unused `async` for function with no await statements when function is used in contexts requiring async

[leighmcculloch]

Summary

Functions with the async keyword get the following error, even when the functions must have async to be used in contexts that are async.

unused `async` for function with no await statements

Lint Name

unused_async

Reproducer

I tried this code:

[package]
name = "pro"
version = "0.1.0"
edition = "2021"

[dependencies]
axum = { version = "0.5" }
tokio = { version = "1.0", features = ["full"] }

use axum::{routing::get, Router};
use std::net::SocketAddr;

#[tokio::main]
async fn main() {
    let router = Router::new().route("/", get(home));
    let addr = SocketAddr::from(([127, 0, 0, 1], 3000));
    axum::Server::bind(&addr)
        .serve(router.into_make_service())
        .await
        .unwrap();
}

async fn home() -> &'static str {
    "Hello, World!"
}

I expected to see this happen:

No warnings.

Instead, this happened:

warning: unused `async` for function with no await statements
  --> src/main.rs:15:1
   |
15 | / async fn home() -> &'static str {
16 | |     "Hello, World!"
17 | | }
   | |_^
   |
   = note: `-W clippy::unused-async` implied by `-W clippy::pedantic`
   = help: consider removing the `async` from this function
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_async

Version

rustc 1.64.0 (a55dd71d5 2022-09-19)
binary: rustc
commit-hash: a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52
commit-date: 2022-09-19
host: x86_64-apple-darwin
release: 1.64.0
LLVM version: 14.0.6