8

使用以下代码（尝试使用reqwestcrate 发出 HTTP 请求），编译器说我的值SID_URI没有实现 trait PolyfillTryInto。这里发生了什么？reqwest::Url清楚地实现了 private trait reqwest::into_url::PolyfillTryInto。

#[macro_use]
extern crate lazy_static;
extern crate reqwest;

static R_EMAIL: &str = "example@example.com";
static R_PASS: &str = "password";
static API_PUBKEY: &str = "99754106633f94d350db34d548d6091a";
static API_URI: &str = "https://example.com";
static AUTH_PATH: &str = "/api/v1";

lazy_static! {
    static ref SID_URI: reqwest::Url = reqwest::Url::parse(&(API_URI.to_owned() + AUTH_PATH)).unwrap();
}

fn get_sid() -> Result<reqwest::Response, reqwest::Error> {
    let client = reqwest::Client::new();
    let params = [("ID", R_EMAIL), ("PW", R_PASS), ("KY", API_PUBKEY)];
    let q = client.post(SID_URI).form(&params).send()?;
    Ok(q)
}

fn main() {
    assert!(get_sid().is_ok());
}

error[E0277]: the trait bound `SID_URI: reqwest::into_url::PolyfillTryInto` is not satisfied
  --> src/main.rs:19:20
   |
19 |     let q = client.post(SID_URI).form(&params).send()?;
   |                    ^^^^ the trait `reqwest::into_url::PolyfillTryInto` is not implemented for `SID_URI`
   |
   = note: required because of the requirements on the impl of `reqwest::IntoUrl` for `SID_URI`
4

1 回答 1

24

编译器并没有对您撒谎，您只是跳过了错误消息的相关细节。这是一个独立的示例：

#[macro_use]
extern crate lazy_static;

struct Example;
trait ExampleTrait {}
impl ExampleTrait for Example {}

lazy_static! {
    static ref EXAMPLE: Example = Example;
}

fn must_have_trait<T>(_: T)
where
    T: ExampleTrait,
{
}

fn main() {
    must_have_trait(EXAMPLE);
    must_have_trait(42i32);
}

error[E0277]: the trait bound `EXAMPLE: ExampleTrait` is not satisfied
  --> src/main.rs:19:5
   |
19 |     must_have_trait(EXAMPLE);
   |     ^^^^^^^^^^^^^^^ the trait `ExampleTrait` is not implemented for `EXAMPLE`
   |
   = note: required by `must_have_trait`

error[E0277]: the trait bound `i32: ExampleTrait` is not satisfied
  --> src/main.rs:20:9
   |
20 |         must_have_trait(42i32);
   |         ^^^^^^^^^^^^^^^ the trait `ExampleTrait` is not implemented for `i32`
   |
   = note: required by `must_have_trait`

比较两条错误消息：

the trait bound `EXAMPLE: ExampleTrait` is not satisfied
the trait bound `i32: ExampleTrait` is not satisfied

第二条错误消息不是说不42实现ExampleTrait，而是说i32缺乏实现。此错误消息显示失败的类型，而不是值的名称！这意味着EXAMPLE在同一上下文中指的是一种类型。

惰性静态通过创建一次性类型来包装您的值并提供线程安全的单一初始化保证：

对于给定static ref NAME: TYPE = EXPR;的，宏生成一个唯一类型，该类型实现Deref<TYPE>并将其存储在带有名称的静态中NAME。

此包装器类型不会实现您的特征，只有包装器类型可以。您将需要调用Deref然后可能重新引用它以到达 a &Url，假设对 a 的引用Url实现了您的特征：

must_have_trait(&*EXAMPLE);

此外，使用裸静态变量会尝试将其移出静态位置（这将是一件非常糟糕的事情），因此您始终需要通过引用来使用它。

于 2018-01-05T14:07:53.267 回答