except for intel, if they publish how their hardware and microcode works internally? aka, opensourcing their internal design?
Of course, they can't since it will allow competitors to copy it, but would that work theoretically?
Even if they released absolutely everything, there's no way to verify that the chips they actually make and sell conform to a design that they release without inspecting the actual chip. If you're really paranoid, you'd have to inspect every chip, and that's usually a destructive operation.
And here's the million dollar idea, to verify you'd need to destructively inspect your chips at EOL to verify you haven't been screwed over. Anyone wants to start a business?
it only protects against backdoor injection by the fab (or the company that produces your masks)
And there are other solutions such as logic-locking.
The idea of logic-locking is to add XOR gates (or a more complex type of gate) to the circuit on well-chosen logic paths. To make the circuit behave correctly, it's required to know the value to be sent to each inserted XOR. These values may be generated by an RNG circuit that is seeded by a secret key.
At manufacturing time the key is kept secret, so it's not possible for the fab to reverse engineer your circuit logic to introduce a backdoor.
Once production is complete, the key is loaded into circuits for sale