity>()->MakeNonOwningWaker();
   }
   return Pending{};
 }
@@ -465,7 +465,7 @@ void BasicMemoryQuota::Start() {
             self->reclamation_counter_.fetch_add(1, std::memory_order_relaxed) +
             1;
         reclaimer->Run(ReclamationSweep(
-            self, token, Activity::current()->MakeNonOwningWaker()));
+            self, token, GetContext<Activity>()->MakeNonOwningWaker()));
         // Return a promise that will wait for our barrier. This will be
         // awoken by the token above being destroyed. So, once that token is
         // destroyed, we'll be able to proceed.

src/core/lib/security/credentials/oauth2/oauth2_credentials.cc

@@ -314,7 +314,8 @@ grpc_oauth2_token_fetcher_credentials::GetRequestMetadata(
   auto pending_request =
       grpc_core::MakeRefCounted<grpc_oauth2_pending_get_request_metadata>();
   pending_request->pollent = grpc_core::GetContext<grpc_polling_entity>();
-  pending_request->waker = grpc_core::Activity::current()->MakeNonOwningWaker();
+  pending_request->waker =
+      grpc_core::GetContext<grpc_core::Activity>()->MakeNonOwningWaker();
   grpc_polling_entity_add_to_pollset_set(
       pending_request->pollent, grpc_polling_entity_pollset_set(&pollent_));
   pending_request->next = pending_requests_;

src/core/lib/security/credentials/plugin/plugin_credentials.h

@@ -103,7 +103,7 @@ struct grpc_plugin_credentials final : public grpc_call_credentials {
    private:
     std::atomic<bool> ready_{false};
     grpc_core::Waker waker_{
-        grpc_core::Activity::current()->MakeNonOwningWaker()};
+        grpc_core::GetContext<grpc_core::Activity>()->MakeNonOwningWaker()};
     grpc_core::RefCountedPtr<grpc_plugin_credentials> call_creds_;
     grpc_auth_metadata_context context_;
     grpc_core::ClientMetadataHandle md_;

src/core/lib/security/transport/legacy_server_auth_filter.cc

@@ -124,7 +124,7 @@ class LegacyServerAuthFilter::RunApplicationCode {
       gpr_log(GPR_ERROR,
               "%s[server-auth]: Delegate to application: filter=%p this=%p "
               "auth_ctx=%p",
-              Activity::current()->DebugTag().c_str(), filter, this,
+              GetContext<Activity>()->DebugTag().c_str(), filter, this,
               filter->auth_context_.get());
     }
     filter->server_credentials_->auth_metadata_processor().process(
@@ -152,7 +152,7 @@ class LegacyServerAuthFilter::RunApplicationCode {
  private:
   struct State {
     explicit State(CallArgs call_args) : call_args(std::move(call_args)) {}
-    Waker waker{Activity::current()->MakeOwningWaker()};
+    Waker waker{GetContext<Activity>()->MakeOwningWaker()};
     absl::StatusOr<CallArgs> call_args;
     grpc_metadata_array md =
         MetadataBatchToMetadataArray(call_args->client_initial_metadata.get());

src/core/lib/security/transport/server_auth_filter.cc

@@ -123,7 +123,7 @@ grpc_metadata_array MetadataBatchToMetadataArray(
 struct ServerAuthFilter::RunApplicationCode::State {
   explicit State(ClientMetadata& client_metadata)
       : client_metadata(&client_metadata) {}
-  Waker waker{Activity::current()->MakeOwningWaker()};
+  Waker waker{GetContext<Activity>()->MakeOwningWaker()};
   absl::StatusOr<ClientMetadata*> client_metadata;
   grpc_metadata_array md = MetadataBatchToMetadataArray(*client_metadata);
   std::atomic<bool> done{false};
@@ -136,7 +136,7 @@ ServerAuthFilter::RunApplicationCode::RunApplicationCode(
     gpr_log(GPR_ERROR,
             "%s[server-auth]: Delegate to application: filter=%p this=%p "
             "auth_ctx=%p",
-            Activity::current()->DebugTag().c_str(), filter, this,
+            GetContext<Activity>()->DebugTag().c_str(), filter, this,
             filter->auth_context_.get());
   }
   filter->server_credentials_->auth_metadata_processor().process(

