diff --git a/src/slic3r/GUI/format.hpp b/src/slic3r/GUI/format.hpp
index 51beafff1..15a3f04c3 100644
--- a/src/slic3r/GUI/format.hpp
+++ b/src/slic3r/GUI/format.hpp
@@ -7,12 +7,35 @@
 // though C++20 format uses a different template pattern for position independent parameters.
 // This wrapper also manages implicit conversion from wxString to UTF8 and format_wxstr() variants are provided to format into wxString.
 
-#include <libslic3r/format.hpp>
-
 #include <wx/string.h>
 
-namespace Slic3r { 
-namespace GUI { 
+namespace Slic3r::internal::format {
+	// Wrapper around wxScopedCharBuffer to indicate that the content is UTF8 formatted.
+	struct utf8_buffer {
+		// wxScopedCharBuffer is reference counted, therefore copying by value is cheap.
+		wxScopedCharBuffer data;
+	};
+	inline std::ostream& operator<<(std::ostream& os, const utf8_buffer &v) {
+		os << v.data.data();
+		return os;
+	}
+	// Accept wxString and convert it to UTF8 to be processed by Slic3r::format().
+	inline const utf8_buffer cook(const wxString& arg) {
+		return utf8_buffer{ arg.ToUTF8() };
+	}
+	// Vojtech seemingly does not understand perfect forwarding:
+	// Why Slic3r::internal::format::cook(T&& arg) is taken for non-const wxString reference?
+	inline const utf8_buffer cook(wxString& arg) {
+		return utf8_buffer{ arg.ToUTF8() };
+	}
+	inline const utf8_buffer cook(wxString&& arg) {
+		return utf8_buffer{ arg.ToUTF8() };
+	}
+}
+
+#include <libslic3r/format.hpp>
+
+namespace Slic3r::GUI {
 
 // Format input mixing UTF8 encoded strings (const char*, std::string) and wxStrings, return a wxString.
 template<typename... TArgs>
@@ -42,42 +65,6 @@ inline std::string format(const wxString& fmt, TArgs&&... args) {
     return Slic3r::format(fmt.ToUTF8().data(), std::forward<TArgs>(args)...);
 }
 
-} // namespace GUI
-
-namespace internal {
-	namespace format {
-		// Wrapper around wxScopedCharBuffer to indicate that the content is UTF8 formatted.
-		struct utf8_buffer { 
-			// wxScopedCharBuffer is reference counted, therefore copying by value is cheap.
-			wxScopedCharBuffer data;
-		};
-		// Accept wxString and convert it to UTF8 to be processed by Slic3r::format().
-		inline const utf8_buffer cook(const wxString &arg) {
-			return utf8_buffer { arg.ToUTF8() };
-		}
-		// Vojtech seemingly does not understand perfect forwarding:
-		// Why Slic3r::internal::format::cook(T&& arg) is taken for non-const wxString reference?
-		inline const utf8_buffer cook(wxString &arg) {
-			return utf8_buffer { arg.ToUTF8() };
-		}
-		inline const utf8_buffer cook(wxString &&arg) {
-			return utf8_buffer{ arg.ToUTF8() };
-		}
-	}
-}
-
-} // namespace Slic3r
-
-namespace boost {
-	namespace io {
-		namespace detail {
-			// Adaptor for boost::format to accept wxString converted to UTF8.
-			inline std::ostream& operator<<(std::ostream& os, const Slic3r::internal::format::utf8_buffer& str) {
-				os << str.data.data();
-				return os;
-			}
-		}
-	}
-}
+} // namespace Slic3r::GUI
 
 #endif /* slic3r_GUI_format_hpp_ */