Decorate-sort-undecorate is a simple sorting pattern known for decades, which was additionally popularized by Randall Schwartz in the context of Perl, earning it the moniker Schwartzian transform. While I’ve been aware of the pattern for many years, I only recently realized just how relevant it still is for optimizing everyday sorts.

Consider this code that sorts JSON objects by a field:

/// Sorts a slice of JSON objects by the "age" field.
fn sort_by_age(v: &mut [serde_json::Value]) {
    v.sort_by_key(|o| o["age"].as_i64());
}

// usage:
fn main() {
    let mut v = vec![
        json!({"name": "John", "age": 25}),
        json!({"name": "Abel", "age": 18}),
    ];
    sort_by_age(&mut v);
    assert_eq!(v[0]["name"], "Abel");
    assert_eq!(v[1]["name"], "John");
}

The key function here is not doing anything big or controversial, it consists of a single field lookup followed by cheap extraction of integer out of the Value enum – the kind of thing you’d expect a reasonable key function to do. But running this on a larger slice shows that performance isn’t exactly stellar:

fn large_vec() -> Vec<serde_json::Value> {
    std::iter::repeat_with(|| json!({"name": "John", "age": rand::thread_rng().gen_range(1..=100)}))
        .take(1_000_000)
        .collect()
}

fn main() {
    let mut v = large_vec();
    let t0 = Instant::now();
    sort_by_age(&mut v);
    let n = t0.elapsed().as_secs_f64();
    println!("{}", n);
}

On my laptop, the above code takes 1.01s to sort a slice of 1 million JSON objects. While not exactly a catastrophe, it feels like we should be able to do better than that.

First, we can switch from stable to unstable sort by changing v.sort_by_key(...) to v.sort_unstable_by_key(...). Doing so improves the timing to 0.44s, a more than 2x speed boost. In the use case that inspired this article unstable sort was fine because the comparison function provided fallback ordering when the primary key compared equal, and the initial order was meaningless anyway. We’ll continue to use unstable sort in the rest of the article because that gives best performance, but it’s trivial to revert to stable sort.

Looking at performance further, we notice that accessing a field of a JSON object must perform a hash table lookup. Since a sort has to do n log n comparisons on average, this means that we’re doing multiple lookups per each element. That’s the kind of situation where decorate-sort-undecorate might help, as it caches the extracted keys, reducing the number of lookups to “only” n, at the expense of some memory:

fn sort_by_age(v: &mut [Value]) {
    // extract JSON objects into a new vector, where each object is decorated with age
    let mut decorated: Vec<_> = v
        .iter_mut()
        .map(|o| (o["age"].as_i64(), o.take()))
        .collect();
    // sort with a key function that trivially refers to the pre-fetched age
    decorated.sort_unstable_by_key(|&(age, _)| age);
    // undecorate, i.e. store just the sorted objects back into the slice
    for (dest, (_, o)) in v.iter_mut().zip(decorated) {
        *dest = o;
    }
}

With this modification, my laptop takes only 0.059s to sort the slice of 1 million objects! This is 7.5x faster than the previous version, and 17x faster than the original stable sort. This is a much more radical speedup than I ever expected, and it’s something to definitely consider when your key function does anything more than access a struct/tuple field.

What is the explanation for such a drastic improvement? Let’s modify the key function to keep track of how many times it gets called:

    let mut cnt = 0u32;
    decorated.sort_by_key(|&(age, _)| {cnt += 1; age});
    println!("{}", cnt);

This shows that the key function is invoked 39 million times to sort 1 million elements. This is explained by log2(1_000_000) being approximately 20, so n log n is around 20 million. And for each comparison the key function needs to be called twice – once for the left and once for the right-hand side of the comparison – which adds up to 40 million. So the decoration cuts down the number of hash lookups from 40 million to 1 million, and that certainly makes a difference. Another thing to take into account is that sorting the decorated slice sorts by comparing integers directly present in the array, which allows inner loops of the sorting algorithm to be nicely inlined. On the other hand, sorting by a hashmap lookup requires call into the complex hash table code, which doesn’t inline nearly as nicely.

One downside is that we had to replace an elegant call to sort_unstable_by_key() with a bit of manual twiddling. Fortunately it turns out that the standard library has a method that does all this for us – slice::sort_by_cached_key(). According to its documentation, it sorts the slice and calls the key function at most once per element, “using temporary storage to remember the results of key evaluation”. That is functionally equivalent to our decorate-sort-undecorate, differing only in some implementation details (it doesn’t extract the original values to decorate them, but uses indices instead). With it sort_by_age() again becomes a one-liner:

fn sort_by_age(v: &mut [Value]) {
    v.sort_by_cached_key(|o| o["age"].as_i64().unwrap());
}

On my laptop this takes 0.089s to execute, making it 1.5x times slower than the hand-written decorate-sort-undecorate, but still 11x faster than the original naive implementation. The performance difference compared to the manual implementation seems to come from the fact that sort_by_cached_key() implements stable sorting. If we modify the manual decorate-sort-undecorate to use sort_by_key() instead of sort_unstable_by_key(), sort_by_cached_key() will come out as 1.34x faster, which means it’s reasonably optimized.

In conclusion, if you’re sorting by a non-trivial key function, and you have memory to spare, consider replacing sort_by_key() with sort_by_cached_key() and measure the result. If that helps, and you want to squeeze as much performance as possible, you can further optimize by implementing a manual decorate-sort-undecorate as shown above.

