There is no can of worms.

Hosting US assets actively being used in war vs Iran = being active co-belligerents. Host countries no longer neutral when they don't adhere to duty of abstention (Hague Convention V). This not even Iran using deniable proxies, this is Qatar allowing sovereign territory to facilitate attack on Iran, which unambiguously makes them legitimate target. Ditto with Diego Garcia.

In the same way railway in RU already legitimate target for UKR because in RU soil. If EU sending out sorties from NATO bases to hit RU then they too would be active belligerents. There's no compartmentalizing using territory to shoot someone else.

> The thing is, if we accept the norms that Qatar can be targeted for kinetic action by Iran for hosting US assets or by the US for hosting Iranian assets, then that opens a MASSIVE can of worms.

None of your examples are actually analogous, they are all more distant support than hosting a base from which direct attacks are carried out except for the first one in which the "can of worms" is justifying attacks on a state that it is already a direct belligerent (and in fact the aggressor) because of third-party support, which, on the other hand, is not analogous for the opposite reason—it is very much not necessary to invoke any third-party action to justify that. The direct belligerence already justifies that.

There’s no “precedent” needed, Russia and Ukraine are simply choosing not to do certain things to avoid widening the war in the ways you mention, because they don’t think that would be to their advantage. The precedent is there already, it’s not like either country is looking at Iran and going “oh wow, I didn’t know that was an option!”