> it is not an inherent feature of the language, just the implementation

So what is it now? If the implementation is correct in allowing you to cause UB with data races, then this is very much a feature of the language, making the language not memory-safe. Alternatively, the implementation is buggy here, in which case there should be a bug report somewhere.

Is there such a bug report (github issue)? Is there a line in the spec saying this is the intended behavior?