Meta: Add a script and CMake function to generate a clang module map

In theory the clang module map should not have absolute paths for the
headers. Other Swift projects seem to use the -ivfsoverlay feature of
clang to work around this, but it seems difficult to get to work.

Meta/generate_clang_module_map.py

@@ -0,0 +1,69 @@
+#!/usr/bin/env python3
+"""
+    Generates a clang module map for a given directory
+"""
+
+import argparse
+import pathlib
+import yaml
+import sys
+
+
+def write_file_if_not_same(file_path, content):
+    try:
+        with open(file_path, 'r') as f:
+            if f.read() == content:
+                return
+    except FileNotFoundError:
+        pass
+
+    with open(file_path, 'w') as f:
+        f.write(content)
+
+
+def main():
+    parser = argparse.ArgumentParser(
+                 epilog=__doc__,
+                 formatter_class=argparse.RawDescriptionHelpFormatter)
+    parser.add_argument('directory', help='source directory to generate module map for')
+    parser.add_argument('generated_files', nargs='+', help='extra files to include in the module map')
+    parser.add_argument('-m', '--module-map', required=True, help='output module map file')
+    parser.add_argument('-v', '--vfs-map', required=True, help='output VFS map file')
+    args = parser.parse_args()
+
+    root = pathlib.Path(args.directory)
+    if not root.is_dir():
+        print(f"Error: {args.directory} is not a directory", file=sys.stderr)
+        return 1
+    pathlib.Path(args.module_map).parent.mkdir(parents=True, exist_ok=True)
+
+    header_files = [f for f in root.rglob('**/*.h') if f.is_file()]
+    module_name = root.name
+
+    module_map = f"module {module_name} {{\n"
+    for header_file in header_files:
+        module_map += f"    header \"{header_file.relative_to(root)}\"\n"
+    for generated_file in args.generated_files:
+        module_map += f"    header \"{generated_file}\"\n"
+    module_map += "    requires cplusplus\n"
+    module_map += "    export *\n"
+    module_map += "}\n"
+
+    vfs_map = {
+        "version": 0,
+        "use-external-names": False,
+        "roots": [
+            {
+                "name": f"{root}/module.modulemap",
+                "type": "file",
+                "external-contents": f"{args.module_map}"
+            }
+        ]
+    }
+
+    write_file_if_not_same(args.module_map, module_map)
+    write_file_if_not_same(args.vfs_map, yaml.dump(vfs_map))
+
+
+if __name__ == '__main__':
+    sys.exit(main())