Learn Rust With Entirely Too Many Linked Lists

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NOTE: The current edition of this book is written against Rust 2018, which was first released with rustc 1.31 (Dec 8, 2018). If your rust toolchain is new enough, the Cargo.toml file that cargo new creates should contain the line edition = "2018" (or if you're reading this in the far future, perhaps some even larger number!). Using an older toolchain is possible, but unlocks a secret hardmode, where you get extra compiler errors that go completely unmentioned in the text of this book. Wow, sounds like fun!

I fairly frequently get asked how to implement a linked list in Rust. The answer honestly depends on what your requirements are, and it's obviously not super easy to answer the question on the spot. As such I've decided to write this book to comprehensively answer the question once and for all.

In this series I will teach you basic and advanced Rust programming entirely by having you implement 6 linked lists. In doing so, you should learn:

  • The following pointer types: &, &mut, Box, Rc, Arc, *const, *mut
  • Ownership, borrowing, inherited mutability, interior mutability, Copy
  • All The Keywords: struct, enum, fn, pub, impl, use, ...
  • Pattern matching, generics, destructors
  • Testing
  • Basic Unsafe Rust

Yes, linked lists are so truly awful that you deal with all of these concepts in making them real.

Everything's in the sidebar (may be collapsed on mobile), but for quick reference, here's what we're going to be making:

  1. A Bad Singly-Linked Stack
  2. An Ok Singly-Linked Stack
  3. A Persistent Singly-Linked Stack
  4. A Bad But Safe Doubly-Linked Deque
  5. An Unsafe Singly-Linked Queue
  6. TODO: An Ok Unsafe Doubly-Linked Deque
  7. Bonus: A Bunch of Silly Lists

Just so we're all the same page, I'll be writing out all the commands that I feed into my terminal. I'll also be using Rust's standard package manager, Cargo, to develop the project. Cargo isn't necessary to write a Rust program, but it's so much better than using rustc directly. If you just want to futz around you can also run some simple programs in the browser via play.rust-lang.org.

Let's get started and make our project:

> cargo new --lib lists
> cd lists

We'll put each list in a separate file so that we don't lose any of our work.

It should be noted that the authentic Rust learning experience involves writing code, having the compiler scream at you, and trying to figure out what the heck that means. I will be carefully ensuring that this occurs as frequently as possible. Learning to read and understand Rust's generally excellent compiler errors and documentation is incredibly important to being a productive Rust programmer.

Although actually that's a lie. In writing this I encountered way more compiler errors than I show. In particular, in the later chapters I won't be showing a lot of the random "I typed (copy-pasted) bad" errors that you expect to encounter in every language. This is a guided tour of having the compiler scream at us.

We're going to be going pretty slow, and I'm honestly not going to be very serious pretty much the entire time. I think programming should be fun, dang it! If you're the type of person who wants maximally information-dense, serious, and formal content, this book is not for you. Nothing I will ever make is for you. You are wrong.

An Obligatory Public Service Announcement

Just so we're totally 100% clear: I hate linked lists. With a passion. Linked lists are terrible data structures. Now of course there's several great use cases for a linked list:

  • You want to do a lot of splitting or merging of big lists. A lot.
  • You're doing some awesome lock-free concurrent thing.
  • You're writing a kernel/embedded thing and want to use an intrusive list.
  • You're using a pure functional language and the limited semantics and absence of mutation makes linked lists easier to work with.
  • ... and more!

But all of these cases are super rare for anyone writing a Rust program. 99% of the time you should just use a Vec (array stack), and 99% of the other 1% of the time you should be using a VecDeque (array deque). These are blatantly superior data structures for most workloads due to less frequent allocation, lower memory overhead, true random access, and cache locality.

Linked lists are as niche and vague of a data structure as a trie. Few would balk at me claiming a trie is a niche structure that your average programmer could happily never learn in an entire productive career -- and yet linked lists have some bizarre celebrity status. We teach every undergrad how to write a linked list. It's the only niche collection I couldn't kill from std::collections. It's the list in C++!

We should all as a community say no to linked lists as a "standard" data structure. It's a fine data structure with several great use cases, but those use cases are exceptional, not common.

Several people apparently read the first paragraph of this PSA and then stop reading. Like, literally they'll try to rebut my argument by listing one of the things in my list of great use cases. The thing right after the first paragraph!

Just so I can link directly to a detailed argument, here are several attempts at counter-arguments I have seen, and my response to them. Feel free to skip to the first chapter if you just want to learn some Rust!

Performance doesn't always matter

Yes! Maybe your application is I/O-bound or the code in question is in some cold case that just doesn't matter. But this isn't even an argument for using a linked list. This is an argument for using whatever at all. Why settle for a linked list? Use a linked hash map!

If performance doesn't matter, then it's surely fine to apply the natural default of an array.

