src/rt/rust_task.cpp has generally weird code in rust_task::new_big_stack

[mstewartgallus]

I know that eventually the task stuff will be rewritten in Rust but for now the file src/rt/rust_task.cpp on incoming has generally weird code in the rust_task::new_big_stack method. I found out about this by using clang's static analysis capabilities (which can be used with scan-build configure; scan-build make;)

Here's some of the code which I thought was weird (it starts at https://github.com/mozilla/rust/blob/incoming/src/rt/rust_task.cpp#L614 ):

big_stack->task = this;
big_stack->next = stk->next;
if (big_stack->next)
    big_stack->next->prev = big_stack;
big_stack->prev = stk;
if (stk)
    stk->next = big_stack;

In the code, stk is dereferenced to get to nxt but is then later checked at if (stk) to see if it is null. Either the line of code big_stack->next = stk->next; should be put inside a check for nullity, or there should be an assertion that stk is not null at the top of the method, and stk should not be checked at stk->next = bit_stack;

I tried to figure out the logic here but I have no experience, and so was confused. Hopefully someone else can figure out how to correct this code.