Codegen reimplementation for GraphQL unions (#666)

- reimplement #[derive(GraphQLUnion)] macro to support:
    - both structs and enums
    - generics in type definition
    - multiple #[graphql] attributes
    - external resolver functions
- remove From trait impls generation for enum variants

- reimplement #[graphql_union] macro to support:
    - traits
    - generics in trait definition
    - multiple attributes
    - external resolver functions
    - GraphQLType implemetation for a raw trait object
    - GraphQLTypeAsync implemetation (#549)

- add marker::GraphQLUnion trait

- rewrite "2.5 Unions" section in Book (Juniper user documentation)

- rewrite `codegen` and `codegen_fail` integration tests for GraphQL unions

Additionally:
- re-export `futures` crate in `juniper` for convenient reuse in the generated code without requiring library user to provide `futures` crate by himself (#663)
- use unit type () as default context for EmptyMutation and EmptySubscriptions
- relax Sized trait bound on some GraphQLType and GraphQLTypeAsync definitions, implementations and usages

Author: tyranron

docs/book/content/types/unions.md

@@ -9,6 +9,8 @@ build = "build.rs"
 juniper = { path = "../../../juniper" }
 juniper_iron = { path = "../../../juniper_iron" }
 juniper_subscriptions = { path = "../../../juniper_subscriptions" }
+
+derive_more = "0.99.7"
 futures = "0.3"
 tokio = { version = "0.2", features = ["rt-core", "blocking", "stream", "rt-util"] }
 iron = "0.5.0"

docs/book/tests/Cargo.toml

@@ -0,0 +1,6 @@
+use juniper::graphql_union;
+
+#[graphql_union]
+enum Character {}
+
+fn main() {}

integration_tests/codegen_fail/fail/union/attr_wrong_item.rs

@@ -0,0 +1,7 @@
+error: #[graphql_union] attribute is applicable to trait definitions only
+ --> $DIR/attr_wrong_item.rs:3:1
+  |
+3 | #[graphql_union]
+  | ^^^^^^^^^^^^^^^^
+  |
+  = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)

integration_tests/codegen_fail/fail/union/attr_wrong_item.stderr

@@ -0,0 +1,6 @@
+use juniper::GraphQLUnion;
+
+#[derive(GraphQLUnion)]
+union Character { id: i32 }
+
+fn main() {}

integration_tests/codegen_fail/fail/union/derive_enum_field.rs

@@ -0,0 +1,5 @@
+error: GraphQL union can only be derived for enums and structs
+ --> $DIR/derive_wrong_item.rs:4:1
+  |
+4 | union Character { id: i32 }
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^

integration_tests/codegen_fail/fail/union/derive_enum_field.stderr

@@ -0,0 +1,15 @@
+use juniper::{GraphQLObject, GraphQLUnion};
+
+#[derive(GraphQLUnion)]
+#[graphql(on Human = resolve_fn1)]
+enum Character {
+    #[graphql(with = resolve_fn2)]
+    A(Human),
+}
+
+#[derive(GraphQLObject)]
+pub struct Human {
+    id: String,
+}
+
+fn main() {}

integration_tests/codegen_fail/fail/union/derive_no_fields.rs

@@ -0,0 +1,7 @@
+error: GraphQL union variant `Human` already has external resolver function `resolve_fn1` declared on the enum
+ --> $DIR/enum_external_resolver_fn_conflicts_with_variant_external_resolver_fn.rs:6:15
+  |
+6 |     #[graphql(with = resolve_fn2)]
+  |               ^^^^
+  |
+  = note: https://spec.graphql.org/June2018/#sec-Unions

integration_tests/codegen_fail/fail/union/derive_no_fields.stderr

@@ -0,0 +1,13 @@
+use juniper::{GraphQLObject, GraphQLUnion};
+
+#[derive(GraphQLUnion)]
+enum __Character {
+    A(Human),
+}
+
+#[derive(GraphQLObject)]
+pub struct Human {
+    id: String,
+}
+
+fn main() {}

integration_tests/codegen_fail/fail/union/derive_same_type.stderr

@@ -0,0 +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.
+ --> $DIR/enum_name_double_underscored.rs:4:6
+  |
+4 | enum __Character {
+  |      ^^^^^^^^^^^
+  |
+  = note: https://spec.graphql.org/June2018/#sec-Schema

integration_tests/codegen_fail/fail/union/derive_wrong_item.rs

@@ -0,0 +1,6 @@
+use juniper::GraphQLUnion;
+
+#[derive(GraphQLUnion)]
+enum Character {}
+
+fn main() {}

integration_tests/codegen_fail/fail/union/derive_wrong_item.stderr

@@ -0,0 +1,7 @@
+error: GraphQL union expects at least one union variant
+ --> $DIR/enum_no_fields.rs:4:1
+  |
+4 | enum Character {}
+  | ^^^^^^^^^^^^^^^^^
+  |
+  = note: https://spec.graphql.org/June2018/#sec-Unions

integration_tests/codegen_fail/fail/union/enum_external_resolver_fn_conflicts_with_variant_external_resolver_fn.rs

@@ -0,0 +1,14 @@
+use juniper::{GraphQLEnum, GraphQLUnion};
+
+#[derive(GraphQLEnum)]
+pub enum Test {
+    A,
+    B,
+}
+
+#[derive(GraphQLUnion)]
+enum Character {
+    Test(Test),
+}
+
+fn main() {}

integration_tests/codegen_fail/fail/union/enum_external_resolver_fn_conflicts_with_variant_external_resolver_fn.stderr

