I have now rewritten my code so that I include bytes directly in my src/main.rs like so:

    #[get("/fonts/<fname>")]
    fn fonts<'a> (fname: String) -> Result<Content<Vec<u8>>, NotFound<String>>
    {
        match fname.as_ref()
        {
            "autobahn.woff" => Ok(Content(ContentType::WOFF,
                include_bytes!("../static/fonts/Autobahn/autobahn.woff").to_vec())),

            "autobahn.ttf" => Ok(Content(ContentType::TTF,
                include_bytes!("../static/fonts/Autobahn/autobahn.ttf").to_vec())),

            "grobe-deutschmeister.woff" => Ok(Content(ContentType::WOFF,
                include_bytes!("../static/fonts/Grobe-Deutschmeister/grobe-deutschmeister.woff").to_vec())),

            "grobe-deutschmeister.ttf" => Ok(Content(ContentType::TTF,
                include_bytes!("../static/fonts/Grobe-Deutschmeister/grobe-deutschmeister.ttf").to_vec())),

            _ => Err(NotFound(format!("No such file: /fonts/{}", fname)))
        }
    }

In the meantime since I first wrote my original code, the web framework I was using called Iron [1] has become unmaintained and they now urge people to pick a different framework. I chose to use Rocket because it seemed to have the features I want and the documentation seemed good enough to get started (and it was).

For templating I found Askama [2], which I am using from git master in order to be able to use it together with Rocket. https://github.com/djc/askama/issues/71

Eventually I will generate the routes for the included bytes with build.rs again. My new code is better both short-term and for when I will create said build.rs.

Thank you masklinn for your comment about include_bytes! which made me look at my code again and rewrite it.

[1]: https://rocket.rs/

[2]: https://docs.rs/crate/askama/