Looks like some cases were not tested in #8398 The following code: ``` rust use std::rt::io::Reader; use std::rt::io::extensions::ReaderUtil; fn parse(reader: &mut Reader) { reader.read_byte(); } fn main() { } ``` still yields a compilation error: ``` test.rs:6:4: 6:10 error: cannot borrow immutable argument as mutable test.rs:6 reader.read_byte(); ```