Pass nil as the default indentationWidth

Author: ahoppen

Sources/SwiftBasicFormat/BasicFormat.swift

@@ -49,11 +49,14 @@ open class BasicFormat: SyntaxRewriter {
   private var previousToken: TokenSyntax? = nil
 
   public init(
-    indentationWidth: Trivia = .spaces(4),
+    indentationWidth: Trivia? = nil,
     initialIndentation: Trivia = [],
     viewMode: SyntaxTreeViewMode = .sourceAccurate
   ) {
-    self.indentationWidth = indentationWidth
+    // Default to 4 spaces if no indentation was passed.
+    // In the future, we could consider inferring the indentation width from the
+    // source file to format in case it is already partially formatted.
+    self.indentationWidth = indentationWidth ?? .spaces(4)
     self.indentationStack = [(indentation: initialIndentation, isUserDefined: false)]
     super.init(viewMode: viewMode)
   }

Sources/SwiftSyntaxMacroExpansion/MacroExpansion.swift

@@ -92,7 +92,7 @@ public func expandFreestandingMacro(
   macroRole: MacroRole,
   node: FreestandingMacroExpansionSyntax,
   in context: some MacroExpansionContext,
-  indentationWidth: Trivia = .spaces(4)
+  indentationWidth: Trivia? = nil
 ) -> String? {
   do {
     let expandedSyntax: Syntax
@@ -192,7 +192,7 @@ public func expandAttachedMacroWithoutCollapsing<Context: MacroExpansionContext>
   extendedType: TypeSyntax?,
   conformanceList: InheritedTypeListSyntax?,
   in context: Context,
-  indentationWidth: Trivia = .spaces(4)
+  indentationWidth: Trivia? = nil
 ) -> [String]? {
   do {
     switch (definition, macroRole) {
@@ -320,7 +320,7 @@ public func expandAttachedMacro<Context: MacroExpansionContext>(
   extendedType: TypeSyntax?,
   conformanceList: InheritedTypeListSyntax?,
   in context: Context,
-  indentationWidth: Trivia = .spaces(4)
+  indentationWidth: Trivia? = nil
 ) -> String? {
   let expandedSources = expandAttachedMacroWithoutCollapsing(
     definition: definition,
@@ -341,7 +341,7 @@ public func expandAttachedMacro<Context: MacroExpansionContext>(
 fileprivate extension SyntaxProtocol {
   /// Perform a format if required and then trim any leading/trailing
   /// whitespace.
-  func formattedExpansion(_ mode: FormatMode, indentationWidth: Trivia) -> String {
+  func formattedExpansion(_ mode: FormatMode, indentationWidth: Trivia?) -> String {
     let formatted: Syntax
     switch mode {
     case .auto:
@@ -396,7 +396,7 @@ public func collapse<Node: SyntaxProtocol>(
   expansions: [String],
   for role: MacroRole,
   attachedTo declarationNode: Node,
-  indentationWidth: Trivia = .spaces(4)
+  indentationWidth: Trivia? = nil
 ) -> String {
   if expansions.isEmpty {
     return ""
@@ -421,7 +421,10 @@ public func collapse<Node: SyntaxProtocol>(
       onDeclarationWithoutAccessor = false
     }
     if onDeclarationWithoutAccessor {
-      expansions = expansions.map({ $0.indented(by: indentationWidth) })
+      // Default to 4 spaces if no indentation was passed.
+      // In the future, we could consider inferring the indentation width from
+      // the expansions to collapse.
+      expansions = expansions.map({ $0.indented(by: indentationWidth ?? .spaces(4)) })
       expansions[0] = "{\n" + expansions[0]
       expansions[expansions.count - 1] += "\n}"
     }

Sources/SwiftSyntaxMacroExpansion/MacroSystem.swift

@@ -23,7 +23,7 @@ extension SyntaxProtocol {
   public func expand(
     macros: [String: Macro.Type],
     in context: some MacroExpansionContext,
-    indentationWidth: Trivia = .spaces(4)
+    indentationWidth: Trivia? = nil
   ) -> Syntax {
     // Build the macro system.
     var system = MacroSystem()
@@ -463,11 +463,14 @@ private class MacroApplication<Context: MacroExpansionContext>: SyntaxRewriter {
   init(
     macroSystem: MacroSystem,
     context: Context,
-    indentationWidth: Trivia
+    indentationWidth: Trivia?
   ) {
     self.macroSystem = macroSystem
     self.context = context
-    self.indentationWidth = indentationWidth
+    // Default to 4 spaces if no indentation was passed.
+    // In the future, we could consider inferring the indentation width from the
+    // source file in which we expand the macros.
+    self.indentationWidth = indentationWidth ?? .spaces(4)
     super.init(viewMode: .sourceAccurate)
   }