If you’d like to include custom liquid tags in your site, you can do so by hooking into the tagging system. Built-in examples added by Rustyll include the highlight and include tags. Below is an example of a custom liquid tag that will output the time the page was rendered:

use rustyll::plugin::{Plugin, PluginTag};
use liquid_core::{Runtime, Value, Object, Result};

pub struct RenderTimeTag;

impl PluginTag for RenderTimeTag {
    fn tag_name(&self) -> &str {
        "render_time"
    }

    fn parse(&self, parser: &mut liquid::Parser) -> Result<Box<dyn Fn(&Runtime) -> Result<Value>>> {
        let text = parser.parse_markup()?.into_text();
        Ok(Box::new(move |_runtime| {
            let now = chrono::Local::now();
            Ok(Value::scalar(format!("{} {}", text, now)))
        }))
    }
}

// Register the tag with Rustyll
impl Plugin for RenderTimeTag {
    fn register(&self, registry: &mut liquid::ParserBuilder) {
        registry.register_tag(self.tag_name(), Box::new(self.clone()));
    }
}

At a minimum, liquid tags must implement:

Method	Description
tag_name	Returns the name of the tag.
parse	Parses the tag and outputs a function that renders the content.

You must also register the custom tag with Rustyll’s plugin system:

// In your plugin's main.rs or lib.rs
pub fn register_plugins() -> Vec<Box<dyn Plugin>> {
    vec![Box::new(RenderTimeTag)]
}

In the example above, we can place the following tag anywhere in one of our pages:

<p>{% render_time page rendered at: %}</p>

And we would get something like this on the page:

<p>page rendered at: Tue June 22 23:38:47 –0500 2010</p>

Tag Blocks

The render_time tag seen above can also be rewritten as a tag block:

use rustyll::plugin::{Plugin, BlockTag};
use liquid_core::{Runtime, Value, Object, Result};

pub struct RenderTimeTagBlock;

impl BlockTag for RenderTimeTagBlock {
    fn tag_name(&self) -> &str {
        "render_time"
    }

    fn parse(&self, parser: &mut liquid::Parser) -> Result<Box<dyn Fn(&Runtime) -> Result<Value>>> {
        let inner_template = parser.parse_until(&["endrender_time"])?;
        parser.expect_tag("endrender_time")?;
        
        Ok(Box::new(move |runtime| {
            let inner_content = inner_template.render(runtime)?;
            let now = chrono::Local::now();
            Ok(Value::scalar(format!("<p>{} {}</p>", inner_content, now)))
        }))
    }
}

// Register the block tag with Rustyll
impl Plugin for RenderTimeTagBlock {
    fn register(&self, registry: &mut liquid::ParserBuilder) {
        registry.register_block(self.tag_name(), Box::new(self.clone()));
    }
}

We can now use the tag block anywhere:

{% render_time %}
page rendered at:
{% endrender_time %}

And we would still get the same output as above on the page:

<p>page rendered at: Tue June 22 23:38:47 –0500 2010</p>

In the above example, the tag block and the tag are both registered with the name render_time, but to register a tag and a tag block using the same name in the same project is not recommended as this may lead to conflicts.