Fair point on medium - this was a lazy example.
Substitutibility probably doesn't apply here in the way you're implying and if it did it would likely be hampered by the 9th circuits findings about transformation in sony v connectix. Arguments here likely would look at rust not having a stable ABI, and hence not being inherently substitutable as a libray (grit-lib), less clear as an executable (grit-cli) on that side
basics of copyright law - the fundamental thing being protected is the expression... is a rust program's expression the same expression as a c program? I'd say generally not.
The test suite could test aspects of the architecture/design of the codebase that are not necessary for interoperability and constitute novel expression of a piece of software in a way that is not at all language specific.
By definition a test suite is about testing interoperability with the test suite. An HTTP test suite should likely test for whether response code 418 is implemented a particular way and while humorous it would still be an interop test no?
No, the git test suite is about testing the git codebase. If you want something like that, you need a conformance suite, which does not exist for git.