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.