Some Rust questions are asked over and over, and are therefore met with short and well-rehearsed answers. “Compile in release mode when measuring performance.” “Don’t try to learn Rust with linked lists.” “Use scoped threads.” “That requires specialization.” But there is one response that is delivered in an almost checkmate-like fashion: “You are trying to construct a self-referential type.” This is swiftly explained to be impossible in current Rust, and the asker advised to pursue a different approach. If they absolutely insist on self-references, they’re referred to std::pin, which is unergonomic, hard to understand, and still requires unsafe. Alternatively, they’re given a list of crates that purport to help with creating self-referential structs, but come with disheartening caveats, and some with safety holes which can turn fatal.

Contrary to usual answers, this will show how to create self-referential types manually. A very important subset of such types are straightforward to reason about and, most importantly, can be manipulated through a safe interface. We will not rewrite the borrow checker, we will not use std::pin, but we will use a bit of unsafe. You’ve been warned.

A practical introduction

Just so you don’t think self-referential structs are a theoretical issue that only happen to people trying to implement specialized data structures, consider this task:

Use the zip crate to implement a function that accepts the name of a ZIP file and the name of a member stored inside it, and returns something that implements Read to stream the member.

Streaming the contents of a ZIP file sounds like a reasonable thing to want to do. We’d like to create a function like this:

fn zip_streamer(file_name: &str, member_name: &str) -> impl std::io::Read {
    todo!()
}

Looking at the crate docs, this seems trivial to implement. Open the file, call ZipArchive::new() to create a ZipArchive, then call ZipArchive::by_name() to get a ZipFile, which implements Read. Presto:

pub fn zip_streamer(file_name: &str, member_name: &str) -> impl std::io::Read {
    let file = File::open(file_name).unwrap();
    let mut archive = ZipArchive::new(file).unwrap();
    archive.by_name(member_name).unwrap()
}

But sadly, this fails to compile. The compiler complains that “borrowed value does not live long enough”:

error[E0597]: `archive` does not live long enough
 --> src/main.rs:8:5
  |
8 |     archive.by_name(member_name).unwrap()
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |     |
  |     borrowed value does not live long enough
  |     argument requires that `archive` is borrowed for `'static`
9 | }
  | - `archive` dropped here while still borrowed

The error message is a bit cryptic at first, it’s not clear how exactly “argument requires that archive is borrowed for static” (or what that even means), but looking at the signature of ZipArchive::by_name() reveals the problem. by_name() returns a ZipFile<'a> where 'a is the lifetime of the archive. The ZipFile we’re attempting to return references archive, a local variable. That would produce a dangling reference and obviously can’t be allowed to compile. The only way for that to work would be if archive were itself static, which is where the requirement “that archive is borrowed for ‘static” comes from.

If you’re an experienced Rustacean, at this point you’re developing a sinking feeling about where this is leading to. If you’re new to the language, or even intermediate, this can still look very much fixable: let’s just wrap both archive and its member into a common struct, return it from the function, and implement Read on the struct. That requires a bit of boilerplate, but should resolve the issue of referencing the local variable:

pub fn zip_streamer(file_name: &str, member_name: &str) -> impl std::io::Read {
    struct ZipStreamer<'a> {
        archive: ZipArchive<File>,
        reader: ZipFile<'a>,
    }

    impl Read for ZipStreamer<'_> {
        fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
            self.reader.read(buf)
        }
    }

    let file = File::open(file_name).unwrap();
    let mut archive = ZipArchive::new(file).unwrap();
    let reader = archive.by_name(member_name).unwrap();

    ZipStreamer { archive, reader }
}

You guessed it, this doesn’t compile either, and in fact fails with a very similar error message about argument requiring archive to be borrowed for ‘static, with this addition:

error[E0505]: cannot move out of `archive` because it is borrowed
  --> src/main.rs:22:17
   |
21 |     let reader = archive.by_name(member_name).unwrap();
   |                  ----------------------------
   |                  |
   |                  borrow of `archive` occurs here
   |                  argument requires that `archive` is borrowed for `'static`
22 |     ZipStreamer { archive, reader }
   |                   ^^^^^^^ move out of `archive` occurs here

Now we have a new problem: in order to construct ZipStreamer, we must move our local variable into a field of the ZipStreamer. But this move immediately invalidates reader, because as far as the borrow checker is concerned, use-after-move creates a dangling reference just like use-after-free does.

This looks bad, but there might still be hope. Can’t we box the ZipArchive to heap-allocate it, or wrap it in an Rc or Arc to postpone its destruction? But try as you might, no approach is allowed to compile.

Without ever being aware of it, we’ve been trying to create the dreaded “self-referential struct”. ZipStreamer is self-referential because the reader field refers to the archive field of the same struct. This makes the 'a lifetime nonsensical, because a lifetime refers to some data in the caller, which just doesn’t apply to the reader field. If instead of impl Read we returned the concrete type ZipStreamer<'a>, we simply wouldn’t have a lifetime to use for 'a. While our function does accept two references, neither of those is useful as the lifetime of ZipStreamer is clearly tied to the data inside itself.

This is by no means specific to the zip crate, the same issue pops up in various scenarios: a function that returns a database connection and an open transaction, or a parser that holds the string it parses and the parsed pieces that refer to string data.

