fix(utoipa-gen): degenerate oneOf when using Option<_> + nullable = false + default

Author: Jean-Marc Le Roux

utoipa-gen/src/component.rs

@@ -1358,7 +1358,7 @@ impl ComponentSchema {
                         // TODO: refs support `summary` field but currently there is no such field
                         // on schemas more over there is no way to distinct the `summary` from
                         // `description` of the ref. Should we consider supporting the summary?
-                        let schema = if default.is_some() || nullable || title.is_some() {
+                        let schema = if nullable {
                             composed_or_ref(quote_spanned! {type_path.span()=>
                                 utoipa::openapi::schema::OneOfBuilder::new()
                                     #nullable_item

utoipa-gen/tests/snapshots/schema_derive_test__derive_schema_unnamed_title-2.snap

@@ -4,10 +4,6 @@ expression: enum_value
 snapshot_kind: text
 ---
 {
-  "oneOf": [
-    {
-      "$ref": "#/components/schemas/UnnamedEnum"
-    }
-  ],
+  "$ref": "#/components/schemas/UnnamedEnum",
   "title": "This is enum ref title"
 }

utoipa-gen/tests/snapshots/schema_derive_test__derive_struct_with_default_attr_field.snap

@@ -30,14 +30,10 @@ snapshot_kind: text
       "type": "array"
     },
     "favorite_book": {
+      "$ref": "#/components/schemas/Book",
       "default": {
         "name": "Dune"
-      },
-      "oneOf": [
-        {
-          "$ref": "#/components/schemas/Book"
-        }
-      ]
+      }
     },
     "leases": {
       "additionalProperties": {