flexstr

A flexible, simple to use, immutable, clone-efficient String replacement for Rust. It unifies literals, inlined, and heap allocated strings into a single type.

Overview

Rust is great, but it's String type is optimized as a mutable string buffer, not for typical string use cases. Most string use cases don't modify their contents, often need to copy strings around as if they were cheap like integers, typically concatenate instead of modify, and often end up being cloned with identical contents. Additionally, String isn't able to wrap a string literal without additional allocation and copying.

Rust needs a new string type to unify usage of both literals and allocated strings in these typical use cases. This crate creates a new string type that is optimized for those use cases, while retaining the usage simplicity of String.

This type is not inherently "better" than String, but different. It works best in 'typical' string use cases (immutability, concatenation, cheap multi ownership) whereas String works better in "string buffer" usage cases (mutability, string building, single ownership).

Installation

NOTE: The serde feature is optional and only included when specified.

[dependencies]
flexstr = { version = "0.4", features = ["serde"] }

Examples

use flexstr::{flex_fmt, FlexStr, IntoFlexStr, ToCase, ToFlexStr};

fn main() {
  // Use an `into` function to wrap a literal, no allocation or copying
  let static_str = "This will not allocate or copy".into_flex_str();
  assert!(static_str.is_static());

  // Strings up to 22 bytes (on 64-bit) will be inlined automatically 
  // (demo only, use `into` for literals as above)
  let inline_str = "inlined".to_flex_str();
  assert!(inline_str.is_inlined());

  // When a string can't be wrapped/inlined, it will heap allocate
  let rc_str = "This is too long to be inlined. It will be wrapped in `Rc`".to_flex_str();
  assert!(rc_str.is_heap());

  // You can efficiently create a new `FlexStr` (without creating a `String`)
  // This is equivalent to the stdlib `format!` macro
  let inline_str2 = flex_fmt!("in{}", "lined");
  assert!(inline_str2.is_inlined());
  assert_eq!(inline_str, inline_str2);

  // We can upper/lowercase strings without converting to a `String`
  // This doesn't heap allocate
  let inline_str3: FlexStr = "INLINED".to_ascii_lower();
  assert!(inline_str3.is_inlined());
  assert_eq!(inline_str, inline_str3);

  // Clone is almost free, even when borrowed 
  // (at most it is a ref count increment for heap allocated strings)
  let static_str2 = (&static_str).clone();
  assert!(static_str2.is_static());

  // Regardless of storage type, these all operate seamlessly together 
  // and choose storage as required
  let heap_str2 = static_str2 + &inline_str;
  assert!(heap_str2.is_heap());
  assert_eq!(heap_str2, "This will not allocate or copyinlined");
}

How Does It Work?

Internally, FlexStr uses an enum with these variants:

• Static - A simple wrapper around a static string literal (&'static str)
• Inlined - An inlined string (no heap allocation for small strings)
• Heap - A heap allocated (reference counted) string

The type automatically chooses the best storage and allows you to use them interchangeably as a single string type.

Features

• Optimized for immutability and cheap cloning
• Allows for multiple ownership of the same string memory contents
• Serves as a universal string type (unifying literals and allocated strings)
• Doesn't allocate for literals and short strings (64-bit: up to 22 bytes)
• The same size as a String (64-bit: 24 bytes)
• Optional serde serialization support (feature = "serde")
• Compatible with embedded systems (doesn't use std)
• Efficient conditional ownership (borrows can take ownership without allocation/copying)
• Both single threaded compatible (FlexStr) and multi-thread safe (AFlexStr) options
• It is simple to use!

