Until the code becomes impossible to comprehend as a result.
Let's unpack that. You advocate using non-Turing complete languages for things that don't need it. Great. Something like DTrace does not need a Turing complete language and D isn't Turing complete. Awesome.
But then you say that if you're using a Turing complete language, what's wrong if the type system is Turing complete as well?
Well, does a Turing complete language, C or Rust, need or in any way benefit from a Turing complete type system? Does a low level systems language, C or Rust, need or benefit from a Turing complete language?
Other than an implementation pun, I don't see any point in this needless complexity. The point of a type system is safety. I don't see how a Turing complete type system gets you to heaven.
Unless I'm missing something, having Turing completeness in a type system doesn't mean it's less safe; it just means the type checker might not terminate. In which case, of course, the code will not compile and thus cannot do anything unsafe (of course the type checker itself might, say, allocate infinite memory and crash your machine... But that's a different story)
Do you believe in unit tests? Ultimately, this is using he taget language to type check itself. If you had a well integrated Turing complete type system (c++ does not), then you could start enforcing some of these at the compilation stage
Methinks you are defending pointless complexity. How did we ever write unit tests before this awesomeness was sprung upon us?