Register new snapshots
use emit_tuple_arg while serializing tuples
Add a cfg_attr syntax extension

This extends cfg-gating to attributes.

```rust
 #[cfg_attr(<cfg pattern>, <attr>)]
```
will expand to
```rust
 #[<attr>]
```
if the `<cfg pattern>` matches the current cfg environment, and nothing
if it does not. The grammar for the cfg pattern has a simple
recursive structure:

 * `value` and `key = "value"` are cfg patterns,
 * `not(<cfg pattern>)` is a cfg pattern and matches if `<cfg pattern>`
    does not.
 * `all(<cfg pattern>, ...)` is a cfg pattern and matches if all of the
    `<cfg pattern>`s do.
 * `any(<cfg pattern>, ...)` is a cfg pattern and matches if any of the
    `<cfg pattern>`s do.

Examples:

```rust
 // only derive Show for assert_eq! in tests
 #[cfg_attr(test, deriving(Show))]
 struct Foo { ... }

 // only derive Show for assert_eq! in tests and debug builds
 #[cfg_attr(any(test, not(ndebug)), deriving(Show))]
 struct Foo { ... }

 // ignore a test in certain cases
 #[test]
 #[cfg_attr(all(not(target_os = "linux"), target_endian = "big"), ignore)]
 fn test_broken_thing() { ... }

 // Avoid duplication when fixing staging issues in rustc
 #[cfg_attr(not(stage0), lang="iter")]
 pub trait Iterator<T> { ... }
```
Deprecate `#[ignore(cfg(...))]`

Replace `#[ignore(cfg(a, b))]` with `#[cfg_attr(all(a, b), ignore)]`
Fix rebase fallout
Correct typo in the Iterator adapters section
guide-pointers.md: C sample code should match the Rust version.

Fix rust-lang/rust#17255
Fix typo in unsafe guide (s/xlibrary/library)
Fix free lifetime vars in HashMap's iterators
Relicense shootout-fasta-redux.rs to the shootout license.

Everyone agreed.

Fix #17078
rustdoc: replace DIV inside H1 with SPAN.

W3C HTML5 spec states that H1 must enclose "phrasing content" which
doesn't include DIV.  But SPAN is OK.
http://www.w3.org/TR/html5/sections.html#the-h1,-h2,-h3,-h4,-h5,-and-h6-elements
Correct tense
auto merge of #17495 : alexcrichton/rust/snapshots, r=pcwalton

cc @nick29581
Stabilize mutable slice API

This commit is another in the series of vector slice API
stabilization. The focus here is the *mutable* slice API.

Largely, this API inherits the stability attributes [previously
assigned](https://github.com/rust-lang/rust/pull/16332) to the analogous
methods on immutable slides. It also adds comments to a few `unstable`
attributes that were previously missing them.

In addition, the commit adds several `_mut` variants of APIs that were
missing:

- `init_mut`
- `head_mut`
- `tail_mut`
- `splitn_mut`
- `rsplitn_mut`

Some of the unsafe APIs -- `unsafe_set`, `init_elem`, and `copy_memory`
-- were deprecated in favor of working through `as_mut_ptr`, to simplify
the API surface.

Due to deprecations, this is a:

[breaking-change]
Fallout from deprecation
Add tests for new _mut variants
auto merge of #17504 : danburkert/rust/tuple-serialization, r=alexcrichton

The tuple serialization logic should be using the tuple-specific emit function.  This fixes part of #17158.  The JSON encoder already proxies to `emit_seq_elt` when `emit_tuple_arg` is called, so this should have an effect.
Ignore two I/O tests that are failing on the win32 bot
auto merge of #17549 : brson/rust/morewinfail, r=alexcrichton

32-bit builds pass the full 'make check' on the bots after this.
auto merge of #17494 : aturon/rust/stabilize-mutable-slices, r=alexcrichton

This commit is another in the series of vector slice API stabilization. The focus here is the *mutable* slice API.

Largely, this API inherits the stability attributes [previously assigned](rust-lang#16332) to the analogous methods on immutable slides. It also adds comments to a few `unstable` attributes that were previously missing them.

In addition, the commit adds several `_mut` variants of APIs that were missing:

- `init_mut`
- `head_mut`
- `tail_mut`
- `splitn_mut`
- `rsplitn_mut`

Some of the unsafe APIs -- `unsafe_set`, `init_elem`, and `copy_memory` -- were deprecated in favor of working through `as_mut_ptr`, to simplify the API surface.

Due to deprecations, this is a:

[breaking-change]