First, I was talking about useful definitions of thread safety. That was language-agnostic, not related to Rust. And if absence of data races were all that Rust guaranteed, then Rust indeed wouldn't be much of a help. What Rust actually does is require you to be (fairly) explicit about threads interacting with each other, which is a much stronger type of guarantee [1]. Rust comes with a default setting derived from the general shared-xor-mutable principle that threads don't mess with each other's data without permission, which makes it easier to reason about thread interaction.
Second, the OP was comparing Rust to actor languages. Actor languages also don't have data races and don't require mutexes (because the memory spaces of actors are disjoint and actors themselves are sequential). In fact, actor languages provide even stronger guarantees, as actors can only interact with each other at well-defined points. "Fearless concurrency" hails back to the 1970s. It is not a new invention, nor is it unique to Rust. It is great that Rust supports it, but I really wish people would stop talking about it being novel and unique.
[1] Which is not to say that Rust's choices don't come with tradeoffs, but then, in the area of concurrency, you cannot realistically avoid tradeoffs. In Rust's case, as with actors, the primary tradeoff is performance for safety.
What I really wish people would learn from Actors is that even if your programming language prevents trivial race conditions over shared memory or message passing primitives, none of that guarantees anything about the behavior of the system you are building once you start to compose those primitives. Gul Agha's dissertation was really eye-opening.
Languages like C++ and D treat this as a library design challenge, not a core language feature. Look at std::lock_guard, for example.
The thread safety is built using Send/Sync traits, which libraries for threading and synchronization primitives, atomics, etc. implement accordingly.
Theoretically yeah, but there's a lot of practical gain to be had even with that example. Other languages won't tell you if you forgot to use the mutex somewhere.