Redesign #[graphql_interface] macro (#1009, #1000, #814)

- remove support for `#[graphql_interface(dyn)]`
- describe all interface trait methods with type's fields or impl block instead of `#[graphql_interface]` attribute on `impl Trait`
- forbid default impls on non-skipped trait methods
- support additional nullable arguments on implementer
- support returning sub-type on implementer

Author: ilslv

docs/book/content/types/interfaces.md

@@ -45,32 +45,14 @@ trait Character {
 struct Human {
     id: String,
 }
-#[graphql_interface] // implementing requires macro attribute too, (°ｏ°)!
-impl Character for Human {
-    fn id(&self) -> &str {
-        &self.id
-    }
-}
 
 #[derive(GraphQLObject)]
 #[graphql(impl = CharacterValue)]
 struct Droid {
     id: String,
 }
-#[graphql_interface]
-impl Character for Droid {
-    fn id(&self) -> &str {
-        &self.id
-    }
-}
-
-# fn main() {
-let human = Human { id: "human-32".to_owned() };
-// Values type for interface has `From` implementations for all its implementers,
-// so we don't need to bother with enum variant names.
-let character: CharacterValue = human.into();
-assert_eq!(character.id(), "human-32");
-# }
+#
+# fn main() {}
 ```
 
 Also, enum name can be specified explicitly, if desired.
@@ -90,71 +72,11 @@ struct Human {
     id: String,
     home_planet: String,
 }
-#[graphql_interface]
-impl Character for Human {
-    fn id(&self) -> &str {
-        &self.id
-    }
-}
 #
 # fn main() {}
 ```
 
 
-### Trait object values
-
-If, for some reason, we would like to use [trait objects][2] for representing [interface][1] values incorporating dynamic dispatch, then it should be specified explicitly in the trait definition.
-
-Downcasting [trait objects][2] in Rust is not that trivial, that's why macro transforms the trait definition slightly, imposing some additional type parameters under-the-hood.
-
-> __NOTICE__:  
-> A __trait has to be [object safe](https://doc.rust-lang.org/stable/reference/items/traits.html#object-safety)__, because schema resolvers will need to return a [trait object][2] to specify a [GraphQL interface][1] behind it.
-
-```rust
-# extern crate juniper;
-# extern crate tokio;
-use juniper::{graphql_interface, GraphQLObject};
-
-// `dyn` argument accepts the name of type alias for the required trait object,
-// and macro generates this alias automatically.
-#[graphql_interface(dyn = DynCharacter, for = Human)] 
-trait Character {
-    async fn id(&self) -> &str; // async fields are supported natively
-}
-
-#[derive(GraphQLObject)]
-#[graphql(impl = DynCharacter<__S>)] // macro adds `ScalarValue` type parameter to trait,
-struct Human {                       // so it may be specified explicitly when required 
-    id: String,
-}
-#[graphql_interface(dyn)] // implementing requires to know about dynamic dispatch too
-impl Character for Human {
-    async fn id(&self) -> &str {
-        &self.id
-    }
-}
-
-#[derive(GraphQLObject)]
-#[graphql(impl = DynCharacter<__S>)]
-struct Droid {
-    id: String,
-}
-#[graphql_interface]
-impl Character for Droid {
-    async fn id(&self) -> &str {
-        &self.id
-    }
-}
-
-# #[tokio::main]
-# async fn main() {
-let human = Human { id: "human-32".to_owned() };
-let character: Box<DynCharacter> = Box::new(human);
-assert_eq!(character.id().await, "human-32");
-# }
-``` 
-
-
 ### Ignoring trait methods
 
