go有没有办法分辨结构体的某个字段的零值是赋值而来还是初始化而来？

没有，为什么要这样？

=== update ===

package main

import (
	"encoding/json"
	"fmt"
	"reflect"
)

type Foo struct {
	Bar string
	Baz int
}

func main() {

	fooType := reflect.TypeOf((*Foo)(nil)).Elem()
	var fields []reflect.StructField
	for i := 0; i < fooType.NumField(); i++ {
		field := fooType.Field(i)
		field.Type = reflect.PtrTo(field.Type)
		fields = append(fields, field)
	}
	fooTypeForUpdate := reflect.StructOf(fields)

	j := []byte(`{
		"Baz": 9
	}`)
	data := reflect.New(fooTypeForUpdate)
	if err := json.Unmarshal(j, data.Interface()); err != nil {
		panic(err)
	}

	data = data.Elem()
	for i := 0; i < data.NumField(); i++ {
		field := data.Field(i)
		t := fooType.Field(i)
		if field.IsNil() {
			continue
		}
		field = field.Elem()
		fmt.Printf("%s: %v\n", t.Name, field.Interface())
	}

}