struct Foo {
    v: Vec<u8>,
}

impl<'a> Foo {
    fn do_foo(&'a mut self) -> Result<Option<&'a [u8]>, ()> {
        loop {
            self.v.extend_from_slice(b"  [ eu ]  ");
            if get_section_name(&self.v).is_some() {
                break;
            }
         };
        Ok(get_section_name(&self.v))
    }
}

fn strip(buf: &[u8]) -> &[u8] {
    let is_blank = |ch: &&u8| (**ch as char).is_ascii_whitespace();
    let i = buf.iter().take_while(is_blank).count();
    let j = buf[i..].iter().rev().take_while(is_blank).count();
    &buf[i..buf.len() - j]
}

fn get_section_name(buf: &[u8]) -> std::option::Option<&[u8]> {
    let buf = strip(buf);
    (buf.len() > 2 && buf[0] == b'[' && buf[buf.len()-1] == b']').then(
        || strip(&buf[1..buf.len()-1]))
}

fn main() {
    let mut foo = Foo{v: Vec::new()};
    println!("{:?}", foo.do_foo());
}