Add the ability to completely disable experimental features in release channels

[brson]

In the stable release channel we're probably going to completely disable the ability to use experimental features and libraries. Add this capability to the build.

cc #16677

[brson]

Some notes from @alexcrichton about potential next steps

• nightly beta builds
• get a travis option
• maybe move to betas on the website

[metajack]

If I understand this right, Servo won't be compilable with release compiler? That seems unfortunate. Has anyone done a survey to see how many projects are in the same boat?

[dherman]

Servo should use the unstable channel of Rust, by design; Servo is still a research project and it's helping to experiment with unfinished parts of the language. Over time, as Servo matures and becomes a shipping product, it should switch to release channel, because it -- like any production software -- should be basing itself on more highly QA'ed features and not be shipping with experimental language features. The nice thing about the train model is that experimental features can stabilize and land faster than they do in traditional languages since they aren't blocked on unrelated features, and trains run regularly frequently.

In general, the reason for not making experimental features available to release channel is the same as not allowing them in release browsers (note that prefs are a per-user switch, not a per-developer switch, and this is the key distinction): if we allowed people to ship using experimental features, we'd've lost all the ability to clearly message that they are experimental and will change.

The channels are our strongest, clearest source of messaging what comes with a guarantee of stability and what does not. If we allowed people to opt out, we'd have no levers left to make it possible to ship experimental features without them getting prematurely frozen. And we'd lose the ability to guarantee that successfully compiling with Rust 1.0 means successfully compiling with Rust 1.1. In other words, it would jeopardize both the ability to experiment and the ability to guarantee stability.

[dherman]

Another way to look at this: the release channel is the channel for production users. If most people are using the unstable channel when 1.0 first comes out, that's just fine. We're preparing for production users but it'll take time for that set to grow. :)

[aturon]

cc #17895