extension_trait/
lib.rs

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extern crate proc_macro;

use proc_macro::TokenStream;
use quote::{quote, ToTokens};
use std::borrow::Cow;
use syn::ext::IdentExt;
use syn::parse::{Nothing, Parse, ParseStream};
use syn::spanned::Spanned;
use syn::{
    parse_macro_input, Attribute, FnArg, Generics, Ident, ImplItem, ImplItemConst, ImplItemFn,
    ImplItemType, ItemImpl, Pat, PatIdent, PatReference, PatTuple, PatType, Receiver, Signature,
    Visibility,
};

struct ItemImplWithVisibility {
    attrs: Vec<Attribute>,
    visibility: Visibility,
    impl_item: ItemImpl,
}

impl Parse for ItemImplWithVisibility {
    fn parse(input: ParseStream) -> syn::Result<Self> {
        let attrs = input.call(Attribute::parse_outer)?;
        let visibility = input.parse()?;
        let impl_item = input.parse()?;
        Ok(Self {
            attrs,
            visibility,
            impl_item,
        })
    }
}

/// Declares an extension trait
///
/// # Example
///
/// ```
/// #[macro_use]
/// extern crate extension_trait;
///
/// #[extension_trait]
/// pub impl DoubleExt for str {
///    fn double(&self) -> String {
///        self.repeat(2)
///    }
/// }
///
/// fn main() {
///     assert_eq!("Hello".double(), "HelloHello");
/// }
/// ```
#[proc_macro_attribute]
pub fn extension_trait(args: TokenStream, input: TokenStream) -> TokenStream {
    parse_macro_input!(args as Nothing);
    let ItemImplWithVisibility {
        attrs,
        visibility,
        impl_item,
    } = parse_macro_input!(input as ItemImplWithVisibility);
    let ItemImpl {
        impl_token,
        unsafety,
        trait_,
        items,
        ..
    } = &impl_item;
    let items = items.iter().map(|item| match item {
        ImplItem::Const(ImplItemConst {
            attrs,
            vis: _,
            defaultness: None,
            const_token,
            ident,
            generics: Generics {
                lt_token: None,
                params: _,
                gt_token: None,
                where_clause: None,
            },
            colon_token,
            ty,
            eq_token: _,
            expr: _,
            semi_token,
        }) => quote! { #(#attrs)* #const_token #ident #colon_token #ty #semi_token },
        ImplItem::Fn(ImplItemFn {
            attrs,
            vis: _,
            defaultness: None,
            sig: Signature {
                constness,
                asyncness,
                unsafety,
                abi,
                fn_token,
                ident,
                generics,
                paren_token: _,
                inputs,
                variadic,
                output
            },
            block: _,
        }) => {
            let inputs = inputs.into_iter().map(|arg| {
                let span = arg.span();
                match arg {
                    FnArg::Typed(PatType { attrs, pat, colon_token, ty }) => {
                        let ident = extract_ident(pat).unwrap_or_else(|| Cow::Owned(Ident::new("_", span)));
                        quote! { #(#attrs)* #ident #colon_token #ty }
                    },
                    FnArg::Receiver(Receiver {
                        attrs,
                        reference: None,
                        mutability: _,
                        self_token,
                        colon_token: Some(colon),
                        ty,
                    }) => quote! { #(#attrs)* #self_token #colon #ty },
                    FnArg::Receiver(receiver) => receiver.into_token_stream(),
                }
            });
            let where_clause = &generics.where_clause;
            quote! {
                #(#attrs)*
                #constness #asyncness #unsafety #abi #fn_token #ident #generics (#(#inputs,)* #variadic) #output #where_clause;
            }
        },
        ImplItem::Type(ImplItemType {
            attrs,
            type_token,
            ident,
            generics,
            semi_token,
            ..
        }) => quote! { #(#attrs)* #type_token #ident #generics #semi_token },
        _ => syn::Error::new(item.span(), "unsupported item type").to_compile_error(),
    });
    if let Some((None, path, _)) = trait_ {
        (quote! {
            #(#attrs)*
            #visibility #unsafety trait #path {
                #(#items)*
            }
            #impl_item
        })
        .into()
    } else {
        syn::Error::new(impl_token.span(), "extension trait name was not provided")
            .to_compile_error()
            .into()
    }
}

fn extract_ident(pat: &Pat) -> Option<Cow<'_, Ident>> {
    match pat {
        Pat::Reference(PatReference { pat, .. }) => extract_ident(pat),
        Pat::Ident(PatIdent { ident, .. }) => Some(Cow::Borrowed(ident)),
        Pat::Tuple(PatTuple { elems, .. }) => {
            if elems.len() <= 1 {
                extract_ident(elems.into_iter().next()?)
            } else {
                let span = elems.span();
                let elems = elems
                    .into_iter()
                    .map(extract_ident)
                    .map(|o| o.map(|ident| ident.unraw().to_string()))
                    .collect::<Option<Vec<String>>>()?;
                let joined = elems.join("_");
                Some(Cow::Owned(Ident::new(&joined, span)))
            }
        }
        _ => None,
    }
}