Extend MacroExpansionContext with a build-configuration property

The (optional) build configuration provides information about the
context in which the macro-generated code will be compiled. It allows
one to inspect various aspects of the configuration, including
available attributes and features, the target
architecture/OS/environment/etc., and any custom settings passed via
`-D` on the command line.

Author: DougGregor

Package.swift

@@ -275,7 +275,13 @@ let package = Package(
 
     .target(
       name: "SwiftSyntaxMacros",
-      dependencies: ["SwiftDiagnostics", "SwiftParser", "SwiftSyntax", "SwiftSyntaxBuilder"],
+      dependencies: [
+        "SwiftDiagnostics",
+        "SwiftIfConfig",
+        "SwiftParser",
+        "SwiftSyntax",
+        "SwiftSyntaxBuilder",
+      ],
       exclude: ["CMakeLists.txt"]
     ),
 

Sources/SwiftCompilerPluginMessageHandling/CMakeLists.txt

@@ -25,6 +25,7 @@ target_link_swift_syntax_libraries(SwiftCompilerPluginMessageHandling PUBLIC
   SwiftSyntax
   SwiftBasicFormat
   SwiftDiagnostics
+  SwiftIfConfig
   SwiftParser
   SwiftSyntaxMacros
   SwiftSyntaxMacroExpansion

Sources/SwiftCompilerPluginMessageHandling/CompilerPluginMessageHandler.swift

@@ -21,8 +21,10 @@ import _SwiftSyntaxCShims
 #endif
 
 #if compiler(>=6)
+internal import SwiftIfConfig
 public import SwiftSyntaxMacros
 #else
+import SwiftIfConfig
 import SwiftSyntaxMacros
 #endif
 
@@ -189,12 +191,25 @@ public class PluginProviderMessageHandler<Provider: PluginProvider>: PluginMessa
       let macroRole,
       let discriminator,
       let expandingSyntax,
-      let lexicalContext
+      let lexicalContext,
+      let staticBuildConfigurationString
     ):
+      // Decode the static build configuration.
+      let staticBuildConfiguration: StaticBuildConfiguration?
+      if let staticBuildConfigurationString {
+        var mutableConfigurationString = staticBuildConfigurationString
+        staticBuildConfiguration = mutableConfigurationString.withUTF8 {
+          try? JSON.decode(StaticBuildConfiguration.self, from: $0)
+        }
+      } else {
+        staticBuildConfiguration = nil
+      }
+
       return expandFreestandingMacro(
         macro: macro,
         macroRole: macroRole,
         discriminator: discriminator,
+        staticBuildConfiguration: staticBuildConfiguration,
         expandingSyntax: expandingSyntax,
         lexicalContext: lexicalContext
       )
@@ -208,12 +223,25 @@ public class PluginProviderMessageHandler<Provider: PluginProvider>: PluginMessa
       let parentDeclSyntax,
       let extendedTypeSyntax,
       let conformanceListSyntax,
-      let lexicalContext
+      let lexicalContext,
+      let staticBuildConfigurationString
     ):
+      // Decode the static build configuration.
+      let staticBuildConfiguration: StaticBuildConfiguration?
+      if let staticBuildConfigurationString {
+        var mutableConfigurationString = staticBuildConfigurationString
+        staticBuildConfiguration = mutableConfigurationString.withUTF8 {
+          try? JSON.decode(StaticBuildConfiguration.self, from: $0)
+        }
+      } else {
+        staticBuildConfiguration = nil
+      }
+
       return expandAttachedMacro(
         macro: macro,
         macroRole: macroRole,
         discriminator: discriminator,
+        staticBuildConfiguration: staticBuildConfiguration,
         attributeSyntax: attributeSyntax,
         declSyntax: declSyntax,
         parentDeclSyntax: parentDeclSyntax,

