Add check for unicode replacement characters

pre_commit_hooks/check_unicode_replacement_char.py

@@ -0,0 +1,32 @@
+import argparse
+import fileinput
+from typing import Optional
+from typing import Sequence
+
+
+def main(argv: Optional[Sequence[str]] = None) -> int:
+    parser = argparse.ArgumentParser()
+    parser.add_argument('filenames', nargs='*', help='Filenames to check')
+    args = parser.parse_args(argv)
+
+    retv = 0
+
+    for line in fileinput.input(files=args.filenames, mode='rb'):
+        try:
+            col = line.index(b'\xEF\xBF\xBD')
+        except ValueError:
+            continue
+        retv = 1
+        # Not saying filename:line:col: because that kind of format is usually
+        # used for character offsets, and we have a byte offset which might be
+        # different, emphasize that.
+        print(
+            f'{fileinput.filename()}:{fileinput.lineno()}: '
+            f'UTF-8 Unicode replacement character at byte {col}'
+        )
+
+    return retv
+
+
+if __name__ == '__main__':
+    exit(main())