Rust - semi complex builder pattern sample

lib.rs

use std::marker::PhantomData;

#[allow(dead_code)]
fn main() {
    let go = Builder::up()
    .left(One { _ghost: PhantomData::<Yes> })
    .build();
    println!("{:#?}", go);
}

#[derive(Debug, PartialEq, Clone)]
struct Yes;
#[derive(Debug, PartialEq, Clone)]
struct No;

#[derive(Debug, PartialEq, Clone, Default)]
struct One<MAYBE> {
    _ghost: PhantomData<MAYBE>,
}
#[derive(Debug, PartialEq, Clone, Default)]
struct Two<MAYBE> {
    _ghost: PhantomData<MAYBE>,
}
#[derive(Debug, PartialEq, Clone, Default)]
struct Three<MAYBE> {
    _ghost: PhantomData<MAYBE>,
}

#[derive(Debug, Clone)]
enum Builder<LEFT, MIDDLE, RIGHT> {
    Up,
    On(On<LEFT, MIDDLE, RIGHT>),
}

#[derive(Debug, PartialEq, Clone)]
struct Up;
#[derive(Debug, PartialEq, Clone)]
struct On<LEFT, MIDDLE, RIGHT> {
    one: Option<One<LEFT>>,
    two: Option<Two<MIDDLE>>,
    three: Option<Three<RIGHT>>,
}
#[derive(Debug, PartialEq, Clone)]
struct Go {
    one: One<Yes>,
    two: Option<Two<Yes>>,
    three: Option<Three<Yes>>,
}

impl Builder<No, No, No> {
    pub fn up() -> Self {
        Builder::Up
    }
}

#[allow(unreachable_patterns)]
impl<LEFT, MIDDLE> Builder<LEFT, MIDDLE, No> {
    pub fn right(self, three: Three<Yes>) -> Builder<LEFT, MIDDLE, Yes> {
        match self {
            Builder::Up => Builder::On(On {
                one: None,
                two: None,
                three: Some(three),
            }),
            Builder::On(on) => Builder::On(On {
                one: on.one,
                two: on.two,
                three: Some(three),
            }),
            _ => unreachable!(),
        }
    }
}

#[allow(unreachable_patterns)]
impl<MIDDLE, RIGHT> Builder<No, MIDDLE, RIGHT> {
    pub fn left(self, one: One<Yes>) -> Builder<Yes, MIDDLE, RIGHT> {
        match self {
            Builder::Up => Builder::On(On {
                one: Some(one),
                two: None,
                three: None,
            }),
            Builder::On(on) => Builder::On(On {
                one: Some(one),
                two: on.two,
                three: on.three,
            }),
            _ => unreachable!(),
        }
    }
}

#[allow(unreachable_patterns)]
impl<LEFT, RIGHT> Builder<LEFT, No, RIGHT> {
    pub fn middle(self, two: Two<Yes>) -> Builder<LEFT, Yes, RIGHT> {
        match self {
            Builder::Up => Builder::On(On {
                one: None,
                two: Some(two),
                three: None,
            }),
            Builder::On(on) => Builder::On(On {
                one: on.one,
                two: Some(two),
                three: on.three,
            }),
            _ => unreachable!(),
        }
    }
}

#[allow(unreachable_patterns)]
impl Builder<Yes, Yes, Yes> {
    pub fn build(self) -> Go {
        match self {
            Builder::On(On {
                one: Some(one),
                two,
                three,
            }) => Go { one, two, three },
            _ => unreachable!(),
        }
    }
}

#[allow(unreachable_patterns)]
impl Builder<Yes, Yes, No> {
    pub fn build(self) -> Go {
        match self {
            Builder::On(On {
                one: Some(one),
                two,
                ..
            }) => Go {
                one,
                two,
                three: None,
            },
            _ => unreachable!(),
        }
    }
}

#[allow(unreachable_patterns)]
impl Builder<Yes, No, Yes> {
    pub fn build(self) -> Go {
        match self {
            Builder::On(On {
                one: Some(one),
                three,
                ..
            }) => Go {
                one,
                two: None,
                three,
            },
            _ => unreachable!(),
        }
    }
}

#[allow(unreachable_patterns)]
impl Builder<Yes, No, No> {
    pub fn build(self) -> Go {
        match self {
            Builder::On(On { one: Some(one), .. }) => Go {
                one,
                two: None,
                three: None,
            },
            _ => unreachable!(),
        }
    }
}

#[cfg(test)]
mod tests {
    use std::marker::PhantomData;

    use super::{Builder, One, Three, Two};

    #[test]
    fn build_only() {
        Builder::up()
            .left(One {
                _ghost: PhantomData,
            })
            .right(Three {
                _ghost: PhantomData,
            })
            .middle(Two {
                _ghost: PhantomData,
            })
            .build();
    }

    #[test]
    fn build_from_unknown_options() {
        let maybe_one = Some(One {
            _ghost: PhantomData,
        });
        let maybe_two = Some(Two {
            _ghost: PhantomData,
        });
        let maybe_three = Some(Three {
            _ghost: PhantomData,
        });

        ///////////////////////////////////////
        // following doesn't compile:
        // "if and else have incompatible types"
        ///////////////////////////////////////
        // let builder = Builder::up();
        // let builder = if let Some(one) = maybe_one {
        //     builder.on.left(one)
        // } else { builder };
        // let builder = if let Some(two) = maybe_two {
        //     builder.right(two)
        // } else { builder};
        ///////////////////////////////////////

        let result = match (Builder::up(), maybe_one, maybe_two, maybe_three) {
            (builder, Some(one), None, None) => builder.left(one).build(),
            (builder, Some(one), Some(two), Some(three)) => {
                builder.left(one).right(three).middle(two).build()
            }
            (_, None, None, None) => panic!("no arguments for builder"),
            (_, _, _, _) => panic!("too few argument or invalid combinations (use either only left() or all: left(), right() and middle())"),
        };
        assert_eq!(
            result.one,
            One {
                _ghost: PhantomData
            }
        );
        assert_eq!(
            result.two,
            Some(Two {
                _ghost: PhantomData
            })
        );
    }
}

as experimented on Rust playground