Handling redundant '=' characters in short options.

[karlmcdowall]

Here's the scenario... Rust Coreutils is aiming to be a drop-in re-implementation of GNU Coreutils, including generating identical errors for malformed inputs. One case that is currently not implemented correctly is when a client includes a redundant '=' between a short command option and its argument. e.g.
With GNU...

$ /usr/bin/tail -n=1 ./hello_world.txt
/usr/bin/tail: invalid number of lines: ‘=1’

vs with RustCoreUtils...

$ rust_tail -n=1 ./hello_world.txt
Hello World!

Digging deeper, it doesn't seem obvious how to mimic the GNU behavior even with a custom ValueParser - the redundant '=' is already stripped-off before we receive the string in the ValueParser, so there's no obvious way to detect this and return an error.
e.g.

use clap::{Arg, Command, ValueParser};
#[test]
fn demo_test() {
  let command = Command::new("prog")
    .arg(
      Arg::new("example")
        .short('n')
        .value_parser(dummy_parser)
    );
  // -n option below includes redundant '=' which we'd like to detect so we can generate an error.
  let args = vec!["dummy_exec", "-n=123"];    
  let m = command.try_get_matches_from(args).unwrap();
  let val = m.get_one::<OsString>("example").unwrap();
  assert_eq!(val, "123"); // We've parsed out the 123 and never saw the redundant '=' alas.
}

fn dummy_parser(val: &str) -> Result<OsString, String> {
  assert_eq!(val, "123");   // Can I somehow configure the ValueParser to give me "=123" here?
  Ok(OsString::from(val))
}

Maybe there's a way to already achieve what we're trying to do? Any suggestions?
Thanks!

[epage]

Discussion on this is happening in #3030.