fix(derive): Ensure App help_heading is applied

We normally set all app attributes at the end.  This can be changed but
will require some work to ensure
- Top-level item's doc cmment ins our over flattened
- We still support `Args` / `Subcommand` be used to initialize an `App` when
  creating a subcommand

In the mean time, this special cases `help_heading` to happen first.
We'll need this special casing anyways to address #2803 since we'll need
to capture the old help heading before addings args and then restore it
after.  I guess we could unconditionally do that but its extra work /
boilerplate for when people have to dig into their what the derives do.

Fixes #2785

clap_derive/src/attrs.rs

@@ -106,6 +106,7 @@ pub struct Attrs {
     author: Option<Method>,
     version: Option<Method>,
     verbatim_doc_comment: Option<Ident>,
+    help_heading: Option<Method>,
     is_enum: bool,
     has_custom_parser: bool,
     kind: Sp<Kind>,
@@ -285,6 +286,7 @@ impl Attrs {
             author: None,
             version: None,
             verbatim_doc_comment: None,
+            help_heading: None,
             is_enum: false,
             has_custom_parser: false,
             kind: Sp::new(Kind::Arg(Sp::new(Ty::Other, default_span)), default_span),
@@ -383,6 +385,10 @@ impl Attrs {
                     self.methods.push(Method::new(raw_ident, val));
                 }
 
+                HelpHeading(ident, expr) => {
+                    self.help_heading = Some(Method::new(ident, quote!(#expr)));
+                }
+
                 About(ident, about) => {
                     let method = Method::from_lit_or_env(ident, about, "CARGO_PKG_DESCRIPTION");
                     self.methods.push(method);
@@ -773,7 +779,12 @@ impl Attrs {
     }
 
     /// generate methods from attributes on top of struct or enum
-    pub fn top_level_methods(&self) -> TokenStream {
+    pub fn initial_top_level_methods(&self) -> TokenStream {
+        let help_heading = self.help_heading.as_ref().into_iter();
+        quote!( #(#help_heading)* )
+    }
+
+    pub fn final_top_level_methods(&self) -> TokenStream {
         let version = &self.version;
         let author = &self.author;
         let methods = &self.methods;
@@ -786,7 +797,8 @@ impl Attrs {
     pub fn field_methods(&self) -> proc_macro2::TokenStream {
         let methods = &self.methods;
         let doc_comment = &self.doc_comment;
-        quote!( #(#doc_comment)* #(#methods)* )
+        let help_heading = self.help_heading.as_ref().into_iter();
+        quote!( #(#doc_comment)* #(#help_heading)* #(#methods)* )
     }
 
     pub fn cased_name(&self) -> TokenStream {

clap_derive/src/derives/args.rs

@@ -349,11 +349,13 @@ pub fn gen_augment(
         }
     });
 
-    let app_methods = parent_attribute.top_level_methods();
+    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;
         #( #args )*
         #subcmd
-        #app_var#app_methods
+        #app_var#final_app_methods
     }}
 }
 

clap_derive/src/derives/subcommand.rs

@@ -216,13 +216,15 @@ fn gen_augment(
                     };
 
                     let name = attrs.cased_name();
-                    let from_attrs = attrs.top_level_methods();
+                    let initial_app_methods = parent_attribute.initial_top_level_methods();
+                    let final_from_attrs = attrs.final_top_level_methods();
                     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 = #arg_block;
                             let #subcommand_var = #subcommand_var.setting(::clap::AppSettings::SubcommandRequiredElseHelp);
-                            #subcommand_var#from_attrs
+                            #subcommand_var#final_from_attrs
                         });
                     };
                     Some(subcommand)
@@ -257,12 +259,14 @@ fn gen_augment(
                     };
 
                     let name = attrs.cased_name();
-                    let from_attrs = attrs.top_level_methods();
+                    let initial_app_methods = parent_attribute.initial_top_level_methods();
+                    let final_from_attrs = attrs.final_top_level_methods();
                     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 = #arg_block;
-                            #subcommand_var#from_attrs
+                            #subcommand_var#final_from_attrs
                         });
                     };
                     Some(subcommand)
@@ -271,10 +275,12 @@ fn gen_augment(
         })
         .collect();
 
