Derive a builder from constructors/methods using the typesafe builder pattern!

Use this if your constructors/method has:

• Optional parameters.
• A large number of parameters.
• Collections parameters.

Add the dependency to your Cargo.toml

[dependencies]
buildstructor = "*"

1. Annotate your impl with #[buildstructor::buildstructor].
2. Annotate your fn with #[builder].
3. Use your automatically derived builder.

pub struct MyStruct {
    sum: usize,
}

#[buildstructor::buildstructor]
impl MyStruct {
    #[builder]
    fn new(a: usize, b: usize) -> MyStruct {
        Self { sum: a + b }
    }
    
    #[builder(entry = "more", exit = "add")]
    fn add_more(&mut self, c: usize, d: usize, e: Option<usize>) {
        self.sum += c + d + e.unwrap_or(3);
    }
}

fn main() {
    let mut mine = MyStruct::builder().a(2).b(3).build();
    assert_eq!(mine.sum, 5);
    
    mine.more().c(1).d(2).add();
    assert_eq!(mine.sum, 11);
}

The difference between this and other builder crates is that constructors/methods are used to derive builders rather than structs. This results in a more natural fit with regular Rust code, and no annotation magic to define behavior.

Advantages:

• You can specify fields in your constructor that do not appear in your struct.
• No magic to default values, just use an Option param in your fn and default as normal.
• async constructors derives async builders.
• Fallible constructors (Result) derives fallible builders.
• Special Vec, Deque, Heap, Set, Map support. Add single or multiple items.
• Generated builders can have receiver, self, &self and &mut self are supported.

This crate is heavily inspired by the excellent typed-builder crate. It is a good alternative to this crate and well worth considering.

All of these recipes and more can be found in the examples directory

Just write your rust code as usual and annotate the constructor impl with [builder]

Builders can be generated on methods that have no receiver.

Configuration:

• entry defaults based on fn name:
  • new => builder
  • <name>_new => <name>_builder
  • <anything_else> => cannot be defaulted and must be specified.
• exit defaults to build

struct MyStruct {
    simple: usize
}

#[buildstructor::buildstructor]
impl MyStruct {
    #[builder]
    fn new(simple: usize) -> MyStruct {
        Self { simple }
    }
    #[builder]
    fn try_new(simple: usize) -> MyStruct {
        Self { simple }
    }
    #[builder(entry = "random", exit = "create")]
    fn do_random(simple: usize) -> MyStruct {
        Self { simple }
    }
}

fn main() {
    let mine = MyStruct::builder().simple(2).build();
    assert_eq!(mine.simple, 2);

    let mine = MyStruct::try_builder().simple(2).build();
    assert_eq!(mine.simple, 2);

    let mine = MyStruct::random().simple(2).create();
    assert_eq!(mine.simple, 2);
}

Builders can be generated on methods that take self, &self and &mut self as a parameter.

Configuration:

• entry cannot be defaulted and must be specified.
• exit defaults to call

use buildstructor::buildstructor;

#[derive(Default)]
pub struct MyStruct;

#[buildstructor]
impl MyStruct {
    #[builder(entry = "query")]
    fn do_query(self, _simple: String) -> bool {
        true
    }

    #[builder(entry = "query_ref", exit = "stop")]
    fn do_query_ref(&self, _simple: String) -> bool {
        true
    }

    #[builder(entry = "query_ref_mut", exit = "go")]
    fn do_query_ref_mut(&mut self, _simple: String) -> bool {
        true
    }
}

fn main() {
    MyStruct::default().query().simple("3".to_string()).call(); // self

    let mine = MyStruct::default();
    mine.query_ref().simple("3".to_string()).stop(); // &self

    let mut mine = MyStruct::default();
    mine.query_ref_mut().simple("3".to_string()).go(); // &mut self
}

Fields that are Option will also be optional in the builder. You should do defaulting in your constructor.

struct MyStruct {
    param: usize
}

#[buildstructor::buildstructor]
impl MyStruct {
    #[builder]
    fn new(param: Option<usize>) -> MyStruct {
        Self { param: param.unwrap_or(3) }
    }
}

fn main() {
    let mine = MyStruct::builder().param(2).build();
    assert_eq!(mine.param, 2);
    let mine = MyStruct::builder().and_param(Some(2)).build();
    assert_eq!(mine.param, 2);
    let mine = MyStruct::builder().build();
    assert_eq!(mine.param, 3);
}

Note that if a field is an Option or collection then if a user forgets to set it a compile error will be generated.

