Derive macro for tagged enums (GraphQLUnion) (#618)

* Implemented device macro for GraphQLUnion's

* Updated PR link in CHNAGELOG

* Disabled documentation on enumeration fields

* Disabled skip on fields

* Changed implementation for std::convert::Into since skip is not possible

* Added documentation for GraphQLUnion

* Added tests for GraphQLUnion

* Fixed typos in error messages (as suggested by review)

* Fixed failing documentation example

* Utilized `resolver_code` in `util::GraphQLTypeDefinitionField`.

Simplifies code and provides the idea of using
`util::GraphQLTypeDefinitionField` for different types than objects.

* Removed wrong statement about skip annotation in docs.

Co-authored-by: Christian Legnitto <LegNeato@users.noreply.github.com>

docs/book/content/types/unions.md

@@ -3,10 +3,11 @@
 From a server's point of view, GraphQL unions are similar to interfaces: the
 only exception is that they don't contain fields on their own.
 
-In Juniper, the `graphql_union!` has identical syntax to the [interface
-macro](interfaces.md), but does not support defining fields. Therefore, the same
-considerations about using traits, placeholder types, or enums still apply to
-unions.
+In Juniper, the `graphql_union!` has identical syntax to the
+[interface macro](interfaces.md), but does not support defining
+fields. Therefore, the same considerations about using traits,
+placeholder types, or enums still apply to unions. For simple
+situations, Juniper provides `#[derive(GraphQLUnion)]` for enums.
 
 If we look at the same examples as in the interfaces chapter, we see the
 similarities and the tradeoffs:
@@ -154,7 +155,7 @@ impl GraphQLUnion for Character {
 # fn main() {}
 ```
 
-## Enums
+## Enums (Impl)
 
