serde: Deserialize a sequence into a given vector

de.rs

use core::fmt;

use serde::de;

pub fn deserialize_into_vec<'de, T, D>(vec: &mut Vec<T>, deserializer: D) -> Result<(), D::Error>
where
    T: de::Deserialize<'de>,
    D: de::Deserializer<'de>,
{
    struct Visitor<'a, T: 'a>(&'a mut Vec<T>);

    impl<'de, 'a, T: de::Deserialize<'de>> de::Visitor<'de> for Visitor<'a, T> {
        type Value = ();

        fn expecting(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.write_str("a sequence")
        }

        fn visit_seq<A: de::SeqAccess<'de>>(self, mut seq: A) -> Result<(), A::Error> {
            if let Some(hint) = seq.size_hint() {
                self.0.reserve(hint);
            }

            while let Some(t) = seq.next_element()? {
                self.0.push(t);
            }

            Ok(())
        }
    }

    deserializer.deserialize_seq(Visitor(vec))
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn it_works() {
        let mut v = Vec::<u32>::new();

        deserialize_into_vec(&mut v, &mut serde_json::Deserializer::from_str("[1, 2]")).unwrap();
        assert_eq!(v, [1, 2]);
        deserialize_into_vec(&mut v, &mut serde_json::Deserializer::from_str("[3, 4]")).unwrap();
        assert_eq!(v, [1, 2, 3, 4]);
    }
}