JSON object not unmarshalling, but not throwing errors

I'm just trying to do some basic JSON unmarshalling for this item:

testJson := `{
        "Users": [
          {
                "id": 1,
                "username": "bbrewer"
          },
          {
                "id": 2,
                "username": "rsmith"
          },
          {
                "id": 3,
                "username": "jmorgan"
          },
          {
                "id": 4,
                "username": "dwallis"
          }]
        }` 

I'm trying to do it in such a way that I can work with the individual components for each entry (so like user.ID or user.UserName). I've set up a struct for it like this:

type UserInfo struct {
        ID              int     `json:"id"`
        UserName        string  `json:"username"`
}

However, whenever I try to unmarshal it, while it doesn't throw any errors, it doesn't seem to be parsing it properly. When I try to print using a range, nothing is outputted, and it seems like the object I'm trying to tie it to is just empty. What am I doing wrong?
Here's my full code (note that I've also tried it without the UserList type, with users as the UserInfo struct type):

package main

import "fmt"
//import "net/http"
//import "io/ioutil"
import "encoding/json"

type UntypedJson map[string][]interface{}

type UserInfo struct {
        ID              int     `json:"id"`
        UserName        string  `json:"username"`
}

type UserList []UserInfo

func main() {
        testJson := `{
        "Users": [
          {
                "id": 1,
                "username": "bbrewer"
          },
          {
                "id": 2,
                "username": "rsmith"
          },
          {
                "id": 3,
                "username": "jmorgan"
          },
          {
                "id": 4,
                "username": "dwallis"
          }]
        }`

        var users UserList
        json.Unmarshal([]byte(testJson), &users)
        for i, v := range users {
                fmt.Println("what do")
                fmt.Println(i)
                fmt.Println(v)
                fmt.Println(v.ID)
                fmt.Println(v.UserName)
        }
}

your code obviously getting an error and you haven't handled it. first handle error like below.

    err := json.Unmarshal([]byte(testJson), &users)
	if err != nil {
		//handle error here.
        log.Fatalln(err)
	}

Answer to your question, you have to create a type including UserInfo array as below. Because your json string has annotated as "Users":[{}]

type UsersInfo struct {
	Users []UserInfo `json:"Users"`
}

And unmarshal your json to the UsersInfo type variable reference.

    var users UsersInfo
	err := json.Unmarshal([]byte(testJson), &users)
	if err != nil {
		log.Fatalln(err)
	}

You can run full code here.