No unreachable warning when duplicate field names on enum after rename attributes

[seam345]

Looking into the rename feature I discovered that rustc does not gives an unreachable warning with serde rename collisions with enums whereas it does on structs as mentioned in #754

Would you still be open to a PR to make clashing renames an error? If so I'm happy to give it a go.

Example

use serde::{Deserialize};

#[derive(Deserialize)]
enum Message {
    #[serde(rename = "Response")]
    Request { id: String},
    #[serde(rename = "Response")]
    Response { id: String},
}

fn main() {
    let json = "{\"Response\": {\"id\": \"...\"}}";
    
    let parsed: Message = match serde_json::from_str(&json) {
        Ok(contact) => contact,
        Err(err) => {
            println!("{:?}", err);
            unimplemented!()
        }
    };

    match parsed {
        Message::Request { id } => println!("request {}", id),
        Message::Response { id } => println!("response {}", id)
    }
    
}

Output

request ...
with no compiler warnings

playgrounds link https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=c6a787d51f1290af999a0e36b9a6d366

[seam345]

or maybe instead of an error it should follow rustc and display a warning

[dtolnay]

This was not fixed by #2562. The code above still compiles with no error or warning.