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: exec.go
    path of file: ./repos/buildkit/client/llb
    
    the code of the file until where you have to start completion: package llb

import (
	"context"
	_ "crypto/sha256" // for opencontainers/go-digest
	"fmt"
	"net"
	"sort"

	"github.com/moby/buildkit/solver/pb"
	"github.com/moby/buildkit/util/system"
	digest "github.com/opencontainers/go
    
func (e *ExecOp) AddMount(target string, source Output, opt ...MountOption) Output {
	m := &mount{
		target: target,
		source: source,
	}
	for _, o := range opt {
		o(m)
	}
	e.mounts = append(e.mounts, m)
	if m.readonly {
		m.output = source
	} else if m.tmpfs {
		m.output = &output{vertex: e, err: errors.Errorf("tmpfs mount for %s can't be used as a parent", target)}
	} else if m.noOutput {
		m.output = &output{vertex: e, err: errors.Errorf("mount marked no-output and %s can't be used as a parent", target)}
	} else {
		o := &output{vertex: e, getIndex: e.getMountIndexFn(m)}
		if p := e.constraints.Platform; p != nil {
			o.platform = p
		}
		m.output = o
	}
	e.Store(nil, nil, nil, nil)
	e.isValidated = false
	return m.output
}