Rust showing expected trait object `dyn Future`, found opaque type when passing function as a param - asynchronous

Question

use std::io::prelude::*;
use std::net::TcpListener;
use std::net::TcpStream;
use std::time::Duration;
// pyO3 module
use pyo3::prelude::*;
use pyo3::wrap_pyfunction;
use std::future::Future;
#[pyfunction]
pub fn start_server() {
let listener = TcpListener::bind("127.0.0.1:7878").unwrap();
let pool = ThreadPool::new(4);
for stream in listener.incoming() {
let stream = stream.unwrap();
pool.execute(|| {
let rt = tokio::runtime::Runtime::new().unwrap();
handle_connection(stream, rt, &test_helper);
});
}
}
#[pymodule]
pub fn roadrunner(_: Python<'_>, m: &PyModule) -> PyResult<()> {
m.add_wrapped(wrap_pyfunction!(start_server))?;
Ok(())
}
async fn read_file(filename: String) -> String {
let con = tokio::fs::read_to_string(filename).await;
con.unwrap()
}
async fn test_helper(contents: &mut String, filename: String) {
// this function will accept custom function and return
*contents = tokio::task::spawn(read_file(filename.clone()))
.await
.unwrap();
}
pub fn handle_connection(
mut stream: TcpStream,
runtime: tokio::runtime::Runtime,
test: &dyn Fn(&mut String, String) -> (dyn Future<Output = ()> + 'static),
) {
let mut buffer = [0; 1024];
stream.read(&mut buffer).unwrap();
let get = b"GET / HTTP/1.1\r\n";
let sleep = b"GET /sleep HTTP/1.1\r\n";
let (status_line, filename) = if buffer.starts_with(get) {
("HTTP/1.1 200 OK", "hello.html")
} else if buffer.starts_with(sleep) {
thread::sleep(Duration::from_secs(5));
("HTTP/1.1 200 OK", "hello.html")
} else {
("HTTP/1.1 404 NOT FOUND", "404.html")
};
let mut contents = String::new();
let future = test_helper(&mut contents, String::from(filename));
runtime.block_on(future);
let response = format!(
"{}\r\nContent-Length: {}\r\n\r\n{}",
status_line,
contents.len(),
contents
);
stream.write(response.as_bytes()).unwrap();
stream.flush().unwrap();
}
I am trying to create a module where I need to pass an async function as an argument. I have passed the element but I am unable to deduce what should I do from the error message. It is telling me that there is some mismatch in type inference.
Here is the error message I am getting on cargo check
error[E0271]: type mismatch resolving `for<'r> <for<'_> fn(&mut String, String) -> impl Future {test_helper} as FnOnce<(&'r mut String, String)>>::Output == (dyn Future<Output = ()> + 'static)`
--> src/lib.rs:124:43
|
124 | handle_connection(stream, rt, &test_helper);
| ^^^^^^^^^^^^ expected trait object `dyn Future`, found opaque type
...
140 | async fn test_helper(contents: &mut String, filename: String) {
| - checked the `Output` of this `async fn`, found opaque type
|
= note: while checking the return type of the `async fn`
= note: expected trait object `(dyn Future<Output = ()> + 'static)`
found opaque type `impl Future`
= note: required for the cast to the object type `dyn for<'r> Fn(&'r mut String, String) -> (dyn Future<Output = ()> + 'static)`
error: aborting due to previous error
Please let me know what change should be made here. Thanks in advance.

Answer 1

You are writing a function type that returns a dyn type, not a reference to it, but the unsized type itself, that is not possible. Every time you want to write something like this, try using a generic instead:
pub fn handle_connection<F>(
mut stream: TcpStream,
runtime: tokio::runtime::Runtime,
test: &dyn Fn(&mut String, String) -> F,
)
where F: Future<Output = ()> + 'static
This now fails with this weird error:
error[E0308]: mismatched types
--> src/lib.rs:19:43
|
19 | handle_connection(stream, rt, &test_helper);
| ^^^^^^^^^^^^ one type is more general than the other
|
= note: expected associated type `<for<'_> fn(&mut String, String) -> impl Future {test_helper} as FnOnce<(&mut String, String)>>::Output`
found associated type `<for<'_> fn(&mut String, String) -> impl Future {test_helper} as FnOnce<(&mut String, String)>>::Output`
But this is expected too, your future is holding a reference to that &mut String you are passing, so it is not 'static anymore. The solution is just to add a lifetime generic parameter:
pub fn handle_connection<'a, F>(
mut stream: TcpStream,
runtime: tokio::runtime::Runtime,
test: &dyn Fn(&'a mut String, String) -> F,
)
where F: Future<Output = ()> + 'a
And now it should compile.