Rust エラー「error[E0308]: mismatched types」が発生した場合の対処法

fn main() {

    let str1: String = "aaa".to_string();
    let str2: String = "bbb".to_string();

    println!( "{}", str1 + str2 );
    
}

error[E0308]: mismatched types
 --> sample.rs:6:28
  |
6 |     println!( "{}", str1 + str2 );
  |                            ^^^^
  |                            |
  |                            expected `&str`, found struct `String`
  |                            help: consider borrowing here: `&str2`

error: aborting due to previous error

fn main() {

    let str1: String = "aaa".to_string();
    let str2: String = "bbb".to_string();

    println!( "{}", format!("{}{}", str1, str2) );
    // aaabbb
    
}