The question is not about actual software locks. The question is "What prevents me from making changes to this variable without breaking everything". So the p

The question is not about actual software locks. The question is "What prevents me from making changes to this variable without breaking everything". So the point is not "What locks guard this data" the point is, how do I determine whether I can currently modify (or even read) this data. This allows the compiler to check that, indeed, you never use the data in a way that would break things. Hence your compiler can then give guarantees about thread-safety. Now it seems likely that, when compilers do this and you write wrong code, your conclusion might be "I need a mutex lock for this", or if performance matters, "I need to rewrite this to allow performant lockless code. But the main question remains "Can I currently modify this data without issues".