Earlier quoted context omitted.
Thats kinda the problem, there are concepts in rust that don't have equivalents in other common languages. In this case, rust's type system models data-race-safety: it prevents data races at compile time in a way unlike what you can do in c or c++. It will prevent getting mutable access (with a compile time error) to a value across threads unless that access is syncronized (atomics, locks, etc)
And from what I can see, rustlang mutability is also a type system construct? I.e. it assumes that all other code is Rust for the purpose of those checks?
Yes
> I.e. it assumes that all other code is Rust for the purpose of those checks?
Not exactly, it merely assumes that you upheld the documented invariants when you wrote code to call/be-called-from other languages. For example that if I have a `extern "C" fn foo(x: &mut i32)` that
- x points to a properly aligned properly allocated i32 (not to null, not to the middle of un-unallocated page somewhere)
- The only way that memory will be accessed for the duration of the call to `foo` is via `x`. Which is to say that other parts of the system won't be writing to `x` or making assumptions about what value is stored in its memory until the function call returns (rust is, in principle, permitted to store some temporary value in `x`s memory even if the code never touches x beyond being passed it. So long as when `foo` returns the memory contains what it is supposed to). Note that this implies that a pointer to the same memory isn't also being passed to rust some other way (e.g. through a static which doesn't have a locked lock around it)
- foo will be called via the standard "C" calling convention (on x86_64 linux this for instance means that the stack pointer must be 2-byte aligned. Which is the type of constraint that is very easy to violate from assembly and next to impossible to violate from C code).
That it's up to the programmer to verify the invariants is why FFI code is considered "unsafe" in rust - programmer error can result in unsoundness. But if you, the programmer, are confident you have upheld the invariants you still get the guarantees about the broader system.
Rust is generally all about local reasoning. It doesn't actually care very much what the rest of the system is, so long as it called us following the agreed upon contract. It just has a much more explicit definition of what that contract is then C.