Rust macro to automatically implement the builder pattern for arbitrary structs. A simple #[derive(Builder)] will generate a FooBuilder for your struct Foo with all setter-methods and a build method.

use derive_builder::Builder;

#[derive(Default, Builder, Debug)]
#[builder(setter(into))]
struct Channel {
    token: i32,
    special_info: i32,
    // .. a whole bunch of other fields ..
}

fn main() {
    // builder pattern, go, go, go!...
    let ch = ChannelBuilder::default()
        .special_info(42u8)
        .token(19124)
        .build()
        .unwrap();
    println!("{:?}", ch);
}

Note that we did not write any definition or implementation of ChannelBuilder. Instead the derive_builder crate acts on #[derive(Builder)] and generates the necessary code at compile time.

This is the generated boilerplate code you didn't need to write. :-)

#[derive(Clone, Default)]
struct ChannelBuilder {
    token: Option<i32>,
    special_info: Option<i32>,
}

#[allow(dead_code)]
impl ChannelBuilder {
    pub fn token<VALUE: Into<i32>>(&mut self, value: VALUE) -> &mut Self {
        let mut new = self;
        new.token = Some(value.into());
        new
    }
    pub fn special_info<VALUE: Into<i32>>(&mut self, value: VALUE) -> &mut Self {
        let mut new = self;
        new.special_info = Some(value.into());
        new
    }
    fn build(
        &self,
    ) -> Result<Channel, ChannelBuilderError> {
        Ok(Channel {
            id: match self.id {
                Some(ref value) => Clone::clone(value),
                None => {
                    return Err(
                        Into::into(
                            ::derive_builder::UninitializedFieldError::from("id"),
                        ),
                    )
                }
            },
            token: match self.token {
                Some(ref value) => Clone::clone(value),
                None => {
                    return Err(
                        Into::into(
                            ::derive_builder::UninitializedFieldError::from("token"),
                        ),
                    )
                }
            },
            special_info: match self.special_info {
                Some(ref value) => Clone::clone(value),
                None => {
                    return Err(
                        Into::into(
                            ::derive_builder::UninitializedFieldError::from("special_info"),
                        ),
                    )
                }
            },
        })
    }
}

Note: This is edited for readability. The generated code doesn't assume traits such as Into are in-scope, and uses full paths to access them.

It's as simple as three steps:

1. Add derive_builder to your Cargo.toml either manually or with cargo-edit:

• cargo add derive_builder

2. Add use derive_builder::Builder;
3. Annotate your struct with #[derive(Builder)]

• Chaining: The setter calls can be chained, because they consume and return &mut self by default.
• Builder patterns: You can opt into other builder patterns by preceding your struct (or field) with #[builder(pattern = "owned")] or #[builder(pattern = "immutable")].
• Extensible: You can still define your own implementations for the builder struct and define additional methods. Just make sure to name them differently than the setter and build methods.
• Documentation and attributes: Setter methods can be documented by simply documenting the corresponding field. Similarly #[cfg(...)] and #[allow(...)] attributes are also applied to the setter methods.
• Hidden fields: You can skip setters via #[builder(setter(skip))] on each field individually.
• Setter visibility: You can opt into private setter by preceding your struct with #[builder(private)].
• Setter type conversions: With #[builder(setter(into))], setter methods will be generic over the input types – you can then supply every argument that implements the Into trait for the field type.
• Setter strip option: With #[builder(setter(strip_option))], setter methods will take T as parameter'type for field of type Option<T>.
• Collection setters: Adding #[builder(setter(each(name = "method_name")))] to fields whose types implement Default and Extend will generate a setter which adds items to the builder collection for that field. It's possible for these setters to be generic over the Into<T> trait too, like so: #[builder(setter(each(name = "foo", into)))].
• Builder field visibility: You can use #[builder(field(private))] or ..(public), to set field visibility of your builder.
• Generic structs: Are also supported, but you must not use a type parameter named VALUE, if you also activate setter type conversions.
• Default values: You can use #[builder(default)] to delegate to the Default implementation or any explicit value via = "..". This works both on the struct and field level.
• Pre-build validation: You can use #[builder(build_fn(validate = "path::to::fn"))] to add your own validation before the target struct is generated.
• Build method suppression: You can use #[builder(build_fn(skip))] to disable auto-implementation of the build method and provide your own.
• Custom build method error types: You can use #[builder(build_fn(error = "path::to::Error"))] to have your builder return an error type of your choosing. By default, the macro will emit an error type alongside the builder.
• Builder derivations: You can use #[builder(derive(Trait1, Trait2, ...))] to have the builder derive additonal traits. All builders derive Default and Clone, so you should not declare those in this attribute.
• Pass-through attributes: Use #[builder_struct_attr(...)], #[builder_impl_attr(...)], #[builder_field_attr(...)], and #[builder_setter_attr(...)] to declare attributes that will be added to the relevant part of the generated builder.
• no_std support: Just add #[builder(no_std)] to your struct, use feature alloc, and add extern crate alloc to your crate.
• No alloc no_std support: Do not use alloc feature and then either add #[builder(no_std, build_fn(error(validation_error = false)))] or #[builder(no_std, build_fn(error = "path::to::Error"))] to your struct.
• Renaming and re-export support: Use #[builder(crate = "...")] to set the root for derive_builder. This is useful if you want to rename derive_builder in Cargo.toml or if your crate is re-exporting derive_builder::Builder and needs the generated code to not directly reference the derive_builder crate.

For more information and examples please take a look at our documentation.

• Tuple structs and unit structs are not supported as they have no field names. We do not intend to support them.
• When defining a generic struct, you cannot use VALUE as a generic parameter as this is what all setters are using.

Detailed explaination of all features and tips for troubleshooting. You'll also find a discussion of different builder patterns.

Yes, we keep a changelog.

Licensed under either of

• Apache License, Version 2.0, (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.