Allow setting attributes on the generated impl block (#9)

This commit adds a new syntax to allow setting attributes directly on
the generated impl block. The main use case I need it for is to set
\#[allow(missing_docs)] on the enum items without needing to
specifically add it to every single variant.

This commit also includes a changelog update and the relevant docs
changes.

CHANGELOG.md

@@ -6,6 +6,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## Unreleased
+### Added
+- `c_enum!` now has some extra syntax to allow directly applying attributes to
+  the generated impl block.
+
 ### Changed
 - It is now only possible to define a single enum instance within a `c_enum!`.
   This is to allow additional syntax in the future that is not supported when

src/lib.rs

@@ -62,6 +62,32 @@
 //! that the variants are converted to constants so macros expecting an enum
 //! variant will not work.
 //!
+//! ## Applying Attributes to Generated `impl` Blocks
+//! Sometimes you need to apply attributes to the generated `impl` blocks (e.g.
+//! to silence warnings). `c_enum!` supports adding an extra `impl` block at the
+//! end and will apply those attributes to the generated `impl` block.
+//! ```
+//! #![deny(missing_docs)]
+//! # //! crate docs...
+//! # use c_enum::c_enum;
+//!
+//! c_enum! {
+//!     /// This crate requires that all items be documented.
+//!     ///
+//!     /// However, we don't want to document the members of this enum for one
+//!     /// reason or another.
+//!     pub enum SomeCEnum : u32 {
+//!         A = 0,
+//!         B
+//!     }
+//!
+//!     // So we attach the #[allow] directive to this extra impl block here
+//!     // and it will be added to the one generated by the macro.
+//!     #[allow(missing_docs)]
+//!     impl {}
+//! }
+//! ```
+//!
 //! # Representation
 //! It is valid to add a `#[repr(C)]` or `#[repr(transparent)]` attribute to the
 //! generated type. The generated type is guaranteed to be a newtype whose only
@@ -222,6 +248,11 @@ macro_rules! c_enum {
                 $field:ident $( = $value:expr )?
             ),* $(,)?
         }
+
+        $(
+            $( #[$iattr:meta] )*
+            impl {}
+        )?
     } => {
         $crate::__c_enum_no_debug! {
             $( #[$attr] )*
@@ -231,6 +262,11 @@ macro_rules! c_enum {
                     $field $( = $value )?
                 ),*
             }
+
+            $(
+                $( #[$iattr] )*
+                impl {}
+            )?
         }
 
         impl ::core::fmt::Debug for $name
@@ -279,11 +315,17 @@ macro_rules! __c_enum_no_debug {
                 $field:ident $( = $value:expr )?
             ),* $(,)?
         }
+
+        $(
+            $( #[$iattr:meta] )*
+            impl {}
+        )?
     } => {
         $( #[$attr] )*
         $vis struct $name(pub $inner);
 
         #[allow(non_upper_case_globals)]
+        $( $( #[$iattr] )* )?
         impl $name {
             $crate::__c_enum_impl!(
                 impl(decl_variants, $name, $inner, 0)