Feature request: make every String a smartstring

[newpavlov]

I am not sure whether implementing the small string optimization in std is a good idea or not, but I think it should be possible to do in a backwards compatible way. Unfortunately, breaking of a bunch of crates which rely on unsound assumptions about String internals along the way.

According to my vague memories, std does not have the optimization because:

• It simplifies the code base.
• Behavior and performance of strings does not depend on compilation target.
• Code does not have branching during string data access.

@Nokel81

I don't think this is possible because the layout of String is already guaranteed and thus cannot be changed due to backwards compatibility guarantees.

Can you link documentation which specifies those guaranties? I don't remember anything like this. If anything, the IP address debacle would point in the other direction. The closest thing is the representation section, but it only says that String has a pointer, a length, and a capacity, it has no restrictions on potentially implementing the small string optimization.

@steffahn

E. g. there's a good amount of crates relying on String having a stable address. stable_deref_trait::StableDeref - Rust

I think such reliance is quite suspicious and AFAIK, strictly speaking, is unsound.

Relevant older topic:

Short String optimization

Has there been any discussion of having String live entirely on the stack for len < size_of::<usize>() * n? Imagine enum StringInner { Stack { len: u8, bytes: [u8; N], }, Heap(Vec<u8>), } Of course, off the top of my head I don’t know how well this plays with existing APIs, but I wanted to put down the idea for discussion before I mull it over more. Note that this is the same size of the current String, if we use StringInner::Heap.0's RawVec's Unique as a niche. I …