Add rule to avoid redundant Swift Testing expectation messages (#371)

Author: calda

README.md

@@ -5092,6 +5092,44 @@ _You can enable the following settings in Xcode by running [this script](https:/
 
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+- <a id='avoid-redundant-expectation-comments'></a>(<a href='#avoid-redundant-expectation-comments'>link</a>) **In Swift Testing, avoid expectation message strings that restate the expectation without adding additional context.** Unlike `XCTAssert`, the Swift Testing `#expect` macro generates detailed failure messages that include the expectation condition.
+
+  <details>
+
+  ```swift
+  // WRONG: Restates what #expect already reports in failure output
+  @Test
+  func `engage warp drive`() {
+    spaceship.engageWarpDrive()
+    #expect(spaceship.isWarpDriveActive, "Warp drive should be active")
+    #expect(spaceship.speed > lightSpeed, "Speed should be greater than light speed")
+  }
+
+  // RIGHT: Omits the message string, or adds valuable context
+  @Test
+  func `engage warp drive`() {
+    spaceship.engageWarpDrive()
+    #expect(spaceship.isWarpDriveActive)
+    #expect(spaceship.speed > lightSpeed, "Spaceship must reach light speed before the warp bubble can form")
+  }
+  ```
+
+  Code comments also work well to add additional context:
+
+  ```swift
+  // ALSO RIGHT
+  @Test
+  func `engage warp drive`() {
+    spaceship.engageWarpDrive()
+    #expect(spaceship.isWarpDriveActive)
+
+    // Spaceship must reach light speed before the warp bubble can form
+    #expect(spaceship.speed > lightSpeed)
+  }
+  ```
+
+  </details>
+
 - <a id='avoid-guard-in-tests'></a>(<a href='#avoid-guard-in-tests'>link</a>) **Avoid `guard` statements in unit tests**. XCTest and Swift Testing have APIs for unwrapping an optional and failing the test, which are much simpler than unwrapping the optionals yourself. Use assertions instead of guarding on boolean conditions.
 
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