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What about side-effects that cannot easily be contained, e.g. HTTP requests sent from your code, DB writes?

I don't think the goal can be achieved pragmatically for many types of modern applications generally. Maybe offering some sort of debug snapshotting and mocking layer for things like that and libraries to implement might be an idea. The debugger should then also be able to identify for which parts preview execution can be done and for which ones it's not possible to contain side-effects.