skip

Applies to: struct fields

Skips generating setters for the member. This hides the member from the generated builder API, so the caller can't set its value.

The value for the member will be computed inside of the finishing function.

Form	How value for the member is computed
#[builder(skip)]	Default::default()
#[builder(skip = expression)]	expression

Example

use bon::Builder;

#[derive(Builder)]
struct User {
    #[builder(skip)] 
    level: u32,

    // Any complex expression is accepted
    #[builder(skip = "anon".to_owned())]  
    name: String,
}

let user = User::builder()
    // There are no `level`, and `name` setters generated
    .build();

assert_eq!(user.level, 0);
assert_eq!(user.name, "anon");

Evaluation context

You can reference other members as variables in the skip expression. All members are initialized in the order of their declaration, and thus only members that are declared earlier (higher) in the code are available for the skip expression.

use bon::Builder;

#[derive(Builder)]
struct Example {
    x1: u32,

    // Note that here we don't have access to `x3`
    // because it's declared (and thus initialized) later
    #[builder(skip = 2 * x1)]
    x2: u32,

    #[builder(skip = x2 + x1)]
    x3: u32,
}

let example = Example::builder()
    .x1(3)
    .build();

assert_eq!(example.x1, 3);
assert_eq!(example.x2, 6);
assert_eq!(example.x3, 9);

Unsupported function syntax

This attribute is not supported with function or method syntax because it's simply unnecessary there and can easier be expressed with local variables.