Question

使用以下代码（尝试使用reqwest crate发出HTTP请求），编译器说我的值SID_URI没有实现特征PolyfillTryInto。这里发生了什么？ reqwest::Url clearly implements the 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`

Answer 1

编译器不是骗你的，你只是跳过错误信息的相关细节。这是一个独立的例子：

#[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

第二条错误消息并未说明42未实施ExampleTrait，而是说 i32 缺乏实施。此错误消息显示失败的类型，而不是值的名称！这意味着同一上下文中的EXAMPLE指的是一种类型。

Lazy-static由creating one-off types工作，它包装你的值并提供线程安全的单初始化保证：

对于给定的static ref NAME: TYPE = EXPR;，宏会生成一个实现Deref<TYPE>的唯一类型，并将其存储在名为NAME的静态中。

此包装类型不实现您的特征，只有包装类型。您需要调用Deref然后重新引用它以获得&Url，假设对Url的引用实现了您的特征：

must_have_trait(&*EXAMPLE);

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

为什么懒惰的静态值声称没有实现它明确实现的特征？

1 个答案: