Generate schema from any serializable value

README.md

@@ -200,6 +200,68 @@ println!("{}", serde_json::to_string_pretty(&schema).unwrap());
 
 `#[serde(...)]` attributes can be overriden using `#[schemars(...)]` attributes, which behave identically (e.g. `#[schemars(rename_all = "camelCase")]`). You may find this useful if you want to change the generated schema without affecting Serde's behaviour, or if you're just not using Serde.
 
+### Schema from Example Values
+
+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. 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
+use schemars::schema_for_value;
+use serde::Serialize;
+
+#[derive(Serialize)]
+pub struct MyStruct {
+    pub my_int: i32,
+    pub my_bool: bool,
+    pub my_nullable_enum: Option<MyEnum>,
+}
+
+#[derive(Serialize)]
+pub enum MyEnum {
+    StringNewType(String),
+    StructVariant { floats: Vec<f32> },
+}
+
+fn main() {
+    let schema = schema_for_value!(MyStruct {
+        my_int: 123,
+        my_bool: true,
+        my_nullable_enum: Some(MyEnum::StringNewType("foo".to_string()))
+    });
+    println!("{}", serde_json::to_string_pretty(&schema).unwrap());
+}
+```
+
+<details>
+<summary>Click to see the output JSON schema...</summary>
+
+```json
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "title": "MyStruct",
+  "examples": [
+    {
+      "my_bool": true,
+      "my_int": 123,
+      "my_nullable_enum": {
+        "StringNewType": "foo"
+      }
+    }
+  ],
+  "type": "object",
+  "properties": {
+    "my_bool": {
+      "type": "boolean"
+    },
+    "my_int": {
+      "type": "integer",
+      "format": "int32"
+    },
+    "my_nullable_enum": true
+  }
+}
+```
+</details>
+
 ## Feature Flags
 - `derive` (enabled by default) - provides `#[derive(JsonSchema)]` macro
 - `impl_json_schema` - implements `JsonSchema` for Schemars types themselves

docs/3-generating.md

@@ -20,6 +20,8 @@ If you want more control over how the schema is generated, you can use the [`gen
 
 See the API documentation for more info on how to use those types for custom schema generation.
 
-<!-- TODO: 
+<!-- TODO:
 create and link to example
+
+Generating schema from example value
 -->

docs/_includes/examples/from_value.rs

@@ -0,0 +1,24 @@
+use schemars::schema_for_value;
+use serde::Serialize;
+
+#[derive(Serialize)]
+pub struct MyStruct {
+    pub my_int: i32,
+    pub my_bool: bool,
+    pub my_nullable_enum: Option<MyEnum>,
+}
+
+#[derive(Serialize)]
+pub enum MyEnum {
+    StringNewType(String),
+    StructVariant { floats: Vec<f32> },
+}
+
+fn main() {
+    let schema = schema_for_value!(MyStruct {
+        my_int: 123,
+        my_bool: true,
+        my_nullable_enum: Some(MyEnum::StringNewType("foo".to_string()))
+    });
+    println!("{}", serde_json::to_string_pretty(&schema).unwrap());
+}

docs/_includes/examples/from_value.schema.json

@@ -0,0 +1,24 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "title": "MyStruct",
+  "examples": [
+    {
+      "my_bool": true,
+      "my_int": 123,
+      "my_nullable_enum": {
+        "StringNewType": "foo"
+      }
+    }
+  ],
+  "type": "object",
+  "properties": {
+    "my_bool": {
+      "type": "boolean"
+    },
+    "my_int": {
+      "type": "integer",
+      "format": "int32"
+    },
+    "my_nullable_enum": true
+  }
+}

