book: Removed platform compatibility table, link to the forge

The content is duplicated, and it doesn't need to be in this location.
It's mostly trivia that doesn't apply to most of the audience.

The forge is up to date.

r? @steveklabnik cc @alexcrichton

Add missing urls on io structs

r? @steveklabnik

Match guessing game output to newest language version

Cargo now informs that it has finished, and there is new error format.

Add error note to illegal code snippet

Mark intentionally invalid code snippet in documentation as such with a comment. Similar comments used elsewhere in this file.

r? @steveklabnik

Make it clear that the reference isn't normative

Any time someone edits the reference, it has to be taken very seriously,
since it's the closest thing we have to a specification. This commit
adds language which indicates that this is not a normative document,
which makes it easier to make tweaks without worrying about forever
harming the future of Rust by painting ourselves in a corner.

r? @aturon

Stabilize `..` in tuple (struct) patterns

I'd like to nominate `..` in tuple and tuple struct patterns for stabilization.
This feature is a relatively small extension to existing stable functionality and doesn't have known blockers.
The feature first appeared in Rust 1.10 6 months ago.
An example of use: https://github.com/rust-lang/rust/pull/36203

Closes https://github.com/rust-lang/rust/issues/33627
r? @nikomatsakis

fix silent overflows on `Step` impls

Part of https://github.com/rust-lang/rust/issues/36110

r? @EddyB

rustbuild: support MIPS host builds

There is a *little* code duplication, but primarily for sake of "match exhaustiveness". Let's blame Linux/MIPS for not exposing endianness explicitly in `uname -m` (that's user-space interface and as such is frozen).

Currently the build won't work as we have to wait for a new stage0 for the MIPS host compilers, but this paves the way to self-hosted Rust on MIPS. The cross-compiled MIPS binaries are confirmed to work on the Loongson 3A2000 (MIPS64r2-compatible) so we have plenty of confidence that they'll work on other MIPS platforms too, as Linux/MIPS user-space ABI is consistent across machines of the same bitness.

r? @alexcrichton

Remove mention of mipsel target_arch

This is the only remaining instance in rustc. All others were in the libc repo, removed with rust-lang/libc#445.

Actually there's more to clean in `libsyntax/abi.rs`, but let's save that for another commit...

Fix Arc::clone()'s MAX_REFCOUNT check (off-by-one)

Before, the strong count of an `Arc` could be set to
`MAX_REFCOUNT + 1`, because when this happened, `old_size` would
be exactly `MAX_REFCOUNT`. `Arc::clone()` would not abort.

This commit changes the check in `Arc::clone()` to also abort if
the old value is equal to `MAX_REFCOUNT`, because then the new
value will be equal to `MAX_REFCOUNT + 1`.

A test would require allocating memory for `isize::MAX` pointers.
This would probably crash any machine, so no test is submitted
with this commit.

There is a *little* code duplication, but primarily for sake of "match
exhaustiveness". Let's blame Linux/MIPS for not exposing endianness
explicitly in `uname -m` (that's user-space interface and as such is
frozen).

Currently the build won't work as we have to wait for a new stage0 for
the MIPS host compilers, but this paves the way to self-hosted Rust on
MIPS. The cross-compiled MIPS binaries are confirmed to work on the
Loongson 3A2000 (MIPS64r2-compatible) so we have plenty of confidence
that they'll work on other MIPS platforms too, as Linux/MIPS user-space
ABI is consistent across machines of the same bitness.

The `mipsel` `target_arch` was introduced with the initial MIPSel
support (rust-lang/rust@82ec1aef293ddc5c6373bd7f5ec323fafbdf7901),
but was subsequently removed with implementation of the Flexible Target
Specification (Rust RFC 0131,
rust-lang/rust@3a8f4ec32a80d372db2d02c76acba0276c4effd0).
This is the only remaining instance in rustc. All others are in the libc
repo, and are fixed in rust-lang/libc@b3676593f6930c32d947c59e210789bbfcb30960.

macros: improve shadowing checks

This PR improves macro-expanded shadowing checks to work with out-of-(pre)order expansion.

Out-of-order expansion became possible in #37084, so this technically a [breaking-change] for nightly.
The regression test from this PR is an example of code that would break.

r? @nrc

use arm abi blacklist for aarch64 fuchsia

r? @alexcrichton

Force static linking of LLVM

Run `llvm-config` with `--link-static` if available, to force static linking of LLVM.
This option was added in LLVM 3.8.

This is my first pull request, any feedback is welcome!

Fixes #36854
See also: #36996

The constant name `MAX_REFCOUNT` suggests that the value is a
_hard_ limit on the amount of references to an `Arc`. This is
a more soft limit however. This commit adds a comment to the
constant to annotate this.

See also: PR #37605

Add test for issue 18060.

Closes #18060

Run llvm-config with "--link-static" if available, to force static linking of LLVM.
This option was added in LLVM 3.8.

Fixes #36854
See also: #36996

[4/n] rustc: harden against InferOk having obligations in more cases.

_This is part of a series ([prev](https://github.com/rust-lang/rust/pull/37402) | [next](https://github.com/rust-lang/rust/pull/37408)) of patches designed to rework rustc into an out-of-order on-demand pipeline model for both better feature support (e.g. [MIR-based](https://github.com/solson/miri) early constant evaluation) and incremental execution of compiler passes (e.g. type-checking), with beneficial consequences to IDE support as well.
If any motivation is unclear, please ask for additional PR description clarifications or code comments._

<hr>

This adds more asserts that `InferOk` results have no obligations, pending completion of #32730.

Each of these could accidentally drop obligations on the floor if they start getting produced by unification, and a future change does just that, in order to produce a "shallow success" (hopefully leading to ambiguities during trait selection), _without_ the possibility of an eventual success - mostly guarded by ICEs for now.

