Add the TracingMacros module and the @Traced macro

This is defined as a separate product so that users who don't want to
pay the compile-time cost of macros don't have to use it, you opt-in to
that cost by depending on the TracingMacros module.

The @Traced macro is only available in Swift 6.0 compilers since
function body macros were introduced in Swift 6.0.

This adds minimum OS versions for Apple platforms in order to be able to
depend on SwiftSyntax. This applies to the whole package since there are
no per-target platform specifications. Most notably: This raises the
macOS minimum deployment target from the (implicit) 10.13 to 10.15.

Author: porglezomp

Package.swift

@@ -1,14 +1,24 @@
 // swift-tools-version:5.9
 import PackageDescription
+import CompilerPluginSupport
 
 let package = Package(
     name: "swift-distributed-tracing",
+    platforms: [
+        .macOS(.v10_15),
+        .iOS(.v13),
+        .tvOS(.v13),
+        .watchOS(.v6),
+        .macCatalyst(.v13),
+    ],
     products: [
         .library(name: "Instrumentation", targets: ["Instrumentation"]),
         .library(name: "Tracing", targets: ["Tracing"]),
+        .library(name: "TracingMacros", targets: ["TracingMacros"]),
     ],
     dependencies: [
-        .package(url: "https://github.com/apple/swift-service-context.git", from: "1.1.0")
+        .package(url: "https://github.com/apple/swift-service-context.git", from: "1.1.0"),
+        .package(url: "https://github.com/swiftlang/swift-syntax.git", from: "600.0.0-latest"),
     ],
     targets: [
         // ==== --------------------------------------------------------------------------------------------------------
@@ -43,5 +53,32 @@ let package = Package(
                 .target(name: "Tracing")
             ]
         ),
+
+        // ==== --------------------------------------------------------------------------------------------------------
+        // MARK: TracingMacros
+
+        .target(
+            name: "TracingMacros",
+            dependencies: [
+                .target(name: "Tracing"),
+                .target(name: "TracingMacrosImplementation"),
+            ]
+        ),
+        .macro(
+            name: "TracingMacrosImplementation",
+            dependencies: [
+                .product(name: "SwiftSyntaxMacros", package: "swift-syntax"),
+                .product(name: "SwiftCompilerPlugin", package: "swift-syntax"),
+            ]
+        ),
+        .testTarget(
+            name: "TracingMacrosTests",
+            dependencies: [
+                .target(name: "Tracing"),
+                .target(name: "TracingMacros"),
+                .target(name: "TracingMacrosImplementation"),
+                .product(name: "SwiftSyntaxMacrosTestSupport", package: "swift-syntax"),
+            ]
+        ),
     ]
 )

Sources/TracingMacros/TracedMacro.swift

@@ -0,0 +1,25 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift Distributed Tracing open source project
+//
+// Copyright (c) 2020-2024 Apple Inc. and the Swift Distributed Tracing project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of Swift Distributed Tracing project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+@_exported import ServiceContextModule
+import Tracing
+
+#if compiler(>=6.0)
+@attached(body)
+public macro Traced(
+    _ operationName: String? = nil,
+    context: ServiceContext? = nil,
+    ofKind kind: SpanKind? = nil,
+    span spanName: String? = nil
+) = #externalMacro(module: "TracingMacrosImplementation", type: "TracedMacro")
+#endif

Sources/TracingMacrosImplementation/TracedMacro.swift

@@ -0,0 +1,50 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift Distributed Tracing open source project
+//
+// Copyright (c) 2020-2024 Apple Inc. and the Swift Distributed Tracing project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of Swift Distributed Tracing project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+import SwiftCompilerPlugin
+import SwiftSyntax
+import SwiftSyntaxBuilder
+import SwiftSyntaxMacros
+
+#if compiler(>=6.0)
+public struct TracedMacro: BodyMacro {
+    public static func expansion(
+        of node: AttributeSyntax,
+        providingBodyFor declaration: some DeclSyntaxProtocol & WithOptionalCodeBlockSyntax,
+        in context: some MacroExpansionContext
+    ) throws -> [CodeBlockItemSyntax] {
+        guard let function = declaration.as(FunctionDeclSyntax.self),
+              let body = function.body
+        else {
+            throw MacroExpansionErrorMessage("expected a function with a body")
+        }
+
+        let operationName = StringLiteralExprSyntax(content: function.name.text)
+        let withSpanCall: ExprSyntax = "withSpan(\(operationName))"
+        let withSpanExpr: ExprSyntax = "\(withSpanCall) { span in \(body.statements) }"
+
+        return ["\(withSpanExpr)"]
+    }
+}
+#endif
+
+@main
+struct TracingMacroPlugin: CompilerPlugin {
+#if compiler(>=6.0)
+    let providingMacros: [Macro.Type] = [
+        TracedMacro.self,
+    ]
+#else
+    let providingMacros: [Macro.Type] = []
+#endif
+}

Tests/TracingMacrosTests/TracedTests.swift

@@ -0,0 +1,91 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift Distributed Tracing open source project
+//
+// Copyright (c) 2020-2024 Apple Inc. and the Swift Distributed Tracing project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of Swift Distributed Tracing project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+import XCTest
+import SwiftSyntaxMacrosTestSupport
+
+import Tracing
+import TracingMacros
+import TracingMacrosImplementation
+
+#if compiler(>=6.0)
+
+final class TracedMacroTests: XCTestCase {
+    func test_tracedMacro_requires_body() {
+        assertMacroExpansion(
+            """
+            @Traced
+            func funcWithoutBody()
+            """,
+            expandedSource: """
+            func funcWithoutBody()
+            """,
+            diagnostics: [
+                .init(message: "expected a function with a body", line: 1, column: 1),
+            ],
+            macros: ["Traced": TracedMacro.self]
+        )
+    }
+
+    func test_tracedMacro_sync_nothrow() {
+        assertMacroExpansion(
+            """
+            @Traced
+            func syncNonthrowingExample(param: Int) {
+                print(param)
+            }
+            """,
+            expandedSource: """
+            func syncNonthrowingExample(param: Int) {
+                withSpan("syncNonthrowingExample") { span in
+                    print(param)
+                }
+            }
+            """,
+            macros: ["Traced": TracedMacro.self]
+        )
+    }
+
+    func test_tracedMacro_accessSpan() {
+        assertMacroExpansion(
+            """
+            @Traced
+            func example(param: Int) {
+                span.attributes["param"] = param
+            }
+            """,
+            expandedSource: """
+            func example(param: Int) {
+                withSpan("example") { span in
+                    span.attributes["param"] = param
+                }
+            }
+            """,
+            macros: ["Traced": TracedMacro.self]
+        )
+    }
+}
+
+// MARK: Compile tests
+
+@Traced
+func syncNonthrowingExample(param: Int) {
+    print(param)
+}
+
+@Traced
+func example(param: Int) {
+    span.attributes["param"] = param
+}
+
+#endif