src/core/lib/surface/call_trace.cc

@@ -56,22 +56,22 @@ const grpc_channel_filter* PromiseTracingFilterFor(
                 gpr_log(
                     GPR_DEBUG,
                     "%s[%s] CreateCallPromise: client_initial_metadata=%s",
-                    Activity::current()->DebugTag().c_str(),
+                    GetContext<Activity>()->DebugTag().c_str(),
                     source_filter->name,
                     call_args.client_initial_metadata->DebugString().c_str());
                 return [source_filter, child = next_promise_factory(
                                            std::move(call_args))]() mutable {
                   gpr_log(GPR_DEBUG, "%s[%s] PollCallPromise: begin",
-                          Activity::current()->DebugTag().c_str(),
+                          GetContext<Activity>()->DebugTag().c_str(),
                           source_filter->name);
                   auto r = child();
                   if (auto* p = r.value_if_ready()) {
                     gpr_log(GPR_DEBUG, "%s[%s] PollCallPromise: done: %s",
-                            Activity::current()->DebugTag().c_str(),
+                            GetContext<Activity>()->DebugTag().c_str(),
                             source_filter->name, (*p)->DebugString().c_str());
                   } else {
                     gpr_log(GPR_DEBUG, "%s[%s] PollCallPromise: <<pending>>",
-                            Activity::current()->DebugTag().c_str(),
+                            GetContext<Activity>()->DebugTag().c_str(),
                             source_filter->name);
                   }
                   return r;
@@ -84,35 +84,35 @@ const grpc_channel_filter* PromiseTracingFilterFor(
                 call->client_initial_metadata().receiver.InterceptAndMap(
                     [source_filter](ClientMetadataHandle md) {
                       gpr_log(GPR_DEBUG, "%s[%s] OnClientInitialMetadata: %s",
-                              Activity::current()->DebugTag().c_str(),
+                              GetContext<Activity>()->DebugTag().c_str(),
                               source_filter->name, md->DebugString().c_str());
                       return md;
                     });
                 call->client_to_server_messages().receiver.InterceptAndMap(
                     [source_filter](MessageHandle msg) {
                       gpr_log(GPR_DEBUG, "%s[%s] OnClientToServerMessage: %s",
-                              Activity::current()->DebugTag().c_str(),
+                              GetContext<Activity>()->DebugTag().c_str(),
                               source_filter->name, msg->DebugString().c_str());
                       return msg;
                     });
                 call->server_initial_metadata().sender.InterceptAndMap(
                     [source_filter](ServerMetadataHandle md) {
                       gpr_log(GPR_DEBUG, "%s[%s] OnServerInitialMetadata: %s",
-                              Activity::current()->DebugTag().c_str(),
+                              GetContext<Activity>()->DebugTag().c_str(),
                               source_filter->name, md->DebugString().c_str());
                       return md;
                     });
                 call->server_to_client_messages().sender.InterceptAndMap(
                     [source_filter](MessageHandle msg) {
                       gpr_log(GPR_DEBUG, "%s[%s] OnServerToClientMessage: %s",
-                              Activity::current()->DebugTag().c_str(),
+                              GetContext<Activity>()->DebugTag().c_str(),
                               source_filter->name, msg->DebugString().c_str());
                       return msg;
                     });
                 call->server_trailing_metadata().sender.InterceptAndMap(
                     [source_filter](ServerMetadataHandle md) {
                       gpr_log(GPR_DEBUG, "%s[%s] OnServerTrailingMetadata: %s",
-                              Activity::current()->DebugTag().c_str(),
+                              GetContext<Activity>()->DebugTag().c_str(),
                               source_filter->name, md->DebugString().c_str());
                       return md;
                     });

