You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: diff.go
    path of file: ./repos/buildkit/client/llb
    
    the code of the file until where you have to start completion: package llb

import (
	"context"

	"github.com/moby/buildkit/solver/pb"
	digest "github.com/opencontainers/go-digest"
)

type DiffOp struct {
	MarshalCache
	lower       Output
	upper       Output
	output      Output
	constraints Constraints
}

func NewDiff(lower, upper State, c Constraints) *DiffOp {
	addCap(&c, pb.CapDiffOp)
	op := &DiffOp{
		lower:       lower.Output(),
		upper:       upper.Output(),
		constraints: c,
	}
	op.output = &output{vertex: op}
	return op
}

func (m *DiffOp) Validate(ctx context.Context, constraints *Constraints) error {
	return nil
}

func (m *DiffOp) Marshal(ctx context.Context, constraints *Constraints) (digest.Digest, []byte, *pb.OpMetadata, []*SourceLocation, error) {
	if m.Cached(constraints) {
		return m.Load()
	}
	if err := m.Validate(ctx, constraints); err != nil {
		return "", nil, nil, nil, err
	}

	proto, md := MarshalConstraints(constraints, &m.constraints)
	proto.Platform = nil // diff op is not platform specific

	op := &pb.DiffOp{}

	op.Lower = &pb.LowerDiffInput{Input: pb.InputIndex(len(proto.Inputs))}
	if m.lower == nil {
		op.Lower.Inpu
    
func (m *DiffOp) Marshal(ctx context.Context, constraints *Constraints) (digest.Digest, []byte, *pb.OpMetadata, []*SourceLocation, error) {
	if m.Cached(constraints) {
		return m.Load()
	}
	if err := m.Validate(ctx, constraints); err != nil {
		return "", nil, nil, nil, err
	}

	proto, md := MarshalConstraints(constraints, &m.constraints)
	proto.Platform = nil // diff op is not platform specific

	op := &pb.DiffOp{}

	op.Lower = &pb.LowerDiffInput{Input: pb.InputIndex(len(proto.Inputs))}
	if m.lower == nil {
		op.Lower.Input = pb.Empty
	} else {
		pbLowerInput, err := m.lower.ToInput(ctx, constraints)
		if err != nil {
			return "", nil, nil, nil, err
		}
		proto.Inputs = append(proto.Inputs, pbLowerInput)
	}

	op.Upper = &pb.UpperDiffInput{Input: pb.InputIndex(len(proto.Inputs))}
	if m.upper == nil {
		op.Upper.Input = pb.Empty
	} else {
		pbUpperInput, err := m.upper.ToInput(ctx, constraints)
		if err != nil {
			return "", nil, nil, nil, err
		}
		proto.Inputs = append(proto.Inputs, pbUpperInput)
	}

	proto.Op = &pb.Op_Diff{Diff: op}

	dt, err := proto.Marshal()
	if err != nil {
		return "", nil, nil, nil, err
	}

	m.Store(dt, md, m.constraints.SourceLocations, constraints)
	return m.Load()
}