SwiftLint/Source/SwiftLintBuiltInRules/Rules/Style/ImplicitGetterRule.swift

import SwiftSyntax

@SwiftSyntaxRule
struct ImplicitGetterRule: Rule {
    var configuration = SeverityConfiguration<Self>(.warning)

    static let description = RuleDescription(
        identifier: "implicit_getter",
        name: "Implicit Getter",
        description: "Computed read-only properties and subscripts should avoid using the get keyword.",
        kind: .style,
        nonTriggeringExamples: ImplicitGetterRuleExamples.nonTriggeringExamples,
        triggeringExamples: ImplicitGetterRuleExamples.triggeringExamples
    )
}

private enum ViolationKind {
    case `subscript`, property

    var violationDescription: String {
        switch self {
        case .subscript:
            return "Computed read-only subscripts should avoid using the get keyword"
        case .property:
            return "Computed read-only properties should avoid using the get keyword"
        }
    }
}

private extension ImplicitGetterRule {
    final class Visitor: ViolationsSyntaxVisitor<ConfigurationType> {
        override func visitPost(_ node: AccessorBlockSyntax) {
            guard node.accessorsList.count == 1,
                  let getAccessor = node.getAccessor,
                  getAccessor.effectSpecifiers == nil,
                  getAccessor.modifier == nil,
                  getAccessor.attributes.isEmpty == true,
                  getAccessor.body != nil else {
                return
            }

            let kind: ViolationKind = node.parent?.as(SubscriptDeclSyntax.self) == nil ? .property : .subscript
            violations.append(
                ReasonedRuleViolation(
                    position: getAccessor.positionAfterSkippingLeadingTrivia,
                    reason: kind.violationDescription
                )
            )
        }
    }
}