[tialaramex]

Very often if you have text, which this does, you can make huge savings by being intelligent with the text.

Rust intentionally provides the simplest possible growable string buffer String, which is literally (under the hood, you can't poke this legitimately) Vec<u8> plus the promise that this is UTF-8 text.

But you might find your needs better served by one (or several) of:

Box<str> -- you don't need capacity, so, don't store it => length == capacity

CompactString -- use the entire 24 bytes for SSO, up to 24 bytes of UTF-8 inline, obviously doesn't make sense if all or the vast majority of your strings are 25 bytes or longer

ColdString -- same idea but for 8 bytes, and also not storing capacity, this only makes sense over Box<str> if you have plenty of <= 8 byte strings

[ben-schaaf]

There's really an endless list of these optimizations. A few I've used (though not necessarily in rust):

Atoms: Each string can be referenced with a single u32 or even u16, and they're inherently deduplicated.

Bump allocator: your strings are &str, allocation is super fast with limited fragmentation.

Single pointer strings (this has a name, I can't think of it right now): you store the length inside the allocation instead of in each reference, so your strings are a single pointer.

[tialaramex]

ColdString is both your "Single pointer string" and a Small String Optimisation on top.

First, on the heap we have a self-indicating length prefix, basically we use the bottom 7 bits of each byte to indicate 7 bits of length and the top bit indicates there are more bits in the next byte. So "ben-schaaf" would be 0x0A then the ASCII for "ben-schaaf"

But, we avoid even having a heap allocation if we have 8 or fewer UTF-8 bytes to encode the text, that's our Small String Optimisation.

To pull this off we specify that our heap allocations will have 4 byte alignment even though they don't need it. This shouldn't be a problem, in fact many allocators never actually deliver smaller alignments anyway.

This means our pointer now has two spare bits, the least significant bits are now always zero for a valid heap pointer. We rotate these bits to the top of the first byte (this varies depending on whether the target is big-endian or little-endian) and we mask them so that for these valid pointers they are 0b10xxxxxx

So, now we can look at the "single pointer" and figure out

If it begins 0b10xxxxxx it really will be a valid pointer, rotate it, mask out that flag bit and dereference the pointer to find the length-prefixed text.

If it begins 0b11111AAA there's a short string here but it didn't need all 8 bytes, the next AAA bytes of the "pointer" are just UTF-8 and conveniently AAA is enough binary for 0 through 7 to be signalled, the exact length we have

If it has any other value the entire 8 bytes of "pointer" is a UTF-8 encoded string

[VorpalWay]

Storing the length in the allocation has potential performance tradeoffs too! The most obvious is that taking a substring/string view will need a copy (or use a different type that store the length outside the allocation).

But it also means the CPU has to follow the pointer (and potentially get a cache miss or pipeline stall) to find the length. Having a fat pointer of ptr+length makes a lot of sense for string views, and for owned string buffers with capacity it can mean avoiding a cache miss when appending to the buffer.

It's complicated in other words.

[Rendello]

> Bump allocator

You can build an ad-hoc bump allocator by using a String and indexing into it. You can't use &str references though, as a growing String may reallocate elsewhere and invalidate your references (Rust won't even let you try this), so you have to use your own indices. This is the same thing that bump allocator libraries usually do, too. It can be tricky but have great performance gains.

I recently 100x-d the speed of an XML/HTML builder I use internally by rewriting it to only have one thing on the heap, a single String. Every push happens right at the call site linearly, and by passing data through closures the formatting (indentation, etc.) is controllable. My first iteration was written in the least efficient way possible and had thousands of tiny allocations in nested heap objects, it was painfully slow.

[MrBuddyCasino]

Atoms: is this similar to interned strings?

[locknitpicker]

> Atoms: is this similar to interned strings?

Yes. It is exactly how they are described.

https://docs.rs/string_cache/latest/string_cache/struct.Atom...

> Represents a string that has been interned.

[ben-schaaf]

The names of these things are hazy and inconsistent. In Java and C# an interned string is the same type as other strings. Others describe atoms as interned strings, some call them symbols. At my work we call the u16/u32 atoms and interned strings are the single pointer strings described above.

[locknitpicker]

> There's really an endless list of these optimizations.

These aren't really optimizations. They are specialized implementations that introduce design and architectural tradeoffs.

For example, Rust's Atom represents a string that has been interned, and it's actually an implementation of a design pattern popular in the likes of Erlang/Elixir. This is essentially a specialized implementations of the old Flyweight design pattern, where managing N independent instances of an expensive read-only object is replaced with a singleton instance that's referenced through a key handle.

I would hardly call this an optimization. It actually represents a significant change to a system's architecture. You have to introduce a set of significant architectural constraints into your system to leverage a specific tradeoff. This isn't just a tweak that makes everything run magically leaner and faster.