src/core/lib/surface/server.cc

@@ -525,7 +525,7 @@ class Server::RealRequestMatcherPromises : public RequestMatcherInterface {
               "Too many pending requests for this server"));
         }
         auto w = std::make_shared<ActivityWaiter>(
-            Activity::current()->MakeOwningWaker());
+            GetContext<Activity>()->MakeOwningWaker());
         pending_.push(w);
         return OnCancel(
             [w]() -> Poll<absl::StatusOr<MatchResult>> {

src/core/lib/surface/wait_for_cq_end_op.h

@@ -41,7 +41,7 @@ class WaitForCqEndOp {
       } else {
         auto not_started = std::move(*n);
         auto& started =
-            state_.emplace<Started>(Activity::current()->MakeOwningWaker());
+            state_.emplace<Started>(GetContext<Activity>()->MakeOwningWaker());
         grpc_cq_end_op(
             not_started.cq, not_started.tag, std::move(not_started.error),
             [](void* p, grpc_cq_completion*) {

src/core/lib/transport/batch_builder.cc

@@ -55,8 +55,7 @@ BatchBuilder::PendingCompletion::PendingCompletion(RefCountedPtr<Batch> batch)
 
 BatchBuilder::Batch::Batch(grpc_transport_stream_op_batch_payload* payload,
                            grpc_stream_refcount* stream_refcount)
-    : party(static_cast<Party*>(Activity::current())->Ref()),
+    : party(GetContext<Party>()->Ref()), stream_refcount(stream_refcount) {
-      stream_refcount(stream_refcount) {
   batch.payload = payload;
   batch.is_traced = GetContext<CallContext>()->traced();
 #ifndef NDEBUG
@@ -70,7 +69,7 @@ BatchBuilder::Batch::~Batch() {
   auto* arena = party->arena();
   if (grpc_call_trace.enabled()) {
     gpr_log(GPR_DEBUG, "%s[connected] [batch %p] Destroy",
-            Activity::current()->DebugTag().c_str(), this);
+            GetContext<Activity>()->DebugTag().c_str(), this);
   }
   if (pending_receive_message != nullptr) {
     arena->DeletePooled(pending_receive_message);

src/core/lib/transport/batch_builder.h

@@ -202,7 +202,7 @@ class BatchBuilder {
     ~Batch();
     Batch(const Batch&) = delete;
     Batch& operator=(const Batch&) = delete;
-    std::string DebugPrefix(Activity* activity = Activity::current()) const {
+    std::string DebugPrefix(Activity* activity = GetContext<Activity>()) const {
       return absl::StrFormat("%s[connected] [batch %p] ", activity->DebugTag(),
                              this);
     }

src/core/lib/transport/promise_endpoint.h

@@ -80,7 +80,7 @@ class PromiseEndpoint {
                              data.c_slice_buffer());
       // If `Write()` returns true immediately, the callback will not be called.
       // We still need to call our callback to pick up the result.
-      write_state_->waker = Activity::current()->MakeNonOwningWaker();
+      write_state_->waker = GetContext<Activity>()->MakeNonOwningWaker();
       completed = endpoint_->Write(
           [write_state = write_state_](absl::Status status) {
             ApplicationCallbackExecCtx callback_exec_ctx;
@@ -124,7 +124,7 @@ class PromiseEndpoint {
               static_cast<int64_t>(num_bytes - read_state_->buffer.Length())};
       // If `Read()` returns true immediately, the callback will not be
       // called.
-      read_state_->waker = Activity::current()->MakeNonOwningWaker();
+      read_state_->waker = GetContext<Activity>()->MakeNonOwningWaker();
       if (endpoint_->Read(
               [read_state = read_state_, num_bytes](absl::Status status) {
                 ApplicationCallbackExecCtx callback_exec_ctx;

src/core/lib/transport/transport.h

@@ -203,7 +203,7 @@ class CallSpineInterface {
   // and this construction supports that (and has helped the author not write
   // some bugs).
   GRPC_MUST_USE_RESULT absl::nullopt_t Cancel(ServerMetadataHandle metadata) {
-    GPR_DEBUG_ASSERT(Activity::current() == &party());
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &party());
     auto& c = cancel_latch();
     if (c.is_set()) return absl::nullopt;
     c.Set(std::move(metadata));
@@ -216,7 +216,7 @@ class CallSpineInterface {
   }
 
   auto WaitForCancel() {
-    GPR_DEBUG_ASSERT(Activity::current() == &party());
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &party());
     return cancel_latch().Wait();
   }
 
