auto merge of #5887 : jdm/rust/stackbounds, r=brson

This is needed to allow GC to work in SpiderMonkey.

src/libcore/gc.rs

@@ -73,6 +73,7 @@ pub mod rustrt {
         pub unsafe fn rust_gc_metadata() -> *Word;
 
         pub unsafe fn rust_get_stack_segment() -> *StackSegment;
+        pub unsafe fn rust_get_c_stack() -> *StackSegment;
     }
 }
 

src/rt/rust_builtin.cpp

@@ -549,6 +549,11 @@ rust_get_stack_segment() {
     return rust_get_current_task()->stk;
 }
 
+extern "C" CDECL stk_seg *
+rust_get_c_stack() {
+    return rust_get_current_task()->get_c_stack();
+}
+
 extern "C" CDECL void
 start_task(rust_task *target, fn_env_pair *f) {
     target->start(f->f, f->env, NULL);

src/rt/rust_task.h

@@ -374,6 +374,7 @@ rust_task : public kernel_owned<rust_task>
     void call_on_c_stack(void *args, void *fn_ptr);
     void call_on_rust_stack(void *args, void *fn_ptr);
     bool have_c_stack() { return c_stack != NULL; }
+    stk_seg *get_c_stack() { return c_stack; }
 
     rust_task_state get_state() { return state; }
     rust_cond *get_cond() { return cond; }

src/rt/rustrt.def.in

@@ -49,6 +49,7 @@ rust_task_is_unwinding
 rust_get_task
 rust_try_get_task
 rust_get_stack_segment
+rust_get_c_stack
 rust_log_str
 start_task
 vec_reserve_shared_actual