SwiftLint/Source/SwiftLintBuiltInRules/Rules/Idiomatic/PreferConditionListRule.swift

import SwiftLintCore
import SwiftSyntax

@SwiftSyntaxRule(foldExpressions: true, explicitRewriter: true, optIn: true)
struct PreferConditionListRule: Rule {
    var configuration = SeverityConfiguration<Self>(.warning)

    static let description = RuleDescription(
        identifier: "prefer_condition_list",
        name: "Prefer Condition List",
        description: "Prefer a condition list over chaining conditions with '&&'",
        rationale: """
            Instead of chaining conditions with `&&`, use a condition list to separate conditions with commas, that is,
            use

            ```
            if a, b {}
            ```

            instead of

            ```
            if a && b {}
            ```

            Using a condition list improves readability and makes it easier to add or remove conditions in the future.
            It also allows for better formatting and alignment of conditions. All in all, it's the idiomatic way to
            write conditions in Swift.

            Since function calls with trailing closures trigger a warning in the Swift compiler when used in
            conditions, this rule makes sure to wrap such expressions in parentheses when transforming them to
            condition list elements. The scope of the parentheses is limited to the function call itself.
            """,
        kind: .idiomatic,
        nonTriggeringExamples: [
            Example("if a, b {}"),
            Example("guard a || b && c {}"),
            Example("if a && b || c {}"),
            Example("let result = a && b"),
            Example("repeat {} while a && b"),
            Example("if (f {}) {}"),
            Example("if f {} {}"),
        ],
        triggeringExamples: [
            Example("if a ↓&& b {}"),
            Example("if a ↓&& b ↓&& c {}"),
            Example("while a ↓&& b {}"),
            Example("guard a ↓&& b {}"),
            Example("guard (a || b) ↓&& c {}"),
            Example("if a ↓&& (b && c) {}"),
            Example("guard a ↓&& b ↓&& c else {}"),
            Example("if (a ↓&& b) {}"),
            Example("if (a ↓&& f {}) {}"),
        ],
        corrections: [
            Example("if a && b {}"):
                Example("if a, b {}"),
            Example("""
                if a &&
                   b {}
                """): Example("""
                if a,
                   b {}
                """),
            Example("guard a && b && c else {}"):
                Example("guard a, b, c else {}"),
            Example("while a && b {}"):
                Example("while a, b {}"),
            Example("if a && b || c {}"):
                Example("if a && b || c {}"),
            Example("if (a && b) {}"):
                Example("if a, b {}"),
            Example("if a && (b && c) {}"):
                Example("if a, b, c {}"),
            Example("if (a && b) && c {}"):
                Example("if a, b, c {}"),
            Example("if (a && b), c {}"):
                Example("if a, b, c {}"),
            Example("guard (a || b) ↓&& c {}"):
                Example("guard a || b, c {}"),
            Example("if a && (b || c) {}"):
                Example("if a, b || c {}"),
            Example("if (a ↓&& f {}) {}"):
                Example("if a, (f {}) {}"),
            Example("if a ↓&& (b || f {}) {}"):
                Example("if a, b || (f {}) {}"),
            Example("if a ↓&& !f {} {}"):
                Example("if a, !(f {}) {}"),
        ]
    )
}

private extension PreferConditionListRule {
    final class Visitor: ViolationsSyntaxVisitor<ConfigurationType> {
        override func visitPost(_ node: ConditionElementSyntax) {
            if case let .expression(expr) = node.condition {
                collectViolations(for: expr)
            }
        }

        private func collectViolations(for expr: ExprSyntax) {
            if let opExpr = expr.unwrap.as(InfixOperatorExprSyntax.self),
               let opToken = opExpr.operator.as(BinaryOperatorExprSyntax.self)?.operator,
               opToken.text == "&&" {
                violations.append(opToken.positionAfterSkippingLeadingTrivia)
                collectViolations(for: opExpr.leftOperand) // Expressions are left-recursive.
            }
        }
    }