@@ -225,7 +225,7 @@ class CallSpineInterface {
   // The resulting (returned) promise will resolve to Empty.
   template <typename Promise>
   auto CancelIfFails(Promise promise) {
-    GPR_DEBUG_ASSERT(Activity::current() == &party());
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &party());
     using P = promise_detail::PromiseLike<Promise>;
     using ResultType = typename P::Result;
     return Map(std::move(promise), [this](ResultType r) {
@@ -349,13 +349,13 @@ class CallInitiator {
       : spine_(std::move(spine)) {}
 
   auto PushClientInitialMetadata(ClientMetadataHandle md) {
-    GPR_DEBUG_ASSERT(Activity::current() == &spine_->party());
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
     return Map(spine_->client_initial_metadata().sender.Push(std::move(md)),
                [](bool ok) { return StatusFlag(ok); });
   }
 
   auto PullServerInitialMetadata() {
-    GPR_DEBUG_ASSERT(Activity::current() == &spine_->party());
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
     return Map(spine_->server_initial_metadata().receiver.Next(),
                [](NextResult<ClientMetadataHandle> md)
                    -> ValueOrFailure<ClientMetadataHandle> {
@@ -365,7 +365,7 @@ class CallInitiator {
   }
 
   auto PullServerTrailingMetadata() {
-    GPR_DEBUG_ASSERT(Activity::current() == &spine_->party());
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
     return Race(spine_->WaitForCancel(),
                 Map(spine_->server_trailing_metadata().receiver.Next(),
                     [spine = spine_](NextResult<ServerMetadataHandle> md)
@@ -376,19 +376,19 @@ class CallInitiator {
   }
 
   auto PullMessage() {
-    GPR_DEBUG_ASSERT(Activity::current() == &spine_->party());
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
     return spine_->server_to_client_messages().receiver.Next();
   }
 
   auto PushMessage(MessageHandle message) {
-    GPR_DEBUG_ASSERT(Activity::current() == &spine_->party());
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
     return Map(
         spine_->client_to_server_messages().sender.Push(std::move(message)),
         [](bool r) { return StatusFlag(r); });
   }
 
   void FinishSends() {
-    GPR_DEBUG_ASSERT(Activity::current() == &spine_->party());
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
     spine_->client_to_server_messages().sender.Close();
   }
 
@@ -398,7 +398,7 @@ class CallInitiator {
   }
 
   void Cancel() {
-    GPR_DEBUG_ASSERT(Activity::current() == &spine_->party());
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
     std::ignore =
         spine_->Cancel(ServerMetadataFromStatus(absl::CancelledError()));
   }
@@ -430,7 +430,7 @@ class CallHandler {
       : spine_(std::move(spine)) {}
 
   auto PullClientInitialMetadata() {
-    GPR_DEBUG_ASSERT(Activity::current() == &spine_->party());
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
     return Map(spine_->client_initial_metadata().receiver.Next(),
                [](NextResult<ClientMetadataHandle> md)
                    -> ValueOrFailure<ClientMetadataHandle> {
@@ -440,13 +440,13 @@ class CallHandler {
   }
 
   auto PushServerInitialMetadata(ServerMetadataHandle md) {
-    GPR_DEBUG_ASSERT(Activity::current() == &spine_->party());
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
     return Map(spine_->server_initial_metadata().sender.Push(std::move(md)),
                [](bool ok) { return StatusFlag(ok); });
   }
 
   auto PushServerTrailingMetadata(ServerMetadataHandle md) {
-    GPR_DEBUG_ASSERT(Activity::current() == &spine_->party());
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
     spine_->server_to_client_messages().sender.Close();
     spine_->CallOnDone();
     return Map(spine_->server_trailing_metadata().sender.Push(std::move(md)),
@@ -454,19 +454,19 @@ class CallHandler {
   }
 
