serde default value from function

[DrLuke]

When defining a serde default value for a field using a function results in the generated value being set as the default value for the schema.

This is troublesome for fields that contain random data, like Uuids, as you probably don't want to define a default value. Also the value will be different each time the schema is generated.

There probably needs to be an attribute to ignore the default value but still make the field optional in the schema.

Minimal example:

use schemars::{schema_for, JsonSchema};
use serde::Deserialize;
use uuid::Uuid;

#[derive(Deserialize, JsonSchema)]
pub struct ExampleStruct {
    #[serde(default = "Uuid::new_v4")]
    uid: Uuid,
}

pub fn main() {
    let schema = schema_for!(ExampleStruct);
    println!("{}", serde_json::to_string_pretty(&schema).unwrap());
}

Output:

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "title": "ExampleStruct",
  "type": "object",
  "properties": {
    "uid": {
      "default": "919c5b9d-db38-4c25-9ecb-4193fc08c78e",
      "type": "string",
      "format": "uuid"
    }
  }
}

[GREsau]

The easiest way to achieve this in schemars 1.0.0-alpha.3 is with the transform attribute, which can apply arbitrary modifications to generated schemas e.g.

#[derive(Deserialize, JsonSchema)]
pub struct ExampleStruct {
    #[serde(default = "Uuid::new_v4")]
    #[schemars(transform = remove_default)]
    uid: Uuid,
}

fn remove_default(schema: &mut Schema) {
    if let Some(obj) = schema.as_object_mut() {
        obj.remove("default");
    }
}

which then produces:

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "title": "ExampleStruct",
  "type": "object",
  "properties": {
    "uid": {
      "type": "string",
      "format": "uuid"
    }
  }
}