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[alexcrichton]

This in theory enables uncommenting IndexMut implementations, but upon doing so
the compiler immediately segfaulted in stage1, so I'll leave those to a later
time.

[alexcrichton]

I first attempted to land alexcrichton@e3dac1e, but that commit segfaulted instantly with this backtrace:

$ gdb --args ./x86_64-unknown-linux-gnu/stage1/bin/rustc src/libcore/lib.rs
GNU gdb (Ubuntu 7.7-0ubuntu3.1) 7.7
Copyright (C) 2014 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
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There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
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<http://www.gnu.org/software/gdb/bugs/>.
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Reading symbols from ./x86_64-unknown-linux-gnu/stage1/bin/rustc...(no debugging symbols found)...done.
(gdb) r
Starting program: /home/alex/code/rust3/x86_64-unknown-linux-gnu/stage1/bin/rustc src/libcore/lib.rs
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7ffff2bff480 (LWP 18546)]

Program received signal SIGILL, Illegal instruction.
[Switching to Thread 0x7ffff2bff480 (LWP 18546)]
0x00007ffff7e84278 in bitv::Bitv::set::h693713f7b138c463Yla () from /home/alex/code/rust3/x86_64-unknown-linux-gnu/stage1/bin/../lib/librustrt-4e7c5e5c.so
(gdb) bt
#0  0x00007ffff7e84278 in bitv::Bitv::set::h693713f7b138c463Yla () from /home/alex/code/rust3/x86_64-unknown-linux-gnu/stage1/bin/../lib/librustrt-4e7c5e5c.so
#1  0x00007ffff7e88574 in bitv::BitvSet.MutableSet$LT$uint$GT$::insert::h437317dbaeae41b5i8a () from /home/alex/code/rust3/x86_64-unknown-linux-gnu/stage1/bin/../lib/librustrt-4e7c5e5c.so
#2  0x00007ffff4313f72 in attr::first_attr_value_str_by_name::h23b9b3a677724c717XC () from /home/alex/code/rust3/x86_64-unknown-linux-gnu/stage1/bin/../lib/./libsyntax-4e7c5e5c.so
#3  0x00007ffff434d0bc in parse::parser::Parser$LT$$x27a$GT$::eval_src_mod::hac338816627dce54NaQ () from /home/alex/code/rust3/x86_64-unknown-linux-gnu/stage1/bin/../lib/./libsyntax-4e7c5e5c.so
#4  0x00007ffff433ee8e in parse::parser::Parser$LT$$x27a$GT$::parse_item_or_view_item::h56365d466ced5143sEQ ()
   from /home/alex/code/rust3/x86_64-unknown-linux-gnu/stage1/bin/../lib/./libsyntax-4e7c5e5c.so
#5  0x00007ffff4346846 in parse::parser::Parser$LT$$x27a$GT$::parse_items_and_view_items::h6c430a0fa80b713eT6Q ()
   from /home/alex/code/rust3/x86_64-unknown-linux-gnu/stage1/bin/../lib/./libsyntax-4e7c5e5c.so
#6  0x00007ffff434ae70 in parse::parser::Parser$LT$$x27a$GT$::parse_mod_items::hf066dcd3ed21aeb821P () from /home/alex/code/rust3/x86_64-unknown-linux-gnu/stage1/bin/../lib/./libsyntax-4e7c5e5c.so
#7  0x00007ffff43549ef in parse::parser::Parser$LT$$x27a$GT$::parse_crate_mod::h4947cca241ec7fec9bR () from /home/alex/code/rust3/x86_64-unknown-linux-gnu/stage1/bin/../lib/./libsyntax-4e7c5e5c.so
#8  0x00007ffff43acabd in parse::parse_crate_from_file::h100116cb38a295e7NsV () from /home/alex/code/rust3/x86_64-unknown-linux-gnu/stage1/bin/../lib/./libsyntax-4e7c5e5c.so
#9  0x00007ffff71658f0 in driver::driver::phase_1_parse_input::closure.142527 () from /home/alex/code/rust3/x86_64-unknown-linux-gnu/stage1/bin/../lib/librustc-4e7c5e5c.so
#10 0x00007ffff715c5a2 in driver::driver::phase_1_parse_input::h4f0913c2647baa03ibA () from /home/alex/code/rust3/x86_64-unknown-linux-gnu/stage1/bin/../lib/librustc-4e7c5e5c.so
#11 0x00007ffff7159feb in driver::driver::compile_input::hd529ab663911c532g6z () from /home/alex/code/rust3/x86_64-unknown-linux-gnu/stage1/bin/../lib/librustc-4e7c5e5c.so
#12 0x00007ffff71b99cf in driver::run::closure.147104 () from /home/alex/code/rust3/x86_64-unknown-linux-gnu/stage1/bin/../lib/librustc-4e7c5e5c.so
#13 0x00007ffff71c4327 in task::TaskBuilder$LT$S$GT$::try_future::closure.147673 () from /home/alex/code/rust3/x86_64-unknown-linux-gnu/stage1/bin/../lib/librustc-4e7c5e5c.so
#14 0x00007ffff71c42d8 in task::TaskBuilder$LT$S$GT$::spawn_internal::closure.147667 () from /home/alex/code/rust3/x86_64-unknown-linux-gnu/stage1/bin/../lib/librustc-4e7c5e5c.so
#15 0x00007ffff7fc287f in task::NativeSpawner.Spawner::spawn::closure.8465 () from /home/alex/code/rust3/x86_64-unknown-linux-gnu/stage1/bin/../lib/libnative-4e7c5e5c.so
#16 0x00007ffff7edd6ec in rust_try_inner () from /home/alex/code/rust3/x86_64-unknown-linux-gnu/stage1/bin/../lib/librustrt-4e7c5e5c.so
#17 0x00007ffff7edd6d6 in rust_try () from /home/alex/code/rust3/x86_64-unknown-linux-gnu/stage1/bin/../lib/librustrt-4e7c5e5c.so
#18 0x00007ffff7e7d0c3 in unwind::try::hc8101bb7ca759647imd () from /home/alex/code/rust3/x86_64-unknown-linux-gnu/stage1/bin/../lib/librustrt-4e7c5e5c.so
#19 0x00007ffff7e7ed5c in task::Task::run::hb7a787d36e5bed95WBc () from /home/alex/code/rust3/x86_64-unknown-linux-gnu/stage1/bin/../lib/librustrt-4e7c5e5c.so
#20 0x00007ffff7fc26f8 in task::NativeSpawner.Spawner::spawn::closure.8406 () from /home/alex/code/rust3/x86_64-unknown-linux-gnu/stage1/bin/../lib/libnative-4e7c5e5c.so
#21 0x00007ffff7e7cec8 in thread::thread_start::h9a3d162da155dd197Vc () from /home/alex/code/rust3/x86_64-unknown-linux-gnu/stage1/bin/../lib/librustrt-4e7c5e5c.so
#22 0x00007ffff6633182 in start_thread (arg=0x7ffff2bff480) at pthread_create.c:312
#23 0x00007ffff6bdffbd in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:111