You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: uattrs.rs
    path of file: ./repos/webrtc/stun/src
    
    the code of the file until where you have to start completion: #[cfg(test)]
mod uattrs_test;

use std::fmt;

use crate::attributes::*;
use crate::error::*;
use crate::message::*;

// UnknownAttributes represents UNKNOWN-ATTRIBUTES attribute.
//
// RFC 5389 Section 15.9
pub struct UnknownAttributes(pub Vec<AttrType>);

impl fmt::Display for
    
    fn add_to(&self, m: &mut Message) -> Result<()> {
        let mut v = Vec::with_capacity(ATTR_TYPE_SIZE * 20); // 20 should be enough
                                                             // If len(a.Types) > 20, there will be allocations.
        for t in &self.0 {
            v.extend_from_slice(&t.value().to_be_bytes());
        }
        m.add(ATTR_UNKNOWN_ATTRIBUTES, &v);
        Ok(())
    }