They have O(1) split-append-insert-remove if you have a pointer there

Yep! Although as Bjarne Stroustrup notes this doesn't actually matter if the time it takes to get that pointer completely dwarfs the time it would take to just copy over all the elements in an array (which is really quite fast).

Unless you have a workload that is heavily dominated by splitting and merging costs, the penalty every other operation takes due to caching effects and code complexity will eliminate any theoretical gains.

But yes, if you're profiling your application to spend a lot of time in splitting and merging, you may have gains in a linked list.

I can't afford amortization

You've already entered a pretty niche space -- most can afford amortization. Still, arrays are amortized in the worst case. Just because you're using an array, doesn't mean you have amortized costs. If you can predict how many elements you're going to store (or even have an upper-bound), you can pre-reserve all the space you need. In my experience it's very common to be able to predict how many elements you'll need. In Rust in particular, all iterators provide a size_hint for exactly this case.

Then push and pop will be truly O(1) operations. And they're going to be considerably faster than push and pop on linked list. You do a pointer offset, write the bytes, and increment an integer. No need to go to any kind of allocator.

How's that for low latency?

But yes, if you can't predict your load, there are worst-case latency savings to be had!

Linked lists waste less space

Well, this is complicated. A "standard" array resizing strategy is to grow or shrink so that at most half the array is empty. This is indeed a lot of wasted space. Especially in Rust, we don't automatically shrink collections (it's a waste if you're just going to fill it back up again), so the wastage can approach infinity!

But this is a worst-case scenario. In the best-case, an array stack only has three pointers of overhead for the entire array. Basically no overhead.

Linked lists on the other hand unconditionally waste space per element. A singly-linked lists wastes one pointer while a doubly-linked list wastes two. Unlike an array, the relative wasteage is proportional to the size of the element. If you have huge elements this approaches 0 waste. If you have tiny elements (say, bytes), then this can be as much as 16x memory overhead (8x on 32-bit)!

Actually, it's more like 23x (11x on 32-bit) because padding will be added to the byte to align the whole node's size to a pointer.

This is also assuming the best-case for your allocator: that allocating and deallocating nodes is being done densely and you're not losing memory to fragmentation.

But yes, if you have huge elements, can't predict your load, and have a decent allocator, there are memory savings to be had!

I use linked lists all the time in <functional language>

Great! Linked lists are super elegant to use in functional languages because you can manipulate them without any mutation, can describe them recursively, and also work with infinite lists due to the magic of laziness.

Specifically, linked lists are nice because they represent an iteration without the need for any mutable state. The next step is just visiting the next sublist.

However it should be noted that Rust can pattern match on arrays and talk about sub-arrays using slices! It's actually even more expressive than a functional list in some regards because you can talk about the last element or even "the array without the first and last two elements" or whatever other crazy thing you want.

It is true that you can't build a list using slices. You can only tear them apart.

For laziness we instead have iterators. These can be infinite and you can map, filter, reverse, and concatenate them just like a functional list, and it will all be done just as lazily. No surprise here: slices can also be coerced to an iterator.

But yes, if you're limited to immutable semantics, linked lists can be very nice.

Note that I'm not saying that functional programming is necessarily weak or bad. However it is fundamentally semantically limited: you're largely only allowed to talk about how things are, and not how they should be done. This is actually a feature, because it enables the compiler to do tons of exotic transformations and potentially figure out the best way to do things without you having to worry about it. However this comes at the cost of being able to worry about it. There are usually escape hatches, but at some limit you're just writing procedural code again.

Even in functional languages, you should endeavour to use the appropriate data structure for the job when you actually need a data structure. Yes, singly-linked lists are your primary tool for control flow, but they're a really poor way to actually store a bunch of data and query it.

Linked lists are great for building concurrent data structures!

Yes! Although writing a concurrent data structure is really a whole different beast, and isn't something that should be taken lightly. Certainly not something many people will even consider doing. Once one's been written, you're also not really choosing to use a linked list. You're choosing to use an MPSC queue or whatever. The implementation strategy is pretty far removed in this case!

But yes, linked lists are the defacto heroes of the dark world of lock-free concurrency.

Mumble mumble kernel embedded something something intrusive.

It's niche. You're talking about a situation where you're not even using your language's runtime. Is that not a red flag that you're doing something strange?

It's also wildly unsafe.

But sure. Build your awesome zero-allocation lists on the stack.

Iterators don't get invalidated by unrelated insertions/removals

That's a delicate dance you're playing. Especially if you don't have a garbage collector. I might argue that your control flow and ownership patterns are probably a bit too tangled, depending on the details.

But yes, you can do some really cool crazy stuff with cursors.

They're simple and great for teaching!

Well, yeah. You're reading a book dedicated to that premise. Well, singly-linked lists are pretty simple. Doubly-linked lists can get kinda gnarly, as we'll see.

Take a Breath

Ok. That's out of the way. Let's write a bajillion linked lists.

On to the first chapter!