Introduce builder trait and patterns

[soareschen]

Summary

A context-generic builder pattern allows context-generic providers to build a generic context without direct access to the concrete type.

pub trait HasBuilder {
  type Builder;

  fn builder() -> Self::Builder;
}

pub trait CanBuildField<Tag, Value> {
  type Next;

  fn build(self, _tag: PhantomData<Tag>, value: Value) -> Self::Next;
}

pub trait MapType {
  type Map<T>;
}

pub struct Filled;

impl MapType for Filled {
  type Map<T> = T;
}

pub struct Empty;

impl MapType for Empty {
  type Map<T> = ();
}

Example

#[derive(Builder)
pub struct Foo {
  pub bar: u64,
  pub baz: String,
}

would have the following derived:

pub struct FooBuilder<F1: MapType, F2: MapType> {
  pub bar: F1::Map<u64>,
  pub baz: F2::Map<String>,
}

impl HasBuilder for Foo {
  type Builder = FooBuilder<Empty, Empty>;

  fn builder() -> Self::Builder {
    FooBuilder {
      bar: (),
      baz: (),
    }
  }
}

impl<F2: MapType> CanBuild<symbol!("bar"), u32> for FooBuilder<Empty, F2> {
  type Next = FooBuilder<Filled, F2>;

  fn build(self, _tag: PhantomData<Tag>, value: u32) -> Self::Next {
    FooBuilder {
      bar: value,
      baz: self.baz,
    }
  }
}


impl<F1: MapType> CanBuild<symbol!("baz"), String> for FooBuilder<F1, Empty> {
  type Next = FooBuilder<F1, Filled>;

  fn build(self, _tag: PhantomData<Tag>, value: String) -> Self::Next {
    FooBuilder {
      bar: self.bar,
      baz: value,
    }
  }
}

impl From<FooBuilder<Filled, Filled>> for Foo {
  fn from(builder: FooBuilder<Filled, Filled>) -> Self {
    Self {
      bar: builder.bar,
      baz: builder.baz,
    }
  }
}