Serialize maps (HashMap, BTreeMap) with complex keys (tuples, etc) [Rust]

serde_patch.rs

#[derive(Serialize, Deserialize, Debug, Clone, Default, PartialEq, Eq)]
struct Attributes {
    #[serde(with = "attr")]
    pub attr: HashMap<Request, u32>,
}

mod attr {
    use serde::{Deserializer, Serializer};

    type Attr = std::collections::HashMap<super::Request, u32>;

    pub(super) fn serialize<S: Serializer>(attr: &Attr, ser: S) -> Result<S::Ok, S::Error> {
        let attr: Vec<_> = attr.iter().collect();
        serde::Serialize::serialize(&attr, ser)
    }

    pub(super) fn deserialize<'de, D: Deserializer<'de>>(des: D) -> Result<Attr, D::Error> {
        let attr: Vec<_> = serde::Deserialize::deserialize(des)?;
        Ok(attr.into_iter().collect())
    }
}
      
// source: https://users.rust-lang.org/t/serde-hashmap-serialization-key-error/49776/8