问题
I use the cassandra of the c/c++ driver to query, and then return the data. Therefore, both cass (LinkedList) and cass_it (Vec) can show the result of the query. However, I want to display the results to the web using the json format, so I chose to reassemble the data using vec. However, there is a problem:
error[E0434]: can't capture dynamic environment in a fn item
--> src/main.rs:306:42
|
306 | let out = serde_json::to_string(&cass_it).unwrap();
| ^^^^^^^
|
= help: use the `|| { ... }` closure form instead
What is wrong? Formatting issues?
Code:
fn main() {
let mut cass = unsafe { cass_connect() };
let mut cass_it = Vec::new();
for cc in cass.iter() {
cass_it.push(cc);
}
println!("{:?}", cass_it);
let mut router = Router::new();
let localhost = "localhost:3009".to_string();
fn handler(req: &mut Request) -> IronResult<Response> {
// convert the response struct to JSON
let out = serde_json::to_string(&cass_it).unwrap();
let content_type = "application/json".parse::<Mime>().unwrap();
Ok(Response::with((content_type, status::Ok, out)))
}
router.get("/", handler, "index");
info!("Listening on {}", localhost);
Iron::new(router).http(localhost).unwrap();
}
Complete code
回答1:
The error says it all:
can't capture dynamic environment in a fn item
The fn
item in question is handler
. Even though you have defined this function inside another method, a function declared with fn
(a fn
item) is compiled just like any other function on the top level of a module. Functions can't capture free variables from their environment; they can only access their explicit arguments and static variables.
The error goes on to say exactly which variable is the problem:
306 | let out = serde_json::to_string(&cass_it).unwrap(); | ^^^^^^^
The variable cass_it
is defined in the enclosing function and cannot be accessed from handler
.
The note at the end of the error message then gives you a suggestion for how to fix the problem:
= help: use the `|| { ... }` closure form instead
A closure can capture variables from its environment. So you can try replacing the fn
with a closure instead:
let handler = move |req: &mut Request| {
// convert the response struct to JSON
let out = serde_json::to_string(&cass_it).unwrap();
let content_type = "application/json".parse::<Mime>().unwrap();
Ok(Response::with((content_type, status::Ok, out)))
};
The move
keyword will cause the closure to take ownership of cass_it
instead of trying to reference the variable in the outer function.
来源:https://stackoverflow.com/questions/54342492/cant-capture-dynamic-environment-in-a-fn-item-about-iron-lib