Linker error calling StackSegment::start()

[kmcallister]

After applying #8475 I can access current_stack_segment but I get a linker error calling .start() on it:

use std::rt::local::Local;
use std::rt::task::Task;

fn main() {
    do Local::borrow |task: &mut Task| {
        fmt!("%?", task.coroutine.get_ref().current_stack_segment.start());
    }
}

$ ~/proj/servo/servo/build/src/compiler/rust/x86_64-unknown-linux-gnu/stage2/bin/rustc foo.rs
error: linking with `cc` failed with code 1
note: cc arguments: -L/home/keegan/proj/servo/servo/build/src/compiler/rust/x86_64-unknown-linux-gnu/stage2/lib/rustc/x86_64-unknown-linux-gnu/lib -m64 -o foo foo.o -L/home/keegan/proj/servo/servo/build/src/compiler/rust/x86_64-unknown-linux-gnu/stage2/lib/rustc/x86_64-unknown-linux-gnu/lib -lstd-6c65cf4b443341b1-0.8-pre -lrustrt -lrt -lpthread -L/tmp/.rust -L/tmp -lrt -ldl -lm -lmorestack -lrustrt -Wl,-rpath,$ORIGIN/../home/keegan/proj/servo/servo/build/src/compiler/rust/x86_64-unknown-linux-gnu/stage2/lib/rustc/x86_64-unknown-linux-gnu/lib -Wl,-rpath,/home/keegan/proj/servo/servo/build/src/compiler/rust/x86_64-unknown-linux-gnu/stage2/lib/rustc/x86_64-unknown-linux-gnu/lib -Wl,-rpath,/usr/local/lib/rustc/x86_64-unknown-linux-gnu/lib
note: foo.o: In function `main::anon::expr_fn_5280':
foo.rc:(.text+0xe832): undefined reference to `rt::stack::__extensions__::meth_31856::start::_bf49874ae71d3b80::_0$x2e8$x2dpre'
collect2: error: ld returned 1 exit status

error: aborting due to previous error

Possibly something's wrong with my locally patched rustc but I did do a clean build after applying that patch.

[brson]

Hm. This is probably because mod stack is not pub!

[kmcallister]

Added a fix to #8475. @brson: r? again?