Given how much memory and CPU time is burned compiling Rust projects I'm guessing it is pretty complex.

I'm not deep enough into the Rust ecosystem to have solid opinions on the rest of that, but I know from the specification alone that it has a lot of work to do every time you execute rustc. I would hope that the strict implementation would reduce the number of edge cases the compiler has to deal with, but the sheer volume of the specification works against efforts to simplify.