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//! Mocking / spying library based on lightweight function proxies.
//!
//! Mocking in Rust is somewhat hard compared to object-oriented languages. Since there
//! is no implicit / all-encompassing class hierarchy, [Liskov substitution principle]
//! does not apply, making it generally impossible to replace an object with its mock.
//! A switch is only possible if the object consumer explicitly opts in via
//! parametric polymorphism or dynamic dispatch.
//!
//! What do? Instead of trying to emulate mocking approaches from the object-oriented world,
//! this crate opts in for another approach, somewhat similar to [remote derive] from `serde`.
//! Mocking is performed on function / method level, with each function conditionally proxied
//! to a mock that has access to function args and can do whatever: call the "real" function
//! (e.g., to spy on responses), maybe with different args and/or after mutating args;
//! substitute with a mock response, etc. Naturally, mock logic
//! can be stateful (e.g., determine a response from the predefined list; record responses
//! for spied functions etc.)
//!
//! [Liskov substitution principle]: https://en.wikipedia.org/wiki/Liskov_substitution_principle
//! [remote derive]: https://serde.rs/remote-derive.html
//!
//! # Overview
//!
//! 1. Define the state to hold data necessary for mocking / spying and derive
//! [`Mock`] for it. Requirements to the state are quite lax; it should be
//! `'static` and `Send`.
//! 2. Place [`mock`] attrs referencing the state on the relevant functions, methods
//! and/or impl blocks.
//! 3. Define mock logic as inherent methods of the mock state type. Such methods will be called
//! with the same args as the original functions + additional first arg for the mock state
//! reference. In the simplest case,
//! each mocked function / method gets its own method with the same name as the original,
//! but this can be customized.
//! 4. If the state needs to be mutated in mock logic, add a `#[mock(mut)]` attr on the state.
//! In this case, the mock method will receive `&`[`Mut`]`<Self>` wrapper as the first arg
//! instead of `&self`. If the mocked function / method is `async`, the mock implementation
//! will receive [`MockRef`]`<Self>` as the first arg.
//! 5. If the mock logic needs to be shared across threads, add a `#[mock(shared)]` attr
//! on the state. (By default, mocks are thread-local.)
//! 6. Set the mock state in tests using [`Mock::set_as_mock()`]. Inspect the state during tests
//! using [`MockGuard::with()`] and after tests using [`MockGuard::into_inner()`].
//!
//! # Features and limitations
//!
//! - Can mock functions / methods with a wide variety of signatures, including generic functions
//! (with not necessarily `'static` type params), functions returning non-`'static` responses
//! and responses with dependent lifetimes, such as in `fn(&str) -> &str`, functions with
//! `impl Trait` args etc.
//! - Can mock methods in `impl` blocks, including trait implementations.
//! - Single mocking function can mock multiple functions, provided that they have compatible
//! signatures.
//! - Whether mock state is shared across functions / methods, is completely up to the test writer.
//! Functions for the same receiver type / in the same `impl` block may have different
//! mock states.
//! - Mocking functions can have wider argument types than required from the signature of
//! function(s) being mocked. For example, if the mocking function doesn't use some args,
//! they can be just replaced with unconstrained type params.
//!
//! ## Downsides
//!
//! - You still cannot mock types from other crates.
//! - Even if mocking logic does not use certain args, they need to be properly constructed,
//! which, depending on the case, may defy the reasons behind using mocks.
//! - Very limited built-in matching / verifying (see [`Answers`]). With the chosen approach,
//! it is frequently easier and more transparent to just use `match` statements.
//! As a downside, if matching logic needs to be customized across tests, it's (mostly)
//! up to the test writer.
//!
//! # Crate features
//!
//! ## `shared`
//!
//! *(Off by default)*
//!
//! Enables mocks that [can be used](Shared) across multiple threads.
//!
//! # Examples
//!
//! ## Basics
//!
//! ```
//! use mimicry::{mock, CallReal, RealCallSwitch, Mock};
//! # use std::cell::Cell;
//!
//! // Mock target: a standalone function.
//! #[cfg_attr(test, mock(using = "SearchMock"))]
//! # fn eat_attr() {}
//! # #[mock(using = "SearchMock")]
//! fn search(haystack: &str, needle: char) -> Option<usize> {
//! haystack.chars().position(|ch| ch == needle)
//! }
//!
//! // Mock state. In this case, we use it to record responses.
