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Comment by gpanders | Hacker News Reader

>Rust also hates hidden allocations

Does it? Rust seems happy to allocate silently all the time.

    let x = String::new(“hi”);
    let y = vec![];

Do either of these allocate? As the writer or reader of this code, how do I know if either of these statements result in a heap allocation, or if the data is strictly on the stack?

Zig’s requirement of explicitly passing around an Allocator type removes any ambiguity completely.

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Sure, any arbitrary function (or macro) logic can allocate. It's more a philosophy, not something that's language-enforced[0] in Rust- if you're creating a mutable, variable-size data structure like a String or a Vec or a HashMap you're not going to be very surprised that it allocates at some point (though technically zero-length Vecs don't allocate on construction, they wait until an item is added)

But closures don't require allocation, iterators don't require allocation, async doesn't require allocation. Copy semantics also don't allow allocation- implicit copies can only happen for data structures that are bitwise-copyable, which is enforced by the compiler. For copy-with-allocation you have to implement the Clone trait, and then invoke it explicitly with the .clone() method

But the original context was a question of philosophy, so I was only speaking to Rust's overall philosophy

[0] Technically I think if you're using no_std you won't have access to any standard constructs that allocate (which obviously will prevent their use at compile-time), though I believe you're still allowed to eg. call out to foreign functions manually that would allocate. And of course, this still isn't as granular as Zig's allocation-control.

> Technically I think if you're using no_std you won't have access to any standard constructs that allocate

Indeed, because you only have core, and not alloc, which is where the allocator lives.

Mostly, core looks very similar to the functionality you can get of the same types in std - except that of course types which allocate are missing (Vec, HashMap, Box, String, etc.) and all the types which reflect operating system services are likewise missing (UdpSocket, File, Mutex, etc.)

However there are deviations. For example, Rust's slices in core don't have a sort() method. Why not? Rust provides a stable sort which uses an allocator because that's much faster, it doesn't bother providing a not-so-good stable sort that can work without using an allocator, if you can't afford an allocator but you want stable sort that's a rather niche case, Rust won't solve it. Rust's sort_unstable() doesn't need an allocator and so that is provided on slices even with only the core library.

Rust For Linux, the Linux kernel's Rust, has core, plus its own somewhat customised twist on alloc, plus an entirely custom kernel library. In Rust For Linux, the allocators are all explicit because Linus hates implied allocation. So for example in Rust For Linux a Vec doesn't have a push() method, because push() on a Vec can cause the Vec to grow, which is an allocation - instead Rust For Linux provides Vec::try_push() which fails if you'd need to grow the Vec before it could successfully push.

The Rust docs are very clear that initializing an empty vector (or hashmap) does not cause an allocation and is thus equivalent to using `Vec::with_capacity(0)`. The same applies to strings because strings are vectors of bytes with additional methods. You can also use the SmallVec library to store the first n items in an array on the stack and spill onto the heap once that length is exceeded.

Whether a clearly visible `vec.push(x)` call is silent to you depends on your viewpoint. I'd say it's recognizeable as potential allocation, it has to be when pushing to a dynamic structure of arbitrary size - but I also understand how this can be seen differently. This is, I think, where Rust and Zig differ. Rust makes most expensive operations explicit but still allows things like this or macros, Zig tries to avoid even that.

> Zig’s requirement of explicitly passing around an Allocator type removes any ambiguity completely.

Not really...

    var list = std.ArrayList(u21).init(std.testing.allocator);
    try list.append('a');

Does creating the ArrayList allocate memory? Is appending to the array list reallocates? The only thing you know is that ArrayList will sometime use the allocator, you can't tell when. And this gets worse once the abstraction level increases, if you have your own type which accepts an allocator and has 100 methods it's impossible to tell which methods allocate.

In both languages you still need to know the specifics of the data structures you are working with, the only real problem that Zig solves is to actually allow you to use custom allocators with the std collections, which is something that is still unstable in Rust.

You can tell when, because if it uses the allocator it will return an error. So the first line definitely doesn't allocate, and the second definitely does.

That is, unless you explicitly handle OOM conditions inside your construct, e.g. 'crash if you're OOM', which isn't typical in zig code. All code I interact with will return an allocator error if allocation fails.

For what it's worth (I can't tell if you knew) vec![] doesn't allocate. Actually in this context it likely won't even compile, although it depends.

vec![] says I want a Vec with nothing in it. That doesn't require an allocation. In effect you are asking for three things, a pointer, of the correct type but not actually pointing at anything, and two integers (length and capacity) which are both zero. These three things don't live on the heap, they're a local variable on your stack.

Rust's first question here is: A vec of what? If we wrote something in those square brackets, it could guess the type of that, but we didn't. It may be able to infer the type from what is pushed into this Vec named y later, except we didn't make it mutable so we can't push anything into it - or if it's returned, from the return type (Rust does not allow function parameter or return types to be inferred). If Rust isn't able to infer the type, that's an error during compilation.

  let mut y: Vec<()> = vec![];

That says I want a mutable but initially empty Vec of the empty tuple. Rust is fine with that, and the result is a Vec which can hold up to isize::MAX of the empty tuple. It won't allocate, because the empty tuple doesn't take up any actual space - it's empty. Since empty tuples are indistinguishable in some sense Rust is really destroying them when you put them in the Vec and then making fresh ones to order when you remove them and there's just no way to tell.

  let mut y: Vec<u8> = vec![];

Now we're making a mutable, initially empty, Vec of bytes. This still doesn't allocate... yet. However if we try to push a byte into it, or we ask to ensure there is space for one or more bytes in it, that will allocate.

  let y: Vec<u8> = Vec::with_capacity(1);

This allocates immediately. Even though we promised we won't actually mutate this Vec, we insisted it be created with capacity for at least one byte, which will mean an allocation. Rust may end up allocating for a modest capacity larger than one byte, since it's likely the underlying system works in larger "chunks".

(You’re forgetting a “new” in the string example)

Thanks, I fixed it :)

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