Comment by chlorion

It also supports running in freestanding setups without an OS, and quite a lot of the language's features still work.

I was extremely surprised by how much functionality is packed into "core", and runs without an OS when using freestanding rust. Even stuff that requires an allocator can run provided you provide your own heap!

A cool example of this was implementing fmt::Write for a memory mapped uart console thing. Then implementing a kprintln! macro that supports all of the formatting machinery that you are use to. This worked without even a heap available.