Deserialise unit as emtpy list

cargo.toml

[package]
name = "desertest"
version = "0.1.0"
authors = ["Keir Lawson <[email protected]>"]
edition = "2018"

[dependencies]
serde = { version = "1.0", features = ["derive"] }
serde_yaml = "0.8"

main.rs

use serde::{Serialize, Deserialize, de, de::{Visitor, SeqAccess}, Deserializer};
use std::fmt;

#[derive(Debug, PartialEq, Serialize, Deserialize)]
struct Foo {
    #[serde(deserialize_with = "deserialize_point_list")]
    foo: Vec<Point>
}

#[derive(Debug, PartialEq, Serialize, Deserialize)]
struct Point {
    x: i8,
    y: i8,
}

struct PointVisitor;

impl<'de> Visitor<'de> for PointVisitor {
    type Value = Vec<Point>;

    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        formatter.write_str("a list of points")
    }

    fn visit_unit<E>(self) -> Result<Self::Value, E>
    where
        E: de::Error,
    {
        println!("visiting unit");
        Ok(Vec::new())
    }

    fn visit_seq<A>(self, seq: A) -> Result<Self::Value, A::Error>
    where 
        A: SeqAccess<'de>, 
    {
        Deserialize::deserialize(de::value::SeqAccessDeserializer::new(seq))
    }
}

fn deserialize_point_list<'de, D>(deserializer: D) -> Result<Vec<Point>, D::Error>
where
    D: Deserializer<'de>,
{
    deserializer.deserialize_any(PointVisitor)
}

fn main() -> Result<(), serde_yaml::Error> {
    let list = "
foo:
  - x: 1
    y: 2
  - x: 3
    y: 4
    ";

    let no_list = "
foo:
    ";

    let deserialized_point: Foo = serde_yaml::from_str(&no_list)?;
    dbg!(deserialized_point);
    Ok(())
}