fix(derive): Improve Kind conflict errors

This makes it better scale for the future

clap_derive/src/item.rs

@@ -855,13 +855,10 @@ impl Item {
     }
 
     fn set_kind(&mut self, kind: Sp<Kind>) {
-        if let Kind::Arg(_) = *self.kind {
-            self.kind = kind;
+        if let (Some(old), Some(new)) = (self.kind.name(), kind.name()) {
+            abort!(kind.span(), "`{}` cannot be used with `{}`", new, old);
         } else {
-            abort!(
-                kind.span(),
-                "`subcommand`, `flatten`, `external_subcommand` and `skip` cannot be used together"
-            );
+            self.kind = kind;
         }
     }
 
@@ -1094,6 +1091,19 @@ pub enum Kind {
     ExternalSubcommand,
 }
 
+impl Kind {
+    fn name(&self) -> Option<&'static str> {
+        match self {
+            Self::Arg(_) => None,
+            Self::FromGlobal(_) => Some("from_global"),
+            Self::Subcommand(_) => Some("subcommand"),
+            Self::Flatten => Some("flatten"),
+            Self::Skip(_) => Some("skip"),
+            Self::ExternalSubcommand => Some("external_subcommand"),
+        }
+    }
+}
+
 #[derive(Clone)]
 pub struct Method {
     name: Ident,

tests/derive_ui/skip_flatten.stderr

@@ -1,5 +1,5 @@
-error: `subcommand`, `flatten`, `external_subcommand` and `skip` cannot be used together
-  --> $DIR/skip_flatten.rs:17:18
+error: `flatten` cannot be used with `skip`
+  --> tests/derive_ui/skip_flatten.rs:17:18
    |
 17 |     #[clap(skip, flatten)]
    |                  ^^^^^^^

tests/derive_ui/skip_subcommand.stderr

@@ -1,5 +1,5 @@
-error: `subcommand`, `flatten`, `external_subcommand` and `skip` cannot be used together
-  --> $DIR/skip_subcommand.rs:17:24
+error: `skip` cannot be used with `subcommand`
+  --> tests/derive_ui/skip_subcommand.rs:17:24
    |
 17 |     #[clap(subcommand, skip)]
    |                        ^^^^

tests/derive_ui/subcommand_and_flatten.stderr

@@ -1,5 +1,5 @@
-error: `subcommand`, `flatten`, `external_subcommand` and `skip` cannot be used together
-  --> $DIR/subcommand_and_flatten.rs:16:24
+error: `flatten` cannot be used with `subcommand`
+  --> tests/derive_ui/subcommand_and_flatten.rs:16:24
    |
 16 |     #[clap(subcommand, flatten)]
    |                        ^^^^^^^