Add long comment to pybind11/compat/pybind11_conduit_v1.h

include/pybind11/compat/README.txt

@@ -8,3 +8,8 @@ The C++ code here
 and nothing else.
 
 DO NOT ADD CODE WITH OTHER EXTERNAL DEPENDENCIES TO THIS DIRECTORY.
+
+Read on:
+
+pybind11_conduit_v1.h — Type-safe interoperability between different
+                        independent Python/C++ bindings systems.

include/pybind11/compat/pybind11_conduit_v1.h

@@ -1,5 +1,70 @@
 // Copyright (c) 2024 The pybind Community.
 
+/* The pybind11_conduit_v1 feature enables type-safe interoperability between
+
+* different independent Python/C++ bindings systems,
+
+* including pybind11 versions with different PYBIND11_INTERNALS_VERSION's.
+
+The naming of the feature is a bit misleading:
+
+* The feature is in no way tied to pybind11 internals.
+
+* It just happens to originate from pybind11 and currently still lives there.
+
+* The only external dependency is <Python.h>.
+
+The implementation is a VERY light-weight dependency. It is designed to be
+compatible with any ISO C++11 (or higher) compiler, and does NOT require
+C++ Exception Handling to be enabled.
+
+Please see https://github.com/pybind/pybind11/pull/5296 for more background.
+
+The implementation involves a
+
+def _pybind11_conduit_v1_(
+    self,
+    pybind11_platform_abi_id: bytes,
+    cpp_type_info_capsule: capsule,
+    pointer_kind: bytes) -> capsule
+
+method that is meant to be added to Python objects wrapping C++ objects
+(e.g. pybind11::class_-wrapped types).
+
+The design of the _pybind11_conduit_v1_ feature provides two layers of
+protection against C++ ABI mismatches:
+
+* The first and most important layer is that the pybind11_platform_abi_id's
+  must match between extensions. — This will never be perfect, but is the same
+  pragmatic approach used in pybind11 since 2017
+  (https://github.com/pybind/pybind11/commit/96997a4b9d4ec3d389a570604394af5d5eee2557,
+  PYBIND11_INTERNALS_ID).
+
+* The second layer is that the typeid(std::type_info).name()'s must match
+  between extensions.
+
+The implementation below (which is shorter than this comment!), serves as a
+battle-tested specification. The main API is this one function:
+
+auto *cpp_pointer = pybind11_conduit_v1::get_type_pointer_ephemeral<YourType>(py_obj);
+
+It is meant to be a minimalistic reference implementation, intentionally
+without comprehensive error reporting. It is expected that major bindings
+systems will roll their own, compatible implementations, potentially with
+system-specific error reporting. The essential specifications all bindings
+systems need to agree on are merely:
+
+* PYBIND11_PLATFORM_ABI_ID (const char* literal).
+
+* The cpp_type_info capsule (see below: a void *ptr and a const char *name).
+
+* The cpp_conduit capsule (see below: a void *ptr and a const char *name).
+
+* "raw_pointer_ephemeral" means: the lifetime of the pointer is the lifetime
+  of the py_obj.
+
+*/
+
 // THIS MUST STAY AT THE TOP!
 #include "pybind11_platform_abi_id.h"