SwiftLint/Source/SwiftLintFramework/Rules/Performance/EmptyCollectionLiteralRule.swift

import SwiftSyntax

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

    public init() {}

    public static let description = RuleDescription(
        identifier: "empty_collection_literal",
        name: "Empty Collection Literal",
        description: "Prefer checking `isEmpty` over comparing collection to an empty array or dictionary literal.",
        kind: .performance,
        nonTriggeringExamples: [
            Example("myArray = []"),
            Example("myArray.isEmpty"),
            Example("!myArray.isEmpty"),
            Example("myDict = [:]")
        ],
        triggeringExamples: [
            Example("myArray↓ == []"),
            Example("myArray↓ != []"),
            Example("myArray↓ == [ ]"),
            Example("myDict↓ == [:]"),
            Example("myDict↓ != [:]"),
            Example("myDict↓ == [: ]"),
            Example("myDict↓ == [ :]"),
            Example("myDict↓ == [ : ]")
        ]
    )

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

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

        override func visitPost(_ node: TokenSyntax) {
            guard
                node.tokenKind.isEqualityComparison,
                let violationPosition = node.previousToken?.endPositionBeforeTrailingTrivia,
                let expectedLeftSquareBracketToken = node.nextToken,
                expectedLeftSquareBracketToken.tokenKind == .leftSquareBracket,
                let expectedColonToken = expectedLeftSquareBracketToken.nextToken,
                expectedColonToken.tokenKind == .colon || expectedColonToken.tokenKind == .rightSquareBracket
            else {
                return
            }

            violationPositions.append(violationPosition)
        }
    }
}

private extension TokenKind {
    var isEqualityComparison: Bool {
        self == .spacedBinaryOperator("==") ||
            self == .spacedBinaryOperator("!=") ||
            self == .unspacedBinaryOperator("==")
    }
}