aksr

A Rust derive macro that automatically generates Builder Lite pattern methods for structs, supporting both named and tuple structs with extensive customization options.

Builder Lite Pattern

aksr implements the Builder Lite pattern, where the struct itself acts as the builder. Unlike traditional builders that require a separate builder object, Builder Lite is an incremental, zero-cost evolution from the new() or Default::default() method.

Requirements:

• Struct must implement Default trait, OR
• Struct must have a new() method

This allows you to use the builder pattern without creating a separate builder type, making it especially useful for rapidly evolving application code.

Features

• 🚀 Builder Lite pattern - Zero-cost, incremental builder evolution

• 🎯 Field aliases for more intuitive API

• 📦 Vector extension methods for incremental updates

• 🔧 Flexible customization via attributes

• 💡 String optimization with &str → String and &[String] support

• 🎁 Smart Option handling for Vec<T> with empty check

• 🔄 Ownership methods - into_*() and take_*() for efficient value extraction

• ⚡ Zero-cost abstractions following Rust best practices

Installation

Add aksr to your Cargo.toml:

[dependencies]
aksr = "0.0.5"

Examples

Run the examples to see all features in action:

cargo run --example rect        # Named struct
cargo run --example color       # Tuple struct

To see the generated code:

cargo install cargo-expand      # Install expand tool
cargo expand --example rect     # View generated code

• examples/rect.rs - Named struct with all features

• examples/color.rs - Tuple struct with all features

Supported Attributes

Ownership Methods: into_*() and take_*()

aksr automatically generates ownership methods for fields that own their data, allowing efficient value extraction without cloning.

into_*() Methods

Generated for:

• ✅ Owned types: Vec<T>, String, HashMap<K, V>, HashSet<T>, BTreeMap<K, V>, BTreeSet<T>, etc.
• ✅ Collections: VecDeque<T>, BinaryHeap<T>
• ✅ Smart pointers: Box<T>, Rc<T>, Arc<T>, Weak<T>
• ✅ Wrappers: RefCell<T>, Mutex<T>, RwLock<T>, Cow<'a, T>, OsString, PathBuf
• ✅ Option<T> (any inner type)
• ✅ Custom owned types (non-primitive)
• ✅ Arrays and tuples
• ✅ Reference types (moves the reference itself)

NOT generated for:

• ❌ Primitive/Copy types: usize, bool, char, i32, u8, f64, etc. (getters already return by value)
• ❌ Fields with #[args(except(into))] attribute

Behavior:

• Consumes self and moves the field value out
• Zero-cost: no cloning, just ownership transfer
• Follows Rust naming convention (like into_iter(), into_inner())

Example:

#[derive(Builder, Default)]
struct Config {
    items: Vec<String>,
    name: String,
    #[args(except(into))]
    metadata: String,  // No into_metadata() generated
}

let config = Config::default()
    .with_items(&["a", "b"])
    .with_name("test");

let items = config.into_items();  // Moves Vec<String> out
// config is now consumed and cannot be used

take_*() Methods

Generated for:

• ✅ Option<T> - Uses Option::take(), leaves None (no Default required)
• ✅ Standard collections (require Default):
  • Vec<T>, String, HashMap<K, V>, HashSet<T>
  • BTreeMap<K, V>, BTreeSet<T>, VecDeque<T>, BinaryHeap<T>
• ✅ Smart pointers: Box<T>, Rc<T>, Arc<T>, Weak<T>
• ✅ Wrappers: RefCell<T>, Mutex<T>, RwLock<T>, Cow<'a, T>, OsString, PathBuf

NOT generated for:

• ❌ Primitive/Copy types
• ❌ Custom types (unless they implement Default)
• ❌ Types not in the whitelist (to avoid forcing Default constraints)

Behavior:

• Takes the field value and replaces it with Default::default()
• Does NOT consume self - you can continue using the struct
• For Option<T>, leaves None instead of requiring Default

Example:

#[derive(Builder, Default)]
struct Data {
    items: Vec<String>,
    opt_value: Option<String>,
}

let mut data = Data::default()
    .with_items(&["a", "b"])
    .with_opt_value("test");

let items = data.take_items();  // Moves Vec, leaves empty Vec
assert!(data.items().is_empty());  // Can still use data

let opt = data.take_opt_value();  // Moves Option, leaves None
assert_eq!(data.opt_value(), None);  // Can still use data

Customizing into_*() Prefix

If into_*() conflicts with your custom methods, you can change the prefix:

#[derive(Builder, Default)]
struct Example {
    #[args(into_prefix = "extract")]
    items: Vec<String>,  // Generates extract_items() instead of into_items()
}

License

This project is licensed under LICENSE.