Add footer attribute

Provides a way to add extra context to the bottom of the help message.
The doc comment appears at the top, and `#[footer = "..."]` is a nice
complement.

examples/full-derive/src/main.rs

@@ -11,6 +11,10 @@ use std::{fmt::Write as _, path::PathBuf, process::ExitCode};
 /// A basic argument parsing example with `onlyargs_derive`.
 /// Sums a list of numbers and writes the result to a file or standard output.
 #[derive(Clone, Debug, Eq, PartialEq, OnlyArgs)]
+#[footer = "Please consider becoming a sponsor 💖:"]
+#[footer = "  - https://github.com/sponsors/parasyte"]
+#[footer = "  - https://ko-fi.com/blipjoy"]
+#[footer = "  - https://patreon.com/blipjoy"]
 struct Args {
     /// Your username.
     username: String,

onlyargs_derive/Cargo.toml

@@ -14,5 +14,5 @@ license = "MIT"
 proc-macro = true
 
 [dependencies]
-myn = "0.2"
+myn = "0.2.1"
 onlyargs = { version = "0.1", path = ".." }

onlyargs_derive/src/lib.rs

@@ -9,8 +9,16 @@
 //! use onlyargs_derive::OnlyArgs;
 //!
 //! /// Doc comments will appear in your application's help text.
-//! /// Multi-line comments are also supported.
+//! ///
+//! /// Features:
+//! ///   - Supports multi-line strings.
+//! ///   - Supports indentation.
 //! #[derive(Debug, OnlyArgs)]
+//! #[footer = "Footer attributes will be included at the bottom of the help message."]
+//! #[footer = ""]
+//! #[footer = "Features:"]
+//! #[footer = "  - Also supports multi-line strings."]
+//! #[footer = "  - Also supports indentation."]
 //! struct Args {
 //!     /// Optional output path.
 //!     output: Option<std::path::PathBuf>,
@@ -111,7 +119,7 @@ mod parser;
 
 /// See the [root module documentation](crate) for the DSL specification.
 #[allow(clippy::too_many_lines)]
-#[proc_macro_derive(OnlyArgs, attributes(default, long, short))]
+#[proc_macro_derive(OnlyArgs, attributes(footer, default, long, short))]
 pub fn derive_parser(input: TokenStream) -> TokenStream {
     let ast = match ArgumentStruct::parse(input) {
         Ok(ast) => ast,
@@ -312,6 +320,11 @@ pub fn derive_parser(input: TokenStream) -> TokenStream {
     } else {
         format!("\n{}\n", ast.doc.join("\n"))
     };
+    let footer = if ast.footer.is_empty() {
+        String::new()
+    } else {
+        format!("\n{}\n", ast.footer.join("\n"))
+    };
     let bin_name = std::env::var_os("CARGO_BIN_NAME").and_then(|name| name.into_string().ok());
     let help_impl = if bin_name.is_none() {
         r#"fn help() -> ! {
@@ -350,6 +363,8 @@ pub fn derive_parser(input: TokenStream) -> TokenStream {
                     {options_help:?},
                     {positional_help:?},
                     "\n",
+                    {footer:?},
+                    "\n",
                 );
 
                 /// The application name and version.

onlyargs_derive/src/parser.rs

@@ -8,6 +8,7 @@ pub(crate) struct ArgumentStruct {
     pub(crate) options: Vec<ArgOption>,
     pub(crate) positional: Option<ArgOption>,
     pub(crate) doc: Vec<String>,
+    pub(crate) footer: Vec<String>,
 }
 
 #[derive(Debug)]
@@ -87,13 +88,19 @@ impl ArgumentStruct {
             .map(trim_with_indent)
             .collect();
 
+        let footer = get_attr_strings(&attrs, "footer")
+            .into_iter()
+            .map(|line| line.trim_end().to_string())
+            .collect();
+
         match input.next() {
             None => Ok(Self {
                 name,
                 flags,
                 options,
                 positional,
                 doc,
+                footer,
             }),
             tree => Err(spanned_error("Unexpected token", tree.as_span())),
         }