Comment by raphlinus

Jake has already mentioned how the type system enforces single-threading (using similar mechanisms as Rc in Rust), but it's not necessarily the case that read-write races from multiple threads are UB.

In particular, Java allows aggressive multithreaded access, and the memory model has some pretty strong guarantees. Informally stated, if a read and a write race, then the read is guaranteed to observe either the old or new value.

Go is something of a middle ground [1]. Races on simple scalar values are not UB, but "fat pointers" (slices and interfaces) can tear and can lead to "arbitrary memory corruption."

When I was reading about "stable shape" I was wondering if it might be something similar. You easily get the same UB problems when dealing with sum types, as a tear across tag and variant payload can cause all kinds of things to go wrong.

[1]: https://go.dev/ref/mem