Add ability to focus on a specific test example

CHANGELOG.md

@@ -48,6 +48,10 @@
   environments, such as in Swift Package Manager plugins.  
   [Juozas Valancius](https://github.com/juozasvalancius)
 
+* Add ability to run only one (focused) example
+  [PaulTaykalo](https://github.com/PaulTaykalo)
+  [#3911](https://github.com/realm/SwiftLint/issues/3911)
+
 #### Bug Fixes
 
 * Extend `class_delegate_protocol` to correctly identify cases with the protocol

CONTRIBUTING.md

@@ -55,7 +55,6 @@ $ xcodebuild -scheme swiftlint test
 $ swift test
 $ make docker_test
 ```
-
 ## Rules
 
 New rules should be added in the `Source/SwiftLintFramework/Rules` directory.
@@ -72,6 +71,19 @@ over time. This way adding a unit test for your new Rule is just a matter of
 adding a test case in `RulesTests.swift` which simply calls
 `verifyRule(YourNewRule.description)`.
 
+For debugging purposes examples can be marked as `focused`. If there are any
+focused examples found, then only those will be run when running tests.
+```
+nonTriggeringExamples: [
+    Example("let x: [Int]"),
+    Example("let x: [Int: String]").focused()   // only this one will be run in tests
+],
+triggeringExamples: [
+    Example("let x: ↓Array<String>"),
+    Example("let x: ↓Dictionary<Int, String>")
+]
+```
+
 ### `ConfigurationProviderRule`
 
 If your rule supports user-configurable options via `.swiftlint.yml`, you can

Source/SwiftLintFramework/Models/Example.swift

@@ -32,6 +32,9 @@ public struct Example {
     /// pathological use cases which are indeed important to test but not helpful for understanding can be
     /// hidden from the documentation with this option.
     let excludeFromDocumentation: Bool
+
+    /// Specifies whether the test example should be the only example run during the current test case execution.
+    var isFocused: Bool
 }
 
 public extension Example {
@@ -55,6 +58,7 @@ public extension Example {
         self.file = file
         self.line = line
         self.excludeFromDocumentation = excludeFromDocumentation
+        self.isFocused = false
     }
 
     /// Returns the same example, but with the `code` that is passed in
@@ -69,6 +73,13 @@ public extension Example {
     func removingViolationMarkers() -> Example {
         return with(code: code.replacingOccurrences(of: "↓", with: ""))
     }
+
+    /// Makes the current example focused.
+    func focused() -> Example {
+        var new = self
+        new.isFocused = true
+        return new
+    }
 }
 
 extension Example: Hashable {

Tests/SwiftLintFrameworkTests/TestHelpers.swift

@@ -356,7 +356,11 @@ extension XCTestCase {
                            requiresFileOnDisk: ruleDescription.requiresFileOnDisk,
                            file: file, line: line)
         }
+        func makeViolations(_ example: Example) -> [StyleViolation] {
+            return violations(example, config: config, requiresFileOnDisk: ruleDescription.requiresFileOnDisk)
+        }
 
+        let ruleDescription = ruleDescription.focused()
         let triggers = ruleDescription.triggeringExamples
         let nonTriggers = ruleDescription.nonTriggeringExamples
         verify(triggers: triggers, nonTriggers: nonTriggers)
@@ -369,10 +373,6 @@ extension XCTestCase {
             verify(triggers: triggers.map(addShebang), nonTriggers: nonTriggers.map(addShebang))
         }
 
-        func makeViolations(_ example: Example) -> [StyleViolation] {
-            return violations(example, config: config, requiresFileOnDisk: ruleDescription.requiresFileOnDisk)
-        }
-
         // Comment doesn't violate
         if !skipCommentTests {
             XCTAssertEqual(
@@ -401,6 +401,8 @@ extension XCTestCase {
 
     func verifyCorrections(_ ruleDescription: RuleDescription, config: Configuration,
                            disableCommands: [String], testMultiByteOffsets: Bool) {
+        let ruleDescription = ruleDescription.focused()
+
         parserDiagnosticsDisabledForTests = true
 
         // corrections
@@ -509,3 +511,32 @@ extension XCTestCase {
         }
     }
 }
+
+private struct FocusedRuleDescription {
+    let nonTriggeringExamples: [Example]
+    let triggeringExamples: [Example]
+    let corrections: [Example: Example]
+
+    init(rule: RuleDescription) {
+        let nonTriggering = rule.nonTriggeringExamples.filter(\.isFocused)
+        let triggering = rule.triggeringExamples.filter(\.isFocused)
+        let corrections = rule.corrections.filter { _, value in value.isFocused }
+        let anyFocused = nonTriggering.isNotEmpty || triggering.isNotEmpty || corrections.isNotEmpty
+
+        if anyFocused {
+            self.nonTriggeringExamples = nonTriggering
+            self.triggeringExamples = triggering
+            self.corrections = corrections
+        } else {
+            self.nonTriggeringExamples = rule.nonTriggeringExamples
+            self.triggeringExamples = rule.triggeringExamples
+            self.corrections = rule.corrections
+        }
+    }
+}
+
+private extension RuleDescription {
+    func focused() -> FocusedRuleDescription {
+        return FocusedRuleDescription(rule: self)
+    }
+}