Add support of dot.case and SCREAMING.DOT.CASE rename on variant and field

[qboileau]

Add support of Dotcase and CapitalDotcase for compatibility with scala libs enumeratum serialization (using circe for example).

Idea is to add dot.case and SCREAMING.DOT.CASE to rename_all RenameRule for Serialize and Deserialize fields and variants.

use serde::Serialize;

#[derive(Serialize)]
#[serde(rename_all = "dot.case")]
struct Person {
    first_name: String,
    last_name: String,
}

fn main() {
    let person = Person {
        first_name: "Graydon".to_string(),
        last_name: "Hoare".to_string(),
    };

    let json = serde_json::to_string_pretty(&person).unwrap();

    // Prints:
    //    {
    //      "first.name": "Graydon",
    //      "last.name": "Hoare"
    //    }
    println!("{}", json);
}

I'm not sure if that's all that is need to do or if it's missing additional changes.

[qboileau]

@dtolnay do you need something else for this PR to be merged ?

[Property404]

This would be nice to have. I was going to push pretty much the same PR but I guess you beat me to it

[dtolnay]

Thanks for the PR!

I would prefer not to build this into serde_derive. But I would be open to someone making an attribute macro (in a different crate) that supports applying a broader set of rename rules to a serde data structure beyond the ones built into serde_derive, by expanding to multiple individual serde(rename = "...") attributes in the right places.

[Mingun]

What about to allow specify path to the const fn that would perform rename?

const fn rename(name: &str) -> &str { ... }

#[serde(rename_all = path::to::rename)]
struct MyStruct { ... }

Then almost all needs would be covered.