No more worrying whether the build call on your builder will return Ok or not. Maybe you forgot to set a field? typesafe-builders solves this by using the Rust type-system to ensure correct usage.

Example

fn example() {
	#[derive(Builder)]
	struct Point {
		x: u8,
		y: u8,
	}

	let builder = Point::builder();
	let partial = builder.with_x(5);
	// These do not compile:
	// partial.with_x(8);
	// partial.build();
	
	// Infallibly construct an instance:
	let result = partial.with_y(8).build();

	assert_eq!(result.x, 5);
	assert_eq!(result.y, 8);
}

How does it work?

Const generic one-hot bitfields. What you get is similar to this:

pub struct Builder<const x_set: bool, const y_set: bool> {
	x: Option<u8>,
	y: Option<u8>,
}

impl<const y_set: bool> Builder<false, y_set> {
    fn set_x(self, x: u8) -> Builder<true, y_set,> {
        …
    }
}

impl<const x_set: bool> Builder<x_set, false> {
    fn set_y(self, y: u8) -> Builder<x_set, true> {
        …
    }
}

// The build function is only available once all fields are set:
impl Builder<true, true> {
    fn build() {

    }
}

TODOS

• Code quality is horrible 🙈
• Add optional truly fields.