> I would assume trivial means the default approach works for most cases.

I mean, it does. I'm not sure what you consider the default approach, but to me it would be to wrap the data in a Mutex struct so that any thread can access it safely. That works great for most cases.

> Perhaps mutable global variables are not a common use case.

I'm not sure how common they are in practice, though I would certainly argue that they shouldn't be common. Global mutable variables have been well known to be a common source of bugs for decades.

> Unsafe might make it easier, but it’s not obvious and probably undesired.

All rust is doing is forcing you to acknowledge the trade-offs involved. If you want safety, you need to use a synchronization mechanism to guard the data (and the language provides several). If you are ok with the risk, then use unsafe. Unsafe isn't some kind of poison that makes your program crash, and all rust programs use unsafe to some extent (because the stdlib is full of it, by necessity). The only difference between rust and C is that rust tells you right up front "hey this might bite you in the ass" and makes you acknowledge that. It doesn't make that global variable any more risky than it would've been in any other language.