Mock instance inside serde implementation - unit-testing

Question

I'm trying to implement custom deserialization function/method which uses some external functionality. The function creates an instance and uses its methods. It's working fine, but I can't figure out how to mock the service in tests.
More general question is: how to provide a state to deserialization function/method?
The code below illustrates what I mean.
MagickBook is an external service which holds a state and contains some essential logic in MagickBook::find method.
Scroll is a deserializable data structure, which should be deserialized using the logic from MagicBook.
I'd like to have a way to provide particular instance of MagicBook from the outside, at the moment of deserialization. For example in tests.
Rust Playground
use serde::de::{Deserialize, Deserializer}; // 1.0.82
use serde_derive::Deserialize; // 1.0.82
use serde_json; // 1.0.33
struct MagickBook;
// Some service which I want to mock in the test
impl MagickBook {
fn new() -> Self {
Self {}
}
fn find(&self, _kind: &str) -> isize {
let effect = 42;
// Here we do some logic depending on input parameter
// -- snip --
return effect;
}
}
#[derive(Deserialize, PartialEq, Debug)]
struct Scroll {
#[serde(rename = "kind")]
#[serde(deserialize_with = "deserialize_effect")]
effect: isize,
}
fn deserialize_effect<'de, D>(deserializer: D) -> Result<isize, D::Error>
where
D: Deserializer<'de>,
{
let book = MagickBook::new();
Ok(book.find(&String::deserialize(deserializer)?))
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn main() {
let scroll: Scroll = serde_json::from_str("{\"kind\":\"wind\"}").unwrap();
assert_eq!(scroll, Scroll { effect: 42 });
}
}

Answer 1

I would recommend accessing the mock through an internally mutable thread-local instance.
use serde::{Deserialize, Deserializer};
use std::cell::RefCell;
struct MagickBook {
value: isize,
}
thread_local! {
static MAGICK_BOOK: RefCell<MagickBook> = RefCell::new(MagickBook::new());
}
impl MagickBook {
fn new() -> Self {
MagickBook { value: 0 }
}
fn find(&self, _kind: &str) -> isize {
let effect = self.value;
// -- snip --
effect
}
}
#[derive(Deserialize, PartialEq, Debug)]
struct Scroll {
#[serde(rename = "kind", deserialize_with = "deserialize_effect")]
effect: isize,
}
fn deserialize_effect<'de, D>(deserializer: D) -> Result<isize, D::Error>
where
D: Deserializer<'de>,
{
let kind = String::deserialize(deserializer)?;
Ok(MAGICK_BOOK.with(|book| book.borrow().find(&kind)))
}
#[test]
fn test_deserialize() {
MAGICK_BOOK.with(|book| book.borrow_mut().value = 42);
let scroll: Scroll = serde_json::from_str(r#"{"kind":"wind"}"#).unwrap();
assert_eq!(scroll, Scroll { effect: 42 });
}