Why Rust rejects self-reference

Before showing how to fix this issue, let’s once more look at the reasons why Rust prevents us from creating a self-referential structure. What would go wrong if we could create one? For example:

// imaginary self-referential struct where `current_token` points into `data`
struct ParseState {
    data: String,               // the string we're parsing
    current_token: &'data str,  // current token pointing into the string
}

There are two problems with current_token pointing into data.

First, there is no way to name the lifetime of current_token. In the above example we used a lifetime named the same as the field, inspired by a Polonius talk. In current Rust the only way to provide a lifetime on a struct member is to either make it 'static or take a lifetime declared on the struct, as in the ZipStreamer<'a> attempt above. But that lifetime is not useful, as it always corresponds to some data outside the type.

Second, Rust must assume that any move of ParseState invalidates current_token. Imagine if, ParseState were declared like this:

struct ParseState {
    data: [u8; 4096],            // the fixed-size buffer we're parsing
    current_token: &'data [u8],  // current token pointing into the buffer
}

Moving ParseState from one place to another will obviously invalidate the reference. This is not the case when the data is a String, or simply a Box, because those types heap-allocate their content, and moving the string or box has no effect on their content. But Rust doesn’t know that, it treats String the same as an array of u8, and assumes that moves are detrimental to borrows. This is the issue that prevents self-referential types from working in scenarios that would be considered perfectly safe and reasonable in C++.

This really bears repeating: there is nothing in the Rust abstract machine that prevents self-referential types from working, as long as you are willing to box them, because then you don’t run afoul of the move semantics. The remaining questions are how practical the implementation is, and whether one can expose a safe interface to it. Let’s investigate that.

Implementation of a self-referential struct

Armed with the above insight, we can sketch what we must do to make ZipStreamer work:

• box the ZipArchive in ZipStreamer::archive, so we don’t need to worry about moves breaking the ZipFile reference
• figure out a lifetime for the ZipFile stored in reader
• ensure that the references in ZipFile stored reader are valid for the whole lifetime of ZipStreamer.

Here is the result:

pub fn zip_streamer(file_name: &str, member_name: &str) -> impl std::io::Read {
    #[allow(dead_code)]
    struct ZipStreamer {
        // actually has lifetime of `archive`
        // declared first so it's dropped before `archive`
        reader: ZipFile<'static>,
        // safety: we must never move out of this box as long as reader is alive
        archive: AliasableBox<ZipArchive<File>>,
    }

    impl Read for ZipStreamer {
        fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
            self.reader.read(buf)
        }
    }

    let file = File::open(file_name).unwrap();
    let mut archive = AliasableBox::from_unique(Box::new(ZipArchive::new(file).unwrap()));
    let reader = archive.by_name(member_name).unwrap();
    let reader = unsafe { std::mem::transmute(reader) };

    ZipStreamer { archive, reader }
}

Wait, this is it?

Explanation

Let’s examine the above code in some detail.

How does it work?

It creates a self-referential type that is safe to move around because the referenced data is behind a box, so its memory location doesn’t change when the top-level type is moved. Since the fields of ZipStreamer are private, no external code can mutate archive, or move it out of its box. When the struct is dropped, the documented drop order ensures that the reader field is dropped before archive field, so the reference never dangles, even when unused.

Is it really safe?

The above function is obviously not safe in the sense of “not using any unsafe”. But it exposes a fully safe external API to functionality that uses unsafe internally – much like many parts of the standard library. In other words, I believe that zip_streamer() is sound, but as always with unsafe code, caveat emptor.

What is the meaning of the ‘static lifetime on ZipFile?

The 'static lifetime is the only lifetime we can use in that position. Any other named lifetime would have to be introduced on the type, and would make it unusable. The use of 'static does mean that we are lying to the borrow checker, as the actual lifetime is the scope of the archive member in the box, which cannot be expressed in today’s Rust. This false lifetime is why we must use unsafe below. The important thing is that this lie cannot result in incorrect code being generated – as ZipFile<'a> is generic over any lifetime and cannot misuse 'static in any way (even the notoriously experimental specialization doesn’t allow specializing on lifetimes). Lifetimes are used to reject unsound code, not to modify the generated assembly.

Some examples that show self-referential types are demonstrated on a reference, and get around the lifetime issue by storing a pointer instead. But that trick wouldn’t work here, as we don’t work with a naked reference, but with the ZipFile<'a> type which contains references. Of course, the same approach would work for a reference.

What is the purpose of transmute()?

The call to transmute() admittedly looks scary, as it’s, quoting actual documentation, incredibly unsafe.

But one of the uses of transmute() is to extend the lifetime of a reference, and that’s exactly what we’re doing here. Transmuting between two different types is fraught with pitfalls because their layouts must match precisely, which in some cases requires forcing a non-standard representation. Transmuting between the same type to extend lifetime is still unsafe, but much easier to reason about from the perspective of correct usage of transmute().

What’s the deal with AliasableBox, wouldn’t Box work as well?

When a Box is moved, Rust assumes (and the borrow checker verifies) that the Box has unique access to its contents. Every code along the lines of:

let b = Box::new(...);
let r = &b.some_field;
let b2 = b;
// use "r"

…will fail to compile because b is moved while there are outstanding references into it. But that’s the moral equivalent of what our code does when it creates archive, creates the reader that refers into it, and then moves archive into the ZipStreamer. It seems okay to do this because moving a box doesn’t change the memory address of its content. But it technically creates a dangling reference because moving the box is defined to invalidate any references into it. A reference surviving the move is something the optimizer is allowed to assume won’t happen and is therefore UB. Running the above example under MIRI reports UB due to an invalidated reference.