Sources/SwiftCompilerPluginMessageHandling/Macros.swift

@@ -13,13 +13,15 @@
 #if compiler(>=6)
 internal import SwiftBasicFormat
 internal import SwiftDiagnostics
+internal import SwiftIfConfig
 internal import SwiftOperators
 internal import SwiftSyntax
 @_spi(MacroExpansion) @_spi(ExperimentalLanguageFeature) internal import SwiftSyntaxMacroExpansion
 @_spi(ExperimentalLanguageFeature) internal import SwiftSyntaxMacros
 #else
 import SwiftBasicFormat
 import SwiftDiagnostics
+import SwiftIfConfig
 import SwiftOperators
 import SwiftSyntax
 @_spi(MacroExpansion) @_spi(ExperimentalLanguageFeature) import SwiftSyntaxMacroExpansion
@@ -53,6 +55,7 @@ extension PluginProviderMessageHandler {
     macro: PluginMessage.MacroReference,
     macroRole pluginMacroRole: PluginMessage.MacroRole?,
     discriminator: String,
+    staticBuildConfiguration: StaticBuildConfiguration?,
     expandingSyntax: PluginMessage.Syntax,
     lexicalContext: [PluginMessage.Syntax]?
   ) -> PluginToHostMessage {
@@ -67,7 +70,8 @@ extension PluginProviderMessageHandler {
         operatorTable: .standardOperators,
         fallbackSyntax: syntax
       ),
-      expansionDiscriminator: discriminator
+      expansionDiscriminator: discriminator,
+      staticBuildConfiguration: staticBuildConfiguration
     )
 
     let expandedSource: String?
@@ -113,6 +117,7 @@ extension PluginProviderMessageHandler {
     macro: PluginMessage.MacroReference,
     macroRole: PluginMessage.MacroRole,
     discriminator: String,
+    staticBuildConfiguration: StaticBuildConfiguration?,
     attributeSyntax: PluginMessage.Syntax,
     declSyntax: PluginMessage.Syntax,
     parentDeclSyntax: PluginMessage.Syntax?,
@@ -143,7 +148,8 @@ extension PluginProviderMessageHandler {
         operatorTable: .standardOperators,
         fallbackSyntax: declarationNode
       ),
-      expansionDiscriminator: discriminator
+      expansionDiscriminator: discriminator,
+      staticBuildConfiguration: staticBuildConfiguration
     )
 
     // TODO: Make this a 'String?' and remove non-'hasExpandMacroResult' branches.

Sources/SwiftCompilerPluginMessageHandling/PluginMacroExpansionContext.swift

@@ -12,12 +12,14 @@
 
 #if compiler(>=6)
 internal import SwiftDiagnostics
+internal import SwiftIfConfig
 internal import SwiftOperators
 internal import SwiftParser
 internal import SwiftSyntax
 internal import SwiftSyntaxMacros
 #else
 import SwiftDiagnostics
+import SwiftIfConfig
 import SwiftOperators
 import SwiftParser
 import SwiftSyntax
@@ -262,6 +264,10 @@ class PluginMacroExpansionContext {
   /// to produce unique names.
   private var expansionDiscriminator: String
 
+  /// The static build configuration, if any, that will be used for the
+  /// macro-expanded code.
+  private var staticBuildConfiguration: StaticBuildConfiguration?
+
   /// Counter for each of the uniqued names.
   ///
   /// Used in conjunction with `expansionDiscriminator`.
@@ -271,10 +277,16 @@ class PluginMacroExpansionContext {
   /// macro.
   internal private(set) var diagnostics: [Diagnostic] = []
 
-  init(sourceManager: SourceManager, lexicalContext: [Syntax], expansionDiscriminator: String = "") {
+  init(
+    sourceManager: SourceManager,
+    lexicalContext: [Syntax],
+    expansionDiscriminator: String = "",
+    staticBuildConfiguration: StaticBuildConfiguration?
+  ) {
     self.sourceManager = sourceManager
     self.lexicalContext = lexicalContext
     self.expansionDiscriminator = expansionDiscriminator
+    self.staticBuildConfiguration = staticBuildConfiguration
   }
 }
 
@@ -321,4 +333,8 @@ extension PluginMacroExpansionContext: MacroExpansionContext {
     }
     return AbstractSourceLocation(location)
   }
+
+  public var buildConfiguration: (any BuildConfiguration)? {
+    staticBuildConfiguration
+  }
 }

