> and I really don't like how the author is so stubbornly in favour of unused-variables-as-errors

FWIW, they also have a goal to emit as much output as possible, even in the face of compilation errors. They have stated that even syntax errors should have the compiler exit with a non-zero exit code, but still produce an executable that will give you a syntax error at runtime. The point of this being to allow you to iterate quickly, but force things like CI to fail.