package main

import (
    "flag"
    "fmt"
    "reflect"
)

type AppConfig struct {
    Pg string `cli:"pg" env:"PG" default:"host=host.local dbname=db user=user password=password" description:"Connection to PostgreSQL"`
}

func main() {
    config := &AppConfig{}
    GetConfig(config)
    fmt.Println(config.Pg)
}
func GetConfig(config interface{}) interface{} {
    e := reflect.ValueOf(config).Elem()
    t := e.Type()
    flag.StringVar(
        e.Field(0).Interface().(*string),
        t.Field(0).Tag.Get("cli"),
        t.Field(0).Tag.Get("default"),
        t.Field(0).Tag.Get("description"))
    flag.Parse()
    return config
}

panic: interface conversion: interface {} is string, not *string

main.go:22: cannot take the address of e.Field(0).Interface().(string)