[9rx]

Ideally, but if a project wasn't written with tests at the time then finding a working time machine can be a challenge. If you try to add them later you won't capture all the nuance that went into the original program. After all, if the implementation code was expressive enough to capture that nuance, you'd already have your test suite, so to speak. Tests are written to fill in the details that the rest of the code isn't able to express.

[giancarlostoro]

Tests are written for various goals: integration testing, to prevent regressions, and in the same effort to prevent regressions to protect mission critical / business logic code. If all those nuances are captured by good tests, you arguably have "100%" test coverage, you don't need to test every single line of code ever written to have 100% test coverage in my eyes. But then when you go to translate your project to a new language, you port the tests first, then test against those tests.

This is my personal belief on this anyway.

[squeefers]

he has no answer for this