//! #[derive(Default, Mock, CallReal)]
//! struct SearchMock {
//! called_times: Cell<usize>,
//! switch: RealCallSwitch,
//! // ^ Stores the real / mocked function switch, thus allowing
//! // to call `Delegate` trait methods.
//! }
//!
//! impl SearchMock {
//! // Mock implementation: an inherent method of the mock state
//! // specified in the `#[mock()]` macro on the mocked function.
//! // The mock impl receives same args as the mocked function
//! // with the additional context parameter that allows
//! // accessing the mock state and controlling mock / real function switches.
//! fn search(
//! &self,
//! haystack: &str,
//! needle: char,
//! ) -> Option<usize> {
//! self.called_times.set(self.called_times.get() + 1);
//! match haystack {
//! "test" => Some(42),
//! short if short.len() <= 2 => None,
//! _ => {
//! let new_needle = if needle == '?' { 'e' } else { needle };
//! self.call_real().scope(|| search(haystack, new_needle))
//! }
//! }
//! }
//! }
//!
//! // Test code.
//! let guard = SearchMock::default().set_as_mock();
//! assert_eq!(search("test", '?'), Some(42));
//! assert_eq!(search("?!", '?'), None);
//! assert_eq!(search("needle?", '?'), Some(1));
//! assert_eq!(search("needle?", 'd'), Some(3));
//! let recovered = guard.into_inner();
//! assert_eq!(recovered.called_times.into_inner(), 4);
//! ```
//!
//! Mock functions only get a shared reference to the mock state; this is because
//! the same state can be accessed from multiple places during recursive calls.
//! To easily mutate the state during tests, consider using the [`Mut`]
//! wrapper.
//!
//! ## On impl blocks
//!
//! The `mock` attribute can be placed on impl blocks (including trait implementations)
//! to apply a mock to all methods in the block:
//!
//! ```
//! # use mimicry::{mock, CheckRealCall, Mock};
//! struct Tested(String);
//!
//! #[mock(using = "TestMock")]
//! impl Tested {
//! fn len(&self) -> usize { self.0.len() }
//!
//! fn push(&mut self, s: impl AsRef<str>) -> &mut Self {
//! self.0.push_str(s.as_ref());
//! self
//! }
//! }
//!
//! #[mock(using = "TestMock", rename = "impl_{}")]
//! impl AsRef<str> for Tested {
//! fn as_ref(&self) -> &str {
//! &self.0
//! }
//! }
//!
//! #[derive(Mock)]
//! struct TestMock { /* ... */ }
//! // Since we don't use partial mocking / spying, we indicate
//! // this with an empty `CheckRealCall` impl.
//! impl CheckRealCall for TestMock {}
//!
//! impl TestMock {
//! fn len(&self, recv: &Tested) -> usize {
//! // ...
//! # 0
//! }
//!
//! fn push<'s>(
//! &self,
//! recv: &'s mut Tested,
//! s: impl AsRef<str>,
//! ) -> &'s mut Tested {
//! // ...
//! # recv
//! }
//!
//! fn impl_as_ref<'s>(&self, recv: &'s Tested) -> &'s str {
//! // ...
//! # ""
//! }
//! }
//! ```
//!
//! ## What can('t) be mocked?
//!
//! ```
//! # use mimicry::{mock, CheckRealCall, Mock};
//! # use std::sync::atomic::{AtomicU32, Ordering};
//! struct Test;
//! impl Test {
//! #[mock(using = "CountingMock::count")]
//! fn do_something(&self) {}
//!
//! #[mock(using = "CountingMock::count")]
//! fn lifetimes(&self) -> &str {
//! "what?"
//! }
//!
//! #[mock(using = "CountingMock::count")]
//! fn generics<T: ToOwned>(value: &T) -> Vec<T::Owned> {
//! (0..5).map(|_| value.to_owned()).collect()
//! }
//!
//! #[mock(using = "CountingMock::count")]
//! fn impl_methods(value: &impl AsRef<str>) -> &str {
//! value.as_ref()
//! }
//! }
//!
//! impl Iterator for Test {
//! type Item = u8;
//!
//! #[mock(using = "CountingMock::count")]
//! fn next(&mut self) -> Option<Self::Item> {
//! Some(42)
//! }
//! }
//!
//! #[derive(Default, Mock)]
//! struct CountingMock(AtomicU32);
//!
//! impl CheckRealCall for CountingMock {}
//!
