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,
		#[optional]
		z: Option<u8>,
	}

	let builder = Point::builder();
	let partial = builder.x(5);
	// These do not compile:
	// partial.x(6); 		// `x` is already set
	// partial.build();		// `y` is not set

	// Set all required fields to enable the `build` function:
	let complete = partial.y(8);
	let result = complete.build();

	assert_eq!(result.x, 5);
	assert_eq!(result.y, 8);
	assert_eq!(result.z, None);
}

Field Attributes

• #[optional] - WIP: to be changed to builder(optional) to prevent name clashes.

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 fields.