 ```rust
 #[derive(juniper::GraphQLObject)]
@@ -187,3 +188,32 @@ impl Character {
 
 # fn main() {}
 ```
+
+## Enums (Derive)
+
+This example is similar to `Enums (Impl)`. To successfully use the
+derive macro, ensure that each variant of the enum has a different
+type. Since each variant is different, the device macro provides
+`std::convert::Into<T>` converter for each variant.
+
+```rust
+#[derive(juniper::GraphQLObject)]
+struct Human {
+    id: String,
+    home_planet: String,
+}
+
+#[derive(juniper::GraphQLObject)]
+struct Droid {
+    id: String,
+    primary_function: String,
+}
+
+#[derive(juniper::GraphQLUnion)]
+enum Character {
+    Human(Human),
+    Droid(Droid),
+}
+
+# fn main() {}
+```

integration_tests/juniper_tests/src/codegen/derive_union.rs

@@ -0,0 +1,268 @@
+// Test for union's derive macro
+
+#[cfg(test)]
+use fnv::FnvHashMap;
+
+#[cfg(test)]
+use juniper::{
+    self, execute, DefaultScalarValue, EmptyMutation, EmptySubscription, GraphQLType, RootNode,
+    Value, Variables,
+};
+
+#[derive(juniper::GraphQLObject)]
+pub struct Human {
+    id: String,
+    home_planet: String,
+}
+
+#[derive(juniper::GraphQLObject)]
+pub struct Droid {
+    id: String,
+    primary_function: String,
+}
+
+#[derive(juniper::GraphQLUnion)]
+#[graphql(description = "A Collection of things")]
+pub enum Character {
+    One(Human),
+    Two(Droid),
+}
+
+// Context Test
+pub struct CustomContext {
+    is_left: bool,
+}
+
+impl juniper::Context for CustomContext {}
+
+#[derive(juniper::GraphQLObject)]
+#[graphql(Context = CustomContext)]
+pub struct HumanContext {
+    id: String,
+    home_planet: String,
+}
+
+#[derive(juniper::GraphQLObject)]
+#[graphql(Context = CustomContext)]
+pub struct DroidContext {
+    id: String,
+    primary_function: String,
+}
+
+/// A Collection of things
+#[derive(juniper::GraphQLUnion)]
+#[graphql(Context = CustomContext)]
+pub enum CharacterContext {
+    One(HumanContext),
+    Two(DroidContext),
+}
+
+// #[juniper::object] compatibility
+
+pub struct HumanCompat {
+    id: String,
+    home_planet: String,
+}
+
+#[juniper::graphql_object]
+impl HumanCompat {
+    fn id(&self) -> &String {
+        &self.id
+    }
+
+    fn home_planet(&self) -> &String {
+        &self.home_planet
+    }
+}
+
+pub struct DroidCompat {
+    id: String,
+    primary_function: String,
+}
+
+#[juniper::graphql_object]
+impl DroidCompat {
+    fn id(&self) -> &String {
+        &self.id
+    }
+
+    fn primary_function(&self) -> &String {
+        &self.primary_function
+    }
+}
+
+// NOTICE: this can not compile due to generic implementation of GraphQLType<__S>
+// #[derive(juniper::GraphQLUnion)]
+// pub enum CharacterCompatFail {
+//     One(HumanCompat),
+//     Two(DroidCompat),
+// }
+
+/// A Collection of things
+#[derive(juniper::GraphQLUnion)]
+#[graphql(Scalar = juniper::DefaultScalarValue)]
+pub enum CharacterCompat {
+    One(HumanCompat),
+    Two(DroidCompat),
+}
+
+pub struct Query;
+
+#[juniper::graphql_object(
+    Context = CustomContext,
+)]
+impl Query {
+    fn context(&self, ctx: &CustomContext) -> CharacterContext {
+        if ctx.is_left {
+            HumanContext {
+                id: "human-32".to_string(),
+                home_planet: "earth".to_string(),
+            }
+            .into()
+        } else {
+            DroidContext {
+                id: "droid-99".to_string(),
+                primary_function: "run".to_string(),
+            }
+            .into()
+        }
+    }
+}
+
+#[tokio::test]
+async fn test_derived_union_doc_macro() {
+    assert_eq!(
+        <Character as GraphQLType<DefaultScalarValue>>::name(&()),
+        Some("Character")
+    );
+
+    let mut registry: juniper::Registry = juniper::Registry::new(FnvHashMap::default());
+    let meta = Character::meta(&(), &mut registry);
+
+    assert_eq!(meta.name(), Some("Character"));
+    assert_eq!(
+        meta.description(),
+        Some(&"A Collection of things".to_string())
+    );
+}
+
+#[tokio::test]
+async fn test_derived_union_doc_string() {
+    assert_eq!(
+        <CharacterContext as GraphQLType<DefaultScalarValue>>::name(&()),
+        Some("CharacterContext")
+    );
+
+    let mut registry: juniper::Registry = juniper::Registry::new(FnvHashMap::default());
+    let meta = CharacterContext::meta(&(), &mut registry);
+
+    assert_eq!(meta.name(), Some("CharacterContext"));
+    assert_eq!(
+        meta.description(),
+        Some(&"A Collection of things".to_string())
+    );
+}
+
+#[tokio::test]
+async fn test_derived_union_left() {
+    let doc = r#"
+        {
+            context {
+                ... on HumanContext {
+                    humanId: id
+                    homePlanet
+                }
+                ... on DroidContext {
+                    droidId: id
+                    primaryFunction
+                }
+            }
+        }"#;
+
+    let schema = RootNode::new(
+        Query,
+        EmptyMutation::<CustomContext>::new(),
+        EmptySubscription::<CustomContext>::new(),
+    );
+
+    assert_eq!(
+        execute(
+            doc,
+            None,
+            &schema,
+            &Variables::new(),
