impl-new-derive

The impl-new-derive procedural macro generates a new constructor for Rust structs. This macro automatically creates a constructor that initializes:

• Public fields from provided arguments (added as parameters to new).
• Private fields with default values using either:
  • An expression specified by #[default(...)], or
  • Default::default() if no custom default expression is provided.

Features

• Automatically generates a new constructor for structs.
• Handles public fields: The new function takes all public fields of the struct as arguments.
• Handles private fields:
  • If the private field has a #[default(...)] attribute, that expression is used for initialization.
  • Otherwise, the private field is initialized with Default::default().
• Supports generic types: The macro works with both generic and non-generic structs.

Usage

1. Add the macro to your project by including it in your Cargo.toml:

   [dependencies]
   impl_new_derive = "0.1"

2. Annotate your struct with #[derive(ImplNew)] to automatically generate a new constructor. Optionally, you can also derive Default if needed elsewhere in your code.

Example for a Non-Generic Struct

use impl_new_derive::ImplNew;

#[derive(ImplNew, Default)]
struct MyStruct {
    pub name: String,
    pub age: u32,

    // Private field; no custom default attribute,
    // so it will be initialized using Default::default()
    secret: String,
}

fn main() {
    // The generated constructor requires arguments
    // for each PUBLIC field in the struct.
    let my_struct = MyStruct::new("John".to_string(), 30);
    println!("Name: {}, Age: {}", my_struct.name, my_struct.age);

    // 'secret' is private and gets a default value of "" (the Default for String).
}

Using #[default(...)] for Private Fields

use impl_new_derive::ImplNew;

#[derive(ImplNew)]
struct Credentials {
    pub username: String,

    // Private field with a custom default value.
    // This field doesn't appear as a parameter in the `new` method.
    // Instead, it will be automatically set to "empty_token".to_string().
    #[default(\"empty_token\".to_string())]
    token: String,
}

fn main() {
    // The `new` function is generated only for public fields,
    // so we pass just `username`.
    let creds = Credentials::new(\"alice\".to_string());
    println!(\"Username: {}, Token: {}\", creds.username, creds.token);
    // Prints: Username: alice, Token: empty_token
}

Example for a Generic Struct

use impl_new_derive::ImplNew;

#[derive(ImplNew, Default)]
struct MyStruct<T> {
    pub value: T,

    // Private field without custom default, so it uses Default::default()
    count: usize,
}

fn main() {
    // In a generic struct, only the public fields appear in `new`.
    let my_struct = MyStruct::new(42);

    // 'count' is private, and we didn't give it a `#[default(...)]`,
    // so it's initialized with `Default::default()`, i.e. 0.
    println!(\"Value: {}, Count: {}\", my_struct.value, my_struct.count);
    // Prints: Value: 42, Count: 0
}

How It Works

When you annotate a struct with #[derive(ImplNew)], the macro performs the following actions:

1. It iterates over the fields of the struct.
2. For each public field, it adds a corresponding parameter to the generated new method.
3. For each private field, it checks if a #[default(expr)] attribute is present:
   • If yes, it uses expr to initialize that field.
   • Otherwise, it uses Default::default() to initialize that field.
4. If the struct contains generics, the macro automatically handles them in the generated impl.

Limitations

• Only works for structs with named fields.
• If a private field doesn't implement Default and does not have a #[default(...)] attribute, the macro fails to compile.
• If you do use #[default(...)], the expression inside must be a valid Rust expression for that field's type.

Contributing

Feel free to open issues or pull requests if you have any suggestions or improvements.

License

This project is licensed under the MIT License.