Add example using Self to reference

When I first came across `Self` I had a hard time finding references to it in the docs (and it's also been asked about on [StackOverflow](http://stackoverflow.com/questions/32304595/whats-the-difference-between-self-and-self).

I hope this example provides someone who comes across it for the first time a little more help.  If there is a better way to show an example actually using `Self`, I'm happy to modify this.  It was just the simplest place to start I could see.

Rollup of 24 pull requests

- Successful merges: #37255, #37317, #37408, #37410, #37422, #37427, #37470, #37501, #37537, #37556, #37557, #37564, #37565, #37566, #37569, #37574, #37577, #37579, #37583, #37585, #37586, #37587, #37589, #37596
- Failed merges: #37521, #37547

Add error when proc_macro_derive is used not on functions

Fixes #37590

std: Track change to cprng syscall signature (Fuchsia)

The mx_cprng_draw syscall has changed signature to separate the status
and size return values, rather than multiplexing them into a single
value with errors interpreted as a negative value. This patch tracks
that change.

Remove recursive call from Cow::to_mut

It seems to prevent it from being inlined.

fix #37559: update compiler-rt

Fixes #37559

r? @alexcrichton

Change `Into<Vec<u8>> for String` and `Into<OsString> for PathBuf` to From

Fixes #37561. First contribution, happy with any and all feedback!

Add `-Z hir-stats` for collecting statistics on HIR and AST

The data collected will be printed to the commandline and looks like the following:

```
// stats for libcore

PRE EXPANSION AST STATS

Name                Accumulated Size         Count     Item Size
----------------------------------------------------------------
TypeBinding                    2_280            57            40
Mod                            3_560            89            40
PathListItem                   6_516           181            36
Variant                        7_872            82            96
LifetimeDef                   21_280           380            56
StructField                   22_880           260            88
Lifetime                      23_800         1_190            20
Local                         30_192           629            48
ForeignItem                   31_504           179           176
Arm                           42_880           670            64
Mac                           46_960           587            80
FnDecl                        57_792         1_204            48
TraitItem                     69_504           362           192
TyParamBound                  98_280           945           104
Block                        108_384         2_258            48
Stmt                         144_720         3_618            40
ImplItem                     230_272         1_028           224
Item                         467_456         1_826           256
Pat                          517_776         4_623           112
Attribute                    745_680        15_535            48
Ty                         1_114_848         9_954           112
PathSegment                1_218_528        16_924            72
Expr                       3_082_408        20_279           152
----------------------------------------------------------------
Total                      8_095_372

POST EXPANSION AST STATS

Name                Accumulated Size         Count     Item Size
----------------------------------------------------------------
MacroDef                       1_056            12            88
Mod                            3_400            85            40
TypeBinding                    4_280           107            40
PathListItem                   6_516           181            36
Variant                        7_872            82            96
StructField                   24_904           283            88
ForeignItem                   31_504           179           176
TraitItem                     69_504           362           192
Local                         85_008         1_771            48
Arm                          100_288         1_567            64
Lifetime                     123_980         6_199            20
LifetimeDef                  126_728         2_263            56
TyParamBound                 297_128         2_857           104
FnDecl                       305_856         6_372            48
Block                        481_104        10_023            48
Stmt                         535_120        13_378            40
Item                       1_469_952         5_742           256
Attribute                  1_629_840        33_955            48
ImplItem                   1_732_864         7_736           224
Pat                        2_360_176        21_073           112
PathSegment                5_888_448        81_784            72
Ty                         6_237_168        55_689           112
Expr                      12_013_320        79_035           152
----------------------------------------------------------------
Total                     33_536_016

HIR STATS

Name                Accumulated Size         Count     Item Size
----------------------------------------------------------------
MacroDef                         864            12            72
Mod                            2_720            85            32
TypeBinding                    3_424           107            32
PathListItem                   5_068           181            28
Variant                        6_560            82            80
StructField                   20_376           283            72
ForeignItem                   27_208           179           152
WherePredicate                43_776           684            64
TraitItem                     52_128           362           144
Decl                          68_992         2_156            32
Local                         89_184         1_858            48
Arm                           94_368         1_966            48
LifetimeDef                  108_624         2_263            48
Lifetime                     123_980         6_199            20
Stmt                         168_000         4_200            40
TyParamBound                 251_416         2_857            88
FnDecl                       254_880         6_372            40
Block                        583_968        12_166            48
Item                       1_240_272         5_742           216
ImplItem                   1_361_536         7_736           176
Attribute                  1_620_480        33_760            48
Pat                        2_073_120        21_595            96
Path                       2_385_856        74_558            32
Ty                         4_455_040        55_688            80
PathSegment                5_587_904        87_311            64
Expr                       7_588_992        79_052            96
----------------------------------------------------------------
Total                     28_218_736
```

reference full path DefaultHasher

Since `DefaultHasher` does not in scope, it's not obvious where to find it.

Shrink `hir::Expr` slightly

r? @EddyB

Fix issues with the Add/AddAssign impls for Cow<str>

* Correct the stability attributes.
* Make Add and AddAssign actually behave the same.
* Use String::with_capacity when allocating a new string.
* Fix the tests.

macros: improve expansion performance

This PR fixes that regression, further improves performance on recursive, `tt`-heavy workloads, and makes a variety of other improvements to parsing and expansion performance.

Expansion performance improvements:

| Test case      | Run-time | Memory usage |
| -------------- | -------- | ------------ |
| libsyntax      | 8%       | 10%          |
| librustc       | 15%      | 6%           |
| librustc_trans | 30%      | 6%           |
| #37074         | 20%      | 15%          |
| #34630         | 40%      | 8%           |

r? @EddyB