Rust Typed Builder

Creates a compile-time verified builder:

```rust

[macro_use]

extern crate typed_builder;

[derive(TypedBuilder)]

struct Foo { // Mandatory Field: x: i32,

// #[builder(default)] without parameter - use the type's default
#[builder(default)]
y: Option<i32>,

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

} ```

Build in any order: rust 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]): rust Foo::builder().x(1).build()

But you can't omit non-optional arguments - or it won't compile: rust Foo::builder().build(); // missing x Foo::builder().x(1).y(2).y(3); // y is specified twice

Features

• Custom derive for generating the builder pattern.
• All setters are accepting Into values.
• Compile time verification that all fields are set before calling .build().
• Compile time verification that no field is set more than once.
• Ability to annotate fields with #[builder(default)] to make them optional and specify a default value when the user does not set them.
• Generates simple documentation for the .builder() method.

Limitations

• No custom build error - if you neglect to set a field or set a field twice you'll get regular no method error that doesn't tell you what you did wrong.
  • If there is a way to generate proper errors I'll gladly implement it.
• The generated builder type has ugly internal name and many generic parameters. It is not meant for passing around and doing fancy builder tricks - only for nicer object creation syntax(constructor with named arguments and optional arguments).
  • For the that reason, all builder methods are call-by-move and the builder is not cloneable. Saves the trouble of determining if the fields are cloneable...
  • If you want a builder you can pass around, check out derive-builder. It's API does not conflict with typed-builder's so you can be able to implement them both on the same type.

Alternatives - and why typed-builder is better

• derive-builder - does all the checks in runtime, returning a Result you need to unwrap.
• safe-builder-derive - this one does compile-time checks - by generating a type for each possible state of the builder. Rust can remove the dead code, but your build time will still be exponential. typed-builder is encoding the builder's state in the generics arguments - so Rust will only generate the path you actually use.