If the compiler docs aren't going to include the test crate then it may as well be included with std.

…e feature list

These are already stabilized in 1.26.

Fixes rust-lang#49631

In intercrate mode, if we determine that a particular `T: Trait` is
unknowable, we sometimes also go and get extra causal information. An
errant `?` was causing us to propagate an error found in that process
out as if `T: Trait` was not unknowable but rather not provable. This
led to an ICE.

…st-lang#49973)

This builder is starting to time out frequently causing PRs to bounce and
otherwise doesn't seem to be catching too many bugs, so this commit removes it
entirely. We've had a number of timeouts in the last few weeks related to this
builder:

* https://travis-ci.org/rust-lang/rust/jobs/360947582
* https://travis-ci.org/rust-lang/rust/jobs/360464190
* https://travis-ci.org/rust-lang/rust/jobs/359946975
* https://travis-ci.org/rust-lang/rust/jobs/361213241
* https://travis-ci.org/rust-lang/rust/jobs/362346279
* https://travis-ci.org/rust-lang/rust/jobs/362072331

On a good run this builder takes about 2h15m, which is already too long for
Travis and the variable build times end up pushing it beyond the 3h limit
occasionally.

The timeouts here are somewhat expected in that an incrementally compiled rustc
compiler isn't optimized like a normal rustc, disallowing inlining between
codegen units and losing lots of optimization opportunities.

This commit papers over rust-lang#49889 (introducing a fixme pointing at rust-lang#50125) for a
bug that was introduced with rust-lang#49695. This workaround is taken from rust-lang#49891.