Introduced a chapter that describes how to perform callbacks from C to Rust
Further details on channel idea
Support linking of rlib files in rustpkg
Fix indention
Update guide-ffi.md

Simplified the first examples to demonstrate callbacks without other threads involved and shortened the elaboration about async callbacks.
syntax::ext: replace span_fatal with span_err in many places.

This means that compilation continues for longer, and so we can see more
errors per compile. This is mildly more user-friendly because it stops
users having to run rustc n times to see n macro errors: just run it
once to see all of them.
Fixed typos in comments of librustc backend.
handle zero-size allocations correctly

The `malloc` family of functions may return a null pointer for a
zero-size allocation, which should not be interpreted as an
out-of-memory error.

If the implementation does not return a null pointer, then handling
this will result in memory savings for zero-size types.

This also switches some code to `malloc_raw` in order to maintain a
centralized point for handling out-of-memory in `rt::global_heap`.

Closes #11634
Actually force a / in the path for ext crates

If the library is in the working directory, its path won't have a "/"
which will cause dlopen to search /usr/lib etc. It turns out that Path
auto-normalizes during joins so Path::new(".").join(path) is actually a
no-op.
Make WindowsPath::new("C:foo").root_path() return Some("C:")
Expose platform independent path separators
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.