Better developer experience when using State in error code paths

[matze]

While the current transformation of Err(impl IntoResponse) into a valid response works well for API-like use cases it is less suitable for old school multi page applications. In those use cases both successful and error response contain rendered HTML which often requires access to the state in order to render appropriately (e.g. required templates, template data etc.).

As of now, handlers would have to do something along these lines to use ergonomic ? together with template rendering:

pub async fn some_page(State(state): State<AppState>) -> Result<impl IntoResponse, impl IntoResponse> {
    async {
        // some fallible operation
        let _ = tokio::fs::read_to_string("foo").await?;
        Ok::<_, anyhow::Error>(GoodPage::from(state))
    }
    .await
    .map_err(|err| {
        ErrorPage::from(state, err)
    })
}

This is an eye sore and just boilerplate. I know of HandleErrorLayer but I have a hard time getting it to work, i.e. extract the State and return some rendered page.