    private final class Rewriter: ViolationsSyntaxRewriter<ConfigurationType> {
        override func visit(_ node: ConditionElementListSyntax) -> ConditionElementListSyntax {
            var elements = Array(node)
            var modifiedIndices = Set<Int>()
            var index = 0

            while index < elements.count {
                let element = elements[index]
                guard case let .expression(expr) = element.condition else {
                    index += 1
                    continue
                }
                if let opExpr = expr.as(InfixOperatorExprSyntax.self),
                   let opToken = opExpr.operator.as(BinaryOperatorExprSyntax.self)?.operator,
                   opToken.text == "&&" {
                    numberOfCorrections += 1

                    elements[index] = ConditionElementSyntax(
                        condition: .expression(opExpr.leftOperand.with(\.trailingTrivia, [])),
                        trailingComma: .commaToken(),
                        trailingTrivia: opToken.trailingTrivia
                    )
                    modifiedIndices.insert(index)

                    elements.insert(
                        ConditionElementSyntax(
                            condition: .expression(opExpr.rightOperand.with(\.trailingTrivia, [])),
                            trailingComma: index == elements.count - 1 ? nil : .commaToken(),
                            trailingTrivia: .space
                        ),
                        at: index + 1
                    )
                    modifiedIndices.insert(index + 1)
                    // Don't increment the index to re-evaluate `elements[index]`.
                } else if expr.is(TupleExprSyntax.self) {
                    // Unwrap parenthesized expression and repeat the loop for the inner expression (i.e. without
                    // incrementing the index).
                    let unwrappedExpr = expr.unwrap
                    elements[index] = element.with(\.condition, .expression(unwrappedExpr))
                    if unwrappedExpr != expr {
                        modifiedIndices.insert(index)
                    }
                } else {
                    index += 1
                }
            }
            for (index, element) in elements.enumerated() where modifiedIndices.contains(index) {
                if case let .expression(expr) = element.condition {
                    // If the expression contains function calls with trailing closures, we need to wrap them in
                    // parentheses. That might not be exactly how the author created the expression, but it is
                    // necessary to ensure no compiler warning appears after the transformations.
                    elements[index] = element.with(
                        \.condition,
                        .expression(ParenthesizedTrailingClosureRewriter().visit(expr))
                            .with(\.leadingTrivia, expr.leadingTrivia)
                            .with(\.trailingTrivia, expr.trailingTrivia)
                    )
                }
            }
            return super.visit(ConditionElementListSyntax(elements))
        }
    }
}

private extension ExprSyntax {
    var unwrap: ExprSyntax {
        `as`(TupleExprSyntax.self)?.elements.onlyElement?.expression
            .with(\.leadingTrivia, leadingTrivia)
            .with(\.trailingTrivia, trailingTrivia)
        ?? self
    }
}

private final class ParenthesizedTrailingClosureRewriter: SyntaxRewriter {
    override func visitAny(_ node: Syntax) -> Syntax? {
        if let opToken = node.as(InfixOperatorExprSyntax.self)?.operator.as(BinaryOperatorExprSyntax.self)?.operator,
           ["&&", "||"].contains(opToken.text) {
            nil
        } else if let opToken = node.as(PrefixOperatorExprSyntax.self)?.operator,
                  ["!"].contains(opToken.text) {
            nil
        } else if node.is(FunctionCallExprSyntax.self) {
            nil
        } else {
            node
        }
    }

    override func visit(_ node: FunctionCallExprSyntax) -> ExprSyntax {
        if node.trailingClosure != nil || node.additionalTrailingClosures.isNotEmpty {
            return ExprSyntax(TupleExprSyntax(
                elements: LabeledExprListSyntax([
                    LabeledExprSyntax(label: nil, expression: node.with(\.trailingTrivia, []))
                ])
            ))
            .with(\.leadingTrivia, node.leadingTrivia)
            .with(\.trailingTrivia, node.trailingTrivia)
        }
        return super.visit(node)
    }
}