chore: import upstream snapshot with attribution

rules/rust/testing.md

@@ -0,0 +1,154 @@
+---
+paths:
+  - "**/*.rs"
+---
+# Rust Testing
+
+> This file extends [common/testing.md](../common/testing.md) with Rust-specific content.
+
+## Test Framework
+
+- **`#[test]`** with `#[cfg(test)]` modules for unit tests
+- **rstest** for parameterized tests and fixtures
+- **proptest** for property-based testing
+- **mockall** for trait-based mocking
+- **`#[tokio::test]`** for async tests
+
+## Test Organization
+
+```text
+my_crate/
+├── src/
+│   ├── lib.rs           # Unit tests in #[cfg(test)] modules
+│   ├── auth/
+│   │   └── mod.rs       # #[cfg(test)] mod tests { ... }
+│   └── orders/
+│       └── service.rs   # #[cfg(test)] mod tests { ... }
+├── tests/               # Integration tests (each file = separate binary)
+│   ├── api_test.rs
+│   ├── db_test.rs
+│   └── common/          # Shared test utilities
+│       └── mod.rs
+└── benches/             # Criterion benchmarks
+    └── benchmark.rs
+```
+
+Unit tests go inside `#[cfg(test)]` modules in the same file. Integration tests go in `tests/`.
+
+## Unit Test Pattern
+
+```rust
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn creates_user_with_valid_email() {
+        let user = User::new("Alice", "alice@example.com").unwrap();
+        assert_eq!(user.name, "Alice");
+    }
+
+    #[test]
+    fn rejects_invalid_email() {
+        let result = User::new("Bob", "not-an-email");
+        assert!(result.is_err());
+        assert!(result.unwrap_err().to_string().contains("invalid email"));
+    }
+}
+```
+
+## Parameterized Tests
+
+```rust
+use rstest::rstest;
+
+#[rstest]
+#[case("hello", 5)]
+#[case("", 0)]
+#[case("rust", 4)]
+fn test_string_length(#[case] input: &str, #[case] expected: usize) {
+    assert_eq!(input.len(), expected);
+}
+```
+
+## Async Tests
+
+```rust
+#[tokio::test]
+async fn fetches_data_successfully() {
+    let client = TestClient::new().await;
+    let result = client.get("/data").await;
+    assert!(result.is_ok());
+}
+```
+
+## Mocking with mockall
+
+Define traits in production code; generate mocks in test modules:
+
+```rust
+// Production trait — pub so integration tests can import it
+pub trait UserRepository {
+    fn find_by_id(&self, id: u64) -> Option<User>;
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use mockall::predicate::eq;
+
+    mockall::mock! {
+        pub Repo {}
+        impl UserRepository for Repo {
+            fn find_by_id(&self, id: u64) -> Option<User>;
+        }
+    }
+
+    #[test]
+    fn service_returns_user_when_found() {
+        let mut mock = MockRepo::new();
+        mock.expect_find_by_id()
+            .with(eq(42))
+            .times(1)
+            .returning(|_| Some(User { id: 42, name: "Alice".into() }));
+
+        let service = UserService::new(Box::new(mock));
+        let user = service.get_user(42).unwrap();
+        assert_eq!(user.name, "Alice");
+    }
+}
+```
+
+## Test Naming
+
+Use descriptive names that explain the scenario:
+- `creates_user_with_valid_email()`
+- `rejects_order_when_insufficient_stock()`
+- `returns_none_when_not_found()`
+
+## Coverage
+
+- Target 80%+ line coverage
+- Use **cargo-llvm-cov** for coverage reporting
+- Focus on business logic — exclude generated code and FFI bindings
+
+```bash
+cargo llvm-cov                       # Summary
+cargo llvm-cov --html                # HTML report
+cargo llvm-cov --fail-under-lines 80 # Fail if below threshold
+```
+
+## Testing Commands
+
+```bash
+cargo test                       # Run all tests
+cargo test -- --nocapture        # Show println output
+cargo test test_name             # Run tests matching pattern
+cargo test --lib                 # Unit tests only
+cargo test --test api_test       # Specific integration test (tests/api_test.rs)
+cargo test --doc                 # Doc tests only
+```
+
+## References
+
+See skill: `rust-testing` for comprehensive testing patterns including property-based testing, fixtures, and benchmarking with Criterion.