Move use of 'default fn' to its own module

[Centril]

This is the proper way to do cfg(...) gating when dealing with an unstable feature that introduces syntax.

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This change is 

[emilio]

Can you elaborate? Why is it the right way, or better than the current code?

This moves one of the specialized functions to its own module, but not the rest, so now you need to look at two different files that interact with each other.

[Centril]

Can you elaborate? Why is it the right way, or better than the current code?

The way this is encoded right now assumes that rustc's parser will always be able to parse default fn even on stable. That's not an assumption you should make because it will force rustc do parse default fn forever even if we decide to remove specialization or change the syntax somehow. By moving the use of the unstable syntax into its own mod foobar; and using #[cfg(CONDITIONALLY_FALSE)] you will make libsyntax avoid parsing the contents of foobar if CONDITIONALLY_FALSE does not hold.

[emilio]

That seems like a pretty common footgun, isn't it? Is that something that rustc should warn about somehow?

[Centril]

Not that common I think but I'm working on doing more pre-expansion gating so you don't get into this situation in most cases.

[emilio]

I guess I can't stamp it, though not in love with this :)

[emilio]

@bors-servo r+

[bors-servo]

📌 Commit 92122e2 has been approved by emilio

[bors-servo]

⌛ Testing commit 92122e2 with merge 3ee6d1e...

[bors-servo]

☀️ Test successful - checks-travis
Approved by: emilio
Pushing 3ee6d1e to master...