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> this is very different from Rust, which requires unsafe even for things like benign write/write races, which are fairly common, and various basic data structures

I know this paper [0] is quite old at this point, but the mention of benign data races reminded me of it. Would you happen to know how applicable it is to modern memory models?

[0]: https://www.usenix.org/legacy/event/hotpar11/tech/final_file...

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Benign write/write races (when multiple threads do unordered writes of the same value to the same address) are quite common and useful, both in parallel algorithms and in lazy initialisation. Useful benign read/write races are far more rare to the point I'd say it's ok to assume they don't (or shouldn't) exist.

However, in C and C++ (and Rust) benign non-atomic write/write races are UB (indeed, LLVM also treats them as potential causes of UB). In C# and in Java they are safe (although Java currently only has non-atomic writes on 32-bit machines, but soon they'll be more common when value types are enhanced). LLVM even has a specific construct to support the Java-style memory model (https://llvm.org/docs/Atomics.html#unordered), and Zig lets you use it (https://ziglang.org/documentation/master/#atomicStore).

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