fix(derive): Don't leak help headings

Before, when `flatten`ing, the struct you flattened in could set the
help heading for all the following arguments.  Now, we scope each
`flatten` to not do that.

I had originally intended to bake this into the initial / final methods
but that would have required some re-work to allow capturing state
between them that seemed unnecessary.

Fixes #2803

clap_derive/src/derives/args.rs

@@ -198,13 +198,18 @@ pub fn gen_augment(
             | Kind::ExternalSubcommand => None,
             Kind::Flatten => {
                 let ty = &field.ty;
+                let old_heading_var = format_ident!("old_heading");
                 if override_required {
                     Some(quote_spanned! { kind.span()=>
+                        let #old_heading_var = #app_var.get_help_heading();
                         let #app_var = <#ty as clap::Args>::augment_args_for_update(#app_var);
+                        let #app_var = #app_var.help_heading(#old_heading_var);
                     })
                 } else {
                     Some(quote_spanned! { kind.span()=>
+                        let #old_heading_var = #app_var.get_help_heading();
                         let #app_var = <#ty as clap::Args>::augment_args(#app_var);
+                        let #app_var = #app_var.help_heading(#old_heading_var);
                     })
                 }
             }
@@ -358,10 +363,10 @@ pub fn gen_augment(
     let initial_app_methods = parent_attribute.initial_top_level_methods();
     let final_app_methods = parent_attribute.final_top_level_methods();
     quote! {{
-        let #app_var = #app_var#initial_app_methods;
+        let #app_var = #app_var #initial_app_methods;
         #( #args )*
         #subcmd
-        #app_var#final_app_methods
+        #app_var #final_app_methods
     }}
 }
 

clap_derive/src/derives/subcommand.rs

@@ -170,13 +170,18 @@ fn gen_augment(
                 Kind::Flatten => match variant.fields {
                     Unnamed(FieldsUnnamed { ref unnamed, .. }) if unnamed.len() == 1 => {
                         let ty = &unnamed[0];
+                        let old_heading_var = format_ident!("old_heading");
                         let subcommand = if override_required {
                             quote! {
+                                let #old_heading_var = #app_var.get_help_heading();
                                 let #app_var = <#ty as clap::Subcommand>::augment_subcommands_for_update(#app_var);
+                                let #app_var = #app_var.help_heading(#old_heading_var);
                             }
                         } else {
                             quote! {
+                                let #old_heading_var = #app_var.get_help_heading();
                                 let #app_var = <#ty as clap::Subcommand>::augment_subcommands(#app_var);
+                                let #app_var = #app_var.help_heading(#old_heading_var);
                             }
                         };
                         Some(subcommand)
@@ -221,10 +226,10 @@ fn gen_augment(
                     let subcommand = quote! {
                         let #app_var = #app_var.subcommand({
                             let #subcommand_var = clap::App::new(#name);
-                            let #subcommand_var = #subcommand_var#initial_app_methods;
+                            let #subcommand_var = #subcommand_var #initial_app_methods;
                             let #subcommand_var = #arg_block;
                             let #subcommand_var = #subcommand_var.setting(::clap::AppSettings::SubcommandRequiredElseHelp);
-                            #subcommand_var#final_from_attrs
+                            #subcommand_var #final_from_attrs
                         });
                     };
                     Some(subcommand)
@@ -264,9 +269,9 @@ fn gen_augment(
                     let subcommand = quote! {
                         let #app_var = #app_var.subcommand({
                             let #subcommand_var = clap::App::new(#name);
-                            let #subcommand_var = #subcommand_var#initial_app_methods;
+                            let #subcommand_var = #subcommand_var #initial_app_methods;
                             let #subcommand_var = #arg_block;
-                            #subcommand_var#final_from_attrs
+                            #subcommand_var #final_from_attrs
                         });
                     };
                     Some(subcommand)
@@ -278,7 +283,7 @@ fn gen_augment(
     let initial_app_methods = parent_attribute.initial_top_level_methods();
     let final_app_methods = parent_attribute.final_top_level_methods();
     quote! {
-            let #app_var = #app_var#initial_app_methods;
+            let #app_var = #app_var #initial_app_methods;
             #( #subcommands )*;
             #app_var #final_app_methods
     }

clap_derive/tests/help.rs

@@ -1,4 +1,4 @@
-use clap::{Args, IntoApp, Parser};
+use clap::{Args, IntoApp, Parser, Subcommand};
 
 #[test]
 fn arg_help_heading_applied() {
@@ -67,6 +67,12 @@ fn app_help_heading_flattened() {
 
         #[clap(flatten)]
         options_b: OptionsB,
+
+        #[clap(subcommand)]
+        sub_a: SubA,
+
+        #[clap(long)]
+        should_be_in_default_section: Option<u32>,
     }
 
     #[derive(Debug, Clone, Args)]
@@ -83,6 +89,31 @@ fn app_help_heading_flattened() {
         should_be_in_section_b: Option<u32>,
     }
 
+    #[derive(Debug, Clone, Subcommand)]
+    #[clap(help_heading = "SUB A")]
+    enum SubA {
+        #[clap(flatten)]
+        SubB(SubB),
+        #[clap(subcommand)]
+        SubC(SubC),
+        SubAOne,
+        SubATwo {
+            should_be_in_sub_a: Option<u32>,
+        },
+    }
+
+    #[derive(Debug, Clone, Subcommand)]
+    #[clap(help_heading = "SUB B")]
+    enum SubB {
+        SubBOne { should_be_in_sub_b: Option<u32> },
+    }
+
+    #[derive(Debug, Clone, Subcommand)]
+    #[clap(help_heading = "SUB C")]
+    enum SubC {
+        SubCOne { should_be_in_sub_c: Option<u32> },
+    }
+
     let app = CliOptions::into_app();
 
     let should_be_in_section_a = app
@@ -96,4 +127,35 @@ fn app_help_heading_flattened() {
         .find(|a| a.get_name() == "should-be-in-section-b")
         .unwrap();
     assert_eq!(should_be_in_section_b.get_help_heading(), Some("HEADING B"));
+
+    let should_be_in_default_section = app
+        .get_arguments()
+        .find(|a| a.get_name() == "should-be-in-default-section")
+        .unwrap();
+    assert_eq!(should_be_in_default_section.get_help_heading(), None);
+
+    let sub_a_two = app.find_subcommand("sub-a-two").unwrap();
+
+    let should_be_in_sub_a = sub_a_two
+        .get_arguments()
+        .find(|a| a.get_name() == "should-be-in-sub-a")
+        .unwrap();
+    assert_eq!(should_be_in_sub_a.get_help_heading(), Some("SUB A"));
+
+    let sub_b_one = app.find_subcommand("sub-b-one").unwrap();
+
+    let should_be_in_sub_b = sub_b_one
+        .get_arguments()
+        .find(|a| a.get_name() == "should-be-in-sub-b")
+        .unwrap();
+    assert_eq!(should_be_in_sub_b.get_help_heading(), Some("SUB B"));
+
+    let sub_c = app.find_subcommand("sub-c").unwrap();
+    let sub_c_one = sub_c.find_subcommand("sub-c-one").unwrap();
+
+    let should_be_in_sub_c = sub_c_one
+        .get_arguments()
+        .find(|a| a.get_name() == "should-be-in-sub-c")
+        .unwrap();
+    assert_eq!(should_be_in_sub_c.get_help_heading(), Some("SUB C"));
 }