Unmarshal JSON with some known, and some unknown field names

Unmarshal twice

One option is to unmarshal twice: once into a value of type Foo and once into a value of type map[string]interface{} and removing the keys "a" and "b":

type Foo struct {
    A int                    `json:"a"`
    B int                    `json:"b"`
    X map[string]interface{} `json:"-"` // Rest of the fields should go here.
}

func main() {
    s := `{"a":1, "b":2, "x":1, "y":1}`
    f := Foo{}
    if err := json.Unmarshal([]byte(s), &f); err != nil {
        panic(err)
    }

    if err := json.Unmarshal([]byte(s), &f.X); err != nil {
        panic(err)
    }
    delete(f.X, "a")
    delete(f.X, "b")

    fmt.Printf("%+v", f)
}

Output (try it on the Go Playground):

{A:1 B:2 X:map[x:1 y:1]}

Unmarshal once and manual handling

Another option is to unmarshal once into an map[string]interface{} and handle the Foo.A and Foo.B fields manually:

type Foo struct {
    A int                    `json:"a"`
    B int                    `json:"b"`
    X map[string]interface{} `json:"-"` // Rest of the fields should go here.
}

func main() {
    s := `{"a":1, "b":2, "x":1, "y":1}`
    f := Foo{}
    if err := json.Unmarshal([]byte(s), &f.X); err != nil {
        panic(err)
    }
    if n, ok := f.X["a"].(float64); ok {
        f.A = int(n)
    }
    if n, ok := f.X["b"].(float64); ok {
        f.B = int(n)
    }
    delete(f.X, "a")
    delete(f.X, "b")

    fmt.Printf("%+v", f)
}

Output is the same (Go Playground):

{A:1 B:2 X:map[x:1 y:1]}