Attacking China means condemning all the countries that depend on it for serious economic impact, turmoil, or even disaster. The list can even include the likes of
Japan, India, South Korea, as where else will they get a chance to make money off of selling their products to a billion Chinese consumers' increasing purchasing power, or, to get relatively cheap goods from for their own country's needs? Not to mention Russia with their new modernization drive & need for investment/development, i.e. the recent upping of bilateral ties & cooperation established between Russia & China. So, if US brazenly engages in nuclear conflict with China, it is almost like an indirect attack on all the countries that benefit from China, including Russia (the country with the largest nuclear stockpile I believe?). Plus, how is Russia going to tolerate additional presence of American troops in China right below it? They don't even like them in the CARs.
Russia will know very well if it lets China fall, then itself will be next in line in the eyes of US, which it knows all too well from the past experience of Cold War era. Sans
China, Russia will be alone, and much more easily made to fall. So Russia is really not going to take this laying down lightly. In fact, if things do start to escalate a bit too much, I suspect there will be attempts made by Russia to ensure reducing the likelihood of conflict between US & China for reconciliation, or to simply ward off the US from attacking China.
So, if that all doesn't work out, then it will most certainly be in Russian interest, as China is nuking away 1/3 of the US population, to nuke away 99% of the remaining 2/3 so as not to be under severe US threat later on.
I guess in the meantime China had better be upping her nuclear arsenal from 1000 to 10,000.