rust type-builder 方便处理可选参数

https://crates.io/crates/typed-builder

typed-builder = "0.5.1"

Rust Typed Builder

Creates a compile-time verified builder:

#[macro_use]
extern crate typed_builder;

#[derive(TypedBuilder)]
struct Foo {
    // Mandatory Field:
    x: i32,

    // #[builder(default)] without parameter - use the type's default
    // #[builder(setter(strip_option))] - wrap the setter argument with `Some(...)`
    #[builder(default, setter(strip_option))]
    y: Option<i32>,

    // Or you can set the default
    #[builder(default=20)]
    z: i32,
}

Build in any order:

Foo::builder().x(1).y(2).z(3).build();
Foo::builder().z(1).x(2).y(3).build();

Omit optional fields(the one marked with #[default]):

Foo::builder().x(1).build()

But you can't omit non-optional arguments - or it won't compile:

Foo::builder().build(); // missing x
Foo::builder().x(1).y(2).y(3); // y is specified twice