OP is using unsafe to store a mut pointer to a boxed value so that it can be mutated in two places. So the statement is just referring to data races.
(I tried to run miri on it, but I'm on Windows, and this is unix-only.)
EDIT: even with porting some of the code, I can’t get the current repository to build, there’s a borrow checker error...)
$ rustc --version
rustc 1.32.0 (9fda7c223 2019-01-16)
$ cargo --version
cargo 1.32.0 (8610973aa 2019-01-02)
$ cargo build --release
$ cargo run --release --bin nes-emulator
`1. I can send you a patch later, but if you use std::io::stdin() and stdout, instead of opening the file descriptors, it should compile on Windows.
2. The borrow error was coming from miri; it detected some UB! Compiling with cargo does work. It’s not the bit I expected; I can file a bug with the error once I’m back at my laptop.
Anyway, this is a cool project, and I’m glad you’re doing it; don’t let my worries bug you. I’ve been trying to learn more unsafe stuff and so the UB bits are just on my mind more lately.
EDIT: actually it's just from building on nightly rust, not stable. No miri needed. I wonder why stable is okay with this... anyway I filed some github issues, please feel free to ignore me or close them, but if you wanna keep talking about this, let's do that there :)