Also, for a toy rust project, I wrote an implementation of a copy-on-write B+tree and I used a fuzzer (honggfuzz) to generate automatically all the test cases.

Ii was insanely effective, there were so much more edge cases than I thought. The fuzzer found all of them and got me a 100% code coverage in no time.

All I had to write as test code was a function mapping an array of random data to a series of btree instructions, apply those to both my implementation and a reference and then check that the two structures had the same data.