Rename C-like enum to Field-less enum

There is no need to reference the C programming language to explain this concept.

compiletest: account for `ui` reference files when deciding to skip

The stamp files for compiletest were ignoring `.stderr` and `.stdout` files. This was driving me crazy.

r? @alexcrichton

Stabilize abi_sysv64

Closes #36167, stabilizing the use of the "sysv64" ABI on x64 platforms where it is not the default ABI.

FCP on this is complete in the tracking issue.

rustc_mir: promote references of statics from other statics.

Fixes #46522 by also allowing `STATIC_REF` in MIR const-qualification, not just AST rvalue promotion.

Display `\t` in diagnostics code as four spaces

Follow up to #44386 using the unicode variable width machinery from #45711 to replace tabs in the source code when displaying a diagnostic error with four spaces (instead of only one), while properly accounting for this when calculating underlines.

Partly addresses #44618.

MIR borrowck: implement union-and-array-compatible semantics

Fixes #44831.
Fixes #44834.
Fixes #45537.
Fixes #45696 (by implementing DerefPure semantics, which is what we want going forward).

r? @nikomatsakis

Rollup of 7 pull requests

- Successful merges: #46136, #46378, #46431, #46483, #46495, #46502, #46512
- Failed merges:

Give compile_error macro examples

I cannot get Rust to build at all with it complaining about GCC not being a valid C compiler or something, so letting TravisCI be my tester...

Fixes #46171

Improve search style

Fixes #46494.

r? @QuietMisdreavus

Update old link

The CONTRIBUTING.md page currently links to an old wiki page in rust-lang/rust-wiki-backup. There is a more up-to-date page in-tree so I changed the link to point there so new contributors can find it more easily.

Document behavior of `ptr::swap` with overlapping regions of memory.

Fixes https://github.com/rust-lang/rust/issues/44479.

Mention the name of ? in Result's docs

Fixes #42725

or at least, this is the best we can really do. #35946 is tracking
better errors already, so that should cover the other part of it.

Fix use of rand in liballoc benches

Clarify what `-D warnings` or `-F warnings` does

They set all lints currently on the warning level to `deny` or `forbid`,
respectively.

coherence: fix is_knowable logic

A trait-ref that passes the orphan-check rules can still be implemented in a crate downstream from our crate (for example, `LocalType for LocalTrait<_>` might be matched by a `LocalType for LocalTrait<TypeFromDownstreamCrate>`), and this should be known by the `is_knowable`  logic.

Trait selection had a hackfix for this, but it's an hacky fix that does not handle all cases. This patch removes it.

fixes #43355.

r? @nikomatsakis

Needs a crater run

rustc_trans: don't write discriminants for uninhabited variants

Fixes #46519.

Patch as suggested by eddyb: https://github.com/rust-lang/rust/issues/46519#issuecomment-349443519

Update rls and rustfmt and set them testing

r? @alexcrichton

Change "Types/modules" title of search tab to be more accurate

From issue #45787. Used "In name" as per suggestion from @Seeker14491.

Fixes #46519.

Patch as suggested by eddyb:
https://github.com/rust-lang/rust/issues/46519#issuecomment-349443519

Tabs are "In Names", "In Parameters", and "In Return Types".

This fixes the tests for issue #29793

I'm not sure how correct it this, but it gets whatever needs to compile
to compile.

This improves performance on large functions.