1. Open the Cargo.toml file that has been generated earlier for you.

2. In the bin folder, create a file called retain_string.rs.

3. Add the following code, and run it with cargo run --bin retain_string:

1   #![feature(string_retain)]
2   
3   fn main() {
4     let mut some_text = "H_el_l__o_ ___Wo_r__l_d_".to_string();
5     println!("Original text: {}", some_text);
6     // retain() removes all chars that don't fulfill a
7     // predicate in place, making it very efficient
8     some_text.retain(|c| c != '_');
9     println!("Text without underscores: {}", some_text);
10    some_text.retain(char::is_lowercase);
11    println!("Text with only lowercase letters: {}", some_text);
12  
13    // Before retain, you had to filter the string as an iterator 
      over chars
14    // This will however create a new String, generating overhead
15    let filtered: String = "H_el_l__o_ ___Wo_r__l_d_"
16   ...