[Feature Request] store validator function for z.refine

[soc221b]

Hi, thank you for building this powerful validation library~

Feature Request

Could we store the user-defined validator in somewhere like schema._def.effect:

https://github.com/colinhacks/zod/blob/890556e/src/types.ts#L322

Use case

I'm implementing a zod-schema-faker library to generate fake data for zod.

Recently I am trying to support fake data generation for custom schema (.custom) and custom validation logic (.refine), the strategy is to identify the schema/validator and execute user-defined faker to achieve the goal.

For example:

// user may have this validator:
const validator = (val: string) => val.length <= 255
const schema = z.string().refine(validator, {
  message: "String can't be more than 255 characters",
})

// they could generate data by register a custom validator:
function validatorFaker(fakeVal: string): string {
  return fakeVal.slice(0, 255)
}
installRefine(validator, validatorFaker)
const fakeData = fake(schema)

Further info about how custom schema to be supported:

import * as z from 'zod'
import { fake, installCustom, ZodTypeFaker, runFake, install } from 'zod-schema-faker'

const pxSchema = z.custom<`${number}px`>(val => {
  return typeof val === 'string' ? /^\d+px$/.test(val) : false
})

class pxFaker extends ZodTypeFaker<typeof pxSchema> {
  fake(): `${number}px` {
    return `${runFake(faker => faker.number.int({ min: 0 }))}px`
  }
}
install()
installCustom(pxSchema, pxFaker)
const data = fake(pxSchema)
// > '10px'

Thank you 😄

[colinhacks]

Yes! This is the plan for Zod 4.