How to convert a `serde_json::Value ` to a `prost_types::Struct` in Rust?

fn to_struct(json: serde_json::Value) -> prost_types::Struct {
  // How to implement it?
}

Is there any crate to do it?

This is not necessarily something you need a crate for. You do however have your types mixed: Not all serde_json::Values can be converted to a prost_type::Struct, as serde_json::Values can also be lists or strings or bools or numbers or null. If you do want to convert to a struct, you'll need to start from a serde_json::Map:

fn to_struct(json: serde_json::Map<String, serde_json::Value>) -> prost_types::Struct {
    prost_types::Struct {
        fields: json
            .into_iter()
            .map(|(k, v)| (k, serde_json_to_prost(v)))
            .collect(),
    }
}

Likely though, you only need the following

fn serde_json_to_prost(json: serde_json::Value) -> prost_types::Value {
    use prost_types::value::Kind::*;
    use serde_json::Value::*;
    prost_types::Value {
        kind: Some(match json {
            Null => NullValue(0 /* wat? */),
            Bool(v) => BoolValue(v),
            Number(n) => NumberValue(n.as_f64().expect("Non-f64-representable number")),
            String(s) => StringValue(s),
            Array(v) => ListValue(prost_types::ListValue {
                values: v.into_iter().map(serde_json_to_prost).collect(),
            }),
            Object(v) => StructValue(to_struct(v)),
        }),
    }
}

Rustexplorer

https://crates.io/crates/prost-wkt-types can generate prost_wkt_types::Struct, which supports Deserialize trait.

Adding onto @Caesar's answer, I just wanted to show an example of how to go back and forth between serde_json::Value and prost_types::Value:

fn to_struct(json: serde_json::Map<String, serde_json::Value>) -> prost_types::Struct {
    prost_types::Struct {
        fields: json
            .into_iter()
            .map(|(k, v)| (k, serde_json_to_prost(v)))
            .collect(),
    }
}

fn serde_json_to_prost(json: serde_json::Value) -> prost_types::Value {
    use prost_types::value::Kind::*;
    use serde_json::Value::*;
    prost_types::Value {
        kind: Some(match json {
            Null => NullValue(0 /* wat? */),
            Bool(v) => BoolValue(v),
            Number(n) => NumberValue(n.as_f64().expect("Non-f64-representable number")),
            String(s) => StringValue(s),
            Array(v) => ListValue(prost_types::ListValue {
                values: v.into_iter().map(serde_json_to_prost).collect(),
            }),
            Object(v) => StructValue(to_struct(v)),
        }),
    }
}

fn prost_to_serde_json(x: prost_types::Value) -> serde_json::Value {
    use prost_types::value::Kind::*;
    use serde_json::Value::*;
    match x.kind {
        Some(x) => match x {
            NullValue(_) => Null,
            BoolValue(v) => Bool(v),
            NumberValue(n) => Number(serde_json::Number::from_f64(n).unwrap()),
            StringValue(s) => String(s),
            ListValue(lst) => Array(lst.values.into_iter().map(prost_to_serde_json).collect()),
            StructValue(v) => Object(
                v.fields
                    .into_iter()
                    .map(|(k, v)| (k, prost_to_serde_json(v)))
                    .collect(),
            ),
        },
        None => panic!("todo"),
    }
}