Function IpoptClearNonlinearVariables 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn IpoptClearNonlinearVariables(
    ipopt_problem: IpoptProblem,
) -> Bool

Drop a subset declared by IpoptSetNonlinearVariables, restoring the default (every variable treated as nonlinear).

Safety

ipopt_problem must be a valid IpoptProblem or NULL.