Add new `direct_return` opt-in rule (#4717)

CHANGELOG.md

@@ -40,6 +40,10 @@
   [Moly](https://github.com/kyounh12)
   [#4655](https://github.com/realm/SwiftLint/pull/4655)
 
+* Add new `direct_return` rule that triggers on `return` statements returning
+  variables that have been declared in the statement before only.  
+  [SimplyDanny](https://github.com/SimplyDanny)
+
 * Add `period_spacing` opt-in rule that checks periods are not followed
   by 2 or more spaces in comments.  
   [Julioacarrettoni](https://github.com/Julioacarrettoni)

Source/SwiftLintFramework/Models/PrimaryRuleList.swift

@@ -1,4 +1,4 @@
-// Generated using Sourcery 1.9.2 — https://github.com/krzysztofzablocki/Sourcery
+// Generated using Sourcery 2.0.0 — https://github.com/krzysztofzablocki/Sourcery
 // DO NOT EDIT
 
 /// The rule list containing all available rules built into SwiftLint.
@@ -35,6 +35,7 @@ let builtInRules: [Rule.Type] = [
     CustomRules.self,
     CyclomaticComplexityRule.self,
     DeploymentTargetRule.self,
+    DirectReturnRule.self,
     DiscardedNotificationCenterObserverRule.self,
     DiscouragedAssertRule.self,
     DiscouragedDirectInitRule.self,

