I know that there are libraries and a couple operating systems and end-user applications written in Rust, but what is remaining to get a system that runs entirely on components written in Rust?
https://www.redox-os.org/ is probably the most ambitious and further along total desktop replacement, written in rust, from the ground up. It boots, has a basic windowing system, and some pretty good fundamentals. Once you have a capable Servo browser running in Redox, you are getting close to an end-use environment. Give it a couple of years. 
Note that Servo currently relies on a lot of C++ code. For example, the javascript engine, Firefoxes Spidermonkey, is written C++.
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We have an OS and a browser written in rust, but there is no database written in rust yet.