在 Rust 中取消引用字符串和 HashMap

我试图理解HashMaps在Rust中是如何工作的，我给出了这个例子.

use std::collections::HashMap;

fn main() {
    let mut roman2number: HashMap<&'static str, i32> = HashMap::new();
    roman2number.insert("X", 10);
    roman2number.insert("I", 1);

    let roman_num = "XXI".to_string();
    let r0 = roman_num.chars().take(1).collect::<String>();
    let r1: &str = &r0.to_string();
    println!("{:?}", roman2number.get(r1)); // This works

    // println!("{:?}", roman2number.get(&r0.to_string())); // This doesn't
}

当我试图在最后一行未注释的情况下编译代码时，我得到了以下错误

error: the trait bound `&str: std::borrow::Borrow<std::string::String>` is not satisfied [E0277]
println!("{:?}", roman2number.get(&r0.to_string()));
                                            ^~~
note: in this expansion of format_args!
note: in this expansion of print! (defined in <std macros>)
note: in this expansion of println! (defined in <std macros>)
help: run `rustc --explain E0277` to see a detailed explanation

docs的Trait实现部分给出了fn deref(&self) -> &str的解引用

那么这里发生了什么？