Use raw pointers to avoid making any assertions about the data field.

Fixes rust-lang#72590

With PR rust-lang#70551, observing a `ty::Error` guarantees that compilation is
going to fail. Therefore, there are no soundness impliciations to
continuing on when we encounter a `ty::Error` - we can only affect
whether or not additional error messags are emitted.

By not bailing out, we avoid incorrectly determining that types are
`!Sized` when a type error is present, which allows us to avoid emitting
additional spurious error messages.

The original comment mentioned this code being shared by coherence -
howver, this change resulted in no diagnostic changes in any of the
existing tests.

confusing diagnostics, issue rust-lang#72253

add test for confusing error message, issue-72253


remove is_multiline check, refactor to self.expect(&token:Semi)


update issue-72253 tests


return Ok

Fix confusing error message for comma typo in multiline statement

Fixes rust-lang#72253.  Expands on the issue with a colon typo check.

r? @estebank

cc @ehuss

…matsakis

Make `PolyTraitRef::self_ty` return `Binder<Ty>`

This came up during review of rust-lang#71618. The current implementation is the same as a call to `skip_binder` but harder to audit. Make it preserve binding levels and add a call to `skip_binder` at all use sites so they can be audited as part of rust-lang#72507.

Resolve UB in Arc/Weak interaction (2)

Use raw pointers to avoid making any assertions about the data field.

Follow up from rust-lang#72479, see that PR for more detail on the motivation.

@RalfJung I was able to avoid a lot of the changes to `Weak`, by making a helper type (`WeakInner`) - because of auto-deref and because the fields have the same name, the rest of the code continues to compile.

…=nikomatsakis

Add test for old compiler ICE when using `Borrow`

The original issue was caused by implementing `Borrow` on a local type and using the tokio-reactor crate which had this impl: https://github.com/tokio-rs/tokio/blob/tokio-0.1.4/tokio-reactor/src/poll_evented.rs#L547-L577

This causes an ICE on Rust 1.27.0:

```console
$ RUSTUP_TOOLCHAIN=1.27.0 rustc src/test/ui/issues/issue-50687-ice-on-borrow.rs
error: internal compiler error: librustc/traits/structural_impls.rs:180: impossible case reached

thread 'main' panicked at 'Box<Any>', librustc_errors/lib.rs:554:9
note: Run with `RUST_BACKTRACE=1` for a backtrace.
error: aborting due to previous error

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.27.0 (3eda71b 2018-06-19) running on x86_64-apple-darwin
```

Closes rust-lang#50687

… r=Dylan-DPC

Small cell example update

r? @Dylan-DPC

…=nikomatsakis

Don't bail out of trait selection when predicate references an error

Fixes rust-lang#72590

With PR rust-lang#70551, observing a `ty::Error` guarantees that compilation is
going to fail. Therefore, there are no soundness impliciations to
continuing on when we encounter a `ty::Error` - we can only affect
whether or not additional error messags are emitted.

By not bailing out, we avoid incorrectly determining that types are
`!Sized` when a type error is present, which allows us to avoid emitting
additional spurious error messages.

The original comment mentioned this code being shared by coherence -
howver, this change resulted in no diagnostic changes in any of the
existing tests.

…olnay

Add remark regarding DoubleEndedIterator

While reviewing rust-itertools/itertools@14293bd#diff-2c16d2ada06ad2fd1fc754679646d471, I realized that a `DoubleEndedIterator` may yield different elements depending on whether it is traversed forwards or backwards. (Not only the *order*, but possibly also the yielded values.)

I found this remarkable, but could not find anything in the current docs, so I thought it may be worth mentioning this explicitly.

Unfortunately, I could not test these changes locally (`rustdoc` complains about `unresolved import`). Sorry if this causes headache.

If I should change something, please let me know. If it seems too trivial, feel free to just close this PR.