+            &CustomContext { is_left: true }
+        )
+        .await,
+        Ok((
+            Value::object(
+                vec![(
+                    "context",
+                    Value::object(
+                        vec![
+                            ("humanId", Value::scalar("human-32".to_string())),
+                            ("homePlanet", Value::scalar("earth".to_string())),
+                        ]
+                        .into_iter()
+                        .collect(),
+                    ),
+                )]
+                .into_iter()
+                .collect()
+            ),
+            vec![]
+        ))
+    );
+}
+
+#[tokio::test]
+async fn test_derived_union_right() {
+    let doc = r#"
+        {
+            context {
+                ... on HumanContext {
+                    humanId: id
+                    homePlanet
+                }
+                ... on DroidContext {
+                    droidId: id
+                    primaryFunction
+                }
+            }
+        }"#;
+
+    let schema = RootNode::new(
+        Query,
+        EmptyMutation::<CustomContext>::new(),
+        EmptySubscription::<CustomContext>::new(),
+    );
+
+    assert_eq!(
+        execute(
+            doc,
+            None,
+            &schema,
+            &Variables::new(),
+            &CustomContext { is_left: false }
+        )
+        .await,
+        Ok((
+            Value::object(
+                vec![(
+                    "context",
+                    Value::object(
+                        vec![
+                            ("droidId", Value::scalar("droid-99".to_string())),
+                            ("primaryFunction", Value::scalar("run".to_string())),
+                        ]
+                        .into_iter()
+                        .collect(),
+                    ),
+                )]
+                .into_iter()
+                .collect()
+            ),
+            vec![]
+        ))
+    );
+}

integration_tests/juniper_tests/src/codegen/mod.rs

@@ -2,5 +2,6 @@ mod derive_enum;
 mod derive_input_object;
 mod derive_object;
 mod derive_object_with_raw_idents;
+mod derive_union;
 mod impl_union;
 mod scalar_value_transparent;

juniper/CHANGELOG.md

@@ -22,6 +22,11 @@ See [#419](https://github.com/graphql-rust/juniper/pull/419).
 
 See [#569](https://github.com/graphql-rust/juniper/pull/569).
 
+- GraphQLUnion derive support ("#[derive(GraphqQLUnion)]")
+  - implements GraphQLAsyncType
+
+See [#618](https://github.com/graphql-rust/juniper/pull/618).
+
 ## Breaking Changes
 
 - `juniper::graphiql` has moved to `juniper::http::graphiql`

juniper/src/lib.rs

@@ -116,7 +116,7 @@ extern crate bson;
 // functionality automatically.
 pub use juniper_codegen::{
     graphql_object, graphql_subscription, graphql_union, GraphQLEnum, GraphQLInputObject,
-    GraphQLObject, GraphQLScalarValue,
+    GraphQLObject, GraphQLScalarValue, GraphQLUnion,
 };
 // Internal macros are not exported,
 // but declared at the root to make them easier to use.

juniper_codegen/src/derive_union.rs

@@ -0,0 +1,122 @@
+use proc_macro2::TokenStream;
+use quote::quote;
+use syn::{self, Data, Fields};
+
+use crate::util;
+
+pub fn build_derive_union(ast: syn::DeriveInput, is_internal: bool) -> TokenStream {
+    let enum_fields = match ast.data {
+        Data::Enum(data) => data.variants,
+        _ => {
+            panic!("#[derive(GraphQLUnion)] can only be applied to enums");
+        }
+    };
+
+    // Parse attributes.
+    let attrs = match util::ObjectAttributes::from_attrs(&ast.attrs) {
+        Ok(a) => a,
+        Err(e) => {
+            panic!("Invalid #[graphql(...)] attribute for enum: {}", e);
+        }
+    };
+
+    if !attrs.interfaces.is_empty() {
+        panic!("#[derive(GraphQLUnion)] does not support interfaces");
+    }
+
+    let ident = &ast.ident;
+    let name = attrs.name.unwrap_or_else(|| ident.to_string());
+
+    let fields = enum_fields.into_iter().filter_map(|field| {
+        let field_attrs = match util::FieldAttributes::from_attrs(
+            field.attrs,
+            util::FieldAttributeParseMode::Object,
+        ) {
+            Ok(attrs) => attrs,
+            Err(e) => panic!("Invalid #[graphql] attribute for field: \n{}", e),
+        };
+
+
+        if field_attrs.skip {
+            panic!("#[derive(GraphQLUnion)] does not support #[graphql(skip)] on fields");
+        } else {
+            let variant_name = field.ident;
+            let name = field_attrs
+                .name
+                .clone()
+                .unwrap_or_else(|| util::to_camel_case(&variant_name.to_string()));
+
+            let resolver_code = quote!(
+                #ident :: #variant_name
+            );
+
+            let _type = match field.fields {
+                Fields::Unnamed(inner) => {
+                    let mut iter = inner.unnamed.iter();
+                    let first = match iter.next() {
+                        Some(val) => val,
+                        None => unreachable!(),
+                    };
+
+                    if iter.next().is_some() {
