Redesign #[derive(GraphQLScalar)] and #[graphql_scalar] macros (#1017)

- `#[derive(GraphQLScalar)]`:
    - support generic scalars
    - support structs with single named field
    - support for overriding resolvers
- `#[graphql_scalar]`:
    - support `transparent` argument

Co-authored-by: Kai Ren <tyranron@gmail.com>

Author: ilslv

docs/book/content/types/scalars.md

@@ -52,7 +52,7 @@ Often, you might need a custom scalar that just wraps an existing type.
 This can be done with the newtype pattern and a custom derive, similar to how
 serde supports this pattern with `#[serde(transparent)]`.
 
-```rust,ignore
+```rust
 # extern crate juniper;
 #
 #[derive(juniper::GraphQLScalar)]
@@ -70,7 +70,7 @@ struct User {
 `#[derive(GraphQLScalar)]` is mostly interchangeable with `#[graphql_scalar]` 
 attribute:
 
-```rust,ignore
+```rust
 # extern crate juniper;
 # use juniper::graphql_scalar;
 #
@@ -91,7 +91,7 @@ That's it, you can now use `UserId` in your schema.
 
 The macro also allows for more customization:
 
-```rust,ignore
+```rust
 # extern crate juniper;
 /// You can use a doc comment to specify a description.
 #[derive(juniper::GraphQLScalar)]
@@ -112,7 +112,7 @@ All the methods used from newtype's field can be replaced with attributes:
 
 ### `#[graphql(to_output_with = <fn>)]` attribute
 
-```rust,ignore
+```rust
 # use juniper::{GraphQLScalar, ScalarValue, Value};
 #
 #[derive(GraphQLScalar)]
@@ -129,7 +129,7 @@ fn to_output<S: ScalarValue>(v: &Incremented) -> Value<S> {
 
 ### `#[graphql(from_input_with = <fn>)]` attribute
 
-```rust,ignore
+```rust
 # use juniper::{GraphQLScalar, InputValue, ScalarValue};
 #
 #[derive(GraphQLScalar)]
@@ -164,7 +164,7 @@ impl UserId {
 
 ### `#[graphql(parse_token_with = <fn>]` or `#[graphql(parse_token(<types>)]` attributes
 
-```rust,ignore
+```rust
 # use juniper::{
 #     GraphQLScalar, InputValue, ParseScalarResult, ParseScalarValue, 
 #     ScalarValue, ScalarToken, Value
@@ -226,7 +226,7 @@ Instead of providing all custom resolvers, you can provide path to the `to_outpu
 Path can be simply `with = Self` (default path where macro expects resolvers to be), 
 in case there is an impl block with custom resolvers:
 
-```rust,ignore
+```rust
 # use juniper::{
 #     GraphQLScalar, InputValue, ParseScalarResult, ParseScalarValue,
 #     ScalarValue, ScalarToken, Value
@@ -253,7 +253,7 @@ impl StringOrInt {
     {
         v.as_string_value()
             .map(|s| Self::String(s.to_owned()))
-            .or_else(|| v.as_int_value().map(|i| Self::Int(i)))
+            .or_else(|| v.as_int_value().map(Self::Int))
             .ok_or_else(|| format!("Expected `String` or `Int`, found: {}", v))
     }
 
@@ -271,7 +271,7 @@ impl StringOrInt {
 
 Or it can be path to a module, where custom resolvers are located.
 
-```rust,ignore
+```rust
 # use juniper::{
 #     GraphQLScalar, InputValue, ParseScalarResult, ParseScalarValue, 
 #     ScalarValue, ScalarToken, Value
@@ -303,7 +303,7 @@ mod string_or_int {
     {
         v.as_string_value()
             .map(|s| StringOrInt::String(s.to_owned()))
-            .or_else(|| v.as_int_value().map(|i| StringOrInt::Int(i)))
+            .or_else(|| v.as_int_value().map(StringOrInt::Int))
             .ok_or_else(|| format!("Expected `String` or `Int`, found: {}", v))
     }
 
@@ -321,7 +321,7 @@ mod string_or_int {
 
 Also, you can partially override `#[graphql(with)]` attribute with other custom scalars.
 
