Add slide to show the Mockall crate in action

src/SUMMARY.md

@@ -172,6 +172,7 @@
   - [Test Modules](testing/unit-tests.md)
   - [Other Types of Tests](testing/other.md)
   - [Useful Crates](testing/useful-crates.md)
+  - [Mocking](testing/mocking.md)
   - [Compiler lints and Clippy](testing/lints.md)
   - [Exercise: Luhn Algorithm](testing/exercise.md)
     - [Solution](testing/solution.md)

src/testing/Cargo.toml

@@ -4,7 +4,14 @@ version = "0.1.0"
 edition = "2021"
 publish = false
 
+[[example]]
+name = "mockall-example"
+crate-type = ["staticlib"]
+path = "mockall.rs"
+test = true
+
 [[bin]]
 name = "luhn"
 path = "exercise.rs"
 
+mockall = "0.11.4"

src/testing/mockall.rs

@@ -0,0 +1,13 @@
+use std::time::Duration;
+
+#[mockall::automock]
+pub trait Pet {
+    fn is_hungry(&self, since_last_meal: Duration) -> bool;
+}
+
+#[test]
+fn test_robot_pet() {
+    let mut mock_dog = MockPet::new();
+    mock_dog.expect_is_hungry().return_const(true);
+    assert_eq!(mock_dog.is_hungry(Duration::from_secs(10)), true);
+}

src/testing/mocking.md

@@ -0,0 +1,58 @@
+---
+minutes: 10
+---
+
+# Mocking
+
+If you want to do mocking, we recommend [Mockall]. You need to refactor your
+code to use traits, which you can then quickly mock:
+
+```rust,ignore
+{{#include mockall.rs}}
+```
+
+[Mockall]: https://docs.rs/mockall/
+
+<details>
+
+- Note that *mocking is controversial*: mocks allow you to completely isolate a
+  test from its dependencies. The immediate result is faster and more stable
+  test execution. On the other hand, the mocks can be configured wrongly and
+  return output different from what the real dependencies would do.
+
+  If at all possible, it is recommended that you use the real dependencies. As
+  an example, many databases allow you to configure an in-memory backend. This
+  means that you get the correct behavior in your tests, plus they are fast and
+  will automatically clean up after themselves.
+
+  Similarly, many web frameworks allow you to start an in-process server which
+  binds to a random port on `localhost`. Always prefer this over mocking away
+  the framework since it helps you test your code in the real environment.
+
+- Mockall is not part of the Rust Playground, so you need to run this example in
+  a local environment. Use `cargo add mockall` to quickly add Mockall to an
+  existing Cargo project.
+
+- Mockall has a lot more functionality. In particular, you can set up
+  expectations which depend on the arguments passed:
+
+    ```rust,ignore
+    let mut mock_cat = MockPet::new();
+    mock_cat
+        .expect_is_hungry()
+        .with(mockall::predicate::gt(Duration::from_secs(3 * 3600)))
+        .return_const(true);
+    mock_cat
+        .expect_is_hungry()
+        .return_const(true);
+    assert_eq!(mock_cat.is_hungry(Duration::from_secs(1 * 3600)), false);
+    assert_eq!(mock_cat.is_hungry(Duration::from_secs(5 * 3600)), true);
+    ```
+
+- You can use `.times(n)` to limit the number of times a mock method can be
+  called to `n` --- the mock will automatically panic when dropped if this isn't
+  satisfied.
+
+- Mockall is available for use in AOSP.
+
+</details>