Emit compile-time error if the derive interface produces duplicate short/long arguments

[nsunderland1]

Please complete the following tasks

• I have searched the discussions
• I have searched the open and rejected issues

Clap Version

3.1.3

Describe your use case

Not much special context for the use case, reasoning behind the feature is given below.

Describe the solution you'd like

Given the following code (playground):

use clap::Parser;

#[derive(Parser)]
struct Args {
    #[clap(short)]
    arg1: String,

    #[clap(short)]
    arg2: String,
}

fn main() {
    let _ = Args::parse();
}

clap will successfully detect at runtime that arg1 and arg2 have the duplicate short form -a:

thread 'main' panicked at 'Command playground: Short option names must be unique for each argument, but '-a' is in use by both 'arg1' and 'arg2'', /playground/.cargo/registry/src/github.com-1ecc6299db9ec823/clap-3.1.3/src/build/debug_asserts.rs:100:17
note: [run with `RUST_BACKTRACE=1` environment variable to display a backtrace](https://play.rust-lang.org/#)

With clap's derive interface, it seems like this could be caught at compile time when the proc macro executes on Args, by producing a compiler error if it picks up on duplicate short (or long) names.

This would give much quicker feedback for programmer error. Also, since clap only checks for argument uniqueness in debug mode, if you're only ever running a release build of your binary you won't actually hit this panic.

Alternatives, if applicable

I personally wouldn't mind it if the assertions were enabled in release builds, but the compile-time approach seems better as it avoids the runtime overhead.

Additional Context

We were mostly only running our binary in release mode, and we went a few days with duplicate short arguments. When we hit the panic, it wasn't immediately obvious what was going on.

[epage]

FYI this has previously been reported as #3133

[nsunderland1]

Ah thanks, went searching with "unique" as my query, shoulda tried flipping it :)