This pun originates from Limbo, I think. Go inherits the colon-equals for the inferred-type definition, but not the colon for type ascription, yet another way in which it could have been beautiful but isn’t.
It's slightly older than Limbo, and comes from Newsqueak. The difference is that Newsqueak used keywords for the other non-variable declarations. `x: int = 123`, `const X: int = 123`, `type My_Int: int`, etc.
