Function render_diff 

pub fn render_diff(
    headline: Option<&str>,
    actual: &VmValue,
    expected: &VmValue,
) -> Option<String>

The addressed, per-leaf diff naming what differs between the two values and where. None when they are equal.

headline is the caller’s framing (e.g. Some("assert_eq failed")), or None to render the diff on its own for a caller that supplies its own context.