To fix the issue, instead of Box we can define a box-like type that internally uses a raw pointer. This has the same semantics as a Box, but prevents the optimizer from assuming that moving it invalidates references to its contents. We use AliasableBox from the aliasable crate for that purpose.

This issue was spotted by /u/Darksonn on reddit.

If this works, why doesn’t async Rust use it instead of pinning?

The approach shown here requires the implementation to adhere to a very specific unsafe contract, such as “you must not move out of this Box” or “reader must be declared before archive otherwise you invoke UB.” Async Rust aims to provide a safer experience by limiting the unsafe portion to the creation of the pin, typically abstracted by the standard library or a crate, and the rest of the code being completely safe and sound.

There are ways to abstract this approach, though, but they come at the cost of ergonomics, and is not yet agreed to be sound in all cases. This is what the self-referential crates provide, and is shown below.

Safe version with ouroboros

Ouroboros is a crate that aims to encapsulate the unsafe contract like the one in the above code into abstractions implemented with a mixture of proc macros and custom types. Here is what the above code looks like using it:

use ouroboros::self_referencing;

pub fn zip_streamer(file_name: &str, member_name: &str) -> impl std::io::Read {
    #[self_referencing]
    struct ZipStreamer {
        archive: ZipArchive<File>,
        #[borrows(mut archive)]
        #[not_covariant]
        reader: ZipFile<'this>,
    }

    impl Read for ZipStreamer {
        fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
            self.with_reader_mut(|reader| reader.read(buf))
        }
    }

    let file = File::open(file_name).unwrap();
    let archive = ZipArchive::new(file).unwrap();
    ZipStreamerBuilder {
        archive,
        reader_builder: |archive| archive.by_name(member_name).unwrap(),
    }
    .build()
}

Under the hood ouroboros generates a very similar struct to the one we’ve created manually:

pub(super) struct ZipStreamer {
    reader: ZipFile<'static>,
    archive: ::ouroboros::macro_help::AliasableBox<ZipArchive<File>>,
}

The 'static lifetime is there, as is a reexport of AliasableBox. Ouroboros tries to provide only safe interfaces – if the code compiles, it should be sound, and require no unsafe. For example it puts the above type definition in a module, which makes reader private, and the only way to access it is using with_reader_mut(). This prevents safe code from ever observing the fictitious 'static lifetime.

As mentioned above, the benefit of using a crate like ouroboros is that it provides a safe abstraction to self-referential type. However, there is a number of downsides as well:

• It generates a lot of code (500+ lines for the above simple example) which affects compilation times. It also adds convenience constructors on the type, which you can’t turn off, and which sometimes actually get in the way.
• The generated code contains a lot of unsafe hidden behind abstractions that give a sense of safety, but might turn out to be unsound, and have in the past. (I’ll refrain from linking to individual issues, but they’re easy to find on the tracker, and there’s the example of rental which was abandoned due to soundness issues.)
• It has a lot of hidden complexity, as it modifies the definition of the type in ways that are sometimes hard to reason about.

In summary, it probably makes sense to use a crate like ouroboros if it works for your use case and if you don’t mind all the extra code it generates. But I’d advise at least being aware what it does for you, as it feels like a bit of a leaky abstraction.

Limitations

The approach shown above, which includes both the hand-written unsafe and ouroboros, does have limitations. An interesting use case of self-referential structs is wrapping foreign data, such as C++ strings and vectors with small string/vector optimization. The approach in this article would technically work for such types, but would not be useful because it requires unconditional allocation, and the whole point of such types is efficiency and avoiding allocations. (They work in C++ by implementing a “move constructor” that fixes up the self-references.)

Another situation where the above doesn’t help is if safe code needs to be able to mutably access parts of the self-referential type. Since it’s safe code, you can’t just prohibit it from moving the reference out of it, you need to prevent it using the type system. This is remarkably achieved through std::pin, although many people have a hard time wrapping their heads around the concept in prctice.

Today self-referential types requires either reasoning about unsafe code, which most people shy away from for very good reason, or using crates with very serious hit on ergonomics. This is what we have to work with until Polonius comes to save us.

You read about faster hash functions and switch to one. Most of your code gets the expected speed boost, but some parts mysteriously get slower – much slower, especially when dealing with large hashmaps. If this sounds familiar, you might have encountered the stable HashMap trap.

Background

Rust’s HashMap is an implementation of Google’s SwissTable. While the hash table provides excellent performance of, the SipHash hash function used by default is designed to protect against a large class of DoS attacks at the price of performance. When you care about performance and maliciously crafted keys are not an issue, switching to a more efficient hash function can boost performance by a significant amount. Rust hash maps make it fairly easy to change the hash function by switching to a different one, and there are crates that offer high-performance hash functions. The Rust Performance Book recommends, among others, the use of the brutally effective FxHasher from rustc-hash crate and the higher-quality ahash crate. This recommendation, however, comes at a price that is rarely if ever mentioned – some O(n) hash map operations, including deserialization, degrade to O(n**2)!

