generational-arena VS bumpalo

In my experience it is often easier to use indexes instead of copying Rc's. If you want to mutate the graph, then look in to slab and generational-arena

Regarding ECS: yes, but in a deflationary sense. The game has entities, stored in something very similar to GenerationalArena, and it has "systems", which are just functions that operate on these entities. The components themselves are just fields of the Entity megastruct. Having an ECS in the narrow sense doesn't really make a lot of sense for this game, because a lot of its rules are dependent on each other, and there's very little chance to extract parallelism. Also, even the current largest levels have less than 10k entities, so simulation performance is not a bottleneck yet.

If you cannot inplace construct the slice-dst on heap and your slice is too large to be copied, then I think there are two solutions: - Using Box>, this adds another level of indirection but avoids the copying - Use an arena like slotmap, slab, generational_arena or concurrent_arena to store the Box<[u8]>. It still needs heap allocation, but it allocates in chunks, thus less fragmentation and performs better.

I think this is less of a good fit though: the word "arena" doesn't imply iteration is possible to me. I think arenas also conventionally will reuse previously-released handles (unless you implement akin to a generational arena), so the term might be a bit misleading in that regard.

Might wanna check out https://github.com/fitzgen/generational-arena / https://github.com/ArnaudValensi/vec-tree/blob/master/tests/tests.rs.

This is far from the only arena allocator written in Rust.

From the same author, a zero-unsafe arena allocator: https://github.com/fitzgen/generational-arena

There are many, many arena implementations available with varying characteristics. It's disingenuous to act like Rust requires the author of an arena library to write "unsafe" everywhere.

If someone wants to adapt it to create computergames, then it would probably be useful to find a way to introduce generational generational indexes

Long story short, heap allocation is painfully slow. Any sort of malloc will always be slower than a custom pool or a bump allocator, because it has a lot more context to deal with.

Rust makes it especially hard to use custom allocators, see bumpalo for example [0]. To be fair, progress is being made in this area [1].

Theoretically one can use a "handle table" as a replacement for pools, you can find relevant discussion at [2].

[0] https://github.com/fitzgen/bumpalo

There are ways to accomplish this as well. Different allocator libraries exist for this kind of scenario, namely bumpallo which allocates a larger block of memory from the kernel, and allocates quickly thereafter. That would amortize the cost of memory allocations in the way I think you're after?

Meaning, storing a lot of things in the same block of allocated memory? Vec is a thing, you know. There's also a bump allocator library.

There are data structures like slotmap, and relatively low-level crates like bumpalo. This is not to say that either fits your use case, just that you definitely have access to the necessary parts to fit what you describe.

Sometimes you can put everything in a bump allocator, then when you're done, free the entire bump allocator in one go. https://docs.rs/bumpalo/

This is useful with crates like bumpalo which give you bump-allocation arenas whose lifetimes are tied to the objects they allocate.

TSC doesn't need to "stick around", right? Just a run-once and the program is over?

In those cases, https://github.com/fitzgen/bumpalo works amazingly as an arena. You can pretty much forget about reference counting and have direct references everywhere in your graph. The disadvantage is that it's hard to modify your tree without leaving memory around.

We use it extensively in http://github.com/dioxusLabs/dioxus and don't need to worry about Rc anywhere in the graph/diffing code.

Probably bumpalo, but then its Box will have a lifetime parameter - bumpalo::boxed::Box<'a, dyn MyTrait>

Strictly speaking, Rust doesn't need this as a built-in language feature, because its design allows it to be implemented as a third-party library: https://docs.rs/bumpalo

The biggest problem is that there's some awkwardness around RAII; I'm not sure whether that could have been avoided with a different approach.

Of course, ideally you'd want it to be compatible with the standard-library APIs that allocate. This is implemented, but is not yet at the point where they're sure they won't want to make backwards-incompatible changes to it, so you can only use it on nightly. https://doc.rust-lang.org/stable/std/alloc/trait.Allocator.h...

Or are you suggesting that the choice of allocator should be dynamically scoped, so that allocations that occur while the bump allocator is alive automatically use it even if they're in code that doesn't know about it? I think it's not possible for that to be memory-safe; all allocations using the bump allocator need to know about its lifetime, so that they can be sure not to outlive it, which would cause use-after-free bugs. I'm assuming that Odin just makes the programmer responsible for this, and if they get it wrong then memory corruption might occur; for a memory-safe language like Rust, that's not acceptable.