Constants

Developers write constants in their code all the time. For example, the following code has two constants 2 and 4:

#![allow(unused)]
fn main() {
let x = 2 + y;
assert_eq(x, 4);
}

It is important that constants are tracked differently than CellVars for several reasons:

• It is sometimes useless to constrain them directly. For example, in let x = 3 + 7; you can see that we should not constrain 3 and 7 separately, but rather the result 10.
• It is sometimes useless to constrain them at all. For example, boolean constants are never constrained because you never need to.
• They can be cached to avoid creating several constraints for the same constant.

Currently a constant appears in the circuit only when CircuitWriter::add_constant is called. It uses the generic gate to constrain the value, and is not cached (so calling it several times with the same constant will create multiple constraints):

#![allow(unused)]
fn main() {
pub fn add_constant(&mut self, value: Field, span: Span) -> CellVar {
    let var = self.new_internal_var(Value::Constant(value), span);

    let zero = Field::zero();
    self.add_gate(
        GateKind::DoubleGeneric,
        vec![Some(var)],
        vec![Field::one(), zero, zero, zero, value.neg()],
        span,
    );

    var
}
}

Note that the Value keep track of the constant as well.

Warning: gadgets must all handle constants gracefully. That is, they must constrain constants themselves (by calling CircuitWriter::add_constant).