Fix #[schemars(with = ...)] alongside #[serde(transparent)] (#67)

Co-authored-by: Graham Esau <gesau@hotmail.co.uk>

CHANGELOG.md

@@ -6,10 +6,11 @@
 
 ### Changed:
 - Minimum supported rust version is now 1.37.0
-
-### Changed:
 - Deriving JsonSchema on enums now sets `additionalProperties` to false on generated schemas wherever serde doesn't accept unknown properties. This includes non-unit variants of externally tagged enums, and struct-style variants of all enums that have the `deny_unknown_fields` attribute.
 
+### Fixed
+- Fix use of `#[serde(transparent)]` in combination with `#[schemars(with = ...)]` (https://github.com/GREsau/schemars/pull/67)
+
 ## [0.8.0] - 2020-09-27
 ### Added:
 - `visit::Visitor`, a trait for updating a schema and all schemas it contains recursively. A `SchemaSettings` can now contain a list of visitors.

schemars/tests/expected/schema_with-transparent-newtype.json

@@ -0,0 +1,5 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "title": "schema_fn",
+  "type": "boolean"
+}

schemars/tests/schema_with_struct.rs

@@ -42,3 +42,12 @@ pub struct Newtype(#[schemars(schema_with = "schema_fn")] DoesntImplementJsonSch
 fn struct_newtype() -> TestResult {
     test_default_generated_schema::<Newtype>("schema_with-newtype")
 }
+
+#[derive(Debug, JsonSchema)]
+#[schemars(transparent)]
+pub struct TransparentNewtype(#[schemars(schema_with = "schema_fn")] DoesntImplementJsonSchema);
+
+#[test]
+fn struct_transparent_newtype() -> TestResult {
+    test_default_generated_schema::<TransparentNewtype>("schema_with-transparent-newtype")
+}

schemars_derive/src/lib.rs

@@ -38,10 +38,11 @@ fn derive_json_schema(mut input: syn::DeriveInput) -> TokenStream {
     if let Some(transparent_field) = cont.transparent_field() {
         let (ty, type_def) = schema_exprs::type_for_schema(transparent_field, 0);
         return quote! {
-            #[automatically_derived]
-            impl #impl_generics schemars::JsonSchema for #type_name #ty_generics #where_clause {
+            const _: () = {
                 #type_def
 
+                #[automatically_derived]
+                impl #impl_generics schemars::JsonSchema for #type_name #ty_generics #where_clause {
                     fn is_referenceable() -> bool {
                         <#ty as schemars::JsonSchema>::is_referenceable()
                     }
@@ -69,6 +70,7 @@ fn derive_json_schema(mut input: syn::DeriveInput) -> TokenStream {
                     }
                 };
             };
+        };
     }
 
     let mut schema_base_name = cont.name();