Add private field to SchemaSettings to prevent struct initialization.

This means future additions to SchemaSettings will not be breaking changes.

schemars/examples/custom_settings.rs

@@ -8,19 +8,16 @@ pub struct MyStruct {
 }
 
 #[derive(JsonSchema)]
-pub enum MyEnum {    
+pub enum MyEnum {
     StringNewType(String),
-    StructVariant {
+    StructVariant { floats: Vec<f32> },
-        floats: Vec<f32>,
-    }
 }
 
 fn main() {
-    let settings = SchemaSettings {
+    let settings = SchemaSettings::draft07().with(|s| {
-        option_nullable: true,
+        s.option_nullable = true;
-        option_add_null_type: false,
+        s.option_add_null_type = false;
-        ..SchemaSettings::draft07()
+    });
-    };
     let gen = settings.into_generator();
     let schema = gen.into_root_schema_for::<MyStruct>();
     println!("{}", serde_json::to_string_pretty(&schema).unwrap());

schemars/src/gen.rs

@@ -2,7 +2,7 @@ use crate::schema::*;
 use crate::{JsonSchema, Map};
 
 /// Settings to customize how Schemas are generated.
-/// 
+///
 /// The default settings currently conform to [JSON Schema Draft 7](https://json-schema.org/specification-links.html#draft-7), but this is liable to change in a future version of Schemars if support for other JSON Schema versions is added.
 /// If you require your generated schemas to conform to draft 7, consider using the [`draft07`](#method.draft07) method.
 #[derive(Debug, PartialEq, Clone)]
@@ -29,6 +29,7 @@ pub struct SchemaSettings {
     ///
     /// Defaults to `"http://json-schema.org/draft-07/schema#"`.
     pub meta_schema: Option<String>,
+    _hidden: (),
 }
 
 /// Controls whether trivial [`Bool`](../schema/enum.Schema.html#variant.Bool) schemas may be generated.
@@ -57,6 +58,7 @@ impl SchemaSettings {
             bool_schemas: BoolSchemas::Enabled,
             definitions_path: "#/definitions/".to_owned(),
             meta_schema: Some("http://json-schema.org/draft-07/schema#".to_owned()),
+            _hidden: (),
         }
     }
 
@@ -71,9 +73,27 @@ impl SchemaSettings {
                 "https://spec.openapis.org/oas/3.0/schema/2019-04-02#/definitions/Schema"
                     .to_owned(),
             ),
+            _hidden: (),
         }
     }
 
+    /// Modifies the `SchemaSettings` by calling the given function.
+    ///
+    /// # Example
+    /// ```
+    /// use schemars::gen::{SchemaGenerator, SchemaSettings};
+    ///
+    /// let settings = SchemaSettings::default().with(|s| {
+    ///     s.option_nullable = true;
+    ///     s.option_add_null_type = false;
+    /// });
+    /// let gen = settings.into_generator();
+    /// ```
+    pub fn with(mut self, configure_fn: impl FnOnce(&mut Self)) -> Self {
+        configure_fn(&mut self);
+        self
+    }
+
     /// Creates a new [`SchemaGenerator`] using these settings.
     pub fn into_generator(self) -> SchemaGenerator {
         SchemaGenerator::new(self)
@@ -100,6 +120,12 @@ pub struct SchemaGenerator {
     definitions: Map<String, Schema>,
 }
 
+impl From<SchemaSettings> for SchemaGenerator {
+    fn from(settings: SchemaSettings) -> Self {
+        settings.into_generator()
+    }
+}
+
 impl SchemaGenerator {
     /// Creates a new `SchemaGenerator` using the given settings.
     pub fn new(settings: SchemaSettings) -> SchemaGenerator {

schemars/src/json_schema_impls/core.rs

@@ -199,11 +199,10 @@ mod tests {
 
     #[test]
     fn schema_for_option_with_nullable() {
-        let settings = SchemaSettings {
+        let settings = SchemaSettings::default().with(|s| {
-            option_nullable: true,
+            s.option_nullable = true;
-            option_add_null_type: false,
+            s.option_add_null_type = false;
-            ..Default::default()
+        });
-        };
         let schema = custom_schema_object_for::<Option<i32>>(settings);
         assert_eq!(
             schema.instance_type,

schemars/src/json_schema_impls/maps.rs

@@ -52,10 +52,7 @@ mod tests {
     #[test]
     fn schema_for_map_any_value() {
         for bool_schemas in &[BoolSchemas::Enabled, BoolSchemas::AdditionalPropertiesOnly] {
-            let settings = SchemaSettings {
+            let settings = SchemaSettings::default().with(|s| s.bool_schemas = *bool_schemas);
-                bool_schemas: *bool_schemas,
-                ..Default::default()
-            };
             let schema = custom_schema_object_for::<BTreeMap<String, serde_json::Value>>(settings);
             assert_eq!(
                 schema.instance_type,
@@ -72,10 +69,8 @@ mod tests {
 
     #[test]
     fn schema_for_map_any_value_no_bool_schema() {
-        let settings = SchemaSettings {
+        let settings =
-            bool_schemas: BoolSchemas::Disabled,
+            SchemaSettings::default().with(|s| s.bool_schemas = BoolSchemas::Disabled);
-            ..Default::default()
-        };
         let schema = custom_schema_object_for::<BTreeMap<String, serde_json::Value>>(settings);
         assert_eq!(
             schema.instance_type,
@@ -96,10 +91,7 @@ mod tests {
             BoolSchemas::Disabled,
             BoolSchemas::AdditionalPropertiesOnly,
         ] {
-            let settings = SchemaSettings {
+            let settings = SchemaSettings::default().with(|s| s.bool_schemas = *bool_schemas);
-                bool_schemas: *bool_schemas,
-                ..Default::default()
-            };
             let schema = custom_schema_object_for::<BTreeMap<String, i32>>(settings);
             assert_eq!(
                 schema.instance_type,