Sources/SwiftCompilerPluginMessageHandling/PluginMessages.swift

@@ -25,7 +25,8 @@ public enum HostToPluginMessage: Codable {
     macroRole: PluginMessage.MacroRole? = nil,
     discriminator: String,
     syntax: PluginMessage.Syntax,
-    lexicalContext: [PluginMessage.Syntax]? = nil
+    lexicalContext: [PluginMessage.Syntax]? = nil,
+    staticBuildConfiguration: String? = nil
   )
 
   /// Expand an '@attached' macro.
@@ -38,7 +39,8 @@ public enum HostToPluginMessage: Codable {
     parentDeclSyntax: PluginMessage.Syntax?,
     extendedTypeSyntax: PluginMessage.Syntax?,
     conformanceListSyntax: PluginMessage.Syntax?,
-    lexicalContext: [PluginMessage.Syntax]? = nil
+    lexicalContext: [PluginMessage.Syntax]? = nil,
+    staticBuildConfiguration: String? = nil
   )
 
   /// Optionally implemented message to load a dynamic link library.

Sources/SwiftSyntaxMacroExpansion/BasicMacroExpansionContext.swift

@@ -12,11 +12,13 @@
 
 #if compiler(>=6)
 public import SwiftDiagnostics
+public import SwiftIfConfig
 internal import SwiftOperators
 public import SwiftSyntax
 public import SwiftSyntaxMacros
 #else
 import SwiftDiagnostics
+import SwiftIfConfig
 import SwiftOperators
 import SwiftSyntax
 import SwiftSyntaxMacros
@@ -62,6 +64,9 @@ public class BasicMacroExpansionContext {
     ///
     /// Used in conjunction with `expansionDiscriminator`.
     var uniqueNames: [String: Int] = [:]
+
+    /// The build configuration that will be applied to the expanded code.
+    var buildConfiguration: (any BuildConfiguration)? = nil
   }
 
   /// State shared by different instances of the macro expansion context,
@@ -82,12 +87,14 @@ public class BasicMacroExpansionContext {
   public init(
     lexicalContext: [Syntax] = [],
     expansionDiscriminator: String = "__macro_local_",
-    sourceFiles: [SourceFileSyntax: KnownSourceFile] = [:]
+    sourceFiles: [SourceFileSyntax: KnownSourceFile] = [:],
+    buildConfiguration: (any BuildConfiguration)? = nil
   ) {
     self.sharedState = SharedState()
     self.lexicalContext = lexicalContext
     self.expansionDiscriminator = expansionDiscriminator
     self.sharedState.sourceFiles = sourceFiles
+    self.sharedState.buildConfiguration = buildConfiguration
   }
 
   /// Create a new macro evaluation context that shares most of its global

Sources/SwiftSyntaxMacros/CMakeLists.txt

@@ -29,5 +29,6 @@ add_swift_syntax_library(SwiftSyntaxMacros
 )
 
 target_link_swift_syntax_libraries(SwiftSyntaxMacros PUBLIC
+  SwiftIfConfig
   SwiftSyntaxBuilder
   )

Sources/SwiftSyntaxMacros/MacroExpansionContext.swift

@@ -12,10 +12,12 @@
 
 #if compiler(>=6)
 public import SwiftDiagnostics
+public import SwiftIfConfig
 public import SwiftSyntax
 import SwiftSyntaxBuilder
 #else
 import SwiftDiagnostics
+import SwiftIfConfig
 import SwiftSyntax
 import SwiftSyntaxBuilder
 #endif
@@ -70,6 +72,13 @@ public protocol MacroExpansionContext: AnyObject {
   /// This array can be empty if there is no context, for example when a
   /// freestanding macro is used at file scope.
   var lexicalContext: [Syntax] { get }
+
+  /// Returns the build configuration that will be used with the generated
+  /// source code. Macro implementations can use this information to determine
+  /// the context into which they are generating code.
+  ///
+  /// When the build configuration is not known, this returns nil.
+  var buildConfiguration: (any BuildConfiguration)? { get }
 }
 
 extension MacroExpansionContext {
@@ -187,3 +196,8 @@ public enum SourceLocationFilePathMode {
   /// e.g., `/home/taylor/alison.swift`.
   case filePath
 }
+
+extension MacroExpansionContext {
+  /// Default implementation that produces no build configuration.
+  public var buildConfiguration: (any BuildConfiguration)? { nil }
+}