Bind form-data request with custom struct

The follow example using custom struct:

type StructA struct {
    FieldA string `form:"field_a"`
}
type StructB struct {
    NestedStruct StructA
    FieldB string `form:"field_b"`
}
type StructC struct {
    NestedStructPointer *StructA
    FieldC string `form:"field_c"`
}
type StructD struct {
    NestedAnonyStruct struct {
        FieldX string `form:"field_x"`
    }
    FieldD string `form:"field_d"`
}
func GetDataB(c *gin.Context) {
    var b StructB
    c.Bind(&b)
    c.JSON(200, gin.H{
        "a": b.NestedStruct,
        "b": b.FieldB,
    })
}
func GetDataC(c *gin.Context) {
    var b StructC
    c.Bind(&b)
    c.JSON(200, gin.H{
        "a": b.NestedStructPointer,
        "c": b.FieldC,
    })
}
func GetDataD(c *gin.Context) {
    var b StructD
    c.Bind(&b)
    c.JSON(200, gin.H{
        "x": b.NestedAnonyStruct,
        "d": b.FieldD,
    })
}
func main() {
    r := gin.Default()
    r.GET("/getb", GetDataB)
    r.GET("/getc", GetDataC)
    r.GET("/getd", GetDataD)
    r.Run()
}

Using the command curl command result:

$ curl "http://localhost:8080/getb?field_a=hello&field_b=world"
{"a":{"FieldA":"hello"},"b":"world"}
$ curl "http://localhost:8080/getc?field_a=hello&field_c=world"
{"a":{"FieldA":"hello"},"c":"world"}
$ curl "http://localhost:8080/getd?field_x=hello&field_d=world"
{"d":"world","x":{"FieldX":"hello"}}

NOTE: NOT support the follow style struct:

type StructX struct {
    X struct {} `form:"name_x"` // HERE have form
}
type StructY struct {
    Y StructX `form:"name_y"` // HERE hava form
}
type StructZ struct {
    Z *StructZ `form:"name_z"` // HERE hava form
}

In a word, only support nested custom struct which have no form now.