The issue

Since the issue is easy to stumble upon when deserializing HashMaps, let’s use that as example. We’ll construct a HashSet for simplicity, but HashMap would exhibit the same behavior. This code creates a set of 10m random values, serializes it to a Vec<u8> using bincode, and deserializes it back into a HashSet:

use std::time::Instant;
use rand::Rng;

type TestHashSet<K> = std::collections::HashSet<K>;
//type TestHashSet<K> = rustc_hash::FxHashSet<K>;

fn main() {
    let mut rnd = rand::thread_rng();
    let nums: Vec<_> = (0..10_000_000).map(|_| rnd.gen::<u64>()).collect();

    let t0 = Instant::now();
    let h: TestHashSet<u64> = nums.into_iter().collect();
    let t1 = Instant::now();
    println!("create: {}", (t1 - t0).as_secs_f64());

    let out = bincode::serialize(&h).unwrap();
    let t2 = Instant::now();
    println!("serialize: {}", (t2 - t1).as_secs_f64());

    let h2: TestHashSet<u64> = bincode::deserialize(&out).unwrap();
    let t3 = Instant::now();
    println!("deserialize: {}", (t3 - t2).as_secs_f64());

    println!("{}", h2.len());
}

Output on my machine:

create: 1.440201369
serialize: 0.071765342
deserialize: 1.114031976

Now let’s swith to fxhash to speed it up. We only need to point the TestHashSet alias to rustc_hash::FxHashSet<K> and we’re good to go. Note that FxHashSet is itself an alias for std::collections::HashSet whose hasher is FxHasher, so it only affects the hash function, not the implementation of the table. With that change, I get this output:

create: 1.391839734
serialize: 0.081116361
deserialize: 5.2052695

Creation is slightly faster, serialization takes about the same time, but deserialization is almost 5x slower than with the stdlib hash! Tested against different set sizes, the deserialization time is close to quadratic – it takes 5s with 10m elements, 20s with 20m elements, 69s with 40m elements, and 219s with 80m elements. We’ve seen the trap.

When does it happen?

First, let’s make it clear that the problem has nothing to do with serde or bincode. Serde just happens to create the set from values obtained by iterating it. The same slowdown can be easily reproduced without serde:

// just as slow as the serde version when TestHashSet is FxHashSet
let mut h2: TestHashSet<u64> = Default::default();
for &n in &h {
    h2.insert(n);
}

Second, this is not a consequence of fxhash being of lower quality or hitting some bad edge case. The same quadratic behavior can be observed using any other stable hash function, including the default SipHash when randomization is omitted. To try it, define TestHashSet<K> as std::collections::HashSet<K, BuildHasherDefault<std::collections::hash_map::DefaultHasher>> (quite a mouthful), which uses the default hash function with a non-random initial state – you’ll get a similar slowdown.

The third thing to be aware of is that you can’t reproduce the issue with a simple clone() or collect() – things like let h2 = h.clone() and let h2: TestHashSet<u64> = h.iter().copied().collect() will be O(n), and almost 10x faster with fxhash than with the default hasher. It also doesn’t happen when you pre-allocate the set using with_capacity(). To reproduce, you need to start with a HashSet that must resize at least once to accommodate the data, such as an empty one. Both clone() and collect() preallocate the HashSet which avoids this performance degradation.

This is why serde hits it – although serde stores the size of every container as a “size hint”, it also caps this hint during deserialization to a low number, for security reasons.

In summary, to reproduce quadratic complexity of construction, all of the following conditions must be met:

• the HashMap/HashSet must resize to accommodate the values we’re inserting
• it uses a stable hash function
• it is built from keys in iteration order of the same HashMap/HashSet

Why does it happen?

This bug is thoroughly described in this 2016 article, which I encourage you to read to understand it. In short, copying a larger table into a smaller one assigns the keys from the first half of the larger table to consecutive locations. Then the keys from the second half of the larger table must fit between the keys which were already densely packed, which forms large collision chains. Although this is a temporary situation, fixed on the next resize, it affects performance visibly. But do refer to the article for details.

The article mentions Robin Hood hashing which is no longer used by Rust hash tables, but don’t let that throw you off. Although the implementation switched to a different probing strategy, it still uses open addressing with power-of-two hash table sizes.

The quadratic behavior was fixed for the default hasher by introducing randomization, but the issue is still open because the underlying bug is still present, visible with non-randomized hashers, as in the above code.

Workarounds

All fixes for this bug boil down to shuffling the items in order to avoid the construction in unfavorable order. Since the actual bug that leads to the issue is in the implementation of the hashmap, I refer to these as workarounds rather than proper fixes.

I do hope this bug gets more visibility and gets fixed, as it hurts people who try to use fxhash to get a faster HashMap, and it’s not immediately obvious what’s going on. In general, a nice property of hash tables is that, unlike unbalanced binary trees or quicksort, they’re normally not sensitive to key order. It’s a bit of a shame that that doesn’t apply to Rust’s hash tables with deterministic hashers.

Pre-allocate the HashSet/HashMap

This is by far the simplest fix when you don’t control the exact type of the map, but do control it is created. In the non-serde case, that means initialize the h2 hash map with capacity given upfront:

// fast because h2 is of the correct size to begin with
// note: can't just call with_capacity() because it assumes RandomState from stdlib
let mut h2: TestHashSet<u64> = TestHashSet::with_capacity_and_hasher(h.len(), Default::default());
for &n in &h {
    h2.insert(n);
}

This can be made to work with serde, but requires writing a custom deserializer.

