Make tests/tools/run-bindgen.py check expectations if the rust path already exists

Author: fitzgen

tests/tools/run-bindgen.py

@@ -19,7 +19,7 @@
 
 flags = ["--no-unstable-rust"]
 
-with open(sys.argv[2]) as f:
+with open(c_path) as f:
   for line in f:
     if line.startswith(BINDGEN_FLAGS_PREFIX):
       flags.extend(line.strip().split(BINDGEN_FLAGS_PREFIX)[1].split(" "))
@@ -40,11 +40,30 @@
 # https://forums.developer.apple.com/thread/9233
 if "DYLD_LIBRARY_PATH" not in env and "LIBCLANG_PATH" in env:
     env["DYLD_LIBRARY_PATH"] = env["LIBCLANG_PATH"]
-subprocess.check_call(base_command, cwd=os.getcwd(), env=env)
 
+# If the rust file already exists, read it now so we can compare its contents
+# before and after.
+original_rust_contents = None
+if os.path.isfile(rust_path):
+  with open(rust_path) as f:
+    original_rust_contents = f.read()
+
+subprocess.check_call(base_command, cwd=os.getcwd(), env=env)
 
 name = None
 with tempfile.NamedTemporaryFile(delete=False) as tests:
   name = tests.name
   subprocess.check_call(["rustc", "--test", sys.argv[3], "-o", tests.name])
 subprocess.check_call([tests.name])
+
+if original_rust_contents is not None:
+  new_rust_contents = None
+  with open(rust_path) as f:
+    new_rust_contents = f.read()
+  if new_rust_contents != original_rust_contents:
+    print("Generated rust bindings do not match expectation!")
+    print("Expected rust bindings:")
+    print(original_rust_contents)
+    print("Actual rust bindings:")
+    print(new_rust_contents)
+    sys.exit(1)