"Inlining" enum into containing struct

[jonas-schievink]

I'm trying to (de)serialize a TOML config which basically allows the user to choose a mode. It essentially looks like this:

struct Config {
    name: String,
    other_common_stuff: u64,
    mode: ConfigMode,
}

enum ConfigMode {
    Url {
        url: String,
    }
    
    Dummy {
        print: bool,
    }
}

I want the config file to look somewhat like this:

name = "bla"
other_common_stuff = 456
mode = "url"
url = "http://example.com"

Or, for selecting the Dummy mode:

name = "bla"
other_common_stuff = 456
mode = "dummy"
print = "Hello, World!"

So I basically need to "inline" the ConfigMode into the Config struct for (de)serialization. Since serde supports untagged enums, this doesn't seem too hard to implement to me, right?

[dtolnay]

We are tracking better support for this case in #119. For now there are two possible workarounds but neither one is great. You can deserialize as one of these and then immediately convert it to your better representation for the rest of your program to use.

#[derive(Serialize, Deserialize)]
#[serde(untagged)]
enum Config {
    Url {
        name: String,
        other_common_stuff: u64,
        url: String,
    }
    
    Dummy {
        name: String,
        other_common_stuff: u64,
        print: bool,
    }
}

or

#[derive(Serialize, Deserialize)]
struct Config {
    name: String,
    other_common_stuff: u64,

    #[serde(skip_serializing_if = "Option::is_none")]
    url: Option<String>,

    #[serde(skip_serializing_if = "Option::is_none")]
    print: Option<bool>,
}

Once we implement #119 it will look like this:

#[derive(Serialize, Deserialize)]
struct Config {
    name: String,
    other_common_stuff: u64,

    #[serde(flatten)]
    mode: ConfigMode,
}

#[derive(Serialize, Deserialize)]
#[serde(untagged)]
enum ConfigMode {
    Url {
        url: String,
    }
    
    Dummy {
        print: bool,
    }
}

[jonas-schievink]

Thanks, that's exactly what I'm looking for!

[rofrol]

Working example https://play.rust-lang.org/?gist=6457ea3e0824e80c0f1270cacb052e57

Prints:

Config { name: "bla", other_common_stuff: 456, mode: Url { url: "http://example.com" } }
Config { name: "bla", other_common_stuff: 456, mode: Dummy { print: "Hello, World!" } }

How to nicely remove url and print and have this instead?

Config { name: "bla", other_common_stuff: 456, mode: Url { "http://example.com" } }
Config { name: "bla", other_common_stuff: 456, mode: Dummy { "Hello, World!" } }

[vallentin]

Technically, using untagged is a bit misleading for OPs case. Yes, mode is still in the config, but nothing is "chosen" based on it (as OP said). So if there's multiple variants with the same fields, e.g. a Url and ShortUrl both with a url: String field, then this solution falls flat.

Additionally, if the containing struct is tagged with deny_unknown_fields then even having mode in the config causes deserialization to fail. Which is undesired, if deny_unknown_fields is used to catch unexpected fields.

Yes, untagged is working as expected in itself. I just wanted to point out, that technically, based on the issue then it isn't a proper solution, it just works.