+                        panic!("#[derive(GraphQLUnion)] all members must be unnamed with a single element e.g. Some(T)");
+                    }
+
+                    first.ty.clone()
+                }
+                _ => panic!("#[derive(GraphQLUnion)] all fields of the enum must be unnamed"),
+            };
+
+            if field_attrs.description.is_some() {
+                panic!("#[derive(GraphQLUnion)] does not allow documentation of fields");
+            }
+
+            Some(util::GraphQLTypeDefinitionField {
+                name,
+                _type,
+                args: Vec::new(),
+                description: None,
+                deprecation: field_attrs.deprecation,
+                resolver_code,
+                is_type_inferred: true,
+                is_async: false,
+            })
+        }
+    });
+
+    let fields = fields.collect::<Vec<_>>();
+
+    // NOTICE: This is not an optimal implementation. It is possible
+    // to bypass this check by using a full qualified path instead
+    // (crate::Test vs Test). Since this requirement is mandatory, the
+    // `std::convert::Into<T>` implementation is used to enforce this
+    // requirement. However, due to the bad error message this
+    // implementation should stay and provide guidance.
+    let all_variants_different = {
+        let mut all_types: Vec<_> = fields.iter().map(|field| &field._type).collect();
+        let before = all_types.len();
+        all_types.dedup();
+        before == all_types.len()
+    };
+
+    if !all_variants_different {
+        panic!("#[derive(GraphQLUnion)] each variant must have a different type");
+    }
+
+    let definition = util::GraphQLTypeDefiniton {
+        name,
+        _type: syn::parse_str(&ast.ident.to_string()).unwrap(),
+        context: attrs.context,
+        scalar: attrs.scalar,
+        description: attrs.description,
+        fields,
+        generics: ast.generics,
+        interfaces: None,
+        include_type_generics: true,
+        generic_scalar: true,
+        no_async: attrs.no_async,
+    };
+
+    let juniper_crate_name = if is_internal { "crate" } else { "juniper" };
+    definition.into_union_tokens(juniper_crate_name)
+}

juniper_codegen/src/lib.rs

@@ -15,6 +15,7 @@ mod derive_enum;
 mod derive_input_object;
 mod derive_object;
 mod derive_scalar_value;
+mod derive_union;
 mod impl_object;
 mod impl_union;
 
@@ -63,6 +64,13 @@ pub fn derive_object_internal(input: TokenStream) -> TokenStream {
     let gen = derive_object::build_derive_object(ast, true);
     gen.into()
 }
+
+#[proc_macro_derive(GraphQLUnion, attributes(graphql))]
+pub fn derive_union(input: TokenStream) -> TokenStream {
+    let ast = syn::parse::<syn::DeriveInput>(input).unwrap();
+    let gen = derive_union::build_derive_union(ast, false);
+    gen.into()
+}
 /// This custom derive macro implements the #[derive(GraphQLScalarValue)]
 /// derive.
 ///

juniper_codegen/src/util/mod.rs

@@ -1255,6 +1255,211 @@ impl GraphQLTypeDefiniton {
             #subscription_implementation
         )
     }
+
+    pub fn into_union_tokens(self, juniper_crate_name: &str) -> proc_macro2::TokenStream {
+        let juniper_crate_name = syn::parse_str::<syn::Path>(juniper_crate_name).unwrap();
+
+        let name = &self.name;
+        let ty = &self._type;
+        let context = self
+            .context
+            .as_ref()
+            .map(|ctx| quote!( #ctx ))
+            .unwrap_or_else(|| quote!(()));
+
+        let scalar = self
+            .scalar
+            .as_ref()
+            .map(|s| quote!( #s ))
+            .unwrap_or_else(|| {
+                if self.generic_scalar {
+                    // If generic_scalar is true, we always insert a generic scalar.
+                    // See more comments below.
+                    quote!(__S)
+                } else {
+                    quote!(#juniper_crate_name::DefaultScalarValue)
+                }
+            });
+
+        let description = self
+            .description
+            .as_ref()
+            .map(|description| quote!( .description(#description) ));
+
+        let meta_types = self.fields.iter().map(|field| {
+            let var_ty = &field._type;
+
+            quote! {
+                registry.get_type::<&#var_ty>(&(())),
+            }
+        });
+
+        let matcher_variants = self
+            .fields
+            .iter()
+            .map(|field| {
+                let var_ty = &field._type;
+                let resolver_code = &field.resolver_code;
+
+                quote!(
+                    #resolver_code(ref x) => <#var_ty as #juniper_crate_name::GraphQLType<#scalar>>::name(&()).unwrap().to_string(),
+                )
+            });
+
+        let concrete_type_resolver = quote!(
+            match self {
+                #( #matcher_variants )*
+            }
+        );
+
+        let matcher_expr: Vec<_> = self
+            .fields
+            .iter()
+            .map(|field| {
+                let resolver_code = &field.resolver_code;
+
+                quote!(
+                    match self { #resolver_code(ref val) => Some(val), _ => None, }
+                )
+            })
