Rust 为什么我不能参考try的结果！和 ＆

这不是一个典型的问题，而是一个典型的问题.鉴于以下代码:

fn foo() -> Result<String, i32> {
    return Ok("world".to_string());
}

fn bar() -> Result<String, i32> {
    let mut value = String::new();
    value.push_str(&try!(foo())); // this line here

    return Ok("Hello ".to_string() + &value);
}

fn main() {
    match bar() {
        Ok(message) => println!("{}", message),
        _ => return,
    }
}

Rust返回错误:

<std macros>:3:43: 3:46 error: mismatched types:
expected str,
found collections::string::String
(expected str,
found struct collections::string::String) [E0308]
<std macros>:3 $ crate:: result:: Result:: Ok ( val ) => val , $ crate:: result:: Result::
<std macros>:1:1: 6:48 note: in expansion of try!
<std macros>:3:43: 3:46 help: run rustc --explain E0308 to see a detailed explanation
error: aborting due to previous error

如果我捕获try!的结果，并分别将&应用于结果，它会工作(并打印出Hello world):

fn foo() -> Result<String, i32> {
    return Ok("world".to_string());
}

fn bar() -> Result<String, i32> {
    let mut value = String::new();
    let foo_result = try!(foo()); // capture the result of try!
    value.push_str(&foo_result); // now use the result with &

    return Ok("Hello ".to_string() + &value);
}

fn main() {
    match bar() {
        Ok(message) => println!("{}", message),
        _ => return,
    }
}

为什么let foo_result = try!(foo()); value.push_str(&foo_result);行而value.push_str(&try!(foo()));不行？从我天真的Angular 来看，它们似乎是等价的，所以我不确定我不了解 rust 迹的哪个关键部分.