exports: support `CXX_MODULES_DIRECTORY`

This directory will be used to store build-discovered information about
targets such as the modules provided by the files in the relevant
`FILE_SET` types.

A directory is used because basing the name on a `<FILE_NAME>-*.cmake`
pattern makes it end up being globbed in the configuration-dependent
information mechanism. Since old modules and targets may be around,
unconditionally including them may refer to targets that do not actually
exist.

1 files changed, 14 insertions, 2 deletions

diff --git a/Help/command/export.rst b/Help/command/export.rst
index dc69645..6785b05 100644
--- a/Help/command/export.rst
+++ b/Help/command/export.rst
@@ -25,7 +25,8 @@ Exporting Targets
 .. code-block:: cmake
 
   export(TARGETS <target>... [NAMESPACE <namespace>]
-         [APPEND] FILE <filename> [EXPORT_LINK_INTERFACE_LIBRARIES])
+         [APPEND] FILE <filename> [EXPORT_LINK_INTERFACE_LIBRARIES]
+         [CXX_MODULES_DIRECTORY <directory>])
 
 Creates a file ``<filename>`` that may be included by outside projects to
 import targets named by ``<target>...`` from the current project's build tree.
@@ -52,6 +53,16 @@ The options are:
   in the export, even when policy :policy:`CMP0022` is NEW.  This is useful
   to support consumers using CMake versions older than 2.8.12.
 
+``CXX_MODULES_DIRECTORY <directory>``
+
+.. note ::
+
+  Experimental. Gated by ``CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API``
+
+  Export C++ module properties to files under the given directory. Each file
+  will be named according to the target's export name (without any namespace).
+  These files will automatically be included from the export file.
+
 This signature requires all targets to be listed explicitly.  If a library
 target is included in the export, but a target to which it links is not
 included, the behavior is unspecified.  See the `export(EXPORT)`_ signature
@@ -95,7 +106,8 @@ Exporting Targets matching install(EXPORT)
 
 .. code-block:: cmake
 
-  export(EXPORT <export-name> [NAMESPACE <namespace>] [FILE <filename>])
+  export(EXPORT <export-name> [NAMESPACE <namespace>] [FILE <filename>]
+         [CXX_MODULES_DIRECTORY <directory>])
 
 Creates a file ``<filename>`` that may be included by outside projects to
 import targets from the current project's build tree.  This is the same