+            .collect();
+
+        let resolve_into_type = self.fields.iter().zip(matcher_expr.iter()).map(|(field, expr)| {
+            let var_ty = &field._type;
+
+            quote! {
+                if type_name == (<#var_ty as #juniper_crate_name::GraphQLType<#scalar>>::name(&())).unwrap() {
+                    return executor.resolve(&(), &{ #expr });
+                }
+            }
+        });
+
+        let resolve_into_type_async = self.fields.iter().zip(matcher_expr.iter()).map(|(field, expr)| {
+            let var_ty = &field._type;
+
+            quote! {
+                if type_name == (<#var_ty as #juniper_crate_name::GraphQLType<#scalar>>::name(&())).unwrap() {
+                    let f = async move {
+                        executor.resolve_async(&(), &{ #expr }).await
+                    };
+                    use futures::future;
+                    return future::FutureExt::boxed(f);
+                }
+            }
+        });
+
+        let mut generics = self.generics.clone();
+
+        if self.scalar.is_none() && self.generic_scalar {
+            // No custom scalar specified, but always generic specified.
+            // Therefore we inject the generic scalar.
+
+            generics.params.push(parse_quote!(__S));
+
+            let where_clause = generics.where_clause.get_or_insert(parse_quote!(where));
+            // Insert ScalarValue constraint.
+            where_clause
+                .predicates
+                .push(parse_quote!(__S: #juniper_crate_name::ScalarValue));
+        }
+
+        let (impl_generics, _, where_clause) = generics.split_for_impl();
+
+        let mut where_async = where_clause.cloned().unwrap_or_else(|| parse_quote!(where));
+        where_async
+            .predicates
+            .push(parse_quote!( #scalar: Send + Sync ));
+        where_async.predicates.push(parse_quote!(Self: Send + Sync));
+
+        let async_type_impl = quote!(
+            impl#impl_generics #juniper_crate_name::GraphQLTypeAsync<#scalar> for #ty
+                #where_async
+            {
+                fn resolve_into_type_async<'b>(
+                    &'b self,
+                    _info: &'b Self::TypeInfo,
+                    type_name: &str,
+                    _: Option<&'b [#juniper_crate_name::Selection<'b, #scalar>]>,
+                    executor: &'b #juniper_crate_name::Executor<'b, 'b, Self::Context, #scalar>
+                ) -> #juniper_crate_name::BoxFuture<'b, #juniper_crate_name::ExecutionResult<#scalar>> {
+                    let context = &executor.context();
+
+                    #( #resolve_into_type_async )*
+
+                    panic!("Concrete type not handled by instance resolvers on {}", #name);
+                }
+            }
+        );
+
+        let convesion_impls = self.fields.iter().map(|field| {
+            let variant_ty = &field._type;
+            let resolver_code = &field.resolver_code;
+
+            quote!(
+                impl std::convert::From<#variant_ty> for #ty {
+                    fn from(val: #variant_ty) -> Self {
+                        #resolver_code(val)
+                    }
+                }
+            )
+        });
+
+        let mut type_impl = quote! {
+            #( #convesion_impls )*
+
+            impl #impl_generics #juniper_crate_name::GraphQLType<#scalar> for #ty #where_clause
+            {
+                type Context = #context;
+                type TypeInfo = ();
+
+                fn name(_ : &Self::TypeInfo) -> Option<&str> {
+                    Some(#name)
+                }
+
+                fn meta<'r>(
+                    info: &Self::TypeInfo,
+                    registry: &mut #juniper_crate_name::Registry<'r, #scalar>
+                ) -> #juniper_crate_name::meta::MetaType<'r, #scalar>
+                    where
+                        #scalar: 'r,
+                {
+                    let types = &[
+                        #( #meta_types )*
+                    ];
+                    registry.build_union_type::<#ty>(
+                        info, types
+                    )
+                    #description
+                    .into_meta()
+                }
+
+                #[allow(unused_variables)]
+                fn concrete_type_name(&self, context: &Self::Context, _info: &Self::TypeInfo) -> String {
+                    #concrete_type_resolver
+                }
+
+                fn resolve_into_type(
+                    &self,
+                    _info: &Self::TypeInfo,
+                    type_name: &str,
+                    _: Option<&[#juniper_crate_name::Selection<#scalar>]>,
+                    executor: &#juniper_crate_name::Executor<Self::Context, #scalar>,
+                ) -> #juniper_crate_name::ExecutionResult<#scalar> {
+                    let context = &executor.context();
+
+                    #( #resolve_into_type )*
+
+                    panic!("Concrete type not handled by instance resolvers on {}", #name);
+                }
+            }
+        };
+
+        if !self.no_async {
+            type_impl.extend(async_type_impl)
+        }
+
+        type_impl
+    }
 }
 
 #[cfg(test)]