[dryarzeg]

> everything run magically leaner and faster

In my opinion, there's no magic in the software engineering. Everything (or almost everything) is a system that can be described, explained, modified and so on. Applications, libraries, operating systems, kernels, CPUs/RAM/GPU/NPU/xPU/whatever silicon there is, ALUs/etc, transistors, electricity, physics... That's nowhere near "magic". There's always some trade-offs, it's just that you may not be aware of them initially.

[ben-schaaf]

You might want to refresh your understanding of the word optimisation. Changing a system to be more effective/efficient is optimisation, how big that change is makes no difference.

[SkiFire13]

> String, which is literally (under the hood, you can't poke this legitimately) Vec<u8>

`String::as_vec_mut` kinda implies that, since it gives you access to that underlying `Vec` which must then exist somewhere.

[_flux]

I looked it up: https://doc.rust-lang.org/std/string/struct.String.html#meth...

In case anyone else was wondering it, yes, it's "unsafe".

[tialaramex]

The thing they were gesturing at, correctly, is the naming. This is of course a convention and not a promise, but by convention Goose::as_crow would be a function that is cheap and gets you say &Crow instead of the &Goose you might have now, whereas Goose::to_donkey suggests that although we can have a Donkey instead of this Goose it's expensive to do that.

Commonly as... conversions are actually no-ops at runtime (the type changes but the data does not, no CPU instructions are emitted) whereas to... conversions might do quite a lot, especially if they bring into existence an actual thing at runtime -- maybe Goose::to_donkey actually needs to go allocate memory for a Donkey and destroy the Goose.

Yes it's unsafe because the Vec doesn't enforce the promise we made about this being UTF-8 text whereas String did, so now that promise is ours to keep and `unsafe` is how we signify that you the programmer took on the responsibility for safety here.

[SkiFire13]

Yes, naming does play a role here, but the biggest hint is `as_vec_mut` returning a reference. For that to work a `Vec<u8>` needs to exist somewhere, and continue to exist after this function returns. For comparison, `to_` conversions generally just return the new data, so this reasoning doesn't apply to them.

[eldenring]

CompactStr doesnt have any additional runtime overhead iirc right? So in theory you can drop it in everywhere even when you expect > 25 chars. Maybe an extra branch in the >25 char case?

[kibwen]

SSO does have overhead. Firstly, on every access you have a branch. Secondly, and more severely, the "most general" umbrella type that all string methods are defined on is a string slice, and whereas conversion from `String` to `&str` is literally a no-op, SSO strings require work to be done to convert them to string slices. Furthermore, note that in the (surprisingly common) case where the string is zero-length, String already skips the allocation, same as an SSO string.

[lenkite]

I wish Box and Option got specialized specialized shorthand syntax in Rust say `^`/ `? or something like that.

   Option<Box<SmithyTraits> -> SmithyTrait^?

[ninkendo]

Box<T> used to be ~T early on in rust… (then it became a `box` keyword, before being removed entirely.) They got rid of it because they wanted to move more things into libraries and have a less opinionated compiler.

I think I agree though, especially with Option. Swift’s option syntax (and kotlin’s which is similar) is so much better, a simple question mark in the type. Options are important enough that dedicated syntax makes so much sense. Rust blew their chance here with ? meaning “maybe early return”, it would have been a lot more useful as an Option indicator.

[kibwen]

I used to think this too, but nowadays I think this would have been a mistake. Result is actually the more important and fundamental type, not Option, and giving Option special syntax would cause people to want to favor it over Result even when it shouldn't be. If anything, I think that `?` should have been reserved as a suffix for function names (not types) that return Result, so that instead of `fn try_foo()` and `let x = try_foo();` we could have `fn foo?()` and `let x = foo?();`, and then the current `?` operator could just be spelled `.try`, akin to `.await`. (And then we maybe could go further and reserve `!` as a suffix for functions that can panic...)

[foresterre]

The Try trait (representing the ? the operation) is super cool though! I wish it was marked stable so you could implement it for types without using the nightly compiler.

Note that both Option and Result implement that same trait.

Perhaps if try blocks ever become a thing... we can finally use it for our own types ;)

https://doc.rust-lang.org/std/ops/trait.Try.html

[tialaramex]

The modern Try trait (try_v2) is indeed wonderful and I hope to see it stabilized one day.

AIUI a key innovation is ControlFlow, reifying the Break/ Continue choice as a sum type in the type system. This is already stable and is a useful piece of vocabulary even without its contribution to understanding the Try trait.

Knowing that Bob's CircusPerformance trait and Sarah's SeaLion type both use ControlFlow to decide whether we should keep going or halt ASAP means you don't have to write fraught adaptor code because Bob thought obviously the boolean "true" means keep going while Sarah's understanding was that it's a signal about being finished, so "true" means stop.

For Try what ControlFlow did was unlock the difference between "Success / Failure" as encoded by Result::Ok and Result::Err and the "Halt / Carry on" distinction ControlFlow::Break and ControlFlow::Continue. Often we want to stop when there's an error, but sometimes we mean the exact opposite, carry on trying things until one of them succeeds.

[arijun]

What does Box<str> give you that &str doesn’t?

[h4x0rr]

Ownership