clap bug?

This program defines a clap command that does nothing. You would run it like this:

$ cargo run -- one two
...
Cli { my_arg: None, command: One(OneSubcommands { command: Two }) }

If you forget the second subcommand, you get a useful usage message:

$ cargo run -- one
Usage: clap-message one [MY_ARG] <COMMAND>

Commands:
  two
  help  Print this message or the help of the given subcommand(s)

Arguments:
  [MY_ARG]  [env: MY_ARG=]

Options:
  -h, --help  Print help

If you forget the second subcommand but also specify the optional environment variable `MY_ARG`, you get an error message with a more abbreviated help message:

dap@zathras clap-bug $ MY_ARG= cargo run -- one
error: 'clap-message one' requires a subcommand but one was not provided
  [subcommands: two, help]

Usage: clap-message one [MY_ARG] <COMMAND>

For more information, try '--help'.

With this diff:

diff --git a/src/main.rs b/src/main.rs
index f00b34f..959b3bf 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -4,7 +4,7 @@ use clap::Subcommand;

 #[derive(Debug, Clone, Parser)]
 struct Cli {
-    #[clap(env = "MY_ARG", global = true)]
+    #[clap(env = "MY_ARG")]
     my_arg: Option<String>,

     #[command(subcommand)]

you don’t get that error message in that case, even though logically we’re still passing the same argument:

$ MY_ARG= cargo run -- one
...
Usage: clap-message one <COMMAND>

Commands:
  two
  help  Print this message or the help of the given subcommand(s)

Options:
  -h, --help  Print help