Shuffle the items

Shuffling the elements also fixes the issue. For example:

// fast because h2 is built of elements which are not in iteration order of h
let mut elems: Vec<_> = h.iter().copied().collect();
elems.shuffle(&mut rnd);
let mut h2: TestHashSet<u64> = TestHashSet::default();
for n in elems {
    h2.insert(n);
}

This is a good “quick fix” when you don’t control how the table is constructed, but do control the data that flows into it. It is not that easy to apply to the serde case, where it would require a custom serializer.

Randomize the hasher

This is the most general solution, and is how the issue was worked around in the standard library. The standard library uses the RandomState to build the default hasher.

If you use another hasher, you just need to make sure that you’re using the randomizing one. (This is what AHashMap uses by default, so you don’t need to do anything special.) If your hasher doesn’t provide a random hasher builder, you can always create one using the public API. For example, for FxHasher we can create an FxRandomState that implements BuildHasher:

#[derive(Copy, Clone, Debug)]
pub struct FxRandomState(usize);

impl BuildHasher for FxRandomState {
    type Hasher = FxHasher;

    fn build_hasher(&self) -> FxHasher {
        let mut hasher = FxHasher::default();
        hasher.write_usize(self.0);
        hasher
    }
}

Ideally build_hasher() would just return FxHasher::with_seed(self.0), but with_seed() isn’t provided, so we get equivalent functionality by creating a hasher and mixing the seed into it. The remaining part is to implement Default for our hasher builder, which will obtain the actual seed. To make it efficient, we won’t get a random seed every time, but only when a new thread is created. Then within a thread we’ll assign each FxRandomState (and therefore each hash table) a new consecutive seed.

impl Default for FxRandomState {
    fn default() -> Self {
        thread_local! {
            static SEED: Cell<usize> = Cell::new(rand::thread_rng().gen())
        }
        let seed = SEED.with(|seed| {
            let n = seed.get();
            seed.set(n.wrapping_add(1));
            n
        });
        FxRandomState(seed)
    }
}

With this in place, we can alias TestHashSet<K> to HashSet<K, FxRandomState>, and the quadratic behavior goes away.

In the last several years async-friendly languages and APIs have received a large amount of attention. One contentious point in the language design space are the “colored functions”, or division of functions to async and non-async ones. The term was introduced by the now-famous 2015 article titled What Color is Your Function?, which uses color as a metaphor for the often painful mismatch between sync and async functions in JavaScript and other languages with explicitly async functions. Since 2015 many more languages have jumped on the aysnc bandwagon, so many more programmers are now getting familiar with the metaphor. Given that some languages managed to provide async IO without colored functions, such as Go, Zig, and in the future likely Java, the discussion around function colors is picking up once again and is raised in the context of Rust. Some people have even tried to argue that the bad rap of colored function doesn’t apply to Rust’s async because it’s not colored in the first place. An article from several days ago is titled “Rust’s async isn’t f#@king colored!”, and similar arguments have appeared on reddit. I’m not picking on any specific post, but I’d like to provide a response to that sort of argument in general.

In this article I will show that Rust async functions are colored, by both the original definition and in practice. This is not meant as an criticism of Rust async, though – I don’t see function colors as an insurmountable issue, but as a reflection of the fundamental difference of async and sync models of the world. Languages that hide that difference do so by introducing compromises that might not be acceptable in a systems language like Rust or C++ – for example, by entirely forbidding the use of system threads, or by complicating the invocation of foreign or OS-level blocking calls. Colored functions are also present in at least C#, Python, Kotlin, and C++, so they’re not a quirk of JavaScript and Rust. And additional features of Rust async do make it easier to connect async code with traditional blocking code, something that is just not possible in JavaScript.

Colored functions

“What Color is Your Function?” starts off by describing an imaginary language that perversely defines two types of functions: red and blue. The language enforces a set of seemingly arbitrary rules regarding how the two are allowed to interact:

1. Every function has a color.
2. The way you call a function depends on its color.
3. You can only call a red function from within another red function.
4. Red functions are more painful to call.
5. Some core library functions are red.

Without knowing the details, a reasonable person would agree that the described language is not particularly well designed. Of course, readers of this article in 2021 will not find it hard to recognize the analogy with async: red functions are async functions, and blue functions are just ordinary functions. For example, #2 and #4 refers to the fact that calling an async function requires either explicit callback chaining or await, whereas a sync function can just be called. #3 refers to the fact that await and callback resolution work only inside async functions, and JavaScript doesn’t provide a way to block the current non-async function until a promise (async value) is resolved. The article portrays async functions as a leaky abstraction that profoundly and negatively affects the language, starting with the above rules.

The rules of async make async code contagious, because using just one async function in one place requires all the callers up the stack to become async. This splits the ecosystem into async and non-async libraries, with little possibility to use them interchangeably. The article describes async functions as functions that operate on classic JavaScript callbacks, but further argues that async/await, which was novel at the time, doesn’t help with the issue. Although await constitutes a massive ergonomic improvement for calling async from async (#4), it does nothing to alleviate the split – you still cannot call async code from non-async code because await requires async.

