声明 `String` 类型的变量不起作用

Question

我刚刚阅读 the Rust documentation about string data types，其中指出：

Rust has more than only &strs though. A String is a heap-allocated string. This string is growable, and is also guaranteed to be UTF-8.

麻烦：我想像下面这样显式声明变量类型：

let mystring : &str = "Hello"; // this works
let mystring : String = "Hello"; // this does not. Why?

Answer 1

因为第二个mystring不是String，而是&'static str，即静态分配的字符串字面量。

为了以这种方式（从文字）创建 String，您需要编写 let mystring = String::from("Hello") (Rust docs).

Answer 2

因为 &str 不是 String。

虽然有几种方法可以使该字符串文字成为 String 实例：

let mystring = String::from("Hello");
// ..or..
let mystring: String = "Hello".into();
// ..or..
let mystring: String = "Hello".to_string();