From 7d6f6cfa17ee484bc3cb288ddcba0a4cfcf64a80 Mon Sep 17 00:00:00 2001
From: Muhammad Zeeshan <mzeeshanid@yahoo.com>
Date: Sat, 9 Dec 2023 15:11:25 +0500
Subject: [PATCH] Add new `one_declaration_per_file` rule (#5377)

---
 .swiftlint.yml                                |  1 +
 CHANGELOG.md                                  |  6 ++
 .../Models/BuiltInRules.swift                 |  1 +
 .../Idiomatic/OneDelarationPerFileRule.swift  | 75 +++++++++++++++++++
 Tests/GeneratedTests/GeneratedTests.swift     |  6 ++
 5 files changed, 89 insertions(+)
 create mode 100644 Source/SwiftLintBuiltInRules/Rules/Idiomatic/OneDelarationPerFileRule.swift

diff --git a/.swiftlint.yml b/.swiftlint.yml
index 965ef4afd..d58e73904 100644
--- a/.swiftlint.yml
+++ b/.swiftlint.yml
@@ -38,6 +38,7 @@ disabled_rules:
   - no_fallthrough_only
   - no_grouping_extension
   - no_magic_numbers
+  - one_declaration_per_file
   - prefer_nimble
   - prefer_self_in_static_references
   - prefixed_toplevel_constant
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 599240bfb..6502094eb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,12 @@
 
 #### Enhancements
 
+* Add new `one_declaration_per_file` rule that allows only a
+  single class/struct/enum/protocol declaration per file.
+  Extensions are an exception; more than one is allowed.  
+  [Muhammad Zeeshan](https://github.com/mzeeshanid)
+  [#2802](https://github.com/realm/SwiftLint/issues/2802)
+
 * Rewrite the following rules with SwiftSyntax:
   * `identifier_name`
   * `let_var_whitespace`
diff --git a/Source/SwiftLintBuiltInRules/Models/BuiltInRules.swift b/Source/SwiftLintBuiltInRules/Models/BuiltInRules.swift
index 63749cbd0..4615c17ca 100644
--- a/Source/SwiftLintBuiltInRules/Models/BuiltInRules.swift
+++ b/Source/SwiftLintBuiltInRules/Models/BuiltInRules.swift
@@ -135,6 +135,7 @@ public let builtInRules: [any Rule.Type] = [
     NotificationCenterDetachmentRule.self,
     NumberSeparatorRule.self,
     ObjectLiteralRule.self,
+    OneDelarationPerFileRule.self,
     OpeningBraceRule.self,
     OperatorFunctionWhitespaceRule.self,
     OperatorUsageWhitespaceRule.self,
diff --git a/Source/SwiftLintBuiltInRules/Rules/Idiomatic/OneDelarationPerFileRule.swift b/Source/SwiftLintBuiltInRules/Rules/Idiomatic/OneDelarationPerFileRule.swift
new file mode 100644
index 000000000..82b918e00
--- /dev/null
+++ b/Source/SwiftLintBuiltInRules/Rules/Idiomatic/OneDelarationPerFileRule.swift
@@ -0,0 +1,75 @@
+import SwiftSyntax
+
+@SwiftSyntaxRule
+struct OneDelarationPerFileRule: OptInRule {
+    var configuration = SeverityConfiguration<Self>(.warning)
+
+    static let description = RuleDescription(
+        identifier: "one_declaration_per_file",
+        name: "One Declaration per File",
+        description: "Only a single declaration is allowed in a file",
+        kind: .idiomatic,
+        nonTriggeringExamples: [
+            Example("""
+                    class Foo {}
+                    """),
+            Example("""
+                    class Foo {}
+                    extension Foo {}
+                    """),
+            Example("""
+                    struct S {
+                        struct N {}
+                    }
+                    """)
+        ],
+        triggeringExamples: [
+            Example("""
+                    class Foo {}
+                    ↓class Bar {}
+                    """),
+            Example("""
+                    protocol Foo {}
+                    ↓enum Bar {}
+                    """),
+            Example("""
+                    struct Foo {}
+                    ↓struct Bar {}
+                    """)
+        ]
+    )
+}
+
+private extension OneDelarationPerFileRule {
+    final class Visitor: ViolationsSyntaxVisitor<ConfigurationType> {
+        private var declarationVisited = false
+        override var skippableDeclarations: [any DeclSyntaxProtocol.Type] { .all }
+
+        override func visitPost(_ node: ActorDeclSyntax) {
+            appendViolationIfNeeded(node: node.actorKeyword)
+        }
+
+        override func visitPost(_ node: ClassDeclSyntax) {
+            appendViolationIfNeeded(node: node.classKeyword)
+        }
+
+        override func visitPost(_ node: StructDeclSyntax) {
+            appendViolationIfNeeded(node: node.structKeyword)
+        }
+
+        override func visitPost(_ node: EnumDeclSyntax) {
+            appendViolationIfNeeded(node: node.enumKeyword)
+        }
+
+        override func visitPost(_ node: ProtocolDeclSyntax) {
+            appendViolationIfNeeded(node: node.protocolKeyword)
+        }
+
+        func appendViolationIfNeeded(node: TokenSyntax) {
+            if declarationVisited {
+                violations.append(node.positionAfterSkippingLeadingTrivia)
+            }
+            declarationVisited = true
+        }
+    }
+}
diff --git a/Tests/GeneratedTests/GeneratedTests.swift b/Tests/GeneratedTests/GeneratedTests.swift
index 919fd63dd..078c6f614 100644
--- a/Tests/GeneratedTests/GeneratedTests.swift
+++ b/Tests/GeneratedTests/GeneratedTests.swift
@@ -800,6 +800,12 @@ class ObjectLiteralRuleGeneratedTests: SwiftLintTestCase {
     }
 }
 
+class OneDelarationPerFileRuleGeneratedTests: SwiftLintTestCase {
+    func testWithDefaultConfiguration() {
+        verifyRule(OneDelarationPerFileRule.description)
+    }
+}
+
 class OpeningBraceRuleGeneratedTests: SwiftLintTestCase {
     func testWithDefaultConfiguration() {
         verifyRule(OpeningBraceRule.description)