Function render_to_string 

pub fn render_to_string(
    template_path: &Utf8Path,
    source: &Utf8Path,
    config: &Config,
    yui: &YuiVars,
) -> Result<Option<String>>

Render a single template and return the body it would produce right now, or Ok(None) if {# yui:when … #} / a config rule when would skip it for the current host. Used by yui diff to compute what the rendered file should contain so the drift can be diffed against what’s actually on disk.

Mirrors process_template’s skip / render decisions exactly, but doesn’t touch the report or the on-disk rendered output.