Introduced a chapter that describes how to perform callbacks from C to Rust
Further details on channel idea
Update guide-ffi.md

Simplified the first examples to demonstrate callbacks without other threads involved and shortened the elaboration about async callbacks.
Ignore all newline characters in Base64 decoder

Ignore all newline characters in Base64 decoder to make it compatible
with other Base64 decoders.
Added benchmark for pow and pow_with_uint
Replace old pow_with_uint with the new pow func

There was an old and barely used implementation of pow, which expected
both parameters to be uint and required more traits to be implemented.
Since a new implementation for `pow` landed, I'm proposing to remove
this old impl in favor of the new one.

The benchmark shows that the new implementation is faster than the one
being removed:

test num::bench::bench_pow_function               ..bench:      9429 ns/iter (+/- 2055)
test num::bench::bench_pow_with_uint_function     ...bench:     28476 ns/iter (+/- 2202)
Consistent formatting for args and attrs
xfail more external syntax extension tests on android
Pass the correct --target flag when type checking pretty-printed code in tests

This makes pretty print tests that have aux crates work correctly on Android.
Without they generate errors ICEs about incorrect node ids. Not sure why.
rustc: remove the explicit count from the lang_item macro.

We can use a secondary macro to calculate the count from the information
we're already having to pass to the lang items macro.
Avoid unused variable warning in quote_*!

The provided span isn't used in all cases (namely primitives).
rustc: check instantiability of fixed length vectors properly.

Previously, they were treated like ~[] and &[] (which can have length
0), but fixed length vectors are fixed length, i.e. we know at compile
time if it's possible to have length zero (which is only for [T, .. 0]).

Fixes #11659.
Add span_note to ExtCtxt

It was the only span_* missing.
Fix misuse of character/byte in std::path.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
auto merge of #11649 : FlaPer87/rust/pow, r=cmr

There was an old and barely used implementation of pow, which expected
both parameters to be uint and required more traits to be implemented.
Since a new implementation for `pow` landed, I'm proposing to remove
this old impl in favor of the new one.

The benchmark shows that the new implementation is faster than the one being removed:

```
    test num::bench::bench_pow_function               ..bench:      9429 ns/iter (+/- 2055)
    test num::bench::bench_pow_with_uint_function     ...bench:     28476 ns/iter (+/- 2202)
```
auto merge of #11652 : hdima/rust/base64-padding-newlines, r=alexcrichton

Ignore all newline characters in Base64 decoder to make it compatible with other Base64 decoders.

Most of the Base64 decoder implementations ignore all newline characters in the input string. There are some examples:

Python:

```python
>>> "
A
Q
=
=
".decode("base64")
'\x01'
```

Ruby:

```ruby
irb(main):001:0> "
A
Q
=
=
".unpack("m")
=> [""]
```

Erlang:

```erlang
1> base64:decode("
A
Q
=
=
").
<<1>>
```

Moreover some Base64 encoders append newline character at the end of the output string by default:

Python:

```python
>>> "".encode("base64")
'AQ==
'
```

Ruby:

```ruby
irb(main):001:0> [""].pack("m")
=> "AQ==
"
```

So I think it's fairly important for Rust Base64 decoder to accept Base64 inputs even with newline characters in the padding.
Add operator trait constraints to std::num::{Zero, One} and document their appropriate use

Zero and One have precise definitions in mathematics. Documentation has been added to describe the appropriate uses for these traits and the laws that they should satisfy.

For more information regarding these identities, see the following wikipedia pages:

- http://wikipedia.org/wiki/Additive_identity
- http://wikipedia.org/wiki/Multiplicative_identity
Improve std::num::pow implementation

The implementation has been made more succinct and no longer requires Clone. The coverage of the associated unit test has also been increased to check more combinations of bases, exponents, and expected results.
auto merge of #11654 : korenchkin/rust/doc_guide-testing_format, r=cmr
auto merge of #11656 : brson/rust/omgandroid, r=cmr