Function substitute 

pub fn substitute(
    resource: &str,
    path: &Utf8Path,
    committed: &str,
    applied: &[AppliedReplacement],
) -> Result<String>

Apply already-resolved replacements as one simultaneous batch, enforcing each rule’s declared match count against the input.

Also the recomputation behind drift detection: the expected on-disk content of a rendered file is exactly this, run over the same committed content with the replacements the binding record remembers.