Comment by abronan
Adding a quick opinion here: I worked for one of these companies (working with libraries written by all of these) and it's just a matter of personal taste. Docker, CoreOS, Hashicorp, etc. are "successful" (which is hard to confirm for now) because they can gather a lot of open source contributions thanks to the easy Go learning curve. I don't think this would be the case if they used Rust instead. Go is definitely not a play toy and you can build scalable and reliable software with it. The problem with Go in my opinion is also its ease of use, it's incredibly easy to write sloppy/buggy code with panics/data races that become increasingly hard to debug. I've had my fair share of frustrating experiences debugging concurrent code with hundreds of goroutines or investigating random data races on complex distributed systems code. I've yet to see that happening with Rust after four months of using it, the compiler does an incredible job at preventing these.While Go's frustration often comes from debugging sloppy concurrent code, the frustration with Rust is often about having your code to compile. But once it compiles, it runs beautifully. Some prefer the former, I personally prefer the latter. Also, I think the argument of increased productivity with Go should not be the main argument, because the time you save writing a large program, you will probably lose trying to debug a deadlock (which could still happen in Rust but harder to make happen) or a random data race that the race detector couldn't catch (or in a vendored library).
If I had to define the experience with both languages, Go is the instant gratification option, you feel productive quickly, the language is easy to learn with a forgiving syntax sometimes at the expense of correctness. Rust is the delayed gratification option, frustrating at times, very hard to learn and master but offering strong safety guarantees with an elegant syntax (which is a personal opinion).
Also it's a controversial topic in the Go community but the lack of generics...
Rust has its downsides too, for example the slow compile times or the lack of maintained libraries for some core functionalities but overall, programming with Rust is a much more enjoyable experience once you start getting ahead the learning curve.
One last thing: It is hopeless trying to learn Rust in four days without very good resources/books. I recommend "Programming Rust" from OReilly if you really want to dig into the language. It's incomplete at the time of writing but still one of the most useful resource out there (with the free Rust ebook). My knowledge of Rust dramatically increased after reading it and I felt much more confident writing larger programs.