#string-literal #string #inline #refcount #heap-allocated #replace #arc-str

no-std flexstr

A flexible, simple to use, immutable, clone-efficient String replacement for Rust

26 releases (8 breaking)

0.9.2 Apr 18, 2022
0.9.1 Mar 24, 2022
0.8.1 Mar 14, 2022

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

Table of Contents


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 forcing a choice between efficiency and storing two different types.

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


String constants are easily wrapped into the unified string type. String data is automatically inlined when possible otherwise allocated on the heap.

See documentation or Usage section for more examples.

use flexstr::{local_str, LocalStr, ToLocalStr};

fn main() {
  // Use `local_str` macro to wrap literals as compile-time constants
  const STATIC_STR: LocalStr = local_str!("This will not allocate or copy");

  // Strings up to 22 bytes (on 64-bit) will be inlined automatically
  // (demo only, use macro or `from_static` for literals as above)
  let inline_str = "inlined".to_local_str();

  // When a string is too long to be wrapped/inlined, it will heap allocate
  // (demo only, use macro or `from_static` for literals as above)
  let rc_str = "This is too long to be inlined".to_local_str();


Optional features:

  • fast_format = enables local_ufmt! and shared_ufmt! format!-like macros for very fast formatting (with some limitations)
  • fp_convert = Convert floating point types directly into a FlexStr
  • int_convert = Convert integer types directly into a FlexStr
  • serde = Serialization support for FlexStr
  • std = enabled by default (use default-features=false to enable #[no_std])
version = "0.9"
features = ["fast_format, fp_convert", "int_convert", "serde"]

How Does It Work?

Internally, FlexStr uses a union with these variants:

  • Static - A simple wrapper around a static string literal (&'static str)
  • Inline - 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.


  • 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 inline size as a String (64-bit: 24 bytes)
  • Optional serde serialization support (feature = "serde")
  • Compatible with embedded systems (supports #[no_std])
  • Efficient conditional ownership (borrows can take ownership without allocation/copying)
  • Both single threaded compatible (LocalStr) and multi-thread safe (SharedStr) options
  • All dependencies are optional and based on feature usage
  • It is simple to use!


NOTE: Both types are identical in handling both literals and inline strings. The only difference occurs when a heap allocation is required.

  • LocalStr - ultra-fast usage in the local thread
    • Heap storage based on Rc
  • SharedStr- provides Send / Sync for multithreaded use
    • Heap storage based on Arc


Hello World

use flexstr::local_str;

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

  println!("Hello {world}");

Creation Scenarios

use flexstr::{local_str, LocalStr, IntoSharedStr, IntoLocalStr, ToLocalStr};

fn main() {
  // From literal - no runtime, all compile-time
  const literal: LocalStr = local_str!("literal");

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

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

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

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

  // From LocalStr wrapped literal - no copying or allocation
  let literal2 = literal.into_shared_str();

  // From LocalStr inlined string - no allocation
  let inlined = inlined.into_shared_str();

  // From LocalStr `Rc` wrapped `str` - copies into `str` wrapped in `Arc`
  let counted = counted.into_shared_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. By passing as &LocalStr instead of &str, you retain the option for very fast multi ownership.

use flexstr::{local_str, IntoLocalStr, LocalStr};

struct MyStruct {
  s: LocalStr

impl MyStruct {
  fn to_own_or_not_to_own(s: &LocalStr) -> Self {
    let s = if s == "own me" {
      // Since a wrapped literal, no copy or allocation
    } else {
      // Wrapped literal - no copy or allocation
      local_str!("own me")

    Self { s }

fn main() {
  // Wrapped literals - compile time constant
  const S: LocalStr = local_str!("borrow me");
  const S2: LocalStr = local_str!("own me");

  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);

Make Your Own String Type

All you need to do is pick a storage type. The storage type must implement Deref<Target = str>, From<&str>, and Clone. Pretty much all smart pointers do this already.


Custom concrete types need to specify a heap type with an exact size of two machine words (16 bytes on 64-bit, and 8 bytes on 32-bit). Any other size parameter will result in a runtime panic error message on string creation.

use flexstr::{FlexStrBase, Repeat, ToFlex};

type BoxStr = FlexStrBase<Box<str>>;

fn main() {
  // Any need for a heap string will now be allocated in a `Box` instead of `Rc`
  // However, the below uses static and inline storage...because we can!
  let my_str = BoxStr::from_static("cool!").repeat_n(3);
  assert_eq!(my_str, "cool!cool!cool!");

Performance Characteristics

  • Clones are cheap and never allocate
    • At minimum, they are just a copy of the union 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_local_str() and into_shared_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_local_str() and to_shared_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 SharedStr and LocalStr using into() are cheap when using wrapped literals or inlined strings
    • Inlined strings and wrapped literals just create a new union wrapper
    • Reference counted wrapped strings will always require an allocation and copy for the new Rc or Arc


In general, inline/static creates are fast but heap creates are a tiny bit slower than String. Clones are MUCH faster and don't allocate/copy. Other operations (repeat, additions, etc.) tend to be about the same performance, but with some nuance depending on string size.

Full benchmarks


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 union wrapper, every string operation has the overhead of an extra branching operation
  • Since LocalStr is not Send or Sync, there is a need to consider single-threaded (LocalStr) and multi-threaded (SharedStr) use cases and convert accordingly


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


This project is licensed optionally under either: