xo-cmake: setup to make share target available via cmake install

xo-cmake/cmake/xo_macros/xo_cxx.cmake

@@ -1227,6 +1227,17 @@ macro(xo_export_cmake_config projectname projectversion projecttargets)
         PERMISSIONS OWNER_READ GROUP_READ WORLD_READ
         DESTINATION ${CMAKE_INSTALL_PREFIX}/lib/cmake/${projectname}
     )
+    # generate Share file that recreates share_${projectname} in consuming projects
+    file(WRITE "${PROJECT_BINARY_DIR}/${projectname}Share.cmake"
+        "if(NOT TARGET share_${projectname})\n"
+        "    add_custom_target(share_${projectname})\n"
+        "    set_property(TARGET share_${projectname} PROPERTY path\n"
+        "        \"\${CMAKE_CURRENT_LIST_DIR}/../../../share/${projectname}\")\n"
+        "endif()\n")
+    install(
+        FILES "${PROJECT_BINARY_DIR}/${projectname}Share.cmake"
+        PERMISSIONS OWNER_READ GROUP_READ WORLD_READ
+        DESTINATION ${CMAKE_INSTALL_PREFIX}/lib/cmake/${projectname})
 endmacro()
 
 # ----------------------------------------------------------------

xo-cmake/docs/history/2026/cmake-facet-export.rst

@@ -0,0 +1,81 @@
+cmake-facet-export
+==================
+
+:date: 2026-02
+
+Background
+----------
+
+``xo_add_genfacetimpl()`` in ``xo_cxx.cmake`` uses ``genfacet`` to regenerate facet
+implementation boilerplate from ``.json5`` IDL files.  When ``FACET_PKG`` is specified
+(e.g. ``FACET_PKG xo_gc``), the macro resolves the IDL directory via a cmake custom target::
+
+    get_target_property(_facet_dir share_${GF_FACET_PKG} path)
+
+``share_${PROJECT_NAME}`` is a custom target created by ``xo_cxx_toplevel_options2()`` for
+every xo satellite, with its ``path`` property set to ``${PROJECT_SOURCE_DIR}`` (the package
+source root). Subdirectory ``idl/`` within that root holds the facet IDL files.
+
+Problem
+-------
+
+Custom cmake targets are not exportable through the standard ``install(TARGETS ... EXPORT ...)``
+mechanism.  As a result, ``share_xo_gc`` and similar targets do not exist when a satellite is
+consumed as an installed dependency (e.g. in a standalone nix build or any build that uses
+``find_package(xo_gc)`` rather than building xo-gc in the same cmake invocation).
+
+This caused ``nix-build -A xo.object2`` to fail at cmake configure time with::
+
+    CMake Error: get_target_property() called with non-existent target "share_xo_gc".
+
+xo-gc was unaffected because it uses only ``xo_add_genfacet()`` (which does not reference
+``share_*`` targets), while xo-object2 uses ``xo_add_genfacetimpl()`` with ``FACET_PKG``.
+
+Immediate fix (2026-02)
+-----------------------
+
+A guard was added to ``xo_add_genfacetimpl()`` to skip target creation gracefully when the
+``share_*`` target is absent, since all callers annotate these as
+``# note: manual target; generated code committed to git``::
+
+    if(NOT TARGET share_${GF_FACET_PKG})
+        message(STATUS "xo_add_genfacetimpl: share_${GF_FACET_PKG} not available; skipping")
+        return()
+    endif()
+
+This unblocked the nix build while leaving a forward path to full support.
+
+Long-term design
+----------------
+
+To make ``xo_add_genfacetimpl()`` fully functional in standalone builds:
+
+1. **Export ``share_`` targets universally** — ``xo_export_cmake_config()`` generates and
+   installs a ``${PROJECT_NAME}Share.cmake`` file for every satellite.  This file recreates
+   ``share_${PROJECT_NAME}`` pointing to the installed data root::
+
+       if(NOT TARGET share_xo_gc)
+           add_custom_target(share_xo_gc)
+           set_property(TARGET share_xo_gc PROPERTY path
+               "${CMAKE_CURRENT_LIST_DIR}/../../../share/xo_gc")
+       endif()
+
+   Doing this universally (not only for facet-providers) keeps ``*Config.cmake.in`` templates
+   consistent and provides future-proofing for other attributes that may be added to
+   ``share_`` targets.
+
+2. **Include the Share file** — every ``cmake/${pkg}Config.cmake.in`` adds::
+
+       include("${CMAKE_CURRENT_LIST_DIR}/@PROJECT_NAME@Share.cmake")
+
+3. **Install IDL files** — packages whose IDL is consumed by ``xo_add_genfacetimpl()``
+   install their ``idl/`` tree::
+
+       install(DIRECTORY idl/
+           DESTINATION share/${PROJECT_NAME}/idl
+           FILES_MATCHING PATTERN "*.json5")
+
+   This maps to the same relative path that ``genfacet`` would find under the source root.
+
+The skip-if-absent guard is retained as graceful degradation for older installed packages
+that predate this change.