Support body macros that don’t return any statements

Previously, when a body macro returned no statements, we would return an empty expansion and not add the braces in `collapse`. Fix that.

Author: ahoppen

Sources/SwiftSyntaxMacroExpansion/MacroExpansion.swift

@@ -463,15 +463,14 @@ public func collapse<Node: SyntaxProtocol>(
   attachedTo declarationNode: Node,
   indentationWidth: Trivia? = nil
 ) -> String {
-  if expansions.isEmpty {
-    return ""
-  }
-
   var expansions = expansions
   var separator = "\n\n"
 
   // Wrap the expansions in a set of braces.
   func wrapInBraces() {
+    if expansions.isEmpty {
+      expansions = [""]
+    }
     // Default to 4 spaces if no indentation was passed.
     // In the future, we could consider inferring the indentation width from
     // the expansions to collapse.

Tests/SwiftSyntaxMacroExpansionTest/BodyMacroTests.swift

@@ -125,4 +125,32 @@ final class BodyMacroTests: XCTestCase {
       indentationWidth: indentationWidth
     )
   }
+
+  func testEmptyBodyMacro() {
+    struct EmptyBodyMacro: BodyMacro {
+      public static var formatMode: FormatMode { .disabled }
+
+      public static func expansion(
+        of node: AttributeSyntax,
+        providingBodyFor declaration: some DeclSyntaxProtocol & WithOptionalCodeBlockSyntax,
+        in context: some MacroExpansionContext
+      ) throws -> [CodeBlockItemSyntax] {
+        return []
+      }
+    }
+
+    assertMacroExpansion(
+      """
+      @EmptyBody
+      func f() {}
+      """,
+      expandedSource: """
+        func f() {
+
+        }
+        """,
+      diagnostics: [],
+      macros: ["EmptyBody": EmptyBodyMacro.self]
+    )
+  }
 }