It sounds like they are talking about a semi-space collector, which I can imagine might in theory be faster than scoped allocation since allocation is a simple pointer bump (just like stack allocation) but when garbage collecting you only have to traverse the live set. However, semi-space collectors can't collect non-memory resources without additional metadata, and they have a memory overhead of at least 2x the maximum live set. I have a hard time imagining even a semispace collector outperforming stack allocation.

Of course if your program is short-lived you can just allocate a sufficiently large heap, pointer bump the whole way and free all non-memory resources on program termination, which ought to be cheaper (but probably not that much in practice due to cache effects).

In Rust, you often use stack allocation, which, as you pointed out, is much more efficient. There's some heap allocation in Rust, but in my experience it's actually a minority of all allocations.

Rust also has an Arena library (http://doc.rust-lang.org/0.11.0/arena/index.html), which can be used for cases where you know that you have enough memory for a given operation and are willing to throw out the entire thing when the operation (or program ;) ) is done.

That link is outdated, see http://doc.rust-lang.org/arena/.

Arena allocation has similar efficiencies as GC.

Except for not having the mark phase (also not having to copy). The mark phase can take a lot of time.

Not necessarily the same. For example for arena allocation, you can drop lots of related objects at the same time. If you don't have to run destructors on them, the cost will be much lower than any other way of managing them.