Universes
This PR transitions the compiler to use **universes** instead of the **leak-check**. It is marked as [WIP] for a few reasons:
- The diagnostics at present are terrible =)
- This changes the behavior of coherence, regressing some things that used to compile
The goals of this PR at present are:
- To start getting some eyes on the code
- To do a crater run
- To see the full travis results (due to https://github.com/rust-lang/rust/issues/52452, I am not able to run the full test suite locally anymore at present)
The first few commits in the PR are changing how `evaluate` treats regions. We now track whether region comparisons occurred, reverting the "staticized" query approach that @arielb1 put in. The problem with "staticized" queries is that it relied on the leak-check to get higher-ranked things correct, and we are removing the leak-check in this PR series, so that caused problems.
You can see at the end a collection of test updates. Mostly we behave the same but with atrocious diagnostics, but there are a number of cases where we used to error and now no longer do, as well as single case where we used to **not** error but we now do (the coherence-subtyping change).
(Note: it would be possible to do a version of leak-check that propagates universe information and recover the old behavior. I am reluctant to do so because I'd like to leave us room to get more precise -- e.g., I want to eventually handle things like `exists<'a> { for<'b> { if ('a: 'b) { 'a: 'b } } }` which presently the leak-check cannot cope with etc. Also because it seems more consistent to me: most folks I've talked to expect the new behavior and are surprised to learn that binding sites were so significant before when it comes to coherence. One question is, though, to what extent are people relying on this in the wild?)
I noticed a duplicated "be" somewhere in the code. A search for it
manifested a couple more locations with the same problem. This change
removes one of the "be"s.
When we coerce `dyn Foo` to `dyn Bar`, that is OK as long as `Foo` is
usable in all contexts where `Bar` is usable (hence using the source
must be a subtype of the target).
This is needed for the universe-based code to handle
`old-lub-glb-object`; that test used to work sort of by accident
before with the old code.
Previously, evaluation ignored outlives relationships. Since we using
evaluation to skip the "normal" trait selection (which enforces
outlives relationships) this led to incorrect results in some cases.
