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" use 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 is too long to be wrapped/inlined, it will heap allocate
  // (demo only, use `into` for literals as above)
  let rc_str = "This is too long to be inlined".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);

  // Concatenation doesn't even copy if we can fit it in the inline string
  let inline_str4 = inline_str3 + "!!!";
  assert!(inline_str4.is_inlined());
  assert_eq!(inline_str4, "inlined!!!");
  
  // Clone is almost free, and never allocates
  // (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

• 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

Benchmarks

Summmary: Creates are fairly expensive (yet) compared to String, but clones are MUCH cheaper.

Keep in mind even though creates are more expensive that depending on your workload you may earn that back via clones and it will save memory as well.

Create

create_static_normal    time:   [3.6473 ns 3.6613 ns 3.6782 ns]
create_inline_small     time:   [9.4807 ns 9.4990 ns 9.5192 ns]
create_heap_normal      time:   [13.597 ns 13.620 ns 13.647 ns]
create_heap_large       time:   [19.031 ns 19.062 ns 19.095 ns]
create_heap_arc_normal  time:   [18.617 ns 18.640 ns 18.664 ns]
create_heap_arc_large   time:   [24.490 ns 24.532 ns 24.578 ns]
create_string_small     time:   [7.2761 ns 7.2809 ns 7.2860 ns]
create_string_normal    time:   [7.6338 ns 7.6401 ns 7.6475 ns]
create_string_large     time:   [13.227 ns 13.318 ns 13.406 ns]

Clone

clone_static_normal     time:   [3.8704 ns 3.8750 ns 3.8799 ns]
clone_inline_small      time:   [4.5057 ns 4.5090 ns 4.5125 ns]
clone_heap_normal       time:   [4.4501 ns 4.4546 ns 4.4597 ns]
clone_heap_arc_normal   time:   [10.701 ns 10.717 ns 10.735 ns]
clone_string_small      time:   [10.986 ns 11.074 ns 11.164 ns]
clone_string_normal     time:   [12.817 ns 12.828 ns 12.842 ns]
clone_string_large      time:   [14.659 ns 14.780 ns 14.889 ns]

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)