Rust エラー「error: character literal may only contain one codepoint」が発生した場合の対処法

fn main() {

    let mut str: String = "a".to_string();
    
    str.push_str('bc');

    println!("{}", str);

}

error: character literal may only contain one codepoint
 --> sample.rs:5:18
  |
5 |     str.push_str('bc');
  |                  ^^^^
  |
help: if you meant to write a `str` literal, use double quotes
  |
5 |     str.push_str("bc");
  |                  ~~~~

error: aborting due to previous error

fn main() {

    let mut str: String = "a".to_string();
    
    str.push_str("bc");

    println!("{}", str); // abc

}