It panics due to lack of the string validity check in async-h1 and due to unwrap in http-types. Also, there is second unwrap that will panic if two fields with similar values exists, but second has valid UTF-8 string.

Minimal code for replication

#[async_std::main]
async fn main() -> tide::Result<()> {
    let mut app = tide::new();
    app.at("/").get( |_| async {
        Ok(tide::Response::new(200))
    });
    app.listen("0.0.0.0:8080").await?;
    Ok(())
}

Note, header value should be an valid UTF-8 string.

curl --header "Test: тест" http://0.0.0.0:8080
# or
curl --header "Test: test" --header "Test: тест" http://0.0.0.0:8088

Sorry if this is wrong repo. I don't really know to which repo this issue related to, because tide uses async-h1 which uses http-types. And I have got this error while using tide.