The point, which he outlines in the post, is to be able to mix high level programming with lower level languages. This is already a common pattern in the industry where you write your application in python/ruby and speed up the tight loops with C/C++/Rust. The lingua franca of the interop is C. Which means that the boundary between the two is very stark and unsafe. You can't use advanced typesystems or seamlessly share datatypes. I'm working on something similar these days (C++ and Rust though, no high level language). You lose all template/generics info at the C bindings boundary. Bindgen programs let you get it back to some degree, but forcing monomorphization/etc are all not easy problems to solve without making the compilers talk to each other (also not easy to do).

Will's idea seems to be to use Rust as a target for a high level GCd language, which can then (mostly) seamlessly interop with "pure" Rust code, and the interop won't have an unsafe boundary.