bug: Custom builders don't construct deferred constraints

[matthew-a-klein]

Description

Gnark allows for custom builders, allowing to build a constraint system without a circuit structure.
This inherits the frontend.API interface, allowing one to call deferred constraints such as with the lookup table interface.

Expected Behavior

When we call builder.Compile() we should construct the deferred constraints. Just like the Compile() function does for stnadard circuit construction. So if we constract a binary API, the constraints do not appear in the compiled constraint system.

Actual Behavior

All constraints attempted by the builder should be compiled in the constraint system.

Possible Fix

In the Compile function fo the r1cs and scs custom builders, we should call the equivalent of the callDeferred() function of the circuit Compile() function.

Steps to Reproduce

1. Construct a new builder as follows builder, err := r1cs.NewBuilder[E](ecc.BN254.ScalarField(), frontend.CompileConfig{ CompressThreshold: 300, })
2. Construct a binary field api as follows uapi, err := uints.New[uints.U64](builder)
3. Now uapi.AssertEq(uints.NewU64(0), uints.NewU64(1)) will not fail, because the constraints are never called

Context

Attempting to write a custom gnark circuit without having to use the circuit struct as above.

Your Environment

• gnark version used 0.14.0
• gnark-crypto version used: 0.19.0
• go version (e.g. 1.20.6): go1.24.4 darwin/arm64
• Operating System and version: macOS Sequoia 15.6.1

[ivokub]

I'm not understanding it exactly what you pass to the uints builder. Are you calling r1cs.NewBuilder in-circuit (as in inside the Define method?) - this indeed creates a completely new builder which is not associated with the main builder (the frontend.API argument to the Define method).

What is the end-goal? Do you want to change the compress threshold parameters dynamically?

[ivokub]

It would be better if you could provide some minimal example I could reproduce. Otherwise I'm afraid I don't have full context to understand the issue.

[matthew-a-klein]

func TestDefer(t *testing.T) {
	type E = constraint.U64
	builder, err := NewBuilder[E](ecc.BN254.ScalarField(), frontend.CompileConfig{
		CompressThreshold: 300,
	})
	if err != nil {
		t.Fatal(err)
	}
	uapi, err := uints.NewBinaryField[uints.U64](builder)
	if err != nil {
		t.Fatal(err)
	}
	uapi.AssertEq(uapi.And(uints.NewU64(7), uints.NewU64(2)), uints.NewU64(2))

	r1cs, err := builder.Compile()
	if err != nil {
		t.Fatal(err)
	}

	if r1cs.GetNbConstraints() < 10 {
		t.Fatalf("Deferred constraints have not been constructed")
	}

}

[matthew-a-klein]

This test fails under currrent functionality. The end goal is to write a groth16 backend for Noir using gnark. WE have been using the builder strcut to manually create the constraints without needing a circuit.

[ivokub]

Ah, I see now. So you would like to parse Noir source code, transpile it to gnark circuit and then use gnark for proving.

What you could do instead is to have the transpiler inside the circuit Define method. Or construct the R1CS directly from compiled R1CS from Noir. There are a few projects actually trying to use gnark as Noir backend, see for example https://github.com/lambdaclass/noir_backend_using_gnark.

[ivokub]

And we really cannot make it work outside of Define method of a circuit. The variables inside a circuit (the elements of type frontend.Variable) are essentially references to particular location in the witness vector, but for that we need to know what type of variable it is (public, secret, internal) and it is only deduced inside the frontend.Compile method by walking the circuit struct.