//! impl CountingMock {
//! // All functions above can be mocked with a single impl!
//! // This is quite extreme, obviously; in realistic scenarios,
//! // you probably wouldn't be able to unite mocks of functions
//! // with significantly differing return types.
//! fn count<T, R: Default>(&self, _: T) -> R {
//! self.0.fetch_add(1, Ordering::Relaxed);
//! R::default()
//! }
//! }
//!
//! let guard = CountingMock::default().set_as_mock();
//! Test.do_something();
//! assert_eq!(Test.lifetimes(), "");
//! assert_eq!(Test.next(), None);
//! let count = guard.into_inner().0;
//! assert_eq!(count.into_inner(), 3);
//! ```
//!
//! Finally, `async` functions can be mocked as well, although they require a bit more complex
//! setup. See [`MockRef`] docs for examples.
// Documentation settings.
#![cfg_attr(docsrs, feature(doc_cfg))]
#![doc(html_root_url = "https://docs.rs/mimicry/0.1.0")]
// Linter settings.
#![warn(missing_debug_implementations, missing_docs, bare_trait_objects)]
#![warn(clippy::all, clippy::pedantic)]
#![allow(clippy::must_use_candidate, clippy::module_name_repetitions)]
use once_cell::sync::OnceCell;
use core::{cell::RefCell, fmt, ops};
mod answers;
#[cfg(feature = "shared")]
mod shared;
mod tls;
mod traits;
#[cfg(feature = "shared")]
pub use crate::shared::Shared;
pub use crate::{
answers::{Answers, AnswersGuard, AnswersSender},
tls::ThreadLocal,
traits::{CallReal, CheckRealCall, GetMock, RealCallGuard, RealCallSwitch},
};
pub use mimicry_derive::{mock, CallReal, Mock};
use crate::traits::{Guard, LockMock, SetMock, Wrap};
/// Wrapper that allows creating `static`s with mock state.
#[derive(Debug)]
pub struct Static<T> {
cell: OnceCell<T>,
}
impl<T> Static<T> {
/// Creates a new instance.
pub const fn new() -> Self {
Self {
cell: OnceCell::new(),
}
}
}
impl<'a, T, S> GetMock<'a, T> for Static<S>
where
S: GetMock<'a, T> + Default,
{
type Ref = S::Ref;
fn get(&'a self) -> Option<Self::Ref> {
let cell = self.cell.get_or_init(S::default);
cell.get()
}
}
/// State of a mock.
///
/// This trait should be implemented via the corresponding derive macro; parts of it are
/// non-documented and subject to change.
pub trait Mock: Sized {
#[doc(hidden)]
type Base: Wrap<Self> + CheckRealCall;
/// Wrapper around [`Self::Base`] allowing to share it across test code and the main program.
#[doc(hidden)]
type Shared: GetMock<'static, Self::Base>
+ SetMock<'static, Self::Base>
+ 'static
+ Default
+ Send
+ Sync;
/// Returns the shared wrapper around this state.
#[doc(hidden)]
fn instance() -> &'static Static<Self::Shared>;
/// Sets the mock state and returns an exclusive guard to the shared state.
#[must_use = "mock is only set until the returned `MockGuard` is dropped"]
fn set_as_mock(self) -> MockGuard<Self> {
let cell = Self::instance().cell.get_or_init(<Self::Shared>::default);
MockGuard {
inner: cell.set(self.into()),
}
}
/// Locks write access to the mock state without setting the state. This is useful
/// for [shared mocks](Shared) to ensure that tests not using mocks do not observe mocks
/// set by other tests.
fn lock() -> EmptyGuard<Self>
where
Self::Shared: LockMock<'static, Self::Base>,
{
let cell = Self::instance().cell.get_or_init(<Self::Shared>::default);
EmptyGuard {
_inner: cell.lock(),
}
}
}
/// Exclusive guard to set the mock state.
///
/// A guard can be used to check / adjust the mock state during the test.
/// Dropping the guard also unsets the mock state, so that targeted functions are no longer mocked.
///
/// In case of [shared mocks](Shared), guards also provided synchronization across concurrently
/// executing tests: until a guard is dropped, other threads attempting
/// to call [`Mock::set_as_mock()`] will block. Unfortunately, this is not always sufficient
/// to have good results; see [`Shared`] docs for discussion.
