Introduce #[structopt(external_subcommand)]

.gitignore

@@ -1,5 +1,6 @@
 target
 Cargo.lock
+expanded.rs
 *~
 
 .idea/

src/lib.rs

@@ -972,22 +972,15 @@ pub trait StructOpt {
     // ** SUBJECT TO CHANGE WITHOUT NOTICE!!! **
 
     #[doc(hidden)]
-    fn is_subcommand() -> bool {
-        unimplemented!()
-    }
+    fn augment_clap<'a, 'b>(_app: clap::App<'a, 'b>) -> clap::App<'a, 'b>;
 
     #[doc(hidden)]
-    fn augment_clap<'a, 'b>(_app: clap::App<'a, 'b>) -> clap::App<'a, 'b> {
-        unimplemented!()
-    }
+    fn subcommand_settings<'a, 'b>(_app: clap::App<'a, 'b>) -> clap::App<'a, 'b>;
 
     #[doc(hidden)]
     fn from_subcommand<'a, 'b>(_sub: (&'b str, Option<&'b clap::ArgMatches<'a>>)) -> Option<Self>
     where
-        Self: Sized,
-    {
-        unimplemented!()
-    }
+        Self: Sized;
 }
 
 impl<T: StructOpt> StructOpt for Box<T> {
@@ -1005,8 +998,8 @@ impl<T: StructOpt> StructOpt for Box<T> {
     }
 
     #[doc(hidden)]
-    fn is_subcommand() -> bool {
-        <T as StructOpt>::is_subcommand()
+    fn subcommand_settings<'a, 'b>(app: clap::App<'a, 'b>) -> clap::App<'a, 'b> {
+        <T as StructOpt>::subcommand_settings(app)
     }
 
     #[doc(hidden)]

structopt-derive/src/attrs.rs

@@ -20,6 +20,7 @@ use syn::{self, ext::IdentExt, spanned::Spanned, Attribute, Expr, Ident, LitStr,
 #[derive(Clone)]
 pub enum Kind {
     Arg(Sp<Ty>),
+    ExternalSubcommand(Sp<Ty>),
     Subcommand(Sp<Ty>),
     FlattenStruct,
     Skip(Option<Expr>),
@@ -266,11 +267,17 @@ impl Attrs {
                 }
 
                 Subcommand(ident) => {
-                    let ty = Sp::call_site(Ty::Other);
+                    let ty = Sp::call_site(Ty::Other); // will be patched later
                     let kind = Sp::new(Kind::Subcommand(ty), ident.span());
                     self.set_kind(kind);
                 }
 
+                ExternalSubcommand(ident) => {
+                    let ty = Sp::call_site(Ty::Other); // will be patched later
+                    let kind = Sp::new(Kind::ExternalSubcommand(ty), ident.span());
+                    self.set_kind(kind);
+                }
+
                 Flatten(ident) => {
                     let kind = Sp::new(Kind::FlattenStruct, ident.span());
                     self.set_kind(kind);
@@ -361,10 +368,12 @@ impl Attrs {
             );
         }
         match &*res.kind {
-            Kind::Subcommand(_) => abort!(res.kind.span(), "subcommand is only allowed on fields"),
-            Kind::FlattenStruct => abort!(res.kind.span(), "flatten is only allowed on fields"),
-            Kind::Skip(_) => abort!(res.kind.span(), "skip is only allowed on fields"),
-            Kind::Arg(_) => res,
+            Kind::Subcommand(_) => {
+                abort!(res.kind.span(), "`subcommand` is only allowed on fields")
+            }
+            Kind::FlattenStruct => abort!(res.kind.span(), "`flatten` is only allowed on fields"),
+            Kind::Skip(_) => abort!(res.kind.span(), "`skip` is only allowed on fields"),
+            Kind::Arg(_) | Kind::ExternalSubcommand(_) => res,
         }
     }
 
@@ -398,6 +407,10 @@ impl Attrs {
                     );
                 }
             }
+            Kind::ExternalSubcommand(_) => abort!(
+                res.kind.span(),
+                "`external_subcommand` must be placed on an enum variant, not on field"
+            ),
             Kind::Subcommand(_) => {
                 if res.has_custom_parser {
                     abort!(