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//! This crate provides the [`ReflectMessage`](https://docs.rs/prost-reflect/latest/prost_reflect/derive.ReflectMessage.html) derive macro
//!
//! For documentation, see the example in the [`prost-reflect` crate docs](https://docs.rs/prost-reflect/latest/prost_reflect/index.html#deriving-reflectmessage).
#![doc(html_root_url = "https://docs.rs/prost-reflect-derive/0.12.0/")]

use proc_macro::TokenStream;
use proc_macro2::Span;
use quote::{quote, ToTokens};
use syn::spanned::Spanned;

/// A derive macro for the [`ReflectMessage`](https://docs.rs/prost-reflect/latest/prost_reflect/trait.ReflectMessage.html) trait.
///
/// For documentation, see the example in the [`prost-reflect` crate docs](https://docs.rs/prost-reflect/latest/prost_reflect/index.html#deriving-reflectmessage).
#[proc_macro_derive(ReflectMessage, attributes(prost_reflect))]
pub fn reflect_message(input: TokenStream) -> TokenStream {
    let input = syn::parse_macro_input!(input as syn::DeriveInput);

    match reflect_message_impl(input) {
        Ok(tokens) => tokens.into(),
        Err(err) => err.to_compile_error().into(),
    }
}

struct Args {
    args_span: Span,
    message_name: Option<syn::Lit>,
    descriptor_pool: Option<syn::LitStr>,
    file_descriptor_set: Option<syn::LitStr>,
}

fn reflect_message_impl(input: syn::DeriveInput) -> Result<proc_macro2::TokenStream, syn::Error> {
    match &input.data {
        syn::Data::Struct(_) => (),
        syn::Data::Enum(_) => return Ok(Default::default()),
        syn::Data::Union(_) => return Ok(Default::default()),
    };

    let args = Args::parse(input.ident.span(), &input.attrs)?;

    let name = &input.ident;
    let descriptor_pool = args.descriptor_pool()?;
    let message_name = args.message_name()?;

    Ok(quote! {
        impl ::prost_reflect::ReflectMessage for #name {
            fn descriptor(&self) -> ::prost_reflect::MessageDescriptor {
                #descriptor_pool
                    .get_message_by_name(#message_name)
                    .expect(concat!("descriptor for message type `", #message_name, "` not found"))
            }
        }
    })
}

fn is_prost_reflect_attribute(attr: &syn::Attribute) -> bool {
    attr.path().is_ident("prost_reflect")
}

impl Args {
    fn parse(input_span: proc_macro2::Span, attrs: &[syn::Attribute]) -> Result<Args, syn::Error> {
        let reflect_attrs: Vec<_> = attrs
            .iter()
            .filter(|attr| is_prost_reflect_attribute(attr))
            .collect();

        if reflect_attrs.is_empty() {
            return Err(syn::Error::new(
                input_span,
                "missing #[prost_reflect] attribute",
            ));
        }

        let mut args = Args {
            args_span: reflect_attrs
                .iter()
                .map(|a| a.span())
                .reduce(|l, r| l.join(r).unwrap_or(l))
                .unwrap(),
            message_name: None,
            descriptor_pool: None,
            file_descriptor_set: None,
        };

        for attr in reflect_attrs {
            attr.parse_nested_meta(|nested| {
                if nested.path.is_ident("descriptor_pool") {
                    args.descriptor_pool = nested.value()?.parse()?;
                    Ok(())
                } else if nested.path.is_ident("file_descriptor_set_bytes") {
                    args.file_descriptor_set = nested.value()?.parse()?;
                    Ok(())
                } else if nested.path.is_ident("message_name") {
                    args.message_name = nested.value()?.parse()?;
                    Ok(())
                } else {
                    Err(syn::Error::new(
                        nested.path.span(),
                        "unknown argument (expected 'descriptor_pool', 'file_descriptor_set_bytes' or 'message_name')",
                    ))
                }
            })?;
        }

        Ok(args)
    }

    fn descriptor_pool(&self) -> Result<proc_macro2::TokenStream, syn::Error> {
        if let Some(descriptor_pool) = &self.descriptor_pool {
            let expr: syn::Expr = syn::parse_str(&descriptor_pool.value())?;
            Ok(expr.to_token_stream())
        } else if let Some(file_descriptor_set) = &self.file_descriptor_set {
            let expr: syn::Expr = syn::parse_str(&file_descriptor_set.value())?;

            Ok(quote!({
                static INIT: ::std::sync::Once = ::std::sync::Once::new();
                INIT.call_once(|| ::prost_reflect::DescriptorPool::decode_global_file_descriptor_set(#expr).unwrap());
                ::prost_reflect::DescriptorPool::global()
            }))
        } else {
            Err(syn::Error::new(
                self.args_span,
                "missing required argument 'descriptor_pool'",
            ))
        }
    }

    fn message_name(&self) -> Result<proc_macro2::TokenStream, syn::Error> {
        if let Some(message_name) = &self.message_name {
            Ok(message_name.to_token_stream())
        } else {
            Err(syn::Error::new(
                self.args_span,
                "missing required argument 'message_name'",
            ))
        }
    }
}