They could be implemented as builtins, like channels, slices, and maps were.

Though what's really missing is generic set functions, and a decision as to which "hashmap shape" is the blessed set (between map[T]struct{} and map[T]bool), a "set" builtin can just be an alias to that in the same way "any" is an alias for "interface{}".

The go language has had generics since inception, just not user defined ones.

If they wanted a set data structure they could have had it like they do maps and slices.

Set is just a Map though where you don't care about the values.

var m[string]struct{} implements this for you.

What you're really missing is the usual suite of helper methods which make sets ergonomic - and for that you need proper generics.

Fwiw lots of folks dislike m[K]struct{}, because while it’s efficient checking for presence is horrible:

    _, present = m[k]

It looks weird and is not an expressions. Obviously you could paper over this with a dedicated builtin function but that’s not great.

A common alternative is m[string]bool, memory density is worse but contains/add/remove are easier to read and more convenient.