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Harjun751's questions

Martin Hope
Harjun751
Asked: 2024-02-22 22:09:05 +0800 CST
  • 10

我目前正在学习 Rust，我想从拥有字符串的项目向量中收集字符串 (&strs) 的引用。

struct Channel {
    cid: usize,
    name: String,
}

fn main() {
    let channels: Vec<Channel> = vec![Channel {
        cid: 1,
        name: "Hello".to_string(),
    }];
    let names: Vec<&str> = channels.iter().map(|x| &x.name).collect();
}

此代码无法编译，出现以下错误：

[E0277]: a value of type `Vec<&str>` cannot be built from an iterator over elements of type `&String`
    --> src/main.rs:11:61
     |
11   |     let names: Vec<&str> = channels.iter().map(|x| &x.name).collect();
     |                                                             ^^^^^^^ value of type `Vec<&str>` cannot be built from `std::iter::Iterator<Item=&String>`
     |
     = help: the trait `FromIterator<&String>` is not implemented for `Vec<&str>`
     = help: the trait `FromIterator<&str>` is implemented for `Vec<&str>`
     = help: for that trait implementation, expected `str`, found `String`
note: the method call chain might not have had the expected associated types
    --> src/main.rs:11:44
     |
7    |       let channels: Vec<Channel> = vec![Channel {
     |  __________________________________-
8    | |         cid: 1,
9    | |         name: "Hello".to_string(),
10   | |     }];
     | |______- this expression has type `Vec<Channel>`
11   |       let names: Vec<&str> = channels.iter().map(|x| &x.name).collect();
     |                                       ------ ^^^^^^^^^^^^^^^^ `Iterator::Item` changed to `&String` here
     |                                       |
     |                                       `Iterator::Item` is `&Channel` here
note: required by a bound in `collect`
    --> /playground/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/iter/traits/iterator.rs:2050:19
     |
2050 |     fn collect<B: FromIterator<Self::Item>>(self) -> B
     |                   ^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `Iterator::collect`

我可以通过调用 as_str() 来提取字符串切片来使其工作：

let names: Vec<&str> = channels.iter().map(|x| x.name.as_str()).collect();

但是，我想了解为什么需要这样做。根据我的理解，由于 deref 强制， a&String可以转换为 a &str。为什么我必须明确使用as_str()？此外，就性能而言，获取as_str()与引用某个项目相同吗？

rust