Function SCIPexpriterGetExprUserData 

pub unsafe extern "C" fn SCIPexpriterGetExprUserData(
    iterator: *mut SCIP_EXPRITER,
    expr: *mut SCIP_EXPR,
) -> SCIP_EXPRITER_USERDATA

gives the iterator specific user data of a given expression

@note The expression iterator mode must be DFS or another mode with allowrevisit=FALSE