Clean up ScalarValue transparent derive argument handling and documentation.

Author: theduke

docs/book/content/types/scalars.md

@@ -41,11 +41,10 @@ crates. They are enabled via features that are on by default.
 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 `#[transparent]`.
+serde supports this pattern with `#[serde(transparent)]`.
 
 ```rust
 #[derive(juniper::GraphQLScalarValue)]
-#[graphql(transparent)]
 pub struct UserId(i32);
 
 #[derive(juniper::GraphQLObject)]

juniper/CHANGELOG.md

@@ -6,13 +6,12 @@ See [#345](https://github.com/graphql-rust/juniper/pull/345).
 
 The newtype pattern can now be used with the `GraphQLScalarValue` custom derive
 to easily implement custom scalar values that just wrap another scalar, 
-similar to serdes `#[transparent]` functionality.
+similar to serdes `#[serde(transparent)]` functionality.
 
 Example:
 
 ```rust
 #[derive(juniper::GraphQLScalarValue)]
-#[graphql(transparent)]
 struct UserId(i32);
 ```
 

juniper_codegen/src/derive_scalar_value.rs

@@ -5,17 +5,17 @@ use syn::{self, Data, Fields, Ident, Variant};
 
 use crate::util;
 
-#[derive(Debug)]
+#[derive(Debug, Default)]
 struct TransparentAttributes {
-    transparent: bool,
+    transparent: Option<bool>,
     name: Option<String>,
     description: Option<String>,
 }
 
 impl syn::parse::Parse for TransparentAttributes {
     fn parse(input: syn::parse::ParseStream) -> syn::parse::Result<Self> {
         let mut output = Self {
-            transparent: false,
+            transparent: None,
             name: None,
             description: None,
         };
@@ -37,7 +37,7 @@ impl syn::parse::Parse for TransparentAttributes {
                     output.description = Some(val.value());
                 }
                 "transparent" => {
-                    output.transparent = true;
+                    output.transparent = Some(true);
                 }
                 other => {
                     return Err(content.error(format!("Unknown attribute: {}", other)));
@@ -62,9 +62,7 @@ impl TransparentAttributes {
                 }
                 Ok(parsed)
             }
-            None => {
-                panic!("Missing required attribute: #[graphql(transparent)]");
-            }
+            None => Ok(Default::default()),
         }
     }
 }

juniper_codegen/src/lib.rs

@@ -68,9 +68,7 @@ pub fn derive_object(input: TokenStream) -> TokenStream {
 ///
 /// ```rust
 /// // Deriving GraphQLScalar is all that is required.
-/// // Note the #[graphql(transparent)] attribute, which is mandatory.
 /// #[derive(juniper::GraphQLScalarValue)]
-/// #[graphql(transparent)]
 /// struct UserId(String);
 ///
 /// #[derive(juniper::GraphQLObject)]