Function string_width 

pub fn string_width(
    font: ObjRef,
    codes: &[u8],
    r: &impl Resolve,
    limits: &Limits,
    diags: &mut Diagnostics,
) -> f64

The advance of codes in the font font names, in thousandths of an em.

Falls back to half an em a code — the proportions of a typical Latin face — when the dictionary carries no usable metrics.