Stardew Valley is built on Monogame. Also Chucklefish is the publisher, not the developer...

Their new game is in Rust, not Stardew Valley. And you’re right that they were only the publishers for it.

Is there anything approaching the XNA/Monogame content pipeline in the Rust world? Something like Stardew Valley is hugely data-driven, and being able to leverage that tooling, not to mention solid cross-platform support, looks like a huge force multiplier.

I don’t think so, but it’s not an area I’m always the most up to date with. I’m guessing if it existed I’d have heard of it, though.

“Amethyst” and “piston” are the two engine projects. I’m not sure if studios are using them or I’d they're building something in-house.

We don’t have a white paper but Ready At Dawn is moving all new stuff to 100% Rust as well, excited to see where it goes.

What GUI library would you even use in Rust? Is the package ecosystem mature enough to support a game?

Most games that are writing their own engines aren't using traditional GUI libraries anyway, it will typically be something custom.

Well that's unfortunate. I was hoping Rust could get a graphics library out of that.

There's a all-Rust graphics library in the works, but projects like ggez are using a SDL wrapper for now which works OK.