Auto merge of #798 - nabijaczleweli:feat/778-simple-cargo-app, r=kbknapp

Implement crate_description!, crate_name! and app_from_crate! macros

Includes documentation.

src/macros.rs

@@ -448,6 +448,82 @@ macro_rules! crate_authors {
     };
 }
 
+/// Allows you to pull the description from your Cargo.toml at compile time.
+///
+/// # Examples
+///
+/// ```no_run
+/// # #[macro_use]
+/// # extern crate clap;
+/// # use clap::App;
+/// # fn main() {
+/// let m = App::new("app")
+///             .about(crate_description!())
+///             .get_matches();
+/// # }
+/// ```
+#[cfg(not(feature="no_cargo"))]
+#[macro_export]
+macro_rules! crate_description {
+    () => {
+        env!("CARGO_PKG_DESCRIPTION")
+    };
+}
+
+/// Allows you to pull the name from your Cargo.toml at compile time.
+///
+/// # Examples
+///
+/// ```no_run
+/// # #[macro_use]
+/// # extern crate clap;
+/// # use clap::App;
+/// # fn main() {
+/// let m = App::new(crate_name!())
+///             .get_matches();
+/// # }
+/// ```
+#[cfg(not(feature="no_cargo"))]
+#[macro_export]
+macro_rules! crate_name {
+    () => {
+        env!("CARGO_PKG_NAME")
+    };
+}
+
+/// Allows you to build the `App` instance from your Cargo.toml at compile time.
+///
+/// Equivalent to using the `crate_*!` macros with their respective fields.
+///
+/// Provided separator is for the [macro.crate_authors.html](`crate_authors!`) macro,
+/// refer to the documentation therefor.
+///
+/// # Examples
+///
+/// ```no_run
+/// # #[macro_use]
+/// # extern crate clap;
+/// # fn main() {
+/// let m = app_from_crate!().get_matches();
+/// # }
+/// ```
+#[cfg(not(feature="no_cargo"))]
+#[macro_export]
+macro_rules! app_from_crate {
+    () => {
+        $crate::App::new(crate_name!())
+            .version(crate_version!())
+            .author(crate_authors!())
+            .about(crate_description!())
+    };
+    ($sep:expr) => {
+        $crate::App::new(crate_name!())
+            .version(crate_version!())
+            .author(crate_authors!($sep))
+            .about(crate_description!())
+    };
+}
+
 /// Build `App`, `Arg`s, `SubCommand`s and `Group`s with Usage-string like input
 /// but without the parsing.
 #[macro_export]