Working with C in Rust

It's really common to have some functionality in C that you want to use from Rust. It's quite common for this to be part of an effort to port the code over to Rust using a pattern like this:

1. Get the C linked in and working.
2. Write some unit tests to ensure that the C is doing what you think it's doing.
3. Write a Rust version, possibly line-by-line porting.
4. Run the same unit tests on the Rust version.
5. Stop using C, announce to the Ministry of Defence that your code is now safe, and roll around in a bathtub of grant money.

So for this section, we'll act like we're porting some simple C to Rust - and get a feel for the process, and some of the pitfalls you might not have expected.