fix(assert): Prevent arguments from requiring self

It's non-sensical for an argument to require itself, so it must be a
mistake, and should be prevented.

This is arguably a breaking change, but of the spacebar heating kind.

Signed-off-by: Omer Tuchfeld <omer@tuchfeld.dev>

clap_builder/src/builder/debug_asserts.rs

@@ -140,6 +140,12 @@ pub(crate) fn assert_app(cmd: &Command) {
 
         // requires, r_if, r_unless
         for (_predicate, req_id) in &arg.requires {
+            assert!(
+                &arg.id != req_id,
+                "Argument {} cannot require itself",
+                arg.get_id()
+            );
+
             assert!(
                 cmd.id_exists(req_id),
                 "Command {}: Argument or group '{}' specified in 'requires*' for '{}' does not exist",

tests/builder/require.rs

@@ -1480,10 +1480,9 @@ For more information, try '--help'.
 }
 
 #[test]
-/// This test demonstrates existing broken behavior, ideally it should panic
+#[should_panic = "Argument flag cannot require itself"]
 fn requires_self() {
-    let result = Command::new("flag_required")
+    let _result = Command::new("flag_required")
         .arg(arg!(-f --flag "some flag").requires("flag"))
         .try_get_matches_from(vec![""]);
-    assert!(result.is_ok());
 }