Remove special root for @

[flaper87]

This patch removes the special auto-rooting for @ from the borrow checker. With @ moving into a library, it doesn't make sense to keep this code around anymore. It also simplifies trans by removing root checking from there

@nikomatsakis

Closes: #11586

[flaper87]

I believe this is a breaking change for scenarios using managed pointers. I'll update the commits accordingly.

[nikomatsakis]

@flaper87 yes, I was going to mention that.

[nikomatsakis]

@flaper87 so -- we definitely should see compilation errors for those tests. The reason we weren't is that my suggested changes to borrowck were incorrect. We basically want to treat @T pointers the same way we treat ~T. Longer term, we'd like to remove all knowledge of how to deref @T from the compiler. I tried to prototype that but ran into a bit of trickiness around coercions, so for the time being I think you want to make changes like these:

nikomatsakis@823f7bb

[nikomatsakis]

Note: that patch I pointed you at leaves some dead code that can be cleaned up. And I think we can remove the moved_variables_set altogether.

[nikomatsakis]

@flaper87 this looks great; however, can you do a bit of cleanup:

1. remove the [breaking-change] annotation except for on one commit that explains what is happening and what people can do to fix it.
2. squash together the test and borrowck changes (I don't care if my authorship is preserved) so that we don't have intermediate, invalid states where the code was wrong.

[flaper87]

@nikomatsakis roger that!

re [breaking-change] annotation, I mentioned in the email thread that it would probably be worth keeping it in all the commits of the same change. I thought that was fine. I'll get rid of it.

Unfortunately, this change is not ready to be merged, stage2 syntax fails to compile due to some borrowed values not living long enough. I'll fix that