 We may want to omit some trait methods to be assumed as [GraphQL interface][1] fields and ignore them.
@@ -176,12 +98,6 @@ trait Character {
 struct Human {
     id: String,
 }
-#[graphql_interface]
-impl Character for Human {
-    fn id(&self) -> &str {
-        &self.id
-    }
-}
 #
 # fn main() {}
 ```
@@ -278,24 +194,6 @@ struct Human {
     id: String,
     name: String,
 }
-#[graphql_interface]
-impl Character for Human {
-    fn id(&self, db: &Database) -> Option<&str> {
-        if db.humans.contains_key(&self.id) {
-            Some(&self.id)
-        } else {
-            None
-        }
-    }
-
-    fn name(&self, db: &Database) -> Option<&str> {
-        if db.humans.contains_key(&self.id) {
-            Some(&self.name)
-        } else {
-            None
-        }
-    }
-}
 #
 # fn main() {}
 ```
@@ -309,119 +207,50 @@ This requires to explicitly parametrize over [`ScalarValue`][3], as [`Executor`]
 
 ```rust
 # extern crate juniper;
-use juniper::{graphql_interface, Executor, GraphQLObject, LookAheadMethods as _, ScalarValue};
+use juniper::{graphql_interface, graphql_object, Executor, LookAheadMethods as _, ScalarValue};
 
 #[graphql_interface(for = Human, Scalar = S)] // notice specifying `ScalarValue` as existing type parameter
 trait Character<S: ScalarValue> {             
     // If a field argument is named `executor`, it's automatically assumed
     // as an executor argument.
-    async fn id<'a>(&self, executor: &'a Executor<'_, '_, (), S>) -> &'a str
-    where
-        S: Send + Sync; // required by `#[async_trait]` transformation ¯\_(ツ)_/¯
+    fn id<'a>(&self, executor: &'a Executor<'_, '_, (), S>) -> &'a str;
 
     // Otherwise, you may mark it explicitly as an executor argument.
-    async fn name<'b>(
+    fn name<'b>(
         &'b self,
         #[graphql(executor)] another: &Executor<'_, '_, (), S>,
-    ) -> &'b str
-    where
-        S: Send + Sync;
+    ) -> &'b str;
+    
+    fn home_planet(&self) -> &str;
 }
 
-#[derive(GraphQLObject)]
-#[graphql(impl = CharacterValue<__S>)]
 struct Human {
     id: String,
     name: String,
+    home_planet: String,
 }
-#[graphql_interface(scalar = S)]
-impl<S: ScalarValue> Character<S> for Human {
-    async fn id<'a>(&self, executor: &'a Executor<'_, '_, (), S>) -> &'a str
+#[graphql_object(scalar = S: ScalarValue, impl = CharacterValue<S>)]
+impl Human {
+    async fn id<'a, S>(&self, executor: &'a Executor<'_, '_, (), S>) -> &'a str 
     where
-        S: Send + Sync,
+        S: ScalarValue,
     {
         executor.look_ahead().field_name()
     }
 
-    async fn name<'b>(&'b self, _: &Executor<'_, '_, (), S>) -> &'b str
-    where
-        S: Send + Sync,
-    {
+    async fn name<'b, S>(&'b self, #[graphql(executor)] _: &Executor<'_, '_, (), S>) -> &'b str {
         &self.name
     }
-}
-#
-# fn main() {}
-```
-
-
-### Downcasting
-
-By default, the [GraphQL interface][1] value is downcast to one of its implementer types via matching the enum variant or downcasting the trait object (if `dyn` macro argument is used).
-
-However, if some custom logic is needed to downcast a [GraphQL interface][1] implementer, you may specify either an external function or a trait method to do so.
-
-```rust
-# extern crate juniper;
-# use std::collections::HashMap;
-use juniper::{graphql_interface, GraphQLObject};
-
-struct Database {
-    droids: HashMap<String, Droid>,
-}
-impl juniper::Context for Database {}
-
-#[graphql_interface(for = [Human, Droid], context = Database)]
-#[graphql_interface(on Droid = get_droid)] // enables downcasting `Droid` via `get_droid()` function
-trait Character {
-    fn id(&self) -> &str;
-
-    #[graphql(downcast)] // makes method a downcast to `Human`, not a field 
-    // NOTICE: The method signature may optionally contain `&Database` context argument.
-    fn as_human(&self) -> Option<&Human> {
-        None
-    }
-}
-
-#[derive(GraphQLObject)]
-#[graphql(impl = CharacterValue, Context = Database)]
-struct Human {
-    id: String,
-}
-#[graphql_interface]
-impl Character for Human {
-    fn id(&self) -> &str {
-        &self.id
-    }
-
-    fn as_human(&self) -> Option<&Self> {
-        Some(self)
-    }   
-}
-
-#[derive(GraphQLObject)]
-#[graphql(impl = CharacterValue, Context = Database)]
-struct Droid {
-    id: String,
-}
-#[graphql_interface]
-impl Character for Droid {
-    fn id(&self) -> &str {
-        &self.id
+    
+    fn home_planet<'c, S>(&'c self, #[graphql(executor)] _: &Executor<'_, '_, (), S>) -> &'c str {
+        // Executor may not be present on the trait method  ^^^^^^^^^^^^^^^^^^^^^^^^
+        &self.home_planet
     }
 }
-
-// External downcast function doesn't have to be a method of a type.
-// It's only a matter of the function signature to match the requirements.
-fn get_droid<'db>(ch: &CharacterValue, db: &'db Database) -> Option<&'db Droid> {
-    db.droids.get(ch.id())
-}
 #
 # fn main() {}
 ```
 
