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: wordlist.rs
    path of file: ./repos/trezor-firmware/core/embed/rust/src/trezorhal
    
    the code of the file until where you have to start completion: use super::ffi;
use core::cmp::Ordering;
use cstr_core::CStr;

/// Holds all the possible words with the possibility to interact
/// with the "list" - filtering it further, getting their count, etc.
pub struct Wordlist {
    words: &'static [*const cty::c_char],
    /// Holds the length of prefix which was used to filter the list
    /// (how many beginning characters are common for all words).
    prefix_len: usize,
}

impl Wordlist {
    pub fn new(words: &'static [*const cty::c_char], prefix_len: usize) -> Self {
        Self { words, prefix_len }
    }

    /// Initialize BIP39 wordlist.
    pub fn bip39() -> Self {
        Self::new(unsafe { &ffi::BIP39_WORDLIST_ENGLISH }, 0)
    }

    /// Initialize SLIP39 wordlist.
    pub fn slip39() -> Self {
        Self::new(unsafe { &ffi::SLIP39_WORDLIST }, 0)
    }

    /// Returns all possible letters from current wordlist that form a valid
    /// word. Al
    
    fn test_get_available_letters() {
        let result = Wordlist::bip39()
            .filter_prefix("ab")
            .get_available_letters()
            .collect::<Vec<_>>();
        let expected_result = vec!['a', 'i', 'l', 'o', 's', 'u'];
        assert_eq!(result, expected_result);

        let result = Wordlist::bip39()
            .filter_prefix("str")
            .get_available_letters()
            .collect::<Vec<_>>();
        let expected_result = vec!['a', 'e', 'i', 'o', 'u'];
        assert_eq!(result, expected_result);

        let result = Wordlist::bip39()
            .filter_prefix("zoo")
            .get_available_letters()
            .collect::<Vec<_>>();
        let expected_result = vec![];
        assert_eq!(result, expected_result);

        let result = Wordlist::bip39()
            .get_available_letters()
            .collect::<Vec<_>>();
        let expected_result = vec![
            'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q',
            'r', 's', 't', 'u', 'v', 'w', 'y', 'z',
        ];
        assert_eq!(result, expected_result);
    }