How to convert json to mlua::Value like c-json.decode

[cppcoffee]

Example code, it will compile with an error:

fn main() {
    let s = r#"{"id":123,"name":"foo"}"#;
    let value: mlua::Value = serde_json::from_str(&s).unwrap();
    println!("{:?}", value);
}

The error messsage:

error[E0277]: the trait bound `LuaValue<'_>: serde::de::Deserialize<'_>` is not satisfied
    --> src/main.rs:3:30
     |
3    |     let value: mlua::Value = serde_json::from_str(&s).unwrap();
     |                              ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `serde::de::Deserialize<'_>` is not implemented for `LuaValue<'_>`

[cppcoffee]

This usage is to register the json_decode function on the Lua side, so it can be used similarly to the c-json library:

local value = app.json_decode('{"id":123, "name":"foo"}')
print(value.name)

[cppcoffee]

Finally, I solved the problem with a layer of intermediate transformations:

fn main() {
    let s = r#"
    {
        "id": 123,
        "name": "foo"
    }
    "#;

    let lua = mlua::Lua::new();
    let tmp: serde_json::Value = serde_json::from_str(&s).unwrap();
    let value = json_to_lua_value(&lua, tmp).unwrap();

    println!("{:?}", value);
}

fn json_to_lua_value(lua: &mlua::Lua, json_value: serde_json::Value) -> mlua::Result<mlua::Value> {
    match json_value {
        serde_json::Value::Null => Ok(mlua::Value::Nil),
        serde_json::Value::Bool(b) => Ok(mlua::Value::Boolean(b)),
        serde_json::Value::Number(n) => {
            if let Some(i) = n.as_i64() {
                Ok(mlua::Value::Integer(i))
            } else if let Some(f) = n.as_f64() {
                Ok(mlua::Value::Number(f))
            } else {
                Err(mlua::Error::RuntimeError("Invalid JSON number".into()))
            }
        }
        serde_json::Value::String(s) => Ok(mlua::Value::String(lua.create_string(s)?)),
        serde_json::Value::Array(arr) => {
            let table = lua.create_table()?;
            for (i, v) in arr.iter().enumerate() {
                table.set(i + 1, json_to_lua_value(lua, v.clone())?)?;
            }
            Ok(mlua::Value::Table(table))
        }
        serde_json::Value::Object(obj) => {
            let table = lua.create_table()?;
            for (k, v) in obj.iter() {
                table.set(k.as_str(), json_to_lua_value(lua, v.clone())?)?;
            }
            Ok(mlua::Value::Table(table))
        }
    }
}

[cppcoffee]

Isn't there a better interface for this usage? That is, it can be converted directly, without the need for a middle layer conversion

[khvzak]

Isn't there a better interface for this usage? That is, it can be converted directly, without the need for a middle layer conversion

Yes, use can use LuaSerdeExt::to_value.

An example: https://github.com/mlua-rs/mlua/blob/master/examples/serialize.rs#L49