Runtime services, including the task scheduler and I/O dispatcher

The rt module provides the private runtime infrastructure necessary to support core language features like the exchange and local heap, the garbage collector, logging, local data and unwinding. It also implements the default task scheduler and task model. Initialization routines are provided for setting up runtime resources in common configurations, including that used by rustc when generating executables.

It is intended that the features provided by rt can be factored in a way such that the core library can be built with different 'profiles' for different use cases, e.g. excluding the task scheduler. A number of runtime features though are critical to the functioning of the language and an implementation must be provided regardless of the execution environment.

Of foremost importance is the global exchange heap, in the module heap. Very little practical Rust code can be written without access to the global heap. Unlike most of rt the global heap is truly a global resource and generally operates independently of the rest of the runtime.

All other runtime features are task-local, including the local heap, the garbage collector, local storage, logging and the stack unwinder.

The relationship between rt and the rest of the core library is not entirely clear yet and some modules will be moving into or out of rt as development proceeds.

Several modules in core are clients of rt:

• std::task - The user-facing interface to the Rust task model.
• std::local_data - The interface to local data.
• std::gc - The garbage collector.
• std::unstable::lang - Miscellaneous lang items, some of which rely on std::rt.
• std::cleanup - Local heap destruction.
• std::io - In the future std::io will use an rt implementation.
• std::logging
• std::comm