schemars/examples/from_value.rs

@@ -0,0 +1,24 @@
+use schemars::schema_for_value;
+use serde::Serialize;
+
+#[derive(Serialize)]
+pub struct MyStruct {
+    pub my_int: i32,
+    pub my_bool: bool,
+    pub my_nullable_enum: Option<MyEnum>,
+}
+
+#[derive(Serialize)]
+pub enum MyEnum {
+    StringNewType(String),
+    StructVariant { floats: Vec<f32> },
+}
+
+fn main() {
+    let schema = schema_for_value!(MyStruct {
+        my_int: 123,
+        my_bool: true,
+        my_nullable_enum: Some(MyEnum::StringNewType("foo".to_string()))
+    });
+    println!("{}", serde_json::to_string_pretty(&schema).unwrap());
+}

schemars/examples/from_value.schema.json

@@ -0,0 +1,24 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "title": "MyStruct",
+  "examples": [
+    {
+      "my_bool": true,
+      "my_int": 123,
+      "my_nullable_enum": {
+        "StringNewType": "foo"
+      }
+    }
+  ],
+  "type": "object",
+  "properties": {
+    "my_bool": {
+      "type": "boolean"
+    },
+    "my_int": {
+      "type": "integer",
+      "format": "int32"
+    },
+    "my_nullable_enum": true
+  }
+}

schemars/src/flatten.rs

@@ -2,6 +2,9 @@ use crate::schema::*;
 use crate::{Map, Set};
 
 impl Schema {
+    /// This function is only public for use by schemars_derive.
+    ///
+    /// It should not be considered part of the public API.
     #[doc(hidden)]
     pub fn flatten(self, other: Self) -> Schema {
         if is_null_type(&self) {

schemars/src/gen.rs

@@ -1,7 +1,7 @@
 /*!
 JSON Schema generator and settings.
 
-This module is useful if you want more control over how the schema generated then the [`schema_for!`] macro gives you.
+This module is useful if you want more control over how the schema generated than the [`schema_for!`] macro gives you.
 There are two main types in this module:two main types in this module:
 * [`SchemaSettings`], which defines what JSON Schema features should be used when generating schemas (for example, how `Option`s should be represented).
 * [`SchemaGenerator`], which manages the generation of a schema document.
@@ -11,6 +11,7 @@ use crate::flatten::Merge;
 use crate::schema::*;
 use crate::{visit::*, JsonSchema, Map};
 use dyn_clone::DynClone;
+use serde::Serialize;
 use std::{any::Any, collections::HashSet, fmt::Debug};
 
 /// Settings to customize how Schemas are generated.
@@ -314,6 +315,70 @@ impl SchemaGenerator {
         root
     }
 
+    /// Generates a root JSON Schema for the given example value.
+    ///
+    /// If the value implements [`JsonSchema`](crate::JsonSchema), then prefer using the [`root_schema_for()`](Self::root_schema_for())
+    /// function which will generally produce a more precise schema, particularly when the value contains any enums.
+    pub fn root_schema_for_value<T: ?Sized + Serialize>(
+        &mut self,
+        value: &T,
+    ) -> Result<RootSchema, serde_json::Error> {
+        let mut schema = value
+            .serialize(crate::ser::Serializer {
+                gen: self,
+                include_title: true,
+            })?
+            .into_object();
+
+        if let Ok(example) = serde_json::to_value(value) {
+            schema.metadata().examples.push(example);
+        }
+
+        let mut root = RootSchema {
+            meta_schema: self.settings.meta_schema.clone(),
+            definitions: self.definitions.clone(),
+            schema,
+        };
+
+        for visitor in &mut self.settings.visitors {
+            visitor.visit_root_schema(&mut root)
+        }
+
+        Ok(root)
+    }
+
+    /// Consumes `self` and generates a root JSON Schema for the given example value.
+    ///
+    /// If the value  implements [`JsonSchema`](crate::JsonSchema), then prefer using the [`into_root_schema_for()!`](Self::into_root_schema_for())
+    /// function which will generally produce a more precise schema, particularly when the value contains any enums.
+    pub fn into_root_schema_for_value<T: ?Sized + Serialize>(
+        mut self,
+        value: &T,
+    ) -> Result<RootSchema, serde_json::Error> {
+        let mut schema = value
+            .serialize(crate::ser::Serializer {
+                gen: &mut self,
+                include_title: true,
+            })?
+            .into_object();
+
+        if let Ok(example) = serde_json::to_value(value) {
+            schema.metadata().examples.push(example);
+        }
+
+        let mut root = RootSchema {
+            meta_schema: self.settings.meta_schema,
+            definitions: self.definitions,
+            schema,
+        };
+
+        for visitor in &mut self.settings.visitors {
+            visitor.visit_root_schema(&mut root)
+        }
+
+        Ok(root)
+    }
+
     /// Attemps to find the schema that the given `schema` is referencing.
     ///
     /// If the given `schema` has a [`$ref`](../schema/struct.SchemaObject.html#structfield.reference) property which refers

