Remove feature gated examples from the reference

[steveklabnik]

Fixes#24573

[rust-highfive]

r? @pcwalton

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[alexcrichton]

I can see where we may want to remove as many feature-gated examples as possible, but some of these seem like they may still be useful? For example the one showing off slice patterns is probably good to keep.

[pnkfelix]

maybe we should move them to an isolated section of the documentation? That way the examples are preserved, without having them in the middle of the text that users on the beta or release channels are reading?

[steveklabnik]

maybe we should move them to an isolated section of the documentation?

Yes, so the reason I just killed these examples is that we have a specific unstable section of the book for describing unstable features. And I'm not sure it's worth time finding alternate examples of things when the reference needs a general cleaning up anyway, which is where I'd go over things like 'does this deserve an example'

[alexcrichton]

Ok, but some of these I believe can be updated to not use feature gates, so perhaps they could be tweaked to stay wherever possible?

[steveklabnik]

@alexcrichton seems legit. I've fixed up the examples, and moved the slice patterns one out. Only one or two got totally axed, because they were about box patterns, primarily.

[alexcrichton]

@bors: r+ 0f20c9d rollup