lib/codegen-meta moved into lib/codegen. (#423)

* lib/codegen-meta moved into lib/codegen.

* Renamed codegen-meta and existing meta.

lib/codegen/meta-python/gen_build_deps.py

@@ -0,0 +1,43 @@
+"""
+Generate build dependencies for Cargo.
+
+The `build.py` script is invoked by cargo when building lib/codegen to
+generate Rust code from the instruction descriptions. Cargo needs to know when
+it is necessary to rerun the build script.
+
+If the build script outputs lines of the form:
+
+    cargo:rerun-if-changed=/path/to/file
+
+cargo will rerun the build script when those files have changed since the last
+build.
+"""
+from __future__ import absolute_import, print_function
+import os
+from os.path import dirname, abspath, join
+
+try:
+    from typing import Iterable  # noqa
+except ImportError:
+    pass
+
+
+def source_files(top):
+    # type: (str) -> Iterable[str]
+    """
+    Recursively find all interesting source files and directories in the
+    directory tree starting at top. Yield a path to each file.
+    """
+    for (dirpath, dirnames, filenames) in os.walk(top):
+        yield dirpath
+        for f in filenames:
+            if f.endswith('.py'):
+                yield join(dirpath, f)
+
+
+def generate():
+    # type: () -> None
+    print("Dependencies from meta language directory:")
+    meta = dirname(abspath(__file__))
+    for path in source_files(meta):
+        print("cargo:rerun-if-changed=" + path)