Set type on enum schemas (#28)

schemars/tests/expected/doc_comments_enum.json

@@ -4,6 +4,7 @@
   "description": "This is the enum's description.",
   "anyOf": [
     {
+      "type": "string",
       "enum": [
         "UndocumentedUnit",
         "DocumentedUnit"

schemars/tests/expected/enum-external.json

@@ -3,6 +3,7 @@
   "title": "External",
   "anyOf": [
     {
+      "type": "string",
       "enum": [
         "unitOne",
         "unitTwo"

schemars/tests/expected/schema-2019_09.json

@@ -358,6 +358,7 @@
   "definitions": {
     "InstanceType": {
       "description": "The possible types of values in JSON Schema documents.\n\nSee [JSON Schema 4.2.1. Instance Data Model](https://tools.ietf.org/html/draft-handrews-json-schema-02#section-4.2.1).",
+      "type": "string",
       "enum": [
         "null",
         "boolean",

schemars/tests/expected/schema-openapi3.json

@@ -294,6 +294,7 @@
   "definitions": {
     "InstanceType": {
       "description": "The possible types of values in JSON Schema documents.\n\nSee [JSON Schema 4.2.1. Instance Data Model](https://tools.ietf.org/html/draft-handrews-json-schema-02#section-4.2.1).",
+      "type": "string",
       "enum": [
         "null",
         "boolean",

schemars/tests/expected/schema.json

@@ -358,6 +358,7 @@
   "definitions": {
     "InstanceType": {
       "description": "The possible types of values in JSON Schema documents.\n\nSee [JSON Schema 4.2.1. Instance Data Model](https://tools.ietf.org/html/draft-handrews-json-schema-02#section-4.2.1).",
+      "type": "string",
       "enum": [
         "null",
         "boolean",

schemars/tests/expected/skip_enum_variants.json

@@ -3,6 +3,7 @@
   "title": "MyEnum",
   "anyOf": [
     {
+      "type": "string",
       "enum": [
         "Included2"
       ]

schemars_derive/src/schema_exprs.rs

@@ -89,6 +89,7 @@ fn expr_for_external_tagged_enum<'a>(
 
     let unit_names = unit_variants.iter().map(|v| v.name());
     let unit_schema = schema_object(quote! {
+        instance_type: Some(schemars::schema::InstanceType::String.into()),
         enum_values: Some(vec![#(#unit_names.into()),*]),
     });