Previously, `BufWriter::write` would just return an `std::io::OtherIoError` if someone attempted to write past the end of the wrapped buffer. This pull request changes the error to support partial writes and return a `std::io::ShortWrite`, or an `io::io::EndOfFile` if it's been fully exhausted.

 I've also optimized away a bounds check inside `BufWriter::write`, which should help shave off some nanoseconds in an inner loops.

Seems to be blocking forever

Test will be fixed in #19588

Seems to be blocking forever

Conflicts:
	src/libcollections/trie/set.rs

Closes #19543

Adds the ability to use a custom allocator heap by passing either --cfg
external_crate and --extern external=<allocator_crate_name> or --cfg
external_funcs and defining the allocator functions prefixed by 'rust_'
somewhere.

This is useful for many applications including OS/embedded development,
and allocator development and testing.

Fixes https://github.com/rust-lang/rust/issues/19510

Part of #18424

Adds `capacity()` function to VecMap, as per the collections reform.

(Salvaged from #19516, #19523, while we await an RFC regarding `reserve`/`reserve_index` for `VecMap`)

Fixes #19402.

Fixes #19335. (or at least, the actionable parts)

Right now, `DerefMut` is not `for Sized?`, so you can't impl `DerefMut<T> for Foo` where `Foo` is unsized.  However, there is no reason that it can't be `for Sized?`, so this pull request fixes the issue.

Closes #19493.

Implement the `BitOr`, `BitAnd`, `BitXor`, and `Sub` traits from `std::ops` for TrieSet.  The behavior of these operator overloads is consistent with [RFC 235](https://github.com/rust-lang/rfcs/blob/master/text/0235-collections-conventions.md#combinations).

Fixes #17332.

We heavily rely on queries and fragments in the URL structure, so it is desired to preserve them even in the redirects. The generated redirect pages try to preserve them with scripts, which take precedence over the original `Refresh` metadata. Non-scripting browsers would continue to work (with no queries and fragments).

(This in turn solves a number of semi-broken links to the source code, which are actually linked to redirect pages.)

Added the example from [this Reddit thread][1], reworked to be more robust with correct logic (first link skipped the 0th and 1st Fibonacci numbers, second forgot about the last two valid values before overflow). Will yield all Fibonacci numbers sequentially in the range `[0, <u32 as Int>::max_value())`.

If the example is too complicated I can change it to a more naive version, perhaps using signed integers to check for overflow instead of `Option` and `.checked_add()`.

Also reworded the doc comments to clarify the usage and behavior of `Unfold`, as the thread suggested that it wasn't really clear how `Unfold` worked and when one should use it.

This change is in the `core` crate but I based the example on `std` since that's where most readers will find the example. I included a note about `core` for clarity. Edit: removed.

Tested with `rustdoc src/libcore/lib.rs`. Rebased against latest master as of the creation of this PR.

[1]: http://www.reddit.com/r/rust/comments/2ny8r1/a_question_about_loops/cmighu4?context=10000

This series of commits deals with broken links to the source code. It also refactors some repetitive codes from Rustdoc. The most important commit, 1cb1f00d, describes the rationale; this will fix a half of #16289. Other commits are reasonably independent to each other and can be made into indiviudal PRs at the request.

### Notes on the broken source links

As of bda97e85 (I've used this to check the PR works as intended), there are 281 (!) such broken links. They can be further classified as follows:

* 178 links to incorrect item types. This is the first half of #16289, and this PR fixes all of them.
* 89 links to redirect pages. They are not technically "broken" but still doesn't give a source code. I have a fix for this in mind, which would make a redirect page slightly *fat*.
* 14 links to incorrect `DefId` in the `gotosrc` parameter. This is #15309, and affects many `liballoc` reexports in `libstd` but *nothing else* (curiously). I'm yet to track this down; might be a metadata bug (not sure).
* 0 links to the crate reexported as a different name. This is the second half of #16289, and seems not hard to fix but I'm running out of time.

Prevalence of this kind of bugs calls for a full link verifier integrated into the testing process. :S

As an example of what this changes, the following code:

```rust
let x: [int ..4];
```

Currently spits out ‘expected `]`, found `..`’. However, a comma would also be valid there, as would a number of other tokens. This change adjusts the parser to produce more accurate errors, so that that example now produces ‘expected one of `(`, `+`, `,`, `::`, or `]`, found `..`’.

(Thanks to cramer on IRC for pointing out this problem with diagnostics.)

https://github.com/rust-lang/rust/pull/19472#issuecomment-65370278

/cc @alexcrichton

deriving encodable + using json::PrettyEncoder removes the only ToJson trait implementation in the rust repository outside of libserialize

@pcwalton does this agree with your FIXME comment?

First half of bootstrapping https://github.com/rust-lang/rfcs/pull/446

Fixes #19339.