[Bug] async_trait underscore variable ignore cannot find value __arg1

[Lukas-Heiligenbrunner]

Hey, thanks for this great lib!
When using async_trait and ignoring a variable with underscore in an implementation your trait fails with:

cannot find value `__arg1` in this scope
  --> src/main.rs:25:5
   |
25 |     #[logcall]
   |     ^^^^^^^^^^ not found in this scope

Code to reproduce:

#[tokio::main]
async fn main() {
    env_logger::builder()
        .parse_env(Env::default().filter_or("LOG_LEVEL", "trace"))
        .init();

    let test = Test {};
    test.test("Hello".to_string()).await;
}

#[async_trait]
pub trait TestTrait {
    async fn test(&self, mystr: String) -> String;
}

struct Test {}

#[async_trait]
impl TestTrait for Test {
    #[logcall]
    async fn test(&self, _: String) -> String {
        "test".to_string()
    }
}

Note, when giving the string of the async test function an explicit name it works fine.