Top-Level Attributes

These attributes are placed on top of a struct or fn declaration.

Attribute	Short description
builder_type	Overrides name, visibility and docs for the builder struct
const	Marks all generated builder functions and methods as const fn
crate	Overrides path to bon crate referenced in the generated code
derive	Generates additional derives for the builder struct itself
finish_fn	Overrides name, visibility and docs for the finishing function
on	Applies member attributes to all members matching a type pattern
start_fn	Overrides name, visibility and docs for the starting function
state_mod	Overrides name, visibility and docs for the builder's typestate API module

Member Attributes

These attributes are placed on a struct field or fn argument.

Attribute	Short description
default	Makes the member optional with a default value
field	Defines a private field on the builder without setters
finish_fn	Makes the member a positional argument on the finishing function
getter	Generates a getter method for a member
into	Changes the signature of the setters to accept impl Into<T>
name	Overrides the name of the member used in the builder's API
overwritable 🔬	Allows calling setters for the same member repeatedly
required	Disables Option<T> special handling, makes the member required
setters	Overrides name, visibility and docs for setters
skip	Skips generating setters for the member
start_fn	Makes the member a positional argument on the starting function
with	Overrides setters' signature and applies a custom conversion

Examples

Struct

use bon::Builder;

#[derive(Builder)]
#[builder(finish_fn = finish)] // <-- this is a top-level attribute
struct Example {
    #[builder(default)] // <-- this is a member attribute
    field: u32
}

Function

use bon::builder;

#[builder(finish_fn = finish)] // <-- this is a top-level attribute
fn example(
    #[builder(default)] // <-- this is a member attribute
    arg: u32
) { }

Method

use bon::bon;

struct Example;

#[bon]
impl Example {
    #[builder(finish_fn = finish)] // <-- this is a top-level attribute
    fn example(
        #[builder(default)]  // <-- this is a member attribute
        arg: u32
    ) { }
}