What is this parenthesis enclosed variable declaration syntax in Go?

var (...) (and const (...) are just shorthand that let you avoid repeating the var keyword. It doesn't make a lot of sense with a single variable like this, but if you have multiple variables it can look nicer to group them this way.

It doesn't have anything to do with exporting. Variables declared in this way are exported (or not) based on the capitalization of their name, just like variables declared without the parentheses.

This code

// What's this syntax ? Is it exported ? 
var (
    rootDir = path.Join(home(), ".coolconfig")
)

is just a longer way of writing

var rootDir = path.Join(home(), ".coolconfig")

However it is useful when declaring lots of vars at once. Instead of

var one string
var two string
var three string

You can write

var (
    one string
    two string
    three string
)

The same trick works with const and type too.