I don't get your comment. The GC is part of the runtime, to it
user code is data.
But the JIT compiler and other internal details of the runtime are its code, and there are very real cross-cutting concerns, like the JIT compilers output should take into account what memory representation the GC expects, where are barriers, when one is run etc. So I'm talking about a project such as the JVM.
> have millions of lines of code you have to trust that your code works in isolation without knowing the details.
More like hope. This is where good design and architecture helps, as well as strong invariants held up by the language. But given that most applications can't really escape global state (not even internally, let alone external state like the file system), you can never really know that your code will work the way you expect it to - that is, it's not trivially composable to any depth.
