When Go was announced it had a spec, a significantly larger standard library than Rust, two compilers (and a parser in the standard library), and much, much better documentation. While the language had changed until the Go 1 release from a few months ago, it had changed less in two years than Rust changed in a few months. Go also introduced a tool that would automatically rewrite your old code to adapt to the new language changes.
All that means is that Go was announced much later in the development process than Rust was. If Rust had been developed like Go, it wouldn't have been made public yet. But that's not the way Mozilla does things.