Types

• FlexStr - regular usage
  • Heap storage based on Rc
• AFlexStr- provides Send / Sync for multi-threaded use
  • Heap storage based on Arc

Usage

Hello World

use flexstr::IntoFlexStr;

fn main() {
  // From literal - no copying or allocation
  let world = "world!".into_flex_str();

  println!("Hello {world}");
}

Conversions

use flexstr::{IntoAFlexStr, IntoFlexStr, ToFlexStr};

fn main() {
  // From literal - no copying or allocation
  // NOTE: `to_flex_str` will copy, so use `into_flex_str` for literals
  let literal = "literal".into_flex_str();

  // From borrowed string - Copied into inline string
  let owned = "inlined".to_string();
  let str_to_inlined = (&owned).to_flex_str();

  // From borrowed String - copied into `str` wrapped in `Rc`
  let owned = "A bit too long to be inlined!!!".to_string();
  let str_to_wrapped = (&owned).to_flex_str();

  // From String - copied into inline string (`String` storage released)
  let inlined = "inlined".to_string().into_flex_str();

  // From String - `str` wrapped in `Rc` (`String` storage released)
  let counted = "A bit too long to be inlined!!!".to_string().into_flex_str();

  // *** If you want a Send/Sync type you need `AFlexStr` instead ***

  // From FlexStr wrapped literal - no copying or allocation
  let literal = literal.into_a_flex_str();

  // From FlexStr inlined string - no allocation
  let inlined = inlined.into_a_flex_str();

  // From FlexStr `Rc` wrapped `str` - copies into `str` wrapped in `Arc`
  let counted = counted.into_a_flex_str();
}

Passing FlexStr to Conditional Ownership Functions

This has always been a confusing situation in Rust, but it is easy with FlexStr since multi ownership is cheap.

use flexstr::{IntoFlexStr, FlexStr};

struct MyStruct {
  s: FlexStr
}

impl MyStruct {
  fn to_own_or_not_to_own(s: &FlexStr) -> Self {
    let s = if s == "own_me" {
      // Since a wrapped literal, no copy or allocation
      s.clone()
    } else {
      // Wrapped literal - no copy or allocation
      "own_me".into()
    };

    Self { s }
  }
}

fn main() {
  // Wrapped literals - no copy or allocation
  let s = "borrow me".into_flex_str();
  let s2 = "own me".into_flex_str();

  let struct1 = MyStruct::to_own_or_not_to_own(&s);
  let struct2 = MyStruct::to_own_or_not_to_own(&s2);

  assert_eq!(s2, struct1.s);
  assert_eq!(s2, struct2.s);
}

Performance Characteristics

NOTE: No benchmarking has yet been done

• Clones are cheap and never allocate
  • At minimum, they are just a copy of the enum and at max an additional reference count increment
• Literals are just wrapped when used with into() and never copied
• Calling into() on a String will result in an inline string (if short) otherwise copied into a str wrapped in Rc/Arc (which will allocate, copy, and then release original String storage)
• into_flex_str() and into_a_flex_str() are equivalent to calling into() on both literals and String (they are present primarily for let bindings so there is no need to declare a type)
• to_flex_str() and to_a_flex_str() are meant for taking ownership of borrowed strings and always copy into either an inline string (for short strings) or an Rc/Arc wrapped str (which will allocate)
• to_string always copies into a new String
• Conversions back and forth between AFlexStr and FlexStr using into() are cheap when using wrapped literals or inlined strings
  • Inlined strings and wrapped literals just create a new enum wrapper
  • Reference counted wrapped strings will always require an allocation and copy for the new Rc or Arc

Negatives

There is no free lunch:

• Due to usage of Rc (or Arc), when on-boarding String it will need to reallocate and copy
• Due to the enum wrapper, every string operation has the overhead of an extra branching operation
• Since FlexStr is not Send or Sync, there is a need to consider single-threaded (FlexStr) and multi-threaded (AFlexStr) use cases and convert accordingly

Status

This is currently beta quality and still needs testing. The API may very possibly change but semantic versioning will be followed.

License

This project is licensed optionally under either:

• Apache License, Version 2.0, (LICENSE-APACHE or https://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or https://opensource.org/licenses/MIT)