-The attribute syntax `#[graphql_interface(on ImplementerType = resolver_fn)]` follows the [GraphQL syntax for downcasting interface implementer](https://spec.graphql.org/June2018/#example-5cc55).
-
 
 
 
@@ -445,25 +274,13 @@ struct Human {
     id: String,
     home_planet: String,
 }
-#[graphql_interface(scalar = DefaultScalarValue)]
-impl Character for Human {
-    fn id(&self) -> &str {
-        &self.id
-    }   
-}
 
 #[derive(GraphQLObject)]
 #[graphql(impl = CharacterValue, Scalar = DefaultScalarValue)]
 struct Droid {
     id: String,
     primary_function: String,
 }
-#[graphql_interface(scalar = DefaultScalarValue)]
-impl Character for Droid {
-    fn id(&self) -> &str {
-        &self.id
-    }   
-}
 #
 # fn main() {}
 ```

examples/actix_subscriptions/src/main.rs

@@ -10,7 +10,7 @@ use actix_web::{
 
 use juniper::{
     graphql_object, graphql_subscription, graphql_value,
-    tests::fixtures::starwars::schema::{Character as _, Database, Query},
+    tests::fixtures::starwars::schema::{Database, Query},
     EmptyMutation, FieldError, RootNode,
 };
 use juniper_actix::{graphql_handler, playground_handler, subscriptions::subscriptions_handler};

integration_tests/codegen_fail/fail/interface/additional_non_nullable_argument.rs

@@ -0,0 +1,19 @@
+use juniper::{graphql_interface, graphql_object};
+
+pub struct ObjA {
+    id: String,
+}
+
+#[graphql_object(impl = CharacterValue)]
+impl ObjA {
+    fn id(&self, is_present: bool) -> &str {
+        is_present.then(|| self.id.as_str()).unwrap_or("missing")
+    }
+}
+
+#[graphql_interface(for = ObjA)]
+trait Character {
+    fn id(&self) -> &str;
+}
+
+fn main() {}

integration_tests/codegen_fail/fail/interface/additional_non_nullable_argument.stderr

@@ -0,0 +1,7 @@
+error[E0080]: evaluation of constant value failed
+  --> fail/interface/additional_non_nullable_argument.rs:14:1
+   |
+14 | #[graphql_interface(for = ObjA)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the evaluated program panicked at 'Failed to implement interface `Character` on `ObjA`: Field `id`: Argument `isPresent` of type `Boolean!` isn't present on the interface and so has to be nullable.', $DIR/fail/interface/additional_non_nullable_argument.rs:14:1
+   |
+   = note: this error originates in the macro `$crate::panic::panic_2015` (in Nightly builds, run with -Z macro-backtrace for more info)

integration_tests/codegen_fail/fail/interface/argument_double_underscored.rs

@@ -1,19 +1,8 @@
-use juniper::{graphql_interface, GraphQLObject};
-
-#[derive(GraphQLObject)]
-#[graphql(impl = CharacterValue)]
-pub struct ObjA {
-    test: String,
-}
+use juniper::graphql_interface;
 
 #[graphql_interface]
-impl Character for ObjA {}
-
-#[graphql_interface(for = ObjA)]
 trait Character {
-    fn id(&self, __num: i32) -> &str {
-        "funA"
-    }
+    fn id(&self, __num: i32) -> &str;
 }
 
 fn main() {}

integration_tests/codegen_fail/fail/interface/argument_double_underscored.stderr

@@ -1,19 +1,7 @@
 error: All types and directives defined within a schema must not have a name which begins with `__` (two underscores), as this is used exclusively by GraphQL’s introspection system.
-  --> fail/interface/argument_double_underscored.rs:14:18
-   |
-14 |     fn id(&self, __num: i32) -> &str {
-   |                  ^^^^^
-   |
-   = note: https://spec.graphql.org/June2018/#sec-Schema
-
-error[E0412]: cannot find type `CharacterValue` in this scope
- --> fail/interface/argument_double_underscored.rs:4:18
+ --> fail/interface/argument_double_underscored.rs:5:18
   |
-4 | #[graphql(impl = CharacterValue)]
-  |                  ^^^^^^^^^^^^^^ not found in this scope
-
-error[E0405]: cannot find trait `Character` in this scope
-  --> fail/interface/argument_double_underscored.rs:10:6
-   |
-10 | impl Character for ObjA {}
-   |      ^^^^^^^^^ not found in this scope
+5 |     fn id(&self, __num: i32) -> &str;
+  |                  ^^^^^
+  |
+  = note: https://spec.graphql.org/June2018/#sec-Schema