How can I put an async function into a Vec in Rust? - rust

Question

I need to put some futures in a Vec for later joining. However if I try to collect it using an iterator, the compiler doesn't seem to be able to determine the type for the vector.
I'm trying to create a command line utility that accepts an arbitrary number of IP addresses, communicates with those remotes and collects the results for printing. The communication function works well, I've cut down the program to show the failure I need to understand.
use futures::future::join_all;
use itertools::Itertools;
use std::net::SocketAddr;
use std::str::from_utf8;
use std::fmt;
#[tokio::main(flavor = "current_thread")]
pub async fn main() -> Result<(), Box<dyn std::error::Error>> {
let socket: Vec<SocketAddr> = vec![
"192.168.20.33:502".parse().unwrap(),
"192.168.20.34:502".parse().unwrap(),];
let async_vec = vec![
MyStruct::get(socket[0]),
MyStruct::get(socket[1]),];
// The above 3 lines happen to work to build a Vec because there are
// 2 sockets. But I need to build a Vec to join_all from an arbitary
// number of addresses. Why doesn't the line below work instead?
//let async_vec = socket.iter().map(|x| MyStruct::get(*x)).collect();
let rt = join_all(async_vec).await;
let results = rt.iter().map(|x| x.as_ref().unwrap().to_string()).join("\n");
let mut rvec: Vec<String> = results.split("\n").map(|x| x.to_string()).collect();
rvec.sort_by(|a, b| a[15..20].cmp(&b[15..20]));
println!("{}", rvec.join("\n"));
Ok(())
}
struct MyStruct {
serial: [u8; 12],
placeholder: String,
}
impl fmt::Display for MyStruct {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
let serial = match from_utf8(&self.serial) {
Ok(v) => v,
Err(_) => "(invalid)",
};
let lines = (1..4).map(|x| format!("{}, line{}, {}", serial, x, self.placeholder)).join("\n");
write!(f, "{}", lines)
}
}
impl MyStruct {
pub async fn get(sockaddr: SocketAddr) -> Result<MyStruct, Box<dyn std::error::Error>> {
let char = sockaddr.ip().to_string().chars().last().unwrap();
let rv = MyStruct{serial: [char as u8;12], placeholder: sockaddr.to_string(), };
Ok(rv)
}
}

Answer 1

This line:
let async_vec = socket.iter().map(|x| MyStruct::get(*x)).collect();
doesn't work because the compiler can't know that you want to collect everything into a Vec. You might want to collect into some other container (e.g. a linked list or a set). Therefore you need to tell the compiler the kind of container you want with:
let async_vec = socket.iter().map(|x| MyStruct::get(*x)).collect::<Vec::<_>>();
or:
let async_vec: Vec::<_> = socket.iter().map(|x| MyStruct::get(*x)).collect();