fix(codegen): Discard type defaults from impl generics

serde_codegen/src/bound.rs

@@ -7,6 +7,20 @@ use syntax::ext::base::ExtCtxt;
 use syntax::ptr::P;
 use syntax::visit;
 
+// Remove the default from every type parameter because in the generated impls
+// they look like associated types: "error: associated type bindings are not
+// allowed here".
+pub fn without_defaults(generics: &ast::Generics) -> ast::Generics {
+    ast::Generics {
+        ty_params: generics.ty_params.map(|ty_param| {
+            ast::TyParam {
+                default: None,
+                .. ty_param.clone()
+            }}),
+        .. generics.clone()
+    }
+}
+
 pub fn with_bound(
     cx: &ExtCtxt,
     builder: &AstBuilder,

serde_codegen/src/de.rs

@@ -85,7 +85,8 @@ fn build_impl_generics(
     item: &Item,
     generics: &ast::Generics,
 ) -> ast::Generics {
-    let generics = bound::with_bound(cx, builder, item, generics,
+    let generics = bound::without_defaults(generics);
+    let generics = bound::with_bound(cx, builder, item, &generics,
         &deserialized_by_us,
         &["serde", "de", "Deserialize"]);
     let generics = bound::with_bound(cx, builder, item, &generics,

serde_codegen/src/ser.rs

@@ -93,9 +93,11 @@ fn build_impl_generics(
     item: &Item,
     generics: &ast::Generics,
 ) -> ast::Generics {
-    bound::with_bound(cx, builder, item, generics,
+    let generics = bound::without_defaults(generics);
+    let generics = bound::with_bound(cx, builder, item, &generics,
         &serialized_by_us,
-        &["serde", "ser", "Serialize"])
+        &["serde", "ser", "Serialize"]);
+    generics
 }
 
 // Fields with a `skip_serializing` or `serialize_with` attribute are not

serde_tests/tests/test_macros.rs

@@ -1,3 +1,4 @@
+use std::marker::PhantomData;
 use token::{Token, assert_tokens, assert_ser_tokens, assert_de_tokens};
 
 /*
@@ -143,6 +144,19 @@ pub enum GenericEnum<T, U> {
     Map { x: T, y: U },
 }
 
+trait AssociatedType {
+    type X;
+}
+
+impl AssociatedType for i32 {
+    type X = i32;
+}
+
+#[derive(Debug, PartialEq, Serialize, Deserialize)]
+struct DefaultTyParam<T: AssociatedType<X=i32> = i32> {
+    phantom: PhantomData<T>
+}
+
 #[test]
 fn test_named_unit() {
     assert_tokens(
@@ -601,3 +615,19 @@ fn test_generic_enum_map() {
         ]
     );
 }
+
+#[test]
+fn test_default_ty_param() {
+    assert_tokens(
+        &DefaultTyParam::<i32> { phantom: PhantomData },
+        vec![
+            Token::StructStart("DefaultTyParam", Some(1)),
+
+            Token::StructSep,
+            Token::Str("phantom"),
+            Token::UnitStruct("PhantomData"),
+
+            Token::StructEnd,
+        ]
+    );
+}