@@ -0,0 +1,17 @@
+error[E0277]: the trait bound `Test: juniper::types::marker::GraphQLObjectType<juniper::value::scalar::DefaultScalarValue>` is not satisfied
+ --> $DIR/enum_non_object_variant.rs:9:10
+  |
+9 | #[derive(GraphQLUnion)]
+  |          ^^^^^^^^^^^^ the trait `juniper::types::marker::GraphQLObjectType<juniper::value::scalar::DefaultScalarValue>` is not implemented for `Test`
+  |
+  = note: required by `juniper::types::marker::GraphQLObjectType::mark`
+  = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: the trait bound `Test: juniper::types::marker::GraphQLObjectType<__S>` is not satisfied
+ --> $DIR/enum_non_object_variant.rs:9:10
+  |
+9 | #[derive(GraphQLUnion)]
+  |          ^^^^^^^^^^^^ the trait `juniper::types::marker::GraphQLObjectType<__S>` is not implemented for `Test`
+  |
+  = note: required by `juniper::types::marker::GraphQLObjectType::mark`
+  = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)

integration_tests/codegen_fail/fail/union/enum_name_double_underscored.rs

@@ -0,0 +1,9 @@
+use juniper::GraphQLUnion;
+
+#[derive(GraphQLUnion)]
+enum Character {
+    A(u8),
+    B(u8),
+}
+
+fn main() {}

integration_tests/codegen_fail/fail/union/enum_name_double_underscored.stderr

@@ -0,0 +1,10 @@
+error: GraphQL union must have a different type for each union variant
+ --> $DIR/enum_same_type_pretty.rs:4:1
+  |
+4 | / enum Character {
+5 | |     A(u8),
+6 | |     B(u8),
+7 | | }
+  | |_^
+  |
+  = note: https://spec.graphql.org/June2018/#sec-Unions

integration_tests/codegen_fail/fail/union/enum_no_fields.rs

@@ -1,4 +1,6 @@
-#[derive(juniper::GraphQLUnion)]
+use juniper::GraphQLUnion;
+
+#[derive(GraphQLUnion)]
 enum Character {
     A(std::string::String),
     B(String),

integration_tests/codegen_fail/fail/union/enum_no_fields.stderr

@@ -0,0 +1,10 @@
+error[E0119]: conflicting implementations of trait `<Character as juniper::types::marker::GraphQLUnion>::mark::_::{{closure}}#0::MutuallyExclusive` for type `std::string::String`:
+ --> $DIR/enum_same_type_ugly.rs:3:10
+  |
+3 | #[derive(GraphQLUnion)]
+  |          ^^^^^^^^^^^^
+  |          |
+  |          first implementation here
+  |          conflicting implementation for `std::string::String`
+  |
+  = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

integration_tests/codegen_fail/fail/union/enum_non_object_variant.rs

@@ -0,0 +1,18 @@
+use juniper::{GraphQLObject, GraphQLUnion};
+
+#[derive(GraphQLUnion)]
+enum Character1 {
+    A { human: Human },
+}
+
+#[derive(GraphQLUnion)]
+enum Character2 {
+    A(Human, u8),
+}
+
+#[derive(GraphQLObject)]
+pub struct Human {
+    id: String,
+}
+
+fn main() {}

integration_tests/codegen_fail/fail/union/enum_non_object_variant.stderr

@@ -0,0 +1,15 @@
+error: GraphQL union enum allows only unnamed variants with a single field, e.g. `Some(T)`
+ --> $DIR/enum_wrong_variant_field.rs:5:5
+  |
+5 |     A { human: Human },
+  |     ^
+  |
+  = note: https://spec.graphql.org/June2018/#sec-Unions
+
+error: GraphQL union enum allows only unnamed variants with a single field, e.g. `Some(T)`
+  --> $DIR/enum_wrong_variant_field.rs:10:6
+   |
+10 |     A(Human, u8),
+   |      ^^^^^^^^^^^
+   |
+   = note: https://spec.graphql.org/June2018/#sec-Unions

integration_tests/codegen_fail/fail/union/enum_same_type_pretty.rs

@@ -0,0 +1,18 @@
+use juniper::{GraphQLObject, GraphQLUnion};
+
+#[derive(GraphQLUnion)]
+#[graphql(on Human = __Character::a)]
+struct __Character;
+
+impl __Character {
+    fn a(&self, _: &()) -> Option<&Human> {
+        None
+    }
+}
+
+#[derive(GraphQLObject)]
+pub struct Human {
+    id: String,
+}
+
+fn main() {}

integration_tests/codegen_fail/fail/union/enum_same_type_pretty.stderr

@@ -0,0 +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.
+ --> $DIR/struct_name_double_underscored.rs:5:8
+  |
+5 | struct __Character;
+  |        ^^^^^^^^^^^
+  |
+  = note: https://spec.graphql.org/June2018/#sec-Schema

integration_tests/codegen_fail/fail/union/derive_same_type.rs renamed to integration_tests/codegen_fail/fail/union/enum_same_type_ugly.rs

@@ -0,0 +1,6 @@
+use juniper::GraphQLUnion;
+
+#[derive(GraphQLUnion)]
+struct Character;
+
+fn main() {}

integration_tests/codegen_fail/fail/union/enum_same_type_ugly.stderr

@@ -0,0 +1,7 @@
+error: GraphQL union expects at least one union variant
+ --> $DIR/struct_no_fields.rs:4:1
+  |
+4 | struct Character;
+  | ^^^^^^^^^^^^^^^^^
+  |
+  = note: https://spec.graphql.org/June2018/#sec-Unions