-    let app_methods = parent_attribute.top_level_methods();
+    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;
             #( #subcommands )*;
-            #app_var #app_methods
+            #app_var #final_app_methods
     }
 }
 

clap_derive/src/parse.rs

@@ -41,6 +41,7 @@ pub enum ClapAttr {
     // ident = arbitrary_expr
     NameExpr(Ident, Expr),
     DefaultValueT(Ident, Option<Expr>),
+    HelpHeading(Ident, Expr),
 
     // ident(arbitrary_expr,*)
     MethodCall(Ident, Vec<Expr>),
@@ -100,6 +101,15 @@ impl Parse for ClapAttr {
                         Ok(Skip(name, Some(expr)))
                     }
 
+                    "help_heading" => {
+                        let expr = ExprLit {
+                            attrs: vec![],
+                            lit: Lit::Str(lit),
+                        };
+                        let expr = Expr::Lit(expr);
+                        Ok(HelpHeading(name, expr))
+                    }
+
                     _ => Ok(NameLitStr(name, lit)),
                 }
             } else {
@@ -107,6 +117,7 @@ impl Parse for ClapAttr {
                     Ok(expr) => match &*name_str {
                         "skip" => Ok(Skip(name, Some(expr))),
                         "default_value_t" => Ok(DefaultValueT(name, Some(expr))),
+                        "help_heading" => Ok(HelpHeading(name, expr)),
                         _ => Ok(NameExpr(name, expr)),
                     },
 

clap_derive/tests/help.rs

@@ -0,0 +1,99 @@
+use clap::{Args, IntoApp, Parser};
+
+#[test]
+fn arg_help_heading_applied() {
+    #[derive(Debug, Clone, Parser)]
+    struct CliOptions {
+        #[clap(long)]
+        #[clap(help_heading = Some("HEADING A"))]
+        should_be_in_section_a: Option<u32>,
+
+        #[clap(long)]
+        no_section: Option<u32>,
+    }
+
+    let app = CliOptions::into_app();
+
+    let should_be_in_section_a = app
+        .get_arguments()
+        .find(|a| a.get_name() == "should-be-in-section-a")
+        .unwrap();
+    assert_eq!(should_be_in_section_a.get_help_heading(), Some("HEADING A"));
+
+    let should_be_in_section_b = app
+        .get_arguments()
+        .find(|a| a.get_name() == "no-section")
+        .unwrap();
+    assert_eq!(should_be_in_section_b.get_help_heading(), None);
+}
+
+#[test]
+fn app_help_heading_applied() {
+    #[derive(Debug, Clone, Parser)]
+    #[clap(help_heading = "DEFAULT")]
+    struct CliOptions {
+        #[clap(long)]
+        #[clap(help_heading = Some("HEADING A"))]
+        should_be_in_section_a: Option<u32>,
+
+        #[clap(long)]
+        should_be_in_default_section: Option<u32>,
+    }
+
+    let app = CliOptions::into_app();
+
+    let should_be_in_section_a = app
+        .get_arguments()
+        .find(|a| a.get_name() == "should-be-in-section-a")
+        .unwrap();
+    assert_eq!(should_be_in_section_a.get_help_heading(), Some("HEADING A"));
+
+    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(),
+        Some("DEFAULT")
+    );
+}
+
+#[test]
+fn app_help_heading_flattened() {
+    #[derive(Debug, Clone, Parser)]
+    struct CliOptions {
+        #[clap(flatten)]
+        options_a: OptionsA,
+
+        #[clap(flatten)]
+        options_b: OptionsB,
+    }
+
+    #[derive(Debug, Clone, Args)]
+    #[clap(help_heading = "HEADING A")]
+    struct OptionsA {
+        #[clap(long)]
+        should_be_in_section_a: Option<u32>,
+    }
+
+    #[derive(Debug, Clone, Args)]
+    #[clap(help_heading = "HEADING B")]
+    struct OptionsB {
+        #[clap(long)]
+        should_be_in_section_b: Option<u32>,
+    }
+
+    let app = CliOptions::into_app();
+
+    let should_be_in_section_a = app
+        .get_arguments()
+        .find(|a| a.get_name() == "should-be-in-section-a")
+        .unwrap();
+    assert_eq!(should_be_in_section_a.get_help_heading(), Some("HEADING A"));
+
+    let should_be_in_section_b = app
+        .get_arguments()
+        .find(|a| a.get_name() == "should-be-in-section-b")
+        .unwrap();
+    assert_eq!(should_be_in_section_b.get_help_heading(), Some("HEADING B"));
+}