Function ggez::timer::check_update_time

pub fn check_update_time(ctx: &mut Context, target_fps: u32) -> bool

This function will return true if the time since the last update() call has been equal to or greater to the update FPS indicated by the target_fps. It keeps track of fractional frames, so if you want 60 fps (16.67 ms/frame) and the game stutters so that there is 40 ms between update() calls, this will return true twice, and take the remaining 6.67 ms into account in the next frame.

The intention is to for it to be called in a while loop in your update() callback:

fn update(&mut self, ctx: &mut Context) -> GameResult<()>
    while(timer::check_update_time(ctx, 60)) {
        update_game_physics()?;
    }
    Ok(())
}