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.
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>
document file!
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 #11656 : brson/rust/omgandroid, r=cmr
Fix documentation typos
auto merge of #11657 : huonw/rust/less-lang-duplication, r=cmr

We can use a secondary macro to calculate the count from the information
we're already having to pass to the lang items macro.
auto merge of #11660 : sfackler/rust/quote-unused-sp, r=huonw

The provided span isn't used in all cases (namely primitives).
auto merge of #11661 : huonw/rust/fixed-length-instantiation, r=thestinger

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.
auto merge of #11670 : sfackler/rust/extctxt-span-note, r=alexcrichton

It was the only span_* missing.
Scan the entire crate for exported macros

It previously missed anything in an inner module.