LightClone

Compile-time enforcement for O(1) clone operations in Rust.

Overview

LightClone is for codebases that embrace immutable data structures. It provides a marker trait and derive macro that guarantees cloning is cheap by only allowing types where cloning involves:

• Atomic refcount increments (Arc)
• Non-atomic refcount increments (Rc)
• Bitwise copies (Copy types)
• Persistent data structures (im, imbl, rpds with structural sharing)

Types like String, Vec, or HashMap that perform deep copies are rejected at compile time.

crates.io | docs.rs

Usage

use light_clone::LightClone;
use std::sync::Arc;

#[derive(LightClone)]
struct Config {
    name: Arc<str>,
    max_connections: u32,
    timeout_ms: u64,
}

let config = Config {
    name: "production".into(),
    max_connections: 100,
    timeout_ms: 5000,
};

// .light_clone() or .lc() for short
let clone = config.lc();

Compile-Time Safety

Invalid types are caught at compile time:

#[derive(LightClone)]
struct Invalid {
    data: String,  // Error: String does not implement LightClone
}

Ergonomic Strings

Use LightStr as a cheap-to-clone string type:

use light_clone::{LightStr, IntoLightStr};

let s: LightStr = "hello".into_light_str();
let clone = s.lc();  // O(1) - just increments refcount

Features

Optional integrations with persistent collection libraries:

[dependencies]
light_clone = { version = "0.2", features = ["imbl"] }

Supported Types

Primitives

All primitive types: i8-i128, u8-u128, f32, f64, bool, char, ()

Smart Pointers

• Arc<T> where T: ?Sized
• Rc<T> where T: ?Sized
• std::sync::Weak<T> where T: ?Sized
• std::rc::Weak<T> where T: ?Sized

Containers

• Option<T> where T: LightClone
• Result<T, E> where T: LightClone, E: LightClone
• PhantomData<T>
• Tuples up to 12 elements

Enums

#[derive(LightClone)]
enum State {
    Idle,
    Loading { progress: u8 },
    Ready(Arc<Data>),
}

When to Use Immutable Data Structures

LightClone enforces that your types use immutable data structures (Arc, Rc, persistent collections) which enable O(1) cloning through structural sharing. This approach shines when:

• Clone-heavy workloads - Sharing state across threads, event sourcing, undo/redo systems, functional pipelines with pure transforms
• Cloning large or nested data - A 10KB string clone copies 10KB; an Arc<str> clone increments a counter
• Concurrent code - Clone and send freely without worrying about data races or locks
• Structural sharing matters - Persistent collections share unchanged portions between versions

The trade-offs to consider:

• Mutation is still faster than cloning - LightClone enforces cloning is cheap, not free. In-place mutation avoids refcount operations entirely, so prefer mutation for hot paths
• Memory overhead - Arc/Rc add pointer indirection and allocation overhead. Persistent collections trade memory for structural sharing

Where LightClone fits: Once you've committed to immutable data structures, LightClone provides compile-time enforcement that cloning is cheap. It catches accidental String or Vec fields that would silently introduce expensive deep clones.

Performance

LightClone has zero runtime overhead—.light_clone() compiles to identical code as .clone().

The real performance benefit comes from using immutable data structures:

Mutation has trade-offs—persistent collections are slower for small, mutation-heavy workloads but catch up as data grows. See BENCHMARKS.md for detailed comparisons.

Minimum Supported Rust Version

Rust 1.70.0. The rpds feature requires Rust 1.85+ due to upstream dependencies.

License

Licensed under either of:

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

at your option.

Created By

Hamilton Greene