This is not real source code. The extension here is only for minimal highlighting.

Context.rs

Context :: struct
{
    allocator                 : Allocator;
    assertion_failure_handler : Assertion_Failure_Handler;
    logger                    : Logger;
}

// 'context' is a reserved keyword.
// It refers to the currently bound context.
// 'action' doesn't need to know the context type.
// What to do in case it's called without the required context? compilation error! -_-
action :: procedure ()
{
    print("Mode: {}\n", context.mode);
}

ActionContext :: struct
{
    using base: Context;
    mode: String;
}

main :: ()
{
    action_context : ActionContext = { ...context, "Fast" };

    using action_context
    {
        return action();
    }
}