Replace joined(separator:) with custom logic in collapse to avoid adding unnecessary separators

Author: mateusrodriguesxyz

Sources/SwiftSyntaxMacroExpansion/MacroExpansion.swift

@@ -527,6 +527,16 @@ public func collapse<Node: SyntaxProtocol>(
   default:
     break
   }
-
-  return expansions.joined(separator: separator)
+    
+  // Join the expansions ensuring `separator` between them.
+  var collapsed = ""
+  for expansion in expansions {
+    if (collapsed.isEmpty || expansion.hasPrefix(separator)) {
+      collapsed.append(expansion)
+    } else {
+      collapsed.append(separator + expansion)
+    }
+  }
+  
+  return collapsed
 }