`+` cannot be used to concatenate two `&str` strings

The `+` operator can be used to concatenate two `&str` strings in Rust. The `+` operator performs the concatenation by converting one of the `&str` strings into an owned `String` type and then concatenating the two strings together.

Here's an example:

fn main() {
    let str1 = "Hello,";
    let str2 = " world!";
    
    let result = str1.to_string() + str2;
    
    println!("{}", result);
}

Output:

Hello, world!

In this example, the `str1` is first converted into an owned `String` using the `to_string()` method. Then the `+` operator is used to concatenate the converted string with `str2`. The resulting string is stored in the `result` variable and printed to the console.