Immutable Strings

This crate offers a cheaply cloneable and sliceable UTF-8 string type. It is inspired by the bytes crate, which offers zero-copy byte slices, and the im crate which offers immutable copy-on-write data structures. It offers a standard-library String-compatible API.

Internally, the crate uses a standard library string stored in a smart pointer, and a range into that String. This allows for cheap zero-copy cloning and slicing of the string. This is especially useful for parsing operations, where a large string needs to be sliced into a lot of substrings.

TL;DR: This crate offers an ImString type that acts as a String (in that it can be modified and used in the same way), an Arc<String> (in that it is cheap to clone) and an &str (in that it is cheap to slice) all in one, owned type.

This crate offers a safe API that ensures that every string and every string slice is UTF-8 encoded. It does not allow slicing of strings within UTF-8 multibyte sequences. It offers try_* functions for every operation that can fail to avoid panics. It also uses extensive unit testing with a full test coverage to ensure that there is no unsoundness.

Features

Efficient Cloning: The crate's architecture enables low-cost (zero-copy) clone and slice creation, making it ideal for parsing strings that are widely shared.

Efficient Slicing: The crate's architecture enables low-cost (zero-copy) slice creation, making it ideal for parsing operations where one large input string is slices into many smaller strings.

Copy on Write: Despite being cheap to clone and slice, it allows for mutation using copy-on-write. For strings that are not shared, it has an optimisation to be able to mutate it in-place safely to avoid unnecessary copying.

Compatibility: The API is designed to closely resemble Rust's standard library String, facilitating smooth integration and being almost a drop-in replacement. It also integrates with many popular Rust crates, such as serde, peg and nom.

Generic over Storage: The crate is flexible in terms of how the data is stored. It allows for using Arc<String> for multithreaded applications and Rc<String> for single-threaded use, providing adaptability to different storage requirements and avoiding the need to pay for atomic operations when they are not needed.

Safety: The crate enforces that all strings and string slices are UTF-8 encoded. Any methods that might violate this are marked as unsafe. All methods that can fail have a try_* variant that will not panic. Use of safe functions cannot result in unsound behaviour.

Example

use imstr::ImString;

// Create new ImString, allocates data.
let mut string = ImString::from("Hello, World");

// Edit: happens in-place (because this is the only reference).
string.push_str("!");

// Clone: this is zero-copy.
let clone = string.clone();

// Slice: this is zero-copy.
let hello = string.slice(0..5);
assert_eq!(hello, "Hello");

// Slice: this is zero-copy.
let world = string.slice(7..12);
assert_eq!(world, "World");

// Here we have to copy only the part that the slice refers to so it can be modified.
let hello = hello + "!";
assert_eq!(hello, "Hello!");

Optional Features

Optional features that can be turned on using feature-flags.

Feature	Description
serde	Serialize and deserialize ImString fields as strings with the serde crate.
peg	Use ImString as the data structure that is parsed with the peg crate. See peg-list.rs for an example.
nom	Allow ImString to be used to build parsers with nom. See nom-json.rs for an example.

Similar

There are several crates similar to this, which are listed in the Rust String Benchmarks. You may want to check the other crates out as well.

This is a comparison of this crate to other, similar crates. The comparison is made on these features:

• Cheap Clone: is it a zero-copy operation to clone a string?
• Cheap Slice 🍕: is it possibly to cheaply slice a string?
• Mutable: is it possible to modify strings?
• Generic Storage: is it possible to swap out the storage mechanism?
• String Compatible: is it compatible with String?

Here is the data, with links to the crates for further examination:

Crate	Cheap Clone	Cheap Slice	Mutable	Generic Storage	String Compatible	Notes
imstr	✔️	✔️	✔️	✔️	✔️	This crate.
tendril	✔️	✔️	✔️	✔️	❌	Complex implementation. API not quite compatible with String, but otherwise closest to what this crate does.
immut_string	✔️	❌	🟡 (no optimization)	❌	❌	Simply a wrapper around Arc<String>.
immutable_string	✔️	❌	❌	❌	❌	Wrapper around Arc<str>.
arccstr	✔️	❌	❌	❌	❌	Not UTF-8 (Null-terminated C string). Hand-written Arc implementation.
implicit-clone	✔️	❌	❌	🟡	✔️	Immutable string library. Has sync and unsync variants.
semistr	❌	❌	❌	❌	❌	Stores short strings inline.
quetta	✔️	✔️	❌	❌	❌	Wrapper around Arc<String> that can be sliced.
bytesstr	✔️	🟡	❌	❌	❌	Wrapper around Bytes. Cannot be directly sliced.
fast-str	✔️	❌	❌	❌	❌	Looks like there could be some unsafety.
flexstr	✔️	❌	❌	✔️	❌
bytestring	✔️	🟡	❌	❌	❌	Wrapper around Bytes. Used by actix. Can be indirectly sliced using slice_ref().
arcstr	✔️	✔️	❌	❌	❌	Can store string literal as &'static str.
cowstr	✔️	❌	✔️	❌	❌	Reimplements Arc, custom allocation strategy.
strck	❌	❌	❌	✔️	❌	Typechecked string library.

License

MIT, see LICENSE.md.