schemars/src/json_schema_impls/core.rs

@@ -25,6 +25,7 @@ impl<T: JsonSchema> JsonSchema for Option<T> {
                     schema
                 }
                 schema => SchemaObject {
+                    // TODO technically the schema already accepts null, so this may be unnecessary
                     subschemas: Some(Box::new(SubschemaValidation {
                         any_of: Some(vec![schema, <()>::json_schema(gen)]),
                         ..Default::default()

schemars/src/lib.rs

@@ -195,9 +195,71 @@ println!("{}", serde_json::to_string_pretty(&schema).unwrap());
 
 `#[serde(...)]` attributes can be overriden using `#[schemars(...)]` attributes, which behave identically (e.g. `#[schemars(rename_all = "camelCase")]`). You may find this useful if you want to change the generated schema without affecting Serde's behaviour, or if you're just not using Serde.
 
+### Schema from Example Values
+
+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. 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
+use schemars::schema_for_value;
+use serde::Serialize;
+
+#[derive(Serialize)]
+pub struct MyStruct {
+    pub my_int: i32,
+    pub my_bool: bool,
+    pub my_nullable_enum: Option<MyEnum>,
+}
+
+#[derive(Serialize)]
+pub enum MyEnum {
+    StringNewType(String),
+    StructVariant { floats: Vec<f32> },
+}
+
+fn main() {
+    let schema = schema_for_value!(MyStruct {
+        my_int: 123,
+        my_bool: true,
+        my_nullable_enum: Some(MyEnum::StringNewType("foo".to_string()))
+    });
+    println!("{}", serde_json::to_string_pretty(&schema).unwrap());
+}
+```
+
+<details>
+<summary>Click to see the output JSON schema...</summary>
+
+```json
+{
+    "$schema": "http://json-schema.org/draft-07/schema#",
+    "title": "MyStruct",
+    "examples": [
+        {
+            "my_bool": true,
+            "my_int": 123,
+            "my_nullable_enum": {
+                "StringNewType": "foo"
+            }
+        }
+    ],
+    "type": "object",
+    "properties": {
+        "my_bool": {
+            "type": "boolean"
+        },
+        "my_int": {
+            "type": "integer",
+            "format": "int32"
+        },
+        "my_nullable_enum": true
+    }
+}
+```
+</details>
+
 ## Feature Flags
-- `derive` (enabled by default) - provides `#[derive(JsonSchema)]` macro.
-- `impl_json_schema` - implements `JsonSchema` for Schemars types themselves.
+- `derive` (enabled by default) - provides `#[derive(JsonSchema)]` macro
+- `impl_json_schema` - implements `JsonSchema` for Schemars types themselves
 - `preserve_order` - keep the order of struct fields in `Schema` and `SchemaObject`
 
 ## Optional Dependencies
@@ -236,6 +298,7 @@ pub type MapEntry<'a, K, V> = indexmap::map::Entry<'a, K, V>;
 
 mod flatten;
 mod json_schema_impls;
+mod ser;
 #[macro_use]
 mod macros;