Add slide to show the Mockall crate in action (#1532)

As more and more Android teams start using Rust, the question of "How to do mocking?" has come up more frequently. Right now, we don't have a good answer to those teams, AOSP doesn't have any mocking framework available. That will change soon with the import of Mockall and this slide is part of the supporting documentation that we can point teams to.

Cargo.lock

@@ -468,6 +468,12 @@ dependencies = [
  "syn 1.0.109",
 ]
 
+[[package]]
+name = "difflib"
+version = "0.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6184e33543162437515c2e2b48714794e37845ec9851711914eec9d308f6ebe8"
+
 [[package]]
 name = "digest"
 version = "0.10.7"
@@ -478,6 +484,12 @@ dependencies = [
  "crypto-common",
 ]
 
+[[package]]
+name = "downcast"
+version = "0.11.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1435fa1053d8b2fbbe9be7e97eca7f33d37b28409959813daefc1446a14247f1"
+
 [[package]]
 name = "dtoa"
 version = "1.0.9"
@@ -499,6 +511,12 @@ version = "0.6.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "3a68a4904193147e0a8dec3314640e6db742afd5f6e634f428a6af230d9b3591"
 
+[[package]]
+name = "either"
+version = "1.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a26ae43d7bcc3b814de94796a5e736d4029efb0ee900c12e2d54c993ad1a1e07"
+
 [[package]]
 name = "elasticlunr-rs"
 version = "3.0.2"
@@ -575,6 +593,15 @@ dependencies = [
  "windows-sys",
 ]
 
+[[package]]
+name = "float-cmp"
+version = "0.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "98de4bbd547a563b716d8dfa9aad1cb19bfab00f4fa09a6a4ed21dbcf44ce9c4"
+dependencies = [
+ "num-traits",
+]
+
 [[package]]
 name = "fnv"
 version = "1.0.7"
@@ -605,6 +632,12 @@ dependencies = [
  "percent-encoding",
 ]
 
+[[package]]
+name = "fragile"
+version = "2.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6c2141d6d6c8512188a7891b4b01590a45f6dac67afb4f255c4124dbb86d4eaa"
+
 [[package]]
 name = "fsevent-sys"
 version = "4.1.0"
@@ -1031,6 +1064,15 @@ dependencies = [
 name = "iterators"
 version = "0.1.0"
 
+[[package]]
+name = "itertools"
+version = "0.10.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b0fd2260e829bddf4cb6ea802289de2f86d6a7a690192fbe91b3f46e0f2c8473"
+dependencies = [
+ "either",
+]
+
 [[package]]
 name = "itoa"
 version = "1.0.9"
@@ -1258,6 +1300,33 @@ dependencies = [
  "windows-sys",
 ]
 
+[[package]]
+name = "mockall"
+version = "0.11.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4c84490118f2ee2d74570d114f3d0493cbf02790df303d2707606c3e14e07c96"
+dependencies = [
+ "cfg-if",
+ "downcast",
+ "fragile",
+ "lazy_static",
+ "mockall_derive",
+ "predicates",
+ "predicates-tree",
+]
+
+[[package]]
+name = "mockall_derive"
+version = "0.11.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "22ce75669015c4f47b289fd4d4f56e894e4c96003ffdf3ac51313126f94c6cbb"
+dependencies = [
+ "cfg-if",
+ "proc-macro2",
+ "quote",
+ "syn 1.0.109",
+]
+
 [[package]]
 name = "native-tls"
 version = "0.2.11"
@@ -1282,6 +1351,12 @@ version = "1.0.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e4a24736216ec316047a1fc4252e27dabb04218aa4a3f37c6e7ddbf1f9782b54"
 
+[[package]]
+name = "normalize-line-endings"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "61807f77802ff30975e01f4f071c8ba10c022052f98b3294119f3e615d13e5be"
+
 [[package]]
 name = "normpath"
 version = "1.1.1"
@@ -1617,6 +1692,36 @@ version = "0.1.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "925383efa346730478fb4838dbe9137d2a47675ad789c546d150a6e1dd4ab31c"
 
+[[package]]
+name = "predicates"
+version = "2.1.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "59230a63c37f3e18569bdb90e4a89cbf5bf8b06fea0b84e65ea10cc4df47addd"
+dependencies = [
+ "difflib",
+ "float-cmp",
+ "itertools",
+ "normalize-line-endings",
+ "predicates-core",
+ "regex",
+]
+
+[[package]]
+name = "predicates-core"
+version = "1.0.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b794032607612e7abeb4db69adb4e33590fa6cf1149e95fd7cb00e634b92f174"
+
+[[package]]
+name = "predicates-tree"
+version = "1.0.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "368ba315fb8c5052ab692e68a0eefec6ec57b23a36959c14496f0b0df2c0cecf"
+dependencies = [
+ "predicates-core",
+ "termtree",
+]
+
 [[package]]
 name = "pretty_env_logger"
 version = "0.5.0"
@@ -2210,11 +2315,18 @@ dependencies = [
  "windows-sys",
 ]
 
+[[package]]
+name = "termtree"
+version = "0.4.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3369f5ac52d5eb6ab48c6b4ffdc8efbcad6b89c765749064ba298f2c68a16a76"
+
 [[package]]
 name = "testing"
 version = "0.1.0"
 dependencies = [
  "googletest",
+ "mockall",
 ]
 
 [[package]]

src/SUMMARY.md

@@ -173,6 +173,7 @@
   - [Other Types of Tests](testing/other.md)
   - [Useful Crates](testing/useful-crates.md)
   - [GoogleTest](testing/googletest.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

@@ -10,9 +10,16 @@ crate-type = ["staticlib"]
 path = "googletest.rs"
 test = true
 
+[[example]]
+name = "mockall-example"
+crate-type = ["staticlib"]
+path = "mockall.rs"
+test = true
+
 [[bin]]
 name = "luhn"
 path = "exercise.rs"
 
 [dependencies]
 googletest = "0.10.0"
+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: 5
+---
+
+# 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>