Function dopadding 

pub fn dopadding(
    s: &str,
    prenum: usize,
    postnum: usize,
    preone: Option<&str>,
    postone: Option<&str>,
    premul: Option<&str>,
    postmul: Option<&str>,
) -> String

Pad string to specified width (from subst.c dopadding lines 892-1332)