expose 0.1.0

Expose your rust API for use in a C++ codebase.
Documentation 
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extern crate proc_macro;
extern crate quote;
extern crate syn;
extern crate libc;

use proc_macro::TokenStream;
use quote::quote;

/// Exposes a function for use in C++
///
/// # Example
/// ```
/// #[expose::cpp_fn]
/// pub fn test() -> i32 {
///     42
/// }
/// ```
///
#[proc_macro_attribute]
pub fn function(_attr: TokenStream, item: TokenStream) -> TokenStream {
    let input = syn::parse_macro_input!(item as syn::ItemFn);
    match input.vis {
        syn::Visibility::Public(_) => compose_func(&input, &input.vis),
        _ => panic!("Exposed member must be public")
    }
}

/// Exposes an enum for use in C++
///
/// # Example
/// ```
/// #[expose::cpp_enum]
/// enum Test {
///     One,
///     Two,
///     Three
/// }
/// ```
#[proc_macro_attribute]
pub fn enumeration(_attr: TokenStream, item: TokenStream) -> TokenStream {
    let input = syn::parse_macro_input!(item as syn::ItemEnum);
    match input.vis {
        syn::Visibility::Public(_) => compose_enum(&input),
        _ => panic!("Exposed member must be public")
    }
}

fn compose_enum(input: &syn::ItemEnum) -> TokenStream {
    let result = quote! {
        #[repr(C)]
        #[no_mangle]
        #input
    };
    result.into()
}

fn compose_func(input: &syn::ItemFn, vis: &syn::Visibility) -> TokenStream {
    let name = &input.ident;
    let args = &input.decl.inputs;
    let out = &input.decl.output;
    let block = &input.block;

    let out = match &out {
        syn::ReturnType::Type(_, _) => out,
        _ => panic!("Must specify return type on exposed functions")
    };

    let result = quote! {
        #[no_mangle]
        #vis extern fn #name(#args) #out {
            #block
        }
    };
    result.into()
}