Function Ei 

pub fn Ei(x: NonZero<Finite<f64>>) -> Result<Approx, Error>

Original C code

int gsl_sf_expint_Ei_e(const double x, gsl_sf_result * result)
{
  /* CHECK_POINTER(result) */

  {
    int status = gsl_sf_expint_E1_e(-x, result);
    result->val = -result->val;
    return status;
  }
}

Errors

If x is so large that floating-point operations will fail down the line (absolute value of just over 710).