///
/// # Examples
///
/// ```
/// # use mimicry::{mock, CheckRealCall, Mock, MockGuard};
/// #[mock(using = "ValueMock")]
/// fn answer() -> usize { 42 }
///
/// #[derive(Default, Mock)]
/// struct ValueMock(usize);
///
/// impl CheckRealCall for ValueMock {}
///
/// impl ValueMock {
/// fn answer(&self) -> usize {
/// self.0
/// }
/// }
///
/// assert_eq!(answer(), 42);
/// let mut guard: MockGuard<_> = ValueMock::default().set_as_mock();
/// assert_eq!(answer(), 0);
/// guard.with(|mock| { mock.0 = 23; });
/// // ^ updates mock state without releasing the guard
/// assert_eq!(answer(), 23);
/// ```
pub struct MockGuard<T: Mock> {
inner: <T::Shared as SetMock<'static, T::Base>>::Guard,
}
impl<T: Mock> fmt::Debug for MockGuard<T> {
fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
formatter.debug_struct("MockGuard").finish_non_exhaustive()
}
}
impl<T: Mock> MockGuard<T> {
/// Performs an action on the mock state without releasing the guard. This can be used
/// to adjust the mock state, check or take some parts of it (such as collected args
/// or responses).
pub fn with<R>(&mut self, action: impl FnOnce(&mut T) -> R) -> R {
self.inner.with(|wrapped| action(wrapped.as_mut()))
}
/// Returns the enclosed mock state and releases the exclusive lock.
pub fn into_inner(self) -> T {
Guard::into_inner(self.inner).into_inner()
}
}
/// Exclusive guard to set the mock state without an attached state.
pub struct EmptyGuard<T: Mock>
where
T::Shared: LockMock<'static, T::Base>,
{
_inner: <T::Shared as LockMock<'static, T::Base>>::EmptyGuard,
}
impl<T: Mock> fmt::Debug for EmptyGuard<T>
where
T::Shared: LockMock<'static, T::Base>,
{
fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
formatter.debug_struct("EmptyGuard").finish_non_exhaustive()
}
}
/// Reference to a mock state used when mocking async functions / methods.
///
/// A separate reference type is required because it would be unsound to spill a direct state reference
/// or a reference to [`Mut`] across `await` boundaries. (Internally, such a reference
/// is produced using interior mutability primitives like [`RefCell`].) Instead, `MockRef`
/// provides access to the state using [`with()`](Self::with()) or [`with_mut()`](Self::with_mut())
/// methods that do not overly restrict the lifetime of the state reference.
///
/// # Examples
///
/// ## Mock with immutable state
///
/// ```
/// # use mimicry::{mock, CheckRealCall, Mock, MockRef};
/// # use std::sync::atomic::{AtomicU32, Ordering};
/// #[mock(using = "CountingMock")]
/// async fn answer() -> u32 { 42 }
///
/// #[derive(Default, Mock)]
/// struct CountingMock(AtomicU32);
///
/// impl CheckRealCall for CountingMock {}
///
/// impl CountingMock {
/// async fn answer(r: MockRef<Self>) -> u32 {
/// r.with(|this| this.0.fetch_add(1, Ordering::Relaxed))
/// }
/// }
/// ```
///
/// ## Mock with mutable state
///
/// Also demonstrates spying logic.
///
/// ```
/// # use async_recursion::async_recursion;
/// # use async_std::task::block_on;
/// # use mimicry::{mock, CallReal, Mock, MockRef};
/// #[mock(using = "CountingMock")]
/// async fn answer() -> u32 { 42 }
///
/// #[derive(Default, Mock)]
/// #[mock(mut)]
/// struct CountingMock {
/// captured_value: Option<u32>,
/// }
///
/// impl CountingMock {
/// #[async_recursion] // workaround for `async fn` recursion
/// async fn answer(r: MockRef<Self>) -> u32 {
/// let captured = r.with_mut(|this| this.captured_value);
/// if let Some(captured) = captured {
/// captured
/// } else {
/// let value = r.call_real().async_scope(answer()).await;
/// r.with_mut(|this| this.captured_value = Some(value));
/// value
/// }
/// }
/// }
///
/// # block_on(async {
/// let guard = CountingMock::default().set_as_mock();
/// assert_eq!(answer().await, 42);
/// let captured = guard.into_inner().captured_value;
/// assert_eq!(captured, Some(42));
/// # })
/// ```
#[derive(Debug)]
pub struct MockRef<T: Mock> {
instance: &'static Static<T::Shared>,
}
impl<T: Mock> Clone for MockRef<T> {
fn clone(&self) -> Self {
Self {
instance: self.instance,
}
}
}
impl<T: Mock> Copy for MockRef<T> {}
impl<T: Mock> MockRef<T> {
#[doc(hidden)] // used by the `mock` macro
pub fn new(instance: &'static Static<T::Shared>) -> Self {
Self { instance }
}
}
impl<T: Mock<Base = T>> MockRef<T> {
/// Accesses the underlying mock state.
