Potential compile time improvements?

[TheNuclearNexus]

In my project right now, I'm dealing with extremely slow compile times (30s+) for a simple template change (in the raw html), which is unbearably slow. This led me to think, what if instead of directly doing #askama_writer.write_str(lit), the literals were stored as static references within the template struct? Then it would be possible to cache the rest of the codegen and only update the literals.

An example of the codegen:

impl<'a> askama::Template for HelloWorld<'a> {
   // Update these separate
   const LIT_0: &'static str = "Hello, ";
   const LIT_1: &'static str = "1";

    // Cache this, invalidate when anything other than a literal is changed
    fn render_into<AskamaW>(&self, __askama_writer: &mut AskamaW) -> askama::Result<()>
    where
        AskamaW: core::fmt::Write + ?Sized,
    {
        __askama_writer.write_str(LIT_0)?;
        match (
            &((&&askama::filters::AutoEscaper::new(
                &(self.name),
                askama::filters::Html,
            ))
                .askama_auto_escape()?),
        ) {
            (expr2,) => {
                (&&askama::filters::Writable(expr2)).askama_write(__askama_writer)?;
            }
        }
        __askama_writer.write_str(LIT_1)?;
        Ok(())
    }
    const SIZE_HINT: usize = 11usize;
}

[GuillaumeGomez]

It's pretty good idea. Interested into sending a patch and see some comparison numbers in your crates?

Also, which version did you use?

[TheNuclearNexus]

I'm using a commit from a few months ago, 6f4a799e7772b7541fd273d1343f089dfb0246a0.

I'd love to make a patch but have no clue where to start 😅

[GuillaumeGomez]

The idea would be to declare same literals only once and refer to them, so for that we need to store that across proc-macro calls, we already have something similar iirc. But anyway, every time you encounter a Lit in askama_derive, instead of writing it right away, we would instead check if it already exists, if not add it into a map and then pass the name of the const where the literal would be stored.

That was the very high level explanations. Check already how we generate code for literals around and I think you might have a better idea what to do. ;)

[TheNuclearNexus]

🫡 I will give it an attempt

[TheNuclearNexus]

I got it working!
The codegen time for my largest template went from 350ms to 175ms when only a literal is changed.

I also found the source of my slow compile time, which is the template expanding to 212k lines!

[GuillaumeGomez]

So seems like it's definitely worth it! Waiting for your PR then. 😉