Can you directly call MapPreventDuplicates::deserialize_as() to get a HashMap with no duplicates?

[drewlwhitney]

Hello!

I've got a CustomProps struct that wraps a HashMap. I'm using a custom deserialization method so I can check at deserialization time whether there are any custom properties, and print something if there are. For example,

pub struct CustomProps(HashMap<String, String>);

impl<'de> Deserialize<'de> for CustomProps {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: de::Deserializer<'de>,
    {
        // how can I also check for duplicates using `MapPreventDuplicates`, if at all?
        let custom_props: HashMap<String, String> = HashMap::deserialize(deserializer)?;
        if !custom_props.is_empty() {
            println!("Note! You have custom properties");
        }
        Ok(Self(custom_props))
    }
}

I also want to ensure there are no duplicate values using MapPreventDuplicates. Is there any way to call MapPreventDuplicates::deserialize_as() directly? I tried something like

let custom_props: HashMap<String, String> = MapPreventDuplicates::deserialize_as(deserializer)?;

but I can't figure out how the type annotations for MapPreventDuplicates should work. I'm also not sure if this is how the serde_as system works; I've been going through the docs but I'm still a little confused.

Thanks!

[jonasbb]

You can use As to make the DeserializeAs types compatible with the plain serde(with=...). You have to take care of all the actions the serde_as macro does, especially replacing _ with Same.

So I think this should work here:

let custom_props: HashMap<String, String> = As::<MapPreventDuplicates<Same, Same>>::deserialize(deserializer)?;

[drewlwhitney]

Mmmmm so you are a genius and this crate is brilliant.

I ended up with this

let custom_props: HashMap<String, String> = MapPreventDuplicates::<Same, Same>::deserialize_as(deserializer)?;

since I'm calling the function directly, so I don't need it to map to serde(with = ...).

Cheers!

[drewlwhitney]

For anyone who sees this, note that trying to use MapPreventDuplicates with something like

use serde_json::json;

let json = json!({
    "foo": 0,
    "foo": 1,
    "bar": 2
});

let thing: [...] = serde_json::from_value().unwrap();

will actually succeed because the json!() macro removes duplicates from the input before serde_json::from_value() is called, so MapPreventDuplicates won't ever see them. This doesn't have anything to do with serde_with, but it is something I ran into