Regression: Subcommand attribute not supported for unit type

[dunnock]

Please complete the following tasks

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

Rust Version

rustc 1.57.0 (f1edd0429 2021-11-29)

Clap Version

3.0.0-rc.7

Minimal reproducible code

use clap::{Parser, Subcommand};                                                                                                                        
  
  #[derive(Parser)]                                                                                                                        
  struct Opt {                                                            
      #[clap(subcommand)]                                                 
      cmd: CmdOptions,
  }                                                                       
                                                              
 #[derive(Subcommand)]   //  ■ proc-macro derive produced unparseable tokens                                                                                                                        
  enum CmdOptions {
      CmdStruct {          
          #[clap(subcommand)]                                                                                                                                                                                                               
          cmd: Cmd2      
      },                                                                                                                                                                                                                 
     CmdUnit(#[clap(subcommand)] Cmd2)   //  ■ expected type, found `#`  expected type                                                                                                                               
  }                                                                  
                                                                                                                                                                                              
  #[derive(Subcommand)]                                              
  enum Cmd2 {                                                                                                                        
      Command
  }

fn main() {}

Steps to reproduce the bug with the above code

cargo build produces compilation errors

Actual Behaviour

❯ cargo build
   Compiling clap-derive-bug v0.1.0 (/home/maxim/src/rust/clap-derive-bug)
error: expected type, found `#`
  --> src/main.rs:15:13
   |
15 |     CmdUnit(#[clap(subcommand)] Cmd2)
   |             ^ expected type

error: proc-macro derive produced unparseable tokens
 --> src/main.rs:9:10
  |
9 | #[derive(Subcommand)]
  |          ^^^^^^^^^^

error: could not compile `clap-derive-bug` due to 2 previous errors

Expected Behaviour

#[clap(subcommand)] macro is supported for unit structs according to #3094

Additional Context

No response

Debug Output

No response

[epage]

We support the attribute on the variant, not on the tuple's element, similar to flatten.

This was missing from the derive reference but I just added it, see https://github.com/clap-rs/clap/blob/master/examples/derive_ref/README.md#app-attributes

[epage]

Also, that linked discussion links out to a PR with an example: #2587

[epage]

I'm assuming what I point to resolves this and am closing this out. Feel free to say otherwise!