diff --git a/include/pybind11/detail/type_caster_base.h b/include/pybind11/detail/type_caster_base.h
index 9979421ad..c8b2b4f01 100644
--- a/include/pybind11/detail/type_caster_base.h
+++ b/include/pybind11/detail/type_caster_base.h
@@ -11,6 +11,7 @@
 
 #include "../gil.h"
 #include "../pytypes.h"
+#include "../trampoline_self_life_support.h"
 #include "common.h"
 #include "descr.h"
 #include "dynamic_raw_ptr_cast_if_possible.h"
@@ -474,52 +475,6 @@ inline PyObject *make_new_instance(PyTypeObject *type);
 // SMART_HOLDER_WIP: Needs refactoring of existing pybind11 code.
 inline bool deregister_instance(instance *self, void *valptr, const type_info *tinfo);
 
-PYBIND11_NAMESPACE_END(detail)
-
-// The original core idea for this struct goes back to PyCLIF:
-// https://github.com/google/clif/blob/07f95d7e69dca2fcf7022978a55ef3acff506c19/clif/python/runtime.cc#L37
-// URL provided here mainly to give proper credit. To fully explain the `HoldPyObj` feature, more
-// context is needed (SMART_HOLDER_WIP).
-struct trampoline_self_life_support {
-    detail::value_and_holder v_h;
-
-    trampoline_self_life_support() = default;
-
-    void activate_life_support(const detail::value_and_holder &v_h_) {
-        Py_INCREF((PyObject *) v_h_.inst);
-        v_h = v_h_;
-    }
-
-    void deactivate_life_support() {
-        Py_DECREF((PyObject *) v_h.inst);
-        v_h = detail::value_and_holder();
-    }
-
-    ~trampoline_self_life_support() {
-        if (v_h.inst != nullptr && v_h.vh != nullptr) {
-            void *value_void_ptr = v_h.value_ptr();
-            if (value_void_ptr != nullptr) {
-                PyGILState_STATE threadstate = PyGILState_Ensure();
-                v_h.value_ptr() = nullptr;
-                v_h.holder<pybindit::memory::smart_holder>().release_disowned();
-                detail::deregister_instance(v_h.inst, value_void_ptr, v_h.type);
-                Py_DECREF((PyObject *) v_h.inst); // Must be after deregister.
-                PyGILState_Release(threadstate);
-            }
-        }
-    }
-
-    // For the next two, the default implementations generate undefined behavior (ASAN failures
-    // manually verified). The reason is that v_h needs to be kept default-initialized.
-    trampoline_self_life_support(const trampoline_self_life_support &) {}
-    trampoline_self_life_support(trampoline_self_life_support &&) noexcept {}
-
-    // These should never be needed (please provide test cases if you think they are).
-    trampoline_self_life_support &operator=(const trampoline_self_life_support &) = delete;
-    trampoline_self_life_support &operator=(trampoline_self_life_support &&) = delete;
-};
-
-PYBIND11_NAMESPACE_BEGIN(detail)
 PYBIND11_NAMESPACE_BEGIN(smart_holder_type_caster_support)
 
 struct value_and_holder_helper {
diff --git a/include/pybind11/trampoline_self_life_support.h b/include/pybind11/trampoline_self_life_support.h
index ee809b9c5..47f623953 100644
--- a/include/pybind11/trampoline_self_life_support.h
+++ b/include/pybind11/trampoline_self_life_support.h
@@ -1,8 +1,61 @@
-// Copyright (c) 2024 The Pybind Development Team.
+// Copyright (c) 2021 The Pybind Development Team.
 // All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #pragma once
 
-// BAKEIN_WIP: Needs refactoring of existing pybind11 code.
-#include "detail/type_caster_base.h"
+#include "detail/common.h"
+#include "detail/smart_holder_poc.h"
+#include "detail/value_and_holder.h"
+
+PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
+
+PYBIND11_NAMESPACE_BEGIN(detail)
+// SMART_HOLDER_WIP: Needs refactoring of existing pybind11 code.
+inline bool deregister_instance(instance *self, void *valptr, const type_info *tinfo);
+PYBIND11_NAMESPACE_END(detail)
+
+// The original core idea for this struct goes back to PyCLIF:
+// https://github.com/google/clif/blob/07f95d7e69dca2fcf7022978a55ef3acff506c19/clif/python/runtime.cc#L37
+// URL provided here mainly to give proper credit. To fully explain the `HoldPyObj` feature, more
+// context is needed (SMART_HOLDER_WIP).
+struct trampoline_self_life_support {
+    detail::value_and_holder v_h;
+
+    trampoline_self_life_support() = default;
+
+    void activate_life_support(const detail::value_and_holder &v_h_) {
+        Py_INCREF((PyObject *) v_h_.inst);
+        v_h = v_h_;
+    }
+
+    void deactivate_life_support() {
+        Py_DECREF((PyObject *) v_h.inst);
+        v_h = detail::value_and_holder();
+    }
+
+    ~trampoline_self_life_support() {
+        if (v_h.inst != nullptr && v_h.vh != nullptr) {
+            void *value_void_ptr = v_h.value_ptr();
+            if (value_void_ptr != nullptr) {
+                PyGILState_STATE threadstate = PyGILState_Ensure();
+                v_h.value_ptr() = nullptr;
+                v_h.holder<pybindit::memory::smart_holder>().release_disowned();
+                detail::deregister_instance(v_h.inst, value_void_ptr, v_h.type);
+                Py_DECREF((PyObject *) v_h.inst); // Must be after deregister.
+                PyGILState_Release(threadstate);
+            }
+        }
+    }
+
+    // For the next two, the default implementations generate undefined behavior (ASAN failures
+    // manually verified). The reason is that v_h needs to be kept default-initialized.
+    trampoline_self_life_support(const trampoline_self_life_support &) {}
+    trampoline_self_life_support(trampoline_self_life_support &&) noexcept {}
+
+    // These should never be needed (please provide test cases if you think they are).
+    trampoline_self_life_support &operator=(const trampoline_self_life_support &) = delete;
+    trampoline_self_life_support &operator=(trampoline_self_life_support &&) = delete;
+};
+
+PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)