Add a render_with_values method?

[GuillaumeGomez]

It could allow to create variables with given values before generating the template. Could be useful if there are variables outside of the type.

[GuillaumeGomez]

Not possible, however we can add a new API to the Template proc-macro to allow to add values at compile-time.

[ronnodas]

Do you mean it's not possible because the type of the value passed needs to be available at compile time? Because right now, given a template type T and a value type V you can create a wrapper struct (oversimplified)

#[derive(Template(...))]
struct WithValue {
    template: &T,
    values: &V,
}

except the fields of T are not available without prefixing template.. I was imagining an extension to the Template derive that would do this automatically and be used like:

#[derive(Template(with_value = V, ...))]
struct T {
   ...
}

Making it generic over V seems harder, although one possibility is to essentially replace the type V with impl ValueForT for a new trait.

[GuillaumeGomez]

The problem is that we only have access to the fields of the current struct, no other information. So for this feature to work, if it's at runtime, it means that all unknown fields like {{ x }} will need to be generated differently. Let's say we take a hashmap into the render_with_values() method. Then for each unknown variable, we'd replace them with getting a value from the hashmap. I'm not sure if it's a good idea in this case.

The other approach I suggested is to instead list values in the template proc-macro directly. Like that, we can still generate the best possible code. However, since it's in a proc-macro, the kind of values which can be used will be limited to booleans, integers, strings, tuples and maybe slices. So I'm not sure if it's a good idea either. It's a tricky feature.

[GuillaumeGomez]

I have a use case for such a method in docs.rs where we have a csp_nonce variable which is only generated when rendering the item. So it's a "dead" field until then. Now I'm thinking that we could maybe introduce a "run-time" kind of variable/struct. Like rinja_vars.get::<TYPE>("VAR_NAME")? What do you think @Kijewski ?

[Kijewski]

So, it's like a Cell<Option<_>> that is filled when first used?

Sorry, I'm on vacation right now, and I won't give any programming question a deeper thought until next Sunday. :) If you want to, maybe you could tell me / link the lines of code that could use that feature and I'll have a look when I'm back home.

[GuillaumeGomez]

Nah, please don't answer until you're back from holiday. We can discuss when you're back, there is no hurry (I'm in holiday too, yeay).