Add Contract for generating separate serialize/deserialize schemas (#335)

docs/3-generating.md

@@ -28,15 +28,14 @@ let my_schema = generator.into_root_schema_for::<MyStruct>();
 
 See the API documentation for more info on how to use those types for custom schema generation.
 
+### Serialize vs. Deserialize contract
+
+Of particular note is the `contract` setting, which controls whether the generated schemas should describe how types are serialized or how they're *de*serialized. By default, this is set to `Deserialize`. If you instead want your schema to describe the serialization behaviour, modify the `contract` field of `SchemaSettings` or use the `for_serialize()` helper method:
+
+{% include example.md name="serialize_contract" %}
+
 ## Schema from Example Value
 
 If you want a schema for a type that can't/doesn't implement `JsonSchema`, but does implement `serde::Serialize`, then you can generate a JSON schema from a value of that type using the [`schema_for_value!` macro](https://docs.rs/schemars/1.0.0--latest/schemars/macro.schema_for_value.html). However, this schema will generally be less precise than if the type implemented `JsonSchema` - particularly when it involves enums, since schemars will not make any assumptions about the structure of an enum based on a single variant.
 
-```rust
-let value = MyStruct { foo = 123 };
-let my_schema = schema_for_value!(value);
-```
-
-<!-- TODO:
-create and link to example
--->
+{% include example.md name="from_value" %}

docs/_includes/examples/serialize_contract.rs

@@ -0,0 +1,29 @@
+use schemars::{generate::SchemaSettings, JsonSchema};
+use serde::{Deserialize, Serialize};
+
+#[derive(JsonSchema, Deserialize, Serialize)]
+// The schema effectively ignores this `rename_all`, since it doesn't apply to serialization
+#[serde(rename_all(deserialize = "PascalCase"))]
+pub struct MyStruct {
+    pub my_int: i32,
+    #[serde(skip_deserializing)]
+    pub my_read_only_bool: bool,
+    // This property is excluded from the schema
+    #[serde(skip_serializing)]
+    pub my_write_only_bool: bool,
+    // This property is excluded from the "required" properties of the schema, because it may be
+    // be skipped during serialization
+    #[serde(skip_serializing_if = "str::is_empty")]
+    pub maybe_string: String,
+    pub definitely_string: String,
+}
+
+fn main() {
+    // By default, generated schemas describe how types are deserialized.
+    // So we modify the settings here to instead generate schemas describing how it's serialized:
+    let settings = SchemaSettings::default().for_serialize();
+
+    let generator = settings.into_generator();
+    let schema = generator.into_root_schema_for::<MyStruct>();
+    println!("{}", serde_json::to_string_pretty(&schema).unwrap());
+}

docs/_includes/examples/serialize_contract.schema.json

@@ -0,0 +1,27 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "title": "MyStruct",
+  "type": "object",
+  "properties": {
+    "definitely_string": {
+      "type": "string"
+    },
+    "maybe_string": {
+      "type": "string"
+    },
+    "my_int": {
+      "type": "integer",
+      "format": "int32"
+    },
+    "my_read_only_bool": {
+      "type": "boolean",
+      "default": false,
+      "readOnly": true
+    }
+  },
+  "required": [
+    "my_int",
+    "my_read_only_bool",
+    "definitely_string"
+  ]
+}