   auto PullMessage() {
-    GPR_DEBUG_ASSERT(Activity::current() == &spine_->party());
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
     return spine_->client_to_server_messages().receiver.Next();
   }
 
   auto PushMessage(MessageHandle message) {
-    GPR_DEBUG_ASSERT(Activity::current() == &spine_->party());
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
     return Map(
         spine_->server_to_client_messages().sender.Push(std::move(message)),
         [](bool ok) { return StatusFlag(ok); });
   }
 
   void Cancel(ServerMetadataHandle status) {
-    GPR_DEBUG_ASSERT(Activity::current() == &spine_->party());
+    GPR_DEBUG_ASSERT(GetContext<Activity>() == &spine_->party());
     std::ignore = spine_->Cancel(std::move(status));
   }
 

test/core/filters/filter_test.cc

@@ -219,7 +219,7 @@ bool FilterTestBase::Call::Impl::StepOnce() {
           events().ForwardedMessageServerToClient(call_, *p->value());
         }
         next_server_to_client_messages_.reset();
-        Activity::current()->ForceImmediateRepoll();
+        GetContext<Activity>()->ForceImmediateRepoll();
       }
     }
 
@@ -229,7 +229,7 @@ bool FilterTestBase::Call::Impl::StepOnce() {
           server_to_client_messages_sender_->Push(
               std::move(forward_server_to_client_messages_.front())));
       forward_server_to_client_messages_.pop();
-      Activity::current()->ForceImmediateRepoll();
+      GetContext<Activity>()->ForceImmediateRepoll();
     }
   }
 
@@ -251,7 +251,7 @@ bool FilterTestBase::Call::Impl::StepOnce() {
           events().ForwardedMessageClientToServer(call_, *p->value());
         }
         next_client_to_server_messages_.reset();
-        Activity::current()->ForceImmediateRepoll();
+        GetContext<Activity>()->ForceImmediateRepoll();
       }
     }
 
@@ -261,7 +261,7 @@ bool FilterTestBase::Call::Impl::StepOnce() {
           pipe_client_to_server_messages_.sender.Push(
               std::move(forward_client_to_server_messages_.front())));
       forward_client_to_server_messages_.pop();
-      Activity::current()->ForceImmediateRepoll();
+      GetContext<Activity>()->ForceImmediateRepoll();
     }
   }
 

test/core/filters/filter_test_test.cc

@@ -64,7 +64,7 @@ class DelayStartFilter final : public ChannelFilter {
         [args = std::move(args), i = 10]() mutable -> Poll<CallArgs> {
           --i;
           if (i == 0) return std::move(args);
-          Activity::current()->ForceImmediateRepoll();
+          GetContext<Activity>()->ForceImmediateRepoll();
           return Pending{};
         },
         next);

test/core/promise/activity_test.cc

@@ -49,7 +49,7 @@ class Barrier {
       if (cleared_) {
         return Result{};
       } else {
-        return wait_set_.AddPending(Activity::current()->MakeOwningWaker());
+        return wait_set_.AddPending(GetContext<Activity>()->MakeOwningWaker());
       }
     };
   }
@@ -80,7 +80,7 @@ class SingleBarrier {
       if (cleared_) {
         return Result{};
       } else {
-        waker_ = Activity::current()->MakeOwningWaker();
+        waker_ = GetContext<Activity>()->MakeOwningWaker();
         return Pending();
       }
     };

test/core/promise/context_test.cc

@@ -42,6 +42,45 @@ TEST(Context, WithContext) {
   EXPECT_TRUE(test.done);
 }
 
+class BaseContext {
+ public:
+  virtual int Answer() = 0;
+
+ protected:
+  ~BaseContext() = default;
+};
+
+class CorrectContext final : public BaseContext {
+ public:
+  int Answer() override { return 42; }
+};
+
+class IncorrectContext final : public BaseContext {
+ public:
+  int Answer() override { return 0; }
+};
+
+template <>
+struct ContextType<BaseContext> {};
+template <>
+struct ContextSubclass<CorrectContext> {
+  using Base = BaseContext;
+};
+template <>
+struct ContextSubclass<IncorrectContext> {
+  using Base = BaseContext;
+};
+
+TEST(Context, ContextSubclass) {
+  CorrectContext correct;
+  IncorrectContext incorrect;
+  EXPECT_EQ(42,
+            WithContext([]() { return GetContext<BaseContext>()->Answer(); },
+                        &correct)());
+  EXPECT_EQ(0, WithContext([]() { return GetContext<BaseContext>()->Answer(); },
+                           &incorrect)());
+}
+
 }  // namespace grpc_core
 