Function colors in Rust async

How does all this apply to Rust? Many people believe that it applies only in part, or not at all. Several objections have been raised:

1. Rust async functions are in essence ordinary functions that happen to return values that implement the Future trait. async fn is just syntactic sugar for defining such a function, but you can make one yourself using an ordinary fn as long as your function returns a type that implements Future. Since async functions, the argument goes, are functions that return Future<Output = T> instead of T and as such are not “special” in any way, any more than functions that return a Result<T> instead of T are special – so rule #1 (“every function has a color”) doesn’t apply.
2. Unlike JavaScript, Rust async executors provide a block_on() primitive that invokes an async function from a non-async context and blocks until the result is available – so rule #3 (“you can only call a red function from within another red function”) doesn’t apply.
3. Again unlike JavaScript, Rust async provides spawn_blocking() which invokes a blocking sync function from an async context, temporarily suspending the current async function without blocking the rest of the async environment. This one doesn’t correspond to a rule from the original article because JavaScript doesn’t support blocking sync functions.
4. Rule #5 (“some core library functions are red”) doesn’t apply because Rust’s stdlib is sync-only.

If these arguments are correct, the only color rule that remains is rule #4, “red functions are more painful to call”, and that part is almost completely alleviated by await. The original JavaScript problems where async functions “don’t compose in expressions because of the callbacks” or “have different error-handling” simply don’t exist with await, in either JavaScript or Rust. Taking these arguments at face value, it would seem that the whole function-color problem is made up or at least wildly exaggerated from some exotic JavaScript problems that Rust async doesn’t inherit. Unfortunately, this is not the case.

First, the split between the sync and async ecosystems is immediately apparent to anyone who looks at the ecosystem. The very existence of async_std, a crate with the explicit purpose to provide an “async version of the Rust standard library”, shows that the regular standard library is not usable in an async context. If function colors weren’t present in Rust, the ordinary stdlib would be used in both sync and async code, as is the case in Go, where a distinction between “sync” and “async” is never made to begin with.

Then what of the above objections? Let’s go through them one by one and see how they hold up under scrutiny.

Aren’t Rust async functions just ordinary functions with a wacky return type?

While this is true in a technical sense, the same is also true in JavaScript and almost all languages with colored async (with the exception of Kotlin) in exactly the same way. JavaScript async functions are syntactic sugar for functions that create and return a Promise. Python’s async functions are regular callables that immediately return a coroutine object. That doesn’t change the fact that in all those languages the caller must handle the returned Promise (coroutine object in Python, Future in Rust) in ways that differ from handling normal values returned from functions. For example, you cannot pass an async function to Iterator::filter() because Iterator::filter() expects a function that returns an actual bool, not an opaque value that just might produce a bool at some point in the future. No matter what you put in the body of your async function, it will never return bool, and extracting the bool requires executor magic that creates other problems, as we’ll see below. Regardless of whether it’s technically possible to call an async function from a sync context, inability to retrieve its result is at the core of function color distinction.

Ok, but doesn’t the same apply to Result? Functions that need a u32 aren’t particularly happy to receive a Result<u32, SomeError>. A generic function that accepts u32, such as Iterator::min(), has no idea what to do with Result<u32, SomeError>. And yet people don’t go around claiming that Result somehow “colors” their functions. I admit that this argument has merit – Result indeed introduces a semantic shift that is not always easy to bridge, including in the example we used above, Iterator::filter(). There is even a proposal to add 21 new iterator methods such as try_filter(), try_min_by_key(), try_is_partitioned(), and so on, in order to support doing IO in your filter function (and key function, etc.). Doing this completely generically might require Haskell-style monads or at least some form of higher-kinded types. All this indicates that supporting both Result and non-Result types in fully generic code is far from a trivial matter. But is that enough to justify the claim that Result and Future are equivalent in how they affect functions that must handle them? I would say it’s not, and here is why.

If the recipient of a Result doesn’t care about the error case, it can locally resolve Result to the actual value by unwrapping it. If it doesn’t want to panic on error, it can choose to convert the error to a fallback value, or skip the processing of the value. While it can use the ? operator to propagate the error to its caller, it is not obliged to do so. The recipient of a Future doesn’t have that option – it can either .await the future, in which case it must become async itself, or it must ask an executor to resolve the future, in which case it must have access to an executor, and license to block. What it cannot do is get to the underlying value without interaction with the async environment.

Verdict: Rule #1 mostly applies to Rust – async functions are special because they return values that require async context to retrieve the actual payload.

Doesn’t block_on() offer a convenient way to invoke an async function from a non-async context?

Yes, provided you are actually allowed to use it. Libraries are expected to work with the executor provided by the environment and don’t have an executor lying around which they can just call to resolve async code. The standard library, for example, is certainly not allowed to assume any particular executor, and there are currently no traits that abstract over third-party executors.

But even if you had access to an executor, there is a more fundamental problem with block_on(). Consider a sync function fn foo() that, during its run, needs to obtain the value from an async function async fn bar(). To do so, foo() does something like let bar_result = block_on(bar()). But that means that foo() is no longer just a non-async function, it’s now a blocking non-async function. What does that mean? It means that foo() can block for arbitrarily long while waiting for bar() to complete. Async functions are not allowed to call functions like foo() for the same reason they’re not allowed to call thread::sleep() or TcpStream::connect() – calling a blocking function from async code halts the whole executor thread until the blocking function returns. In case of that happening in multiple threads, or in case of a single-threaded executor, that freezes the whole async system. This is not described in the original function color article because neither block_on() nor blocking functions exist in stock JavaScript. But the implications are clear: a function that uses block_on() is no longer blue, but it’s not red either – it’s of a new color, let’s call it purple.

