//! Utility macros for utilizing cargo features.

#![deny(clippy::all)]
#![deny(clippy::pedantic)]
#![deny(missing_docs)]

use proc_macro::TokenStream;
use quote::quote;
use syn::{parse2, Item, LitStr};

/// Specifies that a item is only available with a specific feature enabled. This will also
/// document that item is only available with the specified feature enabled by using
/// the [`doc_cfg`] feature.
///
/// All this macro does is to add the following attributes:
/// ```text
/// #[cfg(feature = {FEATURE})]
/// #[cfg_attr(doc_cfg, doc(cfg(feature = {FEATURE})))]
/// ```
///
/// With `{FEATURE}` being replaced with the feature argument.
///
/// # Arguments
/// - The feature. For example: `"parsing"`
///
/// # Panics
/// If not attributed to a item or if arguments is missing.
///
/// # Examples
/// ```
/// # use feature_macros::feature_specific;
/// #
/// #[feature_specific("cool-things")]
/// pub fn do_some_cool_thing() {
///     // ...
/// }
/// ```
///
/// [`doc_cfg`]: https://doc.rust-lang.org/unstable-book/language-features/doc-cfg.html
#[proc_macro_attribute]
pub fn feature_specific(args_stream: TokenStream, item_stream: TokenStream) -> TokenStream {
    create_feature_specific(args_stream.into(), item_stream.into())
        .unwrap()
        .into()
}

fn create_feature_specific(
    args_stream: proc_macro2::TokenStream,
    item_stream: proc_macro2::TokenStream,
) -> Result<proc_macro2::TokenStream, Box<dyn std::error::Error>> {
    let item = parse2::<Item>(item_stream)?;

    let feature = parse2::<LitStr>(args_stream).unwrap().value();

    Ok(quote! {
        #[cfg(feature = #feature)]
        #[cfg_attr(doc_cfg, doc(cfg(feature = #feature)))]
        #item
    })
}

#[cfg(test)]
mod tests {
    use std::error::Error;

    use super::*;

    #[test]
    fn create_feature_specific_works() -> Result<(), Box<dyn Error>> {
        assert_eq!(
            create_feature_specific(
                quote! {"extra-stuff"},
                quote! {
                    pub mod extra;
                },
            )?
            .to_string(),
            quote! {
                #[cfg(feature = "extra-stuff")]
                #[cfg_attr(doc_cfg, doc(cfg(feature = "extra-stuff")))]
                pub mod extra;
            }
            .to_string()
        );

        Ok(())
    }
}