Add testable_import opt-in rule

Source/SwiftLintFramework/Models/PrimaryRuleList.swift

@@ -178,6 +178,7 @@ public let primaryRuleList = RuleList(rules: [
     SwitchCaseOnNewlineRule.self,
     SyntacticSugarRule.self,
     TestCaseAccessibilityRule.self,
+    TestableImportRule.self,
     TodoRule.self,
     ToggleBoolRule.self,
     TrailingClosureRule.self,

Source/SwiftLintFramework/Rules/Lint/TestableImportRule.swift

@@ -0,0 +1,91 @@
+import SourceKittenFramework
+import SwiftSyntax
+
+public struct TestableImportRule: ConfigurationProviderRule, OptInRule, AutomaticTestableRule {
+    public var configuration = SeverityConfiguration(.error)
+
+    public init() {}
+
+    public static let description = RuleDescription(
+        identifier: "testable_import",
+        name: "Testable Import",
+        description: "@testable import should only be used in test files",
+        kind: .lint,
+        nonTriggeringExamples: [
+            Example("import Foo"),
+            Example("""
+            @testable import Foo
+            import XCTest
+            """),
+            Example("""
+            @testable import Foo
+            import class XCTest.XCTestCase
+            """),
+            Example("""
+            @testable import Foo
+            import TestUtils
+
+            class FooTests: XCTestCase {}
+            """),
+        ],
+        triggeringExamples: [
+            Example("↓@testable import Foo"),
+        ]
+    )
+
+    public func validate(file: SwiftLintFile) -> [StyleViolation] {
+        guard let tree = file.syntaxTree else { return [] }
+
+        let visitor = TestableImportRuleVisitor()
+        visitor.walk(tree)
+
+        if visitor.isTestFile {
+            return []
+        }
+
+        return visitor.positions.map { position in
+            StyleViolation(ruleDescription: Self.description,
+                           severity: configuration.severity,
+                           location: Location(file: file, byteOffset: ByteCount(position)))
+        }
+    }
+}
+
+private final class TestableImportRuleVisitor: SyntaxVisitor {
+    private(set) var positions: [AbsolutePosition] = []
+    private(set) var isTestFile = false
+
+    override func visitPost(_ node: ImportDeclSyntax) {
+        if node.isTestableImport {
+            positions.append(node.positionAfterSkippingLeadingTrivia)
+        }
+
+        let testImports: Set = ["XCTest", "Quick", "Nimble"]
+        let components = node.path.withoutTrivia().map(\.name.text)
+        if !testImports.isDisjoint(with: components) {
+            isTestFile = true
+        }
+    }
+
+    override func visitPost(_ node: ClassDeclSyntax) {
+        let inheritedTypes = node.inheritanceClause?.inheritedTypeCollection.map {
+            $0.withoutTrivia().typeName.description
+        } ?? []
+        let testClasses: Set = ["XCTestCase", "QuickSpec"]
+        if !testClasses.isDisjoint(with: inheritedTypes) {
+            isTestFile = true
+        }
+    }
+}
+
+private extension ImportDeclSyntax {
+    var isTestableImport: Bool {
+        guard let attributes = self.attributes else {
+            return false
+        }
+
+        return attributes.contains { syntax in
+            syntax.as(AttributeSyntax.self)?.attributeName.tokenKind == .identifier("testable")
+        }
+    }
+}

Tests/SwiftLintFrameworkTests/AutomaticRuleTests.generated.swift

@@ -785,6 +785,12 @@ class TestCaseAccessibilityRuleTests: XCTestCase {
     }
 }
 
+class TestableImportRuleTests: XCTestCase {
+    func testWithDefaultConfiguration() {
+        verifyRule(TestableImportRule.description)
+    }
+}
+
 class ToggleBoolRuleTests: XCTestCase {
     func testWithDefaultConfiguration() {
         verifyRule(ToggleBoolRule.description)