bug: R1CS.ReadFrom() panics in cbor code

[Scratch-net]

Description

Reading serialized R1CS panics

Expected Behavior

Serialization and deserialization should work as expected

Actual Behavior

panic: reflect: reflect.Value.Set using unaddressable value

Possible Fix

N/A

Steps to Reproduce

import (
	"github.com/consensys/gnark-crypto/ecc"
	"github.com/consensys/gnark/backend/groth16"
	cs "github.com/consensys/gnark/constraint/bn254"
	"github.com/consensys/gnark/frontend"
	"github.com/consensys/gnark/frontend/cs/r1cs"
	"github.com/consensys/gnark/std/math/uints"
)


type Circuit struct {
	Val  uints.U32
	Val1 uints.U32
}

func (c *Circuit) Define(api frontend.API) error {
	uapi, err := uints.New[uints.U32](api)
	if err != nil {
		return err
	}
	uapi.AssertEq(c.Val, c.Val1)
	return nil
}

func main() {
	witness := Circuit{
		Val:  uints.NewU32(10),
		Val1: uints.NewU32(10),
	}

	curve := ecc.BN254.ScalarField()
	r1css, err := frontend.Compile(curve, r1cs.NewBuilder, &witness)
	if err != nil {
		panic(err)
	}

	buf := &bytes.Buffer{}
	r1css.WriteTo(buf)

	r1css = cs.NewR1CS(0)
	r1css.ReadFrom(bytes.NewBuffer(buf.Bytes()))
	fmt.Println(r1css.GetNbConstraints())
}

Context

I was trying to load compiled circuit from file

Your Environment

• gnark v0.9.1
• gnark-crypto v0.12.2
• go1.21.0

[gbotrel]

Hi -- replacing

r1css = cs.NewR1CS(0)

by

// import "github.com/consensys/gnark/backend/groth16"
r1css = groth16.NewCS(ecc.BN254)

Will work. The cs.NewR1CS is indeed confusing now that it is exposed and not documented...