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