我正在使用Rocket创建一个Web服务器,我正在尝试围绕 Responder
trait创建一个包装器,以便我的路由方法可以返回任何结构 .
下面的代码不能编译,因为我不完全理解有关生命周期的错误 . 错误是not listed in the error index;它从E0492跳到E0496 .
由于此代码使用Rocket,因此需要夜间编译器 .
main.rs
#![feature(custom_attribute, proc_macro_hygiene, decl_macro)]
#[macro_use]
extern crate rocket;
extern crate rocket_contrib;
use rocket::{http::Status, response::Responder, Request};
use rocket_contrib::templates::Template;
fn main() {
rocket::Rocket::ignite().mount("/", routes![route]).launch();
}
#[get("/")]
fn route<'a>() -> DynamicResponder<'a> {
DynamicResponder::from(Template::render("template", ()))
}
struct DynamicResponder<'a> {
inner: Box<dyn Responder<'a> + 'a>,
}
impl<'r> DynamicResponder<'r> {
pub fn from<T: 'r>(responder: T) -> DynamicResponder<'r>
where
T: Responder<'r>,
{
DynamicResponder {
inner: Box::new(responder),
}
}
}
impl<'r> Responder<'r> for DynamicResponder<'r> {
fn respond_to<'b>(
self,
request: &'b Request,
) -> Result<rocket::response::Response<'r>, Status> {
self.inner.respond_to(request)
}
}
Cargo.toml
[package]
name = "rocketing_around"
version = "0.1.0"
[dependencies]
rocket = "0.4.0"
[dependencies.rocket_contrib]
version = "0.4.0"
default_features = false
features = [ "handlebars_templates" ]
编译器消息:
error[E0495]: cannot infer an appropriate lifetime for lifetime parameter `'r` due to conflicting requirements
--> src/main.rs:15:5
|
15 | DynamicResponder::from(Template::render("template", ()))
| ^^^^^^^^^^^^^^^^^^^^^^
|
note: first, the lifetime cannot outlive the lifetime 'a as defined on the function body at 14:10...
--> src/main.rs:14:10
|
14 | fn route<'a>() -> DynamicResponder<'a> {
| ^^
= note: ...so that the expression is assignable:
expected DynamicResponder<'a>
found DynamicResponder<'_>
= note: but, the lifetime must be valid for the static lifetime...
= note: ...so that the types are compatible:
expected rocket::response::Responder<'_>
found rocket::response::Responder<'static>
1 回答
类型:
表示该函数保证返回的
DynamicResponder
至少与'a
一样有效 . 但是'a
是由调用者选择的,因此它必须在任何生命周期内都有效,即'a: 'static
. 这就是为什么消息中的第二个音符指的是静态生命周期 .我不完全确定为什么
'a
不仅仅被推断为'static
,但是自己做出改变会解决它:遗憾的是,您仍然会遇到此代码的问题,因为
respond_to
消耗self
,但需要在未知大小的特征对象上调用它 .