bon Builder Patterns

Name: bon
Rating: 87
Author: drewdawg7

Quick Reference

Pattern	bon Attribute
Required constructor param	`#[builder(start_fn)]`
Optional with default	`#[builder(default = value)]`
Accept `impl Into<T>`	`#[builder(into)]` or `#[builder(on(String, into))]`
Vec accumulator	`#[builder(field)]` + custom method
Multi-param setter	Custom method with typestate

Simple Builder Pattern

rust

use bon::Builder;

#[derive(Builder)]
#[builder(on(String, into))]  // All String fields accept impl Into<String>
pub struct Widget {
    #[builder(start_fn)]      // Required in constructor
    pub name: String,

    #[builder(default = 16.0)]
    pub size: f32,

    #[builder(default)]       // Uses Default::default()
    pub enabled: Option<bool>,
}

// Usage: Widget::builder("name").size(20.0).build()

Accumulator Pattern (Vec fields)

Use #[builder(field)] for Command::arg()-style APIs:

rust

#[derive(Builder)]
pub struct Config {
    #[builder(field)]  // Lives on builder, not struct
    items: Vec<String>,

    #[builder(default)]
    limit: Option<u32>,
}

use config_builder::State;

impl<S: State> ConfigBuilder<S> {
    pub fn item(mut self, item: impl Into<String>) -> Self {
        self.items.push(item.into());
        self
    }
}

// Usage: Config::builder().item("a").item("b").limit(10).build()

Custom Multi-Param Setter

rust

use widget_builder::State;

impl<S: State> WidgetBuilder<S> {
    pub fn with_position(self, x: f32, y: f32) -> Self {
        self.position(Some((x, y)))
    }
}

Migration Checklist

When adding #[derive(Builder)]:

• Add use bon::Builder;
• Mark required params with #[builder(start_fn)]
• Add #[builder(default = ...)] for optional fields
• Use #[builder(on(String, into))] for String fields
• Use #[builder(field)] for Vec accumulators
• Delete manual impl block with new() and with_*() methods
• Update call sites: .new() → .builder().build()

API Changes

Before	After
`Foo::new(x)`	`Foo::builder(x).build()`
`.with_bar(y)`	`.bar(y)`
`Option<T>` field	Also generates `.maybe_bar(Option<T>)`

References

See references/migration-examples.md for codebase-specific examples.