heap: rm out-of-date FIXMEs
heap: optimize EMPTY to avoid relocations

Sized deallocation makes it pointless to provide an address that never
overlaps with pointers returned by an allocator. Code can branch on the
capacity of the allocation instead of a comparison with this sentinel.

This improves the situation in #8859, and the remaining issues are only
from the logging API, which should be disabled by default in optimized
release builds anyway along with debug assertions. The remaining issues
are part of #17081.

Closes #8859
remove the closure_exchange_malloc lang item
Change the last linux 64-bit snapshot

I'm rotating in some CentOS 5.10 bots so we *actually* build on Linux 2.6.18
like we advertise doing so. Currently the snapshots are incompatible with CentOS
5.10 due to snapshots requiring glibc 2.6 and CentOS 5.10 having glibc 2.5.

It turns out that rustc only requires *one* symbol from glibc 2.6, which is
`futimens`. The rust distribution itself does not use this symbol, but LLVM
conditionally detects it and then uses it. This symbol isn't even called as part
of the compilation process, so we don't even need it!

The new snapshot was generated following these instructions [1]:

1. Download the current x86_64 linux snapshot and unpack it.
2. Open the rustc binary in a hex editor.
3. Change the linkage against glibc 2.6 from strong to *weak*
4. Write changes and re-run src/etc/make-snapshot.py
5. Upload new tarball to S3

On CentOS 5.10 a warning is printed each time the snapshot runs that the symbol
cannot be found (anyone with glibc 2.6+ does not have this warning printed). The
key part is that we can *bootstrap* on CentOS 5.10 at this point. The next
snapshot will be naturally compatible with glibc 2.3 (even older!) and will not
need to be manually edited.

[1]: http://jamesbond3142.no-ip.org/wiki/wiki.cgi/NewAppsOnOldGlibc
auto merge of #17288 : alexcrichton/rust/change-snap, r=brson

I'm rotating in some CentOS 5.10 bots so we *actually* build on Linux 2.6.18
like we advertise doing so. Currently the snapshots are incompatible with CentOS
5.10 due to snapshots requiring glibc 2.6 and CentOS 5.10 having glibc 2.5.

It turns out that rustc only requires *one* symbol from glibc 2.6, which is
`futimens`. The rust distribution itself does not use this symbol, but LLVM
conditionally detects it and then uses it. This symbol isn't even called as part
of the compilation process, so we don't even need it!

The new snapshot was generated following these instructions [1]:

1. Download the current x86_64 linux snapshot and unpack it.
2. Open the rustc binary in a hex editor.
3. Change the linkage against glibc 2.6 from strong to *weak*
4. Write changes and re-run src/etc/make-snapshot.py
5. Upload new tarball to S3

On CentOS 5.10 a warning is printed each time the snapshot runs that the symbol
cannot be found (anyone with glibc 2.6+ does not have this warning printed). The
key part is that we can *bootstrap* on CentOS 5.10 at this point. The next
snapshot will be naturally compatible with glibc 2.3 (even older!) and will not
need to be manually edited.

[1]: http://jamesbond3142.no-ip.org/wiki/wiki.cgi/NewAppsOnOldGlibc
auto merge of #17280 : thestinger/rust/heap, r=pcwalton