Implement #[derive(GraphQLUnion)] for enums

- add marker::GraphQLUnion trait in 'juniper'

Additionally:
- re-export 'futures' crate in 'juniper' for convenient reuse in generated code without requiring library user to provide 'futures' crate by himself

Author: tyranron

juniper/src/lib.rs

@@ -111,6 +111,10 @@ extern crate uuid;
 #[cfg(any(test, feature = "bson"))]
 extern crate bson;
 
+// This one is required for use by code generated with`juniper_codegen` macros.
+#[doc(hidden)]
+pub use futures;
+
 // Depend on juniper_codegen and re-export everything in it.
 // This allows users to just depend on juniper and get the derive
 // functionality automatically.
@@ -179,7 +183,7 @@ pub use crate::{
     types::{
         async_await::GraphQLTypeAsync,
         base::{Arguments, GraphQLType, TypeKind},
-        marker,
+        marker::{self, GraphQLUnion},
         scalars::{EmptyMutation, EmptySubscription, ID},
         subscriptions::{GraphQLSubscriptionType, SubscriptionConnection, SubscriptionCoordinator},
     },

juniper/src/types/marker.rs

@@ -23,6 +23,30 @@ pub trait GraphQLObjectType<S: ScalarValue>: GraphQLType<S> {
     fn mark() {}
 }
 
+/// Maker trait for [GraphQL unions][1].
+///
+/// This trait extends the [`GraphQLType`] and is only used to mark [union][1]. During compile this
+/// addition information is required to prevent unwanted structure compiling. If an object requires
+/// this trait instead of the [`GraphQLType`], then it explicitly requires [GraphQL unions][1].
+/// Other types ([scalars][2], [enums][3], [objects][4], [input objects][5] and [interfaces][6]) are
+/// not allowed.
+///
+/// [1]: https://spec.graphql.org/June2018/#sec-Unions
+/// [2]: https://spec.graphql.org/June2018/#sec-Scalars
+/// [3]: https://spec.graphql.org/June2018/#sec-Enums
+/// [4]: https://spec.graphql.org/June2018/#sec-Objects
+/// [5]: https://spec.graphql.org/June2018/#sec-Input-Objects
+/// [6]: https://spec.graphql.org/June2018/#sec-Interfaces
+pub trait GraphQLUnion: GraphQLType {
+    /// An arbitrary function without meaning.
+    ///
+    /// May contain compile timed check logic which ensures that types are used correctly according
+    /// to the [GraphQL specification][1].
+    ///
+    /// [1]: https://spec.graphql.org/June2018/
+    fn mark() {}
+}
+
 /// Marker trait for types which can be used as output types.
 ///
 /// The GraphQL specification differentiates between input and output

juniper_codegen/src/derive_enum.rs

@@ -145,6 +145,7 @@ pub fn impl_enum(
         include_type_generics: true,
         generic_scalar: true,
         no_async: attrs.no_async.is_some(),
+        mode: is_internal.into(),
     };
 
     let juniper_crate_name = if is_internal { "crate" } else { "juniper" };

juniper_codegen/src/derive_input_object.rs

@@ -145,6 +145,7 @@ pub fn impl_input_object(
         include_type_generics: true,
         generic_scalar: true,
         no_async: attrs.no_async.is_some(),
+        mode: is_internal.into(),
     };
 
     let juniper_crate_name = if is_internal { "crate" } else { "juniper" };

juniper_codegen/src/derive_object.rs

@@ -132,6 +132,7 @@ pub fn build_derive_object(
         include_type_generics: true,
         generic_scalar: true,
         no_async: attrs.no_async.is_some(),
+        mode: is_internal.into(),
     };
 
     let juniper_crate_name = if is_internal { "crate" } else { "juniper" };

juniper_codegen/src/derive_scalar_value.rs

@@ -127,7 +127,7 @@ fn impl_scalar_struct(
                 executor: &'a #crate_name::Executor<Self::Context, __S>,
             ) -> #crate_name::BoxFuture<'a, #crate_name::ExecutionResult<__S>> {
                 use #crate_name::GraphQLType;
-                use futures::future;
+                use #crate_name::futures::future;
                 let v = self.resolve(info, selection_set, executor);
                 Box::pin(future::ready(v))
             }

juniper_codegen/src/derive_union.rs

@@ -1,4 +1,5 @@
 use proc_macro2::TokenStream;
+use proc_macro_error::ResultExt as _;
 use quote::quote;
 use syn::{self, ext::IdentExt, spanned::Spanned, Data, Fields};
 
@@ -10,12 +11,9 @@ use crate::{
 const SCOPE: GraphQLScope = GraphQLScope::DeriveUnion;
 
 pub fn expand(input: TokenStream, mode: Mode) -> syn::Result<TokenStream> {
-    let is_internal = matches!(mode, Mode::Internal);
-
-    let ast =
-        syn::parse2::<syn::DeriveInput>(input).unwrap_or_else(|e| proc_macro_error::abort!(e));
-
+    let ast = syn::parse2::<syn::DeriveInput>(input).unwrap_or_abort();
     let ast_span = ast.span();
+
     let enum_fields = match ast.data {
         Data::Enum(data) => data.variants,
         Data::Struct(_) => unimplemented!(),
@@ -66,10 +64,7 @@ pub fn expand(input: TokenStream, mode: Mode) -> syn::Result<TokenStream> {
             let _type = match field.fields {
                 Fields::Unnamed(inner) => {
                     let mut iter = inner.unnamed.iter();
-                    let first = match iter.next() {
-                        Some(val) => val,
-                        None => unreachable!(),
-                    };
+                    let first = iter.next().unwrap();
 
                     if iter.next().is_some() {
                         SCOPE.custom(
@@ -140,7 +135,7 @@ pub fn expand(input: TokenStream, mode: Mode) -> syn::Result<TokenStream> {
         SCOPE.not_empty(ast_span);
     }
 
-    if name.starts_with("__") && !is_internal {
+    if name.starts_with("__") && matches!(mode, Mode::Public) {
         SCOPE.no_double_underscore(if let Some(name) = attrs.name {
             name.span_ident()
         } else {
@@ -180,8 +175,8 @@ pub fn expand(input: TokenStream, mode: Mode) -> syn::Result<TokenStream> {
         include_type_generics: true,
         generic_scalar: true,
         no_async: attrs.no_async.is_some(),
+        mode,
     };
 
-    let juniper_crate_name = if is_internal { "crate" } else { "juniper" };
-    Ok(definition.into_union_tokens(juniper_crate_name))
+    Ok(definition.into_union_tokens())
 }

juniper_codegen/src/graphql_union/attribute.rs

@@ -0,0 +1 @@
+