SwiftLint/Source/SwiftLintFramework/Rules/Lint/IBInspectableInExtensionRule.swift

import SwiftSyntax

public struct IBInspectableInExtensionRule: ConfigurationProviderRule, SwiftSyntaxRule {
    public var configuration = SeverityConfiguration(.warning)

    public init() {}

    public static let description = RuleDescription(
        identifier: "ibinspectable_in_extension",
        name: "IBInspectable in Extension",
        description: "Extensions shouldn't add @IBInspectable properties.",
        kind: .lint,
        nonTriggeringExamples: [
            Example("""
            class Foo {
              @IBInspectable private var x: Int
            }
            """)
        ],
        triggeringExamples: [
            Example("""
            extension Foo {
              ↓@IBInspectable private var x: Int
            }
            """)
        ]
    )

    public func makeVisitor(file: SwiftLintFile) -> ViolationsSyntaxVisitor? {
        Visitor(viewMode: .sourceAccurate)
    }
}

private extension IBInspectableInExtensionRule {
    final class Visitor: SyntaxVisitor, ViolationsSyntaxVisitor {
        private(set) var violationPositions: [AbsolutePosition] = []

        override func visitPost(_ node: VariableDeclSyntax) {
            let containsIBInspectable = node.attributes?.contains { attribute in
                attribute.as(AttributeSyntax.self)?.attributeName.tokenKind == .identifier("IBInspectable")
            } ?? false

            if containsIBInspectable {
                violationPositions.append(node.positionAfterSkippingLeadingTrivia)
            }
        }

        override func visit(_ node: ClassDeclSyntax) -> SyntaxVisitorContinueKind {
            .skipChildren
        }

        override func visit(_ node: ProtocolDeclSyntax) -> SyntaxVisitorContinueKind {
            .skipChildren
        }

        override func visit(_ node: StructDeclSyntax) -> SyntaxVisitorContinueKind {
            .skipChildren
        }

        override func visit(_ node: EnumDeclSyntax) -> SyntaxVisitorContinueKind {
            .skipChildren
        }
    }
}