Type annotation?

main.rs

#[post("/new")]
async fn new(req_body: String) -> impl Responder {
    match serde_json::from_str(req_body.as_str()) {
        Err(why) => HttpResponse::BadRequest().body(
            json!({
                "err": "Unable to parse input as request json",
            })
            .to_string(),
        ),
        Ok(t) => HttpResponse::Ok().body(format!("{:?}", t)),
    }
}

Error:

error[E0282]: type annotations needed
  --> src/main.rs:30:42
   |
30 |         Ok(t) => HttpResponse::Ok().body(t.to_string()),
   |                                          ^ cannot infer type
   |
   = note: type must be known at this point