-```rust,ignore
+```rust
 # use juniper::{GraphQLScalar, InputValue, ParseScalarResult, ScalarValue, ScalarToken, Value};
 #
 #[derive(GraphQLScalar)]
@@ -348,7 +348,7 @@ impl StringOrInt {
     {
         v.as_string_value()
             .map(|s| Self::String(s.to_owned()))
-            .or_else(|| v.as_int_value().map(|i| Self::Int(i)))
+            .or_else(|| v.as_int_value().map(Self::Int))
             .ok_or_else(|| format!("Expected `String` or `Int`, found: {}", v))
     }
 }

integration_tests/codegen_fail/fail/scalar/derive_input/attr_invalid_url.rs

@@ -0,0 +1,6 @@
+use juniper::graphql_scalar;
+
+#[graphql_scalar(specified_by_url = "not an url", transparent)]
+struct ScalarSpecifiedByUrl(i32);
+
+fn main() {}

integration_tests/codegen_fail/fail/scalar/derive_input/attr_invalid_url.stderr

@@ -0,0 +1,5 @@
+error: Invalid URL: relative URL without a base
+ --> fail/scalar/derive_input/attr_invalid_url.rs:3:37
+  |
+3 | #[graphql_scalar(specified_by_url = "not an url", transparent)]
+  |                                     ^^^^^^^^^^^^

integration_tests/codegen_fail/fail/scalar/derive_input/attr_transparent_and_with.rs

@@ -0,0 +1,6 @@
+use juniper::graphql_scalar;
+
+#[graphql_scalar(with = Self, transparent)]
+struct Scalar;
+
+fn main() {}

integration_tests/codegen_fail/fail/scalar/derive_input/attr_transparent_and_with.stderr

@@ -0,0 +1,5 @@
+error: GraphQL scalar `with = <path>` attribute can\'t be combined with `transparent`. You can specify custom resolvers with `to_output`, `from_input`, `parse_token` attributes and still use `transparent` for unspecified ones.
+ --> fail/scalar/derive_input/attr_transparent_and_with.rs:3:25
+  |
+3 | #[graphql_scalar(with = Self, transparent)]
+  |                         ^^^^

integration_tests/codegen_fail/fail/scalar/derive_input/attr_transparent_multiple_named_fields.rs

@@ -0,0 +1,9 @@
+use juniper::graphql_scalar;
+
+#[graphql_scalar(transparent)]
+struct Scalar {
+    id: i32,
+    another: i32,
+}
+
+fn main() {}

integration_tests/codegen_fail/fail/scalar/derive_input/attr_transparent_multiple_named_fields.stderr

@@ -0,0 +1,8 @@
+error: GraphQL scalar expected exactly 1 field, e.g., Test { test: i32 } because of `transparent` attribute
+ --> fail/scalar/derive_input/attr_transparent_multiple_named_fields.rs:4:1
+  |
+4 | / struct Scalar {
+5 | |     id: i32,
+6 | |     another: i32,
+7 | | }
+  | |_^

integration_tests/codegen_fail/fail/scalar/derive_input/attr_transparent_multiple_unnamed_fields.rs

@@ -0,0 +1,6 @@
+use juniper::graphql_scalar;
+
+#[graphql_scalar(transparent)]
+struct Scalar(i32, i32);
+
+fn main() {}

integration_tests/codegen_fail/fail/scalar/derive_input/attr_transparent_multiple_unnamed_fields.stderr

@@ -0,0 +1,5 @@
+error: GraphQL scalar expected exactly 1 field, e.g., Test(i32) because of `transparent` attribute
+ --> fail/scalar/derive_input/attr_transparent_multiple_unnamed_fields.rs:4:1
+  |
+4 | struct Scalar(i32, i32);
+  | ^^^^^^^^^^^^^^^^^^^^^^^^

integration_tests/codegen_fail/fail/scalar/derive_input/attr_transparent_unit_struct.rs

@@ -0,0 +1,6 @@
+use juniper::graphql_scalar;
+
+#[graphql_scalar(transparent)]
+struct ScalarSpecifiedByUrl;
+
+fn main() {}