Externally tagged enum variants where all fields have `#[serde(default)]` should allow being written as a unit variant.

[Scripter17]

I'm building a program that takes URLs and strips all the tracking parameters and handles redirects and so on. One of the mappers is ExpandShortLink, which just sends a request to the URL being cleaned and returns the URL the server responded with.

I'm trying to support the case where a user wants to specify HTTP headers for a specific ExpandShortLink without requiring that all uses of ExpandShortLink have a default header map (I expect it to be a niche feature).

Basically, I have this code:

use serde::Deserialize;
use serde_json::from_str;
use std::collections::HashMap;

#[derive(Deserialize)]
struct Rule {
    // ...
    mapper: Mapper
}

#[derive(Deserialize)]
enum Mapper {
    // ...
    ExpandShortLink {
        #[serde(default)]
        headers: HashMap<String, String>
    }
}

fn main() {
    assert!(from_str::<Rule>("{\"mapper\": {\"ExpandShortLink\": {\"headers\": {}}}}").is_ok());
    assert!(from_str::<Rule>("{\"mapper\": \"ExpandShortLink\"}").is_ok());
}

I would like to have {"mapper": "ExpandShortLink"} and {"mapper": {"ExpandShortLink": {"headers": {}}}} both deserialize to Rule {mapper: Mapper::ExpandShortLink {headers: HashMap::new()}}.

[Scripter17]

Great so if I go to a different issue on github mobile and close it it has a chance of closing a random issue

Got it

Great

[iTitus]

Related to #2233 #2508 #2706 Will this get fixed by #2509 or #2520? #2295 also looks promising.

[Scripter17]

The first and third PRs would definitely fix this. I'm not quite awake enough to make heads or tails of the second one but it seems to also handle this.