 int main(int argc, char** argv) {

test/core/promise/for_each_test.cc

@@ -138,7 +138,7 @@ class MoveableUntilPolled {
   }
 
   Poll<absl::Status> operator()() {
-    Activity::current()->ForceImmediateRepoll();
+    GetContext<Activity>()->ForceImmediateRepoll();
     ++polls_;
     if (polls_ == 10) return absl::OkStatus();
     return Pending();

test/core/promise/map_pipe_test.cc

@@ -47,7 +47,7 @@ class Delayed {
   explicit Delayed(T x) : x_(x) {}
 
   Poll<T> operator()() {
-    Activity::current()->ForceImmediateRepoll();
+    GetContext<Activity>()->ForceImmediateRepoll();
     ++polls_;
     if (polls_ == 10) return std::move(x_);
     return Pending();

test/core/promise/party_test.cc

@@ -284,10 +284,10 @@ TEST_F(PartyTest, CanSpawnAndRun) {
   party->Spawn(
       "TestSpawn",
       [i = 10]() mutable -> Poll<int> {
-        EXPECT_EQ(Activity::current()->DebugTag(), "TestParty");
+        EXPECT_EQ(GetContext<Activity>()->DebugTag(), "TestParty");
         gpr_log(GPR_DEBUG, "i=%d", i);
         GPR_ASSERT(i > 0);
-        Activity::current()->ForceImmediateRepoll();
+        GetContext<Activity>()->ForceImmediateRepoll();
         --i;
         if (i == 0) return 42;
         return Pending{};
@@ -331,12 +331,12 @@ TEST_F(PartyTest, CanSpawnFromSpawn) {
   party->Spawn(
       "TestSpawn",
       [party, &n2]() -> Poll<int> {
-        EXPECT_EQ(Activity::current()->DebugTag(), "TestParty");
+        EXPECT_EQ(GetContext<Activity>()->DebugTag(), "TestParty");
         party->Spawn(
             "TestSpawnInner",
             [i = 10]() mutable -> Poll<int> {
-              EXPECT_EQ(Activity::current()->DebugTag(), "TestParty");
+              EXPECT_EQ(GetContext<Activity>()->DebugTag(), "TestParty");
-              Activity::current()->ForceImmediateRepoll();
+              GetContext<Activity>()->ForceImmediateRepoll();
               --i;
               if (i == 0) return 42;
               return Pending{};
@@ -363,8 +363,8 @@ TEST_F(PartyTest, CanWakeupWithOwningWaker) {
   party->Spawn(
       "TestSpawn",
       [i = 0, &waker, &n]() mutable -> Poll<int> {
-        EXPECT_EQ(Activity::current()->DebugTag(), "TestParty");
+        EXPECT_EQ(GetContext<Activity>()->DebugTag(), "TestParty");
-        waker = Activity::current()->MakeOwningWaker();
+        waker = GetContext<Activity>()->MakeOwningWaker();
         n[i].Notify();
         i++;
         if (i == 10) return 42;
@@ -389,8 +389,8 @@ TEST_F(PartyTest, CanWakeupWithNonOwningWaker) {
   party->Spawn(
       "TestSpawn",
       [i = 10, &waker, &n]() mutable -> Poll<int> {
-        EXPECT_EQ(Activity::current()->DebugTag(), "TestParty");
+        EXPECT_EQ(GetContext<Activity>()->DebugTag(), "TestParty");
-        waker = Activity::current()->MakeNonOwningWaker();
+        waker = GetContext<Activity>()->MakeNonOwningWaker();
         --i;
         n[9 - i].Notify();
         if (i == 0) return 42;
@@ -416,8 +416,8 @@ TEST_F(PartyTest, CanWakeupWithNonOwningWakerAfterOrphaning) {
       "TestSpawn",
       [&waker, &set_waker]() mutable -> Poll<int> {
         EXPECT_FALSE(set_waker.HasBeenNotified());
-        EXPECT_EQ(Activity::current()->DebugTag(), "TestParty");
+        EXPECT_EQ(GetContext<Activity>()->DebugTag(), "TestParty");
-        waker = Activity::current()->MakeNonOwningWaker();
+        waker = GetContext<Activity>()->MakeNonOwningWaker();
         set_waker.Notify();
         return Pending{};
       },
@@ -437,9 +437,9 @@ TEST_F(PartyTest, CanDropNonOwningWakeAfterOrphaning) {
       "TestSpawn",
       [&waker, &set_waker]() mutable -> Poll<int> {
         EXPECT_FALSE(set_waker.HasBeenNotified());
-        EXPECT_EQ(Activity::current()->DebugTag(), "TestParty");
+        EXPECT_EQ(GetContext<Activity>()->DebugTag(), "TestParty");
-        waker =
+        waker = std::make_unique<Waker>(
-            std::make_unique<Waker>(Activity::current()->MakeNonOwningWaker());
+            GetContext<Activity>()->MakeNonOwningWaker());
         set_waker.Notify();
         return Pending{};
       },
@@ -458,8 +458,8 @@ TEST_F(PartyTest, CanWakeupNonOwningOrphanedWakerWithNoEffect) {
       "TestSpawn",
       [&waker, &set_waker]() mutable -> Poll<int> {
         EXPECT_FALSE(set_waker.HasBeenNotified());
-        EXPECT_EQ(Activity::current()->DebugTag(), "TestParty");
+        EXPECT_EQ(GetContext<Activity>()->DebugTag(), "TestParty");
-        waker = Activity::current()->MakeNonOwningWaker();
+        waker = GetContext<Activity>()->MakeNonOwningWaker();
         set_waker.Notify();
         return Pending{};
       },
@@ -574,9 +574,9 @@ class PromiseNotification {
       if (done_) return 42;
       if (!polled_) {
         if (owning_waker_) {
-          waker_ = Activity::current()->MakeOwningWaker();
+          waker_ = GetContext<Activity>()->MakeOwningWaker();
         } else {
-          waker_ = Activity::current()->MakeNonOwningWaker();
+          waker_ = GetContext<Activity>()->MakeNonOwningWaker();
         }
         polled_ = true;
       }

