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This is missing a conversation about https://lib.rs/crates/compact_str https://lib.rs/crates/compact_str (and a few alternatives like it). TL;DR: String takes

This is missing a conversation about https://lib.rs/crates/compact_str https://lib.rs/crates/compact_str (and a few alternatives like it). TL;DR: String takes the space of three pointers, that is, 24 bytes on 64-bit archs. compact_str fits up to 24 byte strings in the same space and reverts to String for longer strings. ADD: that is, avoids heap allocation for those, unlike both Box<str> and String.

tialaramex 4y ago
Box<str> is still going to be smaller if you know how big the text is because (unlike CompactString and String) it doesn't need to carry a capacity value. In exchange of course you can't append things to it (without re-allocating) CompactString is a very clever† SSO implementation, and I'll remember it is there if I run into a situation where it might help but I firmly agree with Rust's choice not to implement the SSO optimisation in the standard library's String type. † Storing 23 UTF-8 codepoints as one of several representations in a 24 byte data structure makes sense, you can see how to write a fairly safe SSO optimisation for Rust which does that, but the CompactString scheme relies on the fact Rust's strings are by definition UTF-8 encoded to squeeze the discriminant into the same space as the last possible byte of an actual UTF-8 string, so it can store a 24 byte value like "ABCDEFGHIJKLMNOPQRSTUVWX" inline despite also distinguishing the case where it needs a heap pointer for larger strings. That's very clever.
- rtfeldman 4y ago
  > I firmly agree with Rust's choice not to implement the SSO optimisation in the standard library's String type. Out of curiosity, why is that? I don't know much about how or why that decision was made, but I'm curious.
  - lifthrasiir 4y ago
    SSO means that pretty every string operation has multiple code paths, which can be highly unpredictable. Basically it is a trade-off between memory usage and performance, and the standard library is not really a good place to make that trade-off. By comparison many C++ codes (still) copy strings all over the place for no good reason, so SSO in the standard library has a much greater appeal.
pornel 4y ago
A nice thing is that all string types have &str as the lowest common denominator, so even if you use SSO or on-stack or any other fancy string type, it's automatically compatible with almost everything.

tialaramex 4y ago

Box<str> is still going to be smaller if you know how big the text is because (unlike CompactString and String) it doesn't need to carry a capacity value. In exchange of course you can't append things to it (without re-allocating) CompactString is a very clever† SSO implementation, and I'll remember it is there if I run into a situation where it might help but I firmly agree with Rust's choice not to implement the SSO optimisation in the standard library's String type. † Storing 23 UTF-8 codepoints as one of several representations in a 24 byte data structure makes sense, you can see how to write a fairly safe SSO optimisation for Rust which does that, but the CompactString scheme relies on the fact Rust's strings are by definition UTF-8 encoded to squeeze the discriminant into the same space as the last possible byte of an actual UTF-8 string, so it can store a 24 byte value like "ABCDEFGHIJKLMNOPQRSTUVWX" inline despite also distinguishing the case where it needs a heap pointer for larger strings. That's very clever.

rtfeldman 4y ago
> I firmly agree with Rust's choice not to implement the SSO optimisation in the standard library's String type. Out of curiosity, why is that? I don't know much about how or why that decision was made, but I'm curious.
- lifthrasiir 4y ago
  SSO means that pretty every string operation has multiple code paths, which can be highly unpredictable. Basically it is a trade-off between memory usage and performance, and the standard library is not really a good place to make that trade-off. By comparison many C++ codes (still) copy strings all over the place for no good reason, so SSO in the standard library has a much greater appeal.

pornel 4y ago

A nice thing is that all string types have &str as the lowest common denominator, so even if you use SSO or on-stack or any other fancy string type, it's automatically compatible with almost everything.