Add rule to prefer a generated Equatable implementation when possible (…

…#292)

Package.swift

@@ -42,8 +42,8 @@ let package = Package(
 
     .binaryTarget(
       name: "swiftformat",
-      url: "https://github.com/calda/SwiftFormat/releases/download/0.55-beta-12/SwiftFormat.artifactbundle.zip",
-      checksum: "8783cefc0837416759f81064df7907cc60ddca6d3f8c3b301b123a2193a8585b"),
+      url: "https://github.com/calda/SwiftFormat/releases/download/0.55-beta-15/SwiftFormat.artifactbundle.zip",
+      checksum: "5da82a61d4a29e77acc5a2e8668c40168970e1d6b2078bdfcb377b6f6ef35039"),
 
     .binaryTarget(
       name: "SwiftLintBinary",

README.md

@@ -3677,6 +3677,74 @@ _You can enable the following settings in Xcode by running [this script](resourc
 
     </details>
 
+* <a id='redundant-equatable-implementation'></a>(<a href='#redundant-equatable-implementation'>link</a>) **Prefer using a generated Equatable implementation when comparing all properties of a type.** For structs, prefer using the compiler-synthesized Equatable implementation when possible. [![SwiftFormat: redundantEquatable](https://img.shields.io/badge/SwiftFormat-redundantEquatable-7B0051.svg)](https://github.com/nicklockwood/SwiftFormat/blob/main/Rules.md#redundantEquatable)
+
+    <details>
+
+    ### Why?
+
+    Manually-implemented Equatable implementations are verbose, and keeping them up-to-date is error-prone. For example, when adding a new property, it's possible to forget to update the Equatable implementation to compare it.
+
+    ```swift
+    /// WRONG: The `static func ==` implementation is redundant and error-prone.
+    struct Planet: Equatable {
+      let mass: Double
+      let orbit: OrbitalElements
+      let rotation: Double
+
+      static func ==(lhs: Planet, rhs: Planet) -> Bool {
+        lhs.mass == rhs.mass
+          && lhs.orbit == rhs.orbit
+          && lhs.rotation == rhs.rotation
+      }
+    }
+
+    /// RIGHT: The `static func ==` implementation is synthesized by the compiler.
+    struct Planet: Equatable {
+      let mass: Double
+      let orbit: OrbitalElements
+      let rotation: Double
+    }
+
+    /// ALSO RIGHT: The `static func ==` implementation differs from the implementation that 
+    /// would be synthesized by the compiler and compared all properties, so is not redundant.
+    struct CelestialBody: Equatable {
+      let id: UUID
+      let orbit: OrbitalElements
+
+      static func ==(lhs: Planet, rhs: Planet) -> Bool {
+        lhs.id == rhs.id
+      }
+    }
+    ```
+
+    In projects that provide an `@Equatable` macro, prefer using that macro to generate the `static func ==` for classes rather than implementing it manually.
+
+    ```swift
+    /// WRONG: The `static func ==` implementation is verbose and error-prone.
+    final class Planet: Equatable {
+      let mass: Double
+      let orbit: OrbitalElements
+      let rotation: Double
+
+      static func ==(lhs: Planet, rhs: Planet) -> Bool {
+        lhs.mass == rhs.mass
+          && lhs.orbit == rhs.orbit
+          && lhs.rotation == rhs.rotation
+      }
+    }
+
+    /// RIGHT: The `static func ==` implementation is generated by the `@Equatable` macro.
+    @Equatable
+    final class struct Planet: Equatable {
+      let mass: Double
+      let orbit: OrbitalElements
+      let rotation: Double
+    }
+    ```
+
+    </details>
+
 * <a id='void-type'></a>(<a href='#void-type'>link</a>) **Avoid using `()` as a type**. Prefer `Void`.
 
   <details>

Sources/AirbnbSwiftFormatTool/airbnb.swiftformat

@@ -67,6 +67,7 @@
 --rules redundantGet
 --rules redundantFileprivate
 --rules redundantRawValues
+--rules redundantEquatable
 --rules sortImports
 --rules sortDeclarations
 --rules strongifiedSelf