Optionally add paw support for structopt as a feature (#192)

This adds a feature that can be turned on that will include the
ParseArgs impl when generating the struct and enums

CHANGELOG.md

@@ -1,3 +1,5 @@
+* Add optional feature to support `paw` by [@gameldar](https://github.com/gameldar)
+  ([#187](https://github.com/TeXitoi/structopt/issues/187))
 * Support optional vectors of arguments for distinguishing between `-o 1 2`, `-o` and no option provided at all
   by [@sphynx](https://github.com/sphynx)
   ([#180](https://github.com/TeXitoi/structopt/issues/188))

Cargo.toml

@@ -21,6 +21,7 @@ lints = ["clap/lints"]
 debug = ["clap/debug"]
 no_cargo = ["clap/no_cargo"]
 doc = ["clap/doc"]
+paw = ["structopt-derive/paw"]
 
 [badges]
 travis-ci = { repository = "TeXitoi/structopt" }

src/lib.rs

@@ -21,6 +21,18 @@
 //! structopt = { version = "0.2", default-features = false }
 //! ```
 //!
+//!
+//! Support for [`paw`](https://github.com/rust-cli/paw) (the
+//! `Command line argument paw-rser abstraction for main`) is disabled
+//! by default, but can be enabled in the `structopt` dependency
+//! with the feature `paw`:
+//!
+//! ```toml
+//! [dependencies]
+//! structopt = { version = "0.2", features = [ "paw" ] }
+//! paw = "1.0"
+//! ```
+//!
 //! ## How to `derive(StructOpt)`
 //!
 //! First, let's look at an example:

structopt-derive/Cargo.toml

@@ -20,6 +20,7 @@ heck = "^0.3.0"
 
 [features]
 nightly = ["proc-macro2/nightly"]
+paw = []
 
 [lib]
 proc-macro = true

structopt-derive/src/lib.rs

@@ -433,6 +433,23 @@ fn gen_from_subcommand(
     }
 }
 
+#[cfg(feature = "paw")]
+fn gen_paw_impl(name: &Ident) -> TokenStream {
+    quote! {
+        impl paw::ParseArgs for #name {
+            type Error = std::io::Error;
+
+            fn parse_args() -> Result<Self, Self::Error> {
+                Ok(<#name as ::structopt::StructOpt>::from_args())
+            }
+        }
+    }
+}
+#[cfg(not(feature = "paw"))]
+fn gen_paw_impl(_: &Ident) -> TokenStream {
+    TokenStream::new()
+}
+
 fn impl_structopt_for_struct(
     name: &Ident,
     fields: &Punctuated<Field, Comma>,
@@ -441,6 +458,7 @@ fn impl_structopt_for_struct(
     let basic_clap_app_gen = gen_clap_struct(attrs);
     let augment_clap = gen_augment_clap(fields, &basic_clap_app_gen.attrs);
     let from_clap = gen_from_clap(name, fields, &basic_clap_app_gen.attrs);
+    let paw_impl = gen_paw_impl(name);
 
     let clap_tokens = basic_clap_app_gen.tokens;
     quote! {
@@ -456,6 +474,8 @@ fn impl_structopt_for_struct(
             #augment_clap
             pub fn is_subcommand() -> bool { false }
         }
+
+        #paw_impl
     }
 }
 
@@ -469,6 +489,7 @@ fn impl_structopt_for_enum(
     let augment_clap = gen_augment_clap_enum(variants, &basic_clap_app_gen.attrs);
     let from_clap = gen_from_clap_enum(name);
     let from_subcommand = gen_from_subcommand(name, variants, &basic_clap_app_gen.attrs);
+    let paw_impl = gen_paw_impl(name);
 
     let clap_tokens = basic_clap_app_gen.tokens;
     quote! {
@@ -484,6 +505,8 @@ fn impl_structopt_for_enum(
             #from_subcommand
             pub fn is_subcommand() -> bool { true }
         }
+
+        #paw_impl
     }
 }