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

Author: mateusrodriguesxyz

Sources/SwiftSyntaxMacroExpansion/MacroExpansion.swift

@@ -527,6 +527,14 @@ public func collapse<Node: SyntaxProtocol>(
   default:
     break
   }
-
-  return expansions.joined(separator: separator)
+  // Join the expansions with the given separator between them.
+  return expansions.reduce("") { (partialResult, expansion) in
+    // We don't add the separator before the first expansion or
+    // if the expansion already starts with the separator.
+    if (partialResult.isEmpty || expansion.hasPrefix(separator)) {
+      partialResult + expansion
+    } else {
+      partialResult + separator + expansion
+    }
+  }
 }