Depends on the scale and confidence you want.
In systems where I've taken complex inputs and transformed it into complex outputs, I'd make it as easy to bring each particular pass under test, have snapshots that the same transformations resulted in the same outputs (insofar as I did want that consistency).
I'd think with a compiler, though, it can still be practical to describe the input you depend on as a value.
In distributed systems, your service might rely on another service which might be broken, might return different values than you expect, might return slower than you expect, etc. and so testing the parts of your system which depend on other services can be tricky.
Point is, you can arrange a lot of your code into "it's easy for me to ensure this part works", and you can see wins from that, even if as a whole the full system can be difficult to bring under test.
In systems where I've taken complex inputs and transformed it into complex outputs, I'd make it as easy to bring each particular pass under test, have snapshots that the same transformations resulted in the same outputs (insofar as I did want that consistency).
I'd think with a compiler, though, it can still be practical to describe the input you depend on as a value.
In distributed systems, your service might rely on another service which might be broken, might return different values than you expect, might return slower than you expect, etc. and so testing the parts of your system which depend on other services can be tricky.
Point is, you can arrange a lot of your code into "it's easy for me to ensure this part works", and you can see wins from that, even if as a whole the full system can be difficult to bring under test.