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

import SwiftSyntax

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

    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↓ == [ : ]"),
        ]
    )
}

private extension EmptyCollectionLiteralRule {
    final class Visitor: ViolationsSyntaxVisitor<ConfigurationType> {
        override func visitPost(_ node: TokenSyntax) {
            guard
                node.tokenKind.isEqualityComparison,
                let violationPosition = node.previousToken(viewMode: .sourceAccurate)?.endPositionBeforeTrailingTrivia,
                let expectedLeftSquareBracketToken = node.nextToken(viewMode: .sourceAccurate),
                expectedLeftSquareBracketToken.tokenKind == .leftSquare,
                let expectedColonToken = expectedLeftSquareBracketToken.nextToken(viewMode: .sourceAccurate),
                expectedColonToken.tokenKind == .colon || expectedColonToken.tokenKind == .rightSquare
            else {
                return
            }

            violations.append(violationPosition)
        }
    }
}