Types automatically have into conversion if:

• the type is not a scalar.
• the type has no generic parameters. (this may be relaxed later)
• the type is a generic parameter from the impl or constructor method.

This is useful for Strings, but also other types where you want to overload the singular build method. Create an enum that derives From for all the types you want to support and then use this type in your constructor.

You can use generics as usual in your constructor. However, this has the downside of not being able to support optional fields.

struct MyStruct {
    param: String   
}

#[buildstructor::buildstructor]
impl MyStruct {
    #[builder]
    fn new<T: Into<String>>(param: T) -> MyStruct {
        Self { param: param.into() }
    }
}

fn main() {
    let mine = MyStruct::builder().param("Hi").build();
    assert_eq!(mine.param, "Hi");
}

To create an async builder just make your constructor async.

struct MyStruct {
    param: usize
}

#[buildstructor::buildstructor]
impl MyStruct {
    #[builder]
    async fn new(param: usize) -> MyStruct {
        Self { param }
    }
}

#[tokio::main]
async fn main() {
    let mine = MyStruct::builder().param(2).build().await;
    assert_eq!(mine.param, 2);
}

To create a fallible builder just make your constructor fallible using Result.

use std::error::Error;
struct MyStruct {
    param: usize
}

#[buildstructor::buildstructor]
impl MyStruct {
    #[builder]
    fn new(param: usize) -> Result<MyStruct, Box<dyn Error>> {
        Ok(Self { param })
    }
}

fn main() {
    let mine = MyStruct::builder().param(2).build().unwrap();
    assert_eq!(mine.param, 2);
}

Collections and maps are given special treatment, the builder will add additional methods to build the collection one element at a time.

struct MyStruct {
    addresses: Vec<String>
}

#[buildstructor::buildstructor]
impl MyStruct {
    #[builder]
    fn new(addresses: Vec<String>) -> MyStruct {
        Self { addresses }
    }
}

fn main() {
    let mine = MyStruct::builder()
        .address("Amsterdam".to_string())
        .address("Fakenham")
        .addresses(vec!["Norwich".to_string(), "Bristol".to_string()])
        .build();
    assert_eq!(mine.addresses, vec!["Amsterdam".to_string(), 
                                    "Fakenham".to_string(), 
                                    "Norwich".to_string(), 
                                    "Bristol".to_string()]);
}

Collections are matched by type name:

If your type does not conform to these patterns then you can use a type alias to trick buildstructor into giving the parameter special treatment.

Use the plural form in your constructor argument and buildstructor will automatically try to figure out the singular form for individual entry. For instance:

addresses => address

In the case that a singular form cannot be derived automatically the suffix _entry will be used. For instance:

frodo => frodo_entry

Adding a singular entry will automatically perform an into conversion if:

• the type is not a scalar.
• the type has no generic parameters. (this may be relaxed later)
• the type is a generic parameter from the impl or constructor method.

This is useful for Strings, but also other types where you want to overload the singular build method. Create an enum that derives From for all the types you want to support and then use this type in your constructor.

There had to be some magic somewhere.

To provide more control over generated builders and allow builders for methods with receivers the top level annotation has changed:

#[buildstructor::builder] => #[buildstructor::buildstructor]

1. Annotate the impl with: #[buildstructor::buildstructor]
2. Annotate methods to create a builders for with: #[builder]

• Transfer docs from the constructor to the generated builder methods.
• Better error messages.
• More testing.

PRs welcome!

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

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 licensed as above, without any additional terms or conditions.