feat: expose the precompiled-mode sources to non-CMake builds

Adds pybind11.get_source_dir() / python -m pybind11 --srcdir and a
srcdir variable in pybind11.pc, so build systems such as Meson can
compile src/pybind11_combined.cpp with PYBIND11_PRECOMPILED defined.

Assisted-by: ClaudeCode:claude-fable-5

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index ea1b3a1..f53a646 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -343,8 +343,8 @@
     install(DIRECTORY ${pybind11_INCLUDE_DIR}/pybind11 DESTINATION "${SKBUILD_HEADERS_DIR}")
   endif()
   install(DIRECTORY ${pybind11_INCLUDE_DIR}/pybind11 DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
-  install(DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/src/
-          DESTINATION "${CMAKE_INSTALL_DATAROOTDIR}/pybind11/src")
+  set(pybind11_install_srcdir "${CMAKE_INSTALL_DATAROOTDIR}/pybind11/src")
+  install(DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/src/ DESTINATION "${pybind11_install_srcdir}")
   set(PYBIND11_CMAKECONFIG_INSTALL_DIR
       "${CMAKE_INSTALL_DATAROOTDIR}/cmake/${PROJECT_NAME}"
       CACHE STRING "install path for pybind11Config.cmake")
@@ -355,9 +355,9 @@
     set(pybind11_INCLUDEDIR "\$\{PACKAGE_PREFIX_DIR\}/${CMAKE_INSTALL_INCLUDEDIR}")
   endif()
   if(IS_ABSOLUTE "${CMAKE_INSTALL_DATAROOTDIR}")
-    set(pybind11_SRCDIR "${CMAKE_INSTALL_DATAROOTDIR}/pybind11/src")
+    set(pybind11_SRCDIR "${pybind11_install_srcdir}")
   else()
-    set(pybind11_SRCDIR "\$\{PACKAGE_PREFIX_DIR\}/${CMAKE_INSTALL_DATAROOTDIR}/pybind11/src")
+    set(pybind11_SRCDIR "\$\{PACKAGE_PREFIX_DIR\}/${pybind11_install_srcdir}")
   endif()
 
   configure_package_config_file(
@@ -410,6 +410,7 @@
     endif()
   endif()
   join_paths(includedir_for_pc_file "\${prefix}" "${CMAKE_INSTALL_INCLUDEDIR}")
+  join_paths(srcdir_for_pc_file "\${prefix}" "${pybind11_install_srcdir}")
   configure_file("${CMAKE_CURRENT_SOURCE_DIR}/tools/pybind11.pc.in"
                  "${CMAKE_CURRENT_BINARY_DIR}/pybind11.pc" @ONLY)
   install(FILES "${CMAKE_CURRENT_BINARY_DIR}/pybind11.pc"

pybind11/__init__.py

diff --git a/pybind11/__init__.py b/pybind11/__init__.py
index 3882b2b..1d66e11 100644
--- a/pybind11/__init__.py
+++ b/pybind11/__init__.py
@@ -8,12 +8,13 @@
 
 
 from ._version import __version__, version_info
-from .commands import get_cmake_dir, get_include, get_pkgconfig_dir
+from .commands import get_cmake_dir, get_include, get_pkgconfig_dir, get_source_dir
 
 __all__ = (
     "__version__",
     "get_cmake_dir",
     "get_include",
     "get_pkgconfig_dir",
+    "get_source_dir",
     "version_info",
 )

pybind11/__main__.py

diff --git a/pybind11/__main__.py b/pybind11/__main__.py
index ce597c7..98e9184 100644
--- a/pybind11/__main__.py
+++ b/pybind11/__main__.py
@@ -17,6 +17,7 @@
     get_include_dirs,
     get_ldflags,
     get_pkgconfig_dir,
+    get_source_dir,
 )
 
 
@@ -51,6 +52,12 @@
         help="Print the pkgconfig directory, ideal for setting $PKG_CONFIG_PATH.",
     )
     parser.add_argument(
+        "--srcdir",
+        action="store_true",
+        help="Print the directory containing the library sources for the optional"
+        " precompiled mode.",
+    )
+    parser.add_argument(
         "--extension-suffix",
         action="store_true",
         help="Print the extension for a Python module",
@@ -101,6 +108,8 @@
         print(quote(get_cmake_dir()))
     if args.pkgconfigdir:
         print(quote(get_pkgconfig_dir()))
+    if args.srcdir:
+        print(quote(get_source_dir()))
     if args.extension_suffix:
         print(ext_suffix)
 

pybind11/commands.py

diff --git a/pybind11/commands.py b/pybind11/commands.py
index 8bd0a9b..573b7cf 100644
--- a/pybind11/commands.py
+++ b/pybind11/commands.py
@@ -52,6 +52,24 @@
     return installed_path if os.path.exists(installed_path) else source_path
 
 
+def get_source_dir() -> str:
+    """
+    Return the path to the pybind11 library sources, for the optional
+    precompiled mode. Compile ``pybind11_combined.cpp`` (or the individual
+    ``.cpp`` files) with ``PYBIND11_PRECOMPILED`` defined, and define that
+    macro for every translation unit that includes pybind11.
+    """
+    installed_path = os.path.join(DIR, "share", "pybind11", "src")
+    source_path = os.path.join(os.path.dirname(DIR), "src")
+    if os.path.exists(installed_path):
+        return installed_path
+    if os.path.exists(source_path):
+        return source_path
+
+    msg = "pybind11 library sources not found (pybind11 not installed?)"
+    raise ImportError(msg)
+
+
 def get_cmake_dir() -> str:
     """
     Return the path to the pybind11 CMake module directory.

tests/extra_python_package/test_files.py

diff --git a/tests/extra_python_package/test_files.py b/tests/extra_python_package/test_files.py
index 58dbca4..a14f057 100644
--- a/tests/extra_python_package/test_files.py
+++ b/tests/extra_python_package/test_files.py
@@ -33,6 +33,7 @@
 PKGCONFIG = """\
 prefix=${{pcfiledir}}/../../
 includedir=${{prefix}}/include
+srcdir=${{prefix}}/share/pybind11/src
 
 Name: pybind11
 Description: Seamless operability between C++11 and Python

tools/pybind11.pc.in

diff --git a/tools/pybind11.pc.in b/tools/pybind11.pc.in
index 402f0b3..8b5af0a 100644
--- a/tools/pybind11.pc.in
+++ b/tools/pybind11.pc.in
@@ -1,5 +1,6 @@
 prefix=@prefix_for_pc_file@
 includedir=@includedir_for_pc_file@
+srcdir=@srcdir_for_pc_file@
 
 Name: @PROJECT_NAME@
 Description: Seamless operability between C++11 and Python