Consider using a drop guard when enabling tracking.

[tobz]

In certain cases, leaving tracking enabled before main exits can lead to reentrancy of the tracking infrastructure as cleanup routines as triggered. As a concrete example, if println! is used in the allocation tracker implementation, this causes a panic at program exit:

• implicit cleanup routine for stdio runs at end of main
• this routine takes a lock and mutably borrows some internal state from a RefCell
• a deallocation happens, which drops us into allocation tracking
• as we println! in the allocation tracker, this drops us back into the print infrastructure
• the locks guarding stdout/stderr are reentrant, so we still able to take them
• the code again tries to mutably borrow some internal state, but alas, it is already borrowed
• gasp! le panic.

While printing directly in the allocation tracker implementation is not advised, this still represents a general issue where we likely shouldn't be tracking anything past actual program code, which using a drop guard should achieve: we can clear the tracker implementation right before any runtime boilerplate cleanup runs.