r"""Check that every file ends with a single \n.

def _no_trailing_whitespace(relative_path, contents, linter_options):
    r"""Check that every file ends with a single \n."""
    del relative_path
    del linter_options

    trailing_whitespace = re.compile(r"(?<![ \t\r\n])[ \t]+$")

    for index, line in enumerate(contents):
        for space in trailing_whitespace.finditer(line):
            assert space.end() == len(line) - 1
            return LinterFailure("Trailing whitespace",
                                 index + 1,
                                 line[:space.start()] + line[space.end():])