If this looks like it’s changing the landscape, that’s because it is. And it gets worse. Consider another async function, xyzzy(), that needs to call foo(). If foo() were a blue/non-async function, xyzzy() would just call it and be done with it, the way it’d call HashMap::get() or Option::take() without thinking. But foo() is a purple function which blocks on block_on(bar()), and xyzzy() is not allowed to call it. The irony is that both xyzzy() and bar() are async and if xyzz() could just await bar() directly, everything would be fine. The fact that xyzzy() calls bar() through the non-async foo() is what creates the problem – foo‘s use of block_on() breaks the chain of suspensions required for bar() to communicate to xyzzy() that it needs to suspend until further notice. The ability to propagate suspension from the bottom-most awaitee all the way to the executor is the actual reason why async must be contagious. By eliminating async from the signature of foo() one also eliminates much of the advantage of bar() being async, along with the possibility of calling foo() from async code.

Verdict: rule #3 applies because block_on() changes a blue function into something that is neither red nor callable from red.

Doesn’t spawn_blocking() resolve the issue of awaiting blocking functions in async contexts?

spawn_blocking() is a neat bridge between sync and async code: it takes a sync function that might take a long time to execute, and instead of calling it, submits it to a thread pool for execution. It returns a Future, so you can await spawn_blocking(|| some_blocking_call()) like you’d await a true async function without the issues associated with block_on(). This is because the Future returned by spawn_blocking() is pending until until the thread pool reports that it’s done executing the submitted sync function. In our extended color metaphor, spawn_blocking() is an adapter that converts a purple function into a red function. Its main intended use case are CPU-bound functions that might take a long time to execute, as well as blocking functions that just don’t have a good async alternative. The example of the latter are functions that work with the file system, which still don’t have a good async alternative, or legacy blocking code behind FFI (think ancient database drivers and the like).

Problems arise when code tries to avoid multiple function colors and use block_on() or spawn_blocking() to hide the “color” of the implementation. For example, a library might be implemented using async code internally, but use block_on() to expose only a sync API. Someone might then use that library in an async context and wrap the sync calls in spawn_blocking(). What would be the consequences if that was done across the board? Recall that the important advantage of async is the ability to scale the number of concurrent agents (futures) without increasing the number of OS threads. As long as the agents are mostly IO-bound, you can have literally millions of them executing (most of them being suspended at any given time) on a single thread. But if an async function like the above xyzzy() uses spawn_blocking() to await a purple function like foo(), which itself uses block_on() to await an async function like bar(), then we have a problem: the number of xyzzy() instances that can run concurrently and make progress is now limited by the number of threads in the thread pool employed by spawn_blocking(). If you need to spawn a large number of tasks awaiting xyzzy() concurrently, most of them will need to wait for a slot in the thread pool to open up before their foo() functions even begin executing. And all this because foo() blocks on bar(), which is again ironic because bar(), being an async function, is designed to scale independently of the number of threads available to execute it.

The above is not just a matter of performance degradation; in the worst case spawn_blocking(|| block_on(...)) can deadlock. Consider what happens if one async function behind spawn_blocking(|| block_on(...)) needs data from another async function started the same way in order to proceed. It is possible that the other async function cannot make progress because it is waiting for a slot in the thread pool to even begin executing. And the slot won’t free up because it is taken by the first async function, which also runs inside a spawn_blocking() invocation. The slot is never going to change owner, and a deadlock occurs. This can’t happen with async functions that are directly executed as async tasks because those don’t require a slot in a fixed-size pool. They can all be in a suspended state waiting for something to happen to any of them, and resume execution at any moment. In an async system the number of OS threads deployed by the executor doesn’t limit the number of async functions that can work concurrently. (There are executors that use a single thread to drive all futures.)

Verdict: spawn_blocking() is fine to use with CPU-bound or true blocking code, but it’s not a good idea to use it with block_on() because the advantages of async are then lost and there is a possibility of deadlock.

But Rust’s stdlib is sync-only.

That’s technically true, but Rust’s stdlib is intentionally minimal. Important parts of functionality associated with Rust are delegated to external crates, with great success. Many of these external crates now require async, or even a specific executor like tokio. So while the standard library is async-free, you cannot ignore async while programming in Rust.

Verdict: technically true but not useful in a language with a minimalistic standard library.

Dealing with a two-colored world

Again, the above is not a criticism of Rust async, but merely of the claim that it’s not colored. Once we accept that it is, it becomes clear that, unlike JavaScript, Rust actually does provide the tools we need to deal with the mismatch. We can:

1. Accept that sync and async are two separate worlds, and not try to hide it. In particular, don’t write “sync” interfaces that use block_on() to hide async ones, and the other way around with spawn_blocking(). If you absolutely must hide the async interfaces behind sync ones, then do so at immediately at the entry point, document that you’re doing so, and provide a public interface to the underlying native call.
2. Respecting the above, use block_on() and spawn_blocking() in application-level code on the boundaries between the two worlds.
3. In more complex scenarios, create clear and documented boundaries between the two worlds and use channels to communicate between them. This technique is already used for both multi-threaded and async code, so it should come to no surprise to future maintainers. Ideally you’d use channels that provide both a sync and an async interface, but if those are not available, use async channels with block_on() on the sync side.