syn fails to parse async fn inside None-delimited group

[Aaron1011]

The following code (repository: https://github.com/Aaron1011/async-syn):

// src/lib.rs
use syn::ItemFn;
use proc_macro::TokenStream;

#[proc_macro_attribute]
pub fn parse_fn(_attr: TokenStream, item: TokenStream) -> TokenStream {
    syn::parse_macro_input!(item as ItemFn);
    TokenStream::new()
}

// src/main.rs
macro_rules! make_fn {
    ($input:item) => {
        #[async_syn::parse_fn]
        fn outer() {
            $input
        }
    }
}

make_fn!(async fn inner() {});

fn main() {
    println!("Hello, world!");
}

gives the following error:

error: unexpected token
  --> src/main.rs:10:16
   |
10 | make_fn!(async fn inner() {});
   |                ^^

I think this is caused by the way syn does lookahead when parsing async fn. The error goes away if you change async fn inner() {} to fn inner() {}

Minimized from rust-lang/rust#78629

[Aaron1011]

@dtolnay Thanks for the fast fix!