dispatch: attempt to use a VFS overlay

Use a VFS overlay to avoid polluting the source tree.

.gitignore

@@ -29,4 +29,3 @@ config
 configure
 libtool
 .dirstamp
-/dispatch/module.modulemap

dispatch/CMakeLists.txt

@@ -1,4 +1,13 @@
 
+if(CMAKE_SYSTEM_NAME STREQUAL Darwin)
+  set(DISPATCH_MODULE_MAP ${PROJECT_SOURCE_DIR}/dispatch/darwin/module.modulemap)
+else()
+  set(DISPATCH_MODULE_MAP ${PROJECT_SOURCE_DIR}/dispatch/generic/module.modulemap)
+endif()
+configure_file(dispatch-vfs.yaml.in
+  ${CMAKE_BINARY_DIR}/dispatch-vfs-overlay.yaml
+  @ONLY)
+
 install(FILES
           base.h
           block.h
@@ -16,19 +25,8 @@ install(FILES
         DESTINATION
           "${INSTALL_DISPATCH_HEADERS_DIR}")
 if(ENABLE_SWIFT)
-  set(base_dir "${CMAKE_CURRENT_SOURCE_DIR}")
-  if(NOT BUILD_SHARED_LIBS)
-    set(base_dir "${CMAKE_CURRENT_SOURCE_DIR}/generic_static")
-  endif()
-
-  get_filename_component(
-    MODULE_MAP
-    module.modulemap
-    REALPATH
-    BASE_DIR "${base_dir}")
-
   install(FILES
-            ${MODULE_MAP}
+            ${DISPATCH_MODULE_MAP}
           DESTINATION
             "${INSTALL_DISPATCH_HEADERS_DIR}")
 endif()

dispatch/dispatch-vfs.yaml.in

@@ -0,0 +1,11 @@
+---
+version: 0
+case-sensitive: false
+use-external-names: false
+roots:
+  - name: "@CMAKE_CURRENT_SOURCE_DIR@"
+    type: directory
+    contents:
+      - name: module.modulemap
+        type: file
+        external-contents: "@DISPATCH_MODULE_MAP@"

src/swift/CMakeLists.txt

@@ -18,18 +18,6 @@ target_include_directories(DispatchStubs PRIVATE
 set_target_properties(DispatchStubs PROPERTIES
   POSITION_INDEPENDENT_CODE YES)
 
-
-if(CMAKE_SYSTEM_NAME STREQUAL Darwin)
-  add_custom_command(OUTPUT ${PROJECT_SOURCE_DIR}/dispatch/module.modulemap
-    COMMAND ${CMAKE_COMMAND} -E copy_if_different ${PROJECT_SOURCE_DIR}/dispatch/darwin/module.modulemap ${PROJECT_SOURCE_DIR}/dispatch/module.modulemap)
-else()
-  add_custom_command(OUTPUT ${PROJECT_SOURCE_DIR}/dispatch/module.modulemap
-    COMMAND ${CMAKE_COMMAND} -E copy_if_different ${PROJECT_SOURCE_DIR}/dispatch/generic/module.modulemap ${PROJECT_SOURCE_DIR}/dispatch/module.modulemap)
-endif()
-add_custom_target(module-map ALL
-  DEPENDS ${PROJECT_SOURCE_DIR}/dispatch/module.modulemap)
-
-
 add_library(swiftDispatch
   Block.swift
   Data.swift
@@ -45,6 +33,8 @@ target_compile_options(swiftDispatch PRIVATE
   "SHELL:-Xcc -fmodule-map-file=${PROJECT_SOURCE_DIR}/dispatch/module.modulemap"
   "SHELL:-Xcc -I${PROJECT_SOURCE_DIR}"
   "SHELL:-Xcc -I${PROJECT_SOURCE_DIR}/src/swift/shims")
+target_compile_options(swiftDispatch PUBLIC
+  "SHELL:-vfsoverlay ${CMAKE_BINARY_DIR}/dispatch-vfs-overlay.yaml")
 set_target_properties(swiftDispatch PROPERTIES
   Swift_MODULE_NAME Dispatch
   Swift_MODULE_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/swift
@@ -54,7 +44,6 @@ target_link_libraries(swiftDispatch PRIVATE
   BlocksRuntime::BlocksRuntime)
 target_link_libraries(swiftDispatch PUBLIC
   dispatch)
-add_dependencies(swiftDispatch module-map)
 
 get_swift_host_arch(swift_arch)
 install(FILES