test/core/promise/promise_fuzzer.cc

@@ -304,10 +304,10 @@ class Fuzzer {
           if (!called) {
             if (config.owning()) {
               wakers_[config.waker()].push_back(
-                  Activity::current()->MakeOwningWaker());
+                  GetContext<Activity>()->MakeOwningWaker());
             } else {
               wakers_[config.waker()].push_back(
-                  Activity::current()->MakeNonOwningWaker());
+                  GetContext<Activity>()->MakeNonOwningWaker());
             }
             return Pending();
           }

tools/doxygen/Doxyfile.c++.internal

@@ -2413,6 +2413,7 @@ src/core/lib/gprpp/crash.cc \
 src/core/lib/gprpp/crash.h \
 src/core/lib/gprpp/debug_location.h \
 src/core/lib/gprpp/directory_reader.h \
+src/core/lib/gprpp/down_cast.h \
 src/core/lib/gprpp/dual_ref_counted.h \
 src/core/lib/gprpp/env.h \
 src/core/lib/gprpp/examine_stack.cc \

tools/doxygen/Doxyfile.core.internal

@@ -2191,6 +2191,7 @@ src/core/lib/gprpp/crash.cc \
 src/core/lib/gprpp/crash.h \
 src/core/lib/gprpp/debug_location.h \
 src/core/lib/gprpp/directory_reader.h \
+src/core/lib/gprpp/down_cast.h \
 src/core/lib/gprpp/dual_ref_counted.h \
 src/core/lib/gprpp/env.h \
 src/core/lib/gprpp/examine_stack.cc \