Add new one_declaration_per_file rule (#5377)

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,

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
+        }
+    }
+}