Add async_without_await rule

.swiftlint.yml

@@ -15,6 +15,7 @@ opt_in_rules:
   - all
 disabled_rules:
   - anonymous_argument_in_multiline_closure
+  - async_without_await
   - conditional_returns_on_newline
   - contrasted_opening_brace
   - convenience_type

CHANGELOG.md

@@ -13,6 +13,10 @@
 
 * Add Xcode command plugin allowing to run SwiftLint from within Xcode.  
   [SimplyDanny](https://github.com/SimplyDanny)
+
+* Add new `async_without_await` opt-in rule that checks if an `async` declaration contains at least one `await`.  
+  [Jan Kolarik](https://github.com/jkolarik-paylocity)
+  [#5082](https://github.com/realm/SwiftLint/issues/5082)
 
 #### Bug Fixes
 

Source/SwiftLintBuiltInRules/Models/BuiltInRules.swift

@@ -7,6 +7,7 @@ public let builtInRules: [any Rule.Type] = [
     AccessibilityTraitForButtonRule.self,
     AnonymousArgumentInMultilineClosureRule.self,
     ArrayInitRule.self,
+    AsyncWithoutAwaitRule.self,
     AttributeNameSpacingRule.self,
     AttributesRule.self,
     BalancedXCTestLifecycleRule.self,

Source/SwiftLintBuiltInRules/Rules/Lint/AsyncWithoutAwaitRule.swift

@@ -0,0 +1,158 @@
+import SwiftLintCore
+import SwiftSyntax
+
+@SwiftSyntaxRule
+struct AsyncWithoutAwaitRule: SwiftSyntaxCorrectableRule, OptInRule {
+    var configuration = SeverityConfiguration<Self>(.warning)
+
+    static let description = RuleDescription(
+        identifier: "async_without_await",
+        name: "Async Without Await",
+        description: "Declaration should not be async if it doesn't use await",
+        kind: .lint,
+        nonTriggeringExamples: AsyncWithoutAwaitRuleExamples.nonTriggeringExamples,
+        triggeringExamples: AsyncWithoutAwaitRuleExamples.triggeringExamples,
+        corrections: AsyncWithoutAwaitRuleExamples.corrections
+    )
+}
+private extension AsyncWithoutAwaitRule {
+    private struct FuncInfo {
+        var containsAwait = false
+        let asyncToken: TokenSyntax?
+
+        init(asyncToken: TokenSyntax?) {
+            self.asyncToken = asyncToken
+        }
+    }
+
+    final class Visitor: ViolationsSyntaxVisitor<ConfigurationType> {
+        private var functionScopes = Stack<FuncInfo>()
+        private var pendingAsync: TokenSyntax?
+
+        override func visit(_ node: FunctionDeclSyntax) -> SyntaxVisitorContinueKind {
+            guard node.body != nil else {
+                return .visitChildren
+            }
+
+            let asyncToken = node.signature.effectSpecifiers?.asyncSpecifier
+            functionScopes.push(.init(asyncToken: asyncToken))
+
+            return .visitChildren
+        }
+
+        override func visitPost(_ node: FunctionDeclSyntax) {
+            if node.body != nil {
+                checkViolation()
+            }
+        }
+
+        override func visit(_: ClosureExprSyntax) -> SyntaxVisitorContinueKind {
+            functionScopes.push(.init(asyncToken: pendingAsync))
+            pendingAsync = nil
+
+            return .visitChildren
+        }
+
+        override func visitPost(_: ClosureExprSyntax) {
+            checkViolation()
+        }
+
+        override func visitPost(_: AwaitExprSyntax) {
+            functionScopes.modifyLast {
+                $0.containsAwait = true
+            }
+        }
+
+        override func visitPost(_ node: FunctionTypeSyntax) {
+            if let asyncNode = node.effectSpecifiers?.asyncSpecifier {
+                pendingAsync = asyncNode
+            }
+        }
+
+        override func visit(_ node: AccessorDeclSyntax) -> SyntaxVisitorContinueKind {
+            guard node.body != nil else {
+                return .visitChildren
+            }
+
+            let asyncToken = node.effectSpecifiers?.asyncSpecifier
+            functionScopes.push(.init(asyncToken: asyncToken))
+
+            return .visitChildren
+        }
+
+        override func visitPost(_ node: AccessorDeclSyntax) {
+            if node.body != nil {
+                checkViolation()
+            }
+        }
+
+        override func visitPost(_: PatternBindingSyntax) {
+            pendingAsync = nil
+        }
+
+        override func visit(_ node: InitializerDeclSyntax) -> SyntaxVisitorContinueKind {
+            guard node.body != nil else {
+                return .visitChildren
+            }
+
+            let asyncToken = node.signature.effectSpecifiers?.asyncSpecifier
+            functionScopes.push(.init(asyncToken: asyncToken))
+
+            return .visitChildren
+        }
+
+        override func visitPost(_ node: InitializerDeclSyntax) {
+            if node.body != nil {
+                checkViolation()
+            }
+        }
+
+        override func visitPost(_: TypeAliasDeclSyntax) {
+            pendingAsync = nil
+        }
+
+        override func visitPost(_ node: ForStmtSyntax) {
+            if node.awaitKeyword != nil {
+                functionScopes.modifyLast {
+                    $0.containsAwait = true
+                }
+            }
+        }
+
+        override func visitPost(_ node: VariableDeclSyntax) {
+            if node.bindingSpecifier.tokenKind == .keyword(.let),
+                node.modifiers.contains(keyword: .async) {
+                functionScopes.modifyLast {
+                    $0.containsAwait = true
+                }
+            }
+        }
+
+        override func visit(_: FunctionParameterSyntax) -> SyntaxVisitorContinueKind {
+            .skipChildren
+        }
+
+        override func visitPost(_: ReturnClauseSyntax) {
+            pendingAsync = nil
+        }
+
+        private func checkViolation() {
+            guard
+                let info = functionScopes.pop(),
+                let asyncToken = info.asyncToken,
+                !info.containsAwait
+            else {
+                return
+            }
+
+            violations.append(
+                at: asyncToken.positionAfterSkippingLeadingTrivia,
+                correction: .init(
+                    start: asyncToken.positionAfterSkippingLeadingTrivia,
+                    end: asyncToken.endPosition,
+                    replacement: ""
+                )
+            )
+        }
+    }
+}