///
/// # Panics
///
/// Panics if the mock state has gone missing. This is a sign that test code is ill-constructed
/// (e.g., the mock is removed before all mocked calls are made).
pub fn with<R>(&self, action: impl FnOnce(&T) -> R) -> R {
if let Some(mock_ref) = GetMock::get(self.instance) {
action(&*mock_ref)
} else {
panic!("mock state is gone");
}
}
}
impl<T: Mock<Base = Mut<T>>> MockRef<T> {
/// Accesses the underlying [`Mut`]able mock state.
///
/// # Panics
///
/// Panics if the mock state has gone missing. This is a sign that test code is ill-constructed
/// (e.g., the mock is removed before all mocked calls are made).
pub fn with_mut<R>(&self, action: impl FnOnce(&mut T) -> R) -> R {
if let Some(mock_ref) = GetMock::get(self.instance) {
let base: &Mut<T> = &mock_ref;
action(&mut base.borrow())
} else {
panic!("mock state is gone");
}
}
}
impl<T> CallReal for MockRef<T>
where
T: Mock,
T::Base: CallReal,
{
fn access_switch<R>(&self, action: impl FnOnce(&RealCallSwitch) -> R) -> R {
if let Some(mock_ref) = GetMock::get(self.instance) {
mock_ref.access_switch(action)
} else {
panic!("mock state is gone");
}
}
}
/// A lightweight wrapper around the state (essentially, a [`RefCell`]) allowing to easily
/// mutate it in mock code.
///
/// Besides access to the state, `Mut` implements [`CallReal`], thus allowing
/// partial mocks / spies.
///
/// # Examples
///
/// ```
/// # use mimicry::{mock, Mock, MockGuard, Mut};
/// #[mock(using = "CounterMock")]
/// fn answer() -> usize { 42 }
///
/// #[derive(Default, Mock)]
/// #[mock(mut)] // indicates to use `Mut`
/// struct CounterMock(usize);
///
/// impl CounterMock {
/// fn answer(this: &Mut<Self>) -> usize {
/// // Note a custom "receiver" instead of `&self`
/// this.borrow().0 += 1;
/// this.borrow().0
/// }
/// }
///
/// let guard = CounterMock::default().set_as_mock();
/// assert_eq!(answer(), 1);
/// assert_eq!(answer(), 2);
/// assert_eq!(answer(), 3);
/// assert_eq!(guard.into_inner().0, 3);
/// ```
#[derive(Debug, Default)]
pub struct Mut<T> {
inner: RefCell<T>,
switch: RealCallSwitch,
}
impl<T> Mut<T> {
/// Returns an exclusive reference to the underlying mock.
///
/// Beware that while the reference is alive, further calls to functions in the same mock
/// (including indirect ones, e.g., performed from the tested program code)
/// will not be able to retrieve the state via this method; this will result
/// in a panic. To deal with this, you can create short lived state refs a la
/// `this.borrow().do_something()`, or enclose the reference into an additional scope.
///
/// # Panics
///
/// Panics if a reference to the same mock state is alive, as described above.
pub fn borrow(&self) -> impl ops::DerefMut<Target = T> + '_ {
self.inner.borrow_mut()
}
}
impl<T> From<T> for Mut<T> {
fn from(inner: T) -> Self {
Self {
inner: RefCell::new(inner),
switch: RealCallSwitch::default(),
}
}
}
impl<T> Wrap<T> for Mut<T> {
fn into_inner(self) -> T {
self.inner.into_inner()
}
fn as_mut(&mut self) -> &mut T {
self.inner.get_mut()
}
}
impl<T> CallReal for Mut<T> {
fn access_switch<R>(&self, action: impl FnOnce(&RealCallSwitch) -> R) -> R {
action(&self.switch)
}
}
#[cfg(doctest)]
doc_comment::doctest!("../README.md");