Function fmt_ipopt 

pub fn fmt_ipopt(v: f64) -> String

Format a number in Ipopt’s scientific notation: 16-digit mantissa, signed 2-digit exponent (e.g. 3.7952009505566139e+03). Rust’s {:.16e} is close but emits a 1-digit exponent without leading sign, which makes side-by-side diffs against ipopt output messy.