Function IpoptSetNonlinearVariables 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn IpoptSetNonlinearVariables(
    ipopt_problem: IpoptProblem,
    num_nonlin_vars: Index,
    pos_nonlin_vars: *const Index,
) -> Bool

Declare which variables enter the problem nonlinearly (gh#624).

This is the C-API face of Ipopt’s TNLP::get_number_of_nonlinear_variables / get_list_of_nonlinear_variables pair, which upstream exposes only to C++ callers. It exists so a frontend that already knows the structure of its model — CasADi’s pass_nonlinear_variables, an algebraic modeling language, a hand-written driver — can hand that knowledge to pounce.

Effect is confined to the limited-memory Hessian: curvature is approximated over the declared subset only, and the Hessian is exactly zero for every other variable. Exact-Hessian solves ignore the declaration entirely, and so does any solve that never calls this function — the default remains “all variables are nonlinear”. The subset takes precedence over the num_linear_variables option, matching Ipopt’s own ordering.

pos_nonlin_vars holds num_nonlin_vars variable indices in the problem’s index style (the index_style passed to CreateIpoptProblem). The subset may be arbitrary and noncontiguous; order does not matter. Passing num_nonlin_vars == n is equivalent to not calling this at all.

Returns FALSE (leaving any previous declaration untouched) on a NULL problem, a negative or oversized count, a NULL array with a positive count, or an index outside the problem’s variable range.

Note the deliberate signature choice: the issue that requested this suggested a const Bool* mask, but Bool in the Ipopt C API is a C99 bool, and a array of those would be a per-element data-layout contract that is easy to get wrong from a caller with a different boolean width. The count-plus-index-list shape is the one the TNLP callbacks already use.

Safety

ipopt_problem must be a valid IpoptProblem or NULL. pos_nonlin_vars, when non-NULL, must point at num_nonlin_vars readable ipindex values.