Using subcmd field name in subcommand derive

[omar25h]

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• I have searched the discussions
• I have searched the existing issues

Rust Version

rustc 1.50.0 (cb75ad5db 2021-02-10)

Clap Version

master

Minimal reproducible code

use clap::Clap;

fn main() {
    let _opts = Opts::parse();
}

#[derive(Clap)]
struct Opts {
    #[clap(subcommand)]
    subcmd: SubCommand,
}

#[derive(Clap)]
enum SubCommand {
    Test,
}

Steps to reproduce the bug with the above code

cargo build

Actual Behaviour

error[E0308]: mismatched types
  --> src/main.rs:10:13
   |
10 |     subcmd: SubCommand,
   |             ^^^^^^^^^^ expected enum `Option`, found `&mut SubCommand`
   |
   = note:           expected enum `Option<(&str, &ArgMatches)>`
           found mutable reference `&mut SubCommand`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.

Expected Behaviour

The program compiles.

Additional Context

This issue is initially discovered in #2375. Filing as a separate issue because that's different from the original issue described there.

Debug Output

No response