Use compiletest to make sure unsafe things do not compile

[jonas-schievink]

I only added 2 tests I found in tests.rs, it would be nice to gather more. If anyone finds something unsafe that we should test against, please leave a comment.

[jonas-schievink]

Oh yeah, Travis fails since this only works on nightly. Give me a second.

[jonas-schievink]

Dev-dependencies are not allowed to be optional

UGH. Why isn't there a more elegant and usable solution for compile-fail tests? They're pretty important for all sorts of crates...

[kyren]

I ended up working off of what you had and managed to get compiletests_rs working on travis as of 1426bdb. I didn't put compiletest_rs in [dev-dependencies], but it is optional?

I'm not sure if I have it set up in the most optimal way.

[jonas-schievink]

@kyren Very nice! The only issue with making compiletest a regular dependency is that downstream crates can now request the compiletest_rs feature to be enabled, too. I don't think that's too big of a deal, though.

Eventually I'll revisit my plans to make compile-fail tests more accessible, probably after rust-lang/cargo#4734 lands or rust-lang/rfcs#2318 is accepted and implemented.

Also closing, since this is implemented now.