Derive Macro smart_default::SmartDefault

#[derive(SmartDefault)]
{
    // Attributes available to this derive:
    #[default]
}

Smart Default

This crate provides a custom derive for SmartDefault. SmartDefault is not a real trait - deriving it will actually impl Default. The difference from regular #[derive(Default)] is that #[derive(SmartDefault)] allows you to use #[default = "..."] attributes to customize the ::default() method and to support structs that don’t have Default for all their fields - and even enums!

Examples

use smart_default::SmartDefault;

#[derive(SmartDefault)]
enum Foo {
    Bar,
    #[default]
    Baz {
        #[default = 12]
        a: i32,
        b: i32,
        #[default(Some(Default::default()))]
        c: Option<i32>,
        #[default(_code = "vec![1, 2, 3]")]
        d: Vec<u32>,
        #[default = "four"]
        e: String,
    },
    Qux(i32),
}

assert!(Foo::default() == Foo::Baz {
    a: 12,
    b: 0,
    c: Some(0),
    d: vec![1, 2, 3],
    e: "four".to_owned(),
});

• Baz has the #[default] attribute. This means that the default Foo is a Foo::Baz. Only one variant may have a #[default] attribute, and that attribute must have no value.
• a has a #[default = 12] attribute. This means that it’s default value is 12.
• b has no #[default = ...] attribute. It’s default value will i32’s default value instead - 0.
• c is an Option<i32>, and it’s default is Some(Default::default()). Rust cannot (currently) parse #[default = Some(Default::default())] and therefore we have to use a special syntax: #[default(Some(Default::default))]
• d has the ! token in it, which cannot (currently) be parsed even with #[default(...)], so we have to encode it as a string and mark it as _code = .
• e is a String, so the string literal “four” is automatically converted to it. This automatic conversion only happens to string (or byte string) literals - and only if _code is not used.
• Documentation for the impl Default section is generated automatically, specifying the default value returned from ::default().