Add new prefer_key_path rule

.swiftlint.yml

@@ -37,6 +37,7 @@ disabled_rules:
  - no_grouping_extension
  - no_magic_numbers
  - one_declaration_per_file
+ - prefer_key_path
  - prefer_nimble
  - prefer_self_in_static_references
  - prefixed_toplevel_constant

CHANGELOG.md

@@ -61,6 +61,11 @@
  [SimplyDanny](https://github.com/SimplyDanny)
  [#70](https://github.com/realm/SwiftLint/issues/70)
 
+* Add new `prefer_key_path` rule that triggers when a trailing closure on a function 
+ call is only hosting a (chained) member access expression since the closure can be
+ replaced with a key path argument. 
+ [SimplyDanny](https://github.com/SimplyDanny)
+
 * Warn when `--fix` comes together with `--strict` or `--lenient` as only `--fix`
  takes effect then. 
  [SimplyDanny](https://github.com/SimplyDanny)

Source/SwiftLintBuiltInRules/Models/BuiltInRules.swift

@@ -146,6 +146,7 @@ public let builtInRules: [any Rule.Type] = [
  OverrideInExtensionRule.self,
  PatternMatchingKeywordsRule.self,
  PeriodSpacingRule.self,
+ PreferKeyPathRule.self,
  PreferNimbleRule.self,
  PreferSelfInStaticReferencesRule.self,
  PreferSelfTypeOverTypeOfSelfRule.self,

Source/SwiftLintBuiltInRules/Rules/Idiomatic/PreferKeyPathRule.swift

@@ -0,0 +1,71 @@
+import SwiftLintCore
+import SwiftSyntax
+
+@SwiftSyntaxRule
+struct PreferKeyPathRule: OptInRule {
+ var configuration = SeverityConfiguration<Self>(.warning)
+
+ static var description = RuleDescription(
+ identifier: "prefer_key_path",
+ name: "Prefer Key Path",
+ description: "Use a key path argument instead of a closure with property access",
+ kind: .idiomatic,
+ minSwiftVersion: .fiveDotTwo,
+ nonTriggeringExamples: [
+ Example("f {}"),
+ Example("f { $0 }"),
+ Example("f() { g() }"),
+ Example("f { a.b.c }"),
+ Example("f { a in a }"),
+ Example("f { a, b in a.b }"),
+ Example("f { (a, b) in a.b }")
+ ],
+ triggeringExamples: [
+ Example("f ↓{ $0.a }"),
+ Example("f ↓{ a in a.b }"),
+ Example("f ↓{ a in a.b.c }"),
+ Example("f ↓{ (a: A) in a.b }"),
+ Example("f ↓{ (a b: A) in b.c }"),
+ Example("f ↓{ $0.0.a }"),
+ Example("f(a: ↓{ $0.b })"),
+ Example("f { 1 } a: ↓{ $0.b }"),
+ Example("let f: (Int) -> Int = ↓{ $0.bigEndian }")
+ ]
+ )
+}
+
+private extension PreferKeyPathRule {
+ final class Visitor: ViolationsSyntaxVisitor<ConfigurationType> {
+ override func visitPost(_ node: ClosureExprSyntax) {
+ if case let .expr(expr) = node.statements.onlyElement?.item,
+ expr.accesses(identifier: node.onlyParameter) {
+ violations.append(node.positionAfterSkippingLeadingTrivia)
+ }
+ }
+ }
+}
+
+private extension ExprSyntax {
+ func accesses(identifier: String?) -> Bool {
+ if let base = `as`(MemberAccessExprSyntax.self)?.base {
+ if let declRef = base.as(DeclReferenceExprSyntax.self) {
+ return declRef.baseName.text == identifier ?? "$0"
+ }
+ return base.accesses(identifier: identifier)
+ }
+ return false
+ }
+}
+
+private extension ClosureExprSyntax {
+ var onlyParameter: String? {
+ switch signature?.parameterClause {
+ case let .simpleInput(params):
+ return params.onlyElement?.name.text
+ case let .parameterClause(params):
+ let param = params.parameters.onlyElement
+ return param?.secondName?.text ?? param?.firstName.text
+ case nil: return nil
+ }
+ }
+}

Tests/GeneratedTests/GeneratedTests.swift

@@ -865,6 +865,12 @@ final class PeriodSpacingRuleGeneratedTests: SwiftLintTestCase {
  }
 }
 
+final class PreferKeyPathRuleGeneratedTests: SwiftLintTestCase {
+ func testWithDefaultConfiguration() {
+ verifyRule(PreferKeyPathRule.description)
+ }
+}
+
 final class PreferNimbleRuleGeneratedTests: SwiftLintTestCase {
  func testWithDefaultConfiguration() {
  verifyRule(PreferNimbleRule.description)

Tests/IntegrationTests/default_rule_configurations.yml

@@ -393,6 +393,8 @@ pattern_matching_keywords:
  severity: warning
 period_spacing:
  severity: warning
+prefer_key_path:
+ severity: warning
 prefer_nimble:
  severity: warning
 prefer_self_in_static_references: