In between consulting contracts and job applications, I am building out a small personal Rust project that uses minijinja. It works extremely well and is quite fast, and I have well over a decade of experience building static sites with Jinja and Nunjucks templates, so this is a reasonable approach (trying not to shave the yak by implementing my own templating layer… at least for now).

It is possible to work with complex types across minijinja functions using the ViaDeserialize trait (though there is a fair bit of implicit conversion necessarily happening there, as implied by the name!). However, that constraint indirectly requires that the type you apply it to implement serde::Deserialize, and the errors show up not at the site where you try to define a function in terms of a type which does not implement serde::Deserialize, but where you try to call minijina’s Environment::add_function helper using the function you have defined:

error[E0277]: the trait bound `for<'a, 'b, 'c> fn(ViaDeserialize<&'a page<'b>>, ViaDeserialize<&'c data::config::config>) -> std::string::String {url_for}: Function<_, _>` is not satisfied
   --> src/templates/functions.rs:66:32
    |
66  |    env.add_function("url_for", url_for);
    |        ------------            ^^^^^^^ the trait `Function<_, _>` is not implemented for fn item `fn(ViaDeserialize<&page<'b>>, ViaDeserialize<&config>) -> String {url_for}`
    |        |
    |        required by a bound introduced by this call
    |
note: required by a bound in `Environment::<'source>::add_function`
   --> /Users/chris/.cargo/registry/src/index.crates.io-6f17d22bba15001f/minijinja-2.5.0/src/environment.rs:740:12
    |
736 |     pub fn add_function(&mut self, name: N, f: F)
    |            ------------ required by a bound in this associated function
...
740 |         F: functions::Function
    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `Environment::<'source>::add_function`

Not only is this extremely noisy (despite the Rust team’s best efforts: there is just a lot going on here), it doesn’t really tell you what is wrong or what you need to do to fix it. Why is the trait Function<_, _> not implemented for this call? Who could say?

The answer is that one of the parameters in the function (in this case url_for) does not implement serde::Deserialize, but I had claimed it did by defining url_for like this:

fn url_for(
   ViaDeserialize(path): ViaDeserialize<Page>,
   ViaDeserialize(config): ViaDeserialize<Config>,
) -> String {
   // ...
}

In this case, the Page type was the culprit: it does not (and for reasons not germane to this particular post, cannot) implement Deserialize. I can work around that here, by using a narrower type that can implement Deserialize, but only once I know that that’s the problem!

So, long story short: if you see that gnarly “does not implement Function” error, go check all the types you are using with ViaDeserialize: one of them is not implementing Deserialize!

Bonus: I opened a PR that fixes this, at the cost of some type system shenanigans internally in minijinja; we’ll see what the maintainer (the excellent Armin Ronacher) thinks about it. Hopefully this tip will end up outdated!