unit tests can be co-located with code

Generally test suites live in separate files from the code they test. Historically, this was advantageous from the perspective of compiler development. Files that compile file individually and then link the resulting files together because it allows excluding test code from release executables. However, it would be advantageous to co-locate tests with code because:

tests act as documentation, making the code easier to understand
it makes relocating top level functions easier, there is no need to also move the test
it makes tests in context for LLMs by default

In order to make this feasible it is necessary to either:

use conditional compilation¹
link-time dead code elimination
accept the fact that your executable will be a little larger.

I do this in wiki-language-server and I like it quite a bit.

conditional compilation makes linting/compiling code in an ide environment difficult