Convert memory_region/boxed_region to Rust

[brson]

These implement the local heap allocator. Can probably all live in std::rt::local_heap.

[eamartin]

I've spent a little while looking at the code for this and am hoping to tackle this issue over the weekend.

I have one question so far about the code.

The Rust code (in std::rt:local_heap) contains:

struct Env { priv opaque: () }

struct BoxedRegion {
    env: *Env,
    backing_region: *MemoryRegion,
    live_allocs: *raw::Box<()>,
}

while the C++ boxed_region struct looks like

class boxed_region {
private:
    bool poison_on_free;
    memory_region *backing_region;
    rust_opaque_box *live_allocs;
// some code here ommitted
}

Does the the pointer to the Env struct in BoxedRegion exist just for alignment purposes? It seems a bit odd to use an unsafe pointer to align with a bool (why not just use a Rust bool). Since I'm porting this to Rust, can I just get rid of the Env struct and replace the pointer to env with a poison_on_free field?

edit: Just noticed the rust_env empty struct in both memory_region.h and boxed_region.h but the type is never used. Now I'm a bit more confused. Is there just a remnant from some previous refactoring?

[brson]

rust_env is a declaration left over from previous refactoring, yes. It no longer exists. I can't explain the Env type in the BoxedRegion struct - it looks completely bogus to me and should mirror the C++ decl.