optional-default

A Helper macro to allow specifying default values for some fields of a rust struct while requiring some to be initialized manually.

Usage

Add optional-default to your crate's dependencies: cargo add optional-default

1. Annotate your struct with the OptionalDefault derive macro.
2. Annotate any optional fields with #[optional].
3. If the field should have a default value other than Default::default(), or its type does not implement the Default trait, you can specify your own default value within the #[optional(default = <value>)].
4. The macro will generate a second macro with the same name as your struct. Use this macro to initialize the struct with your specified default values

Example

use optional_default::OptionalDefault;

#[derive(Debug, OptionalDefault)]
struct Example {
    foo: i32, // Required field
    #[optional] 
    bar: i32 // Optional, default = i32::default() = 0
    #[optional(default = 10)]
    baz: i32, // Optional, default = 10
    
}

fn example() {
    // Use the macro as if it was a struct declaration
    let example1 = Example! {
        foo: 1 
        // The other fields are set to their default values
    };

    println!("{:?}", example1); // Example { foo:1, bar: 0, baz: 10 }

    let example2 = Example! {
        foo: 1,
        bar: 5
    };

    println!("{:?}", example2); // Example { foo:1, bar: 5, baz: 10 }

    let example3 = Example! {
        foo: 20,
        baz: 0 // You can override the default values
    };

    println!("{:?}", example1); // Example { foo:1, bar: 0, baz: 20 }

    let does_not_work = Example! {
        baz: 0  
    }; // Error: missing required field foo

}

Limitations

Currently, the macro can only be placed on structs. While it would be possible to implement this approach for enums as well, the initialisation syntax would be inconsistent with regular enum initialisations as Enum::Variant would not be a valid macro name.