warn on unconstrained values

[David Wong]

the compiler should warn us that yy is not constrained to anything and that it's useless:

fn main(private_input: Field, x2: Field, pub public_input: Field) {
    let xx = private_input + public_input;
    assert_eq(xx, 2);
    let yy = x2 + 1;
}

to run:

cargo run test --backend kimchi-vesta --path examples/arithmetic.no --private-inputs '{"private_input": "1", "x2": "3"}' --public-inputs '{"public_input": "1"}' --debug

[David Wong]

there's different ways to tackle that...

• frontend: not sure :]
• r1cs backend: If we tracked all the linear combinations created in a map in the R1CS backend then we could see that some are not used in constraints
• kimchi backend: I'm not sure