How to call an Ethereum contract from Go?

I've tried to follow the go-ethereum guide for this but it seems out of date. I've tried looking around for a web3 library in Go but everything tries to point back to "use geth."

From Go, how do I call a contract in the Ethereum block chain? How different would it be if I'm targeting testnet or testrpc?

Here's a simple example of how you can interact with a smart contract in Go. Assuming you have solc and abigen installed.

solc --abi Store.sol > Store.abi
solc --bin Store.sol > Store.bin
abigen --bin=Store.bin --abi=Store.abi --pkg=store --out=Store.go

Store.sol

pragma solidity ^0.4.24;
contract Store {
event ItemSet(bytes32 key, bytes32 value);
string public version;
mapping (bytes32 => bytes32) public items;
constructor(string _version) public {
version = _version;
}
function setItem(bytes32 key, bytes32 value) external {
items[key] = value;
emit ItemSet(key, value);
}
}

main.go

package main
import (
"context"
"crypto/ecdsa"
"fmt"
"log"
"math/big"
"github.com/ethereum/go-ethereum/accounts/abi/bind"
"github.com/ethereum/go-ethereum/common"
"github.com/ethereum/go-ethereum/crypto"
"github.com/ethereum/go-ethereum/ethclient"
store "./contracts" // for demo
)
func main() {
client, err := ethclient.Dial("https://rinkeby.infura.io")
if err != nil {
log.Fatal(err)
}
privateKey, err := crypto.HexToECDSA("fad9c8855b740a0b7ed4c221dbad0f33a83a49cad6b3fe8d5817ac83d38b6a19")
if err != nil {
log.Fatal(err)
}
publicKey := privateKey.Public()
publicKeyECDSA, ok := publicKey.(*ecdsa.PublicKey)
if !ok {
log.Fatal("error casting public key to ECDSA")
}
fromAddress := crypto.PubkeyToAddress(*publicKeyECDSA)
nonce, err := client.PendingNonceAt(context.Background(), fromAddress)
if err != nil {
log.Fatal(err)
}
gasPrice, err := client.SuggestGasPrice(context.Background())
if err != nil {
log.Fatal(err)
}
auth := bind.NewKeyedTransactor(privateKey)
auth.Nonce = big.NewInt(int64(nonce))
auth.Value = big.NewInt(0)     // in wei
auth.GasLimit = uint64(300000) // in units
auth.GasPrice = gasPrice
address := common.HexToAddress("0x147B8eb97fD247D06C4006D269c90C1908Fb5D54")
instance, err := store.NewStore(address, client)
if err != nil {
log.Fatal(err)
}
key := [32]byte{}
value := [32]byte{}
copy(key[:], []byte("foo"))
copy(value[:], []byte("bar"))
tx, err := instance.SetItem(auth, key, value)
if err != nil {
log.Fatal(err)
}
fmt.Printf("tx sent: %s", tx.Hash().Hex()) // tx sent: 0x8d490e535678e9a24360e955d75b27ad307bdfb97a1dca51d0f3035dcee3e870
result, err := instance.Items(&bind.CallOpts{}, key)
if err != nil {
log.Fatal(err)
}
fmt.Println(string(result[:])) // "bar"
}

For more examples, check out the Ethereum Development with Go book.