entity.id, a unique, persistent ID

[mk-fg]

Bit of a necro-post, but as this topic comes up in google with no apparent solutions, here's one "entity.id"-like value provided by factorio:
entity.unit_number - https://lua-api.factorio.com/latest/Lua ... nit_number

Afaik every valid EntityWithOwner and its children have it, which in practice means that things like trees, rocks, ore, don't have that unit_number.
Hence it's good enough for wide range of purposes where tracking of any kind of player buildings or units it needed.