Source/SwiftLintFramework/Rules/Style/DirectReturnRule.swift

@@ -0,0 +1,232 @@
+import SwiftSyntax
+
+struct DirectReturnRule: SwiftSyntaxCorrectableRule, ConfigurationProviderRule, OptInRule {
+    var configuration = SeverityConfiguration(.warning)
+
+    init() {}
+
+    static var description = RuleDescription(
+        identifier: "direct_return",
+        name: "Direct Return",
+        description: "Directly return the expression instead of assigning it to a variable first",
+        kind: .style,
+        nonTriggeringExamples: [
+            Example("""
+                func f() -> Int {
+                    let b = 2
+                    let a = 1
+                    return b
+                }
+            """),
+            Example("""
+                struct S {
+                    var a: Int {
+                        var b = 1
+                        b = 2
+                        return b
+                    }
+                }
+            """),
+            Example("""
+                func f() -> Int {
+                    let b = 2
+                    f()
+                    return b
+                }
+            """),
+            Example("""
+                func f() -> Int {
+                    { i in
+                        let b = 2
+                        return i
+                    }(1)
+                }
+            """)
+        ],
+        triggeringExamples: [
+            Example("""
+                func f() -> Int {
+                    let ↓b = 2
+                    return b
+                }
+            """),
+            Example("""
+                struct S {
+                    var a: Int {
+                        var ↓b = 1
+                        // comment
+                        return b
+                    }
+                }
+            """),
+            Example("""
+                func f() -> Bool {
+                    let a = 1, ↓b = true
+                    return b
+                }
+            """),
+            Example("""
+                func f() -> Int {
+                    { _ in
+                        let ↓b = 2
+                        return b
+                    }(1)
+                }
+            """),
+            Example("""
+                func f(i: Int) -> Int {
+                    if i > 1 {
+                        let ↓a = 2
+                        return a
+                    } else {
+                        let ↓b = 2, a = 1
+                        return b
+                    }
+                }
+            """)
+        ],
+        corrections: [
+            Example("""
+                func f() -> Int {
+                    let b = 2
+                    return b
+                }
+            """): Example("""
+                func f() -> Int {
+                    return 2
+                }
+            """),
+            Example("""
+                struct S {
+                    var a: Int {
+                        var b = 2 > 1
+                            ? f()
+                            : 1_000
+                        // comment
+                        return b
+                    }
+                    func f() -> Int { 1 }
+                }
+            """): Example("""
+                struct S {
+                    var a: Int {
+                        // comment
+                        return 2 > 1
+                            ? f()
+                            : 1_000
+                    }
+                    func f() -> Int { 1 }
+                }
+            """),
+            Example("""
+                func f() -> Bool {
+                    let a = 1, b = true
+                    return b
+                }
+            """): Example("""
+                func f() -> Bool {
+                    let a = 1
+                    return true
+                }
+            """),
+            Example("""
+                func f() -> Int {
+                    { _ in
+                        let b = 2
+                        return b
+                    }(1)
+                }
+            """): Example("""
+                func f() -> Int {
+                    { _ in
+                        return 2
+                    }(1)
+                }
+            """)
+        ]
+    )
+
+    func makeVisitor(file: SwiftLintFile) -> ViolationsSyntaxVisitor {
+        Visitor(viewMode: .sourceAccurate)
+    }
+
+    func makeRewriter(file: SwiftLintFile) -> ViolationsSyntaxRewriter? {
+        Rewriter(
+            locationConverter: file.locationConverter,
+            disabledRegions: disabledRegions(file: file)
+        )
+    }
+}
+
+private class Visitor: ViolationsSyntaxVisitor {
+    override var skippableDeclarations: [DeclSyntaxProtocol.Type] { [ProtocolDeclSyntax.self] }
+
+    override func visitPost(_ statements: CodeBlockItemListSyntax) {
+        if let (binding, _) = statements.violation {
+            violations.append(binding.positionAfterSkippingLeadingTrivia)
+        }
+    }
+}
+
+private extension CodeBlockItemListSyntax {
+    var violation: (PatternBindingSyntax, ReturnStmtSyntax)? {
+        guard count >= 2, let last = last?.item,
+              let returnStmt = last.as(ReturnStmtSyntax.self),
+              let identifier = returnStmt.expression?.as(IdentifierExprSyntax.self)?.identifier.text,
+              let varDecl = dropLast().last?.item.as(VariableDeclSyntax.self) else {
+            return nil
+        }
+        let binding = varDecl.bindings.first {
+            $0.pattern.as(IdentifierPatternSyntax.self)?.identifier.text == identifier
+        }
+        if let binding {
+            return (binding, returnStmt)
+        }
+        return nil
+    }
+}
+private class Rewriter: SyntaxRewriter, ViolationsSyntaxRewriter {
+    private(set) var correctionPositions: [AbsolutePosition] = []
+    let locationConverter: SourceLocationConverter
+    let disabledRegions: [SourceRange]
+
+    init(locationConverter: SourceLocationConverter, disabledRegions: [SourceRange]) {
+        self.locationConverter = locationConverter
+        self.disabledRegions = disabledRegions
+    }
+
+    override func visit(_ statements: CodeBlockItemListSyntax) -> CodeBlockItemListSyntax {
+        guard let (binding, returnStmt) = statements.violation,
+              !returnStmt.isContainedIn(regions: disabledRegions, locationConverter: locationConverter),
+              let bindingList = binding.parent?.as(PatternBindingListSyntax.self),
+              let varDecl = bindingList.parent?.as(VariableDeclSyntax.self),
+              let initExpression = binding.initializer?.value else {
+            return super.visit(statements)
+        }
+        correctionPositions.append(binding.positionAfterSkippingLeadingTrivia)
+        var newStmtList = Array(statements.dropLast(2))
+        let newBindingList = bindingList
+            .filter { $0 != binding }
+            .enumerated()
+            .map { index, item in
+                if index == bindingList.count - 2 {
+                    return item.withTrailingComma(false)
+                }
+                return item
+            }
+        if newBindingList.isNotEmpty {
+            newStmtList.append(CodeBlockItemSyntax(
+                item: .decl(DeclSyntax(varDecl.withBindings(PatternBindingListSyntax(newBindingList))))
+            ))
+            newStmtList.append(CodeBlockItemSyntax(
+                item: .stmt(StmtSyntax(returnStmt.withExpression(initExpression)))
+            ))
+        } else {
+            let leadingTrivia = (binding.trailingTrivia ?? .zero) + (returnStmt.leadingTrivia ?? .zero)
+            newStmtList.append(CodeBlockItemSyntax(
+                item: .stmt(StmtSyntax(returnStmt.withExpression(initExpression).withLeadingTrivia(leadingTrivia)))
+            ))
+        }
+        return super.visit(CodeBlockItemListSyntax(newStmtList))
+    }
+}

Tests/GeneratedTests/GeneratedTests.swift

@@ -1,4 +1,4 @@
-// Generated using Sourcery 1.9.2 — https://github.com/krzysztofzablocki/Sourcery
+// Generated using Sourcery 2.0.0 — https://github.com/krzysztofzablocki/Sourcery
 // DO NOT EDIT
 @_spi(TestHelper)
 @testable import SwiftLintFramework
@@ -193,6 +193,12 @@ class DeploymentTargetRuleGeneratedTests: XCTestCase {
     }
 }
 
+class DirectReturnRuleGeneratedTests: XCTestCase {
+    func testWithDefaultConfiguration() {
+        verifyRule(DirectReturnRule.description)
+    }
+}
+
 class DiscardedNotificationCenterObserverRuleGeneratedTests: XCTestCase {
     func testWithDefaultConfiguration() {
         verifyRule(DiscardedNotificationCenterObserverRule.description)