rust - 为什么 String::from(*d) 在 &&str 上给出的结果与 *d.to_string() 不同？

Question

我有点困惑为什么取消引用 &&str 在第二种情况下似乎不起作用:

use std::collections::HashSet;

fn main() {
    let days = vec!["mon", "tue", "wed"];
    let mut hs: HashSet<String> = HashSet::new();

    for d in &days {
        // works
        hs.insert(String::from(*d));

        // doesn't
        hs.insert(*d.to_string());
    }
    println!("{:#?}", hs);
}

str 确实实现了 ToString 特征，但它仍然给我错误:

error[E0308]: mismatched types
  --> src/main.rs:12:19
   |
12 |         hs.insert(*d.to_string());
   |                   ^^^^^^^^^^^^^^ expected struct `std::string::String`, found str
   |
   = note: expected type `std::string::String`
              found type `str`

我在这里弄错了什么语法？

Rust Playground Link

Answer 1

to_string被称为 d在它被取消引用之前，所以你将取消引用 String ，结果为 str .

改成

hs.insert(d.to_string());

这是有效的，因为 d自动取消引用为 str , 将转换为 String然后。这叫做 Deref coercions .

If you have a type U, and it implements Deref<Target=T>, values of &U will automatically coerce to a &T

...

Deref will also kick in when calling a method

这是 exactly the case here : impl Deref<Target = str> for String .参见 here for an example :

A value of type &&&&&&&&&&&&&&&&Foo can still have methods defined on Foo called, because the compiler will insert as many * operations as necessary to get it right. And since it’s inserting *s, that uses Deref.

这example证明了这一点:

struct Foo;

impl Foo {
    fn foo(&self) { println!("Foo"); }
}

let f = &&Foo;

// prints "foo"
f.foo();

顺便说一下，

hs.insert((*d).to_string());

还将work , 因为它首先被取消引用为 &str .